CONGRESS OF THE UNITED STATES

CONGRESSIONAL BUDGET OFFICE

A

CBO

STUDY

JULY 2004

Alternatives for 

Boost-Phase

Missile Defense

The Congress of the United States O Congressional Budget Office

CBO

Alternatives for Boost-Phase

Missile Defense

July 2004

A

STUDY
Notes

Numbers in the text and tables of this report may not add up to totals because of rounding.

Unless otherwise indicated, all dollar figures are in 2004 dollars.

The cover shows an intercontinental ballistic missile (a U.S. Minuteman III being test 

launched from Vandenberg Air Force Base in California) in the first seconds of the boost 

phase of its flight. (Photo by the U.S. Air Force)
Preface

A prominent part of the Bush Administration’s strategy for national defense is developing and fielding defenses against ballistic missiles. To that end, the Department of Defense’s Missile Defense Agency (MDA) is pursuing a layered defense composed of various

systems capable of intercepting ballistic missiles at different points in their flight. For the past 

several years, work has primarily focused on intercepting long-range missiles during their midcourse phase (after their booster rockets have burned out but before their warheads have reentered the atmosphere). MDA plans to field initial elements of a midcourse system this year. 

Recently, the agency also began an effort to develop interceptors capable of hitting intercontinental ballistic missiles during their boost phase (the first few minutes after launch, before 

their booster rockets burn out). 

This Congressional Budget Office (CBO) study—prepared at the request of the Subcommittee on Emerging Threats and Capabilities of the Senate Armed Services Committee—looks at 

technical, operational, and cost issues related to using a boost-phase intercept (BPI) system to 

defend the United States against intercontinental ballistic missiles. The study compares the 

strengths, weaknesses, and costs of five alternative designs for a BPI system—three surfacebased and two space-based—that span a range of performance characteristics. In keeping with 

CBO’s mandate to provide objective, impartial analysis, this study makes no recommendations. 

David Arthur and Robie Samanta Roy of CBO’s National Security Division wrote the study 

under the general supervision of J. Michael Gilmore. Raymond Hall of CBO’s Budget Analysis Division prepared the cost estimates and wrote Appendix A under the general supervision 

of Jo Ann Vines. Adrienne Ramsay and Robert Schingler helped review the manuscript for 

factual accuracy. Eric Wang, Barbara Edwards, David Moore, Elizabeth Robinson, and Christopher Williams provided thoughtful comments on early drafts of the study, as did Maile 

Smith of the Institute for Defense Analyses and Professor Daniel Hastings of the Massachusetts Institute of Technology. (The assistance of external participants implies no responsibility 

for the final product, which rests solely with CBO.)

Christian Spoor and Leah Mazade edited the study, and Christine Bogusz proofread it. 

Maureen Costantino produced the cover and graphics and prepared the study for publication. 

Lenny Skutnik printed the initial copies, and Annette Kalicki prepared the electronic versions 

for CBO’s Web site (www.cbo.gov).

Douglas Holtz-Eakin

Director

July 2004

Summary xi

1 Ballistic Missile Defenses and Threats 1

A Brief History of U.S. Ballistic Missile Defenses 1

Layered Defenses and Boost-Phase Intercept 3

Characteristics of ICBMs Important for BPI 4

Representative Threats for Comparing BPI Systems 5

2 Performance Needed for an

Operationally Effective BPI System 7

Commit Time, Interceptor Speed, and Location 9

Sensors and Battle Management Components 10

Interceptors 12

3 Alternative Designs for BPI Systems 21

Assumptions About Sensors and

Battle Management Components 21

Designs for Interceptors 22

Other Design Considerations for a BPI System 27

The BPI Options Examined in This Analysis 29

4 Comparison of BPI Options 31

Costs 31

Areas of the World Covered 33

Capability and Costs to Counter 

Solid-Fuel ICBMs 34

Reliance on Access to Foreign Basing 35

Vulnerability of the BPI System

to Attack 35

Ability to Counter Increasing Numbers

of ICBMs 35

Strategic Responsiveness 36

CONTENTS
vi ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

A CBO’s Estimates of the Costs of Alternative BPI Systems 39

B Kinetic Kill Vehicles for Boost-Phase Interceptors 51

Glossary of Abbreviations 53
CONTENTS vii

Tables

S-1. Funding for the Missile Defense Agency’s Ballistic Missile 

Defense System Interceptors Program, 2004 to 2009 xi

S-2. Characteristics, Cost, and Effectiveness of Alternative Systems 

for Boost-Phase Intercept xvii

1-1. Implications of Intercepting Ballistic Missiles During Different 

Phases of Their Flight 3

1-2. Funding for the Missile Defense Agency’s Ballistic Missile 

Defense System Interceptors Program, 2004 to 2009 4

1-3. Nations with Long-Range Ballistic Missiles 5

1-4. Characteristics of North Korean and Iranian Ballistic Missiles 

and of Representative ICBMs 6

2-1. Characteristics of Radar and Infrared Sensors 10

3-1. Characteristics of Various Current or Proposed Kill Vehicles 24

3-2. Potential Trade-Offs in Designing Surface-Based Boost-Phase 

Interceptors 25

3-3. Comparison of CBO’s Options for a Surface-Based BPI System 28

3-4. Comparison of CBO’s Options for a Space-Based BPI System 29

4-1. Summary of Costs for Boost-Phase Intercept Systems 33

4-2. Effects on CBO’s Options of Targeting Solid-Fuel ICBMs 34

A-1. Estimated Costs of Surface-Based BPI Systems 40

A-2. Estimated Costs of Space-Based BPI Systems 42

A-3. Estimated Costs of the First Production Units for Surface-Based 

BPI Systems 44

A-4. Estimated Costs of the First Production Units for Space-Based 

BPI Systems 45

Figures

S-1. Trajectory of a Notional ICBM x

S-2. Number of Surface-Based BPI Sites Needed for Full Coverage of 

Iran, by Commit Time and Interceptor Speed xiii

S-3. Characteristics of SBI System Needed for Full Coverage of North 

Korea and Iran Against a Single Liquid-Fuel ICBM xiv

S-4. Characteristics of SBI System Needed for Full Coverage of North 
viii ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Korea and Iran Against a Single Solid-Fuel ICBM xv

2-1. Series of Events in a Boost-Phase Intercept 8

2-2. Interceptor Reach Versus Commit Time for Interceptor Speeds 

Between 6 km/sec and 10 km/sec 9

2-3. Time Needed for a BPI Sensor to Detect a Notional Liquid-Fuel 

ICBM 11

2-4. Interceptor Reach Needed Against a Notional Solid-Fuel ICBM,

by ICBM Trajectory 12

2-5. Interceptor Reach Needed Against a Solid-Fuel ICBM Launched 

from Iran or North Korea 13

2-6. Impact of Commit Time and Interceptor Speed on BPI Coverage 

of Iran from a Site in Iraq 14

2-7. Coverage of Iran Possible from BPI Sites in Different Locations 15

2-8. Number of Surface-Based BPI Sites Needed for Full Coverage 

of Iran, by Commit Time and Interceptor Speed 16

2-9. Impact of Commit Time and Interceptor Speed on BPI Coverage 

of North Korea from a Site in the Sea of Japan 17

2-10. Ground Track of a Satellite in a 45-Degree Inclined Low-Earth Orbit 18

2-11. Number of SBIs Needed for Full Coverage of North Korea and Iran, 

by Commit Time and Interceptor Speed 19

3-1. Effect of Interceptor Speed and Payload Mass on the Mass of a

Surface-Based Interceptor 23

3-2. Effect of Interceptor Speed and Payload Mass on the Mass of a

Space-Based Interceptor 26

3-3. Total Mass of SBI Constellation Needed to Counter Liquid-Fuel 

ICBMs, by Interceptor Speed 27

3-4. Probability of a Successful Intercept, by Number of Shots Taken 27

4-1. Location of Surface-Based BPI Sites Needed to Defend Against 

Liquid-Fuel ICBMs in CBO’s Options 36

4-2. Size of Surface-Based BPI Interceptors Relative to the Cargo Bay

of a C-17 Aircraft 37

Box

4-1. The Role of Uncertainty in CBO’s Estimates of the Costs 

of Boost-Phase Intercept Systems 32
Summary

Developing and fielding defenses against ballistic 

missiles are significant priorities of the Department of 

Defense (DoD) under the current Administration. Missile defenses are considered important protection against 

a growing threat that rogue nations might obtain weapons of mass destruction and use ballistic missiles to fire 

them at U.S. forces abroad, U.S. allies, or the United 

States itself. DoD’s Missile Defense Agency (MDA) is 

pursuing a layered strategy for ballistic missile defense, in 

which different defensive systems would target threat 

missiles at different phases in their flight. The premise is 

that even if technological limitations prevented any single 

layer from offering the desired level of protection, multiple layers could together provide an effective defense. 

In the case of intercontinental ballistic missiles (ICBMs) 

that might be launched at the United States, MDA has 

focused on developing a system to intercept ICBM warheads in the midcourse phase of their flight. It plans to 

field such a system in late 2004. Last year, MDA also began an effort to develop defenses that would intercept 

ICBMs during their boost phase—the first few minutes 

of flight. The agency’s current plans for such a boostphase intercept (BPI) system focus on building mobile, 

surface-based defenses that would operate from sites on 

land or on ships, but it also plans to investigate spacebased systems.1

The Congressional Budget Office (CBO) estimates that 

a BPI system that could counter liquid-fuel ICBMs 

launched from anywhere in both North Korea and Iran 

—the representative threats used in this analysis—would 

cost between $16 billion and $37 billion (in 2004 dollars) to develop, produce, and operate for 20 years if the 

system used surface-based interceptors. If the interceptors 

were based on orbiting satellites, those costs would range 

from $27 billion to $78 billion, CBO estimates. If the 

system was scaled back to defend against missiles from 

only one of those countries, costs would be lower. Conversely, if the system needed to counter solid-fuel missiles—which have a shorter boost phase than liquid-fuel 

missiles do—costs could rise. 

DoD has not articulated the specific capabilities required 

for a BPI system. This analysis looks at how various levels 

of system performance translate into different levels of 

operational effectiveness. On the basis of those relationships, this study defines five alternative BPI systems—

three surface-based and two space-based—that might be 

fielded to defend the United States against attack by 

ICBMs and compares their operational strengths and 

weaknesses as well as their costs.

The alternative systems span a range of capability, from 

the current state-of-the-art performance to moreadvanced performance characteristics that have been proposed but not yet developed. Comparing the potential effectiveness of each option illustrates the operational benefits that might be realized by assuming greater technical 

risk in pursuit of higher system performance. Of course, 

the value of better performance depends on the kinds of 

threats to be countered. This analysis is not based on a 

specific prediction about future threats. Rather, it assesses 

operational effectiveness against a range of potential 

threats that reflects the uncertainty in how hostile countries’ ballistic missile capabilities might evolve in coming 

years.

Ballistic Missiles and 

Boost-Phase Intercept

Ballistic missiles consist of a warhead and a guidance system mounted on a rocket motor, or booster. When an 

ICBM is launched, the rocket propels it up and out of the 

atmosphere at a high speed (typically 6 to 7 kilometers 

per second). Once the missile is out of the atmosphere

1. MDA is also developing another BPI system, called the Airborne 

Laser. Although that system may have some capability against 

ICBMs, it is primarily intended to counter shorter-range ballistic 

missiles, which are not the focus of this study.
x ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Summary Figure 1.

Trajectory of a Notional ICBM

(Altitude in kilometers)

Source: Congressional Budget Office.

Note: ICBM = intercontinental ballistic missile.

and its rocket motor has shut down, the booster is jettisoned. The warhead then travels an unpowered, ballistic 

trajectory to its target (much like a basketball’s trajectory 

to the hoop after it leaves a player’s fingertips).

Ballistic missile trajectories are typically divided into 

three phases. The first is the boost phase, which lasts 

from the missile’s launch until the rocket booster shuts 

down, or “burns out” (see Summary Figure 1). For 

ICBMs, which typically have two- or three-stage boosters, the boost phase lasts until the final stage burns out. 

The second, or midcourse, phase lasts from the end of 

boost until the warhead reenters the atmosphere. (The 

early portion of the midcourse phase, after the booster 

burns out but before the warhead is deployed, is sometimes called the ascent or early-ascent phase.) The final, 

or terminal, phase lasts from reentry until the warhead 

reaches its target.

The objective of midcourse and terminal defenses is to hit 

and destroy the warhead. In the case of boost-phase 

defenses, however, destroying or damaging the booster 

alone may prevent the warhead from achieving enough 

velocity to reach its target. Of course, destroying the warhead is preferable because a warhead that falls short of its 

intended target may still cause serious damage elsewhere.2

An intercept occurring in the ascent phase will need to 

destroy the warhead or prevent the warhead from separating from the booster.

Boost-phase intercept is an attractive missile defense alternative because, during boost, a ballistic missile is comparatively easy to detect and track. The hot plume put 

out by the booster rocket presents a large target for infrared sensors, and the rocket body itself is larger and more 

visible to infrared sensors and radar than is the much 

smaller warhead that remains once the booster is jettisoned. In addition, countermeasures on a missile, such as 

decoys designed to distract defensive systems, are more 

difficult to deploy before the booster burns out.

This year, the Missile Defense Agency is spending $118 

million, or 1.5 percent of its budget, on developing kinetic-energy boost-phase interceptors to counter ballistic 

missiles, including ICBMs. Those interceptors would 

employ the “hit to kill” concept, using precise homing to 

fly a “kill vehicle” into a target (akin to a bullet hitting a 

bullet). Because of the velocities involved, such collisions 

can release far more kinetic energy than the chemical energy of a similarly sized explosive warhead. Under MDA’s 

five-year budget plan, annual funding for that effort 

would grow to $2.2 billion by 2009 (in 2004 dollars), or 

28 percent of the agency’s projected budget in that year 

(see Summary Table 1). Most of that money would go toward building mobile, surface-based systems, but a total 

of about $700 million over the next five years would be 

devoted to investigating the feasibility of a space-based 

BPI system.

The challenge of boost-phase intercept is the very short 

time available for the engagement: typically four to five 

minutes in the case of liquid-fuel ICBMs and three minutes or less in the case of solid-fuel ICBMs, which accel1,600

1,400

1,200

1,000

800

600

400

200

0

0 5,000 10,000 15,000

Burnout

(180-320 sec.)

Apogee

(Less than 1,000 sec.)

Impact

(2,000 sec. or more)

Terminal

Boost

Kilometers from Launch Site

Midcourse

Midcourse

Ascent

2. Contrary to popular impression, a ballistic missile intercepted in 

its boost phase usually will not fall on the country that launched 

it. However, if the boost-phase interceptor cannot reliably destroy 

the warhead, uncertainty about how long the booster is supposed 

to burn and when the warhead is supposed to separate from the 

booster can make it difficult to ensure that the warhead does not 

fall on a location short of its target but still within the United 

States or another friendly country.
SUMMARY xi

Summary Table 1.

Funding for the Missile Defense Agency’s Ballistic Missile Defense System 

Interceptors Program, 2004 to 2009

(Billions of 2004 dollars)

Source: Congressional Budget Office based on a briefing by staff of the Missile Defense Agency, March 4, 2004.

erate faster.3 The time available for intercept—coupled 

with the distance that an interceptor must travel to reach 

its target, which results from the geography of a particular 

scenario—determines the response time and interceptor 

speed needed for a BPI system.

In general, more-technologically advanced ICBMs and 

larger threat countries require higher-performance surface-based BPI systems. More-advanced ICBMs usually 

have shorter burn times (that is, boost phases). And 

against a larger country, interceptors will have to fly farther to reach ICBMs launched from deep inside the 

country’s borders. The performance requirements of a 

system can be eased by locating BPI sites in the general 

path that a threat ICBM would fly to reach the United 

States, because such locations would improve the geometry of the engagement.

The feasibility of boost-phase intercept is a subject of 

wide disagreement. That disagreement stems in part from 

differing views of what would constitute an operationally 

useful system. Some people argue that the potential proliferation of more-challenging targets, such as solid-fuel 

ICBMs with short burn times, necessitates a very capable—and thus technically challenging and expensive—

BPI system. In their view, a less-capable BPI system that 

could not counter such threats might be obsolete before it 

was fielded. That position was emphasized in a 2003 report by the American Physical Society (APS).4 Others argue that solid-fuel ICBMs are far in the future for any 

country other than a highly developed one. In that view, 

less-capable BPI systems would be useful to counter 

longer-burning liquid-fuel ICBMs, which might proliferate in the meantime.

This study does not make assumptions about a specific 

future threat or about how an adversary might react to 

U.S. deployment of boost-phase missile defenses. Instead, 

it considers what capability would be provided—and at 

what cost—by several alternative BPI systems against 

both liquid- and solid-fuel ICBMs under a variety of engagement conditions.

Operational Effectiveness 

of BPI Systems

A BPI engagement can be conceptually divided into two 

stages. The first is the commit stage, which lasts from 

when the threat missile is launched until a boost-phase 

interceptor is fired. During the commit stage, the system 

must detect its target, track it, and decide to commit an 

2004 2005 2006 2007 2008 2009

Total,

2004-2009

Budget for BMDS Interceptors 

Program 0.1 0.5 1.1 1.6 2.0 2.2 7.6

Total MDA Budget 7.6 9.0 8.3 9.6 7.9 7.9 50.3

Memorandum:

BMDS Interceptors as a 

Percentage of MDA’s Budget 1.5 5.6 12.9 16.8 25.5 27.8 15.2

3. Those times are typical ones available for intercept. Actual boost 

times will vary depending on the characteristics of a particular 

ICBM. More time may be available if the BPI system can engage 

the ICBM in its early-ascent phase. Conversely, less time may be 

available if the system has to hit the booster before the ICBM has 

reached top speed. Recognizing those considerations, this report 

for simplicity refers to intercepts as occurring “at booster burnout.”

4. Report of the American Physical Society Study Group on Boost-Phase 

Intercept Systems for National Missile Defense: Scientific and Technical Issues (Washington, D.C.: American Physical Society, July 

2003), available at www.aps.org/public_affairs/popa/reports/

nmd03.cfm.
xii ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

interceptor to an engagement. The second stage is the flyout stage, which lasts from when the interceptor is 

launched until it reaches (and, if all goes well, destroys) 

its target.

Dividing the overall engagement time differently between 

the commit and flyout stages could yield different BPI 

systems with similar overall effectiveness. For example, 

designers could compensate for a system that took longer 

to commit by producing faster interceptors, or they could 

make up for slower interceptors by speeding up a system’s 

commit time. Alternatively, the total time available for an 

intercept might be extended by incorporating the capability to hit a missile in its early-ascent phase. 

CBO evaluated those trade-offs in performance by comparing the number of BPI launch locations that would be 

needed to provide a complete defensive layer (full coverage) against a threat country under different combinations of commit times and interceptor speeds. A BPI system that provided full coverage of a threat country would 

be able to engage an ICBM launched at any target in the 

United States from anywhere inside that country. Full 

coverage would also mean that a BPI system could not be 

easily circumvented by mobile ICBM launchers. The 

number of launch locations needed for such a system can 

be used to compare the cost and operational complexity 

of particular BPI designs.

In its analysis, CBO assessed the capability of potential 

BPI systems against both liquid- and solid-fuel ICBMs. 

The representative liquid-fuel ICBM used in this analysis 

has a similar performance—about a five-minute burn 

time—to that of the 1960s-era U.S. Titan II or Russian 

SS-12 missile. The representative solid-fuel ICBM—with 

about a three-minute burn time—is comparable to the 

current U.S. Minuteman III or Russian SS-25 missile. 

Those representative times are slightly longer than the 

ones assumed in much of the 2003 APS report (about 

four minutes for liquid-fuel ICBMs and 2.8 minutes for 

solid-fuel ICBMs). Although those five-minute and 

three-minute values are booster burn times, there is uncertainty about whether an intercept would always occur 

when a booster was burning, and CBO’s analysis implicitly incorporates that uncertainty. For example, the results 

for engaging a missile within a five-minute burn time 

would be similar (to a reasonable approximation) to the 

results for engaging a missile within a four-minute burn 

time plus a one-minute early-ascent phase.5

The size and location of potential threat countries play a 

role in determining the effectiveness of a BPI system by 

determining the distance that an interceptor must fly to 

reach its target. Thus, comparisons of alternative systems 

must be made in the context of specific potential threats. 

This study evaluates the effectiveness of different BPI systems against a representative threat: ICBMs fired at the 

United States from Iran and North Korea. Those countries were chosen because their geographic characteristics 

pose challenges to different kinds of BPI systems and because they are known to be pursuing long-range-missile 

capability.

Surface-Based BPI Systems

Depending on their commit time and interceptor speed, 

the range of surface-based BPI systems that CBO analyzed would need two to four launch sites to provide full 

coverage against a liquid-fuel ICBM fired from Iran (see 

Summary Figure 2).6

 If two launch sites were needed, 

they could be located in Iraq to the east and Afghanistan 

to the west. If additional sites were necessary, they might 

be located in Turkmenistan to the north and on a ship in 

the Persian Gulf or the Gulf of Oman to the south.

CBO evaluated a range of commit times from zero to 90 

seconds. A zero commit time—which means that an interceptor takes off at precisely the same moment as the 

ICBM it is targeting—is an ideal case that could not be 

achieved in practice. CBO included it to provide a lower 

limit. The upper end of the range, 90 seconds, represents 

a long delay in committing, which places stringent demands on the interceptor portion of a BPI system. 

In addition, CBO looked at interceptor speeds ranging 

from 6 kilometers per second to 10 km/sec (under the assumption that interceptors would accelerate to those top 

speeds in 60 seconds). The defensive coverage provided 

by a BPI interceptor decreases dramatically below the 

low-end speed of 6 km/sec. The main reason is that

5. Although similar, the results would differ because an ICBM with a 

four-minute burn time would have a different acceleration profile 

than that of an ICBM with a five-minute burn time. However, 

that difference is smaller than the uncertainty about what the burn 

time of future threat ICBMs might be.

6. At a minimum, a surface BPI site would consist of some number 

of interceptors and their launchers plus communications equipment for linking the site to sensor information from other parts of 

the missile defense system and for communicating guidance information to the interceptor while it was in flight.
SUMMARY xiii

Summary Figure 2.

Number of Surface-Based BPI Sites 

Needed for Full Coverage of Iran, 

by Commit Time and Interceptor Speed

Source: Congressional Budget Office.

Notes: The figures assume that interceptors have a burn time of

60 seconds.

* = full coverage not possible; BPI = boost-phase intercept; 

ICBM = intercontinental ballistic missile; km/sec = kilometers per second.

ICBMs reach speeds of 6 to 7 km/sec as they near burnout, and an interceptor moving significantly slower than 

its target can hit that target only under a narrow set of engagement conditions. Interceptors faster than the highend speed of 10 km/sec are too large to make their deployment to a theater practical.

In the case of CBO’s representative solid-fuel ICBM, 

with its shorter burn time, the trade-offs inherent in designing an effective BPI system are more constrained. In 

general, because less time is available to reach the target, 

more BPI sites are needed so interceptors can have a 

shorter flyout distance. However, under several of the 

combinations of system characteristics that CBO examined, full coverage would not be possible against a solidfuel ICBM launched from Iran. For example, at no speed 

between 6 and 10 km/sec could interceptors engage such 

an ICBM if the system’s commit time was 90 seconds (see 

Summary Figure 2). Even with a 60-second commit time, 

a system with 10-km/sec interceptors would require four 

sites to fully cover Iran, and one with 8-km/sec interceptors would need seven sites.7 A system with 6-km/sec interceptors would require a commit time of less than 30 

seconds as well as a large number of sites to cover Iran. 

North Korea is much smaller than Iran, so it would pose 

fewer difficulties for a surface-based BPI system. CBO’s 

analysis indicates that a single site with 6-km/sec interceptors located on a ship in the Sea of Japan could defend 

against a liquid-fuel ICBM launched from anywhere in 

North Korea, even with commit times as long as 90 seconds. Against a solid-fuel ICBM, the commit time would 

need to be 30 seconds (in the case of 6-km/sec interceptors) or 60 seconds (in the case of 8-km/sec interceptors) 

for that site to provide full coverage.

Space-Based BPI Systems

The factors that determine coverage for a space-based BPI 

system are different, although commit time and interceptor speed still play a role. A space-based system would 

most likely be a constellation of interceptor satellites located in low-Earth orbit at an altitude of about 250 km 

to 300 km. Higher orbits would require greater launch 

costs and be farther from the intended targets. At lower 

orbits, satellites would have shorter life spans because of

None 30 sec. 60 sec. 90 sec.

0

1

2

3

4

5

6

7

8

6 km/sec

8 km/sec

10 km/sec

Interceptor Speed

Commit Time

Against a Liquid-Fuel ICBM

None 30 sec. 60 sec. 90 sec.

0

1

2

3

4

5

6

7

8

6 km/sec

8 km/sec

10 km/sec

Interceptor Speed

* *

Commit Time

* * *

Against a Solid-Fuel ICBM

7. Although it used slightly different assumptions, the APS study 

also concluded that interceptors with velocities of around 10 km/

sec might be able to counter Iranian solid-fuel ICBMs.
xiv ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Summary Figure 3.

Characteristics of SBI System Needed 

for Full Coverage of North Korea and 

Iran Against a Single Liquid-Fuel ICBM

Source: Congressional Budget Office.

Notes: The figures are based on using a two-shot salvo against a 

single liquid-fuel ICBM. They assume that interceptors have 

an acceleration of 10g and that kill vehicles have a mass of 

30 kilograms.

SBI = space-based interceptor; ICBM = intercontinental 

ballistic missile; km/sec = kilometers per second.

atmospheric drag. Satellites in inclined low-Earth orbits 

are not fixed over one spot but instead follow a sinusoidal 

ground track as they move over the Earth. Thus, providing full coverage of a specific threat country requires having a constellation of space-based interceptors (SBIs) with 

their orbits positioned such that at least one SBI is capable of reaching the threat at any given time. 

That orbital reality is at the root of both the main advantage and disadvantage of space basing for boost-phase intercept. On the positive side, space basing can provide 

BPI access to any point on Earth, including the interiors 

of very large countries that could never be reached with a 

surface-based interceptor launched from an adjacent 

country. On the negative side, although SBI constellations can be tailored to focus on specific latitude bands, 

they cannot be concentrated against individual countries.

The number of space-based interceptors needed to cover 

a threat country depends on the performance of the system (which determines the coverage area, or footprint, of 

each satellite) and the latitude of the country. Covering 

higher latitudes generally requires more satellites because 

their orbits must traverse more of the Earth’s surface, 

forcing the satellites to spend more time over locations 

other than the threat country.

Depending on commit time and interceptor speed, a constellation capable of providing full coverage against both 

North Korean and Iranian liquid-fuel ICBMs would require anywhere from about 130 to 1,800 interceptors (see 

Summary Figure 3). Those results assume that two interceptors would be available to be fired at each ICBM to 

increase the probability of intercept—a tactic consistent 

with MDA’s plans for its BPI system.8 (The analysis of 

surface-based systems above did not specify two-shot 

salvo tactics because the results of that analysis were presented in terms of number of launch locations, each of 

which could have one or more interceptors available.) 

Additional SBIs would be needed if the threat country 

launched more than one ICBM almost simultaneously.

CBO examined the same range of commit times for a 

space-based system as it did for a surface-based system because both types of systems would probably rely on a similar infrastructure of missile defense sensors. However, 

CBO used a lower range of interceptor speeds in the SBI

None 30 sec. 60 sec. 90 sec.

0

500

1,000

1,500

2,000

2 km/sec

4 km/sec

6 km/sec

Commit Time

Interceptor Speed

Number of SBIs Needed

None 30 sec. 60 sec. 90 sec.

0

50

100

150

200

250

2 km/sec

4 km/sec

6 km/sec

Interceptor Speed

Commit Time

Mass of SBI Constellation Needed

(Thousands of kilograms)

8. “KEI Contractors Borrow from Other MDA Programs to Meet 

Schedule,” Inside the Pentagon, April 15, 2004, p. 1.
SUMMARY xv

Summary Figure 4.

Characteristics of SBI System Needed 

for Full Coverage of North Korea and 

Iran Against a Single Solid-Fuel ICBM

Source: Congressional Budget Office.

Notes: The figures are based on using a two-shot salvo against a 

single solid-fuel ICBM. They assume that interceptors have 

an acceleration of 10g and that kill vehicles have a mass of 

30 kilograms.

SBI = space-based interceptor; ICBM = intercontinental 

ballistic missile; km/sec = kilometers per second.

a. Actual number of SBIs is 10,909.

b. Actual mass is 1,208,000 kilograms.

analysis—2 km/sec to 6 km/sec—to minimize the total 

mass of the hardware that would have to be launched into 

orbit, an important consideration because of launch 

costs. A smaller number of faster SBIs does not necessarily result in a lower total mass than a larger number of 

slower SBIs. Whether it would be more cost-effective to 

deploy a lighter constellation with many moderateperformance SBIs or a heavier constellation with fewer 

higher-performance SBIs would depend on the relative 

production and launch costs.

Providing full coverage against solid-fuel ICBMs requires 

a larger number of SBIs than is the case against liquidfuel ICBMs. The shorter burn time of solid-fuel ICBMs 

results in a smaller effective footprint for each SBI. To 

maintain coverage, the separation between interceptors 

must shrink, which means that the size of the constellation must increase. Constellations as large as 5,000 SBIs 

(with a 60-second commit time and 2-km/sec interceptors) or as heavy as about 1,000 metric tons (with a 90-

second commit time and 6-km/sec interceptors) might be 

necessary (see Summary Figure 4). On the basis of mass 

alone, a constellation of 1,000 metric tons would require 

more than 40 launches on Delta-IV (Heavy) or Atlas-V 

(Heavy) rockets, the largest expendable launch vehicles 

being planned.

The BPI Options Examined 

in This Analysis

For this study, CBO developed five alternative systems—

three surface-based and two space-based—to compare the 

potential effectiveness and cost of different approaches to 

boost-phase intercept by kinetic-energy interceptors. 

Each system has the following capabilities:

B The ability to counter a representative liquid-fuel ICBM 

from North Korea or Iran. Although neither country is 

known to possess ICBM-class ballistic missiles today, 

both are developing long-range missiles based on 

liquid-fuel technology. North Korea has tested (with 

only partial success) the Taep’o-dong 1, a missile 

thought to have a range of 5,000 km. Iran is believed 

to be working on extended-range versions of its Shahab-series ballistic missiles (although they appear to 

remain short of intercontinental range). In addition, 

both North Korea and Iran have expressed their intention to develop space-launch systems. Such systems 

would be suitable for use as ICBMs.

None 30 sec. 60 sec. 90 sec.

0

1,000

2,000

3,000

4,000

5,000

6,000

2 km/sec

4 km/sec

6 km/sec

Interceptor Speed

Commit Time

Number of SBIs Needed

a

None 30 sec. 60 sec. 90 sec.

0

200

400

600

800

1,000

2 km/sec

4 km/sec

6 km/sec

Interceptor Speed

Mass of SBI Constellation Needed

(Thousands of kilograms)

b

Commit Time
xvi ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

B Coverage of all possible ICBM launch locations in North 

Korea and Iran. The ability to cover all locations in a 

country means that a BPI system could not be easily 

circumvented by mobile ICBM launchers. A system 

able to fully cover Iran and North Korea would be 

highly capable against most other potential threat 

countries employing similar ICBM technology. Iran 

poses a particular challenge for a surface-based BPI 

system because it is the 16th largest country by area. 

(Many of the larger countries are considered unlikely 

to prove a threat to the United States.) North Korea 

poses a particular challenge for a space-based system 

because its high latitude necessitates a large constellation of interceptors. A space-based system with orbits 

capable of covering North Korea could cover about 75 

percent of the world’s countries and about 90 percent 

of those that might now be considered potential 

threats. 

The first step in boost-phase intercept is detecting the 

launch of a threat missile. CBO’s alternative systems are 

assumed to all use the same set of sensors to detect 

launches—a sensor architecture based on the one that 

DoD is planning to deploy to support missile defense and 

other requirements. For BPI, the important sensors are 

the system of space-based infrared satellites (called 

SBIRS-High) now being developed. CBO assumed that 

such space-based sensors could allow for BPI commit 

times on the order of 60 seconds (on the basis of the APS 

study’s assessment of the potential performance of notional satellite sensors employing current technology).9

Sensors such as surface or airborne radar could be effective against small countries like North Korea. However, 

because of horizon limitations, an ICBM launched from 

the interior of a large country such as Iran might not be 

visible to surface or airborne sensors for more than two 

minutes—resulting in too long a commit time for BPI.

Because the sensor architecture is common to all of the 

alternatives, it is not explicitly included in the comparison below. Nonetheless, the analysis assumes that SBIRSHigh or a system with similar capability would be fielded 

in time to support these illustrative BPI systems.

Option 1: 6-km/sec Surface-Based Interceptors

Option 1 is representative of a system that might be developed if getting defenses into the field as soon as possible was a critical consideration. This system is similar in 

booster performance to the one MDA is developing.10

Like the other alternatives, Option 1 provides the capability to handle liquid-fuel ICBMs fired from a large fraction of potential threat countries (see Summary Table 2). 

Although its interceptors have high performance—a 

speed of 6 km/sec and peak acceleration of around 22g 

(22 times the Earth’s gravitational pull)—they should not 

require great advances in booster technology. Likewise, 

the 140-kilogram (kg) kill vehicle should not require 

technically challenging miniaturization. Moreover, the 

interceptors, which weigh just over 3,000 kg each, can 

be transported by air.

Option 2: 8-km/sec Surface-Based Interceptors

Option 2 fields faster interceptors than Option 1. A 

lighter kill vehicle enables the interceptors to fly faster 

without a dramatic increase in booster size. Option 2 assumes that additional research and development will take 

place to produce a significantly lighter kill vehicle. Because of that additional research, Option 2’s BPI system 

might take longer to develop and field than Option 1’s, 

for a given rate of investment spending.

Option 2 is representative of a system that might be developed if solid-fuel ICBMs were considered a likely 

threat or if reducing the number of sites needed to cover 

large threat countries was an important consideration. 

Additionally, with their higher speed, Option 2’s interceptors might be suitable for use in midcourse defenses.

Option 3: 10-km/sec Surface-Based Interceptors

Option 3 represents a system designed primarily to counter solid-fuel ICBMs. It fields the fastest interceptors of 

any of the surface-based systems considered—10 km/

sec—because of the short engagement times available 

against solid-fuel ICBMs. These interceptors use the 

same lightweight kill vehicle as in Option 2, but they 

require a significantly larger and higher-performance 

booster to achieve their higher speed. As a result, they 

have nearly five times the mass of the interceptors in 

Option 2: over 17,000 kg each. The technical challenges

9. That study estimated that current space-based missile warning 

sensors (the Defense Support Program satellites) could also support BPI detection but with longer commit times.

10. Terry Little, “Kinetic Energy Interceptors Overiew” (unclassified 

briefing by the Missile Defense Agency to Congressional Budget 

Office staff, November 13, 2003).
SUMMARY xvii

Summary Table 2.

Characteristics, Cost, and Effectiveness of Alternative Systems 

for Boost-Phase Intercept

Source: Congressional Budget Office.

Note: ICBM = intercontinental ballistic missile; n.a. = not applicable.

a. From a country the size of Iran (in the case of Options 1 through 3) or countries between 25 and 45 degrees of both north and south latitude (in the case of Options 4 and 5).

of achieving the additional interceptor speed could result 

in a longer development time for this alternative than for 

Option 2, given the same rate of investment spending.

Options 4 and 5: Space-Based Interceptors

CBO’s last two alternatives consist of constellations of 

SBIs in low-Earth orbit. Both constellations are sized to 

defend against liquid-fuel ICBMs launched from locations between 25 degrees north latitude (southern Iran) 

and 45 degrees north latitude (northern North Korea). 

Both would also provide a two-shot engagement for increased probability of a successful intercept. 

Although the two systems offer similar effectiveness, they 

achieve that effectiveness with different combinations of 

interceptor performance and constellation size. In terms 

of interceptor performance, Option 4 is on the low end 

with a speed of 4 km/sec and peak acceleration of 15g. 

The interceptors in Option 5 have a speed of 6 km/sec 

and peak acceleration of 35g. Because of its faster interceptors, the constellation in Option 5 can be much 

smaller than the one in Option 4: 156 SBIs (with a total 

launch mass of about 83 metric tons) compared with 368 

satellites (and a total launch mass of 468 metric tons) for 

Option 4.11 Since the effectiveness of the BPI systems is 

similar at both interceptor speeds, the decision about 

Surface-Based Systems Space-Based Systems

Option 1 Option 2 Option 3 Option 4 Option 5

Technical Characteristics

Interceptor Speed at Burnout 

(Kilometers per second) 6 8 10 4 6

Interceptor Peak Acceleration (g) 22 27 39 15 35

Kill-Vehicle Mass (Kilograms) 140 30 30 140 30

Total Interceptor Mass (Kilograms) 3,088 3,469 17,160 847 442

Cost (Billions of 2004 dollars)

Research and Development 7-10 9-13 13-20 7-10 9-13

Initial Production 3-4 4-5 6-7 16-22 5-7

Operations Over 20 Years 6-10 6-10 6-10 33-46 13-20

Total 16-24 18-28 25-37 56-78 27-40

Operational Effectiveness

Able to Counter Liquid-Fuel ICBMs Yes Yes Yes Yes Yes

Able to Counter Solid-Fuel ICBMs

From a small country Yes Yes Yes No No

From a large country No Yes Yes No No

Additional Cost to Counter Solid-Fuel ICBMsa

(Billions of 2004 dollars) n.a. 0 0 107-146 30-40

Vulnerable to Denial of Basing Access Yes Yes Yes No No

Has Potential for Worldwide Coverage No No No Yes Yes

11. The combinations of system characteristics in those options are 

not among the ones shown in Summary Figure 3.
xviii ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

which option is more cost-effective could hinge on future 

launch costs.

An additional difference between the two space-based 

systems is the mass of each kill vehicle: 140 kg in Option 

4 and 30 kg in Option 5. As noted earlier, additional research and development would be needed to produce a 

kill vehicle as light as 30 kg, which means that the system 

in Option 5 might take longer to develop and field than 

the one in Option 4, given the same rate of investment 

spending.

Comparison of Capability 

Under Different Alternatives

All of the alternative BPI systems described above are able 

to provide full coverage against liquid-fuel ICBMs from 

Iran and North Korea, but they do not have identical capabilities. Each design has inherent advantages and disadvantages in such matters as cost, potential area of coverage, capability against solid-fuel ICBMs, dependence on 

access to foreign bases, vulnerability to being attacked or 

to exhausting their supply of interceptors, and strategic 

responsiveness. Not surprisingly, the greatest differences 

exist between the space-based systems and the surfacebased ones.

Costs

Developing and fielding the surface-based BPI system in 

Option 1 and then operating it for 20 years would cost a 

total of $16 billion to $24 billion (in 2004 dollars), CBO 

estimates. The system in Option 2 would cost $18 billion 

to $28 billion, and the one in Option 3 would cost $25 

billion to $37 billion (see Summary Table 2). Those 

ranges reflect the possibility of cost growth comparable 

to what similar defense programs have experienced in the 

past.

The cost estimates for the three surface-based systems assume procurement of equipment for 10 BPI launch sites. 

Each site would include six interceptors plus a set of communications and battle management equipment. Using 

two interceptor shots per engagement, each launch site 

could engage three targets with that allotment of interceptors. For those options, 10 sites’ worth of equipment 

would be enough to defend against liquid-fuel ICBMs 

fired from Iran and North Korea and also provide some 

equipment for use against other countries of concern. Additionally, the estimate for each surface-based system includes costs to purchase, operate, and maintain three 

cargo ships on which a BPI site could be located to provide sea-based capability (such as against North Korea). 

Of the space-based systems, the one in Option 5 would 

cost $27 billion to $40 billion (similar to the high-performance surface-based system in Option 3). The lowerspeed SBI system in Option 4 would cost more—$56 billion to $78 billion—because of higher costs per interceptor and higher launch costs to put more mass into orbit. 

The space-based options are more expensive than the surface-based options because they need more interceptors 

to cover Iran and North Korea at all times and because 

they require paying for launch services. The high operations costs for those options reflect the need to periodically buy and launch replacement SBIs. CBO assumes 

that each interceptor in orbit would have a life span of 

seven years—a typical length for satellites in low-Earth 

orbit—compared with at least 20 years for a surfacebased interceptor.

The costs of the various BPI systems would be lower if 

planners accepted less capability. For example, five surface 

sites could provide coverage just of Iran and North Korea 

(with nothing left over for testing or other coverage). The 

cost of that capability over 20 years could be as low as 

$14 billion to $21 billion for Option 1, $16 billion to 

$26 billion for Option 2, and $22 billion to $35 billion 

for Option 3, CBO estimates. Similarly, if the spacebased systems used a single shot against each ICBM, the 

cost of Option 4 could drop to $36 billion to $51 billion 

and the cost of Option 5 could decline to $20 billion to 

$31 billion. Although those less-capable systems would 

probably be considered insufficient as a stand-alone defense, they might be adequate as complements to other 

layers of a multilayer ballistic missile defense system.

Areas of the World Covered

The space-based systems in Options 4 and 5 would provide much greater global coverage than the surface-based 

systems would. Those SBI constellations could fully cover 

the surface of the Earth between 25 and 45 degrees of 

both north and south latitude—a total area of about 145 

million square kilometers. In principle, an SBI constellation could be designed to provide full global coverage, 

but at far greater cost than for Options 4 and 5. Of 

course, much of the area that would be covered in those 

options does not need coverage. Proponents of basing interceptors in space, however, argue that a space-based BPI 

capability offers a hedge against uncertainty about the 

identity and nature of future threats.
SUMMARY xix

The surface-based systems in Options 1 through 3, by 

contrast, would cover only the countries near which they 

were deployed. North Korea and Iran, for example, total 

an area of about 1.8 million square kilometers. Defending against liquid-fuel ICBMs launched from those countries would use five of the 10 sets of equipment purchased 

in Option 1 and three of the 10 purchased in Options 2 

and 3.

Capability and Cost to Counter Solid-Fuel ICBMs

The challenge to BPI systems will be greater if threat nations can develop or acquire more-advanced ICBMs, 

such as solid-fuel types, that have shorter boost phases. 

Against a representative solid-fuel ICBM, Option 1 

would provide coverage only of smaller countries and 

then only if favorable basing locations were available and 

commit times were short. For instance, Option 1 could 

cover relatively small North Korea only if commit times 

could be held to less than 45 seconds or if several launch 

sites could be located in China.

Greater interceptor speeds would give Options 2 and 3 

better performance against ICBMs with short burn times. 

CBO estimates that even with commit times longer than 

60 seconds, both options could provide full coverage 

from a single site in the Sea of Japan against a solid-fuel 

ICBM with a three-minute burn time fired from North 

Korea. Against larger countries, the greater interceptor 

speed of Option 3 has the potential to provide full coverage with fewer sites than in Option 2. With a 60-second 

commit time, Option 3 could cover Iran using four 

launch sites, whereas seven sites would be needed for Option 2. Costs would not rise for that additional capability, 

because the 10 sites’ worth of equipment purchased under the surface-based options would be adequate to provide coverage against solid-fuel ICBMs from both North 

Korea and Iran. However, fewer systems would be available for other scenarios.

The space-based interceptors used in Options 4 and 5 

could also provide coverage against solid-fuel ICBMs—

but only if the number of interceptors in the constellations more than tripled. Because the footprint of each 

SBI would be smaller against a target with a shorter burn 

time, Option 5 would need a constellation of 516 satellites, and Option 4 would require 1,308 satellites. Those 

extra SBIs would add about $30 billion to $40 billion to 

the cost of Option 5 and $107 billion to $146 billion to 

the cost of Option 4, CBO estimates.

Reliance on Access to Foreign Basing

The ability to position surface-based BPI systems where 

necessary will always be subject to geopolitical constraints. In general, the fewer launch sites a surface-based 

system needs to counter a threat, the less vulnerable it will 

be to other countries’ denial of access to basing sites. 

Thus, of the surface-based alternatives, Option 3 is the 

least vulnerable to denial of basing access because it is 

the most capable, requiring the fewest sites to counter a 

potential threat. Option 2 and Option 1 follow, in that 

order. 

A higher-speed surface-based system is generally less subject to basing constraints, but actual differences will depend on the specific scenario. In the case of North Korea, 

all three options would need only one launch site in international waters, so none would be vulnerable to denied 

access. In the case of Iran, Option 1’s additional vulnerability could be more significant than the simple need for 

additional sites. The 8-km/sec and 10-km/sec interceptors in Options 2 and 3, respectively, would provide coverage against liquid-fuel ICBMs from sites in Afghanistan 

and Iraq, where access to basing might be available (at 

least when this report was written). The additional sites 

that Option 1, with its 6-km/sec interceptors, would 

need would be in places such as Turkmenistan or Azerbaijan, where access is less assured.

By operating in space, the systems in Options 4 and 5 

would be free from the problem of access to basing. 

Vulnerability of the BPI System to Attack

Besides limitations on their location, another way in 

which BPI systems could be prevented from fulfilling 

their mission is if they were attacked by the threat country they were covering. CBO’s analysis assumed that all 

surface-based BPI systems would be located about 100 

km from the border of the threat country—out of range 

of artillery or unguided rockets. However, the sites might 

still require their own defenses against short-range missiles or attack aircraft from that country. (The costs of 

those defenses are not included in CBO’s cost estimates.) 

Space-based systems, for their part, would be potentially 

vulnerable to antisatellite weapons, should threat countries develop such weapons.

Ability to Defeat Increasing Numbers of ICBMs

If threat countries had multiple ICBMs—or decoys, such 

as first-stage rockets with dummy upper stages, that 

might draw interceptor fire—their simultaneous launch 
xx ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

could saturate the defenses of a BPI system. Additional 

interceptors could help counter that capability.

CBO’s options allow for six interceptors per surface site 

or two SBIs over a threat area at all times. Six surfacebased interceptors are enough to engage three targets, and 

two SBIs are enough to engage one target, under the assumption of two-shot salvo tactics. However, the number 

of those targets that could be engaged at the same time 

would depend on how many interceptors in flight a site 

could simultaneously guide.

The number of interceptors needed to engage additional 

ICBMs (or decoys) grows in proportion to the number of 

launch sites needed to defend against the first target. Options 2 and 3 would need the fewest additional interceptors because they would require the fewest sites. Options 

4 and 5 would require many additional space-based interceptors to defeat increasing numbers of ICBMs because 

additional interceptors would be needed throughout the 

entire constellation to counter multiple simultaneous 

launches. If the threat ICBMs were not launched simultaneously, however, other SBIs in orbit—which would 

move into position over the threat in about 10 minutes 

during the normal course of their orbits—could be fired.

Strategic Responsiveness

In terms of reacting quickly to a “bolt from the blue” type 

of attack, Options 4 and 5 would be the most responsive 

against threats that arose between 25 and 45 degrees of 

latitude. Once in orbit, their interceptors would always 

be deployed and on alert. In contrast, the surface-based 

systems would require time for interceptors to be deployed to forward locations on the perimeter of a threat 

country. However, if a threat arose outside the 25- to 45-

degree latitude bands, moving surface-based interceptors 

might be faster than expanding an SBI constellation by 

launching more satellites into orbit.

Among the surface-based systems, those in Options 1 and 

2 would be more responsive than the one in Option 3 because their interceptors could be deployed more easily by 

air. Even with a very light kill vehicle, a 10-km/sec interceptor would weigh more than 17,000 kg. Only one interceptor could be mounted on each launch vehicle that 

would be transportable by a C-17 aircraft, increasing the 

number of transport flights needed for each BPI site. In 

addition, interceptors of that size are best suited to basing 

in fixed silos (as is the case for the Ground-Based Midcourse Defense system that MDA plans to locate in 

Alaska and California). Although such silos are practical 

on home territory, permanent BPI installations in foreign 

countries would be likely to pose greater access problems 

than mobile BPI systems.

Sea-based BPI sites could be less responsive than landbased ones because ships have longer transit time than 

aircraft do. That difference could be lessened if the BPI 

ships were prepositioned in forward areas or if a landbased site could be flown in and temporarily placed on a 

local ship until the dedicated BPI ship arrived.
1

Ballistic Missile Defenses and Threats

Possible attack by ballistic missiles has been a concern to U.S. military planners ever since Germany struck 

Great Britain with V-2 rockets 60 years ago. Unlike powered cruise missiles (such as the Tomahawk) or winged 

glide missiles (such as the Joint Standoff Weapon), ballistic missiles are unpowered and unassisted by aerodynamic 

lift forces for most of their trajectory. Much as a fly ball in 

baseball is only under power while it is in contact with 

the bat, a pure ballistic missile is only under power while 

its booster burns at the beginning of its flight.1

 Nevertheless, intercontinental ballistic missiles (ICBMs) can travel 

more than 10,000 kilometers (6,000 miles).

Ballistic missiles’ flight can be divided into distinct 

phases:

B The boost phase, from launch until the missile’s 

booster burns out;

B The midcourse phase, as the warhead coasts toward its 

target;2

 and

B The terminal phase, the warhead’s final descent to its 

target.

In the past, U.S. work on missile defenses has focused 

mainly on the midcourse and terminal phases. But today, 

efforts to defend against ballistic missiles emphasize 

building layered defenses, with each layer targeting missiles in a different phase of their flight. Thus, interest has 

grown in developing systems to intercept ICBMs in the 

boost phase, during the first few minutes after they are 

launched.

A Brief History of U.S. Ballistic 

Missile Defenses

Efforts to defend against attacks by ballistic missile are 

nearly as old as ballistic missiles themselves. As early as 

1945, for example, the Army’s Project Thumper (which 

sought to develop a high-altitude defense against aircraft) 

examined how Allied forces could defend against Germany’s new V-2 rockets. (Project reports concluded that 

such defenses were beyond the capability of existing technology.) By the time the Soviet Union deployed its first 

intercontinental ballistic missiles in 1960, the Army, 

Navy, and Air Force were pursuing a variety of ballistic 

missile defense (BMD) programs. Much of that research 

was consolidated in Project Defender under the newly 

established Advanced Research Projects Agency. In 1962, 

President Kennedy assigned Project Defender to the 

highest category of national priorities for research and 

development.3

Throughout the 1960s and early 1970s, BMD efforts 

progressed amid debate about their technical feasibility, 

their affordability, and whether ballistic missile defense fit 

into the nation’s strategic defense posture. The Army’s 

first BMD program, called Nike-Zeus, went through sevCHAPTER

1. Some advanced ballistic missiles have delivery buses that provide 

additional maneuvering power later in the missile’s trajectory. 

However, that power is usually intended to fine-tune the warhead’s aim or to complicate missile defense efforts rather than to 

contribute substantially to the missile’s flight.

2. The initial portion of the midcourse phase, after the missile’s 

booster burns out but before the warhead is deployed, is sometimes considered a separate phase, known as ascent or early ascent. 

In early ascent, although the ICBM is no longer actually burning, 

the large, hot (relative to the warhead) booster body is still present 

for defensive systems to track.

3. The White House, National Security Action Memorandum No. 

191, October 1, 1962.
2 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

eral iterations: Nike-X, Sentinel, and finally Safeguard, a 

system that was briefly operational in 1976 to defend the 

ICBM fields at Grand Forks Air Force Base in North Dakota. That deployment complied with a 1974 protocol to 

the Anti-Ballistic Missile (ABM) Treaty, which limited 

ballistic missile defenses to a single fixed site for defending either the national capital or an ICBM field.

After the ABM treaty was ratified in 1972, the United 

States expended little effort on developing ballistic missile 

defenses until the early 1980s, when concern grew about 

a Soviet first-strike capability that might be able to attack 

both U.S. strategic forces and most metropolitan areas in 

the United States. In 1983, the Joint Chiefs of Staff recommended that the United States put more emphasis on 

strategic missile defense. Soon after, the Strategic Defense 

Initiative Organization was established to pursue an expanded program of missile defense research.

Earlier BMD systems such as Safeguard relied on nuclear 

warheads to destroy incoming ICBMs. By the early 

1980s, however, technological improvements in sensors, 

guidance systems, boosters, and command-and-control 

systems had revived interest in “hit to kill,” a concept that 

had been explored by Project Defender as early as 1960. 

A hit-to-kill interceptor is itself a missile. But instead of 

having an explosive warhead, it uses precise homing to fly 

a “kill vehicle” into a target (akin to a bullet hitting a bullet). Relative velocities at impact can reach many kilometers per second, which means that the kinetic energy of 

such a collision can be much greater than the chemical 

energy of a similarly sized explosive warhead. The feasibility of hit to kill was demonstrated in 1984 by the 

Army’s Homing Overlay Experiment when a kill vehicle 

launched from Kwajalein Atoll in the Pacific intercepted 

a dummy ICBM warhead launched from Vandenberg Air 

Force Base in California.

The Strategic Defense Initiative was originally focused on 

defending against a large-scale Soviet attack, so concepts 

for BMD systems included large numbers of interceptors. 

For example, scientists and engineers at Lawrence Livermore National Laboratory proposed a constellation of 

small, smart, space-based interceptors—called Brilliant 

Pebbles—that were intended to destroy target ICBMs 

during their boost phase. By attacking an ICBM in that 

phase, before its multiple independently targeted reentry 

vehicles could be deployed, a single Brilliant Pebbles interceptor could potentially destroy as many as 10 Soviet 

warheads. Although early concepts to defend against 

worst-case scenarios envisioned deploying as many as 

100,000 Brilliant Pebbles, later estimates were reduced to 

around 7,000.4

The end of the Cold War brought fundamental changes 

in the rationale for a strategic missile defense system. The 

focus shifted from countering a large-scale Soviet attack 

to two other objectives: defending the United States 

against accidental or limited ICBM strikes and defending 

deployed U.S. forces against attacks by theater (shorterrange) ballistic missiles. Some defense planners argued 

that limited strikes against the United States could be a 

threat if rogue elements in the former Soviet Union seized 

control of strategic nuclear systems or if ICBM technology proliferated to other hostile countries. The threat 

posed to deployed forces by theater ballistic missiles was 

demonstrated during Operation Desert Storm when an 

Iraqi Scud missile killed 28 soldiers in Al Khobar, Saudi 

Arabia.

In 1991, lawmakers enacted the Missile Defense Act, 

which defined the goal of deploying a system to defend 

against limited attacks by ballistic missiles while still complying with the ABM treaty. With that treaty’s restriction 

on developing defenses against ICBMs, and in the wake 

of the Scud attacks during Desert Storm, BMD efforts 

focused on developing theater-level defenses against missiles in their terminal phase. Those efforts resulted in systems such as Patriot Advanced Capability-3 (or PAC-3) 

and the Theater High-Altitude Air Defense (known as 

THAAD).5

 Those systems were permitted under a 1997 

agreement among the parties to the ABM treaty because 

they lacked the performance necessary to defeat longrange ballistic missiles.

By the mid-1990s, intelligence estimates of threats to the 

United States prompted greater interest in national missile defense (NMD). The Department of Defense announced a program in 1996 that called for three years of 

development of an NMD system and then—if the system’s components had been tested successfully and 

threats to the United States warranted its use—three 

more years to deploy an operational system. That system

4. D.R. Baucom, “The Rise and Fall of Brilliant Pebbles” (paper presented at the North Carolina First Flight Centennial Commission 

International Flight Symposium, “They Taught the World to Fly: 

The Wright Brothers and the Age of Flight,” October 23, 2001).

5. The “T” in THAAD has recently been changed from “Theater” to 

“Terminal.”
CHAPTER ONE BALLISTIC MISSILE DEFENSES AND THREATS 3

Table 1-1.

Implications of Intercepting Ballistic Missiles During Different Phases 

of Their Flight

Source: Congressional Budget Office.

as envisioned in the late 1990s would have included a 

new tracking radar and 20 midcourse interceptors based 

in Alaska, upgrades to existing missile defense radars, 

space-based sensors, and a command-and-control system. 

Officials recognized that pursuing that system would 

eventually require additional modifications to or withdrawal from the ABM treaty. However, in September 

2000, President Clinton decided against deploying an 

NMD system.

The Bush Administration subsequently withdrew the 

United States from the ABM treaty and broadened BMD 

efforts to develop and deploy integrated systems to defend against ballistic missiles of all ranges in all phases 

of flight. To meet that goal, the Missile Defense Agency 

(MDA) is working on a variety of sensors, weapons, and 

command-and-control infrastructure that will be integrated into a layered ballistic missile defense system 

(BMDS).

Layered Defenses and 

Boost-Phase Intercept

Defending against ballistic missile attacks is a challenging 

technical undertaking. In the case of ICBMs, a defensive 

system may need to hit a warhead smaller than an oil 

drum that is traveling above the atmosphere at speeds 

greater than 13,000 miles per hour. Countermeasures 

such as decoy warheads that may be carried by ICBMs 

further complicate the problem of intercepting targets. 

To achieve a high probability of destroying ballistic missiles in flight, MDA is pursuing a layered defensive approach. Each layer is designed to exploit the particular 

vulnerabilities and overcome the particular challenges 

that a ballistic missile presents during a phase of its flight: 

boost phase, midcourse phase, or terminal phase (see Table 1-1). Layered defenses are built on the premise that 

although technological limitations might keep any one 

layer from having an adequate chance of successfully inPhase Advantages Disadvantages

Boost Missile’s thermal signature is large

Booster is large physical target

One interceptor can destroy multiple warheads

Decoys are difficult to deploy

Time available for intercept is short (about three to

five minutes)

Interceptor must be positioned close to country from

which missile is launched

Rocket plume can obscure the missile’s body

Missile’s acceleration complicates the tracking

solution

Hitting the booster can leave a live warhead that falls

short of its target

Ascent/Early Ascent Missile is still large and hot

Extends the time available for intercept

One interceptor can destroy multiple warheads 

Missile is probably flying a predictable ballistic

trajectory

Warhead separation on the missile being targeted

may be very rapid

Interceptor must be positioned close to country from

which missile is launched

Interceptor must destroy warhead because warhead

has enough speed to reach its target

Midcourse Longest time is available for intercept

Missile is probably flying a predictable ballistic

trajectory

Defenses can be positioned in North America or on 

the oceans

Missile’s thermal signature is small, making it difficult 

to detect and track

Warhead is small physical target 

Decoys can dilute defenses

Terminal Most decoys are stripped away during atmospheric

reentry

Forward deployment is unnecessary

Time available for intercept is very short

Debris from the intercept may fall on defended

territory
4 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Table 1-2.

Funding for the Missile Defense Agency’s Ballistic Missile Defense System 

Interceptors Program, 2004 to 2009

(Billions of 2004 dollars)

Source: Congressional Budget Office based on a briefing by staff of the Missile Defense Agency, March 4, 2004.

tercepting its target, multiple layers could together provide an effective defense.

For the past several years, midcourse intercept has been 

the primary focus of efforts to defend the United States 

against attack by ICBMs. The Department of Defense 

plans to field the initial elements of a midcourse defense 

by the end of 2004, including the Ground-Based Midcourse Defense system and portions of the sea-based Aegis BMD system. Boost-phase and terminal-phase efforts 

—such as the Airborne Laser and THAAD, respectively—have focused more on shorter-range ballistic missiles, such as Scuds.

Recently, however, MDA initiated a new effort to develop 

hit-to-kill interceptors capable of engaging ICBMs in the 

boost phase. Interest in boost-phase intercept of ICBMs 

dates to the 1950s, when the Air Force’s Ballistic Missile 

Boost Intercept program looked at using a system of 

space-based interceptors to deploy large wire-mesh structures that would destroy ICBMs in the boost phase. The 

Brilliant Pebbles and Global Protection Against Limited 

Strikes programs of the late 1980s and early 1990s were 

also primarily envisioned as boost-phase intercept (BPI) 

systems. 

The previous focus on space-based BPI was necessitated 

by the large size of the Soviet Union. Only an orbiting 

platform would have access to the interior of that country, the area from which ICBMs would be launched. Because today’s concerns center around smaller countries, 

attention has shifted to terrestrial BPI systems—either 

land-based, sea-based, or airborne—that could be deployed to the borders of a nation considered a threat. 

Such systems have become a potentially attractive means 

of providing a boost-phase defense layer against ICBMs 

fired at the United States. In December 2003, MDA 

awarded a contract to Northrop Grumman to develop an 

initial (or Block 10) surface-based BPI system.

MDA’s five-year budget plan envisions that funding for 

kinetic-energy boost- and ascent-phase intercept systems—currently called BMDS Interceptors—will grow 

from $118 million in 2004 to $511 million in 2005 and 

reach nearly $2.2 billion by 2009 (see Table 1-2). Although the BMDS Interceptors program accounts for 

only about 6 percent of MDA’s proposed 2005 budget, 

that share grows to nearly 28 percent by 2009. Over the 

2004-2009 period, that program averages 15 percent of 

the agency’s budget, or a total of about $7.6 billion. Most 

of the program’s current funding is focused on mobile terrestrial systems, although about $10 million of the 2005 

budget request is slated for initial analysis of a spacebased system. Through 2009, MDA plans to allocate a 

total of about $700 million for work on a space-based 

BPI system.

Characteristics of ICBMs 

Important for BPI

A ballistic missile is composed of a guidance system and 

one or more warheads mounted on a rocket booster. To 

achieve intercontinental range—approximately 10,000 

kilometers (km) or more—ICBM boosters typically accelerate their payloads to a speed of about 6 to 7 km per 

second.

2004 2005 2006 2007 2008 2009

Total,

2004-2009

Budget for BMDS Interceptors 

Program 0.1 0.5 1.1 1.6 2.0 2.2 7.6

Total MDA Budget 7.6 9.0 8.3 9.6 7.9 7.9 50.3

Memorandum:

BMDS Interceptors as a 

Percentage of MDA’s Budget 1.5 5.6 12.9 16.8 25.5 27.8 15.2
CHAPTER ONE BALLISTIC MISSILE DEFENSES AND THREATS 5

Table 1-3.

Nations with Long-Range Ballistic

Missiles

Source: Congressional Budget Office based on Duncan Lennox, 

ed., Jane's Strategic Weapons Systems, vol. 39 (Coulsdon, 

Surrey: Jane's Information Group, July 2003).

Note: The countries listed above all have missiles with ranges 

greater than 500 kilometers.

a. As part of its recent rapprochement with the United States and 

the United Kingdom, Libya has indicated that it will give up its 

missiles that have ranges greater than 300 kilometers.

Boosters can be grouped into two types: liquid-fuel and 

solid-fuel. Liquid-fuel boosters are the older and simpler 

technology. A liquid propellant and a liquid oxidizer are 

used to fuel the missile’s rocket motor. Although liquidfuel missiles are easier to build than solid-fuel missiles, 

they require complicated and potentially dangerous fuelhandling activities and thus can be more difficult to operate and maintain.

Solid-fuel boosters use a propellant and an oxidizer that 

are molded into a solid motor core with a binding agent. 

The solid-fuel motor, which is ignited in its hollow interior and burns from the inside out, generates thrust by 

expelling the combustion products from its nozzle. Producing large solid-fuel boosters is challenging both technologically and industrially, but such boosters are relatively easy to handle after production.

The type of booster used in an ICBM is particularly important to designers of boost-phase intercept systems. 

Solid-fuel ICBMs usually have shorter boost phases than 

liquid-fuel ICBMs do. Thus, a BPI system designed to 

counter solid-fuel ICBMs will need higher performance 

because its interceptors will have less time to reach their 

targets. (Performance requirements for BPI systems are 

discussed in Chapter 2.)

Representative Threats 

for Comparing BPI Systems

As many as 35 countries are thought to possess ballistic 

missiles, although only 13 of them have missiles with 

ranges greater than 500 kilometers (see Table 1-3). Just 

four nations—China, Russia, the United States, and the 

United Kingdom—are known to possess ICBMs.6

 But 

other countries, including some that have had less-thanfriendly relations with the United States over the years, 

are believed to be pursuing ICBM capability. For example, North Korea and Iran could have such capability by 

2015, according to the December 2001 National Intelligence Estimate.7 Specific intelligence estimates are subject to debate, but if a nation is developing space-launch 

capability, it is in effect gaining the ability to field 

ICBMs. Both North Korea and Iran have said they plan 

to develop space-launch systems.8

To compare the effectiveness of alternative designs for 

BPI systems, this analysis uses North Korea and Iran as 

representative threats. North Korea is believed to be developing two long-range ballistic missiles—the Taep’odong 1 SLV (space-launch vehicle), with a range of about 

5,000 km, and the Taep’o-dong 2, with a range of about 

6,000 km, according to unclassified estimates (see Table 

1-4).9

 In August 1998, North Korea launched a Taep’odong 1 SLV with the stated goal of orbiting a small test 

satellite. Although the launch was unsuccessful, it clearly 

demonstrated North Korea’s pursuit of long-range-missile 

capability. Iran has Shahab 3 missiles with an estimated

Longest Range 

(Kilometers) Built or Bought

China 10,000+ Built

France 4,500 Built

India 2,500 Built

Iran 1,200 Built

Israel 3,000 Built

Libyaa 700 Bought

North Korea 2,000 Built

Pakistan 2,300 Built

Russia 10,000+ Built

Saudi Arabia 2,500 Bought

Syria 700 Built

United Kingdom 10,000+ Bought

United States 10,000+ Built

6. The United Kingdom’s long-range ballistic missiles are submarinelaunched Tridents and hence are technically considered submarine-launched ballistic missiles (SLBMs) rather than ICBMs.

7. National Intelligence Council, Unclassified Summary of a National 

Intelligence Estimate: Foreign Missile Developments and the Ballistic 

Missile Threat Through 2015 (December 2001).

8. Duncan Lennox, ed., Jane’s Strategic Weapons Systems, vol. 39 

(Coulsdon, Surrey: Jane’s Information Group, July 2003), p. 122.

9. Ibid., pp. 121-123.
6 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Table 1-4.

Characteristics of North Korean and Iranian Ballistic Missiles 

and of Representative ICBMs

Source: Congressional Budget Office based on Duncan Lennox, ed., Jane's Strategic Weapons Systems, vol. 39 (Coulsdon, Surrey: Jane's 

Information Group, July 2003).

Note: ICBMs = intercontinental ballistic missiles; SLV = space-launch vehicle. 

a. Only one Taep’o-dong 1 (an SLV version) is known to have been tested. However, additional missiles are thought to be available, either for 

operational use or for additional testing.

range of about 1,300 km. It is thought to be developing a 

Shahab 4 with a range of about 2,000 km.10

Those North Korean and Iranian ballistic missiles have 

much longer ranges than do tactical ballistic missiles, 

such as the Scud. But their ranges fall far short of the 

more than 10,000 km distance typical of ICBMs that 

have been fielded in the past (see Table 1-4).

Besides the characteristics of missiles that might be targeted, geography is an important factor with respect to 

the performance needed from a BPI system. Iran and 

North Korea are also good representative threats for assessing the implications of geography. Surface-based BPI 

interceptors must fly farther to reach ICBMs fired from 

the interior of large countries and thus usually need 

higher speeds if deployed against such countries. Iran is 

the world’s 16th largest country by area, so it provides a 

case study that could stress surface-based systems. Geography plays a role in space-based systems as well. Orbital 

dynamics require that the higher the latitude of the country to be covered, the more interceptors must be deployed. A space-based system with orbits that provided 

coverage of North Korea (the northern tip of which is 

located above 42 degrees latitude) could cover about 75 

percent of the world’s countries—and about 90 percent 

of the countries that, at least currently, might be considered potentially hostile to the United States.

The next chapter assesses the performance characteristics 

that a BPI system would need to defend the United States 

against ICBMs fired from those two representative countries. Chapter 3 examines alternative BPI designs that 

would meet the needed performance, and Chapter 4 

compares each alternative’s strengths, weaknesses, and 

costs.

Missile Type

Number of 

Booster

Stages Booster Fuel

Launch Weight

(Kilograms)

Length 

(Meters)

Range

(Kilometers)

Estimated

Status

North Korean and Iranian Ballistic Missiles

North Korea

Taep’o-dong 1 2 Liquid 21,700 27.0 2,000 Flight testinga

Taep’o-dong 1 SLV 3 Liquid (2 stages)

Solid (1 stage)

25,700 32.0 5,000 Flight testinga

Taep’o-dong 2 2 Liquid 64,000 35.0 6,000 In development

Iran

Shahab 3 1 Liquid 16,250 16.0 1,300 Operational

Shahab 4 1 Liquid 42,000 22.8 2,000 In development

Representative ICBMs

Former Soviet Union

SS-17 2 Liquid 71,100 20.9 11,000 Fielded in 1975

SS-25 3 Solid 45,100 20.5 10,500 Fielded in 1988

United States

Titan II 2 Liquid 149,700 31.3 15,000 Fielded in 1963

Minuteman III 3 Solid 34,500 18.2 13,000 Fielded in 1970

10. Ibid., pp. 97-98.
2

Performance Needed for an

Operationally Effective BPI System

A kinetic-energy boost-phase intercept consists of 

a series of events that culminate with a kill vehicle (basically a self-guided, nonexplosive warhead) hitting and destroying a target ballistic missile. Those events can be divided into two stages (see Figure 2-1):

B The commit stage, which lasts from when a threat missile is launched until a BPI interceptor is fired. Functions in the commit stage include detecting the target, 

tracking it, and deciding to fire (that is, committing 

an interceptor to the engagement).

B The interceptor flyout stage, which lasts from when the 

interceptor is launched until it reaches (and, if all goes 

well, destroys) its target.

The challenge of BPI is the short time available for the 

engagement. Both the commit and flyout stages must be 

executed while the threat missile is in its boost phase. 

Typically, that phase lasts four to five minutes for a 

liquid-fuel intercontinental ballistic missile and only 

about three minutes for a solid-fuel ICBM.

More time may be available for the engagement if the BPI 

sensors can continue tracking the target into its earlyascent phase (after the booster burns out but before the 

warheads separate from the rest of the missile). How 

much time can be gained depends on the design of the 

specific ICBM being targeted. Early ascent may last on 

the order of 60 seconds, but it can be much shorter with 

ICBMs designed to deploy their warheads early. Because 

of those uncertainties, the engagement times used in this 

study could represent early ascent as well as boost. For 

simplicity, however, intercepts are referred to as occurring 

no later than “at booster burnout.” If the intercept occurs 

in the early-ascent phase, the kill vehicle must be able to 

selectively hit and destroy an ICBM warhead (as opposed 

to the missile as a whole) because the warhead will already 

have enough velocity to reach its target.

For this analysis, the Congressional Budget Office assessed the capability of potential BPI systems against representative liquid-fuel ICBMs (a lower-technology threat) 

and solid-fuel ICBMs (a more advanced threat). The representative liquid-fuel ICBM has a burn time of about 

five minutes, similar to that of U.S. Titan II or Russian 

SS-12 missiles of the 1960s. The representative solid-fuel 

ICBM has a burn time of about three minutes, making it 

comparable to current U.S. Minuteman III or Russian 

SS-25 missiles. 

The burn time of an ICBM’s booster, coupled with the 

distance that an interceptor must travel to reach its target 

(which results from the geography of a particular scenario), determines the response time and interceptor 

speed necessary for a BPI system. In general, more technologically advanced ICBMs require higher-performance 

BPI systems because those ICBMs usually have shorter 

burn times and faster acceleration. Defending against 

larger threat countries also requires higher-performance 

surface-based BPI systems because interceptors will have 

to fly farther to reach ICBMs launched from deep inside 

a country. Locating surface-based BPI systems in the general path that a threat ICBM might fly to reach the 

United States can ease performance needs by allowing 

better geometry for the engagement.

CHAPTER
8 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Figure 2-1.

Series of Events in a Boost-Phase Intercept

Source: Congressional Budget Office.

Note: BPI = boost-phase intercept.
CHAPTER TWO PERFORMANCE NEEDED FOR AN OPERATIONALLY EFFECTIVE BPI SYSTEM 9

Figure 2-2.

Interceptor Reach Versus Commit Time for Interceptor Speeds 

Between 6 km/sec and 10 km/sec

(Kilometers)

Source: Congressional Budget Office.

Notes: The figure assumes that interceptors have a burn time of 60 seconds.

ICBM = intercontinental ballistic missile; km/sec = kilometers per second.

The performance of space-based systems is less sensitive 

to those geographic factors. Nevertheless, geography—in 

particular, the latitudes of threat countries to be covered—is an important factor in determining the number 

of space-based interceptors needed in a defensive system.

Commit Time, Interceptor Speed, 

and Location

A system’s ability to meet short BPI timelines hinges on 

its having some combination of a fast commit time, high 

interceptor acceleration and speed, and launchers that 

can be positioned close to potential threats. Different 

combinations of those characteristics can yield systems 

with similar overall effectiveness. 

The first two characteristics—commit time and interceptor speed—can be varied to provide a particular interceptor reach (the maximum distance from its launcher that 

an interceptor can travel in the available engagement 

time). Interceptor reach determines the coverage area provided by a BPI system. For a given commit time, faster 

interceptors will provide greater reach. Likewise, for a 

given interceptor speed, shorter commit times will provide greater reach. For example, a BPI system with 6-kilometer/second interceptors and a 60-second commit time 

could provide a reach of about 500 km against a solidfuel ICBM (see point A in Figure 2-2). If the system had 

a 100-second commit time, a 10-km/sec interceptor 

would be needed to provide the same reach (point B in 

the figure). For a given commit time and interceptor 

speed, the interceptor reach is greater against liquid-fuel 

ICBMs than against solid-fuel ICBMs because the liquidfuel missiles burn longer and thus allow more time for an 

intercept.

The interceptor reach needed for a given scenario depends in turn on the ability to position launchers in advantageous places. This study analyzes the interactions 

between those BPI characteristics to determine how a 

2,500

2,000

1,500

1,000

500

0

0 50 100 150 200 250 300

vs. Liquid-Fuel

ICBM

vs. Solid-Fuel

ICBM

Arrows show effects of increasing interceptor speed

(from 6 km/sec to 10 km/sec)

Commit Time (Seconds)

A B
10 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Table 2-1.

Characteristics of Radar and Infrared Sensors

Source: Congressional Budget Office.

range of performances would translate into operational 

effectiveness in the context of actual engagement geometries.

The length of the commit stage is primarily a function of 

the defense’s sensors and battle management components. The length of the flyout stage depends mainly on 

interceptor speed and launcher position relative to the 

ICBM’s launch point and trajectory. Speed and acceleration are functions of an interceptor’s design. The ability 

to put launchers in appropriate locations hinges on the 

mobility of the system and access to basing in other countries, in the case of surface-based BPI systems, and on the 

separation between satellites, in the case of space-based 

interceptors.

Sensors and Battle Management 

Components

The timeline of the commit stage for a boost-phase intercept consists of the time to detect a threat ICBM, the 

time to establish tracking, and the decision time before an 

interceptor is launched. Radar that detects the body of an 

ICBM or infrared sensors that detect the hot plume of 

the booster can be used to acquire the target in its boost 

phase. Either type of sensor might be located on the surface (land or ship), on an aircraft, or on a satellite. Each 

type and location of sensor entails advantages and disadvantages when used to detect ballistic missiles (see Table 

2-1). 

Satellite-based sensors are usually preferred for detecting 

ballistic missiles because satellites in high orbits do not 

have their view (or horizon) limited by the curvature of 

the Earth to the extent that lower sensors do. Horizon 

limits can cause significant delays in detecting ICBMs. 

For example, to detect a liquid-fuel ICBM within 60 seconds of its launch, a surface radar would have to be 

within about 300 km of the launch site if the missile’s trajectory was straight over the sensor (see Figure 2-3). The 

delay would be longer if—as would almost certainly be 

the case—the trajectory did not pass directly over the sensor. Airborne radar can allow for detection from farther 

away—up to about 800 km within the same 60-second 

time frame—because its altitude gives it an expanded horizon. Infrared sensors on a constellation of satellites 

Sensor Location Radar Infrared Sensors

Surface (Land or ship) Detection delays because of horizon limits are likely to 

be too long for all but the smallest threat countries

Cloud cover does not affect detection

Access to necessary locations could be constrained

Technology is easy to deploy and maintain

Detection delays because of horizon limits are likely to 

be too long for all but the smallest threat countries

Sensors are not useful for tracking if they are located 

beneath clouds

Airborne Horizon is greater than with surface radar

Cloud cover does not affect detection

Access to necessary locations could be constrained

Several aircraft per orbit are needed for around-theclock coverage

Stratospheric airship platforms may lack enough power 

for operations

Horizon is greater than with surface radar

Need for high-altitude position limits the choice of 

platform to high-altitude unmanned aerial vehicles 

or stratospheric airships

Several aircraft per orbit are needed for around-theclock coverage

High-altitude airships operating for long periods are 

unproven

Space Horizon is not limited if enough satellites are in orbit

Cloud cover does not affect detection

Access is not constrained

Technology is unproven and could be expensive to 

deploy

Horizon is not limited if enough satellites are in orbit

Cloud cover can delay detection

Access is not constrained

Some strategic warning constellation (representing a 

sunk cost) will be required with or without boostphase intercept

Technology must have tracking capability
CHAPTER TWO PERFORMANCE NEEDED FOR AN OPERATIONALLY EFFECTIVE BPI SYSTEM 11

Figure 2-3.

Time Needed for a BPI Sensor to 

Detect a Notional Liquid-Fuel ICBM

(Seconds)

Source: Congressional Budget Office.

Notes: In the case of surface and airborne radar, detection is 

affected by horizon limits caused by the curvature of the 

Earth. In the case of infrared sensors on satellites, detection 

may be affected by cloud cover, which hides the ICBM from 

view until it breaks through the clouds. These data are for 

ICBM trajectories straight over the surface and airborne sensors.

BPI = boost-phase intercept; ICBM = intercontinental ballistic missile.

would offer the fastest detection: delays of no more than 

30 to 45 seconds, and then only if cloud cover above the 

ICBM launch site hid the missile from view until it broke 

through the clouds (at an altitude of around 7 km). “Seeto-ground” sensors located in orbit might avoid the 

cloud-cover delay suffered by infrared sensors, but such 

technology is unproven.

As described later in this chapter, a BPI system that is effective under a broad range of operational conditions will 

need a commit time of about 60 seconds. Surface sensors 

farther than 400 km from the launch site of a liquid-fuel 

ICBM or 550 km from the launch site of a solid-fuel 

ICBM cannot meet that timeline. Surface radars could be 

effective against small countries such as North Korea, but 

an ICBM launched from the interior of a large country 

such as Iran might not be visible to a surface radar for 

more than two minutes—a significant delay given BPI 

engagement times of three to six minutes (including potential ascent-phase engagements).

Both infrared and radar can be used for the second commit-stage function: establishing the track of the target 

ICBM in order to guide an interceptor to the missile. 

Both types of sensors establish a track by taking successive 

snapshots of the target and determining its change in position as a function of time. That information is used to 

calculate where the ICBM is heading and hence where 

the interceptor should be aimed (a process known as finding a tracking and firing solution). Radar is excellent for 

that task because it can take rapid snapshots of the target 

to provide a high-resolution track. Moreover, since radar 

is an active sensor, a single site can establish range to the 

target.1 Although radar is ideal, an infrared sensor should 

be able to track an ICBM in the boost phase if the sensor 

is designed to take snapshots quickly enough to rapidly 

establish a track and if there are enough sensors that at 

least two can see the target to determine its range.

Once a track is established, the BPI system must calculate 

an aim point for the interceptor and then receive permission to fire. Given the short timelines involved, it is likely 

that the BPI system will have to operate automatically. 

Even if time was available, little additional information 

could be provided to assist a human operator with the 

decision to launch or hold fire. Rather, the decision 

would have to be made in advance as part of an engagement doctrine (for example, stating that if a ballistic missile is launched from within a given area, it will be assumed to pose a threat and will be engaged). Delegating 

to a computer the authority to fire a missile at an airborne 

target over a foreign country is an important policy issue 

involved in fielding a BPI system.

The total commit times that a system ultimately achieves 

will depend on the performance of the sensors, the speed 

with which the system’s tracking algorithms can generate 

a tracking and firing solution from the sensors’ data, and

Time When ICBM Reaches Burnout

0

50

100

150

200

250

300

0 500 1,000 1,500 2,000 2,500 3,000 3,500

Satellite Infared Sensor

Kilometers Between ICBM Launch Site and BPI Sensor

Surface Radar Airborne Radar

(At about 40,000 feet)

1. Active sensors such as radar generate pulses of energy that are 

transmitted toward the target. The radar determines range by 

measuring the time required for reflected energy to return from 

the target. Passive sensors such as infrared arrays detect energy 

emitted by the target itself. Determining range with passive sensors usually requires triangulation with multiple sensors, although 

a general knowledge of the target’s trajectory can be used to provide a rough estimate of range from a single sensor.
12 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Figure 2-4.

Interceptor Reach Needed Against a Notional Solid-Fuel ICBM,

by ICBM Trajectory

(Kilometers)

Source: Congressional Budget Office.

Notes: These data assume that intercept occurs when the ICBM reaches burnout.

BPI = boost-phase intercept; ICBM = intercontinental ballistic missile.

the amount of decision delay (if any) inherent in the 

engagement doctrine. With space-based infrared sensors, 

commit times could probably be on the order of 60 seconds, assuming no decision delay. The 60-second window would include 30 to 45 seconds for the ICBM to 

break through any cloud cover plus 15 to 30 seconds to 

establish a tracking and firing solution. Earlier detection 

would be possible if the ICBM was launched on a clear 

day (a clear night is actually the most favorable for infrared sensors) or if space-based see-to-ground sensors such 

as radar were available to penetrate any cloud cover. 

Even near-instantaneous detection might not shorten the 

commit time significantly, however, because obtaining a 

tracking solution requires that the ICBM start to pitch 

over and commit itself to a down-range direction. Very 

early in flight, an ICBM is primarily moving upward 

rather than down range. For example, after 30 seconds, 

the notional liquid-fuel ICBM described at the beginning 

of this chapter has reached an altitude of about 3 km but 

has only traveled about 0.6 km down range. If a BPI interceptor is committed too soon, it may be vulnerable to 

evasive maneuvers by the ICBM, which are easier to implement earlier in a missile’s flight.

Interceptors

After the commit stage, the remaining time before the 

ICBM’s booster burns out is available for the interceptor 

to fly out to its target. The ability of the interceptor to 

reach its target in time depends on its speed and acceleration, the relative locations from which the ICBM and the 

interceptor were launched, and the trajectory that the 

ICBM follows. However, surface-based and space-based 

interceptors differ in the way in which they depend on 

those factors.

Surface-Based Interceptors 

How great a reach a surface-based interceptor needs 

against a particular type of ICBM is determined by the 

initial separation between the ICBM and interceptor 

launch sites and the ICBM’s direction, or azimuth ( ),

BPI

Site

ICBM

Site

Engagement Geometry

ICBM

Trajectory

ICBM

Azimuth ( )

0

30

60

90

180

2,500

2,000

1,500

1,000

500

0

200 800 1,400 2,000

Kilometers Between BPI and ICBM Launch Sites
CHAPTER TWO PERFORMANCE NEEDED FOR AN OPERATIONALLY EFFECTIVE BPI SYSTEM 13

relative to the line between those sites (see Figure 2-4). 

For example, if the launcher for a representative solid-fuel 

ICBM (with a three-minute burn time) and a BPI 

launcher were 1,000 km apart, the interceptor would 

need a reach of about 600 km if the ICBM was fired directly over the BPI site ( = 0 in the figure). The required 

reach would double to about 1,200 km if the ICBM’s azimuth was perpendicular to the line between the launchers ( = 90 in the figure).

For a given surface-based BPI site, a threat country will 

contain potential ICBM launch locations that span a 

range of launcher separation distances and ICBM azimuths. Because of Iran’s large size, a BPI system to defend 

against missile launches from that country would need 

greater reach in the time available than a system covering 

smaller North Korea. A BPI system based in Iraq to defend against Iranian solid-fuel ICBMs would need a maximum interceptor reach of more than 2,000 km (the 

highest point of the shaded area in Figure 2-5), compared 

with a maximum of about 700 km for a BPI system based 

in the Sea of Japan to defend against North Korean solidfuel ICBMs.

Once an interceptor has been committed, its reach depends on its speed and acceleration. With the same maximum speed, a missile that accelerates more quickly will 

have greater reach than one that accelerates more slowly 

because it will attain its top speed after a shorter booster 

burn time. The discussion that follows assumes burn 

times of 60 seconds for surface-based interceptors. (The 

design implications resulting from higher or lower accelerations are discussed in Chapter 3.)

A useful measure of the effectiveness of a BPI system is 

the coverage it provides against a threat country. In this 

analysis, an area within a threat country is considered to 

be covered by a BPI system if the system is capable of engaging an ICBM fired from that area toward any location 

in the United States (including Alaska and Hawaii). As 

described above, the coverage of a BPI system varies with 

the length of time it takes to commit and the speed of its 

interceptors (see Figure 2-6). For an illustrative case that 

involved using a single BPI site in eastern Iraq to intercept an Iranian liquid-fuel ICBM, the contours in the 

first panel of Figure 2-6 represent the coverage area possible with various commit times, ranging from zero (no delay) to 120 seconds, assuming an interceptor speed of 8 

km per second. The no-delay contour, which offers the 

greatest coverage, corresponds to simultaneous launch of

Figure 2-5.

Interceptor Reach Needed Against a 

Solid-Fuel ICBM Launched from Iran 

or North Korea

(Kilometers)

Source: Congressional Budget Office.

Notes: Shaded areas indicate the range of potential engagement 

conditions for each country. These data assume that intercept occurs when the ICBM reaches burnout.

BPI = boost-phase intercept; ICBM = intercontinental ballistic missile.

200 800 1,400 2,000

0

500

1,000

1,500

2,000

2,500

Kilometers Between BPI and ICBM Launch Sites

Iranian Launch (BPI site in Iraq)

0

60

90

180

 ICBM Azimuth ( ) 

200 800 1,400 2,000

0

500

1,000

1,500

2,000

2,500

Kilometers Between BPI and ICBM Launch Sites

North Korean Launch (BPI site in Sea of Japan)

0

60

90

180

 ICBM Azimuth( ) 
14 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Figure 2-6.

Impact of Commit Time and Interceptor Speed on BPI Coverage of Iran 

from a Site in Iraq

Source: Congressional Budget Office.

Notes: Contour lines show the limits of launch areas that could be defended for the BPI commit times (in seconds) or interceptor speeds (in 

kilometers per second) shown. The arrows in the bottom lefthand corners indicate the spread of trajectories at which an Iranian

liquid-fuel ICBM could be launched against the United States. The dot in Iraq indicates the notional BPI site. These figures assume a 

BPI system intended to defend the entire United States, with an interceptor burn time of 60 seconds.

BPI = boost-phase intercept; ICBM = intercontinental ballistic missile.

the ICBM and the interceptor—a theoretical lower 

bound that could not be achieved in practice. As the 

commit time increases, the area covered by a given BPI 

site decreases. 

The second panel of Figure 2-6 shows the effects of varying interceptor speed rather than commit time. The contours represent the coverage limits for interceptor speeds 

ranging from 5 km/sec to 10 km/sec, given a commit 

time of 60 seconds. The coverage offered by a BPI system 

decreases dramatically below interceptor speeds of 6 km/

sec, primarily because ICBMs reach speeds of 6 to 7 km/

sec as they near burnout, and an interceptor moving 

much more slowly than its target can only hit that target 

under a narrow set of favorable engagement conditions.

Covering a country the size of Iran would be difficult 

with a single surface BPI site and modest system performance. Even with the ideal of no commit time, a single 

site with 8-km/sec interceptors could not cover all launch 

locations in the country. However, multiple sites could 

operate together to provide greater coverage (see Figure 2-

7). In fact, the whole coverage offered by multiple sites 

could be greater than the sum of the individual coverages. 

For example, a particular ICBM launch location in Iran 

might not be covered by a BPI site to its east (such as in 

Afghanistan) for missiles shot toward the west, nor would 

it be covered by a BPI site to its west (such as in Iraq) for 

missiles shot toward the east. But between them, both of 

those BPI sites might cover such a location. One drawback of a multiple-site approach, however, is that it 

would require greater reliance on access to foreign bases.

6

8

10 5

IRAQ

AFG.

TURKMENISTAN

AZER.

SAUDI

ARABIA

Persian

Gulf

UAE

ARM. UZB.

Caspian

Sea

PAK.

0 100 200 km Gulf of Oman

100 200 mi

40

30 30

50 60

KUW.

50

120

60

30

0

IRAQ

AFG.

TURKMENISTAN

AZER.

SAUDI

ARABIA

Persian

Gulf

UAE

ARM. UZB.

Caspian

Sea

PAK.

0 100 200 km Gulf of Oman

100 200 mi

40

30 30

50 60

KUW.

50
CHAPTER TWO PERFORMANCE NEEDED FOR AN OPERATIONALLY EFFECTIVE BPI SYSTEM 15

Figure 2-7.

Coverage of Iran Possible from BPI Sites in Different Locations

Source: Congressional Budget Office.

Notes: Contour lines show the limits of launch areas that could be defended for the BPI commit times (in kilometers per second) shown. The 

arrows in the bottom lefthand corners indicate the spread of trajectories at which an Iranian liquid-fuel ICBM could be launched 

against the United States. The dots indicate notional BPI sites. These figures assume a BPI system intended to defend the entire 

United States, with an interceptor burn time of 60 seconds and an interceptor speed of 6 kilometers per second.

BPI = boost-phase intercept; ICBM = intercontinental ballistic missile.

0

30

60

90

30 0

0 30

60

IRAQ AFG.

TURKMENISTAN

AZER.

SAUDI

ARABIA

Persian

Gulf

UAE

ARM. UZB.

Caspian

Sea

PAK.

0 100 200km Gulf of Oman

100 200 mi

40

30 30

50 60

KUW.

50

30

60

90

90 60

IRAQ AFG.

TURKMENISTAN

AZER.

SAUDI

ARABIA

Persian

Gulf

UAE

ARM. UZB.

Caspian

Sea

PAK.

0 100 200km Gulf of Oman

100 200 mi

40

30 30

50 60

KUW.

50

0

30

IRAQ AFG.

TURKMENISTAN

AZER.

SAUDI

ARABIA

Persian

Gulf

UAE

ARM. UZB.

Caspian

Sea

PAK.

0 100 200km Gulf of Oman

100 200 mi

40

30 30

50 60

KUW.

50

0

30

60

90 IRAQ AFG.

TURKMENISTAN

AZER.

SAUDI

ARABIA

Persian

Gulf

UAE

ARM. UZB.

Caspian

Sea

PAK.

0 100 200km Gulf of Oman

100 200 mi

40

30 30

50 60

KUW.

50

60

0

30

IRAQ AFG.

TURKMENISTAN

AZER.

SAUDI

ARABIA

Persian

Gulf

UAE

ARM. UZB.

Caspian

Sea

PAK.

0 100 200km Gulf of Oman

100 200 mi

40

30 30

50 60

KUW.

50

0

30

60

90

30 0

IRAQ AFG.

TURKMENISTAN

AZER.

SAUDI

ARABIA

Persian

Gulf

UAE

ARM. UZB.

Caspian

Sea

PAK.

0 100 200 km Gulf of Oman

100 200 mi

40

30 30

50 60

KUW.

50
16 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Figure 2-8.

Number of Surface-Based BPI Sites 

Needed for Full Coverage of Iran, 

by Commit Time and Interceptor Speed

Source: Congressional Budget Office.

Notes: The figures assume that interceptors have a burn time of

60 seconds.

* = full coverage not possible; BPI = boost-phase intercept; 

ICBM = intercontinental ballistic missile; km/sec = kilometers per second.

Two to four BPI sites would be necessary to fully defend 

against liquid-fuel ICBMs launched from all possible Iranian locations, given commit times between 30 and 90 

seconds and interceptor speeds between 6 and 10 km/sec 

(see Figure 2-8). With no commit delay, one site with 10-

km/sec interceptors could cover Iran, but no commit delay implies advance knowledge of the exact location, 

time, and direction of the ICBM launch. With that level 

of information, a cruise missile strike on the launcher 

could be a better defensive measure.

In the case of Iranian solid-fuel ICBMs, a greater number 

of surface sites would be needed to provide full coverage 

(see Figure 2-8). Even with short commit times (below 30 

seconds), a system with 6-km/sec interceptors would require many sites to fully cover Iran. Coverage would be 

better with 8-km/sec interceptors, although the number 

of sites needed would climb rapidly with increasing commit time. Even with very fast interceptors (10 km/sec), 

the system would require four sites if the commit time 

was 60 seconds.

The effectiveness of a surface-based BPI system could be 

much greater in the case of North Korea because of the 

shorter distances involved. A system firing 6-km/sec interceptors from a single site (a ship in the Sea of Japan) 

could provide full coverage against North Korean liquidfuel ICBMs given a 90-second commit time (see Figure 

2-9). That system could provide full coverage against 

solid-fuel ICBMs from North Korea if the commit time 

could be reduced to about 45 seconds. With faster interceptors—say, 8 km/sec—commit times against solid-fuel 

ICBMs could lengthen to about 75 seconds and still allow for full coverage.

Space-Based Interceptors 

The coverage provided by a BPI system using space-based 

interceptors (SBIs) is also driven by commit time, interceptor speed, and interceptor acceleration. But various 

additional factors that do not come into play for surfacebased systems help determine that coverage as well. For 

example, because an entire SBI system will have to be put 

into orbit, minimizing its weight—and hence its launch 

costs—is an important consideration. In addition, although orbiting interceptors are free from constraints on 

foreign basing, their locations are still limited by the laws 

of orbital dynamics.

None 30 sec. 60 sec. 90 sec.

0

1

2

3

4

5

6

7

8

6 km/sec

8 km/sec

10 km/sec

Interceptor Speed

Commit Time

Against a Liquid-Fuel ICBM

None 30 sec. 60 sec. 90 sec.

0

1

2

3

4

5

6

7

8

6 km/sec

8 km/sec

10 km/sec

Interceptor Speed

* *

Commit Time

* * *

Against a Solid-Fuel ICBM
CHAPTER TWO PERFORMANCE NEEDED FOR AN OPERATIONALLY EFFECTIVE BPI SYSTEM 17

Figure 2-9.

Impact of Commit Time and Interceptor Speed on BPI Coverage of North Korea 

from a Site in the Sea of Japan

Source: Congressional Budget Office.

Notes: Contour lines show the limits of launch areas that could be defended for the BPI commit times (in seconds) shown. The arrows in the 

upper lefthand corners indicate the spread of trajectories at which a North Korean ICBM could be launched against the United States. 

The dot in the Sea of Japan indicates the notional BPI site.

BPI = boost-phase intercept; ICBM = intercontinental ballistic missile; km/sec = kilometers per second.

An SBI system would most likely consist of a constellation of interceptor satellites located in low-Earth orbit at 

an altitude of about 300 km. Satellites in higher-altitude 

orbits would require greater launch costs and be farther 

from their intended targets. As with surface-based BPI 

systems, that additional distance would result in the need 

for faster, heavier interceptors. However, maintaining 

lower orbits for long periods of time would require extra 

fuel to periodically counter atmospheric drag, and that 

additional fuel would also result in greater weight and 

launch costs.

Objects in low-Earth orbit are not stationary over one 

point on the Earth. Rather, their location travels a sinusoidal ground track that is centered at the equator (see 

Figure 2-10). The inclination of the orbit measures how 

far north and south of the equator the orbital path 

reaches. The ground track of each successive orbit is 

shifted because of the Earth’s rotation. Over a 24-hour 

period, a low-altitude satellite in a 45-degree inclined 

orbit will generate 16 ground tracks between 45 degrees 

north latitude and 45 degrees south latitude. As an SBI 

travels in its orbit, it can defend against ICBM launches 

that may occur within its footprint.

Because of that orbital behavior, space-based boost-phase 

intercept capability cannot be concentrated against specific threats—any single SBI will spend most of its time 

out of position, either over the ocean or over other countries. Providing full coverage of a particular threat country requires having a constellation of SBIs with their orbits positioned such that one or more interceptors is over 

the threat area at any given time. The orbital inclination 

used for the satellites is determined by the highestlatitude threat area that needs to be covered.

The number of SBIs that a system requires depends on 

the type of threat ICBM, the performance of the system’s 

components, and the lowest-latitude threat area to be 

covered. As in surface-based BPI systems, the first two 

factors determine the reach of each interceptor, which in 

turn determines the maximum spacing between SBIs. 

The lowest latitude to be covered is important because 

SBIs in inclined orbits do not provide equal coverage at

0 100 200 km

100 200 mi

CHINA

SOUTH KOREA

RUSSIA

Sea of Japan

Yellow Sea

Korea Bay

42

40

42

40

38 38

124 126 128 130

126 128 130

100

0 100 200 km

100 200 mi

CHINA

SOUTH KOREA

RUSSIA

Sea of Japan

Yellow Sea

Korea Bay

42

40

42

40

38 38

124 126 128 130

126 128 130

60

90

0 100 200 km

100 200 mi

CHINA

SOUTH KOREA

RUSSIA

Sea of Japan

Yellow Sea

Korea Bay

42

40

42

40

38 38

124 126 128 130

126 128 130

90
18 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Figure 2-10.

Ground Track of a Satellite in a 45-Degree Inclined Low-Earth Orbit

Source: Congressional Budget Office.

all latitudes. The sinusoidal ground track associated with 

low-Earth orbit means that each satellite spends a greater 

amount of time at latitudes approaching the orbital inclination and less time at lower latitudes. For example, a 

constellation of satellites in a 45-degree inclined orbit 

that had enough SBIs to fully cover North Korea—which 

lies between 38 degrees and 45 degrees north latitude—

could have gaps in its coverage as much as 40 percent of 

the time for the southern regions of Iran, at about 25 degrees north latitude.2 (That estimate assumes that the 

space-based system has a 60-second commit time and interceptors with 4-km/sec speed and 40 seconds of burn 

time.)

The gaps in coverage of Iran could be filled by adding 

SBIs to the constellation, essentially decreasing the spacing between each interceptor. That approach would have 

the advantage of providing extra coverage of North Korea—an average of two SBIs at any given time. Alternatively, additional SBIs could be launched into lowerinclination orbits better tailored to Iran’s latitudes. That 

approach might require fewer SBIs but would not provide the benefit of greater coverage of North Korea.

Just as surface-based systems with higher interceptor 

speed and acceleration would tend to require fewer sites 

because of the interceptors’ greater reach, SBIs with 

higher speed and acceleration would result in the need for 

fewer interceptors because each one would have a larger 

footprint. The number of satellites needed to provide full 

coverage against liquid-fuel ICBMs launched from between 25 and 45 degrees north latitude—a band that encompasses Iran and North Korea—would vary from 

about 70 to more than 900 for the various combinations 

of commit time and interceptor speed that the Congressional Budget Office analyzed (see Figure 2-11). (The design implications of different SBI speeds and accelerations are discussed in the next chapter.)

150 120 120 90 90 60 60 30 30 0 150

Degrees West Longitude Degrees East Longitude

60 60

30 30

30 30

60 60

0 0

Iran

North

Korea

2. The northernmost point in North Korea actually lies at about 

43 degrees north latitude. However, intercepts of North Korean 

ICBMs fired toward the continental United States would have to 

occur farther north.
CHAPTER TWO PERFORMANCE NEEDED FOR AN OPERATIONALLY EFFECTIVE BPI SYSTEM 19

Figure 2-11.

Number of SBIs Needed for Full Coverage of North Korea and Iran, 

by Commit Time and Interceptor Speed

Source: Congressional Budget Office.

Notes: The figures assume that interceptors have an acceleration of 10g, that kill vehicles have a mass of 30 kilograms, and that the life jacket 

accounts for 50 percent of an SBI’s total mass. They also assume that the threat country does not fire more than one ICBM at the 

same time and that a single interceptor is shot at an ICBM (which is why these numbers differ from the ones shown in Summary Figures 3 and 4).

SBI = space-based interceptor; ICBM = intercontinental ballistic missile; km/sec = kilometers per second.

a. Actual number of SBIs is 5,455.

Solid-fuel ICBMs would present a greater challenge to a 

space-based BPI system, just as they would to a surfacebased system. The shorter burn time of solid-fuel boosters effectively reduces the footprint of each space-based 

interceptor. Consequently, the spacing between SBIs 

must be reduced, increasing the size of the constellation. 

The number of satellites needed to defend against a solidfuel ICBM launched from the aforementioned latitude 

band would be several times greater than the number 

necessary with a liquid-fuel ICBM (see Figure 2-11).

None 30 sec. 60 sec. 90 sec.

0

500

1,000

1,500

2,000

2,500

3,000

2 km/sec

4 km/sec

6 km/sec

Interceptor Speed

Commit Time

Against a Liquid-Fuel ICBM

None 30 sec. 60 sec. 90 sec.

0

500

1,000

1,500

2,000

2,500

3,000

2 km/sec

4 km/sec

6 km/sec

Interceptor Speed

Commit Time

Against a Solid-Fuel ICBM

a

3

Alternative Designs for BPI Systems

An operational boost-phase intercept system will 

consist of sensors to detect and track target ballistic missiles and interceptors to engage them. Battle management 

and communications equipment will also be necessary to 

calculate tracking and firing solutions, serve as the link 

between sensors and interceptors, and tie the BPI system 

in with other elements of a layered ballistic missile defense system. The previous chapter illustrated how a 

broad range of sensor and interceptor performance could 

be combined to yield systems capable of engaging intercontinental ballistic missiles in their boost phase. 

Any specific system design will not only have operational 

strengths and weaknesses compared with other designs 

but also require different levels of investment for development, procurement, and operation. To compare the potential cost and effectiveness of different approaches to 

BPI, the Congressional Budget Office (CBO) developed 

five alternative system designs that span a range of performance levels and basing options. This chapter describes 

the technical analyses underlying each BPI alternative, 

three of which are surface-based and two of which are 

space-based. Chapter 4 compares the options’ potential 

costs and capabilities from several different operational 

perspectives.

Each design was structured to be able to counter a liquidfuel ICBM fired from any location in North Korea or 

Iran at any target in the United States. CBO chose those 

countries because of their geographic characteristics and 

because many observers consider them to pose the most 

likely ballistic missile threats to the United States. As described in Chapter 1, both nations are developing longrange missiles based on liquid-fuel technology and both 

are believed to possess weapons of mass destruction. 

CBO designed each BPI system to be able to defeat 

ICBMs fired from anywhere in North Korea or Iran because that ability means that the system could not be easily circumvented by mobile ICBM launchers. Although 

the alternative systems are intended to counter liquid-fuel 

ICBMs, the analysis also compares their abilities to 

counter solid-fuel ICBMs. 

Assumptions About Sensors and 

Battle Management Components

The sensors and battle management components of a BPI 

system determine the commit time that can be achieved. 

As Chapter 2 indicated, a system that is effective against 

ICBMs will most likely need commit times of less than 

90 seconds—and preferably no more than 60 seconds. To 

defend against ICBM launches from deep inside a large 

country, such as Iran, sensors would have to be based in 

space to meet that time constraint. However, a BPI system would probably not require its own constellation of 

sensor satellites. Instead, it could rely on elements of the 

sensor architecture that is already being developed for the 

multilayer ballistic missile defense system and for other 

purposes. (Current infrared sensors on satellites offer 

detection capability, but they generally lack the worldwide capability to track targets for the BPI mission.)

For boost-phase intercept, the most relevant BMDS sensors are part of the Space-Based Infrared System (SBIRS) 

now in development. That system, commonly known as 

SBIRS-High, is expected to include a mix of four satellites in geosynchronous orbit (remaining stationary over 

one point on the Earth) and two sensor payloads on other 

satellites in highly elliptical orbits. The improved capability that the new sensors are expected to offer will broaden 

the set of missions that can be accomplished. For this 

study, CBO assumed that the necessary elements of 

SBIRS would be available as planned to support the BPI 

mission. Because those sensor systems are common to all 

of the BPI options, they are not explicitly included in 

comparisons of the options’ costs and effectiveness.

CHAPTER
22 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Besides sensors, a BPI system requires a communications 

architecture to transmit detection and tracking information to the surface BPI sites or in-orbit SBIs in preparation for launching interceptors. Having a direct link between the BMDS sensors and each BPI site helps reduce 

delays in information transfer that might result from 

routing sensor data through the central BMDS system. A 

way to communicate with interceptors in flight is also 

necessary to provide course corrections and other updates 

after the interceptors have been launched.

In the case of surface BPI sites, the communications 

equipment would need to be packaged so that it could be 

deployed to forward locations. For those sites, CBO assumed a support package similar to the one planned by 

the Missile Defense Agency, which includes a direct data 

downlink to the BPI site from orbiting BMDS sensors, a 

two-way data link to communicate with an interceptor 

during flight, and other links to integrate the site into the 

BMDS.1 (Those components are described in more detail 

later in this chapter.) For a space-based BPI system, CBO 

assumed that the corresponding functions would be included as part of each interceptor satellite.

Designs for Interceptors

The considerations involved in selecting an interceptor 

design differ for surface-based systems and space-based 

systems. For example, surface-based interceptors must be 

able to survive the high mechanical and thermal stresses 

associated with flying through the atmosphere at supersonic speeds. Space-based interceptors, by contrast, will 

have little or no interaction with the atmosphere because 

intercepts will usually occur at very high altitudes. CBO 

used the analysis of performance needs outlined in Chapter 2 as a guide to develop various notional interceptor 

designs for comparison. The purpose was not to perform 

a detailed, engineering-level design analysis but to understand the impact of certain characteristics on the cost of 

interceptors and their suitability for use in the field.

Surface-Based Interceptors

From the standpoint of effectiveness, a primary trade-off 

in designing a surface-based interceptor is between speed 

and acceleration on the one hand and size on the other. 

High speed lengthens an interceptor’s reach, improving 

its effectiveness against more-advanced ICBMs, increasing flexibility about where it is based, and potentially reducing the number of sites needed to counter a given 

threat. Small interceptor size allows a BPI system to be 

moved more easily to face emerging threats and, once in 

place, to be a less intrusive presence in a host country. Because the cost of boosters generally increases or decreases 

with size, smaller boosters can have cost advantages as 

well. In general, however, higher burnout velocities and 

shorter burn times—the two parameters that determine 

interceptor reach—require larger boosters with greater 

thrust. 

A number of characteristics affect the details of the tradeoff between interceptor speed and size. They include the 

propulsion efficiency of the booster, the mass of its structures, the number of booster stages, and the mass of the 

payload. 

B The efficiency of the propulsion system is characterized by a single factor known as the specific impulse 

(Isp). Measured in seconds, Isp represents the thrust per 

unit of weight flow of propellant expelled by the 

rocket motor. Typical solid-fuel rocket motors have Isp

values of around 280 seconds. By comparison, the advanced liquid-fuel main engines on the Space Shuttle 

have an Isp of about 450 seconds. 

B A booster’s structural mass, which includes all components other than the propellant and the payload, is an 

important design consideration because it represents 

additional weight that must be accelerated by the 

rocket. In general, as booster acceleration increases, 

additional structural mass is needed to handle increased mechanical stresses on the interceptor. 

B Most large boosters use multiple stages to help reduce 

structural mass. Because each stage is jettisoned after 

its fuel is expended, subsequent stages have less structural mass to accelerate. Two stages are usually adequate for speeds of around 6 kilometers per second. 

Additional stages are usually preferred for higher 

speeds. 

B Like structural mass, payload mass is an important determinant of interceptor size. For a surface-based BPI 

interceptor, the payload consists primarily of the kill 

vehicle and a shroud that gives the interceptor’s nose

1. Terry Little, “Kinetic Energy Interceptors Overiew” (unclassified 

briefing by the Missile Defense Agency to Congressional Budget 

Office staff, November 13, 2003).
CHAPTER THREE ALTERNATIVE DESIGNS FOR BPI SYSTEMS 23

Figure 3-1.

Effect of Interceptor Speed and Payload Mass on the Mass of a

Surface-Based Interceptor

(Kilograms)

Sources: Congressional Budget Office; Terry Little, “Kinetic Energy Interceptors Overiew” (unclassified briefing by the Missile Defense 

Agency to Congressional Budget Office staff, November 13, 2003); and Duncan Lennox, ed., Jane’s Strategic Weapons Systems, 

vol. 39 (Coulsdon, Surrey: Jane’s Information Group, July 2003), p. 388.

Notes: The figure assumes that the interceptors’ boosters have a specific impulse of 280 seconds.

The structural mass fraction is the ratio of an interceptor’s structural mass (the mass of everything other than the propellant and the 

payload) to the interceptor’s total mass.

Point A shows the Missile Defense Agency’s Ground-Based Midcourse Defense system interceptor (speed of 7-8 km/sec, peak acceleration of less than 9g); Point B shows the notional boost-phase interceptor from a 2003 study by the American Physical Society (speed 

of 10 km/sec, peak acceleration of about 60g); Point C shows the notional boost-phase interceptor from the Missile Defense Agency 

(speed of 6 km/sec, peak acceleration of about 20g).

km/sec = kilometers per second; kg = kilograms.

an aerodynamically efficient shape and protects the 

kill vehicle from atmospheric heating.

The impact that those characteristics have on interceptor 

speed and size is quantified by a relationship well known 

in the space-propulsion field: the rocket equation. That 

equation enables users to calculate how interceptor mass 

varies as a function of structural mass, payload mass, and 

interceptor speed and acceleration (booster burn time). 

Interceptor characteristics that provide the desired operational performance can be chosen from the relationships 

established in the rocket equation. The selection of burn 

time is a compromise between the desire for high acceleration (to increase interceptor reach) and the penalty of 

high acceleration (larger and heavier boosters to provide 

greater thrust and withstand greater thermal and mechanical stresses). Additionally, interceptors with shorter burn 

times typically require greater maneuverability on the 

part of the kill vehicle because the ability to make trajectory corrections with steering commands to the booster 

ends earlier in the interceptor’s flight. Trajectory corrections after booster burnout must be made by the kill vehicle. Greater kill-vehicle maneuverability in turn results in 

greater weight.

In general, the need for higher acceleration increases as 

the time available for interceptor flyout decreases. CBO’s 

alternative designs for surface-based interceptors—which

8-km/sec speed, 150-kg payload

10-km/sec speed, 50-kg payload

Structural Mass Fraction

8-km/sec speed, 50-kg payload

6-km/sec speed, 50-kg payload

10-km/sec speed, 150-kg payload

0.08 0.1 0.12 0.14

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

6-km/sec speed, 150-kg payload

B

A

C
24 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Table 3-1.

Characteristics of Various Current or Proposed Kill Vehicles

Source: Congressional Budget Office.

Note: MDA = Missile Defense Agency.

a. Report of the American Physical Society Study Group on Boost-Phase Intercept Systems for National Missile Defense: Scientific and Technical Issues (Washington, D.C.: American Physical Society, July 2003), available at www.aps.org/public_affairs/popa/reports/

nmd03.cfm. 

envision speeds of 6, 8, or 10 km/sec—are based on a 60-

second burn time, which is suitable against the representative liquid- and solid-fuel threat ICBMs. The Missile 

Defense Agency’s planned BPI booster is expected to have 

a similar total burn time for its two primary booster 

stages.2 For CBO’s 6-km/sec design, decreasing the burn 

time to 45 seconds would increase the interceptor’s reach 

by about 35 km (less than 4 percent) at the cost of about 

2,000 kilograms (kg) in additional weight (a 60 percent 

increase) and therefore greater expense. Although the acceleration forces resulting from a 60-second burn time are 

high—especially for the 10-km/sec design, which has acceleration of almost 40g—they are not unprecedented. 

For example, the Sprint missile that was part of the Safeguard antiballistic missile system in the 1970s used a fivesecond burn time that generated approximately 100g acceleration.

For a given interceptor speed, decreasing payload mass 

can provide substantial reductions in total interceptor size 

(see Figure 3-1 on page 23). For example, in the case of a 

6-km/sec interceptor with a 0.1 structural mass fraction 

(the ratio of the interceptor’s structural mass to its total 

mass), reducing the payload mass by a factor of three 

(from 150 kg to 50 kg) would reduce the interceptor 

weight by about a factor of three as well. (The Isp of 280 

seconds used in Figure 3-1 is consistent with efficient 

solid-fuel rockets preferred for military use.)

The main variable in payload mass is the mass of the kill 

vehicle. Existing and proposed kill vehicles vary widely in 

mass, from a few tens of kilograms to well over 100 kg 

(see Table 3-1).3 Kill vehicle masses for BPI could range 

from as high as 140 kg, a figure estimated in a 2003 report by the American Physical Society (APS), to less than 

30 kg for very small interceptors proposed by engineers at 

Lawrence Livermore National Laboratory.4 For its options, CBO selected various kill-vehicle masses from that

Vehicle Phase Used

Mass

(Kilograms)

Divert Velocity

(Kilometers 

per second) Sensors

EKV (Used by MDA’s Ground-Based 

Midcourse Defense system)

Midcourse 68 Less than 1 Dual-band medium-wavelength infrared

Charge-coupled device television

SM-3 (Used by Aegis ballistic missile 

defense system)

Ascent,

Midcourse

20 Less than 1 Single-color long-wavelength infrared

Notional Design from the American 

Physical Society’s Boost-Phase 

Intercept Studya

Boost 140 2.5 Infrared

Visible

Laser detection and ranging (lidar)

Notional Advanced-Technology Design 

from Lawrence Livermore National 

Laboratory

Boost 30 2.5 Short- and medium-wavelength infrared

Ultraviolet/visible

Laser detection and ranging (lidar)

2. “KEI Contractors Borrow from Other MDA Programs to Meet 

Schedule,” Inside the Pentagon, April 15, 2004, p. 1.

3. For more details about the components of kill vehicles, see Appendix B. 

4. See Report of the American Physical Society Study Group on BoostPhase Intercept Systems for National Missile Defense: Scientific and 

Technical Issues (Washington, D.C.: American Physical Society, 

July 2003), available at www.aps.org/public_affairs/popa/reports/

nmd03.cfm. The Lawrence Livermore information was reported 

to the Congressional Budget Office in an unclassified briefing by 

Lawrence Livermore staff in November 2003.
CHAPTER THREE ALTERNATIVE DESIGNS FOR BPI SYSTEMS 25

Table 3-2.

Potential Trade-Offs in Designing Surface-Based Boost-Phase Interceptors

Source: Congressional Budget Office.

a. The ratio of an interceptor’s structural mass (the mass of everything other than the propellant and payload) to the interceptor’s total 

mass.

b. Consists primarily of the mass of the kill vehicle and of the interceptor shroud.

c. CBO’s Option 1.

d. CBO’s Option 2.

e. CBO’s Option 3.

range, recognizing that producing a 30-kg kill vehicle 

with BPI performance would require a technological leap 

in miniaturization. The midcourse-intercept kill vehicle 

on the BMD version of the Navy’s Standard missile (SM3) is lighter than that, but it has significantly less maneuverability (divert velocity) than a kill vehicle designed for 

boost-phase intercept.

CBO chose its alternative designs for surface-based interceptors from a range of characteristics appropriate for the 

BPI mission (see Table 3-2). The designs span the range 

of interceptor speeds analyzed in Chapter 2 and a range 

of payload masses consistent with existing or proposed 

kill vehicles. The bold lines in Table 3-2 represent the interceptors chosen for CBO’s three surface-based BPI options. The interceptor with the slowest speed, 6 km/sec, 

includes a two-stage booster. A three-stage booster design 

is more efficient for the higher-speed options (8 and 10 

km/sec). For those higher speeds, the benefit that the extra stage provides by shedding structural mass earlier is 

worth the added complexity of the booster.

Space-Based Interceptors

As explained in Chapter 2, the number of interceptors in 

a space-based BPI constellation depends on the characteristics of the threat, the commit time, and the speed of the 

interceptors. From the perspective of interceptor design, 

for a given threat and commit time, a constellation can 

consist of either a smaller number of faster interceptors or 

a larger number of slower interceptors. However, the adStructural Mass Fractiona

Payload Massb

(Kilograms)

Launch Mass 

(Kilograms) Length (Meters)

Diameter 

(Meters)

Peak

Acceleration (g)

Interceptor Speed of 6 Kilometers per Second with Two-Stage Booster

0.08c 150 3,088 8.3 0.7 21.7

0.08 300 6,176 10.4 0.9 21.7

0.10 150 3,735 8.8 0.7 21.7

0.10 300 7,470 11.1 0.9 21.7

Interceptor Speed of 8 Kilometers per Second with Three-Stage Booster

0.10d 50 3,469 8.6 0.7 26.8

0.10 150 10,408 12.4 1.0 26.8

0.12 50 4,487 9.4 0.8 26.8

0.12 150 13,462 13.5 1.1 26.8

Interceptor Speed of 10 Kilometers per Second with Three-Stage Booster

0.10 50 11,582 12.9 1.1 39.3

0.10 150 34,745 18.5 1.5 39.3

0.12e 50 17,160 14.7 1.2 39.3

0.12 150 51,480 21.1 1.8 39.3
26 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Figure 3-2.

Effect of Interceptor Speed and Payload Mass on the Mass of a

Space-Based Interceptor

(Kilograms)

Source: Congressional Budget Office.

Notes: The figure assumes that the interceptors have an acceleration of 10g.

kg = kilograms; km/sec = kilometers per second.

vantage that faster SBIs offer in terms of needing fewer 

satellites can come at the price of greater mass per interceptor. As with surface-based systems, SBI mass varies 

with characteristics such as speed, acceleration, and payload mass. With the same payload mass, a single-stage 

4-km/sec interceptor will be considerably lighter than a 

single-stage 6-km/sec interceptor. Even the added efficiency (and complexity) of a second stage cannot drop 

the mass of the 6-km/sec design down to that of the 

single-stage 4-km/sec design (see Figure 3-2).

The cost to launch a constellation into orbit may be the 

most important factor when choosing between interceptor speed and number of interceptors. Launch costs currently average about $20,000 per kilogram of mass delivered to low-Earth orbit. If the cost to launch an SBI was 

comparable to its procurement cost, the most cost-effective constellation could be one that required putting the 

least mass into orbit. As an example, for a constellation 

sized to defend against liquid-fuel ICBMs from North 

Korea and Iran, the lowest-mass design would be one 

with 4-km/sec interceptors, assuming a 30-kg kill vehicle, 

a commit time of 60 seconds, and interceptor acceleration of 10g (see Figure 3-3). Speeds below 4 km/sec 

would require both a larger number of SBIs and larger 

launch masses. Speeds above 4 km/sec would also require 

larger launch masses, despite a smaller number of SBIs 

needed.

The masses shown in Figure 3-3 include only the interceptors themselves. Each SBI will also need an orbital 

support system, or “life jacket,” consisting of shielding to 

protect the interceptor from its environment (if necessary), a propulsion system to control the satellite’s attitude and maintain the proper position in orbit, and 

equipment to communicate with the necessary sensors, 

the interceptor in flight, and the BMDS. The mass of the 

life jacket can vary depending on the functions it is expected to carry out as well as specifics of the constellation’s orbits. That mass is important because it can add 

considerably to the mass that must be put into orbit. 

CBO considered two alternative life-jacket masses: 0.5 

times the total mass of the SBI (a number based on the

23456

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

1-stage booster, 140-kg payload

2-stage booster, 140-kg payload

1-stage booster, 30-kg payload

Speed at End of Booster Burn (km/sec)

2-stage booster, 30-kg payload
CHAPTER THREE ALTERNATIVE DESIGNS FOR BPI SYSTEMS 27

Figure 3-3.

Total Mass of SBI Constellation Needed 

to Counter Liquid-Fuel ICBMs, 

by Interceptor Speed

(Thousands of kilograms)

Source: Congressional Budget Office.

Notes: The figure assumes that interceptors have an acceleration of 

10g, that kill vehicles weigh 30 kilograms each, and that the 

system has a commit time of 60 seconds. It also assumes 

that the threat country does not launch more than one ICBM 

at a time and that only one interceptor is fired at an ICBM.

The total mass shown here reflects only the mass of the 

interceptors, not of the life jackets.

SBI = space-based interceptor; ICBM = intercontinental 

ballistic missile; km/sec = kilometers per second.

APS study’s analysis of what might be needed to support 

an SBI) and 0.2 times the total SBI mass (a number that 

reflects the possibility of a more advanced miniaturized 

design). Life-jacket mass can be expected to be roughly 

proportional to interceptor mass because two of its important functions—shielding and propulsion for maneuvering in orbit—are also proportional to interceptor size.

Other Design Considerations 

for a BPI System

Although sensors, battle management systems, and interceptors will determine the fundamental capability of a 

BPI system, other factors will contribute to its ultimate 

effectiveness once it has been deployed and to its cost. 

Those factors include the number of interceptors fired at 

a given target and the number of transport vehicles and 

ships needed for a surface-based system.

Number of Shots Taken at a Target

A surface-based BPI site could consist of as little as one 

interceptor and its associated command, control, battle 

management, and communications (C2BMC) equipment. Although such a site would offer a basic defensive 

capability, an actual operational site would most likely be 

structured to provide greater capability. At a minimum, 

an engagement against one ICBM would probably include two interceptor shots to increase the likelihood of a 

successful intercept.

The probability that an interceptor will hit its target depends on two other probabilities: that the interceptor will 

function as designed (its reliability) and that it will actually intercept the target (its accuracy). Because of the uncertainties involved in an ICBM’s flight, even a perfectly 

functioning BPI system has some chance of simply missing its target. The overall probability of a successful intercept varies with the single-shot probability of a successful 

intercept and with the number of shots taken (see Figure 

3-4).

Figure 3-4.

Probability of a Successful Intercept, 

by Number of Shots Taken

(Percent)

Source: Congressional Budget Office.

23456

0

10

20

30

40

50

Speed at End of Booster Burn (km/sec)

0 20 40 60 80 100

0

20

40

60

80

100

Probability of Hitting Target with a Single Shot

1 Shot

2 Shots

3 Shots
28 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

The probability of intercept that can be achieved with a 

single shot might not be considered adequate against such 

an important target as an ICBM headed for the United 

States. Consequently, additional BPI shots might be desired to increase the overall probability of a successful intercept. Because of the short engagement times for BPI, 

not enough time would be available to wait and see 

whether the first interceptor was unsuccessful before firing a second shot. Instead, the shots would have to be 

fired in a salvo as soon as a tracking solution was established. Such “shoot-shoot” tactics could also apply to a 

space-based system.

For its surface-based BPI options, CBO assumed that an 

individual site would include one set of C2BMC equipment and six interceptors. Each site would thus be capable of conducting a shoot-shoot engagement against three 

ICBMs. If the site’s C2BMC equipment could handle six 

interceptors simultaneously, all three threats could be engaged at the same time. For the space-based options, 

CBO assumed enough interceptors to provide the opportunity for two shots against an ICBM.

Additional Equipment for 

Surface-Based BPI Systems

Estimates of the costs of CBO’s options for surface-based 

BPI systems include the vehicles needed to transport interceptors and C2BMC equipment. Whenever possible, 

CBO used MDA’s estimates of vehicle requirements. Because those requirements are specific to each option, they 

are presented separately as part of the option. (More details about that and other elements of the cost estimates 

can be found in Appendix A.)

The three options for surface-based BPI all assume that 

some interceptors could be based at sea on governmentowned commercial cargo ships, if necessary. Cost estimates for those options include the purchase and operation of three cargo ships—enough to provide one or two 

sea-based BPI sites at any given time (with the other ships 

in transit or in maintenance). Alternatively, Navy surface 

combat ships might be modified to carry and launch 

boost-phase interceptors. However, because the interceptors envisioned in CBO’s options are too large to fit into 

the vertical missile launchers found on today’s surface 

combatants, considerable modifications would be necessary. MDA’s plans include initial experimentation with 

operating surface-based BPI systems from a converted 

cargo ship, followed by a more substantial research and 

development effort (costing more than $2 billion through

Table 3-3.

Comparison of CBO’s Options for a 

Surface-Based BPI System

Source: Congressional Budget Office.

a. Six per site.

b. Assuming 40 tests over a 20-year period.

2013) to integrate BPI launchers into either commercial 

or Navy ships.

The BPI Options Examined 

in This Analysis

As explained above, CBO developed a range of illustrative BPI systems to investigate how much investment 

might be needed to provide different levels of BPI capability against ICBM threats to the United States (see Tables 3-3 and 3-4). The parameters of each option and the 

rationale behind it are described below, preparatory to the 

comparison of the options’ capabilities and costs in the 

next chapter.

Option 1: 6-km/sec Surface-Based Interceptors

Option 1 represents the sort of system that might be developed if policymakers believed that deploying defenses 

as soon as possible was critical. The interceptors in this 

option provide sufficient performance to engage liquidfuel ICBMs fired from all but the largest countries. They 

Option 1 Option 2 Option 3

Characteristics of Interceptors

Length (Meters) 8.3 8.6 14.7

Diameter (Meters) 0.7 0.7 1.2

Launch Mass (Kilograms) 3,088 3,469 17,160

Kill-Vehicle Mass 

(Kilograms) 140 30 30

Structural Mass Fraction 0.08 0.10 0.12

Speed (Kilometers/second) 6 8 10

Burn Time (Seconds) 60 60 60

Peak Acceleration (g) 22 27 39

Procurement Quantities

Mission Sets (Sites) 10 10 10

Ships for Sea Basing 3 3 3

Operational Interceptorsa 60 60 60

Test and Spare 

Interceptorsb 52 52 52
CHAPTER THREE ALTERNATIVE DESIGNS FOR BPI SYSTEMS 29

are also small enough to be transported by large military 

cargo aircraft.

This system uses a two-stage booster and a heavy (about 

140-kg) kill vehicle. The booster performance and killvehicle size are within the current state of the art.

Each BPI site in Option 1 includes the required C2BMC 

equipment and vehicles as well as six interceptors, 

mounted in pairs on three mobile launchers. That configuration would enable each site to conduct a shoot-shoot 

engagement against three threat ICBMs without reloading its launchers. Overall, the option would involve setting up 10 operational sites (including the purchase of 

three cargo ships for sea basing). The implications for 

both cost and operational effectiveness of different procurement quantities are discussed in Chapter 4.

Option 2: 8-km/sec Surface-Based Interceptors

Option 2 is representative of a system that might be developed if defeating solid-fuel ICBMs was considered a 

priority or if reducing the number of sites needed to cover 

large threat countries was an important consideration. 

This system offers greater interceptor performance than 

the system in Option 1 but maintains comparable size 

and mobility.

Option 2 employs a three-stage booster and a lightweight (30-kg) kill vehicle. A light kill vehicle is necessary 

to keep the interceptors small enough for transport by 

aircraft. The 30-kg mass is similar to that of an advancedtechnology kill-vehicle concept proposed by engineers at 

Lawrence Livermore. The structural mass fraction is 

higher than in Option 1 to account for the interceptor’s 

higher acceleration. 

Option 2 includes the same C2BMC equipment and 

launchers as Option 1, as well as 10 operational sites and 

three ships.

Option 3: 10-km/sec Surface-Based Interceptors

Option 3 illustrates a system designed specifically to 

counter solid-fuel ICBMs. That capability comes at the 

expense of mobility, because the interceptors’ boosters 

and launchers would be large and heavy.

Option 3 uses 10-km/sec interceptors, the fastest considered in this analysis. It employs the same lightweight kill 

vehicle as Option 2 but requires a significantly larger and 

higher-performance booster to achieve that higher speed.

Table 3-4.

Comparison of CBO’s Options for a 

Space-Based BPI System

Source: Congressional Budget Office.

Note: SBI = space-based interceptor.

a. Over a 20-year period.

b. No purchases for testing would be necessary because operational SBIs at the end of their service lives would be used for 

tests.

As a result, Option 3’s launch mass is nearly five times 

greater than that of Option 2. The structural mass fraction is also higher than in Option 2 to account for the interceptors’ higher acceleration. With a speed of 10 km/

sec, the interceptors in this option are similar in performance to the fastest interceptor considered in the APS 

study. However, they are smaller than that interceptor because they carry a lighter kill vehicle.

Option 3 includes the same C2BMC equipment and 

number of sites and cargo ships as the other surface-based 

BPI options. However, because of its large booster, each 

interceptor would require its own launcher.

Options 4 and 5: Two Constellations 

of Space-Based Interceptors

Option 4 and Option 5 are constellations of SBIs in lowEarth orbit. Like the three surface-based options, they enOption 4 Option 5

Characteristics of Interceptors

Length (Meters) 5.4 4.3

Diameter (Meters) 0.4 0.4

Launch Mass (Kilograms) 847 442

Kill-Vehicle Mass (Kilograms) 140 30

Structural Mass Fraction 0.1 0.1

Life-Jacket Mass 0.5 times

SBI mass

0.2 times

SBI mass

Speed (Kilometers/second) 4 6

Burn Time (Seconds) 40 30

Average Acceleration (g) 10 20

Procurement Quantities

Operational Interceptors for 

Constellation 368 156

Interceptors for 

Replenishmenta 848 384

Test and Spare Interceptorsb 37 16
30 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

compass a range of interceptor performance. These two 

alternatives are described together because, unlike with 

the surface-based options, the variables of SBI performance and constellation size in these alternatives tend to 

trade off with one another to yield similar operational effectiveness for the system as a whole. The primary difference is one of cost.

The system in Option 4 consists of 4-km/sec SBIs with 

140-kg kill vehicles (similar to the kill vehicles in Option 

1). These interceptors have an average acceleration of 10g 

and a life-jacket mass equal to half the interceptor mass.

Option 5 uses a higher-performance SBI that takes advantage of potential advances in kill-vehicle weight reduction and general satellite miniaturization. The result 

is an interceptor with a 30-kg kill vehicle and a life-jacket 

mass only 0.2 times the interceptor mass. That high-end 

interceptor has a speed of 6 km/sec and an average acceleration of 20g.

The constellations envisioned in Options 4 and 5 are 

sized to defend against liquid-fuel ICBMs launched from 

locations between 25 degrees north latitude (southern 

Iran) and 45 degrees north latitude (northern North Korea) and to provide a shoot-shoot SBI engagement for increased probability of a successful intercept. That capability requires as few as 156 SBIs in the case of Option 5 and 

as many as 368 SBIs in the case of Option 4. The corresponding masses in orbit range from 83 metric tons 

(tonnes) for Option 5 to 468 tonnes for Option 4.
4

Comparison of BPI Options

Although all of the alternative boost-phase intercept systems described in Chapter 3 are designed to provide full coverage against liquid-fuel intercontinental ballistic missiles launched from Iran and North Korea, they 

do not have identical capabilities. Each alternative has advantages and disadvantages inherent in its design. This 

chapter examines the most important distinctions between the alternatives in the areas of cost, coverage, capability against solid-fuel ICBMs, dependence on access to 

foreign bases, vulnerability to being attacked or to exhausting their supply of interceptors, and strategic responsiveness. 

Not surprisingly, the greatest differences exist between the 

options for surface-based systems and space-based ones. 

Within each group, the alternatives with the slowest interceptors (Options 1 and 4) would present the smallest 

development challenges while still providing coverage 

against liquid-fuel ICBMs launched at the United States 

from countries like Iran and North Korea. Among the 

surface-based systems, the more-advanced designs in Options 2 and 3 would add the ability to defeat solid-fuel 

ICBMs launched from large countries (such as Iran) and 

the general ability to provide coverage with fewer BPI 

sites. However, the trade-off for those benefits would be 

greater technical risks in development and higher costs 

for development and deployment. With the space-based 

systems, by contrast, the more-advanced design in Option 5 would have lower costs than the less-advanced design in Option 4 (although, with the appropriate number 

of satellites, both options would offer similar effectiveness). The reason is that, with the cost assumptions underlying the space-based options, the additional development costs to reduce the weight and increase the speed of 

the interceptors in Option 5 would be more than offset 

by the lower production and launch costs that would result from fielding that option’s smaller and lighter constellation.

Costs

The Congressional Budget Office estimated the costs of 

each BPI system in three areas: research and development 

(R&D), production, and operations. R&D includes the 

engineering activities needed to design and develop the 

booster, kill vehicle, and other components that make up 

the system. R&D costs also cover testing the hardware 

and integrating the BPI system into existing infrastructure and support elements. Production costs include 

those to manufacture surface-based interceptors and their 

associated support equipment or, in the case of a spacebased system, to buy interceptors and launch services for 

the initial constellation. Operations costs cover routine 

efforts to maintain and operate the BPI system and to replenish its components over 20 years.

For each option, CBO calculated a basic, or low, estimate 

and a high estimate that accounts for potential cost 

growth comparable to what defense programs have experienced in the past. The range of costs that those two estimates represent also accounts for uncertainty about such 

factors as the maturity of a given technology and the 

complexity of manufacturing, both of which can affect 

costs (see Box 4-1). CBO used established cost-estimating 

relationships to calculate many production costs, such as 

for boosters and kill vehicles. Some of the estimates for 

R&D and operations costs, by contrast, were based on 

analogies with comparable systems, because of uncertainty in the engineering details of an option’s design coupled with the unique characteristics of many components 

of a BPI system. More details about how CBO produced 

the cost estimates are available in Appendix A. 

The costs of the illustrative BPI systems range from $16 

billion to $37 billion (in 2004 dollars) for the surfacebased systems in Options 1 through 3 and from $27 billion to $78 billion for the space-based systems in Options 

CHAPTER
32 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

4 and 5 (see Table 4-1). One significant reason that the 

space-based systems would be more expensive, on the 

whole, is that those options envision deploying a greater 

number of interceptors than in the surface-based systems 

and include costs for launching those interceptors into 

orbit. Each of the surface-based options would buy 112 

interceptors and deploy 60 of them to operational units 

(with the remaining 52 used for tests and as spares in case 

any of the operational interceptors malfunctioned or were 

damaged). Option 5, by comparison, would deploy 156 

interceptors, and Option 4 would deploy 368.

Another factor that makes the space-based systems more 

expensive is the cost of sustaining the constellation of 

space-based interceptors. CBO assumed that orbiting 

SBIs would last for seven years (a life span typical of satellites in low-Earth orbit). In contrast, it assumed that surface-based interceptors could be designed to function for 

20 years (a typical service life for that class of missile). 

Thus, over a 20-year period, each space-based system 

would need to be replaced about twice. Launch costs for 

the more-numerous and heavier Option 4 SBIs are a key 

reason that the lower-speed system in Option 4 would 

cost more than the higher-speed system in Option 5.

The costs of Options 4 and 5 are for a constellation to defend against liquid-fuel ICBMs from Iran and North Korea using a shoot-shoot engagement doctrine. Those costs 

might be lower if a single-shot doctrine was adopted instead, although that change could significantly decrease 

the constellation’s effectiveness. However, because the 

BPI defensive layer is intended to complement a midBox 4-1.

The Role of Uncertainty in CBO’s Estimates 

of the Costs of Boost-Phase Intercept Systems

In this analysis, the Congressional Budget Office 

(CBO) has not examined the entire range of possible 

configurations of systems for boost-phase missile 

defenses and how they might be developed and deployed. Decisions about what defensive systems to 

deploy and how to field them would depend on a 

number of factors, including the nature and extent 

of the threat that the United States would be likely to 

face in future years, the systems’ potential effectiveness against such threats, and the possible reactions 

of U.S. allies and other nations to a decision to deploy a boost-phase missile defense system.

Besides questions about the structure and goals of a 

boost-phase missile defense system, other factors 

common to many Department of Defense (DoD) 

programs complicate the task of estimating costs. In 

particular, estimates for systems that are defined only 

conceptually or that depend on the development of 

new technologies entail more uncertainty than estimates for well-defined programs based on proven 

technologies.

To account for the possible effects of that uncertainty, CBO has provided a range of cost estimates 

for the boost-phase intercept (BPI) systems discussed 

in this study. For each alternative, the low estimate 

represents what the system might cost if few technical or schedule difficulties arose in making the system fully operational. The high estimate accounts 

for potential technical, schedule, and cost growth 

common for similar types of systems. 

As explained in more detail in Appendix A, most of 

the factors that CBO used to estimate cost growth 

were developed by the RAND Corporation and 

based on unpublished updates of a 1996 report, The 

Defense System Cost Performance Database: Cost 

Growth Analysis Using Selected Acquisition Reports. 

CBO applied the results of that analysis to comparable systems that would be developed, built, and deployed as part of the BPI systems described in this 

study. For example, CBO developed the high estimate for BPI boosters by using cost-growth factors 

for the Minuteman and Trident strategic missiles. In 

addition to the factors developed by RAND, CBO 

developed its own cost-growth factors for launch vehicles and for system operating costs on the basis of 

the actual costs of comparable systems, such as the 

Atlas and Delta launch vehicles, the Patriot missile 

system, and the satellite-based Global Positioning 

System.
CHAPTER FOUR COMPARISON OF BPI OPTIONS 33

Table 4-1.

Summary of Costs for Boost-Phase Intercept Systems

(Billions of 2004 dollars)

Source: Congressional Budget Office.

Notes: For details about the major characteristics and procurement quantities of each option, see Tables 3-3 and 3-4.

n.a. = not applicable; SBI = space-based interceptor.

a. Includes interceptors used for testing, in the case of surface-based systems. (With space-based systems, tests would be performed using 

operational interceptors that had reached the end of their service lives.)

course defensive layer and perhaps eventually a terminal 

defensive layer, it may be acceptable to have a lower BPI 

effectiveness than would be the case if the BPI layer stood 

alone. The range of costs for a single-shot capability 

against liquid-fuel ICBMs would drop to $36 billion to 

$51 billion for Option 4 and to $20 billion to $31 billion 

for Option 5.

The surface-based systems in the first three options might 

be scaled back in a similar fashion. Against the representative threat of liquid-fuel ICBMs launched from Iran 

and North Korea, the 10 mission sets that each option 

would purchase would provide for five extra BPI sites in 

the case of Option 1 and seven extra sites in the case of 

Option 2 or Option 3. Those additional sites could be 

used to cover other threat countries or to provide morerobust coverage of both Iran and North Korea. Alternatively, cutting procurement quantities to five mission sets 

would, in principle, still allow for enough surface BPI 

sites to provide coverage of both Iran and North Korea. 

Costs for those scaled-back options might be as low as 

$14 billion to $21 billion for Option 1, $16 billion to 

$26 billion for Option 2, and $22 billion to $35 billion 

for Option 3. 

Areas of the World Covered

Although the space-based options would cost considerably more than the surface-based options, they would 

also provide much greater coverage. Options 1 through 3 

would cover only the countries against which the BPI systems were deployed (Iran and North Korea, for example, 

a total area of about 1.8 million square km), whereas Options 4 and 5 would cover the entire surface of the Earth 

between 25 and 45 degrees of both north and south latitude (a total area of about 145 million square km). However, because much of the latter area is ocean or countries 

that are not likely to pose a threat, it is difficult to quanSurface-Based Systems Space-Based Systems

Option 1 Option 2 Option 3 Option 4 Option 5

Cost Category Low High Low High Low High Low High Low High

Research and Development 6.7 9.5 8.8 13.1 13.4 19.5 7.1 9.8 8.5 12.9

Initial Production

Interceptorsa 2.8 3.6 3.1 4.0 4.8 6.4 8.7 10.7 3.7 4.6

Surface equipment 0.6 0.7 0.6 0.7 0.8 1.0 n.a. n.a. n.a. n.a.

Space launch n.a. n.a. n.a. n.a. n.a. n.a. 7.3 11.0 1.5 2.3

Subtotal 3.4 4.3 3.7 4.7 5.6 7.4 16.0 21.7 5.2 6.9

Operations Over 20 Years

Routine operations and 

support 4.0 6.5 4.0 6.5 4.0 6.5 2.0 4.0 2.0 4.0

Operational test support 1.6 3.2 1.6 3.2 1.6 3.2 1.6 3.2 1.6 3.2

Replacement SBIs n.a. n.a. n.a. n.a. n.a. n.a. 16.1 19.9 7.2 9.0

Replacement SBI launches n.a. n.a. n.a. n.a. n.a. n.a. 12.8 19.2 2.6 3.9

Subtotal 5.6 9.7 5.6 9.7 5.6 9.7 32.5 46.3 13.4 20.1

Total 15.7 23.6 18.1 27.5 24.6 36.6 55.6 77.8 27.1 39.9
34 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

tify the value of the additional coverage. Proponents of 

space-based interceptors argue that the identity of future 

threats is uncertain and that coverage of the ocean is a 

valuable hedge against ICBMs launched from ships or 

submarines. In principle, a space-based system is also capable of covering very large countries—such as China or 

Russia—that are too big to be covered by surface interceptors located around their borders. However, the constellations in Options 4 and 5 would not cover high 

enough latitudes to defend against missiles launched from 

those countries.

Capability and Costs to Counter 

Solid-Fuel ICBMs

Each of the options analyzed in this report offers sufficient capability—in terms of the performance of the BPI 

system and the quantity of equipment purchased—to 

counter liquid-fuel ICBMs launched at the United States 

from anywhere in North Korea or Iran. As explained in 

Chapter 2, the effectiveness of BPI systems would need to 

be greater if, as some analysts argue might happen, threat 

countries were able to develop or acquire solid-fuel 

ICBMs (which have a shorter burn time). A system that 

would still be effective in that case might be a valuable 

hedge against uncertainties in the evolution of the ICBM 

threat.

To counter missiles with shorter burn times, a BPI system 

would need some combination of better performance 

(shorter commit times, faster interceptors) and more sites 

(to get defensive launchers closer to potential launch sites 

of ICBMs). Better performance can be difficult to 

achieve, however, in a system that has already been 

fielded. And in the case of surface-based BPI, locations 

for additional sites may not be available. (The availability 

of basing is discussed in the next section.)

The increased difficulty of countering solid-fuel ICBMs 

has different implications for each of CBO’s options (see 

Table 4-2). Against a representative solid-fuel ICBM, 

Option 1 would only provide full coverage of smaller 

countries and only then if favorable basing locations were 

available and if commit times were short. For example, 

Option 1 could cover relatively small North Korea only 

if commit times could be held to less than 45 seconds or 

if several sites could be located in China. Although Option 1 would allow for enough sites to surround Iran, its 

6-km/sec interceptors would lack the reach to cover 

Table 4-2.

Effects on CBO’s Options of Targeting 

Solid-Fuel ICBMs

Source: Congressional Budget Office.

Note: ICBM = intercontinental ballistic missile; SBI = spacebased interceptor.

launch locations deep in Iran’s interior, assuming reasonable commit times.

The greater performance inherent in Options 2 and 3 

would give them greater capability against ICBMs with 

shorter burn times. Either alternative, using a site in the 

Sea of Japan, could counter a solid-fuel ICBM with a 

three-minute burn time launched from North Korea even 

if commit times exceeded 60 seconds. For full coverage of 

Iran, the number of sites needed under Option 3 would 

increase from two to four (for a 60-second commit time), 

and the number of sites needed under Option 2 would 

rise from two to seven. Those additional sites would not 

require buying more than the 10 mission sets included in 

each option, but they would leave fewer sites available to 

Option Operational Impact Cost Impact

1 Interceptors lack the reach 

to fully cover large 

countries

Not applicable

2 Seven launch sites needed 

to cover Iran versus five in 

the case of liquid-fuel 

ICBMs, leaving fewer sites 

available for other 

scenarios (if no additional 

interceptors are bought)

No additional purchases 

needed beyond the 

equipment for 10 launch 

sites assumed in the 

option

3 Four launch sites needed to 

cover Iran versus two in 

the case of liquid-fuel 

ICBMs, leaving fewer sites 

available for other 

scenarios (if no additional 

interceptors are bought)

No additional purchases 

needed beyond the 

equipment for 10 launch 

sites assumed in the 

option

4 Constellation of SBIs 

increases from 368 to 

1,308

Additional $107 billion to 

$146 billion needed to 

buy more SBIs

5 Constellation of SBIs 

increases from 156 to 

516

Additional $30 billion to 

$40 billion needed to 

buy more SBIs
CHAPTER FOUR COMPARISON OF BPI OPTIONS 35

cover other areas (in addition to North Korea)—two in 

Option 2 and five in Option 3.

CBO’s space-based options could also provide coverage 

against solid-fuel ICBMs, although a denser constellation 

of interceptors would be needed. With the 4-km/sec, 10g 

SBIs in Option 4, the necessary constellation size would 

grow to 1,308 satellites—more than triple the 368 

needed to counter liquid-fuel ICBMs. In the case of Option 5, with 6-km/sec, 20g interceptors, the constellation 

size would increase to a similar extent, from 156 to 516 

SBIs. For those larger constellations, total costs would 

more than double—to ranges of about $163 billion to 

$224 billion for Option 4 and $57 billion to $80 billion 

for Option 5.

Reliance on Access to Foreign Basing

Surface-based BPI systems would need to be deployed to 

sites in countries adjacent to the threat country being 

covered, which would require permission from the host 

nations. Lack of access to those areas could greatly affect 

operations. For example, in the recent conflict in Afghanistan, lack of transit rights over Iran forced aircraft operating from Kuwait to fly much longer distances down the 

Persian Gulf and up through Pakistan to reach targets in 

Afghanistan. In the case of missile defense, being denied 

basing access could complicate BPI efforts to the point of 

rendering them infeasible.

In general, a surface-based system that requires fewer sites 

will be less vulnerable to constraints on access than a system that needs more sites. Consequently, of the surfacebased systems, the one in Option 3 would be the least 

vulnerable to access constraints, followed by the one in 

Option 2. Option 1 would be the most vulnerable. That 

order matches the relative speeds of the systems, which 

has a bearing on how many sites they need. A higherspeed surface system will never need more sites than a 

lower-speed system, but the specific number will depend 

on the scenario in question.

To defend against North Korean liquid-fuel ICBMs, all 

three options would need only one site in international 

waters (see Figure 4-1). Consequently, none would be 

vulnerable to denial of basing rights. To defend against 

liquid-fuel ICBMs from Iran, Option 1 would require 

four sites (roughly one on each side of the country), compared with two sites for Options 2 and 3. The additional 

vulnerability of Option 1 could be much more significant 

than a simple factor of two extra sites, however. The 8-

km/sec and 10-km/sec interceptors in Options 2 and 3, 

respectively, would provide coverage from sites in Afghanistan and Iraq, where basing access might be available (at least when this report was being written). The 

additional sites needed for the 6-km/sec interceptors in 

Option 1 would be located in countries such as Turkmenistan, where access is less assured. (Of course, not 

long ago, access to sites in Afghanistan and Iraq would 

have been considered unattainable as well. In fact, Iraq 

was considered a potential threat country instead of a 

possible BPI site location.)

The ability to position surface-based systems where necessary will always be subject to geopolitical conditions. 

Space-based systems have the advantage of independence 

from such access constraints.

Vulnerability of the BPI System

to Attack

Besides limitations on their location, another way in 

which BPI systems could be prevented from fulfilling 

their mission is if they were attacked by the threat country they were covering. Because surface-based BPI systems would be positioned close to the threat country, 

such an attack could be made before the country 

launched an ICBM.

For the purposes of this analysis, all surface sites were assumed to be located approximately 100 km from the border of the threat country so they would be out of range of 

artillery or unguided rockets. However, short-range ballistic missiles or cruise missiles, as well as attack aircraft, 

from that country could reach a BPI surface site. Consequently, the site might require its own defenses, such as 

Patriot missiles for air defense and a ground force for perimeter defense. The costs of those defenses are not included in CBO’s cost estimates. For their part, the spacebased options would be potentially vulnerable to antisatellite weapons, should threat countries develop them.

Ability to Counter Increasing 

Numbers of ICBMs

The surface-based options envision deploying six interceptors at each site. With shoot-shoot tactics to improve 

the probability of destroying a target ICBM, each site 

would thus be capable of three BPI engagements. However, the threat country could try to saturate a defensive 
36 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Figure 4-1.

Location of Surface-Based BPI Sites Needed to Defend Against Liquid-Fuel ICBMs 

in CBO’s Options

Source: Congressional Budget Office.

Notes: The arrows on each map indicate the spread of trajectories at which ICBMs could be launched against the United States.

BPI = boost-phase intercept; ICBM = intercontinental ballistic missile; km/sec = kilometers per second.

site by launching more than three ICBMs or some combination of actual ICBMs and decoys intended to draw 

BPI fire. For example, it might launch several single-stage 

rockets on ICBM-like initial trajectories to force the 

defense—which must commit interceptors early to meet 

BPI timelines—to exhaust its supply of interceptors 

before an actual ICBM was launched. To counter that 

possibility, BPI sites could require additional interceptors 

and their associated launchers.

The number of interceptors needed to defend against additional ICBMs increases in proportion to the number of 

sites needed in a scenario. Consequently, the number 

would grow faster in the case of defenses against solid-fuel 

ICBMs because more sites are necessary to provide full 

coverage against those missiles.

For space-based systems, the number of interceptors 

needed to counter multiple simultaneous ICBM launches 

would increase rapidly because the entire constellation 

would have to be “deepened,” so that more interceptors 

were available at any given time. If the threat ICBMs 

were not launched simultaneously, however, the orbital 

motion of the satellites would bring new SBIs into position to fire at them in roughly 10 minutes.

Strategic Responsiveness

In terms of reacting quickly to newly emerging threats, 

Options 4 and 5, the space-based alternatives, would be 

the most strategically responsive if a threat arose between 

25 and 45 degrees of latitude. The reason is that, once 

deployed in orbit, space-based interceptors would always 

be on alert. Surface-based systems, by contrast, would 

require time for deployment (as well as access) to areas on 

the perimeter of the threat country. Consequently, Options 4 and 5 are better suited to defeat a “bolt from the 

blue” type of attack from within the covered latitude 

bands. 

0 100 200km

100 200 mi

CHINA

SOUTH KOREA

RUSSIA

Sea of Japan

Yellow Sea

Korea Bay

42

40

42

40

38 38

124 126 128 130

126 128 130

IRAQ AFG.

TURKMENISTAN

AZER.

SAUDI

ARABIA

Persian

Gulf

UAE

ARM. UZB.

Caspian

Sea

PAK.

0 100 200 km Gulf of Oman

100 200 mi

40

30 30

50 60

KUW.

50

Iran Scenario North Korea Scenario

Option 1 (6-km/sec interceptor speed)

Option 2 (8-km/sec interceptor speed)

Option 3 (10-km/sec interceptor speed)
CHAPTER FOUR COMPARISON OF BPI OPTIONS 37

Figure 4-2.

Size of Surface-Based BPI Interceptors Relative to the Cargo Bay

of a C-17 Aircraft

Sources: Congressional Budget Office; Duncan Lennox, ed., Jane’s Strategic Weapons Systems, vol. 39 (Coulsdon, Surrey: Jane’s Information 

Group, July 2003); and Terry Little, “Kinetic Energy Interceptors Overiew” (unclassified briefing by the Missile Defense Agency to 

Congressional Budget Office staff, November 13, 2003).

Notes: The sizes and weights shown are for an interceptor (or missile) only. The interceptor payload masses include the mass of the kill vehicle and the shroud.

BPI = boost-phase intercept; km/sec = kilometers per second; kg = kilograms; smf = structural mass fraction; tonne = metric ton

(1,000 kg); MDA = Missile Defense Agency; KEI = kinetic-energy interceptor; PAC-3 = Patriot Advanced Capability-3; THAAD = Terminal High Altitude Air Defense; GMD = Ground-Based Midcourse Defense.

If a threat arose at a latitude outside the band for which 

an SBI constellation was designed, surface-based systems 

would be better able to respond, because expanding the 

SBI constellation would be time- and satellite-intensive. 

Substantially changing the constellation’s orbital inclination would not be possible with the fuel available to SBIs 

in orbit.

Of the surface-based systems, those in Options 1 and 2 

would be more responsive than the system in Option 3 

because their smaller size would allow for easier deployment by air. The differences in weight and size between 

the three types of surface-based interceptors in CBO’s options are significant compared with the capacity of a C-17 

aircraft, the Air Force’s main strategic airlifter (see Figure 

4-2). Even with a very light kill vehicle, a 10-km/sec interceptor would weigh nearly five times more than an 8-

km/sec interceptor—over 17 metric tons (17,000 kilograms, or nearly 20 English tons). Although 17 metric 

tons is well below the payload limits of the C-17, the 

number of aircraft needed to deploy all of the equipment 

for a BPI site would still be higher under Option 3 than 

under Options 1 and 2 because each interceptor would 

require its own transport vehicle, only one of which 

would fit on an aircraft at a time. The smaller interceptors of the other surface-based options could be mounted 

in pairs on a similar transport vehicle, enabling an additional interceptor to be delivered in each aircraft. 

(About 20 meters)

Cargo Bay

3.1 tonnes

3.5 tonnes

17.2 tonnes

About 7.5 tonnes

0.32 tonnes

0.61 tonnes

22.5 tonnes

34.5 tonnes

Option 1 (Interceptor with 6-km/sec speed,

150-kg payload, 0.08 smf)

Option 2 (Interceptor with 8-km/sec speed,

50-kg payload, 0.10 smf)

Option 3 (Interceptor with 10-km/sec speed,

50-kg payload, 0.12 smf)

MDA's Notional KEI Design

PAC-3

THAAD

GMD

Minuteman III

C-17 Payloads (Unrefueled)

59 tonnes at 3,200 nautical miles

77 tonnes at 2,300 nautical miles

Existing

Designs
38 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

The issue of air mobility aside, interceptors the size of 

those in Option 3 would be best suited to basing in fixed 

silos. That is the case for the Ground-Based Midcourse 

Defense system that the Department of Defense plans to 

field by the end of 2004 (initially at Fort Greely in Alaska 

and Vandenberg Air Force Base in California). Although 

such silos are practical on home territory, permanent BPI 

installations in foreign countries would be likely to pose 

greater access problems than mobile BPI systems.

The responsiveness of surface-based BPI systems could be 

worse if they used sea bases because of the transit time 

needed to get a ship carrying the system to the correct location. Basing BPI ships overseas, much as the Army and 

Marine Corps preposition equipment overseas, could improve the responsiveness of sea-based systems. Alternatively, equipment for a ground-based BPI site could be 

flown to a theater and temporarily placed on a local ship 

to provide interim coverage until the dedicated BPI ship 

arrived.
A

CBO’s Estimates of the Costs

of Alternative BPI Systems

The Congressional Budget Office (CBO) developed 

five options for boost-phase intercept (BPI) systems to 

compare the systems’ potential effectiveness and costs. As 

a basic requirement, each option’s BPI system had to be 

capable of defending the United States against liquid-fuel 

intercontinental ballistic missiles (ICBMs) launched from 

Iran or North Korea; each alternative, however, featured a 

different interceptor to accomplish that mission. Three of 

the options were assumed to deploy surface-based interceptors; the other two options were assumed to use spacebased interceptors. In developing each alternative, CBO 

assessed the technical characteristics of the chosen interceptor, the elements that the BPI system might comprise, 

and how those elements would work together to intercept 

and destroy an enemy missile. The analysis was based on 

a number of sources, including—when they were available—existing programs’ technical requirements, schedules, and costs.

CBO has not examined all possible configurations of systems for boost-phase missile defense and how those configurations might be developed and deployed. Decisions 

about which defenses to deploy and how to deploy them 

would depend on a number of factors, including the nature and extent of the threats that the United States was 

likely to face in future years, the potential effectiveness of 

any missile defense system against such threats, and the 

possible reactions of allies and other nations to a decision 

to deploy a BPI system.

A Summary of the Options

and Their Costs

CBO estimated how much it would cost, in 2004 dollars, 

to acquire the components for the five options considered 

in this study and to operate the resulting systems for 20 

years. (See Chapter 3 for more-detailed noncost information about those components.) Total costs ranged from 

$16 billion to $37 billion for the surface-based systems 

and from $27 billion to $78 billion for the space-based 

alternatives. 

Not explicitly included in CBO’s comparison of the alternatives are the costs for the sensor architecture used to 

identify and track targets. The different systems are assumed to all use the same sensor architecture, which is 

based on the one that the Department of Defense (DoD) 

is planning to deploy to support missile defense requirements and for other purposes. For BPI, the important 

sensors are the space-based infrared satellites (SBIRSHigh) now being developed by the Air Force. In creating 

its options, CBO assumed that those satellites or a comparable system would be fielded in time to support the 

BPI systems.

Estimates of costs for systems that are defined only conceptually or that depend on the development of new 

technologies entail more uncertainty than do estimates 

for well-defined programs based on proven technologies. 

To account for the potential effects of such uncertainty, 

CBO estimated a range of costs for the BPI systems it 

evaluated. For each alternative, the low estimate represents what the systems might cost if few technical difficulties arose in making them fully operational. The high 

estimate takes into account the growth in costs that has 

been common among such systems.

Option 1: Surface-Based Interceptors 

with a Speed of 6 Kilometers per Second

Option 1 represents the kind of BPI system that might be 

developed if the Administration determined that it was 

critical to deploy boost-phase missile defenses as soon as 

APPENDIX
40 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Table A-1.

Estimated Costs of Surface-Based BPI Systems

(Billions of 2004 dollars)

Source: Congressional Budget Office.

Note: BPI = boost-phase intercept; km/sec = kilometers per second.

a. To produce the high estimates, CBO applied cost-growth factors to the low estimates. Those factors (48 percent for boosters and other 

components, 69 percent for kill vehicles, and 31 percent for communications equipment) were developed by the RAND Corporation on the 

basis of unpublished updates to a 1996 RAND report, The Defense System Cost Performance Database: Cost Growth Analysis Using 

Selected Acquisition Reports. 

b. To produce the high estimates, CBO applied cost-growth factors it had developed (either 50 percent or 100 percent, depending on the relative level of cost uncertainty) to the low estimates.

c. The components for testing are purchased during the production phase.

Option 1 

(6-km/sec interceptors)

Option 2

(8-km/sec interceptors)

Option 3

(10-km/sec interceptors)

Cost Category Low High Low High Low High

Research and Developmenta

Boosters 2.2 3.1 2.5 3.5 5.3 7.4

Kill vehicles 0.5 0.8 1.7 2.9 1.7 2.9

Other components 1.2 1.6 1.2 1.6 1.2 1.6

Test and evaluation 0.9 1.3 0.9 1.3 1.4 2.0

System integration 1.9 2.7 2.5 3.7 3.8 5.6

Subtotal 6.7 9.5 8.8 13.1 13.4 19.5

Productiona

Interceptors 2.8 3.6 3.1 4.0 4.8 6.4

Mobile launchers 0.3 0.4 0.3 0.4 0.5 0.7

Cargo ships 0.2 0.2 0.2 0.2 0.2 0.2

Battle management and

communication sets 0.1 0.1 0.1 0.1 0.1 0.1

Subtotal 3.4 4.4 3.7 4.7 5.6 7.4

Operations (Over 20 years)b

Routine operations 1.8 2.7 1.8 2.7 1.8 2.7

Ship operations 1.2 1.8 1.2 1.8 1.2 1.8

Operational testsc 1.6 3.2 1.6 3.2 1.6 3.2

Operational integration 1.0 2.0 1.0 2.0 1.0 2.0

Subtotal 5.6 9.7 5.6 9.7 5.6 9.7

Total 15.7 23.6 18.1 27.5 24.6 36.6
APPENDIX A CBO’S ESTIMATES OF THE COSTS OF ALTERNATIVE BPI SYSTEMS 41

possible. CBO estimated that acquiring the components 

of such a system and operating it for 20 years would cost 

between $16 billion and $24 billion (see Table A-1).

The interceptor used in this option could engage liquidfuel ICBMs fired from all but the largest threat countries 

and would be small enough to be easily deployed by aircraft. The interceptor would have a two-stage booster and 

a kill vehicle of 140 kilograms (kg).

For all three surface-based options, CBO assumed that 

DoD would deploy 60 interceptors in 10 mobile batteries 

(or launch sites) by 2012 and would purchase and operate three cargo ships for basing batteries at sea.1 Each site 

would comprise six interceptors mounted in pairs on 

three mobile launchers and one set of command, control, 

battle management, and communications (C2BMC) 

equipment—a configuration that would enable each site 

to engage three targets, shooting at each one twice without reloading its launchers. An additional assumption incorporated in the surface-based options was that the Missile Defense Agency (MDA) would purchase 52 more 

interceptors: 40 for operational testing and 12 for use as 

spares.

Option 2: Surface-Based Interceptors 

with a Speed of 8 Kilometers per Second

Option 2 reflects the kind of system that might be developed if DoD’s highest priority was to defeat solid-fuel 

ICBMs or to reduce the number of sites required to cover 

large countries. Acquiring the necessary components of 

this system and operating them for 20 years would cost 

between $18 billion and $28 billion, CBO estimates.

Option 2’s interceptor would be faster than Option 1’s 

but still comparably mobile, with a three-stage booster 

and a lightweight (30-kg) kill vehicle. (That size kill vehicle is similar to a concept developed by the Lawrence Livermore National Laboratory.) Moreover, this option’s interceptor would have a larger structural mass fraction 

than Option 1’s interceptor so as to survive the stresses associated with its greater acceleration.2 However, the 

structure of Option 2’s BPI system would be identical to 

that of Option 1’s: 10 launch sites, each containing six interceptors mounted in pairs on three mobile launchers 

and one set of C2BMC equipment.

Option 3: Surface-Based Interceptors 

with a Speed of 10 Kilometers per Second

The BPI system in Option 3 was designed specifically to 

counter solid-fuel ICBMs. CBO estimated that acquiring 

components for such a system and operating them for 20 

years would cost between $25 billion and $37 billion.

The greater ability to engage solid-fuel ICBMs would 

come at the expense of mobility, because in this option, 

the interceptor’s booster and mobile launcher would be 

larger and heavier than those in the other surface-based 

options. CBO assumed that the interceptor would use 

the same lightweight kill vehicle used in Option 2 but 

would require a significantly larger and higher-performance booster to achieve its faster speed. Indeed, the 

total interceptor mass (launch mass) of the interceptor in 

this option would be nearly five times that of Option 2’s 

interceptor. Moreover, this interceptor would have a 

greater structural mass fraction than any of the other 

alternatives’ interceptors because it would accelerate even 

faster than the interceptor in Option 2.

Option 3 includes the same number of launch sites, 

C2BMC equipment, and cargo ships as the other surface 

options do. Because of its larger booster, however, each 

interceptor in Option 3 would require its own mobile 

launcher.

Option 4: Space-Based Interceptors 

with a Speed of 4 Kilometers per Second

Under Option 4, a constellation of space-based interceptors (SBIs) would be deployed in low-Earth orbit, each 

interceptor having a speed of 4 kilometers per second and 

a 140-kg kill vehicle similar to that described for Option 

1. The interceptor envisioned for this option would have 

an average acceleration of 10g (10 times the Earth’s gravitational pull); its life jacket (which provides support functions such as propulsion for maneuvering in orbit) would 

have half the mass of the interceptor as a whole. CBO 

estimated that the costs to acquire the components included under this option, launch them into orbit, and 

operate them for 20 years would total between $56 billion and $78 billion (see Table A-2).

1. An inventory of three ships would allow for deployment of one or 

two sea-based sites at a given time (with the additional ships in 

transit or in maintenance).

2. The structural mass fraction is the ratio of an interceptor’s structural mass (that of all components other than the propellant and 

the payload) to its total mass.
42 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Table A-2.

Estimated Costs of Space-Based BPI Systems

(Billions of 2004 dollars)

Source: Congressional Budget Office.

Note: BPI = boost-phase intercept; km/sec = kilometers per second.

a. To produce the high estimates, CBO applied cost-growth factors to the low estimates. Those factors (48 percent for boosters, 69 percent 

for kill vehicles and other components, and 31 percent for communication equipment) were developed by the RAND Corporation on the 

basis of unpublished updates to a 1996 RAND report, The Defense System Cost Performance Database: Cost Growth Analysis Using 

Selected Acquisition Reports. 

b. To produce the high estimates, CBO applied cost-growth factors it had developed to the low estimates. It used a factor of 100 percent for 

routine operations, operational tests, and operational integration. The factors it used to increase the costs of replacement interceptors 

were the same as those used to increase production costs. The factor used to increase launch-vehicle costs was 50 percent (based on the 

actual costs of comparable systems).

Option 4

(4-km/sec interceptors)

Option 5

(6-km/sec interceptors)

Cost Category Low High Low High

Research and Developmenta

Boosters 2.7 3.8 2.0 2.9

Kill vehicles 0.5 0.8 1.7 2.9

Other components 1.0 1.3 1.6 2.4

Test and evaluation 0.9 1.1 0.8 1.0

System integration 2.0 2.8 2.4 3.7

Subtotal 7.1 9.8 8.5 12.9

Productiona

Interceptors 8.7 10.7 3.7 4.6

Launch services 7.3 11.0 1.5 2.3

Subtotal 16.0 21.7 5.2 6.9

Operations (Over 20 years)b

Routine operations 1.0 2.0 1.0 2.0

Operational tests 1.6 3.2 1.6 3.2

Operational integration 1.0 2.0 1.0 2.0

Replacement interceptors 16.1 19.9 7.2 9.0

Launch services 12.8 19.2 2.6 3.9

Subtotal 32.5 46.3 13.4 20.1

Total 55.6 77.8 27.1 39.9
APPENDIX A CBO’S ESTIMATES OF THE COSTS OF ALTERNATIVE BPI SYSTEMS 43

The constellation of SBIs that this option would deploy is 

sized to defend the United States against liquid-fuel 

ICBMs launched from locations between 25 degrees 

north latitude (southern Iran) and 45 degrees north latitude (northern North Korea).3

 It would provide a shootshoot engagement by the SBIs (two interceptors would 

engage each target) to increase the probability of a successful intercept. Achieving that capability would require 

making 28 launches with heavy-lift launch vehicles to 

place 368 SBIs in orbit.

Option 5: Space-Based Interceptors 

with a Speed of 6 Kilometers per Second

Like Option 4, Option 5 would consist of a constellation 

of space-based interceptors in low-Earth orbit. However, 

those interceptors would be lighter and faster than the 

SBIs deployed under Option 4. Acquiring and deploying 

the necessary components and operating them for 20 

years would cost between $27 billion and $40 billion, 

CBO estimates.

The interceptors to be deployed under Option 5 would 

have a speed of 6 kilometers per second and a 30-kg kill 

vehicle similar to the ones used in Options 2 and 3. This 

option’s interceptors would have an average acceleration 

of 20g and a life jacket with one-fifth the mass of the 

interceptor as a whole. Like the SBI constellation in Option 4, this option’s constellation would be sized to defend against liquid-fuel ICBMs launched from locations 

between 25 and 45 degrees north latitude and would use 

a shoot-shoot engagement strategy. The capability envisioned for Option 5 would require about 15 medium-lift 

launch vehicles to place 156 SBIs in orbit.

CBO’s Estimating Methods

The costs for the BPI systems in CBO’s options can be 

divided into three categories:

B Research and development (R&D)—the engineering 

activities needed to design and develop the booster, 

kill vehicle, and other components that make up a BPI 

system. The R&D phase also involves testing the 

hardware to ensure that it works and integrating the 

BPI system into the military’s existing infrastructure 

and support elements (for example, making sure the 

communications equipment for the BPI system can 

“talk to” that of other DoD units).

B Production—the manufacturing of interceptors and 

associated support equipment and, for the space-based 

options, the purchasing of launch services.

B Operations—the routine efforts to maintain, operate, 

and replenish a BPI system over 20 years.

The rest of this appendix discusses the methods that 

CBO used to estimate all three types of costs. Production 

costs are described before R&D costs because some of the 

methods for calculating R&D costs use estimates of production costs as inputs.

Production Costs

Production costs for the surface-based options would total between $3.4 billion and $7.4 billion, CBO estimates, 

and costs for the space-based options would total between 

$5.2 billion and $21.7 billion (see Tables A-1 and A-2). 

In general, CBO developed those estimates by calculating 

the number of interceptors that each option would require and tallying their purchase costs. CBO also determined how much the other components of each BPI system would cost, including launch services to place the 

space-based interceptors into orbit. 

Interceptors. An interceptor consists of a booster and a 

kill vehicle with a seeker (basically, one or more sensors).4

Surface-based systems include a canister for each interceptor as well, to protect it from the elements; spacebased systems use the interceptor life jacket. CBO’s estimates of the costs for each option’s interceptors include 

costs to buy those components as well as costs to integrate 

and assemble them into a BPI system.

CBO assumed that for each of the three surface-based options, DoD would purchase a total of 112 interceptors 

(60 for routine operations, 12 for spares, and 40 for system testing). Under Option 4, 405 space-based interceptors would be bought (368 for routine operations and 37 

for spares), and under Option 5, 172 interceptors (156 

for operations and 16 for spares). For the two space-based 

alternatives, CBO assumed that DoD would not pur- 3. The northernmost point in North Korea actually lies at about 

43 degrees north latitude. However, intercepts of North Korean 

ICBMs fired toward the continental United States would have to 

occur farther north.

4. For more information about kill vehicles and their components, 

see Appendix B.
44 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

Table A-3.

Estimated Costs of the First Production Units for Surface-Based BPI Systems

(Millions of 2004 dollars)

Source: Congressional Budget Office.

Notes: To produce the high estimates, CBO applied cost-growth factors to the low estimates. Those factors (38 percent for the booster and 

canister and 19 percent for the kill vehicle and seeker) were prepared by the RAND Corporation on the basis of unpublished updates to 

a 1996 RAND report, The Defense System Cost Performance Database: Cost Growth Analysis Using Selected Acquisition Reports. No 

cost-growth factor was applied to integration and assembly costs, which were estimated at 12 percent of the booster, kill-vehicle, 

seeker, and canister costs for both the low and high estimates.

BPI = boost-phase intercept; km/sec = kilometers per second.

chase any test interceptors (beyond those used during the 

R&D stage) because aging operational interceptors could 

be used for any required testing as they neared the end of 

their service lives.

CBO calculated total production costs for each option’s 

interceptors using a two-step approach. Analysts first estimated the costs of producing the first unit of each of the 

interceptor’s components. They then projected those 

costs for the rest of the purchases planned under the 

option, using learning-curve methods to account for the 

effects of quantity on unit production costs.5

Purchase of the First Interceptor. Various methods were 

used to gauge the costs for producing the first unit of 

each of the interceptor components. (All such estimates 

include a profit margin for the manufacturer and an allowance for systems engineering and program management.)

B For the booster, CBO used a cost-estimating relationship (CER) developed by Technomics that employs a 

booster’s total impulse, expressed in newtons of thrust 

times seconds of burn time, to predict its first-unit 

production costs.6

B For the canister (used only in surface-based interceptors), costs were based on estimates from MDA.

B For the kill vehicle and (in the case of space-based interceptors) the life jacket, CBO used a CER developed 

by Tecolote based on a space vehicle’s weight (in this 

case, the weight of the kill vehicle or life jacket) to separately estimate production costs for those two components.7

B For the costs of the kill vehicle’s seeker, CBO used a 

proxy: actual costs for the seeker that will be deployed 

on the newest version of the Navy’s Standard missile.

Assembling the components would add 12 percent to 

those costs, CBO estimated—a percentage that is on a 

par with Tecolote’s CER for such work.

On the basis of those calculations, CBO estimated that 

for Option 1, the first interceptor off the production line 

would cost about $33 million (before any upward adjustments were made to reflect potential cost risk, the possiOption 1

(6-km/sec interceptors)

Option 2

(8-km/sec interceptors)

Option 3

(10-km/sec interceptors)

Cost Category Low High Low High Low High

Booster 15.1 20.8 17.4 24.1 35.5 49.0

Kill Vehicle 12.2 14.5 12.2 14.5 12.2 14.5

Seeker 1.1 1.4 1.1 1.4 1.1 1.4

Canister 1.4 1.9 1.4 1.9 1.4 1.9

Integration and Assembly 3.6 4.6 3.9 5.0 6.0 8.0

Total 33.4 43.2 36.0 46.9 56.3 74.8

5. Learning-curve methods theorize that as more interceptors are 

built, the unit price of each subsequent production lot falls by a 

fixed percentage. 

6. Technomics, Inc., National Missile Defense Propulsion Cost Estimating Relationships (Santa Barbara, Calif.: Technomics, August 

2000).

7. Tecolote Research, Inc., The Unmanned Space Vehicle Cost Model, 

8th ed. (Goleta, Calif.: Tecolote Research, June 2002).
APPENDIX A CBO’S ESTIMATES OF THE COSTS OF ALTERNATIVE BPI SYSTEMS 45

Table A-4.

Estimated Costs of the First Production Units for Space-Based BPI Systems

(Millions of 2004 dollars)

Source: Congressional Budget Office.

Notes: To produce the high estimates, CBO applied cost-growth factors to the low estimates. Those factors (38 percent for the booster and 19 

percent for the kill vehicle, seeker, and life jacket) were prepared by the RAND Corporation on the basis of unpublished updates to a 

1996 RAND report, The Defense System Cost Performance Database: Cost Growth Analysis Using Selected Acquisition Reports. No 

cost-growth factor was applied to integration and assembly costs, which were estimated at 12 percent of the booster, kill-vehicle, 

seeker, and life-jacket costs for both the low and high estimates.

BPI = boost-phase intercept; km/sec = kilometers per second.

bility that production costs might exceed CBO’s estimates). Using the same methods, CBO calculated that 

the first interceptor for Option 2 would cost $36 million; 

for Option 3, $56 million; for Option 4, $31 million; 

and for Option 5, $29 million (see Tables A-3 and A-4).

To account for cost risk, CBO applied to each of its estimates for the interceptor’s components factors that reflected historical cost growth for comparable systems. For 

example, on the basis of information reported by the 

RAND Corporation, CBO estimated that production 

costs for the booster and canister could grow by about 38 

percent and that production costs for the kill vehicle, 

seeker, and life jacket could rise by about 19 percent.

Subsequent Purchases of Interceptors. Costs for the rest of 

the interceptors that would be purchased under each option were estimated by analyzing the trends in actual costs 

for the Ground-Based Midcourse Defense system interceptor that MDA is now purchasing and plans to deploy 

later this year. CBO’s analysis indicated that by doubling 

the quantity being purchased, unit costs could be reduced 

by about 5 percent. Buying the 112 interceptors called 

for under Option 1 would result in an average cost per interceptor of about $25 million, according to CBO’s low 

estimate. Total interceptor production costs under Option 1 would range from $2.8 billion (without taking cost 

risk into account) to $3.6 billion (with cost risk).

CBO used the same learning-curve method to estimate 

total interceptor production costs for the remaining options. Producing 112 of Option 2’s interceptors would 

cost between $3.1 billion and $4.0 billion, and producing the same number of Option 3’s interceptors would 

cost $4.8 billion to $6.4 billion (see Table A-1). For the 

space-based systems, production costs for the 405 interceptors in Option 4 would total between $8.7 billion and 

$10.7 billion, CBO estimated, compared with $3.7 billion to $4.6 billion for the 172 interceptors in Option 5 

(see Table A-2).

Other Components. In addition to interceptors, each of 

the surface-based options would require either 30 or 60 

mobile launchers, 10 sets of C2BMC equipment (one at 

each site), and three cargo ships. Options 1 and 2 envision using 30 mobile launchers, each configured to 

launch two interceptors; Option 3 would require 60 

mobile launchers because each launcher would be able to 

fire only one of the larger interceptors used in that alternative. 

The space-based options would require not mobile 

launchers but rather launch vehicles to carry the interceptors into orbit. The number of launches necessary to deploy the space-based interceptors under each option 

would depend on the payload lift capability of the launch 

vehicle and the total mass of the constellation of intercepOption 4

(4-km/sec interceptors)

Option 5

(6-km/sec interceptors)

Cost Category Low High Low High

Booster 7.8 10.8 5.8 8.1

Kill Vehicle 12.2 14.5 12.2 14.5

Seeker 1.1 1.4 1.1 1.4

Life Jacket 6.6 7.9 6.6 7.9

Integration and Assembly 3.3 4.1 3.1 3.8

Total 31.1 38.7 28.9 35.7
46 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

tors. By CBO’s estimates, the interceptors in Option 4 

would be most efficiently launched by heavy-lift launch 

vehicles, and those in Option 5 by medium-lift vehicles. 

Option 4’s constellation would have about six times more 

mass than Option 5’s because it would include nearly 

three times as many interceptors and each interceptor 

would weigh nearly twice as much.

Using those weight calculations and lift capacities, CBO 

estimated that Option 4 would require 28 launches using 

heavy-lift launch vehicles, and Option 5 would require 

15 launches using medium-lift vehicles. CBO assumed 

that those options could forgo C2BMC equipment because the interceptors would rely on existing space-related 

command and communications facilities, such as those 

located in Colorado at Cheyenne Mountain or Schriever 

Air Force Base.

In the surface-based options, production costs for components other than interceptors would make up a relatively modest share of each option’s total costs. Costs for 

other components required under both Option 1 and 

Option 2 would total about $0.6 billion ($0.3 billion for 

mobile launchers, $0.2 billion for cargo ships, and $0.1 

billion for C2BMC equipment). Those costs would be 

slightly higher in Option 3 (by about $0.2 billion) because of the 30 additional mobile launchers (see Table A1).

In the space-based options, launch services would cost between $7.3 billion and $11.0 billion for Option 4, CBO 

estimates, and between $1.5 billion and $2.3 billion for 

Option 5 (see Table A-2). CBO based its estimates on the 

price of launch services today, deriving a cost of about 

$11,000 per kilogram of payload lift capacity. Applying 

that factor, CBO estimated that a launch on a mediumlift vehicle—one with a lift capacity of 8,300 kg (8.3 metric tons, or tonnes)—would cost about $100 million. 

(That cost covers both the vehicle itself and its launching 

of the interceptor into orbit). A launch on a heavy-lift vehicle—one with a lift capacity of 23 tonnes—would cost 

about $250 million.

On the basis of information reported by the RAND Corporation, CBO estimated that cost growth for other components of a BPI system could amount to about 38 percent for mobile launchers, about 13 percent for C2BMC 

equipment, and about 20 percent for cargo ships.8 For 

the launch vehicles, CBO calculated a cost-growth factor 

of about 50 percent on the basis of the costs for existing 

vehicles, such as the Delta II, Atlas II, and Titan IV systems. 

Research and Development Costs

CBO estimated that R&D would cost between $6.7 billion and $19.5 billion for the surface-based BPI options 

and between $7.1 billion and $12.9 billion for the spacebased options. To arrive at those numbers, CBO’s general 

method was to estimate how much it would cost to develop the BPI system in Option 1, calculating development costs for each major element (the booster, the kill 

vehicle, and the other components) as well as costs for 

testing, evaluation, and system integration (the top-level 

engineering and management efforts needed to support 

the detailed design work of the R&D phase). CBO then 

estimated costs for the remaining options by adjusting 

the Option 1 figures to reflect technological hurdles inherent in the other alternatives. For options that would 

require significant advances in technology relative to that 

envisioned for Option 1, CBO increased the costs. For 

options that represent less-robust technology, CBO lowered the costs.

Boosters. On the basis of information provided by 

MDA, CBO estimated that R&D costs for the booster 

that Option 1 would deploy would total about $2.2 billion—approximately half of CBO’s recent estimate of 

R&D costs for the Ground-Based Midcourse Defense 

system interceptor. CBO considered MDA’s data a more 

reasonable starting point than its estimate because the 

BPI system would probably be slower than the midcourse 

system and use more “off-the-shelf” components in its 

production.

For boosters in the other surface-based options, CBO assumed that their research and development costs would 

be proportional to their production costs. Using the ratio 

of the first-unit production costs for the surface-based 

boosters as a proxy, CBO estimated that developing the 

booster for Options 2 and 3 would cost $2.5 billion and 

$5.3 billion, respectively (without cost growth).

8. Jeanne M. Jarvaise, Jeffrey A. Drezner, and D. Norton, The 

Defense System Cost Performance Database: Cost Growth Analysis 

Using Selected Acquisition Reports, MR-625-OSD (Santa Monica, 

Calif.: RAND Corporation, 1996). The cost-growth factors that 

CBO used in its analysis were based on unpublished updates of 

that report.
APPENDIX A CBO’S ESTIMATES OF THE COSTS OF ALTERNATIVE BPI SYSTEMS 47

For the SBI boosters, CBO also used the ratio of the firstunit production costs as a proxy. It estimated that booster 

development for Options 4 and 5 would cost $1.2 billion 

and $0.9 billion, respectively. CBO then increased those 

costs by a factor of 2.3, bringing the estimates to $2.7 billion for Option 4 and $2.0 billion for Option 5. That 

factor was applied to account for the cost risk associated 

with developing space-based systems that incorporate 

technologies designed for surface-based operations. It was 

based on the cost growth of analogous hardware—specifically, the estimated R&D costs of a space-based radar 

program (about $9 billion) and the actual costs of the 

JSTARS radar program (about $4 billion).

CBO also derived estimates for booster R&D that accounted for the risk of cost growth. Those estimates 

range from $3.1 billion to $7.4 billion for the surfacebased options and $2.9 billion to $3.8 billion for the 

space-based options. Analysts applied a factor of 41 percent to reflect historical cost growth for earlier booster 

development programs, such as for the Minuteman and 

Trident missiles.

Kill Vehicles. Developing the kill vehicle used in Options 

1 and 4 would cost a total of $0.5 billion to $0.8 billion, 

CBO estimates—a relatively small amount compared 

with the cost of developing the kill vehicle that MDA 

plans to use with its midcourse interceptors. The reason 

for the lower cost is that a BPI system would benefit from 

that previous development effort by using existing or 

modified components to reduce R&D costs.

CBO assumed that Options 2, 3, and 5 would require development of a new, miniaturized kill vehicle. Although 

production costs for that kill vehicle were assumed to be 

the same as for the heavier 140-kg vehicle, CBO estimated that R&D costs would be higher because new 

technologies would be needed to make the kill vehicle 

smaller but just as effective. Using information from 

Lawrence Livermore, CBO estimated that designing a 

miniaturized kill vehicle would cost about $300 million; 

a program of five integrated flight tests and several 

ground tests would boost those costs by $750 million. In 

CBO’s estimation, after adding allowances for profits and 

project management and systems engineering, developing 

a miniaturized kill vehicle would cost about $1.7 billion 

to $2.9 billion (assuming a factor of 69 percent to account for historical cost growth in space programs). 

Other Components. CBO used information from MDA 

to estimate R&D costs for the other components of a BPI 

system. Those costs would total about $1.2 billion (without cost growth) for each of the surface-based options: 

$0.2 billion for the canister, $0.3 billion for the mobile 

launcher, and $0.7 billion for C2BMC equipment. For 

the space-based options, R&D costs for other components would total $1 billion in the case of Option 4 and 

$1.6 billion in the case of Option 5. Those figures include $0.7 billion for C2BMC equipment as well as $0.3 

billion for the life jacket in Option 4 and $0.9 billion for 

the life jacket in Option 5.

To account for cost risk, CBO increased those low estimates by factors ranging from 31 percent to 69 percent—

depending on the historical cost growth for each type of 

component. Those high estimates range from $1.6 billion 

for the surface-based options to between $1.3 billion and 

$2.4 billion for the space-based alternatives.

Test and Evaluation. Costs for testing and evaluation 

cover the hardware needed to conduct integrated flight 

tests and the analysis required to evaluate those tests. On 

the basis of information from MDA, CBO assumed that 

the test and evaluation program for Option 1 would consist of five integrated flight tests and that conducting and 

evaluating those tests would cost a total of about $0.9 billion. For the other options, CBO assumed that test and 

evaluation costs would vary with the first-unit production 

costs of the interceptors and would total between $0.8 

billion and $1.4 billion. The estimates that account for 

cost risk range between $1.0 billion and $2.0 billion for 

all of the options, reflecting the historical cost growth of 

analogous hardware.

System Integration. CBO assumed that system integration would add 40 percent to the total costs for interceptors, other components, and the test and evaluation 

phase—a percentage consistent with the costs for programs that deploy complex state-of-the-art technologies, 

such as MDA’s midcourse interceptor. Costs for system 

integration range from $1.9 billion to $5.6 billion, CBO 

estimates.

Operations Costs

In general, CBO estimated the routine costs for operating 

the BPI systems for a 20-year period as well as the costs 

for conducting periodic tests of the interceptors during 

operations (to ensure that they remained effective and 

safe) and for providing continual engineering support. 
48 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

For the space-based options, CBO also estimated how 

much it would cost to buy replacement interceptors and 

launch them into orbit. Twenty years’ operation of the 

surface-based BPI systems would cost between $5.6 billion and $9.7 billion, compared with $13.4 billion to 

$46.3 billion for the space-based systems.

Routine Operations. For Options 1 through 3, CBO assumed that the surface-based BPI systems would be operated similarly to the Army’s Patriot missile units. Each 

launch site would require about 100 people, operating 

three to six mobile launchers and one set of C2BMC 

equipment. (CBO based those assumptions on information provided by MDA.) Judging from the Patriot units, 

each site would cost about $10 million a year to operate, 

CBO estimates. Over 20 years, operating 10 sites would 

cost a total of about $1.8 billion (without cost growth).

For the space-based options, CBO analyzed the actual 

costs of operating the Global Positioning System 

(GPS)—a constellation of 24 satellites. (Its size makes its 

operational challenges analogous to those confronting the 

operators of a BPI system.) On the basis of that comparison, CBO estimated that operating a constellation of 

space-based interceptors would cost about $50 million a 

year under either Option 4 or Option 5 and would total 

about $1 billion over a 20-year span.

To account for the uncertainty in all such estimates—and 

to capture that uncertainty in the assumptions it used to 

develop the estimates for routine operations costs—CBO 

applied a cost-growth factor of 50 percent for the surfacebased BPI systems and a cost-growth factor of 100 percent for the space-based systems. The resulting high estimates of routine operations costs were $2.7 billion and 

$2.0 billion, respectively. The cost-growth factor was 

larger for the space-based interceptors because of the 

greater uncertainty attached to the assumption that a 

constellation consisting of as many as 368 interceptors 

would cost the same amount to operate as a constellation 

of 24 GPS satellites.

Estimated costs to operate the cargo ships that would deploy some of the surface-based interceptors were based on 

actual costs to operate Navy logistics support ships. In 

CBO’s estimation, an inventory of three ships would be 

necessary to ensure that two were continually available. 

Operating those ships would cost a total of about $90 

million a year, CBO estimates, and $1.8 billion over 20 

years. CBO used those amounts for its high estimates—a 

departure from its estimating method for other components, in which it first established the low estimate and 

then determined the high estimate on the basis of historical cost growth. In the case of the cargo ships, CBO decreased the above costs by 33 percent to arrive at a low estimate that reflected a slower operating tempo than that 

of logistics support ships.

Operational Tests. In general, operational tests have three 

components: the targets that are launched to simulate enemy missiles, the interceptors that are fired at those targets, and the analysis of the data from the test. CBO assumed that for any of the BPI systems, MDA would 

conduct a total of 40 operational tests over the 20-year 

period of this analysis. Using information from MDA, 

CBO estimated that each test would cost about $40 million—about $20 million for the targets and their launch 

services and another $20 million for data analysis. Thus, 

40 operational tests over 20 years would cost $1.6 billion, 

CBO estimates. (Costs for operational testing would be 

the same in the space-based options as in the surfacebased alternatives because the tests of SBIs would use the 

same target set and employ similar analyses as the tests of 

surface-based interceptors.) The costs of the interceptors 

used in the tests are classified as production costs and 

thus are not included here.

In taking cost uncertainty into account, CBO incorporated assumptions that the costs for the targets could increase to $40 million each (to allow MDA to test the interceptor against more-sophisticated threats) and that the 

costs for analyzing the additional data could rise to $40 

million as well. On the basis of those assumptions, CBO 

estimated that the cost of operational testing would increase twofold, to about $3.2 billion.

Ongoing Operational Integration. After the BPI systems 

were deployed, some engineering work would continue 

during their operational life to resolve problems and incorporate new technologies. CBO assumed that MDA 

would retain a cadre of about 200 engineers to provide 

that support. Such ongoing operational integration 

would cost about $50 million a year, CBO estimated, or 

about $1 billion over 20 years. For comparison, CBO 

looked at the average annual spending expected to occur 

during the R&D phase. Under Option 1, CBO estimated 

that MDA would spend $6.7 billion over an eight-year 

span in the R&D phase, or about $800 million a year; 

$50 million is slightly less than 10 percent of that annual 

average. CBO’s higher estimate for ongoing operational 
APPENDIX A CBO’S ESTIMATES OF THE COSTS OF ALTERNATIVE BPI SYSTEMS 49

integration (to account for cost uncertainty) was $2 billion for the 20-year period.

Replacement of Space-Based Interceptors. CBO assumed that the space-based interceptors in Options 4 and 

5 would have a life span of about seven years. Thus, over 

the 20-year period considered in this analysis, they would 

need to be replaced two or three times to sustain the system’s effectiveness.

CBO estimated the costs for replacement interceptors in 

the same way that it calculated the costs for producing 

the first interceptor under each option. For Option 4, replacement interceptors would cost a total of $16 billion 

to $20 billion, and for Option 5, between $7 billion and 

$9 billion. MDA would also need to purchase additional 

launch services to put the new interceptors into orbit. In 

CBO’s estimation, the costs for those services would total 

between $13 billion and $19 billion for Option 4 and between $3 billion and $4 billion for Option 5. CBO’s high 

estimates reflect historical cost growth for satellite systems, boosters, and launch services.

B

Kinetic Kill Vehicles

for Boost-Phase Interceptors

The type of kinetic-energy interceptors described in 

this report consist primarily of a rocket booster and a kill 

vehicle. The booster accelerates its kill vehicle to a high 

velocity and carries it toward the predicted intercept 

point. The booster is jettisoned after its fuel is expended, 

and the kill vehicle completes the engagement. As the intercept point approaches, the kill vehicle must identify its 

target using onboard sensors and then correct its course 

to achieve a hit. This appendix briefly discusses the two 

primary components of a kinetic-energy kill vehicle—

sensors for target identification and tracking, and a divert 

and attitude control system (DACS) for maneuvering.

Sensors

The “eyes” of a kill vehicle typically include seekers (basically, one or more sensors) that “acquire” the target and 

help guide the vehicle to the final intercept point. Seekers 

may be active or passive. Passive seekers exist for a broad 

portion of the electromagnetic spectrum, including 

short-, medium-, and long-wave infrared as well as ultraviolet and visible wavelengths. Active seekers may include 

conventional radar or laser imagers or rangers.

The selection of a seeker depends on the characteristics of 

the target. For example, a midcourse seeker must detect 

relatively small, cold warheads moving above the atmosphere and distinguish them from decoys that might be 

deployed to fool missile defenses. In contrast, a boostphase seeker has the seemingly simple task of homing in 

on a very hot, bright ballistic missile rocket. During the 

boost phase, however, a ballistic missile’s signature comprises both the missile body itself and the large rocket 

plume. At high altitudes, the plume “blooms” around the 

missile—in effect, creating a smoke screen of hot exhaust 

gas that, depending on the kill vehicle’s angle of approach, can obscure the body of the rocket. A kill vehicle 

must be able to detect and hit the missile within the 

plume.

That so-called plume/hard-body problem could be solved 

in several ways. One would be to select a single sensor 

wavelength band and combine it with signal processing 

to distinguish the plume from the missile. Although that 

idea is attractive in principle—a one-color seeker is likely 

to be simpler and less expensive than other alternatives—

in practice, the electromagnetic signature of a ballistic 

missile in the boost phase may be too complex for that 

method to be successful. The Missile Defense Agency 

plans to conduct the Near-Field Infrared Experiment 

(NFIRE) to better understand the characteristics of that 

signature for intercontinental ballistic missiles (ICBMs).

The information that NFIRE might provide would also 

be helpful in relation to a second alternative: the use of a 

multicolor seeker sensitive to two or more wavelength 

bands. Although both a missile and its plume are hot and 

bright, they have different spectral characteristics. If signal processors could operate in two or more wavelength 

bands, they might be able to subtract the plume’s contribution to the image seen by the seeker from that of the 

missile, leaving behind only the missile’s characteristics 

for targeting. However, a multicolor seeker suitable for 

use in boost-phase interceptors would be more difficult 

and costly to produce than a one-color seeker.

A third approach to differentiating between the plume 

and the hard body would be to augment a passive seeker 

APPENDIX
52 ALTERNATIVES FOR BOOST-PHASE MISSILE DEFENSE

with an active seeker to detect the missile body in the 

“end game” (the time just before the kill vehicle hits its 

target). Light detection and ranging (lidar) systems that 

use a laser to penetrate the plume and locate the missile 

body have been proposed for that application. However, a 

lidar system’s potential to improve the probability of hitting the target must be weighed against its disadvantages, 

which include increased complexity, weight, and costs relative to other alternatives.

The Maneuvering System

The other primary component of a kill vehicle is the 

DACS—the propulsion package that not only gives the 

vehicle maneuvering capability for the intercept but also 

keeps it balanced and pointing in the right direction. The 

maneuverability that the DACS provides is typically measured in terms of divert velocity. As with the seekers, the 

capabilities needed from the DACS depend on the behavior of the target and the accuracy of the interceptor’s 

tracking. In general, the greater the total course correction needed to keep a kill vehicle directed toward its target, the greater the divert velocity the vehicle requires. 

The predictable ballistic trajectories of midcourse targets 

allow the use of kill vehicles with relatively low divert velocities—typically less than 1 kilometer per second (km/

sec). During the boost phase, however, a target missile’s 

trajectory may be more uncertain. For boost-phase intercept, conclusions from a study by the American Physical 

Society, as well as similar (unpublished) results from engineers at Lawrence Livermore National Laboratory, support the view that intentional evasive maneuvers (if any) 

by an ICBM or unintentional variations in its thrust can 

require that a kill vehicle have greater divert capability—

perhaps as much as 2.5 km/sec.1 But a higher-performance DACS usually comes at the expense of greater 

weight, because larger thrusters are needed as well as 

more fuel (and the tanks to hold it).

A DACS can be of either the solid- or liquid-fuel type, 

and liquid-fuel designs can be either pressure- or pumpfed. Most existing kill vehicles use pressure-fed liquid 

propellant, although the Navy is testing a solid-fuel 

DACS (which will be compatible with shipboard safety 

restrictions) for its SM-3 missile. Pressure-fed systems are 

simpler than pump-fed designs but require heavier propellant tanks and larger nozzles and combustion chambers. Pump-fed systems, such as those being developed by 

Lawrence Livermore, are more complex mechanically 

than pressure-fed systems of a similar mass, but they also 

offer greater divert velocity. Despite some testing of 

pump-fed concepts, the Department of Defense has 

announced no plans for their operational use. Thus, to 

specify the use of pump-fed systems in liquid-fuel DACS 

designs would pose greater technological challenges than 

would specifying the use of a pressure-fed system.

1. See Report of the American Physical Society Study Group on BoostPhase Intercept Systems for National Missile Defense: Scientific and 

Technical Issues (Washington, D.C.: American Physical Society, 

July 2003), available at www.aps.org/public_affairs/popa/reports/

nmd03.cfm. Engineers at Lawrence Livermore National Laboratory communicated similar but unpublished conclusions to CBO 

staff.
Glossary of Abbreviations

ABM: Anti-Ballistic Missile (Treaty)

APS: American Physical Society

BMD: ballistic missile defense

BMDS: ballistic missile defense system

BPI: boost-phase intercept

C2BMC: command, control, battle management, and 

communications

CBO: Congressional Budget Office

CER: cost-estimating relationship 

DACS: divert and attitude control system 

DoD: Department of Defense

g: a unit of force equal to the Earth’s gravitational pull

ICBM: intercontinental ballistic missile

Isp: specific impulse

kg: kilogram

km: kilometer

MDA: Missile Defense Agency

NFIRE: Near-Field Infrared Experiment 

NMD: national missile defense

PAC-3: Patriot Advanced Capability-3

R&D: research and development

SBI: space-based interceptor

SBIRS: Space-Based Infrared System

SLV: space-launch vehicle

THAAD: Terminal High Altitude Air Defense