Page 1 GAO-05-316R Joint Strike Fighter Refueling

United States Government Accountability Office

Washington, DC 20548 

March 14, 2005 

The Honorable John Warner 

Chairman 

The Honorable Carl Levin 

Ranking Minority Member 

Committee on Armed Services 

United States Senate 

Subject: Air Force Assessment of the Joint Strike Fighter’s Aerial Refueling Method 

The Joint Strike Fighter (JSF) acquisition program is estimated to cost $245 billion to 

develop and produce three variants of stealthy fighter aircraft—a conventional takeoff and landing variant for the Air Force, an aircraft carrier variant for the Navy, and a 

short take-off and vertical landing variant for the Marine Corps and Air Force. A 

major goal of the JSF program is to reduce costs by maximizing commonality among 

variants. However, the Air Force conventional variant is being designed with a 

different aerial refueling method than those used by the two other JSF variants. U.S. 

fighters use two different methods for aerial refueling. Air Force fixed-wing aircraft 

are all currently fueled by a boom that extends from a tanker aircraft and is guided 

into a receptacle. The Navy and Marine Corps fighters use a probe that extends from 

the fighter to receive fuel when inserted into a drogue, which is a basket-like device 

on the end of a hose that extends from the tanker. 

The Senate Armed Services Committee directed that we (1) examine the rationale 

behind the Air Force refueling decision for its JSF version, (2) determine the savings 

if the Air Force decided to change its refueling method on its JSF, and (3) determine 

the operational advantages or disadvantages if the Air Force decided to change its 

refueling method. This letter summarizes the information we provided committee 

staff on December 16, 2004. 

To determine the Air Force rationale for selecting a refueling method that is different 

from the refueling method for the other variants, we reviewed JSF requirements 

documents, an evaluation by the JSF contractor of JSF refueling options, and design 

alternatives for each option. To assess the advantages and disadvantages of each 

method, we reviewed Air Force assessments of operational lessons learned and 

mishap reports for aerial refueling missions. To determine whether there were 

potential savings if the Air Force were to change its refueling, we reviewed an Air 

Force estimate of savings and the cost of impacts to other assets, such as tankers, to 

support this change. We also discussed relevant information with program officials 

from the joint JSF program office, the Air Combat Command, and the Air Mobility 
Page 2 GAO-05-316R Joint Strike Fighter Refueling 

Command. We conducted our review between August 2004 and January 2005 in 

accordance with generally accepted government auditing standards. 

Summary

Revising the Air Force’s conventional JSF aircraft refueling method from a boom to a 

probe and drogue would require modifying most of its current KC-135 tanker aircraft at 

an estimated cost of $2.5-$3.5 billion according to the Air Force. This estimated cost far 

outweighs the cost savings gained from having a common JSF refueling configuration 

estimated to be about $180 million for the JSF program. However, the Air Force estimate 

of costs and savings does not consider the future tanker acquisition and potential cost 

savings if the Air Force JSF was refueled by the probe and drogue method. For example, 

a Rand Corporation study concluded that tanker requirements could be reduced from 17 

percent to 50 percent depending upon specific warfighting scenarios if a tanker has the 

capability to refuel simultaneously two aircraft with the probe and drogue method. 

Using the current inventory of KC-135 tankers as a replacement baseline and a 

Congressional Budget Office1 estimate of $150 million for a new tanker, a 17-percent 

reduction in the number of tankers required could equate to an estimated savings of 

$13.7 billion. The Air Force assessment of JSF refueling requirements did not fully 

address advantages and disadvantages of each method to make it clear whether a change 

is beneficial to DOD. 

Background 

The Air Force’s tanker fleet of KC-10 and KC-135 aircraft provides the bulk of military 

aerial refueling services. These are large, long-range commercial aircraft modified into 

tankers. See figures 1 and 2. 

 

1

Letter to the Honorable John McCain, United States Senate, May 7, 2002, from Dan L. Crippen, Director, Congressional Budget 

Office. 
Page 3 GAO-05-316R Joint Strike Fighter Refueling

Figure 1: KC-135 refuels a flight of F-15 aircraft with a boom 

Source: U.S. Air Force.

Figure 2: KC-10 refuels an F-18 with a probe and drogue 

Source: U.S. Air Force.

Each KC-10 has a centerline boom and a drogue so that it can refuel any U.S. fixed wing 

aircraft. Further, 20 have been modified with drogue and hose on each wing that allows 

two fighters to simultaneously refuel—commonly called “multipoint.” While each KC135 has a boom and can be configured with a drogue adaptor to refuel probe-equipped 

fighters, the configuration must take place on the ground before take-off. There are 
Page 4 GAO-05-316R Joint Strike Fighter Refueling 

currently only enough adapters to configure one-half the KC-135 fleet. The following 

table shows the refueling capability of the current Air Force tanker aircraft. 

Table 1: Current U.S. Air Force Tanker Fleet and Refueling Capabilities 

 KC-10 tanker KC-135 tanker 

Number in fleet 59 534 

Capability • All equipped with centerline 

boom and drogue refueling 

capability 

• 20 modified with wing air 

refueling pod system for 

multipoint drogue refueling 

capability 

• All equipped for centerline 

boom refueling 

• 20 modified with drogue 

multipoint refueling system 

• About half can be configured 

for centerline drogue refueling

Average age 22 years 44 years 

Source: Air Force. 

The Air Force is currently evaluating several alternatives including a new tanker to 

replace its aging KC-135 fleet. According to its mission need statement a new tanker 

should be equipped with a centerline boom and drogue capability so it can refuel both 

types of receiver aircraft on the same mission. Further, it should eventually be capable of 

refueling multiple aircraft simultaneously. The current and future tanker force 

capabilities are important factors in determining the type of refueling method selected 

for the JSF. 

Evaluation Results 

The Air Force plans to continue its reliance on boom refueling for its conventional takeoff and landing JSF variant. The operational requirement document specified that this 

variant would use the standard Air Force refueling technique, which is the boom from its 

existing KC-10 and KC-135 tanker fleet. A JSF program official told us that during the 

requirements development process no evaluation was made on using a common 

refueling method and the requirement was driven by the existing and planned refueling 

assets for each service. 

Adopting the probe and drogue method of refueling for the Air Force conventional takeoff and landing JSF version is technically feasible and would save the JSF program about 

$180 million according to an assessment made by the JSF contractor (Lockheed Martin). 

Of the total, an estimated $176.2 million in production savings would accrue because the 

parts for a probe system are less expensive than a boom refueling system. However, the 

Air Force estimates it would cost between $2.5 billion and $3.5 billion to modify 415 KC135 tankers to support a fleet of 1,763 Air Force JSF variants. The cost estimate is based 

on a budget estimate by Boeing Wichita, the KC-135 contractor, that the cost to add a 

multipoint refueling capability is $6.1 million to $8.4 million per tanker depending on the 

production lot size, 5 or 20 tanker aircraft. The Air Force used a simple ratio to estimate 

the number of tankers that would need modification. It assumed that 70 percent of its 

fighter force in 2020 would be JSFs. Therefore, 70 percent of its tanker force (415 

tankers) should be capable of multipoint refueling. 
Page 5 GAO-05-316R Joint Strike Fighter Refueling

In addition to the cost to modify the KC-135 tanker, Air Force officials stated that the 

boom method is less likely to damage the low observable feature of the JSF aircraft and, 

therefore, reduce its vulnerability to enemy air defenses. Thus, the chance for a 

successful mission is increased. In contrast to a stable boom, air turbulences are more 

likely to move the drogue basket and strike the JSF aircraft, leaving it more vulnerable to 

enemy air defenses because its low observable feature has been degraded. While the 

boom method has several operational advantages, a major disadvantage is its inability to 

simultaneously fuel two fighters with the multipoint capability provided with the probe 

and drogue method. The advantages and disadvantages associated with each method are 

shown in table 2. 

Table 2: Analysis of Advantages and Disadvantages for Using Boom and Drogue 

Refueling Techniques on the Air Force JSF Variant 

Boom Drogue 

Advantages • No modification of existing 

tankers for multipoint 

refueling or purchase of 

additional boom to drogue 

adapters 

• Less chance to damage to JSF 

low observable features 

• Weighs 4.1 pounds less than 

drogue assembly 

• Permits refueling by half the 

KC-135s and the British L1011 

tanker 

• Permits multipoint refueling 

on 35 US tankers 

• Increased JSF variant 

commonality 

• Less time to refuel a 4-aircraft 

JSF flight 

• Avoids $3.19 million in 

developmental cost 

• Improved allied 

interoperability 

• Saves $180 million in JSF 

total cost 

• Could reduce number of new 

tankers 

Disadvantages • No refueling with British 

L1011 tanker 

• Loss of $180 million in JSF 

production and operations 

and maintenance savings 

• No multipoint refueling 

• Less JSF variant commonality 

• Requires $2.5-$3.5 billion 

modification to KC-135s 

• Requires change in joint 

common specifications 

• Potential to damage JSF low 

observable features 

• Adds 4.1 pounds to aircraft 

weight 

• Drogue adapter requires 

preflight reconfiguration of 

KC-135 on the ground 

• Half of KC-135s cannot refuel 

with centerline drogue 

Source: GAO analysis based on DOD data. 
Page 6 GAO-05-316R Joint Strike Fighter Refueling 

In assessing the options for refueling the JSF, Air Force Air Combat Command did not 

consider the potential for reducing the number of new tankers by using multipoint 

refueling. For example, Rand2 identified three comprehensive studies3 that concluded 

that the use of multipoint refueling could reduce tanker requirements by 17 to 50 percent 

depending on a particular scenario. If new tankers with multipoint capability replace all 

KC-135 tankers, a 17-percent reduction would reduce the requirement by 91 aircraft (534 

x .17 = 91). The Congressional Budget Office estimated that a new tanker would cost 

$150 million. Thus, a 91-aircraft reduction would save an estimated $13.7 billion. 

Additionally, the multipoint refueling offers improved operational efficiencies by 

refueling two aircraft at a time. However, quantifying the number of new tankers that 

could be reduced and the improved operational efficiencies would require a detailed 

study. 

In its estimates of the number of KC-135s needed to be modified to support an Air Force 

probe and drogue version, the Air Force did not consider that some of the KC-135s would 

be replaced by the new tanker. By contrast, the JSF contractor, in estimating savings to 

the JSF program, projected that 170 new tankers may be in the inventory by the year 

2020, which would reduce the number of KC-135s requiring modification. Quantifying 

this reduction, however, would require a detailed study. In addition, the Air Force did 

not consider the following factors in its projection. 

• The Air Force recently delayed delivery of the first JSF aircraft by 1 year. 

Additional delays could occur in the JSF program given its early stage in the 

development process. If this happens, the Air Force would not need to modify as 

many KC-135s to support the JSF fleet. 

• In December 2004, Air Combat Command officials told us that the Air Force is 

considering buying about 250 short takeoff and landing variant JSFs and about 

1,300 conventional takeoff and landing JSFs. This would reduce the total number 

of Air Force JSFs to be acquired by 213 and could reduce the total number of 

tankers required to refuel the JSF fleet. 

Also, the Air Force’s belief regarding the adverse low observable effect caused by the 

drogue method lacked a technical engineering assessment on the likelihood and the 

extent of low observable damage, according to Air Combat Command officials. It also 

lacked an operational assessment to determine the impact on mission success caused by 

the damage. 

Additional factors could also influence decisions about the refueling method for the Air 

Force variant of the JSF. First, as the JSF moves forward in development, the decision to 

change to a probe and drogue method will become more costly. Key points when the 

cost of design change increases include the critical design review scheduled in fiscal year 

2006 and production scheduled to start in fiscal year 2007. Also, JSF quantities and 

delivery schedules could change as a result of the Quadrennial Defense Review in 2005. 

 

2

Bowie, C.J., et al., Enhancing USAF Aerial Refueling Capabilities, Santa Monica, Calif.: RAND, R-3801-AF, 1990 (for government 

use only; not available to the public). 3 Copeland, Donald, et al., Multipoint Refueling Program Cost Benefit Analysis Final Report, Santa Barbara, Calif.: Frontier 

Technology, Inc., 1995 (for government use only; not available to the public).George, William L., Lt. Col., USAF, Utility of KC-135 

Multipoint Modification, Washington, D.C.: Air Force Studies and Analyses Agency, 1992 (for government use only; not available to 

the public). 
Page 7 GAO-05-316R Joint Strike Fighter Refueling

Conclusions

The advantages and disadvantages of a decision to change the Air Force variant to the 

probe and drogue method of refueling can change based on a wider range of 

considerations than included in the Air Force’s assessment. If the assessment includes 

only the JSF and other related legacy systems, it appears the cost to modify the 415 KC135 aircraft at as much as $3.5 billion is the overwhelming deciding factor and suggests 

that keeping the Air Force variant as a boom refueled aircraft is cost effective. However, 

if the assessment is expanded to include potential reductions in quantities of a new Air 

Force tanker because of efficiencies gained by multipoint refueling, it is not clear where 

the larger cost benefit to DOD lies. 

Agency Comments and Our Evaluation 

This correspondence is being issued without agency comments because DOD did not 

provide comments within the time allotted. 

We are sending copies of this report to the Secretary of Defense; the Director, Office 

of Management and Budget; and interested congressional committees. We will also 

make copies available to others upon request. In addition, the report will be available 

at no charge on the GAO Web site at http://www.gao.gov. 

Should you or your staff have any questions on matters discussed in this report, please 

contact me on (202) 512-4163 or Michael Hazard on 937-258-7917. Principal contributors 

to this report were Richard Strittmatter, Dayna Foster, Marie Ahearn, and Karen Sloan. 

Michael J. Sullivan 

Director, Acquisition and Sourcing Management

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