Nuclear Technology and the Space Exploration Missions

Henry W. Brandhorst and Ronald J. Sovie

NASA Lewis Research Center

Cleveland, OH 44135

00

u_

I

ABSTRACT

The strategy for a major exploration

initiative leading to permanent human

presence beyond earth orbit described

by President George Bush July 20,

1989 is still being developed;

however enough is known to begin

defining the role of nuclear

technologies. Three broad areas will

be discussed: low power (<I0 kWe)

rover/vehicle power systems,

integrated, evolutionary base power

systems (25-100 kW) and nuclear

energy for electric propulsion (2-

i00 MWe) and direct thermal

propulsion (1000s MW). A phased,

evolutionary approach will be

described for both the moon and Mars,

and the benefits of nuclear

technologies relative to solar and

their integration will be described.

INTRODUCTION

The President of the United States

has established a long term course

for the human exploration of space.

Beginning with the Space Station

Freedom in the 1990s and, in the next

century, returning to the moon,

establishing a permanent presence,

and using the experience and

technologies gained from these

missions to move on to the

exploration and habitation of Mars.

Following the Presidents' speech of

July 20, 1989, the NASA performed

detailed studies of a variety of

mission scenarios and architectures

to accomplish these missions and

identified the key technologies

needed to bring these missions to

fruition. In this paper, we will

discuss the various mission scenarios

and approaches, the key technological

requirements for power and propulsion

and the potential benefits of nuclear

technology for meeting these

requirements.

MISSION SCENARIOS

A variety of Exploration Options were

considered during the 90-day study.

These options considered various

approaches such as vigorous

deployment and early landing on the

moon, the earliest possible landing

on Mars, reduced logistics from

earth, delayed program start, and

paced deployment. The Exploration

Option that received the most study

and that will serve as the reference

for this paper is depicted in figure

i. In this approach, we build upon

our past and present investments in

space such as Apollo, Shuttle and the

Space Station Freedom, employ robotic

and manned craft and emphasize

science. The key point is, however,

that we build a lunar outpost first

and learn to live on planetary

surfaces before moving on to Mars.

The lunar/Mars exploration strategy

will be implemented in three phases:

Emplacement, Consolidation and

Utilization. The key elements and

objectives of a typical evolution

strategy are shown in figure 2. In

this figure, the activities are shown

as a function of time as one moves to

the right. A number of key elements

have been identified as critical for

implementing this strategy and they

are shown in figure 3. The critical
items and technologies of interest in

this paper are the rovers, surface

vehicles, surface power systems,

science outpost power systems and

propulsion systems for Mars. The

role of nuclear energy in fulfilling

these needs will be discussed below.

POTENTIAL ROLE OF NUCLEAR ENERGY

Vehicles: Figure 4 depicts a

pressurized manned rover that is

outfitted for long distance traverse

on the lunar surface. The power

level is nominally 7 kW and the cabin

retains a shirtsleeve environment.

Figure 5 depicts a vehicle used to

haul regolith to an oxygen processing

plant. Here the power requirement is

a nominal 3 kW with 15 kW peaks. The

power source shown for these

applications is a dynamic isotope

power source either on a cart at the

rear of the vehicle or integrated

into the structure. Figure 6 shows

more design details. Other potential

applications include remote

observatories or other remote or

mobile systems. Non-nuclear systems

such as photovoltaics and/or fuel

cells or batteries remain as viable

options where the distances and/or

the storage time is short enough to

keep the system mass low.

Surface Power: Figure 7 outlines

several options for stationary

surface power systems. Many

scenarios include an initial

installation powered by solar cell

and regenerative fuel cell systems

which reach about 75 kW, then a

transition to nuclear reactor-based

systems providing I00-I000 kW. It is

important to note that these hybrid

systems are attractive for the

additional reliability they provide

for the human presence. Nuclear

reactor systems are felt to be

essential for the lunar application

because the 354 hr. night is too long

for mass-effective storage systems.

For Martian applications where the

night is only 12 hours long,

photovoltaic/regenerative fuel cell

systems remain competitive.

propulsion: Nuclear thermal rockets

compare directly with the chemical

propulsion systems but offer

substantially higher specific impulse

(2-10X). Two main approaches have

been taken to the nuclear thermal

propulsion system - solid core and

gas core reactors. Solid core

reactor technology was demonstrated

through the NERVA engine tests of the

'70s, while gas core reactors have

been extensively studied. Power

levels of these systems are on the

order of I000 MWth.

Electric propulsion systems range

from 2-10 MWe and 50-100 MWe. The

primary difference between this

approach and thermal or chemical

systems is that the thrusting is

continuous over the mission duration

and, most importantly, substantial

electric power is available for use

once the destination has been

reached. This power could be used to

perform better science as well as for

higher power communications to

improve quality or to provide power

to the surface. Furthermore, the

life of these systems may well be

such that multiple missions to Mars

could be made with one reactor

system.

NUCLEAR TECHNOLOGY BENEFITS

Vehicles: The primary benefit of a

dynamic power conversion unit is to

conserve Pu 238 and to provide a more

compact unit. Specifically, a 5 kWe

unit with a 16% efficient Brayton

conversion unit weighs 826 kg as

shown in Fig. 6, compared to a 500 We

radioisotope thermoelectric generator

(RTG) system weighing 170 kg. The

Pu 238 inventory is reduced about

threefold over that required by the

RTG of equivalent power. Compared to

solar or fuel cell powered vehicles,
isotope dynamic systems offer

virtually unlimited range as well as

sunlight independence.

Surface Power: Figure 7 also

compares the mass of advanced solar

to nuclear options on the moon.

Power system mass savings for nuclear

systems are a factor of 5 at i00 kWe

and a factor of i0 at 550 kWe. When

one considers the total mass

transported to LEO, the benefits of

these advanced solar systems over

state of the art solar are shown in

figure 8. Mass savings for advanced

solar systems are about Ii0 MT at i00

kWe. Figure 9 shows a rendition of

a lunar base using advanced solar

cells and cryogenic storage of the H 2

and 02 regenerative fuel cell

reactants.

Figure i0 shows that nuclear systems

save over i00 MT at the i00 kWe level

when compared to the advanced solar

systems and over ii00 MT at the 800

kWe level for lunar applications. At

Mars, the savings are somewhat less

because of the reduced night. Figure

ii depicts a hybrid lunar base with

a i00 kWe photovoltaic/regenerative

fuel cell system coupled with an 800

kWe nuclear-Stirling power system.

Propulsion: Figure 12 demonstrates

the mass savings to LEO for a variety

of nuclear propulsion options. Solid

core reactor (SCR) systems save

almost 40% of the mass required for

a chemical/aerobrake system. Adding

an aerobrake to the SCR leads to mass

savings of 50%. This is equivalent

to the benefits of a regeneratively

cooled gas core reactor (GCR).

Radiative cooling gives additional

benefit. These cases all had the

same 434 day trip time. One other

major benefit of nuclear systems is

that trip times can be shortened

substantially. As shown, trip times

of 180 days can be attained with mass

saving of 21% while with an increase

of about 30% in mass to LEO, trip

time can be shortened to 120 days.

KEY TECHNOLOGIES

These ambitious missions to ensure

permanent human presence in space

rely on several key technologies for

success. Some of them have been

outlined above - aerobrakes for

planetary capture, advanced chemical

propulsion for space engines, and

nuclear power for a mature lunar

outpost. Other key technologies

include reusable transfer and

excursion vehicles, closed loop life

support systems, oxygen production

from lunar soil and radiation

protection/warning. These are all

assumed to be successful for the

reference approaches that have been

studied to date.

There are additional high payoff

technologies that pertain to the Mars

mission. These include advanced

nuclear propulsion and reduced

gravity countermeasures. From these

lists it can be clearly seen that

nuclear power plays a critical,

sustaining role in the permanent

human presence in space. There are

significant technological challenges

that must be surmounted before these

systems can come to fruition.

CONCLUSIONS

The lunar/Mars initiatives and their

corollary precursor missions offer

the nation and the world an exciting

new challenge. Numerous mission

scenarios have been studied and no

insuperable barriers have been

identified. As a result of these

studies, it is clear that nuclear

power offers significant potential

benefits from kWe class applications

to 1000s of MWth propulsion missions.

These studies have also pointed the

way to the major technologies that

should be developed to enable these

missions. Development programs for

these key technologies are underway.
Exploration Approach

Build upon past and presenl investments in space

0 Apollo, Viking, etc.

O Space Shuttle

O Space Slalion Freedom

Employ robotic craft along with manned systems

Emphasize science along the way

Build a lunar outpost first

D Research base for science and lechnology

[D Testbed for humans Io Mars

Explore Moon and Mars In phases

O Emplacement _ Consolidation _ Operation

I i

Evolutionary approach to realizing Space Policy goal of

"Expanding humnn presence and activity

beyond Enrlh orbit into lhe ._olnr system"

Figure 1. Exploration Approach

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Figure 2. Space Exploration Elements

ORIGINAL PAGE IS

OF POOR QUALITY
Figure 3. Evolution Strategy

Figure 4.

ORIGINAL PAGE IS

OF POOR QUALITY
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Figure 6. Dynamic Isotope Power Source

ORIGINAL PAGE IS

OF POOR QUALITY
LUNAR/MARS EXPLORATION INITIATIVE

SURFACE POWER SYSTEMS

LUNAR

3 • I_rn_E'MI Sx_'rEvs

• 2SkWe (D) 12..5IzW! (H) el.

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• 100kWe (O&lOU.

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• 2Sk'We_ILN) ' 2Skw_fOaN)-*.

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• _SkWe(I_ ' mkWe(I)&_

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Fiqure 7. Stationary Surface Power Systems

Mass Savings to LEO or Advanced Solar Power

w. SOA Solar Power

Figure 8. Mass Savings of Advanced

Solar Technologies

7

OF,_OiNAL PAGE IS

OF POOR QUALITY
Figure 9. Solar-based Lunar Power System

Mass Savings to LEO of Nuclear Power

vs. Advanced Solar Power

Mass Savings (MT)

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Power (kWe)

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Figure i0. Mass Savings of Nuclear Power Systems

8
Figure ii. Hybrid Powered Lunar Base

PROPULSION PERFORMANCE COMPARISON

SCR AND GCR PILOTED MARS MISSIONS, QU'ICK TRIPS

_1 DAY EXPLORATION INmAI'IV_

E

,aee,

m.

DAY STAY TIME

SAV_4C $1_ |lq&

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GCR CCR GCR GCR

4'11 1000 1000 sr_o_ llooo iooo 11000

434 434 434 434 434 L_ IN)

Figure 12. Nuclear Propulsion Mass Savings to LEO

9
Report Documentation Page Nalional Aeronautics and

Space Adrnmislration

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

NASA TM-103156

4. Title and Subtitle 5. Report Date

Nuclear Technology and the Space Exploration Missions

7. Author(s)

Henry W. Brandhorst and Ronald J. Sovie

9. Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

6. Performing Organization Code

8. Performing Organization Report No.

E-5518

10. Work Unit No.

590-13-11

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15. Supplementary Notes

Prepared for the 25th Intersociety Energy Conversion Engineering Conference cosponsored by the AIChE, SAE,

ACS, AIAA, ASME, and IEEE, Reno, Nevada, August 12-17, 1990.

16. Abstract

The strategy for a major exploration initiative leading to permanent human presence beyond earth orbit described

by President George Bush July 20, 1989 is still being developed; however enough is known to begin defining the

role of nuclear technologies. Three broad areas _ discussed: low power (< 10 kWe) rover/vehicle power

systems, integrated, evolutionary base power systems (25-100 kW) :rod nuclear energy for electric propulsion

(2-100 MWe) and direct thermal propulsion (1000s MW). A phased, evolutionary approach withe described for

both the moon and Mars, and the benefits of nuclear technologies relative to solar and their integration

described.

17. Key Words (Suggested by Author(s))

Nuclear power; Solar power; Propulsion; Mission

scenarios; Vehicles; Isotopes; Dynamic conversion;

Technologies

18. Distribution Statement

Unclassified- Unlimited

Subject Category 44

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages

Unclassified Unclassified 10

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