NASA Technical Memorandum 105164
AIAA-91-3433
Nondestructi
ve
Evaluation Tools
and Experimen
ta
l Studies f
or
Monitoring the Health
of
Space
P
ro
pulsion S
ys
t
ems
Edward
R.
Generazio
Lewis Research Center
Cleveland, Ohio
Prepared for the
Conference on Advanced Space Exploration Initiative
Te
chnologies
cosponsored by the AIAA, NASA, and OAI
Cleveland, Ohio, September
4-
6, 1991
NI\5
/\
NASA Technical Memorandum 105164
AIAA-91-3433
Nondestructi
ve
Evaluation Tools
and Experimen
ta
l Studies f
or
Monitoring the Health
of
Space
P
ro
pulsion S
ys
t
ems
Edward
R.
Generazio
Lewis Research Center
Cleveland, Ohio
Prepared for the
Conference on Advanced Space Exploration Initiative
Te
chnologies
cosponsored by the AIAA, NASA, and OAI
Cleveland, Ohio, September
4-
6, 1991
NI\5
/\
NONDESTRUCTIVE EVALUATION TOOLS AND EXPERIMENTAL STUDIES FOR
MONITORING THE HEALTH OF SPACE PROPULSION SYSTEMS
Edward R. Generazio
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
The next generation
of
space propulsion systems
will be designed
to
incorporate advanced health moni-
toring and nondestructive inspection capabilities. This
report provides an overview
of
background
andinfor-
mation
on
space propulsion systems at both the
programmatic and technical levels. Feasibility experi-
mental studies indicate that nondestructive evaluation
tools suoh as ultrasonic, eddy current and x-ray may
be successfully used to monitor the life limiting
failure mechanisms
of
space propulsion systems.
Encouraging results were obtained for monitoring the
life limiting failure mechanisms for three space pro-
pUlsion systems; the degradation
of
tungsten arcjet
and magnetoplasmadynamic electrodes; presence and
thickness
of
a spaUable eleotrically conducting molyb-
denum films in ion thrusters; and the degradation
of
catalyst
in
hydrazine thrusters.
INTRODUCTION
The next generation
of
space propUlsion systems
will be designed
to
incorporate advanced health moni-
toringand nondestructive inspection capabilities. The
nondestructive evaluation (NDE) community identi-
fied several questions that should be addressed. The
following key questions were raised during the
April 2-5, 1990 meeting
of
the Joint Army-Navy-
NASA-Air Force (JANNAF) Nondestructive Evalua-
tion Subcommittee (NDES):
(
1)
What types
of
space propUlsion systems are
being considered?
(2) What are the principles
of
operation
of
these
systems?
(3) Who
is
developing andlor researching space
propUlsion systems?
(4)
How
are inspections and reliability assess-
ments performed on the ground and in orbit?
(5) Do the space propUlsion systems require
health monitoring?
(6) What
,are
the possible failure modes for these
systems?
(7) Have the reliabilities
of
these space propul-
sion systems been determined?
This report describes teohnological driver mis-
sions suppolting space programs that are developing
chemical, electric and nuclear propulsion systems.
The types
of
propulsion systems being considered,
their principles
of
operation and known failure modes,
and the developers are identified. The propUlsion
systems characteristics are desoribed
in
sufficient
detail to identify life-limiting features and opportuni-
ties for nondestructive testing and health monitoring.
However, the reader should be aware that not all
aspects
of
the propulsion system that required health
monitoring and nondestructive evaluation are covered.
For example, the failure modes
of
space-based nuclear
generators or solar panels that supply power in the
form
of
electric energy for electric propulsion systems
are not discussed. Space propulsion systems are at
various stages
of
development; therefore, many ques-
tions, such as those concerned with reliability and
failure modes, remain unanswered. Three feasibility
experiments were performed for evaluating the capa-
bilities
of
NDE tools for monitoring the health
of
chemical and electrical propulsion systems.
TRANSPORTATION FOR FUTURE SPACE
SCIENCE MISSIONS
The actual vehicles and propulsion systems that
are
to
be used for future space missions have,
in
most
cases, not been determined. The specific propulsion
system and vehicle being considered for a particular
mission changes as the mission develops and matures.
Therefore, these propUlsion systems are not predeter-
mined and fixed but are essentially moving targets.
Before the NDE community can assist and affect the
development
of
these advanced propulsion systems,
they must latch onto these moving targets by under-
standing the programmatic thrusts, the path
of
the
development, and current status
of
these systems.
NONDESTRUCTIVE EVALUATION TOOLS AND EXPERIMENTAL STUDIES FOR
MONITORING THE HEALTH OF SPACE PROPULSION SYSTEMS
Edward R. Generazio
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
The next generation
of
space propulsion systems
will be designed
to
incorporate advanced health moni-
toring and nondestructive inspection capabilities. This
report provides an overview
of
background
andinfor-
mation
on
space propulsion systems at both the
programmatic and technical levels. Feasibility experi-
mental studies indicate that nondestructive evaluation
tools suoh as ultrasonic, eddy current and x-ray may
be successfully used to monitor the life limiting
failure mechanisms
of
space propulsion systems.
Encouraging results were obtained for monitoring the
life limiting failure mechanisms for three space pro-
pUlsion systems; the degradation
of
tungsten arcjet
and magnetoplasmadynamic electrodes; presence and
thickness
of
a spaUable eleotrically conducting molyb-
denum films in ion thrusters; and the degradation
of
catalyst
in
hydrazine thrusters.
INTRODUCTION
The next generation
of
space propUlsion systems
will be designed
to
incorporate advanced health moni-
toringand nondestructive inspection capabilities. The
nondestructive evaluation (NDE) community identi-
fied several questions that should be addressed. The
following key questions were raised during the
April 2-5, 1990 meeting
of
the Joint Army-Navy-
NASA-Air Force (JANNAF) Nondestructive Evalua-
tion Subcommittee (NDES):
(
1)
What types
of
space propUlsion systems are
being considered?
(2) What are the principles
of
operation
of
these
systems?
(3) Who
is
developing andlor researching space
propUlsion systems?
(4)
How
are inspections and reliability assess-
ments performed on the ground and in orbit?
(5) Do the space propUlsion systems require
health monitoring?
(6) What
,are
the possible failure modes for these
systems?
(7) Have the reliabilities
of
these space propul-
sion systems been determined?
This report describes teohnological driver mis-
sions suppolting space programs that are developing
chemical, electric and nuclear propulsion systems.
The types
of
propulsion systems being considered,
their principles
of
operation and known failure modes,
and the developers are identified. The propUlsion
systems characteristics are desoribed
in
sufficient
detail to identify life-limiting features and opportuni-
ties for nondestructive testing and health monitoring.
However, the reader should be aware that not all
aspects
of
the propulsion system that required health
monitoring and nondestructive evaluation are covered.
For example, the failure modes
of
space-based nuclear
generators or solar panels that supply power in the
form
of
electric energy for electric propulsion systems
are not discussed. Space propulsion systems are at
various stages
of
development; therefore, many ques-
tions, such as those concerned with reliability and
failure modes, remain unanswered. Three feasibility
experiments were performed for evaluating the capa-
bilities
of
NDE tools for monitoring the health
of
chemical and electrical propulsion systems.
TRANSPORTATION FOR FUTURE SPACE
SCIENCE MISSIONS
The actual vehicles and propulsion systems that
are
to
be used for future space missions have,
in
most
cases, not been determined. The specific propulsion
system and vehicle being considered for a particular
mission changes as the mission develops and matures.
Therefore, these propUlsion systems are not predeter-
mined and fixed but are essentially moving targets.
Before the NDE community can assist and affect the
development
of
these advanced propulsion systems,
they must latch onto these moving targets by under-
standing the programmatic thrusts, the path
of
the
development, and current status
of
these systems.
Technological challenges have been identified (Ref.
1)
that are
dri
ving the development
of
advanced space
propulsion systems. The following set
of
missions
presents technological challenges that must be
addressed to meet national space transportation needs:
(1) Modern expendable launch systems
of
small
and medium capacity
• Payload weight: 20 000 to 50 000
lb
low
Earth orbit (LEO)
• High reliability
• Low cost
• Improved payload-to-lift mass
(2) Unmanned heavy-lift launch capability to
LEO
• Payload weight: greater than
100
000
lb
• Payload envelop: as unrestricted
as
feasible
• Cost: substantial reduction over current
systems (full or
pattia1
reusability will
be
determined by economic tradeoffs)
(3)
Reusable orbital transfer system
to
raise pay-
loads from LEO to higher altitude, sunsynchronous or
geostationary orbit and to return them
• Geostationary payload weight: greater than
20000lb
• Payload envelope:
as
unrestricted
as
feasible
• Robotics: capable
of
interfacing with
intelligent front-end for routine servicing
operations
(4)
Advanced space transportation system to
replace the space shuttle after the turn
of
the century
• LEO payload weight: from 20 000
lb
to
potentially greater than 100 000
lb
• Payload envelope:
as
unrestricted
as
feasible
• Automation and ropotics: used to reduce
turnaround time and mission costs, with
special emphasis on self diagnostics
• Tradeoffs will be made between "Shuttle
II" and the transatmospheric
• Aerospace Plane
(5) High-energy interplanetary transfer system to
meet objectives
of
the National Commission on Space
• High specific impulse, high-thrust, long-life
propulsion systems to minimize duration
of
trips
to
Mars (e.g.,
IO
000
lb
(44000
N)
or greater thrust, 800-sec specific impulse)
• High specific impulse, long-life propulsion
systems for planetary scientific missions
2
(e.g., very low thrust, greater than
IOOO-sec
specific impulse)
• Nuclear-electric or direct thrust engines are
candidates for these missions
• Hybrid power and propulsion systems are
another attractive option
Some
of
the specific technology-driver missions
for space science for the mid-1990's follow:
The Earth Observing System (EOS) (Fig.
1),
with three EOS platforms in sun-synchronous orbits,
is
designed to study the Earth's atmosphere.
It
is
be-
lieved that automated
or
robotic servicing will
be
required at the operational altitude
of
the platform
during its 20-yr life.
The Large Deployable Array (LDR) (Fig. 2)
is
an
astronomical observatory design that will operate
in the 30-to
IOOO-I..lm
range.
It
is
expected that maintenance will occur on a
3-yr schedule.
During a Mars Sample Return Mission (MSR)
(Fig. 3), samples at several depths and at widely
dispersed sites on the Martian surface will be
obtained and returned to Earth in a pristine condition.
SPACE EXPLORATION INITIATIVE
On February
16,
1990, President Bush approved
policy for the Space Exploration Initiative.
Thle
goal
of
this initiative (Ref. 2)
is
to place Americans
on
Mars by the year 2019. The initiative includes
both
lunar and Mars program elements,
as
well
as
robotic
science missions. The near-term focus will
be
on
technology development. This will be done
by
searching for new and iimovative approaches and
technology, and
by
investing
in
high-leverage, innova-
tive technologies with potential to make major impact
on cost, schedule, and performance. Mission, con-
cept, and analysis studies will be done
in
parallel with
the technology development.
A baseline program architecture will
be
selected
after several years
of
defining two or more
ref(~rence
architectures while developing and demonstrating
broad technologies (Refs. 3 and 4). NASA will be
the principle implementing agency, whereas the
Department
of
Defense and Department
of
Energy
will have major roles in technology development and
concept definition. Some
of
the space programs
discussed below have been absorbed or replaced
by
this Space Exploration Initiative.
Technological challenges have been identified (Ref.
1)
that are
dri
ving the development
of
advanced space
propulsion systems. The following set
of
missions
presents technological challenges that must be
addressed to meet national space transportation needs:
(1) Modern expendable launch systems
of
small
and medium capacity
• Payload weight: 20 000 to 50 000
lb
low
Earth orbit (LEO)
• High reliability
• Low cost
• Improved payload-to-lift mass
(2) Unmanned heavy-lift launch capability to
LEO
• Payload weight: greater than
100
000
lb
• Payload envelop: as unrestricted
as
feasible
• Cost: substantial reduction over current
systems (full or
pattia1
reusability will
be
determined by economic tradeoffs)
(3)
Reusable orbital transfer system
to
raise pay-
loads from LEO to higher altitude, sunsynchronous or
geostationary orbit and to return them
• Geostationary payload weight: greater than
20000lb
• Payload envelope:
as
unrestricted
as
feasible
• Robotics: capable
of
interfacing with
intelligent front-end for routine servicing
operations
(4)
Advanced space transportation system to
replace the space shuttle after the turn
of
the century
• LEO payload weight: from 20 000
lb
to
potentially greater than 100 000
lb
• Payload envelope:
as
unrestricted
as
feasible
• Automation and ropotics: used to reduce
turnaround time and mission costs, with
special emphasis on self diagnostics
• Tradeoffs will be made between "Shuttle
II" and the transatmospheric
• Aerospace Plane
(5) High-energy interplanetary transfer system to
meet objectives
of
the National Commission on Space
• High specific impulse, high-thrust, long-life
propulsion systems to minimize duration
of
trips
to
Mars (e.g.,
IO
000
lb
(44000
N)
or greater thrust, 800-sec specific impulse)
• High specific impulse, long-life propulsion
systems for planetary scientific missions
2
(e.g., very low thrust, greater than
IOOO-sec
specific impulse)
• Nuclear-electric or direct thrust engines are
candidates for these missions
• Hybrid power and propulsion systems are
another attractive option
Some
of
the specific technology-driver missions
for space science for the mid-1990's follow:
The Earth Observing System (EOS) (Fig.
1),
with three EOS platforms in sun-synchronous orbits,
is
designed to study the Earth's atmosphere.
It
is
be-
lieved that automated
or
robotic servicing will
be
required at the operational altitude
of
the platform
during its 20-yr life.
The Large Deployable Array (LDR) (Fig. 2)
is
an
astronomical observatory design that will operate
in the 30-to
IOOO-I..lm
range.
It
is
expected that maintenance will occur on a
3-yr schedule.
During a Mars Sample Return Mission (MSR)
(Fig. 3), samples at several depths and at widely
dispersed sites on the Martian surface will be
obtained and returned to Earth in a pristine condition.
SPACE EXPLORATION INITIATIVE
On February
16,
1990, President Bush approved
policy for the Space Exploration Initiative.
Thle
goal
of
this initiative (Ref. 2)
is
to place Americans
on
Mars by the year 2019. The initiative includes
both
lunar and Mars program elements,
as
well
as
robotic
science missions. The near-term focus will
be
on
technology development. This will be done
by
searching for new and iimovative approaches and
technology, and
by
investing
in
high-leverage, innova-
tive technologies with potential to make major impact
on cost, schedule, and performance. Mission, con-
cept, and analysis studies will be done
in
parallel with
the technology development.
A baseline program architecture will
be
selected
after several years
of
defining two or more
ref(~rence
architectures while developing and demonstrating
broad technologies (Refs. 3 and 4). NASA will be
the principle implementing agency, whereas the
Department
of
Defense and Department
of
Energy
will have major roles in technology development and
concept definition. Some
of
the space programs
discussed below have been absorbed or replaced
by
this Space Exploration Initiative.
SPACE PROGRAMS
T~e
National Aeronautic and Space Administra-
tion (NASA) has several programs that require ad-
vanced, space-based propulsion systems. These pro-
pulsion systems may
be
quite different from those
used in Earth-to-orbit launch vehicles. Each program
has a different set
of
mission requirements that drives
the development
of
different space propulsion systems
(Refs. 5
to
7). For example, the propulsion system
used to keep the Space Station Freedom (Fig. 4) in
orbit will be quite different from that used for a
manned Mars mission. To answer the questions pre-
sented earlier,
we
must examine the NASA space
programs that have advanced space propulsion needs.
Each program identifies specific mission requirements
to be met by the propUlsion system (Ref. 8).
During the development
of
a space transporta-
tion system, propulsion studies and vehicle studies
must be iterated until the propUlsion requirements are
defined for the vehicle. Following the definition
of
the propulsion requirements, mission-focused propul-
sionsystem studies identify the specific required
propulsion system. Depending on the acceptable
mission scenario, very different propUlsion systems
and vehicles can result in successful space transfer.
However, since studies have not matured sufficiently,
we are unable to specify what propulsion system will
be used for an actual mission. Mission scenario stud-
ies indicate .that advanced, reliable, long life, low
weight, efficient, high power, and
variable~thrust
space propulsion systems are needed.
Space propulsion systems may be based on elec-
trical, chemical, or nuclear processes (Table I). The
design, operation, maintainability, reliability, failure
modes, health monitoring, and mission requirements
for these propulsion systems will vary considerably.
Therefore, it is natural to examine each
of
these sys-
tems on the basis
of
the physical process used to
produce thrust. Before the types
of
propulsion sys-
tems being considered, developed, or used are
described, it is appropriate to identify the programs
that support the development
of
these propulsion
systems.
Chemical Propulsion Program
Project Pathfinder (Ref. 9) from the NASA
Office
of
Aeronautics and Space Technologyl
(OAST) is a research and technology program
designed to make new missions in space exploration
possible and strengthen the technology base in sup-
port
of
the civil space program'. Pathfinder has a
distant horizon that is reached
by
building on the
space shuttle
.and
space station programs. Pathfinder
addresses technologies that support a range
of
space
TABLE
I.
- SPACE PROPULSION SYSTEMS
Engine type
Principle
of
'Propulsion system
operation
Chemical
Recomposition Liquid oxygen!
liquid hydrogen
(LOXIH
2
)
thruster
Decomposition Hydrazine thruster
Electrical
Electrostatic Ion thruster
Electrothermal Resistojet, arcjet,
mi~rowave
thruster
.
. ,
Electromagnetic Magnetoplasmadynamic
_:'!:,;~
Nuclear
Nuclear fission
Solid core rocket
!
. Gas core rocket
INow NASA Office
of
Aeronautics and
E](plor~tion
Technology (OAET).
'3
SPACE PROGRAMS
T~e
National Aeronautic and Space Administra-
tion (NASA) has several programs that require ad-
vanced, space-based propulsion systems. These pro-
pulsion systems may
be
quite different from those
used in Earth-to-orbit launch vehicles. Each program
has a different set
of
mission requirements that drives
the development
of
different space propulsion systems
(Refs. 5
to
7). For example, the propulsion system
used to keep the Space Station Freedom (Fig. 4) in
orbit will be quite different from that used for a
manned Mars mission. To answer the questions pre-
sented earlier,
we
must examine the NASA space
programs that have advanced space propulsion needs.
Each program identifies specific mission requirements
to be met by the propUlsion system (Ref. 8).
During the development
of
a space transporta-
tion system, propulsion studies and vehicle studies
must be iterated until the propUlsion requirements are
defined for the vehicle. Following the definition
of
the propulsion requirements, mission-focused propul-
sionsystem studies identify the specific required
propulsion system. Depending on the acceptable
mission scenario, very different propUlsion systems
and vehicles can result in successful space transfer.
However, since studies have not matured sufficiently,
we are unable to specify what propulsion system will
be used for an actual mission. Mission scenario stud-
ies indicate .that advanced, reliable, long life, low
weight, efficient, high power, and
variable~thrust
space propulsion systems are needed.
Space propulsion systems may be based on elec-
trical, chemical, or nuclear processes (Table I). The
design, operation, maintainability, reliability, failure
modes, health monitoring, and mission requirements
for these propulsion systems will vary considerably.
Therefore, it is natural to examine each
of
these sys-
tems on the basis
of
the physical process used to
produce thrust. Before the types
of
propulsion sys-
tems being considered, developed, or used are
described, it is appropriate to identify the programs
that support the development
of
these propulsion
systems.
Chemical Propulsion Program
Project Pathfinder (Ref. 9) from the NASA
Office
of
Aeronautics and Space Technologyl
(OAST) is a research and technology program
designed to make new missions in space exploration
possible and strengthen the technology base in sup-
port
of
the civil space program'. Pathfinder has a
distant horizon that is reached
by
building on the
space shuttle
.and
space station programs. Pathfinder
addresses technologies that support a range
of
space
TABLE
I.
- SPACE PROPULSION SYSTEMS
Engine type
Principle
of
'Propulsion system
operation
Chemical
Recomposition Liquid oxygen!
liquid hydrogen
(LOXIH
2
)
thruster
Decomposition Hydrazine thruster
Electrical
Electrostatic Ion thruster
Electrothermal Resistojet, arcjet,
mi~rowave
thruster
.
. ,
Electromagnetic Magnetoplasmadynamic
_:'!:,;~
Nuclear
Nuclear fission
Solid core rocket
!
. Gas core rocket
INow NASA Office
of
Aeronautics and
E](plor~tion
Technology (OAET).
'3
missions including: a return
to
the Moon to build an
outpost (Fig. 5), piloted .missions to Mars (Fig. 6),
and
continuing
exploration
of
Earth
and
the
other
planets.
Project Pathfinder has four major components:
(1) Exploration Technology, (2) Space Operations,
(3) Humans-in-Space, and (4) Transfer Vehicle Tech-
nology. The Exploration Technology, Space Opera-
tions, and Humans-in-Space components include
planetary rover development, surface power, remote
sample acquisition, optical communications, autono-
mous rendezvous and docking, resource processing,
in-space assembly and construction, cryogenic fluid
depots, space nuclear power, extravehicular suits,
human performance, and closed-loop support systems.
The Transfer Vehicle Technology is
of
particular
interest because it supports transportation to and from
geostationary Earth orbit, the Moon, Mars, and other
planets. Specific goals
of
the Transfer Vehicle com-
ponent include significant reduction in the mass that
missions require for launch into low Earth orbit and
in transit, as· well as reductions in the time required
for transit. The key elements
of
the Transfer Vehicle
Technology thrust are the chemical transfer propul-
sion research, cargo vehicle propulsion development,
high-energy aerobtaking development (Fig. 7), auton-
omous lander development, and fault-tolerant systems.
.The Transfer Vehicle Technology thrust led to
the initiation
of
the NASA OAST Pathfinder
Chemical Transfer Propulsion Program (Refs.
10
and 11). This program was initiated to provide 'the
technology to design and develop highly reliable,
reusable cryogenic transfer vehicle
enginC?s
that are
fault tolerant, and have long lives. They will
be
high-performance, liquid oxygenlIiquid hydrogen
(LOXt'Hz) expander cycle engines for space-based
transfer vehicles and Moon and Mars landers.
Electric PropUlsion Program
NASA OAST's PropUlsion, Power, and Energy
Division supports an electric propulsion program.
(Refs.
12
to 15) for a broad class
of
missions. Three
types
of
electric propUlsion systems are being devel-
oped (Refs.
12
to 29): . electrostatic (ion),electro- -
thermal (resistojet, arcjet, microwave, and radiowave),'
and electromagnetic (magnetoplasmadynaillic,
or
MPD). Resistojets are currently used on geosynchro-
nous communications satellites.
4
Nuclear Propulsion Program
In
1987
the
Air
.Force
Systems
Command
reini-
tiated a Direct Nuclear PropUlsion Program (Refs. I,
30, and 31). The goals
of
this program are to
develop a high-impulse, high-thrust, low-weight pro-
pulsion system. This propulsion system would be
used for orbital transfer vehicles, fast launch intercep-
. tors, intercontinental ballistic missiles, and other mis-
sions. In October 1990 NASA's Propulsion, I'ower
and Energy Division initiated a Nuclear Thermal
PropUlsion Program. A Nuclear
Elect~ic
PropUlsion
Program will begin October 1991.
PROPULSION SYSTEM CHARACTERISTICS
The operating characteristics
of
chemical, electri-
cal and· nuclear propulsion systems are quite different
(Ref. 32). Thrust and specific impulse can be used
for making general comparisons between propUlsion
systems. Table II indicates the range
of
thrust:
T
and specific impulse
Ispfor
electrical,
chemi1cul,
and
nuclear propulsion systems. Thrust is the amount
of
force that a propUlsion system generates. The greater
the thrust, the greater the acceleration
of
the vehicle.
Specific impulse (in seconds) is the thrust (in
Newtons) that can be obtained from an equivalent .
rocket which has.a propellant weight flow
rate~
(in
Newtons per second)
of
unity. (Specific impulse
is
somewhat analogous to the number
of
miles per gal-
lon
of
fuel
for
automobiles.) Electric propulsion
systems have lower thrust capabilities than chc,mical
or nuclear propulsion systems do. Chemical propul-
sion systems yield the highest thrust levels available
to
date. However, direct nuclear propulsion
is,
expected to yield greater thrust levels than
che~mical
propUlsion. The specific impulse for electrical resis-
tojets and arc jets are comparable to chemicalll.OXIH
2
and hydrazine propulsion systems. The ion, MPD,
and nuclear propulsion systems have the
highe~st
spe-
cific impulses, and they can exceed those
of
other
systems by an order
of
magnitude.
Classes
of
propulsion systems that will be
needed to meet mission requirements can be identified
from table
11
and from preliminary mission pmpulsion
requirements. High specific impulse engines, such as
ion, MPD, and nuclear propulsion systems, will be
needed for interplanetary transfer. Low thrust
engines, such as resistoJet, arcjet, and hydrazine
missions including: a return
to
the Moon to build an
outpost (Fig. 5), piloted .missions to Mars (Fig. 6),
and
continuing
exploration
of
Earth
and
the
other
planets.
Project Pathfinder has four major components:
(1) Exploration Technology, (2) Space Operations,
(3) Humans-in-Space, and (4) Transfer Vehicle Tech-
nology. The Exploration Technology, Space Opera-
tions, and Humans-in-Space components include
planetary rover development, surface power, remote
sample acquisition, optical communications, autono-
mous rendezvous and docking, resource processing,
in-space assembly and construction, cryogenic fluid
depots, space nuclear power, extravehicular suits,
human performance, and closed-loop support systems.
The Transfer Vehicle Technology is
of
particular
interest because it supports transportation to and from
geostationary Earth orbit, the Moon, Mars, and other
planets. Specific goals
of
the Transfer Vehicle com-
ponent include significant reduction in the mass that
missions require for launch into low Earth orbit and
in transit, as· well as reductions in the time required
for transit. The key elements
of
the Transfer Vehicle
Technology thrust are the chemical transfer propul-
sion research, cargo vehicle propulsion development,
high-energy aerobtaking development (Fig. 7), auton-
omous lander development, and fault-tolerant systems.
.The Transfer Vehicle Technology thrust led to
the initiation
of
the NASA OAST Pathfinder
Chemical Transfer Propulsion Program (Refs.
10
and 11). This program was initiated to provide 'the
technology to design and develop highly reliable,
reusable cryogenic transfer vehicle
enginC?s
that are
fault tolerant, and have long lives. They will
be
high-performance, liquid oxygenlIiquid hydrogen
(LOXt'Hz) expander cycle engines for space-based
transfer vehicles and Moon and Mars landers.
Electric PropUlsion Program
NASA OAST's PropUlsion, Power, and Energy
Division supports an electric propulsion program.
(Refs.
12
to 15) for a broad class
of
missions. Three
types
of
electric propUlsion systems are being devel-
oped (Refs.
12
to 29): . electrostatic (ion),electro- -
thermal (resistojet, arcjet, microwave, and radiowave),'
and electromagnetic (magnetoplasmadynaillic,
or
MPD). Resistojets are currently used on geosynchro-
nous communications satellites.
4
Nuclear Propulsion Program
In
1987
the
Air
.Force
Systems
Command
reini-
tiated a Direct Nuclear PropUlsion Program (Refs. I,
30, and 31). The goals
of
this program are to
develop a high-impulse, high-thrust, low-weight pro-
pulsion system. This propulsion system would be
used for orbital transfer vehicles, fast launch intercep-
. tors, intercontinental ballistic missiles, and other mis-
sions. In October 1990 NASA's Propulsion, I'ower
and Energy Division initiated a Nuclear Thermal
PropUlsion Program. A Nuclear
Elect~ic
PropUlsion
Program will begin October 1991.
PROPULSION SYSTEM CHARACTERISTICS
The operating characteristics
of
chemical, electri-
cal and· nuclear propulsion systems are quite different
(Ref. 32). Thrust and specific impulse can be used
for making general comparisons between propUlsion
systems. Table II indicates the range
of
thrust:
T
and specific impulse
Ispfor
electrical,
chemi1cul,
and
nuclear propulsion systems. Thrust is the amount
of
force that a propUlsion system generates. The greater
the thrust, the greater the acceleration
of
the vehicle.
Specific impulse (in seconds) is the thrust (in
Newtons) that can be obtained from an equivalent .
rocket which has.a propellant weight flow
rate~
(in
Newtons per second)
of
unity. (Specific impulse
is
somewhat analogous to the number
of
miles per gal-
lon
of
fuel
for
automobiles.) Electric propulsion
systems have lower thrust capabilities than chc,mical
or nuclear propulsion systems do. Chemical propul-
sion systems yield the highest thrust levels available
to
date. However, direct nuclear propulsion
is,
expected to yield greater thrust levels than
che~mical
propUlsion. The specific impulse for electrical resis-
tojets and arc jets are comparable to chemicalll.OXIH
2
and hydrazine propulsion systems. The ion, MPD,
and nuclear propulsion systems have the
highe~st
spe-
cific impulses, and they can exceed those
of
other
systems by an order
of
magnitude.
Classes
of
propulsion systems that will be
needed to meet mission requirements can be identified
from table
11
and from preliminary mission pmpulsion
requirements. High specific impulse engines, such as
ion, MPD, and nuclear propulsion systems, will be
needed for interplanetary transfer. Low thrust
engines, such as resistoJet, arcjet, and hydrazine