A description is given of the nuclear thermal and nuclear electric hybrid. The specifications are described along with its mission performance. Next, the technical status, development requirements, and some cost estimates are provided

Reid, B. D.

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N92-1 
b •
NUCLEAR THERMAL/NUCLEAR ELECTRIC HYBRIDS
B. D. Reid
Battelle Pacific Northwest Laboratory
Reactor Technology Center
First I would like to describe the nuclear thermal and nuclear electric hybrid. The
concept isn't new; as our previous speaker indicated, there had been some work done in
the early 1970's, and I will briefly describe some of that again.
We evaluated a hybrid concept, and I will be describing its specifications and its mission
performance. Then, as requested by our workshop organizers, I will discuss technical
status, development requirements, and we, like everyone else, will provide some
optimistic cost estimates.
Essentially (Figure 1), we see the hybrid working as a concept whereby you have both
thermal propulsion and electric propulsion. We see this concept being used when you
would use a thermal propulsion, high thrust thermal propulsion for your
trans-Mars/trans-Earth injection burns, and using electrical propulsion in transit (Figure
2).
This is all using one reactor. There are differences in the reactor performance when it's
a one mode versus the other. In the thermal propulsion mode, the reactor is operating
at, let's say, 1500 megawatts. In the electric production mode, it's of the order of 2 to 3
percent of that, which translates to about 35 megawatts thermal.
In looking at hybrids there are options in the design. One option uses common heat
transfer passages, while the other uses independent heat transfer passages. There are
pluses and minuses to each of these options. I will show a schematic of each of them to
explain them a little more fully. With the independent heat transfer concept, you do
have the possibility of providing electrical power during your thermal propulsion cycle.
This is a rough schematic of a common cycle hybrid (Figure 3). During open cycle
propulsion, you close two valves and it essentially operates just like a standard NERVA.
In the power production mode, the other two valves are closed. Argon is used both in
the closed cycle power conversion and to provide your propellant for your electric
propulsion; in this case we have chosen MPD thrusters.
Here is an independent cycle (Figure 4). In this schematic for the open cycle propulsion,
the hydrogen flows through the reactor, and again the open cycle propulsion just like a
NERVA. During the power conversion cycle, you can operate this cycle independent of
the open cycle propulsion, using the argon through separate cooling passages and also
through your MPD thrusters.
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With this slide I am going to repeat some information that you already have seen today
(Figure 5). As part of the NERVA/ROVER program, dual mode reactors were looked
at. The primary incentive was to reduce propellant losses which were required to
address decay-heat removal.
In this case the NERVA concept was operating at 365 megawatts in its thermal
propulsion mode. For the electrical propulsion, it was operating at only 1 megawatt with
only approximately 25 kilowatts electricity to an organic Rankine cycle. For this there
were very few engine modifications required, primarily materials.
This is another cartoon of the same concept that you saw previously (Figure 6). During
the thermal propulsion mode, the power conversion cycle is essentially cut off and the
turbine is driven by hydrogen flow through the core support tie-tubes. When the reactor
is not in the thermal propulsion mode, this circuit could be energized for power
conversion.
We evaluated a hybrid concept for the manned Mars mission. It is based on a 1500
megawatt NERVA with 850 seconds specific impulse. We chose Brayton cycle with
argon, with 8 megawatts electricity, and 35 megawatts thermal. And again, we are using
the MPD thruster at 5000 seconds specific impulse. In the concept that we have chosen
here during thermal propulsion (Figure 7), two valves are closed: it operates just like a
standard NERVA. During the power conversion cycle, the other two valves are closed,
and we have a valve in the nozzle.
Now, we recognize there are some significant challenges with this, and we were a little
loath to put this up here because we thought it might fail the "snicker" test. But after
careful study, we think that it's a concepts that will work. Again, the advantage with this
type of cycle is that you can achieve a reasonable power production with this concept.
The earlier concepts were very limited in the power you were able to obtain from the
core because you had very limited heat transfer surfaces. This concept operates at about
1400 Kelvin, which is well below the operating temperature of the core in the thermal
propulsion mode. The Brayton cycle will operate at about 150 psi.
This chart shows some of the performance specifications of our concept (Figures 8 & 9).
These thrust/weight numbers here, I would like to caution you, are only for the base
reactor. The lifetime for this concept in the thermal propulsion mode is similar to
NERVA, or approximately 10 hours. In the electric propulsion mode, we have
convinced ourselves that you have a life of at least 2 years, which is sufficient for the
reference mission. Figure 9 is a mass breakdown. This is for a mission that was less
than 600 days, 555 days to be exact.
One of the advantages of the hybrid concept (Figure 10) is that you do have the ability
to provide electricity for housekeeping loads and, with space power beaming, you have
the ability to provide significant power to meet other mission objectives such as
332
supporting a base on the Martian surface.
As noted, this concept is primarily based on the NERVA technology, and there have
been a lot of improvements in that technology that can be incorporated here, primarily
in fuels and materials for nozzles and core materials. We believe that the primary
uncertainties with this concept are going to lie with the other part of the power
conversion system; with the Brayton cycle.
Figure 11 summarizes in chart form our anticipated technology readiness. As most
speakers have touched on today, the reactor is at readiness level approaching 6. Our
greatest uncertainty lies in integrating the two power conversion cycles, and we have
identified that accordingly (Figure 12).
Here is our cost estimate (Figure 13). It's a rough order of magnitude, and it's as
optimistic as the next guy's.
Summing up, in our mission benefits (Figure 14), we have identified modest reductions
in trip time compared to the nuclear thermal only, at the same masses. Said another
way, you have reduced masses compared to nuclear thermal options with similar trip
times. Now, a key to all this is that incorporation of the dual mode concept into a
thermal reactor, you must not significantly degrade the specific impulse of your thermal
reactor, or you have lost everything that you are going to gain with your electric
propulsion. So, a key in coming up with this design is making sure that you haven't hurt
yourself in Isp.
As to safety issues, these are primarily those associated with NERVA, and other people
can probably address those more capably than I. However, there is a minimal additional
risk associated with incorporating this additional system that we can't ignore. It does
provide some finite increase in risk.
333
Bruce Reid
DUO Mode Hybrid ConcetX
2.
.
BIBLIOGRAPHY
Altscimer, J.H. et al.. "Operating Characteristics and Requirements for the NERVA Flight Engine';
J. Spacecraft, Volume 8, July 1971.
Holman, R.R., and B.L. Pierce. "Development of NERVA Reactor for Space Nuclear Propulsion';
AIAA 86-1582, June 1986.
334
fPNL NTP/NEP CONCEPT DESCRIPTION
"h
Based on 1500 MWth NERVA for thermal propulsion
- 850 lap using hydrogen as propellant
Generation of 8 MWe from Brayton cycle at 35 MWth
during electrical propulsion
. Employs MPD thruster at 5000 lap using argon
as propellant and Braylon cycle working fluid
J
Battelle
Figure 1
_ITP/NEP HYBRID DESCRIPTION
Nuclear thermal propulsion for Till end TEl burns supplemented
by nuclear electric propulsion during transit
Characteristics of NTP/NEP Hybrid
- Considerable power differences between open cycle mode and
CI01NKIcycle mode
- Option for common or Independent heat transfer passages In
reactor
- Pouibillty of providing electrical power while in thermal
propulsion mode
J
Battelle
335 Figure 2
f COMMON CYCLE HYBRID N_EP REACTOR
COMPONENT AND VALVING ARRANGEMENT
Battollo
J
Figure 3
INDEPENDENT CYCLE HYBRID NTP/NEP REACTOR
COMPONENT AND VALVING ARRANGEMENT
Battelle
J
336 Figure 4
HISTORY OF NTP/NEP CONCEPTS
• Dual mode Nuclear Rocket proposed during ROVER Program
In early 1970's
• Based on 365 MWth NERVA
Operating at I MWth generating approxlrnately 25 kWe from
Renkine Cycle
• Concept required few engine modifications
- Change reactor dome design
Change tie tube support line materials
J
Battelle
Figure 5
f EARLY DUAL MODE NUCLEAR ROCKET CONCEPT
',@/ ___
L
1
J
Battelle
337 Figure 6
f PROPOSED HYBRID NTP/NEP REACTOR CONCEPT
J
Battelle
Figure 7
f
NTP/NEP HYBRID PERFORMANCE
Propulsion:
Thermal
Electric
850 Isp 75,000 Ib thrust
5000 lsp 15 Ib thrust
NTP Thrust/Weight
With shield
Without shield
4.7 without electric thrust
7.7 without electric thrust
Lifetime:
Thermal. approximately 10 hours
Electric. greater than 2 years
Battelle
J
338 Figure 8
NTP/NEP HYBRID PERFORMANCE
(CONTD)
IMLEO:
Payload 124 MT
Propulsion Subsystem" 16 MT
Propellent (hydrogen I argon) 384 MT
Total 524 MT
*reactor, shield, radiator, armor, thrusters
J
Battelle
Figure 9
f
MISSION PERFORMANCE OF NTP/NEP HYBRID
• Trip time
• Initial Mass In Low Earth Orbit (IMLEO)
Increased commonality and redundancy
- Electric production for housekeeping loads
- Redundant / eltemste means of propulsion
- Electric production for surface application through
space power beaming
J
Battelle
339 Figure 10
NTP/NEP TECHNOLOGY READINESS
Readiness Level
System Component 1 2 3 4 5 6
Reactor (NERVA) _'--
Power Conversion
Shield / ACF Armor
Hybrid Component
Integrstton
Thrusters ---
ACF Radiator ._
Battelle
J
Figure 11
TECHNICAL STATUS AND ESTIMATED COST
- Reactor technology based on proven
NERVA Technology lrom early 1970's
Therefore:
Principal technology uncertainties and primary
development costs associated with power
conversion and intogrstion of power and
propulsion modes
/
Battelle
340 Figure 12
NTP/NEP
System Component
Reactor Development
Power Conversion /
Hybrid Integration
Integrated System
Tasting
Test Facilities
Development Cost and Schedule
91
Fiscal Year
95 99 01 05
$100 M
$500 M
v
$150 M
$200 M
J
Battelle
Figure 13
f Conclusions ON NTP/NEP HYBRID
Mission Benefits
- Reduced trip time compared to NTP options
with only propulsion mode
- Reduced IMLEO compared to NTP options
offering slmllsr trip times
. Offers redundant means of propulsion with enhanced
mission flexibility
Technical and Developmental Risk
- Reactor technology based on proven NERVA technology
from early 1970's
Safety
- Safety Issues primarily associated with NERVA Technology
- Minimum additional risk compared to NTP alone
. Additional complexity does result In soma increased
safety I relisblllty risk
J
Battelle
341 Figure 14



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