Apart from the Particle Bed Reactor development activity conducted under the Timberwind program (discussed in Chapter IIII), the relatively low funding level and brief duration of the SDI MMW program precluded significant hardware fabrication or test activities. However, evaluations of proposed MMW concepts were conducted under the Space Power Architecture Study (SPAS) contracts.
1- Sandia - 1987-89
Soon after the initiation of the SDI MMW program, Sandia National Laboratory conducted a wide-ranging review of gas-cooled reactor concepts for SDI multi-megawatt burst-mode and steady state power production applications (Figure II-59).(1)
Reactor concepts evaluated by Sandia include NERVA Derivative Reactor, Cermet, NERVA/PLUTO Hybrid, Particle-Bed, Pellet-Bed, Pluto-Derivative, Wire-Core, Foam-Fuel, and UB2 Fuel (information on each of these reactors was provided for the Sandia review by a proponent of the system, and additional technical details on these concepts will be found in the sections devoted to each proponent organization). Although the Sandia evaluation was based on information obtained from concept proponents, the evaluation was not based on specific proposals. Instead, Sandia conducted parametric comparisons based on standard assumptions, which in some cases included reductions (up to an order of magnitude) in the reactor and shield masses originally estimated by concept proponents. The reactor concepts were ranked according to technical risk, development cost, fabrication cost, and modal shift time (power ramp rate). Although some attention was also given to safety, it was concluded that insufficient information was available to permit
Reactor Concept Proponent Steady-State Burst Mode
NERVA Derivative Westinghouse 1 1
NERVA/PLUTO Hybrid Westinghouse & LLNL 1 2
Particle-Bed Brookhaven, Babcock & Wilcox 2 2
Pellet-Bed Science Applications (SAIC) 2 2
Pluto-Derivative LLNL (Lawrence Livermore) 2 2
Cermet General Electric 3 2
Wire-Core Rockwell 3 2
Foam-Fuel Babcock & Wilcox No No
UB2 Fuel LLNL Lawrence Livermore) No No
Ranking
Reactor Concept Steady-State Burst Mode
Tech Dev Fab Mass Strategic Tech Dev Fab Mass Risk Cost Cost Materials Risk Cost Cost
NERVA Derivative G F F G- G G G F G
NERVA/PLUTO G- F F G- G G- F F G
Particle-Bed F F F G- G F F F G
Pellet-Bed F+ F G G- G F+ F G G
Pluto-Derivative F F F G G G- F+ F G
Cermet F- P F- G- F F F F- G
Wire-Core F- P G- G F F F G G
Foam-Fuel P P F F G F- P F G
UB2 Fuel P P F+ F+ G P P G G
G Good
F Fair
P Poor
evaluation on the basis of safety considerations.
The Multi-Megawatt Steady-State operating requirement included production of 10 MW electrical power over a period of one year. The burst mode operating regime included a thermal output of 1,000 MW and an electrical power of 500 MW over a 2,000 second operating period. (It should be noted that these power levels and operating times are generally consistent with SEI nuclear propulsion requirements). This study considered standard outlet temperatures of 1200 K, and concluded that 1500 K outlet temperatures would not alter its conclusions. However, the study noted that a UO2 BeO fuel phase transition near 1,900 K and a low-temperature eutectic slightly above 2,000 K would eliminate the PLUTO reactor from consideration for operations at these outlet temperatures, and that outlet temperatures in the range of 3000 K may mandate use of UC-ZrC fuel elements.
The Sandia study concluded that:
"The NERVA derivative reactor appears to have a substantial lead over the other concepts. The NERVA reactor has demonstrated successful operation for operating conditions in excess of the expected SDI requirements in the NRX A-6 and XE' reactor demonstration tests... Years of technology and engineering development work would be required for other burst mode concepts to reach this stage... The UB2 reactor and foam-fuel reactor appear to be poor candidates... the concepts had to show some benefit to justify the risk and cost of a revolutionary concept, and these did not."
However, this conclusion was not universally endorsed, with particularly vigorous dissent being voiced by proponents of the particle-bed reactor.(2) This analysis contends that:
"The Sandia report concludes that the NERVA Derivative Reactor can now be judged as the best gas-cooled reactor choice for burst and multimegawatt steady state power systems. This conclusion appears premature and wrong."
The assessment notes that in the Sandia analysis:
"Start-up time and load following requirements in the burst mode are not used as discriminants. This is a serious deficiency. Capability for fast start-up and rapid power changes to match load will probably decide which reactor concept will be built...
"The PBR is designed to start fast and load follow on the time scale of a few seconds... Reactors with large solid fuel elements, such as NERVA, must change power over a longer time scale. The statement that ramp-up time can be eliminated by maintaining the reactor in a hot critical condition is wrong. When the burst power reactor ramps up, the reactor fuel element has the inlet end at low temperature and the outlet end at high temperature. In the hot critical condition, the reactor is at some uniform temperature (unless coolant flowed continuously for years, which would be prohibitive.
"It is not established that NERVA can start much more rapidly than has been demonstrated, or that it can rapidly load follow....
"NRX-A6 required [about] 60 seconds to reach full power and [about] 60 seconds to shutdown... A sixty second start-up time means that each platform would lose the opportunity for hundreds of shots against boost phase targets if ME launchers and the maximum fuel rate is required. This would be a very serious limitation on system effectiveness... Conversely, if the target firing rate fluctuated widely, tons of hydrogen would be wasted because the power system would need to operate at full power because of shutdown/start-up time constraints."
In addition, it was noted that:
"... the ability to operate at a higher gas outlet temperature than 1200 K should also be incorporated. The 1200 K temperature value is very fuzzy and weapon technology dependent, and could be much higher... The development of practical high temperature high field superconductors will profoundly affect all electrically powered weapon systems. Augmented EM [electro-magnetic] guns could then achieve electrical efficiencies of 80% or more and Neutral Particle Beam (NPB) and Free Electron Lasers (FEL) could use very low-loss superconducting cavities. Gains in efficiency, plus higher operating temperatures, remove the condition that H2 consumption is determined by the weapon. The H2 consumption is then determined by the power system. There would be great incentive to operate at the highest practical reactor temperature to minimize expendable hydrogen. Reactor systems with low outlet temperatures would be unattractive.
A third issue related to reactor controllability. The analysis noted that in the Sandia report:
"Controllability has also been discarded as a discriminator. For fast starts (on the order of 5 seconds to full power) the reactivity insertion is almost one dollar. For fast starts of epithermal and fast reactors, where the H2 coolant has high reactivity worth, controllability may be a serious problem. In the PBR, a well moderated thermal reactor, the [delta] K worth of the H2 coolant is [less than] one dollar, and the reactor appears readily controllable.
This analysis noted that in the Sandia report:
"The NERVA Derivative Reactor is ranked high because technical risk and development cost are judged low. If ... fast start-up and load following are required, risk and cost will increase.
"NERVA construction is very complex and requires high manufacturing tolerance on components. There are thousands of individually orificed fuel channels, each of which must be coated against attack by hot hydrogen. There is a multitude of rods, seals, support pins, etc. which have to fit together precisely, while accommodating large temperature and dimensional changes.
"Burst power systems will have to demonstrate a failure rate on the order of 1% or less over a typical operating cycle. In a burst power system, failure could occur when a small piece of fuel element wipes out on or more turbine blades. Pieces of fuel elements have been observed to exit NERVA type reactors....
"There is an important difference between reactors with simple construction, e.g. the pellet bed and particle bed, and those with complex construction. e.g. the NERVA Derivative. In the former construction, most of the effort goes into developing and testing fuel and materials. In complex reactors, a major effort must also go into engineering a myriad of interlocking mechanical components and demonstrating their reliability. This is expensive and time consuming. Further, meaningful tests must involve the whole reactor.
"For reactors with simple construction, testing a single fuel element, like the particle bed element, is quick, inexpensive, and provides much of the information necessary for reliability assessments. Complete reactor tests are still necessary, but are simpler and cheaper than those for complex reactors."
The Brookhaven analysis also suggested that the Sandia report had overestimated the technical challenges of particle bed reactors:
"Frit clogging is only a potential issue for long term high burn up MMWSS PBR's. Burst power PBR's operate for short periods with extremely low burn up... Experience and analysis indicate that frit clogging will not be an important issue for MMWSS PBR's. Particle failure rates are very low and the hot frit is unlikely to trap fines. Based on experience in actual HTGR's and irradiation tests of fuel particle samples, the failure rate of particles at the expected operating conditions (maximum of 1500 K and 10% burnup) will be extremely low. Approximately one particle in 104 fails for particles manufactured in the U.S., while approximately one in 105 fails if they are manufactured in Europe. The lower failure rate for European particles is due to better quality control during fabrication."
Prior the publication of the final version of the Sandia report, its' authors responded to some of these issues.
On the question of the time required to bring a NERVA Derivative reactor to full power, it was suggested that:
"... there are many other prebattle activities that may be more limiting than reactor power-up; consequently, the few-second reactor ramp-up time requirement may be overly restrictive. Furthermore, it is not clear at this stage that the alert-mode time could not be used to begin a system power up.
"... the constraints imposed on the original NERVA propulsion reactor do not apply to the burst-mode NERVA derivative. The 60-second start-up time was originally constrained by bootstrap start-up, manual control, and the unfueled support block thermal stresses. The bootstrap start-up would constrain all of the open systems whenever the bootstrap start-up approach is used. An auxiliary pump power system will be required for rapid start-up of all open systems... a design change of the support block or maintaining the reactor hot critical should eliminate the support block constraint."
It was also noted that the NERVA Derivative might exhibit superior safety features relative to the particle bed reactor:
"... the high heat capacity and redundant cooling systems of the NERVA concept could offer important safety advantages over concepts, such as the particle bed, for loss-of-coolant accidents. Since the particle-bed fuel has a low heat capacity and transfers heat, in the absence of coolant, primarily by radiation, ... the particle-bed concept may be vulnerable to loss-of-coolant accidents...
The Sandia assessment of this issue was based on:
"... NERVA and the particle-bed 400-MW(e) burst mode reactors following a loss-of-coolant accident after 5 minutes of operation. Failure was assumed to occur at a fuel temperature of 3300 K for both the particle-bed and the NERVA reactors. The particle bed reached failure temperatures within less than one minute. For the NERVA reactor, if tie-tube cooling is not assumed, failure occurred at about 12 minutes. However, tie-tube cooling is an integral feature of the NERVA derivative reactor, consequently, no failure should occur for the NERVA concept following a loss-of-coolant accident. The particle-bed concept has no redundant cooling system and it is difficult to imagine how one could be incorporated."
The relative heat capacity of the NERVA and particle bed reactors was also held to have other implications:
"The absence of redundant cooling and the low heat capacity of the particle-bed reactor also presents some operational problems. Each time the reactor is powered down between launches, the decay heat must be removed... the particle-bed concept will require either continued hydrogen cooling (with appreciable wasted hydrogen) or a closure of the system to allow some type of closed-cycle cooling to remove the decay heat."
Sandia also responded to the Brookhaven:
"... claims that the NERVA reactor was "very complex" while their particle-bed reactor had "simple" construction. This comparison is inappropriate for the reactor's respective states of development. The NERVA reactor is based on a prior existing and fabricated design that included everything necessary to become operational... [in contrast to Brookhaven's] ... evolving view of the particle-bed reactor with only conceptual component arrangements and tentative material selection. It should be expected that a greater amount of design detail for the particle-bed reactor would also increase the number and complexity of the reactor parts and require well-defined dimensions and tolerances.
"The discussion regarding "simple" or "complex" reactor designs in regards to reliability should be considered superfluous... The degree to which component tests could duplicate reactor tests would depend upon the individual components and experimental conditions, but only full reactor tests could provide the actual operating environments. Also, any synergistic effects may only be revealed by total reactor tests. For example, the test of an individual particle-bed element may not expose the axial restriction in coolant flow (caused by the coolant passing along the cold outer frit between elements or within the moderator block plenum before traversing the particle bed) and the resulting possible fuel temperature increase. Finally, little difference should be expected between the cost or number of full reactor tests required for any reactor concept, whether it be classified as "simple" or "complex" because safety, environmental issues, decontamination, and non-reactor experimental hardware remain similar."
Sandia also took issue with Brookhaven's assessment of the potential for particle-bed fuel element hot frit clogging:
"Collection of fines... within the fuel bed itself could occur in such a way that once begun, deposition of fuel would be enhanced. This could exclude regions from cooling flow and create local hot spots along with the concomitant consequences."
These issues remained unresolved at the conclusion of the Multimegawatt program, and the debate has continued.
2- Space Power Architecture Studies
In early 1986 the Air Force initiated the Space Power Architecture Studies to define operational and power requirements for space-based weapons systems. In August 1986 the Air Force Weapons Laboratory awarded contracts for MMW Space Power Subsystem design to TRW ($1.3 million),(3) Martin Marietta ($1.1 million),(4) and General Electric ($1.4 million).(5)
These contracts were intended to:
+ identify space platform integrated power subsystem conceptual designs;
+ assess their ability to operate in the natural and hostile space environments;
+ identify power subsystem and technology development requirements to guide subsequent planning for advanced power subsystem hardware development;
+ develop a computer based methodology of comparing subsystem and advanced power technology options.
With technology availability assumed to be that at the time of the Preliminary Design Review in 1996, based on a 2005 first launch,(6) the studies addressed SDI multimegawatt power requirements for five types of systems:
1 - Electromagnetic Launcher space-based kinetic energy weapon;
2 - Free-Electron Laser space-based directed energy weapon;
3 - Neutral Particle Beam space-based directed energy weapon;
4 - SDI surveillance platforms;
5 - Orbital Transfer Vehicles for deploying and servicing space-based weapons and surveillance platforms.
According to TRW, the first three of these systems had burst-mode power requirements of hundreds of megawatts, while the later two systems had requirements of tens of megawatts. As previously discussed, the Martin Marietta analysis assumed similar power requirements.
i - System Alternatives
Unlike the Sandia study, which was primarily focused on nuclear power source alternatives, the Space Power Architecture Studies considered integrated power systems, including a variety of combinations of power sources, conversion systems, and waste heat management approaches. The TRW study analyzed a total of 23 technology combinations (Figure II-60), while the Martin Marietta study covered 25 combinations (Figure II-61). Each of these was ranked according to a number of figures of merit.(7)
CLOSED CYCLE SYSTEMS
Rank Concept Power Source Conversion Waste Heat Management
1 * H + O / FC(CC) Hydrogen + Oxygen reaction Closed Fuel Cell Ice Heat Sink
2 * THOR In-core thermionic Storage / radiator
3 * Ti + O / B Titanium + Oxygen combustion Closed Brayton Water / radiator
4 * Li + H / TG Lithium + Hydrogen reaction Turbogenerator Radiator
5 * H + O / MHD(CC) Hydrogen + Oxygen combustion Closed MHD Ammonia Heatsink
6 * NDR / B NERVA Derived Reactor Closed Brayton Storage / radiator
7 * H + O / TG(CC) Hydrogen Oxygen combustion Closed Cycle turbogenerator Ammonia Heatsink
8 Ti + O / Kr Titanium + Oxygen combustion Closed Potassium Rankine Radiator
9 HC + O / B Hydrocarbon + Oxygen combustion Closed Supercritical Water heatsink
10 Li + SDF / R Lithium-Sulfur Hexafluoride Steam Rankine Ammonia Heatsink cycle
11 PBR / B Particle Bed Reactor Closed Brayton Storage / radiator
12 Be + O / B Beryllium + Oxygen combustion Closed Brayton Water / radiator
NR STAR-M / Batt In-core Thermionic Li-metal sulphide battery Radiator
OPEN CYCLE SYSTEMS
Rank Concept Power Source Conversion Effluents
1 * H + O / FC Hydrogen Oxygen reaction Fuel Cell + condenser Hydrogen
2 * NDR / TG NERVA Derived Reactor turbogeneration Hydrogen
3 * H + O + Ti / TG Hydrogen Oxygen combustion Turbogenerator Hydrogen (Ti water removal)
4 * Gel / MHD Gel Beryllium + IRFNA combustion Open MHD Steam, MHD Vent, oxides
5 H + O / MHD Hydrogen + Oxygen combustion Open MHD Steam, MHD Vent, Hydrogen
6 Li + H / TG (O) Lithium + Hydrogen reaction Turbogenerator Hydrogen (80%)
7 PBR / TG Particle Bed Reactor turbogeneration Hydrogen, particle fragments
8 H + O / TG Hydrogen Oxygen combustion Turbogenerator Steam, hydrogen
NR NDR / MHD NERVA Derived Reactor MHD Hydrogen, MHD Vent
NR PBR / MHD Particle Bed Reactor MHD Hydrogen, MHD Vent
CLOSED CYCLE SYSTEMS
Score Concept Power Source Conversion Waste Heat Management
3687 CW-11C Hydrogen Oxygen reaction Fuel Cell Radiator
2514 CW-03B Reactor - Liquid Metal Thermoelectric Radiator
2321 CW-04B Reactor - Liquid Metal Thermionic Radiator
2236 CW-05B Reactor - Liquid Metal Thermoelectric / Thermionic Radiator
2217 CW-02B Reactor - Gas-Cooled Closed Fuel Cell Radiator
OPEN CYCLE SYSTEMS
Score Concept Power Source Conversion Effluents
4093 CW-09B Hydrogen Oxygen combustion Brayton Hydrogen + Water Collector
3963 CW-09A Hydrogen Oxygen combustion Brayton Hydrogen + Water
3701 CW-11B Hydrogen Oxygen reaction Fuel Cell Hydrogen + Water Collect
3582 CW-11A Hydrogen Oxygen reaction Fuel Cell Hydrogen + Water
3390 CW-08C Hydrogen Oxygen combustion Rankine + Brayton Hydrogen + Water
3333 CW-08A Hydrogen Oxygen combustion Rankine Hydrogen + Water
3323 CW-13 N2H4 + Oxygen combustion MHD Hydrogen + MHD Vent
3279 CW-08B Hydrogen Oxygen combustion Rankine Hydrogen + Water + Radiator
3162 CW-01 Reactor - Gas Cooled Brayton Hydrogen
3159 CW-10A Hydrogen Oxygen combustion MHD Hydrogen + MHD Vent + Water
3075 CW-10B Hydrogen Oxygen combustion MHD Hydrogen + MHD + Water Collect
2925 CW-12 C2N4 + Oxygen combustion MHD Hydrogen + MHD Vent
2645 CW-06 Reactor - Liquid Metal Brayton / Thermionic Hydrogen
2586 CW-03A Reactor - Liquid Metal Thermoelectric Hydrogen
2514 CW-04A Reactor - Liquid Metal Thermionic Hydrogen
2322 CW-05A Reactor - Liquid Metal Thermoelectric / Thermionic Hydrogen
2276 CW-02A Reactor - Liquid Metal Rankine Hydrogen
2259 CW-07A Reactor (NERVA) - Gas Cooled MHD Hydrogen + MHD Vent
2240 CW-02C Reactor - Liquid Metal Rankine / Brayton Hydrogen
1922 CW-07B Reactor (PBR) - Gas Cooled MHD Hydrogen + MHD Vent
APPLICATION FEL NPB EML
POWER REQUIREMENT (MWe) 143 - 316 170 - 375 400 -1500
POWER SYSTEM MASS Tons)
Gel / MHD 159 353
NDR / MHD 97 202
NDR / TG 74 142
H + O + Ti / TG 105 230
Li + H / TG 272 464
THOR 225 488
LMR / Bat 405 744
H + O / FC 212 407 411
ii - Power Sources
A broad range of power sources were investigated in the SPAS studies. Nuclear power sources analyzed included:(8)
NERVA-derived gas-cooled solid core nuclear reactors generally have relatively fast start up times of less than 15 seconds, and are among the most attractive of the reactor concepts on the basis of cost, mass and technical risk.
Particle-Bed Reactors with gas cooling have even shorter start up times, but significantly less engineering experience than is the case with NERVA derived systems;
Liquid-Metal indirect cycle reactors with a primary lithium cooling loop and a secondary potassium cooling loop. These reactors require long start up times, ranging from several hours from a cold state to several minutes from a hot state, which is inconsistent with SPAS burst mode power requirements. Although lithium offers lower mass liquid metal pumps, piping and coolant mass relative to the more conventional NaK coolants, the higher melting point of lithium (470oK versus 260oK for NaK requires greater attention to coolant thawing in space, as well as complicating ground testing.
STAR-M in-core thermionic reactor, a fast-spectrum reactor with a electromagnetically-pumped lithium coolant system;
THOR in-core thermionic reactor.
Non-nuclear chemical power sources covered in these studies include:
Hydrogen-Oxygen Combustion in combustors derived from rocket motors;
Hydrocarbon-Oxygen Combustion;
N2H4-Oxygen Combustion;
C2N4-Oxygen Combustion;
Titanium-Oxygen Combustion;
Beryllium-Oxygen Combustion;
Gel Beryllium-IRFNA (Inhibited Red Fuming Nitric Acid) Combustion;
Hydrogen-Oxygen Reaction in fuel cells;
Lithium-Hydrogen Reaction;
Lithium-Sulfur Hexafluoride Reaction;
In contrast to the previously discussed Brookhaven analysis of particle-bed reactors, which suggested that high critical temperature superconductors would render this power system more attractive, the Martin Marietta study proceeded from the assumption that:(9)
"... the recent superconductivity advances will take many years to mature before they will have an impact on SDI weapon concepts... This assessment is based on the fact that it took over 23 years to achieve a high gradient-accelerating cavity using niobium technology."
iii - Conversion Systems
Conversion systems covered in the SPAS studies included both dynamic and static conversion approaches. Dynamic conversion systems included:
Brayton dynamic turbogenerators, as with other dynamic systems, offer very fast startup time of less than two seconds;
Rankine dynamic turbogenerators used in closed cycle systems are approximately 25% heavier than open cycle Brayton generators;
Brayton/Rankine combined dynamic turbogenerators;
Brayton / Thermionic combined dynamic generator and static converters;
Static conversion systems included:
Thermoelectric static converters;
Thermionic static converters;
Thermoelectric / Thermionic combined static converters;
Magnetohydrodynamic (MHD) static generators provide a low volume, high power system, which can be rapidly started, provided superconducting magnets remained continuously cooled;
Fuel Cells are attractive from the standpoint of enhanced platform stability due to static conversion, as well as offering modularity, rapid startup, long life, low maintenance, and a proven space technology base;.
One of the major issues raised by dynamic conversion systems was the potential impact of vibrations from rotating machinery and effluent exhaust (for open systems) on directed and kinetic energy weapon platform pointing accuracy. The TRW analysis concluded that:(10)
"Major contributions to dynamic disturbances are nozzle thrust imbalance and low frequency turbulence of liquid flows. Rotating machinery disturbances are not significant. Can be easily mitigated by integral magnetic suspension. Combustion disturbances are insignificant because of the high frequency spectrum."
iv - Waste Heat Management
The Space Power Architecture Studies included both open and closed cycle systems for waste heat management. Open cycle systems included:
Hydrogen exhaust;
Hydrogen exhaust with Water Steam Collector can reduce the negative effects on spacecraft payloads of water effluent with a mass penalty of about seven percent;
Hydrogen and Water Steam exhaust and Auxiliary Radiator;
Closed cycle options included:
Radiators;
Effluent Storage with Radiator would contain power system effluents in large inflatable bags, with subsequent condensation using radiators.
Water Heat Sink;
Ammonia Heat Sink;
Water Ice Heat Sink thermal management systems have the lowest mass of the three heat sink approaches. Although heat sinks have much lower (up to a factor of 50) volumes that inflatable bags, they have somewhat higher mass requirements. Other advantages of heat sinks include the absence of effluents and thermal signatures as well as reduced vulnerability compared to radiators and bags;
The Martin Marietta analysis concluded:(11)
"Infrared-active gases such as water vapor and carbon dioxide in a contaminant cloud will absorb infrared radiation and will emit infrared radiation if they are hot... Since infrared sensors are looking for radiation from water vapor and carbon dioxide in the target rocket plumes, the cloud interferes with sensor function. A neutral particle beam (NPB) passing through matter will become at least partially ionized, and the ionized portion will not follow the same path as the main beam."
"... even the hydrogen flow necessary to cool the NPB weapon itself is sufficient to cause nearly 100 percent NPB ionization. The tolerable NPB ionization is chosen as 10 percent... Directional venting will be necessary with the NPB system to prevent excessive NPB ionization.
"It is ... assumed that a 1000-fold decrease in gas density can readily be achieved by directional venting."
"... hydrogen is an acceptable effluent for FEL and EML platforms provided that reasonable separation distance is allowed between the vent area and the location of sensors. For the NPB, a minimal directional vent for hydrogen effluents will be required to avoid excess NPB beam stripping. Water vapor, carbon dioxide, and MHD products present progressively increasing problems. For these systems the question becomes how effective a directional venting can be."
However, Martin Marietta conceded that:(12)
"... further contamination analysis may lead to the conclusion that even hydrogen is an unacceptable effluent and only true closed cycle systems are acceptable."
Special attention is needed to the design of exhaust systems for dynamic conversion turbogenerator systems. The TRW analysis noted that:(13)
"High Mach number supersonic nozzles effectively direct effluent away from platforms... Opposing nozzle pairs, in conjunction with small balancing nozzles, eliminate net thrust on platform. Use of a common plenum eliminates thrust imbalance due to failure of an individual generator."
It further observed that the:(14)
"Enclosed nature of platform avoids other contamination threats, with exception of the NDR/MHD system in which cesium can deposit on surfaces such as radiators altering thermal characteristics."
The TRW analysis noted that:(15)
"Open cycle power subsystems with essentially pure hydrogen effluent do not create any significant design problems. Column densities are at least an order of magnitude lower than necessary for acceptable NPB attenuation. Lack of absorption on emission in visible and IR bands avoid optical effects. Contamination is insignificant.
"Column density reduction -- Moderate improvement (factor of 2 or so) can be achieved by locating exhaust systems toward rear of platform. Major reduction (approximately 2 orders of magnitude) is possible with a collar-like plume shield near front of platform... Best location is behind sensors and communication equipment at front of platform... For MHD devices, exhaust flow channels could be extended radially to achieve higher exit Mach number of hence more narrowly directed plume.
"For the Gel/MHD, the higher effluent densities and presence of less benign species does not necessarily constitute an unacceptable situation, but clearly warrants very detailed study... Significant reduction in H2O appears necessary to avoid unacceptable optical effects."
In conclusion, TRW observed that:(16)
"Closed cycle burst power subsystems are about three times heavier than open cycle subsystems. Large quantities of hydrogen are needed for the weapon."
v - Summary
The TRW analysis ranked each power system according to the mass required for each application, the time required to develop the necessary hardware, effluent type, dynamic disturbance sources, and response time.(17) Open cycle systems were preferred, with chemical hydrogen-oxygen fuel cells and combustion systems ranked first and third. The second-ranked NERVA-derived system posed additional safety concerns, as TRW questioned:(18)
"Is nuclear reactor power source viable politically / environmentally? Deployment failure WILL occur with quantities proposed."(19)
The Martin Marietta analysis considered fourteen separate figures of merit,which were given a weighing factor according to the relative importance of the variable, and each of the power systems was assigned a point value for each of this variables (Figure II-63).(20)
With a total weighing multiplier of 50, and 100 points representing the highest score on each figure of merit, a perfect score would equal 5,000. This assessment process was performed separately for the FEL, NPB, EML and OTV applications. Although this process resulted in different scores for each application, the ranking for the EML is illustrative of the results of the assessments for other systems.
Weapon OTV
Figure of Merit Weight Weight
Technical risk 5 5
Operational considerations 5 0
Survivability 5 1
Reliability 3 5
Operating lifetime 3 3
Safety 3 5
Maintainability 3 5
Space assembly requirements 1 1
Integration requirements 5 5
Contamination sources 5 1
Dynamic loads 5 1
Life-cycle costs 3 3
Mass 3 3
Volume 3 3
Point Value Description Total Score
100 Best 5,000
80 Better Than Average 4,000
60 Average 3,000
40 Worse Than Average 2,000
20 Much Worse Than Average 1,000
0 Unacceptable 0
As with the TRW study, Martin Marietta concluded that open-cycle systems were more attractive than closed systems, noting that:(21)
"... the radiator masses for the closed cycle options are extremely high, a fact which should eliminate these concepts from further consideration at this time."
Martin Marietta determined that chemical power sources were preferable for open-cycle systems, with nuclear power sources receiving lower overall scores. For both open and closed cycle system, all but one (NERVA-derived gas cooled open Brayton) scored below "average" (less than 3,000 points total)
3 - National Research Council - 1989
In 1989 the Energy Engineering Board of the National Research Council provided an evaluation of the Multi-Megawatt power program, and a review of the Space Power Architecture Study results.(22) The Committee observed several shortcomings in the SPAS analyses, including the facts that:(23)
"Existing SDI space power architecture system studies do not provide an adequate basis for evaluating or comparing cost or cost-effectiveness among the space power systems examined, do not adequately address questions of survivability, reliability, maintainability, and operational readiness, and do not adequately relate to the design of complete SDI spacecraft."
"... the studies apparently did not make allowance for system survivability...
"... masses for open cycle systems ... do not include the masses of hydrogen needed for cooling the reactor of for burning in the turbine....[based on]... the specific direction given to the SPAS contractors, namely, that the hydrogen for these purposes is available "free" -- for example, with no mass penalty -- form the weapon system.(24) On the other hand, coolant required for the 1,800 second burst is included in the masses for the closed-cycle systems."
The Committee's analysis of the SPAS contractor studies was complicated by the fact that the SPAS contractors:(25)
"... unfortunately, employed and inconsistent set of assumptions. Consequently, there are necessarily differences in the three sets of results... This problem is especially severe in comparing estimates of system mass. In that regard... [there were]... significant differences in assumptions among the contractors -- along with overall limitations in the assumptions -- pertaining to the following technological and packaging considerations, to which the mass estimates are sensitive:
"High-voltage power systems perform well with tube radiofrequency (RF) systems.
"Low-voltage power systems perform well with solid-state RF systems.
"Cryo-cooled power conditioning, if realizable, saves mass, hence conductors, transformers, and other components can be less massive.
"Mass estimates were based on conservative near-term or on optimistic far-term assumptions regarding technology.
"Masses required for thermal management and packaging were not uniformly considered.
"The technology postulated for power conditioning does not exist."
As to the relative merits of Rankine and Brayton conversion cycles, the Committee noted:(26)
"A further difficulty encountered... is the significant difference in reactor masses between the Brayton and Rankine systems. This large discrepancy resulted from the fact that the two sets of result were obtained by separate contractors who used different technical assumptions, some of which may be questionable... These differences contributed significantly to the committee's reluctance to recommend, with any assurance, either the selection or elimination of any candidate space power system(s)... additional study of both cycles is warranted in view of unexplained or inconsistent SPAS analysis results..."
The Committee endorsed additional research on advanced materials for closed cycle power systems:(27)
"... if the technology for carbon-carbon composites were sufficiently advanced so as to provide a material for construction of a Brayton cycle power plant, it is conceivable that the turbine inlet temperature could be raised from the 1500o K [presently assumed] to 2000o K. This temperature increase would reduce the mass of the required radiator by about a factor of three, thereby roughly halving the total mass of such a power system, and would also increase the efficiency of power conversion. Realization of the full potential of such a material, about 2300o K... would reduce power system mass even further."
On the question of the choice between open and closed cycle systems, the Committee observed:(28)
"The ... option that avoids radiators is gross heat rejection from a thermal engine, where the waste heat is simply thrown overboard with the effluent. This is a viable concept if the effluent does not unduly interfere with friendly weapon, sensor, spacecraft or power systems. On the other hand, liberating effluents may hinder hostile action."
"... condensation may occur on cold surfaces of the spacecraft, and... there is also the possibility of creating "snow flakes" such as those reported in the Apollo program... Chemically reacting gases or ionized gases could also emit electromagnetic radiation. For an SDI platform, such a radiant plume could interfere with its sensors, increase its detectability, and increase its vulnerability."
"To understand the impacts of effluent clouds, estimates are required of neutral particle densities about a vehicle due to back-streaming around nozzles... There are order-of-magnitude differences between independent predictions of densities in the region "behind" nozzles.
"... one unresolved issue is whether effluent releases will interfere with propagation of a neutral particle beam.... calculations suggest... that the issue of neutral-particle-beam stripping must be addressed.
"Despite difficulties in comparing SPAS results, however, the dramatic mass differences between open-cycle and closed-cycle power systems, found by all the contractors, were qualitatively adequate [to support the Committee's recommendations]"
Thus the Committee concluded:(29)
"The amount of effluent tolerable is a critical discriminator in the ultimate selection of an SDI space power system. Pending resolution of effluent tolerability, open-cycle power systems appear to be the most mass-effective solution to the burst-mode electrical power needs in the multimegawatt regime. If an open-cycle system cannot be developed, or if its interactions with the spacecraft, weapons, and sensors prove unacceptable, the entire SDI concept will be severely penalized from the standpoint of cost and launch weight...
"To remove a major obstacle to achieving SDI burst-mode objectives, estimate as soon as practical the tolerable on-orbit concentrations of effluents. These estimates should be based -- to the maximum extent possible -- on the results of space experiments, and should take into account the impact of effluents on high-voltage insulation, space-platform sensors and weapons, the orbital environment, and power generation and distribution."
The Committee determined:(30)
"Pending resolution of effluent tolerability, open-cycle power systems (systems whose working fluid is used only once) appear to be the most mass-effective solution to burst-mode electrical power needs in the multimegawatt regime. If an open cycle system cannot be developed, or if its interaction with the spacecraft, weapons and sensors prove unacceptable, the entire SDI concept will be severely penalized from the standpoint of cost and launch weight."
Based on the SPAS analysis, the Committee observed:(31)
"Assuming typical costs per pound for development, production, and launching to orbit, and noting that the power system may range from 20 to 50 percent of the total orbital vehicle mass, these systems appear to be very large -- hence probably prohibitively expensive -- and too massive to lift into orbit with any practical launch vehicle, unless they were launched separately and assembled in orbit...
"Gross estimated masses of SDI space power systems analyzed in existing studies appear unacceptably large to operated major space-based weapons. At these projected masses, the feasibility of space power systems needed for high-power SDI concepts appears impractical from both cost and launch considerations. Avenues available to reduce power system costs and launch weights include: (a) to substantially reduce SDI power requirements; (b) to significantly advance space power technology."
H - STATUS
The fate of the SDI Multimegawatt Power Program has followed that of the Strategic Defense Initiative more generally. The vision that President Reagan presented in March of 1983 as the basis of his Strategic Defense Initiative is a world in which nuclear weapons are "impotent and obsolete." Although this is a somewhat vague and indefinite notion, it was generally taken to mean that the SDI would lead to a virtually perfect defense of populations. Certainly the exuberant rhetoric used in support of the program would be difficult to sustain in support of less exalted goals, such as defense of retaliatory forces.
When the President announced his plan, it was portrayed as a means of protecting the American people from nuclear attack. But this was increasingly regarded as requiring a level of perfection that was not likely to be achievable. Very few proponents of SDI were prepared to claim that it would be 100% effective. But in the face of an attack by tens of thousands of warheads, even a 99.44% perfect defense would be worthless in defending cities.
The skepticism concerning Reagan's vision of a prefect defense was at two levels:
Device level concerns focus on the fact that a highly effective anti-missile system would require significant increases in the levels of performance of a variety of technologies. The SDI program included a number of different projects at widely varying stages of development. Not surprisingly, those projects that offered the greatest promise were also those at the most primitive stage of development, and those which faced the largest number of technological hurdles.
System level concerns were the most critical issues facing SDI and the bulk of the criticism of SDI has focused on such problems. The availability of countermeasures that would fool SDI sensors, the vulnerability of the system to direct attack, and the unreliability of the computer software running the system, increasingly suggested that SDI would not be able to meet these expectations of perfectibility.
If there was any hope of meeting the initial vision of an effectively perfect defense, space-based systems of the type supported by the MMW program would be required. It was increasingly apparent, however, that such device-level innovations would not resolve system-level concerns about perfectibility. It was also apparent that these advanced weapons were unlikely to be deployable in the 20th century. Thus the SDI deployment plan was divided into two phases:
Phase One, which would began deployment in the mid-1990s, would consist of initial ground and space based kinetic energy weapon systems that would provide a less-than-perfect defense against existing threats;
Phase Two, which would began deployment around the year 2005, would consist of highly capable directed or kinetic energy weapon systems that would provide a nearly perfect defense against responsive threats;
By the time the initial deployment plans for the SDI were established in August 1987, the focus of attention had largely shifted to the Phase One deployments. The Defense Acquisition Board, the Pentagon's highest committee dealing with procurement matters, established requirements for a system that could be deployed in the mid-to-late 1990s to defend American land-based ICBM silos from a Soviet counter-force attack.(32) The Phase One system was intended to intercept half of the Soviet's force of 308 SS-18 ICBMs (the core of the Soviet counter-silo capability), as well as thirty percent of all Soviet missile warheads, including those carried on SS-18s.(33) This initial operational Strategic Defense System included both space-based and ground-based weapon and sensor systems. Each year thereafter, the precise number and type of these systems evolved, based on changing perceptions of operational capabilities.(34) Estimates of deployment costs dropped from $115 billion to $69 billion.
And in late 1990 the Strategic Defense Initiative was fundamentally reoriented. Instead of protecting US ICBM silos from a Soviet first strike capability or the US population against a massive nuclear attack, SDI metamorphosed into a Global Protection Against Limited Strikes (GPALS) system. This reorientation was a response to both the political opportunities presented by the war with Iraq, and Congressional budget reductions in light of the receding Soviet threat and seven years of fruitless labor on the original SDI project.
GPALS is intended to defend against tactical and theater missile threats as well as up to 200 strategic missile warheads launched at the continental United States, including missile attacks that result from accidental or unauthorized launches, from both the Soviet Union as well as from some third country. The system is currently planned to be deployed in three stages: an air transportable system which would defend against theater missiles; a continental US system using ground-based interceptors at multiple sites and Brilliant Eyes sensors; and the global system with space-based Brilliant Pebbles interceptors.
What all of these changes in thinking about SDI had in common was a move away from the initial vision of a massive "Peace Shield" against large nuclear attacks -- precisely the vision that animated interest in large space-based weapons, and the large power systems they required.
Thus by 1988 doubts had begun to emerge about the MMW nuclear program. Development costs for non-nuclear burst mode systems, such as rocket-driven generators, were determined to be as much as an order of magnitude less than development costs of nuclear MMW sources.(35) More importantly, changes in the follow-on architectures for space-based weapons and sensors, with preference being given to ground-based systems, reduced the need for MMW power. Due to these program reorientations, the MMW contracts were suspended in 1990, although the contracts were held open in case NASA wishes to continue this work. As a result no SDIO funding was allocated starting in FY1990, and no DOE funding was provided for this program in FY1991.
1. Marshall, A.C., "A Review of Gas-Cooled Reactor Concepts for SDI Applications," SAND87-0558, (Sandia National Laboratory, Albuquerque, NM, August, 1989). This report was issued in draft form in April 1987.
2. Powell, J., Ludewig, H., and Horn, F., "Comments on Sandia Report INF-6511-8702 A Review of Gas Cooled Reactor Concepts for SDI Applications, mimeo, no date.
3. This contract produced eight volumes, of which three were classified SECRET. Two volumes of this study were consulted:
TRW, Inc., Space & Defense, Space Power Architecture Study, Task 3 Report, Volume 3, Executive Summary, Report No. SN48083-FR-05, 4 September 1987.
TRW, Inc., Space & Defense, Space Power Architecture Study, Final Briefing, Report No. SN48083-FR-08, 23 September 1987.
4. Martin Marietta Astronautics Group, Space Power Architecture Study (SPAS), Final Report, Report Number TR-87-16, June 1988.
5. Reports resulting from this contract were not consulted.
6. Martin Marietta, op cit, page 453.
7. The evaluation methodology is discussed below.
8. Martin Marietta, op cit, pages 6-12.
9. Martin Marietta, op cit, page 359.
10. TRW, Task 3 Report, op cit, page 13.
11. Martin Marietta, op cit, pages 117, 126, 123, 92-95.
12. Martin Marietta, op cit, page 208.
13. TRW, Final Report, op cit, page 15.
14. ibid, page 17.
15. TRW, Task 3 Report, op cit, pages 12, 90.
16. ibid, page 15.
17. TRW, Final Report, op cit, page 42.
18. ibid, page 63.
19. emphasis in original.
20. Martin Marietta Astronautics Group, Space Power Architecture Study (SPAS), Final Report, Report Number TR-87-16, June 1988, page 94.
21. Martin Marietta, op cit, page 373.
22. National Research Council, Energy Engineering Board, Committee on Advanced Space Based High Power Technologies, Advanced Power Sources for Space Missions, (Washington, National Academy Press, 1989).
23. ibid, pages 3, 53.
24. This was apparently based on the assumption that liquid hydrogen requirements for weapons cooling would exceed the requirements for power production, and that electrical generation systems could thus use cooling system exhaust hydrogen, which would otherwise be vented as waste. This assumption is not explicitly stated or justified in the SPAS studies, although it is mentioned in passing. See Martin Marietta, op cit, page 263.
25. ibid, page 58.
26. ibid, pages 60, 75.
27. ibid, page 63.
28. ibid, pages 50, 47-48, 63, 70.
29. ibid, page 51.
30. ibid, page 2.
31. ibid, page 60, 67.
32. Gilmartin, Trish, "Pentagon Advisory Panel Chairman Urges Gradual Evolutionary Approach to SDI," Defense News, 25 July 1988, p. 30.
33. Norman, Colin, "Cut Price Plan Offered for SDI Deployment," Science, 7 October 1988, pp 24-25.
34. Zimmermann, Peter, "SDI Performs Annual Mutation," Defense News, 21 January 1991, page 24.
35. "Outlook Dims for Near Term Space Nukes," SDI Monitor, 25 January 1988, page 16-17.