An analysis of a Rocketdyne report on nuclear rockets entitled, "Complete spectrum of nuclear engines and their capabilities"

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This document consists AN ANALYSE OF A ROCKETDYNE REPORT ON NUCLEAR ROCKETS ENTITLED, "COMPLETF SPECTRUM OF NUCLEAR ENGINES A N ^ THEL This document is

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This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

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This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor the Air Force, nor any person acting on behalf of the Commission or the A. Makes any warranty or representation, express or implied, with respect to the accuracy, comple te ness, or usefulness of the information contained in this report, or that the u s e of any information, apparatus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the u s e of any information, apparatus, method, or process disclosed in this report. As used in the above "person acting on behalf of the Commission or Air F o r c e " includes any employee or contractor of the Commission or Air Force to the extent that such employee or contractor prepares, handles, or distributes, or provides access to, any information pursuant to his employment or contract with the Commission or DEPARTMENT OF ENERGY DECLASSIFICATION REVIEW

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AN ANALYSIS OF A ROCKETDYNE REPORT ON NUCLEAR ROCKETS ENTITLED "COMPLETE SPECTRUM OF NUCLEAR ENGINES AND THEIR CAPABILITIES" Classification cancelled (01 ilunpil to. Recently I had the opportunity to study a Rocketdyne report R-1951 entitled "Complete Spectrum of Nuclear Engines and Their Capabilities" published in November of 1959. It is believed that this study was conducted by Rocketdyne with their own funds and while it was not marked proprietary, it is doubtful that this Department will be able to obtain a copy. Nevertheless, sufficient information was obtained from this report such that the essential assumptions and results are known. Two nuclear rocket designs were considered in this study. One was a graphite two-pass system based upon the Condor rocket that Los Alamos Scientific Laboratory (LASL) has proposed as a flight system. While the LASL design was based upon one size, Rocketdyne varied the thrust level from 500, 000 to 3, 200, 000 lbs. but retained the LASL design features. The specific impulse of this system was estimated to be 740 seconds at altitude, which corresponds to a hydrogen gas temperature of 3600°F. The second nuclear system considered (called by Rocketdyne the "Prototype") was based upon an advanced graphite reactor design. This system was estimated to give an altitude specific impulse of 890 seconds at a gas temperature of 4700°F. This Prototype reactor is considered to represent the highest performance in a reactor.that can be realized with graphite fuel elements. An all chemical vehicle was studied to compare its capabilities with that of the two nuclear designs. The chemical rockets were assumed to be a LOX/SF-1 (Para-hydrogen) system with an altitude specific impulse of 420 seconds. This represents just about the ultimate performance that can be expected from When comparing the nuclear and the chemical rockets, Rocketdyne assumed that the nuclear vehicles were one stage systems and the chemical vehicles were assumed to be made up of three stages. This assumption definitely favors the chemical system;'but it is realistic. Because of the high specific impulse of nuclear systems and "consequently a low mass ratio, not much benefit is obtained by staging the vehiclesv. Whereas for chemical systems, the Opposite is true. In addition, single stag£ htidlear vehicles are very desirable from reliability and cost considerations. .

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Since Rocketdyne considered only systems launched from the earth ' s surface, the ratio of the thrust at altitude to the initial gross weights was varied from 1. 3 to 1. 7. A high thrust - t o - weight ratio is desirable because it reduces the losses due to gravity and atmospheric drag ; however, high thrust to weight ratios means high accelerations which leads to increased tankage and structural weight. Therefore, an optimum is reached depending upon the mission and characteristics of the rocket system (whether it i s powered by a nuclear system, liquid, or solid propellants). Generally, nuclear rockets and solid propellant chemical systems optimize at thrust - to-weight ratios higher than liquid propellant systems. Rocketdyne considered various missions by merely varying the velocity increment, Yc» from 30, 000 to 46, 000 f t / sec. However, in order to establish a frame of reference, Table I was prepared to indicate the velocity increments required for various missions. The assumptions that Rocketdyne made in regards to reactor weight, reactor size and propellant tank size, and weight are given in the Appendix in figures 1A to 4A. In addition, certain assumptions were made in regards to the thrust structure weight and the shielding required to limit the hydrogen boil-off due to nuclear heating. These assumptions are given in figures 5A and 6A. Unfortunately, time did not permit obtaining the assumptions that were made a s to pump and nozzle weights. The main criteria that Rocketdyne used in comparing nuclear and chemical systems was production cost. In arriving at the overall vehicle cost, the assumptions given in Table II were used. While the Rocketdyne report did not make mention of U235 cost, it i s believed that this is included in the 400 dollars per pound, since no provision was made for recovering the boosters. To date, no work on the costing of graphite reactors for rockets has been * conducted in the Department.Therefore, it is somewhat difficult to pass judgment on Rocketdyne's assumption. However, detail costing studies have been made on the XMA-1A reactor. While this reactor is made up of different materials, the size is in the range of the reactor sizes (90 - 150 inches) considered by Rocketdyne, and they are both moderated reactors. The results indicated that the XMA-1A reactor including reflector and U235 would cost approximately 320 dollars per pound for the first production power plant. This value would decrease somewhat with an increase in the number of reactors but not a s much a s one might expect. This is because the U235 is a large percentage of the cost and would remain constant with the number of reactors produced. Comparing the cost of the XMA-1A reactor with Rocketdyne's assumption for the nuclear rocket reactor leads one to believe that they may not be too far off. However, an actual costing study of the graphite r e a c t o r should be undertaken.

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300 mile earth satellite LUNAR Probe (Earth Escape) Stationary Orbit (22, 000 miles) or Lunar Satellite Moon Landing & Return

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Reactor Cost $400 perlb. of (Including fuel and reflector) reactor weight Hardware Cost $100 per lb.

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Now that the basic assumptions used in the study have been outlined and commented upon, the results will be discussed. Some of the results of Rocketdyne's study are given in figures 1 to 11. These are not a complete set of the figures given in the original report, but represent the more important ones. Rocketdyne compared the nuclear rockets with the advanced chemical systems on the basis of three criteria : production cost, payload carrying ability, and gross weight. Past studies have usually only considered the latter two criteria and neglected cost. The comparison is made on the basis of these three criteria for various altitude thrust - t o - weight ratios, velocity increments, payloads and nuclear specific impluse (Condor vs. the Prototype design). The results indicate the following: 1. The relative cost of chemical rockets to nuclear rockets increases with an increase in velocity increment. In other words, the economic factor becomes more favorable for a nuclear rocket a s the mission becomes more ambitious. The Rocketdyne results indicate that a velocity increment i s eventually reached where the rate of chemical cost to nuclear cost commences to decrease. This i s due to the fact that only single stages were considered for nuclear systems and hence a point is eventually reached where it will be desirable to increase the number of stage for both the nuclear and chemical systems. 2. At a given velocity increment, the ratio of chemical to nuclear cost at first increases with payload and then upon reaching a maximum, will decrease. It is believed that this effect is also due to the limitations of a single stage for the nuclear rocket. However, it i s believed that a two stage nuclear vehicle will be more expensive for moderately ambitious missions. However, a point will be reached (for high payloads and high velocity requirements) where the two stage nuclear may show an 3. As the altitude thrust - t o - weight ratio is increased, the relative advantages of an all nuclear vehicle over a chemical system decreases. However, this does not mean that the lowest thrust - to-weight ratio should be chosen for a nuclear system. The absolute magnitude of the nuclear payload cost would have to be minimized rather than the ratio of nuclear to chemical cost.

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The choice of a proper thrust - t o - weight ratio for nuclear systems will have to await amore detailed analysis. However, past studies have mostly favored the larger take-off thrust - t o - weight ratios of 1. 5 to 1. 7. 4. For all the thrust - t o - weight ratios, velocity increments, and payloads considered, the Condor engine at a specific impulse of 740 seconds cannot compete with an advanced chemical system on 5. A single - stage prototype engine with a specific impulse of 890 seconds offers a significant advantage over an advanced chemical system on a cost basis at velocity increments in the range of 36, 000 to 46, 000 f t / s e c and for payloads in the range of 150, 000 II. Payload Basis 1. For the range of parameters considered, the Condor engine will yield a payload advantage over that of an advanced chemical system for equal g r o s s weights by a factor of between 1. 5 and 2. 3. 2. The prototype engine will yield a payload advantage o v e r that of an advanced chemical system by a factor of from 2. 7 to 4. 3. III. Gross "Weight Basis 1. For the range of parameters considered, the Condor engine will yield a gross weight advantage over that of an advanced chemical s y s t e m for equal payloads by a factor of from 1, 4 to 2, 4. 2. The prototype engine will yield a g r o s s weight advantage over that of an advanced chemical system for equal payloads by a factor of Unfortunately, the only absolute values for cost and payloads that w e r e obtained from the Rocketdyne report were the values for the Condor engine at an altitude thrust - t o - weight ratio of 1. 3(Figures 10 and 11). However, if one desires, the absolute values for the prototype and the chemical system can be calculated from these two figures and the ratios are given in Figures1 to 9. Comments on Rocketdyne's Study Rocketdyne*s study considered only the production cost of the vehicle. However, to obtain a comparison much closer to reality, the cost of such factors as ground handling, shielding and launching facilities should also be considered. Naturally the nuclear rockets will require much more elaborate handling fac ili - ties because of the pre_sence_pf radiation. However, because of a much smaller

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gross weight, the nuclear systems will not require as many launching pads or much jsmaller ones to launch a given payload than the requirements for a chemical system. These two factors will tend to cancel, but before the r e l a - tive magnitude of each can be ascertained, additional analysis will have to be conducted. These effects are being analyzed by Lockheed, Martin and Convair in the RIFT studies with the results becoming available by the first of the year. One area that would be interesting to explore that was not covered in the Rocketdyne report is the effect of reactor cost, reactor weight and specific impulse on the cost of the over-all system. This information would be of value in evaluating the desirability of decreasing reactor size and weight at the expense of an increase in U ^ ^ fuel loading, or in other words, evaluating the desirability on a cost basis of a fast reactor over a thermal reactor. In order to be able to do this, it is necessary to choose a particular mission and vary the reactor cost and weight from the values used by Rocketdyne. Assuming a mission of carrying 150, 000 pounds of payload to a velocity of 40, 000 ft/sec (earth escape), the results are given in Figure 12. (The results obtained by Rocketdyne for the Condor and Prototype engine are represented by two points in this Figure.) This particular mission was chosen because the Rocketdyne study indicated that its cost would be significantly less (23%) than that for a chemical system. The thrust and power levels required to perform this m i s - sion as a function of reactor weight and specific impulse are given in Figure 13. If a reactor were designed to satisfy the curves in Figure 13 at some point, then the resulting reactor weight, cost and specific impulse can be plotted in Figure 12 to obtain the cost per pound of payload and compared with the graphite Fast reactors studied by LASL are characterized by a weight that is lower than the graphite system by a factor of the order of 2. However, the fuel loading can increase by a factor of the order of 2 or 3; hence, the reactor cost per pound can increase by a factor of between 4 and 6. Referring to Figure 12, the result may be an increase in the cost per pound of payload over that of a graphite system. However, this evaluation will have to await the results of the design effort on such advanced systems as tungsten and carbide reactors. At this point, it should be remembered that cost alone is …