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Space isotopiC power systems With the technology sound and growing, and units already built for missions ranging from 120 days to 5 years, the designer can and should plan appropriate space application of isotopic systems BY CAPT. R. T. CARPENTER, USAF U.S. Atomic Energy Commission A new space power system technology technology, and aerospace nuclear Concurrently, the terrestrial appli­ -isotopic power-has developed to safety technology contributed by the cations-the Snap-7 programs-sus­ the point where it can and should be program and used as a foundation for tained the isotopic power development considered by the space-vehicle de­ follow-on space isotopic power-system program and promoted the fission­ signer for use in many types of mis­ developments. product separations and processing sions. Because of this sound technical capability that exists within the Com­ The' Atomic Energy Commission's basis, the Commission's space-oriented mission today! The interest among isotopic space power program dates isotopic power development program terrestrial power users-the Navy, back to several years before Sputnik has made a steady, although some­ Weather Bureau, and Coast Guard­ I, but the program suffered a severe times slow, comeback through a series was sufficiently strong to sup:port this setback in 1959 when the Snap-1A of events since 1959, so that today a . fuels production program, whe:r:eas the generator development program was program technically comparable to interest in Snap-1A had been inade­ cancelled.' This pioneer program was Snap-1A could once more be under­ quate. At the same time, significant not completed because it may have taken with a high probability of suc­ quantities of the long-lived alpha­ been tt;lO ambitious for its day. The cessful completion. This series of emitter fuel, plutonium-238, were be­ need for isotopic power had .not yet events can help demonstrate the status ing produced so that it could be allo­ become apparent to space program of today's space isotopic power pro­ cated to low-powered space systems. planners; its place and full signifi­ gram. Details of the various systems Relatively little direct radiation haz­ cance in the nuclear space power pro­ have been described many times and ards are associated with alpha-emitter gram were not clearly established; its will not be repeated here. For refer­ fuels compared to beta-emitter fission applicable thermoelectric energy con­ ence purposes, the characteristics of products. version technology was still very new; several. space isotopic power systems Because of these background efforts, large quantities of isotopic fuel ma­ are given in the table appearing on it was possible to fabricate, fuel, test, ter'ials were not readily available; and page 70. and get approval to use the first plu­ the operational safety of large quanti­ While efforts were being made to tonium-238 fueled Snap-3 generator ties of radioactive material in space interest space power-system users in on the Navy's Transit-4A navigation vehicles was a brand new unknown in isotopic systems, the first practical satellite in June 1961, with a lead time a space program already full of un­ radioisotope-fueled thermoelectric gen­ of only five months. The launches of knowns. erator-Snap-3-was being subjected the 2.7-w, Pu-238-fueled Snap-3s on That the demise of the Snap-1A pro­ to exhaustive electrical tests; shock, Transit-4A and Transit-4B (in No­ gram was not due to a lack of tech­ vibration, acceleration and thermal­ vember 1961) were not just "firsts in nical soundness is evident when one vacuum environmental tests; and fil'e, space."5 The experience gained in in­ looks at the thermoelectric generator explosion, impact and re-entry burnup tegrating these units into satellites, fabrication technology, isotopic-fuels nuclear safety tests!" the flight-test data obtained from CAPT. R. T. CARPENTER is Deputy Chief of the AEC's Isotopic Power Branch in the Division of Reactor Development. He has been manager of space and terrestrial power development projects with the Commission's isotopic power program since April 1959. Capt. Carpenter's previous assignments include responsibilities as R&D project engineer at the Chemical Corps Biological Labs and at Wright Air Development Center. He holds a B.S. in chemical engineering and an M.S. in nuclear engineering. 68 orbit, and the interagency procedures aspects. The first electrically heated have been developed under the and policies established prior to their Snap-9A generator was operated six Snap-9A program without an inter­ launch produced in a matter of months months after go-ahead from the Navy vening flight test. a state of the art for isotopic space and award of the contract to Martin Snap 11, a 25-w RTG being devel­ power which probably would have Co. by AEC. The first flight-accept­ oped for use on NASA's Surveyor soft taken several years with a much able generator was fueled with Pu-238 lunar-landing missions, has also con­ larger unit, such as Snap-lAo nine months after go-ahead. It is tributed significantly to isotppic space The generator on the Transit-4A scheduled for use this year. power systems technology.s After a satellite continues to produce enough During the past year, a substantial design study and a preliminary safety power to transmit the low-powered amount of data has been obtained analysis had been completed, NASA Doppler navigation signals to earth from ground tests of the electrically established a requirement for the after being in orbit almost two years. heated and fueled generators. Impor­ Snap-11 generator development pro­ Because of a failure in the satellite's tant results were obtained from long­ gram late in 1961. During the past telemetry system a few weeks after term thermal-vacuum tests of the year, a detailed design was completed launch, quantitative data on the per­ units under simulated space operating that would meet all the interface re­ formance of the generator are no conditions. The generators were found quirements of the Surveyor spacecraft. longer available." Qualitative perform­ to be very stable power sources when These included the electrical, physical, ance of the unit is still being moni­ subject to sunlight and shadow con­ nuclear radiation, and thermal inter­ tored by the Transit tracking stations. ditions for a 600-mi. polar orbit. The face specifications." The electrical out­ Telemetry data from Transit-4B in­ high heat capacity of the compact gen­ put can be easily matched to the pay­ dicated the radioisotope thermoelec­ erator allowed a gradually changing load through a DC-to-DC voltage con­ tric generator (RTG) performed pre­ surface temperature and power out­ verter similar to that used with con­ cisely as expected for about eight put under these conditions, intlicating ventional power supplies. months. In early June 1962, an abrupt the solar input had little effect on the The physical limitations of the ve­ drop in generator output voltage was system. The Pu-238 fuel pr:ovides an hicle naturally dictate the size, weight, oeserved. During the next week, the unalterable source of heat far more and shape of an RTG. For the Sur­ voltage came right back up to the nor­ stable than an electrical heater. Power veyor program, it was decided to ex­ mal operating value of 2.1 Y, dropped degradation was observed during tests tend Snap-11 out from the spacecraft again to millivolts, came back up and of the fueled generators that was too (because of overriding thermal con­ then dropped to and stayed at prac­ small to detect with the variations in siderations) so that an optimized RTG tically zero voltage. This cyclic be­ line voltage, etc., experienced with configuration could be used. The sep­ havior is not characteristic of an RTG electrically heated units. Although ex­ aration distance and provisions for failure. Upon analysis of the data, it tremely small, these changes would be shielding in the design of the curium- was concluded that a capacitor across significant over the five-year design 242 fuel capsule will allow Snap-11 to the input to the DC-to-DC voltage lifetime of the generator. meet the extremely stringent back­ converter had shorted out. The solar The cause of this power degradation ground radiation levels specified for power supplies aboard Transit-4B was quickly diagnosed because similar the sensitive radiation detectors failed soon after the high-altitude nu­ but more easily detected power losses aboard the spacecraft. Thermal inte­ clear test of July 9, 1962, and signals had been observed in the Snap-7 gen­ gration problems were most severe are no longer being received from the erators a few months earlier. The ma­ and caused abandonment, for the pres­ satellite: terials used in the generators were ent, of a design for conducting heat One of the lessons learned from outgassing in a time-temperature de­ to the sensitive payload instruments these early flight tests was the im­ pendent manner, causing a ' greater during the cold lunar night. portance of integrating the nuclear heat loss to bypass the thermoelectrics. A thermal mockup of the Snap-11 unit into the payload as soon as pos­ To counteract this, the generators has been fabricated and is undergoing sible. Because the Snap unit was sub­ have been baked at higher tempera­ tests. Electrically heated prototype stituted for a solar-cell panel at the tures and filled with inert gases at generators will be available for inte­ last moment, only a limited number of higher pressures to reduce the effect gration tests later this year. Because telemetry channels were available to of the outgassing on generator per-' of launch vehicle problems, Snap-11 monitor the generator.
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