Snap-19 Fact Sheet - Snap-19--Nuclear Power Source for the Pioneer F Spacecraft

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I SNAP-lg--Nuclear Power Source for the Pioneer F Spacecraft Y 2A X 9 SNAP-19 i s one of a series of nuclear generators developed 2 by the Atomic Energy Commission under its SNAP (Systems for Nuclear Auxiliary Power) program for use in space. will provide the electrical power for the NASA Pioneer F space - craft to be used in the Jupiter flyby mission scheduled for Cape Kennedy Air Force Station,Florida. electric generators, are modified versions of the two SNAP-19 generators on the NASA Nimbus 111 weather satellite launched in April 1969. They will ' represent the first use of nuclear electrical power on an interplanetary spacecraft. The spacecraft will be launched from the carrying probe past Mars--would be hazardous for solar cells, considering the debris in the asteroid belt and the antic i - pate d high - radiation fields trapped by Jupiter ' s magnetic field.If solar cells were used, enormous numbers of them would be required to generate the required power at Jupiter because the sun ' s energy flux will have dropped 2 S - fold this t ions prevent the use of the large numbers o f them that would be required to complete the mission.(Trip time to the Jupiter environment varies between 645 and 795 days depending upon the specific day and hour of launch.) Those batteries that could be accommodated by the weight allowance would b e rundown before the end of the mission and could not be recharged. For these reasons,it was decided to use four rugged nuclear generators, each generating 30 watts of power, to provide the power needed under the difficult conditions of the mission. Two of the generators will be mounted on t h ' e.out & r end of each of the two booms that are to be deployed Eiom'%e space - craft after launch. the minimum requirements for experiments at the. time the Pioneer encounters Jupiter and for transmitting radio signals across the hundreds of millions of miles that separate Jupiter from the earth. However, a fourth generator has been added to reduce Three generators nou ld ' guff ice to meet the possibility of a power shortage at a crucial moment. 3L

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DES CRI PT I ON The modified SNAP-19 fuel is plutonium-238. Itis made up of17discs, each about2.1inches in diameter and 0.2inches thick. They are chemically and biologically The fuel discs are encased in a refractory metal capsule 4.8inches long and 2.5 inches in diameter. The capsule weighs about7.5 pounds, of which 4.4 pounds represent fuel. The fuel capsule is contained within an hexagonal graphite reentry heat shield. The graphite will provide reentry heating protection in the event of an aborted mis - sion. The graphite assembly i s made up o f an internal support structure of zirconium dioxide end plugs and pyro ly tic graphite sleeves ; rc The graphite assembly is contained within a cylindrical, magnesium generator 11 inches high and 20 inches in diameter, including heat radiating fins. The generator weighs about A radioisotope thermbelectric generator continuously converts heat directly into electrical power without moving parts and without reliance on solar energy.Inthe SNAP-19 within the SNAP-19 fuel capsule generates heat.A thermopile assembly o f 180 thermo electric elements converts this heat - - a t least 3 7 watts o f power atthe time of launch and 30 watts when it reaches Jupiter.There are no moving parts. (Thermocouples are the first devices ever invented to pro - duce electric currents from heat without the help of moving parts.) SNAP-19 is designed to operate for about three years. The output from each of the four generators will go t o a separate inverter, which will be paralleled with the other three inverter s to form an alternating current(ac) bus,.. Most ofthe ac power will be rectified and filtered so supplya main 2 8 - volt dc bus, from which the instruments - " will receive power. Most of the other demands +b,oaid tke space - craft will be met from a central transformer rectifier that will receive power from the ac bus and deliver various dc voltages.A battery will be recharged automatic - ally when enough power is available and used to meet any temporary

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Nuclear power is considered essential to the develop - ment of along - lived,reliable, rugged, relatively small and ‘ lightweight electrical system for a variety of space app li - cations including earth orbital satellites,lunar missions and probes to distant planets. Many present spacecraft power systems depend on the use of solar cells for gener - a ting electrical power for direct use and for recharging however, lead to difficult problems in the design of increasingly larger solar cell panels and associated battery storage systems. sources would be required for any exploration missions traveling great distances from the sun. The size of the solar cell array required for such missions would betoo large to be practical.. ir A SNAP radioisotope generator has the potential of supplying many watts of electricity for several years. Since the operation of a SNAP generator does not depend on exposure to sun rays which solar cells require,onlya small storage battery system is needed for - spacecraft operation. The isotope power source is much more rugged than solar cells and is significantly less susceptible to radiation and solar heat damage. The unique properties of plutonium-238 make it an excell ent isotope for use in space nuclear generators.Atthe end of almost 88 years, plu’tonium-238 is still supplying half of its original heat.In the. decay process, plutonium-238 emits mainly the nuclei of helium(alpha radiation),avery mild type of radiation which requires very little shielding. The generator which houses the isotopic fuel essentially shields this radiation and thus avoids a weight penalty for radiation protection of the spacecraft hardware. Before the use of the SNAP-19 system was authorized,a thorough review was conducted to assure the health and safety of personnel involved in the launch and of the general public. For this review,extensive safety analyses and teSts were conducted and demonstrated the safety of the -qystem for all credible accident conditions.. -?

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DEVELOPMkNT OF SNAP RADIOISOTOPE PROGRAM The first significant step in the SNAP isotopic power introduced to the world by President Eisenhower i n 1959. This proof - o f - principle device was The first isotopic space generator was put into ser- \. Navy navigational satellite to supplement the satellite ' s The AEC subsequently developed an advanced type of which weighed about 25 pounds and.generated 20 watts of power. SNAP-9A generators provided all the power for two Navy navigational satellites launched in 1963. Still further advances i n SNAP systems were made with the development and flight of SNAP-19 aboard Nimbus I11 launched in April 1969. This use of SNAP-19 represented a major milestone in the development of dong - lived,highly reliable isotope power systems for space use by NASA. The combination of nuclear power and solar power on Nimbus I11 resulted in the acquisition of data which has been chara c - . ts rize d as "unprecedented" by the scientific community. packages left on the moon by the Apollo astronauts. The first was deployed in November-1969 during the Apollo 1 2 u is sion ; the second was deployed in February 1971 and the third in July 1971. The operation of these generators for extended periods makes possible the acquisition of new dimensions of lunar data. The SNAP-27 generator was developed to power instrument The SNAP radioisotope program has brought forth new technology which has resulted in the use of radioisotopes as sources of compact, reliable,long - lived power on land, on sea and in space. SNAP generators have provided power beacons,lighthouses,navigational and weather satellites. They are being considered as power sources for deepcsea'-and ocean bottom projects and other manned and unmanned space "SNAP radio isotopic devices are designated by odd numbers ; reactors by even.

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Isotopes,Inc., Timonium, Maryland, a subsidiary of Savannah River Laboratory, Aiken, South Carolina, operated for the AEC by the Du Pont Company, prepared the raw plutonium fuel. Mound Laboratory, Miawisburg, Ohio, operated for the AEC by the Monsanto Research Corporation,prepared the final fuel form and encapsulated the fuel. AEC's Space Nuclear Systems Division, Gerutantawn, Maryland, provided technic - a1dirT c tion for the SNAP-19/ Pioneer development program.