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Aviation History
1961
1961 - 0297.PDF
fiJGHT, 10 March 1961 303 Missiles and Spaceflight . . SPACE TOPICS AT THE IAS DISCUSSIONS on space exploration again formed a sub-stantial part of the programme at the recent annual meet-ing of the Institute of the Aerospace Sciences, held inNew York. Three of the more interesting papers presented on tho occasion are here summarized. Missions to Mars. In the paper A Study of Manned Nuclear-rocket Missions to Mars, given by S. C. Himmel, J. F. Dugan, R. W. Luidens and R. J. Weber of the NASA Lewis ResearchCenter, the return manned Mars mission was examined from a general standpoint. The mission was assumed to begin with a1.35 million lb multi-stage vehicle system in orbit around the Earth. Depending on the weight of the system, the Mars vehiclewould either have been injected into orbit as a single unit or assembled in orbit from separate sections delivered by a seriesof smaller boosters. At the correct time the spacecraft, which was assumed to contain a crew of seven men, would be acceleratedby a high-thrust nuclear rocket on to the transfer (or intercept) trajectory to Mars. Upon arrival at the planet, the vehicle would be deceleratedand would enter a planetary orbit. A Mars landing vehicle would detach itself from the primary spacecraft and, carrying two men,would descend to the surface of the planet, relying upon atmospheric drag to effect the necessary deceleration. After aperiod of exploration the two men would take off by means of a chemical booster and rejoin their comrades in planetary orbit.The return flight to Earth would be accomplished in essentially the reverse manner. As might be suspected, there were some technical problems tobe resolved before the Mars mission could become a reality. The authors first discussed the question of trajectories. For out-and-return trips, there were four parameters to be considered: total trip time, "wait" time at Mars (the exploration period beforereturning home), the transfer time (from Earth orbit to martian orbit), and the departure date. Because of the eccentricity of theplanetary orbits, the results of the trajectory study were direct functions of the synodic period under investigation. The authors chose to analyse the 1970-71 period and selectedfor their initial analysis a mission time of 420 days, which included a 40-day wait period at Mars. For a launch on May 19, 1971,an outbound transfer time of 140 days was optimum and required a velocity increment of 12.29 miles/sec. Applied to the vehiclewhile in Earth orbit, this would be sufficient to propel the space- craft out to Mars. Obviously, since the realization of a certainvelocity increment meant the expenditure of a particular amount of energy, it was desirable to minimize the required increment. Regardless of the specific mission selected, the magnitude ofthe total velocity increment required for the out-and-return trip from orbit to orbit was quite high. Vehicle weight increasedexponentially with propulsive-velocity increment and it was therefore worthwhile to reduce the velocity increment (or decre-ment) which had to be provided by on-board propulsive means. The presence of atmospheres on both Earth and Mars suggestedthat aerodynamic braking be utilized for the deceleration phases of the mission. Protection against Radiation The authors next considered the radiation problem. Mars wasassumed to have a radiation belt similar to the Earth's so-called Van Allen belts. This source, combined with cosmic rays and theradiation from solar flares, would constitute a major hazard. Solar flares presented an especially severe problem. During the past20 years six "giant major" flares had been recorded, each of which emitted about 10,000 times the "usual" intensity of radia-tion. For all except the shortest trips, it was mathematically probable that the space traveller would encounter at least onegiant major flare during planetary explorations. The duration of the flares varied from a few minutes to as long as eight hours. Toprovide crew protection during these times it was necessary to include provision in the space vehicle for a heavily shieldedcompartment. The question of the exact radiation dosage which would be tolerated by crew members as an "acceptable risk"remained unanswered. Nuclear Electric Power. Two representatives of the Jet PropulsionLaboratory of California Institute of Technology, T. W. Koerner and J. Paulson, presented a treatise on Nuclear Electric Power forSpice Missions. Heretofore all spacecraft had relied in the main up »n power derived either from batteries or from solar cells toop rate their electronics equipment. But it was apparent that, wi n space vehicles growing ever larger and demanding morepo ver, some power source other than these two means would be rec aired in the near future. The use of nuclear power sources for spacecraft secondarypower appeared to be largely dependent upon the mutual com- patibility of the nuclear devices themselves with the missions andcorresponding space vehicles. In addition, the nuclear systems would be required to demonstrate sufficient performance advant-ages over other kinds of power systems to warrant the increased hazards associated with their use. The most significant power-source parameter was probablyspecific weight, or the weight per unit power, since this factor determined to a large extent the performance of the system. Onthe basis of curves presented at the meeting, it appeared that the crossover point from solar photo-voltaic (solar cell) panels to thereactor sources occurred at power levels of from 0.7 to 3kW, corresponding to weights in the range of 400-6001b. The cross-over region indicated was based upon missions ranging from Mars to Venus; for missions beyond Mars, the crossover pointoccurred at lower power levels. It appeared unlikely that nuclear reactors would come intouse until the advent of the Saturn booster system, at which time it would be possible to employ power systems weighing 5001b ormore. Missions to Mars, Venus and the Moon held possibilities for reactor systems, particularly since there would undoubtedlybe a requirement for operation during the night portion of the planetary cycle. Missions to Mercury also were candidates fornuclear power sources although here, since the Saturn payload was limited, the use of exotic power sources was marginal. Andit further appeared improbable that nuclear sources could be employed on missions to Jupiter; once again the Saturn boostercould carry only a small payload at that range. Commercial Applications Communication Satellites. Leonard Jaffe of NASA presented apaper on The Application of Satellites for Commercial Commun- ication Purposes. According to the speaker, there were basicallythree kinds of communications satellites: the synchronous-orbit (or "stationary" or 24-hour) active repeater satellite; the non-synchronous-orbit (low-altitude) active repeater satellite; and the non-synchronous passive satellite. Significant by its absencefrom this list was the delayed repeater (e.g. the Courier satellite); little interest had been shown by commercial operators in any-thing other than instantaneous communications. The speaker considered each of the three types in turn. Thesynchronous repeater system would include three satellites spaced 120° apart and placed in equatorial orbit at an altitude of 22,300miles. Having the same 24-hour period as the Earth, the three satellites could provide global coverage with the exception of thepolar regions. An advantage of the system was that the system, being stationary with respect to the Earth, could permit the useof large fixed antennas on the ground. The long propagation path, however, presented a problem.The time delay for a signal sent on a return trip to the satellite was 0.25sec. With slight delays in the ground distribution system,it would probably take 0.3sec for a signal to reach its destination. The time required for the originator to receive a response wastherefore 0.6sec and, if one wished to have a telephone conversa- tion with someone on the opposite side of the Earth, the round-trip delay would be 1.2sec. This system delay approximated to "push-to-talk" telephoneoperations and would not impose any great burden upon the users. The greatest technical problem in the synchronous repeatersatellite was reliability of the control mechanisms necessary to maintain the correct position and attitude relative to the Earth. The speaker foresaw lesser problems in the realm of electronics.Achievement of a synchronous orbit was a formidable launching problem, and it was not anticipated that a commercial communica-tions satellite would be orbited before the Centaur vehicle became operational in 1963.Non-synchronous satellites were attractive because they could easily be placed in orbit by existing means and were small andsimple. But a system of active satellites to provide continuous worldwide coverage would require many vehicles. The reducedpropagation range would permit reasonable power requirements aboard the satellite without imposing attitude or position controls.If the orbitmg altitude were to be 3,000 miles—and it was obviously desirable to stay within the inner Van Allen belt—36 satellites in polar orbit would provide 99 per cent continuous service. Eighteen satellites would allow 90 per cent availability.One disadvantage of the non-synchronous system would be the need for more complex ground systems than with the 24-hour system. DON ADAMS
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