If single-stage pure rocket transports could achieve a specific impulse of around 1200 s, they would be very effective and economical. This requires gaseous-core nuclear rockets, which are currently politically unacceptable or laser-heated rockets using remote power stations. Required orbital power stations are very heavy, but initial operation using ground laser stations and orbital relay mirrors may be attractive for Earth orbital transport rockets. Vast space structures, skyhooks, or star bridges are of questionable utility. Their weights exceed by many orders of magnitude that of orbital power stations, and they only go to one point in space. Rocket versatility cannot be lightly discarded. Orbit-to-orbit transfer is more easily implemented. For low velocity increments, chemical rockets are quite efficient. The use of advanced deep cryogenic chemical systems such as hydrogen-fluorine or dense space-storables such as hydrazine/fluo- rine should be pursued. Since orbit-to-orbit transport can utilize much lower thrust/weight ratios, the weight of orbital power stations is greatly reduced and laser-driven remotely powered rockets may have high utility. Laser-driven lightsails require four orders of magnitude greater power than laser- driven rockets for the same thrust. Since the laser-driven rocket propellant load is modest, it is doubtful if laser-driven lightsails can compete. Direct solar-driven rockets or lightsails require large collectors of the diffuse solar energy at Earth, and cannot compete with their laser-driven counterparts. Orbital momentum manipulation devices such as tethers and carousels, like lightsails, can create orbital transportation systems using no rocket propellant. Their weights and dimensions are vastly reduced from Earth-anchored skyhooks. Even so, a high-performance chemical orbital system may be surprisingly competitive. So long as we remain terrorized by the prospect of nuclear rocketry, one good prospect for space transportation stands immobilized. A combination of laser-driven Earth-orbital hybrid air breather-rocket transports and advanced orbit-to-orbit chemical rockets possibly supplemented by laser-driven rockets, tethers and/or carousels appears appropriate under the circumstances. REFERENCES 1. M.W. Hunter II and D.W. Fellenz, The Hypersonic Transport — The Technology and the Potential. Paper presented at the AIAA 7th Annual Meeting and Technical Display, Houston. Texas, October 19-22, 1970. 2. M.W. Hunter II, Thrust Into Space. Holt, Reinhart and Winston. Inc., New York, 1966. 3. G.H. McLafferty, Approximate Specific Impulse Capabilities of Engines Using Mixtures of Air and Nuclear-Heated Hydrogen, United Aircraft Corporation, UAC Report E-l 10224-3, January 28. 1966. 4. W.S. Jones. J.B. Forsyth, and J.P. Skratt, Laser Rocket System Analysis, NASA Contractor Report CR-159521, Lockheed Missiles and Space Co., Palo Alto. Calif. September 1979. 5. W.S. Jones, L.L. Morgan, J.B. Forsyth and J.P. Skratt. Laser Power Conversion System Analysis. NASA Contractor Report CR 159523. Lockheed Missiles and Space Company. Palo Alto, Calif. September 1978. 6. K.E. Drexler. Design of a High Performance Solar Sail System. MS Thesis, Dept, of Aeronautics and Astronautics, MIT, May 1979. 7. K.E. Drexler. High Performance Solar Sails and Related Reflecting Devices, AIAA Paper 79-1418, Fourth Princeton/AIAA Conference on Space Manufacturing Facilities, Princeton. NJ, 14-17 May 1979. 8. R.L. Forward. Roundtrip Interstellar Travel Using Laser-Pushed Lightsails. Presented at the AIAA Annual Meeting. Baltimore, MD. May 24-27, 1982.
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