DIRECT MOMENTUM TRANSFER The first portion of this paper extolls the virtues of pure rockets, some of which are admittedly difficult to design. One should never pass up a free ride, however, and there are some. There are a number of schemes by which momentum can be directly exchanged between two bodies and utilized to propel one of them in a desirable new direction (Fig. 7). One of these schemes, the use of planetary swingby, has been used extensively in the space program. As a vehicle makes a close approach to a planet, its velocity vector is acted upon by the planet's gravity field and the vehicle accelerates as it moves toward the planet, changes velocity laterally as it swings around the planet, and is eventually decelerated to its original approach velocity at a remote distance from the planet. The net result of this is no change in the magnitude of the vehicle's velocity with respect to the planet, but it is now oriented in a different direction. The whole operation appears as if the vehicle had a perfectly elastic collision with a wall at some position on the other side of the planet from the vehicle's initial position. The momentum change of the vehicle must show up as a change in the momentum of the planet. The momentum exchange mechanism is the gravity field of the planet, and since gravity is a conservative field, this is one of the few completely loss-free operations available. Another method of direct energy transfer is the lightsail. In this case, a stream of photons is reflected from a lightweight mirrored surface. Thrust is obtained from the momentum of the rebounding photons. Lightsails can be made to work either as solar sails using the direct energy of the radiation from the sun or they, too, can be driven by lasers. Lightsails have a tremendous theoretical attractiveness since it is not necessary to carry a working fluid onboard the accelerating vehicle. In the case of direct solar sailing, the most ingenious thinking on the use of extremely thin films (almost certainly fabricated in space) results in an acceleration of sail plus payload on the order of 1% of Earth gravity (6,7). Less than three hours at such acceleration will generate 1 km/s and slightly over 9 h would be required to accelerate from low-Earth orbit to escape velocity. Such acceleration times present almost no penalty when operating within the solar system once Earth orbital velocity has been achieved. A laser-powered lightsail can achieve higher acceleration. Like so many other systems, the acceleration of a laser-driven lightsail is limited by temperature. The sail will absorb some energy which must be radiated away to space. The maximum temperature the material can stand thus limits the energy absorbed and consequently the acceleration. Reference 8 estimates the thermally limited acceleration to be 3.6% of Earth gravity for aluminum sail material. This is only 3.6 times higher than the acceleration of the solar lightsail. The use of laser-driven lightsails at one a.u. from the sun is questionable. The problem with laser-driven sails is the same as that with any photon rocket. In virtually all other rocketry, the inability to generate a high enough exhaust velocity is the problem. In the case of photons, the problem is that the exhaust velocity is entirely too high. The power that must be expended to generate this velocity is extreme. To generate 1 kg of thrust, about 1500 MW of energy must be perfectly reflected from a lightsail. This is why solar lightsails are so large, and why laser lightsails are restrained by temperature. At the optimum /S|1 of 1300 s in Fig. 6, only 125 kW is required for 1 kg of thrust at 50% internal efficiency. Orbit-to-orbit rockets, however, can make do with low accelerations comparable to lightsails as compared to the 1.25 T/W of the orbital transports. Many missions, furthermore, do not require the orbital mission velocity of 10.5 km/sec used in Fig. 6. The power
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