## T

and eventually the minimum acceleration a0 must satisfy the conditions:

T V2

For instance, a 1000-kg payload accelerated by a CW CO2 laser beamed by a D = 5-m mirror, received by a 1-m focusing mirror and assuming a Strehl coefficient of 0.5 needs an acceleration of about 21 m/s2 (T = 21,000 N) to accelerate to lunar escape speed within 140 km of the laser range. The power required, with a coupling coefficient of 1000 N/MW turns out to be P = 21 MW, a rather striking figure at this time, but maybe feasible in a few years from now. In any event, equation (6.8) points to the fact that ''shipping'' payload from the Moon requires significant installed power. As in the case of MagLev systems, powering the directed energy beam can be a combination of stored solar energy and nuclear power plant electrical energy. With less acceleration and the requirement to illuminate the accelerating spacecraft for longer time periods, stored solar energy alone is probably insufficient. Takeoffs and landings are vertical with minimum surface footprint. The basic concept has been demonstrated [Myrabo et al., 1998; Myrabo, 2001]. In terms of potential for deep space acceleration and launching from the lunar surface, this concept has the most potential and the least acceleration load on the spacecraft.

As with all of these schemes, a significant amount of material must be either fabricated on the Moon or lifted from the surface of the Earth and that requires an even greater mass of propellant to reach LEO and the Moon. So the tradeoff question is, does the propellant saved in lunar launches and the propellant required to deliver that propellant to the Moon (or in situ facilities to manufacture the propellant) justify the cost of the facility? With current chemical propellants the answer is no, as we saw how large is the ratio between propellant mass required to deliver a payload to its destination and the payload mass. However, as higher-thrust solar electric and nuclear electric propulsion systems become operational, the cost of propellant will fall dramatically and non-rocket launch facilities on the Moon will in all probability become practical.

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