the designs were made with emphasis on the reaction engine used for asteroid retreival misssions. The drive circuit was changed from three to two phases but remained push-pull and the bucket now contained only two superconducting coils. At this time it was felt that very little improvement could be made and that it was time to build a working model to test this out with as many features in a real mass driver as was possible. This resulted in the Mass Driver Two effort at Princeton and M.I.T. (this report). This was the first full time effort into the design, construction, and further research on mass drivers. During this time (1978-1981) research was done on alternative mass driver designs by the groups at M.I.T. and Princeton. The M.I.T. group developed a new family of arc-commutated mass drivers that does away with SCR's and superconducting coils (in favor of ohmic coils) and which could obtain < 250,000 g's of acceleration in principle. A pulsed induction reaction engine for orbit to orbit transfer missions was developed. This "mass driver" consists of only one drive coil and an aluminum ring (< 1gm) "bucket coil" which is accelerated (> lO^6g's) and vaporized. This new mass driver can be scaled down to relatively small (few tons) payloads and should be competitive with MPD thrusters. Finally, the M.I.T. group developed another new accelerator termed the helical rail accelerator. This accelerator is equivalent to taking the rails and armature of a rail gun and twisting them into coaxial helices. At Princeton a new Pull-Only Mass Driver was developed, see Appendices L, N, and 0. This mass driver abandoned the two phase push-pull drive circuit of Mass Driver Two in favor of a multi phase (~10) drive circuit with a single superconducting bucket coil with small trailing guidance coil. If acceleration were sufficiently high, an ohmic coil
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