As we have previously reported [1-4], it may be possible to launch payloads into low-Earth orbit (LEO) at a per-kilogram cost that is one to two orders of magnitude lower than current launch systems. The capital investment required would be relatively small, comparable to a single large present-day launch. An attractive payload would be large quantities of high-performance chemical rocket propellant (e.g. Liquid Oxygen/Liquid Hydrogen (LO_(2)/LH_(2))) that would greatly facilitate, if not enable, extensive exploration of the moon, Mars, and beyond. The concept is to use small, mass-produced, two-stage, LO_(2)/LH_(2), pressure-fed rockets (without pumps or other complex mechanisms). These small rockets can reach orbit with modest atmospheric drag losses because they are launched from very high altitude (e.g., 22 km). They would reach this altitude by being winched up a tether to a balloon that would be permanently stationed there. The drag losses on a rocket are strongly related to the ratio of the rocket launch mass to the mass of the atmospheric column that is displaced as the vehicle ascends from launch to orbit. By reducing the mass of this atmospheric column to a few percent of what it would be if launched from sea level, the mass of the rocket could be proportionately reduced while maintaining drag loss at an acceptably small level. The system concept is that one or more small rockets would be launched to rendezvous on every orbit of a propellant depot in LEO. There is only one orbital plane where a depot would pass over the launch site on every orbit - the equator. Fortunately, the U.S. has two small islands virtually on the equator in the mid-Pacific (Baker and Jarvis Islands). Launching one on every orbit, approximately 5,500 rockets would be launched every year, which is a manufacturing rate that would allow significantly reduced manufacturing costs, especially when combined with multiyear production contracts, giving a projected propellant cost in LEO of dollar400/kg or less. This paper provides new analysis and discussion of a configuration for the payload modules to eliminate the need for propellant transfer on-orbit. Instead of being a "propellant depot", they constitute a "propulsion depot", where propulsion modules would be available, to be discarded after use. The key observation here is that the only way cryo-propellant can get to orbit is by already being in a tank with a rocket engine, and that careful system engineering could ensure that that same tank and engine would be useful to provide the needed rocket impulse for the final application. Long "arms" of these propulsion modules, docked side-by-side, could boost large payloads out of LEO for relatively low-cost human exploration of the solar system.
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