Low-cost propellant launch to LEO from a tethered balloon - economic and thermal analysis

As we have previously reported, 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, using only a relatively small capital investment (comparable to a single large present-day launch). An attractive payload would be large quantities of high-performance chemical rocket propellant (e.g. LO2/LH2) 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, LO2/LH2, pressure-fed rockets (e.g. without turbopumps, which increase performance but are costly). These small rockets could reach orbit with modest atmospheric drag losses because they are launched from very high altitude (e.g. 22 km). They reach this altitude by being winched up a tether to a balloon that is 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 allows significantly reduced manufacturing costs, especially when combined with multiyear production contracts, giving a projected propellant cost in LEO of $400/kg or less. The configuration of the proposed propellant depot and the manner in which the propellant would be utilized has already been reported. The launch processing facility (a small, modified container ship) and cable-car that moves the rocket on the tether have also been reported. This paper provides new analysis of the economics of low-cost propellant launch coupled with dry hardware re-use, and of the thermal control of the liquid hydrogen once on-orbit. One conclusion is that this approach enables an overall reduction in the cost-per-mission by as much as a factor of five as compared to current approaches for human exploration of the moon, Mars, and near-Earth asteroids.