Low-Cost propellant launch to LEO from a tethered balloon: Recent progress

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 turbo-pumps, which increase performance but are costly). These small rockets can 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 can 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 will 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 multi- year 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. The work described in this progress report focuses on a simplified dynamic simulation of the ascent of the rocket, comparing spin-stabilization with 3-axis stabilization in terms of minimizing the amount of propellant drawn from the payload tank needed for head-end vernier thruster control of the stack during ascent. Implications for the vernier thruster configuration and control algorithm are discussed. This paper describes the derived design, including overall geometry, component configurations, refined balloon-tether architecture, and expected system performance.