The use of liquefied gases in small satellite propulsion systems

Abstract

There is currently a trend to produce spacecraft as small as practical, to get the related cost benefits. More and more of these small satellites (launch mass less than 200 kg) are requiring propulsion for major orbit changing, constellation forming and drag compensation. Traditionally, the propulsion choice would have been cold gas nitrogen, however small spacecraft tend to be more volume constrained than mass constrained. Density Isp (impulse per unit volume of propellant) is as important as Isp. Liquefied gases offer the advantage of storage at higher densities than compressed gases, hence higher density Isp. They also offer lower storage pressures than compressed gases, which give better safety factors. For applications where compressed nitrogen cannot meet specified requirements, the use of liquefied gases can prevent the need for more complex and costly hydrazine systems. Typical liquefied gas propellants considered are ammonia, butane, propane, nitrous oxide, carbon dioxide and water. This paper details some of the research work carried out at the Surrey Space Centre on liquefied gas propellants. It also details some of the specific precautions to be taken when using these propellants in a flight system. Some of the uses of liquefied gases in the UK space programme will be reviewed. Feedback will be given on the current in-orbit liquefied gas propulsion systems produced by Surrey Satellite Technology Ltd in conjunction with Polyflex Aerospace Ltd. Currently flying are a nitrous oxide system on UoSAT-12 and a butane system on SNAP-1. Future liquefied gas propulsion on SSTL spacecraft will also be discussed. Copyright © 2001 by Surrey Satellite Technology Ltd. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. INTRODUCTION Surrey Satellite Technology Ltd has launched 19 small spacecraft into low earth orbit (LEO). Only 2 of these 19 have onboard propulsion systems. However there is a changing trend and now most of SSTL's current and proposed spacecraft will contain propulsion systems. Propulsion is required on these platforms for: • Constellation formation and maintenance • Drag compensation to keep the spacecraft altitude constant • Attitude control • De-orbiting at the end of life • Formation flying SSTL and Polyflex Aerospace have developed a number of unique propulsion systems using liquefied gases rather than traditional cold gas nitrogen. These small spacecraft tend to be more volume limited than mass limited. For example Ariane V ASAP has very strict volume requirements for a secondary payload of 600 mm x 600 mm x 800 mm. The associated mass requirement of 120 kg maximum may seem small, however for a small spacecraft of such a volume, it is usually more than adequate. If a cold gas nitrogen propulsion system cannot supply sufficient delta V, the traditional alternative is a small hydrazine system or other alternative energetic monopropellant. However these types of systems typically have major safety issues associated with them and they can add significant costs to any propulsion system. Hydrazine itself is very toxic and a complete safety infrastructure must be in place before it can be used. These types of systems do not fit into the ethos of small, low cost spacecraft and are not considered as an option in this paper. 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.