Most propulsion systems rely on the use of combustion or electrical power input to impart energy to propellants, for thrust and specific impulse benefits. The sun, however, is a natural heat source, providing over a kilowatt per square meter in earth orbit. Through the use of a concentrating mirror with concentration ratios on the order of 1,000 to 10,000:1, we can focus intense sunlight onto a refractory metal or ceramic cavity filled with propellant and obtain very high performance propulsive capability. Isp figures of 400-450 seconds are believed to be achievable. Since a small satellite can only carry a small amount of fuel, this research provides an opportunity to put a small satellite into a deep space orbit, or escape the gravitational pull of the Earth entirely.
Preliminary analysis has confirmed that a micro-scale solar thermal engine, using a storable, stable propellant such as ammonia (NH3), hydrazine (N2H4), or water, will be capable of moving microsatellites between low earth orbit and geosynchronous orbits, put into lunar orbit, or even interplanetary space. Velocity changes (delta-Vs) on the order of 1,000-3,000 m/s are realisable, at thrust levels of up to several Newtons. Low-cost mirror technologies, analogous to terrestrial telescope optics, and judicious selection of high-temperature cavity materials, will be critical to the cost-effectiveness of the design.
Research into this approach is being conducted in collaboration with The Boeing Company and the U.S. Air Force Research Laboratory, who will provide several key components and access to system test facilities. A proposal for a flight experiment, the Microscale Solar Propulsion Experiment (MSPEx), is currently in development and could fly aboard a Surrey microsatellite as early as May 2005.
Since 2001, the Surrey Space Centre (SSC) has undertaken a comprehensive investigation of microsatellite-based solar thermal propulsion, including likely mission applications, satellite and launch vehicle constraints, trajectory analyses, requirements definition, and design. The resulting microscale STP system is currently undergoing component level ground testing in preparation for flight opportunities arising after 2005.
Two solar cavity receivers (denoted Mk. I and Mk. II) have been built and electrically tested in vacuum at temperatures of up to 2,000 K. The 400-gram Mk. II cavity receiver, an insulated ceramic structure, has survived numerous cycles to 2,000 K with no sign of damage or deformation. This receiver is expected to provide roughly 500 N-s of impulse per engine firing, at a thrust level of between 1 and 5 N. Predicted specific impulse (Isp) with ammonia propellant is between 300 and 400 s. Test results have validated our extensive thermal modelling, with close agreement between simulation and actual test data. Receiver flow testing with inert gas, nitrogen, and ammonia propellants, in high-temperature vacuum, is underway. Thrust and Isp data, as well as material degradation and mass loss information, are being collected in a series of thermal cycling tests, intended to simulate a multiple-kick firing profile.
A purpose-built 56-cm diameter concentrating aluminium mirror has demonstrated high concentration ratio (> 10,000:1) and input powers of up to 150 W, at ground level test on-sun. This concentrator is now being used to characterize concentrator-receiver interactions, testing the full optical path. A number of small (14-cm) mirrors, diamond-turned from a plastic substrate, have also been produced and coated at very low cost. These demonstrate moderate concentration ratios (1,225 to 1,600:1), which could be improved with the addition of small hyperboloidal secondary lenses near the focal point, or more simply through the substitution of aluminium for plastic as the substrate material. Aluminium is susceptible to smaller form errors during machining, and thus allows for higher concentration ratios. Three 14-cm aluminium mirrors are being procured for further testing.
Testing with high numerical aperture optical fibre is underway. Recent success with low-loss high-flux solar radiation transmission over fibre lines has been reported by researchers in Israel, for surgical and nanomaterial production applications. SSC is now attempting to duplicate these results and apply them to solar thermal propulsion. Decoupling the solar receiver from the concentrating mirror's focal point permits a single receiver to accept light from multiple mirrors, which can be made smaller at much less expense. The mirror-fiber scheme is under test, with estimates of end-to-end efficiency approaching 75 per cent.
A space test hardware suite, the Microscale Solar Propulsion Experiment (MSPEx), is being proposed by the Surrey Space Centre for incorporation into a microsatellite slated for launch in the 2005/6 timeframe. Two 20-centimetre mirrors, coupled to a single small refractory ceramic receiver, supplies approximately 60 W of power for heating. Estimated thrust levels range from 20 to 100 milliNewtons. Predicted Isp could range as high as 390 s. If successful, this experiment would represent the first space-based test of solar thermal propulsion, and a stepping stone to larger operational systems.