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Completed TRL 2 (started at 1, targeting 2)
Porous silicon has been identified as an energetic material - when exposed to liquid oxygen in a vacuum it explodes. However, if its rate of detonation were sufficiently slowed, it might be an appropriate material to be used as a propellant. Since the surface of the moon is abundant in silicon, such a propellant would have significant advantages in that it could potentially be produced in-situ on the lunar surface, avoiding the large logistical requirements inherent to launching propellants from Earth. Indeed, silicon-rich minerals would otherwise be waste products from other ISRU projects such as CaRD and GaLORE. One could envision the Lunar Gateway as a refueling station where a vehicle would receive its Moon-derived fuel for a continued transit insertion to distant destinations, thus eliminating the transit vehicle propellant mass requirement from launch. One critical question is whether the reaction rate of porous silicon can be attenuated sufficiently to provide a non-explosive, controlled burn. Thus, the purpose of this project is to passivate the surface of porous silicon with exposure to a controlled dose of gaseous oxygen, and then measure the combustion rate of the partially passivated material. This approach has been used for controlling the rate of detonation for explosive porous silicon. The experimental plan is to measure the reaction rate using a COTS bomb calorimeter with COTS pressure transducer, thus expanding experimental capabilities at KSC.
Advantages of SiLOX propellant include: 1. SiLOX would be storable under Lunar or space vacuum for long periods without active cooling because the Si would not oxidize under vacuum. This is likely why Si has not been explored as a propellant for terrestrial applications – it oxidizes in air rendering it unreactive. 2. The Si fuel would be hypergolic with liquid oxygen, i.e., they would react spontaneously without additional ignition. Note also that liquid oxygen would be produced from the very same abundant Lunar regolith from which the Si is produced. 3. Unlike solid rocket motors, a SiLOX hybrid rocket could be turned off by simply shutting off the liquid oxygen. This is useful for missions that require multiple independent rocket burns. 4. By controlling the burn rate, the same fuel could potentially be used for different applications such as high thrust for launch, versus a slower burn rate for efficiency/higher Isp. 5. Silicates are ubiquitous across the solar system; SiLOX could be produced on Mars, most asteroids, several moons of Jupiter, etc. 6. Lunar SiLOX would allow the Lunar Gateway to be used as a fueling station such that transit vehicles could be launched from Earth without propellant (saving mass), and then equipped with SiLOX rockets on-site
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