Spacecraft Propulsion

As rocket engines open their nozzles and unleash thrust, Newton’s third law comes into effect: for every action taken there will be an equal and opposite reaction.

Sandra Bullock’s fire-extinguisher from Gravity worked due to being pushed against an object; but is that possible in space where there’s nothing for it to push against?

Chemical

Chemical propulsion refers to using chemicals to generate thrust in space. It’s the predominant form of propulsion used by CubeSats and smallsats; however electric propulsion systems are becoming more popular over time.

Chemical rocket propulsion works by burning a mixture of fuel and oxidizer in a reaction chamber, where they react and heat the product and expand, producing thrust for the rocket. Since exhaust from chemical engines carry away momentum changes through exhaust streams, designing them with minimal nozzle divergence at control volume exit is key for maximum thrust output.

Chemical propulsion systems measure their specific impulse by the total mass of propellant released, measured as seconds per weight flow rate of propellant (Isp). Electrical thrusters usually boast higher Isp’s than their chemical counterparts.

Electric

Electric propulsion differs significantly from chemical propulsion systems in that its thrust can be increased up to twenty times faster due to electrically powered acceleration of propellant. As such, spacecraft are capable of reaching higher velocities while using less mass of propulsion system for propulsion.

Solar panels aboard satellites supply the electric energy necessary for operating the Electric Propulsion System (EPS), which then utilizes this energy to ionize, or positively charge, inert gas propellants such as Xenon and Krypton (yes, that is how Superman powers his rocket). Accelerated by various combinations of electric/magnetic fields, these propellants then act like thrusters capable of performing orbital transfer maneuvers, station keeping operations, COLA maneuvers and long duration cruises.

Thrusters of various types are available for these applications, from Fakel and Busek’s HET thruster with very low thrust to the larger 12kW AEPS ion thruster found aboard OneWeb satellites. Each system can be tailored specifically to meet mission and operational requirements – for instance acceleration grids, pointing mechanisms and Quantic Evans “space-grade” capacitors which meet requirements such as high power density, ultra-low ESR resistance, and resilience against harsh space conditions are included as components in any configuration.

Hydrazine

Hydrazine liquid monopropellant has long been used in spacecraft thrusters. Unfortunately, until recently it was considered dangerous: carcinogenic and mutagenous substances must be handled and refurbished with extreme caution.

However, its low exhaust velocity makes hydrazine an ideal choice for small satellite attitude control systems and propulsion systems. Voyager probes use these thrusters for three-dimensional stability as well as orbital adjustments.

A 100-lbf satellite typically requires two positive pressure propellant tanks (fuel and oxidizer) with pressurant tanks, fuel pumps, filters, catalyst beds, latch valves, and nozzles to power its propulsion system. A quad redundant regulator equipped with burst disk and relief valve regulates feed pressure in order to guarantee 200 psia flows to thrusters even if one regulator should fail; thermal analysis and vacuum tests are then conducted in order to assess heater sizing requirements for its engine.

Liquid Oxygen

Spacecraft engines must generate thrust to alter its velocity, or what are known as V maneuvers, with precision in order to follow specific orbital and flight paths.

Chemical rocket propellant is typically composed of both fuel and oxidizer; typically liquid hydrocarbon fuels and either hydrogen or oxygen as the latter are commonly found as the latter is inert and cannot react with other elements in its immediate environment.

Liquid oxygen was produced through fractional distillation in a cryogenic air separation plant for use in the F-1 engine that powered the first stage of Saturn V rocket that launched Apollo lunar missions.

Liquid oxygen has an extremely low boiling temperature of -297degF and must be stored in specially insulated tanks prior to its use in rocket engines. Skin exposure to this cold material could result in frostbite or cryogenic burns; experimental use of other low temperature oxidizers like fluorine or ozone have also been attempted, but their high cost and toxicity make them impractical for operational use.