Invention Allows Shorter Rocket Engine Bursts

Aerospace senior scientist Brian Brady has received a U.S. patent on a device that allows rocket engines to provide smaller pulses of thrust and to use more kinds of fuels.

The patent, No. 7,958,719 B2, is titled “Noncircular Transient Fluid Fuel Injector Control Channels.”

According to Brady, rocket engines are designed to operate within a range of fuel-to-oxidizer ratios. In what are called pulsed bipropellant engines, the difference in density and mass between the fuel and oxidizer means there is a difference in the rate of acceleration of the two fluids in the injector channels.

For short pulses of thrust, this results in a deviation from the targeted fuel-to-oxidizer ratio, and less-than-optimal performance of the rocket engine. In extreme cases, partially reacted propellants accumulate in the engine from pulse to pulse, and subsequent explosions destroy the engine.

These problems result in restrictions on the minimum pulse width — the smallest amount of time that an engine can be firing — for a bipropellant engine. One of the performance parameters of a rocket engine is the minimum impulse bit, the smallest amount of thrust that can be delivered. The minimum impulse bit scales with the minimum pulse width; therefore, a limit on the minimum pulse width limits the performance of the engine.

This invention, by keeping the acceleration of fuel and oxidizer in the injector channels constant, allows the pulse width of a bipropellant engine to be varied without changing the fuel-to-oxidizer ratio.

The device would have advantages for missions where minimum impulse bit is important, such as when making small adjustments in a satellite’s position.

Currently, many spacecraft use dual-mode propulsion systems, with bipropellant engines for large thrust operations, and monopropellant engines for smaller thrust, or when minimum impulse bit is important. This limits the choice of fuel to one that works in both bipropellant and monopropellant engines, namely hydrazine. The ability to use bipropellant engines to provide small amounts of thrust could broaden the variety of useable in-space propellants.