Sierra Space, a commercial space company, has partnered with Agile Space Industries, an in-space propulsion solution provider leveraging additive manufacturing, to design, develop, manufacture, and test a hydrazine-rich preburner for their VRM5500-H engine. This was done on a timeline of only 19 weeks and this is likely the first time a hydrazine-rich preburner has been developed or used in an engine, at least as far as the public record is concerned. The first prototype unit that was produced demonstrated stable operation across a 6:1 throttle range with high combustion efficiency.
Large, high-efficiency rocket engines used on large spacecraft need a power source to drive the pumps that take the low-pressure propellants in the tanks to the high pressures needed to run the engine. The AX19 preburner facilitates this by combining all the engine’s hydrazine fuel with some of the engine’s MON-3 oxidizer to decompose the hydrazine into (mostly) hydrogen, ammonia, nitrogen, and water. This hot gas is used as the power to drive the engine’s high-pressure pumps.
Achieving high-efficiency combustion in a rocket engine requires a very efficient injector. There are several different methods for achieving this, but in general, having one or both propellants injected as a gas instead of as a liquid will greatly improve mixing and create a higher-performing engine. The AX19 preburner enables this capability for hydrazine by converting it into gaseous products.
Hydrazine is a monopropellant, which means that it doesn’t need any oxidizer to combust. This presents a challenge when attempting to design a fuel-rich preburner because partial combustion of the fuel as in a kerosene, methane, or hydrogen engine isn’t possible. Monopropellant combustion is typically difficult to keep stable without running it through a large amount of specialized catalyst, which is both very expensive and difficult to acquire in large quantities.
The AX19 was designed to use as little catalyst as possible while maintaining stable combustion. First, a small fraction of the hydrazine is sent to a catalyst reactor where it is decomposed and sent to the preburner main combustion chamber as a hot mix of ammonia, nitrogen, and hydrogen. Next, the remaining liquid hydrazine, which is most of the fuel flow to the preburner, is injected as a liquid into the preburner combustion chamber. At the same time in the main combustion chamber, a small amount of oxidizer is injected into the chamber, where it is fully combusted with a portion of the liquid hydrazine and decomposed gas. All the heat added to the chamber from both the reactor exhaust gas and oxidizer combustion is used for rapid thermal decomposition of the remaining, now thoroughly atomized, liquid hydrazine.
The AX19 preburner was successfully tested as a standalone component in October of 2022 in Durango, Colorado, on Agile Space’s Animas test stand. After initial runs, it was delivered to Sierra Space for further testing and was operated successfully both standalone and coupled to their full VRM-5500H engine in Q1 and Q2 of 2023. This testing covered the entire throttle range and verified the stable operation of the preburner and the successful operation of the VRM5500-H engine.
