Flying Satellites

hydrazine monopropellant system

In spacecraft applications, hydrazine monoprops are the most common fuel system. The hydrazine molecule spontaneously decomposes into hydrogen and nitrogen, producing a high-pressure, superheated steam. This hot gas is then sucked into a spin turbine and used to power a spacecraft. Hydrazine also powers many attitude control engines on flying satellites.

A conventional hydrazine monoprop is composed of a large propellant tank, a propellant valve, and an injector. Each tank contains about 450 kg of hydrazine. Each tank has one gas valve and a pressure regulator that maintains 200 psia feed pressure to the propellant tanks. These devices are designed for long service life. However, the performance of these systems is somewhat limited.

Monopropellant systems, also called single-molecule propulsion systems (SMPS), can be simple and compact. They can work with a small range of substances and are capable of starting and restarting. Since they have low energy requirements, they are ideal for orbit maintenance. However, they have relatively low performance and require an additional substance. Hydrogen peroxide is an alternative bipropellant.

Bi-propellant thrusters have a much higher impulse than monoprops. However, they do not lend themselves to downscaling, and performance is significantly reduced due to combustion chamber mixing. Because of the difficulty of thoroughly mixing the propellants, smaller-scale bi-propellant thrusters are usually used for applications that require high impulse primary thrust. For example, a jet engine RL10 expander power cycle has a thrust of 16,500 lbf.

Hydrogen peroxide has excellent thermal stability, so it is a good choice as a bipropellant. It has a wide variety of applications, including gas-generator and biowaste gas systems. As with hydrazine, hydrogen peroxide can be reactivated by adding oxygen. If the reactivation is too long, the heat generated is insufficient to produce a propellant flow. On the other hand, if the reactivation is too short, the thrust will be reduced.

One way to overcome this problem is to use a composite solid propellant. The chemical product can be tailored to suit specific enthalpy or molecular weight needs. Also, the catalyst bed can be made with sub-millimeter characteristic dimensions, which increases the proportional wall area. With a larger wall, viscous losses are greater.

The PRECISE Team has had considerable experience with hydrazine multiphase decomposition. A new model of reaction kinetics is needed to describe the catalysis. Additionally, a new fluid model is required to capture the characteristics of a variable ratio of specific heats. And, full 3D fluid model computations are necessary to capture the boundary layer dominance of the combustion process.

While the PRECISE team has a lot of practical experience with hydrazine decomposition, this problem is still not solved. However, the project has developed a two-step mechanism for hydrazine decomposition that is integrated with the WP4.2.

The PRECISE team has also improved nozzle design. This includes developing micro coriolis mass flow sensors and a Prandtl-tube sensor with integrated pressure sensors. Another development was the implementation of a 4.5 x 2.5 mm sensor chip, which incorporates 400 mbar full-scale capacitive pressure sensors.