The proposed rocket engine includes a combustion chamber actively cooled by liquid oxygen: Heat from the combustion chamber vaporizes the flowing liquid oxygen, and the absorption of latent heat of vaporization contributes to cooling of the combustion-chamber wall. The resulting high-vapor-quality (mostly vapor) two-phase flow of oxygen is then injected into the combustion chamber to burn with the fuel. Vaporization of the majority of the oxygen prior to injection renders the engine insensitive to wide variations of inlet conditions.

The need to reduce the sensitivity of the engine to inlet conditions arises because of a generic drawback of cryogenic rocket-engine propellants: In general, liquid oxygen and hydrocarbon fuels can be stored as propellants for rocket engines. Because liquid oxygen and other cryogenic propellants are very volatile, thermal-control devices are necessary for storing them. During operation of a cryogenic-fluid feed system of a rocket engine, there may be some nonuniformity in the level of subcooling, and/or vapor bubbles may appear in some locations. The resulting fluctuations in the quality of the liquid oxygen [that is, in the liquid fraction of the two-phase fluid] delivered to the engine can lead to malfunctions.

In designing and operating the proposed engine, no attempt would be made to inject liquid oxygen into the combustion chamber. On the contrary, most or all of the oxygen is vaporized prior to injection, in recognition of the fact that for oxygen and other highly volatile propellants, the magnitudes of flow-rate fluctuations are much smaller in the high-vapor-quality (mainly gas) regime than they are in the low-vapor-quality (mainly liquid) regime. As a result, the pattern of injection of oxygen into the combustion chamber, which pattern can be expected to exert a major effect on engine performance, is rendered much less sensitive to inlet quality variations than it would be if one were to attempt to inject liquid oxygen.

The proposed engine, denoted a heat-exchanger rocket engine, includes flow channels in its combustion-chamber wall. Liquid oxygen enters the engine through these channels. The rate of transfer of heat from the combustion process to the liquid oxygen flowing in the channels is adjusted so that most or all of the liquid oxygen is vaporized and the combustion-chamber wall temperature remains low. Mixing devices in the heat-exchanger flow passages enhance heat transfer in two-phase flow. The high-vapor-quality flow of liquid oxygen from the channels is collected in an oxygen-injector manifold and distributed in the combustion chamber through annular gaps at the head end of the combustion chamber.

Fuel is injected radially into the combustion chamber from a central injector extension. The fuel jets are broken up and atomized by the cross flow of gas. The injector configuration is selected to obtain a fuel-rich wall zone and adequate atomization; these features control engine performance and the rate of transfer of heat to the walls of the combustion chamber and nozzle. The fuel injector is adjusted to control the amount of fuel landing on the chamber wall and, therefore, control the gas temperature driving the heat exchange.

In an alternative scheme, the gas flowing from the heat exchanger would be collected in tanks or other accumulators, then fed from the accumulators to the engine. In this scheme, the operation of the engine as a whole would be somewhat decoupled from the operation of its heat-exchanger portion. This scheme may present some advantages at an overall system level.

This work was done by Jacky Calvignac of TRW Space & Technology Division for Johnson Space Center. For more information, contact the Johnson Commercial Technology Office at (281) 483-3809.


NASA Tech Briefs Magazine

This article first appeared in the December, 2001 issue of NASA Tech Briefs Magazine.

Read more articles from the archives here.