Thrust-Augmented Nozzles for Rocket Engines

The afterburner concept is adapted from turbojet to rocket engines.

February 1, 2007

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

The thrust-augmented nozzle (TAN) has been invented as a means of obtaining high performance from a rocket engine both during liftoff at sea-level atmospheric pressure and later during flight under near-vacuum conditions. In effect, the TAN rocket engine amounts to a booster rocket engine contained entirely within a sustainer rocket engine, and very little weight is associated with the incorporation of the TAN portion. Heretofore, it has been difficult or impossible to design the same rocket engine to perform well at both extremes of ambient pressure.

In a TAN Rocket Engine, at sea-level atmospheric pressure, thrust is augmented by injection and burning of fuel downstream of the nozzle throat.

A launch rocket engine must operate under both atmospheric and near-vacuum conditions. It is required to generate high thrust at atmospheric pressure at liftoff, when the rocket is heaviest. Most of the ascent to orbit is flown under near-vacuum conditions. To minimize the amounts of propellants, the engine must be designed to obtain the highest practicable mission- average specific impulse (Isp). To obtain a high Isp, it is necessary to use hydrogen as the fuel and to design the nozzle to have a large ratio between the cross-sectional area of the nozzle exit and the cross-sectional area of the nozzle throat ("nozzle area ratio" for short).

These requirements are in conflict in that at sea-level pressure, the large-area-ratio nozzle is less efficient in producing thrust. The reduction in efficiency occurs because the radially outermost part of the exhaust gas expands to a pressure below ambient pressure, thereby causing a portion of the nozzle to generate negative thrust. At an extreme area ratio, the exhaust jet becomes separated from the nozzle, causing destructive forces.

The concept of the TAN is related to the concept of the turbojet- engine afterburner: In the TAN, during operation at atmospheric pressure, thrust is augmented through injection and burning of fuel and oxidizer within the nozzle, downstream of the nozzle throat (see figure). Moreover, the thrustaugmenting propellants can be different from those burned in the primary combustion in the core of the engine, making it possible to obtain the benefits of dual-fuel operation. In a turbojet- engine afterburner, the amount of thrust augmentation is limited by the amount of unburned oxygen available in the exhaust from the main combustor. In the TAN, the thrust augmentation is not so limited because both the fuel and oxidizer are injected in the required amounts. The increase in thrust in TAN operation is nearly proportional to the nozzle area ratio. In a high-area-ratio nozzle, it is imperative to use high augmentation to fill the nozzle with exhaust flow without separation.

In TAN operation, the increase in thrust can be attributed to two effects: One effect is the secondary flow of gases generated by combustion of the TAN propellants. The other effect is the displacement of the flow of primary combustion gases (core flow) by this secondary flow. The TAN exhaust gases flowing in the nozzle reduce the exit flow area available to the core flow, thereby reducing overexpansion and thereby further reducing the core-thrust loss associated with overexpansion.

This work was done by Melvin J. Bulman of GenCorp Aerojet for the Air Force Research Laboratory. For further information, download the free white paper at www.defensetechbriefs.com under the Mechanics/Machinery category. AFRL-0009