Nord 1500 Griffon II, which was powered by a turbojet-ramjet combination, a precursor to later turborocket designs.
The air turborocket is a form of combined-cycle jet engine. The basic layout includes a gas generator, which produces high pressure gas, that drives a turbine/compressor assembly which compresses atmospheric air into a combustion chamber. This mixture is then combusted before leaving the device through a nozzle and creating thrust.
There are many different types of air turborockets. The various types generally differ in how the gas generator section of the engine functions.
Air turborockets are often referred to as turboramjets, turboramjet rockets, turborocket expanders, and many others. As there is no consensus on which names apply to which specific concepts, various sources may use the same name for two different concepts.
The benefit of this setup is increased specific impulse over that of a rocket. For the same mass of fuel as a rocket motor, the overall output of the air turborocket is much higher. In addition, it provides thrust throughout a much wider speed range than a ramjet, yet is much cheaper and easier to control than a gas turbine engine. The air-turborocket fills a niche (in terms of cost, reliability, ruggedness, and duration of thrust) between the solid-fuel rocket motor and gas turbine engine for missile applications.
A turborocket is a type of aircraft engine combining elements of a jet engine and a rocket. It typically comprises a multi-stage fan driven by a turbine, which is driven by the hot gases exhausting from a series of small rocket-like motors mounted around the turbine inlet. The turbine exhaust gases mix with the fan discharge air, and combust with the air from the compressor before exhausting through a convergent-divergent propelling nozzle.
Once a jet engine goes high enough in an atmosphere, there is insufficient oxygen to burn the jet fuel. The idea behind a turborocket is to supplement the atmospheric oxygen with an onboard supply. This allows operation at a much higher altitude than a normal engine would allow.
The turborocket design offers a mixture of benefits with drawbacks. It is not a true rocket, so it cannot operate in space. Cooling the engine is not a problem because the burner and its hot exhaust gases are located behind the turbine blades.
The air turboramjet engine is a combined cycle engine that merges aspects of turbojet and ramjet engines. Air passes through an inlet and is then compressed by an axial compressor. That compressor is driven by a turbine, which is powered by hot, high pressure gas from a combustion chamber. These initial aspects are very similar to how a turbojet operates, however, there are several differences. The first is that the combustor in the turboramjet is often separate from the main airflow. Instead of combining air from the compressor with fuel to combust, the turboramjet combustor may use hydrogen and oxygen, carried on the aircraft, as its fuel for the combustor.
The air compressed by the compressor bypasses the combustor and turbine section of the engine, where it is mixed with the turbine exhaust. The turbine exhaust can be designed to be fuel-rich (i.e., the combustor does not burn all the fuel) which, when mixed with the compressed air, creates a hot fuel-air mixture which is ready to burn again. More fuel is injected into this air where it is again combusted. The exhaust is ejected through a propelling nozzle, generating thrust.
Air turborocket vs. standard rocket motor
In applications which stay relatively in the atmosphere and require longer durations of lower thrust over a specific speed range the air turborocket can have a weight advantage over the standard solid fuel rocket motor. In terms of volumetric requirements, the rocket motor has the advantage due to the lack of inlet ducts and other air management devices.
- Reaction Engines SABRE
- LACE separates oxygen from the air
- Air-augmented rocket normally uses some external air, but can operate without it
- Rocket engine uses no external air
- Turbojet uses the combustion products with the air to drive the turbine
- Ramjet needs no turbine for a compressor
- Heiser and Pratt, p. 457
- Heiser and Pratt, pp. 457-8.
- Kerrebrock, pp. 443-4.
- Heiser and Pratt, p. 458.
- Kerrebrock, Jack L. (1992). Aircraft Engines and Gas Turbines (2nd ed.). Cambridge, MA: The MIT Press. ISBN 978-0-262-11162-1.
- Heiser, William H.; Pratt, David T. (1994). Hypersonic Airbreathing Propulsion. AIAA Education Series. Washington D.C.: American Institute of Aeronautics and Astronautics. ISBN 1-56347-035-7.