RE-HEAT (AFTER BURNING)
Re-heat is a system fitted to a gas turbine engine as a means of increasing the total thrust. As much as twice the thrust can be obtained using reheat. Unfortunately it is extravagant with fuel so is suitable for brief periods of use only; nevertheless, re-heat allows flexibility in handling. The only civil aircraft to have reheat is Concorde.
The principle of re-heat is similar to that of the gas turbine engine itself - i.e. thrust is obtained as a reaction from accelerating a mass of air through the engine. Re-heat obtains extra thrust from accelerating the exhaust gases in the jet pipe behind the turbine.
The exhaust gases contain oxygen provided by the un-burnt cooling air. By adding fuel and burning it, the exhaust gases can be ‘re-heated' to cause an increase in velocity with a substantial gain in thrust.
A ring of fuel burners is mounted in the jet pipe and fed with fuel from the aircraft tanks, so that the exhaust acts like a ram jet.
REVISION OF THRUST
As the air flows through the engine it undergoes many changes in speed, direction and pressure. However, as we learnt in Chapter 1 of this book, the useful thrust depends upon the mass of air passing through the engine and upon the change in velocity between the air at the intake and that at the exit of the propelling nozzle. For a constant mass airflow, anything that increases the difference between the final velocity and the initial velocity will give an increase in thrust. Re-heat does just this; by burning fuel in the exhaust system behind the turbine we are creating a ram jet which increases the final velocity of the airflow; this in turn, increases the effective thrust from the engine.
RE-HEAT AND BY-PASS ENGINES
When re-heat is fitted to a by-pass engine, much greater thrust increase can be obtained. This is because the gas temperature before re-heat is much lower and hence the temperature ratio is much higher. Gains in the region of 70% increase in static thrust are readily obtained, with greater gains in thrust at high forward speeds. The limiting factor is the temperature that the jet pipe can withstand.
THE ADVANTAGE OF RE-HEAT
Re-heat provides the best means of substantially increasing the thrust of an engine for short periods. The advantages are those of improved take-off, rate of climb and air speed. Re-heat can be selected or cancelled at will by moving the throttle lever into or out of the re-heat position.
THE DISADVANTAGES OF RE-HEAT
Because of the additional fittings, the diameter of the re-heat jet pipe is greater than that of a standard jet pipe for the same engine. Therefore, drag may be increased because the overall frontal area of the engine is increased. There is also a small weight penalty and the maximum continuous thrust is slightly reduced by the drag of the re-heat fittings inside the pipe. Re-heat is grossly extravagant with fuel.
The design of the jet pipe and nozzle area has a considerable influence upon the overall useful thrust produced by a gas turbine engine. Generally the jet pipe and the propelling nozzle match the gas flow characteristics of the engine so that the final pressure and velocity of the gas produces the greatest amount of useful thrust. Thus the area of the propelling nozzle is as important it must be designed to match the airflow characteristics of the engine if it is to obtain the desired balance between pressure, temperature and thrust.
A fixed area propelling nozzle, as fitted to non re-heat engines, is a compromise designed to provide an acceptable amount of thrust without being ideal for all engine speeds. The size of a fixed nozzle is chosen to provide its greatest efficiency at high cruising and maximum power but, a variable area nozzle would be more efficient.