Concorde Engine Re-heats
Afterburners are simply a method of making use of the hot exhaust gases once they have passed through the turbines. Fuel is spayed into a ring in the exhaust pipe and burned to increase thrust when it is required, such as on take-off and during acceleration through Mach 1. These are the two most power-demanding periods of the flight. An engine powerful enough to cope with them would be too powerful for the cruise, and probably too heavier as well. Concorde’s afterburners increase thrust on take-off by about 20%, but they do make heavy demands on fuel, causing nearly a ton of fuel to be burned between the start of a heavy-weight take-off and 1000 feet on the climb.
The ORIGINAL design for the reheat was done by SNECMA, but due to them getting into all sorts of trouble with the fuel injection system and flame stabilisation, Rolls-Royce baled them out, and it became a Rolls-Royce/ SNECMA design. (The core engine was a 100% Rolls design, with no French input whatsoever. However some engine sub-assembles was manufactured by SNECMA).
The basic way the afterburner worked was by spraying the fuel FORWARDS initially at high pressure, against the jet stram about one inch, until it hit the anvil. . As the fuel strikes the anvil it is blown back by the jet stram and atomises, passing over the spray ring and the over the flame holder. The ignition operated by passing 15KV across a dual cylindrical tube, the resulting arc was ’swirlied’ into the fuel stream by blowing engine 5th stage HP compressor air into the tube (there were 7 stages in all).
The key to successful ignition was a healthy spark, a good supply of air to the ignitor and accurate scheduling of fuel flow. (This was scheduled against dry engine flow as a function of total temperature). The other important factor (as with any afterburner) was correct and rapid operation of the exhaust nozzle. Fortunately, Concorde used it’s primary nozzle for control of engine N1 anyway, so adapting this to operate as an afterburning nozzle also was a relative walk in the park, and it operated superbly.
During the light up phase of 3.5 seconds, the fuel ratio is a fixed 0.45 (ie. reheat fuel is 45% of dry fuel). After the light up phase the full scheduling commenced. As far as the FLIGHT RATING figures go (not take-off) the ratios were 0.6 at a TAT of 54 deg’s C, falling linearly to 0.3 at 107 deg’s C and above. (Remember that Concorde used afterburning really sparingly, just for take-off and then transonic acceleration; cut off at Mach 1.7 altogether.
The system is connected to the engine fuel supply, it comprises of a single spray ring and flame holder mounted on the tail cone behind the turbines. Fuel, metered electronically in proportion to the engine fuel flow, is switched from the flight deck. When the system is activated, fuel flows to the ring, is sprayed upstream into the gas flow and is lit by a timed run of a single igniter plug. The flame then stabilizes on the flame holder, some 8 inches behind the ring. Utilising residual oxygen in the gas flow, its effect is to further increase jet velocity, adding approximately 22% of thrust at take-off and 30% at climb power for transonic acceleration. Its performance is monitored on the flight deck, first by noting rise in fuel flow at initiation, followed by indication of primary nozzle areas running fully open on light-up.
Reheat is used on every take-off, its light-up sequence taking place on the roll as the engines accelerates up to full power – it needs the mass flow associated with an N1 of 81% or more to function. It is switched off at 500ft on a standard flight or at noise abatement cut-back where needed. For the transonic acceleration it is switched on at climb power at M0.95, then off again at M1.7 – a run of between 10 and 15 minutes dependent upon aircraft weight and outside air temperature
Olympus 593 Reheat System
1: Mounting rods,
2: Flame holder,
4: Spray ring,
5: Reheat flame detector,
6: Jet pipe thermocouple,
7: Fuel connection,
8: Reheat igniter
Stephen de Sausmarez & Heritage Concorde
In the end, there were 67 Rolls-Royce/Snecma Olympus 593 Mk.610 engines, that were manufactured for the aircraft.
Plans were drawn up by the two companies for a quieter and more powerful version of the engine, this would have had an extra turbine section and a larger-diameter air compressor that would have eschewed the reheat system and added sound-deadening to the aircraft. This new engine would have had improved efficiency across the board and permitted rather greater range for Concorde and therefore opened up new routes, particularly across the Pacific as well as transcontinental across America. However, the poor sales of Concorde meant that this plan for a Concorde ‘B’ was never put into practice, which is a shame as the next airframe to be build after 216 G-BOAF, would have been the newer version fitted with the newly designed Olympus engines.
Today years after Concorde made its last flight, versions of the Rolls-Royce Olympus engine are still in use powering everything form large passenger ships such as the Queen Mary 2 and navy ships to power stations.