Reply to post: Re: Supre Cruise

Take-off crash 'n' burn didn't kill the Concorde, it was just too bloody expensive to maintain

Tim 49

Re: Supre Cruise

Bit of background, & a quick ramble. Jets work by ingesting air, compressing it, lighting it & shoving the expanding gas out the back, in a continuous process (unlike pistons, which are discrete such, squeeze, bang & blow of course). You can either accelerate lots of air a bit (modern high-bypass turbofans), or less air a lot (very noisy, uneconomical turbojets). However as you go faster, the efficiency of fans drops, & that of the pure jets increases, in simple terms because you need less 'compression' the faster you go. Also as you go faster, you need faster air coming out the back, & jets push the air out faster than fans (huge simplification here, please accept that!).

So for supersonic, you either need low-bypass fans, or pure jets. You also need loads of thrust. One way to get this big thrust is to use reheat, where you squirt neat fuel into the exhaust, downstream of the turbine, & light it, adjusting the nozzles at the back of the engine to compensate for the pressure changes at the back when you start this additional generation. You also want to keep some lower-speed & higher-pressure air coming out the back as well, in a 'cylinder' around the higher-speed supersonic air, in order to (a) make it quieter, & more importantly (b) to stop the cone of supersonic air expanding (diverging); you want the thrust to go straight back & not be wasted as the air expands in all directions.

Using reheat is horribly thirsty, but it means that you can have a lighter, "less powerful" engine for the majority of time when you don't need it. Generally, engines big enough to sustain supersonic flight would be massively overpowered for other times (and remember twin-engine airlines have loads of excess power too, since they must be able to get off the ground if one fails).

However, with careful magic in the intake & exhaust (and clever wings & fuselage to reduce drag), you can do away with reheat, & 'supercruise' on 'dry' power. There's a lot of extra drag when transitioning through M1.0, but it reduced again by M1.7 in Concorde's case, & she could then supercruise, on full dry power, with no reheat. Reheat all the time would use about 4x more fuel.

Concorde was unique here, with about 8% of the thrust coming from the engine itself, 63% from the intake, & the rest from the convergent-divergent nozzle system at the back. So supercruise is just very clever optimisation of the intake/engine/exhaust system to optimise power & efficiency at supersonic parts of flight.

The really clever thing is the intake on Concorde, where you're slowing the air down by 1500MPH in a length of 15 feet, without expelling a shock wave out of the front of the intake, since to do so would multiply drag, & bad things would happen (this is termed an Unstart). Managing this shock front an the multiple shock waves dynamically is an amazing thing.

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