As I understand it, turbojet thrust decreases both with altitude due to rarefied air reducing massflow, and airspeed due to a smaller speed increase between intake and exhaust relative to the airframe. However aircraft designed for sustained supercruise such as Concorde or Blackbird had intakes that used various amounts of ram-effect pressure recovery, so I was wondering how much that would offset the effects of being fast and high i.e. what was Concorde's mach 2 max thrust compared compared to sea level dry thrust, and/or the Blackbird's mach 3.2 turboramjet thrust compared to take-off wet thrust?
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$\begingroup$ Related: In what way are the Concorde's engines considered efficient? $\endgroup$– minsJan 14, 2017 at 12:42
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$\begingroup$ Thanks for the link that's a pretty interesting read, but I was asking about thrust not efficiency. $\endgroup$– TaliskerJan 17, 2017 at 21:34
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2 Answers
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According to the wiki page of the Rolls Royce/Snecma Olympus for the Concorde:
During cruise at Mach 2.02 each Olympus 593 was producing around 10,000 lbf of thrust... Eames (SAE Transactions 1991) however mentions the cruise thrust of each engine to be 6790 lbf.... The 10,000 lbf is perhaps the maximum thrust available at cruising speed (used during acceleration and climb just before transitioning to cruise).
TO thrust is for the 593-610-14-28 - Final version fitted to production Concorde, is listed as 32,000 lbf (142 kN) dry / 38,050 lbf (169 kN) reheat.
As I understand it, turbojet thrust decreases both with altitude due to rarefied air reducing massflow, and airspeed due to a smaller speed increase between intake and exhaust relative to the airframe.
Yes indeed - however, available thrust increases with airspeed, that is why ramjets work. And drag reduces with altitude, so the required thrust decreases.
The contributions to thrust of the parts of a turbojet installation for supersonic propulsion have been computed and presented in prof. Wittenberg's TUDelft D-32B lecture handout, Fig 3.9 (above). It can be seen that the contribution of the inlet rises exponentially with Mach number M, while that of the engine itself reduces above M = 1.5. Above M = 2 the aeroplane drags the engine along! Reason why the turbine was completely bypassed at the M=3 for the SR-71.
In the same lecture handout, the pic above shows the subdivision over the installation parts of thrust, from "Power for supersonic flight", Journal of the Royal Aeronautical Society, July 1964.
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For Concorde, takeoff thrust:weight was 1:0.373. At cruise, the Concorde had a lift:drag ratio of 7:1. At cruise, thrust = drag. So cruise thrust is 1/7 = 0.1429
So, cruise:ASL is 0.14(rounded down for climb fuel burn):0.373 = around 1:3.8
For the SR-71, takeoff thrust:weight is 0.44:1. Using the Kuchemann supersonic L/D formula, we get
4(Mach(=3)+3)/Mach(=3) =1:8
Now this looks too high. And running this formula for Concorde gave 10:1. So, having no better option (yet), I simply took 1:6 as the value. This would give us a cruise thrust:weight of around 1/6:1 = 0.167:1
0.167/0.44 = around 1:3.8 (again?)
The figures for the SR-71 may well be wrong.
