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Why didn't Concorde use a scramjet engine for supersonic flight, but it is used in the Airbus and Skreemr hypersonic flight concepts to achieve supersonic speeds?

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    $\begingroup$ The scramjet has a little problem: How do you start it, and how do you fly before Mach 4 is achieved? In the video the SJY61 ramjet test engine and its support system are first carried by a B-52 to Mach 1 and 50,000 ft, released and immediately accelerated by a solid rocket engine up to Mach 4.5, at this point the booster is jettisoned, and the SJY61 scramjet is actually fired :-) $\endgroup$ – mins Nov 13 '15 at 13:06
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    $\begingroup$ Were there even working scramjets when the Concorde was designed? Wikipedia thinks the US didn't test one successfully until 2004. The Russians apparently had one in the '90s, but either is decades too late. $\endgroup$ – jamesqf Sep 28 '17 at 4:13
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    $\begingroup$ TIL: B-52s can hit Mach 1! Thanks, @mins! :) $\endgroup$ – FreeMan Sep 28 '17 at 17:55
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    $\begingroup$ @FreeMan: At least M 0.91. $\endgroup$ – mins Sep 28 '17 at 19:04
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A supersonic ram jet (scramjet) requires a fuel with a very high flame speed, so the combustion doesn't take place after the fuel-air mixture has left the engine. Aviation fuel would be completely unsuitable, and only hydrogen gives acceptable results. The low density of gaseous hydrogen, the storage problems associated with cryogenic hydrogen and the creation of a hydrogen infrastructure would had created tremendous problems for the Concorde development.

A scramjet is like wing sweep: It is best avoided if possible, but necessary if you want to go really fast. For the scramjet the speed above which it makes sense is Mach 4 or 5, and Concorde was designed for Mach 2.

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  • $\begingroup$ While not a scramjet, the J58 of the SR71 operated in similar conditions - very high temperatures - which necessitated the use of the very pricey JP7 fuel, and the need for an injection of TEB to ignite the fuel - the characteristic green glow seen when an SR71 fires afterburners. Ben Rich describes a project to power the earlier A-12 Oxcart with hydrogen to increase range, but it was abandoned as the minimal range increase didn't offset the difficulties in using hydrogen as an aircraft fuel. $\endgroup$ – tj1000 Sep 27 '17 at 18:59
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The Skreemr at the present time is only an advertisement idea, so comparisons are not really meaningful.

Nevertheless, the difference with the Concorde is the expected range of speeds: at Mach ~2 a scramjet can hardly work (the air would be too slow at the combustion location).

All jet engines work on the same principles: compress (and slow down) the mass of incoming air in the compressor stage, heat it up in the combustion stage, let it expand (and accelerate) in the turbine/nozzle side. If the compressor stage brings the air down below Mach 1, it is not a SCramjet anymore.

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