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Space/Science » in reply to A scramjet won't get you into orbit (they flame out at Mach 10).

Scramjet 101
Gary — (archived)

To get thrust out of a scramjet, you need to get energy out of hydrogen combustion. It takes about a millisecond for hydrogen and oxygen molecules to dissociate into their atomic species and then recombine into water vapor. It takes even longer for hydrogen fuel to properly mix with air for full composition. If the hydrogen and air have blown out of the scramjet engine before combusting to water vapor then the energy released will not provide thrust. A typical velocity for Mach 10 is about 4 km/sec.

4000 m/sec * 0.001 sec = 4 meters

If your scramjet combuster is 4 meters long or less then it stops producing thrust when the free stream velocity is greater than 4 km/sec or Mach 10. So you retort: Okay make the vehicle very long and slender. The response is: A major source of drag at hypersonic velocities is due to skin fricition. You want the vehicle to be short and slender to minimize skin friction and wake drag. The combuster forces you to make the vehicle long. Also, hydrogen fuel has very low density thus requiring lots of volume. This means instead of short and slender, your vehicle is long and fat. Scramjet alone will not get you to orbit.

Gary

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