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Considering that the reason typically given for launching spacecraft from sea-level as opposed to mountains is that the limiting factor is velocity, not altitude, then why isn't the Pegasus rocket launched from a supersonic airplane?

At ~50,000 lbs, a B-1 could take two of them to Mach 2, even if the plane had to be modified as is the case with the shuttle carriers. Other planes could do, especially the easily-available F-111 or some old Russian hardware such as the Tu-22 or Tu-160. Considering that NASA now launches humans on Russian hardware this is not entirely infeasible.

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Thank you. I don't see any information even suggesting that the M-21 crash was due to to a supersonic separation. –  dotancohen Jun 26 '13 at 15:44

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The "Stargazer" L-1011 Lockhead TriStar aircraft also serves as a mobile lab in addition to being a launch vehicle. It is large enough to support future launch vehicles and also supplies a lot of in-flight testing/monitoring. All that stuff takes space which you wouldn't have on a small craft like a F-111, etc. Separation at lower speeds is also safer for the flight crew.

Since the launch vehicle is used over and over and does a bulk of the fuel burning in the over all mission profile, for practical reasons it was better to let the rocket do the extra delta-v. The aircraft only gets it to about 3% (600 mph) of orbital velocity (20,000 mph) and going supersonic (say Mach 2.5) would only get it to about 8-10% (1900 mph). However for all the time the aircraft would be in flight over the many launches, it would be burning a lot more fuel for that small benefit even if they could fit all their supporting test hardware (and no in-flight support crew) onto something smaller.

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Thank you, I did not realise everything else in the launcher. –  dotancohen Jun 26 '13 at 17:34
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Another reason for using a carrier aircraft is to get above most of the thick air. –  Mike Dunlavey Jun 26 '13 at 17:47

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