A few weeks ago:
Branson, the moneybags behind Virgin Galactic, wants to build hypersonic intercontinental passenger transports once he’s got VG up and running (any day now…).
The idea of hypersonic passenger transport stretches back to the 1940’s, with rocket powered vehicles that would have leapt above the sensible atmosphere, flown a ballistic trajectory, and come screaming back in. Other designs over the years have been “cruisers” that use airbreathing engines to fly at Mach 5+ in the thin upper atmosphere. These certainly sounds even harder than supersonic transport, which has spent nearly fifty years being a financial bust (the US SST never got off the ground, so all the money spent on that effort can be considered a complete negative; the Concorde never made a dime, and the TU-144 was a complete CF of a program).
But a sub-orbital HST might turn out to be eminently practical, perhaps even more so than an SST. And for one simple reason: if it is sub-orbital, that means it’s above the atmosphere… and thus *silent.* No sonic boom. This means that it could overfly populated areas without legal problem. (Well, until the bureaucrats, greenies, Luddites and others try to shut it down in the courts…)
Obviously, takeoff of a large rocket powered vehicle would be impressively noisy. I would guess that the HST would be configured for horizontal takeoff and ladings using conventional jet engines; those would get it to an altitude where it would fire up the rockets and blast up and out. Much of the acceleration would occur above the atmosphere, but for those in the departing “home town,” the noise of rocket engine ignition would probably be impressive. And there would be a pretty impressive sonic boom along the re-entry track, a distance of several hundred miles. So a New-York to Tokyo flight would light up the east coast with launch noise, but the re-entry boom would be expended over the largely empty Pacific. But the return flight would have the re-entry boom over the North East US. That might be a bit troublesome. The best way to deal with that might be to re-enter *hard.* Get it done quick so that the re-entry track is as short as possible. Kinda rough on the passengers, but it’d also shorten the flight substantially. Then all they’d have to worry about is the five-hour wait in line before the flight to have their undies pawed and their junk juggled, and then a two-hour wait after their flight for their luggage.
