Science Forums 2

[CraigD]

I’m a aerospace vehicle enthusiast, so followed the X-43A pretty closely during its development and 2001-2004 test flights.

The X-43A is a small (about 3.7 m long), unmanned aircraft intended only to test its scramjet engine. Later X-43s were to be larger and possibly manned, but the program’s been canceled, and is likely to stay that way, or at best remain restricted to small test models. The X-51, a larger (about 7.9 m), unmanned scramjet. is expected to fly this year.

Though scramjets have been successfully flown only for about 5 years, scramjets have been a staple of hard SF for decades. There was even serious design consideration in the 1970s and 80s of making the Space Shuttle a scramjet/rocket hybrid.

Originally Posted by belovelife

were you aware that is is capable of achieving LEO

No way!

The X-43A is the fastest aircraft that’s ever flown. Its top speed was about 3370 m/s at an altitude of about 29000 m (Mach 9.8). Orbital transfer speed from that altitude, however, is about 7940 m/s (the orbital speed of that altitude, 7887 m/s + the delta V or a transfer orbit to 200000 m, 52 m/s). Even assuming a due east course at the equator, which adds about 466 m/s, the X-43A is only about half as fast as it would need to be to reach LEO.

Any vehicle achieving an 200,000 m altitude orbit must have a “kick” rocket or other vacuum-capable motor capable of about 50 m/s delta V to nearly circularize its orbit at max altitude, a pretty routine and easy engineering feat.

Note that, assuming it could sustain 3370 m/s in a vertical climb to the edge of the atmosphere (around 100,000 m), the X-43A would be capable of a suborbital “zoom” climb to well above LEO, to about 674,000 m. Though high, this isn’t unusual performance for a small high-altitude plane launched vehicle – the USAF’s ASM-135 anti-satellite missile, designed to be launched from F-15 fighter jets (deployed 1986, canceled 1988) could hit an altitude of about 563000 m.

I believe belovelife is confusing the X-43A with some recent designs that call for a scramjet as the lower stage of aircraft launched 2-stage to orbit vehicles, or perhaps some older designs for single stage scramjet/rocket (and in some cases, fanjet/scramjet/rocket) hybrids. The upper stage or motor mode in these designs is a rocket, as is the case with every spacecraft that’s yet been orbited.

Aerospace designers seem to have become skeptical that even a scramjet (with, as mentioned above, a small kick rocket) could be made fast enough to reach LEO. I’m skeptical too – given how difficult it is to make an aircraft that can survive brief flights of about mach 9, a mach 20 aircraft seems impossibly difficult with current technology. I’m optimistic, however, that single-stage, fully reusable space airplanes with both scramjets and rockets (possibly operating a single motor in two different modes) may be possible in the near future.