Once again, scramjet proponents are touting them for space access:
Officials hope the engine eventually will provide a speedier transition between conventional aircraft in the atmosphere and rockets in outer space for deployment of satellites, and reconnaissance or strike missions.
“The long-range goal of this for the Air Force is access to space,” said Charlie Brink, an Air Force Research Laboratory propulsion directorate official who manages the X-51 program from Wright-Patterson Air Force Base.
I wonder if he’s actually done any systems studies to see whether that’s going to pan out? I have.
I’m all in favor of scramjets — they have lots of interesting and useful military applications, but it is very unlikely that they will be helpful for space access. I won’t repeat what I wrote the last time this issue came up (geez was it really five years ago?), but you can go read it here:
Proponents claim that by allowing airbreathing up to high Mach numbers, there is no need to take along as much oxygen for the rocket engines, because they can gather it for “free.” This argument assumes that space transportation is expensive because propellants are, but those aren’t the cost driver. If they were, space would already be affordable, because liquid oxygen is actually about as cheap as milk. Propellant costs are such a tiny fraction of launch costs that they’re down in the noise. If we ever get to the point where they become a real issue (as they are for airlines), we’ll have solved the problem.
Their argument also fails on the grounds that collecting oxygen isn’t really “free.” As the old joke goes, there’s no free launch.
If your space transport were to be single stage, you’d now need three propulsion systems — conventional jet, scramjet, and rocket for when you left the atmosphere (which you must do by definition to go into space). It may be possible to have a scramjet lower stage and a rocket upper stage, but the bottom line is that time spent in the atmosphere (necessary to utilize the scramjet) is time spent fighting drag, defeating the purpose. Rockets want to spend as little time as possible in the atmosphere, and carrying two other kinds of engines along and spending enough time in the air to utilize them, just to save on a propellant as cheap as oxygen, just doesn’t make design sense.
In addition, a scramjet engine is designed to operate at a specific vehicle speed, and has poor performance in “off design” conditions, rendering it a poor propulsion choice for an accelerating vehicle.
Henry Spencer debunked airbreathers to orbit earlier this year as well.