Instapundit links to this story on the successful scramjet test Down Under, which (as usual) overhypes the application:
Butler said they could also slash the cost of sending satellites into space, because their potential payload was much larger than a rocket carrying its own fuel.
Even ignoring the fact (probably attributable to an ignorant reporter) that scramjets still have to carry their own “fuel” (it’s the oxidizer that they can leave at home), he shouldn’t buy it. The laws of physics haven’t changed in the three years since I wrote this piece debunking claims that this would revolutionize space lift:
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.
It’s been said that there are three basic rules to aerospace vehicle design, that many designers continue to forget to their peril:
- If you want to cruise, use an airbreather.
- If you want to turn, use a wing.
- If you want to accelerate, use a rocket.
Even with all of the improvements in jet engines over the years, you can’t beat a rocket engine for thrust/weight ratio, and it doesn’t care much how fast the vehicle is going. Chasing after solutions-looking-for-problems like this distracts us from solving the real problem–getting enough market and enough activity to get operational economies of scale, the lack of which is the real cause of the high cost of space access. And the near-term solution to that problem, despite the class-warfare whines of Eurocrats, is commercial space travel.
[Update]
I should probably add my usual disclaimer (as I did in the TCSDaily piece). I’m not saying that scramjets aren’t useful, or that they shouldn’t be researched and developed. The military definitely needs them for atmospheric cruise applications. I’m just saying that space launch will probably not be one of their applications any time soon, and they’re irrelevant to reducing launch costs in the near term (i.e, over the next two or three decades).
[Afternoon update]
One more thought. I’m not completely down on air breathers in general. I do think that concepts for collecting oxygen in the atmosphere are interesting, if it allows you to get the gross takeoff weight of a vehicle down to the point at which it can take off from a runway. It would be nice to have a system that still had LOX tanks, but took off with them empty, and then collected and separated the air (subsonically) until they were full. At that point, it could rocket to orbit. Andrews Aerospace is looking at a concept like this. But that’s not a scramjet, and scramjet technology doesn’t help with it.
[Update late afternoon]
Well, as usually happens in these theological discussions about technical methods of getting to orbit, some of the comments would indicate that I’m a heretic.