I’m still busy, but I’ll try to get up a few posts today. I’ll see how long I can go before my outrage boiler is about to blow, and I have to vent some steam…

Australia has beaten the US at one of the holy grails of aerospace technology–on a shoestring budget, they’ve demonstrated a supersonic combustion ramjet (scramjet) in flight, for the first time in history. NASA has spent many times as much toward that end and never flown anything.

It’s not that big a deal for space, however, at least in my opinion.

While Leonard David’s article states “scramjet vehicles could launch small space payloads at substantially lower cost” as though it were an established fact, there are actually a lot of reasons to think this is not the case. That people (even otherwise smart engineers) believe this is due to a misunderstanding of the source of the high costs of launch.

If you believe that launch is expensive because rockets have to carry a lot of propellant (needing both oxidizer and fuel), then it makes sense that if you have a vehicle that can get its oxidizer from the atmosphere, it would be much cheaper to operate.

Unfortunately, the underlying premise is false. Rockets aren’t expensive becaue they have to carry a lot of propellant. The propellant costs for a typical rocket is a tiny fraction of the launch cost. Even the fact that the rocket has to be larger in order to carry them is a minor contributor.

As I’ve said repeatedly, the primary driver behind high launch costs is low flight rates, and lack of vehicles specifically designed for high ones. Scramjets are sexy for the “technology uber alles” crowd, but there’s no reason to think that by themselves they can reduce the cost of launch.

Even for a high-flight rate vehicle, it’s likely that their disadvantages will vastly outweigh their benefit of not having to carry oxygen. In order to get their oxygen, they have to spend a lot of time in the atmosphere. Airbreathers moving at hypersonic speeds in the atmosphere have a lot of drag, including the drag of the inlet to the engine itself, and it’s an extremely intense heating environment, as bad or worse in many ways than entry. And once they get out of the atmosphere, they have to fall back on rocket propulsion anyway.

Rockets, on the other hand, get out of the atmosphere as quickly as possible, because they tend to perform better in vacuum, and it reduces the drag and the need for thermal protection during ascent.

Also, airbreathing engines tend to optimize at a certain cruise speed, and perform very poorly in off-design conditions. That’s exactly the propulsion system that you don’t want in a launch system, which is under continuous acceleration. Rocket engines are indifferent to vehicle speed (they’re sensitive only to atmospheric pressure).

Scramjets may have some interesting military applications, but I think that they’re unlikely to play any role in commercial flight, or space launch, for a very long time.

But congratulations to Oz anyway–it’s still a great technical achievement.