Our friends Down Under have beaten the U.S. at one of the holy grails of aerospace technology.

On a shoestring budget, researchers at the University of Queensland in Australia 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.

Why is this a big deal? It's very difficult to burn fuel in a supersonic environment. Imagine trying to light a candle in a hurricane. That's just a hint of what it's like to achieve combustion in a chamber in which the air is moving at supersonic, if not hypersonic speeds. It's hard enough to sustain combustion at just a few hundred miles per hour, let alone at a speed at which you can't even hear the sound of the fire as it burns in the engine.

The problem is that it's not that big a deal for space, at least in my opinion.

A couple of weeks ago, I had a column here that stated, "we don't need no stinkin' technology." Well, hypersonic airbreathing engines are one of the many technologies we probably don't need.

While Leonard David's article states that "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, such as liquid oxygen, 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 because they have to carry a lot of propellant. The propellant costs for a typical rocket is a tiny fraction of the launch cost.

Some argue that, regardless of the costs of the oxidizer itself, the cost of rockets comes from the fact that they have to be much larger, in order to carry it. But it turns out that even that is a minor contributor to the total costs. Oxidizer, particularly liquid oxygen (which is what most rockets, at least ones that are environmentally benign, use) is quite dense, and the volume required to carry it along is quite a bit less than that required for the fuel itself. If that's not persuasive enough, consider that it is one of the cheapest liquids on earth, barring water. It costs less than a few cents per pound.

So even a vehicle that has to carry all of its own oxidizer will be less than twice the mass of one that doesn't.

And any rational cost model will indicate that vehicle costs scale less than linearly with the size of the vehicle, so even if the size was doubled, the cost won't be--it will be less than twice as much. And since we are after orders of magnitude (factors of ten) reduction in cost, it should be clear that getting rid of the oxidizer just won't do the job.

As I've said repeatedly, the primary driver behind high launch costs is not anything intrinsic to the vehicle design itself, but low flight rates and lack of vehicles specifically designed for high ones. Scramjets are sexy for the "technology uber alles" crowd, but for reasons stated above, 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 (where the air comes in) 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, because there's no air to work with any more.

Rockets, on the other hand, get out of the atmosphere as quickly as possible because they tend to perform better in vacuum. Using rocket engines eliminates the inlet drag and greatly reduces the need for thermal protection during ascent into orbit, relative to any airbreathing launch system.

There are other issues. Airbreathing engines tend to optimize at a certain cruise speed, and perform very poorly in what engineers call "off-design conditions." That's exactly the propulsion system that you don't want in a launch system, which is under continuous acceleration--not cruising along at a single speed. In addition, 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 the Aussies anyway--it's still a great technical achievement.

Rand Simberg is a recovering aerospace engineer and a consultant in space commercialization, space tourism and Internet security. He offers occasionally biting commentary about infinity and beyond at his Web log, Transterrestrial Musings.

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