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17 June, 2007

Scramjets Are Taking Off

I've been watching Australia's work on scramjets with interest for a few years now, mildly pleased that the country is doing something vaguely space-related. A recent milestone in this work is a successful test-flight, a couple of days ago, of a scramjet that took off from Woomera, South Australia and flew at Mach 10 at a height of 530 km before falling to the ground (as planned).

Now Mach 10 is fast but not nearly as fast as these things could go. Speeds of up to Mach 25 are considered possible, and this, they say, would dramatically reduce the cost of putting a payload into orbit (for which a speed of Mach 30 is needed – just a bit of an extra nudge from a rocket booster would do the trick) and would revolutionise commercial air travel.

But would it?

Scramjets are essentially simple devices. You push air into one end of a tube at supersonic speeds (between Mach 5 and Mach 7 – so you need to strap on a rocket or a ramjet to get them started), pass it through a constriction to compress it a bit, then burn a fuel with it (hydrogen, say) and vent the exhaust gasses (now moving much faster than the intake speed) out the back. The complexity lies in managing the supersonic flow of air and burning fuel in the engine, ensuring a complete mix and burn of the fuel within the engine during the very brief period that is available, and finding designs, materials and cooling systems that can cope with the extreme heat that is generated by friction with the air. Pushing a scramjet along at Mach 25 generates similar amounts of heat to a spacecraft re-entering the atmosphere – and for considerably longer if the often-quoted trip-times of 2 hours between Sydney and LA are ever achieved.

So, you need to use a rocket to get it airborne and going fast enough to work, when it's working, you're barrelling through the upper atmosphere like a meteorite, and then you need some other kind of engine to get it back to a safe landing (unless you glide it down like a space shuttle – with all the air traffic control problems that would cause!) Even if you weren't considering putting people inside such a vehicle and planned to use it to put payloads in space, you now need two rocket engines (one for take-off and one for orbital insertion) and you have two periods of re-entry-style heating to worry about (a scramjet can't just go straight up like a rocket – it hasn't got the thrust required – so it needs to travel along in the atmosphere until it has built up enough speed). Given the problems NASA has with the shuttle and its ceramic heat shield, I can't see a future scramjet being any less problematic.

Nevertheless, once all these difficulties have been surmounted, scramjets should be able to get into space more cheaply than a rocket could, and they should be able to get from one point on the Earth to another in dramatically shorter times than even the best military jets. Which sort of explains why, despite all the talk of revolutionising commercial air travel, it is the USA's Defence Advanced Research Projects Agency and Australia's Defence Science and Technology Organisation which were the collaborator's on Saturday's test.

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