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Know It All: Scramjet -- SCRAM Snails!

Published in Geare magazine, Issue #30, 2005

US space giant NASA is engaged in yet another competition with a tiny upstart. Last year we looked at SpaceShipOne, which successfully won the Ansari X-Prize by launching a spacecraft into space -- and getting it back down again -- twice within a fortnight. Meanwhile, Australia's own University of Queensland has been developing scramjet technology, under its 'Hyshot' program for amounts of money which probably wouldn't cover the catering bill for NASA's X-43A Scramjet program (each test vehicle is left to sink to the bottom of the ocean after its flight!).

So let's find out what a Scramjet is.

Basically, it's a type of jet engine that has long been considered to be a theoretical possibility, but proved rather difficult to engineer.

Jet engines of all kinds differ from rockets of all kinds in one major respect: they breathe air. Rockets carry their own oxidiser. Remember, all fuels work by burning (combining with oxygen). If you're going to space, you need to take oxygen with you.

Rockets are not very efficient within the atmosphere since part of their payload is wasted on carrying oxygen. Jets get their oxygen from the air around them.

Jet engines have one characteristic in common with the internal combustion engine found in the cars and motorbikes reviewed in these pages. In order for the fuel to burn efficiently, it has to be mixed in the right proportion with air, and then compressed. In a car the compression is done by the upward thrust of the piston, squeezing the mixture into the small remaining space carved into the cylinder head. This approach isn't workable for jets.

A standard jet engine, or turbojet, achieves compression by means of the dozens of fan blades at the inlet of the engine. This spins fast enough to draw air in, and the internal shape of the engine slows up the air's progress, causing it and the fuel that has been injected into the chamber, to compress. It ignites, producing heat and energy to eject the hot gaseous products of the ignition out the back. Along the way this exits through another turbine (a fan with lots of smallish blades) at the back of the engine, spinning it.

In the pure jet engine, the turbines are not there to provide thrust. All the thrust derives from the gas ejections at the rear of the engine. Remember Newton's law: for every action there is an equal and opposite reaction. If hot gases are being pushed out the back of the engine, then the engine -- and the vehicle to which it's attached -- has to go forwards.

The turbines are simply to compress the air/fuel mixture (compression ratios of greater than 40:1 are achieved with some engines). The rear turbine drives the front turbine. All the processes -- compression, ignition, expulsion, are not sequential like those in a car, but continuous, all happening simultaneously.

Where the engine is used to supply rotational energy, as with a turboprop aircraft or a jet-powered helicopter, another turbine is bolted onto the back of the engine and its shaft, spun by the action of the exhaust gases on the turbine blades, drives the propellers or rotor blades. Most subsonic jet aircraft use 'turbofan' engines, which have extra large turbine blades which pump back a lot more air than required for compression/combustion, thereby acting rather like propellers, but that's just one of many complications in this high tech area.

There's a limit to how fast all this can happen. In particular, you can only spin the turbines so fast before the blades fly out under centrifugal force. That in turn imposes a limit on how fast the airplane can fly.

That's where the ramjet comes in. Rather than spinning turbines, the ramjet uses the speed of the engine through the air to compress the air/fuel mixture. This has the ability to enormously simplify the design of the engine. Instead of turbines, the speed of the onrushing air (that's what it looks like to the engine) and the design of the throat of the engine creates the necessary compression.

Unfortunately, ramjets don't work unless the plane is going fast (half the speed of sound) and only achieve full efficiency once they get up to near the speed of sound. And they also have speed limitations. For combustion to take place, the air within the engine has to be reduced in speed to below that of sound. At around eight times the speed of sound, it requires as much energy to slow down the air as is available in the fuel, so a natural speed limit is imposed on ramjets.

Scramjets are ramjets which allow supersonic combustion (providing the 'sc' at the front of the word). The air doesn't have to be slowed, energy isn't wasted, and the aircraft can proceed very much faster.

How fast? The $US1.1 million Hyshot program managed to hold a speed of Mach 7.6 for six seconds in 2002. It needed a rocket to get it there, as did NASA's effort. In November last year, NASA's Hyper-X program got its X-43A test aircraft to 10,600km/h, or nearly Mach 10.

NASA's program cost $US250 million.

© 2002-2009, Stephen Dawson