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Tuesday, 30 July, 2002, 15:19 GMT 16:19 UK
Q&A: Hypersonic jets
Graphic, BBC
A prototype hypersonic jet engine - a scramjet - has been tested in Australia.

BBC News Online explains how this technology works and how it could transform world travel and the space industry.

What is a scramjet?

Scramjet stands for supersonic combustion ramjet.

It is a form of jet engine with a simple mechanical design that has no moving parts. However, getting such an engine to work is anything but simple.

A scramjet operates at hypersonic speeds - in excess of Mach 5 (five times the speed of sound). Some researchers working in the field have even talked about reaching speeds of Mach 10 or greater.

OK. Let's say they manage it. What could it mean for me?

A working scramjet would enable high-speed air travel. One could go coast-to-coast in the US in 30 minutes, or London to Sydney in 90 minutes.

The technology could also allow single-stage-to-orbit space vehicles - spacecraft that fly into space in one piece. This would substantially reduce costs in the space delivery business and - eventually - the prices we pay for all our telecommunications.

It really would make space tourism a practical possibility for more people than just the super-rich.

Let's start at the beginning. How does an engine on a 747 work?

In a conventional turbofan engine - the type you'll find on a commercial jet airliner - air is sucked in and mixed with fuel and ignited. It then combusts and expands rapidly out of the back of the engine producing thrust.

The many bladed fans are used to compress the air prior to ignition, because at slower speeds the air is not compressed enough for efficient combustion.

And if I want to go faster?

Above Mach 2, engineers use a ramjet. Here the air is rammed into the combustion chamber and compressed by virtue of the aircraft's speed. Although this means no moving parts are required, it does mean ramjets will only start to work when they are travelling supersonically.

Ramjets have found applications in missiles that are first accelerated by conventional rockets.

A turbo-ramjet was used in the USAF's SR-71 "Blackbird" spy plane to take it up to Mach 3.3 and an altitude of 85,000 feet (26,000 metres).

And if I want even more speed?

Even though a ramjet will propel an aircraft or missile at supersonic speeds, the air moving through the engine itself must be slowed to subsonic speed to enable efficient combustion.

Above about Mach 4 this efficiency falls away dramatically. To go faster still, you need a scramjet.

Here, the airflow through the whole engine remains supersonic. It is claimed that speeds well in excess of Mach 10 should be possible. But being able to control the combustion process in such an environment is a highly complex engineering challenge.

Is anyone else developing a scramjet?

Yes. There are several groups working on scramjet technology. The US space agency has a project called X-43A. Last year, a scramjet test was aborted because the missile carrying the prototype scramjet veered off course.

Nasa's scramjet project is costing about $185m.

How long do we have to wait for a commercial aircraft powered by one of these new engines?

At current levels of research funding the experts say it will be decades. The problem with ramjets and scramjets is that they provide no static propulsion; other types of engine are needed to get an aircraft moving at a sufficient speed so that these faster technologies can take over.

Developing practical combined-cycle engines that can switch between roles is going to take many years.

Phil Mercer reports from Sydney
"Researchers are confident aerospace history has been made"
Scramjet project leader Dr Allan Paull:
"It's not unlike a normal jet engine"
See also:

30 Jul 02 | Science/Nature
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