Explainer: What is a scramjet?


Australian researchers are moving ahead with developing a hypersonic rocket that could one day allow pollution-free passenger travel into space at rapid speeds. But just what is a scramjet?

Scramjets are air-breathing engines that travel at hypersonic speeds.

According to NASA, scramjets can reach predicted speeds 15 times the speed of sound. That's equivalent to converting an 18-hour trip to Tokyo from New York City to a two-hour flight.

Scientists in Queensland have developed a 1.8m rocket set to reach eight times the speed of sound, 8600km/h, in its test flight at the Andøya Rocket Range in northern Norway. If the test is successful, the rocket could be the world's first operational scramjet.

A $14 million international consortium of partners in five countries, led by the University of Queensland's Centre for Hypersonics, is funding the project.

They say the aim of the project is to offer a reliable and economical way to launch satellites.


The idea of a scramjet arose out of a desire to make faster, but smaller sized rockets.

The key to making this happen is changing the fuel used to power the rocket.

Rockets usually combine a liquid fuel with liquid oxygen to create thrust.

Scientists have been working on ways to remove the need for liquid oxygen to make spacecrafts smaller and capable of carrying heavier loads.

The result is a different propulsion system for rockets -- a Supersonic Combustion Ramjet (scramjet).


Theoretically, the mechanics of the scramjet is simple.

It has no moving parts and takes all of the oxygen it needs to burn hydrogen fuel from the air.

This makes it more efficient than a conventional rocket engine as it does not need to carry its own oxygen supply, meaning that a vehicle using a scramjet could potentially carry a larger payload.

But making sure it all works in reality is what has preoccupied scientists at University of Queensland's Centre for Hypersonics as well as the US Air Force, US Navy, and NASA since the late 1950s.

As the diagram below shows, scramjets gather up air (oxygen), forcing it into a combustion chamber where the fuel (usually hydrogen) is burned, generating thrust that keeps the craft going. The only exhaust gas is steam.

File:Scramjet operation en.svg

Image: Luke490 (Wikimedia Commons)

Scramjets are being promoted as a technology that - when it becomes operational - could eventually lower the cost of sending satellites into orbit.

Passenger travel into space could also be possible, although developers say that scenario is at least 50 years away.

Source SBS

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