Question: hey this is not ryan its his friend i wanted to know about the functionality of scram jet engins and the uisefullness in space travel

Katy Milne answered on 24 Mar 2010:

I am going to talk about ram jets first, then scram jets because scram jets are a bit more complicated than ram jets.

Ram jets are even simpler than jet engines. They have no rotating parts at all. The air is squashed due to the changing cross-sectional area of the inlet nozzle. By the time the air reaches the combustion chamber it is hot and under pressure. Combustion causes the gas to start expanding rapidly and a jet of fast moving air comes out of the back, which causes thrust. The basic problem with ram jets is that they don’t work well if the engine is not moving fast relative to the air. If the engine is stationary on the ground, there is nothing which sucks in the air, like the fan and compressor do (the rotating blades before the combustion chamber) on a jet engine and so no thrust is produced. In fact, ram jets are only efficient at very high velocities. The air is ‘rammed’ into the inlet. They work best at around 3 times the speed of sound (around 1.2km/s at sea level and room temperature). Most designs for aircraft with ram jets have both a jet engine and a ram jet on the same aircraft. The jet engine gets the aircraft up to speed, at which point the ram jet is turned on. The advantage of a ram jet over a jet engine is that it is simpler (few moving parts) and that combustion can be carried out at much high temperatures. This allows more energy to be obtained from the fuel and higher speeds to be achieved. The development of ramjets and scramjets is still limited by the availability of materials that can withstand these temperatures. In our atmosphere, objects moving at such high speeds would also heat up alot due to friction. Therefore, the aircraft would have to be plated with special materials to increase its temperature resistance. The vehicle will therefore be heavier and the thrust-to-weight ratio will be decreased.

The scramjet is a ramjet that goes really fast (>3 times the speed of sound). The air entering the ramjet engine must be slowed down so that combustion can occur. Otherwise the combustion flame will effectively be ‘blown out. If the air at the inlet to a ramjet is going very fast and is slowed down suddenly then there will be a ‘shockwave’. ‘Shockwave’ means that the pressure suddenly changes by a large amount. Shockwaves are very inefficient – we want a continual change in pressure. Therefore ramjets are limited in the speeds that they can achieve. Scramjets use supersonic combustion i.e. the flame is moving supersonically along the engine while combustion occurs. Combustion has to happen very quickly and the airflow, temperatures etc within the engine must be very precisely controlled. These factors make scramjet design difficult. Scramjets are also very difficult to test, since the air must be supersonic when it enters the inlet.

Scramjets need a large oxygen supply and so are termed ‘air-breathing’ engines. They could be used to propel a craft into the lower levels of space but then something else would be required to manoeuvre or propel the craft in the vacuum of space.

Jet engine manufacturers do periodically consider combining ramjets and jet engines on a single craft but I think only organisations like NASA would consider scramjets. The cost of development would be astronomical! However, things like that are highly confidential and so I might be wrong.

There is a brilliant article on Scramjets on Wikipedia. There are a few equations, which require degree level physics or engineering to interpret, but if you just skip them, the rest of the information is very interesting. All the limitations of scramjets are described in great detail.

http://en.wikipedia.org/wiki/Scramjet