Ramjets and Scramjets Explained - Mach 14

so now we've had a look at internal combustion engines and jet engines we know that jet engines can power aircraft and indeed aircraft that can fly at high speeds but how good our jet engines how fast can we get them to go the good thing about a jet engine is that you can use it to take off from a runway so we can start off from zero speed and jet engines are capable of reaching speeds of around mark three point five so they are not capable of getting us above mark five and into the hypersonic regime remember from our previous unit that the Mach number indicates how many times the speed of sound and object is moving so if we're flying at altitude where the air temperature is lower than at the ground level then the speed of sound is around 300 meters per second around that 1000 kilometer kilometer per hour mark therefore Jets are good for getting us up to speeds of around 1,000 meters per second that is one kilometer per second or around 3,500 kilometers per hour but not forgetting us up to hypersonic speeds so what are the limitations with jet engines what could we do to make the jet engines go faster in a jet engine we have very low speeds in the combustion chamber in fact in a jet engine the speed of the flow as it passes through the combustion chamber is very much subsonic so if we're flying at supersonic speeds we must slow the flow down through the intake and the compressor so that it's moving at a low subsonic speed as it passes through the combustor as we go faster in just bring the flow from high speeds as it enters the engine to subsonic speeds in the combustion chamber the pressure and temperature of the gas increase the higher the speed that our engine is operating at the higher other pressure and temperatures that we all get when we slow the flow down we'll find out more about the relationship between Mach number and gas pressure and temperature in the second section of the course which is about isentropic flow now in the jet engine it's the role of the compressor to raise the pressure and temperature of the gas so if we're going fast enough we really don't need the compressor because just slowing the flow down to subsonic conditions will raise the pressure and temperature to levels where we can do a good combustion therefore for a jet engine traveling at very high speeds we don't need a compressor if we don't need the compressor then we don't need the turbine because as we saw in the last video the turbine is just there to drive the compressor so we can throw away the compressor we can throw away the turbine and we'll see what we have left well what's left is a ramjet so a ramjet is a jet engine without a compressor or a turbine the ramjet has an inlet to take the air in and slow it down in so doing the inlet raises the pressure and temperature of the air we then inject some fuel in the combustion chamber and we add the energy of the combustion of the fuel to the flow and then we exhaust the gas through a nozzle and just as with the jet engine if we can produce a flow coming out of the nozzle at a higher momentum that it came into the engine we can potentially produce thrust from the engine so how fast can we get a ramjet to go now here's the first important limitation of ramjets ramjets can't start from zero speed because they need to be moving in order to force the gas into the intake so maybe around Mach 3 would be a good starting point for a ramjet engine and then they can take us up to around Mach 6 then if we want to try to push around get up to higher speeds just the RAM effect of forcing the air into the engine raises the pressure and temperature up to levels where the temperature is just too high to be able to do good combustion so we really don't need to slow the air down to subsonic conditions what we can do instead is slow the flow down just a little bit the stray zhing its pressure and temperature a bit but leave it supersonic and see if we can burn the fuel in the air and an engine that does just that is called a scramjet so a scramjet engine has an inlet where we slow the flow down raising its pressure and temperature we have a combustor where we inject some fuel the flow is still traveling supersonically through the combustion chamber where the fuel burns and releases its heat maybe it slows down a little but it's still stays at a supersonic speed and then we exhaust the gas through a nozzle again if we can get the gas coming out of the engine with a higher momentum than it came into the engine then we can potentially produce some thrust here is an example of a small model scramjet engine that was tested in one of our shock tunnels the t4 stalker tube at the University of Queensland and this scramjet was the first complete scramjet powered vehicle that was demonstrated to be capable of producing enough thrust to overcome its drag the photograph shown here is a version of the engine when it was tested in the shop tunnel this engine consisted of a conical intake and it had six combustion chambers located around a cylindrical centre body and then it has a series of six nozzles and we put this scramjet on a force balance and tested it in the tunnel we injected some fuel into the combustion chambers and we measured the overall force on the engine when it was in a hypersonic flow in the shock tunnel producing more thrust than drag meant that this scramjet would be capable of accelerating you can see that a scramjet engine is very simple there are no moving parts so you can look straight through the engine from the intake to the exit and you don't see any moving parts inside so how fast can a scramjet engine go scramjet engines can potentially operate at speeds as low as around Mach 6 so at around the speed where the ramjet engine cuts out at this lower speed there's a crossover region where we could use either a ramjet or a scramjet what about the upper end we're not sure what the upper Mach number limit is for a scramjet we know we can reach Mach 10 that have been some experimental scramjets flown at Mach 10 conditions mark 12 may be possible you'll be nice if we get up we could get up to Mach 14 but this is still an open question at the moment then what happens is we again get to the conditions where slowing the flow down to we get to two high temperatures for us to be able to burn the fuel and release the energy of the fuel into the flow another problem that we run into is that the aerodynamic drag forces on the vehicle become very high at high Mach numbers and the heating problems become really severe so where the Rockets fit into the picture rockets are not air breathing engines they're completely self-contained in that they must carry both their fuel and an oxidizer with which to burn the fuel so that we can release the chemical energy contained in the fuel so the big difference is that whereas an internal combustion engine a jet engine a ramjet and a scramjet will take their oxygen in from the air through which they're flying a rocket has to carry its oxygen with it the good thing about rockets is that they can operate in almost any environment rockets can operate in the Earth's atmosphere they can operate in space or in the atmospheres of other planets and moons so they're very versatile but because they must carry the fuel and oxidiser and the mass of the oxidizer required can be very large rockets are pretty heavy compared with their air-breathing counterparts therefore they are potentially less efficient than the air breathers so if we're looking to fly through the Earth's atmosphere at hypersonic speeds then the scramjet looks like being a very attractive option let's finish by summarizing the engines and they're operating regimes jet engines can operate from zero speed up to around 3.5 times the speed of sound Ram Jets are good for around Mach 3 to mark 6 and then scram Jets are suitable for the higher hypersonic Mach numbers we're still not sure how far we can push into the hypersonic regime with scram Jets Rockets are less efficient but they can operate over all the Mach number regimes in the next unit we'll derive and use the so called rocket equation to see why we might want to use a scramjet engine for a stage of a launch vehicle

Channel: Edwin Henry Blachford

Views: 1,447,250

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Id: xpOa3B03gYg

Length: 10min 2sec (602 seconds)

Published: Tue Jul 26 2016