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New Jet Engine Tested
SpaceAdmiral writes "A revolutionary new jet engine has recently been tested in Australia. It is hoped that the engine, designed by UK defense firm QinetiQ and capable of Mach 7.6, will pave the way for ultra fast, intercontinental air travel. Scramjet (supersonic combustion ramjet) engines have no moving parts and take all of the oxygen they need (to burn hydrogen fuel) from the air, allowing for larger loads than rockets which must carry oxygen for fuel."
First application for Mach 7+ won't be passenger travel, but military (if not already used) where it will not only be fast, but louder than heck - after all Jet Noise is the Sound of Freedom! ;-)
Hulk SMASH Celiac Disease
Scramjet's are a revolutionary "new" type of engine, they have just been difficult to get from the concept to pratical stage.
The biggest problem is a way to compress enough oxygen at top speeds to feed the fuel reaction without needing to carry oxygen on board (which would be a rocket).
D.O.U.O.S.V.A.V.V.M.
Can anyone tell me why this engine is revolutionary? NASA has been testing these types of engine for some time.
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For example, the X-43 which hit mach 9.6.
[url]http://www.nasa.gov/missions/research/x43-ma
I'm not even sure where to start with this one...
1. Turbines in a jet engine are located after compression and combustion occur. Compression is due to compressors located after the inlet of the engine and before the combustion chamber where fuel is introduced and ignited. From the combustion chamber the high pressure, high temperature exhaust is then fed through the turbines which then generate power for quite a few different things including running the compressors.
Engine Theroy: Suck, Squeeze, Bang, Blow.
2. Jet engines do not work at very high speed not because of stresses in the compressors/turbines but rather because of the problems with supersonic flow. For supersonic aircraft the airflow into the engine is slowed to subsonic speeds using inlet geometry to control the oblique and normal shocks in the flow. Yes, theoretically you could spin the compressor faster than it's mechanical stress limits but that would occur a lot longer after the engine stopped working due to the flow.
3. The reason hydrogen is used as fuel for the scramjet is because the pressure tolerances for the engine are extremely small. The compressed flow must maintain supersonic speed, contain enough heat to ignite the fuel, and have enough time to have initiation and reaction occur inside the combustion chamber before it's ejected out the exhaust nozzle.
The reason they're comparing a Scramjet to a rocket engine is because having a Scramjet would dramatically reduce the weight of orbital flight by not having to carry its own oxidizer. For example: 75% of the weight of the Space Shuttle during launch is stored in LOX used as fuel.
However the feasibility of using a Scramjet engine for a single stage to orbit vehicle poses problems of its own. Way too many to list here. But solutions might be found to these problems as technology increases.
FYI The record for air breathing aircraft (not rocket) is 85,068 feet, set in 1976 by a Lockheed SR-71 jet-powered aircraft. This was broken by the NASA helios solar powered (not air breathing) flying wing 96,500 feet in 2001
Concorde was the highest flying commercial airliner with an operational ceiling of 60,000 feet.
The SR-71(AKA blackbird had very specialised jet-engines which operated in a semi ramjet mode.
The biggest problems with a scram-jet is trying to make it a scram-jet, i.e. the airflow through the engine has to remain supersonic. The shock wave angle decreases with mach speed, at low mach numbers it is too wide. This means that you have to have an impossibly large diameter and short engine to maintain supersonic flow. When you get hypersonic (above mach 5) the shock waves get much closer to parallel to the direction of motion, hence you can have a reasonable length and diameter on your engine. The length is the critical problem, as it is necessary to combust fuel and expand the working mass (air) within the engine in order that it can do any useful work. This is very difficult as mach 5 is 1701.45 m/s so if you have a 2m engine tube you have roughly 1.2 milliseconds in which to compress, burn and expand your fuel! The rate of flame propagation in kerosene is just not high enough to get even close at this kind of scale therefore hydrogen is realistically the only fuel which will work as it has the highest flame propagation rate, even hydrogen is difficult.
Scram-jets are a very interesting technology, but there are others like air breathing rocket motors which use the liquid hydrogen to cool the intake air so that it can be compressed sufficiently to be fed into the rocket combustion chamber. This is a pretty good technique although the heat exchangers are pretty difficult to build. The have the advantage that you just bleed in more and more O2 as you leave the atmosphere until they are running in pure rocket mode.
They do have the disadvantage that they can realistically only operate with a max velocity of about mach 5.5 when air-breathing as above that you get 0 net thrust and they are fairly complex. Air breathing rockets make single stage to orbit possible without ridiculous fuel to weight ratios.
Actually, they haven't lost a passenger in a JET aircraft, but they have had several fatal crashes. The most recent was on July 15, 1951 when a Qantas plane crashed in New Guinea killing all 7 passengers and crew. They also had half a dozen other fatal crashes in the 1920s, 30s, and 40s.
Technicallay, Qantas can still say they've never lost a jet aircraft. Though in 1999 one of their 747s over-ran the runway and ended up in a golf course. Nobody died, but the plane was so damaged that it should have been written off. However, Qantas ended up repairing it at a cost of over $100 million -- the most expensive repair in history. Speculation at the time was that Quantas pressured their insurer not to write off the plane as a total loss so that they could continue to claim they've never lost a jet aircraft.
http://www.nas.nasa.gov/About/Education/SpaceSettl ement/Nowicki/SPBI104.HTM
Simple, on the way up you have air and therefore great resistance. So it is far better to go up at mach 2-3 and then when in space (no air) turn around and rocket propel yourself back towards the atmosphere, doing most of your acceleration before you hit the atmosphere. This way you have to use much less fuel and a much smaller rocket. Air resistance is a far bigger proportion of the expended energy than that of gravity.
This kind of test has been used at Woomera many times before, originally developed by the British for testing reentry systems for warheads in the 50's, the Black Arrow program. Incidentaly this was so successful that the Americans paid to extend the program so they could use the results.
Basically they stuck the test reentry unit on top of a rocket, flew it to space then used a rocket to propel it towards the ground at up to mach 12, to test the materials and telemetry.
Interestingly the British re-entry warhead design was the opposite of the USA in that it came down pointy end first whereas the US models at that time came down blunt end first. The British design was far better as it came down much faster and therefore was much more difficult to intercept.
No, sir, you do not understand conservation of energy. Consider a prop plane. Over some interval of time, a propeller accelerates a relatively large amount of mass (compared to a jet) a relatively small amount, for a given force: F=ma. The energy in that mass of air is given by E = (1/2)mv^2. Now, in the same amount of time, let's say a jet engine moves half the mass of air with twice the acceleration. This gives us the same force as the prop engine, but the energy lost in the exhaust is 1/2(1/2m)(2v)^2 or (1/4)m4v^2 or mv^2, twice the energy as the air moved by the prop engine - for the same thrust. The air-fuel mixture of the jet engine is irrelevant: it's putting too much work into a fast moving, hot stream of air that we don't need, except at high speeds where props are inefficient because the tips get too close to the speed of sound.
Because a lot of development goes into jets these days, and because they carry so many people, they're pretty efficient in absolute terms, but I guarantee you that if you wanted more efficiency you would get it with a prop plane flying slower than a jet. (I'm not saying that we should do this, mind you.)
Disclaimer: IANAL. This post is, however, legal advice, and creates an attorney-client relationship.
Furthermore - a scramjet is nearly useless as the first stage of an orbital launcher, because it wants to cruise at a steady speed. An orbital launcher wants to be steadily accelerating.
That's not really true - or at least, it's highly confusing. All jet engines are accelerating whenever they are not idling: they exert a force on the craft causing it to accelerate at a rate of the force exerted divided by the mass of the craft. The apparent acceleration of the craft is reduced by drag and gravity. An orbital launcher has two requirements: that it gain sufficient height to reduce drag to near-zero, and sufficient velocity to actually be in orbit. There's numerous paths that will get you there and few of them involve 'steady acceleration' - a conventional 'great big rocket' launcher has steady thrust, but apparent acceleration to a ground observer is constantly changing with height, since the effects of drag reduce at higher altitudes.
A scramjet does not cruise at a steady speed. It runs at a fixed level of incoming air pressure. It has to run at that level because a scramjet does not contain moving parts to control the air flow. That means, as the surrounding air pressure decreases, the scramjet goes faster. It effectively operates at a fixed speed for a given altitude, and goes faster as you get higher. This is ideal for an orbital launcher.
However: the first stage of an orbital launcher is the one that gets it off the ground. A scramjet is completely useless as the first stage because it doesn't do anything when you aren't moving.
A scramjet path to orbit looks rather different to the old 'big rocket' system. You start with a conventional turbojet aircraft, which takes off and lands normally, using a horizontal path and wings. That's the first stage. You use it to climb to turbojet cruising altitude, and maybe accelerate to your maximum operating velocity (about mach 2 to mach 3). Then you fire a ramjet engine (or small rocket booster - this can be a solid rocket) to get you up to mach 5, which is the breakeven point for a scramjet. Then you fire the scramjet as the third stage, which carries you from mach 5 up to about mach 10 or 12, and most importantly, to near-orbital altitude.
At this point, the orbital craft that was piggybacking you breaks away, and boosts to orbit on one of the conventional late-stage rocket engines, like those used by the shuttle once it has discarded all its booster engines and is in the final orbiter configuration. It's already nearly there, so it doesn't need much fuel. The conventional aircraft that got it up here descends again and lands under turbojets, just like every other jet craft; the orbital craft has its own crew and operates independently.
The two advantages of this design are that it should be largely reusable (because you haven't discarded half the craft on the way up), and it requires significantly less total thrust to get up there. A 'big rocket' craft has to fight the force of gravity all the way up; an aircraft with wings is supported by aerodynamic lift, and merely has to accelerate. The disadvantages are that jet aircraft have more drag than rockets (but aircraft fly all the time; this isn't a fatal problem, it just reduces the advantage), and nobody knows how to build a useful scramjet aircraft yet (the X-43 testing craft just prove the scramjet concept, they aren't useful in their own right). Whether or not anybody can build such a craft that can lift a useful payload weight to orbit is unknown, but the theory says it should be possible.