Higher and faster on regular unleaded…
February 21st, 2008
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Here is a story which really got my attention. Scramjets are not entirely new. A scramjet is basically a specially designed ramjet which can burn fuel in a supersonic airstream. This allows for extreme speeds beyond those of a traditional ramjet, which tops out around Mach 3.5 to Mach 4. Since a scramjet does not need to slow the air on intake by nearly as much, it can theoretically reach speeds in the “hypersonic” region of Mach 6 or greater. This has been demonstrated before by X-43 as well as in static tests.
But there have been problems with ramjets in real world roles. In addition to the unique stresses and temperatures they must operate in, the only fuel which has been suitable for the rapid compression and combustion in a ramjet has been hydrogen. This is a major drawback because hydrogen fuel has a very low energy to volume ratio, thus requiring a very large amount of the fuel and a huge tank to store it in. Furthermore, since hydrogen has to be kept at extremely low temperatures, the tank must be well insulated. Thus, any scramjet powered vehicle would seem to be stuck carrying an enormous tank which increases weight and drag and maintaining and fueling such a vehicle is always going to be much more complicated than a standard aircraft.
However, recent developments may hold the promise of changing all this. The new HyFly missile is powered by a unique experimental scramjet. Although, it failed the first test, the technology holds great promise because it is the first scramjet which can burn hydrocarbon fuel. In other words, this design, which has been successfully tested in wind tunnel experiments, does not need supercooled hydrogen or any exotic fuel at all. It burns standard Jet-A fuel, which is both avaliable and very easy to load and use. Because it is a liquid and has a much higher energy density the use of standard jet fuel holds the promise to make hypersonic flight a reality for manned aircraft. The use of hydrocarbon fuels is possible due to a unique two-stage burning design.
One thing which is also worth noting is that a scramjet of this type can theoretically reach a theoretical altitude of greater than 50 km and a velocity of Mach 8 or greater. Launching payloads into orbit requires a lot of power and generally uses single-use expendable rockets. These rockets use greater than half of their fuel (and often the first stage) to get to a similar altitude and velocity, burning valuable fuel overcoming the high gravity and atmospheric drag of lower altitudes. Since jet engines are much more effecient than rocket engines and do not require carrying their own oxidizer, the hydrocarbon scramjet technology could be used as the first stage or first phase of a launch system and could make possible the truely “reusable and routine” kind of launcher that NASA has been promising the world since the early 1970’s.
Of course, it also is not really useful in going to the moon, so NASA has about zero interest in developing this kind of technology.
Ps. About the title: it implies the use of gasoline, which I am sure someone would point out is not the same as jet fuel. Jets can actually burn gasoline (or almost anything flammable) but jet fuel is normally more similar to kerosene, with a higher energy density and less volatility. But since gasoline is basically the same, except a “lighter” hydrocarbon you could probably use that too.
This entry was posted on Thursday, February 21st, 2008 at 11:27 am and is filed under Good Science, Space. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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February 21st, 2008 at 12:04 pm
You are talking about what was once nasa’s priority and they gave up on after failing so spectacularly so many times. Once NASA made kids dream “Someday you will go to space” as if it was going to be the future to make it more avaliable. Now their message is “Someday there will be people on the moon again, but it almost certainly won’t be you. We’ll be sending up a couple dozen more.”
This is meaningless to NASA. Maybe a private venture will fill the void eventually but it’s a huge project and risk for any private company. We can hope though.
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February 21st, 2008 at 3:18 pm
Actually Jet fuel is closest to ordinary Diesel fuel with only minor differences in technical characteristics. You can run a jet on Diesel fuel and a Diesel on Jet fuel.
The US used to use a single fuel for arctic bases – called Diesel Fuel Arctic. This ran the generators, trucks and aircraft.
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February 21st, 2008 at 3:24 pm
You can run a jet engine on almost anything as long as it won’t clog up the filters or mess with the fuel system. They run gas turbines on methane all the time and yet it runs on diesel, kerosene and that kind of thing. It can also run on light oils like sewing machine oil. When they mothball jets they run the engine like that to flush out all the fuel and make sure the whole system is lubed up and won’t corrode as much. They can run on alcohol too. Regular aviation fuel is a high octane gasoline and it’s not usually for jets but they can run on it if that’s all that’s avaliable.
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February 21st, 2008 at 3:36 pm
All jet fuel is kerosene based, you can run Diesels on Jet-A/JP-8 however you can’t run a standard turbojet on heaver fractions like heating oil which Diesels are happy to burn.
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February 21st, 2008 at 5:34 pm
I guess it would depend on the engine and I have no idea what a scramjet would need, but I assume that it would do as well with lighter hydrocarbons. Anyway, the M1 Abrams and most turbine driven helicopters will run happily on gasoline if that’s what you have, which could be the situation if you have captured fuel or something in battle. That was one of the design considerations of the M1. The turbine in it will run fine on most petroleum derived fuels.
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February 21st, 2008 at 6:11 pm
Gas turbines will run on anything if they are designed for it. At the very least a stock commercial fan jet would need re-nozzlling and the FCU (fuel control unit) changed if you were moving from a light to heavy fuel or back.
Gas turbine based electric power plants with multi-fuel options like gases, oils, and emulsified solids are a special breed that have separate external support for each fuel regime.
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February 21st, 2008 at 8:18 pm
This is getting totally off topic from the post, but you can fuel a jet engine on an aircraft with gasoline or the more kerosene like fuels, depending on the engine.
Any standard military or civ jet plane will take jet-a or jp-8 and also jet-b which is usually only in very low temperature and is basically gasoline and kerosene and additives. Many military aircraft will run on straight gasoline if need be without modification. If you are talking about civilian jets, then the civilian non-jet fuel is avgas which is very light and high octane.
Avgas is more expensive and less effecient and also has a lower flashpoint so you wont use it in a jet unless it is all you had. The time you might need to is small jets like air taxy if they land at a very small airport that is general aviation then they may only have avgas. Some of the smaller jets have engines which will take avgas fine but some you might have to add lubrication or something.
But to cut to the chase, there’s no reason why a jet engine couldn’t run on regular unleaded if it is designed for that but I don’t know if the scramjet could use gasoline. I guess it probably could.
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February 21st, 2008 at 8:32 pm
Mister Fisk said:
The critical phrase here is ‘depending on the engine’
I’ve spent a lot of time in the gas turbine/jet engine sector
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February 21st, 2008 at 9:12 pm
Alright. Back on topic, if there is ever going to be any kind of “spaceplane” or for that matter hypersonic transportation then using something better than hydrogen is going to make a huge difference. Hydrogen is actually better for energy to mass ratio but that is negated and then some by the much larger volume requiring a big tank as well as the need for insulation. If you can take off on jet-a then it can be more doable.
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February 22nd, 2008 at 12:03 am
The Europeans have been talking about a hypersonic airliner which would be pushed through on government funds based on a non-existent market they want to create. (Didn’t they learn their lesson on the Concord???)
Their plan was for hydrogen, but this might actually make it somewhat workable. But even if hypersonic airliners aren’t going to be coming soon, this still could make the idea of breaking mach-4 or more with an air breathing jet a much more realistic proposition for launches or other applications.
If it ever comes to passenger planes though, I’ll be first in line, assuming I can afford it
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February 26th, 2008 at 3:22 am
The hypersonic airliner is based on the old HOTOL design with the proposed Satan RB67 airbreathing rocket engine, first intended for a SSTO spaceliner. It only works with liquid hydrogen for a reason.
The engine runs as a scramjet only at high altitude; until it reaches that point it burns liquid oxygen from internal tanks like a conventional rocket. At about 30-50km and clocking along at Mach 2 the plane opens an airscoop and starts processing the (very cold) rarified air. The scoop’s ramp compresses the airstream while cooling it via a heat exchanger through which the liquid hydrogen is being pumped. Careful control of air and coolant flow rates allows the system to liquefy the oxygen in the airstream while allowing the nitrogen to stay in gas phase. The LOX is separated out of the airflow using something like Hilsch tubes while the nitrogen is dumped out the back. The LOX is pumped into the rocket motor powering the aircraft along with the slightly warmed-up LH2 from the scoop compressor/refrigerator system. Hey Presto, thrust!
It’s actually quite a neat idea, using the LH2’s existing low temperature to do a lot of the work of fractionally distilling air “on the run”. Sadly as you might expect, it’s a pig to get working and keep working as it pushes all sorts of technological and operational boundaries that nobody has done serious work on yet.
The Satan engine concept was shown to work as a lab-bench experiment at Mach 7 or thereabouts roughly fifteen years ago. No engine flight tests have been attempted or even put forward for funding. They’re a long way from bending metal on a production line.
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February 27th, 2008 at 11:54 am
That sounds very interesting but also like a bitch to get to work. The whole idea of extracting oxygen from the air and compressing it “on the fly” as it were.
Does that kind of system really have that much of an advantage over scramjets or ramjets for speeds under mach 8 or so?
You’d need rockets once you get to an altitude of more than 50 km or so, but before thaty the HOTEL sounds unnecessarily complicated
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