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lprovenLast night I found myself at a bit of a loose end, so I went to a meeting of the Strange Phenomena Investigations group which Camilla had told me about. I missed the start, due to visiting ![[livejournal.com profile]](https://www.dreamwidth.org/img/external/lj-userinfo.gif)
sbisson and marypcb to borrow a telephoto camera which which to photograph mr_flay abseiling down the side of BBC TV Centre in aid of Children in Need. However, when I got there, they'd done it ½hr early and buggered off, so that was a bit of a waste of effort. Still, got me out of the house.
At SPI, I listened to a somewhat soporific talk about a haunted house in Gloucestershire. The speaker was keen to wave his hands and talk about "proof" of the existence of life after death and other things until I could take it no more and stood up to explain about what scientific standards of "proof" and demonstration actually mean, as compared to anecdotal evidence.
This, naturally, got me heckled a lot and turned the quiet, orderly meeting into a shouting match, but two or three people (including the moderator) came up to me afterwards to shake my hand and thank me for my contribution, saying that "we really need more people like you" ( whisper it, kiddies - skeptics!) "at these meetings! You did a good job!"
Hugely entertaining. I shall return.
This led me, later, to be quizzed about the new top-secret spaceships that can go at 5,000 times the speed of sound. Were these "scramjet" things real? What did they do? How did they work?
Someone's clearly seen the coverage of NASA's test flight of the X-43a - perhaps on CNN or Slashdot.
My answer went on a bit, so I've put it here for posterity...
This was all off-the-cuff and unresearched. Corrections welcome.
Yes, scramjets are real. They're experimental and are not a practical form of propulsion yet, though.
5000x the speed of sound? No. Hardly. Not in atmosphere. 5000mph is more like it. They only start working at Mach 5 - 5x SoS - but in theory will accelerate up to Mach 14 or so, and thus reach escape velocity.
Essentially, a scramjet is a jet engine with no moving parts.
There is a class of such jet engines, which are called ramjets. The best-known ramjets are pulse jets, which can be built at home by a keen experimenter. Google for "home made pulsejet" and marvel.
The way a jet engine works is by compressing a fuel/air mixture (FAM) so much that is spontaneously combusts.
When a gas expands, it gets cooler; when you compress it, it gets hot. Compress the fuel-air mixture enough, it heats enough to ignite the fuel, causing sudden massive expansion. This is how diesel engines work.
In a jet engine, the FAM combusts and instead of moving a piston, it's thrown out the back of the engine with great force.
Conventional jet engines are turbojets. A rotor in the exhaust stream is spun by the hot gas as it rushes out. This rotor is connected to a long rod that goes right through to the front of the engine, where it's connected to other rotors. The early ones suck air into the engine and later ones compress it for combustion.
Pulsejets work by sucking in air, adding fuel as it goes, then you "start" it with a spark. The FAM burns, whooshBANG, throws the exhaust out so fast it sucks in another load, the remaining heat ignites it, whooshBANG - and it cycles on its own, making a tremendous noise as all the bangs blur together and making the engine glow red-hot.
This works well and delivers tremendous thrust - I've seen them powering go-karts but they also powered the Nazi V2 rocket, the "doodlebug", from their distinctive drone.
But they don't deliver continuous thrust; the cycling causes a loss of efficiency.
The idea of a scramjet is to make a ramjet run continually, like a turbojet, but without all the mechanical bits in the way.
The way to do this is to compress the fuel/air mix (FAM) /continuously/ so that it's always burning, rather than in discrete bursts.
No mere clever engine shape is enough to do this. You can't get enough compression. But if the inlet is the right shape, as the air enters and is compressed, it sets up a shockwave in the combustion chamber which suddenly and dramatically compresses the FAM, which then burns. This only works if the air enters the engine already moving at supersonic speeds and stays moving faster than sound right the way through the combustion chamber.
(Gases behave in very odd ways at supersonic speeds; the modelling - often by Computational Fluid Dynamics, CFD - is hard but it's necessary because it's almost impossible to generate such speeds in a wind tunnel on the ground. The only way to test it is to do it - to build a model and fly the thing.)
Thus, the Supersonic Combustion Ramjet: the SCRamjet.
The idea is simple. The implementation isn't. One major problem is that scramjets don't work at all at under the speed of sound. Indeed, current experimental models only work at much faster than Mach 1 - like 5 times faster.
However, they offer a major boon for orbital and suborbital craft.
The only way to accelerate a vehicle that fast at the moment is a rocket. Rockets don't have intakes; they carry both fuel and oxidizer, which react.
You typically need much more oxidizer than fuel, incidentally. Even hydrogen. Burning hydrogen makes water:
2 H2 + O2 -> 2 H20
But Oxygen has an atomic weight of 16. Hydrogen weighs 1. You only need 1 oxygen molecule (weight 32) for every 2 hydrogen molecules (weight 4) but the oxygen still weighs 8X as much, per mole at the same temperature and pressure.
With a scramjet, you can get comparable amounts of thrust at very high speeds to a rocket, but *you don't need to carry your own oxidizer.* It's oxygen which you scavenge from the air. You need some other form of propulsion to get you up to Mach 5 - such as cheap solid-rocket boosters, perhaps - but the rest of the way to space, you save an awful lot of weight.
And weight is *the* major problem getting spacecraft off the ground and up to escape velocity. Reduce your fuel load by 8/9ths and that's an awful lot more payload you can carry.
To get really science-fictional, it might be possible to get a scramjet to work as a not-very-efficient rocket if you inject oxidizer as well as fuel, so your engines would work in atmospheres without free oxygen - which is to say, any other known planetary body. It won't really work in the vacuum of space, though.
But we need to get them to work in our atmosphere first.
That's what NASA is trying to do. The Australians have already done it, actually, but only with a tiny scramjet strapped to the nosecone of a rocket-propelled missile, designed to crash into the ground. (The Aussies do have an awful lot of mostly empty ground to play with.) It worked, though. Scramjets do work.
NASA is being a bit more ambitious and trying to get a small scramjet-powered vehicle to fly under its own power.
sbisson and marypcb to borrow a telephoto camera which which to photograph mr_flay abseiling down the side of BBC TV Centre in aid of Children in Need. However, when I got there, they'd done it ½hr early and buggered off, so that was a bit of a waste of effort. Still, got me out of the house.At SPI, I listened to a somewhat soporific talk about a haunted house in Gloucestershire. The speaker was keen to wave his hands and talk about "proof" of the existence of life after death and other things until I could take it no more and stood up to explain about what scientific standards of "proof" and demonstration actually mean, as compared to anecdotal evidence.
This, naturally, got me heckled a lot and turned the quiet, orderly meeting into a shouting match, but two or three people (including the moderator) came up to me afterwards to shake my hand and thank me for my contribution, saying that "we really need more people like you" ( whisper it, kiddies - skeptics!) "at these meetings! You did a good job!"
Hugely entertaining. I shall return.
This led me, later, to be quizzed about the new top-secret spaceships that can go at 5,000 times the speed of sound. Were these "scramjet" things real? What did they do? How did they work?
Someone's clearly seen the coverage of NASA's test flight of the X-43a - perhaps on CNN or Slashdot.
My answer went on a bit, so I've put it here for posterity...
This was all off-the-cuff and unresearched. Corrections welcome.
Yes, scramjets are real. They're experimental and are not a practical form of propulsion yet, though.
5000x the speed of sound? No. Hardly. Not in atmosphere. 5000mph is more like it. They only start working at Mach 5 - 5x SoS - but in theory will accelerate up to Mach 14 or so, and thus reach escape velocity.
Essentially, a scramjet is a jet engine with no moving parts.
There is a class of such jet engines, which are called ramjets. The best-known ramjets are pulse jets, which can be built at home by a keen experimenter. Google for "home made pulsejet" and marvel.
The way a jet engine works is by compressing a fuel/air mixture (FAM) so much that is spontaneously combusts.
When a gas expands, it gets cooler; when you compress it, it gets hot. Compress the fuel-air mixture enough, it heats enough to ignite the fuel, causing sudden massive expansion. This is how diesel engines work.
In a jet engine, the FAM combusts and instead of moving a piston, it's thrown out the back of the engine with great force.
Conventional jet engines are turbojets. A rotor in the exhaust stream is spun by the hot gas as it rushes out. This rotor is connected to a long rod that goes right through to the front of the engine, where it's connected to other rotors. The early ones suck air into the engine and later ones compress it for combustion.
Pulsejets work by sucking in air, adding fuel as it goes, then you "start" it with a spark. The FAM burns, whooshBANG, throws the exhaust out so fast it sucks in another load, the remaining heat ignites it, whooshBANG - and it cycles on its own, making a tremendous noise as all the bangs blur together and making the engine glow red-hot.
This works well and delivers tremendous thrust - I've seen them powering go-karts but they also powered the Nazi V2 rocket, the "doodlebug", from their distinctive drone.
But they don't deliver continuous thrust; the cycling causes a loss of efficiency.
The idea of a scramjet is to make a ramjet run continually, like a turbojet, but without all the mechanical bits in the way.
The way to do this is to compress the fuel/air mix (FAM) /continuously/ so that it's always burning, rather than in discrete bursts.
No mere clever engine shape is enough to do this. You can't get enough compression. But if the inlet is the right shape, as the air enters and is compressed, it sets up a shockwave in the combustion chamber which suddenly and dramatically compresses the FAM, which then burns. This only works if the air enters the engine already moving at supersonic speeds and stays moving faster than sound right the way through the combustion chamber.
(Gases behave in very odd ways at supersonic speeds; the modelling - often by Computational Fluid Dynamics, CFD - is hard but it's necessary because it's almost impossible to generate such speeds in a wind tunnel on the ground. The only way to test it is to do it - to build a model and fly the thing.)
Thus, the Supersonic Combustion Ramjet: the SCRamjet.
The idea is simple. The implementation isn't. One major problem is that scramjets don't work at all at under the speed of sound. Indeed, current experimental models only work at much faster than Mach 1 - like 5 times faster.
However, they offer a major boon for orbital and suborbital craft.
The only way to accelerate a vehicle that fast at the moment is a rocket. Rockets don't have intakes; they carry both fuel and oxidizer, which react.
You typically need much more oxidizer than fuel, incidentally. Even hydrogen. Burning hydrogen makes water:
2 H2 + O2 -> 2 H20
But Oxygen has an atomic weight of 16. Hydrogen weighs 1. You only need 1 oxygen molecule (weight 32) for every 2 hydrogen molecules (weight 4) but the oxygen still weighs 8X as much, per mole at the same temperature and pressure.
With a scramjet, you can get comparable amounts of thrust at very high speeds to a rocket, but *you don't need to carry your own oxidizer.* It's oxygen which you scavenge from the air. You need some other form of propulsion to get you up to Mach 5 - such as cheap solid-rocket boosters, perhaps - but the rest of the way to space, you save an awful lot of weight.
And weight is *the* major problem getting spacecraft off the ground and up to escape velocity. Reduce your fuel load by 8/9ths and that's an awful lot more payload you can carry.
To get really science-fictional, it might be possible to get a scramjet to work as a not-very-efficient rocket if you inject oxidizer as well as fuel, so your engines would work in atmospheres without free oxygen - which is to say, any other known planetary body. It won't really work in the vacuum of space, though.
But we need to get them to work in our atmosphere first.
That's what NASA is trying to do. The Australians have already done it, actually, but only with a tiny scramjet strapped to the nosecone of a rocket-propelled missile, designed to crash into the ground. (The Aussies do have an awful lot of mostly empty ground to play with.) It worked, though. Scramjets do work.
NASA is being a bit more ambitious and trying to get a small scramjet-powered vehicle to fly under its own power.
