Last year, James “Vacuum Cleaner” Dyson said on TV that the fan at the back of a jet is what supplies the thrust. Happily, this year, a history lady on the TV set us straight, telling us the fan at the back only steals some of the thrust for using at the front of the jet. What exactly is going on? It’s an interesting story…
Why they work
It starts out simply, with the basic rocket principle:
We have high-pressure gas inside a container, pressing on the front but not the back.
The pressures on the top and bottom of the container cancel each other out,
…but the container is being pushed forward at the front, but not being pushed back at the back. It therefore ends up being pushed forwards.
(The gas, as a lump, is pushed the other way: backwards, and by the same force.)
In a jet, some of the surrounding air is grabbed as it goes past. (That air wasn’t going anywhere particular; it might have just been sitting still.)
Once inside, fuel is burned in it, which increases the heat and pressure. From that point on, it’s the same as the rocket.
People often wonder at this point, why does the gas only go backwards out of the jet? If it is open near the front, why doesn’t it go out of the front as well?
It would do, and it only works because the air is compressed as it goes in, and to a higher pressure than exists inside the combustion chamber, even after it’s heated up. (Actually, just the compression on its own heats it a bit.)
The reason it’s at a lower pressure in the combustion chamber even though it’s hotter, is because the combustion chamber is much bigger than the space the air is squeezed through when it’s pumped in at the front.
But surely, if you’re using more oomph to get the air in than there is inside, aren’t you using more energy at the front than you’re generating inside?
Well, luckily, there isn’t a law about conservation of pressure.
It’s like a dance club with a couple of doors at the front, where bundles of sober people are forced in by bouncers. (It’s a slightly unusual kind of club.) Once inside, the people get drunk and start moving about much more, like the air molecules in a hotter gas, but they’re never under as much pressure as they were when the bouncers squeezed them in. (They’re drunker/hotter/faster moving, but suffer less average pressure since they don’t get so many hits from other people and walls, even though each hit is harder.) However the other end of the hall has no wall, so people can bounce around off three walls, including the one opposite the wall-less side, but eventually they all fly out through the side with no wall.
There’s three ways to squeeze air in at the front:
The last way they found was by just letting the speed of the jet squash the air in. This is a ramjet, and it can work if the front of the jet is the right shape and you’re going faster than 200mph/325kph.
The next way is just to have a big fan at the front, squeezing the air in (axial compressor: the air moves parallel to the axis of the fan). And the obvious source of power for this is to have another fan at the back being blown round by the exhaust gases, connected by a shaft to the compressor fan at the front. That’s why thrust isn’t generated by the rear fan(s), in fact it’s actually lost there, though of course that sort of jet wouldn’t work at all without the rear fan. The places where the jet’s thrust is generated is actually on the front wall of the combustion chamber, and also some from the front fan(s). In the old days, I believe someone did try powering the front fan with a separate piston engine, but just using the fan in the exhaust was much, much better.
That rear-fan-to-front-fan method was what the German authorities used when they developed their first jets. (Their project that produced the first ever flying jet, by Heinkel, was so secret that not even the German air force or air ministry were told about it at first, and it did use the British approach.) The British used a horrible-looking system where the front fan was replaced by a disk with shallow plates leading from the centre to the edge, looking from the front a bit like the underneath of a mushroom (centrifugal compressor: the air moves centrifugally). The air didn’t go through this plate, but was spun to the edges by “centrifugal” force. It was then channelled to a number of separate combustion chambers around the central shaft, and they converged their exhausts onto the rear fan. The plate was however much easier to make than a fan, and more reliable, though it did mean those engines looked like they were made from old steam engines and vacuum-cleaners. After the war of course, it was possible to start again and do it the “proper”, German way, though the centrifugal system is used in helicopters.
Much of the energy spent squeezing the air in, can be converted to thrust, so it isn’t all wasted. Then you just throw fuel into the combustion chamber and ignite it, because of that kind of one-way valve at the front. In fact, even behind the main combustion chamber, you can still pump fuel into the exhaust and get even more thrust (called afterburning) if you’re desperate (so long as there’s enough oxygen left in the exhaust stream 🙂 ).
Another weird thing
As we mentioned before, the air taken in by the jet wasn’t going anywhere particular; it might have just been sitting still.
This is different from the gas thrown out at the back of the rocket, which had been going forward at the same speed as the rocket. With the jet, since that air had been still, it had effectively been moving backwards compared to the jet.
If the rocket is flying at 100, it accelerates some of its fuel backwards, which had also been going at 100, to get a certain amount of thrust from it. That fuel, now in the form of gas, might still be moving in the same overall direction as the rocket, but now more slowly at say 90. It’s been accelerated backwards by 10. When a steadily firing rocket exhausts that amount of fuel every second, it gets a steady amount of force, usually called thrust.
Here’s a subtle but significant difference: for the jet to get the same thrust by accelerating the same amount of gas backwards per second, it will still have to accelerate it by 10 in a backwards direction… but remember, with a jet, the gas (i.e. stationary air) was already going backwards at 100, with respect to the jet!
“So what?” one might think. The rocket, accelerating its fuel backwards relative to itself from 0 to 10 (in a backwards direction) gives the same thrust (needs the same force) as the jet which accelerates surrounding air backwards relative to itself from 100 to 110 (in a backwards direction). Same job done, eh?
Well, oddly, and very weirdly in my view, it takes 21 times as much energy to accelerate something from 100 to 110 as it does from 0 to 10.
In another illustration, if you’re moving along a canal in a barge at 100, and apples are hanging from apple trees alongside the canal,
…then hitting an apple backwards from 0 to 10 with even a magical tennis racket of zero weight, will need 21 times more energy than throwing an apple which you had been carrying in the barge, backwards at 10 relative to you (i.e., so it now only goes forward at 90).
But WHY?! What on earth’s going on? Let’s take one of those apples up to the top of your house and find out. The rocket’s advantage isn’t always 21 of course, it depends on the speeds etc., but all will now be revealed…
When you drop something through a distance of 2, does it end up with twice the energy as if you’d just dropped it from 1?
This was a question people asked in the old days. They also asked if something moving twice as fast had twice the energy. Maybe both were true, they wondered.
They soon found they couldn’t both be true, because of what happens when something falls under gravity. (…ignoring air resistance 🙂 .)
Gravity is very convenient. It always applies the same force to you however fast you’re going (so long as we’re only talking about house-sized heights!) As a result, every second you add exactly the same amount of speed. It’s very neat and tidy. Let’s assume first that on your planet, gravity adds 1 to the speed, every second.
Let’s assume next that your house has two levels, and you live on a planet where an apple takes one second to drop down the first level, to halfway down the house.
According to our first assumption, after one second, it will have accelerated to a speed of one.
How fast will it be going just before it hits the ground? A speed of 2?
We can find the speed if we first find the time: falling under gravity, speed is equivalent to time. (Here, we’ve selected the numbers so that speed actually equals time.)
We can get the time from the distance using the charts below, where the distance shows up as area under the line, and is equivalent to the square of the time (with our figures, equal to the square of the time).
As the distance depends on the time squared, if it falls through one level in one second, in two seconds it will fall through 2 x 2 levels, and after three seconds it will fall through 3 x 3 levels.
So to find how many seconds it takes to fall 9 levels, working backwards by taking time as the square root of the distance, it must be the square root of 9, so it’s 3 seconds.
And therefore to answer our question of how many seconds it takes to fall 2 levels, we take the square root of 2, which is 1.414… .
For us, it’s the speed as well as the time that’s 1.414… . So the speed isn’t 2 at the bottom after all.
The early investigators must have thought that was a bit odd, if falling through the first level added 1 to the speed but falling through the next level only added 0.414… . Is that extra 0.414 the same energy as the first 1? It looks like it is, because if you somehow store the energy of the apple falling at the 1.414 speed at which it hits the ground, perhaps in some kind of spring, you can actually use that squashed spring to push two apples up through a height of one level.
So it did seem to make sense to invent a term called energy, which does double when falling through twice the height,
…but if so, any calculation of energy requires you to square the speed.
But that energy due to speed – kinetic energy – depends on the eye of the beholder, or at least how fast the beholder’s eye is moving compared to what it’s looking at.
So because jets have to work on air going backwards which uses more energy, but a rocket only pushes against stuff traveling at the same speed, jets are at a bit of a disadvantage, which gets worse the higher the speed.
On the other hand, jets have the advantage of not having just their fuel to throw backwards! Although burnt fuel obviously goes into the exhaust, most of the exhaust comes from the surrounding air it didn’t have to take with it. For that reason jets can go further than rockets, particularly if they don’t go too fast.
(Note, in all these cases, you can get thrust from the stuff you throw out of the back whether it hits anything or not.)
Finally, we now have a reason why slow-moving passenger jets have wide engines with big fans, which accelerate a lot of air back by a little, whereas supersonic jets have narrower engines accelerating a little air back by a lot:
You get the same thrust if you accelerate two apples back by one, or one apple back by 2. But if you choose to work on just one apple, it ends up going faster than either of the two apples on their own. As we’ve seen, pushing against something moving faster away from you needs more energy, so if you can get away with pushing two things a little bit rather than one thing a lot, you save energy.
The big passenger jets can get away with pushing a lot of air a little. But we’ve seen that a jet can only work if its exhaust ends up going in the opposite direction, not just slower in the same direction. So a supersonic plane has to accelerate its exhaust gases a lot or it would get no thrust at all.
(As a supersonic jet is going so fast, it does end up pushing a lot a air a lot… in terms of time, but in terms of distance it might well be less air.)
(With the big passenger jets, they choose to move so much air that they don’t even bother pushing all of it through the combustion chamber: most of it just goes round the outside – a high bypass jet.)