For example, if I took a butane lighter & fired it up, there’s no force in any direction. How does a rocket ignition work & propel itself upwards?
Thanks,
In: Engineering
By throwing mass out the back.
F=dp/dt as per newton’s second law
F=d/dt(mv)
The fuel has mass, and if we throw it out the back very fast, the rocket needs to gain an equal and opposite amount of momentum.
F=dm/dt v where v is the speed of the exhaust and dm/dt is the rate of fuel flow.
A butane lighter absolutely exerts a force – the burning butane expands, is less dense than the air around it, and the flame goes up because of it. In a rocket, the combustion wants to expand as well, but it’s not out on the open, it’s inside a space with only one way out. All that expanding gas going out the back of the rocket pushes the rocket the opposite direction.
There IS a force with a butane lighter – it’s just small enough you won’t notice. I’d have to do a lot of math to figure out how much; but its probably measured in fractions of a gram – maybe grams.
You get more force based on how fast you can expel the gasses, how much energy is in the burning, and how much of the burning energy you can capture. Butane lighters mostly just let the gas go, and while the burning is hot (about 4000F), there isn’t a lot of effort to turn that heat into force. In contrast, rocket engines put the gas out a bit faster; burn slightly hotter (about 5000F) – but do a LOT to make sure as much of the burning energy as possible turns into movement.
And you can see the difference: Butane lighters are open flames, meaning most of the force goes in different directions and is lost. In contrast, rocket thrusters are shaped close to a cone – the exact shape is called a “[de Laval nozzle](https://en.wikipedia.org/wiki/De_Laval_nozzle)” – and are made specifically to turn as much energy as possible from burning to thrust.
Because you need a nozzle.
So you may have learned in high school that Newton’s third law says if you have a force in one direction you have an equal force in the opposite direction.
So how does this apply to rockets? Well, we have to push against something and in a rocket’s case it pushes against its exhaust. The way it does this is by rapidly burning its fuel and oxidizer in its combustion chamber, now these aren’t at particularly high pressure by themselves but when they ignite the sudden increase in temperature boosts their pressure to immense levels.
So this hot gas pushes against everything, mainly the rocket, and we give it only one way out, out through the nozzle. So it pushes against everything inside the combustion chamber, nozzle, and eventually pushes itself out, and pushes the rocket away.
Now your butane lighter has no nozzle, this is actually by design cause otherwise you would need to somehow provide it oxygen, but no nozzle means that energy of that high pressure expansion pushes against the air instead of against the lighter.
Let’s start with that butane lighter; it does exert a force, it’s just miniscule, and unrelated to the flame. The body contains butane under pressure, so when you open the valve you allow atmospheric pressure and the lighter to equalize. Recall that each action causes an opposite reaction, and depending on the size of the valve a force pushing the body of the lighter down compared to the direction of the vale is created.
Now for the rocket, it also contains pressurized fuel (and oxidizer), but the analogy breaks down, because that’s not what causes the (bulk of the) lifting force. In the rocket engine, where fuel and oxidizer meet in a specific ratio and are ignited a huge volume of hot gas is created. Again, equal and opposite reaction, the gas wants to escape because the atmosphere has relatively lower pressure and by the shape of the exhaust it is directed downwards causing an opposite reaction on the rocket lifting it upwards.
A butane lighter IS exerting a force, it’s just the force is going everywhere, left/right/back towards your hand. It’s just that the force is so small you don’t notice it.
A rocket uses a nozzle to make sure as much of the force goes out the back as possible, and as little goes to the side as possible.
It’s also burning a lot faster. A model rocket and burn through its tiny motor in seconds. Your butane lighter will last quite a few minutes.
If you throw a rock, it will push back on your hand.
If you throw lots of rocks really fast, it could push you across the room.
If you have a firehose that shoots molten lava, it could lift you into the air.
If the force of the stream of lava is greater than the weight of the tank of lava strapped to your back, then it could lift you into space.
Stand on a very icy flat frozen lake, carrying something heavy. Throw the heavy thing. You will feel yourself being slid back a bit and your feet will slide back a bit. The harder you can throw it the more you will be flung back. Most of the time we subconsciously plant our feet hard when throwing something, so we don’t notice this effect. But do it somewhere where you have really low friction, like on ice, and you can’t plant your feet hard, then you notice it.
If ice isn’t available, then try floating in a still lake on a rowboat and doing it. take along a supply of rocks, get the boat perfectly still as best you can, and then start throwing the rocks in one direction. You will feel yourself get pushed back the other way (probably rotating the boat a bit as you throw.)
This is also the reason that firing a gun makes the gun kick back. Throw the bullet fast one way, and that pushes the gun back the opposite way.
The hot gasses coming out the back of a rocket is like those thrown rocks or the bullet of a gun. You’re throwing mass one way, causing the platform you’re throwing it from (the rocket) to get shoved the opposite way. The faster you can throw the mass, the more push you will get in the opposite direction for doing so.
The only purpose behind the fiery hotness of the gasses coming out of the rocket is that making a controlled explosion that is constrained in all directions but one is the best way we know of to accelerate a thing up to a high speed very fast. The high speed at which the fuel gets flung is the ends. Exploding the fuel is just the means to that ends.
There is a chamber in the rocket with a hole at one end. High pressure gas, higher pressure than that of the outside of the rocket, pushes in all directions as it expands. Because there is a hole in the back of the rocket, there is no pressure on that end, only on the end near the front of the rocket. Therefore, a net forwards force happens.
It’s no different from a party balloon. High pressure inside a chamber that has a hole at one end and is covered on the other end. The only difference is that the rocket has fuel on board to refill the chamber with high pressure gas, since so much of it is escaping from the hole.
Think of the size difference. That butane lighter is indeed exerting force, just not very much.
Meanwhile those rocket engines are probably the equivalent to a few million butane lighters all going off a once, there’s definitely going to create a force, and rocket fuel is a *tad* more energy dense than something like butane.
Every reaction has an equal and opposite reaction. The easiest ELI5 is to think of it like pushing against a wall. Stand in front of a wall and using one hand, push the wall. The wall doesn’t move, but you get pushed backwards, even though you were exerting force forwards. Opposite reaction, the wall just doesn’t move due to it being strong.
In the simplest explanation, the rocket does the same thing. It’s pushing *hard* downwards, but that also means it’s getting oppositely pushed upwards.
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