If you take a rocket in deep space far from any planet, if you fire the rocket engine, would a rocket continuously accelerate tille burn out or would it end accelerating when its forward speed equals the speed the hot gases leave the nozzle of your rocket engine? thanks
Andrew - In fact, it would just carry on accelerating forever until it ran out of fuel because the reason that it’s accelerating is not so much to do with measuring particular speeds. It’s more just to do with the fact that it’s throwing out lots of mass out of its rear end, to put it nicely, and when it does that, it feels a little push. This is one of Newton’s laws – every force must have an equal and opposite force. It receives that equal and opposite force, and as a result, it accelerates.
Chris - This reminds me of a question we had in the Naked Scientists awhile back which was, how hard would you have to pee to push yourself over. I think the answer we’ve worked out was, you would have to be able to pee and produce a fountain, more than 20 metres high in order to achieve sufficient force that would have any kind of backward propulsion, assuming a modestly weighted man. Similar physics, Different situation.
Tomcat asked the Naked Scientists: Question to the rocket scientists: If you take a rocket in deep space far from any planet, if you fire the rocket engine, would a rocket continuously accelerate tille burn out or would it end accelerating when its forward speed equals the speed the hot gases leave the nozzle of your rocket engine? thanks What do you think? Tomcat, Tue, 11th May 2010
It would keep accelerating. Assuming the rocket exhaust velocity is the same relative to the rocket this is independent of the rocket's speed relative to a stationary observer. Even when the rocket has accelerated to the point where the rocket gases are also moving in the same direction as the rocket relative to the observer, the rocket will still be accelerating. graham.d, Tue, 11th May 2010
When a rocket accelerates it also accelerates its remaining fuel, this means that the speed the rocket can push the exhaust gasses out is relative to the rocket, not any notional stationary reference frame, so it will keep accelerating. It is also why you need such a big rocket to go any distance - the rocket has to lift the fuel to lift the fuel to lift the fuel to lift the payload.
Yes the difference is that the reaction mass for the jet remains 'stationary' where as in a rocket you keep accelerating it. So effectively if there is a maximum speed the jet can take air through its tube, and the jet certainly can't go any faster than this.
That's how I see it too, the fuel is stationary versus its chamber and when it expands it will 'push on that said chamber until it finds it's way out where the resistance is least.As for the difference between a jet and and a rocket I would expect it to be gravity, fuel efficiency, engine construction and air resistance limiting a jet, as compared to a rocket? The principle is otherwise much the same, isn't it? yor_on, Wed, 30th Jun 2010
The jet engine is similar to the rocket, in that the hot gas is expanded out the back to provide the thrust to move the unit forward.
Jets and rockets both produce thrust by accelerating mass. You can either accelerate a small mass by a large amount, or a larger mass by a smaller amount of acceleration to produce the desired amount of thrust.
The equation for net airbreathing jet thrust is: F = m_dot * (c-v)
Yeah I forgot that one, the rocket brings it's own oxygen with it bound in it's fuel while ordinary jets take it from the air around it :)
UHB turbines were retrofitted to early jets as soon as they became viable and had enough life history to prove reliability. They are quieter than straight jet engines, and consume less fuel per pound of thrust than the equivalent jet engine. As the drive engine is smaller and lighter it has lower maintenance costs, as well as being cheaper to overhaul. The only drawback is that the engines are fatter and have less ground clearance on underwing pods. The main driver was lower fuel consumption and noise during takeoff, a good thing to all living near airports.
You know that bit in qi where it goes whoop-whoop, well... you fell into the trap- rockets have no top speed, and their generated power is simply proportional to speed!
Well, yeah... but...
Well, yeah... but...
A rocket in orbit does not need to do any work (well, hardly any) to travel at constant velocity in orbit.
Whenever a rocket is being accelerated work is being done. wolfekeeper, Mon, 5th Jul 2010
Not if that acceleration is to keep it in a circular path at a constant speed. Bored chemist, Mon, 5th Jul 2010
Ok, you have a point about that at least, but here we're talking about rockets accelerating under their own power.
Hint: there's a hidden source of energy, other than chemical energy of the fuel. wolfekeeper, Tue, 6th Jul 2010
Actually energy is really important in rocketry.
Anyway, I guess you've given up.
Ah! Right. As a in f=ma is the amount of acceleration applied to the exhaust relative to the rocket, would f=ma not take the kinetic energy of the propellant into account? Geezer, Wed, 7th Jul 2010
No, it doesn't tell you where the energy ends up. You need to look at the speeds to calculate the kinetic energy, F=ma only gives you the rate of change of speed.
Pretty much, but only if you know the speeds at some point (and if the accelerations are known to sufficient accuracy).
ah, the fuel?
Okay thanks Wolfe, I'm not sure I get it still, the difference between chemical and kinetic energy I mean. You mean that it have a 'potential' kinetic energy before getting burned, well, as I think about it, although I won't promise I do, think, that is :)
In physics, you always pick a frame of reference. It's usually a good idea to pick the centre of mass frame or the 'zero momentum frame' which is usually the same thing.
Thanks, i have a distinct feeling I will need to come back to this one and reread it all :) I should get some sleep first too I think. "In rockets the best frame is usually the one that the rocket takes off from." And that would be the launch pad in this case, right? I really liked the sound of the 'zero momentum frame', I mean it, it made me think :) And that's a hard thing for me.