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Author Topic: Will rockets accelerate until they run out of fuel?  (Read 14567 times)

Tomcat

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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?
« Last Edit: 11/05/2010 14:30:02 by _system »


 

Offline graham.d

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Will rockets accelerate until they run out of fuel?
« Reply #1 on: 11/05/2010 15:34:02 »
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.
 

Offline daveshorts

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Will rockets accelerate until they run out of fuel?
« Reply #2 on: 12/05/2010 10:50:50 »
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.

A jet engine on the other hand takes in air which is 'stationary' and pushes it out the back, so a jet plane can't go faster than it can push the gasses out of the back.
 

Offline Geezer

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Will rockets accelerate until they run out of fuel?
« Reply #3 on: 12/05/2010 17:35:47 »
A jet engine on the other hand takes in air which is 'stationary' and pushes it out the back, so a jet plane can't go faster than it can push the gasses out of the back.

I'm not sure there is much difference between a rocket and a jet Dave. Don't they both develop thrust my accelerating the exhaust gas? After that, the thrust is simply a function of the mass of the exhaust times the acceleration.

With bypass jets, some of the thrust is produced by air that was accelerated around the engine.
 

Offline imatfaal

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Will rockets accelerate until they run out of fuel?
« Reply #4 on: 12/05/2010 18:10:50 »

I'm not sure there is much difference between a rocket and a jet Dave. Don't they both develop thrust my accelerating the exhaust gas? After that, the thrust is simply a function of the mass of the exhaust times the acceleration.


Isnt the case that in practical term jets reach a point where they can no longer make an appreciable difference between the speed of the air going in and the speed coming out.  I always thought that this was the idea of the afterburner; which is a bit like a rocket.

matthew
 

Offline daveshorts

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Will rockets accelerate until they run out of fuel?
« Reply #5 on: 12/05/2010 19:05:42 »
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.

So if a jet plane is going near to the speed the air is coming out of its jet, the amount it accelerates the air is less, hence less force. This is why jet engines are different shapes for different speeds of aircraft. Airliners have big short fat engines which push a lot of air backwards realtively slowly (an efficient process but with a low maximum speed, where as fighter jets push less air out much faster which is less efficient but has a higher maximum speed.

The afterburner is effectively dumping fuel into the back end of the jet, and burning with air that has bypassed the burners and has oxygen left in it. This gets hot and expands, increasing the exit speed of the air. This is particularly useful as noone has got a practical jet engine where the air flows through supersonically.

 

Offline peppercorn

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Will rockets accelerate until they run out of fuel?
« Reply #6 on: 12/05/2010 21:57:27 »
This gets hot and expands, increasing the exit speed of the air. This is particularly useful as noone has got a practical jet engine where the air flows through supersonically.

http://en.wikipedia.org/wiki/Scramjet
- may be practical one day, huh?...
 

Offline Geezer

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Will rockets accelerate until they run out of fuel?
« Reply #7 on: 12/05/2010 23:47:33 »

So if a jet plane is going near to the speed the air is coming out of its jet, the amount it accelerates the air is less, hence less force.


Well, yes. If it's not accelerating the exhaust much, it's not going to develop much thrust. But the limitation of a jet does not lie in its inability to accelerate mass, I think it lies more in its inability to inhale sufficient oxygen to oxidize the fuel. Hence, as Peppercorn points out, the Scram jet.

Rockets solve this problem by schlepping the oxidizer with them.
 

Offline thedoc

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Will rockets accelerate until they run out of fuel?
« Reply #8 on: 29/06/2010 20:34:11 »
We discussed this question on our  show

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.

Click to visit the show page for the podcast in which this question is answered. Alternatively, listen to the answer now or [download as MP3]
« Last Edit: 29/06/2010 20:38:25 by BenV »
 

Offline yor_on

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Will rockets accelerate until they run out of fuel?
« Reply #9 on: 30/06/2010 07:04:13 »
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?
 

Offline SeanB

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Will rockets accelerate until they run out of fuel?
« Reply #10 on: 30/06/2010 21:32:42 »
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.

You very rarely find a pure jet engine nowadays, most commercial airlines use what is called an ultra high bypass turbine, which is a form of ducted fan. This has the advantage of being a lot quieter and using a lot less fuel, as the fan is driven by an engine ( a jet engine inside that provides power to turn a shaft more than thrust for the plane) providing a massive amount of air moving relatively slowly.

The rocket engine has a high pressure gas ( either hot from combustion or cold for low power thrusters) expanding in an open ended chamber. This provides reaction via Newtons law, and is somewhat different from a plane.

The major difference is thae jet takes in air, slows it down and compresses it then adds energy via the fuel to get a higher volume of gas than what entered. This is limited by having to slow the inlet air to below supersonic speed, even though the aircraft is supersonis. Scramjets burn supersonically, but have a small issue in that they cannot start from stationary, and need to be running at well over the speed of sound before they do anything other than burn fuel. Rockets have none of these issues, just needing an almighty amount of fuel per kilogram of payload to perform.

 

Offline Geezer

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Will rockets accelerate until they run out of fuel?
« Reply #11 on: 30/06/2010 23:10:31 »
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.

As Sean says, modern commercial jets opt for the latter approach for several reasons. I suspect this method is also more efficient thermodynamically.
 

Offline wolfekeeper

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Will rockets accelerate until they run out of fuel?
« Reply #12 on: 01/07/2010 14:07:17 »
The equation for net airbreathing jet thrust is: F = m_dot * (c-v)

where v is the speed of the aircraft, c is the exhaust speed, and m_dot is the mass of air going through the engine per second. As you can see if you put c=v, you've get no net thrust.  If v > c then you're slowing down (this is actually used that's what a lot of jet 'reverser' cups do- they throw the air sideways, which has the same effect as setting c to 0).

For rockets the equation is just F = m_dot * c, and it's independent of the rockets mass and speed.

One weird thing about rockets; if you calculate the power the rocket generates- it *increases* with time/speed, even though the thrust is the same (energy is force x distance, and distance per second is increasing), and the propellant flow rate is constant. ??? ;D

Puzzle: explain why this is!
 

Offline yor_on

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Will rockets accelerate until they run out of fuel?
« Reply #13 on: 03/07/2010 09:31:34 »
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 :)
Nice explanation Sean, didn't know about ultra high bypass turbines.
 

Offline Geezer

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Will rockets accelerate until they run out of fuel?
« Reply #14 on: 04/07/2010 01:25:13 »
One weird thing about rockets; if you calculate the power the rocket generates- it *increases* with time/speed, even though the thrust is the same (energy is force x distance, and distance per second is increasing), and the propellant flow rate is constant. ??? ;D

Puzzle: explain why this is!

Power is work done in time. Work is force times distance. The greater the distance in time, the greater the power.

A stationary aircraft might be generating a lot of force (thrust), but if it's not moving, it's not producing power (although it might be consuming a lot of energy).





 

Offline Geezer

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Will rockets accelerate until they run out of fuel?
« Reply #15 on: 04/07/2010 01:27:13 »
The equation for net airbreathing jet thrust is: F = m_dot * (c-v)

where v is the speed of the aircraft, c is the exhaust speed, and m_dot is the mass of air going through the engine per second. As you can see if you put c=v, you've get no net thrust.  If v > c then you're slowing down (this is actually used that's what a lot of jet 'reverser' cups do- they throw the air sideways, which has the same effect as setting c to 0).

For rockets the equation is just F = m_dot * c, and it's independent of the rockets mass and speed.


Is this really any different from Force = mass x acceleration?
 

Offline SeanB

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Will rockets accelerate until they run out of fuel?
« Reply #16 on: 04/07/2010 14:43:54 »
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.

 

Offline Geezer

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Will rockets accelerate until they run out of fuel?
« Reply #17 on: 04/07/2010 16:59:04 »
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.



I thought the UHB turbine was the propfan which is actually unducted.

http://en.wikipedia.org/wiki/Propfan
 

Offline wolfekeeper

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Will rockets accelerate until they run out of fuel?
« Reply #18 on: 05/07/2010 00:19:29 »
One weird thing about rockets; if you calculate the power the rocket generates- it *increases* with time/speed, even though the thrust is the same (energy is force x distance, and distance per second is increasing), and the propellant flow rate is constant. ??? ;D

Puzzle: explain why this is!

Power is work done in time. Work is force times distance. The greater the distance in time, the greater the power.
Yes, but the faster the rocket goes, the more power is being produced, but the chemical energy in the rocket fuel-it is burning it at a constant rate!!!! Where does the extra power come from?

That's the puzzle.  ;D
« Last Edit: 05/07/2010 00:23:46 by wolfekeeper »
 

Offline wolfekeeper

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Will rockets accelerate until they run out of fuel?
« Reply #19 on: 05/07/2010 00:22:50 »
For rockets the equation is just F = m_dot * c, and it's independent of the rockets mass and speed.
Is this really any different from Force = mass x acceleration?
Well, technically it's

F = d/dt (mv)

Where mv is the momentum of the exhaust.

Which is slightly different. The f = ma only applies when m is constant.
 

Offline Geezer

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Will rockets accelerate until they run out of fuel?
« Reply #20 on: 05/07/2010 01:02:10 »
For rockets the equation is just F = m_dot * c, and it's independent of the rockets mass and speed.
Is this really any different from Force = mass x acceleration?
Well, technically it's

F = d/dt (mv)

Where mv is the momentum of the exhaust.

Which is slightly different. The f = ma only applies when m is constant.

At constant aircraft velocity I suppose the mass flow rate would be constant. However, F = ma is not so helpful when the aircraft is accelerating because the accelerated mass is a function of the aircraft's speed.

Hmmm...Rockets are a bit simpler. I wonder why they don't say "it's not jet engine science"?
 

Offline wolfekeeper

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Will rockets accelerate until they run out of fuel?
« Reply #21 on: 05/07/2010 01:30:41 »
At constant aircraft velocity I suppose the mass flow rate would be constant.
Actually it would be going down, because the weight of the fuel is going down, which requires less lift, which in turn means that the aircraft generates less drag, which in turn requires less mass flow rate.

Or I suppose you could gain altitude and keep the mass flow rate constant.

Quote
Hmmm...Rockets are a bit simpler. I wonder why they don't say "it's not jet engine science"?
Say that after you've solved the puzzle!

Where does the extra power come from?
 

Offline Geezer

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Will rockets accelerate until they run out of fuel?
« Reply #22 on: 05/07/2010 01:46:55 »
Where does the extra power come from?

Oh, well that's easy. If the rate of consuption has not changed, but the power has increased, it must be operating more efficiently which is likely because it does not have to do so much work to compress the air on account of it getting stuffed into the front end, or something like that maybe.
 

Offline wolfekeeper

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Will rockets accelerate until they run out of fuel?
« Reply #23 on: 05/07/2010 01:52:28 »
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!

However much energy is being generated from combusting the fuel, or even in the fuel, at some speed, the rocket is generating more power than that!
 

Offline Geezer

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Will rockets accelerate until they run out of fuel?
« Reply #24 on: 05/07/2010 02:44:08 »
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!

However much energy is being generated from combusting the fuel, or even in the fuel, at some speed, the rocket is generating more power than that!


I thought we were talking about jets!

Rockets simply operate on F = ma.
 

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Will rockets accelerate until they run out of fuel?
« Reply #24 on: 05/07/2010 02:44:08 »

 

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