How does a Rocket moves in the space?

[ihpalash]

How does a rocket moves in the space? By newton's third law of physics a force must be used to boost the rocket. In the earth it can be easily done because the rocket;s fuel use force to move on. But in the space there's no air or gas or anything.

So by this law, how does a rocket moves in the space?

[swansont]

How does a rocket moves in the space? By newton's third law of physics a force must be used to boost the rocket. In the earth it can be easily done because the rocket;s fuel use force to move on. But in the space there's no air or gas or anything.

So by this law, how does a rocket moves in the space?

 

The rocket pushes hot gas out of the back of it. It exerts a force on this gas, therefore the gas exerts an equal and opposite force on the rocket.

 

IOW, momentum will be conserved. If you are on a frictionless surface and toss a brick away from you, you will recoil in the opposite direction.

[Greg H.]

How does a rocket moves in the space? By newton's third law of physics a force must be used to boost the rocket. In the earth it can be easily done because the rocket;s fuel use force to move on. But in the space there's no air or gas or anything.

So by this law, how does a rocket moves in the space?

Actually, a rocket doesn't push against the air - which is a good thing, otherwise they wouldn't be able to move in space at all. The only thing the rocket is pushing is it's own exhaust, as swansont points out. Because of the conservation law, the exhaust must push against the rocket with the same force - viola, lift off.

 

You can see the same sort of effect if you watch a group of firemen manning a hose, especially if they let go, such as in this video. The hose is being forced around by the "exhaust" of the water jetting out of the end of it.

 

Edited March 28, 2015 by Greg H.

[Delta1212]

It's also important to note that you only need to push on something to start moving. Once you're under way, you'll just keep going in whatever direction you're already moving at whatever speed you're traveling at until you hit something. On Earth, most things stop after a short while because they hit the air and/or ground, and it slows them down until they're no longer moving (with respect to the Earth.

 

In space, there's not a lot getting in your way, so you'll just keep heading off in whatever direction you're traveling in indefinitely, without needing to do anything or expend any energy unless you want to change speed or direction.

[CasualKilla]

Is there a maximum speed for rockets? I have a recurring argument with one of my friends who claims that the speed of which particles leave the rocket limits the maximum speed. My reasoning is that the thrust particles will always the leave rocket at a backwards velocity relative to the rocket, thus one can achieve acceleration at all speed less than c.

[swansont]

Is there a maximum speed for rockets? I have a recurring argument with one of my friends who claims that the speed of which particles leave the rocket limits the maximum speed. My reasoning is that the thrust particles will always the leave rocket at a backwards velocity relative to the rocket, thus one can achieve acceleration at all speed less than c.

 

You can analyze the system in the rest frame of the rocket. What does that tell you?

[studiot]

Another way to analyse this is to consider a momentum balance.

 

The rocket plus its fuel starts off with zero momentum.

 

As the fuel is burnt the exhaust is expelled backwards and the rocket moves forwards.

 

The momentum of the backward moving exhaust equals the momentum of the forward moving rocket.

 

This balance allow you to calculate the maximum speed attainable for a given fuel load, or how far the rocket can go before it has to turn round if it is going to be able to return.

Edited March 28, 2015 by studiot

[Delta1212]

Is there a maximum speed for rockets? I have a recurring argument with one of my friends who claims that the speed of which particles leave the rocket limits the maximum speed. My reasoning is that the thrust particles will always the leave rocket at a backwards velocity relative to the rocket, thus one can achieve acceleration at all speed less than c.

If I'm interpreting your argument correctly, you are right. There is no speed at which a rocket can travel so that it could not move any faster, regardless of how fast its exhaust is moving.

[Enthalpy]

Launchers are meant for, and often achieve it, fly faster than they expal gas. The best exhaust speed for chemical rockets in service is 4560m/s, but satellites orbit at 7800m/s on the low orbits. Which implies that the rocket continues to accelerate even if the gas it expels moves forward versus the Earth.

 

Which is an excellent thing for the validity of our physics theories, because the rocket should accelerate independently of who observes it, whether he's on Earth or he's flying near the rocket, as for instance astronauts do aboard the space station when a capsule or cargo module joins or leaves them.

[Robittybob1]

Is there a maximum speed for rockets? I have a recurring argument with one of my friends who claims that the speed of which particles leave the rocket limits the maximum speed. My reasoning is that the thrust particles will always the leave rocket at a backwards velocity relative to the rocket, thus one can achieve acceleration at all speed less than c.

There is only limited energy in the chemical reaction so I think you'll find a rocket's efficiency drops off once the exhaust is still blown out backwards and yet it still has forward motion compared to the Earth's rest frame.

http://en.wikipedia.org/wiki/Rocket#Energy_efficiency

 

However, as speeds rise, the resultant exhaust speed goes down, and the overall vehicle energetic efficiency rises, reaching a peak of around 100% of the engine efficiency when the vehicle is travelling exactly at the same speed that the exhaust is emitted. In this case the exhaust would ideally stop dead in space behind the moving vehicle, taking away zero energy, and from conservation of energy, all the energy would end up in the vehicle. The efficiency then drops off again at even higher speeds as the exhaust ends up travelling forwards- trailing behind the vehicle.

[Enthalpy]

Efficiency is a tricky idea for rockets... One would need to define a useful work (relative to what object?), a costly energy - and at the end get a figure not very useful or even misleading.

Efficiency is, as a consequence, never used in space travel. People think in terms of speed - relative to that object, including this gravitation energy, etc.

[CasualKilla]

 

You can analyze the system in the rest frame of the rocket. What does that tell you?

Yes that was my argument right there, just wanting to confirm.

 

There is only limited energy in the chemical reaction so I think you'll find a rocket's efficiency drops off once the exhaust is still blown out backwards and yet it still has forward motion compared to the Earth's rest frame.

http://en.wikipedia.org/wiki/Rocket#Energy_efficiency

Thank you, that probably explains where he got the idea from. Interesting read.

Another way to analyse this is to consider a momentum balance.

 

The rocket plus its fuel starts off with zero momentum.

 

As the fuel is burnt the exhaust is expelled backwards and the rocket moves forwards.

 

The momentum of the backward moving exhaust equals the momentum of the forward moving rocket.

 

This balance allow you to calculate the maximum speed attainable for a given fuel load, or how far the rocket can go before it has to turn round if it is going to be able to return.

Rocket science, nice!

Efficiency is a tricky idea for rockets... One would need to define a useful work (relative to what object?), a costly energy - and at the end get a figure not very useful or even misleading.

Efficiency is, as a consequence, never used in space travel. People think in terms of speed - relative to that object, including this gravitation energy, etc.

Yes, it seems efficiency would need to be defined relative to some other object. Which seems rather strange. I guess takeoff location makes the most sense?

I guess it may be better defined as efficiency towards object A, U is the speed object A is moving towards the rocket, and c is the exhaust speed relative to the rocket?

Edited March 30, 2015 by CasualKilla

[Enthalpy]

The takeoff location isn't very fruitful to measure a gain and an efficiency. A first improvement would account for the gravitational energy in addition to the kinetic one, but even that brings few benefits and big drawbacks.

 

Take a probe heading to the Moon: as Earth rotates, once a day the takeoff location approaches the Moon and once it goes away. Then the probe's kinetic energy (plus gravitational) changes versus the takeoff location without any further propulsive action, and so does the efficiency. And when the probe is nears the Moon enough that it feels its attraction, the probe's kinetic plus gavitational energy changes versus the start location too, without propulsive effort.

 

Or a probe to Jupiter. Once a year, Earth's orbital speed adds to the probe's speed, the other half-year it subtratcs. Again a fluctuation in energy and efficiency.

 

We could choose differently the origin of the speed measure, but every time it would bring bizarre results and interpretations. Earth's centre, Sun's centre... avoid some drawbacks but not always, say if a probe orbits the Moon.

 

As a consequence, mission planners are not interested in efficiency. They look primarily in speed variations, because this is what costs fuel, with a direct relation when the speed increment is measured when the thrust is made.

 

Though, energy is conserved, not speed... if measured properly and defining cleanly what it includes and versus what! So while only speed costs, the effect of a push radically differs, depending on when it is applied. To make it simple, a speed increment brings a bigger benefit if it applies when external factors result in a big speed already - since the kinetic energy depends on the square of the speed - like near a planet or moon that has accelerated the probe or satellite. It's the point of the Oberth effect (Wiki).

 

The Oberth effect and the slingshot effect, or gravitational assistance (Wiki) make a huge difference in both the attained speed and energy. Missions to our solar system are possible only thanks to them with chemical propulsion. But since the result differs so much from the same expense, mission planners avoid the idea of efficiency. They compare speed increments instead, and not even again an absolute or objective value, which doesn't exist - they only compare different scenarios.

 

It's an interesting domain. Accessible to simple math if we simplify, but extremely misleading.

[studiot]

All of the stuff that Enthalpy and others are quoting are interesting but due to external agents, not the mechanism of rocket action.

 

If this question was about how does a rocket work then you need to discount external factors and agents.

 

If the question can be widened to include how to best use that rocket action then rock on.