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Posted: 1/26/2017 7:59:49 PM EDT

I've been trying to figure this out and just can't seem to do so. My question is, If you were on a rocket in space and had an unlimited fuel supply would the rocket only reach the speed of the exhaust gas from the rocket engine or would it be able to accelerate past that speed due to no gravity or air resistance to slow its forward motion. My thought is that it would be able to keep on accelerating as long as you had fuel. I know crazy question but in a vacuum and no gravity things behave differently than they do here on earth.

Posted: 1/26/2017 8:17:42 PM EDT

[#1]

Quoted:
I've been trying to figure this out and just can't seem to do so. My question is, If you were on a rocket in space and had an unlimited fuel supply would the rocket only reach the speed of the exhaust gas from the rocket engine or would it be able to accelerate past that speed due to no gravity or air resistance to slow its forward motion. My thought is that it would be able to keep on accelerating as long as you had fuel. I know crazy question but in a vacuum and no gravity things behave differently than they do here on earth.

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Once it runs out of fuel it no longer will accelerate. Gravitation effects withstanding. Given a bazillion years it would slow down because empty space still has atoms and they will still present drag on the ship. For simple purposes as part of your thought experiment that drag really does not matter.

Runs out of fuel, maintains vector and speed unless acted on by an outside force. Newton figured this one out.

An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Posted: 1/26/2017 9:01:51 PM EDT

[#2]

Quoted:
I've been trying to figure this out and just can't seem to do so. My question is, If you were on a rocket in space and had an unlimited fuel supply would the rocket only reach the speed of the exhaust gas from the rocket engine or would it be able to accelerate past that speed due to no gravity or air resistance to slow its forward motion. My thought is that it would be able to keep on accelerating as long as you had fuel. I know crazy question but in a vacuum and no gravity things behave differently than they do here on earth.

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Given an unlimited fuel supply, over sufficient time, it will approach, but never reach, C.

Exhaust velocity determines the amount of thrust the engine provides, it does not set a speed limit - it will still produce that amount of thrust even when the rocket is moving .999C, but due to relativistic mass effects, that same thrust will produce less and less acceleration the closer the rocket gets to C.

Posted: 1/26/2017 9:16:58 PM EDT

[#3]

A rocket that has an unlimited fuel supply would also have an unlimited mass, which would mean zero acceleration no matter how much thrust was applied.

Posted: 1/26/2017 10:37:34 PM EDT

[#4]

f = m x a

or

As long as there is force, there is acceleration.

This is the simple rigid body Newtonian answer.

Never really could grasp the relativity theories, but it's pretty much an 'all bets are off' as you approach 'c'.

Posted: 1/26/2017 10:43:36 PM EDT

[#5]

Relative to which point of reference? If you are on the rocket, your perception will be constant and continuous acceleration. You will appear to travel at speeds faster than light. That is, you know that light takes one year to get from point A to B, but you could get there sooner than that. Actually what you perceive if you measure the speed of light in your rocket, is that the distance somehow got a whole lot closer.

However, from the point of reference from where you left, you will gradually reduce acceleration to a point where you approach the speed of light.

Posted: 1/27/2017 12:38:41 AM EDT

[#6]

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Quoted:
A rocket that has an unlimited fuel supply would also have an unlimited mass, which would mean zero acceleration no matter how much thrust was applied.

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He didn't posit the fuel was stored onboard and formed part of the mass of the rocket - could be a ramscoop, could be magic beans, etc - just that the fuel supply was unlimited.

Posted: 1/27/2017 8:05:08 AM EDT

[#7]

Thanks for your reply to my question. So what I figured was right you would continue to accelerate as long as you had fuel to the point of C. I find the thought of space travel amazing in that even traveling at the speed of light is not fast enough to travel to places we can see in the universe.

Posted: 1/27/2017 1:43:00 PM EDT

[#8]

Quote History

Quoted:
Thanks for your reply to my question. So what I figured was right you would continue to accelerate as long as you had fuel to the point of C. I find the thought of space travel amazing in that even traveling at the speed of light is not fast enough to travel to places we can see in the universe.

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Some of that light has taken thousands of years to reach us.

At the speed of light.

Posted: 1/27/2017 8:55:25 PM EDT

[#9]

The vastness of the universe is hard to wrap my mind around. I once read about a star that was so huge that if you were in an airplane flying at 900 MPH it would take you 1200 years to circle it once and they said that there are bigger ones than that. I don't know how they figure this stuff out it is beyond my ability to do so. MC

Posted: 1/28/2017 9:50:05 PM EDT

[#10]

This concept of rocket exhaust needing high speed is wrong.

It is a simple momentum problem.

The momentum of the ejected material creates an acceleration on the motor.

The only advantage to higher speed is that it creates more momentum. m * v.

We actually have maneuvering motors that use electric charge to push mass away.

One of the problems that must be dealt with is making sure the vehicle does not build up a charge that attracts back the ejected material.

And many instruments on satellites will not operate correctly if the vehicle has ANY residual charge.

It is hard to measure particles in space when you satellite is already charged up.

Posted: 1/29/2017 9:20:49 AM EDT

[#11]

Quote History

Quoted:
This concept of rocket exhaust needing high speed is wrong.

It is a simple momentum problem.

The momentum of the ejected material creates an acceleration on the motor.

The only advantage to higher speed is that it creates more momentum. m * v.

We actually have maneuvering motors that use electric charge to push mass away.

One of the problems that must be dealt with is making sure the vehicle does not build up a charge that attracts back the ejected material.

And many instruments on satellites will not operate correctly if the vehicle has ANY residual charge.

It is hard to measure particles in space when you satellite is already charged up.

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The advantage to higher speed exhaust is one of the driving factors of rockets. Since you are limited by the amount of mass of fuel you have, the higher the exhaust velocity, the more momentum change (usually referred to as delta V) you can get. For a given mass, you also get a higher thrust by exhausting it at higher velocity. One of electric propulsion's (plasma thruster) greatest advantages is that its exhaust is at a very high velocity, reducing the required mass of fuel for a given amount of performance. A disadvantage of electric propulsion is that the power required generally results in low thrust, which can be used for satellite orientation control and station-keeping, but not for launching from the surface or other high-thrust requirements.

Mike

Posted: 2/4/2017 1:13:20 PM EDT

[#12]

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Quoted:

The advantage to higher speed exhaust is one of the driving factors of rockets. Since you are limited by the amount of mass of fuel you have, the higher the exhaust velocity, the more momentum change (usually referred to as delta V) you can get. For a given mass, you also get a higher thrust by exhausting it at higher velocity. One of electric propulsion's (plasma thruster) greatest advantages is that its exhaust is at a very high velocity, reducing the required mass of fuel for a given amount of performance. A disadvantage of electric propulsion is that the power required generally results in low thrust, which can be used for satellite orientation control and station-keeping, but not for launching from the surface or other high-thrust requirements.

Mike

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Strictly speaking, momentum change is delta P. In most problems, momentum change is essentially equivalent to velocity change, but rockets deal with changing mass since they are burning their fuel.

Posted: 2/4/2017 8:19:23 PM EDT

[#13]

I always think this is interesting. You hear people say even if you can travel at the speed of light it will take however many years. Not for you since the closer to c the less quickly time passes. Is it a linear change? 50% speed of c then age 1/2 as fast?

Posted: 2/4/2017 8:25:47 PM EDT

[#14]

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Quoted:
I always think this is interesting. You hear people say even if you can travel at the speed of light it will take however many years. Not for you since the closer to c the less quickly time passes. Is it a linear change? 50% speed of c then age 1/2 as fast?

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It is not linear at all. The ratio is called the Lorentz Factor.

Posted: 2/4/2017 9:14:54 PM EDT

[#15]

Neat

Posted: 2/4/2017 10:36:32 PM EDT

[#16]

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Quoted:

It is not linear at all. The ratio is called the Lorentz Factor.

https://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Lorentz_factor.svg/591px-Lorentz_factor.svg.png

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Quoted:

Quoted:
I always think this is interesting. You hear people say even if you can travel at the speed of light it will take however many years. Not for you since the closer to c the less quickly time passes. Is it a linear change? 50% speed of c then age 1/2 as fast?

It is not linear at all. The ratio is called the Lorentz Factor.

https://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Lorentz_factor.svg/591px-Lorentz_factor.svg.png

And the Lorentz Contraction also describes the additional energy that is required.

We could show it up in a lab setup with a CRT and the spot brightness on the screen.

You started cranking up the acceleration voltage and measured light intensity off the screen.

It was OK for a while, but all of a sudden it took a HUGE increase in acceleration voltage to make a tiny change in light intensity.

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