Fuel Consumption of a Ship Traveling at Near Light Speed

[guss]

Imagine this scenario: An "airplane" that runs on some type of fuel, let's say, liquid, reaches 90% of the speed of light. Once it achieves this speed, will the ship require less fuel due to time dilation, the same amount of fuel due to time dilation, or more fuel to maintain a constant velocity? The reason I use an airplane is obviously to just supply the problem with some kind of friction, in this case air resistance, so that fuel needs to be consumed to maintain that constant velocity.

The reason that less fuel could be consumed is because of time dilation. Time would move more slowly on the ship, thus (maybe) making less fuel contribute to more thrust.

Anyone have an answer?

 

[cragar]

it would need more fuel because the plane would be more massive at that speed.
And that is also why is is hard to accelerate particles near the speed of light in a particle accelerator.

 

[Drakkith]

Anything traveling that fast NOT in an entirely empty section of space would be destroyed due to friction.

 

[guss]

Alright, I'll change the problem. We are in empty space, and the ship is accelerating contantly.

Note how I'm just asking about if more or less fuel would be required JUST due to time dilation. If less fuel were required, I would assume that the increased mass would increase the force required, thus making the same amount of fuel required.

 

[cragar]

Not to nit pick but if your out in empty space you would need to take your own oxygen,
Maybe you could have some nuclear powered rocket , i don't know if that would work tho

 

[Vanadium 50]

This is a poorly posed question. Less fuel than what? Than required to maintain zero velocity?

 

[Lsos]

it would need more fuel because the plane would be more massive at that speed.
And that is also why is is hard to accelerate particles near the speed of light in a particle accelerator.

If it's going at constant speed, why would mass make a difference?

Anyway, I think OP understands that obviously the faster we go through air, the more fuel we need. But is this need for fuel related to speed by a simple function related to speed, or does, at some ridiculous speed, time dilation come into account, perhaps reducing the expected fuel required?

I think OP also clearly understands that conditions at sea level on Earth would not allow near-light speed travel. Let's Reduce the air density/ friction as necessary to make the problem work...

 

[cragar]

I thought he wanted to keep accelerating to c. But if he's out in free space it wouldn't require any fuel to keep at that speed.

 

[Lsos]

He's not out in free space, he's traveling through a gas. I don't know how thick, but it's there and it's producing drag.

 

[mender]

The ship continues to use fuel at the same rate according to the people on the ship. Due to time dilation, outside observers would say that the fuel is being at a lower and lower rate as the ship approaches light speed.

 

[JaredJames]

He's not out in free space, he's traveling through a gas. I don't know how thick, but it's there and it's producing drag.

The question was changed to being in "empty space".

To maintain constant speed, you wouldn't need fuel.

You would only need fuel to continue acceleration.

 

[ZapperZ]

Imagine this scenario: An "airplane" that runs on some type of fuel, let's say, liquid, reaches 90% of the speed of light. Once it achieves this speed, will the ship require less fuel due to time dilation, the same amount of fuel due to time dilation, or more fuel to maintain a constant velocity? The reason I use an airplane is obviously to just supply the problem with some kind of friction, in this case air resistance, so that fuel needs to be consumed to maintain that constant velocity.

The reason that less fuel could be consumed is because of time dilation. Time would move more slowly on the ship, thus (maybe) making less fuel contribute to more thrust.

Anyone have an answer?

Alright, I'll change the problem. We are in empty space, and the ship is accelerating contantly.

Note how I'm just asking about if more or less fuel would be required JUST due to time dilation. If less fuel were required, I would assume that the increased mass would increase the force required, thus making the same amount of fuel required.

There is a severe misunderstanding of basic SR here.

What time dilation are you talking about, first of all? There is a lack of clarity on the reference frame being used. In fact, I think there is a total confusion here as I suspect that the reference frame are being switched back and forth. There is no time dilation on the ship with respect to those traveling with the ship! Period! So how would there be any change in fuel consumption with respect to those on the ship? Do you notice any change in your fuel consumption? After all, according to some other galaxy, you are moving quite fast yourself!

There is also no "mass increase" here for the ship itself. In fact, the concept of "relativistic mass" is highly outdated. Even Einstein stopped using it later in life as it is a misleading concept.

Zz.

 

[guss]

There is a severe misunderstanding of basic SR here.

What time dilation are you talking about, first of all? There is a lack of clarity on the reference frame being used. In fact, I think there is a total confusion here as I suspect that the reference frame are being switched back and forth. There is no time dilation on the ship with respect to those traveling with the ship! Period! So how would there be any change in fuel consumption with respect to those on the ship? Do you notice any change in your fuel consumption? After all, according to some other galaxy, you are moving quite fast yourself!

There is also no "mass increase" here for the ship itself. In fact, the concept of "relativistic mass" is highly outdated. Even Einstein stopped using it later in life as it is a misleading concept.

Zz.

I think that you are misunderstanding me. I'm bad at wording these things, sorry. Basically:

Would time dilation have any effect on the fuel consumption of a constantly accelerating ship traveling at near-light speed through empty space?

Reading my other reply above might make sense as well. And I see what you are saying, but wouldn't the same amount of fuel be able to make the ship accelerate for a longer period of time for inside observers, because the ship would go faster and faster even though it looked like the same amount of fuel was being used to them?

So, for example, let's say this ship was traveling at 90% of the speed of light. To outside observers, this ship would be about 32% as fast as the rest of the world, but wouldn't that mean the, let's say "jet" of fuel, would be burning for longer, thus contributing force for a longer period of time than it normally would (about 317%)?

 

[JaredJames]

Would time dilation have any effect on the fuel consumption of an accelerating ship traveling at near-light speed through empty space?

No. The ships fuel consumption wouldn't change.

But it would appear lower to someone not on board the ship (someone stood on Earth).

 

[guss]

No. The ships fuel consumption wouldn't change.

But it would appear lower to someone not on board the ship (someone stood on Earth).

For someone on the ship, though, wouldn't they experience a longer period of thrust per, say gallon of fuel?

 

[JaredJames]

For someone on the ship, though, wouldn't they experience a longer period of thrust per, say gallon of fuel?

No. The people on the ship (and the ship and the engines and the fuel) would experience 'normal time'. The time dilation is only apparent to outside observers within a different reference frame.

The only way they see the effects of time dilation is when they view the 'outside world'. People back on Earth would appear to be experiencing time extremely quickly.

 

[guss]

No. The people on the ship (and the ship and the engines and the fuel) would experience 'normal time'. The time dilation is only apparent to outside observers within a different reference frame.

The only way they see the effects of time dilation is when they view the 'outside world'. People back on Earth would appear to be experiencing time extremely quickly.

I know that, but when a time-dilated force that is acting upon something moving significantly faster than normal, in this case the outside world, I would think something would change. But what you are saying makes sense, and that's what I was thinking. I'm just thinking too hard about it I suppose.

 

[JaredJames]

I know that, but when a time-dilated force that is acting upon something moving significantly faster than normal, in this case the outside world, I would think something would change. But what you are saying makes sense, and that's what I was thinking. I'm just thinking too hard about it I suppose.

Time-dilated force? Does such a thing even exist?

 

[guss]

Time-dilated force? Does such a thing even exist?

I mean, there is some kind of jet, fueled by some liquid. For every gallon of liquid burned, it produces a force for a certain period of time. Travel at 90% of the speed of light, and that time the force is "there" increases by 317%. That's what I'm thinking anyway. Know what I'm saying?

 

[JaredJames]

I mean, there is some kind of jet, fueled by some liquid. For every gallon of liquid burned, it produces a force for a certain period of time. Travel at 90% of the speed of light, and that time the force is "there" increases by 317%. That's what I'm thinking anyway. Know what I'm saying?

The engine is under the same time frame as the rest of the ship.

The time on board the ship doesn't increase by any factor so far as the ship is concerned. Period.

The time the engine fires for does not change as far as the ship is concerned. The acceleration does not change as far as the ship is concerned. The force the engine produces does not change whether it is run at 1% light speed or 0.99% light speed.

An important point here, the force produced by the engine is constant unless you change how much fuel is provided. The force is independent of burn time.

If I fire the engine for 10 seconds I get 10kN of thrust for example. If I fire the engine for 100 seconds I still get 10kN of thrust. But, the amount of acceleration that will produce is different. I will accelerate more with the second burst than the first as it is acting on the ship for longer.

What you are confusing is two reference frames. You are mixing the engines thrust in the ships reference frame with the duration of burn of an outside observers reference frame.

If an outside observer (on Earth for example) was to look at the ship it would appear that the ship, it's crew, the engines etc are all in slow motion. But, the force produced, regardless of where it is observed is the same.

 

[wrongusername]

The only way they see the effects of time dilation is when they view the 'outside world'. People back on Earth would appear to be experiencing time extremely quickly.

Actually, wouldn't people back on Earth appear to be experiencing time extremely slowly? As slow as the outside world is perceiving their time to be?

Because they can argue that their frame of reference is as valid as yours if you're on earth, and so from their perspective you are the one who's moving really quick. So to them you would appear to be experiencing time slowly, and to you you would also see them as experiencing time slowly?

 

[wrongusername]

I mean, there is some kind of jet, fueled by some liquid. For every gallon of liquid burned, it produces a force for a certain period of time. Travel at 90% of the speed of light, and that time the force is "there" increases by 317%. That's what I'm thinking anyway. Know what I'm saying?

units of force * time = units of momentum... interesting, so does momentum get increased by 317% than it would have otherwise?

 

[JaredJames]

Actually, wouldn't people back on Earth appear to be experiencing time extremely slowly? As slow as the outside world is perceiving their time to be?

Because they can argue that their frame of reference is as valid as yours if you're on earth, and so from their perspective you are the one who's moving really quick. So to them you would appear to be experiencing time slowly, and to you you would also see them as experiencing time slowly?

Once again we are confusing reference frames here and you've got the quickly / slowly aspect completely backwards.

If the ship is your reference frame, time experienced by everything outside the ship (travelling less than your velocity) will appear to move very quickly. 1 year on the ship will be multiple years on Earth.

If the Earth is your reference frame, time on the ship will appear to move really slowly.

From Earths perspective, you (on the ship) will be experiencing time extremely slowly. Not quickly.

There are only three possible reference frames:

On the ship - everything moving faster than you is going to experience time slower, everything moving slower than you is going to experience time faster.
Off the ship, traveling faster than the ship - the ship appears to experience time faster than you.
Off the ship, traveling slower than the ship - the ship appears to experience time slower than you.

units of force * time = units of momentum... interesting, so does momentum get increased by 317% than it would have otherwise?

Again, confusing two reference frames. The engine force, multiplied by the time applied in the ships reference frame is what counts.

The momentum of the ship is the same regardless of reference frame.

Force itself has a time component. 1N = 1 (kg.m)/s^2. When you convert the engine burn time from ships reference (let's say 10 seconds) to Earth's reference (let's say 100 seconds), you also have to amend the force value to reflect the time dilation.

From the Earth's reference, the engine will provide less force per second than it will in the ships reference frame. Remember, 1 second on the ship could be 10 on the earth. So from the ships point of view, when it gives 10kN of thrust for one second, it has given 10kN of thrust over 10 seconds from the Earth's.

It's a bit simplistic, and it doesn't convert as easy as that. I've just given that as a very, very basic example based on my understanding.

 

[guss]

Force itself has a time component. 1N = 1 (kg.m)/s^2. When you convert the engine burn time from ships reference (let's say 10 seconds) to Earth's reference (let's say 100 seconds), you also have to amend the force value to reflect the time dilation.

Ahh, that makes perfect sense. I somehow forgot about that. Thanks for your replies.

 

[Nabeshin]

Once again we are confusing reference frames here and you've got the quickly / slowly aspect completely backwards.

If the ship is your reference frame, time experienced by everything outside the ship (travelling less than your velocity) will appear to move very quickly. 1 year on the ship will be multiple years on Earth.

If the Earth is your reference frame, time on the ship will appear to move really slowly.

From Earths perspective, you (on the ship) will be experiencing time extremely slowly. Not quickly.

No, sorry... this is a very bad misunderstanding of the twin paradox. Wrongusename is correct, BOTH observers see the other as having time passing very slowly. I suggest you read up on the paradox and its resolution before you mislead others.

 

[JaredJames]

No, sorry... this is a very bad misunderstanding of the twin paradox. Wrongusename is correct, BOTH observers see the other as having time passing very slowly. I suggest you read up on the paradox and its resolution before you mislead others.

To both observers (as I'm pretty sure I've mentioned in my previous posts) observe time passing normally.

The time on board the ship doesn't increase by any factor so far as the ship is concerned.

But, the person on the ship will experience 'less' time passing than the person on the Earth.

The person on the ship may undergo 10 years of travel and return, to find the Earth has undergone 100 years.

Heck, I've just read the entire wiki article and don't see a problem. From the first line:

In physics, the twin paradox is a thought experiment in special relativity, in which a twin makes a journey into space in a high-speed rocket and returns home to find he has aged less than his identical twin who stayed on Earth.

The traveler remains in a projectile for one year of his time, and then reverses direction. Upon return, the traveler will find that he has aged two years, while 200 years have passed on Earth.

If a pair of twins are born on the day the ship leaves, and one goes on the journey while the other stays on Earth, they will meet again when the traveler is 5.14 years old and the stay-at-home twin is 10.28 years old.

This is why I kept mentioning reference frames. I'm not going through it all again, it's there numerous times for people to see and I don't see anything that says what I've previously posted regarding reference frames is wrong.

I really would like to see someone who truly understands this stepping in, my description is based on my understanding and it may / may not be completely wrong.

 

[mack_10]

Travelling at near light speeds would require massive amounts of fuel, that why SF writers invented the ram scoop

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

 

[JaredJames]

Travelling at near light speeds would require massive amounts of fuel, that why SF writers invented the ram scoop

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

Yes, but that doesn't have anything to do with fuel use / thrust due to time dilation as per the op.

 

[Dale]

Here is a very detailed page on the relativistic rocket problem, including fuel consumption assuming a 100% efficient conversion of mass to energy:
http://www.xs4all.nl/~johanw/PhysFAQ/Relativity/SR/rocket.html

 

[Nabeshin]

But, the person on the ship will experience 'less' time passing than the person on the Earth.

The person on the ship may undergo 10 years of travel and return, to find the Earth has undergone 100 years.

You have mentioned twice in this thread that the people on board the ship will observe those back at home on Earth "speeded up". This is simply not the case. The observers on Earth will see time moving more slowly for those on the rocket. Those on the rocket will see time moving more slowly for those on the Earth.

The reason you get the difference in age is that at some point the rocket must ACCELERATE to get back to Earth so the two observers can compare their clocks and see how long they have been gone. But as we already pointed out, we are dealing with constant speeds at this point. Therefore, the situation is completely symmetric when the ship is moving with constant velocity.

 

[JaredJames]

So at constant velocity you don't get time dilation?

Once you are no longer accelerating time moves at the same rate whether you are on the ship or not?

 

[Nabeshin]

That's not what I said. I said that at constant velocity, the people on the ship observe those on Earth to be passing slower through time and the people on Earth observe those on the ship to be passing slower through time. Nobody sees anybody else as moving FASTER through time than as measured by their own (stationary) watch. In more SR speak, the proper time between two events is always shorter than the time between the same two events in a different frame.

One cannot discuss unambiguously "the rate of time" anyways, so your question is rather meaningless without reference to with respect to whom the rate of time is being measured. If you modify the question to say "time moves at the same rate relative to the other observer whether you are the ship or Earth observer", then the answer yes.

 

[JaredJames]

OK, so as per the wiki article, if the ship experiences a 5 year journey time and the Earth 10 years, when observed from the ship the Earth would be experiencing time slower?

Am I understanding this right or way off? (I'm not arguing, I just want to clarify. What I've said so far seems ok, aside from the who's viewing what side of things from what I can tell.)

 

[DaveC426913]

If the ship is your reference frame, time experienced by everything outside the ship (travelling less than your velocity) will appear to move very quickly. 1 year on the ship will be multiple years on Earth.

No. If you look back toward Earth, you will observe everyone moving very slowly. As long as the spaceship and Earth are each in inertial reference frames, each will see the other as slower. That's the apparent paradox.


It is only once the spaceship turns around and begins its return to Earth that it will observe Earth as moving quickly.

 

[JaredJames]

Ah, now that clears it up.

 

[DaveC426913]

In this article http://www.einsteins-theory-of-relativity-4engineers.com/twin-paradox-2.html

This graph shows what a receding observer sees. Look at the left graph, bottom half. Eartyh stays put (Y-axis/time axis), spaceship moves away to right.

A signal sent out from Earth every year is received on the spaceship every two years. i.e. the spaceship on its 4 year outward journey (2007 to 2011) only receives only 2 yearly greetings from Earth.

(Of course, after it turns around, it receives 7 greetings on its 4 year homeward journey.)

 

[wrongusername]

To both observers (as I'm pretty sure I've mentioned in my previous posts) observe time passing normally.


But, the person on the ship will experience 'less' time passing than the person on the Earth.

This is true, imo, only from the person on Earth's point of view. How much slower time is passing shouldn't be absolute.

Speculatively, what if you have another spaceship, traveling at a fraction of the relativistic speed of the first spaceship? If the first spaceship undergoes 10 years of travel and decelerates to the second spaceship's frame, and the second spaceship's captain says he's been traveling for 50 years, and the first spaceship then decelerates further to be stationary with respect to Earth, would the people on Earth perhaps measure 100 years of time to have passed?

The person on the ship may undergo 10 years of travel and return, to find the Earth has undergone 100 years.

Heck, I've just read the entire wiki article and don't see a problem. From the first line:

You don't see a problem = where's the paradox? :P

Yes, but the traveller in space is changing frames. In one frame, he is traveling at whatever fraction of the speed of light relative to Earth. In 10 years or so in his time, he would change to another frame which is stationary to the person on Earth. Now suddenly the time on Earth doesn't seem to be moving slow anymore, and he finds much more time has passed on Earth somehow. I don't know how this would affect the symmetry between the two frames, but it's the explanation I've been given to the twin paradox.

 

[wrongusername]

In this article http://www.einsteins-theory-of-relativity-4engineers.com/twin-paradox-2.html

This graph shows what a receding observer sees. Look at the left graph, bottom half. Eartyh stays put (Y-axis/time axis), spaceship moves away to right.

A signal sent out from Earth every year is received on the spaceship every two years. i.e. the spaceship on its 4 year outward journey (2007 to 2011) only receives only 2 yearly greetings from Earth.

(Of course, after it turns around, it receives 7 greetings on its 4 year homeward journey.)

Wow, thanks! It makes more sense now

But in the right half of the picture, where Pam sends signals back to earth, is Earth also suddenly seeing Pam's time going really quick (4 signals in 2 years)? If so, why?

 

[DaveC426913]

But in the right half of the picture, where Pam sends signals back to earth, is Earth also suddenly seeing Pam's time going really quick (4 signals in 2 years)? If so, why?

Because she is following very closely behind her own light.