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If make a barrel with mirror walls, fill it with light on the Earth, and attach to the ship, the ship will need no reaction to move, it may just open a tiny hole in one end of the barrel, letting the photons escape.
But it would be a lot of photonic mass stored inside, and the barrel walls would be enough strong to withstand the radiation pressure.

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9 hours ago, kerbiloid said:

If make a barrel with mirror walls, fill it with light on the Earth, and attach to the ship, the ship will need no reaction to move, it may just open a tiny hole in one end of the barrel, letting the photons escape.
But it would be a lot of photonic mass stored inside, and the barrel walls would be enough strong to withstand the radiation pressure.

Every time a photon hits one of the mirrors inside your bucket there is a reaction, it just gets cancelled out by the other reflections on the other mirrors(unless you open a hole, then there will be some uncanceled reactions when the light escapes instead of bouncing off the mirror where there is now a hole).

 

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12 minutes ago, Terwin said:

Every time a photon hits one of the mirrors inside your bucket there is a reaction, it just gets cancelled out by the other reflections on the other mirrors(unless you open a hole, then there will be some uncanceled reactions when the light escapes instead of bouncing off the mirror where there is now a hole).

Depends

https://www.quora.com/How-do-photons-get-trapped

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19 hours ago, Cheif Operations Director said:
19 hours ago, Snark said:

Not correct.

It's trading the momentum of the fuel (its mass times its velocity) for the momentum of the ship (its mass times its velocity).

Ok thanks

This also leads to an intuitive understanding of why engines with high exhaust velocity are more fuel-efficient than engines with low exhaust velocity.

On Earth, we talk about fuel efficiency in the context of distance, because an automobile must continually expend energy in order to keep moving against the drag of the air and the ground. With a rocket, however, we talk about efficiency in the terms of fuel consumption rate. The technical term is "thrust-specific fuel consumption", i.e., the amount of fuel/propellant being expended every second in order to produce a specific amount of thrust. Since thrust is just force divided by time, you can cancel out the "per second" on both sides, which gives you the amount of fuel required to produce a specific amount of impulse, or change in momentum. 

Suppose you have a spaceship with a mass of 1 kg and a prop tank containing 10 kg of propellant. The initial mass of the system is 11 kg and the initial momentum of the system is zero. If you throw 1 kg of props out the back at -100 m/s, it has a momentum of -100 kg*m/s. By the conservation of momentum, the spaceship must have an equal and opposite momentum of +100 kg*m/s. The spaceship now masses 10 kg, so dividing its +100 kg*m/s by 10 kg gives you a positive velocity of 10 m/s.

But if your propellant is more volatile (or, perhaps, is being accelerated by an extreme heat source like a nuclear reactor), it can come out the back end of your spaceship with much greater speed. For example, if you can throw propellant out the back of your spaceship at -1,090 m/s, then you only need to use 0.1 kg for the same effect. 0.1 kg * -1,090 m/s = -109 kg*m/s, and so the spaceship has a resulting momentum of +109 kg*m/s, and dividing by the remaining spaceship mass of 10.9 kg gives you a positive velocity of 10 m/s.

9 hours ago, kerbiloid said:

If make a barrel with mirror walls, fill it with light on the Earth, and attach to the ship, the ship will need no reaction to move, it may just open a tiny hole in one end of the barrel, letting the photons escape.
But it would be a lot of photonic mass stored inside, and the barrel walls would be enough strong to withstand the radiation pressure.

You'd also need a way to collimate the photons so that their thrust pushed you in the correct direction rather than dispersing out at angles.

And, as @kerbiloid's link indicates, photons do not actually remain trapped in resonant cavities indefinitely. Even with a perfectly-reflective mirror (which is itself impossible), there is a small but nonzero probability that the wavefunction of the photon will tunnel through, either being absorbed by the mirror substrate and turning into heat, or passing through entirely and escaping. 

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11 minutes ago, sevenperforce said:

You'd also need a way to collimate the photons so that their thrust pushed you in the correct direction rather than dispersing out at angles.

And, as @kerbiloid's link indicates, photons do not actually remain trapped in resonant cavities indefinitely. Even with a perfectly-reflective mirror (which is itself impossible), there is a small but nonzero probability that the wavefunction of the photon will tunnel through, either being absorbed by the mirror substrate and turning into heat, or passing through entirely and escaping. 

Of course. They are not storable propellant.

P.S.
This is a joke idea, just in case.

Upd.
Though, who knows what will they use as mirrors in future.

Edited by kerbiloid
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38 minutes ago, kerbiloid said:

Of course. They are not storable propellant.

P.S.
This is a joke idea, just in case.

Upd.
Though, who knows what will they use as mirrors in future.

There is work with photons in quantum systems where the wavefunctions can be trapped in closed timelike curves, which should remove the capacity for quantum tunneling. Imagine if we had a quantum photon rocket powered by entangled particles in time-traveling curves!

Or of course we could capture a black hole inside a resonating cavity and feed it mass whenever we wanted to fire our photon rocket.

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3 minutes ago, sevenperforce said:

There is work with photons in quantum systems where the wavefunctions can be trapped in closed timelike curves, which should remove the capacity for quantum tunneling. Imagine if we had a quantum photon rocket powered by entangled particles in time-traveling curves!

If @FreeThinker could be so kind to add it.

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On 8/12/2019 at 12:23 AM, kerbiloid said:

If make a barrel with mirror walls, fill it with light on the Earth, and attach to the ship, the ship will need no reaction to move, it may just open a tiny hole in one end of the barrel, letting the photons escape.
But it would be a lot of photonic mass stored inside, and the barrel walls would be enough strong to withstand the radiation pressure.

I feel like your "fuel tank" would have rather poor mass ratio..........

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8 hours ago, sevenperforce said:

I feel like your "fuel tank" would have rather poor mass ratio.

If instead of material mirrors use some immaterial physical effect (for strength), it can be as dense as photons of annihilation in the very beginning, i.e. as dense as solid materials and even denser.
You can treat this propellant as "post-antimatter" or "post-annihilation photons".
Like you don't annihilate antimatter while flying, but doing this in advance, and taking the annihilation results as a fuel.
This engine doesn't need a huge nozzle, just manage its transparency in desired spots on the tank surface.

Edited by kerbiloid
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I have to say thank you to Cheif Operations Director for continuing to argue for what he thinks is right when everyone tells him he is wrong, and for remaining positive when abuse comes his way.  That is the stuff I like to see on this forum, and in other areas of life.  

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On 8/14/2019 at 12:28 AM, kerbiloid said:

If instead of material mirrors use some immaterial physical effect (for strength), it can be as dense as photons of annihilation in the very beginning, i.e. as dense as solid materials and even denser.
You can treat this propellant as "post-antimatter" or "post-annihilation photons".
Like you don't annihilate antimatter while flying, but doing this in advance, and taking the annihilation results as a fuel.
This engine doesn't need a huge nozzle, just manage its transparency in desired spots on the tank surface.

At some point you create a kugelblitz, at which point, why not just go with a black hole starship?

Collimation of the thrust beam is always going to be the challenge.

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  • 1 year later...
On 8/10/2019 at 1:53 PM, steve_v said:

Sure, both rockets are expelling propellant. Adding the thrust vectors gives you a net force on the craft.
 

Redirecting a gas stream is accelerating it. By turning the exhaust stream 90°, you cancel it's velocity along the axis of the craft.
Centripetal force and all that jazz comes into play, but because engine and deflector/magnet are part of the same system, the axial forces generated by the engine-deflector system are simply absorbed by the connecting structure.
Making the exhaust go sideways with magnets is exactly the same as slowing it down with magnets, so far as forward thrust is concerned - you just stopped the exhaust moving in the opposite direction to the one you want to go, so you don't go.

You'll get some miniscule thrust from the propellant moving rearward in relation to the craft, but once that propellant is jettisoned out the side it's gone - what you have is just an incredibly inefficient rocket.

This is all irrelevant to your "30 year burn time" proposal anyway, you were talking about recovering propellant, not expelling it as a super-inefficient craft-squeezing device.
Whatever you do to the exhaust stream, if it's being done by something physically connected to the craft then it can't move the craft - it's happening in the same frame of reference, in a closed system.
That's the fundamental problem - if the reaction mass is not decoupled from the engine system, you get no thrust. If you add velocity/momentum to your propellant opposite your direction of travel, it needs to stay that way.

The only way to get something to move in space is to apply a force relative to something else - i.e. the application of the laws of motion on your reaction mass. A reactionless drive is like lifting yourself by pulling on your own bootlaces.

One of these works, the other has been shown over and over to not work:

preview

Your recirculating propellant is just shifting mass around inside the craft, in the same way the "mechanical rocket" does. It goes against all the fundamental principles of rocketry, and it Does. Not. Work.
If getting free propellant and infinite ΔV was as easy as redirecting the exhaust with magnets, details be damned, we'd all be living on Mars right now.

I'm pretty much done here TBH, your posts are extremely difficult to make sense of and arguing around in circles about force-vectors doesn't change conservation of momentum, no matter how you twist it.

Sorry for old thread revival. I was googling the (impossible) idea of recycling rocket exhaust in a closed system which this topic shows doesn't work to get forward motion.

My thoughts that led me here were a closed cylinder with a device at one end that launches a lump of ice backwards towards the rear of the cylinder. If the rear of the cylinder was open then the lump would exit the craft and there would be a positive thrust (I think).

What would happen if the cylinder was now closed and after launching the lump of ice a series of lasers were fired at it from within the cylinder so it was vaporised before if hit the rear of the cylinder and cancelled out the forward momentum gained from launching it reward.

Is there any possibile configuration of this setup or do you still end up with the same mass of steam instead of ice going backwards (sorry no drawing but hopefully my description is clear).

 

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1 hour ago, Albert ninestein said:

Sorry for old thread revival. I was googling the (impossible) idea of recycling rocket exhaust in a closed system which this topic shows doesn't work to get forward motion.

My thoughts that led me here were a closed cylinder with a device at one end that launches a lump of ice backwards towards the rear of the cylinder. If the rear of the cylinder was open then the lump would exit the craft and there would be a positive thrust (I think).

What would happen if the cylinder was now closed and after launching the lump of ice a series of lasers were fired at it from within the cylinder so it was vaporised before if hit the rear of the cylinder and cancelled out the forward momentum gained from launching it reward.

Is there any possibile configuration of this setup or do you still end up with the same mass of steam instead of ice going backwards (sorry no drawing but hopefully my description is clear).

 

Photons have momentum (this is why solar sails work) so, in principle, you could stop your chunk of ice using lasers, although doing it in practice would be challenging to put it mildly.  However, you then need to consider what happens to all those photons. 
 

Very crudely, imagine trying to stop the ice with a stream of ball bearings.  They bounce off the ice and hit the closed end of your cylinder, transferring momentum to it as they do so.  Likewise for photons. Once you do the maths you end up with no net change in momentum for the whole system.

In practice I think you have it right with the ‘same mass of steam instead of ice going backwards’.

 

Edited by KSK
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1 hour ago, KSK said:

Photons have momentum (this is why solar sails work) so, in principle, you could stop your chunk of ice using lasers, although doing it in practice would be challenging to put it mildly.  However, you then need to consider what happens to all those photons. 
 

Very crudely, imagine trying to stop the ice with a stream of ball bearings.  They bounce off the ice and hit the closed end of your cylinder, transferring momentum to it as they do so.  Likewise for photons. Once you do the maths you end up with no net change in momentum for the whole system.

In practice I think you have it right with the ‘same mass of steam instead of ice going backwards’.

 

Thanks, I'll stick with the day job and leave it to an expert to develop interstellar travel!

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On 12/17/2020 at 9:20 AM, Albert ninestein said:

Sorry for old thread revival. I was googling the (impossible) idea of recycling rocket exhaust in a closed system which this topic shows doesn't work to get forward motion.

My thoughts that led me here were a closed cylinder with a device at one end that launches a lump of ice backwards towards the rear of the cylinder. If the rear of the cylinder was open then the lump would exit the craft and there would be a positive thrust (I think).

What would happen if the cylinder was now closed and after launching the lump of ice a series of lasers were fired at it from within the cylinder so it was vaporised before if hit the rear of the cylinder and cancelled out the forward momentum gained from launching it reward.

Is there any possibile configuration of this setup or do you still end up with the same mass of steam instead of ice going backwards (sorry no drawing but hopefully my description is clear).

 

If you vaporise the ice chunk then its water vapour atoms will impact the rear of the cylinder with the same momentum transfer as the solid lump of ice.

 

If you can draw an imaginary border around your system such that nothing crosses that boundary, then that system has not exchanged momentum with its environment and shall not be moving today.

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On 12/17/2020 at 11:20 AM, Albert ninestein said:

Sorry for old thread revival. I was googling the (impossible) idea of recycling rocket exhaust in a closed system which this topic shows doesn't work to get forward motion.

You do not need to explain a special mechanism. Instead you should explain which part of your mechanism works against known physics and show it by observing such phenomenon. General natural law called as conservation of momentum tells that it is impossible to change momentum of closed system without interacting with environment.

Actually conservation of momentum is a consequence of more fundamental natural law, a translation symmetry of space. It means that if natural laws are the same at every point in space. Also other well known conservation laws are connected to symmetry properties of space (it is quite high end theoretical physics and I am not expert at it).

https://en.wikipedia.org/wiki/Noether's_theorem

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The hardest part of the conservation laws is to realize that it's you... yes, personally you... what it the center of the whole Universe, and throwing anything to the left needs to throw same to the right, because the Universe CoM can't move from you in any direction.

(Say, the ship to the left, the exhaust to the right.)

Edited by kerbiloid
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On 12/18/2020 at 3:47 PM, kerbiloid said:

The hardest part of the conservation laws is to realize that it's you... yes, personally you... what it the center of the whole Universe, and throwing anything to the left needs to throw same to the right, because the Universe CoM can't move from you in any direction.

(Say, the ship to the left, the exhaust to the right.)

An simpler way to look at it is that some reaction less engine like the EM drive who did some magic and interacted with the quantum foam or aether would still be an Perpetuum mobile, if it says that you get constant acceleration out of an constant trust. 

The problem here is that as you accelerate your kinetic energy increases squared, at double the velocity your kinetic energy is 4 times as high, 3 time and 9 times the energy. 
So if you put an EM drive on an rotor in an vacuum chamber you will spin up the rotor, at some velocity and rotation you could use an dynamo to bleed off the acceleration, use a bit of that power to feed the EM drive and the rest as free energy.  This is something  who could be build, main cost would be the vacuum chamber and getting an working EM drive obviously :)  

The issue with having to feed more and more energy to accelerate is not something we notice much, yes it affect cars acceleration but air resistance is more of an issue here but it affect braking distance strongly. 

Now if you was to build an coil gun launcher on the moon this would be an issue. Say you want to accelerate the capsule with 10 g constant force. You provide 1 MW power who gave 10 g initial acceleration who is fine. 
After 1 second you travel at 100 m/s, after 2 its 200 m/s now you need to provide 2 MW for 10 g, 4 seconds and its 400 m/s and 4MW. 
 

 

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7 hours ago, magnemoe said:

An simpler way to look at it is that some reaction less engine like the EM drive who did some magic and interacted with the quantum foam or aether would still be an Perpetuum mobile, if it says that you get constant acceleration out of an constant trust. 

The problem here is that as you accelerate your kinetic energy increases squared, at double the velocity your kinetic energy is 4 times as high, 3 time and 9 times the energy. 
So if you put an EM drive on an rotor in an vacuum chamber you will spin up the rotor, at some velocity and rotation you could use an dynamo to bleed off the acceleration, use a bit of that power to feed the EM drive and the rest as free energy. 

It's interesting that it all goes back to relativity. Distilled to its core, the argument is that power output of a drive with constant thrust increases with forward speed, while power consumption of a reactionless drive must stay constant, as failure to do so would indicate ability to measure an absolute speed of the rocket. So a reactionless drive is an infinite energy source if all frames of reference are equivalent.

The way this is usually presented is from a completely different angle. Conservation laws come from symmetries, easiest to see from Hamilton's Equations. dpx/dt = -∂H/∂x, dE/dt = - ∂H/∂t. If Hamiltonian doesn't depend on coordinates, the momentum is conserved. If Hamiltonian doesn't depend on time, then the energy is conserved. But we know that what's spatial displacement in one coordinate system is also a time displacement in another. Because there is no preferred coordinate system, the above conservation laws must hold in all or none. So if you found a violation of conservation of momentum in one coordinate system, there must exist another in which conservation of energy is violated as well. And, of course, above example demonstrates that perfectly.

I just really like that you can have this convoluted mess of math that you can use to prove equivalence of conservation laws in a very general way, or you can go and build a very simple example of a machine that makes free energy under given assumptions, and both of these are essentially built on the same core assumption that there is no preferred coordinate system.

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As there are some Physics experts on this forum please can someone explain why my idea of a free energy machine doesn't work.

Please consider a helium filled blimp on the ground attached to a tether which is in turn attached to an electrical generator. As the blimp rises the tether turns the generator to produce electricity.

Once the blimp reaches a set altitude a compressor stored on-board the blimp compresses some of the helium into an onboard storage tank so the blimp then sinks slowly back to earth and the tether is wound in during the descent.

Once on the ground the helium is released back into the blimp so it floats upwards again and turns the motor repeatingthe cycle.

The tether contains an electrical cable that powers the onboard compressor from a battery stored on the ground which is charged during the ascent.

Let's assume the blimp is contained within a very tall frame so it only goes up or down.

Would the energy used to compress the gas to make the blimp return to earth and the energy required to wind the tether back in during the descent be greater than any energy produced during the ascent?

 

 

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1 hour ago, Albert ninestein said:

As there are some Physics experts on this forum please can someone explain why my idea of a free energy machine doesn't work.

Please consider a helium filled blimp on the ground attached to a tether which is in turn attached to an electrical generator. As the blimp rises the tether turns the generator to produce electricity.

Once the blimp reaches a set altitude a compressor stored on-board the blimp compresses some of the helium into an onboard storage tank so the blimp then sinks slowly back to earth and the tether is wound in during the descent.

Once on the ground the helium is released back into the blimp so it floats upwards again and turns the motor repeatingthe cycle.

The tether contains an electrical cable that powers the onboard compressor from a battery stored on the ground which is charged during the ascent.

Let's assume the blimp is contained within a very tall frame so it only goes up or down.

Would the energy used to compress the gas to make the blimp return to earth and the energy required to wind the tether back in during the descent be greater than any energy produced during the ascent?

Compressing is an very inefficient process. then you compress an gas it generate lots of heat who increases the pressure even more. 
Its ways to dealing with this but they get heavy fast. You need an heat exchanger and an steam stage running on it so heavy. 

Now compressing gas in an nice way to do attitude control on an balloon or airship. But as you are using an gas turbine to run an airship using that heat to heat up lifting gas is probably an better idea for most uses. 

Note that you can use heat to run this setup, have an black balloon, during the day it heats up and raises into the air, during night it cools down and decent to the ground again. 
Its not efficient in any way compared to wind turbines or solar sells but might be relevant for an balloon probe? 

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