Propellant Recycling

[Cheif Operations Director]

58 minutes ago, mikegarrison said:

Imagine the magnet is your rocket nozzle and the "metal" is what captures the propellant. See the problem? No net force, no net momentum -- no movement.

Ok sure but that is not what I am doing at all. This whole thing stops working when the nuclear reactor runs out of fuel.

The magnet is just their to slow down the exhaust so it does not cause the net force to be 0 

3 hours ago, sh1pman said:

It can cool your reactor. Doesn’t do anything else that is useful.

I mean, no elaborate on why this does not generate thrust.

Edited August 11, 2019 by Cheif Operations Director

[mikegarrison]

2 minutes ago, Cheif Operations Director said:

Ok sure but that is not what I am doing at all.

Yes it is.

What is a rocket nozzle? It is a device for turning pressure into momentum. That momentum of escaping gas then propels the rocket forward.

However, your magic "slowing device" is, in effect, an anti rocket nozzle. It captures the gas and turns the momentum back into pressure (or maybe just loses the energy, whatever). But the point is that when you slow down the exhaust, you are undoing the work of the initial rocket nozzle. And if your "slowing down device" is physically attached to the ship with the rocket nozzle (as it must be if you want to reuse the propellant) then you have no net momentum in the system. So you go nowhere.

[Shpaget]

In the other thread I said:

Quote

The point is: no matter how and what you use to redirect exhaust, the only result is loss of usefull thrust. The redirected portion is wasted. Nothing good will come of it.

As for the topic, yes, spike in radiation is quite telling. Some sort of excursion deffinitely happened.

To which you replied:

Quote

Creating as much friction as possible would most likely help this design if I think is done in the right areas. Again I’m trying to recycle the propellant not the energy, this is not a free energy machine. 

Please note and understand the words "no matter how and what you use". The law of conservation of momentum is universal. Friction does not get a free pass. Magnets don't get a free pass. There are no exceptions.

If you don't mind me asking, how old are you? Have you learned about vectors in school? I ask this because in the other thread I drew your proposal in vector form, and I don't think you fully understood it. That drawing shows exactly why your proposal can not work. Vectors can be very intuitive for this sort of problem solving, but if you haven't learned about them, I suppose the picture is not entirely clear for you.

If you did learn about them, I urge you to revisit the topic and refresh your understanding of them, then draw your own force diagram of the system you propose.

[satnet]

5 hours ago, Cheif Operations Director said:

Heat your fuel in a nuclear reactor to change states from a liquid to a gas.  Put your fuel into a rocket engine,  That creates momentum. Then use a device (let’s just call it magic for now so we can move past those technicalities) that then slows down the propellant after it has been used by the rocket and has left the nozzle. Re-capture the gas and condense it, then re-heat it. Basically a nuclear steam reactor but instead of turning a turbine you turn it into a rocket with the nozzle. 

Putting fuel into a rocket engine does not create momentum. Momentum is conserved, you can't create it though you can exchange it in interesting and useful ways. The overall system has the same momentum it started with. Basically if you were to take the vector sum of the momentum of the rocket and the rocket exhaust it would add up to its starting momentum (at least in a vacuum where we can treat this as the total system we're analyzing). A rocket doesn't work by creating momentum, but by dividing it in a controlled manner between the remaining mass of the rocket and the exhausted mass so that the velocity vector of the remaining rocket mass increases in a specific direction. To recapture all of the propellant you must undo this which means that the rocket and exhaust must end up with a net velocity vector of zero and as long as they remain together this means the rocket will not move.

You could of course recapture a portion of the propellant and exhaust some of it. You would decrease your change in velocity relative to just exhausting all of it, but it would give you a variable thrust engine with consistent cooling characteristics. This gives us the only reason I can think of to pursue this dubious enterprise. You could create an engine that had a relatively high initial thrust until it ran low on propellant which then switched to recapturing its working fluid (no longer propellant, now coolant) and used the waste heat in a photon rocket to continue accelerating for as long as you have nuclear fuel (at an obscenely low rate of acceleration).

[RCgothic]

8 hours ago, Cheif Operations Director said:

Ok sure but that is not what I am doing at all. This whole thing stops working when the nuclear reactor runs out of fuel

What you've just written is exactly equivalent to "the electromagnet stops working when the truck runs out of battery."

In order to generate motion, you must transfer momentum to an expelled fuel. If you recapture the fuel, it is not expelled. The sum of momentum of the craft has not changed.

If you want to preserve your propellant for the life of the nuclear fuel you can use it more sparingly, which will result in it being hotter and faster and carrying more energy away per kg, but that requires the engine and reactor to operate at higher temperatures.

 

 

Rocket engines work on the principal of conservation of momentum.

(DryMass+FuelMass) * InitialVelocity = DryMass*NewVelocity + FuelMass*FinalExhaustVelocity

Bear in mind that velocities are vectors and opposite vectors have opposite signs.

If you recapture the exhaust, doesn't matter how, then FinalExhaustVelocity = NewVelocity = Initial Velocity.

No exceptions.

Edited August 11, 2019 by RCgothic

[sevenperforce]

There are three physical laws which tend to frustrate a lot of people.

One is conservation of momentum.

Another is conservation of energy.

The third is the law of entropy.

As a result of these three laws, some unpleasant truths emerge. You cannot produce a change in momentum in a closed system. You cannot cause a closed system to produce energy indefinitely. And you cannot extract heat from a closed system.

These laws are independent. No matter how much "free" energy you have, it won't change the rules of momentum. No matter how much "free" reaction mass you have, it won't change the rules of energy.

The thing which makes a rocket move is the fact that the exhaust is free to escape. Capture it, in any way, and you no longer have a rocket.

[sevenperforce]

20 hours ago, Cheif Operations Director said:

supposing you could create a tarp that would reduce the force hitting the tarp  (again not possible in this scenario) you would still go slower than without the tarp but it would still be a net gain. This all supposes that the force hitting the tarp can be reduced. If it can not be reduced I agree with you.

If you reduce the force hitting the tarp, you reduce the force acting on the exhaust, and thus the exhaust is not captured.

How do you propose to slow down the thrust without applying force to it?

20 hours ago, Cheif Operations Director said:

You would gain force if the tennis ball did not touch the box car again, ie if you lost propellant. You would also gain speed if the equal and opposite reaction could be turn in equal via the 1st law.

Second law:

In an inertial frame of reference, the vector sum of the forces F on an object is equal to the mass  m of that object multiplied by the acceleration  a of the object: F = ma. (It is assumed here that the mass m is constant.

If the reduce the acceleration of the equal and opposite reaction the force it less.

Reducing the acceleration doesn't change anything. If you hit a tennis ball to your friend with a baseball bat, the ball's acceleration will be very rapid and very high. If your friend catches it with a glove, the acceleration will be much lower and much slower. However, the momentum exchanged remains the same.

[sevenperforce]

17 hours ago, Cheif Operations Director said:

Think in KSP, if I have two of the same engines back to back it goes nowhere right? If I have two engines opposite if each other AND one perpendicular to the two so it forms a T shape it travels in the direction of the tangent of the 3rd engine right? 

Same principle

I take the forward thrust and push it to the sides. I recapture the propellant after the energy has been transferred to the engines that cancel each other out. 

The problem is inherent right there in your explanation. You "push" the thrust to the sides. Let's go back to our tennis ball analogy; our engine is a tennis-ball-throwing machine, and we want to recapture those lost tennis balls so we can "recycle" them. Follow along:

1. In order to move the ship prograde, the tennis balls must be pushed retrograde.
2. We have a magic magnetic mechanism to push the flying tennis balls in whatever direction we like.
3. If we only push the tennis balls in the radial direction, they will head off at an angle, but their retrograde momentum will not be arrested.
4. In order to deflect the tennis balls into a perpendicular direction, we must "push" them at an angle between radial and prograde.
5. If our magic magnet mechanism pushes with any prograde component at all, then the tennis balls push back, by Newton's laws, in the retrograde direction. This produces a retrograde force on the mechanism.
6. If the mechanism is attached to the ship, it transfers that "push" back to the ship, and the ship cannot move. If the mechanism is not attached to the ship, it rapidly recedes into the distance.

17 hours ago, Cheif Operations Director said:

I’m not sure why people think I am proposing a free energy machine here, nothing is free in this since it is a mono prop system at takes heat from a nuclear reactor and converts that heat into a deep space probe propulsion system. 

It's not a free-energy machine, it's a reactionless thruster. Both are prohibited by physics.

 

16 hours ago, Ultimate Steve said:

But what I was saying is that it doesn't matter how many stages you use to show down/redirect/remove y axis velocity from the exhaust, it will always add up to 100 percent if you slow it's y axis velocity to zero, and that energy goes into the craft.

Confusing energy and momentum is one of the reasons this is often so hard to explain....

15 hours ago, Cheif Operations Director said:

we may be at a bit of an impasse. If you push off of a table in space you will go backward as will the table. Your gun analogy is interesting but it has the problem that the bullet is not slowed down with another system. Other than the impact.

So have two springs.

Or a system of springs.

Remember, whatever a magnet can do, a frictionless spring can do. 

If you have a tube with springs at both ends, nothing will change the overall momentum of the system, no matter how much nuclear energy you add. Making the springs angled won't make a difference.

[sevenperforce]

15 hours ago, Cheif Operations Director said:

suppose that you never let go but transferred some of your heat energy into The kinetic energy of the table.

Kinetic energy is half the mass times the square of velocity.

As long as you "never let go" of the table, your velocity remains the same as its velocity, and thus the transfer of kinetic energy is zero.

14 hours ago, Cheif Operations Director said:

14 hours ago, Dragon01 said:

You can, in theory, use far IR radiation as a photon rocket (not really worse than any other kind), but it's probably easier to use a laser.

Why has this not been done yet?

Low thrust and poor energy performance.

You have to use massive amounts of energy to generate even a little thrust. We're talking about accelerating at 100% throttle for decades just to get 5 m/s of dV.

14 hours ago, Cheif Operations Director said:

14 hours ago, ThatGuyWithALongUsername said:

Thrust is way, way too small. Much easier to use larger sources of energy, even in the form of photons- now we're getting into solar sail territory.

Or, it you want more concentrated power, lasers from the ground (so you don't have to carry their tremendous weight). See project Starshot. Still hard, though.

Power the laser with a nuclear reactor

A nuclear reactor uses fission to break apart large nuclei into smaller ones. Severing these nuclear bonds destroys nucleic mass and converts it into energy according to e=mc². So you are converting mass into energy. A nuclear-powered photon rocket loses mass over time.

[kerbiloid]

_{(Oops, my fault. That's the 3rd one mentioned.)}

 

Edited August 11, 2019 by kerbiloid

[mikegarrison]

1 minute ago, kerbiloid said:

And the third one contravenes two others by postulating time irreversibility in simple mechanical systems.

Always you got to be with the complications. Oy vey!

[kerbiloid]

14 minutes ago, mikegarrison said:

Always you got to be with the complications. Oy vey!

I've corrected the post. Puzzle tov!

[Snark]

Hello everyone,

A huge amount of material (many pages' worth) has been split off into this thread from a discussion of the Rosatom rocket failure, due to being off-topic in the original location.

Unfortunately this has scrambled this thread a bit, since the split material predates Cheif Operation Director's original post for this thread.  We apologize for the confusion; can't be helped.  Here's the central idea of this thread:

18 hours ago, Cheif Operations Director said:

Heat your fuel in a nuclear reactor to change states from a liquid to a gas.  Put your fuel into a rocket engine,  That creates momentum. Then use a device (let’s just call it magic for now so we can move past those technicalities) that then slows down the propellant after it has been used by the rocket and has left the nozzle. Re-capture the gas and condense it, then re-heat it. Basically a nuclear steam reactor but instead of turning a turbine you turn it into a rocket with the nozzle. 

 

@Cheif Operations Director, we appreciate that you're earnestly trying to figure out a working system, due to your love of rockets and spaceflight.  Thumbs up for that. 

However, you do need to realize that your idea cannot possibly work.  At all, in any permutation, no matter how cleverly you try to arrange things, even if it uses magnets.  This is because your basic concept is trying to violate the laws of physics-- specifically, the law of conservation of momentum.  We've known about for over 300 years, Newton got it right from the start.

I've been reading through the many pages of people trying to explain why your idea won't work.  They're all correct, please listen to them.  If I may, I notice a general source of confusion in your responses:

You are consistently confusing "energy" with "momentum".  Don't.  Energy and momentum are completely different things.  They both have conservation laws, but they're different things being conserved.

Nobody is suggesting that you're proposing a "perpetual energy machine", because you're not-- you're explicit about that, there's a nuclear reactor providing energy.  That's not why your idea won't work.  The problem is that you are proposing a "perpetual momentum machine", which won't work because momentum is conserved.

Here are some important ways that momentum differs from energy:

• Energy can be stored.  Momentum can't.
• Energy can be converted to different forms.  There's only one "kind" of momentum, so that's not a thing.
• Energy can be moved from place to place without any inherent "cost" other than losses to inefficiencies.  It can be "redirected", in that sense.  Momentum can't. 
• You can't "redirect" momentum.  The only way to change the momentum of one object is to transfer momentum to another object.
• Momentum is a vector quantity; it has a direction.  (Unlike energy, which is a scalar and has no direction associated with it.)I

If you have a body that's in a vacuum with no external forces acting on it-- e.g. a rocket-- then the only way it can accelerate in one direction is to expend reaction mass in the opposite direction.  If you want your rocket to go north, it has to throw reaction mass south.  And the only way for the rocket to keep going north is if the spent reaction mass keeps going south.  If you stop the one, you stop the other.

We appreciate that this thread has been so civil and articulate, so a thank you from us to everyone here-- both Cheif, and the folks debating with him-- by keeping this on friendly collegial terms and not resorting to personal comments.  Well done, everyone. 

And encouragement to @Cheif Operations Director, who's working to better understand the physics of these things.  Yes, it's hard.  It's literally rocket science.    Hope the discussion goes well and that you can gain more understanding of how things work, which after all is pretty much the point of the Science & Spaceflight forum.

We now return you to your lively debate about Newton's laws of motion.  Thank you again for everyone's patience and grim determination to be helpful. 

[Cheif Operations Director]

53 minutes ago, Snark said:

Hello everyone,

A huge amount of material (many pages' worth) has been split off into this thread from a discussion of the Rosatom rocket failure, due to being off-topic in the original location.

Unfortunately this has scrambled this thread a bit, since the split material predates Cheif Operation Director's original post for this thread.  We apologize for the confusion; can't be helped.  Here's the central idea of this thread:

 

@Cheif Operations Director, we appreciate that you're earnestly trying to figure out a working system, due to your love of rockets and spaceflight.  Thumbs up for that. 

However, you do need to realize that your idea cannot possibly work.  At all, in any permutation, no matter how cleverly you try to arrange things, even if it uses magnets.  This is because your basic concept is trying to violate the laws of physics-- specifically, the law of conservation of momentum.  We've known about for over 300 years, Newton got it right from the start.

I've been reading through the many pages of people trying to explain why your idea won't work.  They're all correct, please listen to them.  If I may, I notice a general source of confusion in your responses:

You are consistently confusing "energy" with "momentum".  Don't.  Energy and momentum are completely different things.  They both have conservation laws, but they're different things being conserved.

Nobody is suggesting that you're proposing a "perpetual energy machine", because you're not-- you're explicit about that, there's a nuclear reactor providing energy.  That's not why your idea won't work.  The problem is that you are proposing a "perpetual momentum machine", which won't work because momentum is conserved.

Here are some important ways that momentum differs from energy:

• Energy can be stored.  Momentum can't.
• Energy can be converted to different forms.  There's only one "kind" of momentum, so that's not a thing.
• Energy can be moved from place to place without any inherent "cost" other than losses to inefficiencies.  It can be "redirected", in that sense.  Momentum can't. 
• You can't "redirect" momentum.  The only way to change the momentum of one object is to transfer momentum to another object.
• Momentum is a vector quantity; it has a direction.  (Unlike energy, which is a scalar and has no direction associated with it.)I

If you have a body that's in a vacuum with no external forces acting on it-- e.g. a rocket-- then the only way it can accelerate in one direction is to expend reaction mass in the opposite direction.  If you want your rocket to go north, it has to throw reaction mass south.  And the only way for the rocket to keep going north is if the spent reaction mass keeps going south.  If you stop the one, you stop the other.

We appreciate that this thread has been so civil and articulate, so a thank you from us to everyone here-- both Cheif, and the folks debating with him-- by keeping this on friendly collegial terms and not resorting to personal comments.  Well done, everyone. 

And encouragement to @Cheif Operations Director, who's working to better understand the physics of these things.  Yes, it's hard.  It's literally rocket science.    Hope the discussion goes well and that you can gain more understanding of how things work, which after all is pretty much the point of the Science & Spaceflight forum.

We now return you to your lively debate about Newton's laws of motion.  Thank you again for everyone's patience and grim determination to be helpful. 

Since you are stating what everyone else is saying, I will just reply to this post for simplicity’s sake instead of copy and pasting the same response, 

1. You have pretty much convinced me save for one thing

Im not generally contesting that, but something seems off. If you can not store momentum/ create then how does a rocket move  in a vacuum. I’m not saying it is free here I am just saying that if a rocket is traveling at a rate that whole object (collectively) has momentum when you fire the engines you create pressure in the engine that propels your rocket forward. But if you can not store momentum and only transfer it when are you getting that speed from?

 

momentum is p = m x v right? So a rocket gains velocity, to increase momentum but that would mean that your rocket before it fired its engines got momentum from somewhere, so where? What is it orbiting?

Or is it trading the mass of the fuel for velocity? 

12 hours ago, Shpaget said:

If you don't mind me asking, how old are you? Have you learned about vectors in school? I ask this because in the other thread I drew your proposal in vector form, and I don't think you fully understood it. That drawing shows exactly why your proposal can not work. Vectors can be very intuitive for this sort of problem solving, but if you haven't learned about them, I suppose the picture is not entirely clear for you.

If you did learn about them, I urge you to revisit the topic and refresh your understanding of them, then draw your own force diagram of the system you propose.

I do not seem to remember being taught it, I did look into vectors for your response though. I will need to think on it, but yes I do think you are right.

[mikegarrison]

2 minutes ago, Cheif Operations Director said:

1. You have pretty much convinced me save for one thing

Im not generally contesting that, but something seems off. If you can not store momentum/ create then how does a rocket move  in a vacuum. I’m not saying it is free here I am just saying that if a rocket is traveling at a rate that whole object (collectively) has momentum when you fire the engines you create pressure in the engine that propels your rocket forward. But if you can not store momentum and only transfer it when are you getting that speed from?

0 = 1 + (-1)

Start with zero momentum. If you then throw a bunch of stuff out the back (giving that stuff -1 momentum), you give yourself +1 momentum in compensation, because the whole total must still equal the original zero.

Now, for the case where your rocket is already moving, let's sat it has 4 momentum. 4 = 4 +1 +(-1)

So again, if you throw a bunch of stuff out the back with -1 momentum, you generate a +1 momentum on what's left.

The real trick here is that as long as you let that stuff with the -1 momentum drift away, you don't have to add it back in to the momentum of your ship. But if you ever gather it up and bring it back to your rocket, you turn this into a closed loop and you lose the effect that you got from throwing it out.

[Snark]

1 minute ago, Cheif Operations Director said:

momentum is p = m x v right?

Correct.  p = mv.  However, do realize that momentum is a vector quantity (it's a direction, not just an amount), because velocity is also a vector.

This means differing momentums can cancel each other out (by being in opposite directions).

3 minutes ago, Cheif Operations Director said:

If you can not store momentum/ create then how does a rocket move  in a vacuum.

By transferring momentum.

You can't store momentum, or create it.  But you can transfer it from one thing to another.

In this case, the rocket is transferring momentum to its spent reaction mass.

Let's suppose you have a 1000 kg ship (dry mass), that contains 1 kg of reaction mass.  Let's say that the ship is pointed in the +X direction, and has zero velocity.

So, the net momentum of this system is zero.  Right?

The ship then hurls its 1 kg of reaction mass out the back of the ship (i.e. in the -X direction), at a speed of 1000 m/s.

Result?  You have 1 kg of gas moving in the -X direction at 1000 m/s, and you also have a 1000 kg ship that's moving in the +X direction at 1 m/s.

And... the net momentum of the system is still zero.  They're equal and opposite.  Measured on the X axis, the empty rocket has a momentum of +1000 kg m/s, and the momentum of the spent gas is -1000 kg m/s.  So when you add +1000 to -1000, you get zero.  Which is what you started with.

9 minutes ago, Cheif Operations Director said:

when you fire the engines you create pressure in the engine that propels your rocket forward

And propels the spent reaction mass backwards.  Equal and opposite.

[Cheif Operations Director]

3 minutes ago, mikegarrison said:

0 = 1 + (-1)

Start with zero momentum. If you then throw a bunch of stuff out the back (giving that stuff -1 momentum), you give yourself +1 momentum in compensation, because the whole total must still equal the original zero.

Now, for the case where your rocket is already moving, let's sat it has 4 momentum. 4 = 4 +1 +(-1)

So again, if you throw a bunch of stuff out the back with -1 momentum, you generate a +1 momentum on what's left.

The real trick here is that as long as you let that stuff with the -1 momentum drift away, you don't have to add it back in to the momentum of your ship. But if you ever gather it up and bring it back to your rocket, you turn this into a closed loop and you lose the effect that you got from throwing it out.

 

3 minutes ago, Snark said:

Correct.  p = mv.  However, do realize that momentum is a vector quantity (it's a direction, not just an amount), because velocity is also a vector.

This means differing momentums can cancel each other out (by being in opposite directions).

By transferring momentum.

You can't store momentum, or create it.  But you can transfer it from one thing to another.

In this case, the rocket is transferring momentum to its spent reaction mass.

Let's suppose you have a 1000 kg ship (dry mass), that contains 1 kg of reaction mass.  Let's say that the ship is pointed in the +X direction, and has zero velocity.

So, the net momentum of this system is zero.  Right?

The ship then hurls its 1 kg of reaction mass out the back of the ship (i.e. in the -X direction), at a speed of 1000 m/s.

Result?  You have 1 kg of gas moving in the -X direction at 1000 m/s, and you also have a 1000 kg ship that's moving in the +X direction at 1 m/s.

And... the net momentum of the system is still zero.  They're equal and opposite.  Measured on the X axis, the empty rocket has a momentum of +1000 kg m/s, and the momentum of the spent gas is -1000 kg m/s.  So when you add +1000 to -1000, you get zero.  Which is what you started with.

And propels the spent reaction mass backwards.  Equal and opposite.

 

12 minutes ago, Cheif Operations Director said:

trading the mass of the fuel for velocity? 

Correct?

[Snark]

15 minutes ago, Cheif Operations Director said:

trading the mass of the fuel for velocity?

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).

The ship is giving velocity to the fuel's mass (i.e. giving it momentum).  The result is that the ship's mass also gains velocity (i.e. giving it momentum), in the opposite direction.  Thus moving the ship.

[Cheif Operations Director]

2 minutes 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

Edited August 11, 2019 by Cheif Operations Director

[Snark]

It just now occurs to me that, in the same way that "rapid unplanned disassembly" is great kerbal-speak for "explosion"...

...the phrase "rapid unplanned momentum transfer" would be great kerbal-speak for "collision". 

[Cheif Operations Director]

1 minute ago, Snark said:

It just now occurs to me that, in the same way that "rapid unplanned disassembly" is great kerbal-speak for "explosion"...

...the phrase "rapid unplanned momentum transfer" would be great kerbal-speak for "collision". 

Yes, yes it would

[kerbiloid]

2 hours ago, Snark said:

Energy and momentum are completely different things. 

Were. Since this discussion, the "Energy-Momentum Equivalence law" is formulated.

1 hour ago, Cheif Operations Director said:

rapid unplanned momentum transfe

If you please, "rapid spontaneous momentum redistribution".

[Codraroll]

I read this thread this morning, but didn't get a chance to respond before now. Anyway, I think the briefest way to sum it up would be:

Draw a border around your ship and any operations you carry out around it. Unless something carrying momentum (that is, mass at a velocity) crosses this border, your ship won't accelerate.

 

As six pages of discussion demonstrates, it can be said more elaborately than that, but this should be the very core of it.

[Cheif Operations Director]

2 hours ago, Codraroll said:

I read this thread this morning, but didn't get a chance to respond before now. Anyway, I think the briefest way to sum it up would be:

Draw a border around your ship and any operations you carry out around it. Unless something carrying momentum (that is, mass at a velocity) crosses this border, your ship won't accelerate.

 

As six pages of discussion demonstrates, it can be said more elaborately than that, but this should be the very core of it.

 

5 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).

The ship is giving velocity to the fuel's mass (i.e. giving it momentum).  The result is that the ship's mass also gains velocity (i.e. giving it momentum), in the opposite direction.  Thus moving the ship.

As a note, how exactly would a space craft that gets thrust out of a laser (with the laser getting power from a reactor) meet this criteria? Is it that the laser can not be counted as part of the system?

[Snark]

3 hours ago, Cheif Operations Director said:

As a note, how exactly would a space craft that gets thrust out of a laser (with the laser getting power from a reactor) meet this criteria? Is it that the laser can not be counted as part of the system?

Photons carry momentum, even though they have zero rest mass. Each photon has an effective mass given by its energy, as given by Einstein's famous equation E = mc².

So, the laser (which is attached to the ship) imparts momentum to all those photons that it's spraying out the back end of the ship, thus imparting momentum to the ship in the opposite direction.  It's not any different than a regular rocket, really, and still governed by the rocket equation-- just with a really ridiculously insanely high Isp.

If you ask "but where is the reaction mass coming from?", it's from the ship.  Energy has mass. The ship is generating the energy to create those photons, which means the ship is losing the energy that it's emitting, which means the ship is losing mass.  In the particular case of the nuclear-powered ship, the mass loss happens when the nuclear reactions convert a tiny fraction of their atoms' masses to energy.