Trying to understand conservation of momentum [Split from another topic]

[Arugela]

[Moderator's note:  This thread was originally split from another thread about antimatter propulsion.  It was split to its own thread because it's addressing a completely different topic:  confusion about conservation of momentum and how reaction engines work, rather than antimatter per se.

The executive summary:  OP is wondering why you can't save fuel on a rocket by collecting the exhaust gases.  Other folks are trying to explain why that can't possibly work, due to conservation of momentum.

The moderators decided to split the thread, rather than simply pruning the off-topic content, because it seems like a discussion worth having:  it can help people (including folks who are just reading the thread out of interest) understand how momentum works.

So, please everyone, let's remember to be civil and polite to each other.  Nobody ever convinced anyone of anything by being dismissive or rude, regardless of the merit of their arguments.  If you don't have the patience to stay polite and respectful, best to give this one a pass and just stroll on by rather than pouring gasoline on a fire.

Thank you for your understanding.  -- The moderator team]

 

On 10/19/2020 at 11:41 AM, mikegarrison said:

Not zero momentum, though. Photons produce thrust.

I'm going to take it that light does have mass then but it's not been found yet.

Either way, why not then heat the water with gama to turn a generator to then use photons?

The next question is then there a way to collect all matter released from thrust and make as near 0 loss engines as possible. Could  large area be built around the engine exhaust to put it back into the system and still produce thrust in what is in essence a closed loop thrust system? If so some of the other steps would not be needed.

You could then even potentially build thrusters in any part of the craft for a much wider array of designs. Assuming the area does not adjust thrust a little if it pools a large amount of thrust matter as opposed to quickly moving it back. Assuming it's enough distance to matter or not in the same location. Or even farther ahead. Lots of weird stuff could be done with tanks that never need to be emptied. Or minimally adjusted.

If a system could be made to reproduce fuel in some way you could endlessly recycle it and try to power with solar or something for better efficiency. I'm assuming way more doable in space than in atmosphere. You could then only exhaust fuel as needed in gravity or in atmosphere as needed and then recollect if possible for space exploration.

Or maybe just make an anti matter chamber that produces thrust in a small location. You could even try to have multi vector thrust and other odd things.

Edited October 24, 2020 by Snark
Introductory note added by moderator

[mikegarrison]

25 minutes ago, Arugela said:

I'm going to take it that light does have mass then but it's not been found yet.

It's not that simple.

With our current understanding of physics, it would take infinite energy to accelerate something with mass up to light-speed. So photons don't have "rest mass". But energy is mass and vice versa, and photons have energy. So they have the mass of their energy, but they don't have "rest mass".

At least, that's my vague understanding of it.

[sevenperforce]

1 hour ago, Arugela said:

I'm going to take it that light does have mass then but it's not been found yet.

Photons have mass; they just do not have rest mass. Particles which travel at the speed of light (photons and gluons) do not carry any energy other than their kinetic energy.

We, along with all ordinary objects (that is, objects with rest mass) experience the universe as a combination of unbounded dimensions (space) and a fixed dimension (time). Massless particles, on the other hand, experience a universe in which time is unbounded, but space is fixed. An ordinary particle can slow down or speed up or turn; a massless particle can do none of those things. A massless particle experiences no time at all, just a fixed, eternal path between the point where it is created and the point where it is destroyed.

Of course, we observe a photon as an electromagnetic fluctuation that takes energy from one atom and carries it through space to another atom. Because it has energy, it therefore has mass. It's just a very, very small amount of mass.

2 hours ago, Arugela said:

Either way, why not then heat the water with gama to turn a generator to then use photons?

I'm not quite sure what you're asking here, but keep in mind that you can't just get power out of heat. You have to move something from hot to cold in order to generate power. The average temperature of a closed system can never decrease.

2 hours ago, Arugela said:

is then there a way to collect all matter released from thrust and make as near 0 loss engines as possible. Could  large area be built around the engine exhaust to put it back into the system and still produce thrust in what is in essence a closed loop thrust system?

No, that is not possible. The momentum of the engine exhaust striking the boundary of the "large area" is exactly equal to the momentum that the engine exhaust imparts to the engine when it is originally fired. What you are describing is a "reactionless thruster" and it is not possible to create one in this universe. You can't get a push without pushing against something else.

One common idea for creating a reactionless thruster is to say, "What if you fire your exhaust out really fast, but then slow it down more gradually?" This is a quick way to expend a lot of energy, but because thrust is proportional to the momentum of the propellant, it doesn't really help you. The total momentum gain will be exactly balanced out by momentum loss, every time.

[Arugela]

But you could imperfectly do it to try to recycle thrust materials if you had a type that could be put back into the system. It just has to increase effective fuel over time. Or whatever criteria is important. Assuming you can get more thrust out of it. Or if something else takes priority.

Say you have a thruster and a long set of rods out the back that then collect the thrust material away from the back of the thrust rear opening. This could be designed to catch thrust like a net but in a way that doesn't produce a lot of counter thrust as it's collecting looser material away from the nossel. If it's designed to efficiently collect a some types of matter but let most thrust go through can you get more fuel and stop less thrust to get a positive outcome by recycling it back through in some manner. even if you expend another type of fuel or other energy source to reuse it. could other sacrifices be made to not add too much weight to gain? Say loss of cargo. Maybe it would work for probes with small cargo bays. Or other combo vehicles. Or some sort of limit to adding/adjusting fuel storage space.

One idea could be like a solar sail but made to not impede thrust(like being porous or similar) while collecting materials from the thrust. Maybe the material naturally draws in the thrust material while not stopping the push back producing thrust. Not sure if there is a distance where the thrust material stops producing meaningful push. Of where the collection would not produce enough pull. Are there any odd material combos that can produce this in any way. Would the slowing of the potential material or a limit of collection make this worthwhile? Would ion or anything weird do this. What about plasmas? Or types of fuel with multiple fuel types and you only recollect one type as the other may be in greater amount or near infinite. Then you might be collecting less mass. Maybe it requires a special type of thrust/fuel combo and certain materials to collect in the correct way.

Maybe it would have to be part of an engine where the thruster is not primarily thrust and has another purpose.

Maybe it's good for balance in a design and making new designs workable. Any gain whatsoever could be used to shrink tanks. Or redirect them even at a loss of efficiency so some of the fuel is kept in another location for balancing or other reasons. I'm assuming this exists in fuel loop system or similar, but his could extend the loop to out of the spacecraft farther. Maybe the thrust catching could give variable balance modes also. Then it wouldn't just be with pumps or gravity base fuel lines. complexity aside(assuming).

I was assuming the effects of the thrust happened close to the nozzle. This could then allow something farther back acting as a collector to some extent. Then to avoid adding mass to the craft you could stick the thrusters near the middle to front with open air/space between and simply adjust the geometry a bit. Maybe thrusting into large cone shapes in the hull. And with or without another whole in the rear to more propulsion via squeezing the thrust into a small space over a long hull shape. This could then be the primary/secondary thrust with a lot of collectors and maybe a balancer in the middle or end. That or apply magnetism to aid as a second thrust or something depending on the things being used. Or could this be done in a larger enclosed space.

If the moving of collected materials takes less force or energy it could be viable hypothetically.

 

Quote

 

  2 hours ago, Arugela said:

Either way, why not then heat the water with gama to turn a generator to then use photons?

 

 

And I was assuming something like a turbine in the previous example to then power ion or similar.

https://www.newscientist.com/article/mg23431264-500-plasma-jet-engines-that-could-take-you-from-the-ground-to-space/

This could fun if converted into a plasma combustion or other engine type for cars or bikes. then you could have real hybrid electric air powered vehicles with the option for gases. They could be combustion or any other type of vehicle by design. You could also combine thrust and engine torque or other methods together in a more complex system. The primary fuel could be oxygen with all other things besides electricity being mandatory. I would assume it could use a variable concept with compressing air at minimum to adjust during various speeds or performance marks if needed to propel itself and whatever systems are in it. And it could then only need enough to get up to speed to gain air similar to a jet engine. Although this could be started with electricity to create pressure from a stopping position.

Or as the thread is discussing primarily what about an anti matter power source in a closed loop as a  base electrical generator.

What about something like an antimatter fision/fusion reaction or generator that goes back and forth between the two to create constant energy and utilize the effects of both?

http://ffden-2.phys.uaf.edu/213.web.stuff/Scott Kircher/fissionfusion.html

Edit: I wonder if the collection of mass for recycling fuel in a system could be offset with directional thrust/momentum(maybe via the method of collection). It might cost energy(hopefully without adding mass) but it could make the collection not impede completely in the same direction to reduce the effect. Maybe with passive collection materials. and in a system with other forces using the rest of the thrust material to produce more thrust at the end of a collection chamber. I think this is done in aircraft basically. Why not space craft? Just a matter of efficiency. Or is that offset in aircraft by the need for constant or variable thrust to begin with?

Edited October 21, 2020 by Arugela

[sevenperforce]

22 hours ago, Arugela said:

But you could imperfectly do it to try to recycle thrust materials if you had a type that could be put back into the system. It just has to increase effective fuel over time. Or whatever criteria is important. Assuming you can get more thrust out of it.

Any propellant you capture from the engine and put back into the system will be propellant that does not contribute to thrust.

If you want thrust, you have to let the propellant go. It is that simple.

Still not convinced?

Imagine, for a moment, that you have a very large spaceship with a very large, empty cargo bay. You mount a small rocket engine on one side of the cargo bay.

No matter how much thrust that rocket engine provides, it will not produce any net thrust on the spaceship because it will be counteracted by the equal and opposite force of the exhaust hitting the other side of the cargo bay. You can open up holes on the outside of the cargo bay to get thrust from escaping gases, but the ONLY thrust you will get will be from the gases that escape. You will get NO thrust from gases that remain trapped. Zilch. Zero. Nada.

You cannot lift yourself off the ground by tugging on your own bootstraps.

[WestAir]

15 hours ago, sevenperforce said:

You cannot lift yourself off the ground by tugging on your own bootstraps.

I wonder if you couldn't fire prograde in orbit of a binary black hole, and time it to get the exhaust to push you at the 45 degree X intersection of the figure 8 orbit, since the ship would be moving so much faster in the same direction.

EDIT: I think not. Exhaust moving slower than you in the same direction wouldn't help accelerate you. Damn you, Newton. Foiled my boostrap acceleration device.

Edited October 23, 2020 by WestAir

[sevenperforce]

2 hours ago, WestAir said:

I wonder if you couldn't fire prograde in orbit of a binary black hole, and time it to get the exhaust to push you at the 45 degree X intersection of the figure 8 orbit, since the ship would be moving so much faster in the same direction.

EDIT: I think not. Exhaust moving slower than you in the same direction wouldn't help accelerate you. Damn you, Newton. Foiled my boostrap acceleration device.

If you're near a binary black hole, you can use their gravitational waves to slingshot yourself to ridiculous speeds. A binary black hole is the ultimate gravitational assist machine. Conservation of energy is not violated because you are robbing the system of some minute percentage of its angular momentum.

Of course the accretion disc will shred you but that's beside the point.

[Arugela]

On 10/22/2020 at 4:00 PM, sevenperforce said:

Any propellant you capture from the engine and put back into the system will be propellant that does not contribute to thrust.

If you want thrust, you have to let the propellant go. It is that simple.

Still not convinced?

Imagine, for a moment, that you have a very large spaceship with a very large, empty cargo bay. You mount a small rocket engine on one side of the cargo bay.

No matter how much thrust that rocket engine provides, it will not produce any net thrust on the spaceship because it will be counteracted by the equal and opposite force of the exhaust hitting the other side of the cargo bay. You can open up holes on the outside of the cargo bay to get thrust from escaping gases, but the ONLY thrust you will get will be from the gases that escape. You will get NO thrust from gases that remain trapped. Zilch. Zero. Nada.

You cannot lift yourself off the ground by tugging on your own bootstraps.

Unless there is something else at play. Say if you go beyond a certain distance and it ceases to produce as much thrust. Or can be collected with angular momentum to limit the counter effect. It might be possible. Especially if you are collecting one of multiple fuels. Or if you use the angular momentum somehow like gravity production or something. It could excuse or use the loss of thrust as a secondary product. If collecting fuel for fuel maybe it can help regain a single very small thrust producing fuel to in essence extend a very small tank to a very large effective size. Say there is an additive that is used in a fuel that is added at something like 0.01% of the total fuel and this lets it's fuel last for the entirety of the other fuel types relative to it's small size and have very little. Or a stored vs a produced fuel. The 0.01% fuel is stored and the other produced from something like a fission or fusion batter source that is nearly limitless. Might be good for probes. This lets the 0.01% fuel additive work closer to the time the fission/fusion battery does. Not sure on the other fuels but maybe they don't need to be collected. That or go oddball and say the collected fuel is light and it's collected from solar panels or heat collecting panels across the surface(or from a surface that naturally does it.) this could be used to various effects depending on what is at play.

I think conceptually this is the equivlant of x=y where x is the total thrust vs a more complex formula where you view x in more complexity and only replace part of x giving more room to do things. Hence not taking up all of x as counterforce. I'm still not convinced you can't find a way to reduce the effect of collected gases to some small amount with tricks that don't have a complete conterpull. If you use angular momentum around a sphere, with other factors why can't you to some extend reduce the counterpulls effect. Or if you did something weird like fly in a spiral and collect along the edge that is helping or is pushing the spiral correctly it should count reduce the effect. Then you just have to have a reason to fly in a spiral. There should be oddities in practice where it is applicable one way or the other. It just have to have a reason to excuse the other losses. Which is normal. There are uses for additive that are not fully a matter of thrust. It's easy for other things to be more important and it at minimum part of a system to make up for some other needed functions. And with straight thrust, if anything about it is not perfectly linear then there is a use. Assuming you can get the efficiency. I read something about someone saying if it's not a scientific issue but an engineering issue it can be done ultimately. If there are any nuances in the formula to play off of then it might be possible. Just a matter of when and how. Then you might be able to make some cool stuff out of it. Maybe with specialized fuel combinations or something else that works.

This might work with ships made in space. I had and idea for using ships that can go to space to print other ships in order to make space ships. If you can get an oxygen using plasma ship to near orbit and get it in orbit you can take up small amounts of material for varied space oriented manufacturing. Preferably that make very light weight but orientationally strong material layouts. like composites that are make of lattices of small angular materials or similar printed on a small scale to form what looks like firm material. Then you can use methods to spray material on it for coatings for mirrors or protective layers if needed. Maybe even for electrical signals or other things to be sent. Then you give it a propulsion means and have much lighter weight probes and printers for research and exploration. All of these methods exist currently and basically just need to be applied. The only limit I assume exists is having enough power. You might get more unusual circumstances with these or be able to make ships needing such things. Maybe it would be good for satellites first. Even if you can eak out the smallest thrust or as part of angular controls to go with existing methods it might have a purpose. what if it helps power another function even if it has net 0 gains in thrust or movement in itself. Maybe translation of force via materials while doing other things to aid in overall ship design. Temperatures maybe. cold plasma is useful but could also control temps for various reasons. Who knows what oddities coudl come into play with different long term material uses. Any other quality is usable. And it might work well with long use things like antimatter or fusion/fusion like the one I showed.

Could it act as a stabilizing force to avoid or create an existing counterforce for engines to stop or stop loss of rotation? If you have a certain thrust it could give a collectible fuel used in rotation or to give a force to fight against to stop rotation giving room for other thrusters to not effect as much during manoeuvring potentially. Or if it could gain even the smallest gains it could be upscaled and be recyclable fuel source to counter needed thrust from forward fireing engines. If the collection is only part of the thrust and you only gather back one fuel type then you have positive gains. It then extends the fuel type(maybe needed to fire the other one like an oxidizer), but is not totally needed for thrust. Maybe this could collect needed things to help produce more oxidizer for longer trips. could be part of a longer fuel system keeping oxygen for crew and the thrusters. Maybe part of a system to produce oxidizer or something more exotic where loss of thrust is acceptable. The burning could be part of a system producing heat and using the thrusters heat to do something and then collecting some of the material back. Thrusters that are more than just thrusters. The ability to collect in space or stop at planets and collect materials for on site production without a standing station might outweight the loss. Maybe for colonization ships or whatnot. Could you collect any part of an antimatter system with such a setup.

By smallest gains I don't mean actual gains. I mean not loosing the entirety of x in x=y. (IE, using nuances of it that previously weren't. Assuming x=y equals all potentials.)

The above example could work if are recollecting/recycling/remaking oxidizer for extending fuel tank size/etc. Then you could use magnetics at the end after initial ignition for then propel with another means to produce secondary thrust from a different balance location. Placement would obviously depend on the exact design, but it's doable if needed. It's just a compound engine system. Maybe that would work better with ion or something weird and recollect some exotics put through it that effect more than just thrust. Then it's propelled afterwords and as greater thrust with directional thrust abilities where the initial is only straight forward. Maybe the exotic or additive is for temperature or other controls as part of another system.

If not maybe it could work as a part of an atmospheric thrust effect where gases moving from thrust could be used to play on the variable of the medium it's thrusting against to recollect some fuel of some sort. Maybe if the thrusters create a void area in the thrust that then collects fuel at that point while still maintaining thrust against the other points with heavier gas to push against. At which point why not do the same with magnetics or other things in low-0 atmosphere. They would just be specialized engines. If it's thrusting it's pushing off of something. This means it's variable or could be made to be variable enough to collect with less loss of thrust. this might not be the exact correct method to do this, but the principle is there if you can create any variation. which you can if there is anything there to use to thrust against hypothetically. You are never playing with a perfect medium(or perfectly or even remotely efficiently). So, you are not currently thrusting off of a 100% efficient surface. This means there is room to play off of in one direction or another. It's just a matter of understanding it and using it. If it already does something to loose effiency and the thrust material slows down from it's point of most momentum creating push wouldn't the collection of material be less if it looses momentum after pushing against the medium it's pushing against. It can't be as simple as just x=y. Nothing ever works that way or you are not accounting for the how between x and y. Even when converted into the ship movement there still should be room to do something. Even if it's just collected angularly or as part of an inefficient system as above. There is likely some wiggle room to do something. Especially the longer the thrusters have to be active because of build up of gains from the fuel system. Even if from very small gains somewhere else. Say overall tank size of balance of the ship. The more complex the functions the more room there should be for odd systems. I'm not assuming this would work in the most straight forward and simple rocket.

And going from resting mass from the idea of lights true mass vs momentum.(IE mass technically currently including some momentum or other things as a force when measured.) Does anything else share similar qualities?(assuming not everything.) If resting mass is just the reality of movement adding massive thrust via movement and is true mass. The variable nature of other materials could allow this between fuel types if it's slowed down by a medium not effecting the ships momentum. It would simply be using another medium in an existing system before collecting the fuel or matter. Just have to keep it off the ship at that point. Complex variation in atmosphere could do this naturally as it's in play in various ways potentially. And if it can be done in a dense medium like the atmosphere it could be done in space as it's technically the same thing. Just a matter of how and how much energy has to be used to do it and where the energy comes from. Basically it's translation with an outside system. Therefore not screwing up x=y. Just scaling it outside one ships systems and to a larger perspective. I think the problem is considering the current variables are simpler than they might be. If there is more too them there is more wiggle room potentially. We live in something that has lots of unknowns that could be played upon. Or current knowns not used for various reasons. Antimatter may add enough power to play on such things to start. Maybe using the explosion to shoot against the current medium and breaking it down as thrust then only collecting a small part of it. Or something far more exotic. Obviously that is getting very sci fi potentially. Maybe there are current things that could be used.

More rambling: 8)

Could you in essence create enough angular turbulence to then collect with a more efficient means like magnetics or simple redirection. Energy itself can move things to save energy with more efficiency systems. Can this be done otherwise? Could some be bounced back at the ship so the collection happens in the direction of the ships momentum? Not sure how that would work though in detail. Basically the effect of ground or other surface turbulence using space or atmosphere as the medium instead of harder matter like ground or water. Maybe this could be focused on a single set of things like oxygen particles(or to collect/redirect such particles). Could it be done in a way with variable efficiencies of materials?(a translation system) If it's not all equal and currently simplified. I assume you have to bounce it off the medium in a way as to use the same energy overall without bouncing off the ship prior to collection but making the medium do the work for you and going into the greater system. I'm guessing this would require massive thrust or something more delicate. I'm assuming that would take some very specialize thrusters made to play off of a very light medium. Possibly by very efficiency thrust targeted at hitting the medium efficiently somehow. Either hitting small particles or bouncing with nets of thrust material like using some particle that catches as many counter materials to bounce off of somehow without loosing sufficient efficiency or function.

Even if the smallest differences exist in the medium you are pushing off of or the using to push you can use it's energy hypothetically as a source to collect matter. It's just a matter of knowledge. IE, and engineering problem. Each molecule is different. If it's usable it's usable. Even if it's not currently usable. We might even already know it but can't use it for various reasons like energy. Anti matter might make up for that. Especially if engines like the one I posted can exist.

http://ffden-2.phys.uaf.edu/213.web.stuff/Scott Kircher/fissionfusion.html

Could you make a variable engine that created thrust and combine it with something that uses a "lazer" like the other ones that hit matter and in the process redirect or make it bounce back for collection via the above or a combination of destroying the particle to some degree. If it it's just bouncing ions back for collection. You could even do it if a certain combination makes it efficient for collect some stuff back while not being efficient for thrust overall. As long as it collects something important enough to use the extra energy. Especially if that energy isn't using up something significantly enough or significant enough proportionally(If it already exists as waist or with waist remaining as one example). Solar sails could be fun if combined with an engine pushing stuff at the solar sails. Maybe something long burning that helps use solar sales in deep space to create artificial sources.

Could you push with one medium to create an artificial source(that could produce thrust itself off material in space then push off that material with something like electricity or magnetism or other sources to create a ground effect and bounce some material back for collection? Any combination could be used potentially if you can find a way to do this. This could increase collection of ions for sales or oxidizer materials or be used to push off a turbulent material to reduce momentum without effecting the ship for the ship to collect hypothetically. As long as the energy is spent in the medium without directly effecting the ships momentum. Not sure what energy is spent getting it to do this or loss of overall thrust. But if you can get the effect by carefully direct it and there is existing waste it might be possible. Maybe it's possible to play off space like playing pool and get more efficient than current to not loose overall thrust. There has to be lots of waist in current thrusters. So either make it do this or use things we already do that make this happen on their own or in combination.

A lot of things are useful even if not efficient because of outside factors. so the efficiency is not really the overriding factor. If you can get more with less efficient for money or resources or other issue it can outdo a more efficient design while those factors outweigh the other issue.

Edited October 23, 2020 by Arugela

[Arugela]

11 hours ago, WestAir said:

I wonder if you couldn't fire prograde in orbit of a binary black hole, and time it to get the exhaust to push you at the 45 degree X intersection of the figure 8 orbit, since the ship would be moving so much faster in the same direction.

EDIT: I think not. Exhaust moving slower than you in the same direction wouldn't help accelerate you. Damn you, Newton. Foiled my boostrap acceleration device.

 

A bunch of tiny black holes that are encased in something to protect against the individual explosions or even mass explosions? Black hole batteries? Preferably remake able after the fact. The explosion could even be used if possible. Black hole combustion engines in space could be one concept. If not funny as you could actually fly a car in space then.

You could have a space flyable cadillac with a B8 engine!

I don't know if this is practical, but if you can print space ships with light weight methods like using vacuum and spraying light weight materials onto another lite weight material(some light weight specialized string covered in titanium or other coatings for strength along the needed length and possible the ability to take hits and bounce or other needed things(like a draw bridge.).) Could you then build a cheap generator or battery producer near the sun using it to start a cheap fission or other battery type to then be put in a space ship at some variable efficiency if the ship provides the means to maintain the reaction? Or is that not usable. Maybe it would help with the making of the fission/fusion anti matter concept or similar.

If that could be done you could even do none rigid body ships that move like living organisms.  Space amoeba ships? You could make them big enough to protect things by reducing the odds of being hit while also using other parts of the ship to protect more important parts as needed via moving them(out of the way or in the way for protection) or even producing them or other things. Depending on time and ability. You could even have multi purpose parts that float in the ship doing various tasks(especially if they are easily producable) and then have them move to the rear or other sides to jet out material as thrust of similar. Assuming they use thrust or can't use forces from inside the ship to produce momentum. A lot is doable if you can use momentum combined with other forces. You could translate with forces produced inside the ship and momentum from the body to produce thrust potentially. Therefore never loosing mass for thrust. At least not directly.

BTW, if the ship has internal gel it could also protect things inside via slowing other things momentum. Then you have basically a spaceplane that can push it's momentum vs other things to move and take hits via constant thrust production potentially. At least on demand. It could also be used to collect things in space like space debri in case of the worst case scenario. It just needs a means to repair itself and replace any potential lost materials. Assuming it's not constantly gaining materials. Could be a fun way to mine in space. You literally eat the roids and can use any materials that hit the ship. As long as you can keep repairing it's outside and whatnot.

No idea how much momentum can be taken out of objects in that way though. It might need a combination of forces to protect the ship given potential velocities in space. Maybe if the hull can quickly enough put things in the way along the edge to let the objects hit and slow down on top of anything else. A variety of methods could be used to slow things down. Especially if you see it coming. Not sure if you don't.

https://www.sciencenewsforstudents.org/article/amoebas-are-crafty-shape-shifting-engineers

Then you can start using a wider array of energy and sources like biology. And a much wider array of devices to produce needed ship functions.

Edited October 23, 2020 by Arugela

[sevenperforce]

3 hours ago, Arugela said:

(snip)

I read something about someone saying if it's not a scientific issue but an engineering issue it can be done ultimately. If there are any nuances in the formula to play off of then it might be possible.

(snip)

It is a scientific issue, not an engineering issue. It cannot be done. There are no nuances in the formula to play off of. It is not possible.

The formula is the simplest formula imaginable. It has no nuances. This is the formula:

F₁ = ^-F₂

That's it. That's the formula. There's no room in that formula to play around.

A particle in the exhaust of a rocket ceases to produce thrust when it is no longer in contact with the engine bell. Once it leaves the engine bell, it is lost forever. If you attempt to use ANY structure to recover that exhaust particle, then it will produce an opposite thrust on that structure, thus canceling its initial thrust.

What you are suggesting cannot be done.

[Arugela]

Yes it can, hypothetically, be done. And yes the formula has nuances. If you expand the one side to display all variables possible you have smaller areas to play off of.  That is where design plays in and yes you can catch some things(or a lot depending on chosen design.). And there are lots of inefficiencies to utilize potentially...(or just plane old tradeoffs.) What you are saying is not how real formulas work vs real life. BTW, pure simple efficiency is often or normally over ridden by other things. The efficiencies you are normally talking about either don't apply or are a part of other unstated considerations making up the current circumstance and within it. Which often contradict other generic efficiencies which are also other limited considerations.

IE, you are not looking at the thrust in relationship to the rest of the system(s). And not looking at how you can sacrifice things to produce functions. That is literally as oversimplified as you can get it. It's not an argument in real life. It is not a scientific issue.

You are basically only considering the simplest of rockets(And none of the inefficiencies.). IE there is nothing inherently inefficient about the existing design. And not considering things about the thrusters and more advanced complex designs utilizing those things we don't bother with atm.

You can expand the sides of the formula to consider/represent nuances. Plus when your thrust system engages in an, "outside," system as part of the formula, etc. Which is technically already a part of it. There is no end of things you can do if you want to. There is, but that is the actual limit of reality.  Which is not that simple. Any single small unconsidered factor of that thrust, or it's accompanying systems, means you can do more. You are assuming your thrust takes into account all things possible. That is not the case as you do not know enough about it to see what the current consideration fully is. It's just the limits of your current understanding. Not the actual limits. Yes, the formula technically represents thrust  as it's a simple formula. but it does not represent all the things that formula represents in the real world if you expand the formula to it's natural actual entirety.

Case in point. The current thrust is virtually unguided particles flying against a disorganized(assuming) medium(large mass to hit other small mass in space because of lack of guidance(like WW2 bombs vs modern munitions). If those factors are changed you can produce more or less thrust(IE different). You are literally assuming no inefficiencies or variables. Which means you are flat out wrong. The very design of a bell shooting matter at air or space is by design very disorganized and inefficient. It's sort of the point.(literally simplicity of design and manufacturing over complexity.) There are many factors that could then be used to do this relative to an existing design. Especially as your current design is only a set of inputs. And in a limited variation/consideration of the formula. It's that simple! Else you could literally never produce more efficient engines or any other designs period! This is not complicated. That is apart of the formula by nature. This is how the real world works. If there is more than one efficiency you are wrong. There are more efficiencies than one design. Simple as that.

And if you follow this logic you are also assuming your understanding of mass is correct(Or endless other things.). That is why I brought up or expanded on what someone else said about light and resting mass(or I should say made a point of). If other things have different resting mass than the current measurement you have potential room to utilize it(assuming I'm using resting mass correctly.). Therefore it is an engineering issue. As things are currently constantly in motion when measured if their existing mass measurements includes some other factors including things like/producing momentum. This means anything not using true mass could be utilized to do better. That means your current understanding is simply flawed along with your proposition. Very deeply might I add. You are literally assuming everything as simply stated is as complicated as it gets. That is not the case and never to be assumed to be the case. Particularly as the human brain uses partial sets of data collected over time via sensory information to produce thought. This means the base presumptions is always limited as we are designed physical weak and incapable of holding all information for efficiency reasons by design(This used to be better understood in academic areas and is the natural basis of everything in the world in relationship to our understanding. IE science.). Therefore it's incorrect to argue except as a potential argument. Just like everything else.

IE, if the current measurements are not singular things but the product of multiple variables(or even multiple things/objects/forces by whatever definition that is.) then your viewpoint is completely mistaken on it's face. It's then potentially correct if things happen to line up that way. But they are potentially/literally incorrect if they don't. which given the relative simplicity vs reality naturally inherent in ideas could also appear or exist as one thing at one point and another at a different. And taken farther, if all measurements just happen to be when it is in that form it could have another unmeasured form when it's not we don't know of or haven't measured(Which could still be the produce of one or more things in any variation we can't consider or know about in any variation to the natural extent of the most generic extreme logic(IE, an unkown.).). That is always the case in real world application and the basis of proper logic. IE, accuracy and considering the situation of the idea(IE our brain as a machine processing as the type of machine does.) and our unknowns in it as and any other extremity or circumstance that we can or can't conceive of. Proper logic/science is we don't actually know if matter or anything else even exists. It could be a product of many other things that just happen to do x and y time when measuring and always fit the criteria. No statistic is logical to say x amount of measurements overrides this either as we naturally have no perspective to the extremities to say what it is set against. Any attempt to do so is logically flawed. This is proper, "science." At most we can say we keep doing x and this registered as y. nothing more. Anything could be under the hood.

Realistically, you are looking for if those complex things can be done simply from a human perspective. The more you know the more likely you will find a method fitting those other circumstances. The things we use most are often things that are complex but are made simple as things around us do the hard parts for us.(We live off this concept in the form of plants and animals and all thing making our environment. which are factories that are far more complex than things we produce as humans.) It's not about potential/complexity in the generic sense as it is making something do the work for us sufficiently making it relatively simple. Or within our criteria as other machines(IE, interfacing). This includes materials and all other existing objects/forces/etc and how much we have to do to make them do what we want. This bleeds into economy and it's basis is the knowledge collected by what we gain in sensory data as it produces rarity via things like relative ability and many other factors. Although there are a bunch of nuances no longer understood well that are parts of other formal subjects you have to stick together sufficiently.

And if thrust is not pushing off any medium in space, but the ship itself, you still have the efficiency of it from the space ship to use. There should be lots of inefficiencies there to improve.

BTW, if you are just collecting materials from burnt oxydizer to recycle which could be ignited in a small chamber to start with to produce other thrust why is that problematic. The majority of thrust can be from the other materials. Your just basically filtering out materials to reuse some. It could avoid interfering with most if not all thrust potentially. And obviously ion can go through glass(might be thinking the wrong thing.). So, you could burn something and collect it where not collecting ion. It would just have to burn with insufficient thrust to not override the ion thrust. This could be done and so could other combinations. Or if I'm thinking that out wrong light could be produced with expent, "thrust," and collected for secondary reasons and the light could pass through gas to generate(translation of energy) a second thrusters with more thrust. It's a matter of proportions. This can be done with magnetism and other factors as well. I'm assuming this is more useful if you have a reason to produce a ship that does not adjust mass from thrust. Or to minimize it. Having massively increased production of oxidizer on board could be useful. Does anything allow you to do something simple like burn and oxidizer store it in a solid/burnt state and then recreate it from the mass? can anything collect it that easily or would a low? thrust engine be useful where the collection forces from what is hopefully a small amount of oxidizer be collected and reconverted?

And if you are talking bells and inner chambers you can always design them better and then do more with them like recycling materials instead of increasing thrust. Simplest. This could also be used to control the explosive medium to get better thrust potentially like the concept of using ground turbulance or something. No idea on the lifespace of the microchanels or whatever used. If you could redo them between bursts. Maybe with a coating method using vacuum and some very small materials between thrusts.

I'm assuming the more exotic things won't be useful without more powerful energy sources to power them. Unless we can do it efficiently now.

And maybe the amoeba concept uses magnetics or light or something to push off of. Maybe an inner solar system ship? Even very low thrust would work if unmanned. Maybe using the idea of those toys your rock back and forth onto build momentum. And maybe it's filled with gases instead of gels. Maybe the shell is like a giant solar sail or similar and the motions tries to play off of it. Using multiple types would allow you to stop as you don't always move in one direction. I wonder how scale of the ship to the materials matter. What is the most optimal scale based on the limit of the materials. Would small be better or does increasing size help to reduce relative thickness for less overall mass compared to engines or other thrust.

At that point can you use stuff to mess with the explosion in the chamber? Maybe using magnetics or ion or a combination drive. Maybe at lower thrust? If it's all going out the same hole... Or do soemthing stupid like hold ionic gas in the amoeba like a blimp and use motion to push it against things on the wall to produce thrust or make it escape.. Maybe translate it into something else.

Could you take the concept of lightweight production I mentioned previously(if anyone can find it.) and make a fission fusion or other manufacturing plant around the sun using it's magnetic field and resources to simplify it and then produce antimatter? ( I keep typing fission when I mean fusion. If i said fission previously I meant fusion.)

Just found this: https://www.nasa.gov/vision/universe/solarsystem/rhessi_antimatter.html

https://www.scientificamerican.com/article/how-do-we-know-that-dista/

Here is a stupid idea. What if the middle of the sun and other things contain anti matter that formed a container and then started producing fission or started to gather normal matter around it and it's all under our feet. The earth could be a sun with a hard shell that then mixed materials into the outer layers until it became earth creating lava  and other concentrated elements. Call it the anti matter universe instead of electric universe... 8D (could one theory prove the other?!)

I guess that sort of fits this: https://www.wired.com/2010/07/dark-matter-sun/

Quote

The buildup of dark matter could solve a pressing problem in solar physics, called the solar composition problem. Sensitive observations of waves on the sun's surface have revealed that the sun has a much easier time transporting heat from its interior to its surface than standard models predict it should.

What if it is not being produced at the center but at a higher part of the sun? If it's 5 times higher than predicted it could account for a seeming speed variant. IE, if the center is largely anti matter with a containment shell of some type naturally and then normal matter with the heat starting in essence at the separation for certain purposes.

https://newscenter.lbl.gov/2010/11/17/antimatter-atoms/

Would it help to also build a collider near the sun in orbit and use relatively higher temps to do research as to not need to lower it as much?

Also, what if gamma from solar flares is from escaping gamma from an anti matter core? I was just reading something about the observed anti matter or gamma rays not exiting the sun from flares like expected if magnetics or something were the cause.

Edited October 24, 2020 by Arugela

[sevenperforce]

5 hours ago, Arugela said:

And yes the formula has nuances. If you expand the one side to display all variables possible you have smaller areas to play off of. 

No, the formula does not have nuances.

F1 = -F2

There is nothing to expand. There are no other variables. That is the entire equation. 

If an exhaust particle produces a force on your vehicle (that’s thrust), it experiences an equal and opposite force propelling it away. It is now moving away (and at a high rate of speed too). If you want to stop it, you have to exert force on it. And if you exert force on it, it will exert an equal and opposite force on you.

That is the way reality works. Arguing otherwise is like arguing that you can generate infinite energy by plugging an extension cord into itself.

[SOXBLOX]

7 hours ago, Arugela said:

Yes it can, hypothetically, be done.

*facepalm*

No, except in a universe where physics has no power.

Imagine a really simple rocket. It consists of a man with a bucket of rocks, floating in space. He throws a rock. The rock moves one direction, and he moves in the opposite direction. (Remember how guns have recoil? Same thing.) 

You are asking "What happens when the man catches the rock after he throws it?", say, in a net, or something. Here's what happens. The rock is caught, and stops moving. Because of the recoil, from when the man threw the rock, he was moving the opposite direction. Now, the rock hits his net, and slows him back down by the exact same amount.

I had this same exact problem when I was nine. I drew pictures of ships, powered by electric motors, with generators attached to them. The ship would, I thought, recharge its batteries as it sailed, by tapping its screws for energy. Obviously, there are so many problems with this that I'd have had a better chance of jumping over Mt. Everest than making this work. Kinda embarrassed to put that out there...

[Arugela]

Except that it can be solved by only catching the fumes from the bullet just like with a silencer and then recycled to be used. It's how things work in the real world. You are not thinking it out enough.

Imagine if prior to the bell you have a bunch of tubes in the shape of a helix with maybe tubes going backwards and up if needed to catch materials or something instead of a normal chamber(loose example). You have several injections at the top of the helixes. You then use the walls and chambers or whatever is needed to somehow collect some of the oxidizer and recycle it in the process of thrust reducing the use of oxydizer release and recycling it with the trust itself via it's heating action somehow. either via natural things in the thrust or via the release of other materials into it or whatever works. It's doable and could increase the overall oxidizer via production using the thrust process to recycle while burning.

No idea the exact methods or shapes need. But this is just a sloppy example. The concept of shaping itself can be used to increase efficiency for thrust in general or other things. This is what nobody gets about things like infinite energy concepts. They are all real world examples. They are the natural extremity of designing for efficiency increases. It's a part of real world engineering. You just have to think it out a little. They are always applicable in the most blunt sense. That is literally what they all are naturally. Literally textbook cases.

Edited October 24, 2020 by Arugela

[Snark]

Hi everyone,

Just a note that we've split this thread from the antimatter propulsion thread, since it's completely off-topic there.  This discussion has been about conservation of momentum, which is an interesting topic but has nothing to do with antimatter per se.

Here's the executive summary up to this point:

• @Arugela is proposing that a spaceship could save fuel (or, more properly, reaction mass) by somehow collecting the exhaust gases so they can be re-used.
• Everyone else is trying to explain why the laws of physics-- specifically, conservation of momentum-- means that this idea is completely impossible.

To be clear:  Conservation of momentum is a thoroughly upheld physical law, with centuries of experimental evidence behind it, and has been well understood since Newton's day.  Violating it simply isn't possible-- this is a matter of basic physics, and no amount of engineering will be able to get around it.  That's not even hypothetically possible.

With that in mind:  Please feel free to discuss back and forth, everyone.  Discussion and debate are fun and interesting.     The most important rule to remember is that we're all friends here (right?), and it's never okay to resort to personal attacks, name-calling, abusive behavior, etc.  If you feel frustrated, that's understandable; it's difficult to resist when duty calls.  But please, do not take your frustration out on anyone else.  If you can't be patient and polite, then just take a breather and go elsewhere.

Please remember that being dismissive, rude, or disrespectful never convinced anyone to change their minds, regardless of the merits of the argument in question.  (If anything, it tends to just make them mad and dig in their heels even further.)  So please try to remember that the goal here is to have a productive discussion, not to "win".

So, I trust we can all handle this discussion like friendly adults? 

Thank you for your understanding.

 

 

[Arugela]

I think you are missing the point. You do not have to collect all the gases. You just have to need to collect some of it badly enough to be viable design. Hence you are not stopping the thrust. Just some of it. This is useful when one thing is prioritized over another. This is probably more useful as you get more complex engines overriding simpler ones.

And why can't you design specialized inner engine chambers to recycle an oxidizer or something in the process of burning potentially? It could even be shaped to help with efficiency. It's just a matter of knowing how. Current designs are not the peak of what can be done. I'm assuming you can cake the walls with it via something simple potentially for a possible use with modern rockets. You could then add into that point or design around a process using physical features of the rockets walls and possibly injection to recycle in a simpler manner. Assuming there is a process usable to do so. Or whatever works.

It's not necessarily the gases. It could be anything collected to expand tanks or anything else. It's just a generic example. Whatever allows functional expansion of the tanks or similar. Even more generic than that actually.

Edited October 24, 2020 by Arugela

[Snark]

2 hours ago, Arugela said:

I think you are missing the point. You do not have to collect all the gases. You just have to need to collect some of it badly enough to be viable design. Hence you are not stopping the thrust. Just some of it.

Actually, my impression is that people have gotten that point, and are explaining why it's problematic:

• Rockets have thrust because they expel mass.
• The amount of thrust is, roughly put, the product of exhaust velocity and the amount of mass expelled.
• If you capture some of that mass, then you eliminate the thrust it produces.  For example, if you capture 10% of the mass, you also reduce the thrust by 10%.  If you capture 90% of the mass, you reduce the thrust by 90%. 
• This means that there's basically no point in trying to capture.  The net result of any mass you expel and then capture is zero, due to conservation of momentum.  You will have wasted some energy, but won't save any momentum, so it's not useful as a propulsion technique.

If what you're saying is that you can use some sort of contraption to improve the efficiency of the thrust by ensuring, for example, that all the expelled matter goes perfectly straight backward, and not spraying  out in a cone (basically, some sort of collimator for rocket exhaust), then sure, that could help and would be a matter of engineering rather than basic science.  Is that what you're proposing?  If so, then the way to think about this is "how to focus rocket exhaust better" (which could hypothetically have some benefit, up to a point, by eliminating cosine losses), and not "recapturing exhaust gas" (which would be pointless and counterproductive, due to conservation of momentum).

[Regor]

1 hour ago, Arugela said:

Except that it can be solved by only catching the fumes from the bullet just like with a silencer and then recycled to be used. It's how things work in the real world. You are not thinking it out enough.

Imagine if prior to the bell you have a bunch of tubes in the shape of a helix with maybe tubes going backwards and up if needed to catch materials or something instead of a normal chamber(loose example). You have several injections at the top of the helixes. You then use the walls and chambers or whatever is needed to somehow collect some of the oxidizer and recycle it in the process of thrust reducing the use of oxydizer release and recycling it with the trust itself via it's heating action somehow. either via natural things in the thrust or via the release of other materials into it or whatever works. It's doable and could increase the overall oxidizer via production using the thrust process to recycle while burning.

No idea the exact methods or shapes need. But this is just a sloppy example. The concept of shaping itself can be used to increase efficiency for thrust in general or other things. This is what nobody gets about things like infinite energy concepts. They are all real world examples. They are the natural extremity of designing for efficiency increases. It's a part of real world engineering. You just have to think it out a little. They are always applicable in the most blunt sense. That is literally what they all are naturally. Literally textbook cases.

Please don't tell me you believe in infinite energy machines as well. 

[Spacescifi]

 

To put matters even simpler, "For every action, there is an equal and opposite reaction."

I think Newton said that,  but that is conservation of momentum in a nutshell. If you must go forward you push back or throw something back or vice a versa.

Recapturing spent 'pushes' or 'throws' will actually weaken the thrust you get in any given direction.

[Regor]

1 minute ago, Spacescifi said:

 

To put matters even simpler, "For every action, there is an equal and opposite reaction."

I think Newton said that,  but that is conservation of momentum in a nutshell. If you must go forward you push back or throw something back or vice a versa.

Recapturing spent 'pushes' or 'throws' will actually weaken the thrust you get in any given direction.

Exactly, it will be weakened by exactly the ratio that was recollected.

[Spacescifi]

Just now, mabdi36 said:

Please don't tell me you believe in infinite energy machines as well. 

 

Gravity more or less would be such a thing... if we had portals...or warp drive.

 

[sevenperforce]

7 hours ago, SOXBLOX said:

I had this same exact problem when I was nine. I drew pictures of ships, powered by electric motors, with generators attached to them. The ship would, I thought, recharge its batteries as it sailed, by tapping its screws for energy.

I think we all came up with ideas like this. I had a system in which gravity and magnets and electric motors worked together to produce perpetual motion...of course I didn’t realize that the energy required to operate the electric motor was equal to the potential energy gained by the magnetic field.

A lot of it has to do with a lack of understanding of force, energy, momentum, and mass. You can do a lot of fun things with different forces in different directions but you’re not going to violate the conservation of energy. You can waste a lot of energy but you’re never going to violate the conservation of momentum.

1 hour ago, Arugela said:

Except that it can be solved by only catching the fumes from the bullet just like with a silencer and then recycled to be used. It's how things work in the real world. You are not thinking it out enough.

Well, the example proposed by @SOXBLOX was someone throwing a rock, not shooting a bullet. But let’s use your bullet example because it might get you closer to where you are trying to go.

Over and over you keep talking about trying to capture the gases or some parts of the particles, only somehow without preventing the thrust. This doesn’t work, of course, because the exhaust gas IS the thrust, in any meaningful sense. However, with the bullet example, you have two different components, right? You have the bullet itself and you also have the escaping gas, and they both contribute to recoil. Is there something you can do there? Let’s find out.

A 5.56x45 mm NATO round fired from an AR-15 uses two grams of powder and fires a four-gram bullet. I have a feeling we will get tripped up if we try to talk about force, so instead let’s just focus on momentum. The bullet, escaping from the barrel at 950 m/s, has a momentum of 0.004 kg * 950 m/s = 3.8 kg*m/s. The recoil momentum imparted to the rifle is -3.8 kg*m/s. But, just as implied, we aren’t done yet. We haven’t factored in the exhaust gases from the propellant. The exhaust gases are also escaping from the barrel at 950 m/s and they mass 2 grams, so they have a momentum of 1.9 kg*m/s, making the total recoil momentum imparted to the rifle -5.7 kg*m/s.

But let's say we now want to capture that escaping gas and recycle it somehow, so we put some sort of balloon structure around the muzzle that will allow the bullet to escape but will trap the gases. Let’s say it can trap 100% of the gases. The gases exit the barrel at 950 m/s, but they are immediately slowed down and stopped by the balloon structure. That stop imparts -1.9 kg*m/s to the gases, and their equal and opposite force on the balloon imparts +1.9 kg*m/s on the balloon and the entire rifle. So the gas is trapped, but the total impulse on the rifle is -5.7 kg*m/s + 1.9 kg*m/s = -3.8 kg*m/s, just the same as if the bullet had been fired but the exhaust gases had never escaped at all. So yes, you can capture part of the propellant, but that only makes it as if the propellant had never existed in the first place.

”But what if you added a muzzle brake or a silencer that redirected the exhaust gases out at an angle to make them easier to capture?” And sure, you can do that. Let’s say your muzzle brake directs the gases out to the side at a 45 degree angle. That way, the balloon can expand outward radially instead of expanding backward and half of the forces will cancel out. Good, right? The problem is that directing the gas out at a 45 degree angle also reduces the impulse provided to the rifle by half. So in the end, the recoil imparted to the rifle is still just -3.8 kg*m/s, no matter how the gas is captured.

22 minutes ago, Spacescifi said:

Gravity more or less would be such a thing... if we had portals...or warp drive.

If we had a machine that violated the conservation of energy then we would have a machine that violated the conservation of energy.

[Vanamonde]

I believe the mistake being made here is assuming that the expelled propellant loses its momentum to the environment, leaving it slowed and therefore ready to be collected again for a net gain of momentum for the ship. But in a vacuum, the exhaust retains its momentum until it hits something. And if the thing it hits is an extension of the ship it came from, then the exhaust's momentum goes right back where it came from, on the ship, for a net gain of zero. 

But let's assume that this does work. There's still the problem of collecting the propellant which the ship is leaving behind, when the whole point is to make the ship go one way as fast as it can by making the propellant go the other way as fast as it can. How could the ship reach back in its wake to collect that? It's a bit like the idea of a car discarding cans of gas as it goes and then proposing that the car could burn that gas it has left on the road behind itself. 

[Deddly]

I'll also add that devices exist for guns that divert the propelling gas sideways. These devices work very well at doing that. Why would you want to do that on a rifle? To reduce recoil. That should also help you see why even diverting some of the exhaust off to the side is a bad idea if you want recoil (thrust) on your rocket.

[HebaruSan]

10 hours ago, Arugela said:

You then use the walls and chambers or whatever is needed to somehow collect some of the oxidizer and recycle it in the process of thrust reducing the use of oxydizer release and recycling it with the trust itself via it's heating action somehow. either via natural things in the thrust or via the release of other materials into it or whatever works. It's doable and could increase the overall oxidizer via production using the thrust process to recycle while burning.

No, that also can't work, and not in a trivial way; it's a fundamentally non-viable idea. The whole point of using oxidizer is that it reacts chemically with the fuel when you burn it, releasing energy that helps shoot it out of the engine faster. This means that when it leaves the engine bell, it is no longer "oxidizer" in any meaningful sense; instead it is bound up in exhaust gases.

Consider the example of an engine using hydrogen fuel, as in the Space Shuttle Main Engines. You start out with two tanks, one containing H₂ and the other containing O₂. When they reach the combustion chamber, they burn and release energy, and this reaction turns them into H₂O. As a result, what shoots out the engine bell is no longer two separate streams of H₂ and O₂, but H₂O. Any attempt to extract O₂ from that exhaust will fail.

And before you try to what-if or what-about us with other fuels, the above is true of any and all fuels used in rocketry. Oxidizer, in any form, is used specifically because it reacts chemically with fuel, in any form, to help propel the exhaust out.

(Discussion of oxidizer-rich or fuel-rich mixtures elided to avoid confusing the core problem. These fall under the primary problem of slowing your ship down when parts of it are hit by exhaust.)

Edited October 24, 2020 by HebaruSan