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

[mikegarrison]

Here's something you could do.

Let's say you have a photon drive. You have two ships, one with the drive. Both have big mirrors attached.

You shoot the photons from your ship. They hit the other ship and bounce back to you. They hit your mirror and bounce back to the other ship. They bounce back to you again. Etc.

You get extra thrust from the bouncing and rebouncing photons, but it's not free momentum. Why? Because the other ship is accelerating in the opposite direction. The two accelerations (one "+", one "-") cancel each other. (Now in practice you lose photons because mirrors aren't perfectly reflecting, some of the photons spread out and miss the mirrors, etc. So it's not infinite energy, either.) In essence, THE OTHER SHIP is now your reaction mass!

If you had a regular rocket and collected your spent reaction mass, that would (in principle) work to get your reaction mass back. But your collector would be accelerated away from your spaceship in the opposite direction. So you have no way of getting the reaction mass back to the ship with the rocket. It would all end up in the ship with the collector.

And if your collector was somehow attached to your rocket spaceship, the positive acceleration from shooting the reaction mass out would be cancelled by the negative acceleration of collecting it, and you would go nowhere.

Edited October 25, 2020 by mikegarrison

[Vanamonde]

Some content has been removed. Please do not post just to make fun of fellow forum members. 

[wumpus]

On 10/24/2020 at 7:58 PM, mikegarrison said:

Here's something you could do.

Let's say you have a photon drive. You have two ships, one with the drive. Both have big mirrors attached.

You shoot the photons from your ship. They hit the other ship and bounce back to you. They hit your mirror and bounce back to the other ship. They bounce back to you again. Etc.

You get extra thrust from the bouncing and rebouncing photons, but it's not free momentum. Why? Because the other ship is accelerating in the opposite direction. The two accelerations (one "+", one "-") cancel each other. (Now in practice you lose photons because mirrors aren't perfectly reflecting, some of the photons spread out and miss the mirrors, etc. So it's not infinite energy, either.) In essence, THE OTHER SHIP is now your reaction mass!

If you had a regular rocket and collected your spent reaction mass, that would (in principle) work to get your reaction mass back. But your collector would be accelerated away from your spaceship in the opposite direction. So you have no way of getting the reaction mass back to the ship with the rocket. It would all end up in the ship with the collector.

And if your collector was somehow attached to your rocket spaceship, the positive acceleration from shooting the reaction mass out would be cancelled by the negative acceleration of collecting it, and you would go nowhere.

I'm failing to see how any photons after the first bounce are not "free momentum".  Now granted, it is a special case where two ships want to go in diametrically opposite (or will agree on gaining momentum in opposite directions, to be corrected after they are too far apart to be useful).  If you have to drop a mirror (even a small amount of mylar, forgetting for the moment that you have to keep it aligned to the spacecraft and rigid), you will likely lose every Isp advantage that photons have over ions, without remotely gaining any power advantage over ions.

And once you really get going, even if you can manage to keep the beam focused (you can't), expect to lose energy as each bounce shifts the photon more and more red (the low momentum side of the spectrum).

So think of it as a very silly first stage, using a method of propulsion that should be for last resorts in some theoretical last stage.  But the extra bounces are pretty free.

[mikegarrison]

21 minutes ago, wumpus said:

I'm failing to see how any photons after the first bounce are not "free momentum".  Now granted, it is a special case where two ships want to go in diametrically opposite (or will agree on gaining momentum in opposite directions, to be corrected after they are too far apart to be useful).  If you have to drop a mirror (even a small amount of mylar, forgetting for the moment that you have to keep it aligned to the spacecraft and rigid), you will likely lose every Isp advantage that photons have over ions, without remotely gaining any power advantage over ions.

And once you really get going, even if you can manage to keep the beam focused (you can't), expect to lose energy as each bounce shifts the photon more and more red (the low momentum side of the spectrum).

So think of it as a very silly first stage, using a method of propulsion that should be for last resorts in some theoretical last stage.  But the extra bounces are pretty free.

In terms of conservation of momentum, it's perfectly "legal". Any momentum you gain from the bounce has to be offset by negative momentum (in the opposite direction) from the ship you are bouncing the photons off of.

Consider when SpaceX separates the first and second stages from the Falcon9. They just use some kind of gas piston or spring or some such thing, right? Well, in this case the photons are just the spring in the system, pushing the two ships apart.

It wouldn't even have to be photons. It could be a stream of BBs. But the nice thing about photons is that they always move at the speed of light. However, they wouldn't be able to accelerate you infinitely. I'm sure that some sort of time/space effect would cause them to lose energy compared to you as you sped up. Probably related to "redshift" or something like that.

Edited October 26, 2020 by mikegarrison

[JoeSchmuckatelli]

On 10/24/2020 at 8:45 AM, 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.

...

So - as to this, specifically.  When you have a semi-automatic weapon, you actually siphon off some of the gas that would otherwise be pushing the bullet to do the work of activating and energizing the reload mechanism.  So, let's say that you have two weapons with the same length barrel firing the same caliber of bullet.  With the semi-auto, you get 52,000 psi at the muzzle, but with the single-shot, bolt action you might get 52,500.   The muzzle velocity of the round fired from the semi-auto will be lower, 3100 fps vs 3790.

The simple fact of capturing the exhaust gas robs the bullet of energy, and that energy is used to unlock, extract, eject, feed, chamber and lock the next round into place for the convenience of the gunner.  Either way, its a trade off - you either get the most energy into the killing round, or the ability to reload very, very fast and fire subsequent shots.  You don't get both.

 

So - this analogy does not work to help explain your point.

[Arugela]

No, it does. That was the point. I'm assuming if you want you can improve other aspects to regain efficiencies with more work in other ways or leave as is. Like better barrel muzzling or custom bullets to go through it at extra lengths to achieve the original performance. You could even improve the part where you collect the gas. There should logically be means of improvement if going for pure performance. If you want to go through the hassle. Were not advanced enough to out of means to improve something. Not counting cost. the power provided by anti matter might make room for some functional things. Or bring up new necessities in practice. Considering the increase of the scale of it's uses.

The conversation started in a discussion about dark matter engines. I assumed this was reasonable to do.

Here is an idea. Pre chamber the/a gas. Do so without shooting a bullet. Then use the gas after powering the other features to push behind the bullet to keep the original performance. Or as close as possible.(this could go towards a system with the gas not in the bullet like a pea shooter or paint gun then.) Yes, this adds weight or something(maybe). But you can do it. I don't see where the argument is against this. Cost is a factor but it might be usable by the time you are running much more powerful rockets. You will probably need it when doing manned mission as you need a lot of other things anyway. You can prioritize function as you might be going x-y for mars or something and the safety backups outweigh the basic ship design. Especially if these functions are then part of a system on the planet or in orbit as a safety function in case of problems on the ground. It goes into other areas depending on what it is as the systems scope increases. Current designs are primarily around shorter missions. You will get drastically different realities as we try to go farther than LEO. All sorts of other things will come in changing efficiency to things like raw materials at lower efficiency for instance. We will not be playing with the same rocket designs potentially. If you can mine or reuse things you got more options. Our current environment is from a limited reality of buying or making parts in a very specific limited fashion/ecosystem. That can change in may ways. You could already run into this if someone donated a bunch of materials. You can then use it at lower expenses from weight in a bigger craft. Or other realities depending on the materials that could easily break design models. Priorities change. A lot of things come from existing fab equipment costs or whatever. Those realities can all change drastically. Especially if you start mining in space. You are not considering enough factors and how much of a difference there can be as we expand or any other circumstances. There are probably tons of thing we can do now we are stupidly ignoring. Either from just using existing things because of hassle. Or an assumption we can't do better and not looking farther. Why can't we make light weight fabs in space on the ISS. We have existing methods that can be used and fit the normal paradigm of light weight space construction. We are not trying much. All of them are very mature technologies and decades to centuries old or older. But nobody is doing it. We could send very light weight materials up saving rocket cost or payload weights for that purpose and make probes in space saving many other factors. But we choose not to do so. Ironically, the opposite of traditional realities of scientific experiments. You could literally make small probes with cheap materials and even fabrication parts that could fit in an astronauts pockets... But we choose not to. Over very expensive very heavy probes. I can only imagine the reason.(I assume it's because of things that should not be factors.)

Vacuum chambers are a lot cheaper and lighter in space. They cost and weigh less than a screen door! 8)

If you have small and cheap probes. Assuming size is a factor. You can do lots of things we can't because of money constraints. And yes, we can do this now!

Here's about as cheap as it gets. You want a hull. Take up a bag of hefty trash bags!!! That is an extreme example but NASA in general is no way past doing that in ground experiments... Why do we need only multi ton or 100's of pound gold covered satellites when we could get away with much less and much more. There are things that could last longer than a trash bag too. Take actual spider or silk to the ISS from the silk bugs and use it's tensile strength and spray with titanium to make rods to build with on a very small scale. You can use tiny spools of materials to do it and make a probe. Just make tiny ion engines or something and have a blast. Get live observation over every planet and keep a live map and dedicate our resources to keeping a constant scan on all objects in the solar system. Or for other research projects. We have the ISS. We should probably use it to it's fullest while it's still there. Unless we just want to go back to the stone age.

BTW, we could get much lighter coatings of gold also if fabricated in space. Or I'm assuming we can. If not maybe cheaper. Unless it something about getting very exact application over loosing materials in the process.

Assuming you can drop the hull mass. Hypothetically you can get other qualities including cheaper engines even if the are less efficient. This could save cost or be convenient. Especially if you can use extra cheap materials. I don't understand why we make satellites out of heavy ground base materials like large metal rods. Why do we even need a solid structure. I'm sure some of it is for longevity, but if it's cheap enough you can simply repair. Does it help enough to avoid getting hit by debri with the speeds involved? Maybe stuff that is designed not to spread easily that is super light weigh. Or is likely to contact and connect with itself to be easily collectible. Or something that is just like an airbag that can have stuff go through it. Preferably with low debri or easily collected or destroyed debri. We seem to be making things out of the worst possible materials and criteria because of ground manufacturing and who knows what else.

Here is an idea. Deeps space hubble 2.0. Make super light weight structures like I mentioned above(replace with better if needed.) then make a massive mirror out of super light weight materials and coat in space with titanium or copper or gold. Make an array of these at the top/bottom/other important gravitational places in the solar system and make a massive hubble array for cheap. Or simply a bigger one.

I'm either missing something or we are seriously lacking in ambition and imagination. Is there a quality problem with the coating process? I would imagine you could make good enough equipment to make a massive array in space. Nasa has the budget probably. And the part doesn't need to be big potentially.

 

I imagine a small satellite/probe in space is ideal for this type of manufacturing. Or could be made to be so. You could probably make a better lighter base than that glass also. Maybe even more precise.

Edited October 26, 2020 by Arugela

[SOXBLOX]

[Snip]

2 hours ago, Arugela said:

But nobody is doing it.

Since most of the folks not doing these things are aerospace and physics degrees, and since this field is entirely within their purview, is it not likely that there is a good reason for this?

2 hours ago, Arugela said:

But we choose not to.

Yes. We put expensive alloys on our probes just for fun.

No. Actually, they are what protect the highly optimized, high-tech spacecraft from radiation, something Hefty garbage bags would not do.

2 hours ago, Arugela said:

Then use the gas after powering the other features to push behind the bullet to keep the original performance. 

*mostly back on topic*

That actually wouldn't work. "Using" the gas means pulling energy out of it; that means less energy to push the bullet with.

Spoiler

And remember, whenever you change between forms of energy, you lose some to entropy. 100% efficiency is impossible, both in theory and in fact. It simply can't be done. Thermodynamics is about as flexible and forgiving as a concrete wall.

And, the expanding gas behind the bullet is at a very high pressure. If you want to make your gas go into the chamber behind the bullet, you have to make your gas even higher pressure. Of course, if you can make gas at higher pressures than that cartridge can, you might just want to switch to compressed gas guns.

When I mentioned my "self-refueling cargo ship" from way back when I was nine, I didn't add what my friend's uncle (an engineer of some sort) said about it. He kindly explained that it was impossible, because of something called thermodynamics. I thought he was just not getting the idea.

It never occurred to me that if a random nine-year-old could come up with the idea, then it was silly to suppose that no one else had considered it. Obviously, there must be a reason why someone hadn't yet gotten rich doing that exact thing. I suppose that, had that thought entered my head, I would have dismissed it by declaring myself "special" or else, "bright". Silly old me.

Edited October 26, 2020 by James Kerman
Redacted by a moderator

[JoeSchmuckatelli]

2 hours ago, Arugela said:

No, it does. That was the point. I'm assuming if you want you can improve other aspects to regain efficiencies with more work in other ways or leave as is. Like better barrel muzzling or custom bullets to go through it at extra lengths to achieve the original performance. You could even improve the part where you collect the gas. There should logically be means of improvement if going for pure performance. If you want to go through the hassle. Were not advanced enough to out of means to improve something. Not counting cost. the power provided by anti matter might make room for some functional things. Or bring up new necessities in practice. Considering the increase of the scale of it's uses.

The conversation started in a discussion about dark matter engines. I assumed this was reasonable to do.

Here is an idea. Pre chamber the/a gas. Do so without shooting a bullet. Then use the gas after powering the other features to push behind the bullet to keep the original performance. Or as close as possible.(this could go towards a system with the gas not in the bullet like a pea shooter or paint gun then.) 

Look, I'm not a Jenius or nuttin... but I can tell you this; a bullet has only so much powder innit.  So, whether you take pressure from the powder burn before you send the round, or while you're sending the round... you're still stealing energy from the round.  AFAIK you get only so much pressure, and you can choose how to use it - but it's a zero sum game.  It takes energy to move the bolt, and it takes energy to move the bullet.  If you choose to leach some of the gas pressure from the bullet to shift the bolt - the bullet goes a wee bit slower.

 

 

[Deddly]

1 hour ago, SOXBLOX said:

Since most of the folks not doing these things are aerospace and physics degrees, and since this field is entirely within their purview, is it not likely that there is a good reason for this?

I'm not singling you out here, I just see this kind of statement a lot and wanted to comment on it. Although this is completely true, it's actually not an effective argument in a situation like this. When a person comes up with an idea that seems to revolutionise something, they will never assume the idea is bad just because smart and highly-educated people haven't thought of it first - and more importantly, nobody should ever assume that only smart and highly educated people can think up a great idea. If everyone thought that way, there would be no innovation.

The same goes for just stating that the laws of thermodynamics disprove an idea. Yes, it's true that you can't break the rules of thermodynamics, however, in cases like this, it's worth assuming that the person we are reasoning with believes that the laws of thermodynamics can be broken, and they believe they have found an intuitive way to do so. Just telling a person that their ideas break the laws of thermodynamics does not disprove their idea in their own mind, and I hope that our reason for responding is to help the person see why their idea doesn't work rather than just to make ourselves look more intelligent, right?

Now as to the idea(s) we are discussing here, @Arugela, I think it's wonderful that you are a thinking person. Finding new ways to overcome limitations and stretching human ability is something that I encourage everyone to strive for. However, if you really want to find a breakthrough that would change space travel as we know it, you need to also put every effort into understanding why so many people disagree with you. Just because lots of people disagree does not automatically make you wrong, but it is a strong indication that there is a weakness in your design. Try to understand where that weakness is, and try to phrase your ideas as questions rather than assertions - that way, you'll have people helping you get even more knowledgeable rather than merely arguing against you.

Also, you have a lot of ideas. please try to stick to one idea at a time so we can give each one the attention it deserves without making the thread too confusing.

[Shpaget]

If I understand the core idea Arugela is proposing, it would be something like a monopropellant engine where the catalyst is a liquid or powder that is mixed with the monopropellant and ejected out the back. After that we try to catch it and reuse it. Putting aside the method for separation of catalyst and monopropellant exhaust (magnetic, electrostatic...), if the catalyst/monopropellant ratio is fairly high, then it might be useful to capture and reuse it in order to reduce total mass (no need to haul huge amounts of catalyst). This would not break the laws of thermodynamics and the extraction of catalyst would induce some inefficiency, but if the choice was to eject the catalyst or try to reuse it, it just may be more sensible to try and reuse. That being said, this would make sense only if the catalyst can not be a fixed grid/plate/pebble bed/whatever that remains static inside the engine, which happens to be a standard design with the monopropellants we use.

[Deddly]

7 minutes ago, Shpaget said:

if the catalyst/monopropellant ratio is fairly high, then it might be useful to capture and reuse it in order to reduce total mass (no need to haul huge amounts of catalyst). 

To be clear, you are talking about recycling the catalyst rather the exhaust gases, correct? 

[Clamp-o-Tron]

3 hours ago, Arugela said:

Here's about as cheap as it gets. You want a hull. Take up a bag of hefty trash bags!!! That is an extreme example but NASA in general is no way past doing that in ground experiments... Why do we need only multi ton or 100's of pound gold covered satellites when we could get away with much less and much more. There are things that could last longer than a trash bag too. Take actual spider or silk to the ISS from the silk bugs and use it's tensile strength and spray with titanium to make rods to build with on a very small scale. You can use tiny spools of materials to do it and make a probe. Just make tiny ion engines or something and have a blast. Get live observation over every planet and keep a live map and dedicate our resources to keeping a constant scan on all objects in the solar system. Or for other research projects. We have the ISS. We should probably use it to it's fullest while it's still there. Unless we just want to go back to the stone age.

That gold foil is actually closer than you’d think to a trash bag. It’s actually MLI, a little heavier than tinfoil. But it carries lots of utility: thermal protection, essential for a LEO spacecraft which goes through a quick cold/hot cycle, and rad protection, especially good for probes that will be blasted with solar radiation. The bulk of the mass of those sats is either prop tanks, comms, computers, and power. 
Space station modules using extremely lightweight materials are viable, but spider silk won’t do well with thermals. The tensile strength doesn’t matter much here as it’ll never have to support its own weight once in orbit.

@Shpaget at that point you can just call it a bipropellant

[HebaruSan]

1 hour ago, Deddly said:

When a person comes up with an idea that seems to revolutionise something, they will never assume the idea is bad just because smart and highly-educated people haven't thought of it first - and more importantly, nobody should ever assume that only smart and highly educated people can think up a great idea. If everyone thought that way, there would be no innovation.

But it does speak to whether the ideas being discussed have been fleshed out and investigated sufficiently to establish whether they work, as with actual innovation, or whether they've just been randomly thrown out as an aside in a stream of consciousness rant, as with the OP. It's not a reason to dismiss a suggestion; rather, it's a reason to focus on it, think about it more deeply, to identify what are the real reasons the knowledgeable experts don't do it that way? Not only would this enrich the discussion of the ideas themselves, but it would help to improve the understanding of the one making the suggestions, which in turn would improve the next suggestion that gets discussed.

Edited October 26, 2020 by HebaruSan

[Shpaget]

2 hours ago, Deddly said:

To be clear, you are talking about recycling the catalyst rather the exhaust gases, correct? 

Exactly.

2 hours ago, Clamp-o-Tron said:

@Shpaget at that point you can just call it a bipropellant

I could, but chemical reactions between the two are quite different.

[SOXBLOX]

5 hours ago, HebaruSan said:

But it does speak to whether the ideas being discussed have been fleshed out and investigated sufficiently to establish whether they work, as with actual innovation, or whether they've just been randomly thrown out as an aside in a stream of consciousness rant, as with the OP. It's not a reason to dismiss a suggestion; rather, it's a reason to focus on it, think about it more deeply, to identify what are the real reasons the knowledgeable experts don't do it that way? Not only would this enrich the discussion of the ideas themselves, but it would help to improve the understanding of the one making the suggestions, which in turn would improve the next suggestion that gets discussed.

I would have to agree with this. Thomas Edison knew what he was doing when he invented machines, and when he messed up, either someone more knowledgeable, or reality itself, corrected him. When I was nine, I didn't know anything about how conservation of energy worked, but I still tried to "improve" it anyways. There must be a difference...

6 hours ago, Deddly said:

...more importantly, nobody should ever assume that only smart and highly educated people can think up a great idea. If everyone thought that way, there would be no innovation.

Of course! But it is certainly also wise to assume that there are finer details at play, and that those degrees do spend their time learning something, and to be prepared to alter your idea accordingly.

[sevenperforce]

19 hours ago, mikegarrison said:

19 hours ago, wumpus said:

And once you really get going, even if you can manage to keep the beam focused (you can't), expect to lose energy as each bounce shifts the photon more and more red (the low momentum side of the spectrum).

So think of it as a very silly first stage, using a method of propulsion that should be for last resorts in some theoretical last stage.  But the extra bounces are pretty free.

In terms of conservation of momentum, it's perfectly "legal". Any momentum you gain from the bounce has to be offset by negative momentum (in the opposite direction) from the ship you are bouncing the photons off of.

Consider when SpaceX separates the first and second stages from the Falcon9. They just use some kind of gas piston or spring or some such thing, right? Well, in this case the photons are just the spring in the system, pushing the two ships apart.

It wouldn't even have to be photons. It could be a stream of BBs. But the nice thing about photons is that they always move at the speed of light. However, they wouldn't be able to accelerate you infinitely. I'm sure that some sort of time/space effect would cause them to lose energy compared to you as you sped up. Probably related to "redshift" or something like that.

There are proposals to do this with laser-driven light sails. An ordinary laser-driven light sail uses a sail on the probe and laser emitters in LEO. However, you can add a fresnel lensed mirror to the power station in LEO, allowing the photons to bounce back and forth many times.

Conservation of momentum is preserved not only by the thrust force on the LEO mirror, but the fact that the light is bouncing off a moving object. It is redshifted by the bounce and thus loses momentum. The faster the probe is moving away, the more dramatic the redshift.

16 hours ago, Arugela said:

I'm assuming if you want you can improve other aspects to regain efficiencies with more work in other ways or leave as is. Like better barrel muzzling or custom bullets to go through it at extra lengths to achieve the original performance. You could even improve the part where you collect the gas. There should logically be means of improvement if going for pure performance.

You can always improve efficiency, but those gains are incremental. You cannot defeat conservation of momentum.

14 hours ago, SOXBLOX said:

16 hours ago, Arugela said:

Then use the gas after powering the other features to push behind the bullet to keep the original performance. Or as close as possible.

That actually wouldn't work. "Using" the gas means pulling energy out of it; that means less energy to push the bullet with.

This, this, a thousand times this.

Work is energy. If your gas does work on one application, it has lost energy for ALL other applications. You cannot get that energy back.

[snip]

Edited October 26, 2020 by Snark
Redacted by moderator

[Arugela]

1 hour ago, sevenperforce said:

 

12 hours ago, SOXBLOX said:

That actually wouldn't work. "Using" the gas means pulling energy out of it; that means less energy to push the bullet with.

 

You are missing what I am saying. Yes, if you just put the bullets gas back in it might be a problem. I'm assuming you put more gas from a new source to replace it. IE add to the system. A secondary source of gas or pressure to replace the explosion from the bullets material. I'm taking from a design standpoint. The question then is does going to soley external sources of propellent then outweigh just the bullets and moving the bullets with no inherent powders or something else.  And the sole reason for not doing so it not just efficiency. Hypothetically efficiency doesn't even matter with a gun. You can choose to go with the design regardless for any reason. It can be as simple as public being used to it or the reverse and thinking it's cool or choosing one parameter as better than another. Or the companies not wanting to switch or spend money on new things because of potential unforeseen problems. There are endless reasons.

The reason I'm stating nebulous things is because the realities are nebulous. Like cost of materials etc.

And gains are not necessarily incremental if you consider the current implementation is not near efficient.

Edited October 26, 2020 by Arugela

[sevenperforce]

37 minutes ago, Arugela said:

The reason I'm stating nebulous things is because the realities are nebulous. Like cost of materials etc.

No matter how much money you spend, you cannot overcome the conservation of momentum. F₁ = ^-F₂ is not nebulous.

37 minutes ago, Arugela said:

And gains are not necessarily incremental if you consider the current implementation is not near efficient.

Inefficiency means the utilization of a resource is less than 100%.

Suppose you have a system with three different components: A, B, and C. Let's suppose that A represents propellant purity, B represents combustion efficiency, and C represents nozzle efficiency. The total efficiency of your system is the product of the efficiency of its components. If E stands for efficiency, then we would represent this by saying E = A x B x C.

Let's say that A is currently 99%, B is 95%, and C is 92%. So E = A x B x C = 86.5%.

Now let's talk about incremental gains. We can improve B, or we can improve C, or we can even improve A just a little bit. But no matter how much we improve A, B, or C, we can never ever get E to a number greater than 100%. Ever. Even if we make A = 99.99%  and B = 99.99% and C = 99.99%, E is still only going to be 99.97%. You cannot get E to be over 100% unless A, B, or C are also over 100%.

If you disagree, please do not use words. Show me maths.

[Arugela]

Yes you can. If current design is 80% of max potential from a physics standpoint you can design it to 85%...

[Guest]

2 minutes ago, Arugela said:

Yes you can. If current design is 80% of max potential from a physics standpoint you can design it to 85%...

Wow, it's that simple? Better tell the GE engineers... they've been struggling to make more competitive jet engines. You might just have discovered a trade secret in your hands there, guy.

Edited October 26, 2020 by Guest

[sevenperforce]

1 minute ago, Arugela said:

If current design is 80% of max potential from a physics standpoint you can design it to 85%...

But you cannot design it to 105%.

You are proposing that you can get something over 100% efficiency. You might not recognize it, but you are.

[Snark]

Lots of content removed, from people who should know better, due to violating various forum rules, specifically:

• rule 2.2.d:  making gratuitous personal remarks (e.g. addressing the poster, rather than the post)
• rule 2.2.n:  deliberate trolling in an effort to put down people
• rule 2.2.o:  going off-topic (to argue about arguing, rather than discussing the thread topic)
• rule 3.2:  blatant "backseat moderating" -- folks, you're not moderators, don't try to act like one.  If you see something you think is wrong, just report the post and let the moderators deal with it

None of those things are okay, ever.  As you all know (right?), since you agreed to the rules when you created your account.  And most of you have been around here for a long time and know that this is a friendly place where uncivil behavior is never called for.

But it's especially egregious in this case because this very point was raised by the moderator team, ahead of time.

From the very start of the thread:

On 10/21/2020 at 2:30 PM, Arugela said:

[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]

 

From the start of discussion after the thread was split:

On 10/24/2020 at 9:51 AM, Snark said:

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.

 

In short:  You were all asked nicely to please keep it civil and polite and not resort to personal attacks, and to stroll on by if you can't do that.  (Which shouldn't even be necessary advice, as the reason so many folks like to hang out here is because it's a nice, civil place.)  Most of you follow those rules pretty well, the vast majority of the time, so it's pretty disappointing to see this happen.

What's especially sad is that, mixed in with the vitriol and mud-slinging, there was actually some thoughtful, constructive advice tossed in as well-- which unfortunately had to get removed along with the sludge, since it was quoting & directly responding to it and there was no way to keep it in without keeping in the stuff that had to go.  Which means some kind, well-meaning KSP forum user just wasted a bunch of his time trying to help people... which makes me sad. 

Moral of the story:  This is a place where people are reasonable and civil, and it will remain that way.  When people don't follow that, everyone loses.

The thread has been temporarily locked while cleanup is underway.  Please stay tuned.  Thank you for your understanding.

[Vanamonde]

Considering how many people in the modern world do not care how things work, is it really so bad for someone to be enthusiastic about discussing this kind of thing even if he or she is getting it wrong? I myself have learned a few things from reading an explanation as to why somebody's view was mistaken. I have also reached a couple of insights by taking the time to think through and explain something in detail. This is a useful kind of conversation to have. And if the other party doesn't understand right away, sure, that can be frustrating. But so what? It doesn't harm anyone and it can stimulate further useful discussion. 

And none of that is any reason to be nasty to fellow forum members. Those posts have been removed. The thread will be reopened now, but keep the tone polite, please. 

[Codraroll]

On 10/24/2020 at 8:35 PM, Vanamonde said:

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. 

I think you hit the nail on the head here. Intuitively, it just seems like the mass you expel from the engine should slow down and eventually come to rest. Like if you chuck out rocks from the back of the spacecraft, their velocity would eventually dwindle to zero, and if you had them tied to a line longer than that "braking distance" you could reel them back in and give them another good chuck to use them to accelerate the spacecraft once again. Substitute the rocks with exhaust gases, and the long line with, I don't know, magnets or something, and you've got a very enticing setup.

But the crux of it is, of course, that the exhaust mass doesn't slow down by itself. Ten million miles behind the spacecraft, it will happily coast along at the same speed you chucked it out with, give or take some gravity effects by nearby objects. Slowing it down by any means will require some input of impulse and energy, and that will rob your spacecraft of every single fractional bit of momentum gained by chucking it out in the first place.

[SOXBLOX]

10 hours ago, Arugela said:

A secondary source of gas or pressure to replace the explosion from the bullets material.

In other words, a compressed gas gun. Like this one?

 

9 hours ago, Arugela said:

Yes you can. If current design is 80% of max potential from a physics standpoint you can design it to 85%...

So you can increase the efficiency. Great. But the efficiency can't go over 100%. That would mean that you were getting more work out of the system than you put in.

If you did capture your rocket exhausts, and somehow avoided slowing down (maybe a wizard did it), then your rocket would be >100% efficient. You would be getting free work. Incredibly useful, if it were possible. A shame it has been disproven.

Of course, there are no wizards. When you do catch the exhaust, it slows you back down. So, there is no reason to capture the exhausts.

The other direction I see you going is saying "Well, can't we add more things to the equipment to make it more efficient?" The answer is yes, to an extent. Adding grease to ball bearings reduces friction. Adding high-quality fuel to an engine makes it more efficient. But, adding massive, complicated steps where forms of energy are converted multiple times decreases efficiency. Every time you change between one form of energy and another (say, the potential chemical energy in gunpowder, and the kinetic energy of the bullet, combined with the light and sound energy of the fire), you lose some energy as heat, aka entropy.

Fact of life: all physical processes are less than 100% efficient.