Electric thrusters able to use anything as propellant?

[SomeGuy123]

So, in another thread it was brought up that NASA budgets 0.781 kilograms/day for various things like empty containers, makeup, and other marginally essential items.  

And if you were riding an electric plasma rocket to Jupiter or Saturn you might really wish you had more items along for entertainment, instead of being trapped inside a lightweight hab module probably sandwiched between propellant tanks, where every possible gram has been shaved.  

So...what if you could carry 10,000 kg less propellant, and substitute 10,000 kg of beer kegs and glasses, a piano, various luxuries like paper books and recreational pharmaceuticals and everything else to have a pretty good time on the way to Saturn.

You'd wait til the end of the voyage, then rip everything out (you might only do this if you have to tap into your "reserve fuel") and feed it into your electric plasma rocket for the last bit of the deceleration burn.

Do any of the thruster styles have the kind of flexibility to run on anything?

Edited February 21, 2016 by SomeGuy123

[Streetwind]

It doesn't work that way, unfortunately. While many electric drives are essentially propellant-agnostic, that ability comes with a caveat or two.

For starters, the propellant must enter the engine in the state the engine expects it - which is almost guaranteed to be temperate and gaseous. Not too cold, not too hot, not liquid or solid or (god forbid) already plasma. This means implicitly that you need a propellant which is gaseous around room temperature. Astronaut waste is definitely not gaseous around room temperature, so it is immediately disqualified by default. You literally cannot feed this into an ion thruster.

Now, there are certain exotic thruster types which expect their propellant in solid form. The problem with those is that you generally cannot refuel them in flight. A classic pulsed plasma thruster (PPT) for instance in pretty much a stick of teflon with the engine components assembled around it. It's as large as the stick of teflon is. If you want more impulse out of a PPT, you build a bigger PPT because the stick of teflon inside is bigger. And once it has burned out, that's it. The engine is dead. It cannot be refueled. You would have to design an entirely new kind of engine to be able to place different types of "propellant ingots" into it at will during flight, and even then you'd probably need an EVA mission to do it.

And even when you solved all of these issues, and somehow made an engine that can accept astronaut waste as fuel, then it still isn't going to work all that well. The reason we're using ultra-rare and absurdly expensive stuff like xenon for electric engines is because its atomic properties are very favorable. It is easy to ionize, so the engine expends only small parts of its energy budget towards creating the plasma (or, it can produce more plasma at once). The largest part of its budget is devoted to accelerating the plasma, which is the part that actually creates thrust, and results in such high Isp. Also, as a noble gas, xenon doesn't react all that much with the engine even in an environment of massive latent energies.

Astronaut waste, meanwhile, is going to be a much less favorable fuel. It'll be much more difficult to ionize, leaving less energy to do the thrusting, resulting in less thrust and less Isp. Additionally, the fuel is not homogenous - it's not just a pile of identical atoms, but rather a huge mess of fifty different chemical elements assembled into thousands of different molecules and compounds. That means it'll be a fluctuating, unstable burn. And because the stuff is all present in molecules (and even larger organic structures, like proteins), that means it's probably not going to fully dissociate into atoms even when ionized, resulting in comparatively giant particles in the exhaust. This not only utterly kills your Isp (even further), but also may mean that it'll damage your engine, if it relies on things like gridded electrodes. And while we're on the topic of engine damage: that chaotic mixture of elements is going to include super-aggressive stuff, like alkali metals, oxygen, and halogens - fluorine, for instance, is an essential element without which humans cannot survive, even though its pure form is one of the deadliest things known to us. And you're taking all those super-aggressive elements and pump them full of energy. They're going to corrode the heck out of your delicate electric engine in no time flat.

 

TL;DR: bad idea, don't try this at home (if your home is a spaceship rapidly approaching Saturn ).

Edited February 22, 2016 by Streetwind

[PB666]

3 hours ago, Streetwind said:

It doesn't work that way, unfortunately. While many electric drives are essentially propellant-agnostic, that ability comes with a caveat or two.

For starters, the propellant must enter the engine in the state the engine expects it - which is almost guaranteed to be temperate and gaseous. Not too cold, not too hot, not liquid or solid or (god forbid) already plasma. This means implicitly that you need a propellant which is gaseous around room temperature. Astronaut waste is definitely not gaseous around room temperature, so it is immediately disqualified by default. You literally cannot feed this into an ion thruster.

Now, there are certain exotic thruster types which expect their propellant in solid form. The problem with those is that you generally cannot refuel them in flight. A classic pulsed plasma thruster (PPT) for instance in pretty much a stick of teflon with the engine components assembled around it. It's as large as the stick of teflon is. If you want more impulse out of a PPT, you build a bigger PPT because the stick of teflon inside is bigger. And once it has burned out, that's it. The engine is dead. It cannot be refueled. You would have to design an entirely new kind of engine to be able to place different types of "propellant ingots" into it at will during flight, and even then you'd probably need an EVA mission to do it.

And even when you solved all of these issues, and somehow made an engine that can accept astronaut waste as fuel, then it still isn't going to work all that well. The reason we're using ultra-rare and absurdly expensive stuff like xenon for electric engines is because its atomic properties are very favorable. It is easy to ionize, so the engine expends only small parts of its energy budget towards creating the plasma (or, it can produce more plasma at once). The largest part of its budget is devoted to accelerating the plasma, which is the part that actually creates thrust, and results in such high Isp. Also, as a noble gas, xenon doesn't react all that much with the engine even in an environment of massive latent energies.

Astronaut waste, meanwhile, is going to be a much less favorable fuel. It'll be much more difficult to ionize, leaving less energy to do the thrusting, resulting in less thrust and less Isp. Additionally, the fuel is not homogenous - it's not just a pile of identical atoms, but rather a huge mess of fifty different chemical elements assembled into thousands of different molecules and compounds. That means it'll be a fluctuating, unstable burn. And because the stuff is all present in molecules (and even larger organic structures, like proteins), that means it's probably not going to fully dissociate into atoms even when ionized, resulting in comparatively giant particles in the exhaust. This not only utterly kills your Isp (even further), but also may mean that it'll damage your engine, if it relies on things like gridded electrodes. And while we're on the topic of engine damage: that chaotic mixture of elements is going to include super-aggressive stuff, like alkali metals, oxygen, and halogens - fluorine, for instance, is an essential element without which humans cannot survive, even though its pure form is one of the deadliest things known to us. And you're taking all those super-aggressive elements and pump them full of energy. They're going to corrode the heck out of your delicate electric engine in no time flat.

 

TL;DR: bad idea, don't try this at home (if your home is a spaceship rapidly approaching Saturn ).

 The have magnesium thrusters. When we start talking about isps in the 100000 m/s range its feasible....the process is rather simple, chop the metal into small pieces, a bit tricky in space, roll them into a thin wire. As the noodle enters the back of the thruster its entering a vacuum, its zapped by high voltage, somewhere in the 100000v range a heats to 5000 degrees where the vacuum pressure and vacuum pull small bits of metal off which are accelerated. 

Of course you are going to have bits of the metal stick all over the inside ofvthe device spoiling it,mbut in a long flight you could simply grind up the device. Actually the only real source of metal i see in such flights are gas containers, but i have basically come to the conclusion that for manned electric powered flight one needs to use hyperglolics or liquified lfox propellants, whereas these are refueled at stations, like l1 and l2 of target planets and these are carried by fusion electric or solar electric power and return to LEO to be refueled and serviced, in these schemes the only waste is close to LEO. My second scheme is to use low isp xenon power craft to clean LEO of all these scrap satellites.

[Nuke]

its probibly beneficial to have thrusters capable of running on a few things. ammonia, hydrogen, water (at least the hydrogen part), etc. but this would be for long duration multi target isru missions, where resources found at one place refills the tank for the next place. obviously what resources available would really dictate your next destination. id love to see the end of the time when the mission ends because the probe runs out of gas.

[sevenperforce]

I mean I suppose you could bring along a coilgun and package all your consumables in ferromagnetic cylinders, so you could load your waste into those and eject them that way.

But the dv gain will be minimal.

[kunok]

3 hours ago, sevenperforce said:

I mean I suppose you could bring along a coilgun and package all your consumables in ferromagnetic cylinders, so you could load your waste into those and eject them that way.

But the dv gain will be minimal.

You can use an open cylinder in a mag gun, so you stop them in the final of it.

You can count for no close cycle tech at all and use all the waste as the only fuel. It's crazy but technically it can work.

Edited February 22, 2016 by kunok

[sevenperforce]

20 minutes ago, kunok said:

You ca use an open cylinder in a mag gun, so you stop them in the final of it.

You can count for no close cycle tech at all and use all the waste as the only fuel. It's crazy but technically it can work.

I suppose if you can speed up and then slow down your open cylinder...

SO CRAZY IT JUST MIGHT WORK!

[kunok]

Well more a cup or "glass", if you accelerate a 50kg container with 2 tons of waste, you only need to decelerate that 50 kg in the end of the mag gun.

Of course this has the advantage that you can use literally anything for reaction mass. So intercept an asteroid get a load of dirt and you got more fuel.

 

The mag gun problem can be resolved other day

Edited February 22, 2016 by kunok

[Kerbart]

36 minutes ago, kunok said:

You can use an open cylinder in a mag gun, so you stop them in the final of it.

You can count for no close cycle tech at all and use all the waste as the only fuel. It's crazy but technically it can work.

Technically, yes. But keep in mind that Newton's third law (the one that makes rocket engines work in the first place) still applies. So, if the waste you're railgunning out of your ship is 1% of the mass of your ship, you'll need 100× the exit velocity for the desired DV. You can see where this is going for a required DV of say, 50 m/s—and we all know how modest that is. Not to mention the extra mass required for a railgun that can accelerate trash up to 5000 m/s.

[kunok]

1 minute ago, Kerbart said:

Technically, yes. But keep in mind that Newton's third law (the one that makes rocket engines work in the first place) still applies. So, if the waste you're railgunning out of your ship is 1% of the mass of your ship, you'll need 100× the exit velocity for the desired DV. You can see where this is going for a required DV of say, 50 m/s—and we all know how modest that is. Not to mention the extra mass required for a railgun that can accelerate trash up to 5000 m/s.

I know, I think it never will be viable at all, maybe only if we get huge but light fusion reactors, the thing is that you have like 5m/s every day, and you don't carry fuel at all only life support consumables. Is a trash based equivalent of an ion thruster.

[sevenperforce]

4 minutes ago, kunok said:

I know, I think it never will be viable at all, maybe only if we get huge but light fusion reactors, the thing is that you have like 5m/s every day, and you don't carry fuel at all only life support consumables. Is a trash based equivalent of an ion thruster.

Except that the ion thruster has really incredibly poor exhaust velocity.

[kunok]

14 minutes ago, sevenperforce said:

Except that the ion thruster has really incredibly poor exhaust velocity.

No, it has a ******* huge exhaust velocity and therefore ISP, what it doesn't have is trust. That''s why they operate in continuous acceleration trajectories, not in instantaneous burn ones.

The mag gun probably will have less ISP (it depends of how large you made it), but it would have better trust

3 minutes ago, sevenperforce said:

What I meant was, it's the trash-based equivalent of an ion thruster, only an ion thruster with a crappy exhaust velocity.

But more trust, you need to account that too.

Edited February 22, 2016 by kunok
not making 50000 post for replying

[sevenperforce]

5 minutes ago, kunok said:

No, it has a ******* huge exhaust velocity and therefore ISP, what it doesn't have is trust. That''s why they operate in continuous acceleration trajectories, not in instantaneous burn ones.

The mag gun probably will have less ISP (it depends of how large you made it), but it would have better trust

What I meant was, it's the trash-based equivalent of an ion thruster, only an ion thruster with a crappy exhaust velocity.

[Kerbart]

11 minutes ago, sevenperforce said:

What I meant was, it's the trash-based equivalent of an ion thruster, only an ion thruster with a crappy exhaust velocity.

Well, if the propellant used is what comes out of the toilets... that'd be be very fitting!

“Captain! Sir! We seem to be making more progress than was estimated for today,sir!”
“Ah, yes, ensign, keep in mind... we had chili for dinner yesterday!”