Alternatives to nuclear thermal rockets?

[shynung]

1 minute ago, passinglurker said:

To be fair I did say if the destination mattered to consider it for trips to the moon, mars and venus. You are right there would need to be a transition to nuclear eventually I just think establishing or at least exploring on our closest neighbors without nuclear power first is a necessary stepping stone precluding that. With a farther reaching space presence it's possible that fissionable material could be sourced, and produced in space eliminating the risks and politics of launching refined uranium.

For nearby destinations like Venus, the Moon, Mars, and maybe NEOs, current chemical rockets would do perfectly fine. Ion thrusters could work, but it would take quite some time. Not ideal for time-limited missions, but feasible nevertheless.

Mercury, that's another matter.

[magnemoe]

57 minutes ago, passinglurker said:

Oh my... never has torch drive seemed so... plausible  

I certainly avoids the NTR problem of having to launch enriched uranium which is really where most of the politics would lye

So what's the radioactivity like? is it perfectly clean and happy when the engine is off or like NTR does it irradiate some stuff after firing the first time and from then on  casts a cone of death(or at least early retirement) around the shadow shield necessitating some careful docking?

still with a burn time of only a few days it can inject itself and tumble most of the way to simulate gravity for crew health which is nice, and combined with the mass and volume savings of cold tupor sleep it might really be able to be launched all in one go (though at that point wouldn't it be too small to spin? I suppose there is always andy weir advocated tethers...)

 

Yes the fusion drive looks good, downside is that its pretty complex mechanical and electrical. 
The real nice thing is that you don't need an break even fusion reaction to make an practical fusion engine, you goal is to heat up the reaction mass so it goes fast not to harvest more energy than you use, you can get power from solar panels and reactors not trust. 
You will get some secondary radiation as neutrons hit the drive and make material radioactive, it also require plenty of power so you might need an reactor. 
 

[sevenperforce]

13 hours ago, pincushionman said:

NSWR!

My personal favorite is a lithium-hydride NTR/NSWR fission fragment rocket. When exposed to a sufficient neutron flux (like, thermonuclear bomb or gas core NTR level), lithium-6 hydride fissions cleanly to helium and tritium. One of the highest specific impulses possible from nuclear rockets.

11 hours ago, passinglurker said:

Ouch you are right 44% solar power out at mars is rough though how is this any different from the challenges SEP systems face and mitigate by simply bringing more panels? If anything wouldn't solar thermal still have the advantage because it doesn't have the efficiency loses of photo voltaics?

As you point out mirror design does seem trickier though as they can't conveniently fold up like origami panels. Is the inflatable "moth wings" style a see in a lot of concept art the most practical or is it just a case of artists copying each other?

The primary advantage over SEP is mass; you can get a pretty decent TWR because your engine mass can be very very low. You can actually get a higher specific impulse for a given propellant than an NTR, because your heat exchanger can be made out of a material much more heat-resistant than a solid-core nuclear reactor.

As others have said, SEP has the advantage of higher specific impulse, denser propellants, and a more controllable deployment arrangement. You can always have mirrors that fold out, of course, but that really drives up dry weight because you need fine control of alignment, whereas solar panels can still function even when they aren't all perfectly aligned.

11 hours ago, passinglurker said:

Clearly I should change the thread name to "alternatives to any form of nuclear propulsion at all yes I already know torch ships are a thing thank you"

As I said in the op I'm looking for alternatives to avoid the controversy of nuke engines so "directly propelled by atomic chain reactions" isn't an option either sorry.

Reactor-based nuclear electric propulsion is fairly nice though, and not nearly as controversial.

1 hour ago, passinglurker said:

Well hydrogen I'm not considering a problem as it's something NTR's would have to handle too. Basically looking for NTR advantages without the politics, and solar thermal seems to fit the bill (plus there is always ammonia or methane as propellants. 500-600 isp is still and improvement!).

Though you are right the more I look at this the more the giant parabolic mirror becomes a problem. In eliminating the politics disadvantage of NTR's it picks up the solar collecting mega structure disadvantage of SEP! and without meaningful thrust its even worse off than SEP because it doesn't have the ISP to sustain a continuous burn around an orbit necessitating slower kick burns instead (pretty much spoiling it for trips to the moon) still for a trip to mars or venus it could be assembled in high orbit or a lagrange point with a more modest set of mirrors, and still have enough thrust to cut the burn time to days or weeks (as opposed to ion's long slow "whole trip" burn) letting it tumble the rest of the time to simulate gravity without the need of engine pivots.

Actually, a solar thermal rocket can have a better TWR than an NTR, because it is basically an NTR with the shielding replaced by a heat exchanger and the reactor replaced by a gigantic mirror. A mirror can be much, much more lightweight than a reactor. It does have to be scaled up pretty dramatically to get to that point, though.

One way to "cheat" with solar thermal propulsion would be to use a dual-layer heat exchanger: a solid ceramic core with some kind of ablative or otherwise sacrificial secondary layer. With a much larger surface area, the secondary layer would allow for high thrust during the initial escape while ablating away, adding its mass to the thrust of the rocket. Then, once it ablated/burned away, you would be left with a much more lightweight core for sustained propulsion or course correction maneuvers. Rocket equation likes that.

[RCgothic]

Isn't there a maximum temperature that can be reached by solar light concentration? Thought a recent xkcd what-if covered it. 5000'C. With a max temp that means max exhaust velocity, which limits ISP.

Of course that's probably better than chemical rockets, so maybe no problem.

Solar thermal electric would probably have better ISP for a fuel limited design, even with the corresponding efficiency penalty.

Edited May 26, 2016 by RCgothic

[sevenperforce]

3 minutes ago, RCgothic said:

Isn't there a maximum temperature that can be reached by solar light concentration? Thought a recent xkcd what-if covered it. 5000'C. With a max temp that means max exhaust velocity, which limits ISP.

 

Might have to be solar thermal electric, with corresponding efficiency penalty.

However, there are materials that can handle 5000C without melting. A solid-core nuclear reactor will melt down at around 3000C. So the ISP can be higher than NTR even though it isn't as high as with SEP.

[RCgothic]

Just now, sevenperforce said:

However, there are materials that can handle 5000C without melting. A solid-core nuclear reactor will melt down at around 3000C. So the ISP can be higher than NTR even though it isn't as high as with SEP.

I wasn't actually complaining about a low exhaust temperature! 5000'C is actually quite high as you and my edit have noted. I was just musing that it couldn't be made infinitely high... 

But Solar Thermal Electric could probably better the 5'000'C of pure Solar Thermal (for an additional mass and thermal efficiency penalty).

[sevenperforce]

22 minutes ago, RCgothic said:

I wasn't actually complaining about a low exhaust temperature! 5000'C is actually quite high as you and my edit have noted. I was just musing that it couldn't be made infinitely high... 

But Solar Thermal Electric could probably better the 5'000'C of pure Solar Thermal (for an additional mass and thermal efficiency penalty).

That's debatable. I actually misspoke; there aren't any materials that can handle 5000C and remain solid. Tantalum hafnium carbide melts at 3900C. So that's your upper limit for any thermal rocket of any kind. For liquid hydrogen fuel I'm guessing you're looking at around 1300 seconds of specific impulse, max. Maybe 2000 seconds if you can get full disassociation of the hydrogen.

If your propellant was somehow contained and was opaque to certain laser frequencies, I suppose you could make a solar electric laser-pumped thermal rocket with a higher temperature and better exhaust velocity. But you'd need some sort of nozzle, which is problematic. You'd want a magnetic nozzle, at which point it would seriously be better to just go SEP because your dry mass is entirely ungodly.

A novel idea is to use solid lithium hydride as your propellant and use a solar-electric cyclotron or neutronistor to induce a neutron spray that fissions the lithium directly. Poor thrust but you end up with relativistic exhaust velocities. Scales down pretty well, and could be used with a secondary propellant for high thrust.

[ValleyTwo]

Or an EM-Drive.  You know, thrust without fuel.  Unless somebody already mentioned this.

http://emdrive.com/

 

 

[PB666]

24 minutes ago, ValleyTwo said:

Or an EM-Drive.  You know, thrust without fuel.  Unless somebody already mentioned this.

http://emdrive.com/

 

 

You had to go there, it is not an alternative, although it has an infinitely high ISP, the it requires >30 MW per newton, whereas an 10000 ISP ion drive only requires .05 MW per newton, its a great thing for deep space with a fusion reactor, its not such a good thing for getting humans to mars. It could be used with solar panels as to provide a permant, low variable cost goods a fuel transporter between Earths L2 and Mars L1. 

 

The reason I say low and not costless with regard to its use is that because the mars transfers take so long and because this sucker might take 2years to transfer and its solar panels may only allow 12 transfers or less before the solar panels have to be replaced. So you have about an 8% per cycle replacement cost of solar panels. The drice costs themselves are practically immaterial and could be replaced every cycle. 

[Streetwind]

17 hours ago, PB666 said:

although it has an infinitely high ISP, the it requires >30 MW per newton

Please don't make up numbers and claim them for truth, it's unprofessional and misleading.

There is currently no information on how much power input an EMdrive-style thruster requires to develop a Newton of thrust, on account of nobody knowing how it works. For starters, you can't standardize a measuring system until you know what and how you expect to be measuring, so you can't even compare the measurements of test articles between different research teams. And since nobody knows how it works, nobody knows if the shape they are currently working with is even the best way to build one. As such, trying to make statements about what an EMdrive-style thruster can or cannot do right now is completely pointless.

But yeah, until we figure out if and how it works, it can't be considered an alternative to existing or upcoming propulsion tech.

Edited May 27, 2016 by Streetwind

[peadar1987]

Yeah, also it might not (probably doesn't) work. I'd hold fire on the EmDrive for now.

[p1t1o]

17 hours ago, RCgothic said:

Isn't there a maximum temperature that can be reached by solar light concentration? Thought a recent xkcd what-if covered it. 5000'C. With a max temp that means max exhaust velocity, which limits ISP.

Of course that's probably better than chemical rockets, so maybe no problem.

Solar thermal electric would probably have better ISP for a fuel limited design, even with the corresponding efficiency penalty.

Precisely, it is impossible to heat something up hotter than the source, the surface of the sun is approx 5000K, so thats the max temp. Higher temperatures require a hotter sun. And no, it doesn't matter how big your magnifying glass or mirror is.

On another note, you don't necessarily need a material that can remain solid at 5000K, you just use active cooling. This is pretty common.

[PB666]

1 hour ago, p1t1o said:

Precisely, it is impossible to heat something up hotter than the source, the surface of the sun is approx 5000K, so thats the max temp. Higher temperatures require a hotter sun. And no, it doesn't matter how big your magnifying glass or mirror is.

On another note, you don't necessarily need a material that can remain solid at 5000K, you just use active cooling. This is pretty common.

At 5k K alot of things are simply a gas, the surface starts turning tongas even if you cool it. 

[p1t1o]

11 minutes ago, PB666 said:

At 5k K alot of things are simply a gas, the surface starts turning tongas even if you cool it. 

Whilst there is definitely a limit to current cooling techniques, and I've no idea if 5000k lies within it, it is possible to operate materials above their evaporation point. What you are doing is not just cooling the walls, but also cooling the boundary layer of gas right next to it, so the metal itself is not sitting in temperatures where it will melt/evaporate. Ablative cooling for example, basically takes advantage of the fact that the ablation layer will be a gas.

I couldn't find any specific examples of active cooling methods being used at >=5000K, but I've definitely heard examples where copper being has been used above its boiling point.

[PB666]

38 minutes ago, p1t1o said:

Whilst there is definitely a limit to current cooling techniques, and I've no idea if 5000k lies within it, it is possible to operate materials above their evaporation point. What you are doing is not just cooling the walls, but also cooling the boundary layer of gas right next to it, so the metal itself is not sitting in temperatures where it will melt/evaporate. Ablative cooling for example, basically takes advantage of the fact that the ablation layer will be a gas.

I couldn't find any specific examples of active cooling methods being used at >=5000K, but I've definitely heard examples where copper being has been used above its boiling point.

The heat in this case is transferred by light, which directly heats the boundary layer. Or to put this a different way the molecules of gas inside to pressure chamber are everywhere including next to the container, when light strikes these next to the container to heat them to 5000K the molecules next to them can immediately absorb that heat. This is opporsed to a NTR whose heats the gas at the core and there is a gradient to the outside.

Working with electron microscopes the idea of frequency is everything, because its frequency that determines resolution. If you want to get those really high frequencies you need to use charge, and an electron microscope its between 40,000 and 1 million volts. I don't think we can pretend that light will do the same, even if you fired X-rays at the gas, some of the energy is going to go into ionization, which is great, but ultimately you are going to lose ISP. The difference between UV and IR is just not that great.

The second thing is that reflectors need a certain structure, or are massive like a Fresnel lens. A solar panel loose alot of energy but it can convert that remaining energy to much higher static voltage potentials, and thats what you need to accelerate.

[DDE]

I suggest you stop mucking about and go for a nuclear pulse rocket. 9160s tech, quite ecologically clean, makes Greenpeacers scream themselves to death.

On ‎25‎.‎05‎.‎2016 at 7:23 PM, shynung said:

And then there are ideas like this. In short, burning dimethylmercury (a corrosive neurotoxin that goes through latex gloves like it's not even there) with dioxygen difluoride (a highly sensitive oxidizer that likes to explode at will, burns anything it touches, and is generally unpleasant). And to top it all off, the mercury is a metastable (half-life of 30 minutes) isomer that has been irradiated in a nuclear reactor for months, ready to let all that stored energy out at once with a little boost of X-rays.

Wait, hasn't X-ray-stimulated emission of energy by isomers been disproven as pseudoscience?

[shynung]

3 hours ago, DDE said:

I suggest you stop mucking about and go for a nuclear pulse rocket. 9160s tech, quite ecologically clean, makes Greenpeacers scream themselves to death.

It's just as clean as any open-cycle nuclear rocket, which is to say not very clean at all.

OP wasn't looking for ecologically-clean rockets, but politically-clean ones. Out there, we don't really care what comes out of the nozzle, since the exhaust will be far too spread out to do much damage to practically anything. Unless it's a mass driver, that is.

3 hours ago, DDE said:

Wait, hasn't X-ray-stimulated emission of energy by isomers been disproven as pseudoscience?

You were probably thinking of the Hafnium fiasco.

Edited June 9, 2016 by shynung

[DDE]

8 hours ago, shynung said:

You were probably thinking of the Hafnium fiasco.

Yes, but wouldn't that apply to all isomer tech?

8 hours ago, shynung said:

It's just as clean as any open-cycle nuclear rocket, which is to say not very clean at all.

Nope, data says a whole lot cleaner, because the reaction is a whole lot faster and has a large fusion fraction in it (up to 98% if we use Soviet mining bombs from the Taiga test).

[shynung]

22 minutes ago, DDE said:

Yes, but wouldn't that apply to all isomer tech?

Not sure about that. I might have to dig around when I have the time.

22 minutes ago, DDE said:

Nope, data says a whole lot cleaner, because the reaction is a whole lot faster and has a large fusion fraction in it (up to 98% if we use Soviet mining bombs from the Taiga test).

You're still spraying nuclear reaction products everywhere. That's the problem with open cycles.

[DDE]

Just now, shynung said:

You're still spraying nuclear reaction products everywhere. That's the problem with open cycles.

But they are a whole different affair still. One's a mix of unreacted fuel and fission products, the other is a very small amount of fission products and what's left of the neutron-activated casing. Thermonuclear bombs are a lot more clean than nuclear reactors, of any kind.

[11of10]

What if we used a basic NERVA design, with, instead of using a nuclear reactor, we used some other source of heat, like PBAN, perhaps? Would that even work?

[Spaceception]

On 5/26/2016 at 3:32 AM, Streetwind said:

EDIT: Oh, and I just remembered: Nertea even made a concept implementation for a KSP mod in the past. It never got anywhere, and I think he kind of forgot it while being very busy IRL. But who knows, maybe if I ask him he'll feel inspired and add it to Kerbal Atomics at some point?

@Nertea?

[Streetwind]

A bit late to the party, @Spaceception Discussed that with Nertea in PM and he actually showed me a pretty neat concept render and wasn't opposed to continue working on it at all.

Problem is, "not being opposed" and "having time" are not related. Consider that right now, he's still working on porting the last of his mods over from 1.0.5. Things like that definitely have priority. And then there's 1.1.3 pending release in the very near future, which may or may not require recompiling and updating everything all over again...

Still, if you want, you can try stalking his dev thread. Anything new that isn't part of an existing pack is likely going to show up there first.

[Spaceception]

Just now, Streetwind said:

A bit late to the party, @Spaceception Discussed that with Nertea in PM and he actually showed me a pretty neat concept render and wasn't opposed to continue working on it at all.

Problem is, "not being opposed" and "having time" are not related. Consider that right now, he's still working on porting the last of his mods over from 1.0.5. Things like that definitely have priority. And then there's 1.1.3 pending release in the very near future, which may or may not require recompiling and updating everything all over again...

Still, if you want, you can try stalking his dev thread. Anything new that isn't part of an existing pack is likely going to show up there first.

Oh

[monstah]

On 25/05/2016 at 1:00 PM, passinglurker said:

My question is hopefully pretty simple. What can practically accumulate enough of a thrust to weight to not need to slowly spiral out of a gravity well, gets better isp than chemical rockets, and doesn't come with the political baggage of nuclear power? (emphasis mine)

 

On 25/05/2016 at 10:58 PM, pincushionman said:

NSWR!

I just had to laugh right now.

42 minutes ago, 11of10 said:

What if we used a basic NERVA design, with, instead of using a nuclear reactor, we used some other source of heat, like PBAN, perhaps? Would that even work?

Thing is, beating nuclear power density is hard