Alternatives to nuclear thermal rockets?

[passinglurker]

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?

If destination matters let's say it's for a transfer to lunar, martian, or venusian orbit.

So far what I see for interplanetary propulsion is controversial nuclear engines, and impracticaly slow ion drives with little in between.

[Nibb31]

If you find one, let us know.

[Spaceception]

4 minutes ago, 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?

If destination matters let's say it's for a transfer to lunar, martian, or venusian orbit.

So far what I see for interplanetary propulsion is controversial nuclear engines, and impracticaly slow ion drives with little in between.

VASIMR, Solar sails, electrostatic ion thrusters, among potentially some others that I can't think of right now.

Edited May 25, 2016 by Spaceception

[PB666]

8 minutes ago, 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?

If destination matters let's say it's for a transfer to lunar, martian, or venusian orbit.

So far what I see for interplanetary propulsion is controversial nuclear engines, and impracticaly slow ion drives with little in between.

The problem, if you want to take advantage of low orbital based transfers you need power, burning hard at low orbit allows you to create a single path out without having to circularize to gain energy.

It can be done with ION drives, but you its about 66% efficient byt the higher ISP drives can give 3500 so it might be worth it.

  OR

if you have the time, you burn for about 20 degrees (-10 to +10 degrees) your burn window. (called a kick), then do nothing for 340 degrees, you pass the burn window you kick, with each orbit you waste more time doing nothing.

Finally at some point you have enough momentum to kick to an apogee that if you burn along the prograde vector you can burn out of the system.

This could take months, for an interplanetary mission your initial burn point could be 90' degrees (relative to the suns radial) from where your last kick takes place. During the last burn you could also add a few seconds of RCS to help the kick.

 In most cases the launch will give the ION drive the momentum such that when its ION systems come on it can push out of the system. One way is to push to L2  (for mars) or L1 (for venus) which buys time for the ION drive to gain momentum, once it is passed the L points it then establishes the heading to reach its target.

 

 

 

 

[shynung]

Better chemical rockets. The kind dreamed by actual propellant alchemists like John D. Clark, author of Ignition.

He wrote of tripropellant rocket motors that gave 542 seconds of ISP , and burned cooler than LH2/LOX. Said rocket runs on LH2, liquid Fluorine, and liquid Lithium, substances that are not only expensive, highly toxic and corrosive, but also has wildly different storage temperature requirements. So that's that.

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.

Quote

Sure, it’s a devil’s brew, but who wouldn’t say no to a specific impulse on the order of twelve thousand?

No, I'm not saying we should use these things.

Edited May 25, 2016 by shynung

[sevenperforce]

Devil's brew is putting it lightly.

I wanna see an ion engine that can use the atmosphere as reaction mass, so you can get to orbit with it.

[Bill Phil]

Solar thermal?

Beamed power.

Laser sails.

Or just go all in. Nuclear fission fragment rockets.

6 minutes ago, sevenperforce said:

Devil's brew is putting it lightly.

I wanna see an ion engine that can use the atmosphere as reaction mass, so you can get to orbit with it.

You're gonna need enormous power densities.

[sevenperforce]

5 minutes ago, Bill Phil said:

You're gonna need enormous power densities.

Naturally.
 

[Ralathon]

The problem is that you need lots and lots of energy to accelerate mass. In a rocket you need to carry that energy on board, so you need some kind of high density energy storage. LH2-LOX is pretty close to the limit of chemical energy density, so changing fuels won't gain you much efficiency. The next step after chemical energy is nuclear. Nuclear has an energy density about a million times higher than chemical, but as you said it's pretty controversial. If you want higher efficiency than chemical but don't want to mess with nuclear you'll have to design your own energy storage. You'd need to build some kind of super capacitor bank or a massive superconducting coil. But current tech levels in those fields don't even get close to chemical energy densities.

The only other way to get the energy you need is to send it to the ship. The only realistic way to build something with nuclear efficiency without the nuclear bits is to aim a giant laser or maser at your ship. That's the only feasible method I can see with our current technology. And it would still be a nightmare to build.

[sevenperforce]

The really great thing about chemical fuels is that your power source and your propellant are one and the same, so you get to shed it as you no longer need it. The rocket equation loves that. Your propellant is also the coolant for your power source, which saves additional mass.

If there was a more energy-dense chemical fuel, you could use your chemical fuel to heat a separate propellant...but you really still can't beat hydrolox for raw energy to weight ratio.

If you don't go chemical and beamed power isn't an option, you carry your power supply (and, often, your coolant) with you as dry mass to orbit.

One possibility would be something like a shielded-loop propellant-cooled nuclear-electric pulsed inductive thruster. A pulsed inductive thruster is basically the fluid version of a railgun. You get to use ammonia as your reaction mass, which has great density and can double as your coolant due to its convenient catalyzed endothermic decomposition, and it is quite amicable to ISRU as well. It requires no electrodes, accelerates a charge-neutral plasma with a specific impulse of up to 10,000 seconds, and varies its power consumption by changing pulse frequency without a drop in efficiency.

You just need a way to get it to operate in a non-vacuum...which, if you were pumping it as nuclear reactor coolant, shouldn't be hard. Then you set up with a duct and a supercharger to use the lower atmosphere as reaction mass, and you're set.

[PB666]

2 hours ago, Bill Phil said:

You're gonna need enormous power densities.

Efficient matter antimatter conversion, fusion has two high of a mass fraction. Its not doable. 

Edited May 25, 2016 by PB666

[passinglurker]

2 hours ago, Bill Phil said:

 

Solar thermal?

 

Any body have any more on this? Performance, limitations, and challenges involved? 

2 hours ago, PB666 said:

It can be done with ION drives, but you its about 66% efficient byt the higher ISP drives can give 3500 so it might be worth it.

Can you elaborate on this? As I understand it with electric propulsion your thrust to weight is inversely proportional to your isp but I only ever see talk of the "high isp, low thrust" end of the spectrum.

__________

Finally for beamed power it looks promising as well but are there any proposed methods that can't double as controversial orbit to surface weapons?

[PB666]

14 minutes ago, passinglurker said:

Any body have any more on this? Performance, limitations, and challenges involved? 

Can you elaborate on this? As I understand it with electric propulsion your thrust to weight is inversely proportional to your isp but I only ever see talk of the "high isp, low thrust" end of the spectrum.

__________

Finally for beamed power it looks promising as well but are there any proposed methods that can't double as controversial orbit to surface weapons?

Which means you need a high density power supply. Good for traveling insides venusian circum solar orbit orbit, not so good for whizzing out of martian atmosphere. That could all change with fusion power. 

[sevenperforce]

Solar thermal is not too bad, particularly if you're doing stuff at less than 1 AU, but it's limited by propellant choice. With hydrogen, you can get up to 1000 seconds of impulse, but with something like water you're looking at 190 seconds. Dry mass can be quite low, though; you can use hydrogen or helium to inflate balloon mirrors to get pretty good performance.

[passinglurker]

1 hour ago, PB666 said:

Which means you need a high density power supply. Good for traveling insides venusian circum solar orbit orbit, not so good for whizzing out of martian atmosphere. That could all change with fusion power. 

You said it was possible with ion just less efficient. So why not trade isp for the thrust needed to escape orbit in a single burn? And exactly what type of ion propulsion would be capable of doing this?

7 minutes ago, sevenperforce said:

Solar thermal is not too bad, particularly if you're doing stuff at less than 1 AU, but it's limited by propellant choice. With hydrogen, you can get up to 1000 seconds of impulse, but with something like water you're looking at 190 seconds. Dry mass can be quite low, though; you can use hydrogen or helium to inflate balloon mirrors to get pretty good performance.

Isp wise it sounds the same as nuclear thermal and possibly even better thrust cause you aren't hauling a reactor with you. So what's the catch? (Cause there is always a catch) just less performance out at mars?

What would these balloon mirrors look like?

[sevenperforce]

27 minutes ago, passinglurker said:

Isp wise it sounds the same as nuclear thermal and possibly even better thrust cause you aren't hauling a reactor with you. So what's the catch? (Cause there is always a catch) just less performance out at mars?

What would these balloon mirrors look like?

Disadvantage is just that it's pretty much useless beyond Earth, and almost completely useless beyond Mars. You also have to deal with an inflatable mirror design that will hold up against continual solar radiation. Heat rejection is an issue too, as with any thermal rocket.

Google "Solar moth" for examples. It has a very futuristic feel.

[PB666]

 

16 minutes ago, passinglurker said:

You said it was possible with ion just less efficient. So why not trade isp for the thrust needed to escape orbit in a single burn? And exactly what type of ion propulsion would be capable of doing this?

If you are a human, why indeed.

If you are a space tug delivering non-perishables, you save alot of fuel, which means for instance you could carry fuel, a different kind of fuel for chemical rockets to mars to do something like land or return humans back to earth. If you are being bombarded by cosmic rays, you have space induced cachexia, and your food and oxygen supplies are ticking down, you aren't going to sit around 4 to 6 months for an Ion drive to kick you into a partial transfer so your ion drive and efficiently finish the task. You could however ride to Earth L2, pick up a ride on an ION driven transfer ship, ride to Mars L1, get off ride down to a LMO, pick up fuel from another Mars-LMO/Mars L2 recycling ion-driven tug, land on mars, return to LMO, pick up more fuel from the space tug, go back to L1, catch a ride on the same ship back to earth L2, then refuel from another ion driven refueling station, and finally land on earth. The infrastructure being in place, in mar LMO and L1, you could have deliveries of supplies to LEO that are tugged to L2, Mars L1, Mars LMO so that the transfer ships are always conserved, only the supply pod comes from LEO.  The only scenario where ION drive is not useful is scenarios that involve inefficient transfers (e.g. short than the 6 to 9 months). Even so with higher powered solar panels some of these can be pulled off. 

Before you think of ION drives you have to think, am I launching Kepler ST or am I launching Voyager. If im Kepler ST and I am losing valuable coolant by the second, I need to get to my station and do as much work as possible. If I'm voyager or the like, going with ION drive can allow me to add another RTG or two, I can use the ION drive to push me toward the heliopause and use more RTGs to keep me running longer. Any excess that the RTG produces early in life I can use for pushing my little ship faster outward, I'll be running for 75 years, long after Keplar is space junk. Another reason, there are micro ION drives, these can replace inefficient monoprop driven RCS trusters and can be used for holding course for longer periods. Another reason for ION is that you can vary your ISP, if you need a burst of speed, such as at the beginning of a transfer, you have some liberty to accelerate more gas to a lower exhaust velocity.

Imagine that you really need to get going fast, you don't care how long it takes but you really want some speed. So here is what you can do, using a very high ISP engine, good solar paneling you and a small powerful chemical engine. You first use the efficient 20,000 ISP drive and drive at sunrise termination line out of earths system, working in the direction of mercury and burning at the highest ISP Possible, as you prepare to do a gravity assist around mercury, you then use the high efficiency ION drive to increase speed, passing mercury periapsis at optimum you burn 75% of your chemical fuel, using the oberth affect to really increase speed, once in view of the sun you then hit the ION drives a full 9x Solar irradiance, you then push to Venus, and repeat with the remaining chemical fuel at Venusian Pe. Again you burn out at 2.5x solar irradiance, dropping your chemical engines and fuel tanks, you then burn as hard as you can to mars orbit, supplimenting your power with the RTGs, as you pass close to Jupiter, finally you drop your solar panels, and you are now pushing out on RTG. Finally somewhere beyond saturn you run out of fuel. I had some RL like specs for ION drive payloads that had dV of 70,000. Imagine what you could do with that. Leave the solar system at 60,000 meters per second.

The problem of course are the panels, barreling toward mercury you realize that panels do overheat and age quickly, that they may not be producing 2.5x solar irradiance at Venus, and you might end up with alot of leftover fuel even if you could burn it.

Lets take another scenario, lets say you were the philae lander, rather than using a chemicals thrusters and harpoons, why not slow down with small ion drives, rather than plunging into a  comet (I know it was only going a few meters per second), why not light on the comet and push down ever so lightly with ion drives on the surface using the lightest force to keep the lander down. A delicate circumstance needs a delicate device.

So these are examples were you choose the right golf club for the shot you need to make. Sometimes ION drives are not what you want (Launch, expedite) and sometimes they fit the bill.

 

 

[pincushionman]

NSWR!

[passinglurker]

6 hours ago, sevenperforce said:

Disadvantage is just that it's pretty much useless beyond Earth, and almost completely useless beyond Mars. You also have to deal with an inflatable mirror design that will hold up against continual solar radiation. Heat rejection is an issue too, as with any thermal rocket.

Google "Solar moth" for examples. It has a very futuristic feel.

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?

5 hours ago, PB666 said:

 

If you are a human, why indeed.

<snip>.

 

 

Hm.. I had honestly not considered simply switching space trains at a point with less required escape velocity.

But yes I'm trying to avoid the slow spiral out mostly because of the needs of human passengers, but also I want to avoid the long slow burn of most ion drives out of a need for simulated spin gravity (which admittedly I should have said in the op) and burning once before tumbling the whole ship the rest of the way is a lot simpler than a high voltage 360 degree pivot for the engines. Though without NTR or a viable alternative it may be what needs to be done.

1 hour ago, pincushionman said:

NSWR!

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.

[shynung]

3 hours ago, passinglurker said:

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.

The problem with this approach is that nuclear energy is the only source of power that can be used almost anywhere. Chemicals don't have good energy density, and solar panels are nearly useless past Mars. Unless you're happy with confining your spaceborne adventures to the inner solar system, you will have to transition to nuclear power eventually.

That is, unless you are willing to deal with the horrors of high-energy chemical reactions. LH2/LF2 engines have a better performance than LH2/LOX, but the dangers of liquid fluorine makes nuclear rockets seems safe in comparison.

Edited May 26, 2016 by shynung

[magnemoe]

14 hours ago, passinglurker said:

Any body have any more on this? Performance, limitations, and challenges involved? 

Can you elaborate on this? As I understand it with electric propulsion your thrust to weight is inversely proportional to your isp but I only ever see talk of the "high isp, low thrust" end of the spectrum.

__________

Finally for beamed power it looks promising as well but are there any proposed methods that can't double as controversial orbit to surface weapons?

Solar thermal works much the same way as nuclear thermal except that you use sunlight to heat an ceramic block to 3000 Celsius. then run hydrogen trough it. 
Downside is that you need an giant parabolic mirror for serious trust, benefit is that it can be scaled down well so probably more practica for probes where an nuclear engine is overkill, however here ion engines have better isp and its easier to design large solar panels than the disc, you also have easier to handle fuel than hydrogen.
 

[PB666]

2 hours ago, shynung said:

The problem with this approach is that nuclear energy is the only source of power that can be used almost anywhere. Chemicals don't have good energy density, and solar panels are nearly useless past Mars. Unless you're happy with confining your spaceborne adventures to the inner solar system, you will have to transition to nuclear power eventually.

That is, unless you are willing to deal with the horrors of high-energy chemical reactions. LH2/LF2 engines have a better performance than LH2/LOX, but the dangers of liquid fluorine makes nuclear rockets seems safe in comparison.

Yep, i think this thread is cooked, we can do it again next week/month unless someone stickies it. 

[Streetwind]

Really guys? Nuclear salt water rockets, antimatter and other theoretical stuff, when we have a fusion pulse rocket in development right now?

 

 

Features:
- Pulsed nuclear fusion propulsion - like a mini-Orion, except without literally all of the drawbacks
- Solar panels charge capacitors, which discharge whenever they happen to be full to generate a fusion pulse fired through a magnetic nozzle
- Can run just fine even on low power input, just pulsing less often
- Isp comparable to ion engines, fixed and independent of power input (as each pulse is self-contained)
- Thrust/TWR scales directly with power input (as pulse rate scales with power input)
- No need to get a continuous, energy-positive fusion reaction working - each pulse is self-contained, is energy-positive on its own
- No need to develop exotic power sources, solar is perfectly sufficient - but it will benefit if better power sources are developed
- High density lithium propellant does away with the need for huge, heavy hydrogen tankage & cryocoolers
- Almost no radiation shielding required, as the magnetic nozzle pushes everything but neutrons away, and a small integrated shield blocks the neutrons

Currently in development at MSNW LLC, under a NASA contract. They've been at it for a while already, too. Here's a presentation from 2012 (PDF). It calls for a full-scale ground prototype by 2020, and an in-space demonstration mission by ca. 2023. Of course, who knows if that timeline holds (probably not), but this is not some paper project. People are bending metal today to build laboratory prototypes.

And no, it won't have enough thrust to take off from the ground, like Orion would. But by extrapolating current trends in solar panel and capacitor development, they expect that in the 2025 to 2030 timeframe, such a fusion rocket could make a manned Mars land-and-return roundtrip in 210 days (including a 30 day stay on the surface). The whole spacecraft would weigh less than 135 tons, and is therefore within the capabilities of SLS Block II as a single, monolithic launch (though they might need two smaller launches, just due to spacecraft volume).

They compare this to the existing Mars Design Reference mission using NTRs (which are merely paper concepts today), which would require 9 launches to lift 850 tons of material, complex orbital assembly, and a 1680 day roundtrip time.

 

My takeaway from this? Even if this thing is delayed by 5 years, and ends up being only half as good as they make it sound in that PDF, it'll still end up kicking the butts of solid-core NTRs and ion drives alike. So yeah, I'm keeping my fingers crossed for these guys.

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?

 

Edited May 26, 2016 by Streetwind

[shynung]

1 hour ago, 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?

Maybe you should bug him about it. I'd love to have it in the next release.

[passinglurker]

6 hours ago, shynung said:

The problem with this approach is that nuclear energy is the only source of power that can be used almost anywhere. Chemicals don't have good energy density, and solar panels are nearly useless past Mars. Unless you're happy with confining your spaceborne adventures to the inner solar system, you will have to transition to nuclear power eventually.

That is, unless you are willing to deal with the horrors of high-energy chemical reactions. LH2/LF2 engines have a better performance than LH2/LOX, but the dangers of liquid fluorine makes nuclear rockets seems safe in comparison.

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.

 

5 hours ago, magnemoe said:

Solar thermal works much the same way as nuclear thermal except that you use sunlight to heat an ceramic block to 3000 Celsius. then run hydrogen trough it. 
Downside is that you need an giant parabolic mirror for serious trust, benefit is that it can be scaled down well so probably more practica for probes where an nuclear engine is overkill, however here ion engines have better isp and its easier to design large solar panels than the disc, you also have easier to handle fuel than hydrogen.
 

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.

The less solar power around mars is still a problem though. about the only way to fix that is to put some sort of power beaming station around martian orbit (fortunately there is no orbital weapon controversy when the potential weapon is that far away)

 

3 hours ago, Streetwind said:

Really guys? Nuclear salt water rockets, antimatter and other theoretical stuff, when we have a fusion pulse rocket in development right now?

 

 

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...)

 

Edited May 26, 2016 by passinglurker