New Spacex Engine has to be nuclear

[raketa]

1/Starship to reach any part of solar system need nuclear power plant
2/Nuclear rocket engine give more delta V for available fuel.
3/New nuclear engine could be very low trust engine, using just for everything with exception of landing and launching from planets.(Hydrogen could be produce from methane by steam reforming)
4/Power plant could be used also for producing electricity, replacing solars in distances beyond Mars
5/Cooling of power plant could be done using skin of rocket, that is design to accept and radiate heat during landing
6/Power plant will be turn off during launch and landing, to cool down for landing heat wave

[sferrin]

Let's say SpaceX decides they want to develop a nuclear upper stage (or interplanetary tug).  Logistically, CAN they?  Wouldn't the government (and not just the US's) throw up a bunch of red flags?  Maybe it would be similar to the way companies like GE can design/build/sell nuclear reactors? 

[Hominans Kosmos]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

[cferreir]

Good documentary on the NERVA project

[sevenperforce]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Depends on how much dV you need.

[rsdavis9]

There is also nuclear electric with ion thrusters

[raketa]

Let's say SpaceX decides they want to develop a nuclear upper stage (or interplanetary tug).  Logistically, CAN they?  Wouldn't the government (and not just the US's) throw up a bunch of red flags?  Maybe it would be similar to the way companies like GE can design/build/sell nuclear reactors? 

Nuclear fuel could be deliver by Dragon to orbit and install to SS over-there. Dragon launch abort system, could help to approve it such approach.
Spacex didn't need to rush this development, requirement will become imminent in 10-15 years.

[raketa]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Biggest issue is weight of radiator.
Starship skin could serve as radiator.
SS will be glowing in darkness in infrared. Could not be use to attack alien fleet :-).

[raketa]

There is also nuclear electric with ion thrusters

I think direct conversion of heat propulsion is  more efficient and also way to cool down reactor, that will be one the issue deal with.
Ion thruster could  use just electricity and you have to deal with 95% heat to radiate. If most heat is expel and use for propulsion, it will easier to radiate through skin of SS.

[tleski]

Let's say SpaceX decides they want to develop a nuclear upper stage (or interplanetary tug).  Logistically, CAN they?  Wouldn't the government (and not just the US's) throw up a bunch of red flags?  Maybe it would be similar to the way companies like GE can design/build/sell nuclear reactors? 

I guess it depends on the type of fuel. Much easier to get permission to use low enriched uranium compared to high enriched. In the Space Show podcast linked below Dr Christopher Morrison is discussing the issues (including the regulatory ones) related to nuclear propulsion:
https://www.thespaceshow.com/show/31-mar-2019/broadcast-3292-dr.-christopher-morrison

[DistantTemple]

My Vision for SpaceX's ship development towards the 2040's

OK When will nuclear be needed and what for?

What for: It will be needed to go further into the solar system, more quickly. It will be needed for much larger ships.
When: After initial success on Mars. So by 2028 some expectations and prototypes should be appearing.

How will SpaceX get there?

Initially they will have to work closely with NASA. This risks being slow and expensive. SpaceX could set up a Neclear lab, and employ experts in the field. However strong government oversight would still be there, but ISTM that this stage is needed to get a foothold, and gain core competencies.

Because SX will be the only space transport company able to offer NASA a cheap and flexible platform for trials of nuclear engines, NASA will cooperate and develop partnership arrangements.

Once humans land on Mars in 2024 SX rovers, and satellites will start prospecting in earnest for uranium. (Also metals, and rare ores etc etc). If Mine-able Uranium is found on Mars, SX will be able to set up a large operation without contaminating Earth. Reactor components can be shipped from earth, and fuelled in Mars Orbit.

Marian (or Luna maybe) Uranium will be the critical resource, with a big SpaceX push to discover and mine it. However Steel and making stainless steel on mars will be an important secondary thrust.

Initial ships will be 9m SS cylinders re-purposed. Once prototypes are functioning in 2030, larger cylinders will be fabricated in orbit. Maybe Earth orbit, but once stainless steel is made on Mars, then fabrication will be in Mars orbit. (2035) It will be automated, use cold vacuum welding ("perfectly" clean (no oxidation etc) cut edges brought together, make an outstanding join, with no heat deformation etc), and there will be little limit on size, as there will be no gravity deformation issues, and momentum and kinetic energy will replace weight-being-lifted as the main"force" to cope with in aligning and assembling the vast structures. A 50m body will only be limited by the supply of steel and the time to "weld" it. These ships will be mass produced by 2038

The next limitation will be boosting Martian stainless into Mars orbit. Then a space based stainless factory will be assembled form one of these "50m" tubes, in orbit, and sent to make steel from asteroids. (2040) At this point it will also start the first town there. The factory will ingest parts of asteroid, and generate spaceships, and other metals, and minerals etc.

With this vision in mind, in the 2020's SX will have sent unmanned probes to begin to identify resources.
As soon as there is a nuclear engine, (2024) an out-of-solar system probe will be launched, to satisfy GS's ambition.

SpaceX will also put up (space based) telescopes, arrays and other instruments, largely for prospecting, and furthering their own goals, but also allowing pure research, through academic collaboration. On-orbit ship manufacture and nuclear engines will allow more powerful instruments to be sent relatively quickly to all parts of the solar system and beyond.

There is a significant risk that politics, war, pollution, disease, sea level rise, economics, etc could vastly increase the pressure and attractiveness of living off-earth. Conversely if Earth is cherished, removal of Nuclear activities, and boosting metals from Earths surface, could create an economic raison d'etre for the path I have described.

These developments will not require as many 10's and 100's of $Bns as it might seem, as each stage is a rational development of SpaceX from the previous.

Edit: I forgot to include developments in AI robotics, and Nuralink. These will speed up my timeline significantly. Space will be "unregulated" real estate for manufacturing, once BEO. Therefore as long as you protect your humans, an Elonesque rush to develop and manufacture will be standard.
Edit2: bold uranium on Mars

[envy887]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Water NTR gets you hydrogen ISP but at 3-4x the bulk density. Maybe an advantage but there are challenges with shielding the crew and cooling the reactor.

[DistantTemple]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Water NTR gets you hydrogen ISP but at 3-4x the bulk density. Maybe an advantage but there are challenges with shielding the crew and cooling the reactor.

A ship assembled in orbit can have the reactor on a truss 10's or 100's of metres away from the crew.

[50_Caliber]

There already is a plasma thruster that would fulfill the next-generation role for advanced propulsion -VASIMR-https://www.nasa.gov/audience/foreducators/k-4/features/F_Engine_That_Does_More.html
It would need a MW-class nuclear reactor to power it though.

However; with orbital refueling, this will give every bit as much performance needed for a while without any expensive R&D programs to get the propulsion system in orbit. Considering the incredible cost reduction that a functioning SS/SH system will provide, the economic case for nuclear propulsion may be a tough one to make for a while. Though if anyone has a chance at making such a system within a reasonable economic framework, it would be SpaceX.

[raketa]

There already is a plasma thruster that would fulfill the next-generation role for advanced propulsion -VASIMR-https://www.nasa.gov/audience/foreducators/k-4/features/F_Engine_That_Does_More.html
It would need a MW-class nuclear reactor to power it though.

However; with orbital refueling, this will give every bit as much performance needed for a while without any expensive R&D programs to get the propulsion system in orbit. Considering the incredible cost reduction that a functioning SS/SH system will provide, the economic case for nuclear propulsion may be a tough one to make for a while. Though if anyone has a chance at making such a system within a reasonable economic framework, it would be SpaceX.

Again use just electricity and what you will do with heat of nuclear power plant. It is better use heat to use for propulsion. IF reactor is use just produce electricity, it could run in lower input and heat could be dissipated through skin.

[raketa]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Water NTR gets you hydrogen ISP but at 3-4x the bulk density. Maybe an advantage but there are challenges with shielding the crew and cooling the reactor.

I think Spacex will able to push it here and make comparable to solar.
They could build  Power plant and  engine in test version where heat from nuclear fuel could replace temporally by non nuclear source and lower  government red tag at the beginning of development.

[butters]

Elon's comment about an 18m diameter system as his current vision for what comes after Starship would argue against nuclear propulsion being something that he is interested in pursuing. He's obviously thinking that huge refillable propellant tanks is the way to see the solar system. Maybe there will develop an engineering constituency within SpaceX which will someday convince Elon that nuclear propulsion is worth serious consideration, but at present it seems quite far from what he has in mind.

[kendalla59]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

I've been trying to get more information on this topic. Nuclear engines have a thrust to weight ratio that is about 1/20th that of chemical rockets. But the chemical rockets need to carry a lot more propellant mass. In the end the nuclear rocket wins in a theoretical calculation for a Mars mission.

But there are many other considerations:
1. The total cost of placing the propulsion system into space.
2. Maintaining cryogenic H2 for a long duration.
3. Materials properties when irradiated with neutrons over time.
4. Materials properties when exposed to hot H2 and hydrogen plasma over time.

The basic need is to either carry or acquire energy and reaction mass during the flight. Lasers can be used to transmit energy to the spacecraft while in flight. Reaction mass refills could perhaps be propelled separately to the spacecraft using railguns or chemical rockets.

To carry the entire reaction mass and energy needed for the flight, imagine a spacecraft surrounded by, say five shells of rocket stages. The craft itself has a single engine with propellant. That would be surrounded by four engine stages with propellant. The next layer say has twelve engine stages, then 36, then maybe 108 (!!). Each engine/propellant stage will be able to separate and fly back to Earth (or Mars) for recovery and re-use.

Being able to continuously accelerate the spacecraft produces some amount of "gravity" and also reduces the flight time to days instead of months.

[rakaydos]

Given the current regulatory environment and Shotwells interest in eventually taking SpaceX actually interstellar, I suspect they would leapfrog Nuclear to focus on Antiproton Capture.

[sanman]

Nuclear seems to have a lot of risk and liability associated with it, even just for development.

On the other hand, Elon has been promoting that meme of "Nuke Mars", and it may be that he's only half-joking about it. (Why doesn't he just drop a comet/asteroid onto Mars?)
But none of the technology he's already invested into coincides with nuclear.

[ChrisWilson68]

This is yet another of the many threads I'd put in the category of someone posting "I think X is a good idea, therefor SpaceX must be doing X soon".

No.  Just because you think X is a good idea doesn't mean it's likely SpaceX is doing it soon.

[MATTBLAK]

SpaceX or indeed anyone building a nuclear engine?! That'll be the day...

[RotoSequence]

SpaceX or indeed anyone building a nuclear engine?! That'll be the day...

Wasn't Marshall's contract with BWXT Nuclear to continue developing a nuclear thermal propulsion system renewed?

[envy887]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

I've been trying to get more information on this topic. Nuclear engines have a thrust to weight ratio that is about 1/20th that of chemical rockets. But the chemical rockets need to carry a lot more propellant mass. In the end the nuclear rocket wins in a theoretical calculation for a Mars mission

It's not just the engine mass. Hydrogen NTR used a propellant with a bulk density 14 times worse than water, which means it requires huge and heavy tanks.

[wes_wilson]

Given the current regulatory environment and Shotwells interest in eventually taking SpaceX actually interstellar, I suspect they would leapfrog Nuclear to focus on Antiproton Capture.

I agree, this. 

Or fusion. 

(And I assume OP meant fission when saying nuclear)

[MATTBLAK]

SpaceX or indeed anyone building a nuclear engine?! That'll be the day...

Wasn't Marshall's contract with BWXT Nuclear to continue developing a nuclear thermal propulsion system renewed?

I'd heard something like that. But wake me up when they actually begin testing the thing - either on the ground or in flight...

[KelvinZero]

One thing that Elon Musk has specifically mentioned is multiple depots. An obvious way to exploit high ISP drives without adding any more requirements to your Starship, whether nuclear SEP or whatever, would be to raise depots slowly to the necessary orbits. I like the idea of a highly eliptical orbit that skims past the earth, so a couple of tankers can be used sort of like boosters, getting maximum Oberth effect then detaching just before nearest approach to aerobrake and land while the ship does it's own burn, also exploiting Oberth effect.

[Norm38]

Let's say SpaceX decides they want to develop a nuclear upper stage (or interplanetary tug).  Logistically, CAN they?  Wouldn't the government (and not just the US's) throw up a bunch of red flags?  Maybe it would be similar to the way companies like GE can design/build/sell nuclear reactors? 

Not if you build it on Mars.   

[raketa]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

I've been trying to get more information on this topic. Nuclear engines have a thrust to weight ratio that is about 1/20th that of chemical rockets. But the chemical rockets need to carry a lot more propellant mass. In the end the nuclear rocket wins in a theoretical calculation for a Mars mission

It's not just the engine mass. Hydrogen NTR used a propellant with a bulk density 14 times worse than water, which means it requires huge and heavy tanks, bring number of engine to magic 42 :-)

SS will have lot of methane for landing.
Methane could be used to produce hydrogen, you need heatup to 700-1000C, to produce it.Nuclear plant could easy deliver that.
Nuclear engine using heat fuel was tested in 70, just have to be improved. Something could be done in 10 years, no break through required.
Nuclear engine could be seventh engine on SS and use only on orbit to speed up to Mars, Titan Europe .

[raketa]

Given the current regulatory environment and Shotwells interest in eventually taking SpaceX actually interstellar, I suspect they would leapfrog Nuclear to focus on Antiproton Capture.

I agree, this. 

Or fusion. 

(And I assume OP meant fission when saying nuclear)

My concept didn't required any breake through, just to improve already existing or used to exist technology.

[raketa]

This is yet another of the many threads I'd put in the category of someone posting "I think X is a good idea, therefor SpaceX must be doing X soon".

No.  Just because you think X is a good idea doesn't mean it's likely SpaceX is doing it soon.

I predict Elon call his company boring company, check my thread.
Elon wants to go behind Mars in future, without nuclear power plant will not happen. Every nuclear propostion has biggest obstacle how to radiate heat produce by reactor.
I got idea that SS skin design  to withstand heat during landing, could play additional role to be powerful radiator for nuclear power plant.
This is reason i think next step for Spacex will be go nuclear and i think to get biggest pay off from this technology is use to produce electricity propulsion.
This way you increase %5 efficiency of such source to several 10%.

[john smith 19]

There is also nuclear electric with ion thrusters

I think direct conversion of heat propulsion is  more efficient and also way to cool down reactor, that will be one the issue deal with.
Ion thruster could  use just electricity and you have to deal with 95% heat to radiate. If most heat is expel and use for propulsion, it will easier to radiate through skin of SS.

Direct conversion IE Thermionic converters are less than 10% efficient and need very high temperatures to work.  They score in terms of reliability as they have no moving parts.

Do you mean nuclear thermal, where the coolant is the propellant? Or nuclear electric, where you face the problem that you are dealing with true radiators IE heat exchangers who can only lose heat by radiation? This is a field that could be improved quite a lot. What's needed is a high efficiency emitter at the operating temperature  of the coolant. This suggests some sort of approach tuned to give a peak emission wavelength corresponding to that temperature. My instinct is little tufts of carbon fiber or pits of the right dimension (like those on an old CD) would be options.

[Lampyridae]

Nuclear engines may be fine for government-bankrolled Mars missions where they are dependent on getting access to space with Oldspace pork barrel rockets, but it's pointless for commercial space because nuclear reactors are not cheap. Especially when the alternative is Raptors coming off of a production line.

[MATTBLAK]

In-space, cryogenic fuel transfer and Propellant Depots are literally the future. Anything more advanced is the future also - but the medium-to-far future.

[ZChris13]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

I've been trying to get more information on this topic. Nuclear engines have a thrust to weight ratio that is about 1/20th that of chemical rockets. But the chemical rockets need to carry a lot more propellant mass. In the end the nuclear rocket wins in a theoretical calculation for a Mars mission.

But there are many other considerations:
1. The total cost of placing the propulsion system into space.
2. Maintaining cryogenic H2 for a long duration.
3. Materials properties when irradiated with neutrons over time.
4. Materials properties when exposed to hot H2 and hydrogen plasma over time.

I foresee SpaceX's aversion to all things elemental Hydrogen continuing into the future, even if they go down the nuclear thermal route. The bulk density is just too poor, which results in a large dry mass penalty and poor thrust to weight ratio.

Let's say SpaceX decides they want to develop a nuclear upper stage (or interplanetary tug).  Logistically, CAN they?  Wouldn't the government (and not just the US's) throw up a bunch of red flags?  Maybe it would be similar to the way companies like GE can design/build/sell nuclear reactors? 

Not if you build it on Mars.   

This really depends on the regulatory environment of Mars, which has it's own thread and I don't think revising that topic here would be a good idea.

[sevenperforce]

Nuclear is attractive, but it is not automatically the most efficient option. In addition to the many reasons you don't want to use it as a launch engine (political and public concerns as well as abysmal T/W ratio), there are significant instances where biprop chemical rockets would outperform. Let's consider a fast transit to Mars packing 4 km/s out of LEO. Since we will assume that you don't want to land on the nuke, we'll make an apples to apples comparison and talk about getting into a nominal low Martian orbit. A vehicle like Starship can aggressively aerocapture at Mars and then aerobrake in multiple passes to the desired orbit. A nuclear rocket, on the other hand, cannot. Keeping the nuke well away from your crew during burns will require some sort of long truss or heavy shielding or other superstructure that will weigh as much or more as the heat shield and control surfaces on Starship but not permit any aggressive aerocapture. So while Starship enters Martian orbit for free, a nuke must carry 1.1 km/s in additional dV to brake propulsively. Any subsequent aerobraking will be much less aggressive and take much longer than for Starship, so it actually takes more time. If you want to circularize propulsively as well, rather than spend a week aerobraking, that jumps to 2.7 km/s. Additionally, the dry mass penalty is substantial -- I will say 20% m₁ mass growth to account for both the greater tankage volume and the heavier engine. So even if lifting liquid hydrogen into LEO is no more costly than lifting methalox (it's much more expensive, both in the context of actual price and with respect to the bulk density issues), the nuke will have to be  to be pushing a specific impulse of 780 seconds just to break even. And that's without additional considerations like the cost of getting the nuke into orbit, the cost of the nuke itself, the operating lifespan, the need to service the engine and reprocess the fuel, the extra descent/ascent infrastructure you need at Mars.

It is more efficient for the moon, of course, where propulsive braking is non-negotiable. Even here, however, the descent-ascent infrastructure needs to be considered.

Where nukes become game-changing is if you end up with LH2 production capacity on the moon AND regular shuttle/tug flights between the moon and other destinations.

[Lampyridae]

Where nukes become game-changing is if you end up with LH2 production capacity on the moon AND regular shuttle/tug flights between the moon and other destinations.

...and then there's the possibility of orbital elevators on the moon, slings and even mass drivers.

[savantu]

SpaceX is working on nuclear propulsion since at least 2016. Tom Mueller involvement currently is related to nuclear propulsion.

[Nomadd]

There is also nuclear electric with ion thrusters

I think direct conversion of heat propulsion is  more efficient and also way to cool down reactor, that will be one the issue deal with.
Ion thruster could  use just electricity and you have to deal with 95% heat to radiate. If most heat is expel and use for propulsion, it will easier to radiate through skin of SS.

Direct conversion IE Thermionic converters are less than 10% efficient and need very high temperatures to work.  They score in terms of reliability as they have no moving parts.

Do you mean nuclear thermal, where the coolant is the propellant? Or nuclear electric, where you face the problem that you are dealing with true radiators IE heat exchangers who can only lose heat by radiation? This is a field that could be improved quite a lot. What's needed is a high efficiency emitter at the operating temperature  of the coolant. This suggests some sort of approach tuned to give a peak emission wavelength corresponding to that temperature. My instinct is little tufts of carbon fiber or pits of the right dimension (like those on an old CD) would be options.

Stirlings can be close to 25% efficient.

[kevindbaker2863]

This will be available in ten years

[rsdavis9]

This will be available in ten years

and in ten years it will still be 10 years in the future.

[OldSpaceFan]

A nuclear power plant in space? Especially not this pollution!
A solution should already be found to get rid of the scrap left by the "old space" around the Earth.
Let's help / encourage Musk / SpaceX to improve its solution that looks promising for the future.
Nuclear power will have a future when you first solve the radiation problem.

[ZChris13]

Nuclear is attractive, but it is not automatically the most efficient option. In addition to the many reasons you don't want to use it as a launch engine (political and public concerns as well as abysmal T/W ratio), there are significant instances where biprop chemical rockets would outperform. Let's consider a fast transit to Mars packing 4 km/s out of LEO. Since we will assume that you don't want to land on the nuke, we'll make an apples to apples comparison and talk about getting into a nominal low Martian orbit. A vehicle like Starship can aggressively aerocapture at Mars and then aerobrake in multiple passes to the desired orbit. A nuclear rocket, on the other hand, cannot. Keeping the nuke well away from your crew during burns will require some sort of long truss or heavy shielding or other superstructure that will weigh as much or more as the heat shield and control surfaces on Starship but not permit any aggressive aerocapture. So while Starship enters Martian orbit for free, a nuke must carry 1.1 km/s in additional dV to brake propulsively. Any subsequent aerobraking will be much less aggressive and take much longer than for Starship, so it actually takes more time. If you want to circularize propulsively as well, rather than spend a week aerobraking, that jumps to 2.7 km/s. Additionally, the dry mass penalty is substantial -- I will say 20% m₁ mass growth to account for both the greater tankage volume and the heavier engine. So even if lifting liquid hydrogen into LEO is no more costly than lifting methalox (it's much more expensive, both in the context of actual price and with respect to the bulk density issues), the nuke will have to be  to be pushing a specific impulse of 780 seconds just to break even. And that's without additional considerations like the cost of getting the nuke into orbit, the cost of the nuke itself, the operating lifespan, the need to service the engine and reprocess the fuel, the extra descent/ascent infrastructure you need at Mars.

It is more efficient for the moon, of course, where propulsive braking is non-negotiable. Even here, however, the descent-ascent infrastructure needs to be considered.

Where nukes become game-changing is if you end up with LH2 production capacity on the moon AND regular shuttle/tug flights between the moon and other destinations.

Nuclear Thermal does not require liquid Hydrogen

[hkultala]

Nuclear Thermal does not require liquid Hydrogen

NTR without hydrogen as the propellant has so bad isp that it makes practically no sense at all.

And NTR with gaseous hydrogen - then the tank mass will be insane.

[sevenperforce]

Nuclear Thermal does not require liquid Hydrogen

NTR without hydrogen as the propellant has so bad isp that it makes practically no sense at all.

And NTR with gaseous hydrogen - then the tank mass will be insane.

If you have a specific reason to hate the bulk density of liquid hydrogen then you can get decent performance out of liquid methane.

But why?

[kendalla59]

SpaceX is working on nuclear propulsion since at least 2016. Tom Mueller involvement currently is related to nuclear propulsion.

Mueller no doubt is heavily involved in bringing the Raptor into full production, working on those "100,000 mile" issues that he has talked about before, getting ready for flight of the vacuum version, and incrementally increasing the efficiency of the engine.

Here is what Mueller said about nuclear on 5/2/2017:

Using a high-performance low-density propellant is not the answer. So we’ve gotten everything we’re going to get out of chemical propellants.

So we’re looking, actually, at like electric propulsion for the satellites, and we’re talking to people about nuclear-thermal, you know, the NASA centers are working on nuclear; it’s just prohibitively expensive to test because you can’t; it’s not like the 60s, like when you can just let fission products fly out of your rocket into the desert. You’ve now got to scrub it and clean it and capture it, which is super-expensive. I don’t think SpaceX could really afford to develop that rocket ourselves. If NASA ever gets turned on to develop those test stands, we’d probably want to jump in on that. You can just about double the performance of a rocket to Mars compared to a really-good, like a Raptor system, a chemical system, with fission; nuclear fission. Theoretically, fusion may be ten times better, and antimatter maybe a thousand times better, but I think those are certainly not going to happen in my lifetime. Maybe in your lifetimes.

The warp drive is still a long way away. <laughter> So we’re stuck with chemical propellant for quite a while.

[DJPledger]

SpaceX doesn't need a nuclear engine for colonizing Mars and exploring the solar system. Just scale up Raptor to deliver at least 4x the thrust of the current version and use that for powering SS18 and beyond.

Nuclear engines will never get off the ground at least for leaving the Earth's surface due to radiation issues until SpaceX or someone else devs. an engine powered by nuclear fusion which will be likely at least several decades away. Will need to go beyond chemical propulsion for interstellar travel.

[Hauerg]

1/Starship to reach any part of solar system need nuclear power plant
2/Nuclear rocket engine give more delta V for available fuel.
3/New nuclear engine could be very low trust engine, using just for everything with exception of landing and launching from planets.(Hydrogen could be produce from methane by steam reforming)
4/Power plant could be used also for producing electricity, replacing solars in distances beyond Mars
5/Cooling of power plant could be done using skin of rocket, that is design to accept and radiate heat during landing
6/Power plant will be turn off during launch and landing, to cool down for landing heat wave

No, it does not „have to be“.
Not as simple as it might look.
Even beyond Mars.

[sevenperforce]

Nuclear engines will never get off the ground at least for leaving the Earth's surface due to radiation issues until SpaceX or someone else devs. an engine powered by nuclear fusion which will be likely at least several decades away. Will need to go beyond chemical propulsion for interstellar travel.

A proper nuke doesn't have meaningful radiation issues even in-atmo.

[Ludus]

Or they could go with nuclear pulse propulsion Orion style. That would just involve Elon controlling more nuclear weapons in orbit than the Russians and the US have total combined. It would certainly move him up a bit in the Forbes Most Powerful People ranking. He’s already ahead of Kim Jong Un and Netanyahu. https://www.forbes.com/powerful-people/list/#tab:overall

[DistantTemple]

Or they could go with nuclear pulse propulsion Orion style. That would just involve Elon controlling more nuclear weapons in orbit than the Russians and the US have total combined. It would certainly move him up a bit in the Forbes Most Powerful People ranking. He’s already ahead of Kim Jong Un and Netanyahu. https://www.forbes.com/powerful-people/list/#tab:overall

See my essay above it is much, much TL:DR    (MMTL:DDR) Definately dont read!
https://forum.nasaspaceflight.com/index.php?topic=49104.msg1996932#msg1996932

IMO:

So Elon is taking on several "world changing" technologies. Especially ones that are heavily bogged down in paperwork.

If Elon looks ahead to a need for Nuclear, he may "take on" development of that industry. Doing this on earth would be expensive. I suggest he might start on earth working with NASA etc, but then mine Uranium on Mars (if possible) and thus be free to build empty reactors on Earth, and power ships (some immense) across the solar system and beyond, whilst not polluting Earth or needing Earth based regulatory compliance. (apart from crew safety)

These ships would never land on Earth, and many would never land. (shuttles would dock etc)

We will shortly see SpaceX start some small cooperation with NASA - toe-in-the-water stuff.

And as for power... SpaceX (&Elon) would be THE absolute experts and suppliers of Atomic space propulsion technologies. This would include fusion, and antimatter, in time. The best experts would go to SpaceX to see their discoveries bear fruit!

(Disclosure: I'm generally against nuclear fission power on Earth due to safety, and cost and clean alternatives.)

[spacenut]

If we build and test a nuclear fission engine today, it will probably be in space due to possible radiation issues.  Everything for the nuclear fission engine will be launched with chemical rockets to a location in space, assembled, then started to see what it can do.  Testing on the moon may be an option.  After testing, then a large spaceship could be assembled in space with nuclear engines to travel faster around the solar system. 

[MATTBLAK]

I think nuclear-electric systems have a far better chance of coming to be than nuclear-thermal someday. Once cryogenic propellant transfer is perfected or at least regular; the nuclear-electric engine(s) can continue to increase the delta-v after escape velocity is accomplished. The outer solar system could be ours, then.

[AC in NC]

but then mine Uranium on Mars (if possible) and thus be free to build empty reactors on Earth, and power ships (some immense) across the solar system and beyond, whilst not polluting Earth

(Disclosure: I'm generally against nuclear fission power on Earth due to safety, and cost and clean alternatives.)

Why the concern about Earth-sourced nuclear fuel?

Leaving aside the Earth-regulatory issues, uranium-fuel pellets are relatively safe AIUI.  Mining and enriching uranium off-Earth seems a strange trade.  That part of the cycle isn't really dirty is it?

[Ludus]

Or they could go with nuclear pulse propulsion Orion style. That would just involve Elon controlling more nuclear weapons in orbit than the Russians and the US have total combined. It would certainly move him up a bit in the Forbes Most Powerful People ranking. He’s already ahead of Kim Jong Un and Netanyahu. https://www.forbes.com/powerful-people/list/#tab:overall

See my essay above it is much, much TL:DR    (MMTL:DDR) Definately dont read!
https://forum.nasaspaceflight.com/index.php?topic=49104.msg1996932#msg1996932

IMO:

So Elon is taking on several "world changing" technologies. Especially ones that are heavily bogged down in paperwork.

If Elon looks ahead to a need for Nuclear, he may "take on" development of that industry. Doing this on earth would be expensive. I suggest he might start on earth working with NASA etc, but then mine Uranium on Mars (if possible) and thus be free to build empty reactors on Earth, and power ships (some immense) across the solar system and beyond, whilst not polluting Earth or needing Earth based regulatory compliance. (apart from crew safety)

These ships would never land on Earth, and many would never land. (shuttles would dock etc)

We will shortly see SpaceX start some small cooperation with NASA - toe-in-the-water stuff.

And as for power... SpaceX (&Elon) would be THE absolute experts and suppliers of Atomic space propulsion technologies. This would include fusion, and antimatter, in time. The best experts would go to SpaceX to see their discoveries bear fruit!

(Disclosure: I'm generally against nuclear fission power on Earth due to safety, and cost and clean alternatives.)

Looking at the list of industries Elon tackled: Banking&Payments, Automobile manufacturing, Orbital Rockets, Global Telecommunications, Tunneling & urban infrastructure, Brain surgery & Medical Devices, it doesn’t show much fear of complex bureaucracy and regulation. It seems plausible to me that if he judges Nuclear to be a critical technology that’s not advancing fast enough he might put it on the list and try unusual approaches as you suggest.

I don’t think nuclear has any real technical problems on earth but it might have intractable political perception problems that make exotic approaches necessary.

[Ludus]

but then mine Uranium on Mars (if possible) and thus be free to build empty reactors on Earth, and power ships (some immense) across the solar system and beyond, whilst not polluting Earth

(Disclosure: I'm generally against nuclear fission power on Earth due to safety, and cost and clean alternatives.)

Why the concern about Earth-sourced nuclear fuel?

Leaving aside the Earth-regulatory issues, uranium-fuel pellets are relatively safe AIUI.  Mining and enriching uranium off-Earth seems a strange trade.  That part of the cycle isn't really dirty is it?

Doing it on Mars is the ultimate avoidance of NIMBY being in nobody’s backyard but the settlers who have more important things to worry about. You’re right though. They ought to be able to figure out a way of launching fuel pellets in a world with lot’s of Starship Spaceports that doesn’t trigger too much protest.

Maybe there’s a SciFi plot in there someplace in a future where Mars has a tiny population but has come to have a lot more nuclear weapons capacity than earth. God of War.

[DistantTemple]

but then mine Uranium on Mars (if possible) and thus be free to build empty reactors on Earth, and power ships (some immense) across the solar system and beyond, whilst not polluting Earth

(Disclosure: I'm generally against nuclear fission power on Earth due to safety, and cost and clean alternatives.)

Why the concern about Earth-sourced nuclear fuel?

Leaving aside the Earth-regulatory issues, uranium-fuel pellets are relatively safe AIUI.  Mining and enriching uranium off-Earth seems a strange trade.  That part of the cycle isn't really dirty is it?

I was lazy and didn't check the safety of fuel in transit, before use. I still imagine launching nuclear material could raise fears of accidents etc, although Kilopowers and craft containing RTGs will need to be launched.
Still Musk can transform EV manufacturing, rocket launch, battery storage, hopefully tunnelling, and brain interfaces (having of course also transformed online payments!), but transforming the Nuclear industry, is a different matter, with more (justified) regulation, and extreme costs, safety, arms, and public perception. Since his need to transform this industry is for off planet use, restarting the industry under his control off planet, removes all those immense costs and difficulties in one go. Instead he is starting over on an airless planet!
I think SpaceX will be sufficiently successful that if SpaceX finds reasonably mine-able Uranium on Mars, they will develop, and use it.
It will also allow larger Kilopower units to be built there, and it will "facilitate a city of a million!" on Mars.

[DistantTemple]

snip...

Looking at the list of industries Elon tackled: Banking&Payments, Automobile manufacturing, Orbital Rockets, Global Telecommunications, Tunneling & urban infrastructure, Brain surgery & Medical Devices, it doesn’t show much fear of complex bureaucracy and regulation. It seems plausible to me that if he judges Nuclear to be a critical technology that’s not advancing fast enough he might put it on the list and try unusual approaches as you suggest.

I don’t think nuclear has any real technical problems on earth but it might have intractable political perception problems that make exotic approaches necessary.

ISTM that nuclear, especially including research and development, and changing the way things are done, will be much more difficult than the others. Even NASA had a hill to climb in getting the Kilopower project going. None of his other projects have had anything like the safety issues, weapons concerns, or bureaucracy. (even human and monkey, brain connections don't!)

[ZChris13]

but then mine Uranium on Mars (if possible) and thus be free to build empty reactors on Earth, and power ships (some immense) across the solar system and beyond, whilst not polluting Earth
(Disclosure: I'm generally against nuclear fission power on Earth due to safety, and cost and clean alternatives.)

Why the concern about Earth-sourced nuclear fuel?
Leaving aside the Earth-regulatory issues, uranium-fuel pellets are relatively safe AIUI.  Mining and enriching uranium off-Earth seems a strange trade.  That part of the cycle isn't really dirty is it?

Environmentally fairly clean. Regulatory and politically extremely dirty.

[AC in NC]

Environmentally fairly clean. Regulatory and politically extremely dirty.

Understand totally.   

But that hurdle seems like nothing compared to Finding and Mining sufficient uranium and then producing or shipping Cascades of Highly Sophisticated Gas Centrifuges for Uranium Hexaflouride Enrichment? 

That's a lot of hand-waving about something the regulators and politics would probably be far more concerned about than controlling your supply of pellets limited to 20% rather than whatever the big-boom %'age is.

[DistantTemple]

ah yes I see.... either
1) pellets from earth.... under control.... or

2) however if SX refines its own , unregulated on Mars.... imagined risk of Mars building weapons, or their HE Uranium falling into the wrong hands. (hijack of fuel cargo etc)

Yes I can see regulators/US (and NPT countries) would strongly prefer to have SX source fuel pellets on Earth! - to the extent they would facilitate it with updated regulations...

This would put off a Mars mine.

[envy887]

Nuclear Thermal does not require liquid Hydrogen

NTR without hydrogen as the propellant has so bad isp that it makes practically no sense at all.

Not really. Water or ammonia NTR gets about the same ISP as chemical hydrolox but with 3-4x better bulk density meaning smaller lighter tanks.

[DAZ]

I’m going to jump in here, even though I’m likely to regret it because there appears to be some misconceptions about nuclear energy and power in general.

1st I would like to point out that nuclear energy is the cleanest and simultaneous safest form of generating energy we’ve yet conceived.  This is not just an opinion as there is an abundance of evidence to this point.  It became a point of fear-mongering paid for by the fossil fuel industry that we are now sitting in this problem.

The use of uranium as the base fuel in nuclear systems is not only not necessary but not desirable.  Uranium as nuclear fuel using solid fuel reactor systems was a pragmatic decision made to expedite the construction of a nuclear weapon.  Even at the time of the Manhattan project, it was realized that using other nuclear fuel sources in a liquid-fueled reactor was much more desirable.  But the nuclear weapon needed to be developed before the end of the war and using a liquid-fueled reactor would’ve taken too long.

Work was accomplished by the United States on a liquid-fueled reactor using thorium as the fuel for breeder reactor was accomplished in the 50s.  This path was abandoned mostly for political reasons having to do with the self-interests of the people who were already invested in uranium systems and solid fuel reactors.  If your interest in this subject there are plenty of sources to look up regarding the new 4th generation reactor systems being developed in the United States and multiple countries many of which are based on thorium breeder systems and liquid fuels.

One of the problems with a nuclear fuel economy for Mars or the moon using uranium is not the locating and mining of uranium.  Even if uranium is found and refined (a chemical process) in order to use it in a reactor, you will need to do some isotope separation.  Isotope separation is extremely difficult, expensive, complicated, and large-scale industrial process.  Not only is it the part of the process that leads to the development of a nuclear weapon but does not lend itself to a bootstrapping economic system.

A thorium-based reactor, on the other hand, does not require isotope separation.  There are plenty of sources on the Internet to illustrate how a LFTR works.  The process would require a small amount of uranium 233 for your 1st reactor, but after that the uranium 233 can be separated and used as the basis for additional reactors.  The separation of the uranium 233 is accomplished chemically not by isotope isolation.  You are essentially breeding thorium to create uranium 233.  Extremely large quantities of thorium have been detected on the surface of both the moon but also Mars.  These reactors are not only about is inherently safe is anything you could make but also have much higher energy densities, so require much fewer materials used in their manufacture.

The heat from these reactors would be extremely useful in almost all manufacturing and chemical processes that you could think of due to them operating at much higher temperatures than is possible with a solid-fueled reactor.  Additionally a nuclear thermal rocket based on a LFTR would not only be much lighter, safer but much more efficient as it would operate on much higher temperatures than would be possible with a solid-fueled reactor.

Much work has been happening with these various reactor designs in the past 10 to 15 years.  These new reactor designs are not as far away as one might imagine.  Already test reactors are being built in various countries.  It is even conceivable that these designs might be mature enough to be used on Mars and the moon by 2030.

[tyrred]

Loving how clean and safe nuclear has become.  Simplest, most cost efficient energy production scheme, start to finish, least amount of the lowest hazard waste stream, even a child can appreciate it's simplicity.  Too cheap to meter.  Nary an accident, especially from anything flying overhead.  The future is now.

[savantu]

SpaceX is working on nuclear propulsion since at least 2016. Tom Mueller involvement currently is related to nuclear propulsion.

Mueller no doubt is heavily involved in bringing the Raptor into full production, working on those "100,000 mile" issues that he has talked about before, getting ready for flight of the vacuum version, and incrementally increasing the efficiency of the engine.

Umh, not true. He basically semi-retired and his interests are mainly his Porsche racing team and other private interests ( like a 1000mph land record ). He goes to office rarely, sometimes once a month.

With SpaceX he is involved in nuclear propulsion research and for special topics ( Dragon explosion investigation as example ). For obvious reasons he couldn't detail more.  But, he is not involved in Raptor development and/or mass production. In his own words; he left highly capable people to lead SpaceX propulsion department and they can shape their own path.

[raketa]

1/Starship to reach any part of solar system need nuclear power plant
2/Nuclear rocket engine give more delta V for available fuel.
3/New nuclear engine could be very low trust engine, using just for everything with exception of landing and launching from planets.(Hydrogen could be produce from methane by steam reforming)
4/Power plant could be used also for producing electricity, replacing solars in distances beyond Mars
5/Cooling of power plant could be done using skin of rocket, that is design to accept and radiate heat during landing
6/Power plant will be turn off during launch and landing, to cool down for landing heat wave

No, it does not „have to be“.
Not as simple as it might look.
Even beyond Mars.

1/Tell me your plan beyond Mars, for example Titan excursion.
2/I could imagine some rich guy would like to do first trip to Titan. Like it was at beginning of 20 century to reach South pole.
3/They will buy SS from Spacex and  refuel services at LEO.
4/Without Nuclear rocket engine their round trip  will be something like 8-10 years. Aprox 4 years one way trip, 1 year on surface to refuel.
5/Nuclear engine with power plant producing hydrogen and using to speed to Titan and back, you could short trip to 4-5 years.
6/I t will be great and very risky adventure, but achievable if SS Mars transportation occur viable.
7/I hope it will happen 10-20  years after first Mars landing, to see it live.

[hkultala]

Nuclear Thermal does not require liquid Hydrogen

NTR without hydrogen as the propellant has so bad isp that it makes practically no sense at all.

Not really. Water or ammonia NTR gets about the same ISP as chemical hydrolox but with 3-4x better bulk density meaning smaller lighter tanks.

... but much more expensive, heavier and politically problematic engines. Still doesn't make much sense.

[edzieba]

Anything involving highly enriched nuclear materials (Uranium or otherwise): right out. Proliferation isues are so massive (and internationally agreed and enforced) that the idea can be dismissed out of hand. That means all previous NTR designs (Rover and NERVA) are not usable. Orion is right out.
NASA have been working on low-enriched designs (at least as far back as 2014 ), but this is hampered by lack of test sites - TA-18 is long gone, and its use would no longer be acceptable - and minimal budget. Any independent SpaceX efforts will have the same issues: huge cost to build any test site, hard to find anywhere to put it, and additional cleanup costs for operation.
IF NASA can get the funding, clearance, and space to build and operate an NTR test site, and
IF Any NASA engine is funded for parallel development (to maintain a reason to keep such an expensive test site open and operating)
THEN a SpaceX developed NTR is a viable development. It'd still be expensive, so with Musk's goal-oriented development style SpaceX would also need to have a mission that want to achieve that would only be possible (or only economically viable) with an NTR rather than orbital staging or assembly of a chemical-engined vehicle or a solar-electric vehicle.

And if they do have such a mission, the available funds, and the regulatory environment (or lack thereof on a future Mars site), then I'd expect the Elon thing to do would be eschew NTRs and jump to nuclear-pulse or NSWR.

[rsdavis9]

I'm beginning to think the first nuclear engines or big power reactors may need to be made on mars because of all the problems with enriched fuels.

If I remember correctly the CANDU reactor doesn't need enriched Uranium. It uses heavy water and can get a running reactor. From there chemical separation of left over can get you your first thorium reactors.

[edzieba]

There are plenty of low-enriched reactor designs. The problem is those are intended to sit in big concrete and steel assemblages on the ground, rather than be as lightweight as possible. Even the high-enriched NERVA had a fairly lacklustre thrust:weight ratio (compare NERVA-XE's 7:1 to Merlin's 180:1).

[DAZ]

There are dozens of companies working on fourth-generation nuclear reactor designs right now.  There is about a dozen working on liquid-cooled/fueled reactor designs at this time.  Some of these companies will become the SpaceX and Blue Origins of the nuclear industry.  There’s been so much progress made on the molten salt designs that light water pressurized nuclear reactors are considered passé.  These companies are working through the regulatory problems.  The Netherlands has just completed a molten salt reactor for materials testing.  From the regulatory perspective, it is the materials testing that is the long pole in the tent.  Good progress has already been made on things like proliferation and general safety.  Many countries are advancing on the molten salt reactor designs.  The Chinese are already throwing hundreds of engineers and megabucks at these fourth-generation designs.  Even many environmentalists are starting to get on board and realize that we need fourth-generation nuclear reactors to solve our problems.  Very little of this progress is being reported in the mainstream press as it is not generating controversy.  But a considerable amount of information is out there if you know the keywords to put into a Google search.

Much of the initial design work for liquid salt reactor design was started with the nuclear bomber program.  It was realized to get the required small package to make the bomber work you would need to up the power density considerably.  I’m not referring to the fuel density but the size of the package that can generate the power.  The original Dumbo, Pluto, Rover, and NERVA used solid fuel designs through expediency and experience not to maximize performance.  With the much higher temperatures available with the molten salt design much higher efficiencies could have been obtained from an NTR design.

An overlooked feature of an NTR is that they are relatively agnostic to the propellants being used.  As musk has taught the world, maximizing efficiency may not be the most important feature.  The fact that you can go to many places in the solar system and fill up your propellant tanks with just about anything that is laying around that is liquid (or turned into liquid-like ice) even at lower efficiencies is a big selling point to an NTR.  The fact that your NTR, if designed properly, can also generate the electricity needed to produce these propellants with very little additional fuel needed is an additional selling point.  The lower thrust to weight of an NTR is not near as important on the Moon or Mars as it is on earth.  But if you need to use a nuclear powered system to get off from Earth than something like this would be workable in the decades ahead.  Air Enhanced Nuclear Thermal Rocket by x-SpaceX engineer. https://www.nextbigfuture.com/2017/07/air-enhanced-nuclear-thermal-rocket-by-x-spacex-engineer.html.  This concept was using a solid-fueled reactor but would work even better with a liquid molten salt design.

A LFTR design is a match made for Mars or the moon.  Not only is thorium apparently readily available but the smaller size of the reactors makes them ideal to manufacture the 1st ones on the earth and then ship them via starship.  Additionally, with their higher operating temperatures you do not need to use steam turbines (although you can and some earlier versions probably will) but instead, use the much more compact and efficient supercritical CO2 turbines.  The weight and volume savings for just this feature alone would make these designs desirable.  A LFTR with a properly designed kidney will produce all of the needed Uranium 233 without enrichment for all of your future reactor needs.  You can also get the isotopes needed for many of the possible nuclear medicine treatments.  These isotopes need to be manufactured locally as their half-lives are very short and could not be shipped in from Earth.  Even the short time from Earth to the moon would be problematic.  As much fewer materials will be needed, this lends itself better to locally manufactured reactors.

If you’re thinking long-term in space, there are many advantages to going nuclear.

[Elmar Moelzer]

In this context, has anyone seen this years NIAC presentation on PUFF?
Ignoring various pure fusion engine designs, this one could be really interesting. 37 days to Mars, relatively compact vehicle.
As for nuclear electric, this year's NIAC had a presentation featuring a new thermo-electric material with 20% efficiency.

[rsdavis9]

https://en.wikipedia.org/wiki/Stable_salt_reactor

Sounds like a good design.

[Selenaut]

As it has been pointed out repeatedly in these forums the most attractive use of nuclear power would be in an interplanetary tug-concept. To remain at a save distance to prevent it from falling into earth's atmosphere. Also, to provide its highest value by ferrying hardware and passengers to and from our neighbor planets and asteroids.
What I believe to be a key point is that the tugs could and should be very controlled by some authority in a similar fashion as early European colonial powers did with their fleets. Back then the metropolis had a very limited control over what happened in the colonies but focused on controlling the sea routes instead.
Would this make a (desirable?) political control over weaponizable vehicles? I agree that SpaceX is not necessary the best option here. They don’t even seem to be particularly interested as far as I know.

[Barley]

What I believe to be a key point is that the tugs could and should be very controlled by some authority in a similar fashion as early European colonial powers did with their fleets.

You mean piracy, war, and sinking them all over the place?

Could you please come up with a better model before I join Green Peace?

[gaballard]

What I believe to be a key point is that the tugs could and should be very controlled by some authority in a similar fashion as early European colonial powers did with their fleets.

You mean piracy, war, and sinking them all over the place?

Could you please come up with a better model before I join Green Peace?

Just wait until Boeing starts conscripting SpaceX astronauts!

[gaballard]

I always saw the main advantage of nuclear power (albeit fusion, less fission) the ability to generate a lot of electricity for electric thrusters, giving them appreciable thrust in addition to the crazy high ISP.

I have no idea how feasible that would be, given the mass of a reactor, the mass of the thrusters and fuel, necessary control equipment/wiring, etc., or if it'd even be possible with a fission reactor.

[rsdavis9]

I always saw the main advantage of nuclear power (albeit fusion, less fission) the ability to generate a lot of electricity for electric thrusters, giving them appreciable thrust in addition to the crazy high ISP.

I have no idea how feasible that would be, given the mass of a reactor, the mass of the thrusters and fuel, necessary control equipment/wiring, etc., or if it'd even be possible with a fission reactor.

Add radiators for the waste heat or you can't get any electricity out of it.

[Rei]

Just dropping by and saw this.  A couple points.

People using NERVA to describe nuclear thermal as a whole is like discussing the stats of the Saturn V to describe Starship. Nuclear thermal tech has moved on since NERVA.  NERVA demonstrated an 850sec Isp, but modern designs, such as pebble beds, are expected to achieve 1000sec at a lower mass. Some extreme (but also very immature) concepts, such as thin-film fission fragment reactors and "nuclear lightbulb" plasma / misty core concepts, could theoretically achieve 2000-4000sec.

Modern T:W ratios are expected to be significantly improved over NERVA in general, and there are some concepts to dramatically improve it further. One is LANTR, which uses a LOX "afterburner"; this lowers Isp by about a third when in use, but triples thrust - so you start out in afterburning mode, then transition to pure H2.  There's also bimodal designs, where a higher thrust/lower impulse propellant is used initially, transitioning to pure H2 later for lower thrust / higher impulse.   There are also air-augmented nuclear thermal rockets (NEAR) as well as nuclear scramjet (NTTR) concepts. The airbreathing designs can encompass the whole range of airbreathing systems known to man, from ramjets to compressors to LACE (taking on the advantages and disadvantages of each).

Nuclear thermal rockets are particularly useful as ascent stages from a Landis habitat (Venus aerostat), for a number of reasons.

 * Getting out of Venus's habitable layer with a chemical rocket realistically requires two stages. This means recapturing and remating two separate stages while each are hanging from the underside of the habitat.
 * Chemical stage recovery on Venus realistically requires ballutes-turned-balloons to loft the stages until recovery. Nuclear thermal however offers a variety of potential hover options, from nuclear-electric propellers to compressor-driven airbreathing hover.
 * Hydrogen is a limiting resource on Venus. Nuclear thermal rockets - despite realistically using pure hydrogen propellant - are as good or better than all but the lowest-hydrogen chemical propellants in terms of hydrogen consumption per unit cargo capacity.
 * Low-hydrogen chemical propellants (acetylene, carbon monoxide, cyanogen, etc) involve an unfortunate tradeoff between low Isp and high combustion temperatures. They also often have problematic chemical properties with regards to stability and/or toxicity
 * Per unit payload mass, the wet mass of a nuclear thermal rocket on Venus is much lighter.  This allows for a habitat of a given size to support a vastly higher population.  For a chemical rocket, almost all of the payload capacity of the habitat has to be dedicated to lofting its ascent stage.
 * NTR at Venus is the "least NIMBYable" way to get experience with NTR.  Even a NTR which is never fired on Earth's surface and is only used as an interplanetary transfer stage will still make regular visits to LEO, with a used (dirty) core. But a NTR ascent stage at Venus will never fire its engines anywhere near Earth.   It can even be launched onto a Venus intercept trajectory with its fuel under-enriched, with the reactor breeding up to operational power en-route and/or while docked.

More here:
http://www.venuslabs.org/Rethinking%20Our%20Sister%20Planet%20(prepress).pdf

Realistically... while Venus can be colonized with chemical rockets, it becomes a *lot* easier with nuclear thermal.

[gaballard]

I always saw the main advantage of nuclear power (albeit fusion, less fission) the ability to generate a lot of electricity for electric thrusters, giving them appreciable thrust in addition to the crazy high ISP.

I have no idea how feasible that would be, given the mass of a reactor, the mass of the thrusters and fuel, necessary control equipment/wiring, etc., or if it'd even be possible with a fission reactor.

Add radiators for the waste heat or you can't get any electricity out of it.

Sorry if this is a dumb or obvious question, but is that because you can't create the temperature gradient needed for the thermoelectric effect without the radiators?

[rsdavis9]

I always saw the main advantage of nuclear power (albeit fusion, less fission) the ability to generate a lot of electricity for electric thrusters, giving them appreciable thrust in addition to the crazy high ISP.

I have no idea how feasible that would be, given the mass of a reactor, the mass of the thrusters and fuel, necessary control equipment/wiring, etc., or if it'd even be possible with a fission reactor.

Add radiators for the waste heat or you can't get any electricity out of it.

Sorry if this is a dumb or obvious question, but is that because you can't create the temperature gradient needed for the thermoelectric effect without the radiators?

Yes.
Also for stirling or any other heat engine to generator hookup.

[RonM]

I always saw the main advantage of nuclear power (albeit fusion, less fission) the ability to generate a lot of electricity for electric thrusters, giving them appreciable thrust in addition to the crazy high ISP.

I have no idea how feasible that would be, given the mass of a reactor, the mass of the thrusters and fuel, necessary control equipment/wiring, etc., or if it'd even be possible with a fission reactor.

Add radiators for the waste heat or you can't get any electricity out of it.

Sorry if this is a dumb or obvious question, but is that because you can't create the temperature gradient needed for the thermoelectric effect without the radiators?

Yes.
Also for stirling or any other heat engine to generator hookup.

Nuclear-electric systems need radiators or the core will melt. With NERVA engines most of the heat goes out the nozzle via the heated propellant.