Will SpaceX ever go nuclear propulsion?

[Robotbeat]

Also, while I think Solar Electric Propulsion is awesome, don't become enamored with VASIMR. There are a lot of other electric propulsion solutions out there that are less complicated and even potentially higher performing, not to mention more mature and proven.

Honestly, with the kind of delta-v SpaceX is talking about here... I'm not sure any of the solar or nuclear electric schemes currently contemplated would come out better. All of those assume launch is expensive and leverage greatly increased specific impulse. They hope to decrease trip time by doing more delta-v than we can afford to do with chemical. But, high specific impulse has extraordinary power needs. If launch is cheap, the complexity becomes expensive, and Mars is close enough that there's no time to accrue greater total delta-v at low power.

To get around this a crazy increase in specific power is needed. Nuclear thermal has the issues you mention. Another idea might be zapping the solar panels with a monochromatic laser tuned for their band gap at hundreds of suns radiance. If this could get a few days at high thrust that might be worth it. It's hard to think of anything else that would do it even potentially that doesn't require science fiction technology.

Indeed! I'm as big of an SEP fan as you'll come across, but it may just be too complex and expensive for mass transit to Mars. Maybe cheap (and high specific power) thin-film solar arrays could do it, but abundant-chemical seems to fit the bill much better.

[Norm38]

Nuclear will wait until a permanent Mars colony exists, because that's what will drive the demand.
Once there is a colony and regular two-way trade, people will want to go faster. The money and infrastructure will be there to do the R&D.  Not before.

Mars will be colonized with chemical rockets, it's the only way it's going to work.

What is actually more likely to drive nuclear propulsion is a push beyond Mars. Once we have the tech to live on Mars, people will push to explore the belt, and the moons of Jupiter/Saturn.
A 22nd century problem.

[Ludus]

Also, RE:LFTR: LFTR is a bad fit for in-space propulsion. Too heavy. If you disagree with me, ask foremost LFTR developer and evangelist Kirk Sorenson, who visits this forum regularly and has strong opinions about NTR.

LFTR is a thermal breeder reactor and likely too massive for a space reactor. A simpler molten salt reactor design might be an appropriate space reactor, however.

Very cool! A comment from you on LFTR's is about as definitive as Elon Musk dropping in to comment about SpaceX's future plans.

[ArbitraryConstant]

Exotic science fiction technology will be required to build a society in the vast empty void of the asteroid belt. Not doable without fusion drives IMO. But Mars is near enough to go all chemical.

The delta-v to major asteroids would be tenable given what is inferred of MCT numbers, even though the travel time would suck. The advantages are little landing delta-v and and constant solar power. I think granting enough assumptions to set up shop on Mars gets you to the asteroids without much extra (even though this is ambitious in the extreme).

It's the outer moons that would be untenable... huge extra delta-v due to enormous gas giant gravity wells, non-trivial gravity on top of the great distance and delta-v, day/night cycles... Yup, these require technology we can't even demonstrate in the lab today. Mars and the asteroids MAY be possible with extremely high industrial output per person and a lot of big rockets to get them there.

[Vultur]

At some point nuclear reactors will be needed in Mars. That's almost a sure thing (solar radiation is much weaker in Mars due to distance to the Sun),

Nah. Sunlight is not THAT much weaker at Mars. Aphelion = 1.666 AU so using inverse square law solar constant at Mars's aphelion will be 1.666 squared = 2.776 times weaker at Mars. I see slightly different values for the solar constant at 1 AU but they're all close to 1360 W/m^2, so it would be about 490 W/m^2 at Mars's aphelion.

So you need less than 3 times the area you would need on Earth. With light thin films, that's probably very workable.

(Yes, Mars has dust storms that block light, but Earth has long periods of cloudiness too, in at least moderately wet climates.)

EDIT:

Also, while I think Solar Electric Propulsion is awesome, don't become enamored with VASIMR. There are a lot of other electric propulsion solutions out there that are less complicated and even potentially higher performing, not to mention more mature and proven.

Honestly, with the kind of delta-v SpaceX is talking about here... I'm not sure any of the solar or nuclear electric schemes currently contemplated would come out better. All of those assume launch is expensive and leverage greatly increased specific impulse. They hope to decrease trip time by doing more delta-v than we can afford to do with chemical. But, high specific impulse has extraordinary power needs. If launch is cheap, the complexity becomes expensive, and Mars is close enough that there's no time to accrue greater total delta-v at low power.

To get around this a crazy increase in specific power is needed. Nuclear thermal has the issues you mention. Another idea might be zapping the solar panels with a monochromatic laser tuned for their band gap at hundreds of suns radiance. If this could get a few days at high thrust that might be worth it. It's hard to think of anything else that would do it even potentially that doesn't require science fiction technology.

Indeed! I'm as big of an SEP fan as you'll come across, but it may just be too complex and expensive for mass transit to Mars. Maybe cheap (and high specific power) thin-film solar arrays could do it, but abundant-chemical seems to fit the bill much better.

I agree. High-power electric propulsion  (or something else high-Isp like the proposed MSNW pulsed fusion rocket) will be needed for manned missions to Jupiter's moons and beyond, but chemical should work for Mars (and NEOs and Venus's atmosphere, and maybe the asteroid belt depending on what trip times are acceptable).

Presumably they are growing food in sealed greenhouses; how is it getting in the food supply?

Via nutrients collected from outside.

Assuming you didn't blow up an NTR engine directly next to the greenhouses, would this be enough to matter? We blew up 100 open-air nuclear bombs in Nevada and, while there was a statistical increase of cancer rates (or was predicted to be - don't know if this was actually proven) it's pretty tiny compared to, say, the health effects of fossil fuel air pollution, which our society accepts.

And Mars is big... if they were worried, the tests could be done much farther from the colony than the Nevada Test Site was from Las Vegas. And the amount of radionuclides released would presumably be vastly less, unless they just kept blowing up engines one after another in ways that atomized them into the air effectively. And most nutrients would probably be recycled, with relatively low input for colony growth and whatever losses occurred.

[guckyfan]

Exotic science fiction technology will be required to build a society in the vast empty void of the asteroid belt. Not doable without fusion drives IMO. But Mars is near enough to go all chemical.

The delta-v to major asteroids would be tenable given what is inferred of MCT numbers, even though the travel time would suck. The advantages are little landing delta-v and and constant solar power. I think granting enough assumptions to set up shop on Mars gets you to the asteroids without much extra (even though this is ambitious in the extreme).

I wasn't thinking of a problem to get there. Scientific expeditions will be possible, especially with resupply stops at Mars, when the colony there is well established. Long term I also hope for fuel from phobos or deimos which would make a lot of things easier.

But a colony out there will have the same problem as a colony on Mars. Any autonomous society will need to have many people and most importantly produce all raw materials, all industrial goods and food locally. Getting raw materials and people from other locations in the belt or Mars will have travel times of years unless you are able to use many km/s delta-v. That is why I see the need of exotic propulsion. Unless you can build that colony in a single location and source all required materials locally. Powering such a colony with solar energy may be even feasible. The light is less dense but it is not that hard to build large structures that don't need to withstand any gravity or acceleration.

[Llian Rhydderch]

I know this is quite speculative, but following reading 'The Martian' by Andy Weir, and SpaceX's push for Mars, got wondering, is there any merit to go from chemical rockets to nuclear at some point in the future?

I just finished reading that book.  As far as hard SF goes, it should be required reading for future explorers and hardware architects.

As far as nuclear power for space exploration uses, sure there is merit.  But the nuclear industry is so regulated, not to mention expensive, that it would be hard to see any private company pursue nuclear power for their own use.

The U.S. Government will have to decide that it's in it's best interests to get back into producing nuclear power sources for off-Earth applications...

I concur with the first half of what Ron said here.

And if the first half is correct, perhaps the US government (and the political economic "collective choice" problems of voting and making public policy) simply won't get resolved in a technology-advancing direction for nuclear power anytime in the next many decades.

It is not inconceivable that the next technological advance in nuclear propulsion will be made on some asteroid or extraterrestrial planet.   

And since SpaceX is currently the leading corporate entity worldwide with both plans and actions to get off planet in a sustainable way, it is certainly not inconceivable that SpaceX spaceflight technology will be a key enabler of this, whether SpaceX chooses to directly involve themselves in it or not.

[ncb1397]

Exotic science fiction technology will be required to build a society in the vast empty void of the asteroid belt. Not doable without fusion drives IMO. But Mars is near enough to go all chemical.

The delta-v to major asteroids would be tenable given what is inferred of MCT numbers, even though the travel time would suck. The advantages are little landing delta-v and and constant solar power. I think granting enough assumptions to set up shop on Mars gets you to the asteroids without much extra (even though this is ambitious in the extreme).

I wasn't thinking of a problem to get there. Scientific expeditions will be possible, especially with resupply stops at Mars, when the colony there is well established. Long term I also hope for fuel from phobos or deimos which would make a lot of things easier.

But a colony out there will have the same problem as a colony on Mars. Any autonomous society will need to have many people and most importantly produce all raw materials, all industrial goods and food locally. Getting raw materials and people from other locations in the belt or Mars will have travel times of years unless you are able to use many km/s delta-v. That is why I see the need of exotic propulsion. Unless you can build that colony in a single location and source all required materials locally. Powering such a colony with solar energy may be even feasible. The light is less dense but it is not that hard to build large structures that don't need to withstand any gravity or acceleration.

You don't need a lot of Delta-v capability to operate in the asteroid belt. Dawn is doing Vesta to Ceres with solar electric propulsion in about 2 years. If all you are doing is shipping manufactured goods around, you don't have to worry too much about a 2 year journey. Escape velocity on Ceres, the biggest object in the main belt, is about 500 m/s. Escape velocity is so low that some rifle or tank rounds fired would actually escape. I would concentrate the colony there and mine asteroids that pass by. We don't know much about it, but we will learn a lot more in just a few months time including about potential nuclear resources(Uranium).

[guckyfan]

You don't need a lot of Delta-v capability to operate in the asteroid belt. Dawn is doing Vesta to Ceres with solar electric propulsion in about 2 years. If all you are doing is shipping manufactured goods around, you don't have to worry too much about a 2 year journey. Escape velocity on Ceres, the biggest object in the main belt, is about 500 m/s. Escape velocity is so low that some rifle or tank rounds fired would actually escape. I would concentrate the colony there and mine asteroids that pass by. We don't know much about it, but we will learn a lot more in just a few months time including about potential nuclear resources(Uranium).

Yes, I am aware of that. But it does mean that any groups of humans would  be very isolated from any other group of humans. Each location will need to be very nearly self sufficient. Very hard to do if not impossible unless each group is very large. A big problem for mining locations unless you assume it is all fully automated?

[R7]

Presumably they are growing food in sealed greenhouses; how is it getting in the food supply?

Via nutrients collected from outside.

Assuming you didn't blow up an NTR engine directly next to the greenhouses, would this be enough to matter? We blew up 100 open-air nuclear bombs in Nevada and, while there was a statistical increase of cancer rates (or was predicted to be - don't know if this was actually proven) it's pretty tiny compared to, say, the health effects of fossil fuel air pollution, which our society accepts.

Many other routes than just nutrients; consumables like water, breathing gases are mined from atmosphere/soil. It's a dusty place and some of it inevitably gets inside when people and equipment move in and out.

Earth has processes which disperse and lock in the nasty stuff (rain, seas, biology, sedimentation). Folks near Aral Sea (what's left of it anyway) are in danger of being re-exposed to old USSR nuke tests pollution because the eastern lobe has dried up completely and winds can now blow the dried sediment elsewhere. Mars is one big permanent desert with occasional global dust storms so even if your Jackass Planitia is on the other side of the planet the pollutants from your tests over there are just waiting for transportation until they run out of half-lives.

Got nothing personally against nuclear power per se, molten salt reactors for the win! But would save nuclear propulsion for in-space activity only.

[RonM]

One of the reasons for nuclear propulsion was to reduce the mass of the spacecraft. For example, one of NASA's Mars mission plans had 12-15 launches of a HLV for chemical rockets to be placed in LEO, but only 4-5 launches for nuclear rockets. That would be a big savings in launches to LEO.

Now with the today's concept of reusable rockets, getting the required chemical rocket fuel to orbit will be far cheaper. A reusable BFR launching a chemically propelled MCT will be practical. SpaceX does not need nuclear propulsion for the MCT.

[macpacheco]

At some point nuclear reactors will be needed in Mars. That's almost a sure thing (solar radiation is much weaker in Mars due to distance to the Sun),

Nah. Sunlight is not THAT much weaker at Mars. Aphelion = 1.666 AU so using inverse square law solar constant at Mars's aphelion will be 1.666 squared = 2.776 times weaker at Mars. I see slightly different values for the solar constant at 1 AU but they're all close to 1360 W/m^2, so it would be about 490 W/m^2 at Mars's aphelion.

So you need less than 3 times the area you would need on Earth. With light thin films, that's probably very workable.

Except a Mars colony will be far more energy intensive per capta than earth living. And then you need to account for energy storage that could be mostly avoided with two reliable nuclear reactors.
Recycling Water, Oxygen, CO2, human waste. Hydroponic food growth (with around the clock lighting). Plus heating. Its cold in Mars. A nuclear reactor directly produces heat, while using solar for heating in Mars doesn't sound promising.
Plus placing solar panels exposed to the elements in Mars could risk damage from the sandstorms you mentioned. Nuclear is easy to build indoors, shielded from the elements. Things shipped to Mars must be very rugged.
I'm not anti solar. But I at least recognize its serious limitations.
Nuclear on the other hand is reliable, constant power production.
Some molten salt reactors have been designed for uninterrupted operation for 30 years.
I wouldn't be too happy about living in a Mars colony with a water cooled reactor, as its not walk away safe, but a molten salt reactor would easily be.
Perhaps nuclear + solar. But I surely wouldn't bet on solar alone.

[ArbitraryConstant]

Earth has processes which disperse and lock in the nasty stuff (rain, seas, biology, sedimentation).

That.

I think it's a non-issue though, chemical SSTO is easy on Mars. Launch the nuclear stage all the way to orbit before turning it on, natural isotopes of Uranium aren't much concern by themselves.

Getting raw materials and people from other locations in the belt or Mars will have travel times of years unless you are able to use many km/s delta-v.

Ah, I understand. Under that set of assumptions yes, you would need exotic propulsion, or at least a very long wind-up in production. I don't share those assumptions though.

Unless you can build that colony in a single location and source all required materials locally.

It should be the case that asteroids are available with everything available in one place. For eg Vesta it looks like it has everything a planet has except maybe noble gases. No idea how many asteroids have enough of a mix to do this, but it's not zero...

Yes, I am aware of that. But it does mean that any groups of humans would  be very isolated from any other group of humans. Each location will need to be very nearly self sufficient. Very hard to do if not impossible unless each group is very large.

The larger asteroids are easily big enough to support a scale of activity as big as what's being discussed for Mars.

[Robotbeat]

Mars may not be as energy intensive as suburban/exurban living, commuting in a Hummer 30 miles each way to a climate controlled McMansion. Food production on Mars takes about 1kW per person with high-efficiency methods, oxygen included. If vast majority of people are inside the colony, I don't see it as being particularly more energy-intensive than rich countries on Earth, at least not until the colony is thriving dramatically and people are taking joy-rides in rovers and aircraft to go sightseeing.

[kfsorensen]

I wouldn't be too happy about living in a Mars colony with a water cooled reactor, as its not walk away safe, but a molten salt reactor would easily be.

On Mars, there is no such thing as "walk-away safe".  There is nowhere to walk away too.  If the reactor fails to produce energy, or fails any other way, the crew dies.

[Robotbeat]

I wouldn't be too happy about living in a Mars colony with a water cooled reactor, as its not walk away safe, but a molten salt reactor would easily be.

On Mars, there is no such thing as "walk-away safe".  There is nowhere to walk away too.  If the reactor fails to produce energy, or fails any other way, the crew dies.

Good argument for diverse, overlapping power sources. Didn't think you'd make such an argument.

[macpacheco]

I wouldn't be too happy about living in a Mars colony with a water cooled reactor, as its not walk away safe, but a molten salt reactor would easily be.

On Mars, there is no such thing as "walk-away safe".  There is nowhere to walk away too.  If the reactor fails to produce energy, or fails any other way, the crew dies.

Good argument for diverse, overlapping power sources. Didn't think you'd make such an argument.

Kirk's argument is just as important about solar too. If a large enough share of solar panels fail for any reason, you die sooner or later (lack of oxygen, lack of drinking water, lack of food).
It is interesting to actually understand the detailed differences between Molten Salt Reactors and Water cooled ones.
MSRs have zero risk of loss of coolant accidents, because the fuel and the coolant are mixed together. It is essentially impossible to have a thermal runaway that melts the reactor containment (the fuel can't melt its already molten). With the freeze plug, catch pan, and the thermal characteristics of the core (coolant+fuel) material. The argument walk away safe doesn't make justice to the reactor. Impossible to have either a TMI, Fukushima or Chernobyl type accident. Actually the only kind of accident that could be dangerous to those very close to the reactor is a direct asteroid/comet hit, in that case, the whole Mars base is at risk from loss of atmosphere alone.
When you consider all risks in a settlement in any planet that doesn't have a breathable atmosphere. I think I would prefer an MSR reactor to solar panels any day.

[ArbitraryConstant]

Except a Mars colony will be far more energy intensive per capta than earth living.

That's probably true, but that's not an argument for any particular power source, only that one of the main criteria will be what can be scaled up quickly with a very limited industrial base. Here on Earth we have more sunlight, but we also have incredibly easy to use heat sinks for our nuclear reactors. Everything is easier here. It isn't a trivial assertion that nuclear technology is actually better for this than solar, either 20 years from now or possibly even now. Time favors solar more than nuclear due to much faster pace of improvement. The same is largely true for batteries, but they're about 10 years behind, as solar is about 10 years behind wind.

A step change in nuclear technology like LFTR may provide a large enough improvement, but I think that's a much more vulnerable assumption than a zillion different solar labs figuring out incremental improvements.

Plus placing solar panels exposed to the elements in Mars could risk damage from the sandstorms you mentioned.

Rule of thumb: if terminal velocity under a parachute doesn't give you a survivable landing, the wind probably can't damage your stuff. I don't think this is a concern. At worst dust will have to be removed.

Some molten salt reactors have been designed for uninterrupted operation for 30 years.

A design being able to run as long as it's supposed to isn't always how things work out. Materials act weird in those conditions, and I know of cases where eg a refit is required because something swelled more than expected under neutron bombardment.

[macpacheco]

That's probably true, but that's not an argument for any particular power source, only that one of the main criteria will be what can be scaled up quickly with a very limited industrial base. Here on Earth we have more sunlight, but we also have incredibly easy to use heat sinks for our nuclear reactors. Everything is easier here. It isn't a trivial assertion that nuclear technology is actually better for this than solar, either 20 years from now or possibly even now. Time favors solar more than nuclear due to much faster pace of improvement. The same is largely true for batteries, but they're about 10 years behind, as solar is about 10 years behind wind.

A step change in nuclear technology like LFTR may provide a large enough improvement, but I think that's a much more vulnerable assumption than a zillion different solar labs figuring out incremental improvements.

Some molten salt reactors have been designed for uninterrupted operation for 30 years.

A design being able to run as long as it's supposed to isn't always how things work out. Materials act weird in those conditions, and I know of cases where eg a refit is required because something swelled more than expected under neutron bombardment.

The big problem with solar isn't just having enough panels. Its energy storage. Like on earth, it's not always day in Mars. You must store energy overnight.

In don't know Mars, but the moon case is very interesting. A lunar day is 28 earth days, so its literally impossible to store solar power long enough. For a lunar base nuclear is the sole viable power for anything but a very short mission.

Don't underestimate the power of today's computers. We're already at a point where computers simulate things with extreme accuracy. Real world prototypes confirm computer simulations. You can't compare the predicted vs actual behavior of a Gen II (70s design built in the 80s) with a Gen III+ (designed in the last 10 years, being built now) with Gen IV reactors (being designed right now).

Finally, MSR reactors avoid lots of thermal issues that are very significant in water cooled reactors. There is no thermal barrier between the neutron source (fuel) and coolant. The coolant operates at near atmospheric pressure, meaning it can be allowed to expand and contract thermally, which results in a negative temperature coefficient, which results in a naturally load following reactor, no need for control rods or injection of neutron absorbers to control reactivity (power levels). Those swelling issues mostly don't apply to MSRs.

[Robotbeat]

I wouldn't be too happy about living in a Mars colony with a water cooled reactor, as its not walk away safe, but a molten salt reactor would easily be.

On Mars, there is no such thing as "walk-away safe".  There is nowhere to walk away too.  If the reactor fails to produce energy, or fails any other way, the crew dies.

Good argument for diverse, overlapping power sources. Didn't think you'd make such an argument.

Kirk's argument is just as important about solar too. If a large enough share of solar panels fail for any reason, you die sooner or later (lack of oxygen, lack of drinking water, lack of food).
It is interesting to actually understand the detailed differences between Molten Salt Reactors and Water cooled ones.
MSRs have zero risk of loss of coolant accidents, because the fuel and the coolant are mixed together. It is essentially impossible to have a thermal runaway that melts the reactor containment (the fuel can't melt its already molten). With the freeze plug, catch pan, and the thermal characteristics of the core (coolant+fuel) material. The argument walk away safe doesn't make justice to the reactor. Impossible to have either a TMI, Fukushima or Chernobyl type accident. Actually the only kind of accident that could be dangerous to those very close to the reactor is a direct asteroid/comet hit, in that case, the whole Mars base is at risk from loss of atmosphere alone.
When you consider all risks in a settlement in any planet that doesn't have a breathable atmosphere. I think I would prefer an MSR reactor to solar panels any day.

Why not both? Solar panels already have development paid for and are cheap. That's what my point was. That way, if your MSR stops working for whatever reason, you're fine.