Nuclear thermal rockets

[DerekL1963]

@DDE

That quote fails to support your claim.  ("After a single TMI a nuclear-thermal engine is burned out".)  One full day of full power operation is between four to six round trips to Mars.  Nor does it support your assertion that designs in the 60's used "multiple NERVA stages".

Edited August 15, 2017 by DerekL1963

[MaverickSawyer]

2 hours ago, Racescort666 said:

Also in the link that @shynung shared: http://www.projectrho.com/public_html/rocket/enginelist.php#ntrsolidcore

 

I dunno... isn't hydrogen prone to eating the graphite in solid-core NTRs?

[shynung]

@MaverickSawyer Common NTR design usually includes a protective coating on the reactor elements to prevent the propellant from eating them. However, coatings that work against reducing propellants (hydrogen, ammonia, methane) is useless against an oxidizing propellant (oxygen, water, carbon dioxide), and vice versa. Designing a reactor that can withstand both is a pretty challenging engineering task.

[Kerbart]

5 hours ago, KSK said:

Building an NTR that can run on anything is a hard problem. Designing a fuel element coating that can protect against hot reducing propellants (such as hydrogen) or hot oxidising propellants (such as water) isn't too bad but (or so I remember reading) designing a coating that can deal with both is tough. I suppose you could have two sets of channels through the reactor, each with an appropriate coating for the different propellant types but then you'd likely run into all sorts of thermal problems, not to mention those other interactions that @DDE mentioned. I doubt it's impossible but I can certainly imagine it being challenging, to put it mildly.

It's hardly a challenge. Relatively speaking that is. Compared to the paperwork required to get this thing in space in the first place.

[MaverickSawyer]

1 hour ago, shynung said:

@MaverickSawyer Common NTR design usually includes a protective coating on the reactor elements to prevent the propellant from eating them. However, coatings that work against reducing propellants (hydrogen, ammonia, methane) is useless against an oxidizing propellant (oxygen, water, carbon dioxide), and vice versa. Designing a reactor that can withstand both is a pretty challenging engineering task.

Okay, so you could run an engine on LH2 to depart Earth for Mars and maybe for the MOI burn, then use methane for the return to Earth, then use hydrogen again to flush whatever coking deposits you have out of the engine channels.

[wumpus]

13 hours ago, Kerbart said:

It's hardly a challenge. Relatively speaking that is. Compared to the paperwork required to get this thing in space in the first place.

Any guesses on the availability of Uranium or Thorium in the asteroid belt (like a large rock that can be trivially smelted to one or the other)?  You would still need roughly the fuel for at least one full burn and maybe more, but the idea would be no more fuel flights.

This is more a "decades out" idea, but the politics seems to be going away not toward this type of thing.  Since step 1: build an ion system to find an asteroid and bring it nearby (L2?) is more or less within modern tech (ignoring failures in latching onto comets, and no tests on asteroids) it might be a good place to start.

[shynung]

17 hours ago, DerekL1963 said:

Nor does it support your assertion that designs in the 60's used "multiple NERVA stages".

The staged NERVA design @DDE referred to is the Boeing IMIS.

[KSK]

3 hours ago, wumpus said:

Any guesses on the availability of Uranium or Thorium in the asteroid belt (like a large rock that can be trivially smelted to one or the other)?  You would still need roughly the fuel for at least one full burn and maybe more, but the idea would be no more fuel flights.

This is more a "decades out" idea, but the politics seems to be going away not toward this type of thing.  Since step 1: build an ion system to find an asteroid and bring it nearby (L2?) is more or less within modern tech (ignoring failures in latching onto comets, and no tests on asteroids) it might be a good place to start.

I have no idea how trivial it is to smelt uranium on Earth, let alone on an asteroid. It's not so much the smelting through as the enrichment, processing to a form suitable for fabricating fuel elements and then fabricating those fuel elements that's likely to be the problem. Definitely a decades out project.

[sevenperforce]

3 hours ago, wumpus said:

Any guesses on the availability of Uranium or Thorium in the asteroid belt (like a large rock that can be trivially smelted to one or the other)?  You would still need roughly the fuel for at least one full burn and maybe more, but the idea would be no more fuel flights.

This is more a "decades out" idea, but the politics seems to be going away not toward this type of thing.  Since step 1: build an ion system to find an asteroid and bring it nearby (L2?) is more or less within modern tech (ignoring failures in latching onto comets, and no tests on asteroids) it might be a good place to start.

Very, very low. Earth has a high proportion of heavy elements because we got smashed into by Theia and we have a very high amount of geothermal activity, forcing heavy elements up into the crust. There wouldn't be enough in asteroids to mine.

[mikegarrison]

On 8/14/2017 at 5:52 AM, KSK said:

 

 

 

Launching chunks of enriched nukular fuel into orbit seems unlikely to be acceptable to the general public for the foreseeable future.

Not just the general public. There might be concern from other governments that these are really nuclear weapons disguised as rockets.

[KSK]

1 hour ago, shynung said:

The staged NERVA design @DDE referred to is the Boeing IMIS.

I'm maybe missing something but I didn't see anything in that link to suggest that the NERVAs were ditched after burning through all their fission fuel. It just(?) looked like they were staging the NERVA cores for the usual reason - to get around that pesky rocket equation.

Edited August 15, 2017 by KSK

[MaverickSawyer]

There were also (somewhat justifiable) concerns about being able to safely restart the engines later in the mission, hence the staging of the entire module, instead of just dropping tanks.

[wumpus]

3 hours ago, sevenperforce said:

Very, very low. Earth has a high proportion of heavy elements because we got smashed into by Theia and we have a very high amount of geothermal activity, forcing heavy elements up into the crust. There wouldn't be enough in asteroids to mine.

My understanding is that there are plenty of nickel asteroids, and likely a platinum or iridium asteroid that can be found (those are typically suggested when mining asteroids).  Certainly plenty of things heavier than aluminum (or whatever the heaviest bit of moonrock is), although you have to find asteroid.  Anything flung into space via similar action would have large chunks of heavy materials.

I wasn't thinking: smelting is trivial, especially compared to the energy source needed to move it around.  Enrichment is the problem (assuming uranium is out there, it is still a big step up from nickel and iron), which is why I knew such a program would need one (fueled) nuclear reactor to enrich the material as it is mined and/or lifted to orbit.

This method would be ideal for dealing with spent fuel rods (cooled down "correctly" or simply ejected and sent back to base).  I wonder if it would be possible to ship up low level U238 and enrich in orbit/L2/wherever (and ignore the asteroids until they are mined for other reasons)?  I'm guessing this would have all the same political issues, but it seems sad that such would stop a possible real interplanetary space program.

[sevenperforce]

3 hours ago, mikegarrison said:

Not just the general public. There might be concern from other governments that these are really nuclear weapons disguised as rockets.

Eh, I seriously doubt anyone would worry that the US Government is secretly disguising its nukes as rockets. 

3 minutes ago, wumpus said:

My understanding is that there are plenty of nickel asteroids, and likely a platinum or iridium asteroid that can be found (those are typically suggested when mining asteroids).  Certainly plenty of things heavier than aluminum (or whatever the heaviest bit of moonrock is), although you have to find asteroid.  Anything flung into space via similar action would have large chunks of heavy materials.

I wasn't thinking: smelting is trivial, especially compared to the energy source needed to move it around.  Enrichment is the problem (assuming uranium is out there, it is still a big step up from nickel and iron), which is why I knew such a program would need one (fueled) nuclear reactor to enrich the material as it is mined and/or lifted to orbit.

This method would be ideal for dealing with spent fuel rods (cooled down "correctly" or simply ejected and sent back to base).  I wonder if it would be possible to ship up low level U238 and enrich in orbit/L2/wherever (and ignore the asteroids until they are mined for other reasons)?  I'm guessing this would have all the same political issues, but it seems sad that such would stop a possible real interplanetary space program.

Anything substantially heavier than iron is only formed in trace amounts during core-collapse supernovae or, very rarely, in neutron-star collisions.

[MatterBeam]

On 14/08/2017 at 10:36 PM, sevenperforce said:

I've considered it but my supply of plutonium is a bit low.

EDIT: In all seriousness, one of the big things with NTRs is the design. You can get up into some really wicked exhaust velocities if you're willing to bump up to a pebble-bed reactor.

Actually, I don't think the improvements will be that great. Even the toughest 'pebbles' in a pebble-bed reactor can only handle up to 4200K due to the melting point of Tantalum hafnium carbide. Previous nuclear thermal designs had the hot uranium oxide cores directly exposed to the propellant flow, limiting them to the melting point of the fissile fuel (3138K). This 33% increase in temperature, if we run it through this root mean square gas velocity calculator, would increase the maximum Isp on hydrogen propellant (1g/mol) from 8.8km/s to 10.2km/s, only 16% better. 

Only very large deltaVs would reveal savings in mass ratios. I think the true motivation for the development of pebble-bed technology is the safety factor. In case of a catastrophic failure that causes the rocket engine to explode, you'll only deal with rapidly cooling and inert golf-balls instead of a dust cloud of lethal radioactive particles. 

29 minutes ago, wumpus said:

My understanding is that there are plenty of nickel asteroids, and likely a platinum or iridium asteroid that can be found (those are typically suggested when mining asteroids).  Certainly plenty of things heavier than aluminum (or whatever the heaviest bit of moonrock is), although you have to find asteroid.  Anything flung into space via similar action would have large chunks of heavy materials.

I wasn't thinking: smelting is trivial, especially compared to the energy source needed to move it around.  Enrichment is the problem (assuming uranium is out there, it is still a big step up from nickel and iron), which is why I knew such a program would need one (fueled) nuclear reactor to enrich the material as it is mined and/or lifted to orbit.

This method would be ideal for dealing with spent fuel rods (cooled down "correctly" or simply ejected and sent back to base).  I wonder if it would be possible to ship up low level U238 and enrich in orbit/L2/wherever (and ignore the asteroids until they are mined for other reasons)?  I'm guessing this would have all the same political issues, but it seems sad that such would stop a possible real interplanetary space program.

6 hours ago, wumpus said:

Any guesses on the availability of Uranium or Thorium in the asteroid belt (like a large rock that can be trivially smelted to one or the other)?  You would still need roughly the fuel for at least one full burn and maybe more, but the idea would be no more fuel flights.

This is more a "decades out" idea, but the politics seems to be going away not toward this type of thing.  Since step 1: build an ion system to find an asteroid and bring it nearby (L2?) is more or less within modern tech (ignoring failures in latching onto comets, and no tests on asteroids) it might be a good place to start.

The formation of our solar system concentrated the small quantities of fissionable elements inside the cores of rocky planets and the largest moons. Very little managed to stay in the outer solar system. What we'll need to find is rocky asteroids that are billions of years old, composed of the same undifferentiated dust that planets are made up of. You'll find about 1 atom in 10,00,000 to be Thorium and 1 atom in 100,000,000 to be Uranium for every atom of iron or silicon there is.  

3 hours ago, KSK said:

I have no idea how trivial it is to smelt uranium on Earth, let alone on an asteroid. It's not so much the smelting through as the enrichment, processing to a form suitable for fabricating fuel elements and then fabricating those fuel elements that's likely to be the problem. Definitely a decades out project.

On Earth, geothermal and hydrological processes bring up heavy elements from the mantle and concentrate it in groundwater pockets. We mine 'veins' and 'deposits' of these ores. Despite being handily collected for us by natural processes, it is still a tiny fraction of the ores we mine, in this case pitchblende, so a lengthy process is needed to separate and extract uranium from it. It involves chemical baths and centrifuges.

In space, the asteroids we will mine uranium from will not have any water features or tectonic activity. The uranium is as likely to be distributed at the center as it is to be concentrated on the surface. It is most likely that the uranium will be mixed into the rocky grain on which ices and frozen volatiles stuck to over time, forming the bulk of the asteroid. Mining it will involve digging out the ice crust, extracting the rocky core and running the ores in a high temperature plasma separator. The ores are first melted to remove the lower melting point minerals. The denser remains then have to be turned into plasma at a huge energy cost. Electrostatic or electromagnetic fields then sort the elements by charge to mass ratio. Uranium has one of the lowest charge to mass ratios possible, allowing it to be conveniently scooped out. 

Mining asteroids is not technically a decades-out project; we already have the plasma mass spectrometers which use the same principles. It's the massive energy requirements which will take a long time to be available. 

[StrandedonEarth]

1 hour ago, MatterBeam said:

It's the massive energy requirements which will take a long time to be available. 

With practically unlimited solar power from the sun, massive amounts of electrical and thermal power may be online faster than you think. Of course, the hurdle there is bootstrapping the means to create solar collectors and PV arrays in situ...

[wumpus]

15 hours ago, MatterBeam said:

Mining asteroids is not technically a decades-out project; we already have the plasma mass spectrometers which use the same principles. It's the massive energy requirements which will take a long time to be available. 

It would still take a long time to create the spacecraft that can latch on to an asteroid and either bring the whole thing back or smelt it onsite.  Also expect a year or more to get there and several years back.  It  would still have everything smelted and in ingots long before an enrichment reactor was in space and fueled for operation.

[ChainiaC]

Isn't the asteroid 16 Psyche supposed to be a fragment of a shattered body's inner core? Maybe that would be the best place to look for Uranium and Thorium?

[Shpaget]

The problem is that a shattered inner core is likely a solid lump of steel. How do you mine it?

[MatterBeam]

1 hour ago, ChainiaC said:

Isn't the asteroid 16 Psyche supposed to be a fragment of a shattered body's inner core? Maybe that would be the best place to look for Uranium and Thorium?

That would be an excellent target in theory, yes, but in practice a regular asteroid covered in water ices would be more useful than a dry rock.

Just now, Shpaget said:

The problem is that a shattered inner core is likely a solid lump of steel. How do you mine it?

Melt out the volatiles, centrifuge the slag and vaporize the densest layer to extract the full spectrum of elements. 

[DerekL1963]

54 minutes ago, MatterBeam said:

Melt out the volatiles, centrifuge the slag and vaporize the densest layer to extract the full spectrum of elements. 

That's "4) Profit!" stage.  The problem is stages 1-3.   Hard rock mining is difficult enough here on Earth with gravity and confinement...

[MatterBeam]

12 minutes ago, DerekL1963 said:

That's "4) Profit!" stage.  The problem is stages 1-3.   Hard rock mining is difficult enough here on Earth with gravity and confinement...

I was explaining step 2 in the manufacturing process:

-1) Obtain resources

-2) Convert them into something useful.

-3) Sell your product.

-4) Profit!

Each step implies a vast industry sitting behind the scenes to provide or demand your products (ores/uranium). I only explained the technically straightforward step.

[DerekL1963]

1 hour ago, MatterBeam said:

I was explaining step 2 in the manufacturing process:

The question was about step 1 - the mining process.  That's just as big a question as your step 2, and in reality we don't know how to do either at a useful scale.

[magnemoe]

2 hours ago, MatterBeam said:

That would be an excellent target in theory, yes, but in practice a regular asteroid covered in water ices would be more useful than a dry rock.

Melt out the volatiles, centrifuge the slag and vaporize the densest layer to extract the full spectrum of elements. 

Making highly enchanted uranium is hard. 
Not something you will do in space then its replacing engines every decade. 

[KSK]

1 hour ago, magnemoe said:

Making highly enchanted uranium is hard. 
Not something you will do in space then its replacing engines every decade. 

Yeah - the plutonium pixies get grumpy if they're not fed and start casting counter-enchantments.  

Sorry - I'm guessing that was an unintended autocorrect but it just tickled me!