New Nuclear Thermal Rocket testing planned - decided not to use bomb grade U

[wumpus]

Has anyone looked into all the other unsolved issues with NTR?

Hydrogen is likely the best fuel to go from Earth to Mars.  Unfortunately, it is unlikely to get anyone from Mars to Earth (storing such is formidable, and probably not going to work for over a year).  I suspect that such a plan simply doesn't use the NTR for the way back, but there are [barely] other possibilities.

The reactor is likely good for only one firing.  Assuming you get them started, I am unsure how you start the things.  It certainly doesn't help that when the burn is finished, you stop cooling it (the hydrogen mass is the main cooling.  And the thing is still reacting.  You can eject the fuel if you want to use the reactor again(perhaps waiting *many* years if you are waiting for Earth to come back to its mars-transfer-orbit intersection) and attach some sort of means of getting the fuel to impact on Mars (or perhaps Phobos).

I'd suspect you just stage scram-based control rods through the reactor and stage it while it quickly destroys itself.  Again, send it straight to Mars, and try to come up with the best way to impact (I think Phobos is best, but probably too hard).  If grabbing a crashing reactor is of all interest to "bad guys", I imagine that grabbing a dropped reactor would be more interesting.  Also, this thing isn't in orbit, it is in some Earth-Mars transfer orbit, so other controls might get iffy.  Anyone pondering the other "steal a satellite" thread would love "stealing a spent stage".  Remember, assuming somebody sent (slightly larger than microsat) probe to Mars (read timed a complicated sequences of gravity move to be ejected toward Mars) at the same time, nobody afterwards is going to catch up.  You won't even notice until you try to plot a course into Mars and it fights back.*

You are still stuck with the issue of slowing down at Mars and returning, not to mention getting the reactor and all other gear in orbit.

* Not an exactly serious objection, more a placeholder to include for Bruce Schneier's next "Hollywood plot threat" contest.  It becomes yet another political issue following the NTR.  Also fun fodder for the less serious "steal a satellite" thread.

[DBowman]

@wumpus I believe the NTRs are stoppable and restartable:

• the fuel rods are spewing out neutrons all the time, but not enough to make a spontaneous chain reaction
• around the outside of the engine are some control rods that have neutron absorbing material on one side and neutron scattering material on the other
• if you turn the control rods absorber in then the reactor stays 'cool'
• if you turn the rods scatter side in then many neutrons end up back in the fuel rods and split U generating more neutrons - the 'chain reaction' and it gets 'hot'
• some designs had three materials; maybe low and high 'gear'
• apparently they are 'fiddly' to start/stop since propellant also has some effect on neutron flux ( I don't recall the details, maybe the H2 lets the neutrons 'stay fast' => more likely to split U so cooling the core also makes it more 'active'? )

Many studies plan to use cryo H2 and add 30% to tank mass to account for more insulation and 'chillers' to make it long term storable - but you can run them on many things; CO2, H2O, ...

[wumpus]

10 hours ago, DBowman said:

@wumpus I believe the NTRs are stoppable and restartable:

Many studies plan to use cryo H2 and add 30% to tank mass to account for more insulation and 'chillers' to make it long term storable - but you can run them on many things; CO2, H2O, ...

I'm guessing the start/stop issue is just how long it takes to stop and how long you have to push H2 through the reactor until it stops (I'm guessing this strategy would lead to rather long burns of low thrust).

H2O would necessarily have a lower ISP than LH2/LOX rockets (they are already at the highest temperature we can build a nozzle at, imagine trying that temperature in a reactor (I can't imagine CO2 being any better).  Also they run fuel rich (which might be possible if you have the remnants of a H2 tank).  I'd suspect that helium would be the second choice, and is being used long term on the James Webb telescope.  I strongly suspect that their experience with He cryogenics will greatly determine the technical viability of NTR (I think political viability has long since sailed).

Don't forget that H2 has nasty chemical properties that nobody working with He need consider.  You might find that parts of the telescopes cryogenic system require heavy modification with materials able to withstand H2 (i.e. probably not metalic).

[magnemoe]

11 hours ago, DBowman said:

@wumpus I believe the NTRs are stoppable and restartable:

• the fuel rods are spewing out neutrons all the time, but not enough to make a spontaneous chain reaction
• around the outside of the engine are some control rods that have neutron absorbing material on one side and neutron scattering material on the other
• if you turn the control rods absorber in then the reactor stays 'cool'
• if you turn the rods scatter side in then many neutrons end up back in the fuel rods and split U generating more neutrons - the 'chain reaction' and it gets 'hot'
• some designs had three materials; maybe low and high 'gear'
• apparently they are 'fiddly' to start/stop since propellant also has some effect on neutron flux ( I don't recall the details, maybe the H2 lets the neutrons 'stay fast' => more likely to split U so cooling the core also makes it more 'active'? )

Many studies plan to use cryo H2 and add 30% to tank mass to account for more insulation and 'chillers' to make it long term storable - but you can run them on many things; CO2, H2O, ...

Yes its obviously restartable, they tested some back in the 70s and could stop it. 
You must probably run it down over time, stop it and then reduce hydrogen flow as it cools, another option is to use an cooling loop on it at least for the last part. 
Think you are right about the control setup, you could tune it by how much the control rods was rotated. 

Water would be bad as it will boil generating lots of steam, as steam get superheated it split into hydrogen and oxygen who then can mix and explode. 
CO2 should work but methane would be best option after hydrogen. 
 

[TheSaint]

Starting and stopping the reactor is an easy and simple process, you just withdraw the control rods to start it and insert them to stop it. You would have to continue to run reaction mass/coolant through the reactor after it has been shut down to remove decay heat, but it should be pretty easy to arrange that. No destroying or discarding of reactors would be necessary. (Although early Mars mission designs did call for staging away of nuclear engines, it was simply for staging/weight reduction, not due to any limitation of the engines themselves. I don't think that any modern mission designs do, however.)

13 hours ago, DBowman said:

• the fuel rods are spewing out neutrons all the time, but not enough to make a spontaneous chain reaction

Actually, the fuel assemblies do emit enough neutrons to support a chain reaction. That's why the rods have to stay on the bottom to keep the core shut down.

13 hours ago, DBowman said:

• around the outside of the engine are some control rods that have neutron absorbing material on one side and neutron scattering material on the other
• if you turn the control rods absorber in then the reactor stays 'cool'
• if you turn the rods scatter side in then many neutrons end up back in the fuel rods and split U generating more neutrons - the 'chain reaction' and it gets 'hot'
• some designs had three materials; maybe low and high 'gear'

Hmmm. Do you have diagrams of this design? I'd be curious to see it. 

13 hours ago, DBowman said:

• apparently they are 'fiddly' to start/stop since propellant also has some effect on neutron flux ( I don't recall the details, maybe the H2 lets the neutrons 'stay fast' => more likely to split U so cooling the core also makes it more 'active'? )

Common misconception: You don't want your neutrons to be fast (i.e. high-energy) to support fission. People have this image in their heads that the neutrons are slamming into the uranium atoms at high speed and smashing them apart like billiards balls. This is not the case at all. What's actually going on is that the U235 nucleus absorbs a neutron and then (usually) decays by fission, releasing two semi-random fission product atoms, an average of 3.85 fast neutrons, and a whole lot of energy in the form of gamma radiation. While it is possible for a fast neutron to be absorbed by U235 (which is what goes on in a fast reactor or a nuclear bomb) it is much more likely for a neutron to be absorbed if it is a slow (low-energy, or thermal) neutron. So if you have more thermal neutrons you can sustain a chain reaction with a much lower neutron flux. This is why most reactors are filled with moderator materials, such as water, carbon or (in the case of NTRs) hydrogen, which are designed to slow neutrons down to increase the chances that they will be absorbed by U235 and propagate the chain reaction.

13 hours ago, DBowman said:

Many studies plan to use cryo H2 and add 30% to tank mass to account for more insulation and 'chillers' to make it long term storable - but you can run them on many things; CO2, H2O, ...

The main problem with running a fuel other than LH2 is that you eliminate the primary advantage of using an NTR in the first place: high Isp. The high Isp of an LH2 NTR is primarily derived from the fact that its reaction mass has such a low molecular weight. If you go with water or CO2, your Isp suffers immensely. I also think it would be difficult if not impossible to design an NTR that could switch coolants. You would have to design the reactor to start with one moderator in its early core life, then switch to another moderator in its later core life. Again, I guess anything is possible, that just seems like really extreme engineering to me.

2 hours ago, magnemoe said:

Yes its obviously restartable, they tested some back in the 70s and could stop it. 
You must probably run it down over time, stop it and then reduce hydrogen flow as it cools, another option is to use an cooling loop on it at least for the last part. 
Think you are right about the control setup, you could tune it by how much the control rods was rotated.

Yup. A cooling loop would add a lot of mass. You could just shut the reactor down near the end of your burn and run some reaction mass through to remove most of the decay heat, then bleed some through aux nozzles or something over the next couple of days to remove the rest.

2 hours ago, magnemoe said:

Water would be bad as it will boil generating lots of steam, as steam get superheated it split into hydrogen and oxygen who then can mix and explode. 
CO2 should work but methane would be best option after hydrogen.

What actually splits the water into hydrogen and oxygen is the radiation, radiolysis. Hydrogen buildup is a known issue in PWRs. However, given the pressure and temperature regimes of an NTR, a little oxygen/hydrogen combustion would be the least of your worries, and could easily be calculated and accounted for.

[wumpus]

2 hours ago, TheSaint said:

Starting and stopping the reactor is an easy and simple process, you just withdraw the control rods to start it and insert them to stop it. You would have to continue to run reaction mass/coolant through the reactor after it has been shut down to remove decay heat, but it should be pretty easy to arrange that. No destroying or discarding of reactors would be necessary. (Although early Mars mission designs did call for staging away of nuclear engines, it was simply for staging/weight reduction, not due to any limitation of the engines themselves. I don't think that any modern mission designs do, however.)

If the "decay heat" needs cooling, that will be a problem.  The ISP advantage assumes that the H2 is used a reaction mass, not cooling (with and leaving with a low Ve).

2 hours ago, TheSaint said:

Yup. A cooling loop would add a lot of mass. You could just shut the reactor down near the end of your burn and run some reaction mass through to remove most of the decay heat, then bleed some through aux nozzles or something over the next couple of days to remove the rest.

And watch your ISP advantage disappear.

2 hours ago, TheSaint said:

What actually splits the water into hydrogen and oxygen is the radiation, radiolysis. Hydrogen buildup is a known issue in PWRs. However, given the pressure and temperature regimes of an NTR, a little oxygen/hydrogen combustion would be the least of your worries, and could easily be calculated and accounted for.

That and since the thing was designed to run on hydrogen in the first place, hydrogen buildup should be a non-issue (of course you also have to deal with the oxidizing power of free oxygen bouncing around).

I imagine that the fact that early Mars plans assumed ditching the NTRs after a single burn had more to do with actual NTR research at the time and the issues in dealing with all the cooling issues in vacuum.  Single use NTRs are simple and efficient, at the cost of being expensive political nightmares.  Multiple use ones assume vast technical problems (which almost certainly will inflate costs) with hopes of placating politics and prolonging programs thanks to the sunk costs (not getting your budget cut is key for NASA missions to make it to the launch pad).

https://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf - last I heard the nuclear option had been dropped for this.  Not sure if I heard wrong, that this was a different group, or that such things change back and forth.  I can't imagine NASA being willing to bet their chances on going to Mars that Congress will let them launch a nuclear reactor with bomb-grade fuel.  It might make great papers and good science, and it might even be technically solvable (for ISPs significantly greater than kerolox and methalox), but I don't expect it to fly.

I still think that using ions to position cargo and fuel is the way for manned missions to Mars, and that ions are further along (never mind NASA's "official" readiness values, ions have moved at least 2 probes).  

 

[note]: much of my complaints assumed a base ISP of 800.  This (also used by KSP) seems to have come from the 1960s-1970s NTR research (and I'm assuming was single use).  If modern NTRs have a significantly higher ISP [as some forumites have suggested], using fuel for additional cooling might not kill all of your efficiency (especially if you could somehow get ISPs between >>800 and 400 (and lower, but not too much) while cooling (basically shut down the nuclear reaction well before the "burn" stopped).

Edited December 13, 2016 by wumpus
added note.

[TheSaint]

43 minutes ago, wumpus said:

If the "decay heat" needs cooling, that will be a problem.  The ISP advantage assumes that the H2 is used a reaction mass, not cooling (with and leaving with a low Ve).

And watch your ISP advantage disappear.

Decay heat runs about 6.5% of full reactor power immediately after shut down if you look at it after an instantaneous scram from 100% reactor power to 0%. That's the worst case scenario for decay heat. After an hour it will have dropped to about 1.5%, after a day it's sitting at about 0.4%. But there are plenty of things you can do operationally to minimize decay heat on shutdown, like throttling your burn down over time, or reducing the maximum reactor power of your burns as needed for shorter burns. You're talking minuscule amounts of hydrogen to cool the reactor after the burn, and it would be fairly easy to design the reactor casing to remove the steady state decay heat. It's not as big an issue as you think it is.

43 minutes ago, wumpus said:

That and since the thing was designed to run on hydrogen in the first place, hydrogen buildup should be a non-issue (of course you also have to deal with the oxidizing power of free oxygen bouncing around).

Well, the discussion was concerning a reactor that would use water as reaction mass. That would, by definition, be a different design than a reactor that would use hydrogen as reaction mass. As for oxidation, there are plenty of superalloys that are specifically designed to deal with that issue: Hastelloy, Monel, Inconel, take your pick.

43 minutes ago, wumpus said:

I imagine that the fact that early Mars plans assumed ditching the NTRs after a single burn had more to do with actual NTR research at the time and the issues in dealing with all the cooling issues in vacuum.  Single use NTRs are simple and efficient, at the cost of being expensive political nightmares.  Multiple use ones assume vast technical problems (which almost certainly will inflate costs) with hopes of placating politics and prolonging programs thanks to the sunk costs (not getting your budget cut is key for NASA missions to make it to the launch pad).

I really don't know where you're getting this from. Many of the test NTRs that the U.S. produced in the 1960s were fired multiple times. The NRX was fired over 20 times, IIRC. There was nothing insurmountable about making a multiple-use NTR. The decision to stage NTRs in the early Mars design missions was a decision based on the rocket equation and their more cavalier attitude towards nuclear power, nothing more.

43 minutes ago, wumpus said:

https://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf - last I heard the nuclear option had been dropped for this.  Not sure if I heard wrong, that this was a different group, or that such things change back and forth.  I can't imagine NASA being willing to bet their chances on going to Mars that Congress will let them launch a nuclear reactor with bomb-grade fuel.  It might make great papers and good science, and it might even be technically solvable (for ISPs significantly greater than kerolox and methalox), but I don't expect it to fly.

I still think that using ions to position cargo and fuel is the way for manned missions to Mars, and that ions are further along (never mind NASA's "official" readiness values, ions have moved at least 2 probes). 

[note]: much of my complaints assumed a base ISP of 800.  This (also used by KSP) seems to have come from the 1960s-1970s NTR research (and I'm assuming was single use).  If modern NTRs have a significantly higher ISP [as some forumites have suggested], using fuel for additional cooling might not kill all of your efficiency (especially if you could somehow get ISPs between >>800 and 400 (and lower, but not too much) while cooling (basically shut down the nuclear reaction well before the "burn" stopped).

Project Timberwind, back in the 1990s, was projected to have vacuum Isps up to 1000s. As far as I know, that's as good as they've gotten.

An ion-powered mission would be a nightmare. An ion thruster is great for nudging around a 200-lb probe. But now you want to perform an injection burn for a Mars mission with it? Your thrust-to-weight ratio would be abysmal. Any weight you save in reaction mass you would more than lose again in life support, since your mission time is now forever. Your astronaut exposures are now much longer. And at the end of the day, how are you going to power it? A solar array the size of Manhattan? Or a nuclear reactor, which puts you right back in the same political boat you were in with an NTR solution?

However, I don't disagree that a nuclear-powered Mars mission may never fly because of politics. Nuclear power has been so thoroughly vilified in the minds of the public that it's hard to do anything with it anymore.

[wumpus]

2 hours ago, TheSaint said:

An ion-powered mission would be a nightmare. An ion thruster is great for nudging around a 200-lb probe. But now you want to perform an injection burn for a Mars mission with it? Your thrust-to-weight ratio would be abysmal. Any weight you save in reaction mass you would more than lose again in life support, since your mission time is now forever. Your astronaut exposures are now much longer. And at the end of the day, how are you going to power it? A solar array the size of Manhattan? Or a nuclear reactor, which puts you right back in the same political boat you were in with an NTR solution?

However, I don't disagree that a nuclear-powered Mars mission may never fly because of politics. Nuclear power has been so thoroughly vilified in the minds of the public that it's hard to do anything with it anymore.

You don't use ions to send astronauts.  You send the fuel (which in all likelyhood requires docking with a fully fueled rocket stage, we aren't up to KSP fueling standards).  The astronauts dock with the fuel and fire away.  The biggest problem would be that the "depot locations" would have to pass through the Van Allan belts (in highly eccentric orbits with the Pe ~100km).  One would likely sandwich between the belts, and certainly the last could be well outside of them (i.e. somewhere between the Moon and GTO).

The "tryanny of the rocket equation" is due to the "fact" that you have to burn so much fuel to move fuel.  So don't burn fuel to move fuel, use ions.

The biggest problem is that you would likely have to loop the fuel around the moon (because ions don't *do* highly eccentric orbits.  They do nice circular (spiral) orbits).  So getting the fuel in position will be tricky, but doable.  And any cargo needs to go with ions (and gravity tricks) as well.  You only use kerolox/methalox/hyrdolox for manned missions with life support issues.  Anything else (including that kerlox/methalox/hydrolox for manned vessels) can take the slowboat.

In practice, I suspect this wouldn't be done at all.  The ions would simply move the cargo (Mars surface habitat*, lander, fuel for the return trip) to Mars and also move the transit vessel (something astronauts have to live for months in) in said highly eccentric orbit around the Earth.  You then get to Mars transit vehicle via Orion, Dragon (presumably upgraded, or Soyuz (likely requires a few modifications as well).  Docking with fuel between the Van Allen belts is optional and likely discarded as too complicated.  But you still use ions to move all your cargo and 3/5 of your Mars transit vehicle's delta-v (everything but going from nearly luna intercept (i.e. just out of the Van Allen belts) to Mars Transfer and Mars capture).  But you wind up moving something like 2/3 of you transit delta-v (and that's delta-v, so wildly less fuel) for your transit vehicle and never touch it for your cargo.  Find a way to make big enough ions and you will never want NTR again (until you find a place where this doesn't work: perhaps because escape isn't the biggest part of your delta-v requirement).

* yes, this would likely have to loop around the Moon thanks to the goofy trajectories of ion engines.  But in practice this would be a non-issue because we want it out in a lunar-transit orbit anyway (with Pe being easy to move within LEO for at least the last burn to Mars, gotta get that Oberth.  It would probably keep the Pe much higher until actual docking to keep from losing velocity thanks to extreme velocity through not-so-hard vacuum).  We might let it get down closer to the Van Allen belts (if only for a quicker trip in a tiny capsule to the big-multi-month-transit-vehicle), but that really isn't necessary and won't significant;y change the delta-vs.

[just googled project timberland]: Dear Santa Kerman, I have been a good kerbal and kept my trolling to a minimum.  Please sent a NTR rocket capable of lifting off Earth and taking me to LEO and ISS.  It doesn't even have to land a first stage spacex style, although that would likely make it perfect.

Edited December 14, 2016 by wumpus
included note to Santa.

[TheSaint]

12 minutes ago, wumpus said:

You don't use ions to send astronauts.  You send the fuel (which in all likelyhood requires docking with a fully fueled rocket stage, we aren't up to KSP fueling standards).  The astronauts dock with the fuel and fire away.  The biggest problem would be that the "depot locations" would have to pass through the Van Allan belts (in highly eccentric orbits with the Pe ~100km).  One would likely sandwich between the belts, and certainly the last could be well outside of them (i.e. somewhere between the Moon and GTO).

The "tryanny of the rocket equation" is due to the "fact" that you have to burn so much fuel to move fuel.  So don't burn fuel to move fuel, use ions.

The biggest problem is that you would likely have to loop the fuel around the moon (because ions don't *do* highly eccentric orbits.  They do nice circular (spiral) orbits).  So getting the fuel in position will be tricky, but doable.  And any cargo needs to go with ions (and gravity tricks) as well.  You only use kerolox/methalox/hyrdolox for manned missions with life support issues.  Anything else (including that kerlox/methalox/hydrolox for manned vessels) can take the slowboat.

In practice, I suspect this wouldn't be done at all.  The ions would simply move the cargo (Mars surface habitat*, lander, fuel for the return trip) to Mars and also move the transit vessel (something astronauts have to live for months in) in said highly eccentric orbit around the Earth.  You then get to Mars transit vehicle via Orion, Dragon (presumably upgraded, or Soyuz (likely requires a few modifications as well).  Docking with fuel between the Van Allen belts is optional and likely discarded as too complicated.  But you still use ions to move all your cargo and 3/5 of your Mars transit vehicle's delta-v (everything but going from nearly luna intercept (i.e. just out of the Van Allen belts) to Mars Transfer and Mars capture).  But you wind up moving something like 2/3 of you transit delta-v (and that's delta-v, so wildly less fuel) for your transit vehicle and never touch it for your cargo.  Find a way to make big enough ions and you will never want NTR again (until you find a place where this doesn't work: perhaps because escape isn't the biggest part of your delta-v requirement).

* yes, this would likely have to loop around the Moon thanks to the goofy trajectories of ion engines.  But in practice this would be a non-issue because we want it out in a lunar-transit orbit anyway (with Pe being easy to move within LEO for at least the last burn to Mars, gotta get that Oberth.  It would probably keep the Pe much higher until actual docking to keep from losing velocity thanks to extreme velocity through not-so-hard vacuum).  We might let it get down closer to the Van Allen belts (if only for a quicker trip in a tiny capsule to the big-multi-month-transit-vehicle), but that really isn't necessary and won't significant;y change the delta-vs.

Aaahhh, I see that now in your original post. Sorry, trying to write responses while keeping all the plates spinning at work. Interesting idea. Would you send your return fuel ahead to Mars and dock with it there? That would seem like a massive risk. 

In any case, your pipe dream is as good as mine, at this point.

12 minutes ago, wumpus said:

[just googled project timberland]: Dear Santa Kerman, I have been a good kerbal and kept my trolling to a minimum.  Please sent a NTR rocket capable of lifting off Earth and taking me to LEO and ISS.  It doesn't even have to land a first stage spacex style, although that would likely make it perfect.

LOL

[Temeter]

7 hours ago, TheSaint said:

However, I don't disagree that a nuclear-powered Mars mission may never fly because of politics. Nuclear power has been so thoroughly vilified in the minds of the public that it's hard to do anything with it anymore.

Depends. Probably not in US or EU, but Russia and China don't seem to have qualm to this degree, and are certainly less bound by public opinion. Although I can imagine the US taking a bit of a swing, it's not like NTR's on the mars rover caused problems.

Both countries eastern countries aren't able to do a mars mission in the near future, but I can see NASA taking a loooong time. Ignoring SpaceX, which are crazy*.

 

*in the good way tho

 

Edited December 14, 2016 by Temeter

[magnemoe]

1 hour ago, Temeter said:

Depends. Probably not in US or EU, but Russia and China don't seem to have qualm to this degree, and are certainly less bound by public opinion. Although I can imagine the US taking a bit of a swing, it's not like NTR's on the mars rover caused problems.

Both countries eastern countries aren't able to do a mars mission in the near future, but I can see NASA taking a loooong time. Ignoring SpaceX, which are crazy*.

 

*in the good way tho

This, main issue with an NTR is that its an expensive engine to get space qualified and its an lack of missions. 
Like in KSP its pointless for an small probe, you need an manned mission or huge robotic one for it to make sense. 
That is the manned mars mission, manned asteroid and  probably manned moon mission at least with base. 
 

[Temeter]

4 hours ago, magnemoe said:

This, main issue with an NTR is that its an expensive engine to get space qualified and its an lack of missions. 
Like in KSP its pointless for an small probe, you need an manned mission or huge robotic one for it to make sense. 
That is the manned mars mission, manned asteroid and  probably manned moon mission at least with base.

Yep. But the efficiency in manned missions beyond LEO (especially past EO) is an enourmous advantage. Flying a manned missions to the mars with conventional chemical engines is an insane task.

One thing that Realism Overhaul shows really well (with life support mods!).

Edited December 14, 2016 by Temeter

[magnemoe]

4 hours ago, Temeter said:

Yep. But the efficiency in manned missions beyond LEO (especially past EO) is an enourmous advantage. Flying a manned missions to the mars with conventional chemical engines is an insane task.

One thing that Realism Overhaul shows really well (with life support mods!).

Agree, Musk mars system does not but it require multiple refuelings by an giant rocket and ISRU at mars. 

[Temeter]

3 minutes ago, magnemoe said:

Agree, Musk mars system does not but it require multiple refuelings by an giant rocket and ISRU at mars. 

Yup. Getting to Mars is one thing, flying back the other. The ITS lander can't even get back to orbit w/o refueling.

 

And that thing is still magnitudes bigger than the biggest ever build rocket and requires a boatload of technologies and engineering that has never been done before.

Edited December 14, 2016 by Temeter

[wumpus]

9 hours ago, Temeter said:

Yep. But the efficiency in manned missions beyond LEO (especially past EO) is an enourmous advantage. Flying a manned missions to the mars with conventional chemical engines is an insane task.

One thing that Realism Overhaul shows really well (with life support mods!).

Oddly enough, thanks to internal NASA requirements, NTRs have a step up on other non-chemical systems and have their "readiness levels" achieved thanks to 1960s requirements.  I can't see them taking advantage of that fact, but it does exist.

4 hours ago, Temeter said:

Yup. Getting to Mars is one thing, flying back the other. The ITS lander can't even get back to orbit w/o refueling.

And that thing is still magnitudes bigger than the biggest ever build rocket and requires a boatload of technologies and engineering that has never been done before.

This is why I keep pushing "send the fuel and cargo via ions [and gravity assist]" (and don't leave till it is already there).

15 hours ago, TheSaint said:

Aaahhh, I see that now in your original post. Sorry, trying to write responses while keeping all the plates spinning at work. Interesting idea. Would you send your return fuel ahead to Mars and dock with it there? That would seem like a massive risk. 

In any case, your pipe dream is as good as mine, at this point.

LOL

Only for values of missions that don't involve ISRU.  In such a system you wouldn't leave for Mars until all the fuel was where it was supposed to be and all systems checked out.  I'd hope that any other systems that depended on ISRU were already on Mars and producing fuel at the expected rates (ideally fully ready to refill any rocket before the final go/no-go descent to Mars).

You really aren't going to Mars without at least one "crazy" tech.  Nukes/ions/ISRU/steampunk-sized-rockets (and I think even with chemicals+ISRU would qualify as "steampunk-sized" in my book).

[Temeter]

Just now, wumpus said:

Oddly enough, thanks to internal NASA requirements, NTRs have a step up on other non-chemical systems and have their "readiness levels" achieved thanks to 1960s requirements.  I can't see them taking advantage of that fact, but it does exist.

Saint also posted a nice link to a larger study from the 90's, project timberwind: https://en.wikipedia.org/wiki/Project_Timberwind

Smallest timber engine is a 1.5 ton NTR producing 441.3kn thrust at 1000 ISP. That's insanely effective.

I imgine the biggest challence would be the multi-year zero-boiloff cryogenic tank needed for a mars mission, especially for the return. That's another thing we've never done before.

[TheSaint]

6 minutes ago, wumpus said:

Oddly enough, thanks to internal NASA requirements, NTRs have a step up on other non-chemical systems and have their "readiness levels" achieved thanks to 1960s requirements.  I can't see them taking advantage of that fact, but it does exist.

This is why I keep pushing "send the fuel and cargo via ions [and gravity assist]" (and don't leave till it is already there).

Only for values of missions that don't involve ISRU.  In such a system you wouldn't leave for Mars until all the fuel was where it was supposed to be and all systems checked out.  I'd hope that any other systems that depended on ISRU were already on Mars and producing fuel at the expected rates (ideally fully ready to refill any rocket before the final go/no-go descent to Mars).

You really aren't going to Mars without at least one "crazy" tech.  Nukes/ions/ISRU/steampunk-sized-rockets (and I think even with chemicals+ISRU would qualify as "steampunk-sized" in my book).

Well, ISRU is okay for the Mars-surface-to-Mars-orbit leg. Because then if something happens to that vehicle while your crew is en-route, they just get to Mars, are disappointed because they don't get to land, and then come back. But if you're going to use ISRU or pre-staging for the fuel for the Mars-orbit-to-Earth-orbit leg, then the risk goes up substantially. If something happens to that fuel while the crew is en-route, they have no way to get home.

[Temeter]

4 minutes ago, TheSaint said:

Well, ISRU is okay for the Mars-surface-to-Mars-orbit leg. Because then if something happens to that vehicle while your crew is en-route, they just get to Mars, are disappointed because they don't get to land, and then come back. But if you're going to use ISRU or pre-staging for the fuel for the Mars-orbit-to-Earth-orbit leg, then the risk goes up substantially. If something happens to that fuel while the crew is en-route, they have no way to get home.

And that's assuming you can even generate enough fuel. What if something happens, like a sandstorm or other event that damages your fuel production?

All about the SpaceX plan seems insanely risky. I expect it to evolve for a while, tho. Can't imagine how all of that is really realistic (which is at least not uncommon for pioneering projects).

Edited December 14, 2016 by Temeter

[TheSaint]

8 minutes ago, Temeter said:

And that's assuming you can even generate enough fuel. What if something happens, like a sandstorm or other event that damages your fuel production?

All about the SpaceX plan seems insanely risky. I expect it to evolve for a while, tho. Can't imagine how all of that is really realistic (which is at least not uncommon for pioneering projects).

Honestly, I haven't even looked at the SpaceX plans. I just saw the publicity shots of his spacecraft, thought, "That's completely nutters. It's never going to happen," and moved on.

[Temeter]

5 minutes ago, TheSaint said:

Honestly, I haven't even looked at the SpaceX plans. I just saw the publicity shots of his spacecraft, thought, "That's completely nutters. It's never going to happen," and moved

I thought the spacecraft was still the most realistic part of the plan, assuming they'll be able to fund it.^^

[wumpus]

7 minutes ago, TheSaint said:

Honestly, I haven't even looked at the SpaceX plans. I just saw the publicity shots of his spacecraft, thought, "That's completely nutters. It's never going to happen," and moved on.

I loved the bit about "amortization over 1000 flights (presumably a stage 1 launcher that would replace the falcon 9)".  It might be the start of a plan, but it isn't quite workable yet.

[TheSaint]

12 minutes ago, Temeter said:

I thought the spacecraft was still the most realistic part of the plan, assuming they'll be able to fund it.^^

He wants to fly a spacecraft that carries people to Mars six years from now. That's less time than it takes for Toyota to design a new truck, and even they don't get everything right in the first model year. I just remember looking at it in the renderings and thinking that the proportions were all wrong, like it was one of these ships out of Star Wars that flies halfway across the galaxy on a 10-gallon tank. Now I know why. Because he's planning on refueling it in orbit, and then refueling it again in Mars orbit. We've never, in over fifty years of spaceflight, transferred cryogenic fuels from one vehicle to another in orbit. And Musk thinks he's going to somehow have this so down pat that he can risk the lives of a dozen people on it. In six years. When he doesn't even have a human-rated Dragon capsule yet. I'm normally a pretty optimistic guy when it comes to science and spaceflight. But not that optimistic.

26 minutes ago, wumpus said:

I loved the bit about "amortization over 1000 flights (presumably a stage 1 launcher that would replace the falcon 9)".  It might be the start of a plan, but it isn't quite workable yet.

Maybe his accountants used to work for Enron?

Don't get me wrong, I'd love to see all this work out for him. But I'm not holding my breath.

[magnemoe]

10 hours ago, Temeter said:

Yup. Getting to Mars is one thing, flying back the other. The ITS lander can't even get back to orbit w/o refueling.

And that thing is still magnitudes bigger than the biggest ever build rocket and requires a boatload of technologies and engineering that has never been done before.

This, did not say it was realistic just that it was one of the few serious plans with chemical engines. you could not do it without an cheap large scale launches. 

Still think the train has left for the NTR unless it got lots of focus soon. Vasmir or even pulsed fusion powered by an nuclear reactor sounds more likely. Higher isp, easier to ground test both system and you would need an small space rated reactor at target anyway. 

[DBowman]

On 12/14/2016 at 4:37 AM, TheSaint said:

The main problem with running a fuel other than LH2 is that you eliminate the primary advantage of using an NTR in the first place: high Isp. The high Isp of an LH2 NTR is primarily derived from the fact that its reaction mass has such a low molecular weight. If you go with water or CO2, your Isp suffers immensely.

For sure, LH2 is the ideal fuel (or dissociated atomic H if they figure out how to make it happen; isp 1600). I'm just pointing out that there is some flexibility so you can take advantage of different ISRU opportunities. H2O gives you near cryonic isp, but storable. project rho

On 12/14/2016 at 4:37 AM, TheSaint said:

Hmmm. Do you have diagrams of this design? I'd be curious to see it. 

project rho again...

On 12/14/2016 at 4:37 AM, TheSaint said:

Common misconception: You don't want your neutrons to be fast (i.e. high-energy) to support fission.

ah thanks - that makes more sense - putting the LH2 through the engine slows em and makes more thermal neutrons and amps up the reaction?

Re Chemical vs Nukes - I think the last NASA Design Reference Architecture for Mars (DRA5) had both a chem and a nuke option speced out. I get the impression they's love to go nuke but don't think it will get support.

5 hours ago, wumpus said:

This is why I keep pushing "send the fuel and cargo via ions [and gravity assist]"

I also think this makes a lot of sense.

[kunok]

Well, if children of death earth is a valid reference here (is not in mechanical engineering, that's for sure), hydrogen deuteride or pure deuterium is better than regular LH, more dense but almost as efficient because, the lower specific heat?. In the game using the exactly same engine only changing the propellant you change from an exhaust velocity of 9km/s to 8,63km/s to 8,59km/s (for liquid hydrogen, liquid deuterium and liquid hydrogen deuteride), with a huge improvement in the density of the propellant (77 vs 160 vs 120 kg/m^3) and getting a little bigger boiling point (20k vs 24k vs  22k). (you still need to tweak other values for them to work, but the spacecrafts are pretty better this way than with regular hydrogen)

Probably it isn't really a good reference. Can someone point me why?