DBowman

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

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49 minutes ago, DBowman said:

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

project rho again...

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.

I also think this makes a lot of sense.

Yes, add that frozen H2O is far more common than dirt at Jupiter and outward, here NTR and very simple ISRU owns everything else. 

And yes you want slow neutrons, going slow give more chance of hitting something, Hobble the chicken before it crosses the road and its more likely it impact an car. 
Fusion is different here you want the chickens to move so fast they destroy the cars, its also much harder :)

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12 hours ago, TheSaint said:

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.

Oh, I wouldn't take the timeline serious anyway, they are quite optimistic (might even be internal stuff, less PR?). Falcon Heavy and Dragon 2 are years late as well. Six years sound reaaaaaally absurd.

Transfering cryogenic fuel is a quite researched technology, tho. I imagine they have access to a lot of Nasa's research. Of course, that's not the only new thing: Near zero boiloff tanks (methalox is esasier than hydrolox tho), methalox RCS, near infinitely restartable cryogenic engines for multi year missions, operating at chamber pressures never seen before, reentering a ship of this size on earth and mars, not to mention starting, landing and seamlessly reusing the biggest launcher ever built... Did I mention a giant manned spacecraft that might be bigger than the ISS? And then grow your fuel for the return trip.

Lots of crazy stuff.

Quote

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

Haha, I think that's how we all see it.

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17 hours ago, kunok said:

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?

While children of a dead earth might be impressive, I doubt the rest is any better than your mechanical engineering.  The reason H2 is chosen is low mass, and deuterium would interfere with that.  It is possible that having deuterium in the reactor would assist things (presumably helping neutrons bounce around), but the whole point of H2 is maximum lightness for maximum exhaust velocity.  I can't expect any nuclear benefits would offset this (and that's not even questioning the cost of [hundreds?] of *tons* of deuterium).

It was almost certainly added to look/sound cool.  And imply some sort of fusion tech.  And it might actually help with a relatively small percentage of heavier hydrogen could get the effects, but the point of using hydrogen means that the bulk of the fuel needs to leave the nozzle without neutrons attached.  Isp is directly related to exhaust velocity which effectively depends on mass and temperature (plus a big old nasty equation with plenty of variables, but mass and temperature limit everything).  Heavy hydrogen needs more temperature for higher exhaust velocity and ISP, while the temperature is limited by materials science and keeping everything from melting - it will be the same for all fuels.  So you generally don't want neutrons in your hydrogen going out the nozzle regardless of how much a neutron-rich cooling gas helps your reactor [unless it *really* solves the start/stop issues.  Even then you would probably limit the "fancy fuel" to starts and/or stops.]

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19 hours ago, DBowman said:

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

Water gives you about half the Isp of hydrogen. (Specific impulse is proportional to exhaust velocity.)

19 hours ago, DBowman said:

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

Yes. If you read in the section of Project Rho you linked above, he gives a very good description of what happens when you dump cold liquid hydrogen into a hot critical reactor core. (Although in a modern NTR design I'm sure that the startup process would be computer-controlled.) 

19 hours ago, DBowman said:

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.

Yeah, the politics surrounding nuclear power is just ridiculous.

11 hours ago, Temeter said:

Transfering cryogenic fuel is a quite researched technology, tho.

"Researched" and "Accomplished" are two completely different things. We've done a lot of research about self-driving cars, but I'm still not quite ready to bet my life on taking a nap in one.

11 hours ago, Temeter said:

I imagine they have access to a lot of Nasa's research. Of course, that's not the only new thing: Near zero boiloff tanks (methalox is esasier than hydrolox tho), methalox RCS, near infinitely restartable cryogenic engines for multi year missions, operating at chamber pressures never seen before, reentering a ship of this size on earth and mars, not to mention starting, landing and seamlessly reusing the biggest launcher ever built... Did I mention a giant manned spacecraft that might be bigger than the ISS? And then grow your fuel for the return trip.

Yeah, I think that Elon might want to give up his medical marijuana card. He may have been smoking a little too much dope. :wink:

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@TheSaint I see that quoted allot, but with an NTR you can ramp up the temperature significantly and get some pretty good ISP even out of water right? Sure, you would get more from H2 at the same temperature, but you also get the bonus of not needing to warm up from cryo temperatures. Use the water as a heat sink for your ship components, keep it at a toasty 50 C, and then it is already over 250 K warmer than your starting H2. The exhaust won't be 250 K higher, but it can probably be heated a bit higher at the same flow rate. Besides, the ease of storing water just makes it a great fuel for an NTR over most things that require fancy insolation. And if you need some real umph, use the reactor to generate electricity to split the water into hydrolox, and you can get a huge thrust out of it too. Basically use water as a long term hydrolox storage tank.

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6 hours ago, wumpus said:

While children of a dead earth might be impressive, I doubt the rest is any better than your mechanical engineering.  The reason H2 is chosen is low mass, and deuterium would interfere with that.  It is possible that having deuterium in the reactor would assist things (presumably helping neutrons bounce around), but the whole point of H2 is maximum lightness for maximum exhaust velocity.  I can't expect any nuclear benefits would offset this (and that's not even questioning the cost of [hundreds?] of *tons* of deuterium).

I'm almost sure that the effect is because the lower specific heat of the deuterium. Specific heat of deuterium in the game is 5,2 KJ/(kg*K) and for hydrogen is 14,3. That's a simplification because is a constant value, and not a function of the temp like the real thing. But even if you "normalize" it by doubling the specific heat of deuterium because the double density is still a lot better. If only I had time for research...

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11 hours ago, TheSaint said:

Water gives you about half the Isp of hydrogen.

yep H2O NTR about 412 vs LH2 LOX cryonic abut 450

Edited by DBowman
oops 412

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8 hours ago, todofwar said:

but with an NTR you can ramp up the temperature significantly and get some pretty good ISP even out of water right?

ISP is equivalent to exhaust velocity. In a hot gas there is a distribution of velocity of the molecules, some are faster than others. More mass per molecule 'squashes' the distribution toward the slower end. there is a couple of graphs here So for a given temperature a 'lighter' propellant will have more molecules going fast. But it's not just proportional to mass or H20 NTR would be 1/9 of 900 not a little less than half of it.

7 hours ago, kunok said:

I'm almost sure that the effect is because the lower specific heat of the deuterium.

I guess since it's double density the 30% extra tank mass for keeping the H2 cryonic would be effectively reduced (halving the length of the cylindrical tank say). Maybe somewhat better since the gas would be 'slower' and not need to be so cold? (edit: 162 vs 71 kg/m^3 ) I don't have the math to to work out how much you lose from reduced exhaust velocity. The 'most probable' molecular speed would be 70% of plain old H2 but the distribution has a long tail at the fast end so it's 'less bad' than that I think.

Edited by DBowman

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my queston about a fission drive is:

why dont we use the waste products (like the Krypton) and use it as a working mass?

so , there on the on hand heavy fuel, on the other hand -> a lot of working mass for a ion drive...

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this says that HD disassociates at much lower temperatures than H2 resulting a propellant with lower average molecular mass than H2. I couldn't find anything to substantiate that though. I read this as saying the disassociation energy is about the same - but since it's heavier maybe the same energy is reached at a lower temp? Anyway another interesting fact if you disassociate low pressure H2 via electric discharge and it has trouble reassociating giving you long enough to do something with it maybe (like heat and squirt it out a nozzle).

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13 hours ago, TheSaint said:

Yeah, I think that Elon might want to give up his medical marijuana card. He may have been smoking a little too much dope. :wink:

To be fair, all of this is pioneering work. Just think of the Space Station Wernherr von Braun wanted to build for the moon mission:

Wernher-von-Braun-discusses-space-statio

Real question is, how much will remain of the orginal plan?

Edited by Temeter

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1 hour ago, Sereneti said:

my queston about a fission drive is:

why dont we use the waste products (like the Krypton) and use it as a working mass?

so , there on the on hand heavy fuel, on the other hand -> a lot of working mass for a ion drive...

Because the mass of Krypton generated is negligible. On the order of 1.5% of the fission products, and only a few percent of the uranium fuel actually undergoes fission. For 1kg of uranium fuel you'd generate maybe half a gram of Krypton if what you start with was highly enriched.

Even being generous, all the fission products are maybe 50g per kg U235. And they're heavy nuclei that don't accelerate well compared to H.

Finally a kg of 5% U235 contains enough energy to raise the temperature of more than 100 tonnes of H2 by over 2000K.

Fission products just aren't significant.

Edited by RCgothic

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8 hours ago, Temeter said:

To be fair, all of this is pioneering work. Just think of the Space Station Wernherr von Braun wanted to build for the moon mission:

Real question is, how much will remain of the orginal plan?

Wasn't Braun against the Moon mission (he wanted to go to Mars)?

Also with the efficiency of early rockets, you had to make lower stages *huge* if you wanted any upper stage at all (of course this remains true for large enough delta-v, no matter how efficient you are).  Also *odd* elevator spokes?  How are you supposed to deal with the counterweights?  I'm guessing the elevator is an afterthought and the spokes are the driving factor, but it looks pretty silly.  But it probably made sense if you were trying to get to Mars in the 1940s and not the 1960s.

I'm guessing the Raptor engines (if already designed) will go to Mars.  I doubt the rest of it will fit in any workable plan.  The problems will of course be mass, mass, mass, and sudden astronaut death/danger.

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48 minutes ago, wumpus said:

Wasn't Braun against the Moon mission (he wanted to go to Mars)?

Also with the efficiency of early rockets, you had to make lower stages *huge* if you wanted any upper stage at all (of course this remains true for large enough delta-v, no matter how efficient you are).  Also *odd* elevator spokes?  How are you supposed to deal with the counterweights?  I'm guessing the elevator is an afterthought and the spokes are the driving factor, but it looks pretty silly.  But it probably made sense if you were trying to get to Mars in the 1940s and not the 1960s.

I'm guessing the Raptor engines (if already designed) will go to Mars.  I doubt the rest of it will fit in any workable plan.  The problems will of course be mass, mass, mass, and sudden astronaut death/danger.

No clue about the space station. I imagine that was a veeeeeeeery early concept and von braun was indeed thinking a lot further. Braun definitly wanted to the moon, at least at some point. Spoke about it even before WW2 (his original plans might stem from that time).

Even during the apollo landings, people apparently still thought moonlanding would become something normal. And to be fair, we did actually get space stations, just different ones.

As for the engines, there are already miniatur versions of the raptor, the engine is apparently well into it's development cycle. Even if that's the only thing that survives the original plan, it's gonna be insane (complexity/price are of course important as well).

Edited by Temeter

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18 hours ago, todofwar said:

@TheSaint I see that quoted allot, but with an NTR you can ramp up the temperature significantly and get some pretty good ISP even out of water right? Sure, you would get more from H2 at the same temperature, but you also get the bonus of not needing to warm up from cryo temperatures. Use the water as a heat sink for your ship components, keep it at a toasty 50 C, and then it is already over 250 K warmer than your starting H2. The exhaust won't be 250 K higher, but it can probably be heated a bit higher at the same flow rate. Besides, the ease of storing water just makes it a great fuel for an NTR over most things that require fancy insolation. And if you need some real umph, use the reactor to generate electricity to split the water into hydrolox, and you can get a huge thrust out of it too. Basically use water as a long term hydrolox storage tank.

Well, the problem is that if you're doing it right you're already running your reactor close to a lot of other temperature limits, most notably your fuel element temperature limits. So just ramping up the temperature probably isn't an option.

And starting from a lower temperature is actually (at least from a thermodynamic standpoint) a good thing. The amount of heat you transfer out of your core is (if you hold all other parameters constant) proportional to the differential temperature between your core and your coolant. So if your coolant is warmer when it enters the core, it will wind up removing less heat, and your overall heat transfer will be less. (Or, to put it another way, you will have to increase one of your other parameters, such as the radiative surface area, or the mass flow rate, to remove the same amount of heat from the reactor. None of these are desirable outcomes.)

As for splitting into hydrolox, probably not a good option either. You can't store it in the same tank, so you're going to have to lug along extra tankage to store the hydrogen and oxygen after they have been cracked. The equipment needed to crack the water on an industrial scale isn't insignificant. That's all going to be extra mass. Cracking it on the fly probably isn't going to work, since your reactor is going to be a little busy at that point. I doubt it will have the extra output needed to be cracking a ton of water into hydrogen and oxygen every second. You could try packing a second reactor, but that gets back to extra mass issues again. All this to get to the only advantage that water has over hydrogen: it isn't cryogenic. I don't know, it seems like the hydrogen storage issue would be easier to solve.

12 hours ago, DBowman said:

yep H2O NTR about 412 vs LH2 LOX cryonic abut 450

But, on the other hand, this brings up an excellent point. When folks say, "Oh, but a water-fed NTR doesn't have an Isp any better than chemical rockets!" they're comparing it to chemical rockets that use cryogenic fuels, such as hydrolox or kerolox. These are going to have the same long-term propellant storage issues that an LH2 NTR is going to have. Once you start comparing the water-fed NTR to chemical engines that have a comparable shelf-life, it doesn't look half-bad.

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40 minutes ago, TheSaint said:

But, on the other hand, this brings up an excellent point. When folks say, "Oh, but a water-fed NTR doesn't have an Isp any better than chemical rockets!" they're comparing it to chemical rockets that use cryogenic fuels, such as hydrolox or kerolox. These are going to have the same long-term propellant storage issues that an LH2 NTR is going to have. Once you start comparing the water-fed NTR to chemical engines that have a comparable shelf-life, it doesn't look half-bad.

As a plus, water is quite dense at 1000 kg/m3. Hydrocarbon-based fuels generally have triple-digit densities, LH2 only has 70.

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@TheSaint Still, having a fuel that requires 0 insulation has quite a few advantageous over something like hydrogen especially. Hydrogen embrittles most materials, it almost always finds a way to leak because it's so small, and you need to keep it cryogenic the entire time (which is not that easy even in space). And, it's not terribly difficult to ionize water, which opens up ion thrust systems using the reactor for electricity only. Granted, that is a different system overall from an NTR, but I still think water doesn't get enough credit for being such an easy fuel to keep around. Ammonia might be a good compromise though, doesn't need to be as cold as hydrogen and easier to store but has a better ISP than water.  

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5 hours ago, TheSaint said:

Once you start comparing the water-fed NTR to chemical engines that have a comparable shelf-life, it doesn't look half-bad.

good point - storable combustion propellants will give you only around isp 320 vs 412 for H2O NTR.

 

3 hours ago, todofwar said:

Ammonia might be a good compromise though

NH3 NTR isp 520 - which is okay if the propellant is lifted from Earth but you probably have other uses for the N if you were getting it from ISRU. CH4 NTR isp is 640 and is easy to ISRU on Mars. In fact (once the infrastructure was built) it would be cheaper to ship CH4 from Mars to LEO than to lift it from Earth. 

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On 12/16/2016 at 0:18 PM, TheSaint said:

But, on the other hand, this brings up an excellent point. When folks say, "Oh, but a water-fed NTR doesn't have an Isp any better than chemical rockets!" they're comparing it to chemical rockets that use cryogenic fuels, such as hydrolox or kerolox. These are going to have the same long-term propellant storage issues that an LH2 NTR is going to have. Once you start comparing the water-fed NTR to chemical engines that have a comparable shelf-life, it doesn't look half-bad.

This much I can really agree with. And if you are going on a Mars trip, you would never take your Mars transportation habitat back to LEO.  You would do the first burn with hydrogen (~4000m/s), and all the water-based burns would be in the ~1000m/s range.  I'm sure it would be easier to transfer water from ship to ship as well (although the freezing point might be an issue.  Do you want potable reaction mass or add anti-freeze to it (or possibly make it low-grade vodka and be both)).

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On 17.12.2016 at 8:54 PM, wumpus said:

This much I can really agree with. And if you are going on a Mars trip, you would never take your Mars transportation habitat back to LEO.  You would do the first burn with hydrogen (~4000m/s), and all the water-based burns would be in the ~1000m/s range.  I'm sure it would be easier to transfer water from ship to ship as well (although the freezing point might be an issue.  Do you want potable reaction mass or add anti-freeze to it (or possibly make it low-grade vodka and be both)).

In that case you have an engine that can switch between Water and Hydrogen, which is questionable. Not to mention your ISP will suffer, since you can't optimize for either. Vasic stuff like the nozzle design can't be optimal.

I think it's more realistic to go Hydrogen nuke first. And if zero boiloff isn't viable, then a chemical second stage for braking. If we are talking about regular supply drops, then probably using partial aerobreaking, which should make chemical circularization easier.

Edited by Temeter

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On 12/19/2016 at 4:03 PM, Temeter said:

I think it's more realistic to go Hydrogen nuke first. And if zero boiloff isn't viable, then a chemical second stage for braking. If we are talking about regular supply drops, then probably using partial aerobreaking, which should make chemical circularization easier.

If you are doing "supply drops" why in the world would you need to go nuclear?  You might need something sufficiently stable for ion transport (I'd like to think LOX is, but the amature rocket favorite rubber + nitrous oxide would be seriously stable), but putting them in place with ion engines lets you laugh at the rocket equation's attempt at tyranny.

Nuclear issues go beyond the political aspects of things.  You have an intense heat source that requires cooling inside a hard vacuum (and no handy source of expendable heatsink that doesn't kill your ISP): that is not an easy thing to deal with.

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Why not take other approach? If the NTR is to be used in mars, use it as propellant liquid CO2 so it's easy to refuel in mars. Or nitrogen if is better/easier. It will have  a poor ISP, so bigger tankage and/or dropable tanks but it will make everything else a lot simpler because there is no need for extra infrastructure. Just a unit for liquidizing the martian air that you will need anyway in any other approach. Less development, more viable concept.

How problematic will be to use H2 for earth departure and then use CO2 in mars anyway? You will have big tankage for the low density H2 and then you will need it for the massive amount of CO2 needed because the low isp

Edited by kunok
inexistant grammar, should really sleep more

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5 hours ago, wumpus said:

If you are doing "supply drops" why in the world would you need to go nuclear?  You might need something sufficiently stable for ion transport (I'd like to think LOX is, but the amature rocket favorite rubber + nitrous oxide would be seriously stable), but putting them in place with ion engines lets you laugh at the rocket equation's attempt at tyranny.

Your idea with nukes going hydrogen/water is a one way mission. Kinda obvious to assume this isn't human transport.

Also, I'm not sure why you're pushing the Ion thing like it's actually ever been done, or viable in any sense of the word. I don't think anyone even studied large scale ions till now. We only have some empty promises from the vasimir people.

Not to mention you'd still need a megawatt nuclear reactor.

Quote

Nuclear issues go beyond the political aspects of things.  You have an intense heat source that requires cooling inside a hard vacuum (and no handy source of expendable heatsink that doesn't kill your ISP): that is not an easy thing to deal with.

Not harder than a lot of other things. You can throttle nuclear reactors to minimize heat.

Also, you forgo you were arguing in favor of nuclear engines? :wink:

Edited by Temeter

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True.  If it was a one way mission, most of the nukes [technical] problems are already solved, just stage the thing and forget it (possibly injecting the melting core with U238 to make it useless for nuclear proliferation).  Reusable nuclear rockets are likely going to be trickier than reusable chemical rockets.  Even once you throttle nuclear reactors they are still producing a lot of heat, for quite some time.  And all you have are consumables (that kill your ISP efficiency) and [black body] radiators.

But if you get a nuclear thermal rocket, the bigger question is do you build (and launch into LEO) a second, or do you keep the first around Earth's "SOI" and not bother with the second.

I had been assuming a Nuclear Mars Transport and a chemical means to supply it.  But assuming you were only willing to have one nuclear rocket, you would much more likely have a chemical Mars Transport, and use nukes to supply it (the nuclear tug would then pay for itself by moving satellites from LEO to GTO with >800 ISP).  If your tug stays in "Earth's SOI" you can probably get away easily with only using hydrogen as its fuel.

The big question would be if it would be worth using a second nuclear rocket.  Presumably the political issues would have been solved, so while the justification is wildly harder (it won't have nearly the benefit of the first one), it wouldn't be nearly as hard as sell.  It would come down to the cost, except it would only 'need' roughly 2000 m/s delta-v instead of 5000m/s (assume the tug is far less massive, has roughly the same ISP (or better, since it only uses hydrogen) and carrying the fuel).

You probably would also go out with hydrogen out no matter what brought your fuel*, and water back. Or maybe helium.  I wonder how much a balloon capable of containing massive amounts of helium would weigh (especially if you could pressurize it to some pretty low levels.  Ultra-thin while the volume increases by the cube and the mass of the empty balloon increases by a square.  I wonder if helium can escape from a thin film of mylar.  If for some reason you made a nuclear transit vehicle and no tug, then you would have tough issues about bringing the transit vehicle all the way to LEO for refueling.

My bringing up ions is never about moving astronauts (that would require solving harder nuclear issues *and* solving unknown ion ones), but replacing the "tug".  Such a thing might not be as great for satellites (getting to GTO is slow, and time is money for billion dollar satellites), but I'd expect it to get the fuel in place with a 2 1/2 year schedule (thanks to Mars transfer windows).  Of course, you would likely be even more limited on fuels (expect everything to need roughly twice the "shelf life" of the return fuel), but with four digit ISP that can be dealt with.

* well not if you used ions.  But an ion tug and a nuclear rocket would be require a depressingly long R&D cycle**.  It would be like trying to get to Mars after ignoring the moon for 50 years...
** hope not.  Since we already have two ion engine craft in space, I'd like to think that a craft capable of moving 2000m/s worth of fuel in 2.5 years is within reach.  I'm not entirely certain of it.  You might need a couple more to bring the Mars Transporter to ~3000m/s (where the astronauts board) and another one to get all the cargo to Mars (which gives you several years to construct the Mars Transporter while the cargo winds its way to Mars).

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