ArmchairPhysicist

Nuclear Thermal rockets/the holy grail that is never used?

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1 hour ago, Bill Phil said:

I read somewhere that NTRs can get lunar transfer times down to less than a day and still have performance to carry a well sized payload. Reducing time certainly has major advantages.

Reducing time to the Moon by 2/3s would require something like 2-3 times the fuel needed to go to the Moon.  It would also reduce time going to Mars by an equal ratio (which might well be worth it).  The only reason I could imagine for such a stunt would be to get a cargo carrier back before all the hydrogen vented out.  Not that I would be all that surprised if people decided to take the "fast route" once NTRs were up there, but it isn't a very good reason to dig into all the political and technical problems for NTRs.

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1 hour ago, wumpus said:

Reducing time to the Moon by 2/3s would require something like 2-3 times the fuel needed to go to the Moon.  It would also reduce time going to Mars by an equal ratio (which might well be worth it).  The only reason I could imagine for such a stunt would be to get a cargo carrier back before all the hydrogen vented out.  Not that I would be all that surprised if people decided to take the "fast route" once NTRs were up there, but it isn't a very good reason to dig into all the political and technical problems for NTRs.

It only required an extra 900 m/s, far from requiring 2 to 3 times the fuel. Maybe 12% to 15% more propellant with an NTR, but the design called for a margin with over 900 m/s regardless.

Reducing the time is significant. Less complex/expensive life support is required. Less exposure to the reactor and less exposure to radiation in space. Less isolation for lunar bases and the like, only a day away from Earth. Less problems with boiloff. Certainly advantageous. Chemical rockets would be hard pressed to come close.

This also wasn't intended as a stunt, but was proposed for general use. Time is the enemy in space, once everything else has been sorted out.

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3 hours ago, Bill Phil said:

It only required an extra 900 m/s, far from requiring 2 to 3 times the fuel. Maybe 12% to 15% more propellant with an NTR, but the design called for a margin with over 900 m/s regardless.

Reducing the time is significant. Less complex/expensive life support is required. Less exposure to the reactor and less exposure to radiation in space. Less isolation for lunar bases and the like, only a day away from Earth. Less problems with boiloff. Certainly advantageous. Chemical rockets would be hard pressed to come close.

This also wasn't intended as a stunt, but was proposed for general use. Time is the enemy in space, once everything else has been sorted out.

Do you have a link to this study? I'm not calling you out, I'm interested to see what their assumptions were for this hypothetical vehicle. Full disclosure: I am planning on calculating whether they could get the same dV with the same starting mass except using a traditional Hydrolox engine like the J-2.

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1 hour ago, Racescort666 said:

Do you have a link to this study? I'm not calling you out, I'm interested to see what their assumptions were for this hypothetical vehicle. Full disclosure: I am planning on calculating whether they could get the same dV with the same starting mass except using a traditional Hydrolox engine like the J-2.

It's Nuclear Space Propulsion by Holmes F. Crouch. An excerpt can be found on Atomic Rockets here:

http://www.projectrho.com/public_html/rocket/realdesigns2.php#id--Reusable_Nuclear_Shuttle--Lunar_Ferry

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Posted (edited)

I've rephrased the offensive term in the OP, edited some quotes and removed a few replies that, while justified, were derailing the topic.

Please use the Report button instead of derailing a topic.

Report this post if you have any questions or concerns regarding this decision.

Edited by Val
Grammar and redundancy

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Posted (edited)

NTR is hard to deal with for manned missions, and storing propellant for long missions is a problem. It could be interesting if they made a design that could run on multiple propellants... like Lh2 for the outbound journey, and ISRU+water ice for the return.

I'm more interested in nuclear-electric unmanned concepts, like JUICE:

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

*edit* oops, confused that with this:

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

Which was sadly cancelled

 

Edited by KerikBalm

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Nuclear thermal rockets are good for launching really big payloads really fast.

But not so good if those payloads are squishy humans, because then you need radiation shields, and that grows your dry mass almost beyond their efficiency utility.

So you only need nuclear thermal rockets if you are launching big non-human payloads.

For example, if you were sending an all-expendable manned mission to Mars and needed to send supply/cargo out first.

However, unless you have a REALLY big rocket, then your big cargo and big NTR and big hydrogen tanks will be too much to send up as a monolith, so you need to do it piecemeal on a slightly smaller really big rocket. 

But if you're doing it piecemeal, then you can just do orbital propellant transfer with a reusable rocket and you don't need to bother with nukes at all.

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For reference: current NTR designs push about 925 s Isp and 110 kN.

You wouldn't want more than about an hour of burntime, so for a TMI of about 3.8 km/s for a nominal Mars mission, you'd want an acceleration of around 1.1 m/s2 or 0.11 gees. Your m0 will be around 100 tonnes and your payload will be on the order of 55-60 tonnes. So you'd need SLS Block 1B or BFR just to get it into orbit. There's no need for that kind of payload, short of a manned Mars mission.

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18 hours ago, Bill Phil said:

It only required an extra 900 m/s, far from requiring 2 to 3 times the fuel. Maybe 12% to 15% more propellant with an NTR, but the design called for a margin with over 900 m/s regardless.

Reducing the time is significant. Less complex/expensive life support is required. Less exposure to the reactor and less exposure to radiation in space. Less isolation for lunar bases and the like, only a day away from Earth. Less problems with boiloff. Certainly advantageous. Chemical rockets would be hard pressed to come close.

This also wasn't intended as a stunt, but was proposed for general use. Time is the enemy in space, once everything else has been sorted out.

I really have to question your math.  From everything I can find, it takes at least 3km/s delta-v to get to the Moon (note that NTRs are likely to have issues getting much from Oberth thanks to low thrust and start/stop issues).

Apollo 11 took 51 hours and 49 minutes to get to the Moon.  They used a free-return trajectory so they had a pretty much a standard Hohmann trajectory (or at least similar time expended).  Average distance to the moon is 384,000km.

time: 186540 seconds.  distance: 384000000m.  Effective radial velocity: 2058m/s.

There is no way adding 450m/s (each way) to 2058 is going to cut your time by 2/3s (you'll get an 18% improvement).  You will need upwards of 8km/s delta-v to the moon and another 4km/s for coming home (assuming you are willing to aerobrake at those speeds: most of your incoming speed will be thanks to your LEO delta-v).  A NTR can easily get twice, possibly three times the Isp of a hydrolox engine, but that means that you can get at most 3 times the delta-v with the same mass ratio as hydrolox (and don't forget that NTRs will have some nasty dry mass).  I suspect that either you meant 1/3 (and then forgot about braking) or got said information from someone who made such a mistake.

Brachistochrone trajectories are favored for hype (and distant Sci-Fi flavor).  There is certainly a lot of benefit in a small excess of delta-v for long journeys (especially for Mars and beyond), but the cost adds up quickly.

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1 minute ago, wumpus said:

I really have to question your math.  From everything I can find, it takes at least 3km/s delta-v to get to the Moon (note that NTRs are likely to have issues getting much from Oberth thanks to low thrust and start/stop issues).

Apollo 11 took 51 hours and 49 minutes to get to the Moon.  They used a free-return trajectory so they had a pretty much a standard Hohmann trajectory (or at least similar time expended).  Average distance to the moon is 384,000km.

time: 186540 seconds.  distance: 384000000m.  Effective radial velocity: 2058m/s.

There is no way adding 450m/s (each way) to 2058 is going to cut your time by 2/3s (you'll get an 18% improvement).  You will need upwards of 8km/s delta-v to the moon and another 4km/s for coming home (assuming you are willing to aerobrake at those speeds: most of your incoming speed will be thanks to your LEO delta-v).  A NTR can easily get twice, possibly three times the Isp of a hydrolox engine, but that means that you can get at most 3 times the delta-v with the same mass ratio as hydrolox (and don't forget that NTRs will have some nasty dry mass).  I suspect that either you meant 1/3 (and then forgot about braking) or got said information from someone who made such a mistake.

Brachistochrone trajectories are favored for hype (and distant Sci-Fi flavor).  There is certainly a lot of benefit in a small excess of delta-v for long journeys (especially for Mars and beyond), but the cost adds up quickly.

It's 900 m/s on the TLI leg from what I can find. Total delta-V is in excess of 12 km/s for the trip already. There's over 1 km/s margin. The time reduction is 1 way as well. I can't do the math myself right now but I can once I get home.

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