ArmchairPhysicist

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

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Posted (edited)

This has been really bugging me. In KSP for the most part, once I unlock the Thermal nuke rocket and the kspi thermal rockets (that gas core rocket is dope) I simply never use chemical rockets for space work. The efficiency of the nuke rockets is simply too good to pass up. The thing is that the efficiency of the KSP thermal rockets are the same as the real life ones. The timberwind had an isp of 850 at sea level and 1000 in vacuum. And this performance is normal for most thermal rockets, in fact we could probably increase this greatly given this was 1980’s tech. But if these rockets are so good, why don’t we use them? Is it stuck up politics? Some secret downside that isn’t well known? Maybe an unintelligent populace that hears “nuclear” and begins knee jerk protesting?

Admittedly there is a danger of pollution if you suffer a failure in atmosphere. But accidents happen when you don’t know what you’re doing, and we don’t know how to overcome these issues because of the fear of nuclear science.

Edited by Val
Rephrased.

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They're heavy, although they do provide high isp.

It depends on size of payload and total delta V. There are many circumstances where a nuclear engine is preferred and where a chemical engine performs better. 

As for why not... well, NERVA was almost flight ready by the time it was cancelled. Probably something to do with commitment and money.

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However also in KSP the LV-N comes out short with small payloads and that is that we has been sending into deep space. 
Nerva start to win as payload mass increases. But you will need something like an huge probe mission like Europa sample return or manned moon for it to makes sense. 
Note that an smaller nuclear engine would make them more relevant for probe missions. 

Yes nuclear fear will add an cost, note that NTR are common used also that this does not apply for Russia and China, This can even be an income source selling engines to NASA or prestige in being part of an flagship missions. 
Still the cost will make it cheaper to use an delta heavy for the few flagship missions. 

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15 minutes ago, Bill Phil said:

They're heavy, although they do provide high isp.

It depends on size of payload and total delta V. There are many circumstances where a nuclear engine is preferred and where a chemical engine performs better. 

As for why not... well, NERVA was almost flight ready by the time it was cancelled. Probably something to do with commitment and money.

But with regards to money, we’re dumping money into dumb projects that then get canceled every four years. All those billions could have been used on the already developed technologies and imagine where we would be? Nuclear shuttles between the moon and earth probably.

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Well, here's the wiki link on NTR's. It seems to boil down partly down to political will to push it past the anti-nuke crowd, which objects to the risk of an in-flight accident, and partly to the lack of funds and need after plans for manned Mars missions and more manned Moon missions were shelved. 

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23 minutes ago, ArmchairPhysicist said:

But with regards to money, we’re dumping money into dumb projects that then get canceled every four years. All those billions could have been used on the already developed technologies and imagine where we would be? Nuclear shuttles between the moon and earth probably.

That was the plan. But it never got executed. It was viewed as dependent on the Shuttle program, which didn't live up to expectations, and it was shelved until the Shuttle could support it, which it could never do. You'd need a true super heavy lifter to take full advantage of NTRs (if you can't get high flight rates, that is) and that seems to be on the horizon.

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1 hour ago, ArmchairPhysicist said:

The timberwind had an isp of 850 at sea level and 1000 in vacuum.

ISP doesn't deliver payloads - total vehicle performance does.  And in that respect nuclear powered spacecraft suffer heavily from the weight of the reactor and the dry mass of the LH2 tanks.

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Posted (edited)
6 minutes ago, DerekL1963 said:

ISP doesn't deliver payloads - total vehicle performance does.  And in that respect nuclear powered spacecraft suffer heavily from the weight of the reactor and the dry mass of the LH2 tanks.

Timberwind claimed high T/W ratios, although from what I can find the whole project was very sketchy.

Regardless, a LANTR can increase thrust significantly and allow launches using NTRs. At the cost of specific impulse, and an increase in complexity.

Edited by Bill Phil

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What are you going to use them for?

They are detrimental for getting to Earth orbit (too heavy, too little thrust, and possibly radioactive exhaust). They are too heavy to use for sending probes to other planets -- better to make the probes as small as possible.

About the only possible use for them is for manned missions to other planets, and we have never done one of those.

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Posted (edited)
On 16/04/2018 at 12:33 AM, ArmchairPhysicist said:

..... Maybe an unintelligent populace that hears “nuclear” and begins knee jerk protesting?......

Okay, I chuckled when I thought about this part.

Yeah, Fission byproducts, waste  and alot of propaganda has effectively put the brakes on Nuclear Research.

Edited by Val
Quote edited to reflect OP

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12 minutes ago, mikegarrison said:

What are you going to use them for?

They are detrimental for getting to Earth orbit (too heavy, too little thrust, and possibly radioactive exhaust). They are too heavy to use for sending probes to other planets -- better to make the probes as small as possible.

About the only possible use for them is for manned missions to other planets, and we have never done one of those.

Except you’re forgetting that weight is less of an issue once you are in space. Yes normally a heavier craft needs more fuel to due space things. But NTRs simply get around that by giving you higher efficiency for the same fuel volume. Only downside besides Engineering hurtles(mostly these have been all solved) is a lower twr. Go farther, but not as quickly.

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57 minutes ago, ArmchairPhysicist said:

Except you’re forgetting that weight is less of an issue once you are in space.


0.o   Yes, mass matters once you are in space.

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1. nuclear rockets are mostly good for going to other planets (long time spaceflight)

2. nuclear rockets use hydrogen

Doesn't match up.

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There is simply no Mission where they make any sens: Since they get extremly radioactive you can only use them once on missions permanently leaving the earth, which means probes to other plantes. We need that capability only once every few years, instead of building and testing expensive NTRs you can simply use a bigger rocket...

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There are a few technical reasons that I have looked into where NTRs are simply not better. First, NTRs have a minimum size. We don’t have any launch vehicles that can lift a fully fueled NTR powered interplanetary stage. When you start putting it in terms of “launch vehicle can lift X” “upper stage needs Y dV” “payload must be at least Z kg to get useful science”, NTRs start to lose their edge because the dry mass is so high.

Another thing is that, as far as I’ve seen, NTRs don’t have a shutdown/restart ability. There has been some extensive discussion on this forum about NTR shutdown sequence and how it affects performance. The short version is you either need to eject the engine, waste a bunch of fuel to cool the reactor, or have a closed cycle cooling system. One of those is a strict one-time-use and the other two reduce the performance in a way we haven’t quantified. 

Last comes the problem of fuel storage. ULA has looked into long term hydrogen storage in space but their plans are mostly for cislunar space so “long term storage” means a few weeks. I don’t have the report in front of me as I write this but I think it was around 0.1%/day for hydrogen boil off. This isn’t necessarily a deal breaker for using NTRs and Hydrolox engines have the same issue but it’s an increased complexity that needs to be considered.

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A very similar discussion was had not long ago:

 

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Posted (edited)
On 16/04/2018 at 12:33 AM, ArmchairPhysicist said:

 But if these rockets are so good, why don’t we use them? Is it stuck up politics? Some secret downside that isn’t well known? Maybe an unintelligent populace that hears “nuclear” and begins knee jerk protesting?

Admittedly there is a danger of pollution if you suffer a failure in atmosphere. But accidents happen when you don’t know what you’re doing, and we don’t know how to overcome these issues because of the fear of nuclear science.

The Main problem we have with developing efficient nuclear rockets is the fact that they are usually, by nature, extremely delicate in terms of keeping it safe. If they shatter with anything in the atmosphere, or don't entirely get destroyed, it could spell disaster for any neighboring populations, as a nuclear reaction powerful enough to lift a rocket would surely break up very spectacularly upon failure. Then theres the radioactivity to be considered as well. This is one of the reasons we don't launch nuclear waste into the sun, as it would be disastrous if the launch went wrong.

Edited by Val
Quote edited to reflect OP

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"Hydrogen boiloff problem"

Solution: Do not use that flighty stand-up guy. Use nitrogen, ammonia, methane. Heck, even plain old water will do in a pinch.

"But... but hit to the rocket's Isp!"

Solution: Automated space tankers, fuel depots, ISRU and improvements of nuclear technology.

It's not insurmountable. It just requires work, money and most of all - will to start moving in that direction.

The last part apparently is hardest to find :(

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22 minutes ago, Scotius said:

Automated space tankers, fuel depots, ISRU

Once you got that running you dont need to deal with all the challenges of nuclear engines, just use regular ones...

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1 hour ago, Scotius said:

Solution: Automated space tankers, fuel depots, ISRU and improvements of nuclear technology.


If you're sending a heavy payload to any destination via NTR...  it kind of defeats the purpose to have to deploy infrastructure via conventional means that costs 10x as much.
 

1 hour ago, Scotius said:

The last part apparently is hardest to find


No, the hardest part to find are solutions that are actually practical rather than just hand waving without having actually been thought through.

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Hydrogen is the best fluid to use because it is the lightest, but you don't strictly speaking have to use it. Of course, you start losing that ISP advantage that is the whole reason for the NTR in the first place.

Bottom line though -- the real world is not KSP. KSP gets to ignore a lot of real-world issues that make NTRs a solution in search of a problem.

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Aside from the usefulness with ion drives available for unmanned probes, the challenge is sending what is effectively a nuclear reactor into orbit.

It’s of course perfectly safe but a nuclear industry screwing things up every decade or so doesn’t help. Fukushima was perfectly save, and we were told that there was a 0% chance of a meltdown when, we learned later, the meltdown had already happened. Good luck in an environment like that to sell it to the public that this will be different, and safe.  Why would they believe that this time it really is safe?

The problem is not with the technology but with the inaptitude of those involved in self-regulating themselves and marketing it properly. You can’t fix that.

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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.

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