Why aren't we using nuclear thermal rockets?

[phoenix_ca]

We know how to handle it safely, but some people get ignorant. It's people that are the problem, not the nuclear reactors.

*nods* kinda like guns. They're all just tools, and we do know how to operate them reasonably safely. For the same reasons I don't want everyone and their dog to have a gun, neither do I want everyone and their dog to have a nuclear reactor. Things go wrong, people die. And that's just inefficient.

[GoDores]

Anyone ever read "Voyage" by Stephen Baxter. Neat alternative-history fiction of what the space program would have looked like if the US had committed to a Mars landing immediately after Apollo 11. A key plot point hinges around the use of NERVA in a Saturn V stack. I don't know how to do the hidden spoiler thing on the forum yet so I won't comment on what happens, but it's a good read. I would fully support the use of a NERVA type stage if we had something useful to do with it.

[Tauge]

To do a spoiler tag [ Spoiler=words ]More words[ /spoiler ] Without spaces

Stuff

[GoDores]

Neat, thanks!

In Voyage, they're trying to develop a NERVA 3rd stage for the Saturn V because the US committed to a Mars trip in 1986 right after Apollo 11 landed and chemical rockets would be impractical. On the first manned flight of Apollo-N (the Saturn V with NERVA 3rd stage),

the core cooling system is damaged by pogo oscillations on launch and the reactor blows. The astronauts reenter but die of radiation poisoning. No one else is hurt but the idea of a blown-up reactor in space kills NERVA politically and they have to redesign the mission to get to Mars with chemical rockets.

Anyway, it's relevant to the thread, but it's a great book in general. I would definitely recommend it to anyone who plays KSP.

[Mekan1k]

The largest issue seems to be, (besides all the good ones listed) is that an engine that uses radioactive decay, such as a NERVA engine, would end up with nuclear debris deposits in the propellant gasses.

Scientists are unsure of what would happen if the earth ran into radioactive dust. How much would get through the atmosphere? What would be irradiated if it did? Etc.

However, there is a design called the Nuclear Lightbulb engine (Google it), where the decaying rods are isolated by a 'bulb' from the propellant, and the whole piece acts like either a pressure acceleration stage, or the 'ram' part of 'RamJet', with the 'jet' part being the regular hydrogen/oxygen reaction in the rocket.

[Kryten]

The fission products stay in the fuel rods, they don't end up in the plume; the exception is if you have your fuel as a liquid or gas to allow you to use higher temperatures. The nuclear lightbulb design is meant to hold a core of gaseous uranium, not 'decaying rods'; an interesting design concept, but we probably should try and get solid core engines working before worrying about somehow that exotic.

[mushroombrew]

*insert casual idea of your choice so someone with too much time can troll you to death.

Edited December 20, 2018 by mushroombrew
Because my presence here is no longer relevant

[DDE]

[snip]

4 hours ago, mushroombrew said:

Just park it in a high earth orbit 

And BAM! You’ve lost the key advantage of NTRs over electrics, which is sufficiently high thrust to quickly transit through the Van Allen belts.

4 hours ago, mushroombrew said:

A large craft that has the duration and size to accommodate a human crew for years..maybe even decades on long round trips around the solar system.

NTRs don’t last that long. Most of the researched models have to be expended after a burn due to all the neutron poisons rendering most of the unburnt fuel useless. Which is why many NERVA designs had two-three stages.

4 hours ago, mushroombrew said:

No need to ever use the nuclear engines in the atmosphere

This never was the problem. If a solid-core NTR is shedding fuel, you’ve got bigger problems than radioactive contamination.

Which, by the way, due to likely being in the upper atmosphere would be utterly harmless if not beneficial.

4 hours ago, mushroombrew said:

The crew would probably also thank you for not having to live in something the size of a walk in closet for years at a time.

The walk-in closet is the most efficient way of transporting them, no matter the engine involved. Time to get used to it.

[mushroombrew]

[snip]

Google led me here.  So what if it's old?  And you're not entirely correct about the advantages of this type of propulsion.  It achieves much greater efficiency and can reach higher velocities with the same amount of propellant...usually just hydrogen.  The whole convo was about keeping radiation away from the Earth's atmosphere...hence the high earth orbit thing.  Uh..BAM...

Edited December 20, 2018 by Vanamonde

[DDE]

2 hours ago, mushroombrew said:

I mean, it would be expensive to build, but think of the money saved when we wouldn't be using single use vehicles every time.

Aside from needing to swap out the engine every flight, neutron embrittlement and the general wear-and-tear of deep space is likely to render the vehicle one-use anyway.

2 hours ago, mushroombrew said:

We'll buy more weapons instead.

Footprints on Mars don’t protect you from people you dislike.

Your best chance is to try and sell someone the idea of NTR-propelled weapons, like TIMBERWIND tried.

Or the 9M730...

[mushroombrew]

You act like I'm planning to build this thing in my garage.  I was just thinking something up.  And now that I've gotten the gist of how the community is here im deleting my acount.

[DDE]

1 hour ago, mushroombrew said:

And you're not entirely correct about the advantages of this type of propulsion.  It achieves much greater efficiency and can reach higher velocities with the same amount of propellant...usually just hydrogen.  The whole convo was about keeping radiation away from the Earth's atmosphere...hence the high earth orbit thing.  Uh..BAM...

Compared to the electrics, the Isp is still utterly pathetic, which is why NASASpaceflight forums hate NTRs with a passion. With the higher dry mass and rad shielding factored in, you’re going to have trouble breaking even.

And having to deal with hydrogen is not an advantage, it’s a nasty propellant that’s impossible to store for extended periods of time.

Meanwhile, the choice of high Earth orbit isn’t about abstract radiation - radioactive particles in stratosphere or above are a non-threat, which is why ‘self-cleaning’ airburst nukes are not a radiation hazard - but about the reduced probability of the entire reactor falling out of the sky. Which is again exaggerated, as the reactor is likely to survive a reentry and crash-landing without spilling its contents.

Edited December 20, 2018 by DDE

[MinimumSky5]

@mushroombrew Let me try instead....

Hi, welcome to the forums! 

[snip]

From you actual question, my tldr response is *garbled, garbled* ENVIRONMENTALISM OMG NUKES, and money, and needing a large, cheap rocket (we've had large rockets, and cheap rockets, but not large and cheap rockets.) NTRs may well have their place in space exploration, but we simply don't need them yet. 

[snip]

Edited December 20, 2018 by Vanamonde

[Vanamonde]

Some posts have been removed or edited. This is not the way to make a forum newbie feel welcome, folks. Firstly, there is nothing wrong with posting in an old thread as long as the content of the thread is not obsolete, and secondly, even if that was against our forum rules, please simply hit the report button in a post if you feel there's a problem rather than take it upon yourself to attack and insult the person in question. 

[magnemoe]

 

On 10/12/2012 at 5:26 AM, pwnedbyscope said:

I think the biggest, reason and one previously not mentioned is right now there is no reason we would need to use an NTR for any missions, sure they a efficient but there currently are no missions planned or even thought of that would require what an NTR can give, the farthest we as humans have even thought about traveling is Mars, and that can be reach easily, i use that lightly, with LFEs its already been proven, as soon as we reach the extent of what LFEs and SRBs can offer then we will start looking at other options which would include NTRs, but asking why we dont use NTRs is like asking why we dont use solar sails, the technology is there, but there is no use for them. a question i would like to ask is why isnt there any research going into things such as alcubierre drives

@ roboray, on that argument the largest non-nuclear artificial explosion was caused by the N1 rocket

This, you nailed it, look at the types of missions we does, its small probes to various planets, people to IIS and various satellites. 
For none of this missions the LV-N in KSP is an good engine, its only relevant for heavy payloads and pretty high dV requirements or medium payloads with very high dV requirements. 

Nerva start to get relevant for manned moon missions or heavier probes who require high dV burns. Its very nice for manned mars missions. 
It was discussed as an 3rd stage for saturn 5 as you could increase payload size, but it would require to much development. 
Soviet union also thought of this. 
But for current missions it would not be an good engine even if we had it in inventory. 
It also require plenty of development and testing to make it an operative engine, and the safty requirements for nuclear reactors and an guarantee for no radioactive pollution will make this far more expensive. 

Now an unused Nerva just contains enriched uranium who is not very radioactive so an launch fail N1 style would not release much radioactivity, an RTG is more radioactive at launch. 
it will become radioactive on use both during the burn and fission product will leave it radioactive even after burn so it would be dangerous then you arrive at mars. 
One solution to this is to use it as an disposable stage but that make it less attractive for Mars. 

No treaties against it and Russia has used reactors in space, one even fall down in Alaska. 
something like an vasmir and an reactor might be an better option as in higher ISP and its easier to test an reactor than an open system like an nerva. 

Edited December 20, 2018 by magnemoe

[Green Baron]

Hi, @mushroombrew,

we are far from sending humans outside of earths orbit into interplanetary space due to lack of technology, funding and an immediate scientific need and political will to build such a thing. Maybe in some decades, if the will is still there then or the first fusion reactors come into existence.

That nuclear rocket comes up every now and then. It may be mere lobby talk, but Nasa as well proposes it. It is technology from a past millennium :-). Right now there is no political will to build such a thing, and a disaster with nuclear rocket while on a return trajectory would spread one of most the poisonous stuff we know there where it will stay for a long time and add to the artificial radiation background. The massive core will probably impact and spread its contents as fallout. We already had a plutonium disaster with a small satellite in Canada and others have been dumped into the ocean.

Something cleaner would be the wiser choice. Until then, the probes can provide us with a lot of data until technology has reached a level where we can more safely send people anywhere else, imo.

Edited December 20, 2018 by Green Baron

[DDE]

21 minutes ago, Green Baron said:

disaster

There are vicious disagreements between the parties involved as to how bad Cosmos-954 was, with descriptions of the amount of recovered fragments and the share of recovered fuel diverging by a full order of magnitude. All in all, something that’s cost a mere C$6 mio to clean up - most of it likely due to the large search area - was hardly a disaster.

It’s also notable that, as a thermoelectric liquid metal-cooled reactor, the pile involved had a pretty weak pressure vessel.

Edited December 20, 2018 by DDE

[Green Baron]

2 minutes ago, DDE said:

There are vicious disagreements between the parties involved as to how bad Cosmos-954 was, with descriptions of the amount of recovered fragments and the share of recovered fuel diverging by a full order of magnitude. All in all, something that’s cost a mere C$6 mio to clean up - most of it likely due to the large search area - was hardly a disaster.

Radiation levels of a piece found was more than enough to kill humans, had it hit in an inhabited area that would have been really bad. All the fragments found only count for 1% of the fuel it contained. Yes, this was a disaster, in a few thousand years there will still be the potential to kill if somebody picks up a wrong part.

This and all the other cores that were dumped elsewhere tell us from a time when responsibility was not the main argument ;-)

 

[kerbiloid]

If you have found a strange looking mushroom, first put into water and watch if it's glowing blue.

[DDE]

16 minutes ago, Green Baron said:

Radiation levels of a piece found was more than enough to kill humans, had it hit in an inhabited area that would have been really bad.

If one were to hug that particular piece for several hours. Not as improbable as it sounds, but unlikely.

16 minutes ago, Green Baron said:

All the fragments found only count for 1% of the fuel it contained.

A US DoE source says 90%.

https://www.webcitation.org/65iUoQJNc?url=http://www.nv.doe.gov/library/factsheets/DOENV_1198.pdf

16 minutes ago, Green Baron said:

This and all the other cores that were dumped elsewhere tell us from a time when responsibility was not the main argument ;-)

About two of those, each prompting the addition of yet another core ejection system, for a total of three.

Edited December 20, 2018 by DDE
Link

[kerbiloid]

Spoiler

A little later the tourists will hear from the local elders about a wonderful Hot Blue Spring shining in the night for the tired travellers and killing any infection if clean your clothes in its water.

Like the bath in Wonderwoman

 

[Green Baron]

12 minutes ago, DDE said:

A US DoE source says 90%.

I doubt it. Bring it on.

Edited December 20, 2018 by Green Baron

[DDE]

10 minutes ago, Green Baron said:

I doubt it. Bring it on.

Just edited it in.

19 minutes ago, kerbiloid said:

Like the bath in Wonderwoman

So the movie version isn’t made of clay?

[kerbiloid]

Spoiler

7 minutes ago, DDE said:

So the movie version isn’t made of clay?

I'm not sure, but it glows like a Nuka-Cola,

So, I was calm about Diana's chastity after the who-cares-about-his-name guy took a bath on the Amazons' island.

 

Edited December 20, 2018 by kerbiloid

[Green Baron]

You are right, thanks, if thats a reliable source. But that is by no means an excuse to repeat that and risk more contamination for millions of years.

And it adds to the ones that impacted ocean and all the other reactors that were dumped.

The released reactor cores of the Rorsats form a radiation belt in a debris orbit at 950 km (search Rorsat NaK drops).