Why are NERVAs not yet used?

[A35K]

So, we know that the technology for the NERVA has existed for a long time now, and fully functional test engines were built. Now we all know how much easier Interplanetary travel is with these things, so how come they have never actually been applied to a real space mission? Is it because of radiation concerns?

[Gaarst]

Because treaties from the Cold War severely limits the possibilites of sending nuclear stuff in space.

Also, people won't feel confortable with nukes over their heads. Even if agencies prove the risk to be null, the public opinion will be against.

Oh and the Soviets did put some nuclear reactors in space (not nuclear propulsion though), one of them fell over Canada...

[Scotius]

If we would go to Mars or Venus straight after Moon, then NERVAs probably would be used. As it is now, we don't do long-range manned missions, and probes do not require such engines. If an unmanned mission needs a lot of dV, then ion engines are sufficient. In the future? Maybe we use NERVA's, maybe VASIMR. Maybe something more exotic

[Emperor of the Titan Squid]

Yes, its mainly because of treaties, but space travel would be a lot easier with NTR

[A35K]

But why would the treaties apply to something that is clearly not meant to be a weapon? Do NERVAs have a large enough nuclear material content to cause significant fallout? Also, why are RTGs not banned by the treaty?

[tater]

They are scary to some people.

Never underestimate the power of human stupidity.

[Nibb31]

We don't really need NERVA any more. We have SEP, ion, VASIMIR, and so on. They have much better ISP.

[fredinno]

2 hours ago, Scotius said:

If we would go to Mars or Venus straight after Moon, then NERVAs probably would be used. As it is now, we don't do long-range manned missions, and probes do not require such engines. If an unmanned mission needs a lot of dV, then ion engines are sufficient. In the future? Maybe we use NERVA's, maybe VASIMR. Maybe something more exotic

The only time we will need nuclear is when we go past Mars. IONs for Cargo and Zero-boiloff IVF chemical for manned sections are far better overall in ISP and more likely.

1 hour ago, A35K said:

But why would the treaties apply to something that is clearly not meant to be a weapon? Do NERVAs have a large enough nuclear material content to cause significant fallout? Also, why are RTGs not banned by the treaty?

RTGs are not fissile material. NERVA contains fissile material.

[tater]

Ions are only good for cargo, and VASIMR would require a (large) reactor anyway for a low-I_sp, higher thrust dash. Marshall is doing good work on NTP, and the issues surrounding fuel (boil off) are the same with other high I_sp orbital motors fueled with H₂. Maybe in desperation for SLS payloads they'll finally flight test one.

[fredinno]

3 minutes ago, tater said:

Ions are only good for cargo, and VASIMR would require a (large) reactor anyway for a low-I_sp, higher thrust dash. Marshall is doing good work on NTP, and the issues surrounding fuel (boil off) are the same with other high I_sp orbital motors fueled with H₂. Maybe in desperation for SLS payloads they'll finally flight test one.

For that money, you'd be better off building a lunar space station, and get several flights!

[m4v]

the treaties only ban nuclear detonations, you can put a nuclear reactor in space if you want, the problem of NTR is more a lack of political will (they don't need them for bomb the other side of planet anymore) and the difficulties of protecting people and equipment around an unshielded nuclear reactor.

[DerekL1963]

1 hour ago, Nibb31 said:

We don't really need NERVA any more. We have SEP, ion, VASIMIR, and so on. They have much better ISP.

They have much better ISP, but they all have crap thrust and dodgy T/W ratios and are impractical for all but fairly small spacecraft.   ISP isn't everything.

That being said, there are no treaties preventing NERVA (no treaty prevents launching fissile material into space).   We aren't using NERVA for one and only one reason - they're hideously expensive, and being very heavy they require large and hideously expensive boosters.   The only reason they'd ever be useful is to power large and hideously expensive missions.   And once we'd been to the Moon, Congress lost all remaining interest (of which wasn't much at that point) in paying for anything hideously expensive and space related.   (And no, as such things go, Shuttle wasn't hideously expensive.)

[Nibb31]

16 minutes ago, DerekL1963 said:

They have much better ISP, but they all have crap thrust and dodgy T/W ratios and are impractical for all but fairly small spacecraft.   ISP isn't everything.

Who cares about TWR in space ? TWR is important for launching and landing, neither of which you would really want to use a NERVA for anyway. For interplanetary stuff, you don't really care if your burns go on over several weeks.

NASA plans on scaling SEP for larger payloads for the ARM mission.

Edited February 7, 2016 by Nibb31

[tater]

24 minutes ago, fredinno said:

For that money, you'd be better off building a lunar space station, and get several flights!

A lunar station would need many resupply flights, and NASA can't really afford 1 SLS launch per year, much less several.

NTP is still on the table for manned Mars, so like other propulsion candidates it deserves testing. I want to say the current design is only about 1600kg.

[tater]

18 minutes ago, DerekL1963 said:

That being said, there are no treaties preventing NERVA (no treaty prevents launching fissile material into space).   We aren't using NERVA for one and only one reason - they're hideously expensive, and being very heavy they require large and hideously expensive boosters.   The only reason they'd ever be useful is to power large and hideously expensive missions.

They are not all that big. The NTP group has a version that is about 1600kg, and they are pretty close to being able to test it as I understand it.

 

Quote

 And once we'd been to the Moon, Congress lost all remaining interest (of which wasn't much at that point) in paying for anything hideously expensive and space related.   (And no, as such things go, Shuttle wasn't hideously expensive.)

Manned spaceflight is a stunt. It's a stunt that I think is awesome, but it's a stunt. Everything manned could be done better by robots science-wise, and that is only getting more true as capabilities improve in that area. The Space Race was about the Moon. Having done it, it instantly became routine. Manned missions need to be spectacular or no one cares. The vast majority of people have no idea what is currently happening in space exploration. A new station near the moon would maybe get some time on the news right away, then no one would care.

That's the reason why funding dropped. That said, in constant dollars NASA has been remarkably constant since the end of Apollo in terms of funding.

[GoSlash27]

People don't want nukes in space and there would be a huge political backlash.

Best,
-Slashy

[tater]

A tiny % of people would care and be vocal in the US. The same people didn't like RTGs, either. 

[Bill Phil]

1 hour ago, Nibb31 said:

Who cares about TWR in space ? TWR is important for launching and landing, neither of which you would really want to use a NERVA for anyway. For interplanetary stuff, you don't really care if your burns go on over several weeks.

NASA plans on scaling SEP for larger payloads for the ARM mission.

You still have some gravity losses. Albeit not much at all, but they are present.

[tater]

TWR matters for crewed missions. For unmanned missions, obviously I_sp is king, TWR doesn't matter at all.

SEP transit times (like all ion) to and from Mars are measured in years.

[DerekL1963]

1 hour ago, Nibb31 said:

Who cares about TWR in space?  TWR is important for launching and landing

Launch, and landing, and rendezvous, and planetary orbital insertion, and orbital maneuvering, and... the list goes on and on and on.  People who care about real missions and real space care about TWR.  Unlike people handwaving phantom spacecraft, they lack the luxury of picking propulsion systems based on a single dimension.

 

1 hour ago, Nibb31 said:

NASA plans on scaling SEP for larger payloads for the ARM mission.

NASA plans all kinds of things.  Some of them even happen.

[magnemoe]

2 hours ago, tater said:

A lunar station would need many resupply flights, and NASA can't really afford 1 SLS launch per year, much less several.

NTP is still on the table for manned Mars, so like other propulsion candidates it deserves testing. I want to say the current design is only about 1600kg.

This.
Nerva is not used because of lack of missions, you don't use LV-N for tiny probes or to get into orbit in KSP?
Neither does nasa or others.
Nerva is relevant for manned mars mission, an large probe mission like Europa sample return. Its also relevant for an Earth - moon tug supplying an moon base. 
It was on the table for Pluto flyby but it was overkill as its an limit how much it can be scaled down and expensive to develop. 

Radioactivity in space is not an issue, its an issue trying to do all the ground testing needed to certify it. 

 

[Scotius]

One key word: reusability If you build and send up an expensive, complicated engine, you should want to get it back. And use it again, instead of building, sending and throwing away another one. That's what SpaceX is doing with their reuseable rockets - they're getting back most expensive parts - engines with their complicated, precise plumbing. Probes don't come back usually, manned missions do.

[tater]

You need to demonstrate that reusability actually makes a meaningful cost difference.

An Earth orbital nuclear ferry vehicle would presumably be reused as long as it had nuclear fuel via refueling the LH₂ propellant. So it gets reused perhaps many times before it is retired. Retiring a NTR of course requires some thought. A normal craft would be deorbited, we'd not do that with an NTP design. Perhaps end of life is defined as a function of dv per full tank of propellant. When nuclear fuel degradation/ablation results in total dv below some value for the nominal craft, it is at "end of life." Then, perhaps we dock a probe to it instead of the cislunar vehicles it might usually carry, and send it off to never return. Not just a reuse, but a retask in this case.

 

 

[StrandedonEarth]

Because the American public screams every time anything nuclear is launched, for fear the rocket will fail and pollute the ocean with highly toxic radioactive materials.

[DerekL1963]

5 hours ago, StrandedonEarth said:

Because the American public screams every time anything nuclear is launched, for fear the rocket will fail and pollute the ocean with highly toxic radioactive materials.

No they don't, not anymore at least.   Over time the noise and activity of the usual protesters have markedly declined.   (There was an especially noticeable drop after 9/11 and the start of Desert Storm II, the cynic in me wants to the said drop was due to the usual protesting types being distracted and busy elsewhere.)