Why are NERVAs not yet used?

[sevenperforce]

1 hour ago, DerekL1963 said:

More complexity for very little gain...  especially since the weight is more than just the reactor, you also have to consider the fuel tanks.

Multimodal nuclear engines are a pretty good deal, actually. The coolant loops you already need in order to run safely as a nuclear thermal rocket can be repurposed to provide electricity without much additional weight. 

[DerekL1963]

18 minutes ago, sevenperforce said:

1 hour ago, DerekL1963 said:

More complexity for very little gain...  especially since the weight is more than just the reactor, you also have to consider the fuel tanks.

Multimodal nuclear engines are a pretty good deal, actually. The coolant loops you already need in order to run safely as a nuclear thermal rocket can be repurposed to provide electricity without much additional weight. 

Which still doesn't correct the basic fault in his design - the NTR has a lot of weight that doesn't contribute to making electricity for the ion engine.  Saddling an already performance limited propulsion system with dead weight doesn't strike me as a good idea.

It's just not a good idea to take two different propulsion systems that are good at two very different things and try to use them on one vehicle.

[sevenperforce]

2 hours ago, DerekL1963 said:

Which still doesn't correct the basic fault in his design - the NTR has a lot of weight that doesn't contribute to making electricity for the ion engine.  Saddling an already performance limited propulsion system with dead weight doesn't strike me as a good idea.

It's just not a good idea to take two different propulsion systems that are good at two very different things and try to use them on one vehicle.

What would happen if you injected something like hydrazine or ammonia into the propellant stream of a nuclear-reactor-powered ion engine? For that matter, what if you injected highly-compressed air into the propellant stream? Are there any ion thrusters which can even operate in anything other than vacuum?

[shynung]

47 minutes ago, sevenperforce said:

What would happen if you injected something like hydrazine or ammonia into the propellant stream of a nuclear-reactor-powered ion engine?

Hydrazine decomposes into water and oxygen gas. Hot oxygen gas is hideously corrosive. Not exactly something I'd want to get close to a reactor core.

Mixed up hydrazine and HTP. Sorry.

Edited February 25, 2016 by shynung

[sevenperforce]

20 minutes ago, shynung said:

Hydrazine decomposes into water and oxygen gas. Hot oxygen gas is hideously corrosive. Not exactly something I'd want to get close to a reactor core.

There is neither oxygen nor water in hydrazine. Perhaps you were thinking of hydrogen peroxide?

And it wouldn't be getting close to a reactor core anyway. I asked what would happen if hydrazine (which, BTW, decomposes into diatomic hydrogen and diatomic nitrogen) was injected into the exhaust of an ion thruster that happened to be powered by a (separate) nuclear reactor.

[shynung]

1 minute ago, sevenperforce said:

There is neither oxygen nor water in hydrazine. Perhaps you were thinking of hydrogen peroxide?

And it wouldn't be getting close to a reactor core anyway. I asked what would happen if hydrazine (which, BTW, decomposes into diatomic hydrogen and diatomic nitrogen) was injected into the exhaust of an ion thruster that happened to be powered by a (separate) nuclear reactor.

Edited the post. I was mistaken.

Also, regarding your question, I think the hydrogen and nitrogen molecules would be pulled by the ion thruster's exhaust, much like an air-augmented rocket. You'd trade specific impulse for thrust, while giving a bit more energy into the engine from hydrazine decomposition.

How much of this would happen, though, I have no idea.

[DBowman]

5 hours ago, fredinno said:

I hope you brought shielding for aerobrake

A light weight reactor (therefore nuclear potent and fragile) barreling toward a precision aero brake at super Luna return velocities - what could possibly go wrong...

[fredinno]

4 hours ago, DerekL1963 said:

Which still doesn't correct the basic fault in his design - the NTR has a lot of weight that doesn't contribute to making electricity for the ion engine.  Saddling an already performance limited propulsion system with dead weight doesn't strike me as a good idea.

It's just not a good idea to take two different propulsion systems that are good at two very different things and try to use them on one vehicle.

Bimodial Nuclear engines can also produce electricity for the ION drives.

[DerekL1963]

29 minutes ago, fredinno said:

4 hours ago, DerekL1963 said:

Which still doesn't correct the basic fault in his design - the NTR has a lot of weight that doesn't contribute to making electricity for the ion engine.  Saddling an already performance limited propulsion system with dead weight doesn't strike me as a good idea.

It's just not a good idea to take two different propulsion systems that are good at two very different things and try to use them on one vehicle.

Bimodial Nuclear engines can also produce electricity for the ION drives.

*sigh*  Which part of that's a fault in the design did you miss?  That electricity comes with a huge penalty - the weight of the fuel, fuel tanks, structure, piping and turbopumps, and nozzle.  All dead weight being pushed around by a very low thrust engine that will already be challenged to push more than a minimal payload anyhow.
 

2 hours ago, sevenperforce said:

What would happen if you injected something like hydrazine or ammonia into the propellant stream of a nuclear-reactor-powered ion engine? For that matter, what if you injected highly-compressed air into the propellant stream?

I suspect you'd get a little extra thrust from the nozzle used to expel the added propellant and that's about it.   NAICT the ion stream doesn't have enough energy to usefully heat, decompose, or otherwise react or interact with much in the way of mass.  It is energetic and moving fast, but there's not very much of it in the local exhaust stream at any one time - a few micrograms at the very most I'd think.
 

2 hours ago, sevenperforce said:

Are there any ion thrusters which can even operate in anything other than vacuum?

Nope.

[sevenperforce]

12 hours ago, DerekL1963 said:

I suspect you'd get a little extra thrust from the nozzle used to expel the added propellant and that's about it.   NAICT the ion stream doesn't have enough energy to usefully heat, decompose, or otherwise react or interact with much in the way of mass.  It is energetic and moving fast, but there's not very much of it in the local exhaust stream at any one time - a few micrograms at the very most I'd think.

That's a shame. Someone should design an electrostatically-neutral ion thruster with a propellant that could fire in-atmosphere and could readily interact with diatomic nitrogen, so that the nitrogen could absorb at least some of its kinetic energy.

[fredinno]

33 minutes ago, sevenperforce said:

That's a shame. Someone should design an electrostatically-neutral ion thruster with a propellant that could fire in-atmosphere and could readily interact with diatomic nitrogen, so that the nitrogen could absorb at least some of its kinetic energy.

I would imagine you'd need beamed power for that, for starters.

[sevenperforce]

2 minutes ago, fredinno said:

I would imagine you'd need beamed power for that, for starters.

I was going to go with nuclear power. Power efficiency at high thrust wouldn't be quite as good as with an NTR, but it's a lot safer, and could be used in-atmosphere without giving anyone a hernia.

[Gkirmathal]

Interesting thread. A few months ago I was looking into the Russian RD-0140 project, which ended around 1994. But I found out, research on the fuel elements did continue till the end of the 90'ties.

PDF with short surmised research:

Russian Nuclear Rocket Engine Design for Mars Exploration, by Vadim Zakirov, Vladimir Pavshook

 

One of the more striking things was that Russian scientist tested out a production reactor fuel element, with a 'twisted ribbon design' which increased ISP significantly over more conventional fuel element designs like used in NERVE/DUMBO. This was due to 'surface' contact ratio begin higher compared to NERVA fuel elements.

They tested this in an ingenious way in my opinion: via induction, to simulatia reactor heat. Why test a full reactor when one can simulate the conditions this way. I found it rather ingenious.

Check out the pdf it is a read worthwhile.

[JebKeb]

Thunderf00t told me that neutrons decay releasing lots of energy. Could you use that as a propellant?

who cares about the fact we can't contain it. so what? just produce them in the right place

Also, don't electrons have mass? If we had a generator we could throw electrons out the back.

[fredinno]

14 hours ago, JebKeb said:

Thunderf00t told me that neutrons decay releasing lots of energy. Could you use that as a propellant?

who cares about the fact we can't contain it. so what? just produce them in the right place

Also, don't electrons have mass? If we had a generator we could throw electrons out the back.

You probably could, https://en.wikipedia.org/wiki/Fission_sail

But it'd not even be practical due to the fact its almost impossible to contain Neutronium.