NTR rockets

[Hop_David]

Recently Obama has advocated nuclear power plants as an energy source with no carbon footprint.

I am hoping the winds of public opinion are changing. If anti-nuclear hysteria subsides, pursuing notions like NTR rockets becomes possible.

Googling NERVA I found some people who thought NERVA was ready, all we had to do was bolt them to the Apollo rockets.

Then others seem to believe the project was discontinued because of technical problems and cost over runs.

A high thrust and high ISP rocket would certainly be a game changer. It's worth pursuing in my opinion.

[swampyankee]

Yes, and explain to all those people downwind why everything glowed in the dark. Part of the reason it was discontinued because of its radioactive exhaust. Is Ajo, Arizona downwind of Vandenburgh?

[Siguy]

There is a huge difference between supporting nuclear power plants for powering the grid and supporting nuclear thermal propulsion. For one, there hasn't been enough testing to ensure its reliability, or overcome technical issues. Second, the exhaust is highly radioactive and polluting and there has been little research into preventing this, which further contrasts from relatively clean nuclear reactors.

Also, the ISP may not be all as high as hype would have you believe; I think NERVA was only about 900, which is twice that of LOX/LH2 but there's still the added mass of the reactor.

However, there is still nothing preventing nuclear electric propulsion (nuclear reactors in space) other than financial problems. This is still the same contrast though.

[Ara Pacis]

Yes, and explain to all those people downwind why everything glowed in the dark. Part of the reason it was discontinued because of its radioactive exhaust. Is Ajo, Arizona downwind of Vandenburgh?

Then use it immediately upwind of an ocean. Not that it matters, the NTRs considered for actual spaceflight would have been used on an upper stage, not at ground level.

[Antice]

There are some concepts out there that would do away with having the exhaust full of radionuclides and activation products. they would need quite a lot of funding and R&D time before they are ready for use tho.

[swampyankee]

Then use it immediately upwind of an ocean. Not that it matters, the NTRs considered for actual spaceflight would have been used on an upper stage, not at ground level.

I'm old enough to remember atmospheric nuclear testing. The reason it was stopped was because there was a significant (iirc, close to 100%) increase in the background radiation levels throughout the world, and because there were some rather significant cases of radiation injuries to people downwind. I would not categorize opposition to NTR use in the atmosphere as "hysteria".

[Elukka]

I wasn't aware any NTR designs besides the open gas core Nexus boosters have radioactive exhaust.
You'd launch one assuming it will explode and the winds will be as bad as possible and everything will go wrong. It most likely won't, but if it does the results won't be catastrophic.

[Siguy]

I wasn't aware any NTR designs besides the open gas core Nexus boosters have radioactive exhaust.

There's almost no way to prevent core erosion when you have your reaction fluid flowing at such enormous speeds and pressures.

[Elukka]

There seem to be conflicting accounts on whether Project Timberwind solved or didn't solve the erosion issue and whether it's solvable at all to the point of not being a huge thing. Anyone have an authoritative source?

[scottlowther]

There's almost no way to prevent core erosion when you have your reaction fluid flowing at such enormous speeds and pressures.

Yeah, and? How is this even remotely relevant to the use of NTRs for space exploration? You could filthy up the spacelanes between Earth and Mars all you'd like, and nobody'd ever be able to tell. Space is *vast,* and the exhaust would be moving out and expanding at high speeds. Radiaton from the filthiest NTR would very rapidly be washed out by the existing radiation in space.


Since NTRs are *not* intended for use in the atmosphere (nor would hey be of much value in the air), it is my contention that opposition to them *is* hysterical, and based on willfull ignorance.

[Sharlos]

I think most opposition comes from those who oppose it being used in our atmosphere and then those who oppose anything with the word nuclear in it.

[Elukka]

We were talking about atmospheric use so core erosion is certainly relevant.
It's not really a factor how dirty something is in space as you said.

[Antice]

the nuclear lightbulb concept does away with the problem with there being fission products in the exhaust stream.

True. it needs quite a lot more work before it is can be proven viable but it can be very worthwhile. stuff like this isnt funded due to irrational fears of anything nuclear. Just because Orion and the early NTR attempts are failing the fallout test does not mean that there arent more palatable solutions out there.

[Elukka]

A gas core reactor should make the rocket vastly more efficient, too. I'm guessing we couldn't build them any time soon, though, but I really wouldn't mind a design study to see how much technological and engineering development would be required to build one.

[IsaacKuo]

the nuclear lightbulb concept does away with the problem with there being fission products in the exhaust stream.

True. it needs quite a lot more work before it is can be proven viable but it can be very worthwhile. stuff like this isnt funded due to irrational fears of anything nuclear. Just because Orion and the early NTR attempts are failing the fallout test does not mean that there arent more palatable solutions out there.

The problem with the nuclear lightbulb concept is that it costs too much to develop compared to nuclear electric, and it doesn't seem to offer any practical advantage over nuclear electric. It might offer better thrust, but not by enough to make up for the inferior exhaust velocity and heavier fuel tanks (it would need to use hydrogen fuel, whereas nuclear electric could use far denser fuels which also happen to be space storable).

Solid core NTR offers too little benefit over chemical rockets to be worth the costs, and developing in-space refueling techniques more or less eliminate that potential benefit altogether.

[Elukka]

There seem to be endless plausible concepts for space drives, and the NTR quite possibly loses out to many of them. However, it seems most people neglect launchers. NTRs of various types are amongst the only engines that are possibly capable of reaching orbit. I can't think of a way for nuclear electric propulsion to come anywhere near the required thrust-to-weight ratios.

[Extracelestial]

Yes, and explain to all those people downwind why everything glowed in the dark. Part of the reason it was discontinued because of its radioactive exhaust. Is Ajo, Arizona downwind of Vandenburgh?

Hello Swampyankee,

you're mistaken on this one. The NERVA engine and its kin, didn't have a high radioactive output. The fuel acted as coolant and didn't come in direct contact with the fissionable material but with the graphite of the moderator itself. Not even the mad scientists of the sixties wanted to flood Florida with radioactive material at every launch ;-)

The main reason for cancelling, besides political pressure, was the utilisation of hydrogen as fuel. Theoretically any liquid, water, milk or beer might have been used, but the specific impulse is highest when the fuel's molecular weight is lowest. Handling liquid hydrogen is obscenely tricky and most engineers felt that one complication, either nuclear or liquid hydrogen, is enough.

There was however a project called Orion that would have utilised the detonation of a low yield warhead, Hiroshima class that is, outside of the spacecraft, to propel it forward. These people planned to have some thousand tons of payload to the moon at a time when NASA strained every muscle to get 60 metric tons there! Now these patently mad scientists (!) estimated that even at a monthly launch rate, this Orion craft would add about 2% to the already spilled radioactivity of all tests combined. This was due to the unusual trajectory the craft would have taken - straight up instead of a curved path.
The science of this concept seemed to be sound but they figured that it probably could not be arranged to put a crew atop a pile of warheads.

So, radioactive output of a NTR isn't such a problem as many people do believe. But for the peace of mind of common folk I'd gather that they should be used in a vacuum.

Ex

P.S.

wasn't there an announcement of the RKS Energija to start the development of a nuclear rocket? I read it on Spacedaily I believe but can't find the areticle anymore.

[scottlowther]

We were talking about atmospheric use...

Since NERVA was never meant for atmospheric use, you might as well worry about it's use under water or in your local kindergarten. In other words, it's a strawman arguement.

[Elukka]

Not NERVA. We were talking about using various kinds of nuclear thermal rocket designs in the atmosphere. It's obvious NERVA wouldn't work for launching anything since that is not what it was designed for. (well, except as an upper stage)

[scottlowther]

Not NERVA. We were talking about using various kinds of nuclear thermal rocket designs in the atmosphere.

I suggest re-reading the thread, starting with the very first post which explicitly mentions NERVA, and the second which uses the hyperbolic "glowing in the dark" downwind of Vandenberg.

Use of NTRs within the atmosphere has been a vanishingly small fraction of the total work done on NTRs with the exception of DUMBO and a few very vague, preliminary and handwavy ideas regarding Ehricke's Helios, the gas-core boosted Nexus and a few others, you really have to go all the way back to 1947 to find any serious suggestion for using NTRs for ground-launched boosters.



From here: http://up-ship.com/blog/?p=5550

[Hop_David]

There is a huge difference between supporting nuclear power plants for powering the grid and supporting nuclear thermal propulsion. For one, there hasn't been enough testing to ensure its reliability, or overcome technical issues. Second, the exhaust is highly radioactive and polluting and there has been little research into preventing this, which further contrasts from relatively clean nuclear reactors.

I have to admit I didn't know the exhaust was highly radiactive.

Also, the ISP may not be all as high as hype would have you believe; I think NERVA was only about 900, which is twice that of LOX/LH2 but there's still the added mass of the reactor.

Only 900? Exhaust velocity is the part of the exponent in the rocket equation:

Mass propellent/Mass payload = e^(delta V/V exhaust) -1

So doubling the V exhaust can make a huge difference.

However, there is still nothing preventing nuclear electric propulsion (nuclear reactors in space) other than financial problems. This is still the same contrast though.

Electric propulsion's low thrust makes it harder to exploit the Oberth effect. This can substantially boost the delta V. Something with substantial thrust as well as high ISP is different than ion engines.

However, if the exhaust is highly radiactive, I wouldn't want to do a burn deep in earth's gravity well. So perhaps it wouldn't be able to safely exploit the Oberth effect as well as I had thought.

[Hop_David]

I suggest re-reading the thread, starting with the very first post which explicitly mentions NERVA, and the second which uses the hyperbolic "glowing in the dark" downwind of Vandenberg.

While I had mentioned NERVA in passing, it wasn't my intention to limit the thread to NERVA alone. I'm hoping to reduce my vast ignorance regarding NTRs.

If anyone wants to discuss Timberwind or other schemes to reduce core erosion, I wouldn't regard it off topic.

[Antice]

the only concepts where the exhaust would contain anything radioactive is those where the working fluid is piped trough the reactor core.
it is quite possible to move the heating of the hydrogen to a secondary heat exchanger instead at the cost of some engine complexity and efficiency.
molten salt reactors are the first candidate that comes to mind here.
even if you cant solve the erosion problems 100% this will cause wear on the heat exchanger instead of eroding the primary fuel blanket.

as for nuclear lightbulb. the gaseous nuclear fuel is trapped behind a wall of quartz. only the heat is allowed to escape into the propellant.

this is at least 2 concepts that does away with the radioactive exhaust problem. so I dare say it is a far from unsolvable problem like some would like to believe.

Chamber erosion from high temp plasmas is going to be an issue no matter what kind of high ISP thruster you use. this is even a problem for ION thrusters and there will be a service requirement for changing these parts in any reusable design.
for a single use design it is enough to design it to last for the required timeframe.

[mantiss]

Since NTRs are *not* intended for use in the atmosphere (nor would hey be of much value in the air), it is my contention that opposition to them *is* hysterical, and based on willfull ignorance.

I'm old enough to remember Cosmos 954, and that was just a little wake up call, I wouldn't be comfortable knowing larger nuclear engines would be there just sitting waiting for the odd cosmic debris (Iridium 33 vs Cosmos 2251 anyone?) especially since the situation with space junk is increasing FAST.

I don't think I'm paranoid, but this has the potential to be just as bad for space exploration as a shuttle explosion if you spill a few kg of radioactive material say across the midwest or across Europe.

[Hop_David]

Chamber erosion from high temp plasmas is going to be an issue no matter what kind of high ISP thruster you use.

So an NTR would have a short life span?

this is even a problem for ION thrusters and there will be a service requirement for changing these parts in any reusable design.
for a single use design it is enough to design it to last for the required timeframe.

I had thought ion rockets were long lived.

So semi-reusables would be possible if we could change the propellent chamber and nozzle?

[IsaacKuo]

So an NTR would have a short life span?

We don't have enough experience to know for sure. Hydrogen is pretty reactive stuff, of course.

I had thought ion rockets were long lived.

Older electrostatic ion thrusters were not long lived. I remember reading about electric spacecraft concepts which simply used multiple thrusters in series. After one thruster wore out the next thruster would be used. Obviously this is not the best thing for thrust/mass ratio, and ion thrusters had low thrust to begin with.

Newer ion thrusters have much better lifetimes, which has made interplanetary missions like Hayabusa and Dawn practical.

[Antice]

not short. just not as long as one may think. ION thrusters do have a fairly long lifespan. but it is very clearly limited. the thing is. when you have to factor in wear and tear margins you trade away some performance. the longer the thruster has to be able to operate the more of a performance issue it becomes.
Even tho it takes a long time to wear out an ION thruster it has not delivered all that much DV compared to what we routinely get out of chemical thrusters in one go. the reason is that ION thrusters are pathetic in the thrust departement.

A fully reusable nuclear powered interplanetary ferry is possible if it is designed to have easy replacement of both nuclear fuel and thruster assemblies. the wear issue is ofc quite bigger on a NTP than any other thruster due to higher chamber pressures and higher plasma temps than ordinary chemical rockets. but they work on the same principles.

The bonus tho is that NTP does not depend on using pure hydrogen as fuel. it can use almost any gas or even chemically inert water as a propellant. there is an ISP penalty for using water tho. energy is lost during molecular disasotiation. the final ISP may not be much higher than LH2/LOX, but it is infinately more stable and storable on orbit.

[IsaacKuo]

Even tho it takes a long time to wear out an ION thruster it has not delivered all that much DV compared to what we routinely get out of chemical thrusters in one go.

No, it's a lot more delta-v than what's available with chemical thrusters. If this weren't so, then there would be no reason to use electric thrusters for Hayabusa and Dawn.

Anyway, electrode-less thrusters would not have this electrode erosion issue. VASIMR is an example of such an electrode-less thruster. Generally, electrode-less electric thrusters use microwaves to transfer energy from the power system to the propellant.

The bonus tho is that NTP does not depend on using pure hydrogen as fuel. it can use almost any gas or even chemically inert water as a propellant. there is an ISP penalty for using water tho. energy is lost during molecular disasotiation. the final ISP may not be much higher than LH2/LOX, but it is infinately more stable and storable on orbit.

No. The Isp is much lower than LH2/LOX, and indeed much lower than space storable chemical rocket thrusters.

The problem is that the core temperature is limited by the melting point of the core. If you don't use hydrogen propellant, you get much worse performance than chemical rocket thrusters--where the combustion temperature can (and does) greatly exceed the melting point of the chamber walls (these are actively cooled by the propellant).

If you go with a nuclear lightbulb concept, then you can get better temperatures--but at the great expense and effort of actively cooling the lightbulb walls. This involves a lot of mass and complexity, due to the cooling system and radiators, and generally limits you to low acceleration capabilities. Yes, these accelerations are perhaps an order of magnitude better than nuclear electric propulsion--but not by enough to matter. The acceleration is already too low to get a good benefit from the Oberth effect, and interplanetary travel times are long enough that nuclear electric's superior Isp capability wins out in the end.