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Why are NERVAs not yet used?


A35K
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1 minute ago, sevenperforce said:

Neutrons have a magnetic moment, so a rotating magnetic field can almost certainly be configured to align them. Deflection by the appropriate nuclei is close enough to reflection to work, I think.

I think we have to be pretty gullible to follow some of these thoroughly untested fantasies (or pretty much anything Zubrin says). 0.026c is a complete fantasy, and the skeptics have major problems with his 1M ISP 20% enriched solution also.

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5 hours ago, sevenperforce said:

Neutrons have a magnetic moment, so a rotating magnetic field can almost certainly be configured to align them. Deflection by the appropriate nuclei is close enough to reflection to work, I think.

No, they don't, or at least it's not really possible to confine them in a magnetic field at the moment. Otherwise Commercial Nuclear Fusion would be many times easier.

Edited by fredinno
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4 minutes ago, sevenperforce said:

Would there be any utility in using a monopropellant with a respectable isp of its own as a propellant for a NTR?

Not sure if you want to send something through pipes inside a nuclear reactor while it's basically exploding. Engineers, conservative bunch as they are, might have an issue with that.

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On 2/7/2016 at 6:13 AM, A35K said:

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?

Like everyone has said, radiation concerns. Even though we have better in-space propulsion systems, NTR's would be good for upper stages on rockets to increase payload capacity, and Lunar tugs to get stuff from LEO to LMO faster than SEP/VASIMR, and more efficient (But slower) than chemical (Although, we can wait a few extra days to get stuff to the Moon).

So we may see them  in use around the 2030s as upper stages and (Possibly) Lunar tugs. Who knows? :)

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16 minutes ago, Kerbart said:

Not sure if you want to send something through pipes inside a nuclear reactor while it's basically exploding. Engineers, conservative bunch as they are, might have an issue with that.

I think anything you pump through pipes inside a nuclear thermal rocket is already going to basically be exploding. In fact, I think that's the point.

I'm just thinking about storability of fuel. Liquid hydrogen is rather unpleasant to deal with, but it has a much better ISP with an NTR than anything else. Just thought perhaps something with an inherent ISP could make up for the higher molecular weight.

What about something REALLY simple, like a Godiva device?

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1 hour ago, Spaceception said:

Like everyone has said, radiation concerns. Even though we have better in-space propulsion systems, NTR's would be good for upper stages on rockets to increase payload capacity, and Lunar tugs to get stuff from LEO to LMO faster than SEP/VASIMR, and more efficient (But slower) than chemical (Although, we can wait a few extra days to get stuff to the Moon).

So we may see them  in use around the 2030s as upper stages and (Possibly) Lunar tugs. Who knows? :)

Radiation concerns adds to the cost, note that testing them might be more problematic than using them. 
An reactor is easier to test as it have no exhaust. 
However its mostly an lack of missions, you don't use LV-N for tiny probes is KSP eiter and that is all we have done outside of LEO after Apollo. 

Lunar tugs, manned mars mission or an heavy probe like mars sample return is relevant missions. 
 

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2 hours ago, magnemoe said:

Radiation concerns adds to the cost, note that testing them might be more problematic than using them. 
An reactor is easier to test as it have no exhaust. 
However its mostly an lack of missions, you don't use LV-N for tiny probes is KSP eiter and that is all we have done outside of LEO after Apollo. 

Lunar tugs, manned mars mission or an heavy probe like mars sample return is relevant missions. 
 

Even the biggest probes would be better off using SLS or HLV+rendevous than NTR.

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3 hours ago, sevenperforce said:

Would there be any utility in using a monopropellant with a respectable isp of its own as a propellant for a NTR?

When looking at ISP as your most important metric: no.

ISP is directly tied to exhaust velocity. Actually, real rocket scientists prefer to express specific impulse in km/s. Now you may know that what we call is heat is movement of molecules. At any given temperature, smaller molecules will move faster than big ones. The reason that hydrogen is the preferred propellant for NTRs is that it's the smallest molecule at our disposal, so it gives the highest exhaust velocity. A major jump in ISP could be expected if it became possible to run the NTR so hot that hydrogen dissociates, that is, the molecule breaks up and comes out as two single hydrogen atoms (1) which would be moving even faster.

If you introduce anything that's heavier than hydrogen, thrust may go up but ISP will inevitably go down.

 

(1) actually the two atoms will be stripped of their electon as well and come out as proton. This doesn't really matter for the discussion at hand, I just mention it to preempt nitpickers.

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5 minutes ago, Laie said:

When looking at ISP as your most important metric: no.

ISP is directly tied to exhaust velocity. Actually, real rocket scientists prefer to express specific impulse in km/s. Now you may know that what we call is heat is movement of molecules. At any given temperature, smaller molecules will move faster than big ones. The reason that hydrogen is the preferred propellant for NTRs is that it's the smallest molecule at our disposal, so it gives the highest exhaust velocity. A major jump in ISP could be expected if it became possible to run the NTR so hot that hydrogen dissociates, that is, the molecule breaks up and comes out as two single hydrogen atoms (1) which would be moving even faster.

If you introduce anything that's heavier than hydrogen, thrust may go up but ISP will inevitably go down.

 

(1) actually the two atoms will be stripped of their electon as well and come out as proton. This doesn't really matter for the discussion at hand, I just mention it to preempt nitpickers.

Eh, but hydrogen has a nasty weight cost associated with keeping it in liquid form. So if mass fraction is important to you (which, let's face it, the only reason specific impulse is important is that mass fraction is ultimately important), using a denser fuel with a lower isp can give you a higher mass fraction if you don't have to keep it on cryo.

Hydrazine has more hydrogen per unit mass than almost anything else that is liquid at room temperature...and it has the helpful quality of exploding on its own to preheat or drive turbopumps.

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No putting something hot through the reactor will not help the Isp. A solid core NTR's reactor is already running as hot as they can physically handle without core elements melting, after taking into account it has to heat propellant. In fact it's trivial to make the reactor core even hotter, it's just you know, it will start to melt.

Pretty much anything you can put it through the reactor, the core can heat it up to that "just before the core melt" temperature. Sending a hot gas through the reactor means you will have to turn the rate of fission way down to stop the core from going into meltdown, so very little work can be extracted out of the reactor.

If you want better dry mass, make the reactor be able to run on water without oxidising. Then you burn hydrogen for high Isp on the trip out, and burn water on the way back. In fact depending on where you going you might be able to fill up the water tank at your destination instead of bringing it with you.

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1 hour ago, sevenperforce said:

Eh, but hydrogen has a nasty weight cost associated with keeping it in liquid form.

Hydrazine has more hydrogen per unit mass than almost anything else that is liquid at room temperature...and it has the helpful quality of exploding on its own to preheat or drive turbopumps.

The heating properties of hydrazine don't convey any benefit as the limiting factor is temperature -- you don't want to melt the reactor (ninja'd by Temstar who explained it well). As a heat source, nuclear beats chemical by a lot.

Injecting oxygen after the stuff comes out of the heater *will* let you increase exhaust temp, and would greatly increase thrust, but would still reduce ISP.

To give a few ballpark figures: the NTRs that were built and tested on the ground had ISPs on the order of 800 seconds. With modern-day materials, 950-1000s seems doable if you use hydrogen. LOX injection would reduce this to 650; using Methane as a propellant would also deliver about 600s (just mentioning it because it is a number I happen to know; I'm aware that Methane is cryogenic). Water propellant would yield ISPs under 400 (less than Hydrolox which also creates water, because a combustion chamber can run considerably hotter than a NTR).

I just see that Hydrazine can be turned into a mixture of Hydrogen and Nitrogen gas; that reaction would be endothermic, but who cares? The NTR is a bottomless source of heat. If you can rig it so that H&N don't recombine but come out as their elemental gases, the compound ISP would probably be in the 600-700s range as well. Not bad for a storable propellant, not bad at all.

Edited by Laie
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4 hours ago, sevenperforce said:

Eh, but hydrogen has a nasty weight cost associated with keeping it in liquid form.

This problem is exacerbated in most current mission designs because we need to launch the H2 tanks on long thin rockets, resulting in long thin tanks with high surface area to volume ratios, and often a collection of them. Big spherical tanks would not be hit as hard by the insulation mass penalty.

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The reactions where hydrazine decomposes into nitrogen and hydrogen and ammonia are highly exothermic; the reactions where it decomposes into nothing but nitrogen and hydrogen are still highly exothermic, only slightly less so. Once decomposed, they won't recombine. Too hard to break that nitrogen triple bond.

Now, if you used a tank of anhydrous ammonia and a tank of hydrazine at the right ratio, with the proper catalyst bed you could make the reaction entirely endothermic. There are a few ways you could use that...

12 hours ago, DBowman said:

This problem is exacerbated in most current mission designs because we need to launch the H2 tanks on long thin rockets, resulting in long thin tanks with high surface area to volume ratios, and often a collection of them. Big spherical tanks would not be hit as hard by the insulation mass penalty.

How hard would it be to set up a liquid hydrogen electrolysis station in LEO?

Edited by sevenperforce
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16 hours ago, fredinno said:

Even the biggest probes would be better off using SLS or HLV+rendevous than NTR.

My thought was to replace the centaur upper stage / injection stage with NTR to reduce upper stage weight. 
But yes its cheaper to use an larger rocket than develop an new upper stage for one mission. 

Edited by magnemoe
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A combination between Ion Propulsion Tech and Nuclear Propulsion is a viable as well. Especially after leaving orbit, you want to pass the van Allen belt as soon as possible due to the radiation. Ion tech despite having HIgh efficiency would require a very long spiral out of earth orbit before it can head to Mars. Therefore it would be a good to use nuclear propulsion to boost toward Mars and switch to ion after escaping earth gravity.

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12 minutes ago, FreeThinker said:

Therefore it would be a good to use nuclear propulsion to boost toward Mars and switch to ion after escaping earth gravity.

Not really, it doesn't make much sense to use a (incredibly) low thrust ion engine to boost a (very) heavy nuclear engine - nor does it make much sense to toss the nuke stage away after so little use.    You could use the nuke as a tug (but spend a lot of d/v getting it back down to LEO), or just go ahead and use it to go to Mars.

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12 minutes ago, DerekL1963 said:

Not really, it doesn't make much sense to use a (incredibly) low thrust ion engine to boost a (very) heavy nuclear engine - nor does it make much sense to toss the nuke stage away after so little use.    You could use the nuke as a tug (but spend a lot of d/v getting it back down to LEO), or just go ahead and use it to go to Mars.

Or just use Nukes for precious or time-sensitive cargo (crew) and ION for everything else.

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1 hour ago, DerekL1963 said:

Not really, it doesn't make much sense to use a (incredibly) low thrust ion engine to boost a (very) heavy nuclear engine - nor does it make much sense to toss the nuke stage away after so little use.    You could use the nuke as a tug (but spend a lot of d/v getting it back down to LEO), or just go ahead and use it to go to Mars.

Well, you do not necessarily have to toss the heavy reactor away, it would be most economically to use it in bimoddel mode, using the reactor initially for lauch and then use it for electric power productions to power all the electric engine, which are going to need a lot of power.

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Just now, FreeThinker said:

Well, you do not necessarily have to toss the heavy reactor away, it would be most economically to use it in bimoddel mode

Also you could use an NTR to boost almost to escape and stage it. The NTR would naturally fall back, aero break, circularize, refuel, boost something else, and repeat. The boosted 'payload' could be a lower thrust vehicle that would complete it's transfer burn.

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1 minute ago, DBowman said:

Also you could use an NTR to boost almost to escape and stage it. The NTR would naturally fall back, aero break, circularize, refuel, boost something else, and repeat. The boosted 'payload' could be a lower thrust vehicle that would complete it's transfer burn.

I hope you brought shielding for aerobrake.

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32 minutes ago, FreeThinker said:

Well, you do not necessarily have to toss the heavy reactor away, it would be most economically to use it in bimoddel mode, using the reactor initially for lauch and then use it for electric power productions to power all the electric engine, which are going to need a lot of power.

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

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