Author Topic: VASIMR as Plasma core fission rocket  (Read 3632 times)

Online Robotbeat

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VASIMR as Plasma core fission rocket
« on: 04/18/2017 11:36 PM »
VASMIR originally was a fusion rocket concept but never had a real hope of getting ignition. But what about as a plasma core fission rocket? Line the thing with neutron reflectors... It could be done, if done at a big enough scale. I wonder how high of Isp it could achieve if no external radiators were used and the magnets were cooked with liquid hydrogen propellant.
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Online Elmar Moelzer

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Re: VASIMR as Plasma core fission rocket
« Reply #1 on: 04/19/2017 12:02 AM »
VASMIR originally was a fusion rocket concept but never had a real hope of getting ignition. But what about as a plasma core fission rocket? Line the thing with neutron reflectors... It could be done, if done at a big enough scale. I wonder how high of Isp it could achieve if no external radiators were used and the magnets were cooked with liquid hydrogen propellant.
Hmm, do you mean something like a gas core NTR?

Online Robotbeat

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Re: VASIMR as Plasma core fission rocket
« Reply #2 on: 04/19/2017 12:12 AM »
Sorta, except hotter. Has to be a plasma for the magnetic nozzle to work.
« Last Edit: 04/19/2017 12:25 AM by Robotbeat »
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Offline Stan-1967

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Re: VASIMR as Plasma core fission rocket
« Reply #3 on: 04/19/2017 12:46 AM »
Sorta, except hotter. Has to be a plasma for the magnetic nozzle to work.

just to be clear...

1.  Are you proposing to (a.) expel the fuel in plasma state that sustains the fission out the magnetic nozzle, or (b.) proposing to expel another working fluid/propellant, heated by the fission plasma, out the nozzle?

I'm assuming you are proposing (b).   So you can skip the bulk of the power demands to heat the plasma via RF heating, but are still stuck with the power demands of powering & cooling the superconductors & pumps.  Can you extract power (via MHD) from the exiting plasma to feed power back into the system?   I am also assuming your propellant ( Argon?) will be used for cooling?

Sounds like maybe you want to combine  Zubrin's NSWR with VASIMIR's nozzle,  or hopefully "fix" the safety problem inherent to Zubrins idea.


Online Robotbeat

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Re: VASIMR as Plasma core fission rocket
« Reply #4 on: 04/19/2017 12:56 AM »
Both. Fission products and any extra propellant needed to keep the chamber from melting and keep the coils cool.
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Offline Stan-1967

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Re: VASIMR as Plasma core fission rocket
« Reply #5 on: 04/19/2017 01:31 AM »
Both. Fission products and any extra propellant needed to keep the chamber from melting and keep the coils cool.

Well if it is going to be powered by fission, you need to maintain a very controlled neutron economy, so I don't think this favors allowing the mixing your hydrogen/propellant into the fission core. The core also need a effective reflector to keep the neutrons contained.  How then do the thermodynamics & heat transfer work out to heat the propellant to a plasma?  The transient time in the chamber will be too short for any convective or radiative solution to work. ( the downfall of the GCR scheme)  That seems to leave isolating the fission core, & using it to power the RF couplers if you can extract the power.

Now we are back the VASIMIR needing a magical power source.


Online Robotbeat

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Re: VASIMR as Plasma core fission rocket
« Reply #6 on: 04/19/2017 01:39 AM »
The plasma would be isolated by magnetic pressure. It'd be heated by the process of fission. Fission doesn't need heat to work, in fact it works better if cold as it'd be denser. Residence time would be controlled by controlling the shape of the magnetic bottle/nozzle.

It's possible it'd have to be absurdly huge to work in order to reach criticality with a low density plasma. But I'm cool with that.
« Last Edit: 04/19/2017 01:41 AM by Robotbeat »
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Online Robotbeat

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Re: VASIMR as Plasma core fission rocket
« Reply #7 on: 04/19/2017 01:43 AM »
Also, it's possible you could mix in the hydrogen with the fission products near the nozzle throat so as not to mess up the neutron economy. Ideally, you wouldn't need to add hydrogen, but I'm betting there's going to be a hell of a lot of heat leaking into the VASIMR structure, so you'll need to carry that away with hydrogen.
« Last Edit: 04/19/2017 01:44 AM by Robotbeat »
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Offline Hanelyp

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Re: VASIMR as Plasma core fission rocket
« Reply #8 on: 04/19/2017 02:30 AM »
I'm wondering if there's a magnetic field configuration that would contain the fissile plasma as it radiatively heated a propellant plasma that then escapes through a magnetic nozzle.  Maybe a field-reversed configuration with the fuel in the closed inner field lines as propellant travels the open field lines around it.

Online Elmar Moelzer

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Re: VASIMR as Plasma core fission rocket
« Reply #9 on: 04/19/2017 02:49 AM »
So you would create something that is similar to a gas core reactor, but with even hotter fuel so that the gas becomes a plasma. I don't think that this has ever been done before. I would probably start with research on gas core reactors and then go on from there.

Online Asteroza

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Re: VASIMR as Plasma core fission rocket
« Reply #10 on: 04/19/2017 09:30 AM »
You would probably need to benchmark this against a "conventional" NTER + VASIMR accelerator/nozzle where the turbo-inductor heats to/near-to plasma conditions in the last stages of the heater assembly (turbo-inductor functions as an inductive heater which may be able to replace the VASIMR helicon functionally).

See ESA NTER study, NTER = NTR + turbo-inductor

http://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-1107-LS-NTER.pdf

https://info.aiaa.org/tac/SMG/STTC/White%20Papers/THE%20NUCLEAR%20THERMAL%20ELECTRIC%20ROCKET%20ENGINE%20AIAASTTC.pdf

turbo-inductor is before the throat so can still squeeze the VASIMR accelerator/nozzle stack in the bottom...


Offline Rei

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Re: VASIMR as Plasma core fission rocket
« Reply #11 on: 04/19/2017 12:15 PM »
Well if it is going to be powered by fission, you need to maintain a very controlled neutron economy, so I don't think this favors allowing the mixing your hydrogen/propellant into the fission core. The core also need a effective reflector to keep the neutrons contained.  How then do the thermodynamics & heat transfer work out to heat the propellant to a plasma?

I don't know about this idea in general, but hydrogen (1H) is the most effective neutron moderator (elastic scattering cross section times energy drop per scattering) of all isotopes.  Now, its rate of (n,gamma) reactions is higher than would be preferred, which if you need an improved neutron economy is remedied by using deuterium as a moderator, which is excellent in both moderating abillity (although the cross section is lower than 1H and the mass doubled) an and low rate of neutron absorption.  So if you're going for a slow reactor, hydrogen in the fuel is not only possible, but desired. Now if you want a fast reactor, no, you don't want hydrogen anywhere near it.

Another possibility is 4He.  The cross section is quite low, and the mass 4x that of 1H - but the (n, gamma) rate is zero.  Not "near zero" - literally zero; it will never capture a neutron.

I'm not sure what you're referring to when you talk about neutron reflectors in the same context as heat transfer. Neutron reflectors aren't mirrors, it's not about reflecting electromagnetic radiation (visible, IR, etc, aka how radiative cooling works).  They don't really "reflect" neutrons; they just have low rates of neutron capture, and so a "random walk" leads to neutrons frequently reentering the core (it is technically possible to make an actual neutron mirror, but it requires *extremely* cold neutrons). A moderator can double as a reflector for a fast reactor; otherwise you want your moderator made with heavy elements to minimize the amount of neutron energy transferred per interaction.  It's also popular to make reflectors out of multipliers - that is, isotopes with significant (n, 2n), (n, 3n), etc reactions. The only light element that has this to a significant degree is beryllium, which also lucks into having a rather low (n, 2n) cross section.  Most heavy elements have meaningful (n, 2n) cross sections to varying degrees, and often (n, 3n), (n, 4n), etc of relevance. But they tend to require faster neutrons for it than beryllium.

The big issue with cooling a fissioning plasma is not the plasma itself, but any moderators / reflector involved. All of those elastic and inelastic interactions transfer the energy from the neutrons into the moderator / reflector, and you need to get rid of it.  Aka, huge, heavy radiators.This is where your neutron economy comes into play; the higher the percentage of your neutrons that contribute to fission, the fewer neutrons you need to maintain a given level of output, and thus the amount of their energy that goes into your reflector / moderator.  Now, EM reflection does factor into it to, to the degree that the more of the plasma's radiant energy that you reflect back into it, the less fission output you need to maintain the core temperature. The main challenge is that your mirror becomes less reflective over time in exposure to the core. And yes, everyone wants to research ideas for things like "liquid mirrors" or whatnot that  would maintain reflectivitity, but at present, it's an issue of concern.

Back to the idea in general: what's the advantage of this vs. say a dusty or misty core fission fragment rocket?
« Last Edit: 04/19/2017 12:24 PM by Rei »

Online Robotbeat

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Re: VASIMR as Plasma core fission rocket
« Reply #12 on: 04/19/2017 01:10 PM »
Advantage is higher power and thrust than those concepts, I hope.
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Offline Rei

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Re: VASIMR as Plasma core fission rocket
« Reply #13 on: 04/19/2017 03:06 PM »
Advantage is higher power and thrust than those concepts, I hope.

Power is limited by your ability to radiate the heat from your moderator / reflector more than anything else in both designs. Both eject plasma from a magnetic nozzle - the big difference is that a standard plasma core is Maxwellian (thermalized), while in a fission fragment rocket they're non-Maxwellian (and relativistic). Much higher exhaust velocity in the latter, while the core temperature is lower (dusty = solid, misty = liquid), and thus the radiative heat load is far lower.
« Last Edit: 04/19/2017 03:20 PM by Rei »

Online Robotbeat

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Re: VASIMR as Plasma core fission rocket
« Reply #14 on: 04/20/2017 01:06 AM »
Nah, you can also dump heat into liquid hydrogen. Not just radiating. Of course, that limits your effective Isp, but if you can minimize the heat leakage, it might not be so bad.
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Offline Rei

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Re: VASIMR as Plasma core fission rocket
« Reply #15 on: 04/20/2017 11:29 AM »
Nah, you can also dump heat into liquid hydrogen. Not just radiating.

Not when you're talking about the amount of energy released by nuclear reactions.  The difference between energy stored per kg hydrogen vs. energy released per kg fuel is going to be something like 7-8 orders of magnitude difference, depending on how much you're talking about heating up the hydrogen.  And if you're actually talking about having that high of a ratio between fuel and hydrogen, then you're not going to have any fission of note, almost all of your neutrons will go into creating deuterium.

Online Elmar Moelzer

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Re: VASIMR as Plasma core fission rocket
« Reply #16 on: 04/20/2017 08:27 PM »
I think it also depends on how hot you can run your fission engine. Theoretically, the hotter it can run at its standard operating temperature, the less cooling panels you would need. And the hydrogen should be able to carry away the heat too, if you exhaust it like with a NERVA engine. Otherwise, NERVA engines would not work.

Offline Rei

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Re: VASIMR as Plasma core fission rocket
« Reply #17 on: 04/21/2017 12:36 AM »
I think it also depends on how hot you can run your fission engine. Theoretically, the hotter it can run at its standard operating temperature, the less cooling panels you would need. And the hydrogen should be able to carry away the heat too, if you exhaust it like with a NERVA engine. Otherwise, NERVA engines would not work.

What part are you referring to when you mention "your fission engine"? The plasma? The moderator / reflector? The radiators?

I'm rather certain they're not talking about something like NERVA, because they're talking about containing a plasma and exhausting it through a magnetic nozzle. NERVA (nuclear thermal) doesn't work with plasma, as it would destroy the core.
My assumption (although I could be wrong about this) is that they're talking about a plasma containing both hydrogen and the fissile fuel, with the mixture exhausted through the nozzle.

Another architecture could be to have the fuel on the sides and heat the plasma through, say, neutron bombardment, although that would be A) very difficult (*massive* neutron flux needed, the vast majority of which most of which won't go where you want), and B) give you an orders-of-magnitude worse heating problems (since all of the thermalized fission fragment energy, and a good chunk of the neutron energy, and the radiated heat from the plasma need to be radiated).  You can't heat the plasma radiatively from a fissioning fuel unless that fissioning fuel is also a plasma, since radiative exchange will as a whole transfer heat from the hotter substance (the plasma) to the cooler substance (the fuel), not the other way around.  So the latter architecture would have to be cooling the exterior fuel enough to keep it solid, while it's giving off enough neutrons that the fraction of them that go in the right direction raise the core to plasma-temperatures.  Which is an insane cooling task.

If they do just mean NERVA-style nuclear thermal, however, then a VASIMR architecture, plasma, and magnetic nozzles don't come into the picture at all.  Unless the concept is having the hydrogen capture heat from the moderator / reflector before entering the core and joining a plasma with the fissoning fuel.  Wherein, see my previous post about the order-of-magnitudes differences between energy out and heat capacity in that scenario.  The hydrogen going into the core, in order to capture a meaningful fraction of the energy, would have to outmass the fuel by many orders of magnitude, and as a consequence capture all of the neutron flux and stop fission.

Again, if I'm envisioning the architecture wrong, please elaborate  :)
« Last Edit: 04/21/2017 12:57 AM by Rei »

Online Elmar Moelzer

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Re: VASIMR as Plasma core fission rocket
« Reply #18 on: 04/21/2017 05:50 AM »
I think it also depends on how hot you can run your fission engine. Theoretically, the hotter it can run at its standard operating temperature, the less cooling panels you would need. And the hydrogen should be able to carry away the heat too, if you exhaust it like with a NERVA engine. Otherwise, NERVA engines would not work.

I'm rather certain they're not talking about something like NERVA, because they're talking about containing a plasma and exhausting it through a magnetic nozzle. NERVA (nuclear thermal) doesn't work with plasma, as it would destroy the core.
My assumption (although I could be wrong about this) is that they're talking about a plasma containing both hydrogen and the fissile fuel, with the mixture exhausted through the nozzle.

I hope that Robot will correct me if I am wrong, but I understood his "use hydrogen for cooling" as in " you pass liquid hydrogen through past heat sensitive parts of the engine and use said hydrogen carry away the heat. The hydrogen could then be exhausted through a nozzle to generate additional thrust (similar to a NERVA engine).

Offline as58

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Re: VASIMR as Plasma core fission rocket
« Reply #19 on: 04/21/2017 06:49 AM »
I think it also depends on how hot you can run your fission engine. Theoretically, the hotter it can run at its standard operating temperature, the less cooling panels you would need. And the hydrogen should be able to carry away the heat too, if you exhaust it like with a NERVA engine. Otherwise, NERVA engines would not work.

I'm rather certain they're not talking about something like NERVA, because they're talking about containing a plasma and exhausting it through a magnetic nozzle. NERVA (nuclear thermal) doesn't work with plasma, as it would destroy the core.
My assumption (although I could be wrong about this) is that they're talking about a plasma containing both hydrogen and the fissile fuel, with the mixture exhausted through the nozzle.

I hope that Robot will correct me if I am wrong, but I understood his "use hydrogen for cooling" as in " you pass liquid hydrogen through past heat sensitive parts of the engine and use said hydrogen carry away the heat. The hydrogen could then be exhausted through a nozzle to generate additional thrust (similar to a NERVA engine).

But won't that just effectively make it a nuclear thermal rocket (if a rather contrived one)? Wouldn't the 'additional thrust' from exhausting coolant hydrogen make up most of the thrust.


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