Author Topic: What about Timberwind 250 NTR-based MCT?  (Read 10420 times)

Offline RanulfC

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #40 on: 03/08/2016 05:47 PM »
Bimodal NTR's essentially have a helium "primary" coolant loop and radiator that can run a turbogenerator, with a secondary peak power coolant system being the open cycle once through cooling with the liquid hydrogen propellant. Post burn, you are running the primary cooling hard to keep the reactor from melting. For older NTR designs, the same was accomplished via propellant ullage gas dumping through the reactor without the propellant turbopump, essentially dribbling leftover propellant through the reactor until it calmed down.

Thought I'd addressed this but don't see it so I'll do so now. Bimodal's use various working fluids depending on the design. The Triton Tri-modal planned to use Xenon, the original Bi-modal used helium from the pressurization system.

None of the power generation uses "open-cycle" at all, there's no need to as the peak heating isn't bad enough to require it and the power generation cycle never goes above 50% reactor power at any point.

No NTR required or was designed to have a "primary" cooling post-burn as the residual heat wasn't enough to melt the core elements. Taper down thrust was used for a few seconds at the end of the burn to drop the residual heat AND to clear the propellant lines of propellant. This was sufficient to remove any danger of the core melting. The core design was in fact robust enough that it could in fact be shut down AFTER propellant depletion without significant damage to the matrix and residual heating was never found to be an issue.

(In one test the reactor was allowed to run at full power for a few minutes after all LH2 flow had ceased due to a faulty gauge. While some elements of the core matrix in fact did melt slightly there was no actual structural failure and the core was found to still be "usable" for further burns if it had been required. The matrix had "slumped" somewhat under gravity, which wouldn't have happened in microgravity, but not enough to impede flow or cause issues if the reactor had been brought back up to power)

As for the use of any NTR on the MCT it's very, very unlikely. Mostly because being able to shield the NTR on the vehicle would be problematical at best for the design. Shielding is heavy and you can't really substitute distance for mass in any realistic MCT design. (Extending the NTR away from the main hull by pylon such as suggested for the Pilgrim Explorer is possible but you have to retract the pylon to bring the MCT down on Mars or Earth and the reactor is going to always be somewhat "hot" after use. Bringing it closer to your crew for landing and while on the surface doesn't make much sense) Further methane is a horrible reactor working fluid due to the high carbon content.
Lastly the Timberwind was never shown to be a practical NTR design though overall pebble bed reactors can still be used for non-NTR uses.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline chipguy

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #41 on: 03/08/2016 06:00 PM »
Further methane is a horrible reactor working fluid due to the high carbon content.

Could you please expand on that?

After all graphite moderated reactors were once common and graphite is basically pure carbon and the
AECL WR-1 reactor used organic oil as a coolant and that likely had higher carbon content than methane.

Offline RanulfC

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #42 on: 03/08/2016 06:09 PM »
Aerocapture isn't actually harder than LEO reentry. Or rather, it depends.

Mostly it depends :) The factors you have to get right are more difficult due to it being and interplanetary rather than orbital problem AND reentry is rather more "simple" all around because you're end goal is to land rather than simply "capture" into orbit. As you note it depends :)

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Aerocapture is treated as this magical thing we can never do or that would be impossible to achieve. I'm not sure where that falsehood comes from.

Not at all, as we HAVE done it in a general sense. But doing so with a large vehicle from interplanetary speeds IS a much more difficult problem and we have NOT done that yet and everything we know says it's NOT going to be simple or easy which for the longest time it was assumed it would be. It's one of the major reasons everyone wants to "work-up" from GEO, to Lunar and then interplanetary.

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If you DID have NTR, you'd still want to use aerocapture!

Nope, not at all and it's even considered "better" to use propulsive capture if you have abundant chemical since it removes a lot of the difficulty from the mission plan if you DON'T do aerocapture. But if you're constrained for propellant then it becomes much more attractive to do aerocapture and save what you can. In the case of NTR/NEP/SEP you would normally NOT have the option of aerocapture due to design constraints of the spacecraft anyway so the propellant used in an powered maneuver is more acceptable.

...however. HIAD could be made quite cheap. I talked to some of the folks building HIAD, and they said that something like it could be made for just a few hundred thousand dollars (if you were making like a thousand of them). Even with MCT, if you can get it that cheap, it'd be cheap enough to be worth it even if you threw it away.

However, even HIAD could be reused, though if it's cheap enough, it may be better to just get rid of it.

Might want to check that because HIAD actually can't be reused and has to be expended. Currently there's no practical way to "deflate-and-refold" it and it is damaged in use enough that it won't work a second time. IF you were going to use it multiple times, you'd need multiple sets. At some point they would like to develop a reusable version but there's no money or requirement for that at present.

To bad the mechanical (non-inflated) version never got tested, (launch vehicle failure) as that was designed from the start for multiple reentries.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Robotbeat

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #43 on: 03/08/2016 06:18 PM »
Yes at all.

Aerocapture is a very powerful technique that we're not going to throw away if we have NTR or abundant chemical. I guarantee MCT will use it.

SEP may be used for the overall architecture, but it will not be used for BFS itself. Can't land with significant SEP.
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Offline RanulfC

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #44 on: 03/08/2016 06:29 PM »
Further methane is a horrible reactor working fluid due to the high carbon content.

Could you please expand on that?

After all graphite moderated reactors were once common and graphite is basically pure carbon and the
AECL WR-1 reactor used organic oil as a coolant and that likely had higher carbon content than methane.

I should probably say that it was horrible for use in any standard design NTR, but I should also mention that you can design the core to use different propellants HOWEVER that means you can ONLY use the propellant it was designed for.
(With some slight variation but within some pretty strict guidelines)

Further a power reactor such as WR1 isn't an NTR and vice-versa :)

Testing showed that at NTR temperatures the carbon from the propellant tended to adhere to the reactor elements, which blocked their ability to transfer heat (and caused moderation issues due to the un-predictability of the effect) to the propellant and get cooled. The issues with heating were the most problematically.
Supposedly it's less of an issue with cermet, (ceramic/metal) reactors so you can get away with it for more specially designed reactor cores but the standard NERVA (and for that matter the standard pellet-bed materials) it wouldn't work.

If you design the reactor core specifically to use methane, then it normally does not work well with Liquid Hydrogen and vice versa. You can even design a core to use CO2, (hence Zubrin's concept of a Nuclear Mars Hopper vehicle) but doing so means it won't work well, if at all, with LH2. They tested LH2 and Ammonia through the same reactor core and it worked but obviously the LH2 worked better than the Ammonia, and that's about the closest they came to a mutli-propellant NTR design.

The way the get bi-, (or tri-) model operation out of an NTR is by putting a circulation system for the power system, usually intermixed with the non-structural tie-tubes within the reactor. Since the tie-tubes share the core's heat but the working fluid never comes in contact with the propellant or the actual core materials you have more options for the power system fluid than you do for the actual reaction mass of the NTR. (And in thrust-augmentation mode the LOX is injected into the hot-hydrogen stream neat the beginning of the nozzle so again it doesn't encounter any of the reactor core matrix material)

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline A_M_Swallow

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #45 on: 03/08/2016 07:05 PM »
{snip}
I was thinking of the power system rather than the thrusters. ISS is the closest in scale to an SEP power system, and it has required substantial maintenance to keep the power system operating.  I agree that an array of thrusters with something like HIPEP would be very reliable due to redundancy.
{snip}

If the SEP is equipped with two thrusters it can continue flying with half its solar panels destroyed. Mission Control tells the guidance system to only use one of the thrusters. This will probably double the length of the journey.

Arranging the power system so that any solar array can power any thruster is easy; although the spacecraft needs m*n power relays. Where m = number of thrusters and n = number of solar arrays.

Offline Robotbeat

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #46 on: 03/08/2016 07:27 PM »
Dawn has been incredibly reliable. Its NEXT thrusters have lasted far longer than planned.

ISS is super complicated, designed using 1980s tech. We don't need to use ISS as a baseline of what's possible. The huge number of modern commercial satellites using SEP is a FAR better baseline.
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Offline RanulfC

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #47 on: 03/08/2016 07:44 PM »
Yes at all.

Aerocapture is a very powerful technique that we're not going to throw away if we have NTR or abundant chemical. I guarantee MCT will use it.

SEP may be used for the overall architecture, but it will not be used for BFS itself. Can't land with significant SEP.

Eh, no, SEP won't use aerocapture and neither would NEP, or NTR. None of them gains much for trying to design and build for it. Abundant chemical, means just that and so you don't really worry about how much propellant you "save" if you don't have a seriously compelling reason to do so.

Something to keep in mind is that operationally at any point "aerocapture" can turn into "aerobraking" followed by "lithobreaking" or just "breaking" if your vehicle isn't designed to handle the loading. NTR/NEP nor SEP actually need aerocapture as they have a high enough ISP to not require it for a conservative mission architecture. (In the case of SEP/NEP specifically your propellant fraction is low enough you have a huge number of OTHER mission factors that have a much higher priority)

Having said that I too believe that MCT will in fact use aerocapture on both ends as it isn't actually an "abundant" chemical architecture but more of a significant chemical one. Abundant chemical means you have enough propellant at all points in your mission that you do not take any risks that you can avoid and as noted above "aero"-anything IS a risk despite the advantages it gives you.

If SEP is used on the BFS then it won't aero-anything, saving propellant is not a consideration because you won't use enough to matter in the long run and "aero" won't save enough to matter.

MCT on the other hand is chemically powered and limited on the propellant load it can handle during all phases of the flight. The "risk" is far outweighed by the advantages. (It's GOING to land at some point so if aero-capture becomes aero-braking, etc then you're no worse off than you were from the start)

Of course I'm going to have to point out that by separating BFS and MCT at some point prior to the aerocapture maneuver (which would make sense in context as then you don't have to re-accelerate the BFS for the return trip) you DO realize where we've heard this before right? :)

http://www.wired.com/2014/01/to-mars-by-flyby-landing-excursion-mode-flem-1966/
http://www.astronautix.com/craft/flem.htm

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline gdjacobs

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #48 on: 03/08/2016 08:05 PM »
Dawn has been incredibly reliable. Its NEXT thrusters have lasted far longer than planned.

ISS is super complicated, designed using 1980s tech. We don't need to use ISS as a baseline of what's possible. The huge number of modern commercial satellites using SEP is a FAR better baseline.

In the real world, scale matters. Scale poses significant engineering challenges. You can't just wave your hands and make it go away.

{snip}
I was thinking of the power system rather than the thrusters. ISS is the closest in scale to an SEP power system, and it has required substantial maintenance to keep the power system operating.  I agree that an array of thrusters with something like HIPEP would be very reliable due to redundancy.
{snip}

If the SEP is equipped with two thrusters it can continue flying with half its solar panels destroyed. Mission Control tells the guidance system to only use one of the thrusters. This will probably double the length of the journey.

Arranging the power system so that any solar array can power any thruster is easy; although the spacecraft needs m*n power relays. Where m = number of thrusters and n = number of solar arrays.

I can't help but wonder how complex this would make the mission planning. One big advantage of SEP or NEP for probes is that travel time can almost be a free variable. With crew, time constraints will be very real and will directly translate into additional propulsion requirements or trans hab capability, hence mass and cost.

Offline Oli

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #49 on: 03/08/2016 08:06 PM »
Multi-MW EP saves a super ridiculous amount of propellant and I wouldn't consider it more difficult than MCT.

The panels for Juno, some of the most modern and efficient in use by NASA today, mass about 340 kg. with max output at 1AU being about 14 kW. For those keeping track, that's about 24mT in panels for every megawatt of electrical power. Such a large set of solar absorbers would likely need significant additional mass for power conditioning and thermal control.

I wouldn't be so cavalier about the difficulty or expense of building such a thing.

Here's a good formula from the Oleson paper:

Power system mass: 20kg/kw
Tank mass: 4% of prop
Structure: 27% of tank mass and power system mass.

It approximates well the mass of SEP stages with several 100kws in NASA designs. For example in the hybrid architecture the SEP stage has a mass of 11.1t (400kw + 16t xenon). The formula gives 10.97t. Scaled up to 1MW that would be 27.4t.

Note this is relatively high TRL. There's far more ambitious stuff out there. But you can do manned missions with below MW power and a little help from chemicals (hybrid) and scale it up later.

If you look at cost estimates for Mars missions from NASA there are many more equally or more expensive elements. Like heavy lift, CTV (Orion), Mars EDL, Mars ascent, deep space hab, Mars surface systems.
« Last Edit: 03/08/2016 08:39 PM by Oli »

Offline RanulfC

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #50 on: 03/08/2016 08:25 PM »
If the SEP is equipped with two thrusters it can continue flying with half its solar panels destroyed. Mission Control tells the guidance system to only use one of the thrusters. This will probably double the length of the journey.

"Technically" doubles the length of your acceleration schedule not the "journey" which you'd simply adjust the former to arrive at the same point in the latter :)

Quote
Arranging the power system so that any solar array can power any thruster is easy; although the spacecraft needs m*n power relays. Where m = number of thrusters and n = number of solar arrays.

Pretty much yes, it's a design issue you include. However there's a number of ways you can lose half (or more) of your power system which in effect is what he's suggesting. You can also count in designing in multiple redundant power pathways and conditioning nodes.
Not really much more complex than doing so with any propulsion system.

Your main issue with an electrical propulsion system is actually BECAUSE you tend not to require as much reaction mass there is a tendency to rely on simple, monolithic propellant storage (for those needing quantities thereof) which can be an issue if something happens to that :)

Dawn has been incredibly reliable. Its NEXT thrusters have lasted far longer than planned.

Evidence shows that the main failure of EP systems is not the thruster itself but power or other issues not related directly to the thruster. But again that's type dependent and Ion thrusters like NEXT can and do degrade over time.

Quote
ISS is super complicated, designed using 1980s tech. We don't need to use ISS as a baseline of what's possible. The huge number of modern commercial satellites using SEP is a FAR better baseline.

Actually the SEP on most com-sats isn't the same as those that would be used for deep space propulsion both in size (yes, size does matter :) ) and operation. The ISS power arrays and conditioning system is also more like what would be used for high-power operations rather than those on com-sats.

Finally industry studies are showing there are both advantages and disadvantages to SEP on satellites and it's far from clear if they will fully supplant chemical systems or not. Design and operational requirements are actually leaning towards a mixture of both systems on future platforms as the most efficient option.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Oli

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #51 on: 03/08/2016 08:40 PM »

Eh, no, SEP won't use aerocapture and neither would NEP, or NTR. None of them gains much for trying to design and build for it.

Unless something like magnetoshell aerocapture works out. Aerocapture would significantly lower transfer times and thus power requirements.

Offline gdjacobs

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #52 on: 03/08/2016 09:51 PM »

Eh, no, SEP won't use aerocapture and neither would NEP, or NTR. None of them gains much for trying to design and build for it.

Unless something like magnetoshell aerocapture works out. Aerocapture would significantly lower transfer times and thus power requirements.

Well, NTR will almost certainly use aerocapture (at least initially) LH2 storage is too difficult in the cruise phase, and NH3 imposes significant ISP penalties. I wonder if the NTR stage could be equipped with a small thruster package, cut from the transit vehicle near mars after being used for gravity counterweight, then recovered on the free return.

Offline Oli

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #53 on: 03/08/2016 11:37 PM »
Well, NTR will almost certainly use aerocapture (at least initially) LH2 storage is too difficult in the cruise phase

The NTP stages in DRM 5 have all cryocoolers and the transfer vehicle does propulsive capture and return. The Von Braun plan from the late 60s did everything propulsively as well.

Offline A_M_Swallow

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #54 on: 03/08/2016 11:47 PM »

Eh, no, SEP won't use aerocapture and neither would NEP, or NTR. None of them gains much for trying to design and build for it.

Unless something like magnetoshell aerocapture works out. Aerocapture would significantly lower transfer times and thus power requirements.

Well, NTR will almost certainly use aerocapture (at least initially) LH2 storage is too difficult in the cruise phase, and NH3 imposes significant ISP penalties. I wonder if the NTR stage could be equipped with a small thruster package, cut from the transit vehicle near mars after being used for gravity counterweight, then recovered on the free return.

The radiators on a NTR spacecraft will need to survive aerocapture. I am not certain that heat shields and radiators mix well.

Offline Robotbeat

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #55 on: 03/09/2016 12:01 AM »
Yes at all.

Aerocapture is a very powerful technique that we're not going to throw away if we have NTR or abundant chemical. I guarantee MCT will use it.

SEP may be used for the overall architecture, but it will not be used for BFS itself. Can't land with significant SEP.

Eh, no, SEP won't use aerocapture and neither would NEP, or NTR....
What are you talking about? I never said SEP would use aerocapture! My point is EXACTLY that you wouldn't aerocapture with SEP (though you could aero/brake), and that there's no way that BFS would use any significant SEP because BFS will be landing (aerocapture and entry being quite similar to one another... in fact, a botched hyperbolic entry can turn into aerocapture). HOWEVER, there could be a /separate/ architecture component that uses SEP. BFS would not require SEP (neither would the architecture as a whole), but it may be useful for pushing propellant and cargo around on a separate element that does not land.

Quote
Of course I'm going to have to point out that by separating BFS and MCT at some point prior to the aerocapture maneuver (which would make sense in context as then you don't have to re-accelerate the BFS for the return trip) you DO realize where we've heard this before right? :)...
There's confusion in your terminology, here. MCT and BFS are sometimes used by SpaceX folk as synonyms, though it probably makes sense now to talk just about BFS and either retire "MCT" or rename the WHOLE architecture MCT. There most certainly ISN'T a separate MCT craft and BFS craft, from ANYTHING that SpaceX has said. They're either referring to the same thing, or otherwise you could argue BFS is a subset of MCT.

...I think this confusion may arise due to Impaler and others' proposed ideas. But as it stands, BFS is the only part of the architecture (besides the BFR first stage booster) that we know exists. SEP is something that we know SpaceX has /considered/ but we have no evidence that it is actually part of the architecture at all at this point.

Currently, for all we know, the whole architecture consists of basically just the BFR booster, BFS stage (perhaps variants of BFS serving as tankers) and ground elements on Mars.
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To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Robotbeat

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #56 on: 03/09/2016 12:13 AM »
...
Eh, no, SEP won't use aerocapture and neither would NEP, or NTR. None of them gains much for trying to design and build for it. ...
That's untrue. Aerocapture basically means you only have to provide /half/ the delta-v with whatever your propulsion system is.

If you want to get there fast, then accelerating the whole way with SEP/NEP, then separating off your crew capsule  which does does entry and/or aerocapture (essentially a long skip entry, where you do an aerocapture maneuver and then on the next approach to periapsis, you do an entry). Your SEP/NEP craft doesn't do a "flip and burn" until your crew has detached (which makes the SEP/NEP craft much lighter and it has months to then burn back to Mars, and you could even do some aerobraking to get in a lower orbit if you wanted). This allows MUCH faster travel with electric propulsion than you'd otherwise be capable of, potentially much faster than a conventional NTR/chemical approach.

And if you want a fast trip with NTR, you'd still want to do aerocapture as it'd allow you to spend basically ALL your propellant on the Earth-swingby burn, gaining you a much faster trip (or a much smaller IMLEO, you pick).

In every case, aerocapture can help a lot.

Using /multiple/ techniques for the same mission allows /cumulative/ improvements in trip time or IMLEO reduction or increasing payload (or some combination).
« Last Edit: 03/09/2016 12:19 AM by Robotbeat »
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Offline FreeThinker

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Re: What about Timberwind 250 NTR-based MCT?
« Reply #57 on: 12/06/2018 01:32 PM »
Pebble bed reactors are still considered highly efficient for power plant applications, but no one considers them suitable for NTR use. Timberwind isn't mentioned because it was an utter failure at it's proposed application, but people still stumble upon the concept and since there's more stuff telling how fantastic it is than stuff pointing to the actual experiments...
Alright, so the Timberwind was a failure, but what about more advanced pebble bed designs like rotating fluidized bed reactor, which prevents the local pebble overheating issue.

source: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720017961.pdf

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