Polywell Fusion Reactor passes peer review

[Zixinus]

Water disassociates into oxygen which will corrode your engine bell. Rather large negative there.

Aren't there ways to protect the engine bell from corrosion?

[Admiral Valdemar]

From what I heard, the water breaking down issue is a bigger problem for solid core NTRs because of the heating elements themselves being exposed. I'm sure there are ways to protect the engine bell and other components outside the core.

[Sky Captain]

Why simply not use hydrogen as a propellant and don`t bother with corrosion issues? There are plenty of experience accumulated to handle liquid hydrogen. Most of the costs to operate rocket comes not from fuel, but from rocket itself. Even in case of nuclear powered reusable booster maintenance is most likely to be more expensive than hydrogen to fill the tanks.

If water for propellant is still more preferable then it might be possible to prevent corrosion by covering or making any sensitive components out of ceramics or some other corrosion and high temperature resistant stuff.

[Starglider]

Why simply not use hydrogen as a propellant and don`t bother with corrosion issues?

Hydrogen's very low density means that you need much bigger, heavier tanks. That isn't so bad in a conventional launcher where you drop the tanks as you climb anyway. But in an SSTO those big tanks have to be carried all the way into orbit, protected by an enlarged TPS on reentry and supported by heavier landing gear on touchdown. As I understand it this makes the ISP advantages of H2 marginal for earth-to-orbit operations.

[Admiral Valdemar]

I had heard that for D-T fusion engines, you'd just use cryo-H from the fuel tanks to act as reaction mass, since you need to carry the stuff anyway. Though these designs tend to be interstellar in nature, rather than the kind you'd use for SSTO heavy lift capacity (which would probably be better with air-breathing engines on spaceplanes).

[Starglider]

I had heard that for D-T fusion engines, you'd just use cryo-H from the fuel tanks to act as reaction mass, since you need to carry the stuff anyway.

Using deuterium for reaction mass makes no sense in an indirect design, it's expensive and actually has lower ISP. The amount required to run a fusion reactor is trivial compared to the amount of reaction mass needed for an SSTO. Using tritium makes even less sense given that it's very expensive, has even less ISP and is radioactive. Partitioning the cryo tanks is trivial after all. The only time commonality of fusion fuel and reaction mass is helpful is in a classic sci-fi 'torch ship' where you just vent plasma out the back (through a magnetic nozzle hopefully). However...

Though these designs tend to be interstellar in nature, rather than the kind you'd use for SSTO heavy lift capacity

For interstellar travel you probably want ion engines, they're more efficient than any direct-thrust fusion drive we actually know how to build. You might even be able to use the fusion ash as reaction mass for them. The Daedalus external pulse-fusion concept did use D/He3 pellets as both fuel and reaction mass, but that was another speculative design with no relation to polywell.

(which would probably be better with air-breathing engines on spaceplanes).

It's still somewhat debatable whether air-breathing engines (plus wings plus enlarged heat shield plus extra support structure) are worthwhile on chemically fueled SSTOs. If you're determined to go for wings (e.g. for abort options and takeoff/landing site flexibility) on a fusion SSTO, then my guess is that some kind of LACE would be worth the mass and complexity for grabbing extra reaction mass even in a fusion engine, but I wouldn't count on it.

[Sky Captain]

Quote:
(which would probably be better with air-breathing engines on spaceplanes).


It's still somewhat debatable whether air-breathing engines (plus wings plus enlarged heat shield plus extra support structure) are worthwhile on chemically fueled SSTOs. If you're determined to go for wings (e.g. for abort options and takeoff/landing site flexibility) on a fusion SSTO, then my guess is that some kind of LACE would be worth the mass and complexity for grabbing extra reaction mass even in a fusion engine, but I wouldn't count on it.

Then why not use something like fusion ramjet or scramjet to build up as much speed as possible by using atmosphere as reaction mass. IIRC back in the Cold war US airforce experimented with nuclear ramjet for monster cruise missile.

Also, how about cooling issues since for worthwhile high ISP heavy lift SSTO spacecraft you would need reactor producing several hundred GW of power. Would it be possible to safely manage such power levels inside relatively small spacecraft with realistic technology.

[Starglider]

Then why not use something like fusion ramjet or scramjet to build up as much speed as possible by using atmosphere as reaction mass. IIRC back in the Cold war US airforce experimented with nuclear ramjet for monster cruise missile.

There are two major problems with that. Firstly a fission core can be built as lots of long thin fuel elements that the air can flow between, for good direct heat transfer with low drag. A Polywell core is nothing like that; it's a sphere surrounded by huge magnets and ion guns. You'd need a heat exchange loop which adds mass and complexity and reduces efficiency. Secondly ramjets don't produce significant thrust until you're supersonic - to get around that you need some sort of turboramjet or fan-assisted ramjet and that's even more mass and complexity. Finally scramjets are still restricted to a handful of short-duration unmanned prototypes with a very small working speed range. Making a reliable working engine with variable internal geometry that can handle a wide speed range is beyond our current engineering ability and yes, would be even more mass and complexity.

This is basically why Skylon is such a good idea, LACE engines use air from mach 1 to 6, are relatively simple and light, and operate in rocket mode with the same nozzle and fuel system as air-breathing mode. Fusion power would probably let you expand the upper air intake speed by a few machs because maintaining good combustion conditions is no longer an issue.

Also, how about cooling issues since for worthwhile high ISP heavy lift SSTO spacecraft you would need reactor producing several hundred GW of power. Would it be possible to safely manage such power levels inside relatively small spacecraft with realistic technology.

Depends what you mean be 'heavy lift' The reactor powering Project Pluto's nuclear ramjet produced ~500 MW and the engine produced about 16 tonnes of thrust. The Skylon design uses two engines producing 30 tonnes of thrust each, so you'd need a 2 GW reactor to power it if the efficiency was similar. Payload to LEO is supposed to be 12 tonnes. That suggests that you could have hundreds of tonnes of payload with double-digit GW of reactor power. You probably would need 100 GW for a direct lift rocket the size of Saturn V, which does sound impractical.

[Sky Captain]

Quote:
Also, how about cooling issues since for worthwhile high ISP heavy lift SSTO spacecraft you would need reactor producing several hundred GW of power. Would it be possible to safely manage such power levels inside relatively small spacecraft with realistic technology.


Depends what you mean be 'heavy lift' The reactor powering Project Pluto's nuclear ramjet produced ~500 MW and the engine produced about 16 tonnes of thrust. The Skylon design uses two engines producing 30 tonnes of thrust each, so you'd need a 2 GW reactor to power it if the efficiency was similar. Payload to LEO is supposed to be 12 tonnes. That suggests that you could have hundreds of tonnes of payload with double-digit GW of reactor power. You probably would need 100 GW for a direct lift rocket the size of Saturn V, which does sound impractical.

By "heavy lift" I meant reusable nuclear booster that puts at least 100 tons of cargo in orbit necessary if we want to do some serious space stuff. I assumed ISP around 3000 like some proposed gas core nuclear engines to get better payload fraction and acceleration at liftoff 2 - 3 G that`s how I ended up with several hundred GW. Most likely impractical with current technology.

On the other hand Skylon sounds promising concept which theoretically could provide relatively cheap access to space if it does`t hit some serious problems in development like other SSTO projects had. Also horizontal landing is much safer than trying to land your reusable rocket booster on it`s tail. Although I have no idea how it would be possible to provide heat transfer of necessary magnitude without heavy, bulky heat exchangers from Polywell reactor to SABRE engines if we are considering nuclear powered spaceplane.

[kinnison]

Apparently, one of the attractions of the Polywell design is that with a slight change in design the fast alpha particles produced by the proton=B11 reaction can (instead of being used to create power, at least partially) be used as the exhaust. This leads to extremely high exhaust velocities and hence very high Isp. A true fusion rocket, in other words.

Unfortunately, it appears that this approach won't work for high-thrust applications. Or perhaps fortunately; remember the Kzinti Lesson!

[Sky Captain]

Apparently, one of the attractions of the Polywell design is that with a slight change in design the fast alpha particles produced by the proton=B11 reaction can (instead of being used to create power, at least partially) be used as the exhaust. This leads to extremely high exhaust velocities and hence very high Isp. A true fusion rocket, in other words.

Unfortunately, it appears that this approach won't work for high-thrust applications. Or perhaps fortunately; remember the Kzinti Lesson!

Low thrust high ISP engines could be quite useful for strictly orbit to orbit spacecraft basically interplanetary ferry since it would consume little reaction mass and have high payload fraction. For surface to orbit flight very high ISP is undesirable because of huge power requirements necessary to generate enough thrust. Also true high thrust heavy lift fusion torch rocket would most likely melt it`s launch site because it would spew out few terawats of power.

[Dave]

Also true high thrust heavy lift fusion torch rocket would most likely melt it`s launch site because it would spew out few terawats of power.

In the "Honor Harrington" universe created by David Weber, this was solved by launching and landing on lakes or other bodies of water. It also makes refueling the ship easy; just throw a hose over the side. Distilling and splitting it (via electrolysis) is easy when energy is cheap.

Of course, that creates other problems (you're landing / launching in superheated steam, etc) but it's probably cheaper than rebuilding the spaceport every time you use it.

[Sky Captain]

Also true high thrust heavy lift fusion torch rocket would most likely melt it`s launch site because it would spew out few terawats of power.

In the "Honor Harrington" universe created by David Weber, this was solved by launching and landing on lakes or other bodies of water. It also makes refueling the ship easy; just throw a hose over the side. Distilling and splitting it (via electrolysis) is easy when energy is cheap.

Of course, that creates other problems (you're landing / launching in superheated steam, etc) but it's probably cheaper than rebuilding the spaceport every time you use it.

IIRC the same idea was proposed for launching Orion nuclear pulse rockets because launching from land would demolish and irradiate the launch site, also in water launch any leftover radioactive materials would quickly dissipate in ocean thus avoiding the hot spot problem.

[kinnison]

Ah, good old Orion! Of course, this is the simplest way to directly use nuclear reactions as a reaction drive.

I haven't seen this discussed, but it would appear to me that the really large Orion motors ought to be the most efficient. I'll explain why I think this. Low-mass and therefore low-thrust (comparatively!) Orion needs pure fission bombs to power it, and this means that the molecular mass of the exhaust is high - on average, about 117. Scale up to really large designs (I believe that the biggest studied was around 8MT mass) and the power bombs have to have a big fusion component - and therefore the exhaust molecular mass is much lower at around 4 or so (helium nuclei). This naturally leads to much higher exhaust velocity and hence higher Isp.

It's also quite probable that in bigger vehicles the working parts of the vehicle (bomb-handling mechanisms, for example) are a smaller proportion of the total vehicle mass.

Of course, getting an 8-million ton vehicle built on the ground in the first place might present problems. As a reference, this is about 50% more than the mass of the Great Pyramid.

[Sky Captain]

Ah, good old Orion! Of course, this is the simplest way to directly use nuclear reactions as a reaction drive.

I haven't seen this discussed, but it would appear to me that the really large Orion motors ought to be the most efficient. I'll explain why I think this. Low-mass and therefore low-thrust (comparatively!) Orion needs pure fission bombs to power it, and this means that the molecular mass of the exhaust is high - on average, about 117. Scale up to really large designs (I believe that the biggest studied was around 8MT mass) and the power bombs have to have a big fusion component - and therefore the exhaust molecular mass is much lower at around 4 or so (helium nuclei). This naturally leads to much higher exhaust velocity and hence higher Isp.

It's also quite probable that in bigger vehicles the working parts of the vehicle (bomb-handling mechanisms, for example) are a smaller proportion of the total vehicle mass.

Of course, getting an 8-million ton vehicle built on the ground in the first place might present problems. As a reference, this is about 50% more than the mass of the Great Pyramid.

Yeah it basically goes like that bigger, heavier ship can use more powerful thermonuclear bombs and thus can have higher ISP, also ISP can be increased or decreased by varying amount of reaction mass that goes with each bomb. Adding more reaction mass to bomb means higher thrust, but lover ISP decreasing reaction mass increases ISP assuming bombs have the same yield

However IIRC largest proposed designs massing millions of tons were supposed to be built in space since ground launching something that massive would be very difficult. Also to start space mining operation going several 10 000 - 50 000 ton Orion launches should be plenty.

[Starglider]

I haven't seen this discussed, but it would appear to me that the really large Orion motors ought to be the most efficient. I'll explain why I think this. Low-mass and therefore low-thrust (comparatively!) Orion needs pure fission bombs to power it, and this means that the molecular mass of the exhaust is high - on average, about 117.

Yes, but the effect probably isn't as big as you'd think. Simply allowing a nuclear fireball to expand evenly isn't very efficient, as only a small fraction of the plasma hits the pusher plate (plus it's inconveniently high energy). As I recall the realistic designs used pusher mass to absorb most of the energy, arranged in a kind of 'shaped charge' so that more of it hits the pusher plate. This material can be low molecular weight. That said hydrogen bombs develop a lot more yield/weight anyway, so small fission-only Orions are going to be limited performance regardless of thrust efficiency issues.

It's also quite probable that in bigger vehicles the working parts of the vehicle (bomb-handling mechanisms, for example) are a smaller proportion of the total vehicle mass.

Probably true but the shock absorber challenge gets harder and harder the more impulse there is in each pulse.

[Atlan]

Yes, but the effect probably isn't as big as you'd think. Simply allowing a nuclear fireball to expand evenly isn't very efficient, as only a small fraction of the plasma hits the pusher plate (plus it's inconveniently high energy). As I recall the realistic designs used pusher mass to absorb most of the energy, arranged in a kind of 'shaped charge' so that more of it hits the pusher plate. This material can be low molecular weight. That said hydrogen bombs develop a lot more yield/weight anyway, so small fission-only Orions are going to be limited performance regardless of thrust efficiency issues.

IIRC the pusher mass was mostly tungsten, because for maximum effect the drive mass had to be a high density material arranged in a thin disk.

Probably true but the shock absorber challenge gets harder and harder the more impulse there is in each pulse.

Shock absorbers might not be the most problematic part of the design. Nowadays you can use things like magnetic braking, like on some amusement park rides. We're not limited to hydraulics any more. The pusher plate, that's the real challenge. You have to design something which will stand up to thousands of intensely powerful shocks, and it will have to be totally reliable. An Orion can be big and heavy enough that you can bring spare parts for almost anything, but a second pusher plate...