Fusion Power and weapons questions

[Junghalli]

I'm designing a space warship for a hard (mostly) SF setting and I have some science questions I could use some help on for doing so.

(1)What is the highest realistic output for a fusion reactor? I hear numbers being thrown around going into the high TW in SF tech manuals and sites, but we all know how accurate that tends to be in terms of real science. The reactor must be small enough to fit in a spacecraft 250 meters long.

(2)Which would be a better fusion fuel, deuterium or He3? I know deuterium is ridiculously common, but I've heard He3 would be much better.

(3)What would be the highest realistic yield for A) a turret-mounted neutral particle cannon and B) a big spinal mounted neutral particle cannon that runs the length of the whole ship and may or may not take a while to charge.

[Admiral Valdemar]

I'm designing a space warship for a hard (mostly) SF setting and I have some science questions I could use some help on for doing so.

(1)What is the highest realistic output for a fusion reactor? I hear numbers being thrown around going into the high TW in SF tech manuals and sites, but we all know how accurate that tends to be in terms of real science. The reactor must be small enough to fit in a spacecraft 250 meters long.

See that star in the sky? Oh, I think you know which one, for you couldn't see anything without it. It's called the sun. That is a giant fusion reactor. That is not even an upper limit. The bigger the reactor, the more fuel can be turned to plasma, and the more energy can come about via steam turbines or magnetohydrodynamics.

(2)Which would be a better fusion fuel, deuterium or He3? I know deuterium is ridiculously common, but I've heard He3 would be much better.

He3 is cleaner and gives more energy. It is, however, rare next to any hydrogen isotope, so you'd be mining it from moons or gas giant atmospheres.

(3)What would be the highest realistic yield for A) a turret-mounted neutral particle cannon and B) a big spinal mounted neutral particle cannon that runs the length of the whole ship and may or may not take a while to charge.

How fast can a starship fly? It's an arbitrary question, because unless you tell us what power levels your reactor is giving out, the limit could be anything. With particle beams, it depends on how powerful your EM fields in the accelerators are, so no physical material is subject to the energy within.

[kheegster]

It depends on how 'realistic' you want the technology to be, i.e. an extrapolation of existing technology or something pie-in-the-sky.

Assuming the former, ITER is projected to generate 500MW of power and is expected to have its first burn in 2015:



Note the figure at bottom. This looks like it would fit comfortably in a 250m long vessel, and one would expect successive generations to have improved power output and smaller sizes.

The first generation of fusion reactors would be Deuterium-Tritium fusion, and while Tritium is indeed rare, it can be produced through lithium within the reactor itself. Lithium itself, however, is a limited resource. The next generation of reactors are hoped to run on D-D fusion, which would give true 'unlimited' power.

[drachefly]

true 'unlimited'

...
:lol:

Also keep in mind that ITER is a research prototype, not a production model. It may or may not even produce net energy (so far, every time they think they will, it slips through their fingers for some unexpected reason; the improvements have just made it progressively more frustrating when it does so).

What I'm getting at is there is a LOT of room for improvement.

[Junghalli]

He3 is cleaner and gives more energy. It is, however, rare next to any hydrogen isotope, so you'd be mining it from moons or gas giant atmospheres.

Which one would you see being the preferred general fuel (i.e. like what gasoline and diesel is today)? Although I guess for a high performance warship reactor He3 might be just better regardless, seeing as it's higher energy.

How fast can a starship fly? It's an arbitrary question, because unless you tell us what power levels your reactor is giving out, the limit could be anything. With particle beams, it depends on how powerful your EM fields in the accelerators are, so no physical material is subject to the energy within.

This from the guy whose idea of helping me figure out a realistic reactor output was to point at the sun.
Seriously, it can manage an acceleration of at least six gees (around 60 m/s^2 IIRC). Although it could probably almost certainly be more if not for the fact that the universe lacks any kind of inertial dampening, so after a certain point you have to worry about killing the crew (IIRC most humans couldn't take six gravities for very long, so even this would probably be restricted to combat manuevers and short high intensity burns).

It depends on how 'realistic' you want the technology to be, i.e. an extrapolation of existing technology or something pie-in-the-sky.

It doesn't have to be diamond hard but FTL is pretty much the only really pie in the sky tech the universe has (at least the humans anyway) so something within the range of realistic technological plausibility would be good.

Assuming the former, ITER is projected to generate 500MW of power and is expected to have its first burn in 2015:

Ah, thanks, that helps.
Based on that I'd guess that something in the low GW range would be a good upper limit?
Hmm, calculating from that any energy weapons the ship would carry would definitely be sub-kiloton...

[Junghalli]

I think I'll go for 15 GW. It's rather severe high end and probably somewhat unrealistic, but at least it's somewhat in the ballpark and anything less and I can't see it being enough to power the FTL drive. Even at 15 gigs it'll probably take 4-6 reactors working in concert to power a small to medium ship, and maybe 12 for a big dreadnought.

[Patrick Degan]

Seriously, it can manage an acceleration of at least six gees (around 60 m/s^2 IIRC). Although it could probably almost certainly be more if not for the fact that the universe lacks any kind of inertial dampening, so after a certain point you have to worry about killing the crew (IIRC most humans couldn't take six gravities for very long, so even this would probably be restricted to combat manuevers and short high intensity burns).

Keep in mind the considerations relating to vessel mass and fuel. A ship measuring 250 metres and pushing 6g accelerations is going to require lots of fuel —which means additional mass.

[ClaysGhost]

There will most definitely be a limit, set by your volume constraints* and (if you use magnetic confinement) by the strength and compactness of your magnets - the magnets will set the density of your fuel plasma. Obviously if you are given a kilogramme of fuel then there is a limit to how much energy it can be made to produce via fusion, and by using the density of the fuel plasma and the volume of the reactor in which fusion occurs, you can find how much mass is fusing inside the reactor and hence the energy released. Increasing the density while keeping reactor size the same implies more mass inside the reaction chamber, but also stronger magnets.

What technology are you using for propulsion? 6g for a 250-metre long craft is shockingly good performance. Obviously, chemical drives are out, and the popular ion drives are unlikely to be viable in a realistic setting for this sort of performance. A fusion rocket might be the most plausible option and most believable given your method of power generation (but it is still extreme). The exhaust velocity usually quoted for a fusion rocket is about 10,000 km/sec. Assuming a vehicle mass of ~60,000 tonnes (very uncertain, based on large wet-navy ships, not rockets), then to achieve an acceleration of 6g would require you to expel about 350kg/sec at that exhaust velocity, which is a considerable amount of mass (and volume).

*The region responsible for the sun's energy output is rather large...

[drachefly]

The weapons only need to be sub-kiloton if they run off the ship's main power systems. No reason they couldn't pack bombs, rockets, or guns.

But also, energy could be stored over time and then released quickly during battle. One method would be via capacitors

Right now, we have 40 J / gram capacitors (1/2 C(Vmax)^2 ). There is considerable room for improvement in that -- these only run up to 3V!

This will, sooner or later, run up against ultimate physical limits. I would roughly estimate that to be no more than 1 eV per 12 amu ~= 8 kJ/gram

So to get 1 kiloton (~=4TJ) you would need 500 tons of capacitor.

Okay, that seems a touch unreasonable. Perhaps new techniques will be discovered that bypass the basis of my physical limit guess.

[Admiral Valdemar]

I'm assuming given we're talking fusion, that VASIMR would be used. You could have low Ip, high thrust for getting out of atmosphere, and vice versa for long range space travel.

[kheegster]

true 'unlimited'

...


Also keep in mind that ITER is a research prototype, not a production model. It may or may not even produce net energy (so far, every time they think they will, it slips through their fingers for some unexpected reason; the improvements have just made it progressively more frustrating when it does so).

What I'm getting at is there is a LOT of room for improvement.

While it is true that ITER is a research prototype and is not intended to generate useable power, it would quite certainly produce net energy, because this has already been achieved by several experimental tokamaks (e.g. JET and JT-60). The question is whether it can produce enough energy to sustain a chain reaction within the fusion plasma, and most people in the know I've spoken to seemed pretty confident that this can be done with ITER.

The stereotype of 'fusion power always being 30 years in the future' was due to some over-optimistic statements in the infancy of the field back in the 50s and early 60s. Since the research effort matured in the 70s, the focus has always been achieving small incremental steps on the way to eventual fusion power, rather on 'throws of the dice' to try and achieve it in one go. Most tokamaks to date have achieved what they were intended to do, e.g. JET, and have shed valuable knowledge which would be crucial in making ITER successful.

[Junghalli]

What technology are you using for propulsion? 6g for a 250-metre long craft is shockingly good performance.

I had in mind a plasma drive. Basically you take some sort of gas (probably hydrogen) and run it through and around the reactor until it becomes extremely hot, then eject it out the back as a plume of plasma (hence the name).
BTW keep in mind that 6 gees is only for brief combat manuevers. The ship mostly moves by short rocket burns at sublight, and I don't see sustained accelerations much in excess of 2 gees.

The weapons only need to be sub-kiloton if they run off the ship's main power systems. No reason they couldn't pack bombs, rockets, or guns.

Yeah, the ship is also equipped with tac nuke missiles and railguns. The tac nukes range around 200 kilotons.

This will, sooner or later, run up against ultimate physical limits. I would roughly estimate that to be no more than 1 eV per 12 amu ~= 8 kJ/gram
So to get 1 kiloton (~=4TJ) you would need 500 tons of capacitor.
Okay, that seems a touch unreasonable. Perhaps new techniques will be discovered that bypass the basis of my physical limit guess.

Hmm, let's see. The WWII Yamato battleship, which was a little bit more than 250 m, weighed about 65,000 tons. So actually 500 tons of capacitor doesn't sound so bad, provided we're talking about a big spinal mounted weapon. The turret mounted versions would probably still be piss weak, but I could see them being retained because they can't be evaded or intercepted like missiles or railguns, except maybe at very extreme range.

[Ender]

For realistic reactor tech, particularily in space, the limit to power is not the fuel or mechanism for generating power, but what the support systems can contain and deal with. Realistically, don't expect much beyond a few dozen GW, maybe a little over 100 GW.

[Sea Skimmer]

Yeah, the ship is also equipped with tac nuke missiles and railguns. The tac nukes range around 200 kilotons.

200kt is more powerful then many strategic warheads. Tactical nuclear warheads normally range from subkiloton to a few kilotons.

[Junghalli]

200kt is more powerful then many strategic warheads. Tactical nuclear warheads normally range from subkiloton to a few kilotons.

OK I'll make it the same as a US Navy Katie shell: 20 kt. No shields or uber armor in the universe so that should be sufficient.

[Kuroneko]

It would not do to have a large radiation shield, since this would not only take up space but be costly in terms of having too much mass. Mass is a problem, since it affects maneuvaribility and forces a larger engine output. Thus, it would probably be best to favor a fusion fuel that minimizes the neutrons. A good candinate proton/Boron-11 fusion, which satisfies this criterion and still has fairly abundant components.