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Old 10-02-2017, 07:26 PM   #1
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Default [Spaceships] Describe Pion Drive...

Several books (GURPS and otherwise) talk about fusion and anti-matter pion drives. What do you imagine them looking like? The exhaust. The reaction. How does it behave?
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Old 10-02-2017, 08:16 PM   #2
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Default Re: [Spaceships] Describe Pion Drive...

Fusion and antimatter rockets are very different beasts, and even within spaceships there are a range depending on how realistic you want to be. Both of them are fundamentally operating on the same principle as all other reaction drives, though - heat stuff up and throw it out the back to go forward.

A fusion rocket is using a fusion reactor (as opposed to a chemical reaction fission reaction) to provide that heat. As with many rockets, the hotter the better, and the better ability you have to continue to form a rocket nozzle at high temperatures the better thrust and delta-v efficiency you get. That's the primary difference between the fusion rocket at .005G and the fusion torch at 0.5G - the fusion torch assumes that your rocket can handle exhaust at near the operating temperature of your fusion reactor, rather than what conventional materials science dictates[1]. For a fusion rocket, your exhaust is probably a reasonably focused stream of hot hydrogen with assorted radioactive byproducts in it, which will expand and cool behind the ship. Fusion torch exhaust is similar, but probably charged (see below), hotter, and more apparently focused - it's moving much faster, and so spreads out less with distance. The drive itself probably looks a lot like a fuel tank, a fusion reactor (in whatever configuration you want - torus, sphere, quite possibly buried in the middle of the ship since it's delicate and probably supplies important power to other things), and a rocket nozzle (physical or magnetic) on the other end. A fusion rocket will have relatively small tubes since it's drip-feeding it's reaction mass, a torch will likely have substantial pumps and pipes.

Pion drives are very similar, except that instead of using a fusion reaction they use 1:1 antimatter annihilation to 'heat' their exhaust. You're almost certainly using annihilation reactions that produce charged pions (since you need to 'aim' them before they decay), and that means that the products will decay into muons and neutrinos almost instantly, which in turn turn into electrons within a nanosecond or two. That means the exhaust is a stream of electrons and neutrinos, going generally the opposite direction as the ship. It's not really a particle beam, though, since the direction of the decay products isn't going to be consistent at all - it's just that if you start out going at near light speed in one direction, almost all random vectors from there end up also going that way. The drive itself likely depends on how you are storing and depositing your antimatter - either way you are unquestionably putting very small amounts of mass through your rocket chamber, but you could be doing anything from magnetically guiding anti-hydrogen ions from a tank to the chamber to using quantum coordinates to summon them from somewhere's likely that the bulk of the engine mass will be the containment systems and whatever you're using to aim the pions rather than anything traditionally rocket-like.

If you haven't yet, you should check out Atomic Rockets, particularly and

[1] Note that it doesn't have to be that you have materials that remain solid at those temperatures - magnetic fields look promising if it wasn't for the few % of fusion/fuel interaction byproducts that are neutral particles, so inventing isotopes that produced only charged fusion products would work.
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Old 10-03-2017, 01:20 AM   #3
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Default Re: [Spaceships] Describe Pion Drive...

There is no fusion pion drive. An antimatter pion drive, as described, annihilates a matter-antimatter fuel in a 1:1 ratio and uses the charged particle (pion) byproducts of that annihilation as reaction mass, directed by magnetic fields.

Almost nothing in that process can be confined by material means. You'd probably have particle and anti-particle beams intersecting in a space largely surrounded by field generating hardware.

I imagine you wouldn't be able to see the antimatter pion rocket's exhaust, because while it's very energetic there's very little of it. The antimatter pion torch is a tad different in that respect of course...
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Old 10-03-2017, 11:39 AM   #4
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Default Re: [Spaceships] Describe Pion Drive...

The main naked eye visibility for an antimatter pion drive would probably come from inefficiencies in the process heating up the physical components of the drive; direct annihilation products are all invisible to the naked eye.
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Old 10-03-2017, 12:28 PM   #5
Donny Brook
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Default Re: [Spaceships] Describe Pion Drive...

I imagine a mighty lord would put his pions on a giant treadmill and handlers would beat them to ensure they kept up the pace.
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