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Old 12-07-2017, 06:08 PM   #21
Flyndaran
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Default Re: [Spaceships] Mass-driver realism

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Originally Posted by acrosome View Post
And getting prophylactic thyroidectomies at age 14... :)

I mean, really, detonating nuclear weapons in the atmosphere? Unless you boost to orbit first- like those schemes that used a couple of dozen SRBs- building a space industry around Orion is insane. Cool as hell, but insane.
The danger of a working Orion lift off would be much less dangerous than mundane industries as far as disease and sickness goes. But the OMG, RADIATION! terror would do the killing for it.

Panic and politics would be the true dangers, and very real too.
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Old 12-07-2017, 07:18 PM   #22
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Default Re: [Spaceships] Mass-driver realism

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It depends on the type of device you are using. I am sure that there would have been serious work on pure fusion devices (using something more affordable and less dangerous than tritium) if there had been an economic incentive for such devices.
You can't detonate a thermonuclear device without a primary fission device.
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Old 12-07-2017, 09:36 PM   #23
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Default Re: [Spaceships] Mass-driver realism

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It depends on the type of device you are using. I am sure that there would have been serious work on pure fusion devices (using something more affordable and less dangerous than tritium) if there had been an economic incentive for such devices.
"Pure" fusion of either Tritium+Deunterium or De+De puts out 80% of its' energy in the form of neutrons. It's those neutrons that make it valuable to a thermonuclear device but they'd make for major problems for ground level launches. They're les than ideal for the pusher plate as well.

Fission on the other hand puts out 94% of its' energy in the form of charged particles.

I'm sure that pure fusion is possible in theory but I'm not at all sure that it makes for practical explosions large enough for Orion. The National Ignition Facility

https://lasers.llnl.gov/about/what-is-nif

doesn't make for very large explosions at all and it's bit heavy even for optimistic Orions.
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Old 12-07-2017, 09:43 PM   #24
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Default Re: [Spaceships] Mass-driver realism

But you can detonate a pure fusion device without a fissionable element because it is not thermonuclear. As I said, you want to use something else that tritium (50% of the D-D reactions create tritium, which is consumed by secondary fusion reactions and produces the problematic neutrons). You would want either He-3 + He-3 or p + Be-11 (there are proponents of p + Li-7, but around 10% of the reactions instead produce n + Be-7 instead of 4 He, which means that the n produced by the reaction ends up having an energy equivalent to the n from a D + T reaction). Presumably, the device would depend on compression caused by metallic hydrogen detonated by a conventional explosive (Orion is an external explosion, so you cannot use lasers or ion beams with any reasonable chance of success for detonation).
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Old 12-07-2017, 10:28 PM   #25
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Default Re: [Spaceships] Mass-driver realism

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But you can detonate a pure fusion device without a fissionable element because it is not thermonuclear.
I have 2 things to say about the above statement. The first is that it is not yet proven that you can initiate a significant "pure" fusion explosion at this time. The second is that such fusion will always be thermonuclear because it is being initiated by heat.

Fusion without heat would require something like meson catalysis and we don't have a miraculous source of mesons right now.

I'm generally familiar with aneutronic fusion reactions being a veteran of Transhuman Space but the even in that setting's 2100 AD the fusion pulse drives they use are still quite optimistic and still not useful for launching from Earth's surface. They definitely not something we could have today if we'd spent just a little bit more money.
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Old 12-07-2017, 11:41 PM   #26
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Fusion caused by metallic hydrogen compression, inertial compression, magnetic compression, etc is not generally considered thermonuclear because it does not involve a fission explosion.
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Old 12-08-2017, 02:38 AM   #27
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Default Re: [Spaceships] Mass-driver realism

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Originally Posted by Flyndaran View Post
The danger of a working Orion lift off would be much less dangerous than mundane industries as far as disease and sickness goes. But the OMG, RADIATION! terror would do the killing for it.

Panic and politics would be the true dangers, and very real too.
The calculation at the time was ten extra deaths per vehicle launch (compared with a thousand overall from the general atmospheric testing that was going on). Using better models, it's probably more like a hundred per launch.

If you are interested in Orion, the absolutely necessary book is George Dyson's Project Orion: The True Story of the Atomic Spaceship. Highly recommended.
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Old 12-08-2017, 10:33 AM   #28
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Fusion caused by metallic hydrogen compression, inertial compression, magnetic compression, etc is not generally considered thermonuclear because it does not involve a fission explosion.
I'd say that it was a nuclear reaction initiated by heat. There's your "thermo" and your "nuclear" right there. :)

To my understanding "thermonuclear" exists to distinguish reactions caused by heat from those initiated by neutrons.
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Old 12-08-2017, 11:11 AM   #29
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Default Re: [Spaceships] Mass-driver realism

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Fusion caused by metallic hydrogen compression, inertial compression, magnetic compression, etc is not generally considered thermonuclear because it does not involve a fission explosion.
Even assuming that definition was true, it's somewhat irrelevant. What we care about is the quantity of radioactive material generated, which is a combination of fragments of the bomb itself that are radioactive (this is mostly from the fission component, though you presumably get some tritium release) and neutron activation of materials exposed to the bomb (fusion generally produces more of these than fission, though it depends significantly on fuel mix and completeness of burn -- the cycle n+Li6 -> 4He+T, T+D -> 4He+n doesn't actually produce any net neutrons, but some neutrons always escape).
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Old 12-08-2017, 07:12 PM   #30
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Default Re: [Spaceships] Mass-driver realism

Theoretically, you can build a clean fusion bomb - you use a "burn" of pure He-3, (current thermonuclear bombs use a Tritium( H-3) Deuterium (H-2) mix), and you use non-fission methods of achieving the compression/heating (the difference between them is theoretical at the energies needed) for fusion to initiate. The He-3 He-3 reaction is He-3 + He-3 → He-4+ 2 protons. This means no radiation, just heat. As such, the device would be completely clean in it's primary reaction, making an Orion launch fine and dandy.

However, the difficulty is getting a He-3 He-3 reaction, which requires much higher effective temperatures than H-2 +H-3. Certainly not achievable at current TL-8...
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