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Old 10-28-2014, 03:23 PM   #21
Agemegos
 
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Default Re: [Spaceships] Orion space drive: launch from underwater?

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Originally Posted by Flyndaran View Post
Not rocket-torpedo-lifter stage launching the whole thing to the surface?
I don't see an advantage in engineering it that way. Buoyancy is cheap and simple, and has enormous thrust with terrific endurance.

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Exactly how deep does this vehicle start from?
I'm hoping not to have to be specific about that. In a superscience setting it might have to be a couple of klicks deep for the greeblies' civilisation not to have been detected by "scanners". In any setting in which the sci fi is as hard as flan or harder the greeblies will have their civilisation in the photic zone of depths and will build their spaceship on top of an isolated seamount, but orbital sensors won't be able to casually scan the oceans and find the ship and launcher.
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Old 10-28-2014, 03:27 PM   #22
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Default Re: [Spaceships] Orion space drive: launch from underwater?

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I doubt pusher plates need that much protection and being by far the largest component of an Orion ship...
Indeed. If the greeblies are using Orion technology the adventure will take branch 2. They will assemble the spaceship underwater, and then when they need to launch it they will float it to the surface (probably with giant airbags), light the blue touchpaper, and be in space before the oppo can complete an orbit.
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Old 10-28-2014, 03:50 PM   #23
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Default Re: [Spaceships] Orion space drive: launch from underwater?

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Originally Posted by Flyndaran View Post
Don't real world rockets often have cowlings discarded after exiting the atmosphere? Underwater launches would just require tighter seals.
Not just tighter seals. You'd have to assemble at least that section in a dry-room up to the point of sealing on the cowling. (I imagine clean-room assembly products have similar considerations.)

But you're right, the dry bits could be designed with local sealed containers that into the ship still sealed until conditions during or after launch allow them to be opened.
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I doubt pusher plates need that much protection and being by far the largest component of an Orion ship...
Yeah, I wasn't particularly thinking Orion there.
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Old 10-28-2014, 08:07 PM   #24
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Default Re: [Spaceships] Orion space drive: launch from underwater?

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Originally Posted by Fred Brackin View Post
Nope, If being hit by neutrons does not make a substance radioactive (and this is quite common) they you can't call it a radioactive contaminant.

Radiatioactivity isn't like being tainted by Original Sin.
Tungsten contains five stable isotopes: W-180, W-182, W-183, W-184, and W-186. Neutron capture by W-180 leaves you with W-181, with a 121 day half life (and which normally decays via electron capture to stable Ta-181, so it will only give off x-rays and thus is not that bad). Neutron capture on W-182 and W-183 just gives you another stable iostope of tungsten. Neutron capture by W-184 gives W-185, with a 75 day half-life that beta decays to stable Re-185. Neutron capture by W-186 gives W-187, with a half-life of 1 day which beta decays to stable Re-187. So neutron bombardment of tungsten will give rise to some residual activity. A more complete analysis would look at neutron capture cross sections and the energy spectrum of neutrons within the tungsten shell.

Brett didn't specify what form of fusion is being used, but D-T fusion produces neutrons with 14 MeV of energy - this is really quite energetic and can lead to various nuclear spallation processes as well, such as having the neutron knocking off another neutron or proton or alpha particle. These can make make other radioactive particles, which can be the head end of a decay chain of several isotopes before you reach something stable.

Lead will be similar in general scope to tungsten, although different in the particulars.

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Old 10-28-2014, 08:21 PM   #25
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Default Re: [Spaceships] Orion space drive: launch from underwater?

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Brett didn't specify what form of fusion is being used
Indeed not. I'm trying to cover all possible bases. For what it's worth, D-T seems most plausible. But you might use an expensive and bulky aneutronic launch unit to lob the spacecraft to a safe altitude where its cheap and cheerful D-T pulse units would be not to much of a problem.
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Old 10-29-2014, 07:59 AM   #26
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Default Re: [Spaceships] Orion space drive: launch from underwater?

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Indeed not. I'm trying to cover all possible bases. For what it's worth, D-T seems most plausible. But you might use an expensive and bulky aneutronic launch unit to lob the spacecraft to a safe altitude where its cheap and cheerful D-T pulse units would be not to much of a problem.
You might use an aneutronic reaction for other purposes. D-T is easiest to ignite but putting out 80% of its' energy in 6the form of neutrons makes it hard to sustain.

Indeed, in late generation thermonuclear warheads you start out with a minimal fission stage induced by neutrons which makes enough heat to cause a very small D-T fusion reaction to make more neutrons for more fission to make enough heat for a large fusion stage and then you put some cheap uranium in the outside casing to make use of those fusion stage neutrons.

You could also insert more fission-fusion stages to make very large bombs.

So fusion-only bombs would have to be significantly different. You would need a lot of energy input to bring the whole fusion mass up to temp and you'd really rather not have all those neutrons carrying off 80% of your energy.

Any reaction other than D-T needs more energy to start the fusion process but keeps much more energy in the fusing plasma. For fusing of large masses aneutronic might require a smaller net energy input.
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Old 10-29-2014, 07:52 PM   #27
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Default Re: [Spaceships] Orion space drive: launch from underwater?

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Originally Posted by Agemegos View Post

I'm hoping not to have to be specific about that. In a superscience setting it might have to be a couple of klicks deep for the greeblies' civilisation not to have been detected by "scanners". In any setting in which the sci fi is as hard as flan or harder the greeblies will have their civilisation in the photic zone of depths and will build their spaceship on top of an isolated seamount, but orbital sensors won't be able to casually scan the oceans and find the ship and launcher.
Don't be too sure about that. Even within fairly 'hard' parameters, sensors may theoretically become frighteningly effective, esp. if combined with the right sort of software. It's mostly theoretical, but proposals have been made for things like using satellites to observe the ocean surface for the tiny swells produced by the passage of large objects deep down, or to scan for faintly warmer water heated by machines. Other things have been suggested that are compatible with known physics, and that might just work if you had enough computing power to process the data.
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Old 10-29-2014, 08:41 PM   #28
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Default Re: [Spaceships] Orion space drive: launch from underwater?

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Don't be too sure about that. Even within fairly 'hard' parameters, sensors may theoretically become frighteningly effective, esp. if combined with the right sort of software. It's mostly theoretical, but proposals have been made for things like using satellites to observe the ocean surface for the tiny swells produced by the passage of large objects deep down, or to scan for faintly warmer water heated by machines. Other things have been suggested that are compatible with known physics, and that might just work if you had enough computing power to process the data.
Good point, but detection is not the only issue. With reasonably hard ecology the greeblies' civilisation has to be in the photic zone because they can't grow enough food in the aphotic zone to support a civilisation large enough to build a spaceship.

I'm putting the adventure on a planet on the frontiers of known/settled space where the human population is either a long-lost and technologically backward colony or a fairly recent and not-yet-extensive colony or an outpost, so it's possible that it might not have been thoroughly surveyed with a fine-toothed comb of ultra-tech instruments. And it doesn't matter if people know that the greeblies have an industrial civilisation down there — in fact I rather think that the adventure has to depend on their being known-of and even having contact with the humans.

So if the science is hard enough that greeblies can't have an industrial civilisation in the abyssal depths, then it's probably hard enough that "scanners" aren't magic spaceship detectors that can scour all half-billion square kilometres of a planet's surface in a couple of hours.

And if the players won't suspend disbelief in an abyssal industrial but the GM has already established scanners that will penetrate 200m of sea-water, or if the tech of the setting will casually find and recognise a spaceship underwater, of if the campaign is set in the heart of a well-explored ten-millennium-old empire, or if there are powerful psionics who would detect and recognise the greeblies' thoughts of working on a space program? Well, it's hard to write an adventure that can be dropped into any campaign at all.
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Old 10-30-2014, 06:11 PM   #29
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Default Re: [Spaceships] Orion space drive: launch from underwater?

Really good sensors let you detect things. Purified radioactives are rare anywhere. But large things moving underwater and thermal hot spots can be detected but unless you have a good baseline you don't know if that's industrial activity or a whales and natural thermal vents. So if they haven't been monitoring it will take a while to check all the unusual things they detect.
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Old 10-30-2014, 06:30 PM   #30
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Default Re: [Spaceships] Orion space drive: launch from underwater?

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Really good sensors let you detect things.
That's true. And super-science sensors are in general better in that way than hard science sensors.

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Purified radioactives are rare anywhere.
True. But they are also hard to detect through a hundred metres of seawater using realistic technology. They might emit neutrinos that would make it, but neutrinos are crummy for imaging with.

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But large things moving underwater and thermal hot spots can be detected but unless you have a good baseline you don't know if that's industrial activity or a whales and natural thermal vents. So if they haven't been monitoring it will take a while to check all the unusual things they detect.
True. And besides — the adventure will still work if the humans can detect or even localise the industry of the subaquatic civilisation. I'm trying to present a surprise space program, not a surprise industrial civilisation.
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