[Spaceships] Orion space drive: launch from underwater?

[Agemegos]

Originally Posted by Flyndaran

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.

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.

[Johnny1A.2]

Originally Posted by Agemegos

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.

[Agemegos]

Originally Posted by Johnny1A.2

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.

[dcarson]

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.

[Agemegos]

Originally Posted by dcarson

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.

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.

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.