[Spaceships] Orion space drive: launch from underwater?

[Anthony]

Originally Posted by Fred Brackin

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.

Though fast neutrons may cause heavy elements to break up. Pb-206 can absorb a neutron and turn to Pb-207 without any radiation, but if it splits up both halves will be extremely neutron-heavy and thus radioactive.

[Agemegos]

Thanks everybody. I'm trying to make an adventure that is agnostic as to whether setting is superscience or not, and what sort of spaceship tech is in use, while wasting the fewest possible words on enumerating technological contingencies. The adventure is going to need

1. a branch in which the greeblies can launch their ship from the seabed (which Star Wars, Star Trek, and Lensman pseudotech would all allow);
2. a branch in which they can do final assembly underwater, but have to float the ship to the surface and launch from there;
3. a branch in which they have to launch the ship from above the surface, and therefore would have to work in environment suits and with waldoes to build a facility on a pontoon and assemble the ship on that;
4. and a branch in which the ship cannot be launched.

I reckoned that external pulsed nuclear drive technology would go in the third branch, but wanted to make sure.

[Anthony]

Originally Posted by Agemegos

"External pulsed nuclear" will have to be listed in the "float to surface for launch" branch.

Is there any other branch? Sure, if the backblast isn't too excessive you can launch rockets from a short distance underwater, but nothing will launch from deep underwater and you don't want to launch from significantly underwater with anything.

[Agemegos]

Originally Posted by Anthony

Is there any other branch?

Yeah. I've edited post #12 because I thought that needed to be explained better. Unfortunately I wasn't quick enough to head off your perplexity.

There is a scene in a Lensman book — at least one — in which a ship hidden on the seabed simply turns on its Bergenholms and blasts off. I'm pretty sure that Star Wars or Star Trek would allow something like that too.

If you were using liquid-fuel rockets I'm pretty sure that you would have to get the nozzle clear of the water before you could launch, so floating the ship to the surface with inflated bags would not be sufficient: you'd have to build a floating launch facility and assemble the rocket on top of that, which means vulnerable and conspicuous surface works well in advance of possible launch.

[Anthony]

Originally Posted by Agemegos

If you were using liquid-fuel rockets I'm pretty sure that you would have to get the nozzle clear of the water before you could launch

You would need it to be clear of water, but as long as the exhaust pressure exceeds external pressure, you can lose the seal once the rocket fires, so in principle you can launch from a bit underwater (deep underwater, water pressure will be more than exhaust pressure). However, if it's using hydrogen as fuel, liquid hydrogen is very very bulky, much less dense than water, and needs lots of insulation, all of which argue for tanks that are above water.

[Ulzgoroth]

Originally Posted by Agemegos

Thanks everybody. I'm trying to make an adventure that is agnostic as to whether setting is superscience or not, and what sort of spaceship tech is in use, while wasting the fewest possible words on enumerating technological contingencies. The adventure is going to need

1. a branch in which the greeblies can launch their ship from the seabed (which Star Wars, Star Trek, and Lensman pseudotech would all allow);
2. a branch in which they can do final assembly underwater, but have to float the ship to the surface and launch from there;
3. a branch in which they have to launch the ship from above the surface, and therefore would have to work in environment suits and with waldoes to build a facility on a pontoon and assemble the ship on that;
4. and a branch in which the ship cannot be launched.

I reckoned that external pulsed nuclear drive technology would go in the third branch, but wanted to make sure.

I'm not sure why the distinction between 2 and 3 here. If it's just about lifting the ship clear of the water before launching it, you could assemble the launch gantry and platform underwater and float the whole thing up after.

[Agemegos]

Originally Posted by Ulzgoroth

I'm not sure why the distinction between 2 and 3 here. If it's just about lifting the ship clear of the water before launching it, you could assemble the launch gantry and platform underwater and float the whole thing up after.

Yeah, but if it involves filling tanks with fuels and cryogenic oxidisers, emptying and drying the interiors of rocket combustion chambers?

There is a big difference between (1) being able to stay secret until the opposition's orbit takes it below the horizon, and then pop to the surface and launch to orbit in under forty minutes (i.e. the first thing the PC will know about it is that the greeblies will be in orbit) and (2) having to get the ship to the surface and perform hours or days of prep to get it fuelled and ready for launch, with PCs dropping in and bad guys dropping rocks.

I hadn't thought of building the entire launch complex as a sort of buoyancy-launched pre-first stage that floats to the surface with the ship already on top of its launch gantry, because that would take years of engineering to accomplish and the situation doesn't allow time for the greeblies to re-engineer all that. But now that you point it out I see that that is quite a sensible way for them to engineer it from the start, easier than doing the welding and wiring in airsuits (which would need umbilicuses, I think), obvious to subaquatic engineers, and very plausibly the way they would engineer the operation from the start.

So even in the branch in which the greeblies need 24 hours at the surface to get their ship dried out and fuelled up, there might well be no space launch facility visible at the surface when the PCs arrive, and the whole gigantic thing might bob to the surface in the middle of the adventure.

Thanks.

[Flyndaran]

Not rocket-torpedo-lifter stage launching the whole thing to the surface?

Exactly how deep does this vehicle start from? If we're talking miles down, then buoyancy stage sounds best as well as quieter/subtler for anyone that might have oceanic passive sonar systems in place.

[Ulzgoroth]

Originally Posted by Agemegos

Yeah, but if it involves filling tanks with fuels and cryogenic oxidisers, emptying and drying the interiors of rocket combustion chambers?

There is a big difference between (1) being able to stay secret until the opposition's orbit takes it below the horizon, and then pop to the surface and launch to orbit in under forty minutes (i.e. the first thing the PC will know about it is that the greeblies will be in orbit) and (2) having to get the ship to the surface and perform hours or days of prep to get it fuelled and ready for launch, with PCs dropping in and bad guys dropping rocks.

Yeah, there's certainly room for the ship to require some above-water work before it can launch. Or even a whole lot of work...there are a number of rocket components that should really never be immersed in seawater, I'd think. (They could be manufactured underwater in 'dry chambers', but would have to be transported to the surface in sealed containers.)

Originally Posted by Agemegos

I hadn't thought of building the entire launch complex as a sort of buoyancy-launched pre-first stage that floats to the surface with the ship already on top of its launch gantry, because that would take years of engineering to accomplish and the situation doesn't allow time for the greeblies to re-engineer all that. But now that you point it out I see that that is quite a sensible way for them to engineer it from the start, easier than doing the welding and wiring in airsuits (which would need umbilicuses, I think), obvious to subaquatic engineers, and very plausibly the way they would engineer the operation from the start.

So even in the branch in which the greeblies need 24 hours at the surface to get their ship dried out and fuelled up, there might well be no space launch facility visible at the surface when the PCs arrive, and the whole gigantic thing might bob to the surface in the middle of the adventure.

Thanks.

Glad to be of use

[Flyndaran]

Originally Posted by Ulzgoroth

Yeah, there's certainly room for the ship to require some above-water work before it can launch. Or even a whole lot of work...there are a number of rocket components that should really never be immersed in seawater, I'd think. (They could be manufactured underwater in 'dry chambers', but would have to be transported to the surface in sealed containers.)

Glad to be of use

Don't real world rockets often have cowlings discarded after exiting the atmosphere? Underwater launches would just require tighter seals.

I doubt pusher plates need that much protection and being by far the largest component of an Orion ship...