You're down a very deep rabbit hole making some very wild assumptions about what's possible. Among other things:
- Weldable 100 ksi steel is a rare beast. 50-75 ksi is relatively common, but more than that is a challenge. Sure, you can get 100 ksi tool or spring steel, but you can't make a pressure vessel out of that. High strength steel is commonly heat treated and/or high carbon. The former loses strength when welded, and the latter isn't weldable.
We don't need very much. In any case, 4130 is not rare, and can be heat treated post weld, though that seems like a lot of trouble.
We could also avoid welding, with a bit of cleverness. Look at older, high quality bicycles. Should we make our air tank out of Reynolds 531? Or, perhaps, we could stamp out hemispheres.
In any case, this is all easily evaded by going to aluminum and making it 3 times as thick, for the same total weight. And heat treated if welded, but I think relying on welding shows a certain lack of creativity, unless it's spot welding or ultrasonic something.
Another way to avoid it is to use something like a tiny soda bottle. Or a little Kevlar sphere or cylinder, but that's expensive.
- I don't do pressure vessels, so I'm willing to be corrected, but a factor of safety of 3.33:1 seems light for a 150-psi air canister. I would expect 5:1 to 7:1. You're walking around with a grenade, so you need to treat it as such.
Addressed by someone else. It's not an absurd safety factor. I've been within several feet of a two liter soda bottle that had 140 psi in it. If it was like a grenade, someone would have been injured. We are talking about 5 percent as much volume and little more pressure than that bottle. I admit that, when it burst, the towel to prevent shrapnel actually got a hole ripped in it and that the landlord came upstairs in 5 seconds. Better to have an older, slower landlord living further away. Don't do this at home, folks.
- How are you forming this mythical beastie? You're probably going to have to weld it somewhere, and you can't weld foil. I would guess that the minimum wall thickness is ~1/16" for a one-sided weld. Again, I'm willing to be corrected.
There are different welding processes and alternatives to welding, some addressed above.
- You have to connect tubing to the pressure vessel to do something useful. That means more welding and more stress concentrations requiring thicker walls at the connections.
No you don't. If the pressure vessel is in the right place, you can just release the cap, let two halves separate, or something like that. The larger, quicker opening means less wasted energy. What kind of and how much tubing do they attach to pyro charges?
- You have to support this thing at 100-200G for high performance rockets and for failures. That takes some more wall thickness plus support points.
If it's empty, you've got a problem anyway, and if it's full, it's got considerable support. In any case there are a number of other popular devices which won't handle that many g's. I looked at an Altus Metrum system manual. 4 of the devices listed are rated for 50g's or less, one to only 16. Another is rated to 70. See page 79:
https://altusmetrum.org/AltOS/doc/altusmetrum.pdf
There's a reason the CO2 cylinders are the thickness and weight they are.
Yes. One of the reasons is economizing when weight is not critical. Another is to take so much abuse that it would destroy a number of rockets, such as in a cautious commercial diver's buoyancy compensator. If our pressure vessel is only pressurized when inside the rocket, it's not going to get bashed anywhere near as hard.
When designing something new, it helps to come up with ways to do things before inventing the ways that it can't possibly work. If it's a new problem, the constraints aren't likely to be exactly the same as they have been with other problems.
Within the scope of our hobby not absurd hypotheticals.
Lead azide is NOT an absurd hypothetical. A housemate, years ago, made it and set it off. His was dried out on paper. A roommate, when he was a kid, made some, allowed it to dry out in a glass container, had it go off, and ended up with a little bit of glass in his eye. Fortunately, his eye recovered fully. Sodium azide is used, or maybe was used, in auto airbags. In any case, banning absurd hypotheticals in humor is unnecessary. I even put a smiley in for the stonefaces.
Nitroglycerine isn't so hard to make either. You may already be using it if your pyro charges are made from a double base smokeless powder.
As far as tiny nukes go, give it a couple of centuries. I don't know enough to say that they can't come up with a transuranic element with a reasonable half-life, a tiny critical mass, and a low yield. Time to push the accelerators. ;-)
