The helium pressurization reminds me of how ICBMs are blown out of subs.
They have a small tank filled with water. They fire a small solid rocket motor thrust end into the water which flashes the water into steam. The steam pressurizes the tube blowing the icbm out with great force so it pops out of the sea. Then it’s own rocket engine ignites.
The neat thing about the SSBN launch system, in addition to the simplicity, is that (I’m told) the missile travels up to the surface through the water inside the steam bubble, so it experiences NONE of the hydrostatic pressure!
I’ve got a space rocket launcher design that works on a giant version of that, similar to Tesla’s HyperLoop.