Liquid Fuel Engines A and up...

If you used a bladder, fitted inside the oxidizer tank, to contain the fuel, no piston would be required. The fuel bladder would be compressed by oxidizer tank pressure. No moving parts.

Chuck, I produce products that use "symplistic"(sic) autogenous pressurization schemes, and have also dabbled in similarly simplistic systems using bipropellant schemes.

I cannot see how a casing failure (ultimately from over-pressurization, or detonation) is any different between liquids and solids. We use safe offset distances *because* we expect seriously bad things to happen once in awhile.

If you're never seen a KClO4 boilerplate motor rapidly disassemble itself, you might be lulled into the impression that solid-composite failure modes are relatively benign.

YES, you can have unplanned mixing in liquid bi-propellant motors--this may or may not produce spectacular failures. Using liquid N2O, for example, such unplanned mixing events (on the rare event that they happen) are pretty undramatic.

But catastrophic failure in solid composite motors can also send burning chunks of propellant in all directions--how different a fire hazard is this from burning slugs of liquid propellant?

How does a pressure vessel tell the difference between a sudden overpressure caused by "unexpected mixing", and a sudden overpressure from (for example) nozzle blockage in a composite, or unplanned "erosive" burning, or a big-ass void in the propellant?

Any liquid bipropellant motor that is likely to hit the HPR market would very likely be remote-filled, like 80+% of the current HPR hybrids on the market. N2O operating pressures are generally lower, or overlap, the operating pressures of HPR composite motors. Assuming the usual standoff distances, your worry about a pressure-vessel failure exhibits no meaningful difference between solid composites, and bi-props/hybrids.

That's a seriously cool idea. Maybe this thing could actually be built...

Point.

One is "blessed" and "tolerated" and one is not.

Point.

Oh wait. I said that too.

Use a metal bellows instead of a piston with a dynamic seal and route the O2 line around the outside. Always use static seals if possible. Always carry pressure loads on cold boundaries. Do not trust check valves to function properly.

Brad Hitch

Who's grave??

You are correct, sir . I haven't seen one in.... oooh... 45 years.

Lighten up, Cowboy. It's me. I rarely bust your cogliones.