Peter Fairbrother wrote in message
One thing that might be done would be to introduce a very light piston, to
thermally seperate the drive gas from the pumped fuel/propellant. A disk of
foamed plastic, or the high-tech equivalent.

This idea is stated in the patent.

Quimby expanders are good for tolerance requirements, as the pressure
difference is split between several seals.

What is a Quimby expander?

Greg