This answer to Were there any non-state organizations to organize space flight and colonizations in the second half of the 20th century? mentions OTRAG and links to Astronautix's OTRAG page.
The images below are from the Astronautix page. THe first one shows both fuel (kerosene) and oxidizer tanks (50:50 N2O4, NHO3) about two-thirds full, with (presumably sliding) bulkheads separating the liquid from 600 PSI gas, labeled as ullage.
Questions:
- Do the ullage bulkheads really slide? If so, how do they work? O-rings?
- Why is there an additional "intermediate bulkhead" in the middle of the liquid oxidizer? What is it for, how does it work?