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.


  1. Do the ullage bulkheads really slide? If so, how do they work? O-rings?
  2. Why is there an additional "intermediate bulkhead" in the middle of the liquid oxidizer? What is it for, how does it work?

OTRAG from Astronautix

OTRAG from Astronautix OTRAG from Astronautix


Details here: https://www.opsjournal.org/DocumentLibrary/Uploads/OTRAG_u2_final_LK%20.pdf

Each 3 m tank segment had a tank bottom which was perforated so the tanks could be filled continuously. The fuel tanks are only partially filled, the rest is filled with compressed air at up to 580 PSI [40 bar] initial pressure, which provides for the fuel transport. Due to the emptying of the tanks, the pressure then dropped to 218 PSI [15 bar] at the end of the burn phase. This blowdown feed system is the cheapest and most reliable.

The bulkheads do not move like pistons.

The bulkheads below the ullage air (and the one in the middle of the oxidizer) are simply perforated, and the compressed air actually enters the oxidizer and fuel tanks (called blowdown). Vehicle acceleration keeps the air from entering the feed lines to the motor.

enter image description here

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  • $\begingroup$ Thanks for the speedy answer! I'm not sure I understand how the pressurization alone provides ullage (ensuring the liquid is at the bottom or outlet of the tank), but that's probably a separate question. $\endgroup$ – uhoh Sep 11 '19 at 6:48
  • $\begingroup$ Ullage is provided by virtue of the fact that they're under acceleration (gravitational or thrusted) at all times, and 40-bar air is substantially less dense than kerosene, diesel, or nitric acid. $\endgroup$ – Russell Borogove Sep 11 '19 at 14:22
  • $\begingroup$ @RussellBorogove that of course makes sense, the engines don't need to restart after attaing orbit. But what's confusing me is the labels in the image in the question label the gas as ullage. If I understand the term correctly, the ullage is provided by acceleration not by gas pressure. The gas pressure provides pressure, but not ullage. $\endgroup$ – uhoh Sep 11 '19 at 22:31
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    $\begingroup$ "Ullage" basically just means "the portion of the tank that isn't filled with liquid". The extended rocket-science implications of ullage are implicit in casual rocket-science discussion. The fact that there's gas in there at all provides "ullage" in the most restricted sense; the fact that the booster is always under acceleration from ignition until the propellant runs out provides "ullage" in the rocket-science sense; the fact that it's pressurized provides pressure. $\endgroup$ – Russell Borogove Sep 11 '19 at 22:34
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    $\begingroup$ The word ullage actually comes from Brewing. It was the space at the top of a keg of beer.google.com/amp/s/beerandbrewing.com/dictionary/amp/qJJRUFDEWB $\endgroup$ – Organic Marble Sep 12 '19 at 0:58

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