Jim Lovell is famously quoted for saying, “There’s one whole side of that spacecraft missing” after a routine collapse of the liquid oxygen tank ullage volume caused an explosion that infamously rocked Apollo 13. While the explosion was the result of an overpowered motor switch, the ensuing disaster underscores the importance of ullage volume collapse.

Ullage is defined as the volume of vapor above a liquid in a sealed storage tank. Ullage volume collapse is the process of stirring the contents of a cryogenic liquid tank to reduce pressure. That sentence alone should give pause to those of you who have taken thermodynamics.

Not head scratching yet? Try this one. WSU alumnus Jeff Richards is the Western Sales Manager of a leading industrial gas supplier. They are able to deliver liquid hydrogen, in non-refrigerated tanker trucks, over thousands of miles away from it’s Southern California liquefier.

Jeff once posed to me, “the drivers of our liquid hydrogen tanker trucks are able to drop the tank pressure by speeding up and then hitting the breaks. How does that work?” Now most of us should be thinking. If you shake a fluid in a sealed container, the pressure should go up, not down. In fact, the first law of thermodynamics requires that energy is conserved and never destroyed, so by adding energy to a tank by shaking or stirring, it should have higher pressure. Right? Nope, not that easy.

Cryogenic ullage volumes thermally stratify, that is high temperature vapors collect at the top and the temperature decreases with height until it matches the liquid. A liquid hydrogen ullage volume can easily be over 40 K above a liquid at 21 K (I’ve seen 77 K in bulk storage systems and up to room temperature in special circumstances). That high temperature hydrogen puts significant pressure on the liquid. To reduce this pressure the tank is stirred, splashing cold liquid over the top spaces to cool the high temperature hydrogen and thereby reduce the overall tank pressure. Dual fill ports allow liquid to be added to the top of the tank in the vapor region, which collapses the ullage volume and reduces pressure, or to add directly to the liquid, which decreases the volume for the vapor and increases pressure.

But doesn’t the hot hydrogen boil more liquid and further increase pressure? Not necessarily. The trick to ullage volume collapse is the difference between the energy required to boil the liquid (latent heat) and the energy required to change the temperature of the vapor (sensible heat). The latent heat of hydrogen at 21 K is 442 kJ/kg. The sensible heat of hydrogen vapor between 21 K and 40 K at 1.25 bar is just 210 kJ/kg. So it’s over twice as easy to cool hydrogen from 40 K to 21 K as it is to boil liquid hydrogen. As it turns out, hydrogen has a very large latent heat of vaporization compared to other cryogens. The latent heat is so large that ullage volume collapse is a relatively easy and low cost way to reduce the pressure of liquid hydrogen tanks.