# What does “self-pressurizing” mean in regards to propellant tanks?

I've come across this term a few times. IIRC, liquid hydrogen is "self-pressurizing", but other fuels are not. Apparently, if a propellant is self-pressurizing, we do not need to build any complex plumbing to repressurize the tank as the propellant is used up.

I understand that, normally, a tank would undergo adiabatic cooling as its pressure drops (as propellant is used up by the engine), and thus the tank would eventually crack from the extreme temeperature differential. So it's important to keep the tank pressurized even as fuel is depleted. What I don't understand is how a certain propellant can be "self-pressurizing" even tho we are using up that propellant.

What fuels and oxidizers are self-pressuring, and what ones aren't? And why and how?

P.S. Chemistry and math are fine in your answer. I'm not afraid of either.

Liquid hydrogen will boil off in the tank until the pressure reaches equilibrium.

From this page, the defining feature of a self-pressurizing liquid (also referred to as "Vapor Pressurization", or "VaPak") is that it has a high vapor pressure. Specifically for an oxidizer:

Nitrous oxide (N2O) is the most promising oxidizer that can self-pressurize because it is relatively energetic and its vapor pressure is approximately 750 psi at room temperature.

There's also an experimental setup and some charts at the above link that show how the pressure stabilizes as fuel drains out.

Here's another article that gives a much more in-depth explanation of the concept, benefits, and challenges of using a VaPak system (which are interesting but not really what you're asking for so I won't copy information out). They did have some references to tests that mentioned fuels, which I list below.

To answer your question, I've seen references to the following being self-pressurizing (linking to the reference). I don't know what fuels have been actually used for flight as the links I found are either theoretical or from test firings.

### Oxidizer

• im not sure if that first link is what I was thinking of. It sounds like they're talking about pressurized tanks for the purposes of forcing out the propellant into the combustion chamber, like for small thrusters or anything small enough to avoid a turbopump. In other words pressure-fed systems. What I was thinking of was large tanks for large rockets that need turbopumps, which cannot possible be replaced with pressure-fed systems to get the mass-flow rate needed in large rocket engines. – DrZ214 May 30 '15 at 3:18
• The physics of pressurizing the tank are the same regardless of if you're using that pressure to power your engine or simply to maintain tank integrity. The faster that the pressure drops (such as if you've got a turbopump sucking the fuel out) the faster the fuel will vaporize to maintain equilibrium, so I don't think there's a meaningful distinction between pressure-fed and turbopump engines in terms of tank pressure for this discussion. – 1337joe May 30 '15 at 3:41