A little while ago, Ars Technica did a big story on the Firefly rocket under development.


A key design feature is that it will use its own propellant to do self-pressurization (autogenous). At first I thought I understood this well, but in latter online conversations about this, I found out that this detail is actually highly controversial. Here is the part of the article that raises questions, with the emphasis mine.

heated fuel is turned into hot gas, which is then used to pressurize the fuel tanks. As noted earlier, the end result of this is a fuel tank filled with more burnable fuel, rather than an inert pressurant like helium. This makes Firefly’s aerospike fully self-pressurizing.

No one disputes that the method can pressurize the fuel tank. The problem is the wording "fully". Some people apparently believe that liquid Oxygen tank can't be pressurized by the same method, because this would result in high-temperature Oxygen gas or Oxygen-rich gas (this is my own understanding, which might have its flaws). The logic is that:

Hot oxygen will oxidize practically anything that's not already oxidized.

Checking some literature on the subject, they seem to have a point. Autogenous pressurization is addressed for a hydrogen tank in this article, for an example. But there are still several elements I can't convince myself of.

Would autogenous pressurization necessarily raise the temperature of the tank anyway? Even if the Oxygen tank was being pressurized by an Oxygen-rich gas mixture of CO2 and/or H2O, this doesn't necessarily have to be extremely hot. You already have cryogenic fluids to exchange heat with, so maybe it's below room temperature and generally not a big issue.

If I take the "fully self-pressurizing" statement to be a error of the journalist exclusively, then it seems like the Firefly design doesn't make any sense. If you're going to use Helium to pressurize the Oxygen tank, then using autogenous pressurization of the fuel tank clearly isn't going to decrease complexity. Two different methods of tank pressurization sounds fantastically complicated.

So what's the deal? Is there any credible background behind designs for autogenous pressurization of the liquid Oxygen tank? Is this debacle just poor wording or a misguided startup?

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    $\begingroup$ A mixture of CO2 and/or H2O is not useful, CO2 and H20 vapor would solidify when in contact with liquid oxygen. $\endgroup$
    – Uwe
    Commented Aug 30, 2017 at 13:26
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    $\begingroup$ My comment there had nothing at all to do with autogenous pressurization, it was in response to someone asking about oxygen rich combustion. The "hot oxygen" used to pressurize the LOX tank in autogenous systems is "hot" only in terms of cryogenic fluids, there's clearly a very wide range in both temperature and pressure between a LOX tank pressurized with GOX warmed enough to not immediately liquefy, and an oxygen-rich turbopump preburner. $\endgroup$ Commented Nov 23, 2021 at 0:32
  • $\begingroup$ The Starship autogenous pressurization plans for the oxygen tank plans to use oxygen at a temperature of 600 for the job. To my shame though, I cannot locate the original to remind myself if that is 600C or 600K, two very different things :( ...At 600K (326C) the oxygen gas is quite harmless against the stainless tank and plumbing. At 600C, not quite so much. The main issue with using hot oxygen in a cryo O2 tank is not the flammability of any materials, but the collapse of pressure as the very cold liquid sucks heat from the gas. Especially if the cryo liquid sloshes, splashes or fountains. $\endgroup$ Commented Nov 24, 2021 at 15:37

2 Answers 2


If the question is whether such a system is feasible, the answer is yes, it is 100% feasible, because STS used this exact system. The LOX and LH2 tanks in the ET were pressurized on the pad by GSE-supplied helium. After SSME start the tanks were pressurized by gaseous propellants tapped from the engines.

From the 1985 NASA shuttle press reference:

Once the liquid oxygen is loaded and ready for main engine ignition, the liquid oxygen tank's vent and relief valve is closed, and the tank is pressurized to 21 psig by GSE-supplied helium. During SSME thrusting, liquid oxygen flows out of the external tank through the orbiter/external tank umbilical into the orbiter MPS and to each SSME. Pressurization in the tank is maintained by gaseous oxygen tapped from the three main engines and supplied to the liquid oxygen tank through the orbiter/external tank gaseous oxygen umbilical.


When the liquid hydrogen is loaded and ready for main engine ignition, the liquid hydrogen tank's vent and relief valve is closed, and the tank is pressurized to 42.5 psia by GSE-supplied helium.

Approximately 45 minutes after loading starts, three electrically powered liquid hydrogen pumps in the orbiter begin to circulate the liquid hydrogen in the external tank through the three SSMEs and back to the external tank through a special recirculation umbilical. This recirculation chills down the liquid hydrogen lines between the external tank and the high-pressure fuel turbopump in the SSMEs so that the path is free of any gaseous hydrogen bubbles and is at the proper temperature for engine start. Recirculation ends approximately six seconds before engine start. During engine thrusting, liquid hydrogen flows from the external tank and through the orbiter/external tank liquid hydrogen umbilical into the orbiter MPS and to the main engines. Tank pressurization is maintained by gaseous hydrogen tapped from the three SSMEs and supplied to the liquid hydrogen tank through the orbiter/external tank gaseous hydrogen umbilical.


In describing an instance of an autogenous pressurization system, Appendix E of A Review of United States Air Force and Department of Defense Aerospace Propulsion Needs (downloaded from: http://www.nap.edu/catalog/11780.html) states:

[Vapor Pressurization] utilizes the internal energy of a liquid stored in a closed container to perform the work required to expel the liquid from the container. Before starting, the bulk liquid temperature is adjusted so that the vapor pressure equals the desired tank pressure. The liquid is in thermal equilibrium with the saturated vapors present in the tank ullage (other gases excluded). When the tank valve is opened, draining the liquid or vapor, the tank pressure drops, upsetting the vapor-liquid equilibrium. At this point, the liquid boils, creating additional vapor and tending to counteract the pressure reduction.

This would mean that the gaseous oygen would not be appreciably warmer that the LOX beneath it in the tank.


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