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?