Is presssuized gas absent in a turbopump-driven rocket engines?

Question

For RL-10 rocket engines, are the propellent pumps able to provide the propellent without a source of pressurant gas?

Answer 1

The ACES upper stage (in development at the moment) is being designed to do without an external source of pressurizing gas:

Fundamentally IVF is a Hydrogen/Oxygen auxiliary power unit, that uses free boiloff hydrogen and oxygen to generate electricity (eliminating main vehicle batteries), provide autogenous tank pressurization (eliminating most or all Helium bottles), and feed GH2/GO2 reaction control system thrusters (eliminating all stored hydrazine).

The system uses the heat from the IVF internal combustion engine to heat up the propellant for tank pressurization:

This heat rejection feature is much desired in the IVF engine since we wish to scavenge heat for tank pressurization

IOW the IVF boils some of the hydrogen/oxygen and uses the resulting gas to pressurize the tanks.

Answer 2

No, the Centaur with its RL-10 engines requires tank pressurization.

Helium Supply and Pressurization System - Helium is the main pressurant used for tank pressurization. Typically, four 26-inch diameter high-pressure helium bottles mounted on the aft bulkhead are used for storage. They are made of a graphite/epoxy composite overwrap enveloping a stainless steel or aluminum liner. These bottles are charged to 4000 psia prior to liftoff. Helium in-flight purges are used for the sense lines, LO2 vent valve and the LO2 bubbler line.

The bottles are manifold together and are linked to two pressurization valve modules, one for each propellant tank. Each valve module contains four individual valves, which provide single failure tolerance both to open or close. Helium is used for tank pressurization prior to engine burns. A hydrogen autogenous system bleeds gaseous hydrogen off the engines and is used for LH2 pressurization during the engine burns. A valve module identical to those used for helium pressurization provides control.

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See also How does tank pressurization work? and Disposition of the Oxidizer Tank in Rockets with Autogenous Pressurization