Has any work been done on alternatives to rolling element bearings in rocket turbopumps?

Question

Specifically:

1. Using some high pressure gas (before or after combustion) as a fluid bearing.

2. A superconducting maglev bearing, possibly using fuel as coolant.

Any related work is also welcome.

Answer 1

Yes, plenty of work has been done in that area. The best primer is probably the paper that kicked off the whole hydrostatic bearing discussion (this is what you mean by "some high pressure gas"), "Reddecliff and Vohr, 1969". A lot of work has been done since then. The French had a hydrostatic bearing test stand for the never-completed European staged combustion engine (TPX/TPTech). Pratt & Whitney wanted to replace the pump side bearing in one of the Space Shuttle turbopumps with a hydrostatic one, but nothing came of this either.

IHI, the Japanese rocket turbopump manufacturer, has released information about a turbopump that they want to sell overseas that uses hydrostatic bearings, so they are probably the furthest along. Unless of course SpaceX uses hydrostatic, but for all we know their bearings run on fairy dust.

Superconducting magnetic bearings have been studied as well.

Here are the three largest problems for these bearings:

1. The stiffness is tiny compared to rolling element bearings. Hydrostatic bearings have at most a tenth the stiffness, magnetic bearings maybe only a hundredth or a thousandth. This means that rotordynamic instability is a huge development risk, since it can't be well predicted.
2. They require an additional supply of propellant, which lowers the efficiency of the turbopump and increases the complexity.
3. Rolling element bearings have made huge strides, so expendable and even some reusable rockets have really no need. Silicon nitride bearing balls together with induction-harded Cronidur steel races have made some other components in the Space Shuttle look really bad in terms of life time.