FT: Fuel

This has probably been discussed on the list before, I joined it, but
what do spaceships use for fuel in the "official" GZG universe?

I thought that it would probably be skimmed from gas giants, ala
Traveller, but wouldn't that mean that there would be (at least
partially) streamlined tankers/fleet auxiliaries out there, & I've never
seen a mention of them anywhere.  What do the collective you use in your
histories/campaigns?

Colin