The Old Fuel Stub

There are  5 types of Fuel used by virtually all known Space faring civilizations.

NUC-GAS : That is Plutonium in pure nuclear gaseous form. Used by Kermac, Shiss, Nul and until 2300 by most of the Union Ships.

Molecule compacted Hydrogen: This is simply Hydrogen but the distance between the Atoms is compacted so much more fuel can be transported in the same volume. Used by 99% of all Union ships (With ISAH Pods)

Anti Matter. The direct storage and containment of Anti Matter is considered dangerous and problematic (Both Nuc Gas and Molecule Compacted Hydrogen are used to create Anti Matter at the time of need)  But several civilizations have never taken this step and keep Antimatter contained in Magnetic Containers aboard ( the Dai-Than, the Karthanians and the Togar)

Exotic Matter Drives and Exotic Matter Fuel:  the Wurgus, the old Saresii, Seenians,Y'All

Direct Energy Tab from higher Soruces : Narth, USS Tigershark. The Omega Fleet

Union Engines can use both Nuc Gas and MOCH. MOCH is simply more abundant and can be siphoned from Stars, Jovian Planets. Ice or water planets, even scooped from deep space. MOCH must be refined into molecule compacted gas. Non compacted Hydrogen works too but at 1/10th of the Energy per volume.

NUC GAS ships can not use MOCH.

By 5010 most space faring species have gone to Isah type engines (Either bought or copied) and use MOCH.