The X, Y, and Z series are Xperimental SSTOs. They are heavy, sometimes a little unstable with the older crafts, but they have a much more massive range and capacity than any other SSTO.  THIS REQUIRES THE EXCELLENT  KSPI- Extended mod by Freethinker (and its requirement, Tweakscale) And: Near Future spacecraft, Solar, and Construction. USI-LS and Konstruction also present (I'll try to edit it out later ;_;) Guide to the class: X: Fission powered. Early craft, requires maximu