I've made this kick-ass SSTO that could get up to 20 people to Laythe. In previous tests, I managed to get to LKO (and Minmus' surface for refueling) and re-enter with no problem.
However, after I was done with the ALT-12 Laythe tests, for some reason the Poseidon started to fail.
More specifically, it lost thrust and speed as it got to Mach 1 and never got past 400 m/s during the initial ascent.
For more details regarding the previous Poseidon tests, read my "Neptune Mission Files" post about them.
SPH image of the latest variant
CORRECTION ATTEMPT 1
Tried the "low-angle" approach to get to a high enough velocity so that the engines can't stop working
CORRECTION ATTEMPT 2
Tried to ascend as fast as possible at a 30-degree angle. Backfired miserably
How do I fix this? This plane used to be awesome, and now for some reason it's a "massive flop."