I read this book at some point during the 90s, no idea when it was published.
I forget the specifics, but some group believed there was some sort of secret hidden on the moon, or behind it. In order to prove/reveal/whatever this secret, they hatched a very intricate plan to hijack a US space shuttle flight. They somehow altered the flight profile so that the external liquid fuel tank was not jettisoned during liftoff, and instead was used as additional fuel for the shuttle to begin its trans-lunar injection burn (a la Apollo).
(They may also have repurposed some of the interior space in the external tank for additional equipment that they would need when they got to the moon, but that may also be my memory retconning that in after reading about various proposals that would use these empty tanks for various purposes -- labs, habitation, etc. -- for constructing an orbital space station.)
I believe this group had managed to get the shuttle crewed entirely by people on their "side", but they might have also had to have overpowered some -- at the very least, half the shuttle crew was in on the plan.
The author went into considerable technical detail -- mostly via a character acting as a mouth-piece explaining the plan to another character (the MC?) -- explaining the specifics of how their plan would work; it was either entirely accurate, or grounded in enough accuracy to sound completely plausible, and in either case the author clearly knew what he was talking about, and had done his research.
Much of the story was the posturing of the two competing conspiracies -- the "good guys" that had hijacked the shuttle, and the "bad guys" that were trying to keep whatever it was hidden on the moon, well, hidden; I think the "bad guys" were a government conspiracy, or at least had very high-level government connections, whereas the "good guys" were a relatively "grass roots" conspiracy that still somehow managed to subvert pretty much every level of NASA, from modifying the shuttle and its tanks, to altering its flight profile, and even getting a crew (or at least a part of a crew) of their choosing on this shuttle flight. I'm pretty sure that at one point the "bad guys" had a kill switch of some type, and the "good guys" still on the ground (i.e. the part of that conspiracy that wasn't flying the shuttle) had to stop them, or delay them until the shuttle was safely out of range.
I realize there's not a whole lot of detail in this one, but I'm hoping the "hijacked shuttle flying to the moon and back to find a secret hidden there" aspect is unique and memorable enough that someone will recognize it.