The gyros, trajectories, stabilisation bits - the electronics - are (in a sense, and I don't mean to denigrate at all the guys who do them) almost off-the-shelf technology now. You (almost..) just have to write the software - if the right plumbing is there.
Rocket plumbing still isn't anywhere near being off-the-shelf. One of the best rocket guys I know calls himself a rocket plumber, and I completely understand why. It's the major remaining really hard part.
Of course maybe we're using the word plumbing in an unusual sense - it's not just about putting pipes in the right place, it's about flows and pressures, and flow and pressure control, and startup transients and pogo-ing, and combustion stability, and the RCS rockets and their controls which control attitude, and the hydraulic bits which control the direction of thrust, and turbopumps, and tank design, and tank pressurisation design, and chamber and nozzle design, and cooling, and so on - basically, almost all the engineering bits of a rocket are, or can be, considered plumbing.
And when you need things which have -180 degrees C on one side, and
3,500 degrees C 5 mm away, with volumes larger and pressures higher than those in gas cylinder bottles, but only a few mm of material to contain them - and which use flow rates approaching ridiculous levels and cryogenic, corrosive, explosive and toxic fluids - and twice the shaft horsepower of a F1 car at full throttle being both produced and used in something you can hold in one hand - or 4 shaft HP produced and used in something the size of a 35mm film canister - then it gets difficult.But that's rocket plumbing.
-- Peter Fairbrother