The REAL problem, in my opinion, is this: Back in the early 70s congress did not have the testicular fortitude to fund NASA properly to do the more high $$$ shuttle options (like the reusable fly-back first stage option with an Orbiter on top). It was far cheaper up front to slap a side-saddle Orbiter on the side of an expendable External Tank with two jumbo Solid Rocket Boosters on each side. Little did they know how much money would need to be spent in the following three decades on that cheap-up-front configuration. Oh well....

But to keep this on topic: the Orbiter's APUs have turbines in them!