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Looking at the spectrum of where jet aircraft put their engines, there are examples of almost every place that could conceivably used to carry a jet engine:

Notably absent, however, are jets with engines having anything to do with the horizontal stabilizer.

Why aren't there any jets with engines mounted on/in their horizontal stabilizer?

Vikki
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The two configurations I can think of:

The jet mounted on the horizontal stabilizer (above, below, at the tip):

Unlike the vertical stabilizer, the horizontal stabilizer is trimmable (THS), so all of the engine connections (fuel, hydraulic, pneumatic) will need to be made flexible and/or pivotable, which results in added complexity and points of failure. And, the longer lateral placement of the engines will considerably increase the weight; a standard THS mounting (just two points for pivoting) is not built for transferring the thrust to the airframe. This also impacts the loading flexibility; a very tail heavy plane can't have as many passengers in the back without adding a forward ballast, further increasing the weight.

The jet mounted at the root and doesn't move with the THS:

While certainly achievable, like the L-1011's number two engine wrt the vertical stabilizer, the issue here is not why aren't the jet engines there, it's why the THS is not there.

The horizontal stabilizer of a plane like the DC-9 (and its extended family) is purposefully placed above the tail, creating a T-tail. With the tail swept back, the top of it is farther from the plane's center of gravity, giving the THS a bigger moment arm, and in turn allowing a smaller THS, and the other benefit that comes out of that is the end-plate effect of the T-tail, which also permits a shorter fin for the same control authority.

You'll also notice buried engines are of the older small-diameter type, except for large planes like the L-1011, but still that makes maintenance access a lot harder, than an exposed engine.

  • If the engine is mounted on the THS, wouldn't the tilted thrust vector create a pitching moment in the same direction as the intended trim, allowing the amount of THS movement for a given trim operation to be reduced? – Vikki Dec 19 '21 at 23:25
  • @Vikki: I think you're right, I'll revise it, thanks. –  Dec 20 '21 at 01:56
  • "a very tail heavy plane can't have as many passengers in the back without adding a forward ballast, further increasing the weight" A tail-heavy plane would compensate by having more fuselage ahead of the wings, just like today's tail-heavy planes do. Additionally, would the stabilizer's attachment points really have to be reinforced by that much to carry the engines' thrust? They already have to be pretty beefy to handle the huge amounts of downwards lift the stabilizer can generate. – Vikki Dec 20 '21 at 03:50
  • @Vikki: today's examples aren't that tail heavy, so you would need even longer fuselages that will mess with the fineness ratio; today's examples need ballast when light, and yes they're definitely not built to withstand and transfer some 100 kN longitudinally, but if you want to unaccept, feel free. –  Dec 20 '21 at 04:40