What are the drawbacks for having engines mounted on an aircraft such that their direction can be hydraulically controlled for best performance and efficiency? The idea would be to turn the engine 180 degrees at landing so it can operate as a thrust reverser, even on jumbos like the Airbus A380 and the Boeing 747. That would be some serious thrust reversing power.
I wouldn't worry to much about the mass of the engine. That's actually not the biggest problem. Instead, I'd be scared far more by the angular momentum. That turbine has a large moment of inertia and also a higher angular velocity.To flip it, you have to reverse the angular velocity. It's essentially flipping a gyroscope, but this is a gyroscope weighing several tons.
First drawback is going to be speed of response. The engine is big and heavy, so you aren't going to be able to spin the whole thing around in a tenth of a second. Realistically, it's going to be a few seconds to spin it around. A traditional thrust reverser moves only a very small mass (little flaps, etc), so can react much quicker.
Second drawback will be weight. Any type of rotation system is going to very heavy. You'll be flying thousands of pounds around that are only used for a few seconds during the entire flight.
Third is going to be reliability / safety. What would happen if your engine spun around when you did not want it to? E.g. what happens if the rotation system fails and you spin one engine around during takeoff? That would obviously be bad. You need to design the system such that the probability of an accidental reversal is essentially zero. That's possible, but it is going to drive a lot of cost into the system.
There will be only drawbacks, and some small practical problems such as how does the air stream into the engine when the inlet is facing backwards and we've just touched down at 150 knots.
Current thrust reversers achieve about 50-60% of reverse thrust, and the system weighs between 15 and 20% of the engine dry weight. They are very beneficial on wet and icy runways, but have to be toted around the world for the few times they make a difference.
In fact there has been a NASA report titled Why Do Airlines Want and Use Thrust Reversers? stating all above drawbacks, mentioning that savings on brakes is less than cost of the thrust reversers, and exploring what alternatives the airlines would like. The main interest was the use of variable pitch fans, so that the fan can generate the reverse thrust like propellers can. Weighs less than a classic thrust reverser too.
A 1972 article from Flight Magazine on the variable pitch ducted fan development. It actually worked: it could produce reverse thrust from adjusting the fan blade pitch. A proven working design, always a better option!
The planes would lose the capacity to suddenly abort the landing.
If something unexpected happens just before or even just after touchdown, the pilots have the option to give up, set thrust to maximum, and "take off again". Then try to land again, maybe somewhere else depending on the issue. It's an important safety feature.
If the thrust reversers are much slower to turn off, requiring flipping the engines back to position, that capacity is greatly diminished. So, less safe, unless you can make it as fast as current thrust reversers.
Some aircraft (Vertical Take-Off and Landing craft) can do something relatively similar with variable thrust direction. In the list of VTOL aircraft it should be noted that rotation generally either occurs because of ducting (i.e. the Harrier Jump Jet) or rotation of the wing surfaces (i.e. a Bell V-22 Osprey)
Looking at the wing of a A380, none of these are really feasible (with the 747 being very similar)
The safest way to rotate the engine would be to pitch it backwards so it still provides lift before reversal (yawing it would create all sorts of chaos in the airframe and control). So you would have to rotate the engine on the pylon. That means a larger pylon and some way to move the engine so the housing can complete the reverse pitch. But there's not enough room to do that. You'd have to somehow extend the engine in front of the wing first and then rotate it around its axis. That's a lot of work for a small amount of benefit.
Current thrust reversers allow aircraft to land in the same or less distance that they can also take off, without needing huge landing gear and brakes. Shortening the landing distance isn't going to help with shorter airfields, unless the takeoff distance is also shortened. Otherwise, the plane is going to be at that smaller airport until it is disassembled and trucked away.
Here is a video of a C17 that was headed to MacDill AFB, but accidentally landed at a small uncontrolled regional airport. Bit of a problem, as the airstrip was 3400 feet long, and the C17 has a 3500 foot takeoff run. Offload of cargo, a minimum of fuel, and a very gutsy pilot gets it back off the ground.
So a more powerful thrust reverser would add cost, weight, complexity, and more points of failure, without achieving any real benefit. One could shorten takeoff distance with a STOL design, but that would also shorten the landing distance by virtue of a lower minimum airspeed.
Finally, consider the one aircraft today that does have rotating engines: the V22 Osprey. It had a very difficult development cycle stretching over 20 years. When Bell designed a smaller version for the US Army, the V280 Valor, they opted not to rotate the engines, but just the rotor heads, with a clever bevel gear arrangement driven by fuselage mounted turbines.
There is one class of aircraft that does pretty much exactly this thing, the modern Zepplin NT air ships. They have engines on out riggers that can rotate to provide thrust forward, backwards, up or down (In addition to a fixed pusher prop).
In addition they have a prop at the rear mounted on a rotating collar that can be pointed at right angles to the hull in any direction to provide a means to push the stern around (And also control pitch).
Not by all accounts the easiest things to pilot.
"Serious" thrust reversing power could result in pitch up at slow speeds. Thrust reverse is limited to limit the amount of positive pitch movment that could put an aircraft onto its tail (especially when stationary and applying brakes).