Fuel economy is a shared obsession of aircraft and ship operators. In that regard, ship operators generally have the advantage of running rigs big enough to carry diesel engines, whose fuel efficiency comfortably eclipses that of gas turbines, and which are also cheaper to acquire per unit of power output.
In the extreme, really large ships such as oil tankers and container carriers run two-stroke diesels literally the size of houses, installed in cathedral-sized engine rooms and driving the propellers directly.
However, there are some instances where gas turbines excel at sea. While ship owners value low fuel consumption, they also - like aircraft operators - face severe space constraints within the hull volume and can be attracted to relatively compact gas turbines.
Noise is also a factor, at least on passenger ships, and it is striking to stand in the engine room of a gas turbine-powered ship and hold a comfortable conversation; it is barely possible to endure a few minutes near a ship's diesels. Therefore, a few cruise ships have been built with gas turbines, which have the added benefit, from a passenger marketing perspective, of not pumping out visible black smoke.
But where marine gas turbines, typically derived from aero versions, have made the most headway is in warships, where space constraints, speed and the ability to cruise flat-out for extended periods prevail over fuel economy.
US Navy ships have been exclusively nuclear or gas turbine-powered for several decades. Two of the most-used powerplants are the GE LM2500, derived from the CF6 aero engine, and the Rolls-Royce MT30, derived from the Trent 800 as used on the Boeing 777.
The US Navy's latest selection is noteworthy, though, for an unusually close connection between the aero and marine derivatives. Whereas the MT30 chosen for the US Navy's DDG 1000 destroyer and Littoral Combat Ship and the Royal Navy's aircraft carriers is 80% common to the Trent 800, the MT7 recently chosen to power the US Navy's new "Ship to Shore Connector" (SSC) hovercraft will be, between the intake housing and exhaust, 100% AE1107, developed as the T406 to power the Bell-Boeing V22 Osprey tiltrotor.
Rolls-Royce naval marine president James McFadyen says the only variations will be a new power controller, bleed system and power take-off shaft to suit the engine's positioning in the ship. Rolls-Royce will work with Textron Marine, which is developing the SSC landing craft, to devise intake and exhaust housings, again to meet the needs of ship installation.
The AE1107, says McFadyen, is an ideal choice for several reasons. Marinisation of an aero engine normally requires some special blade coatings so the engine can withstand the salty environment. But the Osprey is designed to fly from ships and is a veteran of sandy conditions in Afghanistan, so there is "very little development risk" in creating the MT7 "variant" of an engine family that has accumulated some 45 million hours. Some trials will have to be run to demonstrate the control system meets American Bureau of Shipping standards - a marine equivalent of US Federal Aviation Administration certification.
To meet the Osprey's requirements, the AE1107 is also a very compact and lightweight engine - factors particularly important on a hovercraft, says McFadyen, to maximise performance and payload.
And, he says, the mother ships carrying SSCs will also host V-22s. With only one engine type to service, spare parts storage space is minimised and maintenance crew training simplified.
The contract awarded was to supply engines - four per ship - for the SSC development programme; first delivery is set for late 2015 and SSC testing should begin in 2016, with low-rate production to begin in 2018. However, the Navy's plan is to replace all its landing craft with 73 SSCs, so Rolls-Royce could end up delivering more than 300 engines, including spares.
Development and production will be carried out at Rolls-Royce's facility in Indianapolis and, coming as the Osprey programme winds down, should extend the life of a particularly versatile engine family.
With 300 MT7s, the total 1107 production run will stretch to 1,500 units. Relatives of the AE1107 include the AE2100 used on the Lockheed Martin C-130 and the AE3007, which powers the Northrop Grumman RQ-4A Global Hawk, the Cessna Citation X business jet and Embraer's ERJ 145 and airborne early warning derivative. As of the end of 2011, Rolls-Royce had produced 1,580 AE2100s and 3,150 AE3007s.