RogerBW's Blog

The Short Mayo Composite was a solution to the range problem of fixed-wing aircraft. The British were keen to get better air communications (for freight and mail as well as passengers) to the far extents of the Empire, airships were no longer acceptable after the failure of the R101, and there was clearly money in a fast trans-atlantic crossing.

The basic problem is a positive feedback loop. More range needs more fuel, more fuel needs more carrying capacity and a larger aircraft, and that in turn needs more fuel. But there are some useful edge cases. For example, an aircraft may be slightly too heavily laden to take off, but if it can be got into the air by other means it may be able to keep flying.

Major Robert H. Mayo took this approach. His design was for a large, heavy aircraft, which would carry a smaller and longer-ranged plane up to a useful altitude; the smaller plane would then make a long crossing, while the launch aircraft would go on to somewhere nearer or return to base. (This is not dissimilar to the concepts of staged rockets and fly-back boosters.) He worked with Arthur Gouge, the chief designer at Short Brothers, under an Air Ministry specification.

The launch aircraft, the S.21 Maia, was a heavily modified version of the Short C-Class Empire, with a pylon and trestle on top. The hull shape was optimised to plane more easily and increase the allowable takeoff weight, the wing area was increased, the four engines were moved outboard to stay clear of the cargo on top, and the rear fuselage was tilted up to raise the tailplane.

The S.20 Mercury was a more conventional four-engine floatplane, something like half the size of the Maia. It had a crew of two, and could carry around 600lb of cargo (in service, this was air mail). There was no means of moving between the two aircraft in flight.

Once Maia had reached a suitable cruising height, Mercury's aerodynamic controls would be unlocked and the trim adjusted so that she was effectively flying in (very) close formation with Maia. The pilots of each aircraft pulled their release switches; Maia would set controls for a gentle dive, and Mercury for a gentle climb. The final fastening would unlock under a strain of 3,000lb force, and the aircraft would immediately separate.

Development seems to have been trouble-free. The first in-flight separation was in February 1938, and the first transatlantic flight in July (a little under 3,000 miles from the west coast of Ireland to near Montréal): this was the first commercial, non-stop, westbound flight of the Atlantic by a heavier-than-air machine. With modifications, the range was extended to over 6,000 miles, a seaplane record.

However, the Composite was a creature of the gaps, and the gaps were closing. In 1939 Short Brothers brought in the S.26 G-Class, with a loaded range over 3,000 miles; the C-Class itself was improved to allow heavier fuel loads to be carried; in-flight refuelling was developed (allowing aircraft to take off with relatively dry tanks and then top up before setting out for the long haul); and the war came. Maia was bombed and sunk in Poole Harbour in 1941; Mercury was used for reconnaissance, then broken up later in the year so that the aluminium could be re-used. By the time the war was over, aircraft had developed far enough that the marginal improvement gained by an air launch was no longer worth having.

Though, of course, one could readily regard White Knight/SpaceShipOne as very much the same idea…

Here's a compilation of contemporary documentary footage.