Peter Garrison
One good aeroplane deserves another.
The first aeroplane to cross the Atlantic was a war surplus Vickers Vimy bomber with a wingspan of 21 metres. The Spirit of St. Louis had a 14-meter wing. In 1975 I made the 3,200-kilometre trip from Gander, Newfoundland to Shannon, Ireland, by then a commonplace for single-engine planes with optimistic pilots, in a homebuilt of 7-metre span. In 1998 a pilotless aeroplane of 3-metre span did it, only to be surpassed in smallness, a few years later, by The Spirit of Butts’ Farm, a model plane of 2-metre span that arrived in Ireland with 40 grams of fuel remaining.
It is an unexpected fact of aeronautics that the range of an aeroplane is unaffected by its size. For all practical purposes, only three factors govern range. They are, first, aerodynamic efficiency, represented by the lift-drag ratio and governed, mainly, by streamlining and wingspan; second, propulsive efficiency, the product of the amount of fuel the engines require to produce their power and their effectiveness in converting that power into thrust; and, finally, the fuel fraction, or how much of the takeoff weight consists of fuel. An aeroplane of ordinary efficiency requires a fuel fraction of around one third to cross the Atlantic.
Unfortunately, there’s a catch. An aeroplane with a sufficiently heavy fuel load might be able to fly a great distance once airborne, but it might be unable to get off the ground or out of ground effect.
These facts were not unknown in the 1930s, when airlines began to eye the commercial possibilities of hauling passengers and cargo over longer and longer distances and across oceans. Fast mail service was particularly attractive; already in 1932 a couple of German passenger ships had been equipped with compressed-air catapults from which single-engine Junkers floatplanes were launched while the ships were still hundreds of kilometres from their destination. The mail beat the ship to land by 24 hours, an advantage evidently considered worth the trouble. The spectacular launchings, which jerked the roaring aeroplanes to 70 knots in a space of 60 metres, must at any rate have been a treat for weary passengers.
Lufthansa took the principle a step further, launching twin-engine Dorniers to Brazil from a catapult-equipped ship parked on the coast of Africa. Twins were considered unequal to the weather of the North Atlantic, however, and the German shipbuilding firm of Blohm & Voss, which had an active aeroplane-manufacturing component, produced a four-engine floatplane, designated Ha 139, with a takeoff weight of 18,000 kg of which – you guessed it – about a third was fuel. It too was catapult-launched from a ship.
Meanwhile, in England, the boffins of Short Brothers, a firm specialising in flying boats, were meditating the same problem when Robert H. Mayo, Technical Manager at Imperial Airways, suggested a novel solution. Why not launch a transatlantic aeroplane not from a shipborne catapult, but from a larger aeroplane already in flight?
Thus was born Mercury/Maia, also known as “the Short-Mayo Composite”. It consisted of a 7,000-kg, 23-metre wingspan aeroplane (“Mercury”) with four 365-hp, 16-cylinder, dual-crankshaft engines, planted atop an Empire flying boat (“Maia”) of about three times the size and horsepower. The unusual arrangement attracted a lot of welcome press attention, and it did succeed, but not to the point that more than one was built.
Air launch offered several advantages, principally that the heavily laden aeroplane did not have to get airborne under its own power. Others were less obvious. Since in theory Mercury would not alight until its fuel was largely exhausted, its pontoons did not have to support its laden weight and so could be relatively small. (The same was true of the Ha 139.) Lightweight fixed-pitch propellers could be used, since the low-speed, high power task of getting off the water and to cruising altitude was eliminated – though, just to be safe, Mercury/Maia did take off with all eight engines running. The long-range aeroplane could be made smaller than would normally be expected for its outsize fuel load.
But there were also disadvantages, one of which was that it is harder and costlier to build two aeroplanes than one. The scheme also suffered from an inability to get back. Lacking a second mothership on the American side, Mercury was humiliatingly obliged to island-hop from New York back to England by way of the Azores. On the other hand, with every empty cubic inch carrying fuel, Mercury did set a distance record for floatplanes, 9,678 km, that has never been surpassed.
The idea of one aircraft carrying another was not new with Robert Mayo. It had already been tried, using both a dirigible and an aeroplane as carriers, in 1916. Sopwith Camels slung beneath gigantic airships resembled flies on watermelons. (It’s unclear to me how those aeroplanes, which had to be hand-propped, got their engines started in flight.)
Fast-forward to the 1950s, and we find a rash of experiments involving so-called “parasite fighters” carried by long-range bombers. One scheme, fathered by the inventive ex-Blohm & Voss designer Richard Vogt, creator of the Ha 139, involved hooking fighters to a B-29’s wingtips to increase its span. The fighters had to navigate the white water of the B-29’s tip vortices to get clamped on, and, when attached, were hand-flown, using their elevators rather than ailerons for roll control. Apart from conceiving this idea, Vogt showed no other symptoms of insanity.
Many of NACA’s jet- and rocket-propelled X-planes, which investigated supersonic flight between 1947 and 1968, were carried aloft by motherships, from B-29s to B-52s. Rocket propulsion in particular benefited from air launch, because a rocket’s fuel load includes its oxidizer, while a jet harvests oxidizer from the atmosphere. Fuel weight, and the amount of fuel required just to lift the fuel itself, is an even more critical factor in orbital rocketry than fuel fraction is in long-distance flight.
It was with this fact in mind that Burt Rutan, eyeing the ten-million-dollar X Prize for space flight by a privately developed vehicle, decided to use a big gangly mothership to carry a tiny rocket plane aloft. The system worked; SpaceShipOne, a plastic homebuilt, surpassed the height reached by its precursor spaceplane, NASA’s hypersonic X-15. (Incidentally, in the Department of Belated Errata, I once reported that Rutan named that mothership White Knight after two X-15 pilots, Robert White and Pete Knight. He later told me that the connection had never occurred to him.)
The $25 million development bill for SpaceShipOne was paid by Microsoft’s late co-founder Paul Allen. Evidently gratified by the result, Allen then funded a new project called Stratolaunch, nicknamed Roc, which Rutan had sketched out before his retirement. The idea was that orbital rockets, no less than suborbital spaceplanes, could be smaller and more efficient if launched from an aeroplane at 35,000 feet than if they had to reach that altitude and speed under their own power. Besides, launches would be freed from the constraints of weather, geography, and elaborate ground launching facilities.
Roc took shape in a gigantic purpose-built hangar at Mojave. Intended to lift a 250,000-kilogram payload, it is powered by six 747 engines and, with a wingspan of 117 metres, is, at least in that respect, the largest aeroplane ever built. It made its first flight in April, 2019. Allen did not see it; he had joined the innumerable caravan the previous October. It has since made a dozen or so flights, some for revenue, but has yet to lift the orbit-bound 250,000 kg rocket for which it was intended.
During the seven years of its design and construction, Roc was associated with various space launch enterprises, including SpaceX and Orbital Sciences, and with various potential space vehicles, but in the end no solid linkages were formed. Shortly after the maiden flight, the parent company ceased operations and put the portentous machine up for sale at an asking price of $400 million.
It is said that space entrepreneur Richard Branson offered, somewhat unkindly, to buy it for one dollar.