Jim Smith had significant technical roles in the development of the UK’s leading military aviation programmes. From ASRAAM and Nimrod, to the JSF and Eurofighter Typhoon. We asked his opinion on what we can learn from the Dornier Do 31. 

As with the Royal Air Force, in the early 1960s, the Luftwaffe became concerned about the vulnerability of aircraft operating from large air bases.  The British developed and eventually deployed the Harrier; the Germans, in a frenzy of innovation, developed and flew, but did not put into service, two potentially supersonic VTOL fast-jets, and a VTOL transport, the Do 31E. They also experimented with a zero-length launch system for the Starfighter, the ZELL.

The two fast-jets were the VFW VAK 191, which was intended as a strike aircraft with a limited supersonic dash capability, and the EWR VJ 101C, which was a Mach 1.8-capable VTOL interceptor. The Do 31, as a production aircraft, was envisaged as supplying tactical logistic support to the fast jets, itself using as forward operating bases the airstrips on which the ZELL Starfighters were expected to land using arrester gear.

The events at the start of the Six Day War between Israel and Egypt in 1967 showed that the concern about the vulnerability of aircraft on the ground at airbases was somewhat justified. Israel essentially destroyed the capability of the Egyptian Air Force in strikes against their airbases in the first hours of the conflict. At that time, the aircraft discussed here were all in development.

Do 31E Configuration

The Dornier 31E3 shown in the heading photo was the first, and to date, the only jet VTOL transport aircraft to fly, doing so in the E3 version in July 1967. In December of that year, the aircraft successfully completed transition from vertical take-off to forward flight, and the reverse transition to vertical landing. To do this, it operated with no less than 10 engines, two 15,000lb thrust Pegasus engines in the mid-span nacelles, and a further 4 Rolls-Royce RB 162-4D lift engines in each wing tip pod, each providing a further 4,400lb thrust. Total thrust was an impressive 65,200lb, providing some margin for hot-gas re-ingestion and suckdown effects at the maximum take-off weight of 60,600lb (Data from Janes All the World’s Aircraft 1966-67).

Payload capability was 36 fully equipped troops, or ‘two or three’ Jeeps, or palleted freight, up to a maximum of 11,000lb, and a range of 1100 miles was expected with maximum payload.

The Problems

Given the quite impressive technical achievements of the design, you may wonder why the Do 31 appears in this list. The real problem with this aircraft, and indeed the associated EWR VJ101C and VFW VAK 191B, was with the totally unachievable operating concept. It turns out that airbases (and aircraft carriers) provide really useful capabilities supporting tactical aircraft.

Without some form of operating base, maintenance, spares, fuel, accommodation, mission preparation, and protection and shelter of aircrew and maintenance personnel become major logistic problems.

When you add to this the impact on cost and complexity, weight and drag, of carrying around 10 engines, only 2 of which are used in forward flight, it becomes clear that the Dornier 31E was really up against it, both operationally and technically.

The Eventual Solution

What was the resolution, beyond cancelling all the related projects? Well, once the difficulty of operating without airbase infrastructure had been considered, the answer was to effectively protect the airbases with capable surface-to-air defences, and to protect the aircraft and support infrastructure with appropriate hardened shelters. This enabled conventional solutions to be found to the tactical requirements of most air forces, noting, however, the extremely effective use of carrier-borne (or forward-based) Harriers in support of operations by the US Marines, RAF and a number of Navies.

The tactical and logistic support of forward air operations, it turns out, can be well supported by another aircraft which was in development at the time – the Fiat G222. This has now been developed into today’s C-27 Spartan, which offers similar payload-range performance to the Dornier 31E, albeit with STOL rather than VTOL capability, at a fraction of the cost, risk and complexity of a production Do 31.

Was it worth the effort

Was the Do-31 (and for that matter, the other related projects) worth the effort? Well, all three aircraft explored the difficult VSTOL regimes, with quite challenging requirements. There is no doubt that German Industry gained significantly in understanding computer-assisted control of some advanced and challenging designs.  This may well have helped the Industry play its part in the later development of the Tornado and Eurofighter.

That said, no operational system eventuated, mainly, I suggest, because technical progress had run ahead of operational analysis, resulting in flawed requirements.

 

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Jim Smith had significant  roles in the development of the UK’s leading military aviation programmes. His latest book is available here.

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