Most fighters are born as one thing and die as one thing. The Saab 21 is the rare aircraft that lived two lives — first with a piston engine and a propeller mounted behind the pilot, then with that whole tail-end torn out and a jet engine bolted in its place. It is one of a mere handful of aircraft ever mass-produced in both piston and jet forms.

And because that propeller sat behind the cockpit, Sweden had to solve a problem no one else had faced at scale: how does a pilot bail out without being fed into his own blades? The answer — an explosive ejection seat — made the odd little Saab a quiet pioneer.

A fighter that pushed instead of pulled

When Saab drew up the J 21 in wartime, neutral Sweden needed a modern fighter and had a licence for the German Daimler-Benz DB 605 inline engine. Rather than hang it on the nose, the designers put it at the back, driving a pusher propeller, with the cockpit out front for a superb view and the guns clustered in the nose. Twin booms carried the tail clear of the propeller arc.

That layout created one genuine danger. A pilot who took to his parachute the old-fashioned way would step straight into a spinning propeller. So the J 21 was fitted with a cartridge-fired ejection seat — among the first operational ejection seats in the world, and one of the first fielded in real numbers. What began as a fix for an awkward propeller helped normalise a technology that has since saved thousands of aircrew.

Rip out the propeller, bolt in a jet

By the mid-1940s the piston fighter was already yesterday’s technology, and Sweden wanted into the jet age quickly. The pusher layout offered an unexpected shortcut: the space behind the cockpit that had held a piston engine and propeller could instead hold a jet. Saab replaced the DB 605 with a British de Havilland Goblin turbojet, and on 10 March 1947 the Saab 21R flew as the company’s first jet aircraft.

It was not a simple swap. More than half the airframe had to be redesigned — the tail raised to clear the hot exhaust, new fuel tanks added, the structure reworked. According to aviation historian Bill Gunston, the 21R was one of only two jet fighters ever successfully converted from a piston-engined design, the other being the Soviet Yakovlev Yak-15.

Overtaken almost immediately

The trouble was that a converted wartime airframe could only do so much. The Goblin was modest in thrust, the aircraft was heavy and thirsty, and its top speed of around 800 km/h was respectable but not revolutionary. Only 64 were built, and they were soon reassigned from fighter to ground-attack duties as the A 21R.

What really doomed the 21R was Saab’s own success. The purpose-built Saab J 29 Tunnan — a proper swept-wing jet — was already flying by October 1948, and it made the twin-boom stopgap look instantly obsolete. The 21R was retired by 1956, having served barely a handful of years.

And yet the strange little Saab matters. It let Sweden step from piston to jet with minimal risk, it blooded Saab as a jet manufacturer on the road to the Tunnan, Draken, Viggen and Gripen, and it helped make the ejection seat an everyday piece of fighter equipment. Not bad for an aircraft that spent its first life flying backwards.

Sources: Bill Gunston, “The Encyclopedia of Modern Warplanes”; Saab; Wikipedia.