Sud-Ouest SO-6000 Triton

Origins and Purpose
The design of France’s first jet-powered aircraft, the Sud-Ouest SO-6000 Triton, was begun as early as 1943 by Lucien Servanty and his team, working secretly during the German occupation of France. On 15 December 1944, SNCASO was awarded contract 4478/44 for the construction of five prototypes and a static test airframe. Construction began at Sud Ouest’s facilities at Suresnes in 1945. Despite the shortages of power and heating as well as the extremely harsh conditions immediately after the war, work on the Triton proceeded at the rate of 75 hours per week.

General description
A stocky aircraft seating a pilot and a flight engineer side by side, the Triton had little to distinguish it from any WW II aircraft except the lack of a propeller, and the jet exhaust at the aft end of the fuselage, just beneath the tail assembly. It was a cantilever monoplane characterized by an extremely-large oval-section fuselage and low-mounted trapezoidal straight wings with rounded tips, trim flaps along 57% of the trailing edge, Frise type ailerons, and a 6° dihedral. The first Triton had a “shark’s mouth” type intake. Later prototypes had side intakes.

Construction, systems and equipment
An all-metal aircraft, the Triton’s single-spar wings were built of a light alloy and were fabric-coated for a very smooth profile.  The tailplane, also fabric-coated, used duralumin spars.  The control surfaces were mechanically actuated by a MAP stick with grip, and pedals operated the rudder. There were no airbrakes on the first Triton, but these were added on latter prototypes.

 The Triton’s cockpit was quite roomy, and fitted with dual controls. It was equipped initially with Heinkel ejection seats for the pilot and the flight engineer. The cabin was heated, with a two-layer windshield, and the crew were provided with an oxygen supply. The instrument panel facing the pilot had standard displays which included a Machmeter and all the hydraulic, pneumatic and engine controls. The flight engineer’s side featured all the test instruments and gauges.

 Equipment carried onboard the Triton included a VHF radio SCR 552 and a radio compass, and a blind landing instrument.  Electricity was provided by a 2,400 – 3000 watt generator and two batteries.  The Triton was equipped with a DOP tricycle landing gear, hydraulically actuated and retractable into the wings. The nose wheel which was steerable and retracted backwards had an anti-shimmy device. The main gear had shock absorbers and a 5 m 16 cm stance.

Powerplant and fuel tankage
A variety of jet engines was used in the Triton program. The first Triton was slated to mount a Rateau SR-1A turbojet of approximately 1,400 kg thrust. This engine was excessively delayed and a Jumo 004 was eventually installed instead. Triton 02, which never flew, was also equipped with a Jumo 004. A Rolls Royce Derwent V was planned for Triton 3, but gave way before the first flight to the more-powerful Nene, rated at more than 2,100 kg of thrust. The Nene also powered Triton 04 and Triton 05.
The Triton’s engine was fed from fuel tanks in the fuselage. These consisted of an insulated main tank (1,250 liters) in the upper center and an auxiliary tank (200 liters).
The jet engine, which had an electrical starter, was fixed at three attachment points in the fuselage.

Triton 01
Weight empty: 2,848 kg
Weight loaded: 3,790 kg
Construction of the first Triton was completed in March 1946. Then it was dismantled and trucked in June to the airfield of Orleans-Bricy in the suburbs of Paris for the forthcoming first flights.

 Lucien Servanty had originally conceived the aircraft around a double-flux Rateau SR-1A turbojet of 1,4630 kg thrust, at the time under construction by engineer René Anxionnaz at the Rateau company. Unfortunately, this powerplant’s development was excessively delayed, and Servanty agreed to use a Junkers Jumo 004B-2 turbojet, from a stock found at a BMW plant inside the French-occupied zone of Germany. Test pilot Daniel Rastel, who became the first French jet-pilot in 1942 on the British Gloster E-28/39 was chosen to fly the Triton, and accordingly made several familiarization sorties on a French-held Me 262.

 A preliminary series of static runs of the Triton-fitted engine began on 19 July 1946. On 20 August, the first taxi run was undertaken with Rastel at the controls, and the aircraft achieved a speed of 150 kmph on the ground after a 1000 m run.
The aircraft was taxied again on 31 August, and in the following weeks. Nose wheel shimmy and other landing gear problems were reported along with engine over-heating and fuel leaks.

 The first flight took place in the afternoon of 11 November 1946. With Daniel Rastel and observer Armand Raimbeau in the cockpit, the aircraft made a difficult take-off after a 1,700m run. Flying at 300 m altitude and with the landing gear still extended, the crew carefully circled the base, keeping their speed at 300 kmph. Rastel observed that the controls were very sensitive to touch, before landing the Triton precisely on the runway, at 250 kmph.

 Climbing out from the aircraft, Rastel and Rimbeaud were loudly cheered by their colleagues. The Triton’s first flight, however, did not indicate the end of the aircraft’s development troubles. The second flight took place five and a half months later, on 29 April 1947 and was intended for general handling investigation and landing gear operation. Strong vibrations and stability deterioration forced the termination of the flight when the landing gear was retracted and flaps spuriously operated. These problems recurred during the third flight, on 22 May 1947 and lead to irreparable damage to the wing and lower aft fuselage. These were replaced by parts taken from the 02 which was struck off charge.

 By the time the aircraft flew for the fourth time, on 10 August, modifications had more or less cured the instability problem, but engine overheating to 250° C, a few minutes after taking-off was becoming a serious problem. After the fifth flight, on 9 September, it was clear that the aircraft cannot be flown before solving the overheating problem. A new exhaust was installed, and it succeeded during ground tests in keeping the temperature at 130°.

 Unfortunately, during the sixth flight, on 1 October, landing gear operation resulted again in serious instability problems and the aircraft had to be landed immediately, Rastel having nearly lost control of the aircraft.
None of the first six flights had lasted more than 12 minutes. Flight 7, on 5 October, was the first to last 15 minutes and it allowed the crew to pinpoint with accuracy the problem areas in the aircraft. As a result, a decision was taken to restrict flying to days when temperature was below 15° and to restrict speed to 500 kmph until the instability problem was completely solved. A new engine, more powerful than the Jumo 004 was required.
Unfortunately, the Triton’s troubles were far from over. During the eighth flight, on 11 October 1947, an emergency landing had to be made after a series of engine blasts.
The ninth flight, on 5 November, turned out to be the last of the aircraft when it was discovered that the gas exhaust heat had melted the jet pipe. Lengthy modifications to make the aircraft flyable again were deemed too expensive and impractical, particularly in view of the Jumo 004’s poor performance. Triton 01 had accumulated an hour and a half in the air during its nine flights.

Triton 02
Weight empty: 3,064 kg
Weight loaded: 3,850 kg
Triton 02 was used in static tests of the wing’s rigidity in January 1947. The aircraft was not immediately scrapped after its wing was transplanted into the Triton 01. It acquired new wings and a Jumo 004, while waiting to receive a Rolls Royce Derwent V. During ground tests, this engine also performed erratically. Triton 02 was struck off charge and its Derwent was installed in the Triton 03.

Triton 03
Weight empty: 3,557 kg
Weight loaded: 4,484 kg
Last Triton to reach the flight line, the Triton 03 embodied some redesign of the forward fuselage, the cockpit windscreen being carried forward and the nose intake being deleted. Triton 03 had been slated originally to receive a Derwent engine. This was replaced by the more-powerful Nene 104 and the aircraft first flew, thus powered, on 4 April 1950, at Orleans-Bricy. The flight was troubled by buffeting, landing gear malfunction and a brisk stall tendency, and it only lasted 10 minutes.
The second flight took place on 3 May and incurred the same landing gear trouble. The aircraft possibly never flew again.
Triton 03 is the only surviving Triton and is preserved in the Musée de l’Air et de l’Espace.

Triton 04
Weight empty: 3,197 kg
Triton 04 was the second aircraft to be completed. Weighing 3,850 kg, it was equipped with a Rolls-Royce Nene 101 turbojet rated at 2,183 kg and nose and lateral intakes and overall wing span was increased from 9.16 m to 9.96 m.. The aircraft first flew on 19 March 1948 at Orleans Bricy, with Rastel and Michel Retif at the controls. Made with the landing gear down, this first flight pointed out the advantages of the new engine. But the early flights also encountered frequent landing gear trouble. Particularly harrowing was the fourth flight on 26 March, when the right wheel collapsed on take-off but the sortie continued. Flights 5 and 6 were used for verification of the repaired landing gear. Flight 7, on 12 May, demonstrated the successful operation of the landing gear but almost came to grief when one of the cockpit’s side doors opened in flight, causing the flight to be aborted.
After Rastel, SNCASO’s Jacques Guignard took over the Triton flight operations following the traditional Me 262 familiarizations. He made his first Triton flight (Flight 9 of Triton 04) on 23 June 1948, and his second on the following day. During his third flight, on 25 June, a fabric-coated aileron tore away at 700 kmph but he managed to land safely.

 CEV test pilot Captain Roger Receveau made his first flight on this aircraft in company of SNCASO engineer Pistrak on 26 July 1948, achieving 700 kmph. This was the aircraft’s Flight 15 and it was followed by a series of modifications on the Triton 04, which included strengthening the doors and the improvement of air flow passage into the jet intakes.
Rastel flew the aircraft again in August. On 7 September, he made an emergency landing on a soft green field near the base after loss of pressure and engine flameout. The anomaly was traced to a pump leak which began just before landing. Damage was minor and the aircraft flew again on 12 October.

 Landing gear trouble was encountered in flight in early November, nonetheless, on 10 November, the aircraft made its longest flight to date, lasting 30 minutes, weighing 4,000 kg on take-off and carrying a 950 liters fuel load. It was Triton 04’s Flight 22.
By 2 December 1948, despite recurring problems with stability, the aircraft was achieving speeds in excess of 840 kmph. By year’s end it had logged 25 flights.

 Repairs and modifications of the side intakes and side doors grounded the aircraft until 12 January 1949 when it was flown to the CEV at Brétigny where flight tests would resume and would include stall investigations. By early spring the aircraft was exceeding 850 kmph regularly, and it made a 35 minute flight on 16 March. Eight days later, during an official demonstration with Rastel at the controls, the aircraft carried out an impressive series of maneuvers including tight turns and rolls.

 Rastel and Pistrak took off again on the Triton 04 on 7 May 1949 for a speed record attempt over a 3 km circuit. Aeroclub observers on the ground put the maximum speed achieved at 897.898 kmph. Rastel insisted that they were mistaken but the officials maintained their verdict.

 On 14 May Rastel flew the Triton 04 at the Paris Air Show in its first public demonstration. Shortly later it was taken in charge by the CEV whose pilot Captain Roger Receveau flew the prototype in stall and maneuverability tests in the following few months and in an official demonstration to a Yugoslav delegation. A new pilot, CEV Captain Jean Sarrail, achieved a speed of approximately 900 kmph on this aircraft around the end of the year, and by year’s end 88 flights had been logged in more than 35 hours of flying time.
After four flights in January 1950, the aircraft was grounded for landing gear checks and general inspection. It flew again in March, bettering its own duration record with a flight of 40 minutes on 22 March. Flight 100 was made on 6 April, with Captain Sarrail at the controls, at an altitude of 2,000 m – 4,000 m.

 The aircraft was used for familiarization flights by foreign pilots that spring then was flown in an airshow at the Orly airport in June.
One of the foreign pilots who flew the Triton 04 had been a Belgian and on 24 June, the Triton 04 was flown to Brussels where it was demonstrated to Belgian officials on 25 June then returned the next day to Brétigny.

 The aircraft spent the following months in lay-up. It flew again on 10 October 1950 (Flight 127) with Lt Sarrail at the controls. On 3 November, the aircraft was taken to Orleans-Bricy on Flight 129 while airstrip expansion work was taking place at Brétigny. This was to be the aircraft’s last flight. After landing, extensive engine and hydraulic system damages were discovered and these were enough to ground the aircraft permanently.

Triton 05
Weight empty: 3,437 kg
Weight loaded: 4,359 kg
This aircraft had been slated to mount a Derwent, but was fitted with a Nene 104. It was the first Triton to be equipped with an ejection seat (designed by Heinkel).
Equipped with lateral intakes aft of the cabin, this aircraft first flew on 23 May 1949, achieving a level run at 1,800 m and 420 kmph. During its Flight 8 on 21 July 1949, with Rastel and Pistrak in the cockpit, Rastel lost contact with the control tower in a low cloud base at 2,000-3,000 m during the return trip to Bricy. Disoriented and derailed because of strong winds, fuel ran out and Rastel initiated an emergency landing on a green field. Touchdown and initial roll proved successful, but the Triton crossed over a ditch at 50 kmph, lost its balance and tripped heavily on one side, sustaining damage which was uneconomical to repair. Triton 05 was written off after 2 hr 45 min of flying time.

There was a time when the French Air Force considered the acquisition of 50 Tritons, designated SO-6005, as trainers. Switzerland, Argentina and several nations evinced interest in purchasing for their flying schools these aircraft, which were never built, and which were designed with wing tip fuel tanks.

 The Triton, understandably, was overtaken by aeronautical progress, and the project was officially terminated on 12 December 1950, the total cost being 470 million francs. However the Triton was a stepping stone in the road to greatness. Almost two decades after designing the Triton, Lucien Servanty began his drawings of the Concorde. The rest is history.

Known Triton pilots
André Rastel
Jacques Guignard
Roland Glavany
Armand Jacquet
Paul Boudier
Jean Sarrail
Roger Receveau
Charles Goujon

© Henry Matthews 2012 – Pictures courtesy Jean Lacroze

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