Direct electrical drives were used to move the flaps and other, usually hydraulically-operated, equipment on the aircraft. Production aircraft were powered by four Avon 201 turbojet engines, with thrust. In addition to providing thrust for flight, the engines also provided bleed air for the pressurization, ice protection, and air conditioning systems. The aircraft's electrical generators were also driven by the engines. Napier Spraymat electric heaters were installed in the engine inlets to prevent engine damage due to ice forming on the duct surfaces and then shedding into the compressors. The shape of the engine inlets were long rectangular slots in the first prototype, while the production Valiants had oval or "spectacle" shaped inlets designed to pass greater airflow for the more powerful engines that were installed. The jet exhausts emerged from fairings above the trailing edge of the wings. For required takeoff performance from short tropical dispersal bases, a jettisonable rocket booster engines pack was developed for the Valiant. Trials were performed with two underwing de Havilland Sprite boosters; these were ultimately deemed unnecessary when more powerful variants of the Avon engine became available. Also, there was an increased risk of accidents if one booster rocket failed on takeoff, resulting in asymmetric thrust. Some Valiant engines had water injection, which increased takeoff thrust by about per engine. The crew of the Valiant were contained in a pressurized compartment in the forward area of the aircraft and consisted of a pilot, co-pilot, two navigators, and air signaller (later called an air electronics officer); the manufacturing of this pressurized section was subcontracted to Saunders-Roe. The pilot and co-pilot were located on an upper level in a side-by-side arrangement, the remaining three crewmen sat at stations set lower in the cockpit to the rear. A crew of five had been enabled by the discontinuation of defensive gun turrets and accompanying air gunners, a design philosophy proved by the successful De Havilland Mosquito bomber of World War II. The pilot and copilot were provided with Martin-Baker Mk.3 ejector seats, while the rear crewmen were expected to bail out of the oval main entrance door. It has been claimed that the survivability of the rear crewmen was substantially reduced due to the ineffectiveness of this method of escape. The fuselage area behind the pressurised crew section and forward of the wing was used to house much of the Valiant's avionics, air conditioning, and the retractable main landing gear.