The engine ran on fumaric acid and aniline which was stored in two 75 gallon under wing drop tanks. The plane was tested in flight in April 1945. The rocket engine could run for about a minute. Similarly, the Messerschmitt Me 262 "Heimatschützer" series used a combination of rocket and jet propulsion to allow for shorter take-offs, faster climb rate, and even greater speeds. Cold War era During 1946, the Soviet Mikoyan-Gurevich I-270 was constructed in response to a Soviet Air Forces requirement issued during the previous year for a rocket-powered interceptor aircraft in the point-defence role. The design of the I-270 incorporated several pieces of technology that had been developed by Sergei Korolev between 1932 and 1943. During 1947, a key milestone in aviation history was reached by the rocket-powered Bell X-1, which became the first aircraft to break the speed of sound in level flight, and would be the first of a series of NACA/NASA rocket-powered aircraft. Amongst these experimental aircraft were the North American X-15 and X-15A2 designs, which were operated for around a decade and eventually attained a maximum speed of Mach 6.7 as well as a peak altitude in excess of 100 km, setting new records in the process. During the 1950s, the British developed several mixed power designs to cover the performance gap that existed in then-current turbojet designs. The rocket was the main engine for delivering the speed and height required for high speed interception of high level bombers and the turbojet gave increased fuel economy in other parts of flight, most notably to ensure that the aircraft was able to make a powered landing rather than risking an unpredictable gliding return. One design was the Avro 720, which was primarily propelled by an 8,000 lbf (36 kN) Armstrong Siddeley Screamer rocket engine that ran on kerosene fuel mixed with liquid oxygen as the oxidizing agent. Work on the Avro 720 was abandoned shortly after the Air Ministry's decision to terminate development of the Screamer rocket engine, allegedly due to official concerns regarding the practicality of using liquid oxygen, which boils at -183 °C (90 K) and is a fire hazard, within an operational environment. Work reached a more advanced stage with the Avro 720's rival, the Saunders-Roe SR.53. The propulsion system of this aircraft used hydrogen peroxide as a combined fuel and oxidiser, which was viewed as less problematic than the Avro