What is the maximum speed of an airplane?

Answer 1

Maximum speed of an airplane depends on a several factors. When an airplane begins to approach speeds around 3 times the speed of sound (approximately) the skin of the airplane begins to heat up. Missiles routinely fly faster than mach 3, but this has never been a problem because of the limited flight time of only a few minutes. It becomes a problem when an airplane has to fly this fast for prolonged periods. Another problem is the engine. A rocket engine will push an airplane to much faster speeds than a jet engine will. This is because the technology for "air-breathing" engines that will fly at sustained speeds of over Mach 3 has not yet been fully developed.

Many military aircraft are designed to fly faster than Mach 2, and two commercial aircraft, the Concorde and the Russian Tu-144 are both capable of flying faster than Mach 1 (they were designed just for this purpose). The U.S. and Russia both designed bombers that would fly in the area of Mach 3 during the Cold War, but neither plane entered service. Currently the fastest known manned aircraft with "air breathing" engines is the American SR-71 reconnaissance plane. This plane entered service in the 1960's. Its engines are specially designed to fly at speeds of over Mach 3 for extended periods (as opposed to quick speed runs for a short periods of a few seconds or a couple of minutes) and to fly at altitudes over 80,000 feet. Its skin is designed to shed heat build-up as quickly as possible. Also, its entire body is designed to compensate for the expansion that occurs when the skin heats up, and then the shrinkage that occurs when the airplane cools down again. The SR-71 was retired from official service in 1989. It was reactivated in 1994, and the de-activated again in 1998. The fact that the Air Force willingly retired its most exotic airplane has led many to believe in speculative theories about top-secret replacements, such as "Aurora", which may fly faster than the SR-71, and which may or may not actually exist.

Non air-breathing aircraft have flown much faster than mach 3. The X-15 experimental rocket plane made several flights near, and past the boundary of space in the 1960's. It was also flown to a top speed of Mach 6.7. As far as anybody knows this was the fastest manned and powered flight of an airplane. A rocket plane can do this because it is able to ignore the technological challenges of air-breathing engines. For these high-speed X-15 flights a special ablative heat shield had to be applied that would burn off during the airplane's brief high-speed flight.

The Australian "Hyshot" experimental aircraft became the first airplane to fly using a new type of engine called a "scramjet". This is disputed by some who claim that the Russians had earlier successes with scramjet engines. The Hyshot was an unmanned flight, but it flew for several seconds in 2002. The American-NASA X-43, which also uses a type of scramjet, flew to a top speed of Mach 9.8 (7, 546 MPH) in 2004. The flight was unmanned and lasted only 10 seconds.

There are still significant challenges to engine design and aircraft bodies (for heat considerations) before an aircraft will fly at such a speed for any sustained period of time. The Space Shuttle flies at around Mach 25 (25 times the speed of sound) when it reenters the atmosphere. However, this is not a powered flight, and the shuttle is continuously losing speed as it descends.