The HTV-2, an unmanned military prototype, briefly reached Mach 20 during a test flight before crashing in 2011. NASA's unmanned X-43A set the record for fastest jet aircraft in 2004 when it flew at Mach 9.6.
In the past, engineering limitations have prevented designers from creating traditional jet engines that can propel an aircraft faster than around Mach 4. However, as of 2015, Lockheed is working on a new kind of hybrid jet engine that can propel a plane to Mach 6.
While Lockheed researches its hybrid engine, Boeing is developing the scramjet X-51 aircraft. In 2013, the X-51 completed a test flight at Mach 5.1. Such engines may be used as part of new missiles, as well as on conventional planes.
Anything greater than Mach 5 is considered a hypersonic speed. The Space Shuttle reaches speeds of up to Mach 25 when it re-enters Earth's atmosphere. At this speed, air becomes an ionized plasma and tremendous heat is generated. This is why the Space Shuttle needs so much insulation.
The rocket-powered Bell X-15 holds the record for fastest manned aircraft, with a top speed of Mach 6.72. The Lockheed SR-71 remains the fastest manned air-breathing jet aircraft, having set its record in 1976.