How fast does NASA's X-43 fly?

Short answer

The uncrewed X-43A reached Mach 9.6 on November 16, 2004—nearly ten times the speed of sound, a record still standing for an air-breathing aircraft. But it no longer flies: there were only three flights, and each unit fell into the Pacific as planned after the attempt.

Published: September 26, 2026 · Updated: September 30, 2026 · Edited by Above Ten Thousand · Editorial policy

X-43A and its booster rocket tucked under B-52B wing

The Record Flight of November 2004

The last of the three flights in the Hyper-X program reached Mach 9.6 on November 16, 2004. The X-43A was uncrewed, only a few meters long, and not reusable. The procedure was complex: a B-52B carried the assembly—made up of a Pegasus rocket booster stage and the test vehicle—to high altitude; the booster stage accelerated it to operating speed, then the X-43A separated, fired its supersonic combustion ramjet engine for a few seconds, and then, in unpowered glide flight, delivered measurement data until it hit the Pacific.

Why a Scramjet Needs to Go Fast to Work

A scramjet has no rotating compressors. Air is compressed solely by the shape of the forward fuselage and the intake, and combustion occurs in a flow that remains supersonic—in the X-43A, using hydrogen as fuel. That only works at several times the speed of sound, so the engine can't start without a carrier aircraft and a booster stage. In exchange, it doesn't need to carry oxidizer on board, making it lighter than a rocket.

Three Flights and Done

The first attempt failed in 2001, when the booster stage went out of control and had to be destroyed. In spring 2004 came the first successful powered flight, and in November the record. Since each unit could only fly once and none was recovered, the program ended there. Work on air-breathing hypersonic engines continued later with other demonstrators running on hydrocarbons instead of hydrogen, with much longer combustion but at lower speeds.