Hypersonic Speed Explained: How Hypersonic Planes Work

Hypersonic speed is five times the speed of sound. Scientists measure speeds this fast with a Mach number, named for Austrian physicist Ernst Mach.

Sound has a speed of Mach 1 and anything Mach 5 or above is considered hypersonic. While the speed of sound changes depending on temperature and altitude, Mach 5 is about 3,800 miles (6,116 km) per hour.

NASA's experimental space plane, the X-43A, set a new speed record for jet-powered aircraft on November 16, 2004. In the uncrewed test flight, the plane reached Mach 10 — 10 times the speed of sound or about 6,600 miles (10,600 kilometers) per hour.

This flight broke the previous speed record of Mach 7, set in March 2004 by the X-43A in an earlier test flight.

What set the X-43A apart from other rocket-powered aircraft is that a scramjet engine powered it. Instead of using onboard oxygen to combust the hydrogen fuel, the scramjet scoops up oxygen as it travels through the atmosphere.

By eliminating the need for onboard oxygen and cutting the weight of the spacecraft, the X-43A could lead to cheaper Earth-to-orbit space travel.

The X-43A prototype looks like a flying surfboard. It’s thin, has a wingspan of 5 feet (1.5 m), measures 12 ft (3.7 m) long and 2 ft (0.61 m) thick and weighs 2,800 pounds (1,270 kg). But the most unique feature of the X-43A is its engine.

The best way to understand an X-43A’s air-breathing engine is to first look at a conventional rocket engine. A typical rocket engine is propelled by the combustion created when a liquid oxidizer and a hydrogen fuel are burned in a combustion chamber.

These gases create a high-pressure, high-velocity stream of hot gases. These gases flow through a nozzle that further accelerates them to speeds of 5,000 to 10,000 mph (8,000 to 16,000 kph) and provides thrust.

The disadvantage of a conventional rocket engine is that it requires a lot of onboard oxygen. For example, the space shuttle needs 143,000 gallons (541,314 liters) of liquid oxygen, which weighs 1,359,000 pounds (616,432 kg). Without the liquid oxygen, the shuttle weighs a mere 165,000 pounds (74,842 kg).

An air-breathing engine requires no onboard oxygen. The X-43A scoops up oxygen as it flies through the atmosphere. In an Earth-to-orbit mission, the vehicle would store extra oxygen onboard, but less than a space shuttle requires.

The scramjet engine is a simple design with no moving parts. The X-43A craft itself is designed to be a part of the engine system: The front of the vehicle acts as the intake for the airflow, and the aft serves as the nozzle that accelerates the exhausted air.The scramjet engine is a simple design with no moving parts. The X-43A craft itself is designed to be a part of the engine system: The front of the vehicle acts as the intake for the airflow, and the aft serves as the nozzle that accelerates the exhausted air.

Combustion occurs in the engine only at supersonic speeds because the air has to be flowing at a high rate to be compressed.

Rather than using a rotating compressor, like a turbojet engine does, the forward velocity and aerodynamics compress the air into the engine. Hydrogen fuel is then injected into the air stream, and the expanding hot gases from combustion accelerate the exhaust air to create tremendous thrust.

As mentioned before, scramjet-powered aircraft don’t carry oxygen onboard. That means that they can’t lift off like conventional spacecraft.

The X-43A requires a booster rocket to get it up to a hypersonic speed, at which point it is released and sent flying on its own. This rocket boost is necessary for the scramjet engine to work.

Here’s a rundown of how the X-43A test flights work:

While the idea of traveling faster than sound is exciting, hypersonic aircraft haven't yet transported cargo or people. They have, however, become the subject of an arm's race.

While regular ballistic missiles can reach hypersonic speed, hypersonic missiles can travel at lower altitudes and are highly maneuverable, allowing them to avoid anti-ballistic missiles (ABM).

The two main hypersonic weapons are hypersonic glide vehicles (HGV) and hypersonic cruise missiles (HCM). HGV glide towards their targets after an initial rocket launch, like a swimmer gliding through a pool after a powerful kick off the wall. HCM are powered throughout their flight path by air-breathing engines [source: GAO].

On April 22, 2010, the Defense Advanced Research Projects Agency (DARPA) tested a hypersonic glide vehicle, the Falcon Hypersonic Technology Vehicle 2 (HTV-2). HTV-2 reached a maximum velocity of Mach 22 but only made it 9 minutes into its 30-minute planned route [source: DARPA].

In February 2023, the U.S. Army successfully tested its Long-Range Hypersonic Weapon (LRHW), which used glide technology [source: U.S. Army].

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