How Hypersonic Planes Work

NASA's experimental space plane, the X-43A, set a new speed record for aircraft on November 16, 2004. In the unmanned 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 a previous test flight.

Photo courtesy NASA The X-43A is the first aircraft to reach hypersonic speeds using an air-breathing engine.

What sets the X-43A apart from other rocket-powered aircraft is that it is powered by a scramjet engine. 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, cutting the weight of the spacecraft, the X-43A could lead to cheaper Earth-to-orbit space travel.

In this article, we'll take a look at hypersonic planes and learn about their air-breathing engines.

Living On Air

The futuristic 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.

Photo courtesy NASA The dimensions and views of the X-43A

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. For more information on rocket engines, check out the article How Rockets Work.

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 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 what a space shuttle requires.

Image courtesy NASA The air-breathing engine system

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.

Photo courtesy NASA Artist's concept of the X-43A in flight, with the scramjet engine firing

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.

Taking Flight

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.

Photo courtesy NASA The X-43A attached to the Pegasus booster rocket

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

1. The X-43A is attached to a Pegasus booster rocket.
2. The X-43A and booster rocket are carried up to about 20,000 feet (6,000 m) by a customized, B-52 aircraft.
3. The B-52 releases the launch vehicle.
4. The booster rocket accelerates to a speed of approximately Mach 5 and flies to an altitude of about 100,000 feet (30,500 m).
5. The X-43A separates from the booster rocket and flies under its own power and preprogrammed control.
6. The X-43A flies over the ocean for a few minutes before splashing down.

Image courtesy NASA

NASA officials say that the scramjet engine would be a major step forward for NASA and would arguably provide a safer, more flexible, less expensive way to get people and cargo to space.