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.
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.
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.