The X‑43A prototype resembles a sleek, low‑profile aircraft: a 12‑foot (3.7 m) long, 5‑foot (1.5 m) wingspan craft that weighs 2,800 lb (1,270 kg). Its most remarkable attribute, however, is the air‑breathing scramjet engine that powers it.

To understand the X‑43A’s engine, it helps to compare it to a conventional rocket. A typical rocket engine generates thrust by combusting a liquid oxidizer with a hydrogen fuel in a combustion chamber.

High‑pressure, high‑velocity hot gases flow through a nozzle, accelerating to speeds of 5,000–10,000 mph (8,000–16,000 kph) and producing thrust.

The drawback of conventional rockets is the massive onboard oxygen requirement. For instance, the Space Shuttle carried 143,000 gallons (541,314 L) of liquid oxygen, weighing 1,359,000 lb (616,432 kg). Without that oxygen, the shuttle would weigh only 165,000 lb (74,842 kg).

Air‑breathing engines eliminate the need for onboard oxygen by scooping atmospheric oxygen during flight. For Earth‑to‑orbit missions, the vehicle would carry additional oxygen, but far less than a space shuttle.

The scramjet engine is a remarkably simple design with no moving parts. The X‑43A itself functions as part of the engine system: the front acts as the intake, and the aft serves as the nozzle that accelerates exhaust air.

Combustion in a scramjet occurs only at supersonic speeds, because the air must flow rapidly enough to be compressed.

Instead of a rotating compressor like a turbojet, the vehicle’s forward velocity and aerodynamic shape compress the incoming air. Hydrogen fuel is then injected into this stream, and the expanding hot gases from combustion accelerate the exhaust to create tremendous thrust.