• ramjet: A ramjet lacks a compressor; it uses the forward speed of the aircraft to compress incoming air. This only works if the aircraft is flying fast enough. Though some ramjets are able to operate at a speed of 160 kilometers per hour, they require supersonic speed to achieve a reasonable efficiency. They are at their best at around 3x the speed of sound and can reach around 6x. Ramjet engines have not yet made the jump from the experimental stage to production, though development has accelerated in the 21st century CE.
• scramjet: Ramjets lower the speed of incoming air to subsonic speeds, but scramjets keep it at supersonic speeds. They need a speed of at least 4x the speed of sound to operate and have no trouble operating at speeds up to 15x the speed of sound. Like ramjets, they are still experimental.
• rocket: Rockets carry both fuel and oxident with them and use them both as a source of energy and propellant. The rocket-powered Messerschmitt Me 163 Komet appeared as early as World War II, though remains the only operational rocket-powered aircraft until today.

Hypersonic aircraft fly high and fast enough to surprise air defense systems and outrun surface-to-air missiles. Most are targeted as spyplanes, though some may be used as bombers.

Many hypersonic aircraft will probably be unmanned drones. The first to take to the skies are likely to be missiles. With their speed, they can strike quickly at very long ranges, rivaling ICBMs. Their kinetic energy, rather than their payload, deals the most damage.