The Reaction Engines Scimitar has been proposed for the LAPCAT hypersonic airliner, and the Reaction Engines SABRE for the Reaction Engines Skylon spaceplane. Nuclear-powered ramjet During the Cold War, the United States designed and ground-tested a nuclear-powered ramjet called Project Pluto. This system, intended for use in a cruise missile, used no combustion; a high-temperature, unshielded nuclear reactor heated the air instead. The ramjet was predicted to be able to fly at supersonic speeds for months. Because the reactor was unshielded, it was dangerous to anyone in or around the flight path of the low-flying vehicle (although the exhaust itself wasn't radioactive). The project was ultimately cancelled because ICBMs seemed to serve the purpose better. Ionospheric ramjet The upper atmosphere above about contains monatomic oxygen produced by the sun through photochemistry. A concept was created by NASA for recombining this thin gas back to diatomic molecules at orbital speeds to power a ramjet. Bussard ramjet The Bussard ramjet is a spacecraft propulsion concept intended to fuse interstellar wind and exhaust it at high speed from the rear of the vehicle. Ramjet mode for an afterburning turbojet An afterburning turbojet or bypass engine can be described as transitioning from turbo to ramjet mode if it can attain a flight speed at which the engine pressure ratio (epr) has fallen to one. The turbo afterburner then acts as a ramburner. The intake ram pressure is present at entry to the afterburner but is no longer augmented with a pressure rise from the turbomachinery. Further increase in speed introduces a pressure loss due to the presence of the turbomachinery as the epr drops below one. A notable example was the propulsion system for the Lockheed SR-71 Blackbird with an epr= 0.9 at Mach 3.2. The thrust required, airflow and exhaust temperature, to reach this speed came from a standard method for increasing airflow through a compressor running at low corrected speeds, compressor bleed, and being able to increase the afterburner temperature as a result of cooling the duct and nozzle using the air taken from the compressor rather than the usual, much hotter, turbine exhaust gas. Aircraft using ramjets Hiller Hornet (a ramjet-powered helicopter) NHI H-3 Kolibrie (helicopter) Focke-Wulf Super Lorin Focke-Wulf Ta 283 Focke-Wulf Triebflügel Leduc experimental aircraft Lockheed D-21 Lockheed X-7, 1950 test vehicles AQM-60 Kingfisher, X-7 derived target vehicles using Marquardt XRJ43-MA ramjet Nord 1500 Griffon Republic XF-103, design, to