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Allison F‑80 Trijet

By Jim Leonard

One of the first jet propulsion technologies was the ramjet. The Russians began ground testing of ramjets in April of 1933, flight tested one on the front of an artillery shell in September of 1933, and began testing them on manned aircraft (starting with a Polikarpov I‑ I 5bis biplane) in December 1939. The Germans flight‑tested ramjets during WWII and the US Navy tested a ramjet‑powered missile in 1945. The United States tested German ramjets on the wing tips of a P‑51 in 1946, but this aircraft was lost in a mishap. Marquardt developed three ramjet engines, two of which were flight tested on two F‑80s and an XF‑83, both of which were powered by J33 engines. The Marquardt ramjets were 20, 30, and 48 inches and diameter. The first flight of the 'F‑80 Trijet', with 20 inch diameter ramjets operating, occurred on December 3, 1947 at Wright Field. The first flight with the 30‑inch diameter ramjets was made on April 1, 1948, at Edward Air Force Base. About 100 flights were made by this aircraft including flights in which the J33 was shut down and flight was sustained by ramjet power alone. It was reported that the aircraft flew at 500 mph on the ramjets The picture below shows the F‑80 flying with the 30 inch diameter ramjets operating. The 48‑inch ramjet was never test flown.

Three ramjet‑powered manned aircraft designs were built and flown in France by the Leduc Company beginning in 1946. Because ramjets produce no static thrust these aircraft were carried aloft on the top of larger aircraft and dropped off before the engine was started. They achieved Mach numbers of about 0.85, but were limited to fairly short flights because of high fuel consumption.

Although ramjet engines will operate at subsonic speeds, subsonic ramjets are notoriously inefficient in that they use a lot of fuel for the thrust they produce. Typically jet engines become more efficient as the compression ratio is increased. Since ramjets rely on converting forward speed into pressure their compression ratios are very low subsonically. The French ASR900 ramjet produced 25 10 lb. of thrust at sea level and 620 mph but had a specific fuel consumption (SFC) of 5.7 lb./hr/lb. of thrust. This is about 8 times the SFC of a good turbojet. Supersonic speed improves the specific fuel consumption, and the ramjet is a good low cost engine for an expendable application. Supersonic ramjets have been successful on these missiles: Boeing Bomarc, Douglas Talos, North American Navajo, and Lockheed D‑21.

Allison was involved in the development of a supersonic ramjet using pyrophoric fuel in about 1960. The fuel was TEA (Tri‑Ethel‑Aluminium), and it would ignite spontaneously when sprayed into the combustion chamber, thus making a typically simple engine even simpler.