The first 15.2 cm (6 in) diameter Cobra ramjet was made of out a P-47 exhaust tube and tested in early 1945. By October 1945 a 25 cm (10 in) diameter model reached a speed of 2250 kph (1400 mph) at an altitude of about 6,000 m (20,000 ft). Cobras were boosted to ramjet ignition speed by clusters of solid-propellant rockets. By 1947 Cobras were reaching speeds of Mach 2.4 .
Historical Essay © Andreas Parsch
Applied Physics Lab PTV-N-4 Cobra-BTV
The primary goal of the U.S. Navy's Bumblebee missile program (see also article on SAM-N-6-RIM-8 Talos) was to develop a ramjet-powered surface-to-air missile. Bumblebee was begun in 1944, and the Applied Physics Lab (APL) of the John Hopkins University and the initial research effort was directed to the propulsion system, because essentially no useful data on supersonic ramjets was available at the time.
The first 15.2 cm (6 in) diameter ramjet model, named Cobra, was made of out a P-47 exhaust tube and first tested in early 1945. The initial Cobras used only dummy ramjets and evaluated launching and flight stability issues. Later that year, development had progressed to larger vehicles with live ramjets, and in October 1945 a 25 cm (10 in) diameter model reached a speed of 2250 km-h (1400 mph) at an altitude of about 6000 m (20000 ft). The Cobra missiles were generally launched with the help of clusters of solid-fueled rocket motors.
By 1947, the Navy was experimenting with larger 46 cm (18 in) ramjets, and by that time the Cobra vehicles were generally referred to as Burner or BTV (Burner Test Vehicle). In September that year, the Cobra-BTV was formally designated as the PTV-4 propulsion test vehicle (amended to PTV-N-4 in early 1948). The PTV-N-4 could reach a speed of Mach 2.4 and an altitude of about 9000 m (30000 ft), and was used to evaluate acceleration characteristics, throttleability and powered range of the ramjet propulsion system. When the BTV flight tests were successfully completed, Bumblebee work continued with even larger (60 cm (24 in)) ramjets in the RTV-N-6 Bumblebee XPM (Experimental Prototype Missile), which was the immediate predecessor of the XSAM-N-6 Talos tactical surface-to-air missile prototype.Specifications
Except for the performance data quoted in the text, I have no further data on the physical characteristics of the PTV-N-4 test vehicles.Main Sources
 James N. Gibson: "Nuclear Weapons of the United States", Schiffer Publishing Ltd, 1996
 Norman Friedman: "US Naval Weapons", Conway Maritime Press, 1983
 Frederick I. Ordway III, Ronald C. Wakeford: "International Missile and Spacecraft Guide", McGraw-Hill, 1960
 Bill Gunston: "The Illustrated Encyclopedia of Rockets and Missiles", Salamander Books Ltd, 1979
 P.J. Waltrup et.al.: "History of U.S. Navy ramjet, scramjet, and mixed-cycle propulsion development", AIAA (American Institute of Aeronautics and Astronautics), 1996
 National Air and Space Museum Website
Status: Cancelled 1946.