Explorer 4

Explorer 4, a high‑altitude research aircraft built by Lockheed Corporation in the mid‑1930s, represented a pivotal step in the evolution of pressurised flight. Conceived after the success of the earlier Explorer prototypes, the Model 4 first flew on 12 April 1936 from Burbank, California, and quickly proved its worth for both scientific and military tasks. Its airframe employed an all‑metal, low‑wing cantilever design with a streamlined fuselage that housed a fully pressurised cabin—one of the earliest operational implementations of cabin pressure control. Powered by a 650 hp Pratt & Whitney R‑1340 radial engine, the aircraft could reach 28 000 ft, maintain a cruising speed of 210 mph, and featured retractable landing gear, split‑flaps, and a suite of meteorological instruments. During its service life, Explorer 4 gathered critical atmospheric data that informed the planning of transcontinental air routes and contributed to the development of high‑altitude bombing tactics in World War II. Its success validated Lockheed’s engineering philosophy and directly influenced later pressurised transports such as the Constellation series, cementing the Explorer 4’s status as a landmark in aviation history. The aircraft remains displayed at the Smithsonian National Air and Space Museum, where its original instrumentation continues to educate engineers about early pressurisation techniques.