C.Owers Hansa-Brandenburg Aircraft of WWI Vol.3: Monoplane Seaplanes (A Centennial Perspective on Great War Airplanes 19)
Heinkel opened a small manufacturing business in Grunbach after the war. Carl Caspar had managed to salvage some of his aeronautical works and approached Heinkel as a designer. According to Heinkel's autobiography Christiansen made the Americans aware of Heinkel's efforts with submarine borne aircraft and the US Navy was interested enough to ask for a design. This was despite the Armistice and Peace Treaty provisions that prevented Germany from manufacturing aircraft for a period then from constructing military aircraft or any aircraft that could be modified into a military aircraft. The fact that the US did not sign the Peace Treaty probably overcame any legal problems that the USN may have otherwise encountered.
Joining the Carl Caspar Werke at Travemunde in 1921, Heinkel designed the single-seat Caspar U.1. While this was developed by Heinkel from his work on submarine carried aircraft during the war the U.1 was a biplane floatplane with cantilevered wings without interplane struts, unlike the Brandenburg W.20 flying boat. Power was supplied by a 55-hp five-cylinder Siemens-Halske radial engine. The wings could be folded for storage onboard a submarine. Constructed in great secrecy, two were purchased by the USN (Bureau Nos. A-5434 and A-6435). Japan also purchased two; these were designated U.2 and were powered by a 80-hp Gnome radial engine. Only Japan continued to develop the concept, operating submarine-borne aircraft in World War II, the Aichi A6A1 Seiran being the ultimate aircraft of its type.
The Caspar Werke had been founded in 1921 and occupied the former Fokker works at Travemunde. Heinkel was the first aircraft designer for the firm and it was here that he received his break back into aircraft design and construction post-Armistice. Due to the restrictions of the Armistice and Peace Treaty forbidding and then limiting the production of aircraft in Germany, components were built in different locations in Germany and smuggled abroad and assembled there, mostly at the Swedish Svenska Aero, the firm established by Heinkel and Carl Bucker in Sweden at Lidingo in 1921.
Heinkel moved to Sweden with German workers who would assemble the aircraft in Sweden. Only a few components, the rudders and pontoons, were actually built in Sweden. The Swedish Navy placed orders for the Caspar S.1, soon to be produced as the Heinkel He.1. Heinkel left Caspar and in December 1922 after the restrictions on the building of aircraft were lifted and established Ernst Heinkel Flugzeugwerke AG located at Warnemunde on a lakeside site near the north German coast. Military aircraft were still forbidden and Svenska Aero was to produce components, the aircraft usually being erected at the Swedish Naval Dockyards at Gashaga.
The Caspar U.1 and S.1 seaplanes were exhibited at the Gothenburg Exhibition in 1923 where the U.1 was described as "designed for stowage on a submarine."
Caspar U.1 Specifications
Span 7.2 m
Length 6.2 m
Height 2.33 m
Wing Area 14 m2
Empty Weight 360 kg
Loaded Weight 510 kg
Speed 148 km/hr
Climb to 1,000 m 6 minutes
Ceiling 1,900 m
Source: Wagner, R. and Nowarra, H., German Combat Planes, Doubleday Si Co., NY, 1971