G.Haddow, P.Grosz The German Giants (Putnam)
The successor of the R.VIII was to have been the SSW R.IX, which was in the project stage at the time of the Armistice. Idflieg had already assigned the military number 204/16 to the project, and although it started as a bomber, SSW changed the design into a passenger transport capable of carrying thirty-six including the crew.
As a passenger transport, the R.IX was divided into two decks; the lower deck contained a nose baggage compartment followed by a large engine-room for eight 300 h.p. Basse und Selve BuS.Iva engine. The tractor and pusher propeller were driven by four engines, each coupled to a common gear-box exactly like the pusher-drive configuration in the R.VIII. A combined buffet and lounge was located behind the engine-room, with steps leading up to the rear of the passenger deck. The seats for twenty-four passengers were placed two abreast, separated by an aisle which provided access to a passenger's observation deck located ahead of the wings above the commander's, navigator's compartment. The fully-enclosed pilots' cabin was situated above the front pair of engines and provided good visibility to all sides. In all other respects the R.IX was basically an enlarged version of the R.VIII. Specifications regarding the R.IX are not available.
SSW Kann Project
In 1918 SSW became interested in a unique project by an engineer named Kann, for an R-plane powered by steam turbines. Two oil-fired steam generators were to be located in the fuselage, and the steam fed to four turbines mounted in the monoplane wings, and driving four tractor propellers at 800 r.p.m. via gearboxes. There must also have been some form of condenser unit to reclaim the steam, but unfortunately this is not shown in the drawing. It was estimated that the four turbines would develop a total of 3000 h.p., and enough oil was to be carried in wing tanks for 51 hours flight. The wings, which had an area of 500 sq. m. also housed bays for 3000 kg. of bombs. Total laden weight of the machine was to be 25,000 kg.
J.Herris Siemens-Schuckert Aircraft of WWI (A Centennial Perspective on Great War Airplanes 12)
The SSW R.IX was designed as an eight-engine successor to the R.VIII. Assigned military serial number 204/16, the design was changed to a large passenger aircraft after the Armistice. The R.IX was designed for the same 300 hp BuS.IVa engines used in the R.VIII. Other than the conversion of the fuselage for carrying passengers, the R.IX was essentially an enlarged R.VIII, but no construction was undertaken.