Siemens-Schuckert designed a number of heavy bombers early in World War I, building a run of seven Riesenflugzeug. Intended to be used in the strategic role in long duration flights, the SSW R-series had three 150 h.p Benz Bz.III engines in the cabin driving two propellers connected to a common gear-box through a combination leather-cone and centrifugal-key clutch in SSW R.I to the SSW R.VII models (the SSW R.VIII utilized four engines). In the case of engine failure, which was extremely common at the time, the bomber could continue flying on two engines while the third was repaired by the in-flight mechanic. Two transmission shafts transferred the power from the gear-box to propeller gear-boxes mounted on the wing struts. Although there were some problems with the clutch system, the gear-box proved to be reliable when properly maintained. The SSW R.1 through the SSW R.VII designs were noted for their distinctive forked fuselage. Several of these aircraft (SSW R.V through the SSW R.VII) fought on the Eastern Front. Although interesting in concept, the cost of these and the R-types from other companies was so great that the air force eventually abandoned the concept until more practical designs arrived later in the war.