A much more serious issue for the Luftwaffe's single-engined fighter force during the Battle was the Bf 109E's limited fuel "capacity" as originally designed. The Bf 109E escorts had a limited fuel capacity resulting in only a 660 km (410 mile) maximum range solely on internal fuel, and when they arrived over a British target, had only 10 minutes of flying time before turning for home, leaving the bombers undefended by fighter escorts. Its eventual stablemate, the Focke-Wulf Fw 190A, was only flying in prototype form in the summer of 1940; the first 28 Fw 190A-0 service test examples were not delivered until November 1940. The Fw 190A-1 had a maximum range of 940 km (584 miles) on internal fuel, 40% greater than the Bf 109E. The Messerschmitt Bf 109E-7 corrected this deficiency by adding a ventral center-line ordnance rack to take either an SC 250 bomb for "Jabo" duties, or a standard 300 litre (66 Imp. gal/80 US gallon) capacity Luftwaffe drop tank to double the range to 1,325 km (820 mi). The ordnance rack was not retrofitted to earlier Bf 109Es until October 1940.