The development of MD-80 series began in the 1970s as a lengthened, growth version of the DC-9-50, with a higher maximum take-off weight (MTOW) and a higher fuel capacity. Availability of newer versions of the Pratt & Whitney JT8D engine with higher bypass ratios drove early studies including designs known as Series 55, Series 50 (refanned Super Stretch), and Series 60. The design effort focused on the Series 55 in August 1977. With the projected entry into service in 1980, the design was marketed as the "DC-9 Series 80". Swissair launched the Series 80 in October 1977 with an order for 15 plus an option for five.