the design team also chose to use externally blown flaps to increase lift. this system uses double-slotted flaps to direct part of the jet exhaust downwards, while the rest of the exhaust passed through the flap and then followed the downward curve due to the coandă effect. although the effects had been studied for some time at nasa, along with similar concepts, until the introduction of the turbofan the hot and concentrated exhaust of existing engines made the system difficult to use. by the time of the amst project, engines had changed dramatically and now provided larger volumes of less-concentrated and much cooler air. for the yc-15, four engines were used, versions of the pratt & whitney jt8d-17 widely used on the boeing 727 and douglas dc-9. the yc-15 borrowed components from other mcdonnell douglas aircraft, with its nose gear coming from the douglas dc-8 and the nose section & cockpit being derived from the douglas dc-10. parts borrowed from other aircraft included the universal aerial refueling receptacle slipway installation (uarrsi), taken from a fairchild a-10, anti-tipover stabilizer struts from the lockheed c-141 starlifter, pumps taken from the mcdonnell douglas f-15 eagle, lockheed c-5 galaxy, dc-9 and c-141 and actuators taken from the c-5 galaxy and dc-10. in addition, the environmental cooling system was composed of components taken from the dc-9, c-141 and boeing kc-135.