 Modifications to the Boeing 707 for the E-3 Sentry included a rotating radar dome (rotodome), uprated hydraulics from 241 to 345 bar (3500–5000 PSI) to drive the rotodome, single-point ground refueling, air refueling, and a bail-out tunnel or chute.The original design had two (one forward, and one aft), but the aft bail-out chute was deleted to cut mounting costs.Engineering, test and evaluation began on the first E-3 Sentry in October 1975.