The existence of the H-53 series can be attributed to the United States Marine Corps. The Corps had pushed to purchase a heavy lift helicopter since 1959, with requirements that included day/night adverse weather capabiliy, rear ramp door for loading equipment and vehicles, and a sealed fuselage for water landings.  Sikorsky speeded development of the H-53 by using the six-bladed rotor and transmission from the United States Army CH-54 (S-64) Tarhe heavy lift crane helicopter, incorporating a new powerplant and watertight hull, stabilized by two lateral sponsons holding the retractable main landing gear.  The result was the CH-53A (S-65) Sea Stallion assault transport helicopter.