However , it was found that the four fuel tanks obstruct the field of fire for the door guns ; thus , the external tank system ( ETS ) , carrying two fuel tanks on the stub wings , was developed .
The ESSS entered service in 1986 .
ESSS was found that the four fuel tanks obstruct the field of fire for the door guns ; thus , the external tank system ( ETS ) , carrying two fuel tanks on the stub wings , was developed .
The ESSS entered service in 1986 .