MUST HEAR!!! Boeing 707 Takeoff: Four JT3D turbofan engines giving their best & loudest! [AirClips] - YouTube
According to Wikipedia, the Pratt & Whitney JT3D engine inlet doors purpose is to provide additional air. Is the thrust higher on more air intake? Why don't modern engines have it anymore? -
TF33 Engine | Pratt & Whitney
Turbojet Model Pratt & Whitney JT3C (J57) | National Air and Space Museum