A view down the air intake of the forward port engine, showing a lift fan in position at the bottom of the shaft. Note also the hinged door in the centre of the shot. This would slam shut in the event of engine failure by means of the air pressure behind it, preventing the escape of air through the intakes and thus allowing the air input to the skirt from the remaining engines to keep the skirt inflated. The SRN4 is capable of hovering and moving on just one engine, although not very fast or efficiently. (James Rowson).