And that's where the first phase of the experiment stands unfinished- Liza closed up like a shutter valve under pres- sure.

The BVM consists of a flexible air chamber (the "bag", about the size of an American football), attached to a face mask via a shutter valve.

To provide stability during its vertical takeoffs the Kestrel had a reaction control system whereby compressor bleeding air was vented through special shutter valves located in the nose tail and wing tips.