BOAC: The 2nd column of the 3rd page (numbered 56) of
http://www.boulder.swri.edu/suborbit...Spacecraft.pdf
states that:
Two main pneumatic 625-psi actuators with a 9.5-in bore and 31-in stroke, change the position of the feather ...
http://en.wikipedia.org/wiki/SpaceShipTwo
has a tiny image of the craft, but it's pretty unreadable even if you click to enlarge.
http://en.wikipedia.org/wiki/File:Sp...al_diagram.jpg