A thought on vanes: The same trick John Carmack used for the four
thruster version of the small lander can be applied using vanes. Use
the type of vanes that dip into the exhaust jet (rather than fixed
vanes), so when not in use they are out of the jet. Orient in pairs,
so that they deflect the jet sideways to produce a torque (I envision
little vanes oriented at 45 degrees to the exhaust flow, on actuators
that allow them to dip into the exhust flow and be withdrawn), so the 4
vanes are oriented to produce torque clockwise, counterclockwise,
clockwise, and counterclockwise as you go round the exit of the bell
(direction not important). This couples pitch, yaw, and roll in the
same way that the four thruster arrangement does, with the same
solution. It also reduces the ISP hit from using vanes (not really
that big of a deal) and reduces the demands on the vane materials.
Hopefully this explanation is sufficient. Another way to think about
it is similar to the limiting case of moving the four thrusters close
together so that their exhaust jets merge. Not an exact analogy, but
hopefully it clarifies my quick 'n dirty explanation above.
......Andrew
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