The top-level autopilot used a nonlinear path controller, which produced position, attitude, and rate commands for an inner-loop controller that used linear feedback and gains computed using LQR and a linearized model of the system.

Because the maneuver the kite must fly is very dynamic (always turning, but not at constant rates), control with feedback alone would lead to pretty sloppy flight, since feedback requires an error to develop before the system can respond to it. Good feed-forward was crucial to flying well; from the desired flight path, wind speed/direction, and desired aerodynamic quantities (airspeed, angle of attack, and angle of sideslip), we could compute the required rates and feed-forward actuator displacements needed to execute the trajectory.

For more details, check out my paper titled "The Makani Autopilot: A Critical Retrospective", available here: https://storage.googleapis.com/x-prod.appspot.com/files/Maka...

The usual disclaimer: these views are my own, and do not represent Makani, abc, etc.

I was terribly disappointed the project was shutdown. The engineering problems were unique and challenging. And the team was absolutely world class.

I felt like we could do it. And I wish we were still at it. But that's purely from the technical side. I wasn't involved in things like budget, and whether it would be economically viable.

This is actually a good question as it addresses the common misconception that the purpose of the kite was just to hold the propeller blades high up in the sky. In fact, having the kite fly like an airplane is a crucial part of the principle of operation. The documenary film (https://makanifilm.com/) might help make this clear.

There are airborne wind turbine concepts that use an aerostat (blimp/balloon). However, these suffer from at least two effects: (1) the largest practical turbine aperture is quite limited, and (2) the drag of the turbine will push the whole balloon/turbine contraption downwind and downward.

Some people stayed in Alphabet, either working at Google, X, or one of the bets. Other people left Alphabet and went to work somewhere else. Some are still searching for jobs (covid made the job search somewhat difficult)