Even if it will never actually be built, a balloon elevator for the virtual beanstalk is worth studying because its design problems are similar to those of the main platform. One of the applicable design problems is how to keep the elevator lift cells level during ascent and descent, regardless of the angle of the tether.

If the elevator lift cells do not remain level, they catch more wind and rub against each other more. Unstable lift cells can also produce secondary drag effects (different force vectors, etc.) that make the ascent and descent much more difficult, particularly near the ground station where the tether is furthest from vertical. The tether cannot be simply tied directly to the keel at any single point, because the elevator's keel will try to rotate about that point. If the tether is tied directly below the elevator's center of pressure, this doesn't immediately exert a rotational torque in itself, but doesn't produce any restoring torque to stop rotation, either. Fixing the tether off center directly on the keel will produce a restoring torque the pushes the keel off level.

One way to keep the elevator level under limited conditions is to use a triangular kite keel.

A Kite-Like Keel

In this case, the keel does move the center of rotation forward, causing the rear of the elevator to try to rise. However, the tether tension is effectively applied at a distance from the keel truss rather than directly to the keel truss, allowing it to act with a lever arm that is not parallel to the keel truss. The tether tension then exerts a counter torque that keeps the keel truss from rotating too large an angle from perpendicular to the tether. The actual angle formed with the tether -- the stable point toward which the keel's restoring torque will try to return the elevator -- is determined by the length of the forward and rear lines of the kite keel.

Unfortunately, that angle is relatively fixed. One size of kite keel cannot keep the elevator level over an ascent where the actual tether angle will vary by as much as 45 degrees. While it is possible to manually adjust the kite keel during the ascent -- the point of connection from the two keel lines to the tether can be a pulley rather than a fixed point -- it's not possible to do this fast enough to correct for changes in wind.

An alternative is to use a trapezoidal keel arrangement for the elevator.

Trapezoidal Keel

Essentially, as can be seen in the picture of the elevator keel when the tether is in a vertical position, this is a triangular kite keel with a second truss added. The upper portion -- the part affixed to the keel truss -- is allowed to rotate. The keel lines on either end will trace out a radius about points on the base truss. However, because the angles change, the torques involved change as well. There is still a substantial applied restoring torque, but a much greater range of motion is allowed for the elevator. An elevator with this type of keel can remain much closer to level during the ascent and descent.

Notice also that the trapezoidal design allows for a center truss. This can be used as a tether guide and docking mechanism.

The trapezoidal keel is promising for the main platform as well because it allows for an elevator dock with its own support structure without having to uncouple the elevator from the tether.