[sclarisse]

> Certainly I would have expected the orbit to go higher the faster the planets moved? Is that not the usual effect of increasing speed?

That rather depends on what you mean. The usual effect of increasing speed in a circular orbit at any given moment is to change it to an elliptical orbit, where the apoapsis is further away from the center of gravity and the object in question moves slower. And if you increase speed again at the apoapsis, you can make the orbit circular again, but the object will be moving much slower than it was to start. Thus two accelerations, both prograde, have the net effect of substantially reducing the prograde speed.

For any given circular orbit, though, lower means faster.