Lock total DV in maneuver editor/flight planner

[zajc3w]

short background:
in RSS/RO/RP-1 early career you have to use solid fuel kick stages to achieve moon flyby/impact, and they don't have shutdown capability.

Adding option to plan with fixed dV and adjust angle to prograde would be great help.
I would_ see it as checkbox that changes 3 sliders to
dV
"pitch" angle from prograde/tangent vector
"yaw" angle from prograde/tangent

or just locks the total dv and existing sliders work so increasing one decrases other 2 to keep total dV constant.

[eggrobin]

This is tracked by #870; but as you point out a polar parametrization of the burn would help even if we don’t figure out—and lock—the fixed Δv ourselves.

[eggrobin]

Duplicate of #870

[zajc3w]

Sorry for duplicate, didn't search well enough.