Is there any way to execute precisely the burn time without control of the ship? I am trying to send a lunar impactor on a rp-1 install and the last stage of my ship is unguided, so I point the ship in the right direction and make it spin before launching the probe, but I can't find a way to cut the engine exactly at the right time, when I click "execute node" in the manoeuver planner or editor, it doesn't do it or it doesn't stop when reaching 0.1 ms

No, I'm thinking about how this might get done eventually but right now the maneuver node editor assumes you have control.