Where to burn and when

[KerbMav]

To get it out of my head:

Facing a several minutes long prograde burn I would want to keep burning at the prograde indicator - not at the maneuver marker all the time - or would I?

Edited September 6, 2013 by KerbMav

[stupid_chris]

To get it out of my head:

Facing a several minutes long prograde burn I would want to keep burning at the prograde indicator - not at the maneuver marker all the time - or would I?

Nope If you set a maneuver node, you keep burning at the maneuver node marker until you completed your burn. Your prograde marker will move along the direction you'Re going at, but the maneuver marker will move along the navball so that all the thrust you apply to your ship goes directly to accomplishing the maneuver you set earlier.

[SRV Ron]

Split the burn time to get as close to the path you are aiming for as possible. If a three minute burn, start 1 minute and 32 seconds before the mark, extra 2 seconds to get the engine to full power and shut down when the burn time approaches zero Ignore the marker as it will drift off at the end of the burn. Do plan on a short correction burn halfway to the destination. Since it will usually be a couple of seconds, you don't need to apply full power. Just watch your path change and delta v marker hitting x when you have reached the desired results.

[Jason Patterson]

A maneuver node has two parts, one that indicates the delta-v you requested (X m/s at Y heading) and the second part that provides a projected route following that burn. While the second part is calculated from the first, the first doesn't know that the second exists.

The game calculates a projected orbit based on an instantaneous burn at the node. For short burns t = 0 is a good approximation, but for longer burns things like the direction of gravity and your heading will change markedly before you finish. The game doesn't compensate for that in any way. Instead it tells you to keep burning to finish the original [X,Y] burn.

The difficulty is that you, the pilot, don't want an [X,Y] burn. You want to get to destination Z, which is described by the second part of the maneuver node, but that spot on the navball just doesn't care. You have to watch your map and try to compensate so that your actual final orbit matches the projected orbit. It should be close, but the longer your burn is, the worse the agreement between projection and reality will be.

For escape orbits it can be fairly easy, particularly for fast escapes (non-Hohmann transfers or going to Eeloo or Moho, for instance.) You just look along the two orbital paths and pitch up or down (toward the zenith or nadir, I mean, I'm assuming the ship to be upright and facing in the direction it's heading) to make them match. In most cases you'll wind up pitching up above the maneuver node toward the end.

Could you wait and do all of the adjustments later? Of course. But if you're going to be burning, you might as well burn in the right direction in the first place to save some fuel.