Kerbal Space Program

By adding a force perpendicular to your direction of travel, your velocity will not remain constant. The change in velocity at that point will affect your orbit dimensions.

In other words, just because you are burning Normal or Anti-Normal, does not mean that the magnitude of your velocity vector does not change- it does. To keep your velocity from increasing, you would also need to add a retrograde component to your maneuver, otherwise the increase in energy will cause your orbit's dimensions to increase. Consider it as vector arithmetic- if you are traveling east at 100m/s, and burn north for 50m/s, your new velocity is not still 100m/s, it is now about 112m/s, in a more northeasterly direction. Even though you burned perpendicular to your velocity vector, you still added velocity.

So, I think the maneuver node is accurately depicting what would happen if you burn in any given direction, including Normal or Anti-Normal. Also, keep in mind that maneuver nodes do not assume a burn which takes several seconds, minutes, or hours, which is what actually happens depending on your engine. Instead, maneuver nodes assume an instantaneous application of force, like an explosion which results in the entire DV requirement being satisfied in an instant.

RexKramer wrote:

By adding a force perpendicular to your direction of travel, your velocity will not remain constant. The change in velocity at that point will affect your orbit dimensions.

If by force you mean impulse (i.e. instantaneous change in momentum), then I agree.

In other words, just because you are burning Normal or Anti-Normal, does not mean that the magnitude of your velocity vector does not change- it does. To keep your velocity from increasing, you would also need to add a retrograde component to your maneuver, otherwise the increase in energy will cause your orbit's dimensions to increase. Consider it as vector arithmetic- if you are traveling east at 100m/s, and burn north for 50m/s, your new velocity is not still 100m/s, it is now about 112m/s, in a more northeasterly direction. Even though you burned perpendicular to your velocity vector, you still added velocity.

Yes, except assuming the burn is not instantaneous, 'north' isn't the direction perpendicular to my velocity vector throughout the entire burn (only at the beginning). If I rotate the direction of the force to remain perpendicular to my velocity vector as I burn, my new velocity (after the burn) will be in a different direction, but remain 100 m/s. The trajectory of my craft will have been a circular arc over the course of the burn (assuming my acceleration is constant).

So, I think the maneuver node is accurately depicting what would happen if you burn in any given direction, including Normal or Anti-Normal. Also, keep in mind that maneuver nodes do not assume a burn which takes several seconds, minutes, or hours, which is what actually happens depending on your engine. Instead, maneuver nodes assume an instantaneous application of force, like an explosion which results in the entire DV requirement being satisfied in an instant.

I'm in full agreement that maneuver nodes do (and should) represent instantaneous changes in velocity. My complaint is that the maneuver node's handle's axes are aligned to the post-maneuver orbit, but adjust the maneuver in the pre-maneuver reference frame that is, they don't adjust the maneuver in the direction they point. If you setup the second example I gave (http://i.imgur.com/mxnXYTu.png), this issue becomes evident: dragging the normal/anti-normal handles of the maneuver node seen in the screenshot affects the north/south component of the maneuver's delta-V, even though those handles are oriented east-west. This seems counter-intuitive, no?

Thanks for the fix! It seems to work. And, to be explicit, I'm affirming that this should be considered a bug, and that it's still present in 1.0.2. TechnocratiK, I think the last paragraph in your comment #2 explains the issue nicely.

While technically correct, the current implementation of the manoeuvre node gizmo can be a frustrating exercise in tweaking parameters. Therefore it cannot be judged as a bug, and will remain a feedback item, and at best a feature request.

Using the supplied fix, I was able to turn a Kerbin equatorial orbit into a Munar polar insertion in one operation - something that I'm sure anyone who has attempted it, will know how difficult that can be.

I commend this fix.

It's not correct as is, because the displayed direction of the handles is different from the direction of the change they actually make to the node's velocity.

If they behaved as they do with the fix, that would be great. If they didn't visually rotate, but they affected the node itself in the same way that they do now, that would also be correct, though I would probably consider it less useful. Right now, there's a mismatch between what they do and what they look like.

Thanks for the update, sal_vager! Is this bug currently getting any attention for 1.2? Shall I create an issue in the prerelease project?

I was away from KSP for a while (mea culpa). It seems that between 1.1.3 (excluding 1.1.3) and 1.3 maneuver gizmos were changed to no longer rotate with the post-maneuver orbit: that is, the first option proposed by sal_vager. The issues sal_vager mention (e.g., difficulty planning a polar orbit) remain as of 1.3, but that the gizmo no longer rotates does, strictly speaking, remedy the incongruity between the orientation of its handles and the change in the maneuver vector. Will the previous gizmo behaviour (i.e., rotating with the post-maneuver orbit) be reintroduced in a future version along with the second option proposed by sal_vager? If there is no intent to do so, it is my opinion that this issue can be closed (although I argue that the current situation is suboptimal).

As an aside, the "Precise Node" mod (previously maintained by blizzy78, now by linuxgurugamer) has incorporated my suggested change to the behaviour of maneuver nodes as of 2015-11-10, and is compatible with KSP 1.3.0.