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Clear Air Turbulence

Changing orbital angle without changing apoapsis

Question

When I follow the instructions for changing orbital angle, i e by playing with normal / anti normal maneuver handles, I notice my orbit does not stay circular. The apoapsis increases. Am I doing something wrong?

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That's just what it does, and it looks incorrect if you don't know any better, but the real question should be this:

"Is this what happens when spacecraft burn normal/antinormal and don't correct for it?"

I'm not good enough with the math to figure this one out, and googling this has proved to be difficult as I don't know how to word it, but we have a lot of smart people here who have not called this out as being wrong yet, and I'd like to hear from someone who knows, or can do the math, does this happen?

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Burning normal/anti-normal doesn't change apoapsis. However, it DOES change which direction normal/anti-normal is in. If you don't rotate your vessel as you burn you will no longer be burning in a purely normal/anti-normal direction and so will be burning a little bit pro/retrograde, which will change your orbital velocity and therefore altitude of periapsis/apoapsis.

Choose one of three solutions:

-Accept the change in orbital velocity and compensate with a later burn to adjust apoapsis/periapsis. (Easy)

-Rotate your vessel as you burn to stay pointing normal/antinormal. You can dynamically compensate for any small error by deliberately burning off-normal yourself. (Moderate)

-Guess which angle will be normal at the midpoint of the burn and burn in that direction. Any pro/retrograde vector added in the first part of the burn will be compensated by the second half. (Hard)

Hope that helps.

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This is more about the nodes than the actual burn though, and the node rotates when you adjust the normal/antinormal, so something else must be going on here.

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Near as I can tell, although it /looks/ like the node turns with the new plane of the post-burn path, the burn direction caused by pulling the node doesn't actually change. As a result, you are indeed increasing your spacecraft's speed, which results in a larger semimajor axis, and eventually, an escape trajectory.

Some of this can be mitigated by making your plane-change burn ,bu pulling the normal handle a bit, then the retrograde handle a bit, and alternating between the two to keep your apoapsis where you want it to be. That seems to work up until about 90° plane changes. For plane changes of more than that, you're going to have to fiddle with four handles (Prograde. Retrograde, Normal, and Antinormal) to keep your apsides where you want them to be.

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Apoapsis increases because your energy increases, which in turn because you have a faster velocity after the burn.

So why burning normal becomes faster? It's because by playing with maneuver nodes, you're burning at the normal vector of your original velocity. Your original velocity, your burn, and your final velocity form a right triangle. That's going to increase your velocity for sure by simple geometry.

Now, this is different from always burning normal. Think of a circle with your original velocity as radius. Always burning normal means your velocity changes along the arc of the circle. This way it is not increasing velocity while changing direction.

But of course, this is not the most efficient way of changing direction in-place. As we can see from the circle-analogy. We can just burn a straight line from your original velocity to desired one. This is going to be normal plus a little retrograde, and that's really what you wants.

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RCgothic's explanation makes sense to me.

Won't the SAS lock to normal/antinormal compensate dynamically for the 'shift' in normal/antinormal as you burn and keep you pointing in the right direction?

Assuming you have the tech unlocked of course.

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OK, I think I understand the physics of it.

At the moment this is just about the only thing I used Mechjeb for. To change angle without changing the apo/peripapses.

@ RC Gothic - the only problem with the easy option is that I am wasting lots of fuel. Needlessly accelerating and then decelerating again.

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You need to know the ships TWR and control authority capabilities. As Sal alluded to, the normal and anti-normal markers om the navball will move as the orbit inclination changes. But, if the TWR of the ship in question can out perform the ships control authority, then the ship can't keep up with the change in direction, resulting in a thrust vector that does not actually burn in the normal or anti-normal direction but instead lags behind a certain amount depending on the severity of the TWR/contorl authority strength ratio, resulting in a changing AP/Pe.

edit : clarification. A manuever node created in the anti-normal/normal direction will not change vector with the normal or anti-normal markers on the navball. The manuever node will display changes in an orbit for a constant direction only. Locking into the normal direction on the navbal with the auto-pilot will result in a burn that cannot be predicted by a manuever node. This is not necessarily a bug, but rather a feature that just hasn't been added yet. And, I still don't know if my answers are making any sense. This is not easy to explain.

Edited by Otis

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The apoapsis increases. Am I doing something wrong?

You can combine your (anti-)normal vector on the manouver node with a retrograde to keep your apoapsis in place. You'll have to fiddle a bit as adding retrograde to (anti-)normal also changes your inclination, but I noticed you're saving dV for bigger inclination changes.

If you don't rotate your vessel as you burn you will no longer be burning in a purely normal/anti-normal direction and so will be burning a little bit pro/retrograde, which will change your orbital velocity and therefore altitude of periapsis/apoapsis.

By rotating your ship to keep up with normal/antinormal, you're actually wasting fuel because you're canceling out half of the cosine part of the angle you turn through of dV of the burn, because you added it in the first half of the burn but removing it again in the second. By adding retro as you set up the node to keep your eccentricity, you can go in the direction you're supposed to be going without wasting that dV. Also, you don't have to do that later (which is also a waste).

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By rotating your ship to keep up with normal/antinormal, you're actually wasting fuel because you're canceling out half of the cosine part of the angle you turn through of dV of the burn, because you added it in the first half of the burn but removing it again in the second. By adding retro as you set up the node to keep your eccentricity, you can go in the direction you're supposed to be going without wasting that dV. Also, you don't have to do that later (which is also a waste).

Simplified explanation to aid understanding. I offered three different solutions in ascending order of difficulty and reduced fuel consumption. As always, use of manoeuvre nodes does help a lot.

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I offered three different solutions in ascending order of difficulty and reduced fuel consumption.

Fair enough, although these solutions seem mostly useful in career mode before you upgrade the tracking station to unlock nodes. After that setting up a node that combines normal and retrograde seems like the simplest solution of all.

Edited by FyunchClick
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