(Another) Orbital Inclination Question

[KydonShadow]

I know where you have to burn to change it, just wondering if anyone has any simple steps in finding the 90 degree angle you need to change your inclination to what is required.

[Drew Kerman]

KSP Orbit Mechanic? That's not me being snarky - I know this tool does plane change calculations but I'm not understanding what you're asking exactly. So hopefully this helps

[KydonShadow]

Ah, ok. My main problem is finding the 90 degree burn you need to changee your orbital inclination.

[Superfluous J]

Make a maneuver node anywhere in your orbit. Drag the purple triangle marker in the direction you are going to want to burn. Then, aim at that marker and you know you'll be 90 degrees.

If you have to go over 5 degrees or so this gets out of whack, and you'll have to delete the maneuver node and make a new one and repeat. This is because the act of burning is changing where Normal is on your orbit.

Another option to the above is Enhanced Navball. It puts all the markers from the maneuver node on your navball so you can aim right at the normal vector. Should be stock, really.

[MadMooMan]

http://forum.kerbalspaceprogram.com/threads/50524-0-23-5-Enhanced-Navball-1-2

In addition to what Gaiiden has said (Looks pretty cool by the way ) The thread I linked you above adds all the nodes like normal and anti-normal to the Navball, so instead of guessing you can just see where you need to go, which in your case I think is exactly what you need.

[FenrirWolf]

I second using Enhanced Navball. I don't know why it isn't stock yet.

[KydonShadow]

Ah, ok. Anyone give me a link to it?

[Kasuha]

Safest way is to place a maneuver node, set up the plane change with desired apo- and periapsis and burn along the burn vector. Unless you decide to do a bielliptic plane change which may be a good idea if you're going to make any substantial change of your orbital plane (~60 degrees or more).

If you just want to point at right angle to your velocity vector, make sure your pro/retrograde marker goes beyond the right or left side of Navball (reason: navball is wider in that direction). At the moment the marker (or any signs of it) disappears on one side and the opposite marker appears on the other side is when you're turned exactly 90 degrees to your velocity.

Edited April 21, 2014 by Kasuha

[Superfluous J]

Ah, ok. Anyone give me a link to it?

Oops. I intended to do that and forgot

Enhanced Navball

[KydonShadow]

Ah ok thank you.

Safest way is to place a maneuver node, set up the plane change with desired apo- and periapsis and burn along the burn vector. Unless you decide to do a bielliptic plane change which may be a good idea if you're going to make any substantial change of your orbital plane (~60 degrees or more).

If you just want to point at right angle to your velocity vector, make sure your pro/retrograde marker goes beyond the right or left side of Navball (reason: navball is wider in that direction). At the moment the marker (or any signs of it) disappears on one side and the opposite marker appears on the other side is when you're turned exactly 90 degrees to your velocity.

There were a few things in that explanation that I didnt quite understand. Please keep in mind Ive only had KSP for about a week now.

[Kasuha]

There were a few things in that explanation that I didnt quite understand. Please keep in mind Ive only had KSP for about a week now.

If there's anything unclear on what I wrote, don't be afraid to ask about it in detail. A statement that you "don't understand something of it" doesn't help neither you to understand, neither me to provide better explanation.

[KydonShadow]

Ah, I meant the bielliptic statement you made.

[BudgetHedgehog]

Ah, I meant the bielliptic statement you made.

A bi-elliptic transfer involves raising your apopasis (the furthest point of your orbit to the surface of the thing you're orbiting) to a ridiculous amount, then burning at the new apoapsis to move your periapsis (the closest point of your orbit to the surface of the thing you're orbiting) where you want, then again at that new periapsis to circularise. It's only more efficient when the target orbit is roughly 12 times (or more) bigger/smaller than your current orbit (this is due to the Oberth Effect - you may have noticed when burning for Minmus, the time it takes to raise your apoapsis to Minmus height is much much shorter than to raise it to Mun height, despite them being roughly the same distance (Kerbin to Mun roughly equals Mun to Minmus). The reason this works is because, as explained in the next paragraph, you'll end up travelling slower at apoapsis which is a good thing in this case). This is because dV changes are much more sensitive at higher speeds. That is the Oberth Effect in a nutshell.

Bi-elliptic inclination transfers use this to their advantage - in contrast to the Oberth Effect, changing your direction costs less the slower you are travelling (because you have less velocity to kill in one direction and less to gain in another). At the apoapsis of a bi-elliptic transfer, you're travelling much more slowly than you would be in a circular orbit so it costs much less to change direction. Then it's just a normal circularising burn at periapsis and you're done, as opposed to burning anti-normal for ages.

For a KSP example, if you're going to Moho, it can be more fuel efficient to burn to Jool height, lower your periapsis when you reach apoapsis and then burn at periapsis for an encounter. You'd end up using less fuel than a stright up Hohmann transfer orbit down to Mohos height (even less if you pull off a Jool gravity assist instead of a burn).

EDIT: if you're wondering where the name 'bi-elliptic' comes from is because you make two elliptical orbits - the first is to raise your apoapsis a lot, the second moves your periapsis. If your periapsis goes to the same height as your apoapsis, it's a Hohmann Transfer orbit.

Edited April 22, 2014 by ObsessedWithKSP
I'm drunk and I made some typos

[Kasuha]

Ah, I meant the bielliptic statement you made.

Short, distilled version of what ObsessedWithKSP wrote above:

Bielliptic inclination change means you burn at the intended inclination point prograde and raise your apoapsis on the opposite side as high as reasonable (the higher the better, usually near the SOI boundary), then coast to that apoapsis, do the inclination change for negligible amount of dv there, then coast back to periapsis and burn retrograde to lower your apoapsis back.

Note: applies to circular and near-circular orbits, things are a bit more complicated for elliptical orbits.

This is the most efficient approach if the inclination change you intend to perform is large, and at about 60 degrees it breaks even with classic inclination change done in single burn. Note that it also takes way longer time due to the two coasts you need to perform.

Why is it effective? To reach escape speed (parabollic orbit) from a circular orbit, you only need to increase your current velocity by about 40%. To raise your apoapsis, you need to add less than that regardless how high that apoapsis is.

At 60 degrees inclination change, you need about 100% of your current orbital speed of dv to perform the change which is more than "40% (raise) + a little (change) + 40% (brake)".

[KydonShadow]

Hmmm... seriously the only problem Im having right now is finding the 90 degree angle required.

In a bi-elliptic orbit, is it still a 90 degree to change inclination?