Orbit inclination issue with contract

[b1ackhat]

Hi guys

I'm somewhat confused about a contract I got. It requests a satellite in an Orbit with (according to contract text) an inclination of 63.4.

It's a bit hard to explain, but if I launch into a polar orbit from KSC, the orbit the contract wants is basically rotated at 90 degrees to my polar orbit (if you view the planet from top-down). I have no idea whatsoever how to get there. Whatever maneuver I try to plan to change the inclination ends up costing 4000+ delta-v and it still only gets me half way rotated.

Another thing I don't understand is if I use MechJeb and launch the autopilot into an orbit inclination of 63.4 it launches successfully, but my orbit is nowhere near the orbit the contract wants. So this must mean that whatever those 63.4 degrees are angled to, it's not the same thing that MechJeb thinks that the inclination is angled to.

I'm pretty confused

Edited May 15, 2015 by b1ackhat

[diomedea]

The inclination is just one of the orbital coordinates, and gives the angle the orbital plane has with the plane of reference. You also need to match the longitude of the ascending node, for the orbital plane to be oriented correctly. While launching at 0° or 180° inclination (equatorial orbit) the orbital plane is already matching the reference plane (so the line of nodes is not defined, therefore not the longitude of the ascending node), when launching with an inclination, you have to wait the moment the vessel (= KSC) is aligned with one of the nodes to launch (you can launch both at the ascending node with the correct inclination, or at the descending node with the opposite inclination).

What is good, in KSP the required orbit with contracts is graphically shown in map mode. So you don't have to make complex calculations about the required value of the longitude of the ascending node, you are good to wait until the vessel is under the required orbit (so, it is on the line going from Kerbin's center to the orbit - and as KSC is almost exactly on the equator, that line is almost exactly the line of nodes).

[RocketPropelledGiraffe]

Keep in mind that you can only maneuver to match an inclined orbit when you perform the burn in the ascending or descending node - otherwise you can never get an exact match. That's also the reason why you should launch when KSC is in one of the nodes, as diomedea mentioned.

As to the huge amount of delta-v required remember that you basically change the direction of all your velocity - this differs from a prograde burn where you just add a set amount to it. As a consequence the radial burn is extremely expensive if your current velocity value is large, which usually is the case in tight orbits. If you have to do a big adjustment it often pays off to raise your orbit first to get a high apoapsis and then perform the inclination change there. That way the velocity value at the maneuver node is smaller which makes the inclination change cheaper. After that you can lower the orbit again.

The procedure sounds counter-intuitive because you raise the orbit first and then lower it again which seems to achieve nothing - but you can actually save a lot of delta-v that way.

[Superfluous J]

I made a tutorial video on it. It was in 0.90 so ship itself may not make it to orbit anymore (or may do better, who knows) and the launch is the old "45 degrees at 10kM) but the concept is the same. If you watch it, pay particular attention to how I launch at a specific time, based on the orbit I'm trying to hit.

[Jaeleth]

an inclination of 63.4 means an angle of 63.4 degrees between the orbital plane and the equator. You must launch as close as possible to this angle, initially, in order for plane adjustments to be minimum aftwards. The solution:

1. Put your rocket on the launchpad

2. Fast forward time until KErbin rotates to a longitude where KSP intersects the desired orbital plane (KSP directly below the orbital diagram, you can see this in MAP view)

3. Check where the small dots in the orbital diagram are flowing to (southwardslike or northwardslike).

4. Launch rocket

5. If the small dots were flowing northwardslike, when it is time to initiate the gravity turn, tilt you rocket towards heading 63 ( more or less ), use the navball. If the dots were flowing southwardslike it's a clockwise orbit, when it's time to do the gravity turn, tilt your rocket towards heading 63+180 = 243.

you should end up with an orbit very similar to the desired one. Minor plane adjustement and you'll be there and then it's just a matter to prograde at Pe so that your Ap is coincident with desired Ap, eventually a little radial / anti-radial burn and tha's it

Edited May 13, 2015 by Jaeleth

[b1ackhat]

Thanks a lot for the detailed explanations and the video link!

Got the hang of it now

Cheers