Launch at longitude of ascending/descending node

[Bobe]

For contracts requiring a satellite in Kerbin orbit, I've just been eyeballing the orbital paths to determine when to launch. Since we're given the longitude of ascending node, is there a way to use that to set the launch time?

I'm using MechJeb and Kerbal Alarm Clock, but I don't think either of those allow you to input a longitude.

Just so I understand correctly, since longitudes are normally fixed to the surface, does the game create an arbitrary set of longitudes relative to space rather than the world?

[Anquietas314]

Doesn't MechJeb have a way to launch into a particular orbit? Or is that just for rendezvous?

Just so I understand correctly, since longitudes are normally fixed to the surface, does the game create an arbitrary set of longitudes relative to space rather than the world?

Longitudes are based on some spherical reference frame. In orbital mechanics that reference is based on the stars, but yes it is somewhat arbitrary.

[Bobe]

Doesn't MechJeb have a way to launch into a particular orbit? Or is that just for rendezvous?

You can set an inclination to launch into, and you can launch into a target's plane (or launch to rendezvous), but you can't launch to match a desired orbit.

With that said though, do the orbits have to match up precisely? Does only the periapsis/apoapsis and inclination matter, or the ascending/descending longitudes also, since surely the longitudes (relative to the surface) would match after a given time? I'll have to test that out.

Tested; the orbit has to be exactly as displayed on the map. The fact that the world rotates beneath the satellite doesn't seem to be of any concern to kerbal scientists. And now I have to deorbit this satellite because I can't revert.

Edited December 23, 2014 by Bobe

[Anquietas314]

You can set an inclination to launch into, and you can launch into a target's plane (or launch to rendezvous), but you can't launch to match a desired orbit.

Okay. Maybe go bug r4m0n to implement that

[SpeedDaemon]

So far, for every Kerbin satellite launch contract I've done (a lot!), eyeballing the launch time has been plenty accurate enough. I don't use the MechJeb ascent module, though, just SmartASS. On the pad, set SmartASS to the proper heading (remembering that a zero deg. inclination is a 90 deg launch, so launching at the ascending node of a 160 deg orbit would be HDG of 290), pitch 90, and then just adjust the pitch for your gravity turn as you go up. Once your navball switches to orbital mode, you'll notice your prograde marker moves off of your intended heading due to Kerbin's spin (for anything other than a straight east or west launch), so you'll need to adjust your heading slightly to put it back where it should be. This should either put you close enough for the contract, or so close that you only need very minor adjustments.

[Bobe]

eyeballing the launch time has been plenty accurate enough.

Yeah it doesn't bother me that much, I just like to be able to do things as neatly and efficiently as possible.

[Laie]

You can set an inclination to launch into, and you can launch into a target's plane (or launch to rendezvous), but you can't launch to match a desired orbit.

Not automatically / via mechjeb you can't.

But if you rotate the map in such a way that you look sideways on the desired orbit (all you see is a single line), then watch Kerbin rotate until your vessel on the launchpad sits directly underneath that line(1), then launch, you can match the desired inclination pretty well. With only a little experience (that is, after three or four attempts) you can match the desired inclination down to about two, three degrees from the get-go, which is about as good as it gets. Further refinements must be made during ascent, just watch how AN/DN move around while you point your vessel slightly to the left and right. Getting to within less than one degree of the desired orbit isn't especially difficult.

(1)The perfect moment is actually a little sooner than that.

[Tsynique]

If you are short on deltaV go with Laie's approach.

However, if you have a surplus of deltaV and the target orbit is not heavily inclined, simply launch into an equatorial orbit (west or east), create a maneuver node right below the AN or DN of the target orbit and adjust the inclination to match the target. It's easy to eyeball it. After burning this node you will have a nearly identical LON of AN and Inclination. This allows to launch any time you want and in the "easiest" direction (unless it's a retrograde orbit) but costs more fuel due to plane change burn.

[thyriel]

The fact that the world rotates beneath the satellite doesn't seem to be of any concern to kerbal scientists

With it's zero inclination, zero axial tilt and equatorial KSC there is just no need to bother much about that

If you are short on deltaV go with Laie's approach.

However, if you have a surplus of deltaV and the target orbit is not heavily inclined, simply launch into an equatorial orbit (west or east), create a maneuver node right below the AN or DN of the target orbit and adjust the inclination to match the target. It's easy to eyeball it. After burning this node you will have a nearly identical LON of AN and Inclination. This allows to launch any time you want and in the "easiest" direction (unless it's a retrograde orbit) but costs more fuel due to plane change burn.

To save deltaV don't correct inclination as soon as possible. Make it right after the hohmann transfer to target orbits apoapsis (so Apo is still wrong inclined). The lowest dV for inclination changes is at high apoapsis and low periapsis.

Edited December 25, 2014 by thyriel

[Anquietas314]

To save deltaV don't correct inclination as soon as possible. Make it right after the hohmann transfer to target orbits apoapsis (so Apo is still wrong inclined). The lowest dV for inclination changes is at high apoapsis and low periapsis.

Doing it separately, even at apoapsis, is actually not always the best approach. You should always try to combine an inclination change with another maneuver - such as your launch trajectory or the circularization burn. It will be far less expensive that way (mostly due to Pythagoras' theorem), provided you can execute it correctly. Of course, if it's a very large inclination change (>30 degrees say) or your ascending node is far away from where you're already going to be doing a burn, then it may be more efficient to raise your apoapsis/periapsis before doing the inclination change separately.

[thyriel]

That was meant for orbital inclination changes. Sure a launch into the correct inclination is the cheapest with max 176 dV. (if correctly done) But already in orbit it's a huge difference. For example a 90° change at LKO costs 1300 dV more then the same change when Ap is 2880km high

http://forum.kerbalspaceprogram.com/threads/103279-Trajectories-right-after-launch-why-is-that/page3

[match]

I just eyeball the launch myself, but I guess you could figure out the time of day based on longitude of ascending node, it remains constant to the parent body. i.e. An orbit around Mun would describe it in reference to Kerbin andan orbit around Kerbin would reference it to its parent. Longitudes of ascending node around the Mun would remain geographically fixed (to Mun) since it is tidally locked to Kerbin.

Sounds like a nice project, let us know how it turns out!

Edit: I am incorrect. Longitude of ascending node is not based off of the parent body, but some other reference point as pointed out earlier. I don't see how defining this point would readily (or easily!) resolve the issue though.

Edited December 28, 2014 by match
mea culpa