Question about when to launch for Rendezvous.

[zhollett]

Question from this excellent post from the When to Launch thread (so I don't renew a necro'd thread from 2013)

I checked voneiden's profile and that person has not been active since december 2014, so I hope someone here will be able to answer my question.

voneiden says this:

"Measure the time (t) in seconds it takes to reach apoapsis from launch."

"Also estimate the angle between apoapsis and KSC (α)."

"For greater accuracy: mark down the elapsed time (tα) when measuring α."

The (ta) is referenced as the time you looked at the angle between you and KSC.

What I'm trying to figure out is in the examples, the "angle between apoapsis and KSC" the time it is measured seems to be some arbitrary time.

In the first example the angle to apoapsis was measured at 7 mins even though reaching apoapsis was 10 mins (the made up example)

In the second example, the angle to apoapsis was measured at ~3 mins but reaching apoapsis was ~8 mins (the real example)

So, all that said, here is my question:

When should you measure your angle to apoapsis? (or more importantly, is there another better way to know when to launch for rendezvous)

Note: I want to be more efficient with my time and launch without having to do many orbits to rendezvous with orbiting crafts. I will also be sending this as a PM to voneiden

Edited May 27, 2015 by zhollett

[DeMatt]

The difference between "time to apoapsis" and "angle to apoapsis" is specifically down to the amount of effort it takes the rocket to accelerate UP to apoapsis. Loosely speaking, KSC will not remain centered under your rocket while it accelerates to apoapsis (which would mean time = angle).

So, to measure "angle to apoapsis", you'd fly a test rocket along your intended trajectory, and when it reaches apoapsis, you'd check its longitude versus that of KSC.

Assuming you can fly the same trajectory every time (or have MechJeb do it), then you'd launch when "angle between target and launchpad" = "angular distance travelled by target during launch time" minus "angular distance travelled by rocket during launch".

[FancyMouse]

I don't think so - that depends on your ascent profile and could vary even on identical rockets.

Instead of looking at angle (in which case you may not have a precise number), I always set target right before launch, and then I launch when the distance is about 300km~400km.

[Mighty1]

I do the same as FancyMouse. When the target is at 400 km launch but save before launching so you can refine your launch time.

[Murph]

If the target is in a 400km circular orbit, you can save yourself a lot of hassle by not worrying about rendezvous until after you are in orbit. If you launch to a 80–100km orbit, you will have a much shorter orbit period than the target at 400km. That means it's not going to take long for a window to appear for the burn from 100km to 400km, and you can use the game's own tools to find that reliably.

Launching to rendezvous only really has a benefit if the target is very low (i.e. 70–100km). Variations in your ascent will always throw the rendezvous off a little, but a transfer from 100 to 400 has far less scope for error.

Edit: Oh, hang on, I've just realised you mean 400km range, not 400km orbit. The above stands, but is less relevant unless you actually are trying to rendezvous with something in a higher orbit.

Edited May 26, 2015 by Murph

[KSK]

Very rough rule of thumb - I launch when the target vehicle is a little off the west coast of the landmass on which KSC is on east coast. I very rarely get a precise rendezvous but I'm normally close enough that a couple of orbits is all I need to catch up.

[tomf]

When trying to rendevous with stranded kerbals (who are normally in a 80km orbit) I normally launch when the target is 20^o behind me. Interestingly this has changed from the 30 degrees that was about right for 0.90.

I use Kerbal engineer to tell the me the phase angle and just warp until it reaches 340

[Starwhip]

I'll pull up an image in a moment, but I say about halfway between 90 degrees on the horizon and directly overhead.

Edit: straight from my tutorial, link in sig

[Pecan]

I can't help thinking that while it's good practice this in one of those things people can get overly hung-up on. It's generally not worth the effort to launch at just the right moment - aim for an easy phasing orbit instead and save yourself the headache. Then just wait until the orbits come into synch for the transfer to rendezvous - the dV required is minimal.

[ajburges]

Ditto on Pecan. Either over engineer and launch to a higher orbit while target is over the mountains, or launch to a lower orbit while target is over the sea. One to two orbits to intercept is not a big deal.

Now if we had rescue contacts where the Kerbal entered the atmosphere...

(Even if we did have such contacts, the orbit would still stay stable until rendezvous because of simulation implementation)

[zhollett]

Those are all great suggestions, thank you for your input.

I guess what I wanted was something that was a little less "guess-y"

Again, sincerest thanks.

Btw, launched and had a 34km separation, best ive had yet.

thanks

Edited May 27, 2015 by zhollett
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