How to time encounters?

[kyriewhatasnakesmh]

I'm fairly new to KSP. I wanna know how you people calculate your launches so that you reach planets or objects in one orbit rather than twenty. Any clues?

Thanks

[Spricigo]

 

welcome to the forums

 

several option exist:

https://alexmoon.github.io/ksp/

http://ksp.olex.biz/

 

[bewing]

If you want to do it without mods or any other information -- then you use a test ship in orbit to calculate sequences of manuever nodes. You can swing the final maneuver node around the orbit to find out when the transfer will have to happen.

If you look around, you will find "phase angle" info for transfers to all the planets. Then all you have to do is judge the angle between the planets on your screen.

Otherwise, as spricigo said, mods will hand you everything you need on a platter.

 

Edited August 24, 2017 by bewing

[A_name]

I'm assuming you know the basics of how to use maneuver nodes and setting targets to plan encounters.

For encountering other planets, I look up the phase angle and eyeball it from the tracking station. The phase angle is the relative angle between the locations of the origin planet and destination planet along their respective orbits. Think of it as their position on a clock face. So for example to get to Duna if Kerbin is at 12 oclock you want Duna at around 10:30 roughly, because the phase angle is ~45°. Google "ksp phase angles", there are several sources ranging from simple diagrams to full-scale transfer calculators.

To get to Kerbin's moons, the phase angle is 90°, so if the moon is at 12 o clock you want to burn at 3:00. For the Mun this can be eyeballed by burning when it rises above the horizon.

As I said, most of this is done by eyeballing the angles. This means you need to leave a margin of extra fuel to do corrections and stuff. if you want to be really efficient, the tools are out there to calculate everything perfectly, but it's not necessary.

For rendezvousing with a craft in orbit when you're already in orbit, the larger the difference in altitude the less time it will take to get in phase but it will also make your rendezvous more expensive. If you don't want to wait, burn into an orbit at least a couple hundred kms higher/lower and you'll usually get an encounter within 5 or so orbits. For rendezvousing from the ground to an orbiting craft, wait until the target craft is almost about to pass overhead and launch. During ascent, check map mode to see if your target is passing you or not. Depending on that, launch into a higher or lower orbit. Since you're already close to begin with, it will usually take no more than one or two orbits to catch up or be caught up with.

Edited August 24, 2017 by A_name

[OhioBob]

6 hours ago, A_name said:

To get to Kerbin's moons, the phase angle is 90°, so if the moon is at 12 o clock you want to burn at 3:00. For the Mun this can be eyeballed by burning when it rises above the horizon.

It's more like 120°, or performing your burn at 4 o'clock.  Remember that when we're are in low orbit we can see far beyond the horizon that an observer can see on the ground.  Mun-rise occurs at a phase angle of 90° for a Kerbal standing at sea level, but for a Kerbal in orbit 80 km above the ground, the phase angle at Mun-rise is 118 degrees.
 

[Snark]

For the stock KSP system, I just use http://ksp.olex.biz ... it's super simple, easy to use, nicely intuitive graphic output.

If I'm playing with a modded system, then I just do the math to figure out when the launch window is.

If you're at a planet that's at distance R₁ from the sun, and you want to go to a planet that's at R₂, the phase angle θ in degrees will be:

θ = 63.64 * (1 + (R₁/R₂))^1.5  - 180

(Assumes circular orbits, which is a pretty good approximation in most cases; you can just use planet's SMA.  If you're wondering where the 63.64 came from, it's 90 / sqrt(2).)

So, for example.  Let's say you're going from Kerbin (R₁ = 13.6M km), and you're going to Duna (R₂ = 20.7M km), just plug those numbers into the formula and it gives a result of -44.25 degrees.   The negative value means that Kerbin should be behind Duna when you launch.  A positive number would mean you need to be ahead (e.g. when you're going from Kerbin to Eve).