first question

[ragusila]

Hello everyone

just started with KSP, great game so far. I am trying to do the mission where I place a satellite over a fixed point in synchronous orbit. I can get it to syncronous orbit, but I cant figure out any good way to get it over that point!!

right now I am starting from a 90km round, equatorial orbit, do a Hoffman to the synchronous altitude, and then round it. Works well, but I cant control where the satellite will be over. Is there something I am missing? I found a great chart for calculating rendezvous between orbits, works great, but I cant use it here since the point is not on orbit, but on ground...

cheers

[pandaman]

Hi ragusila. And welcome to the KSP addiction.

From your starting orbit set up a manoeuvre node for your transfer burn. Position the node on your current orbit so that the Ap for it is over the point you want to be in sync orbit over. Then set up another manoeuvre node at the Ap and perform your circularisation burn. That should get you pretty close.

With gentle tweaking you should be able to fine tune your final orbit if it's not close enough initially. You can use RCS if you have it, or right click the engine and reduce its power to give you finer control.

Hope this helps.

Pandaman.

Edited March 10, 2015 by pandaman

[LordFjord]

Welcome to the forums,

you already have your target orbit, now you can do the following:

- if the target on the ground is ahead, lower your periapsis a bit, do some circulations in order to "catch up". Once it is in sight, raise the periapsis again to the target orbit.

- if the target is behind, raise your periapsis and let the ground target catch up with your sat.

It's a bit like with a normal rendezvous, just with a ground target. Same mechanics.

[rcp27]

The time it takes to complete one orbit is a function of the "semi-major axis". The major axis is the orbital distance between Pe and Ap. If you increase that distance, the orbit takes longer, if you decrease that distance the orbit is faster.

There are probably more efficient ways to do it, but this method gets the job done: First, get onto your desired orbit. It doesn't matter where you are relative to your ground target. Check to see where you are. If you are ahead, you need to slow down. If you are behind, you need to speed up. To slow down, you need to increase your orbital period a bit, so burn prograde to raise your Ap a bit. Now, to get from where you are (now the Pe), to the same point on the next orbit, takes longer than the target orbit, so you will arrive there "behind" where you began. Keep going round until you reach your Pe over the point on the ground that is your target, and circularise. To speed up, burn retrograde, and drop your Pe below the target orbit, and the Ap will remain on it. Each time round you will gain some ground, so keep going round until you arrive at Ap over your target, then circularise.

Be careful not to overdo the burns, though. You need to complete one full orbit to arrive back at your target before you can circularise. If you go too far on your adjustment, you may find you overshoot your target. It's easier to make a small adjustment orbital correction and have to go round a few times before reaching your target than to repeatedly overshoot. With practice you will get a feel for how to fine-tune it.

[Cirocco]

Hello and welcome to the forums!

As the previous posters have pointed out, easiest way is to just get to the desired orbit, and then fine tune the orbit to the desired point. In simple terms:

if your ship is ahead of the desired position, burn prograde a little to raise apoaps and wait a few orbits. The higher the periapsis, the more distance relative to the ground the satellite will be moved per orbit.

if your ship is behind the desired position, burn a little retrograde and wait a few orbits. As before, the lower the periapsis, the more distance relative to the ground the satellite will be moved per orbit. Use longer burns if you're impatient, but then you'll risk overshooting the desired point.

One more addition: always remember that the planet is spinning under you while you are performing a maneuver. While going from a lower orbit to a kerbin-centric orbit, the planet will spin with you. That means that if you use a maneuver node no get you exactly where you want to be, you'll end up behind the position you wanted. Aim ahead of the desired position. The lower your starting orbit, the mor you'll have to compensate. How much exatctly is up to practice.

good luck!

[Streetwind]

It's one of the many epiphanies you're going to have about orbital motions while playing this game. The other posters have already explained it in detail, so I'll just stick to the short and succint variant...

"In space, you need to speed up to go slower."

[Cirocco]

"In space, you need to speed up to go slower."

Hahaha, that is an amzing quote. I may have to steal that one

[pincushionman]

Check the language of the contract as well. It sounds like you're doing one of those "kerbosynchronous" orbit contracts. The ones I've done only require you to be "in view of…" the target…and that's as precise as you need to be. You don't need to be directly overhead, just have a line-of-sight to it.

[Vanamonde]

This question about how to play the game has been moved to Gameplay Questions.

[khearn]

"East takes you out, out takes you west, west takes you in, in takes you east, port and starboard bring you back."

- Larry Niven's "The Smoke Ring"

I prefer the more generic "prograde takes you out, out takes you retrograde, retrograde takes you in, in takes you prograde, up and down bring you back."

If you burn prograde, you'll end up with a larger orbit, which raises your altitude (prograde takes you out).

If you burn outward (radial+), you raise your altitude, which puts you in a orbit with a longer period, which moves you backwards relative to where you were (out takes you retrograde).

If you burn retrograde, you end up with a lower orbit, which lowers you altitude (retrograde takes you in).

If you burn inward (radial-), you lower your altitude, which puts you in a orbit with a shorter period, which moves you forward relative to where you were (in takes you prograde).

If you burn up or down (normal or anti-normal), you just change your plane, and half an orbit later you pass back through the same point. (up and down bring you back).

It's kinda like a gyroscopic effect. Everything you do actually effects you 90 degrees off from what you'd expect.

[Empiro]

Everyone in this thread has some useful general advice, but for your specific instance, I'll suggest the following:

You're doing the correct thing of performing a Hohmann transfer to a synchronous altitude. You are now in what is called a Geo-synchronous Transfer Orbit (GTO, or even KTO if you wish). However, instead of circularizing at AP right away, you should instead wait until the desired point is underneath you when you're at AP, and then circularize. Barring some rare timing coincidences where your period is an exact multiple of Kerbin's, it will eventually happen. If you want to make it happen faster, you can even do a bit of math involving orbital periods, and then performing a prograde burn at AP that doesn't fully circularize, but changes your period so that the desired point will pass under you the next time around.

I believe that in real life, this is exactly how multiple Geo-synchronous satellites are launched at once -- they all get released into a GTO, and each satellite performs circularization at different times to put the satellites over different points over the Earth.