Getting an orbit over a particular site

[ibanix]

So here's a "simple" question: Given an orbit already around a body, how do you change the orbit so that the craft always passes over a particular point on the surface (eg, a landing base) on every orbit?

[FancyMouse]

Trivial case is anywhere on equatorial or two poles, where solution is obvious.

Otherwise, you can let your craft to be on your desired point, then make your orbital period an integer multiple of the sidereal rotation period. In this way when it comes back to the same orbital position, the planet also rotates so that the same site is below you.

Can't think of any other solution so far.

[JohnnyPanzer]

Unless the base is at the equator, you're gonna need a geostationary orbit (as in, the vessel is traveling at the exact same speed as the planets rotation), but that's unlikely to be very fuel efficient. The best thing to do is to make sure you place your base at or near the equator. Even if it's slightly off, you can easily make a small inclanation change that'll cost you much less fuel than going to and from a geostationary orbit.

[Pecan]

Here's a simple answer - if the "point" is on the equator or at the poles it's easy.  Otherwise, it's geostationary.

If equator, just go around and around.  If poles, go over and over.  Otherwise ... 

Your orbital period (time it takes for an orbit) has to match the body's 'day'.  If you don't need it to pass over every orbit then you can get closer to the body, therefore orbiting faster, and pass over the point every second, third, or whatever orbit.  This, geosynchronous rather than geostationary orbital period, is important because in KSP there are several bodies where you can't get a geostationary period within the body's SOI (and therefore just can't within the game physics).

[pincushionman]

34 minutes ago, JohnnyPanzer said:

…you're gonna need a geostationary orbit…

Geosynchronous orbit. Geostationary is always above the equator, and always is above the target. But you can instead force an orbit with the same period as the planet's rotation, and end up over the target at the same point on each orbit, regardless of latitude. A molniya orbit is a good example of such an orbit.

Or, you can get your orbital period such that the rotational period is an integral multiple of it, and pass over the target every X orbits instead.

Edited April 14, 2016 by pincushionman

[Servo]

Since geostationary/synchronous orbits are relatively innefficent (they are farther out and take more fuel to rendezvous), I'd recommend simply waiting until the orbit crosses over the launch site, and then launch directly into the inclination of the orbit. This helps around larger bodies (everything bigger than the Mun), but around small bodies (Minmus and smaller) the gain is pretty small since the gravity is so low. This really only helps if you're trying to transfer from between a station and a ground base. For satellites, synchronous orbits are the way to go.

[ibanix]

Thanks for all the helpful answers. In this case, I have a base on Minmus which is not exactly on the equator, and I was looking to put my orbital station in an orbit which would assure easy launches to resupply the base.

I also wanted to find an orbit that would allow me to do easy landings at the base. 

Looks like I may end up moving the orbital base to a pure equatorial base, since that will be "close enough". 
 

[JohnnyPanzer]

Yeah, anytime you're building a base that you know will be visited on a regular basis by a ship docked in orbit, you're going to save yourself a lot of headache if you try to stick to the equator. Or at the very least close to the equator.

Mining bases are prime examples of this. When I look for a good spot I don't just look for the highest concentration of ore, but for the highest concentration of ore on or close to the equator. If you're doing the fuel runs manually, any forced inclination change is going to drive you nuts sooner or later, and even if you use MechJeb it's unlikely that a higher concentration found far from the equator will be worth it, since you'll burn much more fuel each time you land to pick up ore/fuel for your orbital depot.

Stations that will mostly get visitors from outside the SOI of the planetary body they are on is another matter. I often build science outposts or simple why-the-heck-not bases far from the equator. In such cases, I tend to go for the most visually pleasing spot.