Returning directly to the KSC from an inclined orbit?

[OutInSpace]

This one has been nagging me for a while thanks to the STS challenge. In order to get the best rank, you need to land your shuttle back at the KSC (or other runway structure on Kerbin). When you're tooling around in an equatorial orbit, this is easy enough, since you're always more or less aligned with the position of the KSC. But when you get into the more advanced missions, you need to perform activities out-of-plane and still return to the KSC; I've been solving this by simply doing a plane change burn, but that'll be less viable as I head out to the Mun and beyond.

I've tried to nail the KSC from an inclined orbit a few times, but I can never seem to get it. Thus, my question: how would I directly return to the KSC from an inclined orbit?

[Caerfinon]

I would probably de-orbit on the inclination and slow down my plane as it approached the equator then bank to 90 degrees along the equator and fly to KSC normally.    

[bewing]

Basically, you can't.  At least not directly. Partly it depends on your atmospheric maneuverability. Let's say you can adjust your reentry by 10km to each side -- by either thrusting, using aerodynamic control surfaces, or adjusting the timing of your reentry. Let's say you can manage to complete one full Kerbin orbit during your reentry. So that gives you a 20km-wide strip around the planet. If there's a runway in there, you can land on it, but your odds are basically zero.

Or maybe you can aerobrake into a permanent inclined Kerbin orbit. You're still orbiting once every 30 minutes, which gives you 12 orbits a day, and still your odds of flying directly over a runway are bad.

So pretty much the only thing you can do is to increase your horizontal range on your RV. To be guaranteed to be able to land on a point, you need to have a lateral traverse range of a minimum of about 75km. And then you need to aerobrake into orbit, and wait for that point to be on your orbital path.

 

 

[OHara]

This  might be obvious to you already, but just to be sure, you want to time your de-orbit burn so that Kerbin's rotation is bringing KSC under your orbit when you are coming down.  One low-Kerbin orbit before that time looks like the image at right, so you can see that Kerbin rotates significantly in one orbit of the craft.

You might be interested in the SCANSat mod that shows the track of your orbits above the ground.  You can see at right that Kerbin will have rotated a little more than I want before my next orbit, in blue, takes me over KSC, so I will miss 20km to the West.  If I lower my periapsis, to speed my orbit, I can come closer, but still 10km away if I stay barely above the atmosphere.   Planning further ahead, and from a higher orbit, gives more freedom.

I return to KSC when I land spaceplanes, but these have better glide-ratios than the typical STS-like space shuttle.  I would think you need to adapt the craft design to glide at least 4:1, so that a steady glide has prograde 15° or less below the horizon, in order to land on the runway and enjoy the flight.

It does not look like this will be easy.  But, I think people have done this in the STS challenge, so after trying a little you might look at what they did, figure out what the other important bits are, and decide how to get those bits in your craft in your style.

[king of nowhere]

2 hours ago, bewing said:

Basically, you can't.  At least not directly. Partly it depends on your atmospheric maneuverability. Let's say you can adjust your reentry by 10km to each side -- by either thrusting, using aerodynamic control surfaces, or adjusting the timing of your reentry. Let's say you can manage to complete one full Kerbin orbit during your reentry. So that gives you a 20km-wide strip around the planet. If there's a runway in there, you can land on it, but your odds are basically zero.

 

 

You're making it more tragic than it has to be.

1) you don't need to be so accurate. when doing a challenge where money is an issue, i target for 95% recovery, and that translates to being within around 100 km from ksc. not sure on the exact number, still, the strip is much larger.

2) if you don't pass over the ksc the first time, you will on the second, or third. you are unlikely to be perfectly syncronized with the planet rotation, so each day you will pass on a different strip, until eventually you will have one falling exactly over the ksc.

heck. i managed to pass over crater island on laythe just by this strategy, waiting a few laythe days. i doubt it's much bigger than 10 km.

EDIT: 3) if you need the ksc to be in that 20 km strip, and it's not, you still can align your orbit with it with a very small plane change. and the earlier you make the manuever, the cheaper it's going to be.

Edited November 19, 2020 by king of nowhere

[paul_c]

Didn't the actual space shuttle do a 270deg turn once it was done with its re-entry  (and some S turns in there too). With a 270, you're basically flying a crosswind, downwind and base turn like an airplane would approach a runway. It gives opportunities to adjust and line up (in position, angle to the runway and glideslope/appropriate height) by extending or contracting each leg or turn.

I believe a similar thing is done by airplanes with an emergency of variable/unknown length, for example if they are diagnosing something or a developing medical emergency - they will circle the chosen airport above.

[HansAcker]

12 hours ago, paul_c said:

Didn't the actual space shuttle do a 270deg turn once it was done with its re-entry  (and some S turns in there too).

They did. I just came across this older video which explains some of it:

Spoiler

 

It lacks KSP stock graphics

[vv3k70r]

19 hours ago, OutInSpace said:

I've tried to nail the KSC from an inclined orbit a few times, but I can never seem to get it. Thus, my question: how would I directly return to the KSC from an inclined orbit?

Theoreticly You can count Yor path against rotation of Kerbin (Its much easier without atmosphere - it is why Kerbals want to get away from this planet?) but when You take atmospere in the equasion it is more like guessing.

But changing the inclination to pinpoint landing takes fuel and to predict when the orbit whould match further position of landing pad is time dependent, and ods to do this from a random inclined orbit are worthless. Of course You can wait some orbits to get it more or less acurate, but You gonna changing inclination by burning fuel anyway.

But if You aproach some body and want to land pinpoint in one pass it is not quite imposible, but still will take some manouvers to adjust it closer to the target. Without atmosphere I can get something about 70-300m on inclined (over 30deg) target. With atmosphere I do not even try without wings. Once I get KSC this way by using rest of fuels to radial out, and forcing risky parachutes deployment on a bit high altitude over the KSC to brake. But now I consider it not worth of doing again - droppods are not that expensive, and bringing wings to space cost fuel.

Edited November 20, 2020 by vv3k70r