Predicting Landing Spot?

[BenKerman]

Hi,

How do you predict where a vehicle will land after re-entering atmosphere and then proceeding to the ground?

I'm trying to land my SSTO spaceplane (design for which I got from KSP Player's Guide), and not sure how to do it.  I crash-landed my one plane in the ocean.

Is there a program that will calculate this for me, like Alex Moon's Launch Window Planner?

I'm playing on an Xbox One, with no mods.

 

Thanks,

Ben

 

PS  OK, I remember doing problems in physics where I'd have to calculate lateral displacement for an object following a parabola...but those problems always had a flat surface to land on, and now I'm trying to land this video-game spaceship on a curved (spherical) surface!  Not to mention the air drag.

Edited December 15, 2019 by BenKerman

[proteasome]

Trial and error is the best way to learn for this, but here are some things to keep in mind.

First, depending on where you do your deorbit burn, consider that the planet will continue rotation between the time of the burn, and the time you land. This means that if you deorbit further away from your landing site, you want to have your path to landing be ahead of where you intend to land, since the site will rotate towards your end-point.

Second, the more you burn to deorbit, the easier it will be to see where you might land. If your trajectory indicates a ground impact spot (rather than just a periapsis inside the atmosphere) you have something easy to gauge.

Do your deorbit burn so that your trajectory impact is in front of the site you wish to land at. How far forward will depend on how much you deorbit, and where you do your burn.

Then, once you enter the atmosphere, keep checking the map to see how your updating trajectory looks in comparison to your landing site as you start to aerobrake. If the trajectory is falling too far behind the landing site (you're coming up short) then pitch up slightly to increase the distance your plane will glide. If the trajectory is too far in front, pitch down to decrease the distance.

To play around with it, try quick-saving right before you do your de-orbit burn. Play around with how long to burn, and with your re-entry profile (how much you pitch up or down) until you get a feel for what works.

[bewing]

Spaceplanes are  highly variable, depending on their exact design. Some of them glide forever because they have good lift and low drag, some of them screech to a halt and then fall like a brick. Some of them are easy to maneuver so that you can scrub off a lot of speed quickly, and the rest aren't. And some of them are very sensitive to heat, so you may have to be sure to scrub off a lot of speed in the highest part of the stratosphere.

So, as proteasome said, for any new design you fly, you have to just try it once. Test it to see the heat tolerance. Test it to see how much drag and lift it has. Once you have figured out those things, then you need to look for landmarks on the ground and note what speed you need to be flying as you cross that landmark. If you're going a little too fast, then you know you need to scrub some speed.

This also makes a difference for the Pe value you choose when you do the reentry. Most of my planes are sensitive to heat, low drag, high lift, with good maneuvering. Which means I go for a Pe of about 60km, exactly over KSC. Then I reenter the atmosphere with a 90 degree AoA, and I have a bunch of landmarks I look for.

 

[Streetwind]

There is (or used to be) a mod called Trajectories. It would draw your predicted path through the atmosphere factoring in the drag you experience, and thereby predict your landing spot.

Problem with that mod is, even if it still exists/works, it is not easy to make use with spaceplanes. The reason being that the mod needs to know how much drag your reentry vehicle is creating. To do so, it assumes that said reentry vehicle maintains a specific orientation relative to prograde for the entire descent - like, for example, how a capsule always faces blunt end forward. You can even tell the mod which orientation your vessel will be assuming, and it will calculate off of that.

But a spaceplane doesn't maintain a fixed orientation. It pulls the nose up in the beginning, may then raise it more or lower it some depending on which parts are experiencing critical heating, may fly hypersonic S-turns to bleed off speed, employ wing lift instead of atmospheric drag to arrest vertical velocity, and then potentially even fly the terminal descent under engine power. There is no way for an algorithm to precalculate that for you. It doesn't work.

Best you can do with that mod is to practice reentry with your spaceplane multiple times, each time adjusting which orientation the mod uses for its predictions, until you have found a setting that creates a prediction that has you come out roughly at the same location ten kilometers up as your actual reentry does. Then that orientation setting will be roughly (not perfectly) valid for all future reentries... at Kerbin. You'll have to repeat the exercise for all other planets with atmospheres that you may want to visit.

But if you're already practising reentry with your spaceplane, then you can probably get good enough at flying it to predict your rough destination even without a mod to help you...

 

Edited December 16, 2019 by Streetwind

[RizzoTheRat]

As above practice is the only way really and it'll be different for each vehicle.  For gliders airbrakes are your friend, come in long and use the brakes to steepen your approach (brakes to control descent rate, pitch to control speed).  Personally I always try and keep some fuel spare though and come in short and fly to the runway under power.

Edited December 16, 2019 by RizzoTheRat