Re-entry to land at the KSC?

[MrBonk]

I suspect this has been asked many times, so pls forgive me if that's the case. I'm on mobile right now and, for some reason, can't get searching to work properly. 

How do you guys work out where to set your PE so you can accurately land at the KSC? I can get close most of the time, but I invariably end up too short/long and have to fire up my engines to cover the remaining distance or the overshoot. I'd really like to be able to make a 'proper' glide landing with my ssto, but so far I haven't been able to get this accurate enough.

Closest I've managed is the mountains behind the runway, where I had to fire my engines back up for the little extra speed required to make the strip. All the others have been way further than that though. 

[bewing]

If you want to glide in, then you need to make sure you're always set to overshoot. Then you do a maneuver at the end to scrub your excess speed. So you have to know how high and fast your plane will be when the overheating problems are over -- which is very plane-specific. You also have to know how fast it loses speed when it is gliding -- also very plane-specific.

So, as far as I'm concerned, you need landmarks. Flags at different distances from KSC. Or natural landmarks. (And you have to deal with the fact of the navball switching modes, somehow.)

Then it's a matter of trial and error to know how high and fast you want to be at each landmark. If you do a trial and don't make it to KSC, then you know that you need to be higher and going faster at the previous landmark.

I use the east end of the Great Bay in the desert continent, the far eastern end of the Great Desert, the western edge of the KSC continent at the equator, and the mountains west of KSC as my landmarks. 2200 m/s, 2100 m/s, 1900 m/s, (navball switch), 1500 m/s. 17km to 20km altitude as I cross the mountains.

Then it's just a matter of choosing to fly efficiently or inefficiently depending on whether I'm under or over my target numbers.

 

The other point is that if you're severely undershooting, it probably means you have your Pe set too low. So you increase it a little. As far as where to put it, it's easiest to put it right over KSC to start with, at your chosen altitude. Then if you over/undershoot by x kilometers, you subtract/add x to the initial position of the Pe.

 

Edited March 29, 2017 by bewing

[Foxster]

...Not forgetting that things will be different for each craft due to mass and drag. So there isn't going to be one perfect re-entry profile to land at the KSC. 

[Wanderfound]

As the others said: it varies.

However, what I usually do:

1) Set periapsis to 0km right on top of KSC. I usually do the retrograde burn about 1/3 of Kerbin's circumference west of KSC.

2) Reenter with a fairly shallow AoA, aiming to glide down to just above 10,000m as I cross the KSC mountains.

3) Keep an eye on the map screen. If my trajectory is dropping short, pull up a bit to extend the glide; if I'm overshooting, push the nose down to get into the thicker air and dump some speed. If you're overshooting but carrying too much heat to descend, pull up into a high-AoA stall instead.

I very rarely need to fire up the jets on the way home, except to maintain speed during the last few seconds before landing.

That's for the cruisy version of reentry; for a maximum-pace return, move the deorbit burn closer to KSC and use an extreme AoA stall (45°+) all the way.

Edited March 29, 2017 by Wanderfound

[CoreI]

2 hours ago, Wanderfound said:

 

However, what I usually do:

1) Set periapsis to 0km right on top of KSC. I usually do the retrograde burn about 1/3 of Kerbin's circumference west of KSC.

2) Reenter with a fairly shallow AoA, aiming to glide down to just above 10,000m as I cross the KSC mountains.

 

To add onto what @Wanderfound said: This is a decent way to brute force the landing accuracy. This also allows you to land a bit quicker in real time. However, the reason others, including me, may suggest keeping your periapsis at ~20km is that in order to renter with a periapsis of 0km your plane is going to have to be made of some sturdy stuff if your coming in from orbital velocity. The heat will pile on really quick, especially with a low AoA.

*DISCALIMER* I've never tried coming in with a 0km periapsis myself, but my Mk2 plane starts to get pretty close to the bursting point when I begin to get into the ~18km range.

If you share some images of the craft, we might be able to tell you better how low you can go before burning up.

Edited March 30, 2017 by CoreI
Please disregard and see Jarin's post below.

[Jarin]

18 minutes ago, CoreI said:

However, the reason others, including me, may suggest keeping your periapsis at ~20km is that in order to renter with a periapsis of 0km your plane is going to have to be made of some sturdy stuff if your coming in from orbital velocity.

Really? I tend to have hard-descent re-entries, with my retro-burn at 100km over the east end of the desert, and my orbit line intersecting the ground at KSC. So, a very negative PE value. Never had any real overheating problems on re-entry. I just adjust pitch up or down to push my glide slope to keep the orbit intersecting the ground at about the same point. I've been doing this from Mk1 onward. I get hotter on ascent than I do on re-entry. 

Shot-in-the-dark theory here... do you have any thermal control systems onboard to dump your ascent heat? If you re-enter within a couple LKO orbits, I think you can still be carrying a lot of that internal heat, making re-entry a lot tougher.

[CoreI]

49 minutes ago, Jarin said:

Shot-in-the-dark theory here... do you have any thermal control systems onboard to dump your ascent heat? If you re-enter within a couple LKO orbits, I think you can still be carrying a lot of that internal heat, making re-entry a lot tougher.

You sir, could be an excellent marksman. Your shot-in-the-dark theory is indeed correct. I do not pack thermal control systems on spaceplanes. I shall try putting some on next time I play with fire, I mean, spaceplanes.

In that case, anyone looking for advice on spaceplanes reentry, disregard my previous comment.

[Alshain]

Reading the descriptions here makes it sound complicated.  I set my "touchdown" (for lack of a better term) out off the coast beyond the KSC.  Re-entry will usually bring it back, if it goes too far in front of KSC, pull back to glide a little more (after re-entry of course), if it's not enough you are going to want to do a dive.  With a little practice you will get it routine to the point of coming it at a 20 degree glide slope every time. It's a little un-nerving at first because it feels like you are coming in really steep compared to landing a plane, but that's normal.  Once you get near the end of the runway, pull back hard and flare for a fast brake.

[Wanderfound]

3 hours ago, CoreI said:

To add onto what @Wanderfound said: This is a decent way to brute force the landing accuracy. This also allows you to land a bit quicker in real time. However, the reason others, including me, may suggest keeping your periapsis at ~20km is that in order to renter with a periapsis of 0km your plane is going to have to be made of some sturdy stuff if your coming in from orbital velocity. The heat will pile on really quick, especially with a low AoA.

It does make things simpler if you build for heat tolerance...

 

For a low-tech Mk1 type thing, I'll set the periapsis to 20km a bit east of KSC, but keep the plane in a high-AoA stall during descent to drop more speed before hitting the thick air.

Edited March 30, 2017 by Wanderfound

[MrBonk]

Hmmm. I've been setting my PE *way* higher than 0, coz I just expected that my plane would burn up if I went any lower. I'll give it a crack and see what happens I guess

[Capt. Hunt]

I usually use MJ to setup the initial deorbit burn, and then go manual and adjust it from there, but here's how it usually goes down with my spaceplanes and shuttles. (pun intended)

I'll start by saying I mainly use the MJ guidance for timing and the nav-ball markers, do not attempt to land a spaceplane using MJ's landing autopilot.

From 80-150km, MJ's Landing guidance usually starts the burn over the desert.  It aims to bring the trajectory to the surface a few km off the coast of Cape KSC.  That does place MJ's landing prediction right on the VAB roof, however MJ assumes you're making a powered landing ala Dragon, so a glide-in descent will be several thousand meters above the ground at this point.  Keeping MJ's landing prediction piper on, I then continue to burn retrograde until the piper is sitting on the summit of the mountain just west of the KSC.  If you've done it right, the trajectory line should intersect the surface right off the beach.  You'll need to make a slight course correction on approach to line up on the runway (MJ's spaceplane guidance can help here), but If your spaceplane is a good glider, it should come down right on the numbers at a safe landing speed.

[Alshain]

1 hour ago, MrBonk said:

Hmmm. I've been setting my PE *way* higher than 0, coz I just expected that my plane would burn up if I went any lower. I'll give it a crack and see what happens I guess

It can be quite the opposite actually.  If you set your Pe too high, you spend more time heating and achieve less braking going through the atmosphere.  It's the difference between a flash of extreme heat and prolonged extreme heat exposure.  The parts can withstand heat, but if you leave them in the fire too long, they burn up.  It's better to plow into the atmosphere a bit more and let the thick air slow you down fast than to linger up high and take forever to slow down.

Edited March 30, 2017 by Alshain

[proteasome]

I was getting ok on kerbin by using the same vehicle each time, that really helped with having reliable benchmarks. 

Now that I switched to RSS I feel like I'm learning to do it all over again. If anyone has tips for planning deorbit on larger worlds i'd love to hear about them.

[Razorforce7]

1st thing is to make sure that you don't dive in to shallow half way around the planet.. That way it is hard to pinpoint the exact (Pe) at your target since you'll be spending more time in the atmosphere. And any guesses in that case as to where you'll end up become unreliable.
You will need to have only a little more Delta V and I would recommend that you start your re-entry burn 1/4th the way to your landing spot.
This makes sure you start entering more steeply rather then shallow. And you'll have a better "feel" sense of your vessels ballistics if executed this way.
And estimating your exact landing spot is easier 1/4th distance from your landing spot rather then 1/2th distance from it.

I personally use "wing" parts on almost all my vessels including rocket launched crew pods, but especially my spaceplanes.
I make sure I have atleast a small amount of wing parts on all my vessels oriented with my CoM to allow full attitude control on re-entry.

That means if I use a 70 degrees pitch up attitude in the atmosphere I slow down more.
But with a 30 degrees pitch up attitude I slow down less.
So if you see your overshooting, I just pull the nose up and hold it there and suddenly I deccelerate twice as fast.
Basically it allows me to control my decceleration rate with relatively large variations through attitude control depending on my own piloting skills.
I rarely miss the KSC these days with all of the above included.

 

Edited March 30, 2017 by Razorforce7

[MrBonk]

I tried this for the one re-entry I managed to do tonight and I still came up way short. Although I suspect that had more to do with my wild aerobatics (trying to spread the heat around) than anything else  This is a MK1 plane, so it heats up pretty fast. I over-did the yaw input and ended up way out of shape, so it took me a bit to get it back under control. By the time I did, my orbit was going to end before the mountains, so I obviously scrubbed off a *ton* of speed flying sideways 

I can see how much more accurate it will be though. Up until that point, it was right on the money.