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Best practice to de-orbit near KSC


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Okay, so here is a perpetual problem I have when I play KSP.  

I like to make my vehicles cost-effective, and that means recovery of useful parts.  I have gotten pretty good at getting them back down to Kerbin survivable, but the closer they are to KSC, the more the funds recovered from the craft.  This is especially important with spaceplanes, since they often need a good runway to touchdown on.  Unfortunately, my attempts to land near KSC from an equatorial LKO position have a bad habit of undershooting the target, often leaving my craft coming down a continent away.  Even a spaceplane, that could theoretically make that flight back, often finds itself running out of fuel during the long return flight (fuel budgets are especially tight for such craft.)  

Can anyone share a best practice method for getting craft down near KSC, assuming equatorial LKO as a starting position?  Things like what altitude range above the KSC I should aim for or how far around the globe I should begin my breaking burn would be helpful.  I realize there will always be some approximation involved in this depending on the aerobreaking characteristics of the craft and its orientation, but anything that helps me better evaluate my optimal entry would be valuable.  

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It's probably going to be hard to give any sort of general answer on this since it will depend a lot on the craft being used(and perhaps somewhat on the person flying it as well).  Best bet would be to start off with whatever your "typical" re-entry craft is in a perfectly circular orbit.  Save the game there.  Make your best guess as to where to do your deorbit burn and try that.  If you end up being way off, just reload that save and adjust your deorbit burn accordingly.  If you end up 30 degrees short of the KSC, then try doing your burn 30 degrees earlier in your orbit.  Keep adjusting until you find the right place.  Then you can experiment to see how it changes for different starting orbits or different crafts or different Pe altitudes. 

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If you're in LKO (<300km-ish), try executing a burn such that, when you execute it, your periapsis is a. ~45km and b. pretty much directly over KSC. Exactly how you fly from there depends on your craft's drag profile but you should be able to drop short, land at, or overshoot the KSC by varying your AoA and/or bank.

Edited by foamyesque
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I completely agree with your approach -- everything I launch (and I always launch vertically) looks like a spaceplane, and I always intend to land the upper stage on the runway for a 100% refund.

But yes, it depends a lot on the total wing area, the typical mass (ballistic coefficient), the glide qualities, exactly how you built the nose, and whether you intend to do a braking burn or not, and just how big that burn might be.

My spaceplanes are always quite small. In the early game, they are always all MK1 components, so they have very little drag -- and very poor heat tolerances. So with those early spaceplanes, I prefer the following:

On the other side of the planet from KSC, create a 63km Pe directly over KSC. Have a 90 degree AoA all the way through the atmosphere. Just after I cross the Great Desert, and I'm over the ocean west of KSC (at a little over 60km altitude) do a retro burn to lower my (surface!) speed to 1500 m/s or so -- or burn every last drop of fuel, whichever comes first. But at that speed there is no risk of heatsploding, and it's just a matter of managing the altitude and speed (by adjusting the AoA of the nose). You want to have slowed to below 1300 m/s by the time you get to the mountains west of KSC. I always land vertically on the dirt runway with a couple parachutes.

However, I think a klaw makes the best nosecone of all for spaceplanes. So once I have unlocked the klaw my design changes. Klaws allow you to do incredibly aggressive reentries with no heat problems -- even with MK1 parts. So when I have a klaw on my nose, I set my Pe at 55km directly over KSC, and no retro burn. Again come in at an AoA of 90 degrees, and just hold it until my surface speed is down to 1200 m/s. Then glide the rest of the way. My typical design for that has 2 juno engines. So if I have 20 fuel left, that gives me a 100 km range for slight misses.

 

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I have a method that is pretty much fool proof, which I developed it in .90 I believe, or whenever the career mode was introduced. Just like you, I'm obsessed with landing as close as possible to KSC AND recover stages as well, no recovery mods, just with chutes and good flying. Here it goes.

Before you land, you want to set a Per of 70-71 km just above the desert continent, the one PRECEDING the Africa looking peninsula KSC sits on, assuming you in prograde orbit. Exact location doesn't matter that much, usually I aim for the shoreline, but a bit inland, 1-2 minutes of orbit travel perhaps.

Once Per is set (with practice I normally set it to 70,500m pretty routinely), warp to it. Once there, hit retro until your point of landing reaches about midway, between the KSC continent and the one FOLLOWING it. There is like a "inland sea" there, delimited by 2 continents, I like to call it my "Burning Sea", it comes with a warning, watch for things raining down hehehe. The old airstrip is in that sea, also another island further North. You want your point of impact to be further out than the old landing strip, when you're in orbit, because the planet will rotate until you get there, also trajectory will alter due to reentry friction.

If everything is set correctly, you should hit atmosphere just as you pass the shoreline of the desert I mentioned earlier and get heat effects a bit before reaching the shoreline of Africa continent. During reentry, you are free to aero break to alter your trajectory, if you think you aimed too far, or just stay retrograde and don;t do a thing if you think you aimed short. With some practice, you'll be able to land EVERYTHING at KSC, 100% of the time.

Note: while this tactic works for SSTO's, its not 100% foolproof. For spaceplanes, you have to aim a little short, because you want to reach the runway, just as you pass those mountains west of KSC. Oh, make sure you have some heating protection. While its possible to re enter with un-shielded craft, especially the ones with 2000K skin tolerance, any parts that have less, will burn up, unless you carefully balance CoM of your craft, so the engine points backwards by itself.

Note: If your Apo is too high, this will NOT work. If you come from a 500 km Apo, your speed at Per will be too high, over 2200 m/s, so again you WILL burn up, unless you have a heat shield and recessed engines.

With this method I routinely bring back un-shielded main stages (engines 1st), capsules and such. One exception, Vector engine, although its rated at 2000K, it ain't, will always burn up if you go engine 1st. Oh and there's a 2nd exception, the Mammoth and Twin-boar engines will also burn up - they have smaller Vector engines on the back.

Hope this works for you, let me know how it went !

Edited by Zamolxes77
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10 hours ago, bewing said:

However, I think a klaw makes the best nosecone of all for spaceplanes. So once I have unlocked the klaw my design changes. Klaws allow you to do incredibly aggressive reentries with no heat problems -- even with MK1 parts. So when I have a klaw on my nose, I set my Pe at 55km directly over KSC, and no retro burn. Again come in at an AoA of 90 degrees, and just hold it until my surface speed is down to 1200 m/s. Then glide the rest of the way. My typical design for that has 2 juno engines. So if I have 20 fuel left, that gives me a 100 km range for slight misses.

 

Wouldn't you get a similar effect by slapping a 1.25m heatshield or shielded docking port on the nose, for much less cost?

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A klaw costs 450 -- a shielded docking port is much higher tech and costs 400. And a klaw is more useful, since you can dock to anything. It weighs less than a shielded port, and a quarter of what a full heatshield weighs. I also think it has a fair bit less drag than a heatshield.

Edited by bewing
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I use kOS to do this, so I can't describe my deorbit burn in terms of the map. After a rough calculation followed by some trial and error, I came up with the following for a typical command module + heat shield + a few small components set-up:

Starting from an 85km by 85km orbit, burn retrograde when your longitude is 150°, bringing the periapsis down to 29km. That requires a little under 50m/s. It usually pops the pod down in the sea a few kilometres East of KSC (whose longitude is -74°/286°). Edited to add: the periapsis is at longitude 330° when created, but due to the rotation of Kerbin it would be about 315° when reached if the atmosphere weren't there. So the pod lands about 30° short of the estimated periapsis longitude.

I realise that for a space plane you'll probably want something wildly different. It might also differ by craft unless you can get a similar drag to mass ratio (or ballistic coefficient).

I've not yet tackled a targeted re-entry from a moon return but testing shows a command module with a periapsis of 30km usually ends up at a longitude 20° beyond the (predicted, as if there were no atmosphere) longitude of the periapsis. In comparison, a small return probe with a 1.25m heat shield went only 6° beyond.

Edited to add: for all these re-entries, I point to normal a short interval before reaching the atmosphere and stage off any parts that won't be coming home. I found that staging whilst pointing retrograde or prograde sometimes had too much effect on the periapsis.

Edited by ElWanderer
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Pretty much what @ElWanderer says. My deorbit procedure is maybe a bit laborious but perfectly reproducable:

  • From high altitude, burn to a x × 90km orbit
  • At periapsis, burn to 90×80km orbit
  • At periapsis, burn to 80×80km orbit (doing it in three steps guarantees that you end up with 80×80, not 81×79)
  • Depending on the vessel wait until a certain longitude (I keep a list for my standard vessels) and burn for 80×30 with SAS pointing retrograde
  • At 60km altitude, turn off SAS, and stage (ditching the deorbit engine and activating parachutes that are preset to the right altitude)

This lands me within a few km of the KSC, every time. If you overshoot or undershoot, just adjust the longitude for that vessel class.

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My usual Kerbin deorbit burn is from 70km, targeting KCS, and I begin the burn when I am at zero degrees elevation to where I want to land (ie, when an observer on the surface at KCS would see my vessel rise over the horizon).  I continue to burn until the descent path reaches out to a point just past the island east of KSC (I actually use the mod Trajectories to get a more accurate targeting on top of KSC to get 100% cash back from the StageRecovery mod).  This usually ensures a close splash-down to KSC.

 
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