Describe your "standard" Kerbin re-entry profile...

[Hotel26]

Do you use a "standard" re-entry profile for Kerbin?  What are its parameters?  Explain its rationale, if you like.  How, and under what circumstances, do you vary it?

When/where/how much do you fire the initial de-orbit burn?  What are the steps in your procedure after that?

If you like, explain what you've tried and how your approach has thereby evolved over time.

Do you design your craft with any particular characteristics in order to best use a particular profile?

I have found that the way I return craft to the KSC has indeed evolved over time.  What have you found that suits you best?

By all means, add screenshots of craft if you think it will help the discussion!

I think it would be interesting to compare notes on various approaches.  (I doubt there is any one "right" way, so this topic is more for simple comparison and mutual edification, rather than to be the subject of any strong argument!)

Please do tell!

Edited March 8, 2020 by Hotel26

[Arco123]

When I do it I like to do a fast high angle approach so I get maximum time to slow down. This allows me to slow down fast and not waste much fuel to deorbit. Any area works unless its really high of an angle. But when I do that I like to add a lot of drag parts and a few boosters to slow down. 

Shapes are mostly a tear drop or big rectangle.   

[AHHans]

6 hours ago, Hotel26 said:

Do you use a "standard" re-entry profile for Kerbin?

Nope!

Capsules like the Mk1-3 Command Pod with a heat-shield attached get the Pe set to well below 30 km when still in interplanetary space (or even in orbit around the other planet) and just slam into the atmosphere at interplanetary speeds. (Bill and Bob regularly pass out from that, but Valentina is made of sterner stuff.)

More fragile craft with e.g. with a capsule and a crew cabin get a more relaxed re-entry profile. (In particular when there are tourists on board, they always start complaining when the g-forces get a bit higher.) But that varies between "OMG, I hope they don't burn up" and "Well, I guess they'll do another orbit".

Spaceplanes coming back from high orbit (e.g. from Minmus) get a first Pe at 50 - 60 km and usually do multiple passes to bleed off speed before they do their final entry. Spaceplanes from LKO or on their final entry get their Pe to about 30 km above the KSC, and then I adjust the attitude to  get them to land at the KSC. (High-drag = belly forwards, low-drag = nose forwards, and gradual variation in between.)

[Superfluous J]

As I'm coming in I set my Pe at 100km, and place a maneuver node to burn all my fuel retrograde at a specific location that will put my resultant Pe at 40 km.

So long as my resultant Ap is low enough and/or I have enough heat shield, that works fine. If it doesn't I reload and redo the burn so my Pe is at 50km, then 60. Never had to go higher than 60 which is good, as that ended up taking a dozen passes to slow down to reenter.

Note this is in JNSQ where you're actually in some kind of danger coming down

[Curveball Anders]

Setting Pe at 75, the circularize it, go retrograde and plan for a burn towards 35 at about 150E.

Normally it drops my standard up-down taxi more or less at KSC.

 

[Hotel26]

I'll describe my history on this but, before I do, I regard aerobraking as a kind of pre-maneuver with the objective of reaching a 70 km² orbit before final re-entry.  It might often be possible to finesse it direct to land, but for the sake of clarity/conformity, the starting point for the question is, "given you are in a 70 km² orbit, what do you do to land at KSC?"  Sorry for any confusion.

                                                                                             

In the early days, I hunted around for an initial Pe for the de-orbit.  I guess the most naive approach is to think, "I will depress my trajectory to intersect my target" and perhaps add a bit of extra to compensate for atmo drag.  This turns out to be a pretty rough descent.  I spent a while longer exploring the 20-40s for Pe...

Finally, I came up with a magic number of 50km for the Pe and have been using it with good success for several years.  So, over Netherania, [1] I reduce Pe to 50km, meaning that that Pe is directly over the KSC.  I think the significance of the magic number is that, at 50km and above, you can maneuver easily, without significant drag penalties nor risk of structural damage and -- most importantly -- heat build-up is not too severe at this high altitude.  By maneuver, I mean flip between prograde and retrograde and vice-versa.

After some long while, the common wisdom about starting the final descent over the tip of the western desert [108W] sunk in and I began using this point and the Map View to either fire retrograde or else go Radial Out for drag.

That's what I have been doing up until recently, but I'm starting to notice some anomalies (room for improvement) in the strategy.  For one thing, instead of reducing Pe over the KSC, it should really be lowered over 108W.  Which would mean firing the initial retro burn over 72E.

1 hour ago, Curveball Anders said:

Setting Pe at 75, the circularize it, go retrograde and plan for a burn towards 35 at about 150E.

Gonna try this, and thank you.  This looks like a plan finely tuned for a specific case which may yield excellent results for your vehicle.  I'll see if I can tune it for a couple of my own examples.  It might entail a longer path through the atmo below 50km, however, which might break the deal for craft like Archer and Wangari Maathai, for example.

One thing that has been on my mind recently, (due to flying Mach 7 equipment low in the atmo), is I've realized how much AoA can pile up heat via drag when you have a shuttle-style high attitude through the descent.  Some longer craft I use are borderline on heat and can't take maneuvering pressure which can amount to a kind of 'coffin's corner'.  The drag will vaporize your wings, but if you don't pull out, the magnetite will kill you...

What I have begun experimenting with is a way to get one's speed way down, while still high, before triggering the final plunge.  Phase 1: 70 km² to 50 km², stabilize, then Phase 2: 50 km² to land.

                                                                      

Here's something quite tantalizing to look at:

LKO at 70 km² yields an orbital speed of 2,295 m/s or so.  Descending through 50 km, one has accelerated to 2,327 m/s.  Yet in the shot above, we see Dolphin in a 50 km circular orbit, stabilized at a low 2,137 m/s -- a full 190 m/s slower than the conventional descent from space.  I haven't been able to reproduce this with Nerfjet 2020.  I think the reason for the slower speed is lift being generated by the wings and body, even in the very thin atmo at 50 km.  So it may depend upon wing area (although the Nerfjet has the same configuration).

Aaaah.  You can see the angle of incidence on the wings of Dolphin.  They are generating lift even though the AoA at the nose is zero: minimum drag/heat.  I bet the older-design Nerfjet has no wing incidence and therefore neglible lift.  To be investigated...
 

[1] Netherania is on the equator near Nye Island and is diametrically opposite the KSC.  104.39W.  (What you do here pays dividends over the KSC.)

Edited March 8, 2020 by Hotel26

[Snark]

I just set my Pe for about 30 km, regardless of craft or mission. Works for pretty much everything, is steep enough not to get baked but shallow enough not to black out tourists, works for pretty much any arrival velocity.

[Hotel26]

12 hours ago, Snark said:

I just set my Pe for about 30 km,

I'd like to give this a try!  Starting at 70 km and desirous of a touch-down on KSC R9, where would you perform the de-orbit?  (latitude)

 

[Snark]

1 hour ago, Hotel26 said:

I'd like to give this a try!  Starting at 70 km and desirous of a touch-down on KSC R9, where would you perform the de-orbit?  (latitude)

I assume you mean longitude, and I'm the wrong guy to whom to ask that particular question.  I don't care in the slightest where I come down with respect to KSC, since 1. it's pretty much irrelevant for gameplay purposes, and 2. it's not a challenge I'm especially interested in, and 3. I practically never fly spaceplanes (they're a pointless waste of time for me).  When I reenter, basically my only priority is to survive (which is pretty easy), to which I add a slight preference to coming down on the daytime side of the planet ('coz it's prettier) and on land ('coz it's more interesting to look at).

For folks who do care about exactly where they land, the really big question is whether you're doing a simple ballistic reentry, e.g. space capsule with heat shield (where you just plow through atmosphere until you slow down, then drop vertically on parachutes), or whether you're flying a craft with significant aerodynamic control (like a spaceplane).  The latter is much easier to control the landing location precisely than the former.

On those (very rare) occasions when I'm flying a spaceplane and decide to try to land at KSC (or as close as possible) just for a thrill, I generally do my (fairly light) initial deorbit burn over the eastern part of the big desert area across the western sea from "Kerbafrica" where KSC is, lowering my Pe to around 30 km (far to the east of KSC).  As I approach the western shore of "Kerbafrica", I generally do a heavier burn that lowers my Pe below ground level, with a projected surface impact point in the ocean a little east of KSC.  Then I use aerodynamic control to steer my way in.

The important thing to remember when going for a precise landing using a spaceplane (or other craft with significant control surfaces) is that aerodynamic control is king.  How you steer the plane makes a huge difference in where you land.  It's also worth noting that a steeper descent is easier to predict and control than a very very shallow one.  The thing to do is to move your Pe below ground level with an impact point just slightly east of KSC, and then you can use steering to try to keep your target impact point as close to KSC as possible as you descend and slow.  You get the most control (I believe) at an AoA of around 25 degrees or thereabouts, so aim for that.

• If your projected landing spot is heading westwards of KSC too quickly (due to drag slowing your speed), keep your nose pointed above by 25 degrees, so that you generate efficient lift and extend your glide range.
• If your projected landing spot is overshooting KSC to the east, and not decaying westward quickly enough, then roll 180 degrees to point your belly at the sky, and keep the nose pointed below by 25 degrees, to generate negative lift and bring you down sooner.
• If you're reasonably on-target but still going way too fast when you're down low in the atmosphere, and are at risk of burning up and/or still going way way too fast at ground level, then you can pitch up way past 30 degrees to add lots of drag, and/or you can roll on your side to generate lift sideways (alternating left and right) to make big S-curves like the IRL space shuttle did.

TL; DR:  It's remarkable how much control you can have, if you've got some control surfaces to work with.  Steer your way down.

 

[razark]

Lower Pe to 20-30 km, put blunt end forward.

[Hotel26]

11 minutes ago, Snark said:

I assume you mean longitude

Thanks for your qualification (and correction: yes, longitude!)

I personally am more interested here in spaceplanes but others might be interested in the techniques used to spot land capsules, too, since there are some similarities.

I hope people keep posting and I have a related idea I hope to announce shortly.  Stay tuned.

[Guest]

I don't have a single standard return profile, but I have two general templates for two completely different missions: hot and cold.

The hot return is from Munar, Minmus, or an interplanetary transfer with a heat shield. There I set Pe at 35 km, stage the parachutes set to deploy at 0.5 atmospheric pressure, and once in the atmosphere, disengage SAS. If the return is really hot, I may have to set Pe a little higher and burn retrograde between 60 and 75 km to bring it down to a survivable velocity.

The cold return is from LKO and I typically use it with various SSTO lifters designed for aerodynamic return, either glide or powered. There I set Pe above KSC at 40 km, then control the landing point with attitude -- steeper if it looks like I'm overshooting, shallower if it looks like I'm falling short. I usually get it in the ballpark the first time, then I refine it from there, raising or lower the Pe as needed for the particular craft I'm flying. 

It's quite touchy-feely and I don't know if I could describe it rigorously enough that someone else could replicate it the first time by following instructions.

[Daniel Prates]

I'm with @Snark on this one. As in real life, where dumb boosters can do almost the same job than the space shuttle (only way, way cheaper), in KSP too it seems simpler delivery vehicles are preferable than complex and expensive SSTOs and spaceplanes. 

I did fiddle with the concept, though, of spaceplanes and SSTOs that land on water, all you need is a smart and simple design, good glide capabilities and buoyancy. You have a waaaaaay wider landing field than the KCS runway and less mechanical complexity and weight. The trick is, you can land mostly everywhere (where there is an ocean, that is) so you don't have to get picky with the entry profile. You can risk bouncing in upper the atmosphere with more impunity, to lose more speed that way even if it means a less precise landing location, and still have a large ocean close in reach. If landing in the KSC runway is mandatory, your entry plan is for sure much tighter and less flexible so you can't just do anything you want. 

[Kernel Kraken]

Fast, the faster, than really fast, then too fast, then the heat shield fails, then I click revert :(. 

 

in all seriousness, usually as flat as I manage. My re- entry vehicle is almost always a Mk1 pod, radial parachutes, and a 100% ablator head shield on the bottom. I go flat until km about 10,000 meters up, go steeper, and either the parachute works or I explode.

           

Edited March 9, 2020 by Kernel Kraken

[Snark]

8 minutes ago, Kernel Kraken said:

usually as flat as I manage

I go flat until km about 10,000 meters up

Out of curiosity, what do you mean by "flat" here?

[Kernel Kraken]

14 minutes ago, Snark said:

Out of curiosity, what do you mean by "flat" here?

Less than a 30° angle compared to the ground. Something makes me feel like I'm about to get called out on my brainless re- entry tactics.

Edited March 9, 2020 by Kernel Kraken

[έķ νίĻĻάίή]

My "standard" re-entry profile is usually set the Pe to 30 Km, if from LKO. If arriving from other planets, I set the Pe to 40 Km. If it's a spaceplane, I do a capture burn, if I have the fuel. I if I don't have the fuel, I try to slow down as much as possible whilst having the Pe at 45 Km.

[Snark]

  

14 minutes ago, Kernel Kraken said:

Less than a 30° angle compared to the ground.

Mainly I was curious about whether you were referring to your attitude (i.e. angle your craft makes with respect to the oncoming airflow) or your trajectory (i.e. are you approaching the ground at a steep or shallow angle)-- couldn't really tell from what you wrote.

If you're coming in on a ballistic trajectory, i.e. you're just plowing through the air, like a command capsule with a heat shield (and not something like a spaceplane that relies on lift), then typically you'll just be pointing your nose and it's not super critical exactly what your attitude is, as long as you keep your heat shield between the explodey bits and the hot airflow.

Heat shields in KSP are incredibly effective.  Even without ablator, they're still really good-- very high temperature tolerance, and pretty good insulators as well.  Myself, I almost never run with a 100% load of ablator even for fast interplanetary reentries-- it's just not needed, in my experience.  About the only time I'll ever go with a full load is if I deliberately want to make it extra-heavy so that it'll help my craft stay aerodynamically stable on reentry and not flip over and go kaboom-- which is obviously a suboptimal use case.  Most of the time, when I've properly designed my craft to be stable anyway, I just run a heat shield with like 10% or 20% of ablator at most, and that's absolutely fine.

One common mistake I've seen a lot of folks make is to try to do the shallowest reentry possible-- their thought process typically follows the reasonable (but wrong) path of "well, if I dive right down to 30 km, it'll get really hot and I'll probably go kablooie, so I better make my Ap be really high so I'm just kinda 'dipping my toe in the water' and I'll carefully bleed off my speed really slowly."

That's a perfectly reasonable thought process... but in KSP is generally wrong.    The reason it's wrong is that what you care about is not so much how fast heat is generated, but rather how much heat is generated per amount of velocity lost.  As you get deeper into the atmosphere, heating goes up, yes-- but so does drag.  And the crux of the matter is that drag goes up a lot faster than heating does.  If you try to stay way high up in the atmosphere for a really long time, you will heat up a lot (and bleed off lots of ablator, if you've got it)... but it slows you down hardly at all, so you end up slowly baking yourself without really slowing down much.  Whereas if you just bite the bullet and slam it to the lower altitudes pretty quickly, then your heating gets fast but the drag gets really strong quickly, before you have a chance to heat up too much, and you slow down by a lot and that saves your bacon.

Exactly what the "right" trajectory is, will depend on the characteristics of your ship (how heat-resistant it is, how heavy is it relative to its drag).  But the moral of the story is that heating can be counterintuitive, and diving fairly steeply in will often result in less overall heating.

24 minutes ago, Kernel Kraken said:

Something makes me feel like I'm about to get called out on my brainless re- entry tactics.

Degree of brain inconclusive based on observed data thus far.    However, from what you've described, it sounds like you've got an approach that works for you, so that's what really matters.  As long as you're keeping the heat shield "umbrella" covering the rest of your craft, you're good.  The only thing I might suggest is that you may be lugging around more ablator than you really need to.

[Kernel Kraken]

25 minutes ago, Snark said:

 

 

  However, from what you've described, it sounds like you've got an approach that works for you, 

 

Hahahahahahahahahahahahahahahahahahaaaa

 

1 hour ago, Kernel Kraken said:

Fast, the faster, than really fast, then too fast, then the heat shield fails, then I click revert

But thanks, that was great advice. Also just noticed I spelled 'Then' three different times and 2 of them wrong. That should be enough to make a conclusion in my degree of brain 

Edited March 9, 2020 by Kernel Kraken

[Snark]

1 hour ago, Kernel Kraken said:

the heat shield fails

Just how fast are you going when that happens?  And at what altitude?  I've had plenty of unsuccessful reentries myself... but it's generally because I had an unstable craft that flipped over and thus fell outside of its protective heat shield umbrella.  Having the heat shield itself fail is generally not something I need to worry about, in my experience.

Are you diving straight down, or something?

[Kernel Kraken]

8 minutes ago, Snark said:

Just how fast are you going when that happens?  And at what altitude?  I've had plenty of unsuccessful reentries myself... but it's generally because I had an unstable craft that flipped over and thus fell outside of its protective heat shield umbrella.  Having the heat shield itself fail is generally not something I need to worry about, in my experience.

Are you diving straight down, or something?

I usually go from the 30° drop to around 60°, by which point I'm shattering windows on Minmus with the Sonic boom. I'm starting to see the flaw in my plan. The heat shield still works, but I guess the protective bubble is too small(?) Because the capsule usually Poofs at some point. Playing in 1.3.1, btdubs.

Edited March 9, 2020 by Kernel Kraken

[Snark]

17 minutes ago, Kernel Kraken said:

I usually go from the 30° drop to around 60°, by which point I'm shattering windows on Minmus with the Sonic boom.

Let me put it this way.  As you approach the atmosphere-- i.e. before you fall below 70 km-- how high is your Pe set to?

[Dirkidirk]

burn retrograde, keep some fuel if powered landing is required. loose 0-98% of mass during reentry, depending on speed, shielding, and stability. then loose 0-100% of mass during landing depending on the landing speed, landing surface, amount of parachutes and/or the amount of fuel remaining.

Edited March 9, 2020 by Dirkidirk

[AHHans]

15 hours ago, Snark said:

That's a perfectly reasonable thought process... but in KSP is generally wrong. [...]

Hmmm... I can't find anymore where I got that from, but I have in my mind that this "faster reentry mean higher peak heating but lower total heating" is also true in real life.

15 hours ago, Snark said:

and that saves your bacon.

But what if I want my bacon fried? Who is going to do that for me?

[Hotel26]

OK!  Good discussion and some nice points.  I'm trying out some of these.

Now I want to try to up the ante.

Being simplistic, I could say that capsules/ships benefit from heat shields and, when properly done, profile is not critical and can be tailored for the particular job.

With spaceplanes, they too may benefit from judiciously-placed heat shields but aerodynamic stability -- particularly the ability to hold high angles of attack, for space shuttle-style drag -- are key to design and can, again, render technique pretty much moot.

With (passenger) spaceplanes, you design to get them to orbit and you design to get them back down.  QED.  Of course it works.

But what if you build a VLHR[1] lifter in which the primary objective is to loft the maximum payload to space and the secondary but indispensable objective is to return intact for reuse -- and therefore at e.g. KSC?

                                                                            

Since the OP, as I intended it, anyway, is about 'technique' and not 'design', I am going to post a Mission/Challenge and cross-link it here.  It won't be a Competition per se, no points nor leaderboard and no judges nor winners.

I have selected a heavy lifter that is VLHR; obtained the permission of its Creator; and the Mission, should you choose to accept it, will focus on your technique to perform one lap of Kerbin and return it, intact, to KSC, landing on the runway.

It is a heavy aircraft.  It is a fragile aircraft.  It is, at certain flight regimes, only marginally stable.  The Mission is NOT to critique the craft, but to find and clearly describe a technique that will bring it home reliably -- and will impress others (who will be at liberty to try your technique and comment on it; learn from it; and perhaps improve upon it), one way or another.  Test pilots will be encouraged to endorse your technique and/or to politely question it.  Everyone may learn something together, as a result!  That could be fun.

You will NOT be able to tune the craft to your own taste, so (to participate) you WILL have to develop and exhibit your technique in flying it.

Stay tuned for this post, if it sounds at all interesting to You, The Quintessential Spaceplane Pilot.

[1] Vertical Liftoff Horizontal Return

Edited March 10, 2020 by Hotel26