Aerobraking /Aerocapture in 1.05. Things You've Noticed....

[Geschosskopf]

I've just gotten good started so haven't tried Eve, Jool, or Laythe yet. Them have have gotten that far, please post your observations. Me, I've just noticed some significant differences with aerobraking at Kerbin in 1.05 compared to 1.04. For instance:

[U]Returning from Mun/Minmus[/U]
It used to be my SOP that my 1st pass from either of these places was at 37km. This would usually put me with an Ap of around 1Mm and nothing would get hot. Then I'd do 1 or 2 more passes at the same Pe of 37km, or maybe let it fall naturally down to 36-35km, until my Ap was 150-300km, then circularize. All this was with a final rocket stage still attached. Then I'd do the deorbit burn so I'd land close to KSC, dump the last rocket stage, and come down with just the pod, any science parts I was returning, the heat shield, and chutes.

Nowadays, however, things don't work like that. The 1st pass at 37km, the engine comes close to exploding from heat. IOW, engines are no longer good heatshields. Also, my Ap after the pass is still out at 1.5-2Mm. If I leave the Pe at 37km for the 2nd pass, the atmosphere sucks me down and I land right then, ready or not. Which seems quite strange. If the atmosphere is thinner, so that the same Pe and velocity gives me a significantly higher Ap on the 1st pass, and if wind resistance increases with velocity, then keeping my Pe at 37km for the 2nd pass should NOT cause me to land that pass because I have less resistance than before (from an Ap of 1.5-2Mm compared to out at Minmus or even Mun).

And there's a really sharp division between a Pe of 37km and 38km. After the 1st pass, if I raise my Pe to 38km, then it will take me several more passes to bring my Ap downt to where it costs nearly nothing to circularize just above the atmosphere. But 37km and below and I'm landing right then, usually halfway around Kerbin after a rise back up to about 45km.

Note that all this is leading with an engine. Which, as noted, gets dangerously hot.

Anybody else seeing this?

[OgreMagi]

I played with a career mission to Minmus, I dropped to 45km and noticed it took me several more passes to make orbit. the Heat affect was still siginificant but dropping lower heated up the engines and exposed surfaces too much .

[GoSlash27]

The important thing I'm seeing:
A simple Mk.1 capsule with 'chute is too clean when equipped with a heat shield. Once it gets subsonic, you have to wait forever for safe deployment of the 'chute. You run the risk of smacking into the ground before the 'chute deploys.
I logged a bug report about it. In the meantime, I recommend attaching the heat shield with a decoupler and jettisoning it once the heating subsides.

Best,
-Slashy

[Kerbart]

My experience is that coming from Mun you can easily lower your re-entry Pe to about 20km. The heatshield is quite good (it should wear down quicker*) and will easily hold it.

From what I understand, the atmosphere got marginally thicker and higher (the Mk 1 Pod now starts to stabilize itself at 65km instead of 60) but the heat model has apparently been improved, so maybe that is what is causing the quicker heat-up of the engines (instead of the heat being distributed to the rest of the ship it now sticks to the exposed parts? Just theorizing here)

I'm happy with the improvements. Re-entry should be a white-knuckle ride ridden out with clenched cheeks, and require careful aiming in a re-entry window. I'm glad the "any speed, any angle" days of pre-1.0 are over, but I don't mind when re-entry will get a few more notches unforgiving.

*Right now the goal of reentry is to get below 250m/s before you run out of time to open the chutes. I hope to see the day where the goal of reentry is that, [I]and[/I] getting below 1km/s before your heatshield runs out.

[FancyMouse]

[URL="http://forum.kerbalspaceprogram.com/threads/139094-1-0-5-Reentry-drag-and-heat"]Previous discussion[/URL]
And yes these days I'm always having a shield of about 30%~40% ablator when returning from Mun/Minmus.

[Warzouz]

At Duna, I found a bug : Chutes safely opens at 220m/s instead of nearly 600 or 700m/s in 1.0.4. As the atmo is very thin, this speed can only be achieved by retro burning now.

Trying a landing of a light probe on Eve, I lost all landing struts even after burning to slow down. I think my old ascent vehicle couldn't land safely due to overheating.

Finally I use a lot of recoverable rocket SSTO. I've now a hard time recovering them as the engines burn and airbrakes can't be uses more than few seconds at a time.

[Bev7787]

[quote name='GoSlash27'] In the meantime, I recommend attaching the heat shield with a decoupler and jettisoning it once the heating subsides.
[/QUOTE]
Hmmm... Thanks for the tip, will make a config for personal use that adds a DECOUPLER on heat shields.

[oglommi]

Yeah, things get more hot now.

[stormdot5]

[URL="http://forum.kerbalspaceprogram.com/members/71315-Geschosskopf"][B]Geschosskopf[/B][/URL], Regarding your second pass from a lower apoapsis to the same periapsis resulting in more braking. You would have a lower speed at periapsis, starting from a lower apoapsis, so the instantaneous drag at this point would be less. However, a lower apoapsis also means coming closer to Kerbin throughout your orbit, which means contacting the atmosphere sooner and for longer. This is apparently having a more signifigant effect than where you set the periapsis, probably due to this in the
patch notes:
"Physics:
* Drag coefficient changes based on the same factors as turbulent convection (a Pseudo-Reynolds number). This means higher drag high up in the atmosphere, and slightly lower drag when going very fast very low."

[norup]

So far in 1.0.5, I have returned several times from Minmus with no heat shield and only the Terrier engine protecting the MK1 Lander Can stacked with the new 2-person flight cabin. Typically aerobraking a few times with periapsis around 40-38km. That feels rather silly - an engine like that should easily be destroyed by overheating without any fuel flowing to cool it.

[Reactordrone]

Heat shields appear to work much better than they used to. I haven't destroyed one through overheating (not that I've really tried to).

Front/top sides of capsules are now dangerously unprotected.

Drag on a Mk1 capsule seems the same or very slightly lower at all altitudes than in 1.0.4. I'm not seeing higher drag in the upper atmosphere.

[problemecium]

Yep - things have indeed been burning up a lot more. It turns out airbrakes overheat and explode at hypersonic speeds now, which has given my "old" 1.0.4 reusable rockets some grief (as well as the water changes).

[Warzouz]

[quote name='parameciumkid']Yep - things have indeed been burning up a lot more. It turns out airbrakes overheat and explode at hypersonic speeds now, which has given my "old" 1.0.4 reusable rockets some grief (as well as the water changes).[/QUOTE]

I recovered most of mine with grossly the same reentry profile. But I lost one.

1.0.4 reentry profile
- 60m/s deorbit on the left side of the west kerbin crater (thus targeting KSC)
- Deploying airbrakes in space
- Deploying 2 drogues around 500m/s
- Deploying 6 to 24 regular chutes around 200m/s
- Powered landing in the water 10 to 50km east of KSC

1.0.5 adjustments
- Airbrakes are open and close many times to avoid overheating. Their efficiency is minimal, though.
- If engines overheat, I slowdown by burning fuel at 100% thrust for 1 or 2 seconds
- Sometimes rolling the rocket helped. This is useful when you have some wings control surfaces. As they are reversed, they increase the roll the SAS want to correct, the spinning can be quite high.

I had to increase wing size due to a bug in fairing body lifting making rockets flip. Rockets with big wings tend to flat spin very rapidly (or wings are destroyed). This rapid flat spin protect the whole stage from overheating. The rocket slows down faster that way, making landing prediction quite hard to do...

This is also the method I used to reenter a SSTO space plane from LKO : pitching up and down the most rapidly I could. Flat spinning works also very well on a non aerodynamic (nose cone and intakes destroyed) DOVE stock space plane.

Please note that I didn't test all my launchers (check Cygnus recoverable rocket SSTO in my signature)

[ment18]

I don't like the heat shields, the ablator doesn't ablate fast enough. I accidentally came back from Jool wrong and hit the atmosphere at over 9km/s:D. The heat shield over heated with over 100 units of ablator left and with ignore max temp the heatshield got up to about 130% heat and still had a lot of ablator left on landing. It was a mk1 pod, a 1.25m heat shield and the parachute. Had to use a really low reentry, like 20km. On one reentry I was able to bleed off about 3km/s without cheating, but that would still have me shooting out of the Kerbin system at 6km/s.

Edit: This is stock physics with kerbal engineer and a few other parts mods Edited November 16, 2015 by ment18
Clarity

[LostOblivion]

Since pyrolysis flux is no longer based on the remaining ablator in the heat shield, but the size of the heat shield itself (that is, how much ablator it can have), we can now tweak the ablator resource before launch and bring along just as much ablator that we need, with the same effect, making the RV much lighter for all Kerbin reentries.

Also, as Slashy said, if you have the issue of not losing enough speed before being able to safely open the parachute, jettison the heavy heat shield after the heating phase. That, or fit a drogue. There's a reason RVs like the one used on both MER and MSL jetted their heat shields.

[Geschosskopf]

[quote name='stormdot5'][URL="http://forum.kerbalspaceprogram.com/members/71315-Geschosskopf"][B]Geschosskopf[/B][/URL], Regarding your second pass from a lower apoapsis to the same periapsis resulting in more braking. You would have a lower speed at periapsis, starting from a lower apoapsis, so the instantaneous drag at this point would be less. However, a lower apoapsis also means coming closer to Kerbin throughout your orbit, which means contacting the atmosphere sooner and for longer. This is apparently having a more signifigant effect than where you set the periapsis, probably due to this in the
patch notes:
"Physics:
* Drag coefficient changes based on the same factors as turbulent convection (a Pseudo-Reynolds number). This means higher drag high up in the atmosphere, and slightly lower drag when going very fast very low."[/QUOTE]

I think you misunderstood me. I have the same Pe (37-38km) on both the 1st and 2nd passes. I do the 1st pass, at Ap raise the Pe back to where it was, then make 2nd pass.

On the 1st pass, the Ap is way out near Mun or Minmus so the orbit is very eccentric. On the 2nd pass, the orbit is much more circular although still eccentric (Ap ~1.5Mm). So while on both passes the ship is in the exact same "layers" of the atmosphere (70km down to 37-38km), on the 1st pass the high eccentricity causes it to spend more time in the atmosphere, nearly 1/2 of Kerbin's circumference. Wth the 2nd pass, the ship WOULD only be in the atmosphere for about 1/3-1/4 or so of Kerbin's circumference. However, actually it never exits on this pass and lands about 1/2 to 2/3 of Kerbin's circumference from where it entered the atmosphere).

The ship is the same on both passes (except for a tiny amount less mass from the >5m/s Pe-adjusting burn between the passes). So on both passes, it has the same drag coefficient, (essentially) the same mass, and goes through the same density layers of the atmosphere. On the 2nd pass, the ship's speed at Pe isn't THAT much less than it was on the 1st pass---it's still well over 2000m/s. But it is slower, so in the same atmospheric density and with the same drag coefficient and mass, it should be experiencing somewhat less drag force than on the 1st pass. And its trajectory would have it in the atmosphere for less time, so this lower force SHOULD have less time to act on it. This leads to the expectation, which was supported by observations in all KSP versions from 0.20 to 1.04, that using the same Pe on the 2nd and subsequent passes. somewhere between 35-40km, should result in gradually bringing the Ap down over multiple passes.

But in 1.05, the 2nd pass sucks you right on down to the ground. Why? The only explanation that fits observations now is that the 1.05 aero model imposes more drag on the same ship in the same air if the ship is going slower. Which IMHO is wrong. Further, this increase in drag is grossly disproportionate to the difference in speed. Which also IMHO seems wrong.

Now, I'm no rocket scientist so I could be wrong about the expected behavior in this example. But higher drag for the same ship in the same air at a lower speed strikes me as bogus.

[quote name='Warzouz']At Duna, I found a bug : Chutes safely opens at 220m/s instead of nearly 600 or 700m/s in 1.0.4. As the atmo is very thin, this speed can only be achieved by retro burning now.[/quote]

Oops :). Thanks for that tip.

[quote name='LostOblivion']Since pyrolysis flux is no longer based on the remaining ablator in the heat shield, but the size of the heat shield itself (that is, how much ablator it can have), we can now tweak the ablator resource before launch and bring along just as much ablator that we need, with the same effect, making the RV much lighter for all Kerbin reentries.[/quote]

Well, technically "how much ablator it can have" is an arbitrary limit set by the part definitions. The size (aka diameter) of the heat shield just determines its surface area, which is the important thing for the various equations. But in theory, you could apply any arbitrary thickness of ablator material to the underlying circular plate. You're just subtracting some but I don't see why you shouldn't be able to add some, too. Regardless of the total amount (aka thickness) of ablator, however, the same amount will always be exposed to the heat right up until its all gone, due to the constant surface area.

[LostOblivion]

I know, and agree, however I think it's a nice game limitation restricting the upper limit of how much ablator a heat shield can have for its size, and although a lower limitation of 0 is unrealistic without changing the looks of the heat shield, it makes for more freedom in gameplay.

[Pthigrivi]

After some stage-recovery experiments last night I found you can in fact use airbrakes for re-entry if you come in shallow enough. I came down nose on prograde with a Ap/Pe of 80km/60km and was able to skate through the upper atmosphere for long enough that the brakes never overheated.

I can understand why people have become a little frustrated after getting used to the old style, but from a pure gameplay standpoint I've found the atmo and heat changes really top notch. It's tough but it's manageable, and has forced a lot of really interesting solutions. Edited November 16, 2015 by Pthigrivi

[dantedarkstar]

[quote name='Geschosskopf']I think you misunderstood me. I have the same Pe (37-38km) on both the 1st and 2nd passes. I do the 1st pass, at Ap raise the Pe back to where it was, then make 2nd pass.

On the 1st pass, the Ap is way out near Mun or Minmus so the orbit is very eccentric. On the 2nd pass, the orbit is much more circular although still eccentric (Ap ~1.5Mm). So while on both passes the ship is in the exact same "layers" of the atmosphere (70km down to 37-38km), on the 1st pass the high eccentricity causes it to spend more time in the atmosphere, nearly 1/2 of Kerbin's circumference. Wth the 2nd pass, the ship WOULD only be in the atmosphere for about 1/3-1/4 or so of Kerbin's circumference. However, actually it never exits on this pass and lands about 1/2 to 2/3 of Kerbin's circumference from where it entered the atmosphere).
[/QUOTE]

You are wrong. If both passes have the same periapsis, then the one with higher apoapsis ALWAYS spends less time in atmosphere. Since dynamics in space is reversible, make a thought experiment. You are starting from periapsis, and setting your velocity to some arbitrary V0. If V0 is higher, your trajectory will be always flatter (less curved) and therefore at any point further away from Kerbin than the lower-V0 trajectory, right ? Now, as we all know, the higher the velocity, the higher the apoapsis. Hence the high-apoapsis trajectory is always flatter near periapsis and therefore spends less time in atmosphere. Which means that the higher the starting point of a dive, the less distance will be through the atmosphere, provided the periapsis is the same. Of course it is possible to do higher dive with more atmospheric time, but it will always require lower periapsis.

Higher apoapsis = less distance in atmosphere, given fixed periapsis

[Armisael]

[quote name='LostOblivion']I know, and agree, however I think it's a nice game limitation restricting the upper limit of how much ablator a heat shield can have for its size, and although a lower limitation of 0 is unrealistic without changing the looks of the heat shield, it makes for more freedom in gameplay.[/QUOTE]

That limitation doesn't really exist. You can stack multiple heat shields on top of each other - once all the ablator is gone from the first one, it'll overheat and explode, exposing the second one. It's a little more expensive in cost/parts/weight than a single heat shield with a huge pile of ablator, but it still works just fine.

[Geschosskopf]

[quote name='dantedarkstar']Higher apoapsis = less distance in atmosphere, given fixed periapsis[/QUOTE]

Um, no. You're looking at this from the wrong end (burning at Pe) and assuming the Pe is fixed. It's not. I am actively changing it at Ap on both passes, and the change is in different directions each time. Spaceflight is reversable in total energy needed going in both directions, but the times and places you use that energy changes depending on whether you're going outwards or inwards.

It goes like this

Setting up for 1st pass:
I've just escaped Mun and am at my Ap out there next to Mun's orbit, and my Pe is say 400km. So I do a retrograde burn at Ap, which tightens the radius of the curve near Pe as the Pe comes down. Before I'm not in the atmosphere at all. As my Pe reaches 69km, I'm only in the atmosphere for an instant. As the Pe continues to decrease, the arc of my flight through Kerbin's atmosphere increases, until by 37km it's approaching 1/2 of Kerbin's circumference (say 3/8). The more your lower at Pe, the tighter the radius of the curve in the vicinities of Pe, until it's nearly an instantaneous reversal of direction when the Pe is at Kerbin's center.

Setting up for 2nd pass:
During the 1st pass, my Pe will fall to about 34km while my Ap drops down to about 1.5Mm. The orbit has changed from a comet-like long, skinny ellipse to a fairly circular oval, with a significantly greater proportion of the change in shape happening on the major axis compared to the minor. If this was not so, then you'd never change eccentricity. And the closer the orbit is to a circle, the shorter the distance between the foci, so the closer 1/2 the major axis approximates the radius of a circle centered between the foci and intersection the Ap and Pe. This "squishing" of the ellipse flattens out the curve in the vicinity of Pe, making it more like a circle instead of a hairpin. The Pe has moved in a bit, the Ap has moved in a lot, while the minor axis hasn't changed nearly as much. Because the curve is flatter, the distance in the atmosphere is less than before, like about 1/4 of the circumference of Kerbin. When I burn at Ap again, prograde this time, I raise the Pe back to 37km, it has no noticeable effect on curvature because it's a not enough of a change, in proportion to the ellipse axes, to matter.

[wdsmith3]

using the drouge chutes will help. they can safely open at a higher speed( above sound barier) and helps slow you down higher up. I usually set an action group to cut the drogue and deploy the main chute at the same time, as the drogue chute is not effective at low speed.

from a minmus return, I set the pe at around 25ish, im doing 350km/s at 5k altitube when i open the drouge. at 1k altitude, I cut the drouge and deploy the main

[dantedarkstar]

Geschosskopf, I still hold that you are wrong. Let me explain it again.

Assuming no irreversible processes happen (like atmospheric braking) then spaceflight is *fully* reversible. There is no time arrow defined -- if time is reversed, we couldn't tell (as we can for example for thermodynamic processes like gas escaping from container into space since reverse process is HIGHLY unlikely), since the dynamics would be exactly described by the same physics.
This means that considering going from different APs to same PE is *exactly* equivalent of going from PE to different APs. Of course this ignores atmosphere, but since you are going to the same predicted PE, it ignores atmosphere anyway. So let's assume that Kerbin has no atmosphere for a moment, and just track how much of the trajectory is "below 70km" or any other set altitude. Then my previous claims hold true. The higher velocity at given fixed PE, the higher the AP, but at the same time less curve and therefore trajectory always above the lower-velocity trajectory.
We have 2 trajectories (both are after any burning you did at AP, so both are ballistic trajectories from that point): A) from AP 12Mm to PE 37km, B) from AP 1.5Mm to Pe 37km
This is exactly equivalent to trajectories: C) from PE 37km to AP 12Mm, D) from PE 37km to AP 1.5Mm. It should be obvious to anyone playing KSP that trajectories A and C have the same velocity at periapsis, and that velocity (Va=Vc) is higher than the periapsis velocity of trajectories B and D (Vb=Vd<Va=Vc) (can't cheat conservation of energy without burning engines, which I assume you don't do except at AP to adjust PE). If your velocity is higher, then the curvature of the trajectory (at a given altitude) MUST be smaller. This is because the gravitational force is proportional only to masses and the distance from the body that you are interacting with. And the mass of the object itself cancels out when calculating acceleration, so it only depends on mass of the planet and distance form the planet center -> altitude. Given higher velocity, the curvature must be smaller = larger curvature radius, as described by r = V^2/a (this is simple relation for circular motion, but if we consider local curvature then it works as well), since a = acceleration from gravity which is the same for both A/C and B/D trajectories.
While it is obviously possible to draw an ellipse of higher eccentrity (and higher AP) with same PE, that is also narrower, such ellipse is not a physical trajectory and will not have Kerbin as one of focal points (which is always true for trajectories in point gravitational field, as calculated in KSP).

In the below picture, the low AP orbit (actually lowest possible, since it's circular) is blue. The high AP orbit is NOT the red one that indeed would spend more time in atmosphere, but the green one, which is always above blue one, except at PE. Black points are supposed focal points of the ellipse. Note that the red one does NOT have focal point at the center of Kerbin (cyan circle), since PE is always *closest* point to the focal point which must be (according to physics) the body with gravity field (so by definition PE is closest to focal point). An even higher AP ellipse would be again wider around kerbin and always above green trajectory.
[IMG]http://i67.tinypic.com/24o9b0o.png[/IMG]

[panzer1b]

Sofar, and the only vessels ive tried with are SSTLs (single stage to laythe), i have noticed that aerocapture can be done fairly easily from most places (me personally) would actually go to/return from. At kerbin, you can do a aerocapture with a SSTO from duna, dres, and eve. I have not tried returning from jool yet, so i cant comment on how hard that is. Eve aerocapture IS DOABLE with heat shields (back in 1.0.4 it seems that even was just plain suicidal). Duna is a utter joke as my ships dont even get yellow (the most you will get from say kerbin is orange and you can actually land directly during that aerocapture if you want to). Laythe can be done as before (if you time it so laythe is going away from you as you enter its SOI, if its not, then forget touching atmo at 6km/s and surviving that), direct aerocaptures work, although you wont be able to establish an orbit on the first pass (as expected). Jool also can be done (as before) in a spaceplane, just be careful not to go too deep as it gets very hot very quick. I still say tylo gravity assist is the most reliable and safest option, but it seems like vessels that have everything made of 2000K minimum exterior can survive aerocapture at laythe and jool. Aerobraking at minmus doesnt work apparently (it has bloody clouds near the surface, why the heck do i not slow down when i get near the surface?).

Im gonna try kerbin->eve, and jool->kerbin, and ill report my results (using a spaceplany like vessel).

[Starhawk]

[quote name='dantedarkstar']An even higher AP ellipse would be again wider around kerbin and always above green trajectory.
[URL]http://i67.tinypic.com/24o9b0o.png[/URL][/QUOTE]

You are correct. The physics dictates that the lower the apoapsis, the more time you will spend in atmo assuming the same periapsis which is within the atmo.

Happy landings!

[COLOR=silver][SIZE=1]- - - Updated - - -[/SIZE][/COLOR]

[quote name='panzer1b']Aerobraking at minmus doesnt work apparently (it has bloody clouds near the surface, why the heck do i not slow down when i get near the surface?).[/QUOTE]

Minmus has no atmosphere. Stock, of course, has no clouds so I can't say where your clouds might be coming from.

Happy landings! Edited November 18, 2015 by Starhawk
clarity