Eve aerobraking at 5000m/s

[Galinette]

Dear all,

I am passing close to Eve and would like to loose as much speed as possible using aerobraking. My speed at atmosphere edge is 5000m/s and I can adjust the angle to as low as needed. However, as soon as I pass 90km and the barometer is starting to move to the right, I explode in a few seconds. The speed barely decreases, which means that few energy was dissipated.

Isn't this a bug / non realistic behavior?

My ship has a heat shield (3.5m) and a cylindrical interstage fairing (3.5m too). It has 8 AIRBRAKES at the back for stability. So I can't do much better. Does the shield need to be much larger than the ship beneath?

I know 5000m/s is a lot, and I did not expect to loose all speed in one shot without exploding, but here even if I make a very high pass and loose a couple of m/s, I explode...

Thanks in advance!

[Speadge]

Hi,

 

eve is known for its dense atmosphere.. you have at 200km the same gravity as on kerbin / earth at sea level (for example).

so overheating might happen very fast, but you should also consider the pressure. best check your log WHY all vanished at once.
=> its the same as you fall straight down to kerbin - even with the biggest heatshield you wont reach the surface when u entered the atmosphere at 5-6km/s

beside that, does your heatshield (i think u meant 3.75, not 3.5) cover the whole vessel?

[Warzouz]

I wouldn't dare entering Kerbin atmosphere over 4500m/s. Eve is worse.

You should do a slowdown before trying aerobraking at a more reasonable speed.

BTW how did you end with at 5000m/s ? it seems very high for a Eve encounter (I don't remember well though...)

[Streetwind]

I'll echo the notion to check your log. What is the first part that goes? Is it the heatshield? If so, does the heatshield burn through all of its ablator before exploding, or does it just randomly pop while still having ablator?

What you're encountering is the "Mars Atmosphere Dilemma". You are trying to slow yourself with atmospheric drag, but the atmosphere is so thin that it doesn't slow you down in any useful way... and yet, at the same time, it is thick enough for heat to be a problem. So your issue here is twofold:

1.) The very edges of any atmosphere, even Eve or Jool, are useless for aerobraking. You need to dip down a little further to have useful amounts of air resistance. This is why you're not seeing your velocity change much.

2.) Your vessel has difficulties handling a short-term temperature spike. Both skin temperature and internal temperature are relevant - and if either value exceeds its limit, the part explodes. Now, the heatshield is supposed to keep its skin temperature low by consuming ablator (essentially trading the consumable resource in exchange for losing heat energy). However, the consumption of ablator doesn't happen at an infinite speed. If the heat buildup in the heatshield's skin is especially rapid, then the temperature may spike past it's limit and the part explodes.

This is especially important for high-speed atmospheric entries, because at very high speeds, you get a phenomenon called 'turbulent flow'. That's when the normally smooth, laminar airflow around the spacecraft breaks up into something chaotic, which destroys the protective hypersonic bow shock that normally keeps the really high temperature shock heating zones away from the hull. KSP actually models this by sharply scaling up heat influx above a specific speed.

So it's possible that your craft is encountering turbulent flow here, which spikes the heatshield skin past its maximum temperature tolerance. How to fix this? Chiefly by going slower, I'm afraid. Your tolerances for extreme speeds are unfortunately much tighter than IRL, because KSP needs to greatly fudge shock heating physics in the first place - else you wouldn't even get overly warm on a normal low Kerbin orbit reentry. Ironically, this is where KSP's tiny solar system with its tiny orbital speeds makes things harder, not easier. Your 5 km/s Eve reentry speed corresponds to going something like 20 km/s IRL. That's absurdly high - way faster than anything manmade has ever traveled. Even New Horizons left Earth at "only" 16.25 km/s, and it's the current record holder for fastest spacecraft ever.

Of course, it's also possible that something else is happening, but without knowing more information, we can't tell.

 

Edited December 15, 2015 by Streetwind

[Snark]

Excellent explanation by @Streetwind, +1 to that.

One question-- what's your ship mass?  You're supposed to be able to aerobrake at Eve at 6000 m/s, according to developer NathanKell ... but that's with a fairly lightweight ship.  If you've got hundreds of tons shoving that heatshield along in front of you, you're gonna have problems.

Also, airbrakes used to be da bomb when it came to atmospheric reentry, but these days they're much less useful for that-- they tend to overheat and explode pretty quickly.  So it needs to be the heatshield doing the work.

[Starwaster]

This needs more information. 

Did you run out of ablator?

what altitude did you burn up? Steep or shallow? (What was your periapsis)

what barometer? If you mean the indicator to the right of the navball, that's G-forces. If you saw that move within seconds of hitting atmo, then you might be too steep. 

Edited December 15, 2015 by Starwaster

[Foxster]

45 minutes ago, Snark said:

Excellent explanation by @Streetwind, +1 to that.

One question-- what's your ship mass?  You're supposed to be able to aerobrake at Eve at 6000 m/s, according to developer NathanKell ... but that's with a fairly lightweight ship.  If you've got hundreds of tons shoving that heatshield along in front of you, you're gonna have problems.

Also, airbrakes used to be da bomb when it came to atmospheric reentry, but these days they're much less useful for that-- they tend to overheat and explode pretty quickly.  So it needs to be the heatshield doing the work.

The mass has a lot to do with it, as pointed out above. Below 100t and you might be good, above that and you are going to struggle. 

I disagree on the use of airbrakes though. They are still essential but the secret is to make sure that, when deployed, they are in the occlusion shadow of your heatshield. You might need to do some rotating and clipping to make that possible. This might seem counter-intuitive but that's just how things work in KSP i.e. drag occlusion works just for immediately attached parts, bow wave heating occlusion works for all parts behind the big leading edge (the heatshield in this case).  

Everything must be behind the heatshield; that is a line drawn vertically from the heatshield's edge. Then you need to make sure that your craft stays exactly on prograde, meaning you will need plenty of drag (airbrakes) at the back and a (disposable) reaction wheel bolted to the heatshield. 

Pe is vital. I find 65km is the sweet-spot but be prepared for some F5+F9. You will likely need a couple of passes.  

 

Edited December 15, 2015 by Foxster

[Speeding Mullet]

5 hours ago, Streetwind said:

Even New Horizons left Earth at "only" 16.25 km/s, and it's the current record holder for fastest spacecraft ever.

As a point of order, New Horizons is the current record holder for the fastest (confirmed) object leaving earths gravity well (escape velocity).  I say confirmed, as during the Pascal-B nuclear test as part of operation plumbbob in 1957 a 900kg manhole cover was blasted off the top of a test shaft, and subsequently observed by Dr Robert Brownlee as "going like a bat out of hell".  Calculations based on video evidence clocked the manhole cover at more than 66km/s. 

The manhole cover was obviously never found, almost certainly vaporised by compression heating in the atmosphere, but I like to think somewhere out there there's a manhole cover floating through space .

The fastest ever man made object is the Helios 2 probe which achieved a speed relative to the sun of over 70km/s.

SM

[Galinette]

Hi all,

Some answers to your questions below

• As explained (maybe not clearly enough) in the first post, the burn up altitude is slightly below 90km, maybe 85km but not lower.
• I know the G-Force meter! What I call the barometer is the logarithmic gauge labelled "Atmosphere" just below the altimeter. The burn up happens while it's at the limit between the black range and the dark blue range. I do not know the actual pressure but it's likely very low.
• The ship mass is about 30t
• The heatshield is not much consumed. I will check logs to see what explodes first and give you feedback
• I suspect that parts are slightly outside the heatshield protected volume are in cause. First, the airbrakes are outside, this was on purpose but probably a bad idea (I can retract them though). But the fairing is maybe the biggest problem. It is a 3.75m fairing built cylindrically up to the top 3.75m heatshield (as an interstage fairing). So there is zero margin. It seems also that the fairing end is not really mechanically connected to the heatshield, as they move independently when the ship wobbles. So I suspect that the fairing explodes and causes a reaction chain.

Yes, 5000m/s is not optimal, the window was missed by about 20°, but since I am running several ships in parallel in carrier mode I wanted to try it anyway and I'm ready to loose money on this (no kerbal life is threatened, this is an automatic probe).

The speed after Eve encounter is 3200m/s, but it accelerates up to 5000m/s when I reach the outer atmosphere layer.

Cheers

Edited December 15, 2015 by Galinette

[Geschosskopf]

Kerbin has the only atmosphere that actually fades away to nothing realistically.  All the others went from still reasonably thick to nothing pretty much instantly.  Prior to 1.02, this really didn't make a difference because there was no reentry heat at all, or it wasn't a factor.  But from 1.02 to 1.05, just barely touching any air but Duna's at interplanetary speeds was usually instant death without over-engineering some monstrous heatshield thing out of gobs of parts.  The root cause of this problem was that all atmospheres except Kerbin's are arbitrarily cut off at lower altitudes than they should be if they followed the same gas laws as Kerbin's.  IOW, they're much more like oceans than atmospheres.

In 1.05, in an effort to make interplanetary aerocapturing possible again, Squad thinned out the upper few km of the other planets' atmospheres without changing their heights.  So basically, instead of fixing the real problem, they just moved the point at which it happens from the upper edge of the air to some point further down.  But you still hit a wall of air at some point and I believe that's what's happening to the OP here.  The thinner upper region doesn't slow you down enough to survive the sudden spike in density in the unaltered lower region.

Thus, since 1.02, I have totally stopped even trying to aerocapture at any place except Duna.  It's a lot easier and safer just to carry enough fuel to capture on thrust.  Maybe do some aeroBRAKING from orbital speeds but don't touch air until the orbit is closed and the Ap brought in a fair distance from the SOI edge.

[RocketBlam]

The last time I used Eve for aerobraking (which was about an hour ago, as it turns out), I was coming in from Kerbin, and set my periapsis at 80k. I don't know what my speed was, but it was typical for an arrival from Kerbin. But even at 80k, all of my landing gear blew off within a few seconds. Luckily nothing else did, and after many loops, I managed to land.

Eve has a seriously thick atmosphere, so you have to be really careful. 80k on Eve is probably something like 45k on Kerbin. 

[OhioBob]

13 hours ago, Geschosskopf said:

Kerbin has the only atmosphere that actually fades away to nothing realistically.  All the others went from still reasonably thick to nothing pretty much instantly.  Prior to 1.02, this really didn't make a difference because there was no reentry heat at all, or it wasn't a factor.  But from 1.02 to 1.05, just barely touching any air but Duna's at interplanetary speeds was usually instant death without over-engineering some monstrous heatshield thing out of gobs of parts.  The root cause of this problem was that all atmospheres except Kerbin's are arbitrarily cut off at lower altitudes than they should be if they followed the same gas laws as Kerbin's.  IOW, they're much more like oceans than atmospheres.

I've actually modded my atmospheres to obey the gas laws and to be more lifelike.  Kerbin's atmosphere is basically the same, but the others are significantly different.  I had just completed the mod and was starting to experiment with it when 1.0.5 was released.  Since Squad claimed to have made some atmosphere changes, I switched back to the stock atmospheres.  I want to know how the new stock atmospheres behave so I can have a good comparison when I switch back to my modded atmospheres.  I'll probably eventually make the mod publically available, but I want to first have that comparative experience so that I can intelligently discuss the differences.

[Geschosskopf]

40 minutes ago, OhioBob said:

I've actually modded my atmospheres to obey the gas laws and to be more lifelike.  Kerbin's atmosphere is basically the same, but the others are significantly different.  I had just completed the mod and was starting to experiment with it when 1.0.5 was released.  Since Squad claimed to have made some atmosphere changes, I switched back to the stock atmospheres.  I want to know how the new stock atmospheres behave so I can have a good comparison when I switch back to my modded atmospheres.  I'll probably eventually make the mod publically available, but I want to first have that comparative experience so that I can intelligently discuss the differences.

I eagerly anticipate the public release of your mod.  The ridiculous "wall of air" at other planets is quite annoying.

[Streetwind]

Doesn't FAR do this already?

[OhioBob]

58 minutes ago, Streetwind said:

Doesn't FAR do this already?

I'm not a FAR user, but it's my understanding that FAR uses the same atmospheric properties as the stock game.  That is, air temperature, pressure and density is the same in both FAR and stock, but the way the two models computes things like drag and lift is different.  The mod that I came up with changes the atmospheric properties so that pressure and density fades with increasing altitude in a much more realistic way.

[Streetwind]

Ah, I see. That makes sense. I always had different reentry heating and slowdown behavior with FAR, and I was wondering if the atmosphere was different (it is after all one of the core elements of Realism Overhaul).

[Dafni]

13 hours ago, Geschosskopf said:

I eagerly anticipate the public release of your mod.  The ridiculous "wall of air" at other planets is quite annoying.

It baffles my why SQUAD hasn't fixed this rather essential part of the game by now.

[OhioBob]

On ‎12‎/‎17‎/‎2015‎ ‎5‎:‎15‎:‎34‎, Dafni said:

It baffles my why SQUAD hasn't fixed this rather essential part of the game by now.

They did improve it quite a bit in 1.0.5 by thinning out the upper atmospheres of Eve and Jool.  Eve's atmosphere is unchanged up to about 45 km, but after that it's a little thinner than pre-1.0.5.  For instance, at 80 km the air pressure is 63.5% of what it use to be.  Jool's upper atmosphere is much thinner than pre-1.0.5.  There's not much change up to about 30 km, but at 123 km it is 74% of what it use to be, and at 150 km it is just 20%.

(eidt)  Note that the above indicate reductions in air pressure.  On Jool, the air density is reduced even further because the molecular weight changed from 2.8 to 2.2 kg/kmol.

 

Edited December 18, 2015 by OhioBob

[MaverickSawyer]

Well, I'm no expert at aerocapture (which is what this seems to be), having rarely ventured outside of the Kerbin system, but I would suspect that you're coming at this the wrong way. My PERSONAL suggestion is to do a propulsive capture into a highly to moderately elliptical orbit, then use aerobraking in VERY small amounts to gradually shed the rest of the speed. Granted, I've never been to Eve at all, so I have no idea if that's even workable, But the whole "do a single-pass aerocapture" principle has not been very appealing to me... I lost one of my few missions to Duna doing that. Came in a touch too low, they swapped end for end, and then they went from aerocapture to direct entry. Bob and Bill were NOT happy.

[OhioBob]

On ‎12‎/‎16‎/‎2015 at 3:24 PM, Geschosskopf said:

I eagerly anticipate the public release of your mod.  The ridiculous "wall of air" at other planets is quite annoying.

The mod is now available...

http://forum.kerbalspaceprogram.com/index.php?/topic/128858-105-realistic-atmospheres/

[FancyMouse]

5km/s isn't a problem - I've done 5.5km/s reentry on Eve in 1.0.4, which is even harder than 1.0.5. The trick is to make sure heat shield covers the whole ship, and the ship is capable of pointing prograde.

Fairing shouldn't be a problem. If it's strictly cylindrical, then it should receive zero convective flux and should be fine.

Airbrake is of little use when entering at that speed - they'll explode sooner than they become effective. But since KSP looks at the shockcone, simply being surface attached to the cylinder doesn't mean it will get the heat - you need to check right-click menu with debug option on (or use my AutoThermalData modlet below in my sig).

Better post a picture right before anything explode, and show the log right after something explode - that helps identifying the problem.

[Foxster]

7 minutes ago, FancyMouse said:

Airbrake is of little use when entering at that speed - they'll explode sooner than they become effective.

I gotta disagree with this. 

Air brakes are incredibly useful and can make a lot of difference in how easily you get a craft down on Eve. The "secret" is to make sure that, when deployed, they are in the bow wave thermal shadow of a heat shield. This can often be achieved by rotating them at placement in the VAB. 

Undeployed...

Deployed ready for landing...

Check it is all behind the heat shield...

[FancyMouse]

15 minutes ago, Foxster said:

The "secret" is to make sure that, when deployed, they are in the bow wave thermal shadow of a heat shield.

Oh yeah sure but doesn't that sound cheating, exploiting the difference between KSP heat calculation versus drag calculation? If KSP calculates drag in a better way (maybe it's fixed in future versions), then if airbrake doesn't get heat, it shouldn't get drag, either.

So no at least I won't use it. I agree I should've made myself clear.

[Foxster]

I know where you are coming from but it is not a complete exploit of the mechanics, there is some sense to it. 

The heat generated when moving fast through atmosphere is not caused by drag/friction - it is caused by bow wave compression. So, with the airbrakes out of the compression front, they should not heat much.

Also, the flow of gas around a real craft would not be straight up in a tube, it would slow and curve back behind the craft, interacting with air brakes placed there. 

 

Edited December 31, 2015 by Foxster

[Snark]

3 hours ago, Foxster said:

I know where you are coming from but it is not a complete exploit of the mechanics, there is some sense to it. 

The heat generated when moving fast through atmosphere is not caused by drag/friction - it is caused by bow wave compression. So, with the airbrakes out of the compression front, they should not heat much.

Also, the flow of gas around a real craft would not be straight up in a tube, it would slow and curve back behind the craft, interacting with air brakes placed there. 

 

I gotta say I'm with @FancyMouse on this one.  It feels like an exploit to me.

At hypersonic speeds in the upper atmosphere, the craft isn't slipping through the air; it's punching a hole in it.  There's not laminar flow around the craft, or even turbulent.  It's just slamming the air aside, leaving a vacuum behind it.  The air doesn't curve back behind the craft, because it doesn't have time to-- the gas molecules only move at the speed of sound, and the craft is traveling many times that.  The gas is already far behind the craft by the time it has a chance to flow back into the hole the craft is leaving.

So if you've got any airbrakes (or anything else) that are completely in the shadow of the heatshield, they're basically in a vacuum.  They shouldn't experience any drag at all.

In any case, at those speeds, "drag" in the usual sense is fairly meaningless-- what you have is dynamic pressure from the air slamming into the front of the craft.  Any component on the craft is either exposed to that air flow-- in which case it experiences drag, and bow-wave-compression, and heating-- or else it doesn't, in which case it has neither heat nor drag.  There's not a scenario in which you get to have drag without heating.  (If you could, wouldn't NASA be using it?)

By all means, use this trick if you enjoy it, that's what KSP is all about.    But I won't be using it myself, it just feels like cheating to me.