How to avoid ‘sploding in Eve atmospheric entry

[Lewie]

Today I sent a lander to Eve for the first time. I entered the atmosphere at 6,000 m/s at 62,000 meters. I was using the ten meter inflatable heat shield, but I still blew up. Any advice?

[Snark]

11 hours ago, Lewie said:

Today I sent a lander to Eve for the first time. I entered the atmosphere at 6,000 m/s at 62,000 meters. I was using the ten meter inflatable heat shield, but I still blew up. Any advice?

The inflatable heat shield doesn't have any ablator on it, so 6000 m/s is way too fast for it.

Either hit atmosphere at a lower speed, or use a shield with ablator.

[jimmymcgoochie]

Aim higher. Inflatable shields can take quite a lot of heat, but they’ll still cook if you go bombing into Eve’s atmosphere directly from a transfer orbit. Brake high in the atmosphere to capture into Eve orbit, then keep clipping the atmosphere to slow down and shed speed until you eventually drop down to land. It’s slower but safer.

Did your lander burn up while it was pointing retrograde, with the shield first, or did it flip around and burn up because the soft, squishy bits got exposed to the heat? If it’s the latter, try to shift your centre of mass as close to the shield as possible to keep it stable. Inflatable heat shields generate a lot of drag so if the centre of mass is too far away it becomes unstable and will easily flip the wrong way and things get very burny, very fast.

[Lewie]

The heatsheild went first, the lander went after the heatsheild was gone. 

Thanks for the advice.  I didn’t know that the inflatable couldn’t take that..at what altitude would you recommend aero capturing?

[Snark]

2 hours ago, jimmymcgoochie said:

Aim higher.

I would actually disagree with this recommendation-- or, at the very least, it's a lot more nuanced than that.  The thermodynamics of aerobraking can be... counter-intuitive.

For example:  It's a very common mistake that a lot of folks make who are new to KSP, to think:  "Well, I heat up from entering atmosphere.  And there's more atmosphere lower down.  So if I dive too deep I'll go kaboom, and I'm marginal for surviving.  I want the gentlest possible reentry, so I can survive.  Therefore, I'm just gonna 'dip my toe in the water' and try for a very shallow reentry where I don't go very deep."

That sounds perfectly reasonable and logical... but is actually wrong (at least, in a lot of cases).  Rather counter-intuitively, it's actually often less heating if you dive steeply than shallowly.  That is:  you can have a situation where you go kaboom if you try a shallow reentry, but if you plow more deeply into the atmosphere, you do better.

The reason for this is that two things happen on reentry:

• Drag slows you down
• Friction heats you up

Both of those effects get stronger, the deeper you go into atmosphere.  But as you get deeper, drag builds up faster than heating does.  That's the key issue.

If you go for a really shallow reentry... the problem is that the upper reaches of the atmosphere do heat you up quite a bit, but don't slow you down by any appreciable amount.  There's just not enough drag.  The result is that you get slow-roasted without ever slowing down enough.  By plowing right in, it's true that heat builds up in a hurry-- but drag builds up faster, and you end up slowing down a lot (and therefore heating less) before you can build up too much heat.

That doesn't mean you should reenter heading straight down.    It is possible to go too steep.

Just... beware of thinking that "shallower = more survivable".  My observation has been that people who explode on reentry are more likely to have a problem due to being too shallow instead of too steep.

1 hour ago, Lewie said:

at what altitude would you recommend aero capturing?

On Eve, 62 km (as you say) is actually a pretty good height, maybe even you could aim a bit lower (in the high 50s).  It's low enough to generate significant drag and not slow-roast yourself.  If you're blowing up at 62 km, my guess is that you'd make it worse, not better, if you aimed higher.

I think the real issue, here, is that you're just going too darn fast.  6 km/sec is really fast for something without ablator.  So, I'd suggest that you be going slower than that when you hit atmosphere.  Ways to do that:

1. Have a good transfer window.  If you launch from Kerbin to Eve on a good window, you should only be going about 4600 m/s when you hit Eve's atmosphere.
2. Spend some fuel thrusting to reduce your velocity before hitting atmosphere.

The other thing you didn't mention is, what's the mass of your lander?  The heavier it is, the more of a beating your heat shield will take, because you'll need to dive deeper (while still going fast) until there's enough drag to slow you down.

If it turns out that your problem is that you're super-heavy, then you could either reduce the mass of the lander, or else deploy additional heat shields so that you have extra drag surface to slow you down.  Ships that survive reentry well tend to be light and draggy-- i.e. they have a low ballistic coefficient-- so you can lower yours by adding additional drag without upping your mass much.

 

TL;DR:  You can absolutely re-enter on Eve using the inflatable heat shield, and I think you're doing it at about the right altitude.  Just make sure you're not going too fast, and (if you're super-heavy) consider deploying additional shields to up your drag.

(And, of course, there's always the option of switching to shields that actually have ablator on them-- they can take more punishment.)

 

[Zhetaan]

2 hours ago, jimmymcgoochie said:

Aim higher. Inflatable shields can take quite a lot of heat, but they’ll still cook if you go bombing into Eve’s atmosphere directly from a transfer orbit. Brake high in the atmosphere to capture into Eve orbit, then keep clipping the atmosphere to slow down and shed speed until you eventually drop down to land. It’s slower but safer.

Did your lander burn up while it was pointing retrograde, with the shield first, or did it flip around and burn up because the soft, squishy bits got exposed to the heat? If it’s the latter, try to shift your centre of mass as close to the shield as possible to keep it stable. Inflatable heat shields generate a lot of drag so if the centre of mass is too far away it becomes unstable and will easily flip the wrong way and things get very burny, very fast.

Actually, don't aim higher.  Shallow reentries involve lower peak temperatures, but greater overall heat, and are useful really only when you have a pressing need to manage skin temperatures more than overall heat (as one may need to do with spaceplanes).  You can thoroughly cook a rocket with a shallow Eve reentry.  Steep reentries involve higher thermal spikes, but a lot less overall heat; this is exactly the sort of condition that ablator-loaded heat shields are designed to manage.

 

Edit:  Ninja'd by @Snark

Edited May 28, 2020 by Zhetaan

[Lewie]

Wow, thanks guys! I never would’ve known to do these tips. The total mass of the lander was about three-two tons. I did launch at transfer window, at roughly 54 degrees. I was definitely off a little bit. I was using a nerva for my transfer stage, so I wouldn’t really be able to slow myself down by burning retrograde, as I would’ve been forcing my periapsis too low. Which as Snark said might just be the trick...

thanks again, your advice will be incredibly helpful!  

[Snark]

5 hours ago, Lewie said:

I was using a nerva for my transfer stage, so I wouldn’t really be able to slow myself down by burning retrograde, as I would’ve been forcing my periapsis too low. Which as Snark said might just be the trick...

Note that even a fairly tiny burn or can adjust your Pe height by quite a lot, if you do it when you're still far out on the fringes of the SoI.  So as long as you have the dV to spare to do a substantial burn when you're near Pe to slow yourself down, you totally can.

Just do a (tiny) burn when you're way out on the fringes of the system to raise your Pe an appropriate amount, and then when you're closer in and soon to hit atmosphere, you can do a burn which accomplishes the dual goal of slowing you down (to better survive reentry) and lowering your Pe to what it needs to be for reentry.  (The closer to the planet you can get before doing the burn, the better, in order to take maximum advantage of Oberth effect.)