Re-entry on Eve impossible due to overheating

[BenCushwa]

[quote name='Xyphos'][SIZE=5]Makki's claims are correct, re-entry is now impossible.[/SIZE]

[URL]http://youtu.be/uU3hPkNbeCc[/URL][/QUOTE]

I applaud your efforts, but this is hardly conclusive.

1. You ran your test with physics time warp on. I do things that work normally but blow up in physics time warp all the time. Try it without the warp.

2. You only re-entered your ship on a very shallow trajectory. Try a steeper trajectory. As others have said, shallower isn't always better. Sometimes the increased peak heating of a steeper angle can be offset by the shorter duration of the heating (assuming you have a heat shield of course).

3. You're relying entirely on the drag on a relatively small surface area to slow you down. More drag works like a steeper descent angle: higher peak heating but at shorter duration. Try finding more ways to induce drag to slow you down faster.


EDIT:

[quote name='Xyphos']it's becoming frustrating having to unlearn everything and relearn whenever squad releases an update.[/QUOTE]

I feel your pain sir. Many of my old ships/flight profiles don't work in 1.0.5. I've managed to convince myself that it's part of the fun. ;^) Edited November 18, 2015 by BenCushwa

[Starwaster]

[quote name='Kobymaru']Dear atmospheric Overlords, this is very counter-intuitive. Could you walk us mortals through it? Personally, I'm not opposed to formulas. (I just don't know which are important and what the abstractions for KSP are).

ps.: I asked the same question [URL="http://forum.kerbalspaceprogram.com/threads/139855-PSA-Eve-%28re%29entry-is-impossible-CONFIRMED?p=2302079&viewfull=1#post2302079"]in the other thread[/URL]. Sorry for the spam.[/QUOTE]


I don't think I have anything to add on to what Ferram said but here is some reading material on the subject:

[url]https://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/ame/media/Section%20III.4.1.7%20Returning%20from%20Space.pdf[/url]

[Workable Goblin]

[quote name='magnemoe']Then Apollo returned from the Moon it was an issue going to high thing could make them bounce back with the result that they would run out of air before returning, they would also have no control of the second entry, if really unlucky they could enter to low on that. [/QUOTE]
There was actually a mode in the Apollo Guidance Computer (the Apollo Skip Guidance) which was intended to use precisely this ability to skip out of the atmosphere to allow it to access more return areas than a simple ballistic or even lifting entry would allow. It was never used, but the contemporary Soviet Zond spacecraft routinely used skip reentry to reach Soviet territory from typical reentry zones over the Indian Ocean. It does require fairly precise guidance for a true skip entry, but that's mostly if you actually want to get to a particular area or if you are returning from interplanetary speeds. Otherwise, going shallow without retaining orbital velocity will tend to put you on some kind of skip.

Generally speaking, an overly shallow entry ought to cause you to bounce out of the atmosphere, not overheat. If it's doing it the other way around, it's really quite peculiar, in my opinion.

[OhioBob]

[quote name='sdj64']Now, aerocapture at Eve is another story, one which almost always ends in a big fireball for any ship. Eve missions should plan for propulsive capture now.[/QUOTE]

[FONT=Calibri][SIZE=3][COLOR=#000000]I did a bunch of aerocapture tests at both Eve and Jool and found little problem. [/COLOR][/SIZE][SIZE=3][COLOR=#000000]It is entirely possible to aerocapture at Eve.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] The only serious problem I had was when I got down to an altitude approaching 60 km and the vehicle wanted to flip around. [/COLOR][/SIZE][SIZE=3][COLOR=#000000]This was a stability problem and not a heating problem.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] In most cases I found that an intercept periapsis of about 65 km worked pretty well with neither heating nor stability problems.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] There should be little need to have a periapsis lower than that unless you have a very high entry velocity or a very high ballistic coefficient. [/COLOR][/SIZE][SIZE=3][COLOR=#000000]If you have both a high entry velocity and a high ballistic coefficient, then you’ll likely burn up. [/COLOR][/SIZE][SIZE=3][COLOR=#000000]If you have a vehicle with a very low ballistic coefficient, then you may need a periapsis closer to 70-75 km. [/COLOR][/SIZE][/FONT][FONT=Calibri][SIZE=3][COLOR=#000000]In any case, you’ll definitely need heat shields.
[/COLOR][/SIZE][/FONT][FONT=Calibri][SIZE=3][COLOR=#000000]
I also performed some Eve entry tests.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] Again my biggest problem was having the ship want to flip.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] Above 60 km there wasn’t much problem, but as soon as I got deeper in the atmosphere stability was a big issue.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] Activating RCS helped to stabilize it, but it still flipped when I got <50 km. [/COLOR][/SIZE][SIZE=3][COLOR=#000000]I ended up adding aerobrakes at the top and that solved the problem.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] By the time I needed to deploy the aerobrakes I was slow enough that they didn’t overheat. [/COLOR][/SIZE][/FONT][FONT=Calibri][SIZE=3][COLOR=#000000]In fact, I didn’t have any problems with overheating as long as I could keep the ship stable.
[/COLOR][/SIZE][/FONT][FONT=Calibri][SIZE=3][COLOR=#000000]
By the way, I had the same flipping problem at Jool, which wanted to occur at about 150-155 km. [/COLOR][/SIZE][SIZE=3][COLOR=#000000]Above that altitude, aerocapture was easy.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] For most cases a periapsis altitude of about 160 km worked well. [/COLOR][/SIZE][SIZE=3][COLOR=#000000]For very low ballistic coefficients, 165-170 km would be better.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] Like Eve, a combination of high entry velocity and high ballistic coefficient should be avoided.[/COLOR][/SIZE][/FONT]

[Mister Dilsby]

Powered descent, surface refueling, problem solved. (hey, that's also a six word story! woo hoo!)

[OhioBob]

[quote name='Choctofliatrio']So can we just get an accurate summary? What's the best periapse for Eve aerobraking/aerocapture? Is it high? Low? Medium?[/QUOTE]

[FONT=Calibri][SIZE=3][COLOR=#000000]I think that for most cases the answer is about 65±5 km.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] It varies depending on the entry velocity and the drag characteristics of the vehicle.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] If your entry velocity is slow and/or you have a low ballistic coefficient, then set your periapsis on the high end. [/COLOR][/SIZE][SIZE=3][COLOR=#000000]If your entry velocity is fast and/or you have a high ballistic coefficient, then set your periapsis on the low end.[/COLOR][/SIZE][/FONT][FONT=Calibri][SIZE=3][COLOR=#000000] It will vary case by case; you might have to experiment to find out what works best.
[/COLOR][/SIZE][/FONT][FONT=Calibri][SIZE=3][COLOR=#000000]
The equation for ballistic coefficient incorporates the drag coefficient, which is not easily determined.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] A substitute for ballistic coefficient is what I call the [/COLOR][/SIZE][I][SIZE=3][COLOR=#000000]shield loading[/COLOR][/SIZE][/I][SIZE=3][COLOR=#000000], which is the mass of the vehicle divided by the square area of the heat shield(s). [/COLOR][/SIZE][SIZE=3][COLOR=#000000]I ran experiments using test vehicles with shield loadings ranging from 600 kg/m[/COLOR][/SIZE][SUP][SIZE=2][COLOR=#000000]2[/COLOR][/SIZE][/SUP][SIZE=3][COLOR=#000000] to 8400 kg/m[/COLOR][/SIZE][SUP][SIZE=2][COLOR=#000000]2[/COLOR][/SIZE][/SUP][SIZE=3][COLOR=#000000].[/COLOR][/SIZE][SIZE=3][COLOR=#000000] And I tested entry velocities ranging from about 4600 m/s to 5000 m/s, which is typical for intercepts originating from Kerbin.[/COLOR][/SIZE][/FONT][SIZE=3][COLOR=#000000][FONT=Calibri]
[/FONT][/COLOR][/SIZE][FONT=Calibri][SIZE=3][COLOR=#000000]
The 65±5 km number worked well for most cases in the middle.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] Only at the extremes did I get outside that range. [/COLOR][/SIZE][SIZE=3][COLOR=#000000]For example, when testing the highest shield loading in combination with the highest entry velocity, my ship would flip and explode. It was the only case in which I couldn’t complete a successful aerocapture.[/COLOR][/SIZE][SIZE=3][COLOR=#000000] And when testing the lowest shielding loadings, a periapsis of about 70-75 km was needed.[/COLOR][/SIZE][/FONT]

[Pthigrivi]

[quote name='OhioBob']The only serious problem I had was when I got down to an altitude approaching 60 km and the vehicle wanted to flip around.[/QUOTE]

I had a similar experience. Often this can be solved just by moving the COM as close as possible to the heat-shield. I bet one could also put fins on the tail end on radial decouplers so as not to eff up accent for return missions.

[More Boosters]

[quote name='Starwaster']I don't think I have anything to add on to what Ferram said but here is some reading material on the subject:

[URL]https://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/ame/media/Section%20III.4.1.7%20Returning%20from%20Space.pdf[/URL][/QUOTE]

[quote name='Article']Figure 4.1.7-1. Apollo Capsule Re-entry. This artist’s concept of the Apollo re-entryshows that air friction causes the capsule to glow red hot. The astronauts inside stay cool,thanks to the protective heat shield. (Courtesy of NASA/Johnson Space Center)[/quote]

And now I'm confused. I thought heating was mostly due to compression?

[Streetwind]

It is. The mistake is just really, really common :P