Proper Re-entry question

[Chads]

Hello,

First time poster on this forum.  I didn't see a solid answer, so I'm just curious if anyone has some tips.

I'm re-entering from the Mun, and I was trying a direct re-entry, and I burn up.  (1.25m heat shield, Science Jr, Cargo Bay with other experiments, Mk1 Capsule, 2 parachutes, OCTO Probe on top).

I realize that re-entry is a relatively narrow window of angle and velocity, but I'm not really sure I've seen anything that puts those into perspective.  What's the proper means of re-entry from another body, like the Mun or Minmus?  I took way too much fuel, compared to what I see in Scott Manley videos, but even burning all of it off, I'm way to fast or steep (or both?  I'm not really sure how to tell).

Pointers greatly appreciated!

[Snark]

Hello, and welcome to the forums! 

1 hour ago, Chads said:

What's the proper means of re-entry from another body, like the Mun or Minmus?

Just plow straight in, pretty much.  As long as you have a heat shield (which you do), and as long as your ship's not unreasonably massive (doesn't sound like yours is), it ought to be a piece of cake.

The reentry vehicle you describe sounds entirely reasonable.  As long as you can keep it pointed with the heat shield in front, and don't go flipping around, it ought to be fine.  What's your periapsis (Pe) altitude before you hit Kerbin's atmosphere?

If you aim for a Pe of around, say, 30 km or so, you should be just fine  You could go a bit lower than that, probably.

[HvP]

Hello @Chads and welcome to the forums!

I think we can be of more help if we knew a little more about the circumstances of your reentry. At what height are you putting your periapsis when you reenter Kerbin? Are you coming in with the heat shield below you all the way in, or are you flipping around nose first at some point? What is your approximate speed and altitude when you explode? Is one part overheating before the others, or does it just all explode at once?

The most likely scenario is that you are simply coming down too steeply. You don't want your periapsis to be near or below the surface before you enter the atmosphere. A periapsis of around 25 or 30 km seems to be the preferred reentry method. If you come in too much steeper than that you might still be going too fast when you hit the dense part of the atmosphere and the shock will blow you up. That usually happens if you are still going more than about 3000 m/s below 20km. Unless your craft is very heavy, though, this shouldn't be a problem. I think you're pretty light.

There are a couple of other design considerations that might be causing your problem. The science jr. cannot take much heat before it explodes and it's doors will extend beyond the protection of the heat shield if you leave them open. Always right-click the science jr. and close the doors before you reenter the atmosphere. It's even better to put the cargo bay between the heat shield and the science jr. because some internal heat will still be transferred up the stack and could cause weak parts attached right behind the heat shield to explode.

Also, the octo probe core is very sensitive to heat. You cannot allow it to contact the heating effects if at all possible. If your craft is not coming in very straight with the heat shield down then your probe core may be exposed to too much heating and explode. It's best to put them inside cargo bays if it will fit. If you still need the probe core exposed on top then just make sure that you have enough control to keep it out of the flames of reentry.

All other things being equal, you want the center of mass of your reentry vehicle to be as close to the heat shield as possible. Otherwise you might flip over and come in nose first which could fry your parachutes, science experiments and probe core.

Edited June 17, 2017 by HvP

[Chads]

Thanks very much for the quick responses!

I'm using SAS/Retrograde tracking during the descent.  It seems that Science Junior blows up, without ever being listed as the "hottest component."  I believe I had it adjacent to the capsule, with the cargo bay then between it and the heat shield.

I'm guessing I was coming in too steeply, though, based on your comments.  I do typically aim for about 30km for my periapsis on re-entry.  I was closer to 5km, though.  I thought I needed to be steeper due to the extra speed, so as not to bounce off the atmosphere.  I thought that due to previous failures, where I did indeed bounce off, orbit again, bounce off again, etc., until the heat shield was out of ablator.    Not the worst experience ever, but it may have set me up for failure this time around, as I took the wrong lesson away from that.  I'll be sure to pay closer attention to that as I move forward.

When the Materials experiment would blow up, it would throw the rest of it end over end, and eventually everything would go.  I did manage to keep the capsule and pilot, though.  So, while I lost a little bit of hardware, at the end of the day (and 3-4 restores from save) I had recovered the kerbonaut. 

Again, thanks!  I look forward to both learning more, and ultimately contributing in the future!

 

Chads

[Foxster]

Remember that there is no need to bring anything back to Kerbin other than your capsule, heatshield and parachute.

Your Kerbal can collect the results from the experiments and they will be saved in the capsule when he re-enters. Getting rid of anything else will reduce mass (which increases the rate of descent) and the risk of the re-entry craft flipping or otherwise overheating, as well as obviously saving dV of not returning the experiments to Kerbin. 

A mk1 capsule with a heatshield (about 20% ablator is all you will ever need) can survive direct re-entry from just about anywhere with a Pe of about 25km.   

[Wanderfound]

The Sci Jr is the problem; it, like most science parts, has a very low heat tolerance.

Either hide it in a fairing or cargo bay, or collect the science from it and dump it before reentry.

[Reusables]

You don't need ablators in heatshields during reentry from LKO-scale speed - adjacent parts might be subjected to more heat, but I don't think it will ruin your ship as the part there is the cargo bay.

Imo direct reentry is sometimes not feasible, and it could be better to bounce off and enter again in kerbal universe.

Edited June 17, 2017 by Reusables
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[Chads]

Really appreciate it, thanks much!