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Probe --> landing: heat shield and angle of entry


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HEC core with a simple therm on the side and a few solar panels.  Small fuel tank and terrier engine.  Heat shield just below that.  Pe set to just over 100k with elliptical orbit.  I burn retrograde to get my Pe low enough to land.  Then I dump the stage, leaving only the configuration above, coming in heat shield first.  Complete explosion.  My plan was to ditch the shield once below the worst of it, using the terrier and parachutes to land. 

Questions:
1.  what is the required speed I need to reduce to in order to avoid a melt down?  Or do I just need a bigger shield?  
2.  I'm not using any aerobraking to set up my initial orbit.  So my Ap is several Million, while my Pe is stable at 100K.  I just rotate a few times to make sure I'm likely to hit land. 

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it depends, on the planet, the reentry speed, the reentry angle and a number of other factors, so this is a really hard question to answer with the given info, there is no one "required speed" to reenter at, nor is a bigger heat shield necessarily going to solve your problem.  First off, keep an eye on what the heat shield is doing.  Is it slowly losing ablator until it runs out?  then yes, maybe a bigger heat shield might help you last longer.  Is it shedding ablator quickly or (worse) overheating faster then it can ablate the heat away?  Then you probably are coming in too steep and fast.  Aerobraking may shed some of that speed enough to make it survivable.

Edited by Capt. Hunt
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When you say "heatshield just below that" do you really mean it's below the Terrier engine and somehow not obscuring its thrust? Or actually above the HECS core?

Because if it's attached directly to the HECS, which has a 1200K temp limit, some of the heating will propogate to it and can destroy it while the rest of the craft remains intact. Putting a service bay between the shield and tank, with the HECS and thermometer inside the bay, is more ideal.

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11 hours ago, MPDerksen said:

My plan was to ditch the shield once below the worst of it, using the terrier and parachutes to land. 

Well, using the terrier engine for (or even during) atmospheric entry has two major problems. One - that I hope will get fixed soon - is that it suffers from bug #23924, i.e. there is currently a bug in KSP that prevents the terrier from being protected by heat-shields in most cases. The other issue is that the terrier is a pure vacuum engine! At Kerbin sea level it has less than a quarter of the thrust it has in vacuum. So if you want to land in an atmosphere(*) it is better to e.g. use the spark engine, that one has more thrust in atmosphere than the terrier, and it is lighter.

So while I think that your problem is due to the bug I mentioned, the real question is: Why are you trying to use a terrier in an atmosphere?

And about you other questions: which planet are you trying to re-enter anyhow?

P.S. (*)Here Duna counts as having no atmosphere. But Duna's atmosphere is so thin that if you manage to burn up your craft entering that then you probably deserve some kind of award for that. (Even if it is a Darwin award.:cool:)
P.P.S. Like for other questions here in the last day I'll recommend to the moderators to shift this thread to the Gameplay Questions and Tutorials section of the Forums.

 

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Let's start with some basics, like- what game version? Any mods, specifically those that would affect re-entry heating e.g. FAR or deadly re-entry? What planet, and what version of the planet (stock, rescaled etc.)? What size of heat shield and how much ablator? What re-entry heating settings are you using within KSP itself?

I'm going to have a guess and say you're trying to bring your craft down on Kerbin itself, in which case the terrier is dead weight because it can't function in an atmosphere and parachutes alone will suffice. Service bays are very heat-resistant so stick the probe and any other gubbins either inside or on top; fairings are also quite heat-tolerant if you stick a heat shield directly below them to take the brunt of the energy and can protect the contents before being ditched when you need to deploy your parachutes.

If you're trying to land on either Eve or Laythe, the terrier won't help you much either and chutes alone will do the job; if you're trying to land on Duna, then the air is so thin that you can barely use drogue chutes and the terrier will work at near 100% efficiency, but you'll never get enough heat to burn up. And if you're trying to "land" on Jool, then prepare to say goodbye to your probe as it slowly sinks into oblivion...

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Maybe we can pretend I never asked the question?  LOL.  Sorry.  It was my first Eve landing attempt and I didn't do my homework before the attempt.  I removed the Terrier completely, and upgraded to the inflatable heat shield.  Done.

The plan was to use the shield until I got low enough to ditch it, then have an engine for some control of my landing site.  Overall a very poorly planned mission.  Started it with my head straight and completed it without difficulty.

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