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Reentry Question


Leeman
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Hey guys,

Last time I played KSP I was glad that I am able to orbit a rocket. Now with the new update right out the door, thats promising better performance, I thouhgt I pick it back up.

 

Anyhow now to the outline what leads me to the question

I am not too far into the career (haven't unlocked all of the 2.5m parts) and just did a reentry that kinda got me worried as bigger ships will probably need more time to slow down / ablate more on the heat shild. I came in to Kerbin with an Apoapsis of arround 1 110 500 Meters and a Periapsis of arround 30 100 Meters. Orbit Speed of arroumd 2780m/s; Surface Speed of arround 2620 m/s. My path was counter clockwise arround Kerbin My vessel was perfectly aero dynamic at that speed and didn't drained power during the descend however it took quite long to break and it used arround 120 Ablator of the Heatshield. Now I now I still have arround 80 left so I could have went on longer.

So. Now finally to the question:

If I come in from an higher Appoapsis, like returning from Minmus or even from the orbit of Sol  Kerbol / Sun if I chose a periapis of arround 30 000m I will burn up as I will be way too fast. What would you suggest is a good reentry height?

Edited by Leeman
Kerbal Noob called Kerbol sol.. Pff. :P
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The amount of ablator that a heat shield has can be adjusted.  When you first install a heat shield, it defaults to the maximum amount - 200 kg for a 1.25m heat shield, 800 kg for a 2.5m heat shield, and 1800 kg for a 3.75m heat shield.  When you are in the VAB, you can right-click on the heat shield and there is a slider that allows you to reduce the ablator mass.  For returns from Mun and Minmus, you typically don't need the maximum amount.  What I usually do is install the heat shield on the part that will reenter, and turn the slider all the way down to zero.  I then note the mass and take 5% of that amount.  I then increase the slider until the ablator mass is equal to or just over the 5% amount.  I found that I've never needed more than that when reentering from anywhere at or inside Minmus' orbit.  However, for high-speed interplanetary trajectories, you'll likely need a lot more.
 

Edited by OhioBob
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I'm nowhere as scientific as OhioBob, but a relatively low ablator amount (about a quarter or less of a heatshield) works fine for the Mun and Minmus, maybe a bit higher for return from Duna or Eve. I've tended to err on the side of caution for faster return values.

As for the re-entry height - 28-31 km works well at just about any incoming velocity. It's actually a bit lower than the best re-entry height from low orbit, since you will not slow quickly enough to fall lower until you've actually passed periapsis (whereas from low orbit, you'll slow then fall into the thicker atmosphere far sooner).

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8 minutes ago, OhioBob said:

I set my ablator mass to about 5% of my reentry mass.  Has worked every time for me without any problems.

Sorry but.. What do you mean with that?^^

 

Edit: Oh I am in the VAB right now. So you mean if my reentry stage weights 3,8t.. 5% of it is 190 000 .. to set the Ablator Value to 190? (so in the end I guess you mean 5% of the kg as thats 190?)

Edited by Leeman
Found out what he means.. probably
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6 minutes ago, OhioBob said:

The amount of ablator that a heat shield has can be adjusted.  When you first install a heat shield, it defaults to the maximum amount - 200 kg for a 1.25m heat shield, 800 kg for a 2.5m heat shield, and 1800 kg for a 3.75m heat shield.  When you are in the VAB, you can right-click on the heat shield and there is a slider that allows you to reduce the ablator mass.  For returns from Mun and Minmus, you typically don't need the maximum amount.  What I usually do is install the heat shield on the part that will reenter, and turn the slider all the way down to zero.  I then note the mass and take 5% of that amount.  I then increase the slider until the ablator mass is equal to or just over the 5% amount.  I found that I've never needed more than that when reentering from anywhere at or inside Minmus' orbit.  However, for high-speed interplanetary trajectories, you'll likely need a lot more.
 

Okay, thanks :) 

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4 minutes ago, Plusck said:

I'm nowhere as scientific as OhioBob, but a relatively low ablator amount (about a quarter or less of a heatshield) works fine for the Mun and Minmus, maybe a bit higher for return from Duna or Eve. I've tended to err on the side of caution for faster return values.

As for the re-entry height - 28-31 km works well at just about any incoming velocity. It's actually a bit lower than the best re-entry height from low orbit, since you will not slow quickly enough to fall lower until you've actually passed periapsis (whereas from low orbit, you'll slow then fall into the thicker atmosphere far sooner).

 

Now that you meantioning it, passing periapsis. Does that mean that I could skip atmosphere and on my second way down I might come down to steep? I had a lot of too steep entries lately.. :P 

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1 hour ago, Leeman said:

Now that you meantioning it, passing periapsis. Does that mean that I could skip atmosphere and on my second way down I might come down to steep? I had a lot of too steep entries lately.. :P 

On a return from Mun/Minmus with a periapsis of 30 km, you shouldn't skip.  But you'll likely level off and hold a fairly constant elevation for about a minute or so while you're bleeding off most of your velocity.  It's been a while since I've done a Mun/Minmus return but, from what I recall, I think I usually land about 30 degrees long of the initial periapsis.  If you come in much higher than 30 km, however, you could easily go way long or even do a skip.  I'd probably err on having a periapsis a little below 30 km rather than above it.

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For minmus re entry I'll usually set PE to 45-50km which brings my AP down below 25Mm and then will pass once more to bring it down to roughly 10Mm or so.  At that altitude its less than mun height and I'll lower PE to 30-35km which garentees re-entry in that half orbit or at the very most one or two passes.  This is also only when I have minimal fuel, since I'll usually just start burning retro grade when I'm a few degrees away from PE which will greatly lower AP and still lower PE enough that I don't even need to make a pass to safely reenter. 

I'm also a big fan of airbrakes which turn a 40km PE and 70km AP to a rentry after a pass or two

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25 minutes ago, mrclucks said:

For minmus re entry I'll usually set PE to 45-50km which brings my AP down below 25Mm and then will pass once more to bring it down to roughly 10Mm or so.  At that altitude its less than mun height and I'll lower PE to 30-35km which garentees re-entry in that half orbit or at the very most one or two passes.  This is also only when I have minimal fuel, since I'll usually just start burning retro grade when I'm a few degrees away from PE which will greatly lower AP and still lower PE enough that I don't even need to make a pass to safely reenter. 

I'm also a big fan of airbrakes which turn a 40km PE and 70km AP to a rentry after a pass or two

I did something very similar to that for a while, but I got supremely bored just sitting watching my craft sizzle and spark its way through the atmosphere, then pressing warp (no, you can't warp faster under 120000m.. no, you can't warp faster under 240000... no, you can't warp faster under the age of 25 with a full driver's licence and six years' experience...).

So now I tend to come in excessively hot, sometimes losing bits off the side, and have completely forgotten how to re-enter from low orbit with an unstable craft.

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12 minutes ago, Plusck said:

I did something very similar to that for a while, but I got supremely bored just sitting watching my craft sizzle and spark its way through the atmosphere, then pressing warp (no, you can't warp faster under 120000m.. no, you can't warp faster under 240000... no, you can't warp faster under the age of 25 with a full driver's licence and six years' experience...).

So now I tend to come in excessively hot, sometimes losing bits off the side, and have completely forgotten how to re-enter from low orbit with an unstable craft.

You're aware that alt+warp keys gives you physical warp, in those cases where regular time warp doesn't work?

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16 hours ago, Leeman said:

 

Now that you meantioning it, passing periapsis. Does that mean that I could skip atmosphere and on my second way down I might come down to steep? I had a lot of too steep entries lately.. :P 

You don't actually skip as Kerbal capsules aren't lifting bodies.  However, it's quite possible to dive into the atmosphere and head back out because you didn't bleed off enough speed.  With the periapsis a little high and coming in from one of the moons I've actually come down on the third pass.  With periapsis a bit high each pass through the fire bleeds off apoapsis but leaves the periapsis almost untouched.  Once you are back in space redeploy your panels and warp around until you are approaching the atmosphere again.  Fold your panels again and enjoy the fireworks.  Repeat until your apoapsis has been bled down to near Kerbin, then you'll actually come down.  Beware that this can draw a lot from your batteries--I've run out of power once when my apoapsis ended up at about 55 km so I didn't get to recharge before diving in again.

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16 minutes ago, Kerbart said:

You're aware that alt+warp keys gives you physical warp, in those cases where regular time warp doesn't work?

Oh, sure, but I tend to worry that I won't spot excessive heating quickly enough, or that SAS will drain my batteries, or some part will decide there's too much stress, all of which tends to limit physics warp to well past Pe. And going right through the atmosphere on maximum physics warp still takes almost the same time as slowing down violently and landing on the first pass.

Plus, as Loren Pechtel just mentioned, there's the whole "stowing panels, going through atmosphere, unstowing" process and then de-warping, stowing and realigning for the next pass...

At that stage, I just wanna go home!

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28 minutes ago, Loren Pechtel said:

You don't actually skip as Kerbal capsules aren't lifting bodies.  However, it's quite possible to dive into the atmosphere and head back out because you didn't bleed off enough speed.  With the periapsis a little high and coming in from one of the moons I've actually come down on the third pass.

While I typically don't like to use multiple passes, it is possible to rise a bit in elevation before making a final dive all on the first pass.  I usually don't bother with this in the stock game, but when playing Real Solar System, I use this technique to reduce the g-load.  The technique results in two peaks in the acceleration curve that are both lower than the single peak that you'll get when coming in at a steeper angle (i.e. a lower periapsis).  If I do it right, I can limit my peak acceleration to about 6 g.  The technique works in the stock game as well, but the g-loads are generally low enough that I don't find it worth the trouble.

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There's a few things you can do, as others have mentioned -  come in with a higher Pe and, perhaps, do multiple passes - it's tedious and a problem if you're running with life support mods, but it will do the trick - eventually...

 

Otherwise, other things you can do to lessen the danger is to reduce the weight of your ship as much as possible with it re-enters - the heating isn't just about speed, it's about reduction in speed, and that's also dependant on your kinetic energy - generally, the lighter the ship, the easier it will be to slow down. 

 

OTOH, rocket engines are surprisingly good at absorbing heat on re-entry. For very early missions, I generally don't bother with a heatshield, but instead leave my final stage in place on re-entry, and decouple it only when coming to land if I think the parachute won't slow down the whole stack sufficiently.


Wemb

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