Thread of the Month: Be Kind to your Kerbonauts - Reentry Technique

[SlowThought]

Hi. I'm new to the game, but I have relevant real world experience, and I couldn't help but notice that a straightforward Apollo type re-entry was imposing something like 15 g on my little green guys. They seem to take it with aplomb, but it seems needlessly cruel.

This has been a concern to the KSP before, but with no solutions offered. I have taken to planning my final periapsis at about 60km, just skimming the atmosphere. With each atmospheric encounter (and yes, there are many more than one) the apoapsis is lowered a bit, making the subsequent skip just a bit longer. When final re-entry occurs, peak g is greatly reduced (per the end of mission stats, peak g went from 15 to < 5; I think the peak was during initial launch, as I observed nothing > 3 during re-entry).

If you are in a hurry and you have fuel left, a retrograde burn at periapsis can cut a few of the extra orbits out, or even all if you have enough to circularize.

Do the Kerbonauts care? Not that I can tell, but I feel better.

Edited May 1, 2014 by Rowsdower
Thread of the month. Hooray!

[roblamb98]

According to my studies, kerbals are much more able than humans to withstand excessive G-forces.

[Kerbonautical]

Personally I do similar things without intention. I tend to pack extra fuel which of course makes the landing heavier and prone to explosion so I usually perform controlled re-entries as a means of burning off the excess fuel, plus over time I got used to doing it this way and can now make relatively close returns to the KSC. Downside is that sometimes I seem to return without the speed to get the flashy light show of re-entry. I couldn't say how much this reduces the G forces because I've never really thought to check but I should imagine it cuts out a lot when there's no re-entry effects.

[Bobnova]

I like to roleplay it too.

There's a mod you can get, Deadly Reentry, that adds reentry heat damage and g force damage, then it's a genuine concern rather than just a RP concern.

It makes things more interesting, I like it.

[NecroBones]

My guys don't care what I subject them to. 15G and they keep on smiling.

[Jas1126]

Like others, I tend to bring extra fuel for landing. By the time I'm actually in the atmosphere, my speed is below 500 m/s. I'm always looking for ways to spice up the game. Sometimes I'll land on Kerbin without bothering with parachutes for the extra challenge.

[Zeroignite]

For gentle reentry, setting a periapsis for 20-25km works pretty well. Lower results in a greater peak-g, higher keeps you going fast in-atmo for an uncomfortably long time (with Deadly Reentry, you have ablative shielding burning off). In general, anything below 30km will pretty much guarantee a one-pass aerocapture to landing.

[Sabastian]

I usually just try and keep the g-meter out of the red zone during reentry. A pe of around 10km usually works pretty well even when coming in from interplanetary trajectories. For the purposes of role playing, I typically assume that the kerbonauts are wearing g-suits for reentry and can therefore sustain up ~10g's for short periods of time. As was the case with the Apollo missions, skipping out of the atmosphere is not an option in my space program.

[cantab]

You mean I shouldn't enter Kerbin's atmosphere near-vertically on the way back from Eeloo?

[Madrias]

I tend to try to keep them out of the red-zone but allow gratuitous re-entry flares when bringing a spaceplane in. Give the pilots and crew and passengers something cool to see without them worrying about losing their snacks.

My strategy? A real nice, shallow, but fast, descent into the atmosphere, and as soon as the air has some bite to slow me down, I pop air brakes. Once we're below supersonic, I engage jet engines and fly to landing site, closing the air brakes as needed, and try to put the plane on the ground gently. A few times I've had a plane end up coming down a bit fast, so I've popped air brakes, and once the flares stop, I'll throw open my drogue chutes.

[samiamthelaw]

You mean I shouldn't enter Kerbin's atmosphere near-vertically on the way back from Eeloo?

This thread confuses me. I always thought that subjecting kerbals to tremendous acceleration was my reward for completing a mission to Eeloo.

[zarakon]

I assume that Kerbals enjoy extreme G forces, so I'm doing them a favor by hitting the atmosphere at 5000 m/s

[Felsmak]

Myself, I look forward to the day I make them reenter Kerbin's atmosphere from the apoapsis of Eeloo.

[Dkmdlb]

I'm surprised that you're getting a 15g re-entry. How fast are you going when you re-enter and what is your Pe? You should be able to return from Minmus in one pass without doing more than about 5Gs with a Pe at about 30km.

[InterCity]

Well, i am trying not to kill kerbals, they are too cute.

However, kerbal cruelty is not a concern for me. So why bother switching off the KSP Interstellar warp drives before re-entry...

[Zeroignite]

There's no aerobrake quite like flying to the Joolian system and doing a direct-hit on Laythe for a LLO capture. Doubly so if Laythe happens to be on the oncoming part of its orbit.

The reentry corridor is just a couple hundred meters wide...

[OrtwinS]

I have been ignoring the G-meter for as long as I play (stock), since it has no effect on gameplay.

I wish it would though. And I'd love to see life-support requirements as well...

Until further notice I just regard Kerbals as extreme-G tolerant, feeding of sunlight and capable of entering indefinite stasis to cope with being locked up in an MK1 pod from Kerbin to Jool and back.

[Minarkhaios]

I used to think that the best way was to use a good aero-braking in the upper atmosphere layer.

Since attempting this on moho, I stand corrected.

[rkman]

a straightforward Apollo type re-entry was imposing something like 15 g

Apollo type re-entry is actually not straightforward http://forum.kerbalspaceprogram.com/threads/60525-Apollo-13-questions?p=819485&viewfull=1#post819485

and can be simulated in ksp by putting periapsis at about 32 km. At some point altitude will rise again while staying inside the atmosphere, before the final descent.

Edited April 24, 2014 by rkman

[Tex]

In my experience, the optimal descent trajectory can be achieved by lowering the periapse to 20-25km. I've actually managed to land command pods without them ever glowing orange from re-entry (although the re-entry effects did occur). The G-meter never rose above 3-4gs!

[EdFred]

Apollo type re-entry is actually not straightforward http://forum.kerbalspaceprogram.com/threads/60525-Apollo-13-questions?p=819485&viewfull=1#post819485

and can be simulated in ksp by putting periapsis at about 32 km. At some point altitude will rise again while staying inside the atmosphere, before the final descent.

I always try to aim for 30-35km on reentry. However, on one return (I forget from where), even at that altitude, I had to come around a second time as I skipped out of the atmosphere.

[Zaryulenko]

The way i use to get them back is a 2 maneuver system, Maneuver 1- Transfer to standard 100x100km Orbit 2-direct Reentry no Pe point to a target zone (usually KSC moved to the desert coast)

[Bearsh]

They can be shot by a rocket and slingshotted into orbit on low gravity planets and arise unharmed... Nor burned. These little green dudes are immortal to non-crashing disasters. These little green dudes are the most heroic brave and awesome species i've met!

[rhoark]

If you really want a low-G reentry, wings are the answer.

[Horn Brain]

Don't do the gently skimming the atmosphere repeatedly thing. When you finally dip the apoapsis into the atmosphere you start from too high up and dive too steeply. It would be nice to plot a maximum g-force vs. periapsis altitude graph from running this experiment but I don't have time. I've learned that there is a sweet spot for low g reentry. If you start with a periapsis of zero and raise it by increments, the maximum g force encountered on each reentry will decrease up to a point, and then you'll start to see it increase again once you get to the point where you lose horizontal velocity too fast and too high and end up dropping more like a brick.

Hope that helps. The numbers people are giving you in here are about right. Around 30km to 40km is what I aim for, depending on how fast I'm coming in.