Re-entry

[Steampunked]

So I took a good break from KSP for a while and with 1.0.5 I have restarted a new career. One thing I'm noticing is a kill a lot of Jebs on reentry. Before the aero overhaul I just burned retrograde until PE was down below the surface of Kerbin for return and I'd just ride out re-entry. Now I am having to balance descent angle with how much time I spend at a given altitude for heating vs. deceleration. Unfortunately I have not yet found a comfortable return trajectory envelope. Does anyone have any suggestions or advice?

Many thanks.

~ash

Edited December 9, 2015 by Steampunked

[agent 902]

i often just leave the pe between 28-30 km and then hold retro when i reenter witha normal capsule+heatshield. 

[Streetwind]

The proper reentry strategy is far more craft-dependant than it used to be. Are you blowing up lone pods, or do you have difficulties returning stacks of science jr's? Without knowing details, we can only give you general advice, and things like what agent 902 is saying above may not work for all of your use cases.

General advice:
- Don't fly straight up and down. People often experience reentry issues on their first suborbital hops because their capsules don't slow down in time. Even when not going into orbit, you should have at least a modicum of horizontal velocity so you pass through more air that can slow you down.
- Don't reenter too carefully. For a lot of people, the first response to burning up is trying an even shallower reentry from an even lower apoapsis, because they equate burning up with being too aggressive. This is not always the case. You can also burn up from being not aggressive enough, particularly when your vertical speed is too low at the beginning. Coming in from a higher apoapsis improves your initial vertical speed, while setting your periapsis around 25km will reduce your vertical speed by the time you get into the lower reaches (your vertical speed would usually zero out and reverse when passing periapsis, if you were not inside the atmosphere).
- Mount a heatshield. You said you've been away for a while, which means I have to ask: are you aware that heatshields are a thing now? If you try to reenter without a heatshield, you will frequently burn up.
- Use body lift. If you angle your capsule forwards during reentry, air will deflect downwards and push you up. Similarly, you can angle backwards and gain some extra downforce. That way you can control your vertical speed to some extent (though you need a certain amount of air resistance for this to work).
- Make sure your vehicle has enough area versus mass. A long, thin rod of heavy modules will just not slow down in time. The heavier your reentry vehicle is, the more area you want to expose to the airstream.

[Hupf]

- to focus on one point already mentioned above: the lower the initial apoapsis, the less problems you will have. This is kind of the opposite strategy to "aerobraking", but you might try using any fuel to spare to start your atmospheric reentry from a lower orbit, reducing entry speed and heat

- personally, I really like the A.I.R.B.R.A.K.E.S for initially slowing down

- take note of the different heat tolerances of all parts on your vessel. Some (engines, pods) will be much more robust (2400K) then others (1000K) like science equipment

- try to orient your vessel so that the more sensitive parts are "shielded" by the more robust ones, i.e. so that no airflow touches them

[Streetwind]

25 minutes ago, Hupf said:

- to focus on one point already mentioned above: the lower the initial apoapsis, the less problems you will have. This is kind of the opposite strategy to "aerobraking", but you might try using any fuel to spare to start your atmospheric reentry from a lower orbit, reducing entry speed and heat

Erm no. I stated the opposite of that. A lower apopasis does not reduce atmospheric heating. It reduces your vertical speed in the upper atmosphere, increasing the time you spend there and thus the amount of heat you soak up without actually slowing down in any measurable way. Then your vertical speed increases as you fall into the lower reaches, instead of decreasing, so the amount of shock heating spikes up faster than your forward velocity is reduced and you die a fiery death.

Your horizontal (prograde) speed is almost entirely irrelevant when reentering from below 1000km apopasis altitude. Your vertical speed (your rate of falling towards the planet) in various atmospheric regimes is everything. You want it to be high in the upper atmosphere and low in the lower atmosphere, not the other way around.

 

Edited December 9, 2015 by Streetwind

[Steampunked]

 

Thank you Streetwind, that is very helpful!.

[Snark]

+1 to previous posters' advice.

In practical terms: if you put a heatshield on and set your Pe to 30km for the initial approach, you'll be good for the large majority of cases.

There are still plenty of ifs, ands, or buts for various edge cases involving your design, but in my experience "heatshield and 30km" works for nearly everything. The only question is how much ablator to put on the shield (it almost never needs a full load).

 

Edited December 9, 2015 by Snark

[Hupf]

1 hour ago, Snark said:

The only question is how much ablator to put on the shield (it almost never needs a full load).

Thanks - I never got around to make that connection - fine-tuning LF amounts on my craft but accepting heat-shield weight as it was

[Snark]

1 hour ago, Hupf said:

Thanks - I never got around to make that connection - fine-tuning LF amounts on my craft but accepting heat-shield weight as it was

My experience is that if you're in Kerbin orbit (i.e. not coming back from interplanetary space at very high velocity), then for "normal" ship designs, you almost never need more than a 25% ablator load, and sometimes less than that.  A little goes a long way.

By "normal" I mean it's not a lawn-dart scenario, i.e. you don't have a super long, skinny design with lots of mass stacked behind a small-diameter heat shield.

(About the only time I ever completely used up a 1.25m heat shield with a full load of ablator was just such a case.  I had a contract to take 4 tourists to solar orbit, and I was impatient and didn't want to wait, and the contract reward was high enough that I didn't mind overbuilding and wasting dV.  And I wanted to try an oddball design.  So I made a super long, skinny lawn dart.  From top down:  disposable nosecone for launch, 1.25m heat shield with full load of ablator, 1.25m probe core, two Mk1 crew cabins, two of the 1.25m 4-ton fuel tanks, and a Viktor engine on the bottom. Four AV-R8 winglets on the back for stability.  Came screaming in at faster-than-escape speed, and with a profile like that I had to lower the Pe down to the low 20's in order to aerobrake enough.  It actually used up all the ablator but was otherwise OK, other than burning off some nonessential radially attached solar panels.)

For interplanetary reentry you'll need more, of course, but for non-escape trajectories, a little dab'll do ya.

[OhioBob]

I've just started playing my first game since 1.0.5 was released, so I haven't had a chance to see if my old rules of thumb still work.  However, in 1.0.4 my rules for reentry periapsis at Kerbin were:  42 km from LKO, 30 km from Mun/Minmus, and 25-30 km from deep space.