Wow, heat shields are hardcore...

[RocketBlam]

So I was just coming back to Kerbin from Dres, having again woefully underestimated the fuel I needed to get there and back. I had 200 d/v  left after completing the burn back to Kerbin.

So, there was no way to enter Kerbin orbit like that. My only chance was to just put my periapsis low in the atmosphere (in this case, 25,000m) and hope that the heat shield would hold. Honestly I didn't think it would work, but it did... went into the atmosphere and went straight in, didn't even leave the atmosphere again for another go around. 

Speed on entering Kerbin's atmosphere was something like 3,500 m/s.

[Tig]

woah, 25km peri will aerocapture in 1 pass at basically any speed.  Most ships aren't coming back from a 35km peri...

Did you need to capture in 1 pass? (life support mods, etc)

You might want to check out the mod Trajectories - I really like it, gives you a good idea of the results of aerocapture and aerobraking.  @Youen is the lead on that project.

[RocketBlam]

23 minutes ago, Tig said:

Did you need to capture in 1 pass? (life support mods, etc)

No, it was my second try though. First try I set periapsis at about 37km, and it didn't slow it down enough to put it in orbit. So on the next reload I thought "What the hell, we'll just try 25, and it if it blows up, we'll try something between." But it slowed down in atmosphere so much that it just... went right in.

I'm really surprised at this. My spaceplanes are hell to get on the ground without burning up.

[Tig]

7 minutes ago, RocketBlam said:

"What the hell, we'll just try 25, and it if it blows up, we'll try something between."

LOL.  That quote is like the definition of Kerbal Space Program. 

Kerbal Space Program: Iterate till it stops exploding. 

 

[KerikBalm]

I had a similar setup (no mat bay, but a mk1-2 pod and 2.5m heat shield) coming back from Moho at >4,000 m/s... I forget what periapsis it had, but the heatshield took it like a champ... heat bars did start to show up on the pod, but only to the orange.

My eve lander on the other hand... after some aerobraking in the upper atmosphere to lower apoapsis after capture... its ablator had been used up quite a bit (also to save funds and mass, I launched it without full ablator)... the center shield ran out of ablator, and the red bar filled up to the brim, but it held.

That eve example was with a lot of mass, a rather tall (and marginally stable) stack behind it. A pod will decelerate much quicker due to its lower mass per unit surface area.

You can slam them into jool with a PE quite low (you won't be entering Jool at less than 8 km/s), and they'll survive until parachute deployment (but they won't survive long, of course)

[CaelumEtAstra]

Congrats @RocketBlam, you just earned your spot as a quote in my signature. Enjoy. 

Edited May 25, 2016 by Mrsupersonic8

[Streetwind]

As atmospheric pressure increases, heat scales up only slowly, but drag scales up a whole lot. So as long as you have maximum heat tolerance left, you want to go as low as possible. And thankfully, heatshields not only have absurdly high limits, but also actively cool themselves to prevent that limit from being reached (by spending ablator).

So yes - trying to kill a heatshield that has ablator left is really, really hard, even intentionally. The G-forces on the crew must have been mildly unpleasant, though...

[Brainlord Mesomorph]

In another thread they are discussing how blunt things don't heat up up as fast as pointy things b/c of an air gap

 

[Snark]

It can be counterintuitive (at least, it was for me), but plunging more steeply into atmosphere can be less punishing in terms of heat than trying to ease in gradually, due to the effect that Streetwind points out.

In your case, you were also greatly helped by having a very low mass (basically just the capsule) behind the heatshield. Low mass per area = quicker to slow down. Every kilogram you can shed before reentry will help you.

You can observe the extreme example of this with the inflatable heatshield. If you've got it lightly loaded (e.g. no more than a dozen tons or so), and have your mass arranged so that you don't flip during reentry, you can hit Kerbin's atmosphere at 3600 m/s going STRAIGHT DOWN with no problem. You'll pull an absolutely insane number of gees, but you'll survive unscathed.

The extreme IRL example of this was the Galileo Probe, which hit Jupiter at over 47 kilometers per second and pulled 230 gees of deceleration, slowing to pretty much a dead halt in under two minutes.

[OhioBob]

I use direct entry at Kerbin all the time.  I've never found a need to mess around with aerocapture or any other means to slow down.  Just come straight on in.  I'm always more concerned about where I'll land rather than whether or not I'll burn up (don't want to come down on a steep mountain side).  I ran a bunch of simulations and I found that, for a Mk1-2 command pod, a periapsis altitude of about 23,000 m was a good general purpose number.  The g-load is survivable and neither the peak temperature nor the total heat load are excessive.  Furthermore, a periapsis of 23,000 m works over a very wide range of entry velocities, anywhere from a LKO reentry to 5000+ m/s.  If you are ever uncertain, just pick 23 km and you'll probably be safe (unless you didn't provide enough ablator).  Of course this altitude will likely change for vehicles with different ballistic coefficients.

 

Edited May 30, 2016 by OhioBob

[Cairol]

A shallow re-entry that takes forever can be worse than a steep, quick one. Because when the core heat begins to rise,
you are basicly in much greater danger than when it's just the surface heat.
If your core gets hot during a prolonged decent at high speed, you can still blow up when all heat- and mach-effects have already ceased,
because of ongoing heat redistribtution between the core and the surface.
Core heat catched fom a shallow entry is much more difficult to dissipate than a high surface heat from a steep re-entry.
So my expierience is: "Make it quick."

If it was too quick, it's just the Kerbal way, and not my fault...

[PnDB]

I put 40 units of ablator on my 1.25m heat shield for returns from Mun or Minmus and aim for around 30 km periapsis.  I end up with 3 to 5 units of ablator left.

I'll have to give OhioBob's recommendation of 23 km a try, and see how the ablator fares.

[mrclucks]

This thread has flipped my whole process of re-entry on its head.  Faster is always better.....

[OhioBob]

7 hours ago, PnDB said:

I'll have to give OhioBob's recommendation of 23 km a try, and see how the ablator fares.

Generally speaking, the greater the entry angle (i.e. the lower the periapsis), the higher the peak temperature but the lower the amount of ablator used.

[VaPaL]

@OhioBob How this compares to IRL reentry? This would still work with your mod Realistic Atmospheres?

[cicatrix]

Try DRE at 3.5 km/s and 45 degree entry.

If heat doesn't kill you, high g would. My heat shield was blown in a matter of 10 seconds.

[OhioBob]

1 hour ago, VaPaL said:

@OhioBob How this compares to IRL reentry? This would still work with your mod Realistic Atmospheres?

The stock Kerbin atmosphere and my modded atmosphere are both based on models of Earth, so they are very similar.  For most practical purposes the two atmospheres should perform the same**, so the 23 km periapsis altitude ought to work in both stock and Realistic Atmospheres.

As far as real-life is concerned, I haven't performed the same amount of investigation, so I can't say.  Perhaps there is some handy-dandy perigee altitude that will work in most cases, but if there is, I haven't discovered it.

** If you were to compare the two atmospheres side by side, you would undoubtedly see some small differences in the numbers, but nothing of consequence.  Of course that's true of Kerbin only; the differences between stock and RA for the other planets is significant.

[Renegrade]

Meh, ablator is overrated.  My current re-entry capsules don't bother with it at all, except perhaps as ballast for some of the compound designs (and Eve).  I did a re-entry test at one point at about 4.1 km/s (orbital speed as it crossed the 70km boundary, ~3850 surface vel) without any ablator for a 1.25m capsule, and although the heat bar was stalking the end of the gauge in a dangerous manner, it never reached it.

Funny thing was, it was a Minmus-rated return capsule. Overkill testing might be overkill, but it's also safety~

(NB: A shield is used in all cases, simply stripped of all ablator)

[VaPaL]

@OhioBob Well, I just found this on a Wikipedia page:

Maybe it's more about angle the perigee height (at least for the Space Shuttle.

Also found about skip reentry. The apollo (not sure if all missions) used 1 skip to reentry.

 

@Renegrade I also did a direct return from Minmus reentry, but with the 2,5m capsule and a heatshield full of ablator. Set my periapsis to 25km and not only slowed down enough to safe parachute daployment at a good height but used less than half of my ablator.

[Cunjo Carl]

In addition to being tough against reentry heat, the new inflatable heat shield blocks all radiative heating within a small radius. In the  Extreme Sunbathing competition, we're vying for who can dip deepest into the sun's atmosphere. The answer is not too deep because it's the sun, but it's fun trying! The inflatable shield magic works for kerbals in command chairs, too .

[Streetwind]

13 hours ago, VaPaL said:

Maybe it's more about angle [than] the perigee height

Technically yes. But in practical application, the periapsis (along with your apoapsis) defines the angle at which you reenter in the first place

The deeper your Pe, and the higher your AP, the steeper the angle becomes.

Edited June 1, 2016 by Streetwind

[awfulhumanbeing]

The inflative one is just soooo hot. By far the best way for one-way aerobraking.

[samstarman5]

On 5/25/2016 at 4:42 PM, mrclucks said:

This thread has flipped my whole process of re-entry on its head.  Faster is always better.....

When you can keep it under control in a well-balanced craft. Have anything out of place and it becomes a weak point in the heat protection...and boom.

[magnemoe]

Any tips for handling aerobrake with ships? They tend to flip over so the heatshield end up behind.
No issue with pods, have an 20 man return module who flips in lower atmosphere but that is not an problem 
But I have been testing some eve landers who has to use the inflatable shield and they always flip and burn up even with multiple large reaction wheels. 
In lower atmosphere again an flip can be beneficial as i can use parachutes to get shield away. 

[TrooperCooper]

 

Yeah, G-forces should be meaningful. Hey, why not go down 90 degrees for maximum impact if the shield is holding?