2.5m heatshield doesn't protect hitchhiker pod?

[Mitchz95]

I just lost an expedition (six tourists, one pilot) because the Hitchhiker exploded during reentry on the way back from the Mun, despite having a 2.5m heatshild directly attacked to it. Why did this happen?

[Spraki]

Personally speaking, I always took a plus 1 size due to fears of such a result. So i would've had the 3m variant under the 2.5m. Tweakscale at times. 

However it also depends on your descent angle (sorry, don't know the correct term) but sometimes it is best not to put 25km on periapsis. Better aerobrake a few times.

[OrbitalBuzzsaw]

For craft with fragile parts like Hitchhikers, I usually reenter with my periapsis at 40km. Better safe than sorry.

[Aegolius13]

Two possibilities seem... possible.  First,  if your ship was not facing completely retrograde,  the pod might have been exposed to enough direct heat to cook it.  As mentioned above, using a bigger diameter heat shield can guard against this.   It can also help to design your reentry vehicle for stability (mass at heat shield end,   drag at opposite end), and/or to include more reaction wheels or rcs for attitude control. 

The other possibility is that enough heat leaked through the shield via conduction.   Even with ablation,  the heat shield was probably hotter than the critical temp for the hitchhiker.  You could try putting a high heat tolerance part, like a service bay, in between. 

Ultimately,  the hitchhiker is a somewhat fragile part, and I don't love it for reentry.   The Mk 2 and M3 cabins and the Mk1-2 command pod tend to be more robust. 

 

[Jouni]

Reentry from the Mun with a hitchhiker, a Mk1-2, a 2.5 m heat shield, and some parachutes is barely survivable. Put your periapsis to ~35 km, set navball to surface mode, and tell SAS to hold retrograde. The heat bar on the hitchhiker will probably go red, but it usually won't explode.

There are around three main issues:

1. The heat tolerance of the hitchhiker storage container is low. Using a part with higher tolerance as a heat sink between it and the heat shield will make the reentry easier.
2. The reentry vehicle is unstable, because the hitchhiker is a large part with low mass. Placing more mass between it and the heat shield will make holding retrograde easier. Airbrakes might also work, but I find the idea silly.
3. There is a lot of mass behind the 2.5 m heat shield, and the atmosphere can't slow it down quickly enough. A larger heat shield would help, but it would also make the vehicle even more unstable.

One simple solution is to add a heat shield without ablator between the hitchhiker and the actual heat shield. This increases payload mass from ~8.5 tonnes to ~9 tonnes, but otherwise it's very cheap.

[Mitchz95]

Adding an extra (empty) heat shield did the trick. Thank you guys!

[The Aziz]

Whoa. I recently returned from solar orbit with vessel containing an engine plus tank, then heat shield (2.5m), FOUR Hitchhikers and a capsule. After like 15 F9's I managed to land, so almost no problem here. Just had to allow the engine to go first and then blow up to slow down conduction and keep CoM as low as possible.

[Foxster]

This was discussed before and the conclusion was, IIRC, that it is the handrails running around the Hitchhiker's rims that cause the problem. It means the part exceeds the 2.5m size.  

Just use the mk2 cabin or a couple of mk1s and the issue goes away. Both those weigh 0.5t less too. 

[selfish_meme]

I accidentally staged my heatshield before reentry on a Minmus return in a mk1-2 just the other day and it still made it down

[Mitchz95]

13 hours ago, Foxster said:

This was discussed before and the conclusion was, IIRC, that it is the handrails running around the Hitchhiker's rims that cause the problem. It means the part exceeds the 2.5m size.  

Just use the mk2 cabin or a couple of mk1s and the issue goes away. Both those weigh 0.5t less too. 

Ah, that would do it.

I'm in career mode so I haven't locked the mk2 or the 3.75m heatshield yet. Shouldn't be too long, though.

[Red Shirt]

I've tried several times this morning to intentionally explode the hitchhiker on reentry. I failed. How ironic is that? One time I even forgot the heat shield and at about 25k the can turned prograde and I still managed to land it. I must be losing my Kerbal touch. I am in sandbox using KSP 1.2.2. 

[tater]

It's pretty hard to get anything in stock KSP to have problems reentering, ever, in my experience. Periapsis below 40km? I'd set it below 20.

OK, just tried this in 1.3 on the laptop. HH, a battery, some solar panels, a parachute, and a 2.5m heat shield. I threw a tank and engine on the bottom, and set a 19km periapsis from the Mun, direct. Dumped engine/tank, and reentered. Didn't come close to using all the ablator, and while the HH heated up some it was never at risk. G-load was nominal as well.

You likely set the periapsis too high so you spent too much time slowing down.

On 7/21/2017 at 6:05 PM, OrbitalBuzzsaw said:

For craft with fragile parts like Hitchhikers, I usually reenter with my periapsis at 40km. Better safe than sorry.

This is exactly wrong. Shallow reentry results in longer dwell time in the heating regime, which blows you up. 40km is incredibly high for that entry.

It's a balance between peak heating and constant, lower level heating. Too deep, too fast, and peak heating will kill you. Not deep enough, and you heat without bleeding off velocity.

[OrbitalBuzzsaw]

7 hours ago, tater said:

It's pretty hard to get anything in stock KSP to have problems reentering, ever, in my experience. Periapsis below 40km? I'd set it below 20.

OK, just tried this in 1.3 on the laptop. HH, a battery, some solar panels, a parachute, and a 2.5m heat shield. I threw a tank and engine on the bottom, and set a 19km periapsis from the Mun, direct. Dumped engine/tank, and reentered. Didn't come close to using all the ablator, and while the HH heated up some it was never at risk. G-load was nominal as well.

You likely set the periapsis too high so you spent too much time slowing down.

This is exactly wrong. Shallow reentry results in longer dwell time in the heating regime, which blows you up. 40km is incredibly high for that entry.

It's a balance between peak heating and constant, lower level heating. Too deep, too fast, and peak heating will kill you. Not deep enough, and you heat without bleeding off velocity.

I get the sense that you don't fly many small ships.

[tater]

I just demonstrated that the stated craft, a single HH with a heat shield has zero issues.

Flying small ships? I almost only reenter capsules on kerbin. Always set peri close to 20.

[OrbitalBuzzsaw]

Just now, tater said:

I just demonstrated that the stated craft, a single HH with a heat shield has zero issues.

Flying small ships? I almost only reenter capsules on kerbin. Always set peri close to 20.

If there are ZERO ISSUES, then explain why the OP's craft exploded. Gotcha!

[tater]

Because he renters with a periapsis that is far too high.

Which I explained in the post you quoted.

[Foxster]

I have to say I find the same things as tater. 

Returning from Kerbin orbit with a Ap out by the Mun I can't make a simple craft burn up, even with a hitchhiker and a high Pe. 

Unless we are talking about a non-stock Re-entry Heating setting?

Edited July 24, 2017 by Foxster

[Jouni]

OP had a pilot and six passengers in the craft, which sounds like a Mk1-2 pod in addition to the hitchhiker. Reentry becomes much more deadly, if you double the mass without increasing the cross section.

[OrbitalBuzzsaw]

18 minutes ago, Jouni said:

OP had a pilot and six passengers in the craft, which sounds like a Mk1-2 pod in addition to the hitchhiker. Reentry becomes much more deadly, if you double the mass without increasing the cross section.

Ah... Fair point, that thing is a pain in the ass.

[Kerbal101]

1 hour ago, tater said:

Because he renters with a periapsis that is far too high.

Which I explained in the post you quoted.

"Setting periapsis far too high" applies only from high space over Kerbin orbits. With orbits from Mun and higher, its either aerobreaking (n-loops depending on altitude periapsis) or suicide descend. Or (heatshield) overheat.

I think the issue the OP had something to do with center of mass balance. Having it barely below mid-line, would make retrograde hold difficult, which would cause.. yeah.

Edited July 24, 2017 by Kerbal101

[tater]

I tested a direct entry from mun to 20km. No problems at all. The only sas was a probe core.

if I was aerobraking, I would have set a higher periapsis than 40, but would have required I keep some sort of rcs aboard to tweak periapsis (imho).

You have to try to burn something up coming from within Kerbin SoI, imo.

[Red Shirt]

I didn't catch the return from Mun in my earlier experiments. So tried again. Used a MK1-2 pod with a HH attached below. 2.5 heat shield at bottom. First try had a Pe of 20k. No problem. Heated quickly but slowed quickly. Next try had a 40K Pe. Exited back into space for an extra trip around Kerbin but landed on next entry into atmosphere. No Kabooms. Final attempt set Pe to 0 for a steep reentry. Still did not explode. 

I have no doubt the OP did destroy the ship. I just can't duplicate. My guess is either the craft did not remain retrograde or the Pe was less than 0 and could not slow fast enough.

Edited July 24, 2017 by Red Shirt

[Kerbal101]

@tater I don't see anything in your statements, which I could find contradictory, tater. I have descended directly from Minmus, causing 3/4 HS overheat - but it worked.
But there is a thin nuance of how well the vehicle is kept within retrograde.. to which the CoM placement contributes the central role. Double heatshield would not only extend the vehicle - it would also shift the CoM below, making it more stable. We don't have a picture of OP descend vehicle to see if there was a problem...

Spoiler

paste from elsewhere, just to illustrate what I mean:

(least stable) >>  >>  >> (most stable)

 

[tater]

I tend to not reenter anything that doesn't look like something we have seen actually reenter intact (on purpose) IRL.

As a result, my CMs are usually in the right place. My HH test for OP was a large chute, 1.25m battery, 1.25m probe, a few flat solar panels just in case, the HH, and heat shield. 

Probe sas set to retrograde, didn't touch it.

[hms_warrior]

There is an other, obvious explanation: The Orbital retrograde/prograde is different from the surface/atmosphere prograde/retrograde. Basicly, if you have your spacecraft set to hold prograde/retrograde on your orbital speed, you are at a slight angle towards the movement through the atmosphere. This angle might be enough to expose parts behind your heatshield.

Edited July 25, 2017 by hms_warrior