Fairing heat up abnormally

[KerbalKind]

Howdy,

So today I am on a mission to put a space station around Eve. I put a large fairing over the entire station with many delicate components. However, upon entry and aerobreaking into Eve's atmosphere, the fairing part heats up very quickly and explodes, and I am uncertain of the reasons.

I have been regularly doing aerobreaking into Eve with unmanned probes, so I think I have a good idea of the thermal tolerance of parts in this condition. I never have a fairing even close to explode at 84k km altitude from Kerbin-Eve transfer speed like this before.

https://imgur.com/vRptlee: fairing part getting toasty

https://imgur.com/3ZHU6nF: fairing exploded. All other parts are fine but starts to heat up once the fairing is gone

Help! It's the first manned mission I ever taken to Eve after countless experiments with probes, and THIS decides to be the first one to actually have issues. I have enough fuel left to do a capture burn, and can send rescue later. So I'm more interested in knowing WHY? It just contradicts  my experience from previous flights with fairing.

Edited July 10, 2020 by KerbalKind

[catloaf]

My best guess is that the crewed mission weighs a lot more, causing it to be less susceptible to drag. Thus the craft is moving faster at lower altitudes and gets hotter. I recommend you attach a heat shield to the craft. However, the extra dV used to carry the large heat shield may be more than what's necessary for partial aerocapture (some retroburning, higher periapsis.) I recommend you do a partial aerocapture into an elliptical Eve orbit and use multiple aerobreaking passes to help you circularize. 

[AHHans]

5 hours ago, KerbalKind said:

and I am uncertain of the reasons.

Oh, I'm sure the reason is Eve. Eve just likes to explode your craft.
Especially if you are coming in at interplanetary speeds.

I'm more surprised that you managed to aero-brake your uncrewed probes without the use of a heat-shield. What kind of speeds did they have at similar altitudes in Eve's atmosphere? I also think that the thermal mass of the fairing in relation to the heated area scales differently than you expect.

[KerbalKind]

13 hours ago, catloaf said:

I recommend you do a partial aerocapture into an elliptical Eve orbit and use multiple aerobreaking passes to help you circularize. 

Yes that was exactly what I was doing - only lower periapsis enough to aerobreak into a very high Eve orbit. Sorry if it wasn't clear.

9 hours ago, AHHans said:

Oh, I'm sure the reason is Eve. Eve just likes to explode your craft.
Especially if you are coming in at interplanetary speeds.

I'm more surprised that you managed to aero-brake your uncrewed probes without the use of a heat-shield. What kind of speeds did they have at similar altitudes in Eve's atmosphere? I also think that the thermal mass of the fairing in relation to the heated area scales differently than you expect.

I thought I might include a previous craft of mine (craft B) to explain why I am puzzled at this latest one's performance (craft A). The craft is of similar design: fairings ahead and deployable airbrakes (made with wings facing the airstream attached to robotics hinges). It was a proven design so I was pretty confident before sending my kerbals to their destiny.

I sent both to Eve-Kerbin transfer at the same window (couples of hours apart to switch control), so entry profile should be very similar. I got craft B to an even lower aerocapture periapsis (75km) than craft A (77km). Here is craft B's performance:

- At 84km and 4628m/s (slightly higher than craft A), the fairing barely heats up: https://imgur.com/0CdJqlv

- At periapsis (75km) and 4552m/s (almost the same as craft A at 84km), the foremost fairing heats up significantly but nowhere even near maximum temperature like craft A at 84km: https://imgur.com/tQRqgBi

Craft B made a safe aerocapture to Eve. I am puzzled on why the thermal performance is so drastically different?

- Is it because craft B is composed of 3 fairings instead of one big one?

- Craft A is heavier (46 tons) compared to B (16 tons). But with similar entry speed and altitude, shouldn't atmospheric heating be the same? The majority of drags should occurs at the airbrakes (craft B only have 3 airbrakes, while craft A has 4, and each has larger break area to account for increased mass)

- I had previous design with heat shield before, and could get them to much lower aerocapture periapsis (60-70km) to get captured by Eve. From previous experience, without heatshield the fairings will overheat at periapsis of 71km or below. I experimented with the deployable airbrake design using robotics hinge because: 

                 A/ it always allow aerocapture at 77k periapsis or above, well above the fairing tolerance range, so I don't need heat shield         

                 B/ can be scaled up for higher vessel mass

                 C/ Much finer altitude control in the final circularizing aerobreak

That's why I can't understand how craft A's fairing heats up so rapidly to maximum temperature at 84k altitude - fairings were only in danger zone at 71km or below before, and heating happened slowly over the entire trip, not so rapidly in 3-4s like this.

 

Edited July 10, 2020 by KerbalKind

[James M]

Just tossing ideas here. Perhaps it's got something to do with the longevity of your lander craft. The fairing is equally as long and thus any thermal advantages gained from aerobraking retrograde are more or less offset by the fact that the heated plasma just rides on the sides of the fairing?

I dunno. Eve is hard. 

[catloaf]

Could you tell us what your velocity is at 84 km for craft a and b?

[KerbalKind]

1 hour ago, catloaf said:

Could you tell us what your velocity is at 84 km for craft a and b?

Craft A: 4540 m/s

Craft B: 4629 m/s

They can be seen in the images.

[AHHans]

1 hour ago, KerbalKind said:

At 84km and 4628m/s (slightly higher than craft A), the fairing barely heats up: https://imgur.com/0CdJqlv

I'm surprised that this doesn't just blow up due to overheating. But, you live an learn. (Well, in this case me and not you. )

And, yes, it has a higher velocity at a lower altitude than craft A, so it isn't a case of "just" getting slowed down faster. But if I see it correctly then it is a different sized fairing than in the other craft, so that might be a significant difference. A larger fairing has ha higher thermal mass than a smaller fairing - and it's not just the fairing base, but also the fairing shell - but it also gets heated more. I did have a quick look into it, but couldn't come up with a simple relation of fairing size to thermal mass and to heat load. So I can't really tell you how to optimize a fairing for maximum survivability.

 

[KerbalKind]

53 minutes ago, AHHans said:

I'm surprised that this doesn't just blow up due to overheating. But, you live an learn. (Well, in this case me and not you. )

 

which was why I made lots of unmanned probes to test things. I were so sure the fairings (2600K tolerance) could survive 73km periapsis or above  (I have occasionally pushed to 71km). Until it didn't. 

53 minutes ago, AHHans said:

And, yes, it has a higher velocity at a lower altitude than craft A, so it isn't a case of "just" getting slowed down faster. But if I see it correctly then it is a different sized fairing than in the other craft, so that might be a significant difference. A larger fairing has ha higher thermal mass than a smaller fairing - and it's not just the fairing base, but also the fairing shell - but it also gets heated more. I did have a quick look into it, but couldn't come up with a simple relation of fairing size to thermal mass and to heat load. So I can't really tell you how to optimize a fairing for maximum survivability.

 

Yes the fairing for A is 3.75m, while for B it is 2.5m. I thought it might contribute, but did not think it could make such a large difference on its own. Still I might make another craft with both just to test it.

Edit:

Okay, for what it's worth, here is an experiment with a new fairing for craft A. The craft is practically the same, but I split from a single long 3.75m fairing into two shorter ones.

Old fairing: https://imgur.com/dQgWiIt

New fairing: https://imgur.com/xUEG1C4

This new craft A now behaves very similarly to craft B, with the fairing barely heated up at 84km and surviving at 76km periapsis (some of the airbrake wing sections explodes, but they are a different matter)

At 84 km: https://imgur.com/nD39052

At 76km: https://imgur.com/S1AhEnM

So it's established that the fairing design (shape, length?) is the critical factor. Now, the question I still can't figure out is why they are so different? Just from an (amateur) look, the old fairing looks smoother aerodynamically to me.

Edited July 10, 2020 by KerbalKind