[Deadly Reentry+FAR+B9] Is this a legit way to survive Deadly Reentry?

[Col_Jessep]

Hi!

TL;DR: I'm using air brakes from B9 to increase my drag in the upper atmosphere. This decreases my speed faster and the ship doesn't heat up as much.

I'm using a good number of mods and I'm working on a way to build my Kethane Nomad.

Basically, a science vessel that can fly to another celestial body, scan for Kethane, land on a deposit, drill, refine its fuel and take off again.

I'm also using Mission Commander so money is limited, fuel is very expensive and I want a way to explore a lot without straining my budget too much.

Taking off from bodies with an atmosphere is not ideal with the current design, so no need to think about aerocapture. Aerobraking however would be really nice, especially in the Jool System.

The obvious solution would be ablative plating since the inflatable shield is too large and might spin my Nomad out of control. I wasn't too fond of bringing several ablative heat shields

or to only have a limited amount of aerobrakes. The heat during reentry is mostly caused by the compression of the air in front of the ship, not by friction.

Slowing the vehicle down with increased drag should therefore help to keep the temperatures down, right?

Time for testing!

I brought a 24 ton vehicle into LKO. Its AP is around 90km and PE at 30km before I decoupled the engine. All tests were done from the same quicksave and without any steering/burning on my part.

I point the nose retrograde and disable SAS when I enter the upper atmosphere. The winglets and air brakes keep the vehicle pointed retrograde.

Test 1. A vehicle of comparable size and mass braking in Kerbans atmo without air brakes or ablative shield:

The vehicle spontaneously disassembled itself a split-second after I took the screenshot. This is probably what my Nomad would experience during aerobraking in Jools atmo.

Test 2. Reloaded the quicksave and didn't separate the ablative heat shield before reentry this time:

The heat shield is completely spend but the vehicle survives.

Test 3. This time I deployed the 22 air brakes before I enter atmo:

The vehicle heats up to ~850°C which is still within acceptable parameters. Aerobraking should be doable. A nice side effect is that the increased drag

slows you down before the parachutes deploy. That means fewer chutes... <3

Test 4. Air brakes and ablative shield:

Only a third of the ablative shield is used up instead of 100% without air brakes! The temperature stays under 400°C the whole time.

My question: Is this a legit way to deal with reentry heat? Would it work in real life? Or did I just stumble about a weakness in the way those mods integrate?

My idea was to slow my ship down as much as possible in the upper atmosphere. Scott Manley (who else?) gave me this idea when he turned his space shuttle

in the upper atmosphere left and right to increase drag and kill velocity before dipping into the thicker parts of the atmo :

What are your thoughts?

Edited December 16, 2013 by Col_Jessep

[TechnicalK3rbal]

Use the directional heat shielding (for example, wings and b9 fuel tanks) to survive the heating. Aslo, use structural parts that have a high heat tolerance (=>1200) and just cover the bottom. also, you can mod procedural fairings to have a directional (non-ablative) heat shield.

[Kasuha]

I believe real airbrakes wouldn't survive reentry without shielding, the heat would be too much for them. They are probably not overheating in KSP for some reason, perhaps the extending part is not part of the heat distribition or something. So in my opinion this counts as an exploit if you're going for realism.

But I am no aircraft technician.

[longhornchris]

Yeah, a lightweight drag brake would melt. Worse, in real life the deployed drag brakes would create all sorts of complicated shock flows which would allow high temp plasma to get to the structure.

In real life, the key is to maintain a detached normal shock to keep temps at the hull down. Blunt structures are great for this but sharp points and edges allow the shock to get closer. The edges around deployable drag brakes would likely allow the shock to get too close.

There are exceptions, such as ICBM RVs that can survive an attached shock but they also only have to survive till they are low enough to detonate, rather than actually survive coming to a stop.

[arq]

I'm with the others. The heat tolerance of the airbrakes is higher than it had ought to be. Many parts have tolerances that seem unreasonable, especially structural parts. While I'm willing to accept that for girders and plates I've had stuff come down with a decoupler on the front and do just fine, which I disagreed with (actually I'd intended for it to burn up).

[Taki117]

assuming that the airbrake was coated with a special material (jebs pantented ablative space paint) this is a viable technique for slowing upon reentry. However, this would increase the weight of the airbrake making them less effective. (I too use this techneque, but have increased the weight and decreased the drag to compensate. Alternatively you could add ablative shielding to them as well, simply copy over the code from one of the included heat shields.)

Now, that being said, imho using 22 of them does seem a bit like cheating. I personally never use more than one set of 8x symmetry for the small ones, or one set of 4x symmetry for the large ones. (Except in extreme or special cases)

Edited December 15, 2013 by Taki117

[Motokid600]

.. retexture the airbrakes to look like heatshielding ( which it should be as is ). And what's directional heat shielding? I've never heard that term before. Now I haven't tried DR, but isn't the whole point of a heat shield to cover the entire craft? Meaning the diameter of the hs has to be greater then the craft, no? This way nothing is exposed. Other then the airbrakes why is it your wings or anything protruding out don't get destroyed?

Edited December 15, 2013 by Motokid600

[Taki117]

The point of the heat shield is to re-direct the heat away from your craft, not necessarily cover the entire thing. Therefore the diameter of the heat shield has to be no larger than the largest diameter that is going to be facing reentry. Direction heat shielding basically means that it will only reflect heat if pointed in a specific direction (Think Space Shuttle wings) The wings have heat shielding along the leading edge, which redirects the heat out and away from the rest of the wing.

[Hodo]

I have all three of those mods, and I rarely use airbrakes to slow my craft down on re-entry. The thing is you are coming in at to steep of a re-entry angle. My typical re-entry target is 5km above the ground over KSC, for my space planes, and my crew capsules usually 5-10km above the ground almost a third the way east of KSC.

This way I spend as much time as possible above 30km slowing down before I hit the thicker atmosphere at 27km. If I can get the speed below mach 5 I know it will survive re-entry. If I come in faster than that, there is a fair chance it will burn up or burn through its heat shield. If you want a real challenge try landing anything on Eve with DRE.

[Col_Jessep]

I'm with the others. The heat tolerance of the airbrakes is higher than it had ought to be. Many parts have tolerances that seem unreasonable, especially structural parts. While I'm willing to accept that for girders and plates I've had stuff come down with a decoupler on the front and do just fine, which I disagreed with (actually I'd intended for it to burn up).

I agree that the air brakes are too cheap and too light. But instead of modding the mod I might go for some expensive and heavier visual fluff. Maybe one of the B9 cockpits. If it was just about the costs and weight I would just slap the inflatable heatshield on, it's cheap, light and reusable I think. That would be boring though. I don't want to add a giant part that will be only used very occasionally.

I believe real airbrakes wouldn't survive reentry without shielding, the heat would be too much for them. They are probably not overheating in KSP for some reason, perhaps the extending part is not part of the heat distribition or something.

They heat up but not too badly, 500-600°C, which is quite manageable. The forces on the air brakes miniscule. Each one only provides 0.03kN drag compared to the 30+kN of the large tank (at the time you hit the thicker part of atmo). The trick is that they just give you that little amount of drag in the upper atmosphere that you need to go from Mach 6.5 to 5.8 or something. Doesn't sound like much but it's enough to make reentry survivable. I can even retract the flaps when I hit the thick part of the atmosphere. If I retract them at 32km they already have slowed me down enough to survive reentry. Keeping them deployed after that is just to keep the crew from turning their helmets into barf bags...

If you want a real challenge try landing anything on Eve with DRE.

Now there is a challenge I want to try!

Yeah, a lightweight drag brake would melt. Worse, in real life the deployed drag brakes would create all sorts of complicated shock flows which would allow high temp plasma to get to the structure.In real life, the key is to maintain a detached normal shock to keep temps at the hull down. Blunt structures are great for this but sharp points and edges allow the shock to get closer. The edges around deployable drag brakes would likely allow the shock to get too close.

So if you would design the drag brake like a ring around the ship that increases the radius and has no sharp edges the shock flows would be reflected away from the ship, right?

[Kasuha]

The space shuttle had airbrakes, too - its tail plate can split up to provide braking. But there is no mention about it being used during reentry - I believe it wasn't, because it would get torn off the craft.

[Taki117]

The space shuttle had airbrakes, too... I believe it wasn't, because it would get torn off the craft.

This may be due to the maneuvering required during reentry. The shuttle did a controlled reentry (similar to what Scott does in the video posted earlier) which requires use of the rudder, which would not be possible if it were being used as an airbrake. The shuttle is also an asymmetric craft so using the airbrake high up would possibly result in it pitching nose up, then flipping, exposing the soft topside to the atmosphere.

[ferram4]

Part of the issue is that the airbrakes are designed for use with stock KSP, and it takes a lot of drag to make something draggier in stock KSP. Since FAR doesn't touch that part module (why would it? it has no idea that a 3rd party air brake exists) you're basically summoning higher-than-stock-KSP levels of drag on demand with this. If FAR actually affected those air brakes, they'd make a lot less drag, just like the air brakes that you can create by using FAR to turn regular control surfaces into air brakes.

Now honestly, look at the comparison of situations 1 and 3; how much extra area have you added to the cross-section of the vehicle by deploying all of those air brakes? Maybe increased it by ~50% if we're being generous? But compare the change in drag coefficient, which can still be seen despite the fact that it's hidden behind KER; in situation 1 it's ~1.1, while in situation 3 it's ~6.7. Now, does a ~50% increase in cross-sectional area translating into 6 times the drag really seem that kosher to you?

If you reduced the drag the airbrakes provided to be something closer to a drag coefficient of 0.5 based on their deployed area (which looks to be about 0.1m² for those), then it might not be so broken; remember that the stock drag is equal to maximum_drag*mass*0.008 for a cross-sectional area of 1m², so you'll have to convert it into crazy KSP drag.

TL;DR: Your air brakes are overpowered.

[nickexists]

I've always hated how ridiculously overpowered those airbrakes are. I guess it's just because I'm using FAR. I try to use them for landings but most of the time I slow down too much and just fall out to the ground.

[Col_Jessep]

Now honestly, look at the comparison of situations 1 and 3; how much extra area have you added to the cross-section of the vehicle by deploying all of those air brakes? Maybe increased it by ~50% if we're being generous? But compare the change in drag coefficient, which can still be seen despite the fact that it's hidden behind KER; in situation 1 it's ~1.1, while in situation 3 it's ~6.7. Now, does a ~50% increase in cross-sectional area translating into 6 times the drag really seem that kosher to you?

I completely forgot about the FAR stats! Nope, 600% from those little flaps certainly seems a bit ridiculous. I checked: If I turn the whole ship sideways I get a little bit more than 200% increase of drag - which seems to be correct. The flaps however...

Well, maybe we will get a more reasonable stock aerodynamics model at some point and those crazy OP parts can be adjusted. Not holding my breath though.

Thanks for the answers and explanations guys! You have helped a lot!

And thanks for your awesome mod Ferram! Once you go FAR you never go back. Okay, I'll put a dollar in the Horrible Pun Box now...