Keeping a dead craft steady during reentry

[Kimberly]

I'm playing Better Than Starting Manned, which includes DeadlyReentry. I'm trying to launch a craft with four goo containers up to a height of 250 km, then letting it fall back down for recovery. Due to technology constraints and a little bit of personal pride, I'm trying to get the craft to land deadstick--that is, once I separate the final stage on the way down and activate the parachute, I have no more control over it.

The problem with this is that my craft won't stay pointing retrogade. It flips over so that the parachute is destroyed by the force and heat of reentry, and the heatshield is uselessly pointing upward! (This is long before the parachute properly deploys.) How can I keep my craft pointing up with no active guidance? I've tried making the center of mass lower, but I don't think I can change the design much further. My current final stage:

[capi3101]

This may sound dumb or may not be in the spirit of what you're doing, but have you turned SAS on after you've oriented your craft for re-entry (while you're still in control of the craft)?

[Mystique]

Is there a SAS on that thing at all? By the way, do you have control module on it? I.e. are you focused on above mentioned module during landing or watching from another vessel? If you don't stay focused on it, it will just disappear in atmosphere. If that's not the case, I can't see a way to keep it stable without direct control and use of SAS. Maybe something like spacecraft may hold direction because of wings, but evey they start flipping like crazy without SAS or manual control.

[Soda Popinski]

Try placing a girder between the ones you already have and the chute. That should ensure your center of mass is below the middle of the craft.

Also, if your canisters sick out beyond the heat shield to much, they may burn up. In that case, I would use more girders (rectangular ones if you have unlocked them), and place all 4 parallel and coaxial to the girders. That may reduce the expressed area of the canisters.

Edited December 27, 2013 by Soda Popinski

[brooklyn666]

Why deadstick? Every craft that reenters the atmosphere in a controlled fashion (with intent to recover) has a guidance system that keeps it properly aligned during reentry.

I think the problem is with stock atmospheric model. I'm not sure how DR modifies it, but it doesn't accurately apply friction, lift, and compressive forces, which in real life would a stabilize a pure ballistic reentry.

[Psycix]

Based on the current implementation of aerodynamics, the only factor that can separate your center of drag from your center of mass (and therefor determining aerodynamic alignment) are parts with a non-0.2 drag ratio.

This means you want <0.2 parts at the bottom and >0.2 parts at the top.

I believe the parachute (once activated) has a high drag ratio (activated, but not opened, so that the staging icon is blue) so that should keep it upright.

What is the drag value of that heat shield? If it is larger than 0.2 then that explains everything.

You could also try to add a few fins (near the parachute, as far above the CoM as possible)

I'm not sure when reentry heat kicks in compared to aerodynamic forces, perhaps you will be best off slapping on a probe core for minimal reaction wheel control.

[Champ]

How can I keep my craft pointing up with no active guidance?

I am not an expert, but i think you can't without FAR. Try fins, but with that creepy drag model....

I think your best chance is to change your reentry.

In Deadly ReEntry (like in real life atmospheric reentry), you want to be slowed down as high as possible, so that your velocity is not too high when you reach the thicker layers of the atmosphere). For that you can :

- Add more angle (so that you pass more time in the upper atmosphere)

- Add more drag (counter-intuitive, but realistic).

Don't know for stock chutes (i use real chutes - great mod !), but can't you deploy your chutes higher ? I pre-deploy my chutes at something around 40000m. Pre-deployed chutes (you know, the blue icon) will help you reduce your speed before entering the thicker layers of the atmosphere.

[Champ]

I would need to check, but i think the heat shield has a very high drag. This may be intentional : higher drag means lesser velocity in the thick atmosphere. However, this surely does not help pointing your craft correctly...

[numerobis]

Goo cans also have low drag. It would be nice to have a center if drag indicator in the VAB, but we don't.

[Kimberly]

I have no control module at all on the craft, so no SAS or reaction wheels or anything. The parachute keeps the craft stable when deployed, but it can only activate at 0.01 atmospheres, by which time it will have already burnt up if the craft is misaligned. The goo containers themselves seem immune to heat; they don't go above 50 degrees Celsius or so even when pointing down during reentry.

I don't have any other girders, but I'll try adding an extra octagonal one and see how that goes.

[Tank Buddy]

It has to do with the center of mass. I believe that the center of mass should be as close to the bottom of your craft as possible if you want it to be stable while oriented downwards.

[Taki117]

you could always just stack another heat shield on the bottom. I know it seems like cheating, but think about it. If you burn through one layer of heat shield the second is ready to go to pick up the slack, and you will increase the wight at the bottom of the craft.

[Kimberly]

I added another strut to move the parachute higher up relative to the goo containers and heat shield, but no luck. The craft's preferred orientation still seems to be with the parachute pointing prograde. (I'm not even sure adding a control system would counter the force.) I checked the and the heat shield's drag is 0.2, while the goo containers have drag of 0.1. I tried moving the containers up, but that didn't help, either.

On a different note, do I want my heat shield to be pointing orbit prograde or surface prograde?

[astropapi1]

Surface prograde.

If you feel like it, you can decrease the drag of the heatshield and increase it's mass modifying the .cfg. That should solve the problem.