Eve Descent

[Xandufar]

I've been working on one of the most difficult missions in KSP: Plant a flag on Eve, and return the Kerbal that planted that flag to Kerbin. I finally figured out how to build a huge lander that, when tested from the "set orbit" cheat, descends retrograde through the atmosphere of Eve protected by an inflatable heatshield, lands, and returns to orbit, no problem. That's no small accomplishment! However, when I add boosters mean enough to get it to Eve without cheating, the SAME EXACT lander, descending from the SAME orbit, and the same EVERYTHING as far as I know, burns up in the atmosphere of Eve because it will not stay retrograde. At about 45 km, when the heat is on, it begins to tilt, leaving it unprotected from the heatshield :[ Smart A.S.S. makes no difference. Nothing I know makes a difference. Very aggravating! Any insights would be appreciated.

[overkill13]

Rule #1 when using inflatable heat shields:  Place another inflatable heat shield on the opposite side of the lander.  This will balance the drag forces and should help keep the ship inline while it deorbits.  Jettison the shield before landing so that your ascent vehicle is clear of the extra weight/drag.  This isn't a guarantee and pics would certainly help diagnose your issue, but the grand majority of inflatable heat shield issues are solved most easily in this manner.

[Xandufar]

38 minutes ago, overkill13 said:

Rule #1 when using inflatable heat shields:  Place another inflatable heat shield on the opposite side of the lander.  This will balance the drag forces and should help keep the ship inline while it deorbits.  Jettison the shield before landing so that your ascent vehicle is clear of the extra weight/drag.  This isn't a guarantee and pics would certainly help diagnose your issue, but the grand majority of inflatable heat shield issues are solved most easily in this manner.

Yeah I did all that. Having an inflatable heat shield on top and bottom is essential. Like I said, everything worked fine in the test phase. Thanks for your response though!

[Foxster]

Is the fuel level the same when you use set-orbit and when you fly the mission? Are you descending from the same Eve altitude and have the same Pe? Have you done 'control from here' on the lander's capsule to make sure KSP is correctly working out which way is up (it sometimes gets confused with multiple controllable parts on a starting craft)?

I'd guess that you are on the edge of stability and some small variation is enough to tip the craft into instability. Try adding some more drag to the back end, for instance by adding a bunch of disposable airbrakes. 

 

[Aeroboi]

7 hours ago, Xandufar said:

I've been working on one of the most difficult missions in KSP: Plant a flag on Eve, and return the Kerbal that planted that flag to Kerbin. I finally figured out how to build a huge lander that, when tested from the "set orbit" cheat, descends retrograde through the atmosphere of Eve protected by an inflatable heatshield, lands, and returns to orbit, no problem. That's no small accomplishment! However, when I add boosters mean enough to get it to Eve without cheating, the SAME EXACT lander, descending from the SAME orbit, and the same EVERYTHING as far as I know, burns up in the atmosphere of Eve because it will not stay retrograde. At about 45 km, when the heat is on, it begins to tilt, leaving it unprotected from the heatshield :[ Smart A.S.S. makes no difference. Nothing I know makes a difference. Very aggravating! Any insights would be appreciated.

First of all, a picture with the CoM and CoL in the VAB will help to let us see what might be wrong.

According to your explanation your added boosters might shift the CoM downward making aerodynamics and Mass orient the space craft in a undesirable way. Try to move the Boosters further up or down depending on which way it flips.

[bewing]

My bet would be that the burnoff of fuel on your trip to Eve raised your CoM. So what you would want to try is pumping all your remaining fuel to the lowest-possible tanks, from the highest-possible tanks.

[purpleivan]

Alternatively to using inflatable heat shields at opposite ends of the lander, you can add a detachable tower with some wing elements on it.

Something like this... basically a big lawn dart sticking out the top of the vehicle, that can be detached before liftoff. If the vehicle on entry tends to tip over a bit, away from pointing retrograde, then the drag on the wings elements straightens it up again. It has to be enough drag though, for the design of vehicle.

[Xandufar]

9 hours ago, Foxster said:

Is the fuel level the same when you use set-orbit and when you fly the mission? Are you descending from the same Eve altitude and have the same Pe? Have you done 'control from here' on the lander's capsule to make sure KSP is correctly working out which way is up (it sometimes gets confused with multiple controllable parts on a starting craft)?

I'd guess that you are on the edge of stability and some small variation is enough to tip the craft into instability. Try adding some more drag to the back end, for instance by adding a bunch of disposable airbrakes. 

 

The fuel level is the same: full. The altitude is the same, etc. The "control from here hypothesis" is interesting, but that can also be ruled out.

I experimented by comparing my test lander to the one on my completed craft. I simply subtracted everything from the completed craft that was not on the test lander, until they both had the same part count, mass, configuration, etc. One turns and burns, the other punches through. My theory right now is that the construction process left behind some fatal computer code artifact that's invisible in the VAB. Sounds crazy, but that's what I got. I'm gonna try starting over with the one that doesn't crash, rebuild the complete craft, and see what happens.

[Xandufar]

8 hours ago, Aeroboi said:

First of all, a picture with the CoM and CoL in the VAB will help to let us see what might be wrong.

According to your explanation your added boosters might shift the CoM downward making aerodynamics and Mass orient the space craft in a undesirable way. Try to move the Boosters further up or down depending on which way it flips.

My explanation was probably confusing, but no boosters were added to the actual lander itself. The only boosters I added were for separate, lower stages needed to get the lander to Eve orbit without using the "Set Orbit" cheat.

Edited March 25, 2019 by Xandufar

[Xandufar]

6 hours ago, purpleivan said:

Alternatively to using inflatable heat shields at opposite ends of the lander, you can add a detachable tower with some wing elements on it.

Something like this... basically a big lawn dart sticking out the top of the vehicle, that can be detached before liftoff. If the vehicle on entry tends to tip over a bit, away from pointing retrograde, then the drag on the wings elements straightens it up again. It has to be enough drag though, for the design of vehicle.

It's interesting you bring this up. I've been working on this mission for longer than I care to admit, and one of my earlier attempts included a similar "lawn dart" feature on top of my lander. It helped a lot, but it wasn't enough. An upside down inflatable heat shield on top finally worked for me.

[Xandufar]

6 hours ago, bewing said:

My bet would be that the burnoff of fuel on your trip to Eve raised your CoM. So what you would want to try is pumping all your remaining fuel to the lowest-possible tanks, from the highest-possible tanks.

All the tanks are full, so idk.

[Xandufar]

11 hours ago, Xandufar said:

I'm gonna try starting over with the one that doesn't crash, rebuild the complete craft, and see what happens.

It worked! Mission success! I've been trying to do this on and off for almost as long as I've been playing ksp. It's ironic that I basically threw in the towel and asked for help literally one night before I finally pulled it off. Thanks for the input!