Eve Lander keeps flipping

[Der Anfang]

Not during ascent, but during entry. I have a plant a flag on eve contract, currently, so I've encouraged myself to much around in sandbox mode and design some potential landers (all capable of reaching orbit from sea level). Then, of course, I hyper edit the lander into Evian orbit. At first, entry seems like it will work out. However, I have to keep enabling the ignore heat cheat to test my vehicle (more interested in testing the ascent of the lander). However, once I actually want to do the legit mission, I want to design a frickin method of entering and landing on Eve without burning up (and without relying on cheats, obviously)! The biggest problem with all of my landers is that they all flip over halfway through entry and explode. The heat shield seems virtually useless because of this. I have tried normal ablator shield clusters and the inflatable version. All just flip around and burn up, even with Bad-S SAS... and something tells me I shouldn't need to rely on SAS at all, even.

What am I doing wrong? Does anybody else here have more trouble landing a lander on Eve more than they do with ascent??

This is what my lander (with the testing hardware) looks like. CoM and  CoF (if relevant) is made visible.

Edited June 23, 2016 by Der Anfang

[Zhetaan]

Your heat shield stack looks like a parachute.  When you put it in Eve's atmosphere, it acts like a parachute.  Note the location of your centre of mass:  the heat shields (plus all the extremely draggy struts) are all far away from it, so they have a lot of torque and can wrench your rocket around no matter how much the SAS tries to compensate.

Is there a reason you put those heat shields on such a long boom?  Move them closer to the base of the rocket and see what that does.

Edited June 23, 2016 by Zhetaan

[N_Danger]

 I haven't been to Eve yet, but It looks like to me the CM is too far away from the bottom to be stable in thick air. Maybe try moving the CM down closer to the heat shields or add more drag to the top part?

[Der Anfang]

12 minutes ago, Zhetaan said:

Your heat shield stack looks like a parachute.  When you put it in Eve's atmosphere, it acts like a parachute.  Note the location of your centre of mass:  the heat shields (plus all the extremely draggy struts) are all far away from it, so they have a lot of torque and can wrench your rocket around no matter how much the SAS tries to compensate.

Is there a reason you put those heat shields on such a long boom?  Move them closer to the base of the rocket and see what that does.

The reason behind the long boom: It's experimental, and it clearly did not work. Before, I also did not have the long boom, and it still flipped over. The reason why I tried that in the first place is that I read somewhere from the forums here that if something is top heavy, the vehicle will point in the desired direction -heat shield first- during a re-entry, so I tried to put the heat shield as far away from the CoM as possible to make it "top-heavy".

Does the CoM need to be higher or lower in respect to the heat shield?

[MDZhB]

The CoM needs to be lower to make it not flip. Think about the Apollo command module. It had most of its mass towards the bottom to keep it pointed the right way. The CoM is always going to want to be close to the source of gravity, and the main source of drag will want to be the opposite, so you need to put the CoM lower to compensate.

Edited June 23, 2016 by MDZhB

[the_Demongod]

Imagine a dart (or arrow, badminton birdie, etc), where the center of mass is at the front and the center of lift is at the back. The center of lift becomes the tail and is "dragged" behind the center of mass. You've essentially created a huge parachute in front of your ship. What you want to do is have the heat shields as close to your ship as possible, and then have your center of lift as far behind that as possible for maximum aerodynamic stability. I've even seen people create a big boom with fins on it trailing behind the vessel to act as a tail.

It's not "top heavy" you want, it's "side hitting the atmosphere heavy."

Edited June 23, 2016 by the_Demongod

[Zhetaan]

You had bad information, then.  The general rule for atmospheric rocketry is that you want the heavy bits at the prograde end and the draggy bits at the retrograde end if you want a stable flight.  The idea is that the draggy bits, by dragging through the atmosphere behind the centre of mass, will stabilise the craft so it flies in a straight line without flipping.  For ascent, it's possible to put so much drag on a rocket that it won't turn even when you want it to do so--this is why, for example, a gravity turn ascent requires you to manage the throttle, especially if you're using static fins instead of actuated control surfaces.  Going too fast makes your fins essentially put your rocket in a groove that it can't escape, and that costs you a lot of efficiency.

Now consider re-entry on Eve:  you're going at orbital velocity, and in addition, the atmosphere is thicker.  All of those drag effects I just described are magnified and your rocket flips.  It's unpowered, so it doesn't cost efficiency.  It just costs you your rocket.

Try using F12 and looking at the aerodynamic forces overlay as you drop your rocket into Eve's atmosphere.  The red arrows will tell you your drag vector; the longer the arrow, the more drag from that part.  See which parts make your rocket flip:  I urge you to do this so that you can see the effect in action for yourself, on your own rocket, without needing to rely on anyone else's say-so.

The solution is the same, though:  move your centre of mass down so that your centre of drag is above it.  Just so you know, that will make this an interesting mission to get off the ground back at Kerbin; you'll need to use both fairings and creative construction techniques to be able to fly it (or maybe just fly it upside down).  Ascending from Eve can be made easier if you make the draggy parts with decouplers so you can jettison them and keep them from flipping your rocket on launch thereby.

Edited June 23, 2016 by Zhetaan

[Snark]

So, a few options.

First... I strongly advise using the inflatable heat shield.  Not only does it make the craft much easier to launch from Kerbin, but also that thing is amazingly effective.  It's the equivalent of having SIXTEEN of the 2.5m shields facing into the airflow.

If you want to keep your ship pointed prograde with the heat shield forward, then put your CoM as close behind the heat shield as you can manage, then add fins or airbrakes or something at the extreme opposite end (far from the heat shield) to act like the feathers on an arrow and keep you pointed right.

Another option, if you're using the inflatable shield:  put it on the back of the rocket and let it trail behind you like a big heat-resistant parachute.  Stick a regular heat shield on the front end to prevent that end from getting fried too much.

Or you could stick an inflatable on both ends. 

[Zhetaan]

55 minutes ago, Snark said:

Another option, if you're using the inflatable shield:  put it on the back of the rocket and let it trail behind you like a big heat-resistant parachute.  Stick a regular heat shield on the front end to prevent that end from getting fried too much.

@Snark is absolutely correct and offers a solution so obvious that I completely missed it:  when in doubt, put heat shields on the side that flips to face forward.  That answer is, as I said, obvious, but it's certainly a valid one.  Just don't overdo it:  you can't just plaster your whole rocket in heat shields and not worry at all.

Moar heat shields! 

[KerikBalm]

12 hours ago, Snark said:

then add fins or airbrakes or something at the extreme opposite end (far from the heat shield) to act like the feathers on an arrow and keep you pointed right.

just one comment: controllable fins don't work right when the craft is travelling retrograde... they always deflect the way they should with prograde airflow. In some eve landers, I've put something that can act as a control point (docking port/probe core) facing retrograde, so that I can select to control from there, and get the fins to work right.

Otherwise, just disable their activation on descent (if you want to use them for steering during ascent) so they act as a static fins.

Also I 2nd the idea of using the inflatable heat shield as a sort of heat resistant parachute... my old eve landers before the 10m heat shield:

1 kerbal lander

3 kerbal lander with vectors

Note the heat shields and airbrakes at the back to help stabilize them.

th 3 kerbal lander was barely... just barely stable enough to make it through reentry.. but in many tests it wouldn't

Edited June 24, 2016 by KerikBalm

[Vaporized Steel]

To illustrate the COM problem. Most of your mass are your fuel tanks. The heatshields are taking all the force (momentum) of the blasting wind during your reentry.

Because of the heatshields rotation they do have a earodynamic effect whereby they will want to point in the proper direction. But putting that mass that far backward makes the bowshock created by the heatshield to reach your payload mass easily, creating instability in your point of direction. And you'll only need a little instability for your heatshield to drift only slightly of the prograde marker and then the whole shebang will work like a hinge tumbling your heatshields the other way and then it's gamer over.

As suggested, use the inflatable heatshields. They're alot wider, extending the bowshock, deeclerate faster, are angled by default so you'll have a natural aerodynamic cone. But that suggestion is more additional, the most obvious one is putting that mass closer to the heatshield. No matter what shields your using.

[Foxster]

I think you have most of what you need to fix your craft above but one more thought: all those struts etc. Struts, girders and fuel ducts are incredibly draggy. 

What you will have with your craft is that the rear-end drag of your air brakes will be lost to all the drag of those struts and girders. 

A question...What's the tank and engines on the top for?

Edited June 24, 2016 by Foxster

[Der Anfang]

2 hours ago, Foxster said:

I think you have most of what you need to fix your craft above but one more thought: all those struts etc. Struts, girders and fuel ducts are incredibly draggy. 

What you will have with your craft is that the rear-end drag of your air brakes will be lost to all the drag of those struts and girders. 

A question...What's the tank and engines on the top for?

Yup, I've tested everything that people here have suggested and it has worked, mostly! Still kinda difficult to develop an eve lander design that is bottom heavy, though. As for the tank and engines on top, that is solely a testing thing (since I am using hyper edit) to deorbit my craft without using it's own fuel tanks. The actual eve lander for the legit mission will have a deorbit stage, but not that.

[gilflo]

Take a look at my design, it works perfectly for Eve Entry - second part of my post

-compact, long and aerodynamic form

-ejectable inflatable  shield well centered

-Ejectable speedbrakes and RCS  (ejectable vernors at the COG if possible, if not, it can work, but you have to test it)

-don't warp in atmosphere

a better design I try after this one for a plane to Eve, is to set a big  braking tank with speedbrakes at the top of your lander, Thus you have the cnter of gravity of the whole thing nearer the top of your lander where you can set a lot of RCS Vernors all around the tank (the nearer the COG, the better it is) . The ejectable shield at the base of your lander will protect the whole ship and RCS and speedbrakes will help to keep it centered on retrograde.