Why does this SSTO spin backwards on re-entry?

[Vaporized Steel]

*Playing v1.2 Career mode.

My own answer would be that my COM would drift behind my COL when my tanks are empty and my cargo is jettisoned in orbit.

I built a Panther, Spark and Thud engine SSTO design that can carry about 6 tons to orbit. (I don't have better engines yet)
It takes off and gets into orbit with no problems.

The following screenshot shows the COM and COL on the picture with all fuel tanks emptied.

And a few more images on this link if that could prove any evidence to issue.

I have active connection to the KSC btw.

With all fuel tanks filled the COM will only move forwards about a meter, because I designed the empty mass in such a way that I wouldn't need to worry much about the shifting COM in the first place.

 

Unfortunately it behaves just like the COM is forward even while it isnt during reentry. As soon as I hit the atmosphere and the Reaction Wheel Torque fails (@ about 64km) the vessel starts tumbling backwards with the tail to the retrograde marker.

When I go below 10 Km I can usually get control back somehow and get the nose back down and commence flight.

But because I cannot control my reentry angle I cannot control my aerobraking and I cannot make a precise landing at the KSC.
I never use Aerobrakes on SSTOs but rely on a balance to achieve high angles of attack. I rather not use them, especially because I want it to land at the KSC, and Aerobrakes on the back will make it a projectile.

On this design the plenty of forward and rear pitch control surfaces seem totally ineffective.

Any ideas?

Edited October 17, 2016 by Vaporized Steel

[Palaceviking]

New body lift since 1.1.3 doesn't seem to show up in the sph.

Had same problem, need to move com forward a bit more.

[Vaporized Steel]

Never ran into this issue on my other designs, so I guess I missed the consequences so far of not knowing wrong body lift display.
I'm going to adjust it a bit more and see if it works.

I had hoped that such a troublesome bug would have gotten the deserved attention and be solved in v1.2
Granted there are countless fixes so I'm hardly ranting about this one.

But it doesn't seem like a minor bug and could really annoy people when constructing vessels. I hope it gets fixed ASAP.

[Vaporized Steel]

Issue solved. The wrong lift display must be really far off then. I had to space the COL/COM markers by quite a margin to get it right.

[ForScience6686]

Just because the com and col look properly placed, does not mean it is stable.  My understanding is drag plays a bigger role in this.  If I was flying a craft like that I would separate the two more, having a closer relationship to each other generally means more maneuverable but more prone to spins.  Further apart equates to slower turns but more stable near the stall point.  You'd be surprised how far apart you can have the markers and be stable.  

[luizopiloto]

MK2 fuselage creates a lot of Lift and drag at high AoA... you can try move those canards to the middle of that cargobay... and tweak the main wings a bit...
Do some tests with sufficient fuel for a flight at 5000m...

[Plusck]

Definitely a problem of drag more than body lift.

If you look at the triangular part of your plane (wings and rear half of fuselage, and imagine it getting slammed flat into the air, you can see it'll be pretty stable, with drag fairly balanced around where the CoM marker is.

Then you add that long front section of the fuselage, and it's just never going to be able to counteract that long lever arm. The plane can be stable when pointing straight ahead, but never with a very high AoA, when drag becomes more important than lift.

[ForScience6686]

9 minutes ago, Plusck said:

Definitely a problem of drag more than body lift.

If you look at the triangular part of your plane (wings and rear half of fuselage, and imagine it getting slammed flat into the air, you can see it'll be pretty stable, with drag fairly balanced around where the CoM marker is.

Then you add that long front section of the fuselage, and it's just never going to be able to counteract that long lever arm. The plane can be stable when pointing straight ahead, but never with a very high AoA, when drag becomes more important than lift.

That's a very good point, and one that space planes generally run into since people generally build them with com near the back.  

[Vaporized Steel]

Well, I did manage to get it to reenter just fine. I thought there was a mark solved option somewhere for your thread but haven't used that in a very long time and can't find it in my post when I "edit" it. Either I'm looking badly or it is removed.

@luizopiloto

Thanks for your information and advice. It wouldn't help to test flight this craft under those conditions.
The craft flipped backwards and keeps spinning during reentry up until it decends below 10 through 8km and then I can somehow fly level again. Which confirms @Plusck his explanation about drag versus lift.

So it will fly just normally if I test flight at 5000meter. It did that with level flight or even when trying to quikly pull the nose up. But moving the canards backward was indeed one of the implementations that fixed this craft.

Repositioning the fuel tanks also did wonders, the craft is indeed wider now and with a shorter length. It looks less eye catching if you ask me, but function over form as always.

@Plusck

Makes alot of sense. I know there is aerodynamic overlay while in flight but I do think some indicator or VAB/SPH overlay that shows how drag is going to behave would be very usefull. I can not always tell what's the cause of aerodynamic fail by the overlay function. A VAB/SPH indicator probably allows you to get it right in 1 or 2 tries. But that may make the game easier to a point where perhaps nobody is willing to go, just my idea though.

Now that you explained why my vessel behaved as it did I kinda see it the same way now that I have it working. I probably avoid this problem next time.
 

 

 

Edited October 17, 2016 by Vaporized Steel