Shuttle going backwards when re-entering

[JoseEduardo]

not sure if this is the place to post this, but what am i doing wrong in this design?

whenever i try to make my shuttle re-enter, after it starts to really burn it simply does a backflip and re-enter backwards, and often i loose control, this happens in both, stock and FAR, if i attempt to go directly heading prograde my shuttle steers to the right and go backwards, if i try to enter with a positive AoA after a few seconds it does a backflip and also re-enter backwards, i'm not sure what might be causing it, the wings, tail and control surfaces are all from Bobcat's Buran, and whenever i try the same scenarios with his buran everything works out fine, the only main difference is that, unlike Buran, i don't attach the main structure of the wings and tail to the fuselage, they're set right on the surface of the fuselage... i've attempted putting all the remaining fuel in the front section to put the CG ahead of the CL, in the rear cargo bay to put the CG right above to the CL, i've even reduced the length of the fuselage, but all attempts ended the very same way, with absolutely no difference between them

again, this happened with both atmospheric models, stock and FAR, Buran works fine in both, and even with a positive AoA, but this design doesn't, and i have no idea why...

here's a few pics of it in the SPH

fully loaded of fuel and monopropellant:

now removing fuel from the stations

if i leave fuel in the rear cargo bay instead of the front one the CG goes directly above the CL and looks exactly like Buran, but the result is the same

and here's two KVV screenshot with the fuselage merged with Buran just for fuselage size comparison purposes

it's a little over a single cargo bay bigger than Buran, both cockpits are almost the same size (as it can be seen in the KVV pics), even making it almost the same size as Buran didn't work

mods in it:

Buran - Airlock, Tail, Wings, Control surfaces

MK IV - Fuselage and the two smaller engines

a surface attachable flag that i don't remember right now where i got it

and KOSMOS TKS RCS thrusters

the only stock parts are the main jet engine and a landing gear on the nose

Mechjeb was added through a MM patch in the cockpit

Edited January 29, 2015 by JoseEduardo

[Alshain]

You say it flips AFTER you start to burn? Where is your Center of Thrust, is it out of line with the center of mass?

It could also be the shock cones, they are very far forward and they create a lot of drag which pulls the front end around. Try to keep intakes behind the CoM when possible.

Edited January 28, 2015 by Alshain

[JoseEduardo]

Sorry, when I mean burn I mean atmospheric burn during reentry due to atmospheric drag

[Alshain]

Sorry, when I mean burn I mean atmospheric burn during reentry due to atmospheric drag

Ok, then I'm going to go with the forward drag issue. Those intakes create a drag and it's as if you tied a piece of string to them and pulled from behind the plane, it flips. Put the intakes in the back and it will even hold it steady.

[JoseEduardo]

Unfortunately the intake is fixed to the cockpit... does closing them works?

[Alshain]

Unfortunately the intake is fixed to the cockpit... does closing them works?

You know, I'm really not sure. Try it.

What cockpit is that? I don't remember intakes on the Mk IV cockpit.

EDIT: I see, they were added in a later version. That is not good.

Edited January 28, 2015 by Alshain

[NASAHireMe]

This is a very common problem on these forums. Intakes are a possible issue, but more likely it has to do with your CoM/CoL. Here's a post by user Rune:

"I've answered this for someone else today:

Originally Posted by Rune

Actually, and take it from an aeronautic engineer, it's rather simple: if you lose control, it's because your plane is not aerodynamically stable. Now, what makes a plane aerodynamically stable? Well, picture the most aerodynamically stable stable thing I can think of, an arrow: how does it make it to keep pointing in the direction of travel no matter what happens? Well, the answer is simple: the center of lift is way, way behind the center of mass. Unlike in the rocket pendulum fallacy, you aerodynamic thingy "hangs" from the center of lift, and is pulled down from the center of mass. That means the center of mass will always try to get before the center of lift in the direction of travel. Which is why opening a chute behind it will always work (a parachute gets greats amount of "lift", or better said, aerodynamic forces).

Now you can go a bit too far, and make your plane too aerodynamically stable. Then what will happen is you don't have enough control authority to make it point anywhere but prograde, and "prograde" will start being more and more "downwards"... very gentle way of crashing, and with a beautifully curved trajectory.

All in all, what I can say is: there is a reason planes are long, and have rear stabilizing surfaces at the end of a tail, well behind the center of mass. And why flying wings are very, very tricky to pull off in the real world without fly-by-wire systems.

Rune. Hope that helps.

Intake drag can also be a contributing factor, though. Intakes behind the CoM help with stability.

Rune. Killing two birds with one post."

- - - Updated - - -

I would also recommend reading through these threads:

http://www.reddit.com/r/KerbalAcademy/comments/1kezq8/spaceplane_re_entry_angle/

http://forum.kerbalspaceprogram.com/threads/75768-Space-Plane-Reentry-issue

http://forum.kerbalspaceprogram.com/threads/70480-Stabilizing-SSTO-spaceplanes-during-reentry

http://forum.kerbalspaceprogram.com/threads/85638-Yaw-stability-and-re-entry

Tips I'll highlight:

1. Place more intakes/control surfaces at the rear

2. The landing gears (can't tell if you're using them in the pictures you posted) have no mass/drag in flight, but DO have them in the SPH/VAB (this is a 'bug', if you want to call it that...). So that may be screwing up your CoM/CoL balance. So trim your CoM/CoL balance first, then slap on landing gears at the last minute.

3. Watch your fuel drainage. It may be draining in such a way that it inadvertently flips your CoM/CoL balance

4. There's always workarounds that are inelegant. When I had this problem on one of my very small SSTO planes, I just accepted reentering backwards. I used drogues to slow me down when I get under 20,000 meters, then fired up the jets and started accelerating backwards. Like I said, inelegant.

Edited January 28, 2015 by NASAHireMe

[Starwhip]

Besides what has been posted above, I notice that the Center of Lift is below your Center of Mass. That's not helping.

Try moving it to or above the level of your Center of Mass.

[Alshain]

This is a very common problem on these forums. Intakes are a possible issue, but more likely it has to do with your CoM/CoL.

There is a difference this time. It's the addons fault, Nertea added intakes to the cockpit and they cannot be removed. I've been discussing it in that thread. Hopefully we can talk Nertea into fixing because it really is a nice part pack.

[JoseEduardo]

i'll try the other Mk IV cockpit without the intake and will report back ASAP

This is a very common problem on these forums. Intakes are a possible issue, but more likely it has to do with your CoM/CoL. Here's a post by user Rune:

"I've answered this for someone else today:

http://forum.kerbalspaceprogram.com/images/misc/quote_icon.png Originally Posted by Rune http://forum.kerbalspaceprogram.com/images/buttons/viewpost-right.png

Actually, and take it from an aeronautic engineer, it's rather simple: if you lose control, it's because your plane is not aerodynamically stable. Now, what makes a plane aerodynamically stable? Well, picture the most aerodynamically stable stable thing I can think of, an arrow: how does it make it to keep pointing in the direction of travel no matter what happens? Well, the answer is simple: the center of lift is way, way behind the center of mass. Unlike in the rocket pendulum fallacy, you aerodynamic thingy "hangs" from the center of lift, and is pulled down from the center of mass. That means the center of mass will always try to get before the center of lift in the direction of travel. Which is why opening a chute behind it will always work (a parachute gets greats amount of "lift", or better said, aerodynamic forces).

Now you can go a bit too far, and make your plane too aerodynamically stable. Then what will happen is you don't have enough control authority to make it point anywhere but prograde, and "prograde" will start being more and more "downwards"... very gentle way of crashing, and with a beautifully curved trajectory.

All in all, what I can say is: there is a reason planes are long, and have rear stabilizing surfaces at the end of a tail, well behind the center of mass. And why flying wings are very, very tricky to pull off in the real world without fly-by-wire systems.

Rune. Hope that helps.

Intake drag can also be a contributing factor, though. Intakes behind the CoM help with stability.

Rune. Killing two birds with one post."

- - - Updated - - -

I would also recommend reading through these threads:

http://www.reddit.com/r/KerbalAcademy/comments/1kezq8/spaceplane_re_entry_angle/

http://forum.kerbalspaceprogram.com/threads/75768-Space-Plane-Reentry-issue

http://forum.kerbalspaceprogram.com/threads/70480-Stabilizing-SSTO-spaceplanes-during-reentry

http://forum.kerbalspaceprogram.com/threads/85638-Yaw-stability-and-re-entry

Tips I'll highlight:

1. Place more intakes/control surfaces at the rear

2. The landing gears (can't tell if you're using them in the pictures you posted) have no mass/drag in flight, but DO have them in the SPH/VAB (this is a 'bug', if you want to call it that...). So that may be screwing up your CoM/CoL balance. So trim your CoM/CoL balance first, then slap on landing gears at the last minute.

3. Watch your fuel drainage. It may be draining in such a way that it inadvertently flips your CoM/CoL balance

4. There's always workarounds that are inelegant. When I had this problem on one of my very small SSTO planes, I just accepted reentering backwards. I used drogues to slow me down when I get under 20,000 meters, then fired up the jets and started accelerating backwards. Like I said, inelegant.

that's exactly what was driving me crazy asking myself "wth am i doing wrong in this?", i am pursuing my pilot license and stability was one of the items i had to study, and i know that perfectly well, i saw the thread of the guy explaining how to make a plane in KSP with the nice drawings and infos, and all was making perfect sense to me on his explanation, but at the same time i was thinking "i am doing everything according to the books, what's wrong then? is it the wings that might have issues with FAR? is there something different in KSP or even spaceplanes from actual planes?"

on the highlights:

1 - yeah, i followed Alshain's link and found out there's a legacy version without the intakes that i'll try out and see if it solves, and since i'm planning on having the thing land like a jet plane there will definetively be intakes (i'm thinking about the Mk2 radial ones)

2 - i am, but only 1 stock at the nose and 2 main landing gears from Buran, which is bundled with the wing

3 - as i stated in the OP i paid a lot attention to that, i've tested the fuel drainage in flight, and came to a conclusion that i usually have around 1/3, or 1/2 at most, fuel left when i perform the de-orbit burn, in the pictures above i took screenshots of, respectively, full tanks (never happen even if i start the de-orbit burn full), rear tank empty (needless to say that i need to perform a fuel transfer), monopropellant to a "optimistic" value (in short, more weight than what could have in fact) and center tanks empty (leaving only the front tank in the front cargo bay with fuel, again, pumped) (i have a pilot mind, make sure it works, and make sure it works in the worst scenario, just so i don't have to worry about it when i use it under normal conditions... thats mainly what kept me from making genocides with kerbals and everytime i didn't make stuff according to the "standard procedure", it went terribly wrong, KSP, FSX and RL, that's mainly why i got terrified when i first flown myself, now i want to fly again )

4 - i actually thought about that, because i was able to brake and regain control after a while with it even going backwards, but usually things blew up and i was already too low and too much in trouble for making this a standard procedure... even if i regained full control of it, if i make the slightest turn, it would go nuts

Besides what has been posted above, I notice that the Center of Lift is below your Center of Mass. That's not helping.

Try moving it to or above the level of your Center of Mass.

indeed, but that's how low-wing planes work (airliners aswell), the force it makes isn't (or at least shouldn't be) enough to flip it especially when you consider how wings work, but it's ksp, and symmetrical wings do the same job as asymmetrical wings, so that can be really possible...

i don't remember right now what's the main actual problem with low-wing planes, all i remember is that high-wings use far less runway to land and take-off than a low-wing plane and are more stable, that's why BAe 146-300 can land in short strips like Santos Dumont Airport ones without having reverse and a E-190/195 of the same capacity needs reverse to stop the plane without going for a swim, the main advantage of the low-wing is it's maneuverability though

EDIT: btw, just so you folks don't get this post wrong, i really appreciate the help (if i didn't i wouldn't have come here in the first place, and yeah, i should have said that before editing, that's usual on me), and i really understand why you guys brought up the CoM/CG - CoL issue, not everyone is forced to know flight mechanics and how planes work, that's why i help my flight attendant classmates when possible, i'm just pointing out that i do understand all this from previous training way before KSP

- - - Updated - - -

it was indeed the intakes, thanks for the help guys!

i've downloaded the Mk IV 1.1.0 from Kerbalstuff to get the cockpit without the intakes, and now i was able to control the shuttle amazingly, now it flies amazingly well, i thought it was a conflict with Buran wings and tail with FAR and had it removed for testing, but since that also occurred with stock drag, and wasn't Buran's fault, i can rule it out now and get FAR back

i'll flag it now as answered

Edited January 29, 2015 by JoseEduardo

[Alshain]

Glad we found you a work around.

[JoseEduardo]

well... apparently Bobcat's buran doesn't work with FAR, and unless i missed something in the thread, there's no config for it... i've tested the very same vessel i posted here in stock and it worked, just like Buran, but when i got it to FAR, both, Buran and the shuttle i made, flipped and re-entered backwards, no matter where the CG was... the only way i was able to re-enter was pointing prograde...

oh, and now the thing has two intakes on the back (from the Mk IV pack, i've resized them to 75% just so they don't stick too much on the back), resembles a bit the ones on the Space Shuttles, but with a different function, either open or closed, the result is the same

i'm not re-flagging the thread to unanswered though, as it has been solved in stock, the issue now is with FAR, so no need to "re-open" it

oh well... either i choose to don't use a shuttle or remove FAR, tough choice...

again, thanks for the help guys!

[Alshain]

Try these, they should look good with Mk3 or Mk IV and I'm pretty sure they have FAR support because bac9 doesn't support anything else (even though these are not made by him)

http://forum.kerbalspaceprogram.com/threads/91841-0-90-B9-Style-Shuttle-Wings-V-1-3-%28Updated-at-01-10-2015%29

[JoseEduardo]

Try these, they should look good with Mk3 or Mk IV and I'm pretty sure they have FAR support because bac9 doesn't support anything else (even though these are not made by him)

http://forum.kerbalspaceprogram.com/threads/91841-0-90-B9-Style-Shuttle-Wings-V-1-3-%28Updated-at-01-10-2015%29

MAAAAAAAAAAAAAAAN!!! before i decided to use Buran for this i tried remembering the name of these add-on, i saw it when i didn't need and when i wanted i couldn't find it! a big thank you for it!

i'll definetivelly change for this one, it even has a parachute on the tail!