Help stabilize large planes/shuttles for re-entry.

[archnem]

Hey all:

I've been trying to build a shuttle that can carry 40 tons to orbit and return.  I've also been running into problems with medium to large spaceplanes as well.  My designs tend to flip over during re-entry and become wildly unstable.  I've had some limited success if I re-enter locked to prograde, but then my cockpits tend to explode!  And even then, if I touch the controls at all the plane flips.  I have built larger shuttles in previous versions and never had any real problems.  However, now if I try to re-enter at any AoA above prograde my craft flips and goes out of control!

First off, yes, the CoL is behind the CoM :).  I suspect I just have too much mass towards the back of the shuttle, which is pulling the ass-end down and causing the plane to flip?  Does that sound plausible?  I've tried putting in large numbers of SAS wheels, adding control surfaces, anything I can think of, but anything but the smallest planes and shuttles go wildly insane on re-entry - even though I can fly them off the runway and at lower altitudes perfectly fine.

Here are a couple of pictures of the large shuttle I'm working with now, any help would be appreciated!

[AeroGav]

Size isn't important 

I strongly recommend two mods to help you build stable aircraft  - RCS build aid,  and  CorrectCoL

RCS build aid shows a red ball in SPH which indicates where your CoM will be when empty.

You can do the same thing by manually emptying all the tanks but trust me ,  the number of times you need to check this when building a complex spaceplane, it saves much work.

CorrectCoL   makes the blue ball more accurate.     At the moment, the Blue ball only takes account of parts with a lift rating in their description.   But in reality all parts interact with the airflow and create forces.   You have a lot of fuselage in front of the CoM and it has a longer lever arm than the stuff at the back, it may be that your CoL is further forward than you think it is.

That layout you have there btw, has the cargo in front of the CoM, which means it's always going to be either a bit too nose heavy when laden or a bit too tail heavy (flip happy) when empty.   Best is cargo bay dead centre of CoM, with half your fuel right behind the cargo bay and half right in front, or on sponsons either side.     

 

CorrectCoL has a more sophisticated Static Analysis graph you can bring up too, for pitch and yaw.

Spoiler

 

info -

The horizontal axis is for simulated AoA,  and the vertical shows the pitch torque the airframe generates.    Left on the horizontal axis means negative AoA, right means positive AoA.    Up on the vertical axis means the aerodynamic forces are going to be pitching you up, Down  means the aero forces are pushing your nose down.   What you are looking for is a line that slopes "downhill from left to right", in other words, on the left of the graph, when AoA is negative,  the torque from all the aero forces will be trying to bring the nose back up, but this torque gets weaker as the plane gets closer to prograde and becomes a nose down moment when the nose is above prograde.   .

 

The nuclear option for stability/flipping problems is to check and tweak incidence angles.      

Usually, surfaces are attached at zero incidence angle and all aero surfaces on the ship will have the same AoA.

If they differ however, it is imperative that surfaces ahead of CoM should have greater AoA than ones behind.     That way, wings at the front gain lift slower than wings at the back as AoA increases, as they get into diminishing returns (or even a stall) first.   However, because this increases lift on the front end at low AoA,  it moves the blue ball forward, so you need to slide the wings back a bit.

Short version -  Using fine rotate tool, rotate your front Canards up ever so slightly.    Rotate the elevons , on the trailing edge of the wing , down ever so slightly.    This will cause the blue ball to move forwards in SPH, so you'll need to slide the wings back to keep it behind CoM.  But it will now fight back if you try to stall it.    Its like being held to a railway line by a strong rubber band,  You can steer a few degrees either side of prograde but if you go too far it yanks you back in line. 

[agrasyuk]

As I came to find out it is not enough to have CoL behind CoM, the aft portion that contains the engines is still way to heavy.

what I would try is swapping places between the cargo bay with the middle fuel tank. before entry pump all the remaining fuel into this front tank, perhaps it will make the front heavy enough. or perhaps you may need to add even more "ballast" tanks? the craft under spoiler can maintain the pictured attitude on entry only with full front tanks, otherwise it will tumble exactly as you describe. I am pumping the fuel aft again after more dense atmosphere is reached since the craft cannot maintain level flight when it is that front heavy. 

Spoiler

 

and there it is a much more deep answer above  

Edited March 22, 2017 by agrasyuk

[AeroGav]

25 minutes ago, AeroGav said:

info -

The horizontal axis is for simulated AoA,  and the vertical shows the pitch torque the airframe generates.    Left on the horizontal axis means negative AoA, right means positive AoA.    Up on the vertical axis means the aerodynamic forces are going to be pitching you up, Down  means the aero forces are pushing your nose down.   What you are looking for is a line that slopes "downhill from left to right", in other words, on the left of the graph, when AoA is negative,  the torque from all the aero forces will be trying to bring the nose back up, but this torque gets weaker as the plane gets closer to prograde and becomes a nose down moment when the nose is above prograde.

There, a picture is worth a thousand words. 

Red ball from RCS build aid is right on top of my yellow ball - CG is same full and empty.

Static Stability analysis graph slopes downhill across the AoA range - in other words , the more the nose pitches up, the stronger the plane's stability tries to pitch it down again.   It crosses the axis at about 0.5 degrees AoA, meaning if you let go of the stick and have SAS off, it will settle at an AoA of 0.5.     The thin vertical blue line (hard to see unless you click on the picture) indicates it needs about 5 degrees to maintain level flight in the conditions i simulated - 22km altitude and 1150 m/s.

 

[archnem]

30 minutes ago, AeroGav said:

You can do the same thing by manually emptying all the tanks but trust me ,  the number of times you need to check this when building a complex spaceplane, it saves much work.

This alone is worth the price of admission.  I installed both mods, and sure enough, my CorrectCoL graph shows I'm wildly unstable under alot of conditions.  I'll play around with it and see what I can do.  Thanks, this should help alot!

[archnem]

6 hours ago, AeroGav said:

There, a picture is worth a thousand words. 

Wow, thank you so much!  I started from scratch using CorrectCoL, and just delivered 45 tons to LKO.  Not only is my shuttle way more stable on re-entry, ascent was also rock-solid.  I was used to fighting the orbiter a bit on the way up, and figured that was #justkerbalthings.  This time, even with a big ship, the craft was a joy to fly.  This is going to help my engineering immensely!