Help with spaceplane design

[Zamolxes77]

I build dozens if not hundreds of space planes. I can make them whatever I want. But now I met an impossible situation. Plane should behave one way, but it doesn't.

This is a spaceplane / chemical SSTO hybrid. Is designed to take off as a rocket, deploy its payload in orbit, then glide its way to the runway. As you can see from the picture, when nearly empty, only 200 monoprop left in it, CoL is right behind CoM, slightly above and behind, pointing backwards. It should perfectly aero-break in upper atmosphere, but NO. around 50 km up, when pointing at 90 AoA, it starts flipping backwards, exposing the fragile Vector engines to plasma stream. I tried moving the wings dozens of times, I cannot stomp that behavior. I keep moving wings aft, more and more, until at some point, it performs the aero break beautifully, but then points his ass retrogade at low speed, AGAIN.

I don't know what to do anymore, I'm terribly frustrated, is it some sort of bug or am I missing something ?

[Geschosskopf]

Put a lot of airbrakes on the upper wing surface near the trailing edge and deploy them after the de-orbit burn.  They will help keep your nose up because they tend to act as elevons.  Plus they slow you down faster.

[Zamolxes77]

33 minutes ago, Geschosskopf said:

Put a lot of airbrakes on the upper wing surface near the trailing edge and deploy them after the de-orbit burn.  They will help keep your nose up because they tend to act as elevons.  Plus they slow you down faster.

It doesn't work. I added 6 airbrakes on each wing, 3 on top 3 on bottom, no change. I added 4 of them only on upper wing surface, again, no change.

[Geschosskopf]

11 minutes ago, Zamolxes77 said:

It doesn't work. I added 6 airbrakes on each wing, 3 on top 3 on bottom, no change. I added 4 of them only on upper wing surface, again, no change.

Well, only put them on the top, not the bottom.  But if they don't work, then pump any remaining fuel to the rear and add more torque.

[Zamolxes77]

5 minutes ago, Geschosskopf said:

Well, only put them on the top, not the bottom.  But if they don't work, then pump any remaining fuel to the rear and add more torque.

To the rear ? Did you understand I have problems with the rear shifting forwards and flipping the plane, rear 1st ? Adding more weight in the rear would only make it worse, not solve it !

[Geschosskopf]

Oh, I thought you said the nose was going down.

[Rocket In My Pocket]

 

[Alshain]

It's flipping around backward because the center of mass is so far behind the center of the craft.  It's behaving just like a command pod, only that isn't really what you want.  To make matters worse it has no lift to stabilize it because the wings are incredibly small for it's mass.  My initial thought is that you need way more wing, and if you can bring the Dry CoM a little forward, it wouldn't hurt.

Edited August 6, 2016 by Alshain

[bewing]

Use canards instead of the silly elevons.

[dafidge9898]

5 hours ago, Zamolxes77 said:

I don't know what to do anymore, I'm terribly frustrated, is it some sort of bug or am I missing something ?

A CoL behind the CoM doesn't always guarantee it won't flip over in the upper atmosphere. You have some pretty heavy engines in the back, and when that high up in the atmosphere, the heavy part of the craft always leads. 

[Zamolxes77]

8 hours ago, Alshain said:

It's flipping around backward because the center of mass is so far behind the center of the craft.  It's behaving just like a command pod, only that isn't really what you want.  To make matters worse it has no lift to stabilize it because the wings are incredibly small for it's mass.  My initial thought is that you need way more wing, and if you can bring the Dry CoM a little forward, it wouldn't hurt.

I think you're correct, I never built a spaceplane so heavy before it has a dry mass of 130 tons, my previous heaviest SSTO was something around 120 tons wet and it was built out of mk2 parts.

For starters, I installed this mod that shows true CoL (stock game bungles the job when using parts with no lift rating), and looking into adding more wings, problem is I have no idea what ratio should I aim for. Suggestions ?

5 hours ago, dafidge9898 said:

A CoL behind the CoM doesn't always guarantee it won't flip over in the upper atmosphere. You have some pretty heavy engines in the back, and when that high up in the atmosphere, the heavy part of the craft always leads. 

Correct, but they're only 8 Vector engines, at 4 tons a pop that 32 tons, less than 25% of total mass of the plane, which clocks in at 130 tons. I got tube shaped, rocket main stages, orange tanks, to fly, using nothing but the 4 stabilizing fins and the motor represented almost 40% of total dry mass !

[Alshain]

1 minute ago, Zamolxes77 said:

I think you're correct, I never built a spaceplane so heavy before it has a dry mass of 130 tons, my previous heaviest SSTO was something around 120 tons wet and it was built out of mk2 parts.

For starters, I installed this mod that shows true CoL (stock game bungles the job when using parts with no lift rating), and looking into adding more wings, problem is I have no idea what ratio should I aim for. Suggestions ?

That's one thing I've never been able to write down as a number or even a loose rule of thumb.  I've usually been able to eyeball about how much is good by how much it looks like a real plane, occasionally only needing to add more.  I guess you could call it experience.  You might compare to a real plane, we know the Mk3 is based loosely on the space shuttle.  Look at the ratio of wing to body on that, and don't forget the wing extends under the body.

Or perhaps a more conventional plane....

 

[foamyesque]

Aerodynamics in stall are considerably different than lifting aero; turn on the aerodynamic overlay. I would suggest ripping out the elevons and using a fore-and-aft set of standard canards with maximum deflection range; they will act much more strongly to modify your drag profile in stall conditions and maintain your AoA. The wings themselves can be made of a BigS strake leading into a BigS wing and with BigS tailfins tacked to the outside configured for roll. A set of structural wings can then run along the ventral surface of the exterior tanks, bridging the gap between them and the central core.

 

EDIT:

 

Another possible culprit may be the radially attached docking ports on the nose increasing your drag there.

EDIT:

Example:

Edited August 7, 2016 by foamyesque

[CorvusCorax]

This is an important for both FAR and stock aerodynamics:

You have 4 force centers on a space - or airplane:

1. Center of mass/gravity (COM, COG)  - this is the virtual point where if the craft was hinged on it would be in perfect balance - also usually the rotation center when turning in vacuum

2. Center of lift - this is the virtual point where all the lifting forces applying to an airplane appear to be combined. (Lift= force perpendicular to flight direction)

3. Center of drag - this is the virtual point where all the dragging forces applying to an airplane appear to be combined. (Drag= force opposed to flight direction)

4. Center of all aerodynamic forces (2+3 combined)

Most tutorials don't distinguish between 2 and 3 and just use 4 named as either of them. KSP afaik displays 4 in both SPH and VAB

Depending on shape and angle of attack, a structural part can have low drag and high lift as well as high lift and low drag, so 2 and 3 are NOT identical. However these conditions are both dependent on angle of attack. Which means if you change the angle of attack, points 2, 3 and 4 all shift around.

 

Everybody is being taught that if you have your center of mass (1) in front of the center of aerodynamic forces (4), your craft will be stable. This is not wrong, but it's only half the truth.

In theory, the crafts mass wants to keep its momentum pulling on point 1, while the aerodynamic forces push against it seemingly in spot 4. These forces against each other will rotate the craft. Usually center of drag behind center of mass will turn the nose into flight direction. Center of lift behind center of mass will push the nose down (which is good for autonomous stall recovery. In fact, you counteract that with elevator trim creating a down force at the tail which effectively makes the center of lift identical with center of mass while center of drag is still behind center of mass. As such the craft will remain flying level  in a stable craft, a faster speed will lead to increased lift in the front, center of lift moving forward - pushing nose up, while slower speed will shift the center of lift back pushing the nose back down making it glide nicely)

If you deviate your orientation ever so slightly, this arrangement will force the nose back into flight direction and everything is fine.

Now if you steer extreme angle of attacks, 30° or more, then your aerodynamic shape changes drastically. Some surfaces might stall (increasing drag a lot, while reducing lift) while others - for example the fuselage itself when experiencing airflow from the side - might suddenly become a "lifting surface" (creating unwanted sideways lift)

 

You can test that in KSP by rotating the entire craft around. The calculation for "center of lift" always assumes that the craft is flying towards the hangar door (in the SPH) or up (in the VAB). If you turn the craft nose up in the SPH, KSP will show you where your center of aerodynamic forces shifts to at 90° angle of attack.

If your craft is stable, it will still be more or less at the same spot behind the COG. If so, then the forces would turn your craft back into a nose first position.

However with such a wide-bodied craft as you have, the fuselage will create much more drag when flying at 90° AoA than when flying nose first - especially in the forward section. (Those nosecones really have crappy drag coefficients when flying sideways)

This is almost guaranteed to move your COL towards the nose, above the COG. And that causes your craft to turn tail first. (That doesn't mean that it's actually stable with its tail first. The new COL with tail forward might again be somewhere else, leading to a craft spinning around oddly and never settling in any useful orientation)

 

To have a really stable plane that you can trust during reentry manouvers and high AoA, you need to have the COG in front (as in closer to the nose of the craft) of COL not only when in its default orientation - nose forward, but also when turned 45° and even 90° sideways, up or down. If that is the case, you won't enter stuff like irrecoverable spins and such.

 

(This is a real world issue even with some existing operational airplanes: A so called "flat spin" is exactly that: In a stalled high AoA configuration, the COD shifted so far forward, that the craft remains, spinning, in that configuration and its impossible to move the nose back down, even though the craft flies stable with a nose first orientation)

 

With FAR, this is made further complicated. At high supersonic speeds, the center of drag shifts as well. With delta wings and a pointy nose, it COD usually shifts to the back, but with a blunt shaped nose it might even move forward (detached bow shock and the like forming up)

 

Edited August 12, 2016 by CorvusCorax
clarification on which front is in front when turned sideways

[capi3101]

On 8/7/2016 at 0:16 AM, Zamolxes77 said:

For starters, I installed this mod that shows true CoL (stock game bungles the job when using parts with no lift rating), and looking into adding more wings, problem is I have no idea what ratio should I aim for. Suggestions ?

Quick question - are you flying stock or FAR?

My own self, I fly FAR, and when I go to design a spaceplane, I try to shoot for a wing loading of about half-a-tonne per square meter. The math there is simple - start with your desired take-off mass (in tonnes) and double it - that's your desired horizontal wing area (in square meters). About a quarter of that goes into the tailplane, and the rest goes into the main wing. I then go on to determine the wing semi-span based on the desired aspect ratio for my craft, and from there I can determine the length of the wing root and tips. It's a pretty complicated set of equations and to be wholly honest I haven't been playing the game for the last eight months, so I'm still trying to re-interpret all of my shorthand. Whether or not the same formulas will work in stock is another matter entirely, and I should mention that I do use Procedural Wings as well, so it's relatively easy for me to set wings up the way I want them.

Other thing I typically shoot for with my spaceplanes is the cranked arrow design - which can be emulated with a connector, a strake forward and a wing-wing off the edge, and those are their own sets of formulas.

Stock aero, I have no idea what to tell you. Conventional wisdom used to be 1 lift unit per 2 tonnes of plane, but that no doubt is information that's WAY out of date by now.

[Val]

Very good advice @CorvusCorax

On 8/6/2016 at 8:08 PM, Zamolxes77 said:

... CoL is right behind CoM, slightly above and behind, pointing backwards. It should perfectly aero-break in upper atmosphere, but NO. around 50 km up, when pointing at 90 AoA, it starts flipping backwards, exposing the fragile Vector engines to plasma stream. I tried moving the wings dozens of times, I cannot stomp that behavior. I keep moving wings aft, more and more, until at some point, it performs the aero break beautifully, but then points his ass retrogade at low speed, AGAIN.

I don't know what to do anymore, I'm terribly frustrated, is it some sort of bug or am I missing something ?

Your vertical stabiliser is too small.

You are right to assume it's not a CoL/CoM issue. KSPs CoL/CoM indicators only shows you whether the craft will be stable on the pitch Axis.

It just needs a larger tail fin.