constant roll

[Cadet_BNSF]

Whenever I am flying a winged aircraft, notably my shuttle for the sts challenge, they tend to roll yaw to one side or the other, and as I correct, the oscillations grow bigger and bigger. Can anyone explain this?

https://kerbalx.com/Cadet_BNSF/Shuttle-Alaska-MkII

Here is the craft file

Edited May 29, 2017 by Cadet_BNSF
corrected problematic axis of rotation

[HvP]

Hello @Cadet_BNSF

If you take just your shuttle and lay it horizontally in the spaceplane hangar you can check the centers of mass, lift and thrust. You have some tweaking to do with them.

If you turn on the markers for the center of mass (yellow circle) and center of lift (blue circle) you will notice that your center of lift is well below the center of mass, way down near the belly of the craft at the back. This makes your shuttle very unstable along the axis of the center-line. The blue marker is an indication of where the UPWARDS lifting force will be concentrated, and the yellow marker indicates where the DOWNWARDS force of gravity will be concentrated. So imagine pulling down on the yellow marker and at the same time pulling up on the blue marker. Your craft tries to tip over.

It's of course more complicated than that because other surfaces produce drag and lift that the aero model isn't showing you in the hangar. But there are a few rules of thumb to start with. Keep your center of lift behind the center of mass from front to back, but not too far back or else you won't have enough lift in front to keep your nose up. Keep your center of mass in line with, or very slightly above, your center of mass from top to bottom. Remember, your plane is trying to "hang" from its wings. If the wings are too low on the fuselage then its most stable position is upside down -- hanging from the wings. Imagine how hard it would be to fly a hang glider upside down with the wings at the bottom.

The space shuttle had a highly advanced computer control system constantly adjusting the control surfaces every fraction of a second to keep it flying straight. Jet fighters are configured with low slung wings to make them capable of turning and pitching in very tight circles. They are designed to be aerodynamically unstable on purpose, but it takes a fighter pilot to control them along with sophisticated control software to keep them from falling out of the sky. If you want to keep the low attachment of the wings for aesthetics then you will probably need a couple of big reaction wheels to counteract the tendency of the craft to roll onto its back, and only fly with SAS on. Even so it will be a big handful.

My suggestion would be to raise the wings up some so that they attach more in the mid-line of the fuselage. Also, they are so far back right now that I'd imagine it is nosediving quite severely. I understand that canards may ruin the look you are going for, but perhaps experiment with ways to bring the center of lift forward some, closer to the center of mass of the shuttle (when the shuttle is empty).

I'm going to assume that you only use the vector engines when launching attached to your main tank. Otherwise, there is no way they can drive the shuttle in a straight line in their angled configuration.

You have a nice design that looks like a shuttle. Unfortunately, KSP doesn't give you all of the control you need to fly it like one, so you may have to make some little compromises in the looks department.

Edited May 28, 2017 by HvP

[ForScience6686]

My experience with shuttles says they are not good at handling on the ascent.  You have offset mass, thrust, and control surfaces.  My method is to not fight it, get it in a suitable trajectory and let it go.  Correct once in thinner atmosphere or space.

[Cadet_BNSF]

17 hours ago, HvP said:

Hello @Cadet_BNSF

If you take just your shuttle and lay it horizontally in the spaceplane hangar you can check the centers of mass, lift and thrust. You have some tweaking to do with them.

If you turn on the markers for the center of mass (yellow circle) and center of lift (blue circle) you will notice that your center of lift is well below the center of mass, way down near the belly of the craft at the back. This makes your shuttle very unstable along the axis of the center-line. The blue marker is an indication of where the UPWARDS lifting force will be concentrated, and the yellow marker indicates where the DOWNWARDS force of gravity will be concentrated. So imagine pulling down on the yellow marker and at the same time pulling up on the blue marker. Your craft tries to tip over.

It's of course more complicated than that because other surfaces produce drag and lift that the aero model isn't showing you in the hangar. But there are a few rules of thumb to start with. Keep your center of lift behind the center of mass from front to back, but not too far back or else you won't have enough lift in front to keep your nose up. Keep your center of mass in line with, or very slightly above, your center of mass from top to bottom. Remember, your plane is trying to "hang" from its wings. If the wings are too low on the fuselage then its most stable position is upside down -- hanging from the wings. Imagine how hard it would be to fly a hang glider upside down with the wings at the bottom.

The space shuttle had a highly advanced computer control system constantly adjusting the control surfaces every fraction of a second to keep it flying straight. Jet fighters are configured with low slung wings to make them capable of turning and pitching in very tight circles. They are designed to be aerodynamically unstable on purpose, but it takes a fighter pilot to control them along with sophisticated control software to keep them from falling out of the sky. If you want to keep the low attachment of the wings for aesthetics then you will probably need a couple of big reaction wheels to counteract the tendency of the craft to roll onto its back, and only fly with SAS on. Even so it will be a big handful.

My suggestion would be to raise the wings up some so that they attach more in the mid-line of the fuselage. Also, they are so far back right now that I'd imagine it is nosediving quite severely. I understand that canards may ruin the look you are going for, but perhaps experiment with ways to bring the center of lift forward some, closer to the center of mass of the shuttle (when the shuttle is empty).

I'm going to assume that you only use the vector engines when launching attached to your main tank. Otherwise, there is no way they can drive the shuttle in a straight line in their angled configuration.

You have a nice design that looks like a shuttle. Unfortunately, KSP doesn't give you all of the control you need to fly it like one, so you may have to make some little compromises in the looks department.

Thanks for this, it helps reentry, but on ascent, it still yaws to the north or south.

Here is the new craft file, after making some adjustments:https://kerbalx.com/crafts/29119/edit

Edited May 29, 2017 by Cadet_BNSF
added craft file

[HvP]

1 hour ago, Cadet_BNSF said:

Thanks for this, it helps reentry, but on ascent, it still yaws to the north or south.

Not a problem. I did a little experimenting and I think I've found the main trouble with the launch.

Too much control, believe it or not. But more specifically, all of the control surfaces on your shuttle are trying to do everything all at once and they are fighting each other for control. You have pitch, yaw and roll enabled for your vertical and horizontal stabilizers on the shuttle -- but when they engage all that force is being applied off center because there are fins on only on one half of the craft. Simple solution, your vertical tail fin should just have yaw activated. The inner set of horizontal control surfaces should be set only to pitch. And the outer set of horizontal control surfaces should be set only to control roll.

Also, you have so much thrust from the shuttle's vector engines that they can be turned down to about 60% each, just on the shuttle.  I also turned down the gimbal control on those 3 shuttle vector engines to 70%. That dampens down the over-correction a lot (Edit to add, although maybe not since this causes problems higher up when you begin to run out of fuel in the main tank. Some more experimentation is in order on this point.) After making these changes the whole launch still requires a gentle touch. Try not to over-correct. Ease over during your gravity turn, don't make any sudden movements. When rolling, allow SAS to settle down a bit before rolling back the other way. Remember, since you only have control surfaces on one side of your assembly all of your control surfaces will be dragging on one side of the craft a lot more than the other. Wobbling will be unavoidable, but manageable I think with these minor alterations.

I would also add some physical struts between your shuttle and main fuel tank. Autostrut is fine when your tank is full, but by the time the tank empties it is no longer the heaviest part and you lose the autostrut connection once you stage off the solid rocket boosters. Collisions ensue.

One last thing. If you assign an action group to toggle off your shuttle's vector engines it will make it much easier to switch over to your orbital engines. With the vector engines running your shuttle will just spin head-over-heels without the weight of the main fuel tank.

Edited May 29, 2017 by HvP

[HvP]

Ok, one more idea. I don't think you need to turn down the thrust on the shuttle's vector engines after all. That thrust will be needed once the main tank runs low since the off-center weight will start to cause a spin.

Instead, try assigning an action group to the two inner vector engines of the large fuel tank assembly that will toggle them off when you need to, leaving the outboard engine running. Once the outboard fuel tank gets low on fuel there is less weight for those 3 vectors to push and it becomes uncontrollable. Toggling off two of the three engines will reduce thrust on that side of the launch vehicle to 33% and becomes necessary sometime around the time you stage away your solid fuel boosters. The timing of when you toggle them off depends on your launch profile.

It's a lot to manage while you are trying to maintain a good launch trajectory and not collide with your dropped solid boosters, but with a little practice it might be the way to go.

Edited May 29, 2017 by HvP