There seems to be a bug with center of lift

[Kerbart]

11 minutes ago, thereaverofdarkness2 said:

Being able to fix it by changing the design doesn't mean that the current design isn't bugged. If it should work, and it doesn't work, then it's bugged.

I'm sorry people can't see what I'm talking about. I'm trying to make it as clear as I possibly can. If drag is what's making it unstable, it would be unstable when flipped around. However it is not unstable when flipped around.

I'll try using the aerodynamic indicator and see what I can figure out.

No, if drag is making it unstable, it means COM and COP are not where you claim they are. 

Spoiler

You keep saying that the COM is behind the COP (which is not the center of lift) yet we don't know where the center of pressure is. It's hard to determine if this is a bug, or that there are design issues at play; all we have to go by is a picture (without any indicators) of a topsy-turvy design with a lot of surface area at the top and a lot of heavy parts at the bottom, making me willing to bet you dollars to donuts  that this is a vehicle where a low placed COM (heavy SRB's at the bottom, lots of surface at the top) simply wants to follow it's natural tendency -- COM ahead of COP.

[Plusck]

1 hour ago, Plusck said:

I just tried the second one. With 4x symmetry for the fins on the last stage, it was fine. With 3x symmetry, it wasn't quite enough to cope with full thrust for the final stage, when the heating effects were at their strongest.

However, I certainly didn't find it resolutely facing engine-first into the airflow. The fins are weak, but not that weak.

I edited my last post to include this bit but I may have done it too late to be seen.

After several efforts, I only managed to get it to flip convincingly at about 16km altitude, with the parachutes deliberately pullled a touch outwards so that they emerged from the service bay at the corners, with ablator reduced to a low amount, and with three fins.

Lower down the airflow over the fins is significant enough to leave the rocket aerodynamically balanced. With four fins it is much more stable. Likewise with more ablator. Not sure really whether the parachutes made a difference. However I didn't quite make orbit with it. And this was all with SAS off and virtually no control input.

[thereaverofdarkness2]

On 5/10/2016 at 11:38 AM, Plusck said:

I edited my last post to include this bit but I may have done it too late to be seen.

After several efforts, I only managed to get it to flip convincingly at about 16km altitude, with the parachutes deliberately pullled a touch outwards so that they emerged from the service bay at the corners, with ablator reduced to a low amount, and with three fins.

Lower down the airflow over the fins is significant enough to leave the rocket aerodynamically balanced. With four fins it is much more stable. Likewise with more ablator. Not sure really whether the parachutes made a difference. However I didn't quite make orbit with it. And this was all with SAS off and virtually no control input.

Did you make a gravity turn at around 10km? I find the craft remains pretty stable even into the gravity turn if I waste a bit of fuel by not activating the SRBs right away, but it seems to flip right about when I ditch the last SRB and try to make the gravity turn significant. It also seems dependent on intrinsic velocity, regardless of how thick the air is. I can keep it stable by continuing through high atmosphere at low atmosphere velocities, which will never have a chance at getting into orbit but it does remain stable. On the other hand, it does remain stable pointing its nose into the ground after it runs out of fuel and plummets to the ground--it is not moving fast enough to trigger the instability at such low altitude.

Maybe there's something about center of pressure I need to learn. I'd never even heard that term before I made this post. Center of drag doesn't seem to fully explain my issue though it seems to make a difference. It is possible that these forces are unbalancing the rocket slightly, and that that is all they need to throw off the tiny fins. I'm sure if I modeled a rocket like this in real life, it would indeed fly straight--but it does indeed fly straight in KSP at low velocities.

 

 

In the end most of my frustration is with not only how few parts there are to work with at the beginning of the game, but also how poorly matched they are. The liquid fuel engines are absurdly heavy for their size. I didn't find a good comparison of a similar-size engine but after scaling NASA's J-2X down to 1.25x2 meters, it would have a mass of 0.179 tonnes, while the first two engines in KSP are over five times as massive. The command pod is 800kg, I can't imagine the Kerbal inside plus his suit being over 100kg--yet the command pod isn't quite absurdly heavy enough to make up for the engines. You have to throw on more command pods at the top to balance out the weight, but that messes with the aerodynamic stability by creating drag, which is the last thing you need on a rocket with nearly 50% dry mass combined with engines that have lower than real-world specific impulse values.

I'm fine with inefficient parts, but I don't understand why the engines have to be so much heavier than the command pods. In a real rocket, the command pod is probably the heavier piece.

[Plusck]

43 minutes ago, thereaverofdarkness2 said:

Did you make a gravity turn at around 10km? I find the craft remains pretty stable even into the gravity turn if I waste a bit of fuel by not activating the SRBs right away, but it seems to flip right about when I ditch the last SRB and try to make the gravity turn significant. It also seems dependent on intrinsic velocity, regardless of how thick the air is. I can keep it stable by continuing through high atmosphere at low atmosphere velocities, which will never have a chance at getting into orbit but it does remain stable. On the other hand, it does remain stable pointing its nose into the ground after it runs out of fuel and plummets to the ground--it is not moving fast enough to trigger the instability at such low altitude.

Maybe there's something about center of pressure I need to learn. I'd never even heard that term before I made this post. Center of drag doesn't seem to fully explain my issue though it seems to make a difference. It is possible that these forces are unbalancing the rocket slightly, and that that is all they need to throw off the tiny fins. I'm sure if I modeled a rocket like this in real life, it would indeed fly straight--but it does indeed fly straight in KSP at low velocities.

 

 

In the end most of my frustration is with not only how few parts there are to work with at the beginning of the game, but also how poorly matched they are. The liquid fuel engines are absurdly heavy for their size. I didn't find a good comparison of a similar-size engine but after scaling NASA's J-2X down to 1.25x2 meters, it would have a mass of 0.179 tonnes, while the first two engines in KSP are over five times as massive. The command pod is 800kg, I can't imagine the Kerbal inside plus his suit being over 100kg--yet the command pod isn't quite absurdly heavy enough to make up for the engines. You have to throw on more command pods at the top to balance out the weight, but that messes with the aerodynamic stability by creating drag, which is the last thing you need on a rocket with nearly 50% dry mass combined with engines that have lower than real-world specific impulse values.

I'm fine with inefficient parts, but I don't understand why the engines have to be so much heavier than the command pods. In a real rocket, the command pod is probably the heavier piece.

Gravity turn at 10km is the "old atmosphere" approach and consistently fails in the more recent versions of the game. In 1.0.5 and 1.1.x, gravity turn should be very gentle and virtually immediate off the launchpad. A general rule of thumb tends to be to aim at 10° angle at 100m/s, which should drop all on its own down to 45° at 8-11km and a velocity in the 300-400 m/s range.

For your second rocket, turn off SAS, launch, turn a few degrees right and let it follow prograde on its own. Since you can't use the throttle to make sure it drops to 45° at 8-10km, this will take a few tries to get right. On the LF stage, full throttle gets it too hot a touch too quickly, so about 80% throttle should work. at about 15km, turn SAS on again and nudge it towards the centre of prograde constantly to avoid flipping.

If you try to force a gravity turn with your rockets, they certainly will flip, without a doubt.

Also, if you take off the middle set of fins, and replace the LF stage fins with 4x symmetry, it reduces the chances of flipping and might let you just leave SAS off while you watch the map. As I said though, I never made orbit, and was consistently arriving at 70km with a velocity of about 1800m/s - so about 300 m/s shy of orbit.

As for your criticism of the part masses, I really don't know what to say. I'm not an expert on the innards of rocket engines, but I seriously doubt a 1.25x2m rocket engine, with compression chambers and all the rest, would weigh only 200kg.

[thereaverofdarkness2]

I find there's little use in trying to do the gravity turn before about 3-5km. I do a gentle gravity turn pretty much starting there with most rockets, but this one refuses to turn at all until the final SRB is ditched. I wasn't suggesting making a sharp turn. I asked because if you head straight up, you cut through the atmosphere a lot faster and by the time the rocket is aerodynamically unstable, it just might be so far up that the air doesn't really matter anymore.

I could try playing around with some 4x symmetry on fins. Another idea I had was to re-orient them to have them balanced into the gravity turn.

200kg is a lot. 1.25x2m is about the size of a human. It's a thin metal shell for the bell piece, and the rest is pipes and an attachment surface. If it has a gimbal function, that may require a bit more engineering and mass. 180kg is the mass of two tall men; the engine is the size of one man and it is mostly air. 1250kg for an engine that size without gimbal calls into question what it is made out of. Perhaps a cast-iron engine would be that heavy, but even a cheap steel one would be much lighter.