Issues during Liftoff (non aerodynamic), and Mk1 Capsule COP

[RobFalcon]

While I have no mods installed which should affect flight characteristics, mostly visual mods and utilities, I'm placing my questions here just in case.

First: I've run into a strange issue during liftoff.  At several points of the launch, my rockets will make a sudden and very large course deviation, pretty much starting to roll on all three axes.  With effort, I'm able to get them pointed back in the right direction and continuing along their trajectory.

Normally, I would just chalk this up to poor aerodynamic properties and move my fins, but in addition to mid-launch it's also happened to me when my rocket is passing through 70 km going up, and when lifting off of the Mun.  Most recently, it happened with a pretty standard design which has been proven stable for quite a while.  (Pictures in imgur link.)  For the Mun launch, I think I was well below the low space to high space transition point.

Second: I've also noticed something strange with my center of pressure while using the Mk 1 capsule.  When I make my standard tall rocket design, I have to put on many more fins than usual.  In the past (up to 1.2, at least) I only needed four.  Now, I need about twelve.  (Pictures in imgur link.)  My rockets still flip out, but with the previous issue ongoing I'm not sure if it's purely an aerodynamic concern.

I am not using autostruts with any of the rockets, though given the situations I've been in strutting should not have been an issue.

Any assistance with figuring this out would be greatly appreciated!  Additional files and images can be gathered if needed.

The particulars:
I'm running Version 1.2.1.1604  (Locking there so I can actually finish a playthrough!)

Mods:
Science Here and Now
Better Burn Time
Distant Object Enhancement
Engine Lighting
Stock Visual Enhancements (With EVE and Scatterer)
Kerbin Environmental Institute
Kerbal Joint Reinforcement
Kopernicus
NavyFish Docking Aid
PlanetShine
SCANSat
Science Full Reward
Tac Fuel Balancer
Texture Replacer
Trajectories
Alarm Clock
Transfer Window Planner

If all else fails, I can attempt to update to 1.2.2 and see if that solves the problems.  Not ideal, as I'm trying to maintain a stable setup, but it IS an option.

Link to imgur album showing the aforementioned rockets.

[Harry Rhodan]

18 hours ago, RobFalcon said:

Second: I've also noticed something strange with my center of pressure while using the Mk 1 capsule.  When I make my standard tall rocket design, I have to put on many more fins than usual.  In the past (up to 1.2, at least) I only needed four.  Now, I need about twelve.  (Pictures in imgur link.)  My rockets still flip out, but with the previous issue ongoing I'm not sure if it's purely an aerodynamic concern.

Your design definitely is aerodynamically somewhat questionable. You launch with an extremely high TWR which will make you hit the the air much harder in the lower and denser parts of the atmosphere. And your lower stage is short and heavy while balancing a long and light noodle on top. That'll keep your center of mass low and give the aerodynamic forces a long lever to work against you.

The other design suffers basically from the same problems: low center of mass with a light and wide payload on top. The FL-T100 of the payload weighs about 0.6t full and is probably the heaviest part of the payload while the Swivel on the bottom of your rocket already weighs 1.25t. So with only one tank your CoM will most likely drop even further during ascent while you want it to rise to keep stable.

 

And the blue indicator is not the center of pressure but the center of lift. There's no indicator for CoP in the game to help you with these problems.

[Tex_NL]

1 hour ago, Harry Rhodan said:

And the blue indicator is not the center of pressure but the center of lift. There's no indicator for CoP in the game to help you with these problems.

I was about to say the same before I read through your reply. It is the CoL and not CoP.
In most planes the CoL and CoP are reasonably close together and the difference can usually be ignored. In rockets they can not. The vast majority of rockets consist of large parts that do induce large amounts of drag but not produce lift. The few parts that do produce lift will place the CoL marker far from the CoP and may give the wrong impression.
In the VAB it's best to completely ignore the CoL marker and simply keep mass near the top and drag near the bottom. Large, wide but mostly empty fairings do not fall into that rule of thumb. Instead they just ring the Kraken's dinner bell.

Edited December 11, 2016 by Tex_NL

[RobFalcon]

On CoL vs CoP; understand all.  I was just part of a fluids project involving rockets where CoP was a big thing, so it's kinda stuck in my brain.

I also understand what you mean by the overall configuration.  I took another look at the manned rocket, getting rid of the 2.5 meter section, and was able to control the CoL with a normal amount of fins.  I agree; I made the top stages too long and thin.  I'll call concern #2 solved.

With concern #1: The problem I'm seeing seems independent of any factors pertaining to the rocket.  It's happened at different points during a launch from Kerbin, and when relaunching from the Mun.  The unmanned rocket had one such issue when exiting Kerbin's atmosphere.  The other was a 2.5 meter manned Mun lander (direct ascent style).  In both cases, the rocket suddenly started a three-axis spin, almost like they had hit something but with no damage caused.  I suppose it could be Krakens, but I've never seen that many of them in such a short order without really trying to summon them.

[Shadowmage]

21 hours ago, RobFalcon said:

On CoL vs CoP; understand all.  I was just part of a fluids project involving rockets where CoP was a big thing, so it's kinda stuck in my brain.

I also understand what you mean by the overall configuration.  I took another look at the manned rocket, getting rid of the 2.5 meter section, and was able to control the CoL with a normal amount of fins.  I agree; I made the top stages too long and thin.  I'll call concern #2 solved.

With concern #1: The problem I'm seeing seems independent of any factors pertaining to the rocket.  It's happened at different points during a launch from Kerbin, and when relaunching from the Mun.  The unmanned rocket had one such issue when exiting Kerbin's atmosphere.  The other was a 2.5 meter manned Mun lander (direct ascent style).  In both cases, the rocket suddenly started a three-axis spin, almost like they had hit something but with no damage caused.  I suppose it could be Krakens, but I've never seen that many of them in such a short order without really trying to summon them.

 #1 -- KJR has been known to cause such issues, or did with the initial 1.2 versions.  Try removing it and see if your unexplainable attitude changes continue to occur.

[RobFalcon]

Yup.  That fixed it.

I removed KJR and, once I remembered to turn on the autostruts for the boosters, I followed the same flight path out of the atmosphere with no disruptions.  I'll make another test off the Mun later, but I'm feeling confident that the issues have been resolved.

Harry Rhodan, Tex_NL, Shadowmage; thank you all for the assistance in troubleshooting!