Stability Issues on a Mac Performance Scorpion

[Ddogoo]

I recently got my level 1 certification on a Aerotech H135. The flight was perfectly straight and had no issues.

Shortly after, I launched the same rocket (Mac Performance 3” Scorpion) on a H195. (Video attached) Predicted altitude was 1300 feet based on RockSim However, the rocket took a sharp turn off the pad and continued at a 45 degree angle. Luckily, the parachute deployed at the last second, and the rocket is still fly able.

I purchased a Cesaroni I180 too and I plan on flying it next weekend. I am not sure if it is safe to fly it on this motor without stability issues.

The stability margin of the rocket with the H195 was 2.29 and the margin with the I180 is 1.92.

There are a few things that may have caused this instability to happen.

1. The thrust of the H195 was much greater than the H135 + motor issues?

2. The coupler was not fully snug and seated into the airframe (picture attached). Due to the limited packing space and the large 48 inch parachute.

These are just guesses and I have really never had anything like this happen before. I am not sure what caused this and if I should make changes. Also, I am not sure if it is safe to fly in the I180. If you could give me your thoughts/suggestions that would be great!

Thank you
View attachment IMG_4054.MOV

 

[T-Rex]

This video implies that there was a good amount of wind, and it seemed to be gusty as well. The model could have just weather cocked.
As for the rest of your post, you say you ran the flight through Rocksim. Did you adjust the delay to match the simulation?
You say the sim says the I180 gives a stability margin of 1.92. You are a L1 flyer, you tell me, is that stable.
Learn to fold your parachute a little tighter to fit the tube allowed (or, possibly get a thinner chute like a Top Flight thin mil) (I will also add that I run OpenRocket, not Rocksim. I am assuming that the stability is in calibers, not inches or millimeters or some other unit)
If the coupler needs tightened in the tube, masking tape does a good job.

ETA: I am not trying to be rude, just pushing you to answer your own questions.

 

[Conway Stevens]

I recently got my level 1 certification on a Aerotech H135. The flight was perfectly straight and had no issues.

Shortly after, I launched the same rocket (Mac Performance 3” Scorpion) on a H195. (Video attached) Predicted altitude was 1300 feet based on RockSim However, the rocket took a sharp turn off the pad and continued at a 45 degree angle. Luckily, the parachute deployed at the last second, and the rocket is still fly able.

I purchased a Cesaroni I180 too and I plan on flying it next weekend. I am not sure if it is safe to fly it on this motor without stability issues.

The stability margin of the rocket with the H195 was 2.29 and the margin with the I180 is 1.92.

There are a few things that may have caused this instability to happen.

1. The thrust of the H195 was much greater than the H135 + motor issues?

2. The coupler was not fully snug and seated into the airframe (picture attached). Due to the limited packing space and the large 48 inch parachute.

These are just guesses and I have really never had anything like this happen before. I am not sure what caused this and if I should make changes. Also, I am not sure if it is safe to fly in the I180. If you could give me your thoughts/suggestions that would be great!

Thank you
View attachment 572297

I would ask a couple questions. You are using Rocksim. Your stability numbers are your CG to CP ratio in calibers that you listed i believe. How did you establish CG? Did you establish the CG as the rocket was built and loaded like it would fly without the motor?

A motor making more thrust will be more stable. A motor with less thrust has the ability to be less stable under conditions. How much does the rocket Weigh on the pad fully loaded with motor? What was the thrust ratio with that given weight. A ratio of 4:1 should be a minimum. In winds as long as not over safe levels for flying should be higher. Stability margins in calibers can't be establish unless you have a real measured physical CG not calculated.

 

[kuririn]

What is that black spot in the plume just before it started veering badly?
Motor anomaly?

 

[spigalau]

Is your main parachute in the payload or booster section ?

What size motor mount are you running ? The 2 x H motors are 29mm, the I180 is 38mm.

 

[Ddogoo]

This video implies that there was a good amount of wind, and it seemed to be gusty as well. The model could have just weather cocked.
As for the rest of your post, you say you ran the flight through Rocksim. Did you adjust the delay to match the simulation?
You say the sim says the I180 gives a stability margin of 1.92. You are a L1 flyer, you tell me, is that stable.
Learn to fold your parachute a little tighter to fit the tube allowed (or, possibly get a thinner chute like a Top Flight thin mil) (I will also add that I run OpenRocket, not Rocksim. I am assuming that the stability is in calibers, not inches or millimeters or some other unit)
If the coupler needs tightened in the tube, masking tape does a good job.

ETA: I am not trying to be rude, just pushing you to answer your own questions.

The wind speed the day of the launch was around 10 mph gusts of 15 mph. The simulations were updated with the flight day winds. The optimal delay according to RockSim was 7 seconds, I went with 8. And RockSim simulated a straight flight. I did measure the cg on the range (with the motor) and it was input into RockSim.

Maybe that the high caliber of stability caused the rocket to be more subject to the gust of wind that hit right before launch? I’ve never done a large amount of in depth research into stability though.

The rocket with the I180 should be pretty stable (I think).

The coupler was definitely tight into the airframe but it wasn’t fully flush. I will probably purchase a thin mill parachute or drill a shear pin hole and do some ground testing.

Thank you

 

[Ddogoo]

Is your main parachute in the payload or booster section ?

What size motor mount are you running ? The 2 x H motors are 29mm, the I180 is 38mm.

Yes, I flew two 29mm dms motors and. I am planning on flying a Cesaroni 38mm I180.

The main parachute was in the booster section and was using a JLCR instead of DD.

 

[Ddogoo]

View attachment 572391

What is that black spot in the plume just before it started veering badly?
Motor anomaly?

That’s what I originally though

 

[Ddogoo]

I would ask a couple questions. You are using Rocksim. Your stability numbers are your CG to CP ratio in calibers that you listed i believe. How did you establish CG? Did you establish the CG as the rocket was built and loaded like it would fly without the motor?

A motor making more thrust will be more stable. A motor with less thrust has the ability to be less stable under conditions. How much does the rocket Weigh on the pad fully loaded with motor? What was the thrust ratio with that given weight. A ratio of 4:1 should be a minimum. In winds as long as not over safe levels for flying should be higher. Stability margins in calibers can't be establish unless you have a real measured physical CG not calculated.

The rocket weighed in at 5 pounds with the motor. The thrust to weight ratio was 9:1. I did measure the cg and input it into RockSim. I measured the cg with and without the motor.

 

[SmurfTacoSupreme]

More data will help. Measured CG? Weight? What was ejection delay set to?

Weather cocking into the wind is likely and common.

 

[SmurfTacoSupreme]

The rocket weighed in at 5 pounds with the motor. The thrust to weight ratio was 9:1. I did measure the cg and input it into RockSim. I measured the cg with and without the motor.

What is your CG? Also…which nose cone do you have?

 

[Ddogoo]

More data will help. Measured CG? Weight? What was ejection delay set to?

Weather cocking into the wind is likely and common.

Measured cg for the H195 flight was 37 inches from the tip of the nosecone. It weighed 5 pounds with the motor. The ejection delay was set to 8 seconds. Rocksim told me the optimum delay was 7 seconds (assuming the rocket went straight up).

I am using the standard Mac performance nosecone.

 

[Conway Stevens]

View attachment 572391

What is that black spot in the plume just before it started veering badly?
Motor anomaly?

Looks like what happens when casting tube is ejected out the nozzle. But not anything that should have made a difference and looks to have been right at motor burn out.

 

[Ddogoo]

Looks like what happens when casting tube is ejected out the nozzle. But not anything that should have made a difference and looks to have been right at motor burn out.

There was no physical damage that I could see in the exterior of the case so it probably isn’t that.

 

[Handeman]

Actually, no external damage would be another indication that it probably was a casting tube being ejected.

 

[Ddogoo]

Actually, no external damage would be another indication that it probably was a casting tube being ejected.

Interesting, thanks for your help.

 

[BSNW]

I have the same rocket. 38mm mount and with dual deploy. As built, this kit is well over-stable. I have had flights like you have described because it was under powered. ALSO make sure the travel (distance of your top button to the top of the rail) is long enough. I see in the video it leans into the wind the millisecond the motor burns out. IMHO, This is a rocket not meant for small H motors with 1 second burns.

On breezy days, it needs high thrust motors. A margin of 2.29 would give you this arching over you see as the rocket is "nose heavy" and very over-stable. This rocket needs an 4 grain 38's like the I285 or I211 on not so windy days. If you are leaning the rail as well, this would give you problems. Most folks don't consider how they lean the rocket in regards to how the rocket rests on the rail. I always "hang the rocket" with buttons facing the wind.

However, I ALWAYS fly mine on a straight up rail with an I285, I357 or J350. It sure goes straight then I tell ya'

p.s. My "Hot Rod" 4" Scorpion (photo on Mac website) also "leand in" if I didn't put a proper motor in it. Flew straight as a laser on 4 grain J and K motors. These rockets are meant to "go like hell".

Peace
Andrew

 

[Ddogoo]

Thank you for the advice. I have some larger motors and cases that I am planning on flying.

I thought it would be a better idea to work my way up to them, but might as well just go for it.

Thanks for the tip with leaning the rocket also.

-Dylan

 

[jFlds]

Looked like a little weather cocking to me from first glimpse.

Big thing to remember always that the upper winds don't match the surface winds. The region we mainly fly there are a lot of changing winds on the surface and different upper wind patterns throughout the day. I typically overcompensate for the cocking a few degrees off the rail anyway. Most of my fleet are way over stable as built on purpose. Some I won't fly in higher winds due to it. I also have some ballast weights to use in the boosters to even them out and cut down on altitude.

Stability and what you are comfortable with is something you learn and keep learning as it changes with the field conditions.

And by the way.... MAC kits and that canvas do better with bigger motors! The biggest thing you can stuff in there they can take it!

Good luck with your future flights.