UNSTABLE ROCKET SUGGESTIONS NEEDED

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TOM C

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IMG_0002.JPGBLACK AND YELLOW 14.jpg View attachment BLACK AND YELLOW 14.rkt

This rocket is unstable when launched on a M3700 it lifted off the pad...turned on its side at about 500' and deployed the chutes. Rocksim program and drawing are attached.

Are the transitions in body tube size and the nosecone size a factor?

Would increasing the fin size be a fix to make this fly stable?

Appreciate your input...
 
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A little more information would helps us to help you, to determine what the problem is?
Rocket specs, possible Rocksim or open Rocket file?

JD
 
Fins. More importantly, speed off the rail. What length of rail did you use?-the longer the better for this one.
 
View attachment 181651 View attachment 181652

This rocket is unstable when launched on a M3700 it lifted off the pad...turned on its side at about 500' and deployed the chutes. Rocksim program and drawing are attached.

Are the transitions in body tube size and the nosecone size a factor?

Would increasing the fin size be a fix to make this fly stable?

Appreciate your input...

The overhanging nosecone will cause a lot of base drag at the front of the rocket, moving the center of pressure far forward. That is not something that can be simulated without CFD.
 
How much did it actually weigh on the pad?

I would believe that the fins are too small.
Given all the transitions and different body tube diameters.

JD
 
How windy was it and what was the rail length? I opened it up in Open Rocket and did a simulation with a 75mm Cesaroni M3700 (only M3700 in OR's database). It is important to note that OR ignores the tube fins, as it does not support them. With a 6' rod, it shows it leaving the pad at 53 FPS. with a 12' it shows it going 73 FPS and at 18" it says 91 FPS I don't think OR takes rail button drag in to account. I was always told 45 ft/sec is a minimum. It shows a margin of 1.36 with angle of attack at 0 degrees. At 4 degrees it is 1.04 and at 17 degrees it is at .508.

At 17.75' tall, she is quite large. Maybe that gives the wind a lot of leverage as the bottom button was still on the rail but the top one(s) are free of the rail, making the bottom button a hinge point. What is the relationship between the turn it took, the side of the rail that the rocket was slid into, and the direction of the wind? Using directions can help (Ex. Rocket attached to North side of rail, Wind from the West at xx MPH, rocket turned to the East).
 
The overhanging nosecone will cause a lot of base drag at the front of the rocket, moving the center of pressure far forward. That is not something that can be simulated without CFD.

Didn't think of that. Good point CarVac.
 
Two additional thoughts to what's already been posted.

The aft transition is short and may result in high turbulence at the base of the fins, reducing their effectiveness.
The tubes also mask the base of the fins, further reducing their effectiveness.
 
Two additional thoughts to what's already been posted.

The aft transition is short and may result in high turbulence at the base of the fins, reducing their effectiveness.
The tubes also mask the base of the fins, further reducing their effectiveness.

Basically, it is a perfect storm for the standard stability calculations. Many of the assumptions are very badly violated.
 
IMG_0002.JPGThe length of unistrut the rocket used was 16'
Pad weight 123 pounds
wind was moderate-mild about 5 mph..
this is rocket pictured on rail

appreciate your input
Tom
 
View attachment 181655The length of unistrut the rocket used was 16'
Pad weight 123 pounds
wind was moderate-mild about 5 mph..
this is rocket pictured on rail

appreciate your input
Tom

Cool looking rocket.

It looks like it was about 3' up the unistrut to begin with. The NC transition isn't flat like it shows in the .rkt file. It is definitely not an easy thing to determine. You are obviously experienced. It would be good to figure out why a rocket that looked like it should be stable, wasn't.
 
Cool looking rocket.

It looks like it was about 3' up the unistrut to begin with. The NC transition isn't flat like it shows in the .rkt file. It is definitely not an easy thing to determine. You are obviously experienced. It would be good to figure out why a rocket that looked like it should be stable, wasn't.

I noticed the same thing about the nose cone transition. Have you experimentally determined the rocket weight and the c.g. ? I have found that for large rocket builds the rocket may very well weigh more than what RocSim derives from calculating components. If you use the added mass option in RocSim, then you need to add the mass at a location so that the c.g. is the same as the real c.g. Finding the c.g. location for a large HPR model is difficult. Once I had an odd-looking E or F powered rocket with large clear plastic fins in the base. RocSim said it was stable, but in practice it was clearly not. On another issue tube fins can be difficult to model in RocSim. I remember this past year doing this for tube fins in RocSim. One method appeared to work, but another option did not. Your NC and center body are both tending to move the c.p. forward, which is not a good thing. Larger fins will help, but will they help enough. You could build a smaller model to find out if you can master the situation.
 
I weighed each rocket component separately and know the total weight is accurate. I could not demonstrate the CG and have relied on Rocksim for this...long heavy rocket and a task to physically find the balance point.
The unistrut rail pictured is 20 and the rocket is on a standoff about 48" in length.
The rail angle was approx straight up.
The tube fins are pods...one that contains the drouge chute which deploys in the rear direction at apogee so I would like to keep them.

Making the fins larger on rocksim increases my margin of stability.
It would be very easy to enlarge the fins....bolt on plywood fins to the existing plywood through the wall fins.

Contine to thank you for your opinions.
Tom C.
MARS
 
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How much does that nose cone weigh?
How much of the uni-strut was above the rail buttons....
or in other words how much travel did the rocket have to before it left the rail?

With the picture for more accurate visual; I strongly believe that there isn't enough fin surface to stabilize your rocket.

Added with the fact that a massive shift in CG at motor burn out.


JD
 
The transitions can cause issues with the sims as they complicate things a bunch. Basically, Barrowman equations have some issues with the transitions.

Which calculation did you use in Rocksim- Barrowman or the Rocksim method?

In your sims is you account for all the epoxy, hardware, and associated stuff?


Mark Koelsch
Sent from my iPhone using Rocketry Forum
 
Several issues.

  • The as built rocket does not match your simulation model.
  • Not relevant because hobby sims will not properly calculate the stability of this rocket.
  • You can measure the CG.
    • Get a 2x12 and put it on end.
    • Lay the pad prepared rocket across the 2x12.
    • Put the rocket at its approximate CG across the beam.
    • As you move it back and forth the rocket will tip to the aft or the nc.
    • Split the difference and you have your CG.
  • Increase the fin span to twice the diameter of the largest cylinder on the airframe to get it out into clean air.
  • I believe that will make it stable.

Also despite your 16' rail length, it appears your guidance length is ~8' due to the high mounting position of your launch lugs. This however is not the cause of the instability.

Bob
 
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My meager 2 cents is that in my experience (which is why its only worth 2 cents :)) is that you don't get into trouble when you have transitions up high. However, when you have transitions from larger to smaller close to the fins, you get funny effects. These cause airflow issues over the fins that reduce their effectiveness.

I had a short rocket rocket with an 8" upper body and a 3" lower. The rocket simmed fine and flew fine on H123's. On an H242, it went up and hung a very abrupt right turn then flew off stabily into the field. The only apparent difference was acceleration and velocity and the consensus was the fins were less effective with more turbulence.
 
DSC_5534.JPGChanging the fin size ended in a perfectly stable flight 2 weeks ago at MARS in Geneseo N.Y. sonotube fiberglassed body tubes 1/2"plywood fins nose cone built by PYTHON ROCKETRY www.pythonrocketry.com

This is the rocksim file for the larger fin rocket.View attachment BLACK AND YELLOW.rkt
The flight was on a CTI M4770 and will fly again at MARS in October.

Appreciate the advice.

Tom C.
MARS...Geneseo NY
 

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View attachment 184707Changing the fin size ended in a perfectly stable flight 2 weeks ago at MARS in Geneseo N.Y.

This is the rocksim file for the larger fin rocket.View attachment 184708
The flight was on a CTI M4770 and will fly again at MARS in October.

Appreciate the advice.

Tom C.
MARS...Geneseo NY
Yeah, I just saw this and was going to say that turbulent flow at the last transition and around those pods was greatly reducing fin effectiveness, something that I doubt anything other than computational fluid dynamics could properly simulate.
 
Yeah, I just saw this and was going to say that turbulent flow at the last transition and around those pods was greatly reducing fin effectiveness, something that I doubt anything other than computational fluid dynamics could properly simulate.

Exactly. Barrowman does not deal with transitions all that well. If the rearmost transition were much longer the airflow would have been smoother, but is short and will cause turbulence.


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