Help with CP & CG ...

Fifinella

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Hi Everyone,

I am new here and asking for your help. We have a Piper L-4 we built with bazookas referenced from some photos we have researched. We would like to demo the firing of the bazookas, while on the ground at airshows. Note this is all okay per the FAA.

The help need is if the rocket we have design, scale wise, will be stable in flight. I have a STEP, STL, IGS and other export files on the model rocket if needed for the calculations I can send. Anyone interested in do this? Your help would be greatly appreciated.
Thank you,
KAP
298893888_603352354638709_4085848814254077015_n.jpg
Pic-1.JPG
 
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dhbarr

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Find the CG of the fully-assembled item ( incl. motor ) by balancing it.

Post whichever files you can to this forum, and/or model it in OpenRocket.

After you're satisfied with the stability, do a regular up-and-down flight ( it -does- have a recovery system, yes? )

At a glance, that looks like it will do backflips across the ground in essentially random directions.
 

Steve Shannon

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It looks to me as though it would be unstable without adding nose weight. Make up one of the “bazooka rockets” with an inert mass object in place of the motor and drop it from someplace high up and perhaps even the plane to see if it falls through the air nose down or if it flutters.
 

Fifinella

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I can send anyone the files to do the stability calculation and let me know what we need to do to make it stable. Just PM me and let me know your email and what file format to send.

Picture show Center of Mass using a Aerotech 24mm EnerJet E30T-4 motor. Rocket uses 6 foot streamer 1.50" wide silk.

Spec's:
Mass properties of Rocket assy-3A
Configuration: Default
Coordinate system: -- default --

Mass = 0.5139 pounds

Volume = 150784.1087 cubic millimeters

Surface area = 312.3003 square inches

Center of mass: ( inches )
X = -0.0133
Y = 9.7798
Z = 0.0200

Principal axes of inertia and principal moments of inertia: ( pounds * square inches )
Taken at the center of mass.
Ix = ( 0.0000, 1.0000, 0.0000) Px = 0.2267
Iy = (-1.0000, 0.0000, -0.0022) Py = 32.0479
Iz = (-0.0022, 0.0000, 1.0000) Pz = 32.0480

Moments of inertia: ( pounds * square inches )
Taken at the center of mass and aligned with the output coordinate system.
Lxx = 32.0479 Lxy = 0.0000 Lxz = 0.0000
Lyx = 0.0000 Lyy = 0.2267 Lyz = 0.0002
Lzx = 0.0000 Lzy = 0.0002 Lzz = 32.0480

Moments of inertia: ( pounds * square inches )
Taken at the output coordinate system.
Ixx = 81.1998 Ixy = -0.0666 Ixz = -0.0001
Iyx = -0.0666 Iyy = 0.2269 Iyz = 0.1009
Izx = -0.0001 Izy = 0.1009 Izz = 81.1997

Thank you,
KAP

Rocket assy-3A.jpg
 
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dhbarr

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Does the measured CG of the fully-assembled article match the presented sim?

Are the fins inside the ring angled or straight ( i.e. do you expect it to spin ? ). And how thick are they? The CG is decently far forward, so the only question remaining is how close to the CP it is. Regardless of what any sim says, you'll want to do a purely vertical test launch.

Do remember that if you're launching them very much off vertical, it will not be compliant with either NAR or TRA Model Rocket safety codes. Propellant and overall mass keeps you generally compliant with FAR 101, so you'll just have to make sure you're not creating a hazard to persons or property.

Realize that if you're launching horizontally the insurance situation is between you, the host field, and any other related entities.
 

Fifinella

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dhbarr ....

The CG is from calculation based in Solidworks which matches the actual rocket when assembled with streamer, wading, motor, paint, etc ready to fly within a 1/16" of SW position.

All the requirements meet the FAA regulations and any demo at an airshow is handled on a case -to-case with them. Mostly a ground firing demo of ~ 40° off ground level as seen in the top posted picture of the plane and bazooka angle.

We are looking just to see before the first test firing if we are close to a stable flight or do we need to bring the CG forward more from the CP? That is the question.

Fins (4) are straight and .04" (1mm) thick as is the surrounding ring.

Good questions here.
KAP

Pic-2.JPG
 

Steve Shannon

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The center of gravity must always be forward of the center of pressure. Usually we require “one caliber” or one body diameter forward at minimum. In this case I would suggest the larger diameter of the “warhead.”
 

Fifinella

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The center of gravity must always be forward of the center of pressure. Usually we require “one caliber” or one body diameter forward at minimum. In this case I would suggest the larger diameter of the “warhead.”
Hi Steve,

Thanks for the info. My question on this forum is if someone can help us determine where the CP is located in relaion to the CG based on the designed model posted here. We hope its far enough back from the CG but we do not know or have the means to find out its location.

The dimension on size, diameter wise is fixed. Length of tube can vary as can the fin length.

Thanks for the comment.
KAP
 

Steve Shannon

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Hi Steve,

Thanks for the info. My question on this forum is if someone can help us determine where the CP is located in relaion to the CG based on the designed model posted here. We hope its far enough back from the CG but we do not know or have the means to find out its location.

The dimension on size, diameter wise is fixed. Length of tube can vary as can the fin length.

Thanks for the comment.
KAP
Doesn’t Solidworks predict CP? I am surprised.
How about this:

Another way to estimate it is to do the cardboard cutout method where you trace the silhouette onto a piece of cardboard and find the CG of the cutout. It’s just the centroid of the shape. You probably can do that virtually as well.
 

Fifinella

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Yes it does. We do not have that add-on, cost $18,614.00!

The CG is not the issue and we have done the cardboard cut out too. It matches the SW cal. We need to see where the CP it on the model.

Thanks,
KAP
 

Steve Shannon

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I understand you have predicted where the CG is of your rocket. I mentioned the CG of the cardboard cutout because that is generally where the CP falls on that shape, especially for ring fin rockets.

Unfortunately the files you have don’t work in either of the two most common simulation software packages used by us. There’s OpenRocket, which is free and RockSim which isn’t. I’m getting ready to host a launch tomorrow but maybe someone can throw something together for you just to find the CP, but looking at your pictures I suspect it’s going to be very close to the CG, unless you have added nose weight. If that 3d printed nosecone is hollow then it’s very lightweight and does little to help.
 

Fifinella

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How well does the spininig-on-a-string work for seeing the stability>

Thanks,
KAP
 

cls

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Yeah, do the swing test.

To my eye though, the ring fin is too long, flow through will stagnate, so no lift, so CP will be forward... Experience shows rings should be about 1 caliber long.

Between adjusting the ring fin length, and adding ballast in the nose, you can probably make it fly.
 

lakeroadster

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@Fifinella sent me the CAD file and I created an Open Rocket simulation.

The simulation shown below is with the geometry as provided. Stability is -3.36 calibers

Open Rocket does not calculate the CP correction for a ring fin. There is a ring fin hack, I'll look at that next.

Fifinella Bazooka 2022-10-14 Open Rocket Simulation Rev 00.jpg
 
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lakeroadster

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Change the ballast to 13 ounces in the nose and you have a stability of 1.05 caliber.

Thinking about where all the weight would be on a real bazooka rocket... this seems logical.

Be sure to swing test the rocket though, just to ensure stability. It don't mean a thing, if you ain't done that swing. :headspinning:

Technical Report TR-1, “Rocket Stability”


Fifinella Bazooka 2022-10-14 Open Rocket Simulation Rev Ring Fin Hack and 13 ounces of nose we...jpg
 
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lakeroadster

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An E motor would not lift 13oz + rocket..
Sure it will. An E30-4 yields a 29.4 mph off the rod speed, and that is adequate. Especially with those long silo tubes shown in the first post. The simulation assumes a 39" rod length.

But if it's concerning, a F44-4 will also fit, or upgrade to a 29mm motor but then you'll have to add even more nose weight to maintain stability.


Fifinella Bazooka 2022-10-14 Open Rocket Simulation Rev 02 Ring Fin Hack and 13 ounces of nose...jpg
 
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Steve Shannon

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Nope.. straight up. I was just looking for stability. But your point is valid. In reality, if they are launched horizontally... they'll be skipping along the ground.
Yeah, I was thinking they might launch slightly inclined to avoid the land shark effect, but they’re also not fighting gravity as much.
One thing I do know is that an ejection charge on the ground is a great way to start a fire. 😇
 

Fifinella

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Thanks to you all for the support.

One thing I see in the posted simulation is the MASS WITH MOTOR (total weight) at 18.6 oz. The actual weight as is with no added nose weight and ready to fly is: 5.0 oz. so I assume the 18.6 oz is with the added 13 oz nose weight.

Launch angle on ground firing is ~40° from ground level.

Here is a challenge for all: How would you design this rocket keeping to the overall scale look to fit a tube I.D. of 2.36" and be stable ?


306733744_470715145031009_7682066534036974023_n.jpg 310153404_2625261537642990_7683022089007857956_n.jpg 1390106921_27.jpg
 
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Fifinella

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Yeah, I was thinking they might launch slightly inclined to avoid the land shark effect, but they’re also not fighting gravity as much.
One thing I do know is that an ejection charge on the ground is a great way to start a fire. 😇

One of the airshows we attend. I do not think a rocket motor starting a fire will be an issue .... Mid Atlantic WW-II Weekend.

MAAM-Reading-Air-Show-Flame-Thrower-1024x683.jpg
 

lakeroadster

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Here is the updated model, using ABS plastic for the 3D printed parts.
  1. Launch angle set to 50 degrees from vertical, and using a streamer. (Note that NAR Safety Rules specify a maximum of 30 degrees from vertical)
  2. Launch tube lengths updated to 54-1/2" long.
  3. Rocket now has a 16 ounce nose weight to make it stable, based on these changes.
  4. Ground hit speeds of 60 mph are problematic.

Fifinella Bazooka 2022-10-15 Open Rocket Simulation Rev 02 Ring Fin Hack Streamer.jpg


Thanks to you all for the support.

One thing I see in the posted simulation is the MASS WITH MOTOR (total weight) at 18.6 oz. The actual weight as is with no added nose weight and ready to fly is: 5.0 oz. so I assume the 18.6 oz is with the added 13 oz nose weight.

Launch angle on ground firing is ~40° from ground level.

Here is a challenge for all: How would you design this rocket keeping to the overall scale look to fit a tube I.D. of 2.36" and be stable ?
Above model is 5 ounces without the 16 ounce nose weight.​
How would you design this rocket keeping to the overall scale look to fit a tube I.D. of 2.36" and be stable ?
The rocket simulates as stable with the 16 ounces of nose weight.​
How would I design it?
To make it a recoverable rocket, don't use a streamer, use a 24 inch diameter rip stop nylon parachute as shown in the simulation below. That reduces the ground hit speed to under 15 mph. EDIT: See Post #28 by @Steve Shannon

Fifinella Bazooka 2022-10-15 Open Rocket Simulation Rev 02 Ring Fin Hack Parachute.jpg Fifinella Bazooka 2022-10-15 Open Rocket E30-4 Plot Parachute.jpg
 
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lakeroadster

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I've spent some time thinking about what to use as the recovery arrangement for this rocket. Watching NHRA drag racing and seeing them using parachutes that deploy at 300 mph, I'm confident this rocket can be recovered safely.

I think I would try what I call an "over the top" parachute that utilizes 300# braided kevlar for the parachute lines and rip stop nylon for the chute material. The lines go up and over the rip stop nylon parachute and thus the chute will not be ripped away from the lines when the chute blossoms at high speed.

I did something similar on the 2nd flight of my F-79. On the first flight, using regular parachute lines that attach to the chute, the chute ripped away from the rocket and the rocket came in ballistic. The 2nd chute with the over the top lines didn't shred.

In regard to attaching the chute to the rocket, I would create a bulkhead and attach an eyebolt to that bulkhead in the 3D printed nose cone section, attach a swivel to the eyebolt and then attach the parachute to that swivel, again using 300# braided Kevlar. For the body tube / fin can sub assembly I would secure the recovery device using 300# braided Kevlar attached to the motor mount, and then attach an elastic shock chord to the Kevlar, then attaches that elastic shock chord to the parachute chords.


Kevlar Shock Chord - Replaceable.jpg

Here's some photo's of the over the top chute I made using 100# Kevlar and a plastic chute.

001.JPG 002.JPG 003.JPG
 
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