Pyramid stability =advanced=

[DPatell]

Tomorrow I plan to fly a small (12" on each side) pyramid on a 29-120 Purple G motor. Should equate to about a G100 or so. This pyramid has flown many times on E's and F's, mainly the 24-40 F24W and F20W SU. The problem here is that the rocket was designed for the F20, so the 6" motor casing for the G motor sticks out a little bit....Just a little bit

So here it is. Ask Chuck, it flies great on F20's. Think that it will be okay with the motor hanging out so far?

First, this is a picture of the pyramid, an F20 and the G motor, just to get the relative size of everything.

 

[DPatell]

Secondly, here is a picture of the back of the "rocket." I'm not worried about it holding together...just if it will be stable or not.

This is how far the motor sticks out...


so should I do it or not?

 

[rstaff3]

I've always believed the CP of a pyramid should be 1/3 of its height above the base, and the CG can be tested against this. After all the discsussion on aerodynamics, I now question if that is true. I mean I still believe that that is a safe relationship, but am not sure if the CG can move behind that point.

I have been wondering the same thing about by golf-hat rocket the Sweat Stained Clifton Tracking Station Hat of Death. It was designed for a G35/38. I didn't even have a rule of thumb for that one, which is what the 'C' rack is for I'd now like to use the G64 and G79. In the spring I may have to test this.

One good think about draggy designs...they aren't going to far!

 

[DPatell]

I'm not exactly sure how to go about measuring the CG of this.

And yes, I am going to do it, basically this is a wager to see who knows what they're talking about

I say it works and goes FAST.

 

[lalligood]

Originally posted by DPatell
Secondly, here is a picture of the back of the "rocket." I'm not worried about it holding together...just if it will be stable or not.

This is how far the motor sticks out...


so should I do it or not?

If that is how the motor will be when you launch it, I'll cast my vote for "DON'T do it". Once upon a time I built a much smaller pyramid designed for D12s. The motor was left sticking out maybe 1/2" out the back. Went to a local schoolyard by myself and with no one else around--which I'm really glad I did because in 3 flights it never got more than 25 feet up and pretty much tumbled like a giant 4-sided D&D die in the air (& sometimes on the ground) the entire time. I considered the project a failure & retired it.

I would have to say that the guideline about the CP being 2/3 from the forward tip is probably right based on my experience.

HTH,

 

[Chuck Rudy]

My guess......after looking at Ed Miller's lampshade, I'm guessing the extra motor length should go out the top. The bottom should be flush. Add to that the larger pyramids at LDR$ had the end of the motor well inside the pyramid. But it's just a guess.

Chuck

 

[Stymye]

as with any cone/pyramid ,,, If the motor hangs out the back ... use proper noseweight... to get the 2/3 cg correct..

my 29mm applewhite cone flys great even with an Ellis G hanging way out the back, it has plenty of noseweight to counteract it

 

[Jerry Irvine]

Pyramids are stable?

Merry Jerry

 

[DPatell]

Sure they are...are you?

 

[teflonrocketry1]

According to the attached RockSim version 7 simulation file, the flight should be stable with a 2 inch engine overhang on a G100 motor! I am just begining to understand the aerodynamics of these types of designs. The pyramid itself generates CP 2/3 from its base, but since this is a "short fat design" you have to take into account the base drag or base vortex contribution to the CP. I predict a CP of about 24 inches from the tip of the nose if this phenmenon is taken into account (assuming a 12 inch high pyramid). If you send me more details of the construction I can post a better simulation file. The simulation predicts the G100 motor doesn't gain you much performance over the F20W only about 20 feet in altitude (about 370 feet AGL). Better to go low and slow, a G25 would get you to about 50 feet higher. Use this information with caution since this simulation of a pyramid rocket is purely theoretical and has not been proven by actual data.

Bruce S. Levison, NAR #69055

 

[FredA]

Bruce,

Can you tell us more about your rocsim model.....
What is the justification for the rear (fake) transition?
How did you know how to place and size it?
Why does the model fail for H motors?
Thanks,
FredA

 

[rocket877]

Originally posted by rstaff3
I've always believed the CP of a pyramid should be 1/3 of its height above the base, and the CG can be tested against this. After all the discsussion on aerodynamics, I now question if that is true. I mean I still believe that that is a safe relationship, but am not sure if the CG can move behind that point.

One good think about draggy designs...they aren't going to far!

I concur with Dick. I have flown many variations of a 4 sided (4 sides and a square base) pyramid and a good rule of thumb is the motor should be not stick out below the bottom edge of the sides. This is assuming no nose weight.

If the pyramid is built from light weight materials such as cardstock, foamboard or corrigated cardboard, the CP of a pyramid is about 2/3 from the nose and the CG is behind that because of the weight of the base. Whenever possible the base should be recessed into the pyramid some as you did in you design. The motor's CG is about at it's middle, so to bring the overall CG of the rocket toward the nose, the motor should be as far forward as possible.

From the pictures, it looks like you must have put some sort of motor block in the motor mount. If the 6" motor could slide all the way up it should be far enough forward to fly safely.

I never use any form of internal motor block if I can help it because it always comes back to bite me in the a** when I want to fly a bigger motor, and I ALWAYS want to fly a bigger motor.

As the old saying goes, "I'll fly any rocket again, on a bigger motor, that don't crash the first time on a littler one."

The best way I have found to hold a motor in place is with a 1/4" - 1/2" masking tape thrust ring around the base of the motor and not a motor block up the tube. About 3 wraps of tape is usually all that is necessary.

One other point. Do not assume that a Tetrahedron (3 sides and a triangular base) will be stable just because a 4 sided pyramid of similiar dimensions is stable. For some reason the CG has to be more forward with a Tetrahedron than with a Pyramid. The base drag effect seems to be less.

The choice of motor for a draggy design is a whole nother subject.

BTW: What do the letters stand for on the rocket in the pictures?

 

[DPatell]

The pyramid originally had a 24mm motor mount, and the back would pop off. That is until I tried an F24W in it and the casing ejected and lost. So I ripped out the 24mm, put in a 29mm and used motors with no ejection charges. It's only corrugated cardboard, so it's really light.

The letters are FPOD...or "Flying Pyramid of Death." Alternate names have also included "Flying Pile of Dung."

It's good fun. I'm not too worried about altitude, just speed. The G100 should absolutely tear it off the pad because it is so lite, then it should just about stop, flip over and touch down. Should be cool.

I tried to fly it yesterday, but the core of the grains was too small so it chuffed rather than light. I opened the cores a little, so the Kn should be at a workable range now. I will try again sometime over winter break.

I also have a large plywood pyramid with a 38mm motor mount. The 38-220 will be used with a Red load in it. The motor is flush with the back of the pyramid, so it should definately fly good.

I'll keep you all updated!

 

[rstaff3]

Cardboard pyramids rule. Mine has a 29mm and 3 24mm mounts. Nice slow descent.

 

[teflonrocketry1]

Originally posted by freda
Bruce,

Can you tell us more about your rocsim model.....
What is the justification for the rear (fake) transition?
How did you know how to place and size it?
Why does the model fail for H motors?
Thanks,
FredA

This RockSim file models the pyramid as a cone of the same height with a diameter selected to give an equivalent base area (12 x12 for the pyramid = (13.54/2)^2*pi for the cone, thus same volume.
The rear transition simulates the dynamic base vortex. The rear transition is the same diameter as the equivalent cone but its length is pi times this diameter. This would put the CP at about 2.1 diamters behind the model if it were simply a flat plate. The CP location of the base vortex is based on an extrapolation of the data from John Cipolla's spool rocket report at:

https://www.aerorocket.com/spool/spool.html

The design doesn't fail for H motors; since this is a wide diameter model, putting a high thrust motor in to gain a higher speed and hence higher altitude, is more than offset by the increse in drag at the higher speeds so you really don't get the expected increase in altitude. To get a higher altitude with this short and wide design it is best to select the lower thrust motor e.g. G40 gives a much higher flight than a G80.

Bruce S. Levison, NAR #69055

 

[gregzo]

Hey Dan - how's it going??
Could you send some info on your 38mm plywood Pyramid? I would like to know what you are using for materials, recovery system, dimensions, weights, etc. I am starting the construstion of a plywood Tetrahedron for the NERRF launch. It will have a 54mm mount that I plan on launching light K's in. (And larger 38mm's for economical flights...) Plan is for a pull pin timer for ejection.
Estimates now are for a 24" base traingle and 27 to 28" tall for a slightly stretched tet. I plan on making a cardboard mockup first to see if I can get all the recovery system installed or if I will need more room internally.
Thanks

 

[DPatell]

Umm...recovery system? That would be gravity...

It really isn't anything special, just kind of glued together a bunch of triangular pieces of 1/4" luan plywood... and a square base with a 38mm hole in the middle for the motor tube.

I don't expect it to fly too many times, this is just for a few poops and giggles.

 

[gregzo]

How much does it weight? What general size is it? I was expecting around 10 to 14 pounds for what I had in mind. I think I will need a chute for that......

Good luck with your purple motor. Post some pictures!!!

 

[DPatell]

Lol, you probably will. I'm not exactly sure of the size off the top of my head, I think each of the triangular plates has an 18" base and is maybe 18" tall, not sure. It's 4 sided and probably weighs around 1.5 or 2 pounds.

I will definately post pics of the purple and red motors, they should fly this Friday.

 

[urbanek]

Originally posted by DPatell
I'm not exactly sure of the size off the top of my head, I think each of the triangular plates has an 18" base and is maybe 18" tall, not sure. It's 4 sided and probably weighs around 1.5 or 2 pounds.

I've seen a similar sized Pyramid made from foam-board held together with duct-tap safely fly on an I motor. With this sort of thing, you can go really light since the air pessure is so well spread out.

urbanek

 

[DPatell]

Well, the final argument is in...they both worked perfectly!

Time to re-evaluate all of those pyramid hypothesis, lol.

First up is the small cardboard pyramid on the 29-120 purple. WARPED off the pad to around 300-400'. Very cool.

The pyramid is at the top of the flame in the picture, kind of hard to see. Not very purple, but the flame is 4' long!

 

[DPatell]

Second was the larger pyramid on a 38-220 SrNO3 red. Awesome slow boost, but it tilted a little bit. I'm not sure if the motor mount is perfectly straight on it...but who cares, it was cool

Here it is just before moving. The ground shows the real red color. Very deep and vibrant.

 

[DPatell]

Second picture is of it off the rail and about 10-20' up. Very cool flight!

 

[DPatell]

Off topic, but also flown was an Aerotech Initiator on an F20 for kicks.

 

[MattEx]

Beautiful red there ...
Looks pretty aluminized too.
Could you possibly PM or email me the formula? I'm looking to develop a red... yours is a pretty vibrant red .

 

[rstaff3]

Glad the pyramids worked as expected. Cool motors too!

 

[Hospital_Rocket]

Originally posted by DPatell
Second picture is of it off the rail and about 10-20' up. Very cool flight!

You could submit the photo to the weekly world news as proof of an alien invasion...