Looking for info on a "Push-Pull" rocket

[powderburner]

This is apparently an old, OOP design, but I don't know if it was mod-roc sized or mid-power or something else. I saw a reference to it on another forum and it got me curious. Apparently this rocket had a forward motor pod, an aft motor pod, and (I'm guessing) a minimal structure in between (like maybe a stick?)

This design is purported to somehow be stable under thrust. I would like to understand more about this.

Anybody have any data on this oldie? Perhaps a picture, or maybe even a set of instructions?

Thanks ahead of time for your help!

 

[TheAviator]

I've never seen the design, but I did build a 2x2 13mm motor stack for A Cluster Altitude at NARAM. The two halves were connected by a pair of carbon rods. There were also three small fins on the rear pod. Don't use balsa for the rear cone, it will get eaten by the forward motor exhaust. Estes plastic cones held up just fine though.

 

[powderburner]

Sounds interesting too, Brian, but I understand that this "Push-Pull" rocket design was finless. That's part of what makes me curious what this thing looks like and how it could be stable.

 

[ScrapDaddy]

Sounds interesting too, Brian, but I understand that this "Push-Pull" rocket design was finless. That's part of what makes me curious what this thing looks like and how it could be stable.

I think I have an idea, I think it was like an acme style stabalized

 

[TheAviator]

This is a "D'OH" moment for me. I can think of a rocket in production that fits the bill: the FlisKits Nell! I realize it doesn't have a rear motor, but the theory is the same.

 

[Gnomad]

I've never heard of such a rocket, but I believe the best way to think about stability would be in terms of the classic stick rocket, or bottle rocket.

A stick rocket needs to have the CG just aft of the motor. This is typically achieved with a long stick, length bringing the CG back with minimal overall weight. However, there is nothing preventing the use of a shorter stick with a mass at the end, and there shouldn't be anything preventing that mass from being another motor.

As long as the CG is close to just behind the forward motor you should be stable.

 

[powderburner]

OK guys, a little side note here:

It may be a little-known fact, or a misunderstood principle, but the configuration of the first Goddard rocket (motor in front, cg behind) IS NOT inherently stable.

If the rocket's cp is behind the cg (by a sufficient margin) it should be reasonably stable. The cp can be moved aft through the use of aft-located fins, or the use of base drag from a relatively large base area. These features are related to external geometry.

A "Nell" rocket design might be aerodynamically stable if the net result of cp and drag forces act in a way that is located behind the cg. For the Fliskits model rocket, this is entirely possible if the motor (relatively heavy) is in the nose and the rest of the rocket is made up of lightweight materials. For Goddard's full-scale original rocket, I don't know where the cg was but it was apparently located at or slightly behind the cp, because Goddard himself acknowledges that his design did not exhibit stable flight.

Placement of the motor in the front does not automatically bestow stability unless the motor's thrust axis is actively vectored to correct for any flight deviations. The idea that the rocket's cg somehow "hangs" below the motor and is therefore supposed to be self-correcting makes the mistake of overlooking the effects of a fixed (non vectoring) motor mount: the thrust axis remains acting through the cg of the rocket and there is no correcting thrust effect which tries to turn the rocket (in ANY direction). If a rocket's cg is located behind the motor, it still needs to be in front of the rocket's center of pressure.

Back to the Push-Pull thing: does anyone have any configuration data on that specific rocket design?

 

[MarkII]

Did you look through the r.m.r. Descon archives at EMRR? Or the scratch-build articles? I vaguely recall seeing something like what you are talking about, and it might have been there that I saw it.

 

[ScrapDaddy]

https://www.rocketreviews.com/reviews/all/push_me_pull_me.shtml

 

[ScrapDaddy]

https://www.rocketreviews.com/reviews/all/scratch_push_me_pull_you.shtml

 

[powderburner]

Did you look through the r.m.r. Descon archives at EMRR? Or the scratch-build articles? I vaguely recall seeing something like what you are talking about, and it might have been there that I saw it.

Been there, done that. Unfortunately, neither one of those closely-named designs matches up with the description I read about one motor in front, one motor in the back, a long stick connecting them, and no fins.

https://www.rocketreviews.com/reviews...pull_you.shtml
https://www.rocketreviews.com/reviews..._pull_me.shtml

do not appear to be the design in question, but thanks for the effort, ScrapDaddy

 

[ScrapDaddy]

One last thought, this one?:
https://www.rocketreviews.com/reviews/all/scratch_no_fins.shtml

 

[jflis]

OK guys, a little side note here:

It may be a little-known fact, or a misunderstood principle, but the configuration of the first Goddard rocket (motor in front, cg behind) IS NOT inherently stable.

If the rocket's cp is behind the cg (by a sufficient margin) it should be reasonably stable. The cp can be moved aft through the use of aft-located fins, or the use of base drag from a relatively large base area. These features are related to external geometry.

A "Nell" rocket design might be aerodynamically stable if the net result of cp and drag forces act in a way that is located behind the cg. For the Fliskits model rocket, this is entirely possible if the motor (relatively heavy) is in the nose and the rest of the rocket is made up of lightweight materials. For Goddard's full-scale original rocket, I don't know where the cg was but it was apparently located at or slightly behind the cp, because Goddard himself acknowledges that his design did not exhibit stable flight.

Placement of the motor in the front does not automatically bestow stability unless the motor's thrust axis is actively vectored to correct for any flight deviations. The idea that the rocket's cg somehow "hangs" below the motor and is therefore supposed to be self-correcting makes the mistake of overlooking the effects of a fixed (non vectoring) motor mount: the thrust axis remains acting through the cg of the rocket and there is no correcting thrust effect which tries to turn the rocket (in ANY direction). If a rocket's cg is located behind the motor, it still needs to be in front of the rocket's center of pressure.

Back to the Push-Pull thing: does anyone have any configuration data on that specific rocket design?

Very good points. In fact, the design that Goddard came up with was based in a misunderstanding he himself had with regard to stability. He put the motor on top because he thought that for stability the "Center of Thrust" had to be ahead of the CG...

The instability he noticed during its flight was the result of 2 things:

1. Actual instability... light weight combustion chamber (motor) up top and heavy fuel and oxy tanks below...
2. Flexibility of the fuel/oxy tubes. The bottom of this rocket was free to swing and move about. not good...

As mentioned, the FlisKits Nell gets around these problems by having a heavy (comparatively) motor up top and light weight empty tubes at the bottom. Further, the fuel/oxy lines are stiff (again, by comparison) and keep the overall model much more rigid.

It could actually be an interesting vehicle to test the push-pull idea though. But may be more work than it's worth for just testing an idea...

 

[powderburner]

It could actually be an interesting vehicle to test the push-pull idea though. But may be more work than it's worth for just testing an idea...

I still have not found exactly what this rocket design actually was, but the claimed performance was that it was stable under thrust. It supposedly went unstable during the delay burn and had "barbell" tumble recovery.

I am beginning to suspect that this design was never stable in the first place, and that the longitudinal pitch inertia simply delayed any tumbling until far enough into the flight that it was confused with thrust cut-off. But I was wrong before, once. I would like to learn more about this thing and how it worked, and yes, I would spend the time to flight-test it.

 

[Micromeister]

Powder:
Haven't heard or read about the Push-Pull rocket your discribing but have in the past done a few "finned" stacked clusters That sorta of sound something like your discribing.

I can for sure tell you that in Test flights of the Mirco 2x1/8A motor and 3 motor 1/2a neither were in the least stable without fins 4xA in-line have also been flown with fins pretty well without not at all. As a matter of fact in one early 3x1/2A test flight I burned off the lower Fin section the two remaining motored forward section continued to ejections snaking all over the sky. Somewhere I have a couple pics of the burned off forward section of that 3-1/2A model but can't put my finger on it

 

[MarkII]

Mentioned on another forum...

https://forums.rocketshoppe.com/showpost.php?p=100430&postcount=4

 

[Cluster King]

This is apparently an old, OOP design, but I don't know if it was mod-roc sized or mid-power or something else. I saw a reference to it on another forum and it got me curious. Apparently this rocket had a forward motor pod, an aft motor pod, and (I'm guessing) a minimal structure in between (like maybe a stick?)

This design is purported to somehow be stable under thrust. I would like to understand more about this.

Anybody have any data on this oldie? Perhaps a picture, or maybe even a set of instructions?

Thanks ahead of time for your help!

Powder burner,

I don't know about the rocket you refer to, but I've built push pulls. My 1st effort was the Page 260 (so named after a photo in Tim Van Milligen's book "Designing and building model rockets" 2nd edition. guess what page?!) It had 2-18mm top and 2-24mm. I'm building an up scale using 4" bt and it will have 80 motors (Quest D5's) ready for launch May 2010.

Hope this helps.

Let me know if I can help you.

"High ground should be reserved for memorials for the dead of the last senseless war."

Tsolo Dann