Cg and CP for rockets with wooden stick

[Mumbles]

For what purpose? It just sounds like a lot of work for something a normal shell on top of a rocket already accomplishes. Or an orientation rope.

[OldMarine]

Though I realize they are deadly as hell, I still laugh at the Syrian Elephant rockets that popped up after the video above. Some fly like a shoe thrown across the room and there's no way they're anything other than an area denial weapon. In Beirut they lobbed a few at us and you actually had time to decide whether to duck or not!

[JMan]

Hey guys. There seems to be a lot of misconception here in this thread. First off I noticed once or twice that "center of pressure" was referred to as the pressure in the motor and where it is expelled. This is entirely not true when talking about aerodynamics. The center of pressure is where the average drags is located at.

 

It was also thought at one point in this thread that the center of gravity should be below then center of thrust (the nozzle area) and that the rocket will act as a pendulum. Oh how very wrong this is. Early rocket scientist robbert Goddard thought of this too and made a little pyramid rocket with the center of gravity below the nozzle exit thinking the rocket would swing like a pendulum and the nozzle would act as the fulcrum (point at which the rocket piviots). This is not true. Rockets always pivot on (or very near if affected by other forces) the center of gravity.

 

After this rocket just went up turned over and went back down Goddard realized that the flying object pivitoed on the CG. So if we know the rocket pivots on the CG we would want to find something that causes torque on the motor. Fins do this. When the rocket is not pointed in the direction of its velocity vector (the direction it is moving) the fins can push the rocket left or right. Applying this force infront or behind the CG causes a torque. Putting the fins infront magnifies the change in heading, and behind reduces this change in heading. You can think of them as deflecting air left or right as the rocket picvots on its CG. I don't think it's too hard to imagine this and I'm trying to explain it the best I can I hope this helped

 

BUT. The sticks have a different method to their madness. By extending a force farther away from the fulcrum you will have more torque (as it is measured in force x distance). So the farther away your center of pressure gets from your CG you will get more effective torque. This is the same thinking with fins except sticks monopolize that far distante away from the CG so they are more efficient.

 

You all seem confused by the weights on the end of the sticks. This is very simple to explain how this makes the rocket more stable (early flight and low power atleast). Lifting a milk jug isn't that hard for most people, but lifting a milk jug with your arm full extended is extramly hard for most people. The same goes with that weight on the end of your stick. Your rocket moves around its CG if your rocket isn't going fast enough for aerodynamic forces to work greatly on it adding weight will make it harder for your rocket to turn. But as you add more weight to the end of the stick your CG moves closer to the end of the stick as well. Either: 1.) add some weight to the top of your motor as well to keep it from moving too far back or 2.) do some math.

 

The force needed to turn your rocket is torque (measured in force x distance) so as your CG moves closer to that weight it's distance decreases. But there is a point where your weight and torque is most effecent. Simply add weight (know amount) and measure the distance for weight to CG multiply the weight and distance. Add or decrease weight and repeat. The point is to try and get the biggest number possible so the rocket needs more torque to turn the rocket. Keep in mind once your fuel burns out your CG will move very far backwards with this method. By the end of the burn the motor should still have the center of pressure BEHIND the CG.

 

I'd be happy to explain this in more detail if anyone had a problem understanding it. This is not opinion or trial and error, but math. If I made a mistake please call me out I'd hate to be giving out wrong info for the sake of a rocket turning over and poking someone's eye out. But I hope this helped clear up any problems with terminology, concepts, or math involved in balencing your rocket.

Edited April 14, 2017 by JMan

[OldMarine]

I had a long reply typed up but I realized I would sound like someone else on this forum and deleted.

Condescension is counter-intuitive to constructive conversation.

[JMan]

I apologize if I came off that way I truely mean to help clear up some confusion. I just thought it might help to have all that info in one place for anyone looking to find it

[stix]

Yeah Jman, I think the confusion lies in the difference between rockets with fins and pyrotechnic rockets where the motor is attached to a stick. Let's call them finned rockets and stick rockets.

 

I couldn't quite grasp all what you've posted above - too much text. Some pictures if you could would help.

 

This statement I find a bit confusing:

. . . But as you add more weight to the end of the stick your CG moves closer to the end of the stick as well. Either: 1.) add some weight to the top of your motor as well to keep it from moving too far back or 2.) do some math.

 

Well, apart from the math, instead of adding weight to the top of your motor, wouldn't it be easier to just remove some of your added weight from the end of the stick? - serves the same purpose I would have thought. dunno.

 

First off I noticed once or twice that "center of pressure" was referred to as the pressure in the motor and where it is expelled.

 

Having been a participant of this thread for some time, I don't remember anyone confusing CP with the point of thrust! Which part or posts are you referring to? I must have missed something.

 

@OM

. . . Some fly like a shoe thrown across the room . . .

 

I still laugh about that one. I assume that you are referring to the incident where in a press conference, someone threw a shoe a George Bush Junior. Perhaps not the most astute American President of times past, but nevertheless he still had the wherewithal and time to duck out of the way - Brilliant!!

 

However, one does wonder that if the shoe in question was one of those old 50"s models with a pointy toe - fins on the backside and some weight added to the toe... perhaps an entirely different outcome?

 

btw. JMan, I don't think that OM was referring to your long post - you haven't been on this forum long enough to deserve that reputation. Perhaps the remark was more aimed at my usual long-winded and pointless posts. Well, at least I hope so.

Edited April 13, 2017 by stix

[JMan]

I'm on a phone and I can't find how to quote someone but

 

@Bluecomet24

I had always thought that it's best to have the center of pressure (nozzle) above the center of gravity. If you think of it as pushing/pulling a stick upward, then a stick being pulled from the top (COG below nozzle) is more stable than one being pushed from the bottom (COG above nozzle), which would want to flip upside-down

 

This, to me at least, certainly sounds like he's referring to the center of thrust as the center of pressure. And although he is not wrong, rockets do not behave as through they are being "pulled" or "pushed"

 

But about the weight on the end of the stick. Yes you could reduce the weight, but try picking up a half empty milk jug with your arm extended, and try with the other arm a full milk jug. Point is it take more energy to change its velocity vector when it's got more weight farther away from the pivot point (force x distance) more weight you have on the end the bigger that numbers going to be.

 

But if your rocket is inherently stable with less weight then by all means yes take some weight off. This is just for marking the rocket more stable, if you're happy with how it flys or think it'd still do great with some weight off go ahead and try it probably will. That's just the point at where your rocket will be most stable

[BlueComet24]

Oh, sorry about that; I'm not really a physics guy, so I'll concede.

[JMan]

No that's completely ok. Your input on this is great plus many people think this. It's a common misconception, but mixed up terms can cause rockets not to fly and that's the exact opposite idea of this forum. Im just trying to help completely explain it. Because when I started I certainly didn't understand a lot of the terms, and I just hope no one new gets scared off by something they do not know or are too afraid to ask (or don't want to make an account to ask).