Cg and CP for rockets with wooden stick

[NeighborJ]

I can't provide the math to prove the short stick theory but I can explain why it works on fast rockets.

Idealy on slow motors the weight of the stick will pull the rocket plumb (TRs method), and this is a sure fire way to ensure a stable flight. The short stick method does not work on the same principal and does not require a tangible balance point. Instead the balance point is created in flight, due to velocity and the drag which is created by the stick. As the velocity increases the drag increases which simulates aft weight. This simulated weight moves the cg to the rear of the motor. If the rocket tips to one side the drag increases exponentially on the stick which simulates even greater mass to the aft.

To accomplish this phenomena a large burst of speed is required to get the rocket past the initial instability of a low speed flight. The spindle you mentioned above is capable of using sali whistle which would be a much faster fuel and makes a more suitable platform for the short sticks. If you want to use benzo fuel, it works better with a longer spindle, even as long as a BP spindle.

The double sticks work great but I'm not sure you fully understood David Fs method. He tapes the ends of the sticks together so it looks like a pair of crutches.

[DavidF]

Please bear with me as I try to post pictures. This one seems to have worked, but others don't. I'll post this and try again to post another. This is the jig I made up for taping sticks onto 1lb rocket motors. The first stick is laid in the right side, the motor is laid in the middle, and the second stick goes on the left. The little holder is turned to hold the motor in place. Then the tape can be easily applied since the hands are free. I made a wooden base for it so it can just sit on the table.

 

 

 

[DavidF]

[DavidF]

[DavidF]

Yes, like a pair of crutches look.

Ha, it worked! Next time I'll put them in one post. Here's what I did to add a picture to the post: I compressed the original picture using Windows picture editor. I chose the size for documents. This reduced it to less than 200kb. Then I started a gallery and uploaded my picture to it. Next, I clicked on my name, and selected the gallery. I clicked on the picture I wanted, and it opened from thumbnail to larger size. I clicked it again. Then I right-clicked, and selected COPY. After that, I was able to click the green 'image' icon, which asked for the URL. I right-clicked in the space, and pasted the file in. I clicked OK. It then appeared in the post as you see. Just in case somebody finds this helpful, I thought I'd describe what worked for me.

Edited January 3, 2017 by DavidF

[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. Thus I think a rocket with a very long stick would be more stable, not just because of the extra drag, but because of the lower COG. Ideally, I'd think a very long, very light stick would be preferred.

[stix]

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. Thus I think a rocket with a very long stick would be more stable, not just because of the extra drag, but because of the lower COG. Ideally, I'd think a very long, very light stick would be preferred.

 

That's correct.

 

Everything has a centre of gravity - flip a pencil in the air and you will see it. On earth we have the wind, air and gravity to deal with - in space we don't. Nevertheless the centre of gravity still holds true for both.

 

For these type rockets, the longer the stick is, the more stable it will be. It's not that hard to imagine building a rocket+very long stick, where the rocket barely takes off but always remains stable and vertical. With a longer stick this is achievable but of course there is a limit - weight. Therefore an appropriate amount of thrust is required.

 

Do a physical test. Tie a loop of string (twine) around the rocket until it balances. Go to an open place and swing it around above your head. Hard to do with big heavy ones, but if you can then you will see the CG & CP at play. The aerodynamic forces govern the CP.

 

If the the nose moves away from the centre, then it's unstable. If you've ever seen a weather vain then have a good think about how and why it works.

Edited January 3, 2017 by stix

[Sulphurstan]

Yes I've seen that for finned rockets, didn't know it could also apply to our stick rockets.. Have to try that out.

[dagabu]

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. Thus I think a rocket with a very long stick would be more stable, not just because of the extra drag, but because of the lower COG. Ideally, I'd think a very long, very light stick would be preferred.

 

 

 

That's correct.

 

Everything has a centre of gravity - flip a pencil in the air and you will see it. On earth we have the wind, air and gravity to deal with - in space we don't. Nevertheless the centre of gravity still holds true for both.

 

For these type rockets, the longer the stick is, the more stable it will be. It's not that hard to imagine building a rocket+very long stick, where the rocket barely takes off but always remains stable and vertical. With a longer stick this is achievable but of course there is a limit - weight. Therefore an appropriate amount of thrust is required.

 

Do a physical test. Tie a loop of string (twine) around the rocket until it balances. Go to an open place and swing it around above your head. Hard to do with big heavy ones, but if you can then you will see the CG & CP at play. The aerodynamic forces govern the CP.

 

If the the nose moves away from the centre, then it's unstable. If you've ever seen a weather vain then have a good think about how and why it works.

 

This topic never gets old, does it?

 

Going back 6 years, we were having a discussion about just this subject, the person arguing the "long stick" position was adamant that only rockets with longs sticks would ever fly straight! Damnitt!

 

Here is a picture and video of "Mr. Sparky", a nozzleless rocket with four fins, no stick. The fins are 4" long in a tapered delta shape.

 

 

https://youtu.be/yG3a2lrg7Ww

[BlueComet24]

Mr. Sparky flew because the fins create LOTS of drag if the rocket starts rotating from its vector, so going straight is the path of least resistance.

[dagabu]

Of course these are fins, not sticks but where is the guide rod? Not there...

 

Lets throw a ball shell on top, THAT will make it topple over for sure!

 

Nope...

 

https://www.youtube.com/watch?v=gqkxTemDQ-0

[dagabu]

Mr. Sparky flew because the fins create LOTS of drag if the rocket starts rotating from its vector, so going straight is the path of least resistance.

 

Drag? These are wings or 'surfaces', they knife through the air and only a portion of the fin creates "drag". They steer the rocket using resistance to the air, the drag pulls it back, resistance is a form of drag but is not the same. This is called the restoring force, not the catch all 'drag' term used often.

 

Lets look at that scenario then. The 'drag' is really close to the Cg and CP when using little fins, making it more erratic, less stable, right?

 

Four fins have 16 square inches of surface for buffeting the wind. These are #3 rockets, they typically use a 1/2" by 48" long stick (by those that stick with the 'traditional methods'), or 96 square inches of 'drag' in surface area.

 

Hmmm, something wrong with my math? Nope! Ok... lets use a 48" long stick 1/4" on each side for 48 square inches of surface, we know that the whole "There has gotta be enough weight to balance it at the nozzle!" idea is nonsense since those little 4g fins dont matter much for weight and with only 16 inches of surface, it should just flop around!

 

Even a 1/4" only 24" long has 24 square inches of 'drag'

 

What we really are looking for are the conditions for a stable rocket, such as "the center of pressure must be located below the center of gravity". Sticks of all lengths can meet this requirement.

 

The serious answer is one of experimentation, you have to try it out for yourself and not be a slave to traditional methods if you want to seek the place where the traditional methods fail.

[dagabu]

Bottom line is this: We have a non-standard motor with a payload that is ever changing, thrust levels that vary so widely that comparison is useless.

 

Try what you think may work.

 

I.e. Does it cork screw? What is the grain and stiffness of the stick telling you? To answer this, you have to measure the deflection the stick makes in all four directions as the stick is laid flat on a flat, horizontal surface with only the portion of the stick that would normally be tied/taped/glued to the motor being clamped down. A weight of just an couple ounces attached to the end of the overhanging end and the deflection measured.

 

Do this for all four flats and see what you find. This is one of the reasons that Bass or Poplar are so popular, they have pretty close deflection characteristics, no splitting (pine and cedar along with oak are notorious for breaking along the length upon take off).

[calebkessinger]

Nice info Dave .. and Dave.

 

I use pretty small sticks and usually just add more of them if i need more stability. I've seen Dave F's rocket technique first hand and have used it myself. I really like it on the bigger ones, keeps them nice and straight with no quick changes of direction most of the time.

[ExplosiveCoek]

Thanks Dave, the corkscrewing indeed happened on the rectangular stick, which has therefore different 'bending properties'.

Interesting.

 

But I don't get this: the center of pressure must be located below the center of gravity

Wasn't it the other way around?

 

If I interpret your information right, my rockets were arcing because of not enough resistance, and not due to an in-balance in weight/stick configuration?

How should I act based on this information? I can't make my rocket fuel more powerful, nor my spindle longer.. Which is what I would do normally if my rocket doesn't fly straight.

But I do want these rockets to fly straight . For now I think my best option is to use the larget spindle on a 5'' shell, and see if that does lift it properly. Would be quite a pity tho, as I can't use it for 6'' shell's anymore, and I need to go and buy a mortar to shoot them from too haha.

[stix]

. . . But I don't get this: the center of pressure must be located below the center of gravity

Wasn't it the other way around? . . .

 

Yeah, I still get confused about it too. I think maybe you have to look at things differently for stick rockets compared to a finned model rocket because of where the thrust is coming from. Perhaps with stick rockets you need to worry less about CP and drag and focus more on the CG?

 

On a model rocket you need fins for it to be stable. If you put your finger over the nozzle end and try to balance it vertically, it will likely topple over. Do the same thing with a stick rocket and it should be stable, if not you would have to put more weight on the end of the stick or make the stick longer. Ideally 2 shorter sticks would be more stable.

 

Part of the issue with a rocket arcing over may be wind coming from the side. On a theoretical windless day then you would be relying on aerodynamic force to keep it stable. Any wind from the side would tip the tail end over and point the nose into the wind. That could be the reason why some people swear by thinner sticks, ie. less area for a side wind to push on.

 

So what would happen if there was no air? What happens in space with no air or gravity? Interesting things to think about, which I have no answer for.

[dagabu]

Thanks Dave, the corkscrewing indeed happened on the rectangular stick, which has therefore different 'bending properties'.

Interesting.

 

But I don't get this: the center of pressure must be located below the center of gravity

Wasn't it the other way around?

 

If I interpret your information right, my rockets were arcing because of not enough resistance, and not due to an in-balance in weight/stick configuration?

How should I act based on this information? I can't make my rocket fuel more powerful, nor my spindle longer.. Which is what I would do normally if my rocket doesn't fly straight.

But I do want these rockets to fly straight . For now I think my best option is to use the larget spindle on a 5'' shell, and see if that does lift it properly. Would be quite a pity tho, as I can't use it for 6'' shell's anymore, and I need to go and buy a mortar to shoot them from too haha.

 

Sorry, no. Your rockets are arching for one of two reasons only. Wind or weight. Let me explain.

 

A long stick (and the very reason why I debunk the traditional stick theory so much) acts just like the wind-vane that was spoken of a while back. It steers the lighter portion of the rocket sideways as the pressure of the wind acts upon the stick, it moves the stick in the direction of the wind, causing it to "Steer" into the wind, creating arcing.

 

The second reason is weight. Again, I'll explain. The fuel only has a certain amount of potential energy in which can be turned into thrust. That energy can be measured in many ways and those that wish to get technical (not all that necessary when it comes to pyro because of the many variables from rocket to rocket in the same batch) call it impulse. Lets just call it thrust for now, I'll let the "rocket surgeons" drone on about impulse later. If you have enough thrust to lift and KEEP lifting a shell, in calm wind and a well made rocket, it SHOULD fly straight up if launched straight up. If the potential thrust is not enough to lift and keep lifting that shell, it will arc over. Even a bare motor with a heavy stick, with the motor lacking thrust, or even a light stick for that matter, the rocket will arc over.

 

CAN the stick cause arcing? Maybe... I really don't know but I do know that a cork screwing rocket will eat up the thrust very quickly giving you a low break too.

 

Get more lift, shoot in light wind, reduce your shell weight and size. That is where I would start.

[dagabu]

 

Yeah, I still get confused about it too. I think maybe you have to look at things differently for stick rockets compared to a finned model rocket because of where the thrust is coming from. Perhaps with stick rockets you need to worry less about CP and drag and focus more on the CG?

 

On a model rocket you need fins for it to be stable. If you put your finger over the nozzle end and try to balance it vertically, it will likely topple over. Do the same thing with a stick rocket and it should be stable, if not you would have to put more weight on the end of the stick or make the stick longer. Ideally 2 shorter sticks would be more stable.

 

Part of the issue with a rocket arcing over may be wind coming from the side. On a theoretical windless day then you would be relying on aerodynamic force to keep it stable. Any wind from the side would tip the tail end over and point the nose into the wind. That could be the reason why some people swear by thinner sticks, ie. less area for a side wind to push on.

 

So what would happen if there was no air? What happens in space with no air or gravity? Interesting things to think about, which I have no answer for.

 

Here is where it gets sticky: Finless rockets have no fins, the Shia militants are known to use finless rockets. These are the Ashtar 107mm in Iraq for example. Known as a Burkan Rocket.

 

 

[BlueComet24]

Here is where it gets sticky

I dunno, Dag, that looks pretty stickless to me.

Are those spin-stabilized? If not, I can't imagine they're too accurate.

[NeighborJ]

Spin stabilized or not, I'm runnin if it's pointed in my general vicinity. Looks like it has a riffled guide on the launcher. Edited January 6, 2017 by NeighborJ

[dagabu]

This one flew straight, not spinning either.

 

[calebkessinger]

Pretty neat.

Learn something new everyday!

 

My spinning rockets are usually spinning cause the stick got destroyed due to supersonic flight!

ok.. maybe not that fast. but close.

[NeighborJ]

IDK Dave, I looked close at that vid with a larger screen and the image is grainy but I swear that rocket is spinning. It would either need a riffled guide launcher or a spin vectored nozzle or both. The first set of photos show guide veins in the nozzle.

[dagabu]

Sure, look at the second picture, they are obviously made to spin. The main idea is still the same, no sticks.

[NeighborJ]

Ya know what? Ever since ive seen this rocket ive been thinking about making a spinn stablized launcher for large shells. Im thinking of a horizontal wheel with drivers spinning up a whistle powered shell with no stick. It would be a lot of work and requires an additional element of balance but i think it could be done. I think it could be launched from a girandola but it could go way wrong real quick.