Another question about cg and cp

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blackwing94

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Do you mark the cp on your rockets?

I ordered some cp decals from stickershock and received a page of cp and cg decals. I put the cp decal on my AT Arcas, but why would I add the cg decal? Doesn't the cg change with motor choice, parachute choice, add a chute release or altimeter, or not.... It seems to me (as a beginner) that as long as I know where the cp point is I can quickly field test that the cg is at least 1 cal ahead. So why would I mark a cg on my rocket? What am I missing?

This leads me to ask, do you ever add adjustable weights to a rocket as conditions require it?
 
I marked the CG & CP on my L1 rocket only so I could show my examiner when he asked where they were. The CG will change with each motor. I have rockets with nose weight but nothing adjustable, just based the weight on worse case scenario.
 
I do use adjustable nose weights on larger rockets that can utilize a variety motors. Typically I use a series of lead disks on a threaded rod in the nose cone.
 
I mark CP, but really only for reference in case the RSO/LCO decides to question it. For CG, I weight my rockets, if possible, for largest motor I'm going to use based on OR calculations, and call it good. It limits what it'll do on smaller motors, but ensures a more than adequate margin of safety.

If anything isn't going to work out the way I want it to, I have in my notes for that rocket which motors are a 'no-go'.

On larger rockets I plan on making a series of weights that I can add/subtract easily to a nose bay so that I can maximize performance.
 
Do you mark the cp on your rockets?

I ordered some cp decals from stickershock and received a page of cp and cg decals. I put the cp decal on my AT Arcas, but why would I add the cg decal? Doesn't the cg change with motor choice, parachute choice, add a chute release or altimeter, or not.... It seems to me (as a beginner) that as long as I know where the cp point is I can quickly field test that the cg is at least 1 cal ahead. So why would I mark a cg on my rocket? What am I missing?

This leads me to ask, do you ever add adjustable weights to a rocket as conditions require it?

For cert flights you should have Cp marked and be able to demonstrate where Cg is.
 
For cert flights you should have Cp marked and be able to demonstrate where Cg is.

Bingo. A tiny piece of masking tape, or a little Sharpie dot for CP. If you need to mark CG for some reason, use a tiny piece of masking tape. Once you are past the RSO, you can pull them off.
 
Bingo. A tiny piece of masking tape, or a little Sharpie dot for CP. If you need to mark CG for some reason, use a tiny piece of masking tape. Once you are past the RSO, you can pull them off.

I would remove the Cg mark because it will change. I would make the Cp mark permanent for my own reference. Why not? However, if something happened to change* the Cp, I would re-mark it.

*Old rockets never die, they just get shorter - Dale Emery

Steve
 
I've always marked Cp in some permanent way, generally on the back-side where it's out of sight. My Nike Smoke has one of its rail buttons placed at the Cp so it serves as the mark. For Cg I always mark it with a small piece of blue tape, and leave it on as I go to the RSO table in case there are any questions. It's also on the back side so it can easily be compared to the Cp mark, but it means about half the time I forget to take it off on the pad. I'm typically removing it post-flight but when I remember it comes off before the flight.
 
on one of my rockets I used 1/8" vinyl tape(red) as a quick reminder to me of just how far aft was allowable for the cg. since one can get markers in a variety of colors (including gold and silver) cp markings need not be ugly. I believe the requirement is that the rocketeer be able to show where the cp is, marking it is easier than bringing reams of print outs and a tape measure with you to the RSO table :).
Rex
 
I permanently mark the CP and put another permanent mark where the minimum CG should be at one caliber ahead of the CP for non-mach flights, two calibers for mach.

Once the rocket is loaded and ready to go, I can rest the rocket on the min CG mark. As long as the nose drops, it's stable (wind considerations not withstanding).

For temporary/adjustable nose wight, I use a molley bolt with nuts and large fender washers attached to the bottom of the nosecone.
 
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Thanks for all the great responses and first hand experiences. This forum provides an amazing wealth of knowledge and a friendly atmosphere. :)
 
I do use adjustable nose weights on larger rockets that can utilize a variety motors. Typically I use a series of lead disks on a threaded rod in the nose cone.

I am just getting into some bigger air-frames and this ideas resonates with me.

I also permanently mark the CP on my air-frames, but not the GC, so it is easy for the RSO. Just balance it at the RSO table compare it to the CP mark and you're good to hit the pad.
 
I mark my cp and mark my min cg with sticker shock stickers so in the field I can easily see if my rocket should be stable by balancing it ready for flight.
 
I permanently mark the CP and put another permanent mark where the minimum CG should be at one caliber ahead of the CP for non-mach flights, two calibers for mach.

Once the rocket is loaded and ready to go, I can rest the rocket on the min CG mark. As long as the nose drops, it's stable (wind considerations not withstanding).

For temporary/adjustable nose wight, I use a molley bolt with nuts and large fender washers attached to the bottom of the nosecone.

+1 on this. I took a primed but not painted rocket to a launch as an afterthought. With a pencil, I marked the CP and where the absolute minimum CG had to be for stability so I could check it in the field. It was too big for a swing test (Estes Partizon - my first time launching a 29mm motor), so I wanted to be sure, in case I decide to pick something up from the vendor at the site and fly the thing.

I'm glad I did. It was my only good flight of the day, and I knew it would be stable.

If I had finished painting the rocket, I'd put the CG decal at the safest aft location, so I could check it on the fly.
 
A thought- the Cp is also a variable location. Typically, the faster you go the more it moves forward. So, while useful to know, it is not a static point once you are moving.
 
I always mark CP and use a CG sticker and a marker line around the tube at the 1 caliber mark just for quick reference when doing field balancing. Not just for me but for RSO as well if they want to do a quick check.
 
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