Center Point, Balance & Center of Gravity Marking/Symbols?

[Rockiteer]

Hello everyone, got an interesting one here today. What is the proper marking(s) and/or symbol(s) for center point/midpoint, balance point and/or center of gravity? I attached a drawing so please help me decipher everything; a, b, and c. Now before everyone starts jumping up and down remember I haven't done any serious model rocketry since 1974 and back in my day (1969) we just took our home built model rockets (normally a kitbashing of two or more Estes kits with some parts from the scrap box for good measure), tied some string around the body tube, found the balance point then did the John Wayne "Hatari" thing over our heads a couple of times to insure stability in flight so please take that into account before chastising me for my ignorance on this topic. Your inputs and tutelage is truly appreciated. Reach for the stars and beyond everyone!

 

[Nytrunner]

A) CP
B) CG
C) Meatlover's Pizza

Jokes aside, (MP) is something from Rocksim right? I don't use it so I don't know what names they've given some things. Where does (MP) pop up on its model?

 

[dr wogz]

MP = Military Police
MFP = Main Force Patrol
MSRP = look how much of a discount you're getting!

 

[Rockiteer]

A) CP
B) CG
C) Meatlover's Pizza

Jokes aside, (MP) is something from Rocksim right? I don't use it so I don't know what names they've given some things. Where does (MP) pop up on its model?

Yea, that's where the damn thing keeps popping up. So CP and CG are good, the other is BS. Thanks.

 

[Rex R]

mid point might be useful for putting decals on..., first I'd heard of it.
Rex

 

[Steve Shannon]

The only one that needs marking is Cp. You must be able to identify Cg, but it changes depending on motor mass. Midpoint is meaningless aerodynamically.

 

[Rockiteer]

The only one that needs marking is Cp. You must be able to identify Cg, but it changes depending on motor mass. Midpoint is meaningless aerodynamically.

Yea, CG changes with different mass in the ass end of the rocket. Like the meaningless aerogdynamically... Sounds like my grandmother who was a 43 year school teacher with a degree in rhetoric from Kansas Teacher College in 1923.

 

[rocketguy101]

Hello everyone, got an interesting one here today. What is the proper marking(s) and/or symbol(s) for center point/midpoint, balance point and/or center of gravity? I attached a drawing so please help me decipher everything; a, b, and c. Now before everyone starts jumping up and down remember I haven't done any serious model rocketry since 1974 and back in my day (1969) we just took our home built model rockets (normally a kitbashing of two or more Estes kits with some parts from the scrap box for good measure), tied some string around the body tube, found the balance point then did the John Wayne "Hatari" thing over our heads a couple of times to insure stability in flight so please take that into account before chastising me for my ignorance on this topic. Your inputs and tutelage is truly appreciated. Reach for the stars and beyond everyone!

a= CP b= CG and I think your c is actually two symbols sitting close to each other; (M) for mass object and P for parachute.

In the RS image below, when I originally opened the model, the parachute was further forward so the (M) and P sat next to each other. I moved the parachute away to make the pic. I think whoever modeled this originally was using a mass object to simulate the shock cord.

 

[rocketguy101]

Open Rocket displays parachutes and mass objects a little differently

ps: Welcome back!!!

 

[Nytrunner]

I'm still going with c) Meatlover's Pizza

 

[wighty44]

On my HPRs I mark the CP & CG with vinyl decals from Sticker Shock 23. The CG is placed at the no motor loaded position to give a visual indication of the SM available - may be overkill but it doesn't hurt.

 

[Peartree]

It's probably worth noting that Cp is not shorthand for Center Point but for Center of Pressure. Far from being the middle geometrically, it is the center under which the rocket would pivot equally (where pressure to each "side" of it would be equal) when subjected to a breeze/crosswind.

 

[scsager]

It's good practice to permanently mark the location of the CP on the finished rocket. The CG can change, so it makes little sense to mark the CG on the rocket.

The CP (or Center of pressure) is usually stated as distance from the tip of the nose cone. For rocket kits (like the Madcow Super DX3) the manufacture usually includes the CP in the instructions. Otherwise you will need a very accurate simulation to predict the CP. Carefully inspect and modify the simulation to exactly match the actual rocket dimensions. This is critical for an accurate CP prediction.

The CP never changes *unless* you change the design of the rocket. If you make the rocket longer or shorter. If you increase or decrease the total fin area, you will change the location of the CP.

Because the CP does not change, it can (and should) be marked on the body of the built rocket. This will enable you to check stability of the ready to fly rocket.

When you lay the rocket down and balance it. The point at which it balances is the CG (center of gravity).
A "caliber" is the diameter of the rocket. For a 4" inch (diameter) rocket, 1 caliber=4" inches.

When the RSO checks the stability of your rocket, he is looking for the the rocket to balance *at least* 1 caliber in-front-of the CP marking.

 

[Igotnothing]

Ima gonna take a guess at MP. I propose it means Mach Pressure. Like Center of Pressure, only adjusted to the most forward position it could slide forward to -either at the highest M number the rocket is expected to achieve, or at the max forward position the CP is going to creep forward to in supersonic flight. MP is then the most forward position of CP when it peaks at around M2.7. Sorry, previous poster. CP does change, a lot, above Mach.