vertical grimmace

My Feedback: (1)

I was wondering, how do you determine where the center of gravity is, or should be on a model rocket? I just purchased a couple of large, scale rocket kits and just want to make sure they are balance properly. I am thinking just like aircraft, the CG is very important.
When I was a kid, I designed a few rockets and I remember a couple not flying well, and I am sure they were not balanced properly. Akin to being tail heavy on an aircraft.
Is there a rule of thumb, like 25% from the tail or something like that? After all, this is rocket science, and I just went to the public schools!

SGC

As`a general rule, were the CG is is not overly important- so long as its at least one calibre ahead of the CP (centre of pressure or aerodynamic neutral point), there are other factors as to CG/CP location that affect a rockets overal flyte characteristics, but the one caliber + rule is the main basic requirement. There are rocket design software available that will calculate these for you , Rocksim is an excelant commercial program or Openrocket is a free dounload( http://openrocket.sourceforge.net/ ) . You can imput/design your rockets on these programs and it will give you the CP and proposed CG-(needs confirming after build as amount of glue , density variations of components will vary), also a lot of the comercial kits will have sim files available you can plug ino the software to use to find the CP if the kit instructions dont give it, can also be used to find out the best motor options for your kit and expected altitudes etc.
Stewart

vicman

My Feedback: (10)

I seem to remember a swing on a string test where you tie the rocket to a string and swing it around like a CL plane and depending if it went nose up or down you would need to add weight to the nose.

SGC

Vic, Hiyu,
Ive used the string method in the past myself for small model rockets - it works. But as he mentioned buying large , scale rockets they maybe a bit large or due to scale detail ,difficult to attach a string ,that i mentioned useing the software.
Attached is an Openrocket report on my Binder Aspire which can be printed as a feild reference on launch days to aid in motor selection for the prevailing launch conditions etc , also note if the report is on an own designed rocket the full size fin pattern and fin location guide are included to aid construction.
Stewart

Jim_Purcha

My Feedback: (8)

Thanks for the Openrocket website. I'm currently leading somekids in a local 4H Club Rocket Program.

Jim

AH-64_d

Head to the library and check out "Handbook of Model Rocketry," by Stine and Stine. Chapter 9 answers your question. The rest of the book will answer any other questions you are likely to have about model rockets.

thailazer

When I was a kid, I put a large nose cone on a small Estes rocket and off we went. Big engine of course. The rocket took off doing tight spirals until it landed still firing at a truck yard full of gas tankers. The owner of the place was not impressed. I think it pays to learn where the CG is and to know where the mass/volume is on a rocket if you are into this kind of thing! I got the rocket back but put the original nose cone on.

ckreef

The string test is a crude but reliable test for stability. On a standard designed rocket - find where the rocket balances on your finger or pencil (forwrd to aft.) Tie a string to that position and secure the string with masking tape (a loop of string around the rocket). Hold the string and start spinning around in circles. Slowly let out more string going faster and faster in circles. Do this till about 10' - 20' of string is let out (the bigger the rocket the more string). IF the rocket continues to fly basically straight with nose forward you should be stable enough to maiden. Make sure you do this test w/ motor installed in a ready to launch configuration.

When satisfied w/ the test slowly reel the string back in. Once you have the rocket in hand sit down on the ground before you fall over from being too dizzy. At the end of the test if you are not about to fall down you didn't go in circles fast enough or didn't do the test long enough.

46u

My Feedback: (3)

They have free software to test you rocket. I have never used it yet but most say it is pretty good for free. Has any one here used it?