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Moving CG Forward - Thoughts?

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mattvd

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I have a rather stubby 3" rocket with some heavy duty fins. I plan to launch with a range of motors . . . from a G70 to a J350. I am worried that the CG is about right for the G70, but will be too far AFT for the J350.

I came up with a solution using a system of interchangeable weights added to the nosecone depending on the flight configuration. Using 3 oz. weights, I cut one in half and left two whole. Using 1/4" screws I can now add either 3 oz. or 6 oz. (or nothing) depending on the need. I have not worked out the exact requirements in RockSim yet -- so I can use different weights if necessary.

Thoughts? Does anybody see any fatal flaws with this solution?

Thanks.

BluePhenix Add Weight to Nose Cone_sml.jpg


BluePhenix Add 3 Oz_sml.jpg


BluePhenix Add 6 Oz_sml.jpg
 

MarkM

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Great solution. This is similar to the way I did my 4" Bullpup so I could fly on a wide range of motors without adding any more noseweight than necessary.

One suggestion. Don't use Rocksim to determine your CG! Use it to determine you CP. Your CG should be one tube diameter forward of that. Mark it. Load the rocket up in flight-configuration mode (chutes, cord, etc). Add removable weight to the aft end (a baggie filled with a dense material like fishing sinkers works) equivalent to the wight of the motor+casing. Now, add weights to the nose. When the rocket balances on the edge of your hand at the necessary CG mark, you're good for that motor and rocket should be stable. Repeat for additional motors. I find this a far better method than using Rocksim to predict CG. This method uses the actual rocket, not a virtual one. Again, it's the way I determined the correct amount of noseweight for my Bullpup and always got perfect, stable flights from it.
 

mattvd

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Thanks. And, yes -- I do use the manual balance of the "ready to fly weight" to confirm everything. I will try your bag-o-sinkers trick.

I plan to launch a couple times at the next TTRA launch this weekend. Maybe I will see you out there.
 

terryg

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I always carry extra large quick links to the launch so that I can trim the CG if needed (add to the nose cone's eyebolt --- quick and easy).
 
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judo

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I'm building a 4" stubby with a 29mm motor mount. I built it out of left over pieces from my Photon Probe. The BT measures only 14" and it has an Aerotech nose cone. I cut off about half of the shoulder and Gorilla Glued a bulkhead with an eybolt deep inside the nose. It gave me room enough for the parachute and shock cord and moved the empty CG about half a BT forward. I'll be sure to pick up some quick links myself before first flight.
 

powderburner

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mattvd,

Your design looks good as far as being able to trim the ballast to whatever the launch configuratn requires. MarkM's advice is great also. And it may be too late to try my suggestn (if your rocket is already built), but for future reference:

Short stubby rockets can benefit significantly from putting the trim ballast into the tip of the nose, especially if the NC is a relatively long one (Big Daddy style). If the NC is solid balsa or foam you can bore a hole from the base, or if the NC is hollow plastic you can cut a hole through the base. You can use any number of approaches to insert various removable ballast loads and position them near the tip of the NC. Longer arm (distance from ballast to overall c.g.) means less ballast weight which means higher altitude.
 

mattvd

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Thanks for the advice. Yes -- I would rather get the ballast as far forward as possible. But, you have guessed correctly. The issue became clear after completing the nose cone.

Looks like a few people have used similar approaches -- thanks for the confirmation. I think this should work out well. I'll find out this weekend!

Thanks.
 

GaryT

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I was going to say the same thing by putting the weight as far forward as possible, I did do the same thing you did on rocket but I mounted the bulk head as far up inside the NC as I could, I have 4" Polecat Bullpup that I made DD by cutting the BT (not making it any longer) But the weight is permanent, I have an empty 2nd NC for that bird but I built it to fly mainly on the 38/720 loads J350 J500G etc. Your method should work just fine though, Good Luck!

Pics are J350W and J500G
untitled3[1].jpegBullpupJ350W[1].jpegBullpup J500G.jpg
 

SteelyEyed

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I have a rather stubby 3" rocket with some heavy duty fins. I plan to launch with a range of motors . . . from a G70 to a J350. I am worried that the CG is about right for the G70, but will be too far AFT for the J350.

I came up with a solution using a system of interchangeable weights added to the nosecone depending on the flight configuration. Using 3 oz. weights, I cut one in half and left two whole. Using 1/4" screws I can now add either 3 oz. or 6 oz. (or nothing) depending on the need. I have not worked out the exact requirements in RockSim yet -- so I can use different weights if necessary.

Thoughts? Does anybody see any fatal flaws with this solution?

Thanks.
_______________________________

I don't see any flaws with what you did. It should work great. Here's some photos of an adjustable nose weight technique that I have used on 4 inch and 5.5 inch airframe rockets (that are short/stubby designs requiring me to cut off the back of the nosecone and utilize the nosecone volume to make room for the parachute and shock cord). It's made from a sprinkler head and 8 oz. fishing weights.







 

BB-ROCK

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nice ideal you have there SteelyEyed. I may try that on one my projects.:)
 

mattvd

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SteelyEyed -- Thanks. Very neat idea. What is that fitting attached to the tip of the “canister”? I assume it is used to screw the whole assembly into the tip of the nosecone.
 

SteelyEyed

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SteelyEyed -- Thanks. Very neat idea. What is that fitting attached to the tip of the “canister”? I assume it is used to screw the whole assembly into the tip of the nosecone.
_____________________________________

That fitting in the end of the sprinker head is a brass pipe nipple. It just happened to fit nicely into the tip of the nosecone I was using and it served as a stand-off and added some desired nose weight. I drilled a couple of holes through the pipe nipple 90 degrees apart. Then I roughed up the inside of the nosecone with an awl and put a large blob of JBWeld in the tip of the nosecone and then stuck the brass pipe nipple into the JBWeld. The JBWeld epoxy would then flow into the deep scratches and the threads and holes to give it some "bite" to which to adhere. It worked quite well.

Cheers,
SE
 

AHansom

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How stubby is the rocket? On some really short rockets like my 29mm Big Daddy were the motor is long enough that it extend into the nosecone the CG can actualy move forward in launch ready config. Remember to check for proper ballance after motor burn out. For checking the CG with long motors I use some pieces of all thread or rebar cut to the same lenght as the motor to simulate the weight.
 

MitchR

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The sprinkler head idea is fantastic, I love it! It's on par with my shifter boot in my car (toilet plunger and a bath mat).
 

SteelyEyed

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The sprinkler head idea is fantastic, I love it! It's on par with my shifter boot in my car (toilet plunger and a bath mat).

I think your shifter boot is a great idea, but I think you just insulted my sprinkler head... :)
 
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