Rocksim Question and CG

The Rocketry Forum

Help Support The Rocketry Forum:

AfterBurners

Well-Known Member
Joined
May 8, 2012
Messages
7,840
Reaction score
460
Location
Somewhere on the planet
A lot of time when I would buy a kit or scratch build a rocket I would always make sure that the CG 1.5 ahead of the CP. Recently I purchased a couple kits from Madcow for instance the 4" Patriot.

It states in the instructions that the CG should be 28" from the nose tip. Now in Rocksim when you add mass to the nose cone you have a few selection. I selected "From the Nose Tip" which should match the specifications in the instruction by MADCOW. I know I'm answering my own questions here.

With 14oz of nose weight the puts the CG 29.9 from the tip of the nose cone and a margin of 1.60

Keep in mind I haven't added the nose weight yet. I guess my question is if I'm off a little is it going to make much of a difference or do most of you just go by the 1.5 rule, meaning the CG needs to be ahead of the CP by 1.5 x the airframes diameter in this case 5.5 inches.

I remember when I built the 2.2 Batray I added nose weight based on the 1.5 method or thereabouts and when I check where the CG was from the tip in Rocksim I was right on my calculations (17" from the tip and Madcow suggested 17.75 per instructions). Unfortunately this was after the first flight that corkscrewed without any nose weight. Since then I learned to always double check my numbers. Although with OR the numbers probably vary.

So I'm just looking for some feed back
 
Last edited:

blackbrandt

That Darn College Student
Joined
Mar 18, 2012
Messages
9,281
Reaction score
42
I tend around 1.5. I go slightly higher for anything going stupid fast.
 

RocketFeller

Well-Known Member
Joined
Sep 15, 2009
Messages
1,185
Reaction score
72
If you are dealing with short, fat rockets you don't need 1.5 caliber stability. Rocksim doesn't take base drag into consideration. Our upscale Dragonfly only sims to 1.15, but it it super stable to the point that it backslid momentarily at apogee before deployment.
 

AfterBurners

Well-Known Member
Joined
May 8, 2012
Messages
7,840
Reaction score
460
Location
Somewhere on the planet
If you are dealing with short, fat rockets you don't need 1.5 caliber stability. Rocksim doesn't take base drag into consideration. Our upscale Dragonfly only sims to 1.15, but it it super stable to the point that it backslid momentarily at apogee before deployment.
I wouldn't say that any of the rockets I'm dealing with are actually short and fat like a FATBOY from Polecat or maybe a SUMO. Take for instance the Jayhawk. Anything that has a lot of wood on it or is scaled seems to need a bunch of nose weight. For I just stay between 1.25 and 1.60
 

Rex R

LV2
Joined
Apr 21, 2010
Messages
6,101
Reaction score
147
For the patriot you want the cg 28" or less from the tip of the nose (with the biggest motor you intend to fly), remember to factor in the weight of the epoxy binder when weighing out your ballast (lead shot/bb's/whatever). 14 oz.s sounds about right for a 38mm 2-grain motor.
Rex
 

markkoelsch

Well-Known Member
Joined
Mar 18, 2009
Messages
4,364
Reaction score
149
On many scale kits you will need a fair amount of nose weight. Why you ask. Well, almost all of these were in reality actively guided missiles whereas your rocket is not.
 

RocketFeller

Well-Known Member
Joined
Sep 15, 2009
Messages
1,185
Reaction score
72
Right on. I was just talking in generalities. This is a subject we did a lot of talking about as we were doing simulations last month. Apparently some "stubby" rockets fly on one-half caliber of stability.

https://www.rocketryforum.com/showthread.php?87301-Stubby-rocket-stability


I wouldn't say that any of the rockets I'm dealing with are actually short and fat like a FATBOY from Polecat or maybe a SUMO. Take for instance the Jayhawk. Anything that has a lot of wood on it or is scaled seems to need a bunch of nose weight. For I just stay between 1.25 and 1.60
 

AfterBurners

Well-Known Member
Joined
May 8, 2012
Messages
7,840
Reaction score
460
Location
Somewhere on the planet
For the patriot you want the cg 28" or less from the tip of the nose (with the biggest motor you intend to fly), remember to factor in the weight of the epoxy binder when weighing out your ballast (lead shot/bb's/whatever). 14 oz.s sounds about right for a 38mm 2-grain motor.
Rex
Rex- as always thank you. If I put 17oz of nose weight in the Patriot the CG will be at 28" When I run a flight simulation in RS I notice when the rocket gets about half way up, the nose gets wobbly. I know there is a technical name for this, but I figure someone will chime in. Now this flight is on a J500-10 which is what I plan on using for my certification and largest I plan to fly. Actually a J825-10 gives a slower deployment @ 1.07mph verse 12 mph.
 
Last edited:

AfterBurners

Well-Known Member
Joined
May 8, 2012
Messages
7,840
Reaction score
460
Location
Somewhere on the planet
Not sure how many of you can download this file, but I'll post it anyway. I kept the nose weight at 14oz, because I still have to factor in the weight of the epoxy and wood dowel.

Stupid me I went on Madcow's website and ordered a bulk plate only to realize after I placed the order that I really don't need because the nose cones attached to the payload via plastic snap rivets and there's reason to have one. No biggie I can always use it for something else and I was also able to get a couple 9x9 chute protectors for under $6 each

See file below

View attachment patriot.rkt

Here's a screen shot in Rocksim

2016-07-04.jpg

Loaded with a J500-10

2016-07-04 (1).jpg
 
Last edited:

AfterBurners

Well-Known Member
Joined
May 8, 2012
Messages
7,840
Reaction score
460
Location
Somewhere on the planet
Here is the RS file on my 3" PML Bull Puppy in stock form with no nose weight. The best motor for this seems to be a 29mm H97J-10, which flies excellent and keeps it within a reasonable altitude to visually track it. The CG and CP are in the right locations.

2016-07-04 (4).jpg

Whereas with J500-10 it changes everything. It drops it to .86 but then again the rocket goes over 6100' So I figure the best solution if I want to fly the bigger motors is just to add a payload with DD or do a HD if possible? Just buy another nose cone and use this set up for high flights only.

2016-07-04 (5).jpg

Rocksim File

View attachment Bullpuppy_38mmMAC-8.rkt
 
Last edited:

ksaves2

Lifetime Supporter
TRF Lifetime Supporter
Joined
Nov 25, 2009
Messages
6,136
Reaction score
398
Location
Central Illinois
[video=youtube;aDCFS2JYAa0]https://www.youtube.com/watch?v=aDCFS2JYAa0[/video]

Is what happens with a full scale rocket when one doesn't pay attention to the CG. Occurs right at the beginning of the tape. Rocket flew fine on an M motor for an L3 flight. Went straight up on an N motor but suffered some aft end damage when
one of the two chutes streamered and it hit hard. The O motor made it too butt heavy and the CG wasn't rechecked before the fateful flight shown. Whatever one does, don't get complacent.
If going really stupid fast, use RAS Aero as it shows the CP shift at high mach speeds. For most folks it's reassuring but whoeee, does CP really start moving forward above Mach 4. Kurt
 

ksaves2

Lifetime Supporter
TRF Lifetime Supporter
Joined
Nov 25, 2009
Messages
6,136
Reaction score
398
Location
Central Illinois
Probably should (continue to) add nose weight
Yeah, I thought I had enough nose weight in a 29mm motor powered Jart. Flew off at a funky angle but due to a stout harness and the fiberglass construction the high speed deployment didn't hurt a thing. I used an NC bulkhead with a hole in the middle so I could stuff the harness up in the nosecone too. I added more weight and epoxy and all is well now. Kurt
 

AfterBurners

Well-Known Member
Joined
May 8, 2012
Messages
7,840
Reaction score
460
Location
Somewhere on the planet
When you use the dowel method to hold the mixture of epoxy and BBs is there a preferred method of where the dowel should go through?

Do you want it above the weight or should it pass through the middle portion of the BBs? Is theree difference in the placement?
 

Rex R

LV2
Joined
Apr 21, 2010
Messages
6,101
Reaction score
147
I like to have the skewer(s) covered by the epoxy, pour the mass (w/o epoxy)into the nose, using a strong light mark where the mass ends, add a measured amount of water to the nose until the mass is covered(this will tell you how much epoxy to use), pour out the whole mess and let dry. drill the nose for the skewers (a 12" long bit works wonders). for me the '2-ton' epoxy(30 minute) doesn't get warm enough to heat up both the lead shot and the nose past the warm stage, your epoxy may vary.
Rex
 

AfterBurners

Well-Known Member
Joined
May 8, 2012
Messages
7,840
Reaction score
460
Location
Somewhere on the planet
I pulled up both the ROCKSIM files for the Patriot and Jayhawk kits from MADCOW and downloaded them. It's true the Jayhawk's CG is 19" from the tip loaded with a G76-7 and only has 3 oz of nose weight added, which puts it at .11 margin? I was scratching my head wondering how that is, until I realized that the settings in ROCKSIM was using the "Barrowman Stability Equations" See below.

View attachment jayhawk.rkt

So I went ahead and pulled up the Patriot design from Madcow's website. So I pull up the Patriot and loaded with a J825-10 with 12oz of nose weight using the "Barrowman Stability Equations" it comes in at .03 marginal. The CG is 32" from the nose tip.

View attachment patriot.rkt

I do know that the Barrowman is more like old school as if you were to find the balance point by taking the rocket and balancing it on the edge of something correct? So why is the marginal off by so much? The CG is almost in the same spot. I'm sure with a couple more ozs of weight it would be at 28" Then the Jayhawk only has 3oz of weight to get the CG at 19" from the nose tip whereas using ROCKSIM its takes a lot more weight.

My question is why and is one method better than the other? I've always used the ROCKSIM method and went with 1.5 theory and never had issues.

AB
 
Last edited:

rstaff3

Oddroc-eteer
Joined
Jan 18, 2009
Messages
11,759
Reaction score
14
The Rocksim method uses enhancements to the basic Barrowman equations and always provides more stability margin. At least in my experience. On general tame designs, I generally go for a margin of one with the largest motor I plan to use. On more oddball ones, I will add weight to increase the margin. That comes from a little distrust when I add pods on pods to model the design, and because I often have to cheat on the model (example: Rsim still doesn't do curved fins) to get a sanity check. Note that if you use pods you have to use the Rocksim method.

So which is better. For me I say Rocksim.

My Jayhawk's CG is at 16" without motor. I forget the thought process that got me there, but it is heavier than stock. IIRC the recommended motors stopped at F's? I have flown on a G78 and a G64 and I probably added weight to balance with the larger motor. The prior swooped around a bit but stayed in the up direction for a decent flight. The G64 didn't have that issue.
 

AfterBurners

Well-Known Member
Joined
May 8, 2012
Messages
7,840
Reaction score
460
Location
Somewhere on the planet
So on your basic 3-4 fin rocket design stay 1.0 and above and anything with a lot wood, scaled or awkward design at least 1.5 for me I tend to stay 1.25-1.60 on most designs. I usually base this with the largest motor I intend fly. Maybe I should stick with what I know works best.
 

Rex R

LV2
Joined
Apr 21, 2010
Messages
6,101
Reaction score
147
just to muddy the waters, the cg as suggested by Mad cow will give you a 1 cal. margin.
Rex
 

Rex R

LV2
Joined
Apr 21, 2010
Messages
6,101
Reaction score
147
dunno which method MC uses(have to ask them) they are on record as saying that their numbers will give the racket 1 cal. margin, seems to work :).
Rex
 

rstaff3

Oddroc-eteer
Joined
Jan 18, 2009
Messages
11,759
Reaction score
14
So on your basic 3-4 fin rocket design stay 1.0 and above and anything with a lot wood, scaled or awkward design at least 1.5 for me I tend to stay 1.25-1.60 on most designs. I usually base this with the largest motor I intend fly. Maybe I should stick with what I know works best.
First off, everyone is responsible for ensuring their rockets fly safely. While I sometimes believe a vendor (who of course had flown every kit many times with different motors and weather conditions :eyeroll:) I like to verify things for myself. If that's what you are comfortable with, I'd say 1.5 is fine. Also verify thrust ratios and rod speed etc. Note that if all the recommended motors are Fs, their GG may not be sufficient when loaded with a G.
 

rstaff3

Oddroc-eteer
Joined
Jan 18, 2009
Messages
11,759
Reaction score
14
Oh, just struck me, the Jayhawk Rsim requires the Rocksim method due to the tip winglets. If Rsim allows the Barrowman and gives other values, that is a BUG!
 

AfterBurners

Well-Known Member
Joined
May 8, 2012
Messages
7,840
Reaction score
460
Location
Somewhere on the planet
This is my new ROCKSIM file of the Patriot. Got off the phone with Mike and he cleared things up. Rex you are right. MC's calculations on their instructions is exactly 1.0 Whenever you change between the ROCKSIM and Barrowman Stability Equations the CG stays at 28" from nose cone tip, whereas the CP drops from 36" using RS to 32" using Barrowman. This design file has 23oz of nose weight added to get the 28" from the nose tip. This weight will have to include the BB's and also the epoxy. I figure using water will get me close to the weight of the epoxy when measuring / weighing it out. With a loaded J825-10 CG = 28" CP = 32" Margin is at 1.11

View attachment patriot.rkt
 

Rex R

LV2
Joined
Apr 21, 2010
Messages
6,101
Reaction score
147
Devcon 2-ton epoxy has(when properly mixed) an average density of 1.1g/cm^3(resin = 1.2g/cm^3, hardener = 1g/cm^3). other epoxies may vary.
Rex
 

Rex R

LV2
Joined
Apr 21, 2010
Messages
6,101
Reaction score
147
oh, and a stock MC Jayhawk does very nicely on an F40-7.
Rex
 
Top