Rocksim Question and CG

Rex R said:

oh, and a stock MC Jayhawk does very nicely on an F40-7.
Rex

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I'll try it but I had something bigger in mind for it.

OK I know the rule of thumb is to determine your CG and CP based on the largest motor you plan to fly in your rocket. In this case my 4" Patriot will be my L2 flight. Nothing fancy just motor ejection and get it back in one piece.

All calculations are based from the tip of the nose cone.

With a loaded J500G-10 I get following numbers:

ROCKSIM Calculation

CG - 27.3
CP - 36.3
2.25 Margin

Barrowman Calculation

CG - 27.3
CP - 32.4
1.28 Margin

Now my question is this. If I wanted to fly a smaller motor say a 38/360 it will definitely change the calculations quite a bit. As you can see below:

With a loaded H112J-6

ROCKSIM Calculation

CG - 25.3
CP - 36.3
2.73 OVERSTABLE

Barrowman Calculation

CG - 25.3
CP - 32.4
1.76 Margin

As you can see the CG is ahead of the CP with the H112J-6 motor compared to the J500G-10 motor. Is this something that I should have concern for? In the instructions for the kit when it states that the CG should be 28" from the tip of the nose cone, it doesn't state what motor was installed when calculating the CG and CP. After exchanging some emails with Mike, who by the way was extremely helpful, he mention that both ROCKSIM and BARROWMAN will do their best to calculate these figures, but ROCKSIM is a bit less conservative and when using it the CP will be off a noticeable difference from Barrowman. I doubt it would do much harm, it just mean that the rocket would be able to fly if the wind kicks up. I would like to know at what point does it get questionable?

Before I never really paid much attention to the instructions on the MADCOW kits where the CG is from the tip of the nose cone. I would simply add weight to the nose using Rocksim until I had a marginal of 1.25-1.50 depending on the design. I never even bothered using Barrowman's calculations and I hate to admit this, but didn't know what the difference was up until now.

So I just want to make sure I have my numbers correct before I start mixing the epoxy and adding the weight.

I wish I knew someone who built this kit to offer some insight. 23oz of nose weight seems like a lot, but that's what it takes to get the CG at 28" from the nose tip with a J500-G-10

BTW - I decied for now on moving forward all my first flights on a new rocket will fly in primer. If I get a half dozen good flights then the rockets has earned it's paint.

you mean like this one ? mine is balanced for a two grain 38mm. bigger motors require a payload(a pound of cheese for an I211). provided you can get a high enough air speed off the rail I wouldn't worry much about it being 'over stable' just means you will not need to walk as far to recover, not to mention that with those 'tiny' fins patriots don't tend to weather vane as much as you might think.
Rex

Rex R said:

you mean like this one ? mine is balanced for a two grain 38mm. bigger motors require a payload(a pound of cheese for an I211). provided you can get a high enough air speed off the rail I wouldn't worry much about it being 'over stable' just means you will not need to walk as far to recover, not to mention that with those 'tiny' fins patriots don't tend to weather vane as much as you might think.
Rex

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Exactly like that one. Except mine is primer.

A pound of cheese ... so I guess my numbers are pretty accurate. Thanks for sharing

Over stable is a crock. A 2.73 margin is not excessive. Heck, if the rocket were to go really fast it might be preferable as the Cp moves forward the faster you go.

markkoelsch said:

Over stable is a crock. A 2.73 margin is not excessive. Heck, if the rocket were to go really fast it might be preferable as the Cp moves forward the faster you go.

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That's just what ROCKSIM has.... anyway like I said before just want to make sure everything is right on before I add the weight etc.

Rex R said:

you mean like this one ? mine is balanced for a two grain 38mm. bigger motors require a payload(a pound of cheese for an I211). provided you can get a high enough air speed off the rail I wouldn't worry much about it being 'over stable' just means you will not need to walk as far to recover, not to mention that with those 'tiny' fins patriots don't tend to weather vane as much as you might think.
Rex

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Rex - On your paint job. Is the lower black band painted on or is that a decal. It looks like it goes in between the fins and that's how it appears on the instruction card. I always thought it was a decal?

Did Mike indicate how he came about 28" and what he calculated the Cp to be? I find the Madcow instructions to be very odd in this regard - give an ambiguous CG, but no Cp.

I've built 4 MC rockets(one in the pile) and the all gave the minimum safe cg distance in that fashion, it is just the way MC does instructions. at least the patriot is a 'normal' rocket that can be computer modeled with relative ease.
Re: decals
lol; the only decal I got with mine was the 'US ARMY', everything else is paint, the lower black band actually extends forward of the fins by about an inch or so, and yes it was a pain to mask off. the upper band is easy, it is on the payload section, no pesky fins to get in the way.
Rex