Calculating CP for the Vostok R-7

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grafgulch

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I am in the process of building a 1/50 scale Vostok R-7 . Since this is a model with 4 strap on boosters I'm having trouble understanding of to calculate the CP using the Barrowman Equation. I think RockSim can do this, but I don't have the funds to buy the program right now. Is there an example anywhere on how to calc this? I know I could do the swing test or the cardboard cutout method. But really want to make sure I can build this model and it be stable.

Thanks,
Paul
 

rocketguy101

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Paul
CP is a "geometry" thing, so if everything is scaled accordingly, the CP location will scale. CG differs due mass of the parts, etc. I don't remember exactly what I started with (may have just followed the methodology of the Cosmodrome simulation) but I modeled my 1/52nd scratch built Soyuz and came up with the CP location in the similar position as the Cosmodrome image.

I added weight to the nose to put the CG in the location per Rocksim, then I swing tested the beast. It did OK with and then I've flown it several times...works great! Even taped a cheapo keychain camera on it for on flight (dented booster with a hard landing but it popped out and flew again) here's that video

1638489486533.png
 

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grafgulch

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Paul
CP is a "geometry" thing, so if everything is scaled accordingly, the CP location will scale. CG differs due mass of the parts, etc. I don't remember exactly what I started with (may have just followed the methodology of the Cosmodrome simulation) but I modeled my 1/52nd scratch built Soyuz and came up with the CP location in the similar position as the Cosmodrome image.

I added weight to the nose to put the CG in the location per Rocksim, then I swing tested the beast. It did OK with and then I've flown it several times...works great! Even taped a cheapo keychain camera on it for on flight (dented booster with a hard landing but it popped out and flew again) here's that video

View attachment 492388
Thanks for your advice.. the only reason I thought the CP might change was from seeing this article from Apogee. Newsletter80.pdf (apogeerockets.com)

paul
 

lakeroadster

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Thanks & yes I have seen that file, but I thought I read somewhere that the CP location changes the models scale factor. AM i overthinking this?
Your not overthinking it. It'll change some, but not much if everything is close to scale. But unless you're making everything, the model will likely have subtle changes that will change the CP a little.

Looks like a neat build... have you started a build thread?
 

grafgulch

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Your not overthinking it. It'll change some, but not much if everything is close to scale. But unless you're making everything, the model will likely have subtle changes that will change the CP a little.

Looks like a neat build... have you started a build thread?
I would love to do a build thread, but I work on computers all day and the chore writing a build report and actual building the model keeps me from doing so.
 
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