# Downsizing Question

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#### texasck1

##### Well-Known Member
I am currently working on a 13mm cardstock version of a Sprint.

I calculated that everything would need to be 72.2% of the original dimensions (13/18=.72222). If I build everything to that dimension, can I also assume that the CP will be 72.22% of the original? Will this get me close?

The original CP is approximately 10.7 (from EMRR) and if I calculate it at 72.22% it would be 7.73. Am I making a reasonable assumption?

We all know what ASSUME means!

Thanks

That's what I have done on my downscales and it seems to have worked for me.

Originally posted by texasck1
I calculated that everything would need to be 72.2% of the original dimensions (13/18=.72222). If I build everything to that dimension, can I also assume that the CP will be 72.22% of the original? Will this get me close?

Yes.

Remember that the CG is likley to have moved. You probably knew that though.

Thanks for the answers. I thought I had it right, I just needed to have it confirmed.

I finished the build last night. I just need to figure the paint scheme. It will have to wait because I'm going to A&M vs. OU today. Maybe tomorrow!

Thanks again.

Actually, in one of the Apogee newsletters, Tim Van Milligen shows that the CP does not stay in the same place if you up or down scale a rocket. For a 72% downscale, I wouldn't think that it moves a huge amount, though.

I wish I could remember which newsletter it is.

Not according to the Barrowman equations.

If you scale everything down or up by a given factor, then the CP location will also be scaled by that given factor.

As an example, if you have a rocket 250mm long and the CP is 200mm from the nose, and you scale everything down by one half, then the rocket will be 125mm long and the CP will be 100mm from the nose.

Originally posted by hokkyokusei
Not according to the Barrowman equations.

If you scale everything down or up by a given factor, then the CP location will also be scaled by that given factor.

As an example, if you have a rocket 250mm long and the CP is 200mm from the nose, and you scale everything down by one half, then the rocket will be 125mm long and the CP will be 100mm from the nose.

I can't really debate it with you, but please go <a href=:https://www.apogeerockets.com/education/newsletter_archive.asp">here</a> and click on "Issue #80 - (5-3-02) Does the CP location change when you upscale a rocket?"

While there's hardly any change at all for a relatively minor scale change like the 72% downscale we're talking about here, there *is* a change when you change scale.

Originally posted by JRThro
I can't really debate it with you, but please go <a href=:https://www.apogeerockets.com/education/newsletter_archive.asp">here</a> and click on "Issue #80 - (5-3-02) Does the CP location change when you upscale a rocket?"

While there's hardly any change at all for a relatively minor scale change like the 72% downscale we're talking about here, there *is* a change when you change scale.

That link didn't work for me, I'm afraid.

I don't know what Tim said, so I can't debate it even if I wanted to.

As to what I said - try it for yourself.

From reading the article, I see that Tim is talking about dynamic analysis. He's also talking about a large upscale (factor of 20) and even then, the shift of CP isn't enough to make any practical difference. He also confirms what I said about the Barrowman equations.

So, if you're happy to use static analysis, such as the Barrowman equations, to calculate the CP in the first place, then you can assume that it doesn't change when you scale.

Guy's
I think your both correct, CP stays close to the scale involved but heres the rub. The CG can very wildly depending on the materials used and degree of increase or decrease in size. BT-50 to BT-5 downscale of a Sprint shouldn't cause any trouble however when going several sizes in either direction attention must be paid to the
CP CG relationship.

WOW, lots of great info.

JRThro
That is a great article. I see where there can be changes in the CP with the resizing. Lots of variables that can effect it.

I didn't realize the can of worms I was opening.

I'm going to be conservative and make sure the CG is plenty forward. I plan on launching it on an A10-3T. I'll make sure no ones around in case it becomes embarrassing. Wouldn't want any witnesses!

Thanks for the info. You guys are GREAT!

I'll post pics when it's painted.

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