Center of Pressure Estes Scion

Can anybody help me out and tell me where the center of pressure is for the Estes Scion? I just finished building mine and from the outside it looks like an unadulterated kit but on the inside I added an ejection baffle, metal eye bolts, longer shock cord, and I epoxied pretty much everything. Plus I made it eject half way down the BT and not at the nose cone. I know the changes can alter the CG some but I don't know about the Center of Pressure. I have this beautiful I205 that fits wonderfully inside and I was wondering if it would be too much motor or if it would displace the CG behind the CP. Thank you!

PS... I tried to figure this out by myself and I downloaded Open Rocket but I'm not very computer savy and I can't figure out how to open a .jar file. I don't have Winrar or whatever it needs.

RalPh8 said:

Can anybody help me out and tell me where the center of pressure is for the Estes Scion? I just finished building mine and from the outside it looks like an unadulterated kit but on the inside I added an ejection baffle, metal eye bolts, longer shock cord, and I epoxied pretty much everything. Plus I made it eject half way down the BT and not at the nose cone. I know the changes can alter the CG some but I don't know about the Center of Pressure. I have this beautiful I205 that fits wonderfully inside and I was wondering if it would be too much motor or if it would displace the CG behind the CP. Thank you!

PS... I tried to figure this out by myself and I downloaded Open Rocket but I'm not very computer savy and I can't figure out how to open a .jar file. I don't have Winrar or whatever it needs.

Click to expand...

The .jar file is a Java extension, download the latest version of Java, install it. Double clicking on the Openrocket.jar should open the file, its a pretty simple system. Winrar and other archiving programs won't do anything with a .jar

I've got a .ork file over on my .ork files thread... Post #99

RalPh8 said:

Can anybody help me out and tell me where the center of pressure is for the Estes Scion? I just finished building mine and from the outside it looks like an unadulterated kit but on the inside I added an ejection baffle, metal eye bolts, longer shock cord, and I epoxied pretty much everything. Plus I made it eject half way down the BT and not at the nose cone. I know the changes can alter the CG some but I don't know about the Center of Pressure. I have this beautiful I205 that fits wonderfully inside and I was wondering if it would be too much motor or if it would displace the CG behind the CP. Thank you!

PS... I tried to figure this out by myself and I downloaded Open Rocket but I'm not very computer savy and I can't figure out how to open a .jar file. I don't have Winrar or whatever it needs.

Click to expand...

I made an Estes Scion Rocket Sim model just now. I think it's CP calculation should be pretty good right now. It shows 36.55 inches from the tip of then nosecone.

K'Tesh said:

I've got a .ork file over on my .ork files thread... Post #99

Click to expand...

Nice looking model. I just modified a leviathan model I had, swapped the fins around and lengthened the lower tube to 21.5 inches. Our Cp values match nicely.
From the numbers shown on your image above, it looks like the Scion has less than 1 cal stability with an Estes BP F15 motor?

So you have the CP location of the Scion. The important thing to remember about CP is that it's location is affected by the shape of the thing moving through the air, not the weight of the thing.

If you don't have a sim program available you can still find a CP location like this:

Measure all external dimensions with this handy calculator:
https://physics.gallaudet.edu/tools/rocketcop.html

Go super old school with a cardboard profile:
https://www.apogeerockets.com/education/downloads/Newsletter18.pdf

Hope this helps.

I tried to check out the instructions, but Estes has not put them up on their website yet. Can the fins be mounted reversed (easily, or with a bit of tab trimming) such that they are conventional swept-back profile?

Trident said:

I tried to check out the instructions, but Estes has not put them up on their website yet. Can the fins be mounted reversed (easily, or with a bit of tab trimming) such that they are conventional swept-back profile?

Click to expand...

Yes.

Thanks.

That's about the only issue I have with the Scion, and not a big one as long as you calculate for it, the Cp is a bit far forward for my taste. All the other Cp numbers I'm seeing are pretty close to mine. I ended up with 36.278" back from the tip when I did it up in openrocket. It's not a problem if you build it stock, but if you used the coupler area for an electronics bay you may need some nose weight as the Cg shifts aft. Reversing the fins is no trouble either, as you have to slot the body tube yourself anyway. However you will have to measure new locations for the slots. Another option is leaving the fins forward swept, but adding one more, as the kit comes with 4 of them.

As long as you have the Cg at, or forward of, the bottom edge of the coupler tube, you'll be fine with the stock fin configuration.