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Rocketbuilder
16th May 2012, 12:29 AM
Can anyone give me the CG location on a stable Saturn 1 SA-5 rocket? I would prefer a picture of some sort, since the one I'm building is a little bigger than the Dr. Zooch model.

stantonjtroy
16th May 2012, 01:47 AM
I swear this is not meant as a Smart A&& answer. It's 1.5 calibers ahead of your CP and the CP is where it calculates for your particilar rocket. The change in location vs scale is exponential not linear so you really need to calculate the CP (sim or long hand) and then figure the CG. Not the answer you were looking for. Sorry.

Saluki
16th May 2012, 02:26 AM

http://f1.grp.yahoofs.com/v1/EPyyTz4kZ-tJoQM5tvJACMW2quexq7Wkc78Fj0CaImApl2old29B7KVwX-cwmOgCfYHSMoHF5IVVGCStHanTFDFatdNa/Stabilizing%20Plastic%20Models0001.pdf

Rocketbuilder
16th May 2012, 12:37 PM
I swear this is not meant as a Smart A&& answer. It's 1.5 calibers ahead of your CP and the CP is where it calculates for your particilar rocket. The change in location vs scale is exponential not linear so you really need to calculate the CP (sim or long hand) and then figure the CG. Not the answer you were looking for. Sorry.
Now that you say that, I do recall reading that CP changes with upscales and downscales. Oops. :blush: I don't have Rocksim, so could I sim the body with the multiple fuel tubes as a single tube, without throwing off the CP location?

Rocketbuilder
16th May 2012, 01:30 PM
Update: I found and read the Apogee article about how the CP location changes wen you up/downscale a rocket. Apparently, when you upscale, the CP moves backward. So, since my SA-5 is bigger than the Dr. Zooch SA-5, if I made the CG location about the same relative to the rocket, it will be stable because the CP will be farther back, right?
So I would really appreciate if if someone could post a picture of the CP/CG location of the Zooch A-5 in Rocksim. :)

stantonjtroy
17th May 2012, 12:05 AM
Try here; http://meatballrocketry.com/sa5-2008/. I go here often as I love the Saturn 1 rocket family and Josh has raised building it to an art form. The site has the SA-5 Saturn I in two different scales with CG, CP info for each. based on that you can get in the ball park and just add a little more spread with nose weight to prop up your stability margin. As to the question of siming the eight tubes as one, yes you can. I've built three different S1B's in three drastically different scales and did this for stability calculations for each. All three flew straight as an arrow. Remember also, 1.5 caliber is a rule of thumb minium. You can always add a little more to increase stability. Best of luck and keep us posted.

Rocketbuilder
17th May 2012, 01:01 PM
Thanks Stantonjtroy, I looked at Meatball Rocketry, but unfortunately the 1/59 was a lot bigger than mine, and the little one was a lot smaller.

Could someone with Rocksim please post a picture of the Dr. Zooch SA-5 file? That would be exactly what I need. :)

Meatball 1
27th May 2012, 02:34 PM
The scale shouldn't matter, as long as your SA-5 is proportionally the same. The Meatball Rocketry SA-5 was built true-to-scale based on the drawings by Peter Alway that are at this page: http://meatballrocketry.com/saturn-i-data/

So take my CG/CP Graphic here: http://meatballrocketry.com/wp-content/gallery/scale_models/sa5_2008/sa5_misc/CG_CP.gif

So 3.28" x 59 = 193.52" below the fairing at the top of the first stage on the real SA-5. You can then calculate based on the scale of your model. Of course, there are no guarantees on the CG, especially since I can't account for any scale variations of your model; however, this position worked for me on several test flights and the final flight. On the final flight the CG was maybe slightly forward by ~1/4".

Good luck. :)

DumasBro2
13th June 2012, 01:20 PM
http://georgesrockets.com/GRP/AOL/GCGassaway/SCALE/saturns/Saturn_IB_drwg.GIF

Here's George Gassaway's drawing.

Rocketbuilder
13th June 2012, 09:17 PM
The reason I asked tho question in the first place was because I remembered reading an Apogee Peak of Flight newsletter article that said that the CP moved back when you upscaled a rocket, and forward when you downscaled a rocket. Since my SA-5 is essentially a downscale of the much larger Meatball Rocketry SA-5, I was worried that the CP would be proportionally further forward enough to destabilize the rocket. However, after finding the article and reading it again, I see that the proportional change in the position of the CP was relatively minor at an upscale of 20X (they were using a Mosquito as an example), so I shouldn't need to worry about it on my model, as the it is much more than 1/20th of the size of the Meatball SA-5. I will, however, adjust the CG a little further forward, just to be sure.
Thanks a lot! :) I'm finally motivated to finish the rocket! :fly:

georgegassaway
15th June 2012, 07:02 AM
http://georgesrockets.com/GRP/AOL/GCGassaway/SCALE/saturns/Saturn_IB_drwg.GIF

Here's George Gassaway's drawing.

Thanks for remembering, but that's for the Saturn-IB. Very different from the SA-5.

I swear this is not meant as a Smart A&& answer. It's 1.5 calibers ahead of your CP and the CP is where it calculates for your particilar rocket. The change in location vs scale is exponential not linear so you really need to calculate the CP (sim or long hand) and then figure the CG. Not the answer you were looking for. Sorry.

Have you ever tried to calculate a CP for a rocket as complicated as a Saturn-I? Because it is so complicated, and your answer seems to imply it is "easy", that your answer does come off as partly smart-*** for that reason. :) :) :)

I only know what the "Stable" CG location is for a Little Joe-II, not from fancy calculations, or even rule of thumb methods that don't like complex rockets either. I know the stable CG location thanks to a couple of unstable flights from when the CG was below the location of the "N" in the lettering UNITED STATES. The Saturn-IB is also a notorious "stable one flight, unstable the next" vehicle, if the CG is marginal. The known-to-be-stable CG location I list for that (in the drawing of mine mentioned above) is thanks to Jay Marsh, who found out the hard way that it was not stable (reliably) when the CG was more to the rear of the known to be stable location.

- George Gassaway

stantonjtroy
16th June 2012, 03:34 AM
Actually I did do the calculations long hand (For the Saturn 1B) and didn't find it particularly difficult. Granted there was ALOT of number crunching but none of it too tough. Just time consuming and realy need to keep it organized. I've applied these methods to 4 different 1B's in 4 differing scales from 1.5" SIV-B frame to a 6" SIV-B frame and got numerous flights on all of them. Naturally sims became available shortly after but even then I continued to work the numbers out long hand until I became more confident with the figures the sims were producing. I still do it from time to time with my kids to show them how to apply math to rocketry. My 13yo daughter can do it but prefers the sim; I don't think she gets the point. :)

Thanks for remembering, but that's for the Saturn-IB. Very different from the SA-5.

Have you ever tried to calculate a CP for a rocket as complicated as a Saturn-I? Because it is so complicated, and your answer seems to imply it is "easy", that your answer does come off as partly smart-*** for that reason. :) :) :)

I only know what the "Stable" CG location is for a Little Joe-II, not from fancy calculations, or even rule of thumb methods that don't like complex rockets either. I know the stable CG location thanks to a couple of unstable flights from when the CG was below the location of the "N" in the lettering UNITED STATES. The Saturn-IB is also a notorious "stable one flight, unstable the next" vehicle, if the CG is marginal. The known-to-be-stable CG location I list for that (in the drawing of mine mentioned above) is thanks to Jay Marsh, who found out the hard way that it was not stable (reliably) when the CG was more to the rear of the known to be stable location.

- George Gassaway

Meatball 1
16th June 2012, 03:31 PM
Short rockets like the Saturn I don't do well for standard n-caliber CG/CP relationships because the caliber needed will likely be smaller than that which is typically needed for longer, (http://www.apogeerockets.com/education/downloads/Newsletter154.pdf). In the case of the Saturn I, I suspect the combination of base drag and progressively larger diameters in the lower half of the airframe tends to offset the normal caliber considerations. As for needing to do calculations based on a changing model scale, if I understand correctly, neither Barrowman nor RockSim results will show proportional changes in CP location when a rocket's scale in changed (http://www.apogeerockets.com/education/downloads/Newsletter80.pdf) because Barrowman does not account for actual drag coefficient values. Even so, I think manual stability calculations are way cool, even though I haven't actually tried that since I was 14-15 years old.

But George is correct. There are things that are hard to account for within RockSim, such as odd-shaped surface details, clustered tanks, fin chord profile (probably lots of base drag for square-bottomed Saturn fins). I'm not sure how easy it is in newer versions of RockSim to account for things like that, but back in 2003 Tim Van Milligan shared a Saturn IB RockSim file with me to help me figure out how to make a file for the 1:59 SA-5 test model. The eight tanks were there as internal tubes IIRC, but so was an "invisible" zero-mass outer tube to represent the tank cluster for CP calculations. In my own file I kept the surface details to a minimum, but I'm sure they make some difference with there being so many of them. So at best my CP location shown in the diagram is an approximation. But the good news is that my CG location has worked for all the test flights and NARAM-50 flight, and assuming the CP is more or less correct, that's a .45 or .47 caliber relationship, depending on which diameter one is considering.