Upscale Astron Sprite cg?

[jungmann1]

Hey guys, hope someone can give some idea on this. Building a 4.5X upscale of the old Astron Sprite and need some idea of where to start on the CG placement. Want to put a rail guide right on the cg, figure that would allow easy swing test. Using the Peter Alway plans as a guide but he doesn't mention anything about cg. Hope someone can lend a hand here. Thanks in advance. John.

 

[jungmann1]

That is just a dry fit, nothing glued together yet so I can add hardpoints for the rail guides. Also, still have to make the little pod extensions for the fins.

 

[jungmann1]

A couple more pics now that I have the tip pods cut out. Still everything dry, no glue until the hard points for the rail guides go in.

Some specs: Mad Cow nose cone, Loc 4" main airframe tube, found 10 inch ring fin, 1/4 inch aircore composite fins, 29 mm mount, current weight: 19.25 ounces,

 

[ScrapDaddy]

At 19.25 Oz I wouldn't use tumble recovery

 

[jungmann1]

Yeah, if all goes well this one will be returning under a nice reliable nylon chute! I also probably won't be punching that big vent hole in the side of the body tube but I might have to simulate it with a black disc, like the Roadrunner used to use all the time. This is right at 27 inches tall. Sure wish I could find some CP info on this. Haven't been able to model it in RASaero and Rocksim doesn't work on Vista operating system yet.

 

[Pantherjon]

Rocksim doesn't work on Vista operating system yet.

Rocksim will work on Vista, you just need to do a couple extra steps to get it to work..I can't recall what they all are as I run XP..Something about setting up an emulation box or window, I don't recall right now..But, I do recall seeing some posts either here or over on YORF on getting RS to work on Vista..

 

[jungmann1]

Well, let me put it this way, I can't make it work on Vista. I talked to Tim Van Milligan a few months ago about a Vista version and he gave me some steps that were supposed to let it work but I could never get it to go. Computers and I have never really gotten along that well.

 

[MarkII]

Can you take something like a hardcover book and stand it up between a couple of stacks of heavy books so that it is very stable? And then lay your rocket on the top edge of the upright book and slide it around until it balances? That will be your center of gravity.

I used an example of a book because the top edge of the standing book will probably fit between your fins if it becomes necessary to balance it there.

MK

 

[ScrapDaddy]

Can you take something like a hardcover book and stand it up between a couple of stacks of heavy books so that it is very stable? And then lay your rocket on the top edge of the upright book and slide it around until it balances? That will be your center of gravity.

I used an example of a book because the top edge of the standing book will probably fit between your fins if it becomes necessary to balance it there.

MK

I think he ment where SHOULD it be. Well, you could use a rocksim simulation to find the CP then your CG should be 1 cal in front of that.

 

[cjl]

Well, let me put it this way, I can't make it work on Vista. I talked to Tim Van Milligan a few months ago about a Vista version and he gave me some steps that were supposed to let it work but I could never get it to go. Computers and I have never really gotten along that well.

Rocksim should work in both Vista and 7 - I'm running it on both right now. Try right clicking on the rocksim icon, going to "Properties", then "Compatibility", and then set it to run in compatibility mode for Windows XP, and also select the "Run As Administrator" option. That should fix most compatibility problems.

 

[jungmann1]

I think he ment where SHOULD it be. Well, you could use a rocksim simulation to find the CP then your CG should be 1 cal in front of that.

ScrapDaddy is correct. I need to find the CP so I know how much ballast to add to bring the CG to where it will be stable, approx. one caliber ahead of the CP. Thanks for the input guys, might have to wrestle with Rocksim some more.

 

[MarkII]

The location of the CP should be comparable to the CP's location in the original Sprite, assuming that your build is a true upscale. Once you determine that, then you can adjust the CG to where it should be for a stable flight. I assumed that you had already located the center of pressure on your build.

MK

 

[ScrapDaddy]

ScrapDaddy is correct. I need to find the CP so I know how much ballast to add to bring the CG to where it will be stable, approx. one caliber ahead of the CP. Thanks for the input guys, might have to wrestle with Rocksim some more.

Cha Ching!

Score 7 for SD! :wave:

 

[cjl]

Cha Ching!

Score 7 for SD! :wave:

 

[MarkII]

Uhh, I don't know - ask RockSim.

Anyway, like I said, if you use comparable proportions and straight scaled-ups of the fins in your build, then the CP location should be in about the same proportional location as it was in the original Astron Sprite. It will be the CG that will vary due to the difference in parts densities, etc. In order to adjust the center of gravity to a suitable location, you must always find the location of the center of pressure first. You can adjust the CP's location by changing the length of the airframe or the size or sweep of the fins, but most people find it easier to adjust the location of the center of gravity instead. As a result, the CP's location is usually thought of as being stationary (fixed in place by the overall design), while the CG's location is regarded as movable (changed by shifting the distribution of mass).

Dave Rose posted an article on EMRR about his scratch-built Superscale Sprite, which was also based on LOC 4" tubing. You can find his RockSim file here.

MK

 

[Pantherjon]

But isn't a Sprite in the class of short/fat(height challenged/wide) rocket? And that these type rockets can fly stable and straight with less then 1 caliber of separation from the CP to the CG? I know that I added a TON of not really needed nose weight to get the CG to be one caliber forward of the CP in my 4x18mm Fat Boy..Also, on the Sprite with that ring and other doo-dads creating a ton of drag I think it can get by with less then 1 caliber.:2:

 

[ScrapDaddy]

That's not politicly correct, The Sprite is umm... Well.... A "Horizontaly Gifted" rocket

 

[cjl]

But isn't a Sprite in the class of short/fat(height challenged/wide) rocket? And that these type rockets can fly stable and straight with less then 1 caliber of separation from the CP to the CG? I know that I added a TON of not really needed nose weight to get the CG to be one caliber forward of the CP in my 4x18mm Fat Boy..Also, on the Sprite with that ring and other doo-dads creating a ton of drag I think it can get by with less then 1 caliber.:2:

Yes, you can usually get away with <1 caliber on the stubby rockets. I tend to go with ~15% of the length on stubby rockets (rather than 1 caliber). You could probably get away with 10% of the length if you wanted though.

 

[ScrapDaddy]

We've met on threads, you know I'm not right very often.....

 

[MarkII]

But isn't a Sprite in the class of short/fat(height challenged/wide) rocket? And that these type rockets can fly stable and straight with less then 1 caliber of separation from the CP to the CG? I know that I added a TON of not really needed nose weight to get the CG to be one caliber forward of the CP in my 4x18mm Fat Boy..Also, on the Sprite with that ring and other doo-dads creating a ton of drag I think it can get by with less then 1 caliber.:2:

There is an interesting but brief article by Bruce Levison about the CP of short, fat rockets in Peak of Flight #154.

MK

 

[cjl]

We've met on threads, you know I'm not right very often.....

Heh...

You know, your percentage of correct answers would be improved if you figured out that if you don't know the answer, you can just not answer (or state your opinion with the disclaimer that you don't know/don't have experience with that specific thing).

 

[ScrapDaddy]

There is an interesting but brief article by Bruce Levison about the CP of short, fat rockets in Peak of Flight #154.

MK

Coming from the other end of the spectrum is https://www.apogeerockets.com/education/downloads/Newsletter239.pdf

 

[jungmann1]

My upscale is based on the fact that that I had a ten inch diameter section of tube laying around, begging to become a ring fin so the scaling factor comes from that relationship. The airframe actually scales out to closer to 3.5 inches but the Loc 4 inch was the closest I could come without fabbing my own tubing. Soooo, no not a direct upscale! The fins are very close to exact scale relative to the ring fin and the nose cone is very close in length but a slightly more tapered shape. My build is going to be much lighter than the one in the EMRR link unless I have to get totally crazy with the nose weight! And in that case the 29 mm mount probably wouldn't be enough!
I have never been clear on why stubby rockets can get away with less stability margin. And, I have no idea how a ring fin effects that situation, if at all. I appreciate all your input. And thanks so much for posting that Rocksim file link; now I just need to get the darn thing working on my PC.

 

[jungmann1]

Uhh, I don't know - ask RockSim.

Anyway, like I said, if you use comparable proportions and straight scaled-ups of the fins in your build, then the CP location should be in about the same proportional location as it was in the original Astron Sprite. It will be the CG that will vary due to the difference in parts densities, etc. In order to adjust the center of gravity to a suitable location, you must always find the location of the center of pressure first. You can adjust the CP's location by changing the length of the airframe or the size or sweep of the fins, but most people find it easier to adjust the location of the center of gravity instead. As a result, the CP's location is usually thought of as being stationary (fixed in place by the overall design), while the CG's location is regarded as movable (changed by shifting the distribution of mass).

Dave Rose posted an article on EMRR about his scratch-built Superscale Sprite, which was also based on LOC 4" tubing. You can find his RockSim file here.

MK

Can someone with functional Rocksim take a look at the file linked to above and tell me where the CP is on that rocket? That should at least get me in the ball park. Still not having any luck with my Rocksim; maybe the disc has gotten corrupted or something. Any help would be appreciated.

 

[WillMarchant]

23.63"

 

[jungmann1]

Thanks, Will, that will get me close enough for a swing test. Appreciate the effort!:clap: