Cp/cg

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rocketgroupmike

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Hi, all. My PML Sudden Rush (w/cpr) needs aft weight due to the mods I've done. (no Quick-switch motor mount, 9" payload section) I've heard the rules of thumb, but I'd like to get specific, if y'all can help me out. Rocksim shows the CP to be 56.5" (Barrowman) or 58.5" (Rocksim) so where should I put the CG? (3" airframe diameter) :confused:
 

rstaff3

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I wouldn't add any aft weight just to move the CG back. The rule of thumb is to have at least one caliber (3" in your case) of stability so the CG should be forward of the Cp by that amount. For 'standard' designs, I typicaly trust the Rsim Cp which is (normally) the more optimistic of the two. Over 2 calibers you start getting into the range called overstable. This can lead to a lot of weathercocking and I have heard it can lead to instability in stiff winds. However, I have flown some seriously overstable rockets in moderate winds with no problems. Some weathercocking but nothing that would lead to concern. Just my experience, your milage could vary.
 

rocketgroupmike

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Thanks, Dick. Not what I wanted to hear, but... Maybe I'm being too picky, but I want this rocket to be perfect. What exactly do you mean by Rsim CP being more optimistic?
 

rstaff3

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The Rsim CP is almost always further aft than the Barrowman in the Rsim program. If you trust the prior (which I normally do) it means the CG can be further back, which also often means less nose weight to be added.

If you want a heavier rocket by all means add aft weight to get an even one caliber of stability. Just make sure you do this based on the heaviest motor you will ever want to use, or you may find yourself goingback for more nose weight to readjust.

On a typical rocket I will NEVER add aft weight just for the sake of moving the CG.
 

rocketgroupmike

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Hmmm. The CG is now a full 3 calibers forward of the Rocksim CP... Any other opinions?
 

teflonrocketry1

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Using the RockSim files for the PML Sudden Rush from EMRR:

https://www.rocketreviews.com/cgi-bin/search/searchall.cgi?sudden+rush#RS

I get a CG of 47.5 inches (I211W motor, 48.4 inches with a J350W) with Barrowman CP at 57.2 and RockSim CP at 60.5 (static margin between 3 and 4). This seems to have worked good for the 30 or so flights in the EMRR flight log. I suggest you use a similar margin!

If you post the RockSim file of your rocket including the exact modifications that you made, I will take a look at the design and get back to you with more details.

A rule of thumb is just that; the real flight data serves as a better indicator of the required stability margin. Some long skinny rockets require more than a one caliber margin for stability, do you really want to investigate that issue on your model?

Bruce S. Levison, NAR #69055
 

rocketgroupmike

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Thanks, Bruce. Unfortunately, I only have the demo version of Rocksim, and I can't save anything to post. The only thing I did which affects the CP is add a 9" payload section (made, like the rest of the airframe, of QT) just behind the NC. I left out the PML Kwik-Switch motor adapter, which makes the rocket even more nose-heavy.
 

teflonrocketry1

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Attached is a RockSim version 5 file that you can open with the demo version of the software for the PML Sudden Rush with an additional 9 inch payload section and a 38mm motor mount. Is this close to what your design looks like? Let me know what changes to make to accurately simulate your design.

Bruce S. Levison, NAR #69055
 

scm86

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The best balast that i can think of for the aft end of a rocket is a bigger motor.

i have flown rockets with several calipers stability with no problem, and they are stable on smaler motors and larger motors.

No need to worry, and you now have an excuse to cram a bigger motor into it ;-)

Scott McNeely
 

rocketgroupmike

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That looks like it, Bruce, except that I started with the file from the PML site and added only the 9" tube itself. (couplers, etc. don't affect CP, do they?) Rocksim then shows the CP as 56.341 or 58.592, and says the rocket is overstable. I marked these on my airframe and the balance point is about 3 calibers forward. And Scott, this rocket is built for 54mm J's, so my L1 cert flight will be merely a "test!" ;)
 

bobkrech

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Mike

The interpretation of the CP/CG stability relationship is more involved than simple RockSim static stability analysis would lead you to believe. Your trying to solve a problem that doesn't exist. Your rocket with 3 calibers of stability is not over-stable.

The late Bob Dahlquist wrote an excellent experimental wind tunnel paper on Wind Caused Instability in the the March 1998 issue of High Power Rocketry but it's too long to post here, and not currently on the web.

Robert Galejs's followup article on "Wind Instability - What Barrowman Left Out" in Sport Rcoketry explained some of the phenomenon that Bob observed in the wind tunnel. It's an excellent review of what really determines rocket stability.

https://www.cmass.org/member/Robert.Galejs/sentinel39-galejs.pdf

Your Sudden Rush is a "long/skinny rocket" with an aspect ratio of 22:1. It you read Robert's article, you will see that the "over stability" goes away in a cross wind because the CP moves forward on long rockets because the simple CP calculation do not properly account for the lift of the body tube for non-zero angles of attack, so you don't have the stability margins you think you have based on the simple static calculations of RockSim.

It's always a bad thing to add extra tail weight to the rocket. All it does is reduce the rocket's stability and make the rocket more likely to weather cock in crosswinds.

The real source of the "instability" problem in crosswinds is being underpowered when leaving the launch rod such that the angle of attack of the rocket is large and the rocket is near the aerodynamic stall point. The cure is a longer launch rod and/or a higher thrust to weight ratio motor to obtain a higher velocity before free flight. You shouldn't fly a long/skinny rocket from a short rod or on a motor much under a 10:1 thrust/weight ratio on a windy day if you are concerned about stability.

Bob Krech
 

rocketgroupmike

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Thanks to all who've helped me out here... I guess the consensus is to NOT add weight to the rear end.

I won't add aft weight, but I will admit (finally) that this is my second Sudden Rush that I've added the 9" payload section to. (and left out the Kwik-Switch motor mount) The first had a great launch on an I211W with aft weight added to bring the CG to about 1.5 calibers forward of the CP. It flew great on the way up, and with no added weight, my new one should go even higher!

So why do I have another one? The first never deployed its recovery system due to my having used high-current "motor ignitors" instead of low current "e-matches" attached to my altacc 2c. Unfortunately, I naively thought that those terms in quotes represented a universally recognized distinction, but I now realize that's not the case. The rocket lawn-darted from ~2000 feet into a muddy field and I never found the hole. A very expensive lesson...

Anyway, special thanks to Bob K. for that outstanding article on stability which has finally convinced me, although I further admit to not understanding all the math in it.

Now I will again try for level 1 with my new rocket on an I300T at Frontier Rocketry's August launch. Wish me luck!

Best Regards to all and Thanks Again!

Mike Dean
NAR 64305
 

teflonrocketry1

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Mike,

See attached RockSim version 5 file. Is this close to what you have? With some of the larger K motors intalledl, the margin of stability might only be about 1.8 calibers. I don't mind "fine tuning" this file for you, just let me know what changes need to be made.

Bruce S. Levison, NAR #69055
 

rocketgroupmike

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That looks like my .rkt file Bruce, but I think my stock motor mount will only fit the short 54mm motor hardware. RMS 54/852, maybe 1280 or AMW 54/1050, not sure about PRO54. No K motors fit these cases, I think, but J's are fine with me. Thinking (way) ahead, I'm hoping to score a J180T for my level 2 cert flight. Hmmm, maybe a J315R!
 

teflonrocketry1

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Good luck with your level 2 certification flight. It sounds like you are on top of things since you are asking good questions beforehand and actually running simulations.

A couple of things to look at; run the simulations with only droge deployment just to see where the rocket will land if the main fails. You could also run a completely failed deployment as a worst case senario. If either situation lands the rocket close to the pad or prep areas, consider a different angle to the launch rod. This just might save someone from getting hurt or prevent damage to other property.

Bruce S. Levison, NAr #69055
 
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