Is there a way to find the CP without doing the cardboard cutout test?

adrian said:

(You will, of course, have to enter all the internal components if you want OR or Rocksim to simulate the rocket's flight!)

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Only if you want the software to compute mass the mass for you.

If you do the mass over-ride, then you only need to input a nose cone, body tube, and fins to execute a basic flight simulation.

Buckeye said:

Only if you want the software to compute mass the mass for you.

If you do the mass over-ride, then you only need to input a nose cone, body tube, and fins to execute a basic flight simulation.

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Mass override is a good idea for any sim really. It can be a bit helpful for design work, but the extra mass of paint and epoxy is hard to quantify in advance and accurate pad weight means a much more accurate performance sim.

Buckeye said:

I really don't understand the motivation behind freeware and open source. Yes, I do have my share of free apps and downloads. I just don't understand why developers are willing to do it!

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That's easy.

1) It's a hobby...same as any other.
2) People find value in others using and enjoying their product.
3) People build free versions of apps that others find enjoyable...and may move into paid apps once their name is established (one can also look at DLC/Freeimum/IAP as variations of this).
4) People like the idea of adding their ideas to a program...being able to contribute meaningfully to a group project.

There are things other than money that motivate people.

FC

I used ROCsim tonight to check the Mega Der Red Maxs CP.. googled up a rocsim file and opened it up.

I may tweak the file later as mine is far from stock..but I started with an estes nose cone and body tube so should be pretty accurate for CP .

Also because someone else created the .rkt file..had my CP answer in less than 5 minutes.

Kenny

Buckeye said:

I am raising an old thread and hijacking it....

I agree entirely. The cost of Rocksim is relatively minor in the grand scheme of this hobby. <snip>... When somebody provides me a good or service, I usually expect to pay for it. <snip>

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luke strawwalker said:

Yes, you're right... someone who develops something of use should be able to expect SOME type or kind of return... some do it for fun or for others (that's why we do hobbies like rocketry, or help TARC teams, or whatever), some do things for profit (sometimes a LOT of profit). <snip>

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These sentiments are exactly what I was thinking when I made the decision to pay for a membership to this forum. I use ABP so I don't see ads, but I wanted to support the site so I joined.

Sorry for the hijack of my own with this post. But it seemed to go along with what was being said. I hope nobody minded.

Bat-mite said:

Use Modified Barrowman equations. Try this site. https://my.execpc.com/~culp/rockets/Barrowman.html

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These are very nice, but is their use really what Tripoli specifies as necessary for it's L1 approval? They actually use the word "calculate," which doesn't seem to me the same as either a simulation result or a balancing act of a cutout.

cecil (real new)

There's another way to find the CP, one of the most accurate methods of all....

Wind tunnel.

You can construct a fairly simple one at home, as outlined in this Apogee Rockets newsletter #252:

https://www.apogeerockets.com/downloads/Newsletter252.pdf

First let me address the OP's original question about Cardboard cutouts for various fin and shaped models:
Yes it works well with ANY shape model regardless of fin number or wierd body configuration. The trick is to do Two Views. Top and Side or Bottom and Side whichever give the best representation of the vehicle configuration. Balance both to get the LCP for each view then average the two to get the LCP location for the model. from there simply measure forward ONE Caliber (largest Body Diameter) to locate you vehicles CG. Now add weight the nose to move it to that point on the completed model. After doing the cardboard cutout CP-CG method I Swing Test the model to ensure first flight stablilty. Even on Larger models and MPR's. Try to swing the model starting with the nose down about 5deg.

If your having trouble tracing the model outline it can be done with photos as long as they are taken from exactly the same distance. This way you'll get True Scale cutouts that can then be balanced and meauresed then scaled up to the full size model. Takes a bit more time and math doing it this way but it Works.
The other tracing method is to suspend the model over your cardboard with a very bright light directly above the model. Trace the darkest outline shadow on the cardboard. Hang sideways and repeat to get the side view without moving the distance between model and cardboard. On larger models I do this with the cardboard or paper on the floor with the model suspended on two strings from a gooseneck floor lamp

There is yet another way that I haven't seen anyone bother to suggest.

Back in the Stone AGE BC (Before Computers) Home PC's anyway. We had to find our model CP's the old fashioned way...with Pencil, paper and OMG a Slide-rule!
Then in the very early 1970's Centuri put out a series of Technical Information Reports TIR's that made these calculations Much Simpler.
One of which was "Calculating Model Rocket Center of Pressure". This phamplet was and still is an extremely good source of information and way to calculate just about any models CP. I regularly use this booklet when the currently available sim programs fail misurably in allowing the needed additions to a design to get a reasonable CP.
I'm not sure if these materials are available on-line anywhere, but if they can be found each of the 4 or 5 booklets are more then worth just about any price to download or purchase.

Below are a couple photos of the covers and a few pages from the CP calc phamplet. This is I think what the OP was really looking for in his original post.

Don't let the Math deter you! I'm no Math Wiz but use these calcualtions and Charts all the time. There is a real sense of accomplishment when you have your answer even though it may take a time or two to work it out. After a few runs you'll be able to complete these in just a few minutes. Shoot It take me longer to input a model in Roc-Sim then it does to use the old long hand calculations.
Hope this helps

Micromeister said:

There is yet another way that I haven't seen anyone bother to suggest.

Back in the Stone AGE BC (before Computers) Home PC's anyway. We had to find our model CP's the old fashioned way. On paper.
In the very early 1970's Centuri put out a series of Technical Information Reports TIR's. One of which was "Calculating Model Rocket Center of Pressure"

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The Barrowman equations were suggested. Are the Centuri TIRs a different methodology?

TopRamen said:

then remembered why I despise Math.

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luke strawwalker said:

... I remember why I hate math...

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I hope you guys don't talk like this in front of your kids and grandkids. America needs engineers and scientists.

Unlike painting figurines, rocketry is a hobby of craftmanship and engineering skills. If you can't embrace the mathematics, then please turn in your "I am a rocket scientist" T-shirt. :rant:

Buckeye;
Jim was one of the Authors and a Narhams club member as well

Buckeye said:

I hope you guys don't talk like this in front of your kids and grandkids. America needs engineers and scientists.

Unlike painting figurines, rocketry is a hobby of craftmanship and engineering skills. If you can't embrace the mathematics, then please turn in your "I am a rocket scientist" T-shirt. :rant:

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Oh get over yourself... I can do math but I don't friggin like to.

America needs engineers huh... Tell that to my nephew that graduated with high marks from Purdue with a degree in aeronautical engineering and a minor in nuclear engineering and ended up working in a sporting goods store for a couple years while he couldn't find a job and finally got his alternative education certification to teach high school... Only "professional" job he could get....

So much for the dire need for engineers.... That's all so much rubbish... " oh we gotta have more math and science and engineers and scientists" except there aren't enough jobs in those fields and they don't pay near as good as finance, software, business, etc...

I told my daughter "go where the money is... Go where the jobs are." The rest is just talk...

Later
OL JR

:tongue:

luke strawwalker said:

Oh get over yourself... I can do math but I don't friggin like to.

America needs engineers huh... Tell that to my nephew that graduated with high marks from Purdue with a degree in aeronautical engineering and a minor in nuclear engineering and ended up working in a sporting goods store for a couple years while he couldn't find a job and finally got his alternative education certification to teach high school... Only "professional" job he could get....

So much for the dire need for engineers.... That's all so much rubbish... " oh we gotta have more math and science and engineers and scientists" except there aren't enough jobs in those fields and they don't pay near as good as finance, software, business, etc...

I told my daughter "go where the money is... Go where the jobs are." The rest is just talk...

Later
OL JR

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JR is right...in this job market, what we need are tradesmen. Instead of college grads being crushed by student loan debt, we need people who can do the work needed that keeps modern civilization going.

An example. A friend of mine, pilot type, O-6 (Colonel) in the USAF, flight pay (I think...I know it starts going down once you start getting in the upper ranks), makes not a trivial amount of money. His brother who has never gone to college has always made more money than he has.

As an elevator repairman.

These are the jobs that we can't seem to fill here and which make surprisingly good money. Heck, companies will pay you to get the training you need to fill the position.

"Hey Mav, you got the number to that elevator repair school?"

FC

Blue colar work isn't for everyone. I've done both, including everything from retail to manufacturing. I left a very high paying job as an air traffic control specialist working for the FAA a few years ago to finish a degree in Mechanical Engineering. Why? Because I simply wanted to do "more" with my life. With strengths in math and science, I'd rather be the guy sitting at the desk modeling in SolidWorks than the guy out welding or machining what I designed. With that said, I personally do enjoy fabricating, welding, etc, but I don't wish to do it for a career. Like flying, I'm a licensed pilot, but I never would do it for a career (which is why I went into ATC instead). Getting a college degree isn't for everyone and it is definitely NOT necessary to obtain a good paying career (like elevator repair, which is a good gig).

There is definitely more of pressure on students coming out of high school today who feel they "must" go to college, when in reality, going to a trade school could actually be a much better option. Having said that, I would never tell people they should go straight into industry or attend a trade school rather than seek out a degree in engineering, that's just silly. There ARE a lot of jobs out there for engineers in all areas, even if they're not specifically "engineering" positions.

Nice anecdotes, guys. Bottom line: Engineering is the highest-paying profession for a 22-year-old kid with a BS degree and no experience on Day 1.

Buckeye said:

Nice anecdotes, guys. Bottom line: Engineering is the highest-paying profession for a 22-year-old kid with a BS degree and no experience on Day 1.

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Not if you can't find a job...

Wasn't for lack of trying, either... he put in with at least a half dozen aerospace companies and even Caterpillar when they opened up a new engine plant... in at least a couple cases, he had friends there who suggested he apply. No luck.

Like I said... you gotta do what jobs are available... that's health care, finance, teaching, stuff like that.

Later! OL JR