OpenRocket HELP!

[Jake]

I really didn't know where to post this, so I figured I'd just put it here.

Howdy all!

I have an OpenRocket trouble shooting question for ya.

I've spent the past two months, designing (using OR), building, and flying the biggest rocket I've made so far.

During the designing and building process though, I hit a snag.. and would like to know what I did wrong, and how I can avoid it next time.

Ok. I started off by creating the rocket in OR.
I am attaching the .ork and PDF files.


While I was doing so, I was very careful in making sure that all my components were true to what I would be using in real life.
The dimensions of each component, the material that was being used, the weight, all of that stuff.

A man I know suggested that the distance of the CoG from the CoP should be around 1.5 cal.
So by the time I finished the design, as you can see, I was pretty close to that 1.5 cal.

But, here's where I messed up. I made sure the rocket in the design was stable at around 1.5, assuming OR had the CoP placed properly... But as we will find out later, it didn't.

We built the rocket, and marked the CoP and CoG, where OpenRocket said they should be.



When we tied a string around the CoG, it did balance.

But, when we 'swing tested' it, (tied a piece of string around the CoG and then swung the rocket in a circle) instead of flying nose first into the wind, it flipped around and started to fly fins into the wind.

Well, that obviously means that the CoP has been marked incorrectly right?

We knew that to get this rocket to fly stably, we needed to find the actual CoP, and add weight to the front of the rocket to bring the CoG up in front of the CoP.

So we started by cutting out a to scale 'silhouette' of the rocket, and balanced it on a ruler.



This showed us were the actual CoP was... Which was far in front of where OpenRocket said it would be.

(we preformed this test twice. The second time we cut out the picture of the rocket that can be printed out from OR, and balanced it on a pin. Both tests produced the same results).

We marked the new CoP on the rocket, and added weight to the nose until the CoG was 1.5 cal (4.5") in front of it.



When we tried the swing test again, it flew perfectly, nose into the wind.
And, when we launched it, it flew great.

So what I would like to know is, what do you guys think I did wrong when designing the rocket in OR, to produce such a wonked up CoP?

If any of y'all can help, it would be much appreciated!
thanks, Jake

 

[CarVac]

First of all: there's a Rocketry Electronics and Software forum for posting questions about, well, rocketry software.

Regarding your rocket design: what on EARTH are those mass objects? There's a 17-ounce (!) mass object just forward of the motor mount (why?), and one in the nosecone. What's the point of the one at the back? Instead of using mysterious, unknown, unlabeled mass objects, you should use overrides on individual components to adjust the actual masses of each individual component.

Regarding the CG location: Both locations are correct: the far forward one is with no motors; the aft one is with all 7 motors in.

Regarding stability: The "real" CP is not determined by the cardboard cutout method: that's a very conservative approximation that for 99% of all rockets, gets the CP much farther forward than it actually is.

Regarding the swing test: this rocket is large enough that you have to have a really really long string in order to swing test it properly. Instead of that: When you have a windy day, load all 7 motors in the rocket, hang the rocket balanced from a string, and see if the rocket points into the wind. That is the most reliable test for stability of larger rockets.

 

[Jake]

Hi CarVac.

That's good to know.. Thanks.

Those mass objects were just to get it balance the way we wanted it to be.

Thank you for your information
I was just talking with a few other guys, and they confirmed too that the cut out method was not very accurate.

and thanks for the tip on checking the balance. I will try that next time.
jake

 

[CarVac]

Those mass objects were just to get it balance the way we wanted it to be.

What do you mean by "the way we wanted it to be"? You want it to be stable, so don't put the aft mass object in.

 

[Rex R]

I did notice a few things...1) there was a glitch in the file (no mass for the motor tubes), the air frame (mailing tube?) is actually kraft paper, no chute (or shock cord), and the fact that this is a class 2 rocket (needs a waiver to launch legally)...210 newtons of impulse and 250g of propellant.
Rex

 

[bobkrech]

Jake

From the questions you are asking I'm assuming that you are new to rocketry, and are not high power certified. While this may or may not be true, you should know that your rocket with (7) E-9 engines is an FAA Class 2 high power rocket, not a model rocket, because it contains more than 125 grams (4.4 oz.) of propellant (and it also weighs more than 1500 grams (53 oz.)). https://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&rgn=div5&view=text&node=14:2.0.1.3.15&idno=14

To legally fly this rocket you need to have obtained a written FAA waiver before you launch it, and you need L1 high power certification if you are launching it at a NAR or TRA launch.

Bob

 

[Ironnerd88]

Hey, Jake.

Open Rocket is a pretty good piece of software, but it looks like your still coming up to speed with it. I'm going to assume that the mass item at the front is the actual nose cone weight, since yours says it's made out of office paper. I'm also guessing that the mass item at the back is the weight for the motor tubes, thrust rings, engine hooks, and centering rings (etc...). Since your CG was actually correct, it looks as though you approximated those items pretty well. But after you become a little more familiar with OR, you'll want to start labeling items a little more. This will really help if you have a problem and want to get help from other users.

I attached a file for one of my rockets to show you what I mean about that. Just look at the various parts and you'll see they are all pretty well labelled. This also makes it easier to order parts, since you already have them annotated.

Back to your rocket. I have had a few that would not swing test. Not only did they fly just fine, they were also kits... from Estes! (So I figured they had been flown a few times). The swing test is not 100%.

Also, don't fly this with all of those engines in it if you're not level 2. If you really wanna send it into the air (and do not have a HPR cert), try it with some "C" motors (adapters are really easy to build), or just one "F". The reduced engine weight will move the CG foreward and make the rocket more stable.

Good luck, and have fun!

View attachment Lucky Dragon II.ork

 

[Jake]

Thank you all for your assistance.
I was really just looking for some information, concerning why the CoP points indicated by OpenRocket, and the silhouette balancing, were at different points on the rocket.
I am understanding it all better now, thanks to you guys, and some help from some other rocketeers.

Jake

 

[Ironnerd88]

Jake,
Try this. find the Barrowman equations and work it out "by hand", and compare those results to the cut-out and to OR.

https://my.execpc.com/~culp/rockets/Barrowman.html (there is also a spreadsheet to do the math for you farther down the page).

See what you get.

I really don't know why the CP would be off...

 

[Rex R]

the location of the cp is not static, it is dynamic. it varies with the angle of attack. it will be the furthest forward with an angle of attack of 90 degrees (cutout method) and furthest aft at zero degrees(well speed plays a role too) OR calculates the cp at an angle of 30 degrees. if the weather etc. is likely to cause an angle greater than 30 degrees...then perhaps you shouldn't be trying to launch.
Rex

 

[CarVac]

the location of the cp is not static, it is dynamic. it varies with the angle of attack. it will be the furthest forward with an angle of attack of 90 degrees (cutout method) and furthest aft at zero degrees(well speed plays a role too) OR calculates the cp at an angle of 30 degrees. if the weather etc. is likely to cause an angle greater than 30 degrees...then perhaps you shouldn't be trying to launch.
Rex

OpenRocket calculates CP at Mach 0.3 and 0 degrees, not 30 degrees.

 

[Jake]

Jake,
Try this. find the Barrowman equations and work it out "by hand", and compare those results to the cut-out and to OR.

https://my.execpc.com/~culp/rockets/Barrowman.html (there is also a spreadsheet to do the math for you farther down the page).

See what you get.

I really don't know why the CP would be off...

Thanks! I'll give that a look!