TRA L1 Certification Question...

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TheSamurai

TheSamurai

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This year I think I am going to do my L1 Certification with Tripoli. I had a quick question. Obviously there isn't a written test for L1. I've read over the requirements and I have never calculated the CP before on my rockets. L1 requires that the CP be put on the outside of the rocket. Does anyone have a layman's way of doing that? Is there a youtube video? What other advice do you guys have for doing L1 with TRA?
 
RockSIM works also. RASAero as well...

You can also go to www.rocketreviews.com and see if the CP/CG is listed in their resources.
Cardboard Cutout method is another way - not very good but it's better than nothing.
 
Is it a kit? Most kits have "rocksim" files out there you can download.

Open Rocket is free. Just run the java executable, open the rocksim file that you downloaded for your rocket, and it will show you the CP. Measure and mark your rocket and your ready to fly. Just make sure your center of pressure (CP) is at least on body diameter behind the CG. So, if its a 4 inch air frame, make sure the CP is 4 inches behind the CG mark.
 
Last edited:
neond7 said:
Is it a kit? Most kits have "rocksim" files out there you can download.

Open Rocket is free. Just run the java executable, open the rocksim file that you downloaded for your rocket, and it will show you the CP. Measure and mark your rocket and your ready to fly. Just make sure your center of pressure (CP) is at least on body diameter in front of your CG. So, if its a 4 inch air frame, make sure the CG is 4 inches in front of your CP mark.
Click to expand...

I will have to look for the file online. Not sure if there is or not. Doesn't the CG/CP change with the different motors?

And on the airframe. It needs to be one body diameter in front or one body tube length? When you say diameter are you measuring the bottom of the body tube?
 
Also, it says on the Rockism that the diameter is 2.5 inches. So the CG must be in front of the CP by 2.5 inches. Is that correct?

According to the Rockism link, the CG is 23.1668 inches from the front and the CP is 38.7111 inches from the front. That's 15 inches which is well above the 2.5 minimum. If I am reading and understanding it all correctly of course.
 
I know your going for your L1, which is GREAT, I read a TON here before I made my attempt, that info was priceless!!

Take a look at the L2 exam questions, not saying to study it for your L1, but there is a lot of good knowledge to help you better understand some of the concepts and maybe get a leg up on getting ready for your L2 as well...

https://www.tripoli.org/LinkClick.aspx?fileticket=M8/Gtc4WyGw=&tabid=316
 
correct , cg needs to be the diameter of the kit in front of cp,so if its a 2.5 inch kit yes, the center of gravity needs to be in front of the center of pressure by 2.5 inches,which it sounds like it is,imo most smaller l1 kits the rocket should be stable w no added weight,when you get into bigger kits or short fat ones it comes up,good to check it though, i read that in modern high power rocketry 2 , a great book ,also i ask alot of questions on this forum,lol alot, no stupid questions and i have learned a ton ,was soo green, still learning big time
 
The diameter of the airframe tube is often called a "caliber" for purposes of determining stability. You want the CG at least one caliber noseward of the CP. The CG will change depending on your motor, payload, etc. The CP shouldn't change unless you modify the construction of the rocket. If the CG is more than a few calibers in front of the CP, the rocket will tend to weather-cock into any prevailing wind.

To calculate the CP, you can use a 2D model of your rocket and find where the balance point on the 2D model is located; that's your CP. Another way is to use the barrowman equations. Everyone should do this calculation at least once :). Finally, you can use open rocket or rocksim to calculate CP.
 
Modern High Power Rocketry 2 is a very good book and will enlighten you on many aspects of HPR!!
 
If you are building the Ventris with the standard fins that come with it, you are going to get approx 6.2 calibers of stability, unloaded, which is more than enough for this bird, even taking into account that 1/5th of its diameter is 2" (the pod up front). My RockSim file shows that the CP is right at the front of the fins. Mark this point after you finish and paint your rocket. You can use anything, I like Sharpie or little stickers. This point doesnt change as long as the rocket shape remains the same. You can easily prove CG at any time by balancing the rocket on your finger. This is the measurement that changes with different motors loaded, etc., so a permanent mark for this is a little useless.

Good Luck on the L1, can i suggest a H54 (CTI) as the cert motor. It burns for 3.5 seconds, will get this Ventris to around 3000ft, which isnt too bad on a calm day, and the boost will impress the crowd!

Spooks said:
Modern High Power Rocketry 2 is a very good book and will enlighten you on many aspects of HPR!!
Click to expand...

Seconded!!!! Get it and read it!!
 
Salvage-1 said:
If you are building the Ventris with the standard fins that come with it, you are going to get approx 6.2 calibers of stability, unloaded, which is more than enough for this bird, even taking into account that 1/5th of its diameter is 2" (the pod up front). My RockSim file shows that the CP is right at the front of the fins. Mark this point after you finish and paint your rocket. You can use anything, I like Sharpie or little stickers. This point doesnt change as long as the rocket shape remains the same. You can easily prove CG at any time by balancing the rocket on your finger. This is the measurement that changes with different motors loaded, etc., so a permanent mark for this is a little useless.

Good Luck on the L1, can i suggest a H54 (CTI) as the cert motor. It burns for 3.5 seconds, will get this Ventris to around 3000ft, which isnt too bad on a calm day, and the boost will impress the crowd!



Seconded!!!! Get it and read it!!
Click to expand...

Thanks for the suggestions. I will definitely check that motor out. As far as High Power Rocketry 2 goes, I downloaded it for my Kindle Fire a week and a half ago and I am about 1/4 of the way through it.
 
Excellent!! I read it between my L1 and L2 and continue to look at it today!!
TheSamurai said:
Thanks for the suggestions. I will definitely check that motor out. As far as High Power Rocketry 2 goes, I downloaded it for my Kindle Fire a week and a half ago and I am about 1/4 of the way through it.
Click to expand...
 
TheSamurai said:
I will have to look for the file online. Not sure if there is or not. Doesn't the CG/CP change with the different motors?

And on the airframe. It needs to be one body diameter in front or one body tube length? When you say diameter are you measuring the bottom of the body tube?
Click to expand...

The CG will change slightly depending on the size of the motor..CP should stay fairly constant..

'Caliber' is the diameter of the body tube. A 1 caliber requirement means that the CG be 1 body tube diameter in front of the CP..Example: Rocket is 4" in diameter, CG needs to be at least 4" in front(towards the nose cone) of the CP.
 
Thank you all very much for all the advice. It has been extremely helpful. Hopefully we get to launch this year. It is only March and BLM is already concerned about us launching. It's gonna be a long summer apparently.
 
TheSamurai said:
As far as High Power Rocketry 2 goes, I downloaded it for my Kindle Fire
Click to expand...

DING DING DING!!! The Kindle Fire was the best present my son received from Santa this year! I promptly downloaded HPR2. I've gotten into the Lvl 2 stuff, but will probably go back and read stuff again. Very good reference, and has some of the guys in it I fly with as Mark used to fly with us and in Orangeburg before moving back to Cali.
 
TheSamurai said:
This year I think I am going to do my L1 Certification with Tripoli. I had a quick question. Obviously there isn't a written test for L1. I've read over the requirements and I have never calculated the CP before on my rockets. L1 requires that the CP be put on the outside of the rocket. Does anyone have a layman's way of doing that? Is there a youtube video? What other advice do you guys have for doing L1 with TRA?
Click to expand...
The old fashioned layman's way to determine a rough approximate of the cp is to create a cardboard cutout of the rocket's silhouette and then find the balancing point or center of gravity of that. For basic (3FNC or 4FNC) rockets it works well at subsonic velocities, certainly close enough for most RSOs.
You can get fancier and use math to calculate the center of pressure as explained here on a page that NASA maintains: https://exploration.grc.nasa.gov/education/rocket/rktcp.html
Steve
 
Steve Shannon said:
The old fashioned layman's way to determine a rough approximate of the cp is to create a cardboard cutout of the rocket's silhouette and then find the balancing point or center of gravity of that. For basic (3FNC or 4FNC) rockets it works well at subsonic velocities, certainly close enough for most RSOs.
You can get fancier and use math to calculate the center of pressure as explained here on a page that NASA maintains: https://exploration.grc.nasa.gov/education/rocket/rktcp.html
Steve
Click to expand...

That is one that needs a bookmark!
 

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