Lvl 1 cert on an Aerotech G-force - Stock shock cord good enough?

[Steve Shannon]

Yep, switched to the barrowman method just to test in RockSim. It puts it in line with Open Rocket. However if the Barrowman method is accurate, holy **** this rocket is inherently unstable. Loading a G engine into the file downloaded from Apogee puts the CG and CP just 1.5 inches away from each other (.43 margin). Loading a 3 grain H puts that at .23

They do both fly successfully in the sim, but that seems sketch, no?

Not really. A positive margin is still positive. Anything larger than 0 should be stable. The problem is that it’s an estimate and could be inaccurate. That’s why a margin greater than 1 is usually required.
At 0 margin the rocket will not restore its flight. With negative numbers the rocket will always try to turn 180 degrees. The greater the number the greater the force trying to turn the rocket, whether restoring it to fly directly into the airstream or turning it around.

 

[ThirstyBarbarian]

Here’s a screenshot of my G-Force sim.



Not everything was exactly as I remember it.

My final built weight was 45.3 oz, so 1284 grams, which is really close to your original Rocksim file. But I knew mine was unusually heavy due to a lot of decorative elements which added weight. I thought mine would be heavier than yours, but this is more than I remembered.

OR puts the CP on mine at 41.7”. So that’s even a tiny bit forward of your OR file. But mine shows the CG as 33.8, so the stability is good. Even with H motors loaded, it’s a comfortable margin. The thing is, my file doesn’t look like I have a CG override loaded, so apparently I didn’t do that the way I do now. It was one of my earlier rockets, so maybe I wasn’t into the habit of overriding CG back then and never updated the file once I adopted the practice. Or maybe the calculated CG was accurate enough. I don’t know now.

If your CG is really 40.8 and the CP is really 41.7, then, yeah! That’s close! Did you say your measured CG with no motor loaded came to 37.5? That’s nearly 4 inches aft of mine, so that’s a full caliber right there. I wonder what the difference is. Although, as I said, I can’t be certain of mine because I did not add an override. Your fillets do not look so big as to be shifting the CG very far. How much does the cooling mesh weigh? I did not use it, but I can’t imagine it weighs much. When you balanced the rocket and measured the CG, you had all the recovery gear in the chute bay, correct?

 

[overklock]

Here’s a screenshot of my G-Force sim.

View attachment 550775

Not everything was exactly as I remember it.

My final built weight was 45.3 oz, so 1284 grams, which is really close to your original Rocksim file. But I knew mine was unusually heavy due to a lot of decorative elements which added weight. I thought mine would be heavier than yours, but this is more than I remembered.

OR puts the CP on mine at 41.7”. So that’s even a tiny bit forward of your OR file. But mine shows the CG as 33.8, so the stability is good. Even with H motors loaded, it’s a comfortable margin. The thing is, my file doesn’t look like I have a CG override loaded, so apparently I didn’t do that the way I do now. It was one of my earlier rockets, so maybe I wasn’t into the habit of overriding CG back then and never updated the file once I adopted the practice. Or maybe the calculated CG was accurate enough. I don’t know now.

If your CG is really 40.8 and the CP is really 41.7, then, yeah! That’s close! Did you say your measured CG with no motor loaded came to 37.5? That’s nearly 4 inches aft of mine, so that’s a full caliber right there. I wonder what the difference is. Although, as I said, I can’t be certain of mine because I did not add an override. Your fillets do not look so big as to be shifting the CG very far. How much does the cooling mesh weigh? I did not use it, but I can’t imagine it weighs much. When you balanced the rocket and measured the CG, you had all the recovery gear in the chute bay, correct?

Regarding the weight, remember, mine is not yet painted, filled or decaled, but yes, everything is loaded.

So, the difference as I've discovered is the calculations used to find the center of pressure. Open Rocket uses the Barrowman's method, while Rocksim uses the Rocksim method. That being said, Rocksim allows me to select the barrowman method, which causes the CP's to line up.

Here's the breakdown:

Barrowman Method:

No Motor (unloaded)
Margin: .80 marginal
CG: 38.5
CP: 41.74

With G80 motor loaded
Margin: 0.43 marginal
CG: 39.9
CP: 41.74

With H90 motor loaded
Margin: 0.23 marginal
CG: 40.82
CP: 41.74



RockSim Method:

No Motor (unloaded)
CG: 38.5
CP: 44.88

With G80 motor loaded
Margin: 1.21
CG: 39.99
CP: 44.88

With H90 motor loaded
Margin: 1.01
CG: 40.82
CP: 44.88



These are clearly very different. The sim itself is still successful, even when calculating on the Barrowman method, but it is certainly cutting it close.

Would help a lot if someone with a G-force could measure their CG as well.

 

[wsume99]

Yep, switched to the barrowman method just to test in RockSim. It puts it in line with Open Rocket. However if the Barrowman method is accurate, holy **** this rocket is inherently unstable. Loading a G engine into the file downloaded from Apogee puts the CG and CP just 1.5 inches away from each other (.43 margin). Loading a 3 grain H puts that at .23

They do both fly successfully in the sim, but that seems sketch, no?

Technically as long as the CG is ahead of the CP then the rocket is stable. Moving them further apart simply increases stability.

I got my L1 cert with a G Force. IIRC I added a few ounces of nose weight to push the stability margin in OpenRocket over 1. I did this to avoid any arguments with the RSO over stability margin. Some clubs want stability margin over 1.0 on cert flights. Short of adding lead fillets to your fins I see no way you could have added enough weight to make your rocket unstable for the motors you are planning to fly.

 

[overklock]

Technically as long as the CG is ahead of the CP then the rocket is stable. Moving them further apart simply increases stability.

I got my L1 cert with a G Force. IIRC I added a few ounces of nose weight to push the stability margin in OpenRocket over 1. I did this to avoid any arguments with the RSO over stability margin. Some clubs want stability margin over 1.0 on cert flights. Short of adding lead fillets to your fins I see no way you could have added enough weight to make your rocket unstable for the motors you are planning to fly.

Did you have any issues with separation at apogee with that added nose weight?

 

[tmhorvath]

Hopefully not driving too many people crazy here with how many questions I have, but was curious if anyone here has certified on an Aerotech G-force, and if the stock shock cord setup is good enough.

Primary concern would be the ejection charge being too strong and burning the elastic shock cord up. Although this has never been an issue on any of my other Aerotech models with the cooling mesh, I’ve never launched with an H motor either.

Here’s the setup:

- Elastic shock cord
- Aerotech’s cooling mesh
- parachute mounted on shock cord 1 foot down from nose cone

I plan on using a Cesaroni H90, cutting the delay down to 7 seconds. My understanding here is that the 29mm cesaroni has a 1.3g charge.

I know in the long run that a nomex blanket + nylon cord will be better, but for my cert I want to keep it as simple and stock as possible.

Has anyone here certified on the G-force? Any words of advice?

Edited with rocksim file for those who need it in the future.

I did not certify on the GForce, but could have. I would not use the stock shock cord. I use Kevlar for all my builds. Also, I upgraded to a 38mm and converted mine to dual deploy so I could control the ejection charges. Too many people saying the charge in a stock motor wasn’t enough for the volume and watches it lawn dart. My biggest concern was how flimsy the fins are, but they never fluttered.

 

[KenECoyote]

I don't know if it has been mentioned earlier, but I recalled reading about some cases of the ejection charge being insufficient to separate the G-Force due to the large internal volume, so some advised adding a bit more bp to the ejection charge if possible. Also ensure that the coupler isn't too tight. YMMV & good luck!

 

[ThirstyBarbarian]

I’ve heard about this concern that the G-Force has a large internal volume. It really does not. If you build it as a center break rocket, according to the instructions, the volume of the chute bay is actually a lot smaller than a lot of 4” rockets. It’s only the length of the coupler.

I think where this idea comes from is from people who did not build it that way and used the coupler to join the two body tube sections and set it up for a traditional nose cone instead of center break. Then the volume is very large.

 

[overklock]

I’ve heard about this concern that the G-Force has a large internal volume. It really does not. If you build it as a center break rocket, according to the instructions, the volume of the chute bay is actually a lot smaller than a lot of 4” rockets. It’s only the length of the coupler.

I think where this idea comes from is from people who did not build it that way and used the coupler to join the two body tube sections and set it up for a traditional nose cone instead of center break. Then the volume is very large.

This is my understanding too. My MDRM breaks no problem, the G-force has a much smaller internal space.

 

[beeblebrox]

Just a thought, but you could increase the length of the rocket also by adding another 12-18" of tube with another coupler to the bottom part. This would not add much weight but it would be much more stable anyway, big and slow is perfect for cert flights. The CTI motor you mention is a good choice for that as would be basically any low ranged H-motor. I have seen too many people lose small rockets on cert flights. Get Certified, then go for broke...

 

[wsume99]

Did you have any issues with separation at apogee with that added nose weight?

None. I built mine in a ziperless configuration with the coupler attached to the booster side and I extended the MMT all the way to the end of the coupler. This reduced the internal volume the charge has to pressurize. I also used CA on the coupler and payload tube interface and sanded them to achieve a good fit.