Can anyone with an Aerotech G-force manually test their center of gravity?

[neil_w]

Fully loaded is with motors.

 

[overklock]

Fully loaded is with motors.

Ok, so my CG fully loaded is still pretty low down. Thanks

 

[T-Rex]

Do the instructions talk about balancing and adding weight before flight? Do the instructions give you a CG to shoot for? And I'll ask because you put CG and swing test in same sentence, you measured your CG by just locating the point where the rocket balances, right?

 

[overklock]

Do the instructions talk about balancing and adding weight before flight? Do the instructions give you a CG to shoot for? And I'll ask because you put CG and swing test in same sentence, you measured your CG by just locating the point where the rocket balances, right?

No to both unfortunately on the instruction manual.

And I balanced the rocket on a string loop. Didn’t do the old overhead swing test to find the CP. just balanced it on a looped string to find the CG.

 

[Banzai88]

Indeed.

The same base drag should be present regardless of the length of the rocket, and so the hack should apply no matter what.

As rockets get longer, the effect of the hack on CP diminishes, but I don't see why it should strictly only apply below a certain aspect ratio. If someone could explain why it would, I'd appreciate it. We are investigating building some sort of native support for this into a future version or OR.

I believe that the source info from Apogee said something like anything over the 10:1 there were insignificant returns, or something of that nature.

As a 'hack', I don't think that it's intended to be anything other than a somewhat crude way of tricking the software into predicting the proper CP of less than 10:1 L : D rockets to better fill in the holes in equations or how the software crunches numbers, not as a 'ya gotta add this for any size rocket' to augment the already established base drag that it computes.

In that, applying it to anything over the 10:1 ratio is a waste of time since the equations apparently have much better resolution and reflection of reality beyond that. To know for sure you'd have to run a few comparative sims and maybe pull back the curtain behind the original RS/OR code and/or look up Bruce Levison, the author of the original 3 part paper and see what he has to say about it.

Apogee POF 154, 157, 162 from 2005.

 

[neil_w]

In that, applying it to anything over the 10:1 ratio is a waste of time since the equations apparently have much better resolution and reflection of reality beyond that. To know for sure you'd have to run a few comparative sims and maybe pull back the curtain behind the original RS/OR code.

But the important thing implied here is that it doesn't *hurt* to do it on longer rockets. If, say, the software automatically applied it to the blunt ends of all rockets then it would OK. Does this sound correct?

(We would provide an easy way to disable this feature if not wanted, but it would presumably be on by default).

 

[Banzai88]

But the important thing implied here is that it doesn't *hurt* to do it on longer rockets. If, say, the software automatically applied it to the blunt ends of all rockets then it would OK. Does this sound correct?

No idea, I've never personally used it on anything over 10 : 1, only on things less than that. You'd have to run a series of sims on a less than 10 : 1, a 10 : 1, and a more than 10 : 1 rocket model to see how WITH and WITHOUT the hack adjusted the CP to know for sure if coding in the 'hack' on 100% of sims is detrimental, or if it's even necessary. (I would run a few and start a new thread on it doing the comparison, but I won't have much free time until next week).

The source material from Apogee seems to suggest that it's only for something less than 10 : 1 because of the lack of fidelity of the software for however they've coded it to do the CP equations. By extension, since it works well with OR, that's what I stick with. I would think that, if you could, screen the model in the sim for 10 : 1 or less, then apply the hack as a specific subroutine that could be clicked ON or OFF, as the user needs, but I'm not a software guy so I have no idea if that's feasible.

 

[Steve Shannon]

Ok, so my CG fully loaded is still pretty low down. Thanks

With the exception of hybrids and some minimum diameter rockets that are mostly motor, adding a motor will almost always drive the CG lower.
Technically speaking the only way adding a motor will shift CG forward is if the CG of the motor falls forward of the CG of the rocket.

 

[overklock]

With the exception of hybrids and some minimum diameter rockets that are mostly motor, adding a motor will almost always drive the CG lower.
Technically speaking the only way adding a motor will shift CG forward is if the CG of the motor falls forward of the CG of the rocket.

Of course, just making the point that my CG fully loaded is 40.7" from nosecone, which is still substantially lower than the 38.5 that the other individual was worried about.

Apogee lists the CP as 46" from nosecone which just seems wholly inaccurate, even according to their own software.

 

[Steve Shannon]

Of course, just making the point that my CG fully loaded is 40.7" from nosecone, which is still substantially lower than the 38.5 that the other individual was worried about.

Apogee lists the CP as 46" from nosecone which just seems wholly inaccurate, even according to their own software.

On simple 3FNC or 4 FNC rockets the CP falls just ahead of the forward end of the fin root. A large blunt aft end will shift it slightly rearward but not until the rocket is flying.
I would probably trust the CP that Apogee published. They have a lot of experience.
The Fosse modification to the Barrowman equations has been pretty accurate in my experience. If you’re still nervous hang your rocket on a string and see whether it points into the wind. If it does, just fly the damned thing.

 

[overklock]

On simple 3FNC or 4 FNC rockets the CP falls just ahead of the forward end of the fin root. A large blunt aft end will shift it slightly rearward but not until the rocket is flying.
I would probably trust the CP that Apogee published. They have a lot of experience.
The Fosse modification to the Barrowman equations has been pretty accurate in my experience. If you’re still nervous hang your rocket on a string and see whether it points into the wind. If it does, just fly the damned thing.

Excuse my ignorance, FNC?

Generally speaking, I trust Apogee 99.9% of the time. However the model that they uploaded that they based their CP on is just straight up incorrectly built. Incorrect fin position, extra components etc. Maybe it's for an old model of the G-force, but it's CP is going to be inaccurate due to the fins being an inch and a half higher than stated in their model, see below:

Their model:
Notice the parachute shifted too far forward, fins too far back, motor mount too far back, additional components, etc. The fins being shifted too far literally adds an extra inch to the length of the rocket. Same thing with the extended motor mount.



My model:

Fins shifted forward, motor mount shifted forward, additional components removed.

 

[Steve Shannon]

3FNC = 3 fin and nosecone, the simplest design.
4FNC = 4 fin + nosecone. Second simplest design and one which shifts the CP rearward slightly compared to the 3FNC.
Your model still shows the CP just ahead of the fins 44.8 inches, just over one inch forward from the Apogee model, and the CG nearly 6.5 inches ahead of that at 38.5. Too bad you didn’t insert the picture that has the motor loaded, but it probably doesn’t shift the CG rearward more than 2 or three inches. That’s easy enough to test once you’re loaded.
Mark the CP and test the CG. If it needs nose weight improvise. I had a candidate cut a hole in the base of a nosecone and put in a few rocks, followed by packing it with a sweat sock he had in his car. He flew his cert flight in memory of his father who had died of a brain tumor. He and his mother cried when he succeeded. I think I just had some dust in my eye.
You’ll do just fine.

 

[NateB]

FNC = Fin and Nose Cone. As in 3 or 4 Fin and nosecone rocket describing a very simple rocket.

Your question and concern is understandable, but I would listen experienced advice you have been given and trust the flight. You've built an established kit, I'd trust it to fly. If you want to further ease your mind, do a swing test and then fly it on a G before your cert attempt. If it were my rocket, I'd just relax and fly it.

I built a Darkstar Jr for my L1 amd flew it without the payload and AV bay. Come launch day, I noticed it was windy and other rockets were flying top far away. I took that into consideration and bought a motor from one of the vendors from several I simmmed ahead of time. I showed the rocket to the LCO, answered a few standard questions along with why I had 2 CP points marked (I used decals for CP with and without the payload bay.) It was approved and the rocket flew and was recovered down range by the away pads.

It was an uneventful flight that was almost a letdown from some other people's experience with L1. Whoosh, pop, recover - just like every other nominal low and mid powered flight I have had. It was really nothing to stress or worry about.

 

[overklock]

3FNC = 3 fin and nosecone, the simplest design.
4FNC = 4 fin + nosecone. Second simplest design and one which shifts the CP rearward slightly compared to the 3FNC.
Your model still shows the CP just ahead of the fins 44.8 inches, just over one inch forward from the Apogee model, and the CG nearly 6.5 inches ahead of that at 38.5. Too bad you didn’t insert the picture that has the motor loaded, but it probably doesn’t shift the CG rearward more than 2 or three inches. That’s easy enough to test once you’re loaded.
Mark the CP and test the CG. If it needs nose weight improvise. I had a candidate cut a hole in the base of a nosecone and put in a few rocks, followed by packing it with a sweat sock he had in his car. He flew his cert flight in memory of his father who had died of a brain tumor. He and his mother cried when he succeeded. I think I just had some dust in my eye.
You’ll do just fine.

Here's the same model loaded (my model).

Rocksim's method for calculating CP



Barrowman method:

 

[overklock]

FNC = Fin and Nose Cone. As in 3 or 4 Fin and nosecone rocket describing a very simple rocket.

Your question and concern is understandable, but I would listen experienced advice you have been given and trust the flight. You've built an established kit, I'd trust it to fly. If you want to further ease your mind, do a swing test and then fly it on a G before your cert attempt. If it were my rocket, I'd just relax and fly it.

I built a Darkstar Jr for my L1 amd flew it without the payload and AV bay. Come launch day, I noticed it was windy and other rockets were flying top far away. I took that into consideration and bought a motor from one of the vendors from several I simmmed ahead of time. I showed the rocket to the LCO, answered a few standard questions along with why I had 2 CP points marked (I used decals for CP with and without the payload bay.) It was approved and the rocket flew and was recovered down range by the away pads.

It was an uneventful flight that was almost a letdown from some other people's experience with L1. Whoosh, pop, recover - just like every other nominal low and mid powered flight I have had. It was really nothing to stress or worry about.

Totally get you. A little difficult to do a swing test with a 1kg, 60 inch rocket though...

I did just do a test in front of a fan, although this does not seem accurate. Vortices and whatnot. My Arreaux swung in circles and I know it's stable.

 

[Steve Shannon]

Here's the same model loaded (my model).

Rocksim's method for calculating CP

View attachment 550961

Barrowman method:

View attachment 550963

Based on this I would add a little nose weight.

 

[overklock]

Based on this I would add a little nose weight.

I have a second G-force here literally ready to build (yes I am that paranoid) in case something went wrong with my initial L1 flight.

I think I'm just going to rebuild the damn thing, weigh all the parts first, add some nose weight and HOPE that the additional nose weight does not prevent proper ejection.

 

[Steve Shannon]

I have a second G-force here literally ready to build (yes I am that paranoid) in case something went wrong with my initial L1 flight.

I think I'm just going to rebuild the damn thing, weigh all the parts first, add some nose weight and HOPE that the additional nose weight does not prevent proper ejection.

Ground test. There’s no reason to assume that a little nose weight will interfere with ejection of the nosecone.

 

[overklock]

Ground test. There’s no reason to assume that a little nose weight will interfere with ejection of the nosecone.

Good idea, but I've only ever seen people ground test when using DD, I'll have to look into how to seal everything off and still get a wire in (considering I'm not using an altimeter).

 

[NateB]

If you have a second kit, weigh and measure all of the parts and make your own sim file. You'll now have the dry weights which should be comparable to the one you built already. You'll still have to override for your glue, but it will be close. See how far apart your sim file is from the one provided and go from there.

 

[overklock]

If you have a second kit, weigh and measure all of the parts and make your own sim file. You'll now have the dry weights which should be comparable to the one you built already. You'll still have to override for your glue, but it will be close. See how far apart your sim file is from the one provided and go from there.

That’s the plan. Dry fit everything together as well and test the CG prior to gluing anything. Should tell me a lot

 

[T-Rex]

If you really feel that you need to add nose weight, your sim shows the nosecone shoulder as 2". Cut the forward tube at the bottom of the shoulder. Losing 2" from the end of the body tube is not all that significant. Most likely you would be the only one to notice. Don't re-glue the nose until you are sure......

 

[neil_w]

If you really feel that you need to add nose weight, your sim shows the nosecone shoulder as 2". Cut the forward tube at the bottom of the shoulder. Losing 2" from the end of the body tube is not all that significant. Most likely you would be the only one to notice. Don't re-glue the nose until you are sure......

Why not just cut it s bit lower and then reattach the pieces with a coupler afterwards?

(Iff it is necessary to add nose weight)

 

[T-Rex]

Why not just cut it s bit lower and then reattach the pieces with a coupler afterwards?

(Iff it is necessary to add nose weight)

Because I'm not smart enough to think of that... I like your way better.

 

[overklock]

If you really feel that you need to add nose weight, your sim shows the nosecone shoulder as 2". Cut the forward tube at the bottom of the shoulder. Losing 2" from the end of the body tube is not all that significant. Most likely you would be the only one to notice. Don't re-glue the nose until you are sure......

Why not just cut it s bit lower and then reattach the pieces with a coupler afterwards?

(Iff it is necessary to add nose weight)

This is definitely an option I will look into.

I just did the math. Dry this thing should weigh 970 (current build is 1014). Looks like I added an additional 44 grams to the tail end with all the epoxy/fillets I used. I guess this is why Aerotech calls for cyanoacrylate rather than epoxy.

I taped 45 grams of weight to my nosecone with some gram weights and voila, it brought the CG an entire inch and a half up putting it at 37 inches on the dot unloaded, 39.5 fully loaded.

Stability margin using Barrowman's method is still <1.0 (0.58) but it makes me a little more comfortable having the CG more forward.

Guess I'll add weight to the nose cone on my current build (or just launch it as is), then use only cyanoacrylate on the next to ensure the tail end is as light as possible. Never hurts to have a spare.

 

[rcktnut]

You could drill a small hole into the nosecone a bit from the tip add some lead shot and squeeze in some polyurethane glue.

 

[overklock]

You could drill a small hole into the nosecone a bit from the tip add some lead shot and squeeze in some polyurethane glue.

Heyyy, now that’s a great idea. A small 3mm hole and a syringe should be more than enough and a lot less work. Thanks for the idea!

 

[jmasterj]

AT calls for CA because it makes the (on the fragile side) fins replaceable. Once you add epoxy fillets, good luck.

 

[MikeT]

I got my L1 on the G Force. H128W motor. No added weight at all.

 

[overklock]

I got my L1 on the G Force. H128W motor. No added weight at all.

View attachment 551025

Do you still have it by any chance? Could you do me the favor of checking your CG by manually balancing it? (No motor loaded) nice paint job btw