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

[overklock]

Not sure if I totally screwed up my G-force by adding too much epoxy to the filets and centering rings.

My build is all stock, with the center separation setup.

My CG when the motor isn't loaded is supposed to be somewhere in the ballpark of 36 inches, however I'm closer to 38.5 inches after doing the string test.

Worryingly, this puts the CG and CP incredibly close when using the barrowman's equation.

If anyone with a G-force could manually test their CG (no motor loaded), I'd greatly appreciate it!

 

[neil_w]

Barrowman, based on OR or Rocksim output? What is your stability margin?

If too low, then it would seem that some nose weight is in order, regardless of where others' CG lie.

 

[overklock]

Barrowman, based on OR or Rocksim output? What is your stability margin?

If too low, then it would seem that some nose weight is in order, regardless of where others' CG lie.

Barrowman via rocksim.
Rocksim calculation comes out to a 1.58 margin (unloaded)
Barrowman via rocksim calculation comes out to a 0.80 margin (again unloaded)

Loading the rocket with an H motor brings these two much close as you would expect. Rocksim being 1.01 and Barrowman being 0.23

These numbers are startlingly different.

Nose weight at this point is a little difficult as the nosecone is already cemented on.

 

[neil_w]

Hmm, the difference between Rocksim and Barrowman is indeed startling. I'm not sure, in general, which to trust more. But I'd be awfully nervous flying with Barrownman at .23 cal.

 

[Steve Shannon]

Barrowman, based on OR or Rocksim output? What is your stability margin?

If too low, then it would seem that some nose weight is in order, regardless of where others' CG lie.

This!
It really doesn’t matter how the CG on others’ GForce rockets stacks up except as a point of discussion.

 

[overklock]

This!
It really doesn’t matter how the CG on others’ GForce rockets stacks up except as a point of discussion.

Well generally yes, I'd agree with you. But having someone who says "yeah I launched this rocket on an H and here's my CG" would make me much more comfortable about actually attempting my level 1 with this particular rocket.

 

[rcktnut]

The CP of the G Force built stock is at 45.9. If your CG is at 38.5, your stability margin ( 7.4") is at 1.85. Just using simple math not Barrowmans or RS equations. Load the motor your going to use and then check the CG.

 

[overklock]

The CP of the G Force built stock is at 45.9. If your CG is at 38.5, your stability margin ( 7.4") is at 1.85. Just using simple math not Barrowmans or RS equations. Load the motor your going to use and then check the CG.

I'm assuming you're pulling that CP number from Apogee's/Rocksim's database. That model is completely wrong. Not sure where they pulled it from or who designed it. Fins are in the wrong position, motor mount is set far too low, set up for dual deployment, even the height is wrong.

I had to redesign the thing to get it to look right.

Here's my redesigned file.

 

[teepot]

If you need more nose weight. And the nose cone is already on. Add some large quick links to where the shock cord meets the body. Some of my quick links weight half an ounce or more. Make sure the chute doesn't slid further down than where you have it when you balance it.

 

[overklock]

If you need more nose weight. And the nose cone is already on. Add some large quick links to where the shock cord meets the body. Some of my quick links weight half an ounce or more. Make sure the chute doesn't slid further down than where you have it when you balance it.

Is this possible even on the G-force? Considering it separates halfway down the tube and is blocked off by a bulkhead?

Also, will adding more weight to the upper section prevent it from separating?

 

[ThirstyBarbarian]

I hope someone can measure their CG on their unloaded rocket and reassure you. I wish I could do it, but my rocket was lost to a motor cato, and I don’t seem to have a record of measuring the CG. One thing I would say about the G-Force is that I have never thought of it as an unstable or marginal rocket, and I’ve never heard of anyone describing it that way. I think that it’s reliable, and you won’t have a problem.

So that might be enough for a regular flight, but you are doing a cert flight, and I understand how you want to be as confident as possible. And you also want to be able to answer questions from the person signing off your cert. I’m the same way. I had similar concerns on my cert flight.

Since then, I’ve signed off a few level 1 certs, and I do ask about the CP and CG and why the person thinks the flight will be successful. The main reason I do that is for them to show that they have thought it through, not necessarily that they have a 1.0+ caliber of stability margin. That’s a guideline, and not a hard cutoff where rockets above 1.0 are stable, and those below 1.0 are unstable. It’s really more of a starting point for taking a closer look at whether the rocket will fly well or not. I routinely fly rockets with less than .2 calibers of stability according to some calculations, and they are as stable as a rock. So there are other factors you can consider other than the least favorable algorithms.

To be ready for your cert flight, here are a few things you could do.

• Get someone to measure their own CG position to see if your rocket is unusually balanced toward the aft end. I kind of doubt this is the case, but it would be a good data point just in case.
• Mark the rocket with the various CP points calculated by the various algorithms, including the favorable default Rocksim algorithm, which is supposedly an improved algorithm, plus the less favorable Rocksim algorithms, and the OR algorithm. You’ll be able to discuss that there is a range of CP calculations, they are all stable to varying degrees, and the newest Rocksim shows more than 1.0 cal. If you want, you could even do the “zero mass cone hack” to account for base drag. I rely on that for some of the marginal stability flights I mentioned, and if you want more info, let me know.
• You could do a test flight on a G motor before the cert flight. If it flies straight, you can mention that.
• You can ask for your flight to be on the longest 1010 rail available. The longer the rail, the faster the speed off the rail, and the straighter the flight will be. So if they have a 10’ rail, that would be great, 8’ is good, 6’ is kind of short. Ask what they have and go for the longest.
• You could opt for a different cert motor. “Trust In Thrust!” Low stability rockets do well with higher thrust because the high speed aids stability, same as the longer rail. So instead of an H90, you could go for a higher thrust H, like an H135 or H182. You could save the longer burn motors for after your cert.
• And you can be ready to point out the G-Force is a known kit with a long history and no reputation for stability issues.

As a person who has signed off a few NAR level 1 certs, if I asked you about stability, and you told me, “I did a few simulations using different algorithms, and they gave a range of results. I marked them here on my rocket, and this newest Rocksim algorithm gives the most favorable results. Here’s the balance point for the CG [demostrate]. So the stability is positive for all of the calculated CPs, and the Rocksim is around 1 caliber. Plus, this is a known kit with a good reputation for stable flights. And I’d like to fly it off the longest rail with a high thrust motor, just to be sure.” If you said that, I’d say, “Great! Let’s fly it!” And I would have a high confidence in success.

One of the guys who has chimed in on your G-Force threads is @Steve Shannon , former president of the Tripoli Rocketry Association, and he knows something about certifications. Steve, is there anything else you would want addressed before giving the go-ahead on this flight? Any red flags?

 

[cls]

sheesh... load the motor and balance the rocket. should be somewhat forward of the center of the fins. like 1 caliber, 4 inches? tie a string where it balances and give it a swingntest. use 8 or 10 feet of string. it points forward when swung around you, right? my daughter did this with hers whennshe was 7.

relax, go fly it.

 

[overklock]

I hope someone can measure their CG on their unloaded rocket and reassure you. I wish I could do it, but my rocket was lost to a motor cato, and I don’t seem to have a record of measuring the CG. One thing I would say about the G-Force is that I have never thought of it as an unstable or marginal rocket, and I’ve never heard of anyone describing it that way. I think that it’s reliable, and you won’t have a problem.

So that might be enough for a regular flight, but you are doing a cert flight, and I understand how you want to be as confident as possible. And you also want to be able to answer questions from the person signing off your cert. I’m the same way. I had similar concerns on my cert flight.

Since then, I’ve signed off a few level 1 certs, and I do ask about the CP and CG and why the person thinks the flight will be successful. The main reason I do that is for them to show that they have thought it through, not necessarily that they have a 1.0+ caliber of stability margin. That’s a guideline, and not a hard cutoff where rockets above 1.0 are stable, and those below 1.0 are unstable. It’s really more of a starting point for taking a closer look at whether the rocket will fly well or not. I routinely fly rockets with less than .2 calibers of stability according to some calculations, and they are as stable as a rock. So there are other factors you can consider other than the least favorable algorithms.

To be ready for your cert flight, here are a few things you could do.

• Get someone to measure their own CG position to see if your rocket is unusually balanced toward the aft end. I kind of doubt this is the case, but it would be a good data point just in case.
• Mark the rocket with the various CP points calculated by the various algorithms, including the favorable default Rocksim algorithm, which is supposedly an improved algorithm, plus the less favorable Rocksim algorithms, and the OR algorithm. You’ll be able to discuss that there is a range of CP calculations, they are all stable to varying degrees, and the newest Rocksim shows more than 1.0 cal. If you want, you could even do the “zero mass cone hack” to account for base drag. I rely on that for some of the marginal stability flights I mentioned, and if you want more info, let me know.
• You could do a test flight on a G motor before the cert flight. If it flies straight, you can mention that.
• You can ask for your flight to be on the longest 1010 rail available. The longer the rail, the faster the speed off the rail, and the straighter the flight will be. So if they have a 10’ rail, that would be great, 8’ is good, 6’ is kind of short. Ask what they have and go for the longest.
• You could opt for a different cert motor. “Trust In Thrust!” Low stability rockets do well with higher thrust because the high speed aids stability, same as the longer rail. So instead of an H90, you could go for a higher thrust H, like an H135 or H182. You could save the longer burn motors for after your cert.
• And you can be ready to point out the G-Force is a known kit with a long history and no reputation for stability issues.

As a person who has signed off a few NAR level 1 certs, if I asked you about stability, and you told me, “I did a few simulations using different algorithms, and they gave a range of results. I marked them here on my rocket, and this newest Rocksim algorithm gives the most favorable results. Here’s the balance point for the CG [demostrate]. So the stability is positive for all of the calculated CPs, and the Rocksim is around 1 caliber. Plus, this is a known kit with a good reputation for stable flights. And I’d like to fly it off the longest rail with a high thrust motor, just to be sure.” If you said that, I’d say, “Great! Let’s fly it!” And I would have a high confidence in success.

One of the guys who has chimed in on your G-Force threads is @Steve Shannon , former president of the Tripoli Rocketry Association, and he knows something about certifications. Steve, is there anything else you would want addressed before giving the go-ahead on this flight? Any red flags?

Words can’t describe how grateful I am for this response. Those bullet points are a great help! Longer rail with a higher thrust motor probably sounds like a good idea. A G flight prior was something I was thinking of as well.

I’m going to have to do a search on the zero cone hack. Could definitely use more info on that if you’re willing to spare the time.

Thank you again!

 

[ThirstyBarbarian]

sheesh... load the motor and balance the rocket. should be somewhat forward of the center of the fins. like 1 caliber, 4 inches? tie a string where it balances and give it a swingntest. use 8 or 10 feet of string. it points forward when swung around you, right? my daughter did this with hers whennshe was 7.

relax, go fly it.

You signed off my level 1 cert. My rocket was my modified Mega Der Red Max with the fin extensions and fin pod modifications, and I was concerned about stability and no good way to sim it with the weird modifications. You kind of talked me down a bit, and when I flew it, it was great. I think everyone is a bit nervous about L1.

 

[overklock]

You signed off my level 1 cert. My rocket was my modified Mega Der Red Max with the fin extensions and fin pod modifications, and I was concerned about stability and no good way to sim it with the weird modifications. You kind of talked me down a bit, and when I flew it, it was great. I think everyone is a bit nervous about L1.

I only get to go out to launch maybe 1-2x per year, and it’s quite the trip for me. I wanna make sure everything goes smoothly.

Definitely nervous.

 

[ThirstyBarbarian]

I only get to go out to launch maybe 1-2x per year, and it’s quite the trip for me. I wanna make sure everything goes smoothly.

Definitely nervous.

Everyone is nervous for Level 1, probably more nervous for Level 1 than Level 2 or 3, and that has to do with experience.

Certification is about demonstrating what you have learned and what you know to show you are ready for the next steps, so I personally feel like I’d rather see someone asking questions and making sure they’ve considered everything before their L1 attempt than being cavalier about it.

The questions you are asking show you understand the issues you should be considering. You noticed something looked off, and you are checking on it. That’s a good thing and demonstrates knowledge. With that said, I think the stability question is going to turn out not to be a problem for your flight, and maybe with the suggestions for mitigating stability problems, you can relax a bit.

 

[cls]

You signed off my level 1 cert. My rocket was my modified Mega Der Red Max with the fin extensions and fin pod modifications, and I was concerned about stability and no good way to sim it with the weird modifications. You kind of talked me down a bit, and when I flew it, it was great. I think everyone is a bit nervous about L1.

I remember!! I think I was muttering something about "wow, this is too nice to fly". Your sturdy DRM just keeps going, I'm on like my 3rd four inch one, keep shredding them.

 

[Steve Shannon]

I hope someone can measure their CG on their unloaded rocket and reassure you. I wish I could do it, but my rocket was lost to a motor cato, and I don’t seem to have a record of measuring the CG. One thing I would say about the G-Force is that I have never thought of it as an unstable or marginal rocket, and I’ve never heard of anyone describing it that way. I think that it’s reliable, and you won’t have a problem.

So that might be enough for a regular flight, but you are doing a cert flight, and I understand how you want to be as confident as possible. And you also want to be able to answer questions from the person signing off your cert. I’m the same way. I had similar concerns on my cert flight.

Since then, I’ve signed off a few level 1 certs, and I do ask about the CP and CG and why the person thinks the flight will be successful. The main reason I do that is for them to show that they have thought it through, not necessarily that they have a 1.0+ caliber of stability margin. That’s a guideline, and not a hard cutoff where rockets above 1.0 are stable, and those below 1.0 are unstable. It’s really more of a starting point for taking a closer look at whether the rocket will fly well or not. I routinely fly rockets with less than .2 calibers of stability according to some calculations, and they are as stable as a rock. So there are other factors you can consider other than the least favorable algorithms.

To be ready for your cert flight, here are a few things you could do.

• Get someone to measure their own CG position to see if your rocket is unusually balanced toward the aft end. I kind of doubt this is the case, but it would be a good data point just in case.
• Mark the rocket with the various CP points calculated by the various algorithms, including the favorable default Rocksim algorithm, which is supposedly an improved algorithm, plus the less favorable Rocksim algorithms, and the OR algorithm. You’ll be able to discuss that there is a range of CP calculations, they are all stable to varying degrees, and the newest Rocksim shows more than 1.0 cal. If you want, you could even do the “zero mass cone hack” to account for base drag. I rely on that for some of the marginal stability flights I mentioned, and if you want more info, let me know.
• You could do a test flight on a G motor before the cert flight. If it flies straight, you can mention that.
• You can ask for your flight to be on the longest 1010 rail available. The longer the rail, the faster the speed off the rail, and the straighter the flight will be. So if they have a 10’ rail, that would be great, 8’ is good, 6’ is kind of short. Ask what they have and go for the longest.
• You could opt for a different cert motor. “Trust In Thrust!” Low stability rockets do well with higher thrust because the high speed aids stability, same as the longer rail. So instead of an H90, you could go for a higher thrust H, like an H135 or H182. You could save the longer burn motors for after your cert.
• And you can be ready to point out the G-Force is a known kit with a long history and no reputation for stability issues.

As a person who has signed off a few NAR level 1 certs, if I asked you about stability, and you told me, “I did a few simulations using different algorithms, and they gave a range of results. I marked them here on my rocket, and this newest Rocksim algorithm gives the most favorable results. Here’s the balance point for the CG [demostrate]. So the stability is positive for all of the calculated CPs, and the Rocksim is around 1 caliber. Plus, this is a known kit with a good reputation for stable flights. And I’d like to fly it off the longest rail with a high thrust motor, just to be sure.” If you said that, I’d say, “Great! Let’s fly it!” And I would have a high confidence in success.

One of the guys who has chimed in on your G-Force threads is @Steve Shannon , former president of the Tripoli Rocketry Association, and he knows something about certifications. Steve, is there anything else you would want addressed before giving the go-ahead on this flight? Any red flags?

I think you did a great job of covering it. I would just suggest that a very little bit of nose weight could be like an insurance policy.

 

[overklock]

Everyone is nervous for Level 1, probably more nervous for Level 1 than Level 2 or 3, and that has to do with experience.

Certification is about demonstrating what you have learned and what you know to show you are ready for the next steps, so I personally feel like I’d rather see someone asking questions and making sure they’ve considered everything before their L1 attempt than being cavalier about it.

The questions you are asking show you understand the issues you should be considering. You noticed something looked off, and you are checking on it. That’s a good thing and demonstrates knowledge. With that said, I think the stability question is going to turn out not to be a problem for your flight, and maybe with the suggestions for mitigating stability problems, you can relax a bit.

Thanks for the encouraging words.

BTW, I understand that the zero mass cone hack is generally reserved for "fat stubby rockets" but assuming I did this right (base diameter for aft end of cone + zero mass + 4x the caliber of the rocket for length), then we are looking much more stable when using Barrowman's!

 

[ThirstyBarbarian]

Thanks for the encouraging words.

BTW, I understand that the zero mass cone hack is generally reserved for "fat stubby rockets" but assuming I did this right (base diameter for aft end of cone + zero mass + 4x the caliber of the rocket for length), then we are looking much more stable when using Barrowman's!

View attachment 550902

That looks good, but I’ve usually seen pi x diameter used for the length.

This trick is something that can be used to reassure yourself, and I often will use it to check stability on some of my short, stubby rockets with very low stability numbers according to OpenRocket’s calculations. I kind of wonder if Apogee is using something like that behind the scenes with their current algorithm that is so much more favorable than the Barrowman calculations.

Anyway, the zero mass cone trick is just for reassurance, and you could add it to your discussion with the person who will be witnessing your flight if stability questions come up. You need to remove it from your sim before you do your flight sims because the cone will throw off your projected altitude, optimal delay, etc.

 

[overklock]

That looks good, but I’ve usually seen pi x diameter used for the length.

This trick is something that can be used to reassure yourself, and I often will use it to check stability on some of my short, stubby rockets with very low stability numbers according to OpenRocket’s calculations. I kind of wonder if Apogee is using something like that behind the scenes with their current algorithm that is so much more favorable than the Barrowman calculations.

Anyway, the zero mass cone trick is just for reassurance, and you could add it to your discussion with the person who will be witnessing your flight if stability questions come up. You need to remove it from your sim before you do your flight sims because the cone will throw off your projected altitude, optimal delay, etc.

pi x diameter makes more sense. I read 4x the caliber on another post. Funnily enough, it comes out pretty darn close to Apogee's bunk build they setup on their website that has all the inaccuracies. Who knows what's going on there, but you could very well be right about them taking the cone hack into consideration.

I've created a laundry list of data points that I plan on printing up and taking with me, showing various CG's, CP's and stability margins across all calculation methods.

Thank you again for your help!

 

[ThirstyBarbarian]

Sounds like a good plan. When is your cert flight? This coming weekend? I’m traveling starting tomorrow and likely won’t be checking in on the forum much, so good luck with your flight!

 

[overklock]

Sounds like a good plan. When is your cert flight? This coming weekend? I’m traveling starting tomorrow and likely won’t be checking in on the forum much, so good luck with your flight!

I launch over at Lucerne dry lake, so I usually head out when big Bear isn’t too snowy. Looking at February/march time period. I’ve got some time.

If you ever make it out there, I’d love to shake your hand for all your help!

 

[Banzai88]

Thanks for the encouraging words.

BTW, I understand that the zero mass cone hack is generally reserved for "fat stubby rockets" but assuming I did this right (base diameter for aft end of cone + zero mass + 4x the caliber of the rocket for length), then we are looking much more stable when using Barrowman's!

View attachment 550902

Yes, stubbies ONLY. Anything less than 10:1 Length: Diameter.

Apogee lists the length of the G Force as 60 inches with a diameter of 4 inches, so you're 20 inches OVER the target range of the hack.

 

[overklock]

Yes, stubbies ONLY. Anything less than 10:1 Length: Diameter.

Apogee lists the length of the G Force as 60 inches with a diameter of 4 inches, so you're 20 inches OVER the target range of the hack.

Can you explain what you mean by 20 inches over the hack?

Apogee’s description and Rocksim model are totally wrong btw. The rocket itself comes in at 59.75”, and the design they have on their site has the aft end built completely wrong.

Edit: I see what you mean. Ratio wise im over the limit of being able to use the hack.

 

[Antares JS]

Can you explain what you mean by 20 inches over the hack?

At 4" diameter, a 10:1 ratio would be 40" long, making the 60" long rocket 20" over the limit for using that hack.

 

[ThirstyBarbarian]

I’ve never heard there is a length:diameter limit on using the hack. It’s mostly used for short, stubby rockets, but I’ve never heard there is a “rule” about when to use it.

 

[neil_w]

I’ve never heard there is a length:diameter limit on using the hack. It’s mostly used for short, stubby rockets, but I’ve never heard there is a “rule” about when to use it.

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.

 

[Steve Shannon]

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 agree with you. Base drag always exists for a blunt bottomed rocket.
I think that as the length of a rocket increases, the effect of base drag becomes a smaller contributor to CP compared to the effect on CG of the extra length.

 

[overklock]

For anyone interested, I found an old post from another member who also had their CG at precisely 38.5 "fully loaded". Not sure if that's with or without the motor. Presumably fully loaded implies it is. But if it's without the motor, that matches up perfectly with mine.

Whether or not his flight went well is up in the air.

https://www.rocketryforum.com/threads/need-help-where-is-the-cp-on-the-aerotech-g-force.164279/