Estes Argent with a G74-6: Stability margin of 6.08 caliber a concern?

[WizardOfBoz]

I'm planning on trying for my L1 a week from today, and hope to launch my Madcow Super DX3 on an H180. This will be at the Maryland Delaware Rocketry Association launch site ("the sod farm", which is literally what is is).

I think that I'm ok with the DX3, but I also plan to launch my build of an Estes Argent. That kit is long since sold out but I put one together with parts from the Estes site. It's pretty aligned with the original design, except for the upper (smaller 2" diameter) body tube. Estes did not have any of the stock 13.5 inch long tubes, but they did have that tube slotted for fins (for a different kit). I bought two such tubes and a coupler. I cut the slots off and glued them together. So I have a 19-5/8 forward tube length rather than the 13.5 inch stock design. Layout shown below.

For the Argent, Estes recommended an F15-6, and claims that the rocket will hit 1000 feet on that motor. Openrocket estimates 500 feet. The Tripoli prefect has been very helpful in mentioning that there are many rocket-eating trees near the site, and that less than 1000 or 1500 feet is probably wise (thanks, Dennis!). So I was thinking of launching with a G74-6. This should give me 1300 feet or so at apogee. My build is about 530 grams, while Estes estimated 487 grams for their original design.

In doing some preflight stability analysis and safety prep, I get
1. Speed off the rod: 49.6 ft/s (ok, >45 ft/s)
2. Max Thrust to mass ratio: 94.5 N (/9.81 m/s=) 9.63 kg force compared to 0.530 kg weight, for a ratio of about 18. (ok, >5)
3. Stability margin: 6.08 caliber!

While 6.08 means that the rocket is stable, it's high. The rocket is overstable. I'm worried about weathercocking, high horizontal velocity at deploy, etc. So I have questions:
1) Is a SR of 6 something to be concerned with?
2) There are some comments from Argent owners that smaller motors gives slower velocity which allows for more weathervaning. Perhaps the more powerful G engine will alleviate some of the overstability issue?
3) Would you guys launch this rocket, with a G180?
4) Are there expedient techniques to reduce this margin (e.g. weights added just in front of the forward CR?)
5) Even if I look at OR with the original shorter tube, the stability margin is still really high (5.51). Did Estes not do its homework here?
6) Long term, would adding fins to the forward tube (moving the CP forward) be a sane and appropriate way to move the CP forward?

Thanks for the help.

 

[WizardOfBoz]

Argent is yellow and on the left. The DX3 body is on the right.

One pet peave about the Argent. Admittedly, the Estes paint job on the Argent was very nice. Classical. White, with really dark blue (black?) and orange trim strips. Still, the word "Argent" means silver. (Its why silver's chemical elemental symbol is Ag). Why couldn't they paint it silver? I was orginally going to do that (paint it silver) but decided to name it "Blondie" to honor my wife. But still, Argent? And its white, blue and orange?

 

[Buckeye]

Send it. You have good speed off the rail, so over stability is rarely a problem.

OR Barrowman CP prediction is pretty conservative.

Estes errors on the side of lightweight builds and big fins for MPR.

 

[neil_w]

Your % margin is 24%, which is a bit on the high side but not crazy. Use a higher thrust motor to get it moving and it should be ok.

 

[WizardOfBoz]

Great, thanks guys.

 

[waltr]

Agree, margin a bit high so if windy don't fly.

Also examine the Thrust curves for various motors. There are big differences between Total Impulse, Average thrust and initial thrust.

The G motor I like to get a rocket off the rail with good velocity is the AT G76G (29/40-120 case). It has a high initial thrust that decreases as it burns.
Try thinking of a way to add weight near the motor. This will decrease the margin and altitude.

Good luck with your L1. I did my L2 cert flight at the Sod Farm last year. We had a perfect day with light winds that blew straight down the field and not toward the trees. Flight was to 3300 feet and landed about 500 feet down range.
If the wind is in any other direction then keep it low. A $400 tree climber fee is not as fun.

 

[WizardOfBoz]

Again, thanks - this is all useful info.

One way to add weight near the motor would be use my RMS-29/360 with a couple of spacers, instead of a SU. The only engine I see that is "G" and can be used (180 is the minimum length for the RMS-29/360 with both spacers) is a G75-10. That does move the CG down, but its a more powerful engine than the G74. OR predicts an apogee of about 2200 feet with that motor (as compared to 1300' with the G74). I have a single use G80 that is probably too powerful for the rocket and range, and the G75 gives an even higher apogee. I suspect that I'd better stick with the G74, and fishing weights poured into the body tube before launch. Too, I don't have a G75 and while it only costs about 35 bucks, there's a 50 dollar hazmat associated with it. I hate hazmat fees.

So I suspect I'll have to try the G74-6. Possibly with some weight added to the body tube. With a no-go if the wind is high. I'll depend on the experts on-site for advice.

But what about adding forward fins? What do you guys think?

 

[WizardOfBoz]

A better pick of the rockets for next weekend:
View attachment 1719169481747.jpeg

 

[WizardOfBoz]

If the wind is in any other direction then keep it low. A $400 tree climber fee is not as fun.

Is that what the going rate is? Wow.

As an aside, I saw a video of a rocket with its shock cord draped over some wires causing some arcs. I was thinking: either use a shock cord with a copper wire woven into it, or soak the shock cord in some salt water before launch. Then, if your rocket drapes over the lines, the shock cord bridges the gap and poof! It evaporates. The lines are no longer linked, and the two parts of the rocket on either end of the former shock cord come right down.

As I said, for the flight next weekend I'll listen well to the experts and folks who've flown there before.

 

[ThirstyBarbarian]

It should be fine. And yes, Estes did not do their homework if they recommended an Argent fly on an F15. Originally Estes released these PSII rockets when they were selling rebranded Aerotech composite motors under the Estes brand, so they could recommend those higher thrust composites for this bird. That deal ended, so the only 29mm motors they still sold under their own brand were their F15 and E16 motors, but those really do not have enough thrust for the Argent. That low thrust and the over-stability is a recipe for disaster. I’ve flown mine on AT F20s for a low flight of about 500’. That is actually near marginal, but the motor has a decent thrust spike at the beginning of the burn, and I don’t do it on windy days, so it works out fine. But higher thrust is better for safer flights. If you want to keep it lower, maybe try an F67.

 

[Neutronium95]

One way to add weight near the motor would be use my RMS-29/360 with a couple of spacers, instead of a SU. The only engine I see that is "G" and can be used (180 is the minimum length for the RMS-29/360 with both spacers) is a G75-10.

You can only space the 29/360 down to the 29/240 case.

 

[WizardOfBoz]

You can only space the 29/360 down to the 29/240 case.

Thanks!

I was going by the chart on the Apogee website: https://www.apogeerockets.com/Rocke...ch_29mm_Reload_Adapter_System?cPath=7_33_125&

This shows that with two spacers you can get down to the 180 length. I presumed that with one spacer you get to the 240 sized. There's only 4 reloads that fit that size, so it's not a big deal. But the reload adapter system only allows one to step down one size?

 

[Art Upton]

FYI a G80 is not to powerful for that rocket. Your field ? maybe, but not the rocket.

 

[WizardOfBoz]

@ThirstyBarbarian, thanks for the info - good to know the background. Depending on the flight I can can buy an F67 to try out. As it happens I have a two-pack of G74Ws which gives pretty close performance (about an 8% lower apogee for the F67). Of course, if that 8% higher flight causes you to dangle the rocket in a tree....

 

[WizardOfBoz]

FYI a G80 is not to powerful for that rocket. Your field ? maybe, but not the rocket.

Art, you are spot on. One of the things that my first conversation with Dennis Kingsley (whom I believe is the Prefect for the Maryland-Delaware Tripoli Prefecture (correct terminology) educated me on was that the rocket and the motor have to fit the launch site. Horses for courses if you will. So the sod farm has some rocket-eating trees nearby. I'm sure that on a low-wind day one could arrange for a waiver and shoot something into the proverbial stratosphere (which my 15-year-old self would have been up for). But for normal days, with wind, and wanting to see my rocket again, and being ... a ways away from 15, I'm going low. It should still be fun!

I believe that Dennis said something like "The Argent could be a little hot to do a level 1 on our field". Also "It's probably best to stay lower than 1500 feet". With maturity, one learns to listen. Or rather, learning to listen indicates that you are maturing and not just getting older. I hope!

So, it's encouraging that the work I put into the Argent could be used with a higher power motor (H engine simulations show an apogee of 2200-3300 feet roughly) and I may do this (I may adapt this thing for dual deploy - it has space for drone and main chutes in the compartments). But for now... low, with a higher likelihood of rocket recovery, is what I'm after.

 

[WizardOfBoz]

No comments on adding fins to the small diameter tube forward, to move CP forward?

It would seem a little bit better than melting a bunch of tire weights and casting a one pound ring, and fix it just in front of the forward CR, OR tells me I have a stability margin of 3.5. Interesting, when I look at the sims, this lowers the predicted apogee to 745 feet (from 1300) for the G74. But for larger motors, the apogee doesn't change much.

I could do an experiment. I could buy another two 2 inch tubes and a coupler from Estes, create smaller version of the fins currently on the rocket, and create a "top stage" (in looks only). OR could then be used to "tune" the stability. My day job is in simulation, so this would actually be fun. The design with fins (shown above) has a SM of 3.5 caliber or so!

 

[neil_w]

Canards will pull the CP forward, if you want to do it that way. Aim for stability margin of 10-15% with the heaviest motor you’re considering flying.

 

[GalantVR41062]

My first mid power build as a BAR was a scratch build with canards because they looked cool. The flight profile was exciting, then the canards were shaved down to little nubs and a longer nose cone was made.

I also learned how to calculate the CP, measure CG, what makes a rocket fly up, kind of like rocket science.

I also have a Argent, built and flown on a wide range of motors up to a CTI H160 classic. It's painted primar grey and has some metallic silver on the nose cone, now I know why.

 

[Art Upton]

No comments on adding fins to the small diameter tube forward, to move CP forward?

It would seem a little bit better than melting a bunch of tire weights and casting a one pound ring, and fix it just in front of the forward CR, OR tells me I have a stability margin of 3.5. Interesting, when I look at the sims, this lowers the predicted apogee to 745 feet (from 1300) for the G74. But for larger motors, the apogee doesn't change much.

I could do an experiment. I could buy another two 2 inch tubes and a coupler from Estes, create smaller version of the fins currently on the rocket, and create a "top stage" (in looks only). OR could then be used to "tune" the stability. My day job is in simulation, so this would actually be fun. The design with fins (shown above) has a SM of 3.5 caliber or so!

When it comes to these "transition" rockets, I don't glue in the top section(s) anymore. I make a really tight friction fit since the lower section of the transition is going to be the mount for the recovery it's not a safety issue. Makes for easy repairs or experiments like your Canards a choice you can put on, or remove.

If you don't like friction fit with larger motors, one or two very small screws can hold the tube to the transition section. I make my payload bays always screw together to the tubes, not glue in.

Hey, I have flown my Estes PSII Partizon 2.5" rocket that is made of 3 tubes and couplers with just friction fit masking tape many many times. Twice even on I205s. This was so it could be broken down for shipping in a smaller box* just like the kit comes in. Also could make repair easy if any was needed in the future.

* makes it possible to ship "rockets in a box" to FedEx hold for pickup while you take a plane to a faraway launch site. Then when you leave just ship it back to your home on the way to the airport. You buy your propellent on site from a motor vendor.

 

[loopy]

Argent can also mean white...it doesn't just mean silver.

I love my Argent, and it flies really well, but I don't fly it on gusty days. Mine is black with a red lace pattern on it, and I call it "Argent Provacateur"...lol

 

[WizardOfBoz]

@Art Upton Thanks, this is useful info. I plan to drill holes in the upper tube as well, so that I don't have a pneumatic body tube separation. Also, this will allow me to add an avbay (perhaps) and do dual deploy... But again this info is very useful to me - much appreciated.

 

[WizardOfBoz]

Argent can also mean white...it doesn't just mean silver.

I love my Argent, and it flies really well, but I don't fly it on gusty days. Mine is black with a red lace pattern on it, and I call it "Argent Provacateur"...lol

I work for a French company, and have tired (futilely as it happens) to learn French. "Argent" means money in French - that is, silver. But I now note that argent can mean white as well - thanks for upgrading my knowledge. My Argent is yellow, and is called Blondie, as an homage to my wife. My wife is more stable than the rocket though.

 

[loopy]

I work for a French company, and have tired (futilely as it happens) to learn French. "Argent" means money in French - that is, silver. But I now note that argent can mean white as well - thanks for upgrading my knowledge. My Argent is yellow, and is called Blondie, as an homage to my wife. My wife is more stable than the rocket though.

I'm glad she's very stable...lol...Yeah - I knew about the French meaning for money, I just looked it up to see if there were other definitions as I'd never really thought that much about it (not even thinking about it meaning "silver"). According to Webster and the Cambridge dictionary, it can be silver or white, especially related to a coat of arms. More interesting word than I thought...lol

 

[jmasterj]

Thanks!

I was going by the chart on the Apogee website: https://www.apogeerockets.com/Rocke...ch_29mm_Reload_Adapter_System?cPath=7_33_125&

This shows that with two spacers you can get down to the 180 length. I presumed that with one spacer you get to the 240 sized. There's only 4 reloads that fit that size, so it's not a big deal. But the reload adapter system only allows one to step down one size?

The 29/360 is two sizes bigger than the 29/240. There's just no middle 29/300 case.

I would not modify your Argent. If the winds are high or blowing towards the trees, don't fly it. If they blow toward the farm entrance or are low, let 'er rip!

 

[WizardOfBoz]

Canards will pull the CP forward, if you want to do it that way. Aim for stability margin of 10-15% with the heaviest motor you’re considering flying.

So I think I understand stability caliber. Can you explain percentage? Many thanks.

 

[waltr]

So, it's encouraging that the work I put into the Argent could be used with a higher power motor (H engine simulations show an apogee of 2200-3300 feet roughly) and I may do this (I may adapt this thing for dual deploy - it has space for drone and main chutes in the compartments). But for now... low, with a higher likelihood of rocket recovery, is what I'm after.

Problem with adding DD to this rocket is it will move the CG forward and increase the stability margin. The opposite of what is needed.

Since you are moving into higher power rockets pick and build a different rocket to put DD into.

 

[WizardOfBoz]

The 29/360 is two sizes bigger than the 29/240. There's just no middle 29/300 case.

I would not modify your Argent. If the winds are high or blowing towards the trees, don't fly it. If they blow toward the farm entrance or are low, let 'er rip!

Ahh, so from 180 to 240 is 60 (one spacer), but from 240 to 360 is 120(two spacers). So if I got a 180 case, could I use it to fly engines sized for the 120 and 60 case?

It kind of seems like the RMS-29/40-120 would be the one to get if one were launching the Argent or other smaller rockets using 29mm motor.

I'll keep your flight advice in mind. I think it will be reinforced by the RSO/LSO. Thanks!

 

[jmasterj]

Ahh, so from 180 to 240 is 60 (one spacer), but from 240 to 360 is 120(two spacers). So if I got a 180 case, could I use it to fly engines sized for the 120 and 60 case?

It kind of seems like the RMS-29/40-120 would be the one to get if one were launching the Argent or other smaller rockets using 29mm motor.

Yes to both!

 

[loopy]

Ahh, so from 180 to 240 is 60 (one spacer), but from 240 to 360 is 120(two spacers). So if I got a 180 case, could I use it to fly engines sized for the 120 and 60 case?

It kind of seems like the RMS-29/40-120 would be the one to get if one were launching the Argent or other smaller rockets using 29mm motor.

I'll keep your flight advice in mind. I think it will be reinforced by the RSO/LSO. Thanks!

if you're going to get into mid power on a regular basis at all, you DEFINITELY want the 29/40-120 case.

 

[Steve Shannon]

I just have one pedantic suggestion. In your first post you convert from Newtons to "kilograms of force".
Newtons are the correct units for force in the SI system for both thrust and weight. Kilograms are a unit of mass, not weight. Multiply mass times gravity to get weight in Newtons.