Aerotech G-Force Separation Issue

The maiden flight of my Aerotech G-Force resulted in a separation failure. The tubes only moved apart about 1.5 inches, leaving the coupler still inside the lower section, with no parachute being deployed. The rocket remained intact and fluttered down with only minimal damage (broken fin). My analysis of the problem led me to the fact that a tremendous amount of vacuum pressure is created when the tubes are pulling apart. I know the ejection charge should overcome that, but 4.0 inch tubes and small payload area create a lot of vacuum related resistance. I drilled 4 small "relief" holes in the coupler so that the vacuum is released once the holes are exposed, and it now pulls apart effortlessly without the vacuum resistance. Has anyone else had this issue and used this solution, or a different solution?

Rob , when your ejection charge goes off there is no vacuum . Think in terms of a gun the rapidly expanding gases on the back side of the bullet pushes the bullet out . I would look for other issues like size of deployement charge or when you painted your rocket did you get overspray on the coupler or in the body tube ? Especially in the warm sun the heat can make that paint a bit...Gooey making them harder to push apart . Hope that helps , Jim

I agree with jis2 above. When the ejection charge goes off, there should be no negative pressure inside the recovery bay to overcome. When you pull the rocket apart by hand, you are pulling it from the outside, and that might create some vacuum inside the bay that makes the sections harder to pull apart. But when the ejection charge goes off, it creates a positive pressure inside the bay, pushing the sections apart from the inside, not pulling them from the outside --- there is no negative pressure holding the sections together, only positive pressure pushing them apart. My opinion is that, if anything, the pressure relief holes will allow the release of the positive pressure inside the bay that is required to push the sections apart. In other words, they may hurt more than help.

How many holes are there and how big are they? Maybe post some pictures.

Sometimes, if a rocket is going to go high, like over 2,000 feet, or very fast, you might want a single small vent hole of about 1/8" into the recovery bay to allow pressure equalization. But the purpose is to relieve the build up of positive pressure inside the bay from the rapid increase in altitude. It's not for relieving negative pressure holding the sections together. Without the hole, the positive pressure might force the sections apart prematurely during flight. The small hole is enough to relieve that pressure build up, but not big enough to relieve the big and rapid pressure pulse from the ejection charge. And the relief hole would be drilled directly into the bay --- on the G-Force, that would be above the seam where the two tube sections meet.

Here are some other things you could check that might have caused the problem:

Is there a lot of friction between the coupler and the tube? A good pressure seal is a good thing, but a rough or sticky surface that makes the coupler grab onto the tube is not. I coated the bottom tube inside with CA and sanded it smooth so that there was a very smooth surface for the coupler to slide into. It slides easily but still holds pressure well.

Is the ejection charge strong enough? Any single-use G motor should be manufactured with a strong enough ejection charge to work fine with a G-Force. With a DMS type motor or a RMS type reload, you need to add the charge.

Did the delay grain actually burn, and did the ejection charge actually go off?

Is there any obstruction in the path of the ejection charge? The G-Force has the cooling mesh and the baffle. Is there any chance they have caused an obstruction?

Did the motor retention work? If the motor ejected, then the ejection charge may have been lost to popping the motor out instead of popping the recovery bay open. I know you had made some modifications to the retention to allow longer motors. Did it work ok? Based on a casual glance, it looked ok to me.

Thnx TB and jis2 for all of the good input. Just to answer a few of the issues raised; the delay grain did burn, the ejection charge did go off and the motor remained in the retainer. There was no paint on the coupler, but I did clean-up (sand) the coupler to move a little more smoothly. I will do CA inside the lower tube as well (I have done that on previous projects and it works well). I also put some thin CA inside the coupler to strengthen it as it did flex quite a bit, which may contribute to binding. The mesh is in place and fitted similar to other Aerotech projects I have done - I don't think there is an obstruction, but I will test. I think the key thing I need to consider based on the input received is the strength of the ejection charge - it was launched on an RMS G76-4 with the standard charge. The whole reason I have a concern is that this is not the first G-Force I have seen or read about with a separation issue (the first G-Force I ever saw fly did a nose plant at Moffett due to no separation). I saw on some other blogs (possibly Rocket Reviews) that the G-Force is pushing the limits with a standard ejection charge. I just want to make sure there is not something else endemically wrong with the standard design that requires modification. I think I will do some bench testing with a slightly larger ejection charge and the modifications I have already made and see what happens (I will cover the relief holes and test both ways). TB- you had a great flight at Snow Ranch with your G-Force; what motor/charge did you use and did you make any modifications? Thnx- : )

Link to the Rocket Reviews post I referenced above : https://www.rocketreviews.com/kevin-smiths-g-force-1870.html

Rob, now that you mention it being possibly a known problem, I think I remember Mike at BAR mentioning the possibility of separation problems having to do with the coupler binding inside the other tube due to it getting a little sideways or angled. I don't really remember what he was saying about it.

I was a little concerned about how tight my coupler fit into the lower tube, so I used CA to harden the surfaces and lips of the tubes and then sanded them smooth. The first step was to really soak a lot of thin CA into the lip of the tube. I did that by getting a surface I wasn't concerned about ruining --- it was the lid to a cardboard box of some kind, like maybe a shoebox. I put a layer of wax paper inside the lid and folded up the edges so that liquid could not easily escape the wax paper. Then I squirted a little puddle of thin CA into the center of the wax paper. It wasn't very much, just a fraction of an ounce, and maybe a puddle a couple of inches across. Then I got the lower section of the rocket, flipped it over upside down, and placed the lip of the tube flat on the wax paper over the puddle. By moving the tube around. I was able to get all parts of the rim in good contact with liquid CA, and it just wicked up into the cardboard. I actually did this with the coupler too and also with the top lip of the payload section.

Next, I used a foam brush to "paint" a lot of thin CA onto the inner surface of the lower tube. After it was dry, I sanded the inside of the tube so that it was very smooth. With the lip of the tube reinforced with CA, I was also able to sand a little bit of a bevel or a rounded lip on the inside of the rim of the lower tube. I also painted CA on the inside of the coupler and sanded that too, mostly because I liked how smooth it could be, and also because I thought it might protect against moisture and mud in case of a sloppy landing site. Then I also sanded and rounded the reinforced lip of the coupler. Having both the coupler and the lower tube lips hardened and rounded made it much easier to get the coupler started into the lower tube when mating the parts together --- the rims did not tend to bind on each other as much.

Those improvements made the coupler slide into the tube a lot easier, while still providing a good seal. It was still a bit tight, so the last step was something that I am not sure I can really recommend, but I will tell you about it anyway. I have some silicone spray lubricant called Jig-a-Loo. It's a non-staining lubricant that is safe for use with things like fabrics, and it is moisture resistant and non-greasy. People use it for things like lubricating curtain rods, or zippers, or hardware inside auto interiors where you want lubricant, but don't want to damage the fabrics. I had just a trace amount in the bottom of a can and I sprayed some on the inner surface inside the lower tube. Wow! That made it really TOO slippery! If I tried to pick up the rocket holding only the top section, the bottom section would just fall right off. You really don't want it so loose it will drag separate. To tighten it up again, I had to add a little bit of tape for a tighter fit. I don't know if the lubricant has dried up a bit or soaked in or evaporated, but it is not as slippery now as when I first added it. The reason I would not necessarily recommend it is that it might be too slick, and also I've heard since then that some silicone lubricants are bad for certain paints.

Regarding the motors, at the last launch, I used a G77-4R, and a G40-4W. At an earlier launch, I used a G75-4DM and a G78 or G79. So far I have only flown it on AT single-use motors, so the ejection charges are set at the factory. I've only used 4 second delays so far.

I hope that all helps! Good luck with your testing, and please let us know what you find. I'd hate to crash this rocket. It's one of my favorites now!

As I recall, many of the failures to separate were in cases where the G-Force was modified to separate at the nosecone, which gives a massive volume to fill that is on the borderline for standard Hobbyline ejection charges. Built stock the volume is small and should easily pressurize enough. I would test to see if the cooling mesh is too tightly packed and causing an obstruction, as well as the other advice given here.

Rocket86 said:

As I recall, many of the failures to separate were in cases where the G-Force was modified to separate at the nosecone, which gives a massive volume to fill that is on the borderline for standard Hobbyline ejection charges. Built stock the volume is small and should easily pressurize enough. I would test to see if the cooling mesh is too tightly packed and causing an obstruction, as well as the other advice given here.

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This makes sense to me. The volume built stock is not really very big. It's a 4" tube, but the coupler is only 6" long. Add in the volume of the motor tube/baffle, and it is not much all together. I think the Mega Der Red Max probably has a bigger volume, and those don't seem to have ejection issues. My guess is that there is a problem with binding/friction. Or there is an obstruction or other pathway for the ejection gas to escape.

TB: The coupler strength I think is an issue that requires modification. My initial thought when I was building the rocket was that the coupler seemed mighty thin/flimsy for such a big rocket. Binding of the coupler has got to be one of the contributing factors to what I have read are frequent separation problems with G-Force rockets. Since the maiden flight I have reinforced the inside of the coupler similar to what you describe above. I may even layer in some additional reinforcement in the form of another coupler that will be split and re-sized to conform to the inside of the existing coupler and CA'd in place, or epoxy/cloth. I will also do the CA in the lower body tube as suggested. I will also see if I can spread out the cooling mesh a little with a coat hanger modified into a small hook as suggested by Rocket86. I will then bench test before the next flight. I will post the results of the testing. Thnx for all of the input.

The amount of black powder in the hobby line case reload is marginal for the G-Force and should be supplemented. If it is not in the right position at apogee or the coupler fit is too tight then the result is too often what happened to you. The G77R uses the high power 29/120 case and comes with more black powder in the reload kit. These and other 29/120 reloads are good choices for the G-Force due to this.

terryg; Yes I was using a hobby line RMS case with a G76-4 with apparently the smaller charge. I do have a high-power forward closure from an RMS 29/180 that I will use on the RMS 29/40-120 (I hope it will be the same thread size) and get a G77R for the next flight. Hopefully the larger charge size will do the trick along with the coupler modifications suggested above. *great advice* - Thnx

No, the two cases are not compatible at all. Get a 29/120 (case only) for the G77. It uses the same the front and rear closures as the 29/180 motor. You can get all the way up an I200 by getting longer and longer cases (the larger ones need a seal disk also). You can still use the 29/40-120 hobby line case with G reloads by filling up the red cap with extra black powder. Like I said it is marginal, it does not take that much more to make it reliable. The G77 is a great reload with a long very red flame.

terryg; I have not worked out getting black powder yet, so it looks like Sirius Rocketry will be getting more of my kid's college fund so I can get the 29/120 and the G77's. : )

Your best bet for black powder is to attend a club launch where people are flying dual deploy and talk to the fliers who will be more then willing to give some to you. Another approach is to save any unused black power from one reload to use in another. Smaller diameter rockets take less black powder then 4 inch ones. Be aware that since the rear close on 29/120 case is different then the hobby line case and you might need to modify your motor retention method. Aerotech has just released a screw on retainer that is quite nice. The G79W reload is also a good one and most of the 29 mm line of reloads can ship without hazmat.

RKT_ROB said:

terryg; I have not worked out getting black powder yet

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If no better source, ECK-1 kit from valuerockets. https://www.valuerockets.com/product_details.aspx?pid=14&itemid=67

I'm a little skeptical that lack of BP is the problem -- I don't have a G-force but I have flown similarly-sized 4" rockets without issue with G64/G76 many times -- but maybe.

mikec said:

If no better source, ECK-1 kit from valuerockets. https://www.valuerockets.com/product_details.aspx?pid=14&itemid=67

I'm a little skeptical that lack of BP is the problem -- I don't have a G-force but I have flown similarly-sized 4" rockets without issue with G64/G76 many times -- but maybe.

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It's good to know that is an option, but it's a pretty expensive option on a per gram basis. I'm going to need extra black powder for my Warlock, and I might go for this solution until I can find a larger quantity. One thing that is kind of nice is it's in a premeasred quantity.

ThirstyBarbarian said:

It's good to know that is an option, but it's a pretty expensive option on a per gram basis. I'm going to need extra black powder for my Warlock, and I might go for this solution until I can find a larger quantity. One thing that is kind of nice is it's in a premeasred quantity.

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For 12 of the 1.4 gram vials, each of which is enough to deploy the Warlock if used in addition to the stock charge, it actually seems like a ok deal. The fact they are pre measured is great. 1.4 grams is about what I used on my Warlock (two Econojet charges) and it worked well.

Rob, I've been thinking more about this and I remember you mentioned in another thread wanting to remove the motor block and retainer clip, but were worried about damaging the motor tube. Then at the launch you showed me you had come up with a different retention solution for longer motors. I didn't ask if you had made any attempts to remove the block or clip before coming up with the other solution. If so, is there any chance there is a hole in the motor tube that would allow gasses into the closed off areas of the lower section? If the charge was pressurizing that section too, then it could be a bit diluted and weakened when it gets to the recovery bay. I'm assuming that's not the case, but thought I would ask.

Also, about black powder, there is a store in Dublin that sells FFFG. I know the FFFF is preferred, but thought I would mention it anyway. It's called Guns Fishing and Other Stuff.

SCrocketfan said:

For 12 of the 1.4 gram vials, each of which is enough to deploy the Warlock if used in addition to the stock charge, it actually seems like a ok deal. The fact they are pre measured is great. 1.4 grams is about what I used on my Warlock (two Econojet charges) and it worked well.

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Yeah, it's actually not bad on a flight by flight basis. Just kind of high on a gram by gram basis.

ThirstyBarbarian said:

Yeah, it's actually not bad on a flight by flight basis. Just kind of high on a gram by gram basis.

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Yeah, 6 dual deploy or 12 single deploy flights for $7 isn't bad. I'm trying to find a source of BP for my Vindicator Jr dual deploy for Dairy Aire and these might work well.

mikec; thnx for the referral to valuerockets. When I clicked the link I think I heard a chorus of angels singing ; ) Great solution for small quantities of BP. I will still try to source larger quantities of BP from the suggested store, the on-site vendor at our club events, or as a last resort on-line and pay hazmat. TB: I did not attempt to do any motor tube modifications yet. All is still intact in the original design, but thnx for the insight. I think all of the coupler suggestions will be implemented, I will check the mesh and use the larger charge with the G77 and HPR RMS and *hope* the riddle will be solved. Thnx for all of the great suggestions on this thread.

With ours, we have the upper AF pinned to the coupler with removable plastic rivets, and have the ejection charge kick out the NC and parachute. Has worked well with 29mm CTI charges, as well as 24mm charges with a little extra FFFFg added.


Later!

--Coop

RKT_ROB said:

The whole reason I have a concern is that this is not the first G-Force I have seen or read about with a separation issue (the first G-Force I ever saw fly did a nose plant at Moffett due to no separation).

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When was the nose plant, 3 or 4 years ago? Could have been mine. That was one of my first mid power rockets. My coupler was just too tight, and I probably had a ton of weight in way too much filler primer. Chalk it up to inexperience. I was able to keep booster with a few modifications and rebuild. I dumped the cooling mesh. Sanded the heck out of coupler. Used CA and all of that. There is no question now that coupler is not tight in hot or cool weather. It was a learning experience for me. I learned and have flown it many times on AT G motors. No issue.

Chris,

The attached video captures a little of the event I mentioned and is dated 1.18.2014. You will see a black G-Force launched right at the beginning of the video and then hear the "heads-up" comments and the comments of a "lawn dart" right before before my Aerotech Arcas launches (which is what I was filming). I did not capture the actual impact, but listen to the video and you will understand what happened. I hope this was not yours : )

[video=youtube;r2km_k8Ivfk]https://www.youtube.com/watch?v=r2km_k8Ivfk[/video]

RKT_ROB said:

Chris,

The attached video captures a little of the event I mentioned and is dated 1.18.2014. You will see a black G-Force launched right at the beginning of the video and then hear the "heads-up" comments and the comments of a "lawn dart" right before before my Aerotech Arcas launches (which is what I was filming). I did not capture the actual impact, but listen to the video and you will understand what happened. I hope this was not yours : )

[video=youtube;r2km_k8Ivfk]https://www.youtube.com/watch?v=r2km_k8Ivfk[/video]

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Always tough to watch. Not mine though. Mine was awhile back and was white and red. Straight up, arced over, straight down hard. Nosecone and payload tube nearly disintegrated. Fortunately mine hit the dirt and not the concrete. I don't think that anything would have been left of mine. My first mid power rocket was a Sumo. It crashed on its first flight (about 6 years ago or so) due to me packing the chute wrong. Chute ejected but did not open. Also at Moffett. Hit the concrete pretty good but not a lawn dart. Somehow it survived and I was able to repair it enough to fly. Not pretty, but I still fly it. Someone from LUNAR taught me about the chute burrito. Those were tough lessons for me, but I did learn from them and these things helped me on my way to high power. Good luck with your rocket. Maybe I will see you up there sometime.

I flew my G-Force this past weekend and Snow Ranch and I noticed a few new issues with the coupler that I had not noticed before. The first flight, everything went together as usual with the coupler sliding into the lower BT fairly smoothly. It might have been slightly tighter than in the past, but it still went together fine. I usually give it a light twist back and forth once it is together to make sure it is not binding, and I'll pull it out a little ways and push it back in to see if it seems like it will separate smoothly. I did all that, and it was fine, and the first flight was perfect.

When I went to load it up again for a second flight, the coupler did not want to slide into the BT smoothly. It was hard to get started and seemed to catch while I shoved it and twisted it together. Once it was together, I tried to pull it apart again, and it was definitely stuck. I had to sort of rock it back and forth while pulling on it to work it back out. I think if I had tried to launch it that way, it might have failed to deploy.

Once I got it apart, I inspected everything, and the main problem seemed to be that the inside of the lower BT was fouled with a lot of soot and grit from the previous ejection charge. Just wiping it out was not enough. I actually ended up unhooking the recovery harness and sanding the inside with 220 grit to remove the soot and grit, then wiping out all the dust. It still seemed a little bit tight. I still had a partial piece of tape inside the lower BT from when I had used the spray lubricant and needed to add tape to make the connection tighter, so I removed that. Then I very lightly sanded the coupler to remove some grass stain and a tiny bit of dirt that had got on it from the landing. After that it was fine, and I flew it successfully again.

I started to load it up again for a third flight, and again the coupler was sticky and did not want to go together smoothly. I think it was just fouled with soot again. I decided to move on to another rocket instead of try to clean that one again.

My plan is to try to sand the inside of the lower BT with some 320 or higher grit and get a very smooth surface that will be easy to wipe down and clean up after each flight. If necessary I might add more thin CA inside the area and sand again until it is very smooth and easy to clean.

Maybe this is another clue to the puzzle. Maybe the recovery bay is prone to getting fouled and needs a thorough cleaning each time. It definitely seems like it is worth it to check the fit each and every time.

TB; I was not able to make Snow Ranch on 3/7 but I hope to be there 4/4 with the tweaked G-Force and *hopefully* an L-1 cert attempt rocket. Thanks for the info above. I think the relatively thin coupler may be compounded by the soot issue in the G-Force and is another piece of the puzzle. I still contend that the thin coupler results in more binding than with most rockets due to the heavy payload section and nose cone exerting force on the thin coupler and causes a bind. I just ordered another coupler from e-Rockets and will fit it inside the existing coupler to add strength (similar to an LOC "stiffy" https://www.wildmanrocketry.com/ProductDetail.aspx?product=1828), CA the lower tube and add a larger charge with a G77 HPR. I will take your advice on checking the fit after each flight. I still hope to be able to bench test the G-Force modifications prior to 4/4, but I am also trying to get my L-1 rocket built before the Ranch is closed for the summer : (

RKT_ROB said:

TB; I was not able to make Snow Ranch on 3/7 but I hope to be there 4/4 with the tweaked G-Force and *hopefully* a L-1 cert attempt rocket. Thanks for the info above. I think the relatively thin coupler may be compounded by the soot issue in the G-Force and is another piece of the puzzle. I still contend that the thin coupler results in more binding than with most rockets due to the heavy payload section and nose cone exerting force on the thin coupler and causes a bind. I just ordered another coupler from e-Rockets and will fit it inside the existing coupler to add strength (similar to an LOC "stiffy" https://www.wildmanrocketry.com/ProductDetail.aspx?product=1828), CA the lower tube and add a larger charge with a G77 HPR. I will take your advice on checking the fit after each flight. I still hope to be able to bench test the G-Force modifications prior to 4/4, but I am also trying to get my L-1 rocket built before the Ranch is closed for the summer : (

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I still haven't gotten around to cleaning up that rocket yet, double-checking the fit, and making any changes. I'm pretty sure it just needs to be cleaned and smoothed some, and then find a way to make it easy to clean each time.

I am planning to bring the G-Force to Snow Ranch on 4/4 too. SCrocketfan and I are planning to drag race on CTI H135's if you'd like to get in on that.

Good luck getting your L1 rocket ready! The season is running out fast! I hope you can make it.

Do you guys have the mesh in your G Forces?? I built mine stock, now retired after around 50 flights, pretty beat up, was going to re paint etc. but seeing it can't use HPR 29mm. I'm going to get a new one and modify the MMT for HPR motors. It was a favorite of mine. I never used CA on the coupler, I have it fit that when standing on the floor I can lift up the payload section and it separates, the booster section doesn't move (stays on floor). With the motor inserted I can pick up the whole rocket lifting by the payload section. I "cut"/slightly rounded the inside edge of the booster section BT and outside edge of the coupler. I never had a separation problem ever. I never had a problem with BP residue, I don't remember ever cleaning out the BT or coupler. I never added extra BP to an ejection charge.

TB/RCKTNUT; My motor tube was built stock, and I am now kicking myself. I am going to see if I can do a modification to accept at least a 29/180 so I can do a smaller H motor, but there will be no drag racing for me against an H135 until I can get that done. If I have an opportunity to build another G-Force, I will leave out the motor hook and motor block as a minimum, use an Estes or Aero Pack screw on retainer and eliminate the mesh if needed to accept the longer length 180 or 240 RMS casing. Once I get the L-1 done, and the coupler reinforced on the G-Force, I will start on the motor mount modification. I will post the design I have in mind once I get to that. PS: TB - you are world famous now : )