Need Help on my Graduator

[Mad Rocketeer]

OK, I messed up (mildly, I guess, but frustratingly) on my new LOC Graduator. I'm using J.B. Weld on the places where anything attaches to the MMT and Bob Smith 30-minute epoxy everywhere else. I added milled glass fiber to the 30-minute epoxy at the front of the front centering ring (applied to the body tube before sliding the MMT in). I've left the rear centering ring unglued until I get the internal fillets done.

Well, like a dummy, I read the epoxy instructions (30 minutes to set up, 2 hours before handling) and timed my fin attachments for the first two fins at 2 hour intervals. This morning when I went to attach the third fin, I saw that the first wasn't straight any more. I'd gone to great lengths to get it perfect too, and when I went to do the second fin last night, the first was arrow straight and felt firm. Upon looking at it this morning, I could see that the first fin had drooped while the second was drying in a vertical position overnight. I went back and read the J.B. Weld again, and it said 4 to 6 hours to handle. Rats! I'd have gladly let the first fin harden overnight before doing the next one, had I noticed in time.

Lesson: Wait the full time before going on to the next fin when using J.B. Weld. Even if it feels firm, it can slip if left at an angle long enough.

Now, what? Just eyeballing it, I'd guess that the fin is off by just under 10 degrees. The place where it meets the MMT has probably slid at least the width of the fin (1/8 inch) to the side. Once the third fin has had its full 6 hours of curing time (especially since the garage is cool today), I'll try to get a better estimate of the extent to which the angle of the fin is off. I could figure it with a little math too, I guess, but I'm feeling lazy today.

So, my questions:
1.) Can this be fixed? I hate to leave it like this, but the fin is attached to the MMT (unfilleted as yet) with J.B. Weld, and to the body tube (above and below the tab only, no fillets) with 30-minute epoxy. J.B. Weld has been used (so they say) to repair tractor engines. Any chance I can get this to turn loose or dig through it without risking or destroying the fin and/or MMT? [I'm doubting it.]
2.) How bad is a tilt of this magnitude? Would it:
a.) Interfere with safety, from an RSO's perspective, for a mid-to-low-L1 bird?
b.) Disqualify this rocket for an L1 attempt?
c.) Interfere with the durability / survivability of the rocket?
d.) Interfere significantly with the performance? [I don't intend using this as a competition rocket.]
3.) Will this significantly affect the appearance of the rocket? [Seems like it would be most noticable right now, while the rear centering ring is still uninstalled.]
4.) Now what? Just finish it and ignore the tilt, taking it as a lesson learned?

Thanks in advance for the good advice. I know there will be people here with the experience to answer my questions and guide my next move.

 

[r1dermon]

well, i screwed up a bunch of fins on estes models, and they all flew fine...just because it doesnt look right, doesnt mean that it wont perform the same duties.

JB weld HAS been used to repair cracked engine blocks, in-fact, i repaired MY cracked engine block in my boat with it. it's held up for two seasons.

i think it all depends on how badly screwed up it is...i would think that as long as its at an angle to both other fins, then it should be fine. but im no RSO. heh.

as long as you didnt punch holes through the smooth layer on the tubing, then you should be fine to use an X-acto knife to delaminate it at the joint...wait until its fully set and then try that, i've done it on my aura to fix a cracked fin from a hard landing. then just sand the fillet off the fin and do it again....my suggestion. its best to tack the fin in place with something before using a glue which sets up slowly. some people use CA to tack fins to the BT, and THEN fillet. i use 5 min epoxy on the front of the fin root and on the back. leaving the middle open so the 30min can ooze into it. makes for really straight fins.

good luck with whatever you choose to do.

 

[SwingWing]

If the epoxy is still fairly fresh, there is a chance, albeit slim that you can soften it somewhat with a heat gun. Maybe you can pull the fin straight? Post curing epoxy with heat is a good idea anyway.
Which way is the fin off? Is it still parallel to the BT? If so, this is less of a concern than if it is at an angle to the airflow. (twisted). If it's twisted, I'd start over.

 

[r1dermon]

yeah, i should have said, its hasnt been an issue for me personally because my fins were still straight. if on is twisted then that'll be one heck of a corkscrew.

 

Originally posted by Mad Rocketeer
OK, I messed up (mildly, I guess, but frustratingly) on my new LOC Graduator. I'm using J.B. Weld on the places where anything attaches to the MMT and Bob Smith 30-minute epoxy everywhere else. I added milled glass fiber to the 30-minute epoxy at the front of the front centering ring (applied to the body tube before sliding the MMT in). I've left the rear centering ring unglued until I get the internal fillets done.

Well, like a dummy, I read the epoxy instructions (30 minutes to set up, 2 hours before handling) and timed my fin attachments for the first two fins at 2 hour intervals. This morning when I went to attach the third fin, I saw that the first wasn't straight any more. I'd gone to great lengths to get it perfect too, and when I went to do the second fin last night, the first was arrow straight and felt firm. Upon looking at it this morning, I could see that the first fin had drooped while the second was drying in a vertical position overnight. I went back and read the J.B. Weld again, and it said 4 to 6 hours to handle. Rats! I'd have gladly let the first fin harden overnight before doing the next one, had I noticed in time.

Lesson: Wait the full time before going on to the next fin when using J.B. Weld. Even if it feels firm, it can slip if left at an angle long enough.

Now, what? Just eyeballing it, I'd guess that the fin is off by just under 10 degrees. The place where it meets the MMT has probably slid at least the width of the fin (1/8 inch) to the side. Once the third fin has had its full 6 hours of curing time (especially since the garage is cool today), I'll try to get a better estimate of the extent to which the angle of the fin is off. I could figure it with a little math too, I guess, but I'm feeling lazy today.

So, my questions:
1.) Can this be fixed? I hate to leave it like this, but the fin is attached to the MMT (unfilleted as yet) with J.B. Weld, and to the body tube (above and below the tab only, no fillets) with 30-minute epoxy. J.B. Weld has been used (so they say) to repair tractor engines. Any chance I can get this to turn loose or dig through it without risking or destroying the fin and/or MMT? [I'm doubting it.]
2.) How bad is a tilt of this magnitude? Would it:
a.) Interfere with safety, from an RSO's perspective, for a mid-to-low-L1 bird?
b.) Disqualify this rocket for an L1 attempt?
c.) Interfere with the durability / survivability of the rocket?
d.) Interfere significantly with the performance? [I don't intend using this as a competition rocket.]
3.) Will this significantly affect the appearance of the rocket? [Seems like it would be most noticable right now, while the rear centering ring is still uninstalled.]
4.) Now what? Just finish it and ignore the tilt, taking it as a lesson learned?

Thanks in advance for the good advice. I know there will be people here with the experience to answer my questions and guide my next move.

I'd try to pull it out somehow and grind the JB off the root of the fin and re-do it. At worst, I'd think that you'd pull off the outer layer of paper on the MMT tube.

As for tacking fins in place, here's a few things you can try:

Put a little epoxy on the root of the fin, put the fin in place. Next, run a piece of making tape from the bodytube to the outer edge of the fin, then back to the body tube on the other side. This will hold it in place while the epoxy hardens, and won't let it move.

Another method, (actually my preferred method) is to use Loctite Gel superglue. Yes, Gel Superglue. This stuff is great! Put it on the root of your fin, attach it to the motor tube. It sticks instantly to phenolic tube, and nearly instantly to glassline coated tube. It's plenty strong, too. I tack the fins with the Gel Superglue, then run my 30-epoxy fillets over it. I've built many a rocket that way and never had a problem.

Good Luck!

 

[Mad Rocketeer]

Originally posted by r1dermon
well, i screwed up a bunch of fins on estes models, and they all flew fine...just because it doesnt look right, doesnt mean that it won't perform the same duties.

i think it all depends on how badly screwed up it is...i would think that as long as its at an angle to both other fins, then it should be fine. but im no RSO. heh.

That's what I was hoping too. I think I'm probably OK aerodynamically, I don't know yet aesthetically, and I'm a bit concerned strength-wise. The fin tab meets the MMT at an angle rather than squarely, as it should. If I leave it alone, I'll need to be extra careful about the internal fillets for that fin. I'd even thought about adding a 1" or so width strip of epoxied-on fiberglass cloth to the fin tab - body tube internal joints, lapping half onto the tab and half onto the tube. Might be more needed now. Might need it more on the tab-MMT joints though. Can J.B. Weld be used to glass a joint?

Originally posted by r1dermon
JB weld HAS been used to repair cracked engine blocks, in-fact, i repaired MY cracked engine block in my boat with it. it's held up for two seasons.

Cool! I knew they made the claim on the package about a tractor engine. Sounds tough to remove or grind through.

Originally posted by r1dermon
as long as you didnt punch holes through the smooth layer on the tubing, then you should be fine to use an X-acto knife to delaminate it at the joint...wait until its fully set and then try that, i've done it on my aura to fix a cracked fin from a hard landing. then just sand the fillet off the fin and do it again....my suggestion.

Hmmm. I hadn't considered trying to peel a layer off the MMT. Are they thin enough to do that without weakening it? How does one reach 5 or 6 inches into a 3/4" wide curved space to peel a thin layer from the MMT in a strip just along where the fin attaches? I could probably get my Exacto knife in there, but I'd probably have to try to score the paper with the tip, leaning the knife backward, since there'd be no room to bring the edge into play. I have a curved and pointy pick-like tool too that might work. I'd hate to mess the tube up though. I didn't punch any holes in the MMT, but I did scuff it to get better adhesion.

Originally posted by r1dermon
It's best to tack the fin in place with something before using a glue which sets up slowly. some people use CA to tack fins to the BT, and THEN fillet. i use 5 min epoxy on the front of the fin root and on the back, leaving the middle open so the 30min can ooze into it. makes for really straight fins.

Good luck with whatever you choose to do.

Thanks. I'm always a bit leery of tacking a fin with something weaker, especially in a heat intensive or high stress location. Seems like it just denies the stronger stuff a grip on the tacked portion of the surface. Hmmm. I could see mixing both and applying the quick stuff to the ends and the stronger stuff in the middle, all at once before pushing the fin home.

The J.B. Weld is pretty tarry and thick, so it held in place pretty well as long as the fin was positioned vertically. Hanging out at the side a full 60 degrees away from vertical was too much over time though. If I'd have just left it alone, it would have been perfect. Sigh.

Originally posted by SwingWing
If the epoxy is still fairly fresh, there is a chance, albeit slim that you can soften it somewhat with a heat gun. Maybe you can pull the fin straight? Post curing epoxy with heat is a good idea anyway.
Which way is the fin off? Is it still parallel to the BT? If so, this is less of a concern than if it is at an angle to the airflow. (twisted). If it's twisted, I'd start over.

Originally posted by r1dermon
yeah, i should have said, its hasn't been an issue for me personally because my fins were still straight. if on is twisted then that'll be one heck of a corkscrew.

Well, I'm fortunate there. The tube is slotted, so the fin is still parallel to the tube. Think of it as a door. The hinges are strong, and the door is square in the frame, but it's sitting a bit to the side instead of straight out from the wall.

I don't have a heat gun. The hottest things I have are a blow dryer (for hair) amd a soldering iron. I doubt the former is hot enough, and the latter would probably just make a mess and / or burn my rocket, if I could even get it into the space.

Originally posted by asmqajm
I'd try to pull it out somehow and grind the JB off the root of the fin and re-do it. At worst, I'd think that you'd pull off the outer layer of paper on the MMT tube.

It has some epoxy to the body tube in noseward of and tailward from (but not along) the tab (which runs most of the fin length). The epoxy may also be adhering from the front of the tab to the front centering ring, which I placed to run right at the top of the fin slots for strength. I tried to make that happen and planned to catch any gaps with the internal filleting when I do it.

What's the best way to do this? Just grab the fin and yank on it like I'm a really bad landing? Given how I placed the centering ring, I won't be able to pull the fin downward or upward without damaging things, and pulling side to side runs a higher risk of cracking the fin. Should I score the paper of the MMT first? I sure hate having to do this, but I do want the fin straight if reasonably possible.

Originally posted by asmqajm
As for tacking fins in place, here's a few things you can try:

Put a little epoxy on the root of the fin, put the fin in place. Next, run a piece of making tape from the bodytube to the outer edge of the fin, then back to the body tube on the other side. This will hold it in place while the epoxy hardens, and won't let it move.

Another method, (actually my preferred method) is to use Loctite Gel superglue. Yes, Gel Superglue. This stuff is great! Put it on the root of your fin, attach it to the motor tube. It sticks instantly to phenolic tube, and nearly instantly to glassline coated tube. It's plenty strong, too. I tack the fins with the Gel Superglue, then run my 30-epoxy fillets over it. I've built many a rocket that way and never had a problem.

Good Luck!

I remembered the masking tape thing, but too late of course. I'll probably do that next time. That would have saved me maybe, but there's the chance I'd have thought the J.B. Weld was cured so why not remove the tape before it leaves sticky stuff on my nice tube.

I wouldn't want to use anything that grips right away to tack the fin with. I like to have time to sit on the floor and look across the tube to position the fin just so before it locks in place. As above, I'm also leery of tacking things by putting something weaker but faster along a fin root, as it excludes the slower stronger stuff from having that area to grab onto. I have some slow CA glue, so I'd probably have time to get things straight, but maybe not.

The J.B. Weld was fine actually. I just didn't give it the full setup time, and I paid a price. I need to go look at it to see if surgery is required for it to look good enough.

Let me know on just how to go about trying to disengage the epoxy on the body tube and the J.B. Weld on the MMT. How to peel a tube in such a tight area, etc. The MMT area won't matter cosmetically, and as long as I'm careful to keep any ugly stuff under the area that will be filleted, I can get away with some ugly at the body tube too. I'd rather minimize it though.

Keep those ideas coming, and thanks!

Any RSO types out there with sage advice and/or reassurance?

 

Originally posted by Mad Rocketeer


It has some epoxy to the body tube in noseward of and tailward from (but not along) the tab (which runs most of the fin length). The epoxy may also be adhering from the front of the tab to the front centering ring, which I placed to run right at the top of the fin slots for strength. I tried to make that happen and planned to catch any gaps with the internal filleting when I do it.

What's the best way to do this? Just grab the fin and yank on it like I'm a really bad landing? Given how I placed the centering ring, I won't be able to pull the fin downward or upward without damaging things, and pulling side to side runs a higher risk of cracking the fin. Should I score the paper of the MMT first? I sure hate having to do this, but I do want the fin straight if reasonably possible.

I'd pad the fin in a vise and yank it out. Perhaps you could score the cardboard around the epoxy so it pulls off and stops, peeling as little tube as possible? Just an idea.

I wouldn't want to use anything that grips right away to tack the fin with. I like to have time to sit on the floor and look across the tube to position the fin just so before it locks in place. As above, I'm also leery of tacking things by putting something weaker but faster along a fin root, as it excludes the slower stronger stuff from having that area to grab onto. I have some slow CA glue, so I'd probably have time to get things straight, but maybe not.

Everyone has their own way of doing stuff. I've flown E to K power with fins that were tacked with Gel Superglue. I haven't lost a fin yet. When you have six fillets per fin, the area between the motor tube and fin is small compaired to all those fillets. If you have a fin break off, you most likley had bigger problems than Gel Superglue in the first place. It's a matter of what makes you feel comfortable.

The J.B. Weld was fine actually. I just didn't give it the full setup time, and I paid a price. I need to go look at it to see if surgery is required for it to look good enough

I've done fins tacked with JB Weld before too. I really liked the results - it works well. Where it really shines is on external fillets. I did JB Weld external fillets to die for on my 3" THOR. That thick goo stays put!

 

[r1dermon]

wouldnt want you messing up your MMT, but i think that if you want it to be straight, you're going to have to lose that outside layer of paper. what i'd do is start at the back of the fillet/fin joint. slip a hobby knife just under the fillet, try not to stab into the MMT. after you accomplish this, i'd move the knife side to side, or see what works to get it to move forward up the fillet.once you get far enough along, it will eventually just come off way easier as you use the knife as a wedge. you could try a REALLY hot soldering iron. im sure that after a while it'll be soft enough for you to seperate it.

 

[Mad Rocketeer]

Originally posted by asmqajm
I'd pad the fin in a vise and yank it out. Perhaps you could score the cardboard around the epoxy so it pulls off and stops, peeling as little tube as possible? Just an idea.

I don't have a vice. I may need to get one, but I don't have a work bench as such to bolt one to either. Sigh.

Originally posted by asmqajm
Everyone has their own way of doing stuff. I've flown E to K power with fins that were tacked with Gel Superglue. I haven't lost a fin yet. When you have six fillets per fin, the area between the motor tube and fin is small compaired to all those fillets. If you have a fin break off, you most likley had bigger problems than Gel Superglue in the first place. It's a matter of what makes you feel comfortable.

E to K? Kind of hard to argue with that, isn't it? I've heard others advise this sort of thing too, so that's a lot of votes for it as a good technique. As you say, it's largely a matter of preference and style. The point about the ratio of covered areas is a very good one too. I need to reconsider this one. I don't want to discount a good technique out of hand.

Originally posted by asmqajm
I've done fins tacked with JB Weld before too. I really liked the results - it works well. Where it really shines is on external fillets. I did JB Weld external fillets to die for on my 3" THOR. That thick goo stays put!

When I first put on some of the J.B. Weld, I wasn't sure whether I was going to like it. It's thick and kind of stringy, and I tried to use a cotton tip swab, like I often use with wood glue. It pulled the cotton strands out into a ropy string behind the swab, making it pretty messy to place adhesive with. I quickly figured out that it is better to use the stir stick as an applicator too. I thought of it as a fillet material, but wasn't sure, due to weight and the stringy-sloppy factor. Can one use the same alcohol dipped finger technique for smoothing and shaping it that is used for epoxy? Will that prevent the stringy smearing that might otherwise occur? Good masking, which should be done anyway, would handle the sloppy factor, but without a way to make the surface slick for shaping, it would just grab my glove (or popsicle stick or plastic spoon, etc.) and form a string rather than taking a nice curve. Seems like, heavy though it is, it would still be lighter than epoxy clay. Overall, I think I like it.

I plan to use 30-minute epoxy, with milled fiberglass and microballoons mixed in, (or perhaps as two layers with the milled fiber underneath for strength and the microballoons for sandability over the top before the first layer sets up) for my fillets this time though.

Originally posted by r1dermon
Wouldnt want you messing up your MMT, but I think that if you want it to be straight, you're going to have to lose that outside layer of paper. What I'd do is start at the back of the fillet/fin joint. Slip a hobby knife just under the fillet, try not to stab into the MMT. After you accomplish this, I'd move the knife side to side, or see what works to get it to move forward up the fillet. Once you get far enough along, it will eventually just come off way easier as you use the knife as a wedge. You could try a REALLY hot soldering iron. I'm sure that after a while it'll be soft enough for you to separate it.

The fin tab hits the MMT at enough of an angle that I think I can get under the joint for a little way anyway. It may get tough as I move forward and therefore deeper into the space between the tubes. Man! I'm bummed with myself over this, but I guess it's the hard lessons that are best learned. How thick are the paper layers, typically? How hard would the J.B. Weld be to cut into? Any chance of cutting the adhesive itself as I work forward? I should be so fortunate, no doubt.

I could get all the way down the tab-MMT joint and find out the tab-CR joint still has it held, preventing my adjusting the angle, but I don't think so. If that joint had adhered properly, it would have cured to handling strength in the two hours I gave it and would therefore have prevented my mishap.

When I hold the rocket, with the rear centering ring slipped into place, and look at it from the fin end, it looks OK as long as one of the good fins is pointing upward. When I rotate the first fin upward though, the bad angle stands out like a sore thumb, viewed that way. It's good enough to be stable; I could argue that one with math. But I'll never be satisfied with the look if I leave it that way. That, more than anything else, decides me to try, ever so slowly, to make a fix.

Hmmm. In a pinch, I guess I could try to cut through the tab as close to the MMT as possible, rotate the fin into line, and add square spars on both sides to complete the link to the MMT. That might add a bit of tail weight, and I'd have to recalculate the stability, but I'm betting it would ramain stable with no extra nose weight needed. I'd rather not, but it's an idea to have in reserve, in case it comes to that. Thoughts?

 

[rbeckey]

JB Weld can be disolved with gel type paint strippers. Protect the MMT with masking tape and liberally coat the epoxy with stripper, wait several hours and scrape.

 

[Mad Rocketeer]

Originally posted by rbeckey
JB Weld can be disolved with gel type paint strippers. Protect the MMT with masking tape and liberally coat the epoxy with stripper, wait several hours and scrape.

Excellent! That sounds like it could be an ideal solution. It brings up some questions though.
1.) Will this stuff disperse eventually, allowing me to re-attach the same fin root with J.B. Weld again? If so, how will I know it's ready? When it feels/looks dry?
2.) Will this swell, distort, or weaken the MMT? I don't care about looks, as the MMT is internal, but I need stuff to fit right when I'm done and for the MMT to remain strong, etc.
3.) Do I have any clean-up to do once the J.B. Weld turns loose? Will there be gel to wipe out? Do I need to wipe out the old J.B. Weld, or will it just dissipate the solvent and re-harden in place?

This sounds like it may be the magic solution I'd really like to find. I hope so anyway.

 

[rbeckey]

My experience with this was removing the front sight of a rifle that had been attached with a set screw and JB Weld. I soaked the end of the barrel in a cup of gel stripper and the JB Weld softened enough to be removed with a wooden scraper.

I suggest experimenting with an extra piece of MMT tubing to determine the best way to protect it from the stripper. Perhaps normal epoxy is not affected. In that case, a solution could be to coat the MMT with epoxy, except the JB Weld itself, then paint the stripper on and let soak. Scrape off whatever you can, and do it over and over until the fin can be pulled free. That is just one suggestion. Perhaps a layer of pvc or some plastic that will not dissove in the stripper could be applied to the adjacent areas on the MMT. This may not be a workable solution, it was just a thought I had. If all else fails, I'd peel out the MMT from the inside and start over.

Good luck. I hope it works out for you. I have kicked my own butt a number of times after similar miscalculations.

 

[Art Upton]

I'd cut the jb weld with stripper and/or a knife and redo the fin.
I like aligned fins, lots less spin, higher flights.

Mostly, JB weld is really for metal, mostly.
Like for putting the aero-pac retainer on the motor tube, or sealing your motor's engine block from leaking coolent.

You'll get better results for strength with yellow glue or 30min epoxy for the fins to motor mount when dealing with wood fins of all kinds.

Good idea to leave off the rear centering ring, I build all my fin fillets from the inside out.

Who says you can't fly paper rockets on M motors ?

 

[Mad Rocketeer]

Originally posted by Art Upton
I'd cut the jb weld with stripper and/or a knife and redo the fin.
I like aligned fins, lots less spin, higher flights.

I looked for gel stripper today (Wally World only) and didn't find any yet. I may go ahead and see (carefully) what I can do with a knife and/or a sharp pick while keeping my eyes open for the gel.

Yep. I need the fin aligned. The way it's off wouldn't cause a spin, since it's still parallel to the tube. The tube is slotted. [Great tubing, by the way.] It's more like taking a door that's standing straight out and closing it just a little. As nearly as I can measure it, I'm off by about 4 degrees. It sure looks like a lot to the eye though.

Originally posted by Art Upton
You'll get better results for strength with yellow glue or 30min epoxy for the fins to motor mount when dealing with wood fins of all kinds.

If so, I should have just used the 30-minute epoxy (Bob Smith) for everything. I'd be perfectly aligned and probably filleted too by now. Sigh. My reasoning for the J.B. Weld was that it is formulated to take a lot of heat. Also, it's gooey enough to help hold the fin where I want it while it cures, as long as it's vertical. Next time, I'll probably use masking tape to hold the fin in line too, though what I did worked great, except for rotating the tube to the next fin position too soon.

The kit instructions call for epoxy throughout, but I figured that J.B. Weld is epoxy, just a specialized form. I thought I'd give it just that little bit extra heat resistance where the MMT meets other structural members.

Sigh. I'll get it fixed. The good advice and commiserating helps though. It's good to know that there's hope for me.

Good link for parts, by the way. If I end up having to peel out the MMT, as rbeckey mentioned, I could leave the forward centering ring in place, cut a new piece of 29mm LOC tubing to length (Hmmm. Wish I'd measured it. Easy to do with a tape measure inside the MMT though.), and slip it in from the fin end, gluing it to the CR as I slide it up. Then before the adhesive sets up, I could put stripes of adhesive beside each fin tab on the MMT then rotate the MMT to get the fin tab to MMT butt joints done. I'd sure have to do the masking tape thing though because I'd be doing all three fin tabs at once. After that fully cured, I'd be where I am now (except aligned) and could start the filleting from the MMT outward.

 

[Art Upton]

The bob smith 30 min will soak into the wood parts better then JB weld will.

This will bond better. Yellow glue soaks in even better, Tite bond is the strongest. 30 min sets quicker.

heat really is not an issue with HPR motors, as the motor must remain at a low temperture for the paper motor tube.

Wish you luck on un sticking the part.

A new motor tube can be made as long as you like, I'd use 8 inches myself.

 

[Mad Rocketeer]

Originally posted by Art Upton
The bob smith 30 min will soak into the wood parts better then JB weld will.

This will bond better. Yellow glue soaks in even better, Tite bond is the strongest. 30 min sets quicker.

heat really is not an issue with HPR motors, as the motor must remain at a low temperture for the paper motor tube.

Wish you luck on un sticking the part.

A new motor tube can be made as long as you like, I'd use 8 inches myself.

Live and learn. If I was doing it again, I might use the Bob Smith 30 minute for everything, using milled glass fibers to thicken and strengthen the internal fillets.

I'm also considering using 1" wide strips of 3/4 oz. fiberglass cloth, attached with J-B Weld (MMT) and 30 min. (BT) to lap over the internal fin to tube joints. It might come in handy especially in case I nick the MMT (or worse) while chiseling the J-B Weld joint loose.

I'm thinking I'll do the butt joint (fin to MMT) with the same J-B Weld, just to keep it all symmetrical, but I may do the fin to MMT fillets in Bob Smith with milled fiber. Thoughts?

Of course, first I have to get the current joint loose. I've noticed that the cured J-B Weld, tough as it is, is a lot like a heavy rubber. It's flexible, at least somewhat, especially where I had a thick drop. I can easily see where it would fail to soak in well. In fact, where I did have a loose drop, just sitting on the tube by itself, it wasn't all that difficult to pry loose. The fin to MMT joint is tough though.

Long MMTs are a good idea too. (I just measured the stock MMT at 6".) If I do have to replace the MMT, I might make it longer, as long as it doesn't goof up the stability. (Conceivably, it could even improve it, depending on the current COG location.) If I did that, I could add a centering ring at the front of the MMT to make it act as a stuffer tube. I could even incorporate a baffle.

I'm hoping to save those ideas for a scratch built cousin to the Graduator, though, and just finish this one off stock.

 

[Art Upton]

Originally posted by Mad Rocketeer
Live and learn.

That's what makes rocketry fun !

My first full K powered rocket had the fins break right at the roots. I used that to learn even more. That is what makes the hobby fun.

On a rocket like you are working on, you don't need to put anything in the epoxy at all. Milled glass is not needed, as well as cloth not needed.

In fact, yellow glue is fine for all that rocket can do even on 29mm I motors. But epoxy sets quicker.

You don't need to worry about glassin' and cloth and stuff till you get to average thrusts greater then say 300-350ns.

Even then, I've flown paper, plywood and regular epoxy on J420Rs.

Now a motor like a J570 or the rocket killer K660 will need glass fins with cloth fillets etc..

Tac the far root edges of the fin with CA and kicker to hold it in place while the 30min glue sets on the fin root between the tac points.

Then add fillets after all fins are tac/epoxy in place and dried.

 

[Mad Rocketeer]

Originally posted by Art Upton
That's what makes rocketry fun !

My first full K powered rocket had the fins break right at the roots. I used that to learn even more. That is what makes the hobby fun.

On a rocket like you are working on, you don't need to put anything in the epoxy at all. Milled glass is not needed, as well as cloth not needed.

In fact, yellow glue is fine for all that rocket can do even on 29mm I motors. But epoxy sets quicker.

You don't need to worry about glassin' and cloth and stuff till you get to average thrusts greater then say 300-350ns.

Even then, I've flown paper, plywood and regular epoxy on J420Rs.

Now a motor like a J570 or the rocket killer K660 will need glass fins with cloth fillets etc..

Tac the far root edges of the fin with CA and kicker to hold it in place while the 30min glue sets on the fin root between the tac points.

Then add fillets after all fins are tac/epoxy in place and dried.

Ahhh! Good info! First time I've seen someone put numbers on that kind of advice. I'd like to see something with this kind of information go into the r.m.r. tips archive, the Rocketry Online tips, and/or a sequel to Tim Van Milligan's great (fantastic, actually) Model Rocket Design and Construction book. Thanks!

 

[Mad Rocketeer]

Well, I got the fin loose this morning. I'd been picking away at the joint with my #11 blade a few minutes at a time for a while. This morning, I switched to the rectangular, chisel ended blade and got it all cut through. [J-B Weld actually slices nicely with an Exacto if you can get at it.] The epoxy on the fin root noseward and finward of the fin tab was holding it at the bad angle too, so I just wiggled it until the outer layer of paper turned loose at those points. As I had already surmised, the fin tab had not bonded to the rear of the forward centering ring. Next, I'll pull the fin out, clean everything up, and re-attach it according to the original plan, just executed better. When all is finished, it will be 100%, with no loss of strength or cosmetic damage. Thanks for the advice!

I may poke or drill some small holes partway into the fin material in the areas that will be under the fillets before applying the fillets. That will strengthen the joints against having the epoxy shear off and letting the fin pull through the adhesive. I don't believe that I'll need to poke any rivet holes in the body tube material, since the tube slot will prevent similar shearing there. I don't think I'll put any rivet holes in the MMT either.

 

[Mad Rocketeer]

Well, while cleaning my MMT, I noticed that I'd cut through it while chiseling loose the J-B Weld. On the outside of the MMT, the cut is about 5/8" long, and it shows up on the inside surface as fairly short. The cut runs nearly parallel with the tube and goes in at a fairly flat angle in the other dimension. I.e., if you look at the outside of the MMT, the line is nearly vertical, and if you slid a chisel knife blade into the cut, the knife handle would be nearly parallel to the MMT too.

Question: Does this sound fatal for the MMT?

I'm thinking I can work glue into the cut from the outside of the MMT (not sure how far it will penetrate though) and perhaps apply a bit of fiberglass over that spot on the tube with epoxy or J-B Weld. J-B Weld is probably too gooey for applying fiberglass, isn't it?

Proceeding on the assumption that I can fix this and have it be safe and strong. Let me know if I'm wrong. Thanks.

 

[llickteig1]

Mad Rocketeer,

As Art Upton has said, all the glass and heavy duty adhesives are not required for the Graduator. Your rocket will survive anything you can stuff into that 29mm motor mount built with yellow glue or hobby epoxy.

Don't worry about the little gash in your motor mound unless adhesive gets through it to make it hard to get your motor in.

Up until about 2 or 3 years ago, the Graduator didn't even have fins that go to the motor mount, only the through the wall fins. This illustrates that the new desigh is plenty strong.

Also, I'm now getting concerned that you are getting so much JB Weld and epoxy at the bottom of your rocket that it will become unstable. The more epoxy, fiberglass, and etc. you add and the big motor that marginally would need those reinforcements will probably make the CG of your rocket too far aft and you'll get a loopy flight.

Please find out where the CP of your model is and mark it on the rocket. With the rocket ready to fly (i.e. motor, chute, etc.), make sure the CG is properly ahead of your marked CP.

--Lance.

 

[Art Upton]

Originally posted by Mad Rocketeer
Well, while cleaning my MMT, I noticed that I'd cut through it while chiseling loose the J-B Weld. On the outside of the MMT, the cut is about 5/8" long, and it shows up on the inside surface as fairly short.

I'd not worry about it, it will be fine. put just a tiny bit of yellow glue on the outside only of it if you want to seal it back up.

you don't want to get any glue on the inside of the mmt because that would prevent sliding a motor in.

 

[Mad Rocketeer]

Originally posted by llickteig1
As Art Upton has said, all the glass and heavy duty adhesives are not required for the Graduator. Your rocket will survive anything you can stuff into that 29mm motor mount built with yellow glue or hobby epoxy.

Don't worry about the little gash in your motor mound unless adhesive gets through it to make it hard to get your motor in.

Cool. I heard and absorbed Art's message about the fiberglass and had decided already not to do that, until I saw the gash. That made me wonder if it might need a "Band-Aid". Maybe just some epoxy or wood glue rubbed over that spot on the outside of the tube? So far, the cut doesn't affect the motor fit. I don't have any motors above 24mm or any casings at all yet, but I do have the 29mm to 24mm adapter tube that comes with the kit, and it still fits like a glove. I'll be very careful not to let any adhesive get through to the inside of the MMT.

I hope to (perhaps) use this rocket to do a L1 cert on some day, so I want it strong enough to handle H motors, though they aren't listed for it by the manufacturer. I'm no fan of "cast iron" overbuilt rockets though and don't want to cross that line.

Originally posted by llickteig1
Up until about 2 or 3 years ago, the Graduator didn't even have fins that go to the motor mount, only the through the wall fins. This illustrates that the new design is plenty strong.

I actually ran across that when researching the Graduator before deciding to buy one. It's mentioned in the EMRR reviews. I'm glad they went to the full length tabs.

Originally posted by llickteig1
Also, I'm now getting concerned that you are getting so much JB Weld and epoxy at the bottom of your rocket that it will become unstable. The more epoxy, fiberglass, and etc. you add and the big motor that marginally would need those reinforcements will probably make the CG of your rocket too far aft and you'll get a loopy flight.

So far, I have J-B Weld in the butt joints of fun tab to MMT. I had planned to add a thin fillet of J-B Weld on those same joints then add internal and external fillets of 30-minute hobby epoxy at the fin to body tube joints. I had considered adding a bit of milled glass fiber to the internal fillets and microballoons (and maybe milled glass) to the external ones. The kit instructions call for epoxy in the butt joint and in an external fillet. I'm not sure of the stability margin on a stock Graduator, but of course I'll check the rocket when it's done and add nose weight if necessary.

How far off are my plans? I'll re-do the re-aligned fin butt joint in J-B Weld for symmetry. I'd like to do the fillets in all three places for strength. My thought is to keep the internal fillets small and the external fillet as large as is best for aerodynamics (as specified in G. Harry Stine's book) but relatively light and sandable. I could switch to epoxy instead of J-B Weld for the fin tab to MMT fillets.

Thoughts?

Originally posted by llickteig1
Please find out where the CP of your model is and mark it on the rocket. With the rocket ready to fly (i.e. motor, chute, etc.), make sure the CG is properly ahead of your marked CP.

Oh yeah! You're preaching to the choir now. I sometimes do that even with the Estes A-B-C type rockets. Definitely for anything D and above. EMRR's CP Library lists the Graduator's CP as being 29.36" to 32.87" from the nose. If I don't get at least a caliber and preferably 1.5 to 2 calibers of stability from the more conservative of those numbers, the nose weight will go in until I do. Cross my heart.

Originally posted by Art Upton
I'd not worry about it, it will be fine. put just a tiny bit of yellow glue on the outside only of it if you want to seal it back up.

you don't want to get any glue on the inside of the mmt because that would prevent sliding a motor in.

Thanks. Just want to make sure it's not going to start working apart or something. Good point on not letting glue get inside.

Thanks for keeping me on track, guys.

 

[Stymye]

Mad , the graduator is just a big model rocket ,,, yellow glue or epoxy doesn't matter , glass shouldn't be necessary unless you launch from the desert.ofcourse with an ample motor glass doesn't matter either... no need to worry too much about that

build ,launch ,enjoy

 

[Mad Rocketeer]

Got it. I guess I'm just practicing for the bigger stuff. I do hope to put an H in this thing some day, even if I have to add nose weight before I do.

I had another thought. How often does one need to re-open a fin can for repairs? Is it worth it to provide for attaching the rear CR with a pair of screws so it can be opened if necessary? I'm thinking not, but If I broke a fin in a way that required complete replacement (rather than just reattaching a tip, etc.), how do I re-open the can without major body tube damage?

 

[DynaSoar]

Originally posted by Mad Rocketeer
That's what I was hoping too. I think I'm probably OK aerodynamically, I don't know yet aesthetically, and I'm a bit concerned strength-wise. The fin tab meets the MMT at an angle rather than squarely, as it should. If I leave it alone, I'll need to be extra careful about the internal fillets for that fin. I'd even thought about adding a 1" or so width strip of epoxied-on fiberglass cloth to the fin tab - body tube internal joints, lapping half onto the tab and half onto the tube. Might be more needed now. Might need it more on the tab-MMT joints though. Can J.B. Weld be used to glass a joint?

Aerodynamically you're fine. Aesthetically, that's pure choice.
As far as strength, you've already overbuilt this thing. The strongest kick you're going to be able to put in that thing is an Ellis H275. The I69 has a bit more power, but as a slow burner in such a light rocket, that won't take it nearly as fast as the H275. That *might* crack Mach 1. You have nothing at all to worry about.

That tiny gap along one side of the fin root/MMT joint is nothing. Remember, the JB Weld wasn't just used to glue two pieces of engine block togther, it was actually used to fill a crack.

 

[Mad Rocketeer]

Originally posted by DynaSoar
Aerodynamically you're fine. Aesthetically, that's pure choice.
As far as strength, you've already overbuilt this thing. The strongest kick you're going to be able to put in that thing is an Ellis H275. The I69 has a bit more power, but as a slow burner in such a light rocket, that won't take it nearly as fast as the H275. That *might* crack Mach 1. You have nothing at all to worry about.

That tiny gap along one side of the fin root/MMT joint is nothing. Remember, the JB Weld wasn't just used to glue two pieces of engine block togther, it was actually used to fill a crack.

I fixed the fin for the aesthetic reasons. It's as straight as an arrow (like the others) now. I poked a small cut, at a slant (see above) in the MMT, but that's not an issue, I think. I may dab it with a spot of wood glue (outside of the MMT only) as someone suggested.

I realize that I've doubtless overbuilt this by some margin. I may even end up having to add a bit of nose weight, though that remains to be seen.

I've done J-B Weld at the fin tab to MMT and CR to MMT joints, 30 minute epoxy with some milled glass fiber at the CR to BT, fin tab to CR, and internal fin tab to BT joints, and 30 minute epoxy with some milled glass fiber and a fair bit of microballoons at the external fin to BT fillets. I did the external fillets at a 1/4" radius of curvature, which is at the low end of the 4% to 8% of fin root chord (6") recommended in The Handbook of Model Rocketry, but they still look huge to me. I'll wrap 330 grit sandpaper around a 1/2" dowel (1/4" radius) and sand the external fillets to a circular curve, which will remove much of their current thickness.

From what I have recently been told here, that's substantial overkill for this rocket.

I got here by two paths, I think.
1.) A desire to not only build my first MPR rocket but to perhaps eventually poke my toe barely into the L1 HPR water with the same rocket. Or, failing that, to at least practice some of the HPR building techniques. Sounds like I overdid it at least some, even so,
2.) Ignorance of the (unwritten?) guidelines for when various techniques and materials become reasonable, advisable, and finally necessary.

In retrospect, I'd have probably been fine (A.) Doing the joints and fillets called for in the instructions in epoxy, as directed (without bothering with additives), (B.) Adding other internal fillets and such with a layer or two of wood glue, as I might an LPR fin fillet, and (C.) Building the external fillet to the 1/4" radius using Fill-N-Finish or some other lightweight filler and dripping a little CA over it after the filler dries, for strength.

The stuff I used doesn't seem all that heavy to me, but I guess it is. I'll definitely test the stability.

Since I don't have a casing or reloads right now (or even an SU larger than a D12), I thought I'd look up the initial weights and diameters of the SU and reloadable motors I might be expected to use with this rocket, including some of the more extreme cases, like the H275, and come up with something to distribute the same weight over the same length, starting just aft of the end of the MMT, and get some CG numbers for each to judge stability from. I have the CP from EMRR, and I'd look for the 1 to 2 calibers of stability. If the rocket needs weight only for the H or needs substantially more for the H, then I'll likely stick with E through G. Yep. I'll check the maximum liftoff weights before deciding too. I already know from the EMRR reviews that I'll not be using the D motor that's listed for it, and I never planned to use an I motor with this one.

I'd love to see the r.m.r. archives, Rocketry Online!, Apogee, or someone similar do a chart of recommendations for this kind of thing. At what point should epoxy come into play? At what point should strengtheners be added? Under what weight / size should epoxy fillets not be used? At what point is it useful to do strengthening inside the fin can? Etc. I know there's no way to be exhaustive about it, but some "official" thumb rules would be awesome for guys like me who are looking to transition from all LPR to the next stage or two.

All in all, I'm quite pleased with the way the rocket is going together and looking. I'm learning a lot with this one, and I guess that was my ultimate goal after all.

I really appreciate all the sage advice I'm getting too. I find that the fastest way of all to learn.

 

[loopy]

In my opinion, it's okay to overbuild. I'd rather have a tank where a hummer would do instead of having a hummer when I need a tank (sorry - weird analogy). I usually build my MPR birds with epoxy for aesthetic reasons, plus refining HPR build techniques. As long as you're not seriously overdoing it, I don't see it as a problem.

Loopy

 

[DynaSoar]

Originally posted by Loopy
In my opinion, it's okay to overbuild. I'd rather have a tank where a hummer would do instead of having a hummer when I need a tank (sorry - weird analogy). I usually build my MPR birds with epoxy for aesthetic reasons, plus refining HPR build techniques. As long as you're not seriously overdoing it, I don't see it as a problem.

Loopy

Over building is not a problem, except that it makes the bird heavy. Making something stronger than it needs to be really only makes sure it's less likely to break. And you don't know how strong it needs to be until real life happens and you do stuff like set it on top the car while unloading and the wind blows it off and onto the parking lot. When this happens and the fin doesn't break, you can tell it wasn't underbuilt.

"Too much is always better than not enough." -- J.R. "Bob" Dobbs

Worst that can happen is it won't fly on D or E motors because it's heavy. So it costs a little more to fly.

He was concerned he needed more. He doesn't. And practicing HPR techniques on a MPR bird is a great idea. It means your "first" will be around a while, and when you do get a LPR bird you'll know what you're doing.

 

[Mad Rocketeer]

Thanks for the good words. This stuff is a bit of an art, and you guys are the artists. I, the humble paint bearer, will learn at the feet of the masters.

I see what you're saying about overbuilding being OK (within reasonable bounds, no doubt). I'm, at heart, kind of of the Tim VanMilligan school on liking the idea of strong yet light. I wish I'd gone back and re-read his chapter on LMRs in Model Rocket Design and Construction a few days ago instead of last night. E.g., with fins going all the way to the MMT and with the way I placed the CRs against the fin tabs, the glue joints on the fins really don't have to carry all that much load. Which is what the good people on this thread have been telling me. D'Oh!

But I believe I'm in the range of merely somewhat overbuilt rather than grossly so, so I'll probably just have that good long-lasting MPR LMR that maybe doesn't have the full range of motors it could have.

I'm a fan of rockets with a lot of motor range too though, and it is one of the reasons I chose the Graduator. The stuff I first saw on it advertized that it flew on motors ranging from D to H. The instructions that come with it list motors (one each) in the D to G range. The reviews I've seen say it's a bit scary (even built stock) on a D, ejecting too close to the ground for comfort, so I gave up the idea of using Ds. I'd hoped to make it strong enough for Hs, and I guess I did, as long as I ensure stability, but I'd hate to have weighed it down too much for Es. Oh well. If I did, I did. There may be some higher thrust reloadable Es that would make safe, if low flights. I'm a fan of slow majestic takeoffs to low, visible altitudes, as long as the takeoff speed is fast enough for stability and the altitude is enough for a safe recovery.

Question: How should I attach the shock cord and parachute? I think the stock shock cord is quite likely sufficient, and I'm convinced that the stock attachment is fine. I usually use fishing swivel clips to attach the cord and/or chute to make them interchangable, replacable, externally storable, etc., and to help prevent tangles. This one's too big for those lightweight, somewhat flimsy clips though. I'd like to use quick links if they're not too heavy. What size, metal, weight, etc. should I be looking for for this model? The smallest ones I see at Lowes still seem too large to me. I'd probably loop the shock cord attachment line through a small ring of some kind, like a key ring, before attaching it. Then I'd tie the shock cord to that (so it would be less likely to be cut by the attachment, which looks like Kevlar maybe) and to the nose cone. Then I'd tie a butterfy knot a foot or so from the nose and use the quick link to attach the parachute to that. Thoughts?

Thanks for mentoring me through this one, guys!