Building to last

[Jeff Lassahn]

What kinds of things make a difference for the long-term survival of a rocket?
I'm realizing that although I built a lot of rockets when I was younger, I didn't fly all that often so I probably never had a single rocket that flew more than a dozen or so times. Now that I'm Born Again, I'm building more rockets but I haven't flown any of them many times yet either.

So I'm asking for people's experiences with how rockets age as they fly many times.

What kinds of things do you see wear out or go wrong over time?
What kinds of design and build techniques make a difference in how well a rocket survives into its old age?

I'm mostly interested in low power, classic cardboard, balsa and black powder; I imagine the high power world has some differences...

 

[Spitfire222]

As a BAR with greatly improved building skills since I was into model rocketry as a kid, I also came into it with this attitude. I would replace cardboard centering rings with thin plywood, omit the Estes tri-fold mount for kevlar attached to the motor mount, sometimes fiberglass the fins, and in general, using primer and clear coats during paint. The issue with some of these modifications is that they add weight, and it alters how the rocket flies. I'm finding that some of my rockets need the higher end of the recommended motors as a minimum, and some don't fly well anymore (my overweight Big Daddy come to mind).

The other issue is that the more time and effort I put into a rocket, I get more and more nervous to launch it for fear of a CATO, damaging it, or losing it. (Case in point: first flight of my Black Brant II with fiberglassed fins and exquisite paint job was lost when it drifted much further than expected).

My advice to you, and something I'm going to do going forward, is not worry so much about building for longevity, since you never know when a parachute will fail to eject, when a motor will explode, or if the winds up high are stronger than forecast. I'm still going to try my best to do a good job on building my rockets from now on, but not stress too much about building them to last, since it might be out of my control anyway.

Last anecdote: I fully painted my Estes M104 Patriot missile instead of using decals (even the lettering). It was a beautiful rocket when I completed it. First flight: delay was selected too long, so the parachute ejected a few meters above the ground, and the rocket core sampled. It was repaired, but it was frustrating to say the least. After all that work, and it got damaged on the maiden flight! It has since gone on to become my most flown rocket, since it's now a low-stress, no-longer-perfect rocket!

 

[Sooner Boomer]

Modern commercial rocket kits have some issues. Most of them relate to the fact that the mfgr has to cut costs to sell inexpensive kits. You'll need to replace some of the parts, and modify others if you want a sturdier rocket.
1. Nose cones - almost all of the nose cones that come in kits today are thin-walled blow molded plastic. These will break on a hard impact. They get very brittle in winter. They get brittle with age (if they last that long!). Some paints don't stick, others melt the plastic. Replace them with balsa nose cones from Balsa Machining Service (BMS). (this post is going to sound like an ad for them)
2. Fins and construction - look at the balsa the fins are made from. Learn about the different grain types - C grain is best for common fins. And learn how to paper them. If the design doesn't use them, but the outer body tube allows it, use through-the-wall construction. The tabs at the base of the fins go through the outer body tube and attach (glue) to the motor mount. Ideally the fins would be glued along four lines, the motor mount, the front and rear motor mount disks, and the outer body tube.
3. Get crazy and over-build - upgrade and even double the cardboard disks used for motor mounts (and other places). Go totally nuts and replace them with thin plywood (1/16" or 1/8" for low power). Replace paper cone or plastic transitions with balsa. (again, BMS)
4. Body and engine tubes - BMS makes heavier, foil-lined tubes in some sizes. The foil lining helps them stand up long term to the corrosive effects of burning ejection gasses.
5. Recovery - the Estes (and other) mfgrs use rubber band like material for shock cords. These degrade from exposure to black powder ejection charges. At least replace them with woven elastic (cheap, at WalMart). Estes (and others) use a "Tri-fold) mounting method to attach the shock cord inside the body tube. Instead, use a loop of kevlar that is attached (looped around) the engine mount and goes through the upper engine mount disk. This allows you to replace the shock cord before it goes bad.

More ideas...

 

[rklapp]

Epoxy, lots of epoxy. Just remember that heavier rockets require bigger motors and probably more NC weight requiring bigger motors. It’s a vicious cycle...

Honestly, I look at repairs as badges of honor. Broken fins and busted tubing can be lovingly repaired. I start with small motors and work my way up until the rocket decides to go on an adventure and float away. I’ll always have the video to remember the good times we had.

 

[Blast it Tom!]

1. One does have to watch the CG and total weight, to be sure. If you put extra strengthening back where the motor is, it can decrease your stabilty margin, so be careful - a "string test" (at least) may be called for to make sure it's stable.

2. After a winter launch where my parachutes failed to open well, I'm converting to ripstop nylon, if I can ever get the patterns made for my beloved seamstress wife.

3. For models with excessive volume ahead of the engine mount (such as my Mercury-Redstone), I added a stuffer tube (basically replace the engine tube with a longer one and a couple more centering rings). Tube is slightly stiffer and recovery more certain, so rocket "lives" longer.

4. I wick super-thin CA into the ends of my body tubes.

These are in addition to good recommendations above. Best to you and happy building! I just got back in a year ago, still going pretty slow... but retirement's coming!

 

[ZEDL1]

I'd suggest that any rocket that uses some form of paper or cardboard for the airframe be lined with fireproof paper prior to each flight. This helps a bunch with longetivity, and the material is very inexpensive. I bought a huge roll of it online (I can't remember the vendor, but not a rocket-centric company as I remember). I simply roll a length into a tube form and push it into each model that uses paper/cardboard before each flight, then toss the "liner" and replace with new before the next flight. I haven't done it yet, but I'll also use it for 3D printed as well as phenolic-airframed models too.

 

[Blast it Tom!]

I'd suggest that any rocket that uses some form of paper or cardboard for the airframe be lined with fireproof paper prior to each flight. This helps a bunch with longetivity, and the material is very inexpensive. I bought a huge roll of it online (I can't remember the vendor, but not a rocket-centric company as I remember). I simply roll a length into a tube form and push it into each model that uses paper/cardboard before each flight, then toss the "liner" and replace with new before the next flight. I haven't done it yet, but I'll also use it for 3D printed as well as phenolic-airframed models too.

I hope you can remember, I'd like to get some of that. I think I just read around here somewhere that some are actually foiling inside their tubes. It'd be tough on some of our little tubes, though.

 

[BEC]

I'm going to be a dissenting voice here. I have easily a dozen low power models, most BT-50-based, with over 30 flights each on them. I have two examples of one particular favorite of mine (Nova Payloader), one with 101 flights and one with 75, and my current flyer of that same design will go on its 33rd sortie the next time I go out to fly. Neither the 101-flight model nor the 75-flight model is very pretty any more. Each has been flown on up to composite Ds (and the one with 75 flights had several on the now OOP Aerotech D10-7s which have a blowtorch for an ejection charge...so the tube is soft and the paint bubbled above the motor mount, which is why it was retired at 75 flights though I think it is actually still flyable).

First of all, shock cords will fail and the "upgrades" suggested here will only put that off for a time. Just expect that it's going to happen (well unless you have a removable shock cord AND you replace it before it fails every time). Kevlar looped around the upper centering ring will burn through in ~35 flights. Estes' current white rubber, put in with the regular tri-fold (well away from the motor mount) can last that long if you use wadding correctly (and there's enough room in the model - things as short as the Alpha are harder). Inexpensive elastic from Hobby Lobby or Walmart won't last any longer than the white rubber either.

My current method for doing shock cords is to put a ~12 inch length of Kevlar line in with the good old reliable Estes tri-fold mount, then tie the original kit's shock cord (if it's recent and therefore long enough) or a good length of elastic (2x the model length at least) to that. I also have begun putting short length of 1/16th inch diameter heat shrink tubing over the kevlar where it rubs on the edge of my hardened-with-thin-CA top of the body tube (otherwise the edge of the tube will eventually saw through it).

I've yet to put enough flights on a model done this way to know how long it will really last. The 32-flight Nova Payloader I alluded to earlier has the most flights on this complete combination. So far no replacements of either shock cord or parachute.

But expect to replace the 'chutes after awhile....whether plastic or nylon.

I have never had issues with blow-molded plastic cones, and balsa, while it won't crack the way plastic will, does dent....so that's a trade of one issue for another. I use both....but I've never substituted one for the other in a given model.

As for beefing up the bottom end with epoxy or thicker/alternate materials for centering rings....I don't bother. The only epoxy in any of my low power or mid-power models is there to hold a screw-on motor retainer to the motor tube (24 or 29mm). For 18mm Beacon Fabri-tac does fine. Otherwise, it's all water-based glues or Beacon Foam-Tac/Fabri-Tac for plastic-to-paper joints.

Being aware of the balsa in the kit for the fins and if what is supplied is really soft swapping that out or maybe papering (I've never tried that) should help. One of the artifacts of a failed shock cord is often a hard landing for the body and a broken fin. But even just using wood glues to attach them, the fins don't necessarily come off, but break elsewhere. As I say, I've never tried papering fins, but that is one lightweight strengthening method that at least makes sense to me.

The other failure mode I often have to deal with is repeated strong ejection charges driving the motor aft and causing the top of the motor hook to tear the motor tube. Putting thin CA in the slot to harden it when building seems to help, but does not eliminate this. I normally do add a motor block above the top of the hook if the kit doesn't have one but that doesn't help the hook being driven aft by repeated ejection charges. This is hard to repair (though it can be done with care and thin CA with a long, thin applicator). I plan to use an 18mm Estes screw-on motor retainer on the next Nova Payloader I build to avoid this issue. Time will tell if this has me trading one problem for another.

Blast-it-Tom's suggestion about stuffer tubes in larger models flying on 18mm motors is a good one. My wife and I have built several Big Berthas and occasionally we have had simple failures to eject that probably would have not happened had we had a stuffer tube in the models as early Big Berthas did.

In general "improving" by adding weight, whether it is epoxy or just replacing parts with thicker/heavier ones just makes the crashes, when they come —and they will —harder.

I will be curious to learn about this "fireproof paper" that ZEDL1 speaks of.

 

[Cape Byron]

Honestly, I look at repairs as badges of honor. Broken fins and busted tubing can be lovingly repaired.

Embrace Kintsugi.

Kintsugi - Wikipedia

en.wikipedia.org

 

[BEC]

Embrace Kintsugi.

I like it!

That 101-flight Nova Payloader has some stories to tell, and the 75-flight one spent some time on a power line once....

Yes, let them tell their stories.

 

[Patternflyer]

Not sure if this would work for applying foil or any other liner inside, but when (rarely) I install a trifold shock cord mount, I glue it into place, then run a long balloon down the tube. Then I simply inflate the balloon and it will push outward and apply just the right amount of equally distributed pressure. If there is a chance of glue oozing out of whatever you’re gluing, be sure to run a layer of wax paper between the balloon and the item.

 

[BEC]

Not sure if this would work for applying foil or any other liner inside, but when (rarely) I install a trifold shock cord mount, I glue it into place, then run a long balloon down the tube. Then I simply inflate the balloon and it will push outward and apply just the right amount of equally distributed pressure. If there is a chance of glue oozing out of whatever you’re gluing, be sure to run a layer of wax paper between the balloon and the item.

I've thought of doing this....but have been too lazy to find suitable balloons, so just use my fingers and try to be patient.

 

[Woody's Workshop]

Kind of depends on cost of the rocket is the way I look at it.
I picked up a dozen Vikings at Hobby Lobby for $30 w/ Free Shipping.
Nothing but some arrangements is all that I will alter on them.
When you get up to rockets you want to keep in the flying fleet there is a few things I do.
It's not the only way, it's not a correct way, it's just my way.
I use Kevlar to the Engine Mount, then not in some elastic just below the top of the BT.
I put the chute and nose cone on separate leads using 3 snap swivels.
I may add a centering ring or two or switch to ply, or even upgrade the EM.
Paper the fins and leave mounting tabs, after coating the fins several times with thinned Tite Bond.
Few more coats of unthinned Tite Bond over the paper and sand smooth.
I give the whole body tube a couple coats of Tite Bond and sand smooth. This also fills the wrap joints.
After inserting the EM I take a paint brush and tape it to a dowel and coat the inside of the BT with Tite Band, except where the NC goes.
That gets CA glue to prevent zippering. I'm going to try CA on my next build to try and save some weight there.
Larger fin fillets, even on TTW Fins.
I use Tamiya white putty to correct anything on the nose cone if it's some sort of plastic.
If it's Balsa, several coats of thinned Tite Bond and a couple unthinned coats and sanded smooth.
I never sand Tite Bond finer than 220 grit.
Couple coats of primer, wet sand with 400 grit and paint.
Oh, and I take a small piece of card stock folded over and glue it under the LL.
Keeps things from dragging on the LR.
A little bit of Graphite Powder on the inside of the LL never hurts.
Mine end up being a lot heavier than Factory Specs, but will take several hard landings.
Just have to be mindful of the CP & CG locations to keep it stable since a lot of the weight was added aft.

 

[dwightr]

Rockets with fins that don't extend aft of the body tube seem to last longer.

 

[neil_w]

I would *love* to fly any of my rockets up to the point where they fall apart. Given how often I (don't) fly, plus the fact that I am always adding rockets to my fleet, I am unlikely to ever see one rocket achieve 30+ flights. Well, maybe not entirely true: my no-prep rockets such as the Quinstar and Odd'l Cyclone could get there, since I almost always bring at least one of them along as an extra. The Quinstar I expect to last almost indefinitely, because it doesn't have any of the usual failure points of a normal rocket, and most CATOs shouldn't bother it either. And it comes down right near the pad.

But as far as normal rockets go: I do Kevlar + elastic shock cords; I've had to replace the elastic but not yet the Kevlar on any rocket. Unfortunately, most of my designs preclude @hcmbanjo 's replaceable Kevlar strategy, but I do ponder ways to improve the longevity of the Kevlar connection, and will implement it as soon as I think of it.

I use heavy-walled BT50 for motor mounts and anything structural. I have a lot of rockets with long extended BT50 motor mount tubes, and feel better having more material there to endure the repeated ejection charges.

Of course, I coat all tube edges with CA. I have also considered a light CA coating of the full interior of the rocket, to seal it off from moisture. I only tried this once, and it was a bit of a mess, so I haven't done it again since.

Some designs are inherently more prone to damage either during descent (e.g. chute snagging) or on landing (rear-swept fins, tall fin aspect ratios, or delicate protruding fiddly bits). I try to keep such things out of my designs (well, mostly) but you can't help that with kits.

 

[Blast it Tom!]

I'm going to be a dissenting voice here. I have easily a dozen low power models, most BT-50-based, with over 30 flights each on them. I have two examples of one particular favorite of mine (Nova Payloader), one with 101 flights and one with 75, and my current flyer of that same design will go on its 33rd sortie the next time I go out to fly. Neither the 101-flight model nor the 75-flight model is very pretty any more. Each has been flown on up to composite Ds (and the one with 75 flights had several on the now OOP Aerotech D10-7s which have a blowtorch for an ejection charge...so the tube is soft and the paint bubbled above the motor mount, which is why it was retired at 75 flights though I think it is actually still flyable).

First of all, shock cords will fail and the "upgrades" suggested here will only put that off for a time. Just expect that it's going to happen (well unless you have a removable shock cord AND you replace it before it fails every time). Kevlar looped around the upper centering ring will burn through in ~35 flights. Estes' current white rubber, put in with the regular tri-fold (well away from the motor mount) can last that long if you use wadding correctly (and there's enough room in the model - things as short as the Alpha are harder). Inexpensive elastic from Hobby Lobby or Walmart won't last any longer than the white rubber either.

My current method for doing shock cords is to put a ~12 inch length of Kevlar line in with the good old reliable Estes tri-fold mount, then tie the original kit's shock cord (if it's recent and therefore long enough) or a good length of elastic (2x the model length at least) to that. I also have begun putting short length of 1/16th inch diameter heat shrink tubing over the kevlar where it rubs on the edge of my hardened-with-thin-CA top of the body tube (otherwise the edge of the tube will eventually saw through it).

I've yet to put enough flights on a model done this way to know how long it will really last. The 32-flight Nova Payloader I alluded to earlier has the most flights on this complete combination. So far no replacements of either shock cord or parachute.

But expect to replace the 'chutes after awhile....whether plastic or nylon.

I have never had issues with blow-molded plastic cones, and balsa, while it won't crack the way plastic will, does dent....so that's a trade of one issue for another. I use both....but I've never substituted one for the other in a given model.

As for beefing up the bottom end with epoxy or thicker/alternate materials for centering rings....I don't bother. The only epoxy in any of my low power or mid-power models is there to hold a screw-on motor retainer to the motor tube (24 or 29mm). For 18mm Beacon Fabri-tac does fine. Otherwise, it's all water-based glues or Beacon Foam-Tac/Fabri-Tac for plastic-to-paper joints.

Being aware of the balsa in the kit for the fins and if what is supplied is really soft swapping that out or maybe papering (I've never tried that) should help. One of the artifacts of a failed shock cord is often a hard landing for the body and a broken fin. But even just using wood glues to attach them, the fins don't necessarily come off, but break elsewhere. As I say, I've never tried papering fins, but that is one lightweight strengthening method that at least makes sense to me.

The other failure mode I often have to deal with is repeated strong ejection charges driving the motor aft and causing the top of the motor hook to tear the motor tube. Putting thin CA in the slot to harden it when building seems to help, but does not eliminate this. I normally do add a motor block above the top of the hook if the kit doesn't have one but that doesn't help the hook being driven aft by repeated ejection charges. This is hard to repair (though it can be done with care and thin CA with a long, thin applicator). I plan to use an 18mm Estes screw-on motor retainer on the next Nova Payloader I build to avoid this issue. Time will tell if this has me trading one problem for another.

Blast-it-Tom's suggestion about stuffer tubes in larger models flying on 18mm motors is a good one. My wife and I have built several Big Berthas and occasionally we have had simple failures to eject that probably would have not happened had we had a stuffer tube in the models as early Big Berthas did.

In general "improving" by adding weight, whether it is epoxy or just replacing parts with thicker/heavier ones just makes the crashes, when they come —and they will —harder.

I will be curious to learn about this "fireproof paper" that ZEDL1 speaks of.

Please subscribe me to your newsletter!

For my motor mounts (the very few I've built since restarting), there is usually a hook with a coupler or other wrap about halfway between the thrust block/hook front and the tail. I just glue the daylights out of that (tightbond 3) and hope that the shear strength helps.

 

[Scott_650]

Some of the basic techniques for making any LPR rocket sturdier are pretty much freebies as far as weight is concerned - redoing shock cords either with Kevlar leaders or increased length elastic adds little or no weight, papering fins is weight “agnostic“ depending on much sealer/filler/primer/paint you’d typically use without papering (my opinion is it definitely adds strength though there are some who disagree), wicking thin CA into tube edges adds no weight (at least not enough for my cheapo kitchen scale to detect), swapping out plastic chutes for thin mil nylon can increase launch weight but again, I think the benefits are worth the slight weight penalty. The whole motor mount shock cord versus trifold shock cord is again an “agnostic” topic to me - the length is a bigger factor (other than on skinny rockets where recovery gear clearance is an issue - a motor mount Kevlar leader definitely saves space). One thing that I think nearly everyone can agree on is almost every Estes shock cord is too short.

The “build a LPR rocket like a HPR rocket thing“ is, I think, a result of two factors: first, lots of rocketeers got into the hobby “backwards” - they started out with HPR rockets then circled back to LPR and brought the whole plywood and epoxy philosophy along with them, the second is what we’re doing right now - mass communication via the internet, once something gets embedded into online conversations it tends to metastasize and spread like crazy. I’ve watched something similar happen in the long distance trail running world - once some idea or technique takes root in the interwebs it quickly becomes the only way something is to be done if you really know what you’re doing. Until the next perfect way to do it comes along, of course. Heavily built stability shoes replaced by barefoot running replaced by zero drop minimal shoes replaced by thickly cushioned zero drop shoes...each being proclaimed as the light and the way. When the truth of the matter is the “best” way is the way that works best for you.

I tend to build rockets the way I intend to fly them - if it’s never going up over any motor but one within the recommended power range it pretty much gets built per the instructions with the included parts (longer shock cords, papered fins - I hate sanding! - and edges sealed with CA being my standard mods). If the plan is to fly it over more power than the designer intended then maybe it’ll need a little more “beef” to fly safely and hold up to the increased stresses - so doubling up or replacing the card stock centering rings, thicker or plywood fins, etc might be in the cards (I rarely use epoxy since most water-based PVA adhesive bonds are stronger than the bonded substrates. And epoxy smells lousy). Since this is a hobby, which by definition should be fun, I consider playing around with materials and techniques all part of that fun.

 

[mbeels]

First of all, shock cords will fail and the "upgrades" suggested here will only put that off for a time. Just expect that it's going to happen (well unless you have a removable shock cord AND you replace it before it fails every time). Kevlar looped around the upper centering ring will burn through in ~35 flights.

Lots of good advice from BEC, I think. A rocket with a replaceable shock cord (even one built from cardboard and woodglue) can fly many, many flights.

 

[Pete.D]

Fiberglass is a solution to most rocket aging. Doesn't have to be huge; there are 2" fiberglass kits from Rocketry Warehouse (sold by Mad Cow). I ordered a few on Black Friday, but they haven't come yet. Ballistic recovery will always be an issue, but could mitigate with redundant systems.

 

[jrap330]

Kind of depends on cost of the rocket is the way I look at it.
I picked up a dozen Vikings at Hobby Lobby for $30 w/ Free Shipping.
Nothing but some arrangements is all that I will alter on them.
When you get up to rockets you want to keep in the flying fleet there is a few things I do.
It's not the only way, it's not a correct way, it's just my way.
I use Kevlar to the Engine Mount, then not in some elastic just below the top of the BT.
I put the chute and nose cone on separate leads using 3 snap swivels.
I may add a centering ring or two or switch to ply, or even upgrade the EM.
Paper the fins and leave mounting tabs, after coating the fins several times with thinned Tite Bond.
Few more coats of unthinned Tite Bond over the paper and sand smooth.
I give the whole body tube a couple coats of Tite Bond and sand smooth. This also fills the wrap joints.
After inserting the EM I take a paint brush and tape it to a dowel and coat the inside of the BT with Tite Band, except where the NC goes.
That gets CA glue to prevent zippering. I'm going to try CA on my next build to try and save some weight there.
Larger fin fillets, even on TTW Fins.
I use Tamiya white putty to correct anything on the nose cone if it's some sort of plastic.
If it's Balsa, several coats of thinned Tite Bond and a couple unthinned coats and sanded smooth.
I never sand Tite Bond finer than 220 grit.
Couple coats of primer, wet sand with 400 grit and paint.
Oh, and I take a small piece of card stock folded over and glue it under the LL.
Keeps things from dragging on the LR.
A little bit of Graphite Powder on the inside of the LL never hurts.
Mine end up being a lot heavier than Factory Specs, but will take several hard landings.
Just have to be mindful of the CP & CG locations to keep it stable since a lot of the weight was added aft.

Tite bond wood glue is easily sandible, no roughness, bumps or other irregularities?

 

[shockie]

1. Use heavy aluminum foil glued in with epoxy for the 4" above the engine to protect the body tube from the ejection charge exhaust flame
1A. Attach the kevlar shock cord to the engine mount and run it underneath the aluminum foil and on the side of the baffle....this will protect the shock cord
2, use a old centuri baffle above this with a fine mesh screen on the aft engine facing the engine.... this will allow only the hot exhaust gas thru but prevent the baffle from filling up with BP particulate
3. for minimum diameter models use the lariat loop to attach the kevlar shock cord to the engine itself and this way it's removable
4, replace balsa fins with plywood.
5. use epoxy not white or yellow glues for gluing
6. use epoxy to coat fins and body tubes externally

 

[jrap330]

I would like to see thicker center rings from estes and others, too often after cutting and removing the ring from the cardstock I will put a crease or fold in it which is made worst by the installation. So plywood may add too much weight but a thicker ring material will aid in longevity

 

[Pete.D]

I've been building (and losing) Estes rockets for a long time, and liking their simplicity and light weight. But they just don't last, and they're getting expensive. I just got my L1 and L2 this year, but I still love little rockets as long as they don't go so high I lose sight of them. So now that I'm exposed to the HPR techniques and vendors, I just noticed the Wildman "Mini" series: 1.5'" fiberglass, relatively inexpensive and light (for fiberglass; just under a pound), can fly on anything from 24mm E's to 29mm I's, and offer a DD option. Hmm.. a solution?

 

[Jeff Lassahn]

If I'm understanding correctly, there are two classes of suggestions here:
1. make your rocket stronger so it can survive accidents better
2. make changes to make certain things wear out slower, or make them replaceable when they do wear out

Am I right that most of the suggestions for making the rocket stronger (plywood, epoxy, etc) don't really prevent the problems with normal wear and tear, the intent is purely to survive catastrophe better?

Also, it sounds like by far the biggest failure due to wear from normal flight is the shock cord.

There's also some indications that gradual body tube damage from ejection charges is a problem. How much of a problem is that? What kinds of problems happen? Is it the heat from the charge, the gunk that builds up from combustion residue? Does it depend on what size of engine your using?

 

[neil_w]

Tite bond wood glue is easily sandible, no roughness, bumps or other irregularities?

Speaking at least for TBII: it is sandable, but I wouldn't say "easily" nor is it pleasant. I've used it to seal the edges of papered fins, but that's a very very small amount to sand, so not too bad.

I appreciate that using TB as wood filler works for Woody, but it's not a technique that I think most would enjoy. Nor is it necessary for long-lasting fins.

If I'm understanding correctly, there are two classes of suggestions here:
1. make your rocket stronger so it can survive accidents better
2. make changes to make certain things wear out slower, or make them replaceable when they do wear out

3. Design rockets that eliminate most common failure points (applies to scratch builds only)

Am I right that most of the suggestions for making the rocket stronger (plywood, epoxy, etc) don't really prevent the problems with normal wear and tear, the intent is purely to survive catastrophe better?

Sort of, but I wouldn't call landing a "catastrophe". Also, having stronger fins means you can use smaller parachutes which increases likelihood of getting the rocket back.

That said, I am *not* an enthusiast of ridiculous overbuilding of LPR rockets. I do use epoxy when it is called for, but for me at least fin attachment does not qualify. I try to make my rockets as strong as possible up to the point where it starts adding significant unnecessary weight.

What kinds of problems happen? Is it the heat from the charge, the gunk that builds up from combustion residue? Does it depend on what size of engine your using?

I believe the problem is that BP residue is hygroscopic, so over time moisture absorption weakens the tubes (this is part of the theory of CA-coating the interior of body tubes exposed to ejection charges).

 

[Pete.D]

There's also some indications that gradual body tube damage from ejection charges is a problem. How much of a problem is that? What kinds of problems happen? Is it the heat from the charge, the gunk that builds up from combustion residue? Does it depend on what size of engine your using?

Most of my older Estes rockets have deterioration of the body tube, with many having blown out the upper portion of the motor tube and centering ring. Paint peeling off the body tube and balsa fins. Surface-mounted fins come loose at the junction of fillet and body tube. The fins themselves warp and crack. The shock cords oxidize and break, without even being stretched. The motor mount problems are mainly associated with the 24mm Estes "D" motors, but body tube and centering ring problems seem unrelated to the motor size. I can't tell whether body tube problems are due to heat or BP combustion residue, but it might just be humidity. Nose cones don't seem to deteriorate, but the paint on them does: this happens for both plastic and balsa cones. Paper body wraps loosen and peel off, but plastic wraps are generally OK.

Maybe I expect too much; most of the deteriorated rockets are over 20 years old, and some are 30. I'm guessing much of the deterioration is due to humidity and temperature change. I've seen plenty of old fiberglass boat hulls that are still solid at that age, but maybe it doesn't matter: I probably won't be alive in 20 years.

 

[Dane Ronnow]

The other failure mode I often have to deal with is repeated strong ejection charges driving the motor aft and causing the top of the motor hook to tear the motor tube.

I ordered a motor mount kit a while back from either Discount Rocketry or Rocketarium (can't remember which) that came with an engine hook that was a quarter-inch longer than the Estes-style hooks I have, with a single right-angle bend on both ends. It took me a second to realize that the extra quarter-inch was so the forward end of the hook penetrates the motor tube above the motor block, thus giving it greater resistance to the rearward pull at ejection.

These hooks are 4" long (for an E motor), rather than the 3.75" that Estes hooks are.

 

[jrap330]

Most of my older Estes rockets have deterioration of the body tube, with many having blown out the upper portion of the motor tube and centering ring. Paint peeling off the body tube and balsa fins. Surface-mounted fins come loose at the junction of fillet and body tube. The fins themselves warp and crack. The shock cords oxidize and break, without even being stretched. The motor mount problems are mainly associated with the 24mm Estes "D" motors, but body tube and centering ring problems seem unrelated to the motor size. I can't tell whether body tube problems are due to heat or BP combustion residue, but it might just be humidity. Nose cones don't seem to deteriorate, but the paint on them does: this happens for both plastic and balsa cones. Paper body wraps loosen and peel off, but plastic wraps are generally OK.

Maybe I expect too much; most of the deteriorated rockets are over 20 years old, and some are 30. I'm guessing much of the deterioration is due to humidity and temperature change. I've seen plenty of old fiberglass boat hulls that are still solid at that age, but maybe it doesn't matter: I probably won't be alive in 20 years.

I will say, yes, 20-30 years old what more than anyone expect from fragile paper and balsa. That is an amazing record, 20-30 years.

 

[jrap330]

I ordered a motor mount kit a while back from either Discount Rocketry or Rocketarium (can't remember which) that came with an engine hook that was a quarter-inch longer than the Estes-style hooks I have, with a single right-angle bend on both ends. It took me a second to realize that the extra quarter-inch was so the forward end of the hook penetrates the motor tube above the motor block, thus giving it greater resistance to the rearward pull at ejection.

These hooks are 4" long (for an E motor), rather than the 3.75" that Estes hooks are.

Come with instructions indicating that, because I would assume for a longer engine

 

[Dane Ronnow]

I would assume for a longer engine

Perhaps. But my point was by using a longer hook, which can be cut and ends bent, the forward end of the hook can be located above the motor block, preventing the inevitable tear in the motor tube.