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Who Am I? How’d I Get Here?

For as long as I can remember, I’ve had a strong interest in science, aviation, rockets, all things “techy”, and anything mechanical. Right brained? That’s me in so many ways.

Years ago I was heavily involved in model aviation. I built and flew mostly e-powered planes – everything from mostly pre-built kits all the way to scratch-built sport flyers that I dreamed up on my own or with the help of others. By the time I had a toddler, infant, new job and a house in my life, it became too time consuming and too spendy of a hobby for my young family. So I reluctantly put it aside except for the occasional dalliance in cheap RTFs from time to time; always with the intention of getting back to it “soon”. It’s been a decade, we’re way past soon.

As my son approached the ripe and hyper-kinetic age of 9, I began searching for something we could do together that would be fun, intellectually stimulating and teach him the value of learning to create things with the power of your own mind, hands and effort.

He’s always enjoyed seeing and asking questions about the remainder of my fixed-wing squadron gathering dust in the cellar … none of which are currently airworthy (…but all need “just little work” to get flying again). He does manage to keep those cheap quad-prop drones in the air for a little while, but still doesn’t have the hand-eye coordination to handle control reversal, or the more dynamic flight characteristics of fixed-wing aircraft. Further, decent hobby-level planes are on the pricier end of what I’m willing to spend the cash to buy, and invest the time in building and frequent repair necessary to keep them airworthy right now. Never mind stomaching the inevitable spectacular crashes that come with the process of a young boy learning to control a distant, fast-moving model with no brakes, desperately fighting the irresistible pull of gravity and whims of the wind in 3D space in a delicate dance of thrust, lift and drag.

Then, one day while picking up supplies at a Michael’s Crafts for some project or other at home, I happened to notice they had a small supply of Estes starter kits and motors. Memories of my young-self poring over Estes catalogs at about my son’s age came flooding back. I also remembered with a pang that I never did get to fly any of those rockets I had been pining over on those full-color glossy pages all those years ago. Full-on nostalgia moment … I’m suddenly a boy at the five and dime store with his allowance burning a hole in his pocket.

Without hesitation, I grabbed a $30 starter kit off the shelf, and a few packs of motors @ $10 each from the nearby display and headed for the cashier line – MY boy was NOT going to be deprived of this experience! I’m pretty sure I even remembered to buy whatever it was that I had gone there for originally, but I couldn’t guarantee that…

I knew in my soul that this was going to be the perfect hobby for us to dive into together. Very low bar to entry, and LOTS of room to expand into as we gain skills and experience, and no need for advanced hand-eye coordination to prevent expensive disasters.

When I got home, my son was like a rocket on the pad with a hot igniter in the tube when I showed him what I’d bought. My wife winced at the fact that I had spontaneously spent $60 on “some new toy” (some day she’ll get used to this!), but the excitement displayed by the boy as he tore open the package and started looking at how things go together changed the mood quickly.

That was late June 2018 – just over a year ago as of this writing. Through that summer and until the New England weather turned unfriendly to launching unguided missiles into the Northerly winds, we decorated the trees and rooftops surrounding a schoolyard near our home with misguided missiles as we gained valuable experience in model building, motor selection, flight-path estimation, judging wind conditions both at ground level and aloft, parachute packing and what happens to that parachute when you forget your ejection wadding, or the meaning of the term “lawn dart” – what happens to your rocket when you have a nose cone that’s a bit too snug to eject at all. We discovered all sorts of interesting failure modes – Murphy is a creative guy, and gremlins are definitely as real in rocketry as they are in aviation. Every launch a leap of faith that it’ll ever be seen again. Every launch an opportunity to learn something new.

We learned what a great ice-breaker flying rockets in the park can be. People LOVE watching rockets fly. It was a rare trip to the field that didn’t attract a gaggle of excited local kids asking questions, getting involved in the action, and being reminded to please not step on the rockets and stay 15′ from the pad. Even the parents would occasionally work up the courage to actually talk to a stranger and ask me questions about what we were doing and how we got started in it.

We learned lessons about attachment and how to let go. We learned that it’s OK to buy a kit, put the effort into building and decorating it … only to have it become a dangling ornament 50′ up a tree you don’t dare try to climb, or land gently on the school’s “spaceport” (the roof) after its first launch. It’s gone. It’s final. It’s OK! We learned something and can build another one!

The cost of learning with low-power rockets is pretty minimal. With every build, re-build or repair you’re learning new skills and improving techniques. With every launch, you learn more about what it takes to heave an aerodynamic chunk of flaming mass impossibly high into the sky, and hopefully get it back to do it all over again. When a model makes its last flight, well … it was only a few dollars for the kit, and not a second of wasted time for the build. And now you get to build another one, only better this time! Priceless!

It turns out the modeling skills and techniques I obtained building airplanes carries over well to rocketry. I very quickly have become hooked on the whole experience. I began devouring everything I could about the hobby via YouTube videos, blogs such as this, forum posts, magazines and chatting with other hobbyists. It wasn’t long before I had acquired a (still growing) collection of rocket-oriented build tools, finishing materials, spare parts, a backlog of kits (and even longer wish-list), and an ever-growing list of kit mod and scratch-build ideas. I’ve discovered rocket design and flight-modeling software, and have begun learning about design, dynamic stability and aerodynamics. My nerd-gasm is in full swing.

Now I find myself getting into more advanced kits, scratch builds and kit mods/bashes. We’ve launched our first D and E powered models this flying season, and have even dabbled a little bit in composite fuel motors. We launched our first rocket that was taller than me. We launched a 1/100th scale Saturn V model on the 50th anniversary of the first moon landing. We’ve joined a local club (CMASS), and I’ve just become a NAR member so that I can be more helpful to the club on launch days, and advance my knowledge and involvement in the hobby.

My son’s enthusiasm keeps growing. He has just assembled his first non fin-can model (mostly) by himself in the form of an Estes Mosquito. It’s about the tiniest possible rocket … only a few inches tall, but he’s proud of his accomplishment. He’s got a pretty good understanding of motor selection, and has become fairly handy at launch prep.

Most every launch has him wide-eyed and energized as the model leaps off the pad into the sky, and holding his breath until he can see the parachute successfully deployed near apogee. Then, jabbering-on excitedly about the events of that launch as he races off to collect the safely landed missile, watches forlornly as an errant gust of wind or updraft carries another beloved (they’re ALL beloved) model into the next county, or picks up the pieces of a model that met its demise in a CATO (aka. “unplanned rapid disassembly event”) on the pad … inspecting the damage and theorizing on the cause of the explosion … doing science whether he realizes it or not.

I beam with pride and joy as I see him unabashedly flit from canopy to canopy to workbench at club launches eyeballing the other hobbyist’s tools, toys and rockets. Asking them a seemingly endless stream of questions. Bragging about his exploits on the flight line. Offering advice (solicited or otherwise) from his growing Encyclopedia of Rocketry Knowledge. Forgetting to ask before touching, then apologising sincerely. Making friends young and old, and absorbing knowledge like balsa absorbs CA. I hold back my instincts to jump in every time it seems like he’s being a little bit of a noodge to some hobbyist with his rapid-fire questions and childish indiscretions. This is how he’ll learn, take off the kid gloves, it’s a safe space.

He already knows more of the club members than I do! The flyers we’ve met have all been warm and welcoming, and seem to have endless patience for sharing their knowledge generously with an enthusiastic 10-year-old question-firing machine gun showing so much interest in the hobby that they clearly love, and genuine admiration for the models they’ve brought to the field that day. “Will you fly this one today? How long did it take you to build that one? That has a black powder E motor in it? What would happen if you put in a composite F? Why’d you paint it like that? Does that thing actually fly? My Dad has a rocket that’s taller than he is! What’s that do? Do you need any motors? My dad has a bunch!” … all without hardly stopping for a breath.

He wants to know when we can fly 29mm motors (I got a 2nd hand 29mm powered rocket, but it needs some work and parts to complete). When I’ll go after a Lv 1 cert (Maybe I’ll attempt it next year). Do I think I’ll ever get a Lv 3 cert? (Not in the near-term plan, but who knows? Maybe we’ll move to the vicinity of the Black Rock desert someday…) What would happen if we put X motor into Y airframe? Will we ever shoot a rocket into space? He’s littered the house with old copies of Sport Rocketry magazine that he charmed some generous rocketeer into letting him have. He loves thumbing through them again and again – looking at the ads, reading the articles, admiring the photos. Yeah, enthusiastic.

So … a seemingly mundane, yet fateful trip to the craft store one summer day in 2018 has launched us onto a trajectory of new discoveries and experiences. And I get to see glimpses of the world through the lens of a young boy’s sparkling brown eyes again. Carpe diem.

Mean Machine Mk. II

In an earlier post, I review the Estes Mean Machine (001295) and discuss the benefits and shortcomings of this really great rocket. With this build, I’m going to try and address some of the shortcomings of the “out of the box” build of this model.

Planned mods:

Replace 3/16 launch lugs for 1/4″ lugs + rail guides
Ejection charge baffle
Mid-body separation
Piston ejection with dual chute independent recovery
Payload bay

My construction techniques will tend toward “over built” for several reasons. First, I usually fly on smaller fields, so quicker descents are desirable. “Hard and fast” is far more preferable than waiting for the 40′ tree I’m hung up in to fall down so I can get my rocket back. Second, when I do have the opportunity to fly in larger spaces, it’ll be fun to put some higher thrust composite motors in this rocket and try to make it look small. I’ve simmed up through “F” RMS motors in OpenRocket and all the results look good. Third, this rocket is just plain fun to fly. It looks great on the pad, at launch and in the air. It’s going to see a lot of launches, and I want to minimize turnaround time and repairs.

Design Choices

Launch lug changes
The kit provides 3/16″ launch lugs, which require a Maxi Launch Rod. Personally, I already own a pad that has 18″ rod, and a pad with a 1/4″ rod for mid-power rockets. I don’t really want to have a 3rd rod for the “crossover” class of rockets that the 3/16″ rod is intended for. Also, this rocket “stock” flies on D or E engines, and a 1/4″ inch rod is recommended for anything over a D impulse engine. Finally, this tall rocket really needs all the support it can get. On a thinner rod, this thing really sways around in the slightest breeze. All you need is an unexpected gust to come up at motor ignition, and suddenly you’re dealing with a land shark.

I really prefer to launch this rocket from a rail when one is available, so adding rail guides is a natural choice. I don’t own a rail, and will likely want to launch this rocket when I’m not at a club launch, so in the end, I’ll have 1/4″ lugs on one side, and rail guides on the other. This provides the additional support this rocket needs in all but the calmest conditions and allows me to launch when and where I want.

Ejection Baffle:
Anything over a BT-50 requires quite a lot of wadding to protect your chutes. I spend enough on “expendables” buying motors, so the less I spend on other one-time-use items the better. Yes, I could use “dog barf”, or cellulose insulation from the local Home Depot, but that’s one more thing I have to haul to the launch field and store at home. Also, there have definitely been times where ejection wadding has been forgotten for any of a few different reasons (having a 10 y.o. First Officer is definitely one of them…). Eliminating sources of human error is solid engineering, and one less thing to do at launch prep reduces turnaround time.

Singed parachutes are no fun. So, as a rule I try to use an ejection baffle in any rocket in which one can be reasonably installed. Beyond parachute protection, baffles provide a convenient and robust anchor point for your shock cord … another mark in their favor.

Mid-body Separation:
The stock build of this rocket deploys a single 24″ parachute in traditional fashion behind the nose cone. This is simple and effective, but a 24″ parachute is pretty floaty, and provides ample opportunity for the rocket to drift far beyond the desired LZ.

This recovery choice is fine on larger fields, and provides a dainty enough landing for the relatively delicate airframe. If you want to come down faster, the thin body tubes are liable to kink and the fins are going to take a beating.

One option to ensure in-field recovery would be to use a Jolly Logic Chute Release, but unless you already own one or want to invest in one for future use, that’s a pretty extravagant expense for this model. Another option is to reduce the mass that’s going to be hitting the ground. You don’t really want to build this model any lighter than it already is, so the next option is to break up the mass.

Splitting the rocket in the middle at ejection and having the two parts descend independently turns it into two 3′ rockets coming back to Earth, rather than a big 6′ long rocket. The body tubes will be spared the long bending forces, and the fins won’t be subjected to the full weight of the whole airframe.

Piston Ejection / Dual chute independant recovery:
Splitting the airframe in half at ejection presents some interesting challenges. One is that you don’t want the 3′ long upper section smashing into the lower section. There are certainly many ways to mitigate this problem, but I thought it’d be fun to have the upper and lower sections come down independently under their own ‘chutes.

Parachute deployment of the upper half isn’t a concern. When it falls away from the lower section after separation it’ll pull the chute out on its own. However, this does leave the question of how ensure the chute for the lower section will deploy.

In “traditional” ejection, the nose cone will drag the chute out. It’s possible, maybe even likely, that the ejection charge will push the lower chute out, but that leaves us with no redundancy and a failed deploy will likely result in the lower section coming home ballistic and taking a nice core sample of your flying field.

To resolve this, I’ve opted to go with a piston ejection system. A piston is made from a piece of coupler with a bulkhead and a hole through the middle. The shock cord passes through the bulkhead with a knot on each side to keep it in place and is anchored to the ejection baffle below.

During launch prep, you slide the piston into the body tube, and then you follow it in with the lower chute, and then the upper chute. When the ejection charge fires, the piston will act like a sabot. The pressure wave from the ejection charge will drive the piston forward, forcing the upper body to separate and push the parachutes out ahead of it.

Payload bay:
Now that there’s a bulkhead at the mid-body coupler, this creates a large sealed space in the forward section of the rocket. Since I’m not going to be using the twist-lock connector at the mid-body point, I now have this part to spare.

This provides the opportunity to move the twist-lock coupler to the first body tube junction providing access to a nearly 3′ long payload bay. Of course, this volume is not likely to be necessary, so I’ll add a ‘floor’ with a bulkhead at the bottom of the lower twist-lock connector.

I’ll now be able to give random grasshoppers from the local flying field the ride of their lives in a roomy cabin. Of course jelly beans, action figures, and even boring things like altimeters are also a possibility.

Another advantage of moving the twist-lock connector up to the front of the rocket is that it now breaks down into three pieces, making transport and storage that much easier.

Estes Mean Machine (#001295)

The Estes Mean Machine (#001295) is a fantastic looking sport flier. It’s built up with four lengths of BT-60 body tubes, and stands 79″ tall. You’ll definitely get some attention as you approach the flight line with this rocket over your shoulder, and you’ll keep their attention with its dramatic slow liftoff characteristics and beefy sounding D/E power.

It’s got a 24mm motor mount, and flies great on D12-3 (~500′) or E12-4 (~700′) BP motors. These motors both perform extremely well in this model. Estes also includes the D12-5 and E12-6 as “recommended” motors, but in my experience these delays are way too long, and could definitely result in a relatively high-speed ejection if your trajectory is somewhat flat. The one time I flew an E12-6, the “pucker factor” was definitely high as I watched the rocket nose over after apogee and start coming in ballistic. That’s a looong 6 seconds to wonder if the laundry is gonna come out.

Rod Launch – D12-3

Rail Launch – D12-3
(Note the motor casing nearly hitting me. “Shotgun” ejection – motor issue that tore out the motor hook)
Estes made good on the warranty!

Rail Launch – E12-6 … just a bit too much delay

The Good:

If you want to go with composite motors, there are lots of option there too. I used OpenRocket to do a simulation of this rocket using an Aerotech E30-7 for a predicted apogee of over 1200′.

Great looking model that really has “presence”, whether it’s just sitting on the pad, arcing through the sky or on display with the rest of your collection
Very stable model that flies straight as an arrow, and (despite having over 10 cal. of stability) doesn’t weathercock much in the wind.
Dramatic lift offs and lower apogees when using BP motors make this rocket a lot of fun to watch and suitable for flying in smaller fields.
Many power options available if you decide to fly composite motors.
Kit provides water slide decals.
Relatively easy to build, and excellent instructions.
The rocket breaks into two ~3 foot sections for storage and transport. The provided “twist lock” connection works very well for easy disassembly, and flight prep. This feature makes it much easier to pack the ‘chute and load the motor, its as if you were working with two 3′ rockets instead of a 6’ beanpole. You can then put the two halves together when you reach the flight line and are ready to put it on the pad.

The “Less Good”:

The kit is provided with 3/16″ launch lugs, which require a Maxi Launch Rod. The sheer length of this model alone begs for more support, and if you’re going to be using E or greater you really should be using a 1/4″ rod. If you have access to one, this rocket launches even better from a rail.
The tubing couplers provided with the kit are only 1.5″. I feel like this is somewhat marginal, and you have to take great care to make sure you get the joints straight. A cheap upgrade for this kit is to get some of Apogee’s BT-60 couplers which are 3″ long and don’t add any significant amount weight.
The length of this rocket, coupled with its thin-walled tubing can lead to warping issues if left in the sun. Earlier iterations of this model showed it painted black on the package insert, the current version is sporting blue and white livery. I’m guessing in response to this issue.
The length and weight of this rocket also tends to take its toll on landings. This has led to frequent fin repairs. I definitely recommend adding epoxy fillets to beef up the surface-mounted fins. There’s also a significant bending moment when the rocket contacts the ground … this resulted in a kinked body tube (easily repaired by cutting at the kink and adding a coupler) after one of my launches.

I believe all of these shortcomings are easily addressed with a few modifications. During one of my launches, I had a “shotgun ejection” that tore out the motor hook. This has nothing to do with an issue with the model itself … it was a motor malfunction. I’m still flying it with friction-fit motor retention, but the upshot is that Estes gave me a new kit and pack of motors under warranty when I reported the motor malfunction.

In an upcoming post, I’m going to build “Mean Machine Mk. II” with some mods inspired by my experiences with my first one.

Bertha’s Breech Baby – Part 4

Build Complete

I completed the Breech Baby build late last week and got it filled, primed and painted over Friday and Saturday. I’m very satisfied with the final outcome of this build. The lack of a nose cone seam is a nice look. The only disappointment I had in the finish is that I had tried to have three red stripes just forward of the fins, but a paint reaction with the clear coat forced me to abort that idea. I may pick up some red pin-striping tape to reconstruct that design element.

I used 2 coats of red Krylon Fusion, with an 800 grit wet-sand between coats. After masking the fins and stripes, I followed with a light coat and then a heavier coat of Rustoleum “black night metallic” before stripping the masking and another light 800 grit wet-sand. The rings looked pretty good, but not quite as crisp a line as I had hoped. Still, for an “experimental” model that may end its days on the first flight I was happy.

However, when I shot a coat of Rustoleum “crystal clear enamel” the wheels came off. The red paint in the rings wrinkled badly, and the wrinkling continued up under the black. No problem on the fins though, so there’s that I guess.

This is actually the second time I’ve had a paint reaction with this particular red paint. The first was on an Estes Sterling Silver I built earlier this summer. In that case I had let it sit for about 24 hours before shooting the clear, and I had thought that was the issue. Since I had applied all of the coats in one afternoon/evening, I thought I’d be OK. Maybe it’s bad juju for mixing brands, or perhaps the formulation of this red paint is particularly sensitive. I also have a can of the Krylon Fusion in orange that has never given me issues with mixing brands.

Fortunately, I successfully sanded the wrinkles smooth, and then shot another coat of the Rustoleum black, followed by another coat of clear.

Maiden flight

Sunday evening presented just perfect conditions for Breech Baby to have its first flight. Temps in the mid-70’s, scattered clouds, and virtually no wind to speak of.

I loaded in a B6-4, hit the “go” button and the rocket went straight off the pad, arcing slightly to the North during the coast phase. Ejection appears to have been just a bit before apogee at I guess 400-500′. I’ll have to see how it does on a B6-6. The motor pod ejected and and the canopy deployed perfectly and it floated down for an in-field landing maybe 100′ from the pad.

Boosted Bertha (#1946)

Boosted Bertha
Photo Credit: estesrockets.com

The Boosted Bertha is Estes’ latest addition to their popular Bertha family of rockets. These low-power rockets fly on 18mm engines, and sport fat BT-60 body tubes, retro-futuristic fin shape, and a blunt nose cone. These rockets are a lot of fun to launch, as they have nice, slow liftoffs and are easy to watch in flight and track during descent. Their lower apogees allow you to fly a “big” rocket in smaller fields. Always a crowd pleaser.

The Boosted Bertha takes the Bertha line to a new level both in terms of the design and kitting of this model. The sustainer is essentially a Big Bertha, but has a couple of nice upgrades. Most notably, they’ve included screw-in motor retention instead of the traditional motor hook. They’ve also provided water slide decals; a very nice upgrade from prior Bertha iterations, which include self-adhesive decals instead.

The booster design is probably the most interesting part of this kit. At first glance, you might think they pretty much truncated a Baby Bertha and scaled up the fins. However, there’s quite a lot of engineering that’s gone into this part of the kit. Like the sustainer, they provide screw-in motor retention. However, that’s where the similarities to previous iterations of the Bertha line end.

The first thing you’ll notice is that the booster’s body tube has laser-cut slots for through-the-wall fins. A design feature that will certainly improve the longevity of this rather large tumble-recovery part of the rocket. I’ve certainly expended a few bottles of CA re-attaching fins on other Bertha’s I’ve owned after a few hard landings.

The next thing you’ll notice is the longer motor mount tube for the booster as well as the vented centering rings. That’s right – the Boosted Bertha is gap staged. The motor tube for the booster is 4″ long, and the instructions indicate that you should install the motor block so that it allows the motor to project 1/4″ from the end of the tube. This leaves the forward end of the motor 1 1/2″ from the end of the motor tube.

The forward end of the booster’s motor tube is not vented as is typical in gap staging. My guess is that the motor tube stops just short of the engine retainer on the sustainer, and the vented centering rings do the job of venting the pressure wave, allowing the hot gasses from the expended motor to travel up the tube and ignite the sustainer’s motor. I wonder how this will affect the plastic motor retainer in the sustainer … time will tell. I do plan to coat the end of the booster’s motor tube with epoxy to prevent (forestall?) it burning away over time.

I’ll update this post with more thoughts and impressions as I go through the build process and first few flights.

Estes Black Brant II (#7423)

Estes Black Brant II. Photo Credit: estesrockets.com

This build log will be for the re-released Estes Black Brant II (#7423). I’ve seen some very nice-looking scale builds of this model of the Canadian built sounding rocket. I won’t be going for a scale build, my focus will be more on making this a durable sport flyer. The goal will be to have an attractive-enough looking model that’ll look great in the air and on the flight line, and hopefully be tough enough to survive many launches and potential rough landings. Any place I deviate from the “stock” build will be to either make the build process a little easier or to increase durability, as I hope to put many motors through this model. If it’s a little heavy, fine by me … I like slow launches and lower flights as much as I enjoy sending one 1/4 mile high once in a while. With the range of motors you could fly this rocket on, either objective is definitely a possibility.

Sadly, I neglected to take photos of the two “unique” parts of this build. The motor mount and boat tail, and the antennae. However, Chris Michielssen covers these details nicely in his build log for this model, which he built for Estes. Definitely a “gold standard” build.

The motor mount/boat tail assembly requires that you make three cuts from the blow molded “tail cone” (as they call it). The instructions suggest using a hobby knife for this step. Personally, I thought it’d be a bit tedious to make these cuts in the relatively thick plastic with a knife. Instead I used a sharp razor saw which allowed me to make the required cuts quickly, accurately and cleanly.

Once the cuts are made, and the edges cleaned up a bit, assembly of the motor mount is fairly simple. The only “fiddly” part I encountered is getting the motor tube centered in the boat tail. I’d recommend using an adhesive that will give you a little working time so you can manipulate the tube into the correct orientation before it grabs tight. Once assembled, I found it necessary to sand the motor opening in the boat tail a bit to open it up enough to allow a 24mm motor casing to slide in.

I do wish this model incorporated some sort of motor retention … I’d prefer not to rely on friction fit and/or taping the motor in place. It’d be awesome if someone could 3D print a boat tail similar to the Schrockets by Apogee Ibis which incorporates a slot to allow the use of a spring steel motor hook.

The nose cone build-up is pretty straight-forward. The only real deviation I made from the manual here had to do with the installation of the three “antennae”. The kit provides 3 plastic toothpicks that you cut down to 2″ each. The instructions say you should use a hobby knife to drill out three holes for the toothpicks to go through the body tube. I used a 1/16″ drillbit instead, after having made a small centering hole with the hobby-knife to keep the bit from wandering. I just wrapped some tape around the shank end of the bit and spun it with my fingers to get 3 perfectly sized, clean holes. The instructions also show the butt-ends of the toothpicks meeting at the aft end of the blow-molded nose cone. I found that is really not possible, as the end of the nose cone is too far away to make contact and have the antennae at a “pleasing” swept-back angle. Instead, I had the three antennae meet in open space, and then quickly tacked them with thick CA. Once I was pleased with the installation, I added a blob of epoxy at the meeting point, and on the inside of the body tube where each toothpick passes through.

For the fins, rather than profile them with those beautiful scale knife-edge profiles, I’ve gone for a more rounded airfoil shape. The kit directions show profiling patterns for the “scale” profile with the knife-edge leading and trailing edges, as well a “sport” with just the leading edge beveled. I’m not going to try and go supersonic, and the rounded profile will be more appropriate for slower flights and a bit more rugged than the thinner beveled profile. I’ve also opted to leave the trailing edges square as they project beyond the bottom of the body tube. This should help them survive bumpy landings a little better. Should you decide to go for the chiseled profiles, I’d suggest a good sanding block and a jig to hold a consistent angle.

Once the motor mount is inserted, attaching the fins is a breeze. The laser cutting is perfect, and the aft end of the fin root is cut to match the angle of the boat tail. All you need to do is make sure it’s in firm contact with the boat tail and body tube, and you’re guaranteed to have perfect fore-aft alignment of all 3 fins. One less dimension to worry about, so you can focus on just mounting them plumb and square to the body tube. Additionally the 5/16″ balsa provides a very wide contact surface for glueing, so the fins should be very well bonded. It did take a few passes with the hobby knife to get them out of the sheet though, I think they could’ve turned the power up on the laser just a tad for this thicker material.

After the fins were sanded and shaped, I use Titebond thinned with water and painted all surfaces except the root edge with the thinned glue. I make sure to seal the end grain on the fin tip with a couple of coats so it’ll take paint well, but I avoid sealing the end grain on the root edge to allow for glue penetration into the fibers and a stronger bond when attaching them to the body tube. On thinner stock, it’s definitely a good idea to do both sides of the fin at once to prevent warping. Probably not as much of an issue here with these beefy fins, but I still do it as a matter of habit. Once dry, a quick pass with some 180 grit paper and the grainlines will be mostly filled. Any remaining grain will be easily dealt with during paint prep with CWP and/or filler primer.

I glued and filleted the fins with Titebond, let them fully dry, then came back and added epoxy fillets. The fins on this rocket extend beyond the end of the body tube, so will be taking impact forces on landing. The epoxy fillets should further reinforce the fin attachment, as well as be visually appealing and provide some modest aerodynamic benefits. I first tried employing epoxy fillets on the Bertha’s Breech Baby build, in which I learned that masking first with blue painter’s tape allows you to get very neat fillets very easily.

I used Gorilla Weld Steel Bond epoxy for the fin fillets, which has a 10-minute cure time. Working time is probably 5 minutes or so, so I’d only mix enough to do two fillets at a time. I globbed the epoxy into the fin joint, making sure there was more material than I needed for the final fillet. Then, starting from the middle and using a gloved finger wetted with isopropyl alcohol, smooth the fillet first one direction and then the other; not cleaning the excess off of my finger tip until I’m satisfied with the whole fillet. If you are dissatisfied with the result, just glob on more epoxy and re-pull the whole fillet rather than try and patch things up. I do recommend removing the masking tape before the epoxy cures to prevent it getting glued in place. I removed the tape from each fillet as soon as I finished pulling. This will leave a slightly raised edge at the fin root and body tube which will be easy to sand and/or fill and blend once the epoxy cures.

Once the epoxy hardens enough to sand, but before it’s fully cured, I hit the fillets lightly with some 180 grit paper to take down any high spots. I’ll then use some CWP to further blend and smooth the fillets and sand again with 400 grit. CWP dries quickly and sands very easily, so this is a pretty quick step.

Rough-in work completed, nose cone primed.

Finally, I added the launch lugs. The provided lugs are 3/16″, which requires the Maxi Launch Rod. I’ve personally got a Porta-Pad II with a 1/8″ rod , and a Porta-Pad E with a 1/4″ rod. The last thing I need is a third launch lug size in my fleet, so I swapped out the provided lugs with 1/4″ lugs from Apogee. I used some thick CA to attach the lugs, and then added fillets to strengthen and blend them into the airframe using FIXIT epoxy clay. This material has a ~3 hour cure time, and molds just like clay. It’s very easy to smooth and blend it to a feather edge with a finger wetted with isopropyl alcohol. I may try using this products for fin fillets in the future, but it’ll definitely be a more time-consuming method than using a more fluid type of epoxy. I’ve been finding this stuff a very handy addition to my build bench, and have found it useful for many different applications.

Bertha’s Breech Baby – Part 3

Rough sanding and first coat of prime

I’ve completed assembly of the airframe. It just took a bit of CWP to fill the nose cone to body tube joint. A coat of sanding primer and some sanding with 220 grit paper, and not a spiral or groove to be seen.

I also experimented with using epoxy fillets, both to build up the fillets a bit for appearance/aerodynamics as well as to provide some added strength. I’d hate for the shock cord to interfere with the fins and tear one off during recovery.

It took a little bit of practice to pull them nice and smooth, by the fourth fin I pretty much had it down. It definitely helps to mask off the fillet area with some tape. As you can see in the images, they didn’t come out too bad. A little bit of sanding and CWP will clean them up nicely.

I’ll detail epoxy fillets in detail with my next build; the new version of the Estes Black Brant II (#007243), which you can see getting started in the background in the above photos.

Bertha’s Breech Baby – Part 2

See here for Part 1 of this build

Static Test #1.

This static fire with an A8-3 validates the proof of concept. I’ve attached the parachute directly to the kevlar loop for the purposes of this test so that we’re testing the system with all the important pieces in place.

As you can see, the motor pod ejected quite easily and the parachute unraveled from the motor pod. However, my shock cord is clearly too short as it pretty violently flings around with the force of ejection.

This actually cause some minor damage to the aft end of the body tube. Easily fixed by working it down with my fingernail – I’ll soak some thin CA into the tube to toughen it up.

Before actually flying this ship, I’ll be adding a swivel, maybe another 12″ of Kevlar, and some elastic shock cord between the Kevlar shock cord and the parachute shroud lines. With the parachute provided in the kit, there’s more than enough room to accommodate the elastic shock cord along with the ‘chute without risking making it all to tight and preventing a clean eject.

Bertha’s Breech Baby – Part 1

The Estes “Bertha Family” is a very popular model for flying in smaller fields. Their fin design has a very retro sci-fi look, and they tend to have nice slow liftoffs with lower apogees, giving you your best chance for an in-field landing when those hungry trees are around.

I typically have at least one of these great rockets in my fleet … and sometimes both. My last Baby Bertha ended up on the roof of a local school. I recently picked up a new kit at The Spare Time Shop in Marlboro, MA. I typically like to put a personal touch on my kit builds; this time I’m going to try something a little different.

I dislike using ejection wadding, especially on models with larger diameter body tubes. It just takes forgetting to put in the wadding on one launch, and you end up torching your ‘chute, making for an “interesting” recovery. For this purpose, I have taken to adding ejection baffles in models that support them. Typically an ejection baffle is installed with its bottom end at least one BT diameter away from the end of the motor mount.

The Baby Bertha is just too short to be compatible with a baffle, so I decided to go with rear ejection. With rear ejection, you typically use a longer stuffer tube with centering rings on either end. The recovery device is wrapped around the stuffer tube, and the forward centering ring is positioned far enough aft of the forward part of the stuffer tube to leave a bit of room loosely coil the shock cord around it so it doesn’t tangle at ejection.

ApogeeRockets Peak of Flight newsletter (Issue #439 [PDF]) has an article covering rear ejection, and I used that to guide my design.

Materials

The following additional materials (besides the usual adhesives and such…) not included in the kit are:

Kevlar Shock Cord – very heat resistant and exceptionally strong.
BT-20 body tube (for stuffer tube/motor mount).
An extra BT-60 to BT-20 (CR-18/41.6) centering ring (to create a bulkhead).
An extra BT-50 to BT-20 (CR-18/24) centering ring (to reinforce the forward centering ring on the stuffer tube). I didn’t have one on hand, so I used an 18mm motor hook retention ring (usually included in 18mm motor mount kits) and “FixIt” epoxy clay.
Cardstock

The Build

Step 1:

Using the extra CR18/41.6, cut out a circle of cardstock large enough to cover “punch out” in the middle, and glue in place.

Step 2:

Cover the side of the centering ring that you just applied the cardstock patch to with a thin coat of epoxy. This will provide some heat shielding for the bulkhead, as the hot gasses from the ejection charge will be firing directly at it.

I used “Gorilla Weld” 10 minute epoxy because that’s what I hand on hand, but any quick setting epoxy will do.

Punch a small hole near the edge (see photo below) to allow the shock cord to pass through.

Step 3:

Measure the height of the nose cone from the shoulder to the bottom of the inner cone, and mark the inside of the tube. Use a BT-60 coupler to push the bulkhead down to the line you marked, and add a fillet of TiteBond or white glue to fix it in place.

Cut off a length of Kevlar shock cord. At this point it’s not necessary to have the “right” length, just make sure it’s about twice the length of the body tube so you have enough slack to work with during the build. Tie it off to the loop on the bottom of the nose cone, and pass it through the hole in the bulkhead you punched earlier.

Pull the shock cord through, coat the shoulder of the nose cone with adhesive and glue it in place on the top of the body tube. Set this assembly aside.

Step 4:

Measure the distance from the bulkhead to the aft-end of the body tube. Cut off a length of BT-20 to this measurement. This will be the motor mount/stuffer-tube. Assembly is pretty much the same as described in the kit instructions with the following changes:

Use an 18mm motor casing (A, B or C motor), and place a mark 1/4″ from the end. This will be the motor overhang, and your guide for the depth of the motor block ring.
Smear some glue inside the stuffer tube at the distance inside the tube where the motor block will sit. Use the motor you just marked, and push the ring up inside until you reach the mark on the motor casing and remove it quickly so the glue won’t “grab” it.
Now install the motor clip and motor clip retention ring as per the kit instructions.
Add the aft centering ring at the location indicated in the kit instructions and add the glue fillets.
Punch a small hole near the inner-edge of the forward centering ring for the shock cord to pass through. I made it about a half-circle “nip” in the inner part of the ring.
Make a mark about 1″ from the forward end of the stuffer tube, and glue the forward centering ring in place at that location.

Step 5:

Thread the shock cord through the CR-18/24 centering ring, and then thread the shock cord through the hole you punched earlier. Apply some glue to the stuffer tube forward of the centering ring and then slide the CR-18/24 ring into place to reinforce the forward centering ring (note that I used a motor clip retention ring, as I didn’t have a CR-18/24 on hand). Thread the shock cord through the CR-18/24 centering ring, and then thread the shock cord through the hole you punched earlier. Apply some glue to the stuffer tube forward of the centering ring and then slide the CR-18/24 ring into place to reinforce the forward centering ring.

Make sure to leave yourself several inches of shock cord on the other side of the centering ring to tie a loop in the end, as well as another knot about 1/2″ back from the loop to anchor it into the epoxy we’ll be adding in the next step.

Step 6:

Pull the shock cord back so that the knot is just aft of the hole it passes through. Using putty or clay-like epoxy (I used Fixit epoxy clay, but anything like it – such as JB Weld would work just as well), fix the shock cord on the aft-side of the forward centering ring and smooth it out. Then, fillet the forward side with the epoxy for additional reinforcement.

The completed assembly should resemble the image below. Note how the shock cord is wound around the forward end of the stuffer tube. This should pay out smoothly during ejection, similar to how fishing line pays out off of a spincast reel.

Assemble the rest of the kit as per the the provided instructions.

In my next post on this build, I’ll provide a video of my static testing, and provide recommendations on completing the recovery system.