The epic fin-ale to the fin attachment series! (Pun intended.)
So far, we’ve attached the fins using the “through the wall” method with epoxy at several points: (1) the fin root, where it directly touches the motor mount tube, and (2) all along the inside edges of the fins and motor mount tube using a syringe to inject it. This created an incredibly strong foundation (especially by mixing some chopped carbon fiber into the epoxy), and at last we can turn to (3) the outside of the rocket to create fin fillets.
The first step here is to measure and mark approx. 3/8 inch from the joint, along both the airframe and the fin itself, and draw parallel lines on each. Then follow this up with masking tape along the full length of each line. Using this technique, you can apply the epoxy fillet, and when you remove the tape afterwards, it will leave a very clean edge. Generally you want to wait before removing the tape so the epoxy has a chance to partially cure – but it’s easiest to remove the tape if you do so before it fully cures.
For the epoxy mixture, the difference this time will be the addition of a thickening agent so it’s not quite as runny and it maintains its shape. Specifically, this means first mixing the two part epoxy (resin and hardener), then mixing in some chopped carbon fiber as before, and finally adding the thickener.
As I mentioned earlier, I’m using West System resin and hardener, and also for the thickener. The stuff is extremely lightweight – so much that it’s almost difficult to even take the can’s lid on or off, as the slightest breeze or air movement will cause it to fly up into the air like dust. Of course, this is something that should be done only while wearing a respirator or face mask.
Once the epoxy is sufficiently thickened, it should have the consistency of peanut butter – spreadable, but will more or less hold its shape.
Finally, a round length of wood (e.g. a broomstick) or plastic (e.g. metal pipe or PVC pipe) will be very helpful at this stage, particularly one that has a 3/4″ or 1″ diameter. I happened to have some spare 3/4″ PVC pipe lying around from my earlier project running electrical wire to the shed to build the workshop. The idea here is to spread some epoxy on at the joint where each fin touches the rocket body, and then to use the PVC pipe to run along the full length, creating a nice, smooth, rounded fin fillet.
Above is a picture showing my partial progress with this technique. It can take some practice getting the right epoxy consistency with the thickener, and also using the pipe to create the rounded fillet. But once finished, this will provide the third and final bond of the fins to the rocket body.
As noted above, you can wait before removing the masking tape, but don’t wait so long that the epoxy fully cures.
After one set of fins is complete, as shown above, you can rotate the rocket 120 degrees and repeat (although give the epoxy some time to cure, first, before rotating). Repeat a second time, and then rotate and repeat a third.
Once all the fin fillets are completed, you’re done with the actual rocket build! The rest is cosmetic work or just attaching things to this newly constructed rocket: priming and painting (which, frankly, is optional), attaching recovery harnesses and parachutes (slightly less optional but simple), and conducting ground testing.
Fair warning here: I’m actually splitting the “attaching the fins” information into three separate posts. This is not out of a sense of malice or sadism, but simply because there’s a lot going on with the fins.
The prior post (part 1 of 3 in this epic fin series) was basically the prep work and first few steps to secure the fins to the rocket body by using some epoxy and inserting the fin root through the wall where it can bond against the motor mount tube inside. Along with a printed fin alignment guide and a bunch of heavy objects, this keeps the fins in place and serves as a starting point for the multi-step approach for attaching them.
This post (part 2 of 3) involves epoxy, a syringe, and a bunch of strategically drilled holes. This might sound like the setup to a bad joke, but it’s actually literal and straightforward.
If you’ve been following along so far (whether with your actual fiberglass rocket build, or just conceptually in your head), you know that inside the airframe there is a motor mount tube, held in place and centered with the aptly-named centering rings. The top and bottom of each fin should just barely be touching a centering ring, inside.
The idea here is to take the syringe and inject the epoxy into each hole. There’s two holes per fin (so 12 total in this case for 6 fins), with one hole on either side of each fin, roughly 1/2 inch away from the fin. Using a typical plastic syringe that you can find at a local drugstore, you can inject 10 ml at a time, and you should inject roughly 25 ml for each side of the rocket, split evenly among 4 holes. The rocket should be positioned horizontally and completely level, as in the above picture.
Once the epoxy has been injected into these 4 holes, you tilt the entire rocket forwards and then backwards, slowly, in order to move the epoxy around inside and completely coat the area where each fin touches the motor mount. The centering rings on each side should create a “dam” to prevent the epoxy from going any further past the edge of the fin, if everything is aligned reasonably well.
(If not, well, the epoxy may ooze out some of the other holes below a little. Not a huge deal, but may require a bit of extra cleanup.)
Once the epoxy is spread evenly inside, it needs some time to cure. Come back a few hours later. At this point, you can rotate the rocket 1/3 of the way around and repeat the process for the next 4 holes, and then finally a third time after that. Ensure each time that the rocket is level as the epoxy cures, so it doesn’t slowly ooze and collect in a lopsided fashion. This would not be ideal for a uniform fin attachment, it could also throw the rocket off balance in its weight.
Above is a picture of the two part epoxy mix (resin and hardener) when combined and thoroughly stirred in a small plastic cup. The syringe I used is pictured as well. You may need several since they can get clogged over time.
One final note here: you can also mix some chopped carbon fiber (pictured below) with the epoxy, and again mix thoroughly. The color will darken noticeably. This epoxy mixed with chopped carbon fiber will significantly strengthen the bond as it cures. In other words, those fins are never coming off.
This epoxy injection technique is pretty cool, and it’s been tested and used successfully for many years. Try it!
In my next post (part 3 of 3 in this series on attaching fiberglass fins) I’ll briefly cover the final step: creating external fin fillets where each fin touches the rocket airframe. It gives one final layer of protection to ensure the fins are secured, and also looks more aerodynamic.
A few weeks ago, I received a surprise: a brand new sewing machine showed up at my front door.
It was a surprise because I didn’t order it. I’ve never owned a sewing machine, and I have no overwhelming desire to own one. I’m not even sure what one does, exactly, with such a machine.
And yet, there was a UPS label on the front of this box, and it was very clearly addressed to me; it had my name and my address. Everything was spelled correctly and appeared to be in order. I was unambiguously the intended recipient.
So I assumed, logically enough, that this must either one of two things: (1) a gift from some friend or family member, currently anonymous but soon to surely reveal themselves as the generous benefactor, or (2) a mistaken shipment.
Let me also quickly point out that this was also no ordinary sewing machine. It came in packaging directly from the manufacturer, where the return label said SVP Machines. A quick Google search revealed this to be the parent organization of Singer Sewing Company. This was a Singer Stylist 7258, and a quick glance at the Singer website revealed this machine’s retail price to be about $300. The Rolls Royce of sewing equipment!
Given that these seemed to be the only two possibilities, I left it in the box and waited for someone to contact me. Surely either a friend who was trying to surprise me would coyly ask me if I’ve received a package recently, or (less likely but still possible) the manufacturer would contact me and say they somehow accidentally sent me their product, and would likely ask me to ship it back (hopefully at their own cost).
But days went by, and then a week, and then two. I heard nothing from anyone. This sewing machine was a real mystery. Where did it come from? Was it truly a mistaken shipment from SVP Machines (i.e., Singer) who didn’t even realize anything was wrong?
II. THE MYSTERY DEEPENS
After about two weeks, I happened to be checking my recent credit card transactions online. I do this periodically, usually once a month when I receive my statement and I grow outraged and indignant that my bill could be so high. I’ll glance through the transactions looking for something amiss, only to sheepishly realize that one after another is justifiable and I simply spent more than I realized.
But this time, something was different. I saw a charge from May 20 from “SVP Machines” for $826.
That can’t be right.
I’ve never heard of SVP Machines before receiving the sewing machine package a few weeks before this, so I immediately connected the two. This was clearly a charge from Singer Sewing Co. for the machine I received.
I stared at this charge and tried to mentally re-calibrate the likelihood of my previous two possibilities. This was definitely not a friend or family member offering me a generous gift (though the chances of that had already faded more with every day that had passed). My credit card was being charged. That’s not typically how a gift works. That only left the manufacturer making a grievous error, somehow incorrectly charging me and shipping me their product.
But the dollar amount didn’t add up, either. I checked the Singer website one more time to be sure I was looking at the right machine. The Singer Stylist 7258 was still going for $300. So even if this was somehow a mistake, where was the $826 charge coming from? Was I missing something?
III. THE STORM CLOUDS GATHER
At this point, I decided I should investigate a bit further. I looked more closely at the UPS label on the package and yes, my name and address were correct. But among all the bar codes, tracking numbers, and other information on the label, I noticed in the upper corner that it said “2 of 3.” Did this mean there were two other packages in the shipment? That might explain the $826 charge for what appeared to be a $300 product.
I checked the UPS website and entered the tracking number from the label. Sure enough, this was one of 3 packages in the shipment. In the tracking details, it showed that there was a request from the shipper the day after the order was placed (while the packages were still in transit) to re-route them to another city in Washington state that is approximately a 4 hour drive from the Seattle area, in the central part of the state. The other two packages were successfully delivered the day after mine to that address. UPS confirmed they were left at the front door.
This certainly seemed like credit card fraud, although it is still baffling that someone is committing this crime and going to these extreme lengths solely to purchase sewing machines. And even then, why did one of the machines get delivered to me at all?
But I noticed two other things that chilled my blood and removed any doubt that this was fraud, and part of a bigger scheme. My credit card transactions indicated a brand new charge as of June 8: the United States Postal Service (USPS) was charging me $1 for “change of address.” And my credit card account stated that I ordered a replacement card, and the card was on the way.
I never attempted to change my address with USPS, and I never ordered a replacement card.
IV. CRISIS AVERTED
At this point, I was certain it was fraud and some sort of identity theft scheme, and I started taking action to correct everything with all the parties involved. I spent the better part of a day making phone calls, alternating between navigating automated menus and speaking to real people.
I disputed these unauthorized charges with the credit card company and explained I never ordered a replacement card (someone else must have done so), and so they cancelled the card and issued me a new one. I made sure USPS was aware that I am not changing my address and someone else fraudulently attempted to change it on my behalf, so that my mail would get routed to them.
I spoke to folks at both UPS and Singer, trying to get a name or the exact street address for the location that the other two packages were sent to, but both refused to tell me. All I know is the city. They did say that they can tell law enforcement, and so I also filed a report with the local police, who should get this information and track down the perpetrator.
While I have a new replacement card, and USPS is aware that my address didn’t change, there are still some deep unanswered questions here.
Who was responsible for this unwarranted attack on my person?
Once they obtained my credit card information and decided to make unauthorized purchases, why did they start (and end) with sewing machines?
Why did one of these machines arrive at my door, when the other two were rerouted to the perpetrator?
What are the relative advantages and disadvantages of chainstitch, lockstitch, overlock, and coverstitch?
These are questions that may never be answered, and we may need to leave them to the philosophers. But this concludes the saga of the sewing machine and thwarted attempt at identity theft.
Let me know in the comments if you have any thoughts about this strange occurrence, or if you’ve ever experienced something similar.
If you’ve never put together a rocket before, well, I’m baffled that you are reading this blog. But typically, a rocket will have either 3 or 4 fins, which are placed symmetrically – equally spaced out, in the 360 degrees around the center. If 3 fins, then they’d be 120 degrees apart; if 4 fins, then 90 degree spaces.
This rocket has 3, and then another 3 aligned above them for a total of 6, each spaced out by 120 degrees. I think having 6 in this arrangement is purely aesthetic, as opposed to just having 3. Who knows?
I’ve used this two part epoxy before (resin and hardener) in one or two previous steps with this rocket construction. But this is the first time I’m using it in larger quantities.
Basically, I used a fin alignment guide (which I found online for free and printed out) to ensure that the fins were aligned properly, spaced exactly 120 degrees apart all the way around. I then prepared some of the epoxy and applied it with a popsicle stick (sophisticated technology), applying it to the edges or “roots” of each fin as if I were buttering a piece of toast. I inserted each fin into its slot, where the fin edge or root with the epoxy is pressed up against the motor mount inside. This is the first of several steps to ensure the fins are securely attached, starting with the interior.
To hold everything in place while this initial round of epoxy cures, I had a couple options. I saw some fancier solutions that other people have done involving using jigsaws and drills to cut out holes in large plywood sheets, and lots of vises and clamps.
That seemed like a lot of work, so I just used some rubber bands and propped the rocket/ fins up against some heavy objects like cans of paint or bricks while it cured (making sure nothing could move, and that the fins were aligned perfectly according to the alignment guide).
For a closer look at the epoxy, I included this photo as well, since it’s critical to this and the next few steps. I used West System epoxy as it was highly recommended, and it works great. The 105 is the epoxy resin, and 205 is the hardener. Each comes with a pump, and you just combine one pump of each into a mixing cup, and mix thoroughly for several minutes. It begins to cure pretty quickly, and the chemical reaction causes it to get extremely hot as it cures (to the point where it will burn you if you touch it, even through the plastic mixing cup, and steam is visibly coming off the top).
For most of the fin attachment points, I’m also mixing in some chopped carbon fiber (pictured here as well) which is, in certain places, injected inside with a syringe. The carbon fiber greatly strengthens the epoxy as it cures.
Next, I’ll continue using the epoxy to attach the fins via this injection method, along the inside. After that, a final application of carbon fiber-infused epoxy on the outside of the rocket to create fillets (i.e., just a narrow strip of epoxy along each area where the fin touches the outer rocket body, shaped into a curve to minimize drag).
Things are really coming along – with the fins finally attached, it’s starting to look like a rocket!
Before I jump into the riveting details of rail buttons, I’ll take a step back and explain what this is, and why it matters.
Every rocket has a “center of pressure” and a “center of gravity” (or center of mass). I won’t go into detail about these concepts here, but basically, the relationship between these two things is important for a rocket to remain stable in the air. When it’s moving at a fast speed, the fins help keep it going in a straight line (i.e., up) because of the way the air pushes on them. I’m oversimplifying these concepts, but this is the point:
When the rocket is sitting on the launch pad and first lifts off, it is not moving quickly enough to be stable. If you tried launching a rocket from a pad without any kind of support, there would be a pretty good chance that it would not ascend perfectly vertically. It’s entirely possible it would not ascend at all, as it might tip over and fly horizontally (perhaps into a crowd of spectators). This is not ideal for your rocket, or for the spectators.
The solution to this is to provide just enough support for the rocket to keep going vertically as soon as it launches and just begins to (quickly) gain speed. With small model rockets, a thin metal pole is all you need, just a couple of feet high. The rocket will have a small launch lug (basically like a plastic straw) attached to its side, which slides down over the metal pole, ensuring the rocket takes off using the pole as a guide.
For larger rockets, it’s the same concept but with slightly fancier hardware. Instead of a thin pole, the launch pad will have a much bigger rail standing vertically for support. And instead of a plastic straw glued to the rocket, it will have rail buttons, made from plastic and secured by drilling a hole in the rocket body and attaching with metal screws.
The concept is extremely simple, and installation is fairly simple as well. It just requires measuring where you want the two rail buttons to be located, marking the spots, and drilling to insert the hardware. In general, you want the rail buttons exactly halfway between two fins, with one very close to the bottom (aft) end of the rocket, and another some distance up the side.
Often one or both is drilled and screwed directly into a centering ring. Whether that’s possible or not on your particular rocket, it also helps to add a small amount of epoxy just to make sure it’s secured in place. Here, you can see where I attached the rail buttons on this fiberglass rocket.
And that’s it! Rail buttons installed, and the rocket can be flown from a standard launch rail.
The next step will require slightly more work: attaching the fins, which are of critical importance in achieving that fashionable “rocket” appearance.
As I’ve written extensively about before, it’s absolutely critical that your rocket has a good motor retention system. You don’t want the motor falling out of the bottom of the rocket. For one thing, it may land on someone’s head. More importantly, if the motor falls out, your rocket will not perform as intended.
There are different types of motor retention mechanisms, ranging from the extremely simple (e.g., wrap it with masking tape so it’s a tighter fit) to relatively simple (e.g., metal clips or hooks to keep it in place), to slightly more complicated but significantly more reliable hardware, such as a machined aluminum motor retainer. That’s what I went with for my previous rocket (54mm motor mount) and that’s what I’m using again here (75mm).
The main question for me was: how am I going to attach it? The aluminum motor retainer has pre-drilled holes for screws, but generally you would attach it directly to the closest centering ring, which would be the “aft” one, or the one closest to the bottom of the rocket.
With this rocket, everything is fiberglass, and I wasn’t sure about drilling this many small holes in a fiberglass centering ring (“CR”). The ring is extremely thin, and the screw inserts would extend far beyond it instead of sitting comfortably inside it. Plus, I needed something to fill a gap between where I wanted my aft CR to sit, and where the motor retainer would be located.
So for multiple reasons, I decided to buy and use an additional wooden CR (two, actually, as I needed to fill a 1/2″ space and each wooden CR is exactly 1/4″ in width).
It worked out well. I used wood glue to attach the two CRs together, and then after it dried, I drilled holes and inserted each of the metal screw inserts. I then attached the aluminum motor retainer to the wooden CRs with screws, as pictured here.
I was then able to attach the whole thing – additional CRs and metal retainer – to the motor mount tube using epoxy.
To be honest, this was one of the easier and more straightforward steps in the assembly of this rocket. This type of retainer is reliable, though, and now I have one less reason to worry when I launch.
There are still plenty of other things to worry about, plenty of things that can go wrong – sometimes catastrophically – and I’m sure at least one of them will.
Actually, though, I’m going to split this into two posts, just as I did with my last rocket build. Right now, I’m just putting together the frame of the e-bay – the exoskeleton, if you will – and later I’ll do a separate, longer post once I add all the electronics inside.
The basic components here include the 11 inch fiberglass tube (which acts as a coupler), a vent band or ring around the center of the tube, and an aluminum bulk plate on each end (see above for an unnecessarily zoomed-in picture of the bulk plate). The bulk plates are “stepped” with an edge or lip so that the inner part fits tightly inside the e-bay tube, and the outer edge with its slightly larger diameter prevents the bulk plate from being pushed any further and falling in. Basically the same concept as a sewer lid.
Each bulk plate came with a pre-drilled hole in the center, but I needed to drill several more holes through the aluminum for the all-thread rods which run through the entire e-bay and connect the two ends together. A few tips for drilling through metal:
Use a drop of oil.
Go slowly – there’s no need to rush.
Wear protective eye gear – this process throws around metal shavings that could cause serious eye damage.
Clamp down the metal you are drilling through and make sure it’s secured.
With the drilling complete, I attached the basic hardware. Each side has a forged eyebolt going through the center, with washers on either side of the bulk plate and a nut to secure it. I also added a dab of epoxy on both sides just to hold everything in place permanently.
Then I inserted the all-thread rods through each bulk plate; they also have washers and nuts on either side of the plate. These aren’t meant to be secured permanently, however, as at least one side needs to be removable in order to access the interior of the e-bay and the sled with electronics.
Above is a picture of the completed e-bay. Nothing too complicated – again, this is just the frame or skeleton of the e-bay, without the electronics inside. But you need to start with the outer part.
I’ll put together a more comprehensive article once I figure out the sled, flight computer and battery, switch, and wiring, along with the PVC pipe on the outside for storing black powder charges, just as I did for my previous e-bay.
But first, I’m going to finish building the rocket itself. Next up: the motor retainer.