I gave a brief preview in a recent post, but I’m excited to report that the L3 Fusion rocket is now finished. This is a kit available for pre-order from Scott Binder at SBR, and I was fortunate enough to partner with Scott to do a test build on his latest design.
As mentioned a while back, the L3 Fusion is a larger, upscaled version of his classic Fusion rocket. It has a 5.5″ diameter airframe and is about 90 inches in length, with a 75mm motor mount tube capable of flying on an M motor. What’s particularly great about this rocket is that it combines strength with being lightweight. Its cardboard airframe weighs in at just 11 lbs when fully loaded, minus the motor.
And yet it’s fully double-tubed from top to bottom, and the entire interior is coated with West System epoxy to harden and strengthen it. This thing can take a beating, and it is more than strong enough to handle an M motor.
I plan to fly it for L3 certification on an Aerotech M-1297 motor. Believe it or not, this will be my first time using a reloadable (RMS) motor, and my first time putting one together. I’ve previously just used disposable (DMS) motors since they’re so easy to handle – minimal preparation, and then discard entirely after the single use. But having now built the M-1297 in preparing to fly, I have to admit there’s something satisfying about putting together the motor yourself. Of course, it’s a bit messy and you’ll get your hands dirty – and the casing is not cheap – but the end product speaks for itself.
In addition to building the rocket, we also set up a small studio and filmed the entire project, from start to finish. Throughout the process, I try to explain what I’m doing, though I’m far from an expert (I am, after all, just applying for my L3 certification). It was a lot to film, and as you can imagine, the video editing process is extremely time-consuming (props to Scott for undertaking this). But it should make for a great tutorial on YouTube, and I’ll post the video as soon as it’s ready!
The final chapter in the long saga of building the Darkstar Extreme: ground testing.
Actually, I don’t know whether I should have saved this step for the end. I’ve been looking at what some other folks do, and I’ve seen several people conduct ground testing as soon as the rocket is technically built, but prior to any kind of priming or painting, or adding other finishing touches. To be honest, that probably makes more sense because it’s bound to suffer some minor scratches and scrapes during testing, so better to do it “naked” and paint it later. Fortunately, I don’t care!
Just to take a step back and recap what I’m even doing here: ground testing is important to test the electronics inside the rocket and make sure everything is wired up and working properly. I won’t go into the details of the electronics bay, but there are a lot of things connected to other things, and then there are more things. If even a single connection is loose or comes apart, the whole e-bay could cease to perform its basic function.
And by “its basic function,” I mean the flight computer (circuit board), connected to both a battery and an on/off switch, needs to be powered on and fire a charge at the right moment, which causes an electric match to spark, which causes some carefully packed black powder to explode, which causes the rocket to separate in a pre-determined place. And then it all needs to happen again to cause the rocket to separate in a different place. Parachutes deploy, rocket lands safely.
Aside from generally checking that the electronics and wiring work correctly, ground testing is particularly important to determine the appropriate amount of black powder (BP) to use. There are different types of BP, and there are many online calculators that will tell you, based on the volume of your rocket’s interior space and your desired pounds per square inch (PSI), how much BP you should use exactly. However, actual conditions can vary, and it’s a good idea to test – and test again.
Ultimately, you want to use enough BP to ensure that the rocket separates, forcefully. Really forcefully. With verve. Anything less than this could cause a failure to separate, which means the parachute probably won’t deploy, which could be catastrophic for the rocket (and potentially any hapless bystanders). Of course, you don’t want to use so much BP that the explosion itself destroys your rocket, either. It’s a fine line. Your precious rocket hangs in the balance.
I did multiple rounds of ground testing on different days (note the color variation and also choice of test stands in the different photos). On that note: never use a more elaborate test stand than necessary. Here I was, building a crude stand out of wood, like a sucker! I could have just been using a cardboard box all along. The angle is better anyway.
The calculator told me that I should use about 2.5 grams of BP, but I found that I needed to use closer to 3.0 grams to really separate the rocket forcefully. I also have a backup flight computer with its own separate charges, and for the backup I will use about 4.0 grams of BP. Gotta make sure it gets the job done.
It’s debatable whether this testing is really part of the rocket’s construction, strictly speaking, but in any event this was the final step before launching the rocket out in the field.
Now it’s time to fly!
The recovery system in a high power rocket is extremely important. Needless to say, launching a large rocket extremely high into the air is pretty fun. But that thing is going to come down again, sooner or later, and the recovery system you’ve designed and built for your rocket will determine whether it comes down like a ballistic missile or floats down gently for a soft landing. It could potentially injure or even kill someone. More importantly, your valuable rocket could be completely destroyed.
If the rocket separates in the air, even with no parachute, that’s a good start. A rocket separated into two (connected) parts loses its aerodynamic design; it will fall, but awkwardly and more slowly than if it were in one piece. Of course, if it’s heavy, it’s still going to hit the ground pretty hard.
Drogue vs. main
Better to separate and have a parachute. A single chute can be sufficient – it depends on the rocket’s expected altitude and how heavy it is. Ideally, though, the rocket will be capable of “dual deploy,” which just means deploying two separate parachutes, a drogue and a main.
The drogue chute is smaller and deploys at apogee. The idea here is that, at apogee, the rocket’s speed has ground to a halt. It’s no longer shooting upwards, but it hasn’t yet started falling back down very fast either. The rocket separates at this point and the smaller drogue chute deploys, slowing the rocket’s descent to some degree.
After the rocket continues its descent and is closer to the ground, the airframe separates again and the main parachute deploys. This larger chute slows the rocket’s descent considerably and allows for a softer, gentler landing.
This order is important. You wouldn’t want to release the main (larger) parachute at apogee because even a slight wind would carry it very, very far away by the time it landed. But if you only used a drogue chute during the flight, the rocket’s descent would not slow sufficiently and it’d have a rough landing.
A weighty problem
This brings us to my current predicament. The Darkstar Extreme is a fiberglass rocket, and it’s pretty heavy, at roughly 14 lbs before adding the recovery system or the motor. That’s about what two gallons of milk weigh. Imagine the force it would take to accelerate two gallons of milk vertically, thousands of feet into the air – and likewise, the size parachute you’d need to significantly slow the descent of something that heavy.
There are online calculators that can help determine the right parachute size (diameter) based on the weight of your rocket and how fast (or slow) you want it to land. There are a lot of trade-offs in rocketry, some of which may not be obvious. All else being equal, for example, the softer the landing, the better – but there is a trade-off here. You can slow the descent and get as soft a landing as you’d like by increasing the size of the parachute. But large parachutes take up a lot of space. It’s increasingly difficult to stuff a gigantic parachute into your rocket without increasing the size of the rocket (and its weight, which then requires an even bigger parachute, etc.).
I ended up buying a 2 ft diameter drogue chute and an 8 ft diameter main chute, both from Rocketman Parachutes. In the above pictures, I opened them up as soon as they arrived at my house and ran around in the street trying to catch the wind and fly them like kites – with mild success.
They’re also a very vibrant orange color, which should make the rocket a little easier to see during its descent in the sky, and easier to locate once it touches down.
I’m looking forward to launching the Darkstar Extreme in the near future and seeing these parachutes deploy! I should also be able to capture a lot of data with my flight computer, so I’ll be able to tell how fast the rocket was descending with the drogue, as well as with the main chute.
All the difficult and time-consuming rocket construction steps are basically complete. The drilling and sanding fiberglass is done, the epoxy has cured, and technically, you can fly it “naked” at this point. But a painted rocket just adds that extra touch of class, and we are nothing if not classy.
Before getting started here, a couple of tips and some basic prep. There are a few parts that you might want to cover before spraying anything: things like the aluminum tip on the nose cone, the aluminum motor retainer, and the rail buttons. You can use masking tape to manually cover up these things pretty easily.
Also, keep in mind that where a coupler slides into another section of the airframe with a snug fit, you don’t want to build up multiple layers of primer and paint, or things won’t fit at all anymore, without sanding the paint off. You may want to cover coupler ends with masking tape as well to save yourself trouble later.
So, to begin: lay out the rocket on cardboard or somewhere that you don’t mind getting turned into a rainbow of colors. Spray a thin layer of primer over everything (I chose a simple white primer) and use a quick back and forth motion. Don’t spray too close, and keep it continually moving while spraying, so that paint doesn’t build up too much in any single area. You can always come back and spray again and again, lightly with a thin coat each time.
Of course, since the parts are lying on the ground, you can’t get underneath and will need to wait for them to dry and then rotate them. Depending on how much you’re able to coat the pieces each time, you may need to rotate them just once, or perhaps twice.
Finally, once everything has a nice layer of primer and it’s dry, you can begin spraying paint. The colors and design are totally up to you, but I would certainly recommend a high gloss finish.
In my case, I went with a glossy navy blue for the rocket body. I then used “sunshine yellow,” also glossy, for the fins and the vent ring around the e-bay. Note that it didn’t matter if the layers of navy blue got all over the fins, but once this was finished, I had to use more masking tape to very carefully and thoroughly mark off the fins from the rest of the body. I also used some brown paper grocery bags, tearing them up into approximate sizes to cover the rocket between and around the fins, with masking tape at the edges sealing it off to create sharp and exact lines.
I’m no expert painter; this is only the second rocket I’ve ever painted. But I think once finished, it turned out pretty well.
Of course, this paint job is bound to get scratched, scuffed, and generally deteriorate over time. The rocket gets disassembled and re-assembled, and parts bump and bang into each other, and of course upon landing after even a single flight it will get dirty and a bit dinged, no doubt. I can always touch up the paint in the future when that happens, and it doesn’t really matter – it’s purely cosmetic. But it is fun and it just completes the look.
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.
Exciting times! I’m ready to attach the fins.
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.