How to build a fiberglass rocket, part 7: motor retainer

June 4, 2020 improbableventuresLeave a comment

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).

metal screws in wooden centering ring, on workbench with drill nearby — time to drill

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.

black aluminum motor retainer attached to wooden centering ring — motor retainer

I was then able to attach the whole thing – additional CRs and metal retainer – to the motor mount tube using epoxy.

aluminum motor retainer attached to red fiberglass motor mount — business end of the motor mount

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.

How to build a fiberglass rocket, part 6: e-bay

June 1, 2020June 1, 2020 improbableventuresLeave a comment

On to the electronics bay! My pride and joy.

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.

black circular aluminum bulk plate for e-bay — aluminum bulk plate

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.

black circular aluminum bulk plate clamped to workbench with metal shavings — drilling through metal

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.

bulk plates and all thread rods for e-bay — exoskeleton of the e-bay

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.

red fiberglass e-bay on workbench — looking good

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.

How to build a fiberglass rocket, part 5: nose cone

May 26, 2020May 26, 2020 improbableventuresLeave a comment

Like many fiberglass rocket kits, the Darkstar Extreme has an aluminum-tipped nose cone. The aluminum tip is for more than just show: it has a couple of structural purposes.

One is the manufacturing method of the nose cone itself. The process uses “filament wound fiberglass,” which involves placing resin-impregnated fibers around a mandrel (a gently tapered cylinder). It is difficult to make this come to a point, and instead the manufacturer just shortens the nose cone and puts an aluminum tip on.

Another purpose is that during flight, the tip of the nose cone absorbs the most heat, and aluminum is a better material to use for this specific part of the rocket.

So, the question for me now is: how to connect the nose cone to the rest of the airframe?

You might be asking: how hard can that be? And you’d be right; it isn’t particularly difficult. But some nose cones have a portion that can fit inside the rocket body, as though there’s a built-in coupler. This nose cone, though, is the same diameter as the rocket body and will not fit inside it.

The good news is, the rocket comes with a 6 inch long coupler. Half goes inside the nose cone, half inside the airframe (payload section). On the nose cone side, I just epoxied them together to create a permanent bond. On the other side, I drilled three small holes through the airframe (and coupler) and inserted nylon screws (shear pins). This allows everything to stay together until a large force is applied mid-flight and the airframe separates from the nose cone, deploying a parachute.

The bad news, however, is that I need to attach a kevlar cord to the nose cone somehow, and the best way to do this is to put a bulk plate with a forged eye bolt on one side of the coupler. Either side will do, but it makes sense to put it on the side that goes a few inches into the nose cone, rather than the side that comes a few inches out, since that increases the available storage space inside the rocket for things like the parachute and 25 feet of kevlar cord.

The kit came with a fiberglass bulk plate with no edge or lip (see above picture). It will fit inside the coupler, but I don’t feel too confident that epoxy alone will hold it in place. Instead, I ordered another aluminum bulk plate with an edge or lip – the inner part fits inside the coupler, but there is an outer lip that sits above the coupler so it cannot be pulled through, no matter how hard the cord is yanked.

green aluminum bulk plate attached to red fiberglass coupler — a christmas coupler

I epoxied the aluminum bulk plate to the coupler, and then used more epoxy to attach the forged eye bolt (with a long screw attached) and two nuts to the bulk plate itself. There’s no way this setup is coming loose during flight regardless of the forced applied.

view inside coupler, from above — masking tape dam keeping epoxy near walls

Above is a view of the inside of the coupler. I added some masking tape in an attempt to create a very crude barrier or dam, keeping the additional epoxy a bit closer to the edges to seal them.

red fiberglass coupler secured inside grey nosecone — a perfect fit

That’s it! The coupler and nose cone are in good shape, and I’m ready to move on to the next section: my old friend, the e-bay.

How to build a fiberglass rocket, part 4: motor mount

May 24, 2020 improbableventuresLeave a comment

With the airframe of the rocket nearly complete, I just needed to prep the area where fins will eventually go. The rocket is pre-slotted (i.e., it comes with slots already punched out to insert the edge of the fins), but the slots are all too narrow and needed to be sanded quite a bit to widen them.

In addition, I drilled 12 individual holes (one for each side of all 6 fins). Later in this assembly, I’m going to insert the fins into these slots, where their edges will be up against the motor mount tube inside this airframe. I’ll then inject epoxy with a syringe into each hole, and tilt the rocket back and forth to spread it around, ensuring that the fins are strongly secured in place both internally and externally. But I’m getting ahead of myself.

booster section of rocket airframe, with slots for fins — pre-slotted airframe

On to the motor mount tube!

This 75mm fiberglass tube has a slightly smaller diameter than the 4″ rocket airframe. (To be clear, I have no idea why the motor mount tubes are almost universally measured in metric units – 54mm, 75mm, and 98 mm being fairly common in high power rocketry – while the airframe itself is measured in inches. It’s a mystery for the ages.)

There are 4 beige colored fiberglass centering rings: the inner diameter of each ring fits snugly around the motor mount, and the outer diameter of each ring fits inside the larger airframe. The purpose of these rings, as the name implies, is to center the motor mount inside the airframe.

red motor mount with yellow kevlar recovery harness on workbench — motor mount with kevlar recovery harness

The primary goal here is to secure the yellow kevlar recovery harness to the motor mount. Later, I’ll attach a much longer kevlar cord to this one, and the other end of that cord will attach to one end of the e-bay (with a parachute attached as well).

This basically makes sure that the bottom part of the rocket, including the motor, stays linked to the e-bay in the middle of the rocket – and also makes sure that a parachute can deploy, when these parts separate after apogee. Since there’s nothing obvious to hook or attach this cord to on the motor mount, the solution is to simply epoxy it directly to the motor mount.

I measured the width of the cord (1 inch) and marked it on the top centering ring, and then sanded down a 1 inch width on both sides of the inner part of the ring, to allow just enough space for the cord to fit between the ring and the fiberglass tube. About 6 inches of cord are on each side of the tube.

motor mount with recovery harness tucked inside centering ring — before: tucked inside centering ring

After that, I created some very crude “dams” with masking tape since the epoxy is a bit runny before it cures. I put a generous amount of epoxy underneath the cord to bond it to the tube, and then even more on top of the cord, in order to totally encapsulate it.

Here you can see a “before” and “after” picture. I couldn’t quite get all the masking tape off afterwards because some was sealed and bonded (somehow I did not foresee this). But the cord is totally encapsulated. When the epoxy cures, it becomes incredibly hard and is similar to plastic.

recovery harness sealed in epoxy — after: sealed in epoxy

The recovery harness here is now thoroughly secured to the motor mount.

A few notes on epoxy, as this was my first time ever using it. It’s pretty straightforward, but there’s a slight learning curve. I used West System 105 resin and 205 hardener: these are two separate products that come in separate containers with pumps. You add them together (in a ratio of one pump each) into a mixing cup, and then mix them together (I used a popsicle stick) very thoroughly, for several minutes.

Once mixed, the epoxy begins to harden and cure much faster than I initially realized. It also gets very hot, from the chemical reaction – to the point where it’s literally giving off visible steam, and the heat from touching the outside of the plastic mixing cup will burn your fingers.

It’s also a bit runny when spreading, so it really helps to create a barrier or dam with masking tape to keep the epoxy where you want it, as it cures. The tape can easily be removed later.

I was previously used to working with wood glue for cardboard rocket sections and plywood fins, but fiberglass is a whole new experience. Wherever fiberglass pieces need to be permanently attached (e.g., the fins to the rocket body), this two-part epoxy is used, and it’s amazingly strong.

Next up: the nosecone.

How to build a fiberglass rocket, part 3: airframe prep

May 21, 2020 improbableventuresLeave a comment

My first couple of posts related to the Darkstar Extreme were just recapping my progress in high power rocketry to date, and outlining everything that’s needed in order to build this particular rocket. But now I’m finally ready to begin assembly.

As mentioned previously, the “kit” basically includes all of the rocket airframe parts (shown below), along with some nylon recovery harnesses and miscellaneous hardware (steel screws, nuts, washers, forged eye bolts, and quick links). The first thing I did after unboxing everything was soak the fiberglass pieces in water for 24 hours, to remove any remaining mold release agent. In other words, as the proud parent of a new rocket, one of the first things you should do is to give it a proper bath.

fiberglass rocket airframe parts, soaking in water pre-assembly — exciting, fast-paced action!

After rinsing off and drying each piece, I moved everything out to the workshop. Time to begin construction of the workshop’s inaugural rocket.

Just to provide some overall structure for what I’m planning to do here: the idea is to assemble the fiberglass airframe, but in a way that allows it to separate at multiple key points in the future. In certain places I’ll use epoxy to permanently attach pieces together, but in several other locations I’ll need to measure and drill holes, and then insert small nylon screws (“shear pins”) which are strong enough to hold the pieces of the airframe together, but which also have the ability to shear in half when sufficient force is applied (e.g. a small controlled explosion), allowing the rocket to separate and a parachute to deploy.

fiberglass rocket airframe parts on workbench — marking the airframe for future drilling

To help visualize how all these pieces go together, the major components of this airframe (from top to bottom, when the rocket is standing vertically on the launch pad) are: the nosecone (grey, with aluminum tip) permanently epoxied to a 6″ coupler; a 24″ payload section; an 11″ coupler which serves as an electronics bay; and a 52″ booster section. There is also a 1.5″ band or ring that fits around the e-bay/ coupler, and six fins (three larger, three smaller). Inside the booster section is a motor mount with a smaller 75mm diameter and 4 centering rings.

fiberglass rocket airframe pieces on workbench, pre-assembly — labeled for your viewing pleasure

Here you can see a lot of measuring, marking, and drilling on the airframe. More specifically, there are 3 holes drilled in the nosecone/ coupler and the payload section, which can then be secured together (and later separated) with shear pins. Another 3 holes and shear pins connect the “bottom” of the e-bay/ coupler to the long booster section. And then three more holes – this time plugged with steel screws serving as rivets – connect the “top” of the e-bay/ coupler to the payload section. These steel rivets ensure the payload section does not separate from the e-bay during flight, but they allow diassembly on the ground by removing the rivets, if needed.

fiberglass rocket airframe parts, pre-assembly — crude but effective sanding technique

Finally, in the middle 1.5″ of the e-bay/ coupler, I marked the location of the band or ring that will be secured to the coupler with epoxy, shortly after this.

Aside from measuring, labeling, and drilling, I also needed to sand many parts of the fiberglass airframe. In general, it’s helpful to sand anywhere that epoxy will be used to ensure better bonding. This includes a few external areas (like the one pictured above), as well as the areas on the centering rings where they will touch the motor mount and the booster section; the edges of all six fins, along with the areas that the fins will touch on both the motor mount and booster section, and so on. Lots of sanding here with coarse (60 grit) sandpaper.

So begins the thrilling assembly of the Darkstar Extreme.

How to build a fiberglass rocket, part 2: specs

May 19, 2020May 28, 2020 improbableventuresLeave a comment

I provided a full list of materials that I’ll be using to build the Darkstar Extreme, but just to offer a little preview on what the completed rocket will look like, here are some of the specs. And the picture below is just an example of the finished version – to be clear, I don’t usually post pictures that aren’t my own, but my rocket will look similar to this once it’s done (just probably a different paint job).

completed darkstar rocket, painted red and black, on green grass — note: not my rocket

Length: 101 in. (about 8.5 ft)
Dry weight: 223 oz (about 14 lbs)
Airframe diameter: 4 in.
Motor mount diameter: 75mm
Altimeter/ flight computer: TeleMetrum
Backup altimeter: TBD
Main parachute: 8 ft diameter Rocketman parachute
Drogue parachute: 2 ft diameter Rocketman parachute
Motor: TBD

I’ve already started construction, so I’ll have a lot more updates coming soon.

How to build a fiberglass rocket, part 1: materials

May 15, 2020May 18, 2020 improbableventures1 Comment

As promised, below is the full bill of materials that I’m using to build the Darkstar Extreme. It’s important to note that, aside from this particular kit, many of the other things in this list are optional, depending on your particular rocket design; frequently, parts or materials can be swapped out and replaced with other similar items.

Rocket airframe

Darkstar Extreme kit from Wildman Rocketry, including:
- Fiberglass booster (52″ length, 4″ diameter)
- Fiberglass payload (24″ length, 4″ diameter)
- Fiberglass coupler (11″ length, 4″ diameter)
- Fiberglass coupler (6″ length, 4″ diameter)
- Fiberglass nose cone (4″ diameter) with aluminum tip
- Fiberglass motor mount (75mm diameter)
- Fiberglass vent band (1.5″ length, 4″ diameter)
- Fiberglass centering rings (x4)
- Plywood centering rings (x2)
- Fiberglass fins (3/16″ thick)
- Aluminum bulk plates (stepped, CNC cut) for e-bay and nose cone
- Misc. hardware (stainless steel nuts, washers, forged eye bolts, quick links)

Aluminum motor retainer (75mm) from Aeropack
Rail buttons (1/4″) for 1010 rail
Sandpaper to sand fiberglass (coarse, 60 grit)
Primer and spray paint

Epoxy

Resin (West System 105)
Hardener (West System 205)
Thickener (West System 406, colloidal silica)
Hobby epoxy
Chopped carbon fiber (1/8″ thick, 1/2 lb)
Syringe to inject epoxy

Recovery

Kevlar harness (1″ thick, 8 ft length)
Kevlar cord (3/8″ thick, 25 ft length, with two loops) from One Bad Hawk, for drogue parachute
Kevlar cord (3/8″ thick, 25 ft length, with three loops) from One Bad Hawk, for main parachute
Drogue parachute (2 ft diameter) from Rocketman Parachutes
Main parachute (8 ft diameter)
Fire blanket (18×18″ nomex) x2
E-matches
Black powder (FFFF)
PVC end caps for ejection charges
Nylon shear pins (2-56 screws, 3/8″ length)

Electronics bay

Fiberglass “sled”
TeleMetrum flight computer
LiPo rechargeable battery
Terminal blocks
Button head screws, 2-56
Locknuts, 2-56
Molex connector kit, 4-pin
Molex connector kit, 8-pin
A23 battery for buzzer
A23 battery holder
Piezo buzzer, mountable on bulk plate
Terminal block for buzzer circuit
Push button switch
Wire for connections (20 awg, “bell” wire)

Motor

TBD – will probably use a J motor for the initial flight

How to build a fiberglass rocket: Darkstar Extreme

May 13, 2020 improbableventures1 Comment

With the workshop newly completed, and a seemingly endless quarantine/ lockdown in effect, it’s time to turn my attention to building a new rocket.

fiberglass rocket parts spread across wooden workbench — ready to assemble

So far, I’ve built and flown a couple of low and mid power rockets, and I built one high power rocket – the HyperLOC 835, which is a 4″ diameter rocket made primarily from thick cardboard and plywood, with a 54mm motor mount. It can fly on an H, I, or J motor, and I plan to use it once launch events start up again for my L1 certification and probably for my L2 cert as well. It also gave me the opportunity to build my first electronics bay and learn more about flight computers and telemetry.

My next project is a bigger high power rocket: the Darkstar Extreme. This one also has a 4″ diameter but it’s made entirely from fiberglass (except for the aluminum-tipped nosecone and aluminum bulk plates). Fiberglass is significantly stronger than cardboard, wood, or other similar materials; it’s the strongest building material for rockets aside from aluminum.

The other chief advantage of this rocket is a larger 75mm motor mount. More powerful motors come in larger diameters, and this rocket can technically fly on a K, L, or even M motor. An M motor would require me to get my L3 certification, a daunting goal, though one that I plan to achieve in the not too distant future. But I could fly it on a K or L motor as soon as I get my L2 cert.

After unboxing this kit and soaking the airframe pieces in water for 24 hours, I’ve laid out the pieces on my workbench and am ready to start construction. The kit only comes with the major pieces: the fiberglass airframe and nosecone, a few aluminum bulkplates, some basic hardware (forged eye bolts, nuts, washers, and quick links), and nylon recovery harnesses.

The kit does not include the motor (of course), any parachutes, fire blankets, a motor retainer, or certain other necessary hardware (nylon screws/ shear pins, steel screws/ rivets, additional metal bulk plates, etc), so I bought those separately. I also splurged on some kevlar recovery harnesses rather than using the nylon ones that came with the kit because kevlar can withstand significantly higher temperatures and won’t burn easily.

I’ll post a more comprehensive bill of materials separately in case anyone is interested.

How to build a rocket workshop (part 10: the resurrection)

May 9, 2020 improbableventures1 Comment

The much-anticipated finale to the workshop saga!

The last few posts covered the electrical work, from digging a trench to installing a sub-panel in the shed and running conduit out from the main panel in the house, and then the official inspection. I also briefly covered the similar process needed to run some ethernet cable out there.

With that work complete, it just left cleaning up and furnishing the inside to create a true workshop.

workshop interior view, with work benches and stool — welcome to the rocketry workshop!

Most of this work was mundane – sweeping up a cartoon-like cloud of dust around myself, using a shop vac to get up sawdust and debris, etc. I also set up a second workbench against the back wall, and a few houseplants just to lend some color to the shop. They get plenty of direct sunlight during the day through that window.

I installed a few shelves (see below) to hold bags and cases of tools and equipment, and got some circular holders to tidy up the multiple 100 ft extension cords. The fire extinguishers were already in the shed, but I decided to keep them around in case something catches on fire (extremely likely).

I also put up a second light fixture overhead (not pictured here), installed some additional pegboard for hanging tools, and a few other miscellaneous things.

workshop interior view, facing open door — shop vac helps tidy up

The primary reason I needed a workshop to begin with was just for more space – some workbench or table area to lay out parts, and measure, drill, cut, sand, glue, and generally build things. With that goal in mind, I can say: mission accomplished.

workshop interior view, with workbench and stoool — exit, stage right

Finally! Just a place to sit and assemble rockets. It’s only about 10×10 ft, but it’s a really practical space.

I may spruce up the inside or outside of this shop more over time, like adding some flooring, building a larger exterior deck/ porch, and so on. I have a few other ideas. But the core goal is complete, and I’ve already begun work on my next rocket, the Darkstar Extreme from Wildman Rocketry. Much more to come on that soon!

How to build a rocket workshop (part 9: tropical storm)

April 6, 2020April 6, 2020 improbableventures1 Comment

Since we had already done all the work involved in running copper wire and conduit outside to bring electricity to the shed, we figured we might as well run some cat 5 (ethernet) cable out there, too, for a wired ethernet connection. I mean, we’d already dug the trench – so why not? An excellent yet rhetorical question.

And as long as we’re running one cat 5 cable, might as well run two. Right?

white door on blue exterior wall, with grey pvc conduit above and to the right of the door — double the conduit

In view of the larger project of transforming the shed into a workshop, I have to concede that this step was really more along the lines of “what the hell” than anything else. I can’t say it was absolutely necessary. In fact, one might argue it was totally unnecessary. I can’t say that I have any immediate plans to use a wired internet connection out there. The wifi signal from the house certainly reaches the shed, if I needed it. I cannot imagine why I would need a wired connection.

And yet… we already dug the trench, which is the type of work that I’d never want to do again. It may be totally unforeseeable now, but in the unlikely event I suddenly need a wired internet connection in the workshop, it seems worthwhile to invest in just a small amount of extra time and effort now, instead of undertaking another huge (and avoidable) project later.

And to be honest, compared to running electricity out there, this was much easier.

open metal junction box with cat-5 cable and ethernet jacks — labeling the jacks for future reference

The PVC conduit (3/4″ this time, easily able to hold two ethernet cables) was laid in the trench about 8 inches above the other 1″ conduit with the electrical wires, still about 10 inches below ground level. Next to the house, the conduit runs up vertically along the wall and then over a door and off to the side, right next to the 1″ conduit. I’ll paint them both blue to match the house, eventually, as well. At that end of the conduit, we ran the ethernet cable through the floor and wall of the house, and installed a 4-jack outlet inside, next to the modem/router.

On the other end of the trench, the conduit came up inside the shed to a single junction box, pictured here. It was basically the same process as the electrical wiring, just much simpler inside the shed with a single piece of conduit and single box.

I labeled the jacks and ethernet cable on both ends for future reference and then slapped a metal box cover and 2-jack wall plate on top. Tested both jacks and they are perfectly functional. Success! I may not know why I did this, but I know that the objective was achieved.

two finished ethernet jacks with wall plate cover — ready to use

This means the wiring is complete and I’ve entered a new phase of this project: beautification. Basically, time to clean things up. Outside, there’s an enormous mountain of dirt and rocks that needs to go back into a deep trench and cover up the conduit. I also need to buy a few (hundred) bags of mulch to cover the bare soil, and make the building’s external appearance look at least marginally more presentable (not dissimilar to goals related to my own appearance).

Inside the shop, there’s some general cleanup to do and a few pieces to put back in place. I also need to spend a little time planning the design for the layout, and where tools and equipment should go. It may only be a small 10×10 ft space, but all the more reason that the layout matters: space is at a premium.

I’ll have a few more pictures and a final update once it’s complete. It’ll be ready for building rockets just in time for spring – a.k.a. rocket-building season.