Building the Public Missiles XCalibur for HMC E80

Pictorial Outline of the PML XCalibur instructions, and all modifications necessary for E80, and only one real mistake. 

* also the title is wrong, it's an XCalibur, but the photo below shows a phobos (I don't have any good XCalibur pics), and I was looking at it when I saved the file. Every step in this tutorial applies equally to the Phobos and the XCalibur (or any other PML single-stage non -minimum-diameter non-CPR kits), so it doesn't really matter. 


I suggest reading the entire tutorial once through first, as it is self-referencing in many places. 

Sand both ends of the motor mount tube (MMT), for about an inch. Any sandpaper coarser than 150 grit or so will do just fine. Sand down until the centering rings fit snugly, or until the color changes as in the first image, whichever requires more sanding. 

Take the HAMR and slide it onto one end as far as it goes. DO NOT GLUE IT TOGETHER RIGHT NOW. Instead, use a pencil to mark the extent of the MMT which is taken up by the HAMR. Remove and set aside the HAMR. Label this end as the aft or bottom end of the MMT and mark it, so that you don't screw it up later. 

EDIT: You should probably sand the entire MMT like this, actually. 

Mix up some epoxy-about a full teaspoonfull. Using a Popsicle stick, smoothly butter up the end of the tube (the front end, not the aft end we just marked for the HAMR). 

Thread the fat shock cord (the one in the piston kit) through the notched centering as shown in the first picture; about 2-3 inches should be threaded through. Slide this assembly over the epoxied-end of the mmt as shown in the second image; use the tip of your popsicle stick to make a fillet around the front and back of the ring, as shown in the third image. 


A NOTE OF FILLETS: Fillets are basically corner radii between internal corners. Imagine putting two pieces of wood at 90 degree angles, and dragging the tip of a popsicle stick or some other round object along the corner; the round shape in the corner traced out by the two surfaces and the round tool is the fillet. The aesthetic quality of a fillet is directly related to its strength; smooth, uniform, and graceful fillets have no stress concentrations and maximum surface area for bonding. 

The trick to making a good fillet with a popsicle stick is muscle control and patience. It's difficult to explain. When making a fillet, move your hand in a continuous smooth motion; if you find yourself making erratic or short motions, or jabs, trying to move epoxy around, you're just going to make a mess and a weak and ugly fillet. The last time you touch the fillet should be a single uninterrupted constant-speed-and-pressure stroke with the popsicle stick.  The fillet shown in the third image here is not actually very good; the lumpiness betrays my haste. 

Step 14 has some good pictures of fillets on the fins. 

Smear all of your remaining epoxy under the tail of shock cord that is hanging out. If you have extra, build it up around the shock cord on the front side of the centering ring. Use blue tape to crush the shock cord down onto the MMT. 

IMPORTANT: What I failed to show was how to know how far back to place that front centering ring. What you need to do that I neglected to take pictures of it set the MMT assembly next to the airframe, parallel on a table or something. Line up the HAMR mark with the back end of the airframe (the side with fin slots); the forward (notched) (this one) centering ring should placed such that it just barely doesn't overlap with the fin slots. Said another way, the back of the centering ring needs to be in line with the front of the longest fin slot.   Steps 11 and 16 have illustrations that might be helpful. 

While that is drying, take the payload bay tube, the solid disk of plywood (called a bulkplate), and the piece of brown phenolic coupler tube that is not part of the piston (the longer one). These are assembled together into the payload bay. 

First, sand out the inside of the payload tube, about two inches in. The gray plastic does not chemically stick to epoxy, so the only traction we are going to get is the amount of roughness we give it by sanding. BECAUSE OF THIS, SANDPAPER OF 80 GRIT MAXIMUM IS RECOMMENDED. 60-Grit should be stocked in the E80 lab for this purpose. Rough it up as much as you can. 

In addition, sand down the phenolic coupler. The OD should be sanded until it makes an easy slip fit into the airframe and payload bay. In addition, the ID should be sanded in about half an inch on one end; the bulkplate should fit snugly into this end. 

Take the bulkplate and the eyebolt, with its two nuts and washer. Assemble them as shown here. Tighten the nuts as tightly as you can (wrenches are recommended) (if you go to the shop for wrenches, FOR GODS SAKE PLEASE DON'T WALK OFF WITH THE WRENCH) , but you do not need (and should not) epoxy the nuts in place. They're not going to come loose; the plywood acts as a lock washer. 

Mix up some epoxy, about a large garbanzo bean worth of both parts. Using a popsicle stick, butter up the inside of the phenolic tube. You want epoxy smoothly spread on the entire inside surface of the payload bay that you sanded out already. 

With a twisting motion, slide the coupler tube in. The twisting should spread the epoxy around evenly.

You will be very happy later if you take the opportunity now to clean up the shoulder. Take a piece of paper and use the edge of it to scrape off any epoxy which beads up on the exposed interior corner, as shown in the third image. This will let the two halves of the rocket fit snugly against each other.  

Butter the sanded ID region of the coupler with epoxy. Save about a pea's worth. Take the bulkplate and, holding onto the eyebolt, slide it into the tube until it is slightly recessed. Again, a twisting motion ensures the epoxy spreads out nicely. Insert it such that is is recessed by about 3/16 of an inch. 

Use the tip of your popsicle stick to lay a fillet with the last of your epoxy around the interior corner created by the recessed bulkplate, as shown in the second image. 

If the bulkplate is loose, it will be necessary to hold the bulkplate straight while the epoxy sets up. If it dries crooked, it is likely to break when the parachute deploys (if the parachute deploys roughly), dumping your electronics (and grade) on the desert floor. 

Note: the fillet shown in the second image is an excellent example of what all fillets should look like; smooth and continuous. 

Take the dried motor mount, and stuff the shock cord into the tube. Ideally you can wad it up such that the entire cord is stuffed inside the tube. 

This is where I screwed up, sort of. In the next images, just pretend the shock cord is stuffed inside the MMT. 

Get out the 80 or 60 grit sandpaper again; it's time to sand more! 

Use blue tape to mask off the everything on the body tube except for the area around the fin slots. I'd leave about 1/4 of an inch all the way around the fin slots. I tried to do a before-after set of shots; what I have illustrates the BARE MINIMUM of sanding required for a sturdy bond. As usual, the more the better. In addition, sand the inside of the body tube as much as you can, as deep as you can. I found that the wood rasps in the shop were well suited to scarring up the inside of the body tube for the full depth of the fin slots. 

Note: I masked the living daylights of my model, because it had a clear tube and I wanted it to be transparent when it was done. It's not necessary, but it makes it look much prettier. At the very least, i recommend putting tape along wither side of the fin slot 1/4" away; this will help make the fillets look good later on. 

Tape or otherwise attach a popsicle stick to a thin dowel rod (there should be some in the E80 lab for internal fillets). Mix and pick up a goodly chunk of epoxy like this one. Carefully spread it around inside of the airframe at the forward end of the longest fin slots, where the centering ring will need to go (second image). 

Insert the MMT with wadded up shock cord into the airframe, with a twisting motion to spread around epoxy. Line up the centering ring with the fin slot as shown, like we did back when we glued the centering ring on in the first place. 

(Again, pretend that the shock cord is wadded up inside the MMT.) 

MAKE SURE that the shock cord epoxied on the outside of the MMT is not underneath a fin slot, or the fin won't fit in properly. 

Finally, slip the aft centering ring over the aft end of the MMT (last picture). Leave it exactly as shown; it should be slightly inserted into the airframe, NOT EPOXIED IN PLACE, and most of it should be exposed still. The purpose of this is to hold the motor mount true (concentric and colinear) with the airframe while the front-end epoxy cures, but removable to allow for internal fillets. 

While that dries, let's sand some more. I'm sure by now you're bored with sanding, so let's add a new complication: carcinogens! 

The fins are made of a material called G10/FR4 Garolite. It is a phenolic-resin fiberglass. Sanding fiberglass produces particulates that, depending on their size, can cause cancer or just muck up your respiratory system. That's why it's important that WHENEVER YOU SAND FIBERGLASS, SAND IT UNDER RUNNING WATER. This is called WET-SANDING, and it is a standard safety practice. 

You want to use something a bit finer on G10, probably like a 150-grit, or even a 220-grit sandpaper. First, for your own safety, you want to round the edged of the fins. This is just a splinter-prevention measure, although the rounded smooth edges of the fins add a nice professional touch. Don't try to sharpen the fins; they'll just degrade as it lands repeatedly on the hard desert floor. I tried to capture this process in the first two images. 

Then, we want to sand for epoxy bonding. Masking is not necessary but I like it, it looks good. The tape is laid 1/4" up from where the airframe will meet the fin, which is identified by the corners where the rectangular fin tap is attached to the triangle fins. Sand into the fiberglass until you can see this pattern when you dry the fin off; it doesn't help to go further. 

Take the fins, and dry-fir them in place. If they do not fit (some forcing is ok, but if a good hard wrap with the palm of your hand on the fin doesn't make it settle in place it's too tight), then sand them until they do. 

When you are satisfied with the fit of all fins, mix up some epoxy, like a teaspoon worth. Using the edge of the popsicle stick, try to shove epoxy down the slots. It is also possible to force epoxy down into the slot by spreading epoxy over the slot and then squishing it with the flat of the popsicle stick tip. In addition, use the flat of the popsicle stick to lay a big, healthy bead of epoxy along the bottom of the fin*; this will also help. Do this for all fins and fin slots. Then, insert all the fins, making sure that they are all the way down against the motor mount. 

Sorry, but this was a two-handed job, and it wasn't easy to take pictures of. 

*spread some epoxy on the flat, lay the flat on the bottom edge of the fin, at a 45 degree angle to the plane of the finn and drag the stick both along the length of the fin and toward you across the fin. This should cause epoxy to accumulate in a bead along the edge of the fin. 

Mix up some fresh epoxy. Laying the external fillets is easy. Dip the popsicle stick into the spoxy, and pull out a small glob; carefully lay it down in the corner of a fin and the airframe; and in smooth dragging motions, draw that glob along the corner in both directions, until there is a smooth and continuous fillet between the airframe and the fin side. DO this for all fin side-airframe interfaces (sorry XCalibur teams :( )

I have added a white dye of sorts to the epoxy in these images, to make it easier to see. 

If you have masked off the areas around the fillet, you can reap an added benefit in this step. After your fillets are laid but before the epoxy is dry, you can peel the tape off, leaving perfectly-smooth-sided fillets, as shown in the last images. 

Note: I made these fillets (well, Carlo Vaccari did, but that's a technicality) with a finger. I actually prefer using a popsicle stick, but either work-the popsicle stick is less messy and easier to keep a correct shape with, but it's harder to make the ends look nice. 

Internal fillets are really hard on such a small rocket, but the difference the make is significant, especially if you intend on flying this rocket on larger motors post-E80. Unfortunately and unhelpfully, I have no pictures of the process. Once again, it is hard to photograph a 2-handed job, and this time Carlo wasn't around to model for me. 

First, remove the aft centering ring by grabbing it with your fingernails, or failing that, the edge of a knife, and wedging/pulling it backward until it slips off. 

What I do for internal fillets on such a small rocket is to mix up a bunch of epoxy (like a tablespoon), take a long dowel (like the one pictured glued to a popsicle stick in step 11) , and as quickly as possible, pick up epoxy and shove it into the hollow next to the fins. Twist the dowel as you do this; it will force the epoxy into whichever corner (fin-airframe or fin-MMT) and make nice curved fillets. 

They don't need to be especially large, mind; if they're there at all, it'll help a lot. 

Now, let's glue in that last centering ring. Smear epoxy up into the gap; it doesn't hurt to smear it around the fins themselves also. Then just slide the aft centering ring in until it rests against the fins. The pencil line we made way back in step 1 should now be just visible ; the HAMR will seamlessly press against the aft centering ring, for a neat and professional look. 

Use sandpaper to rough up and clean up the exposed end of the motor mount  in advance of putting the HAMR on. 

Mix the JB-weld. Squeeze out two small beads of JB-weld, one of each part. They should both be about the size of small peas. I mixed WAY TOO MUCH. 

Spread the JB weld smoothly around the exposed MMT, as shown. Take the HAMR (with the actual knurled ring on) and slide it on over the epoxy (you can put some more epoxy in the HAMR as well, where it touches the MMT). Again, a twisting motion will spread the epoxy evenly. It should fit up against the aft CR. It's ok if it doesn't, though, it just doesn't look as good. 

What's important now, though, is to clean up the JB weld. It likely has left a few small lumps inside the MMT. It is important to clean these off now, as the JB weld dries very hard and will be difficult ot remove later. DO NOT SMEAR IT AROUND TRYING TO REMOVE IT. This will make it even more difficult to clean up afterward; it will also potentially make it impossible to put rocket motors in the rocket. Instead, use a knife or the edge of a piece of paper (like in step 6)  to remove the excess epoxy. 

Launch Lugs are pretty straightforward. Take a big giant long piece of blue tape, and lay it down the length of the airframe (between fins, obviously). Peel it up and lay it down again as many times as it takes to get it totally straight; look down along the body to get it straight. 

After that, you can mark the location of the aft lug by placing it on the tape, about an inch above the aft end of the rocket. Tape around it on all four sides, then peel out the original tape inside the rectangle of tape you just added.  Use your 60-grit sandpaper to rough up the tube in the rectangle you have exposed; also use it to rough up the tube inside the tape rectangle. Also rough up the underside of the launch lug. 

Mix up some epoxy, probably like garbanzo-bean sized lumps for each lug. Smear it all over the bottom of the lug; then stick it down to the airframe. Look down the length of the body tube to judge the straightness and alignment of the lugs; if you're not confident in your ability to align both at once, do them one at a time, waiting for the epoxy to cure in between. 

Take a stick and shove the shock cord out of the motor mount. If you hold the airframe upside down, the shock cord should hang down out the forward end of the airframe. 

Take the cord, and thread it through the bulkplate-like thing with the slot in it and the D-ring as shown, and epoxy and clamp as shown. Wait for the epoxy to dry. 

Sand out the inside of the piston coupler tube so that the bulkplate thing from the last step sits pretty snugly inside. You want to now take epoxy and butter up the inside of the phenolic tube as shown; Note how the bulkplate has already been passed through the piston. This is deliberate. 

Using the D-Ring and the shockcord, pull/push the piston cap bulkplate thing into the piston, sliding and twisting up across the epoxied section. It should be recessed by about 3/16 or an inch. A nice bead of epoxy should build up on the inside; this is why we threaded the shock cord through piston in the first place, because now we don't need to make an internal fillet. 

Use the last of your epoxy to make a fillet around the inside of the piston, as shown. Also, make a fillet around the shock cord fold for good measure (not shown.)

There are three knots to be tied to finish the rocket. The method for these knots is illustrated in the PML instructions. It is generally not recommended to epoxy the knots, as it is very nice to be able to take the parts apart again. If you're having trouble getting the knots tight, ask for help from a proctor. 

Knot 1: Skinny shock cord to the piston D-ring, at one end of the skinny shock cord. 

Knot 2:  Shock cord to parachute, about two feet from the far end of the skinny shock cord, away from the piston. 

Knot 3: Skinny shock cord to eyebolt on the payload bay. 


The nosecone will also need to be riveted to the payload bay airframe. Make a 5/32 hole through both the nosecone and the payload bay airframe at the same time (hand drill is the easiest); drive a rivet through the hole (rivets in the E80 lab), use a sharpie to draw a short line from the nosecone over the seam and onto the payload bay; then, leaving the first rivet in, drill two more 5/32 holes, as close as possible to 120 degrees apart, and as close as possible to the same longitudinal position, just so your rocket doesn't look dumb. Unless you use mechanical assistance (which isn't necessary), they're not going to be perfect, and only the orientation that the rivets will fit in is the one you just marked with the sharpie. 

A note about painting: You will need to use masking tape to mask off the launch lugs before you paint anything, or the paint will prevent the rocket from fitting on the launch rail. ALWAYS USE SPECIALTY PLASTIC PRIMER when painting the rockets; otherwise the paint just flakes off. In general, the closer you follow the instructions on the paint, the nicer the paint will come out. Try to avoid mixing paint brands.