Wednesday, January 14, 2015

My First Kit Bash - The Cosmic Explorer (Part 1)

I've started building a new rocket, and it's a rocket I've built before - the Estes Cosmic Explorer, one of my favorite flyers in my fleet.

Instead of doing the standard build, as the kit instructs, I'm doing my first kit bash. For rocket n00bs, a kit bash is when you take an existing rocket kit and modify it to achieve a desired effect. This could be any number of things - making the rocket shorter or longer, adding a payload section, adding things to the exterior to make it look different, or making it look more like a scale model - any number of things.

In my case, I'm going to modify it so it takes a larger motor - in this case, an E-sized motor.
A selection of motors. The second and third from the left are standard 18mm motors, the size that normally go
into this kit. The one on the far right is the 24mm E motor which will go in this rocket when it's done.

The standard motor for most kits of this size is 18mm in diameter and 2.75 inches long. An E is 24mm in diameter and 3.75 inches long. The rocket body is about 1.33 inch in diameter, just big enough to accommodate the larger motor size and still have a little space on the sides.

This going back and forth between metric measurements and English measurements may seem confusing, but it's pretty common in American rocketry. Everything would be easier if it were all metric, I know, but most kits are sold with parts in fractions of inches. Motor diameter, however, is almost always stated in millimeters.

Edit: It has been pointed out to me that what I'm doing here is not, technically, regarded as kitbashing. This is one of those things you won't really find in a dictionary, but depending on whom you ask, a kitbash can be simply modifying the model to perform differently than intended by the manufacturer (this is the definition you'll find on Urban Dictionary). But in model rocketry, kit bashing (it can be one word or two) is generally regarded as taking the parts of one kit and modifying them so that the result is a rocket which looks like a different, existing kit (the Wikipedia definition).

Regardless, I want you to see how you can modify a kit to suit your needs, and that modification can be major or minor.

Part 1 - Parts, and Assembling the Motor Mount

The Package

Parts. Top row: two body tubes and a tube coupler. Middle row: Fins, nose cone, 18mm motor tube.
Bottom row: Centering rings, parachute, motor hook, shock cord and decals. At the
very bottom is an errata correcting critical errors in the kit instructions.
The rocket has an 18-inch long BT-55 airframe. BT-55 is the Estes standard measurement of a tube which is about 1.33 inches in diameter - its designation is now used by other rocketry companies as well. A tube coupler is a slightly narrower piece of paper tubing used to join two lengths of body tube to make a longer rocket. It gets glued with half its length inside both tubes.

The fins have fin tabs on them. If you look at the aft body tube, it has slots in the base. This rocket has what's known as through-the-wall or TTW fins. They go through the wall of the airframe and are glued directly to the motor tube inside. You find this a lot in larger rockets, and it adds strength to the connection between the airframe and the fins - they're not likely to snap off on landing if they're TTW fins.

The blue motor tube will be set aside and saved for another rocket. I'll replace that with a larger diameter tube. Because of this, of course, I'll need to trim those tabs on the fins - they're long enough to reach that narrower tube that comes with the kit, but too long if you have a wider tube inside - they'd stick outside the rocket!

From the bag of stuff, I'll only use the parachute, shock cord and decals. The centering rings and motor hook are for a smaller motor, so I'll save those for something else.

A closer look at the fins

The new motor mount parts
 I'm replacing the 18mm BT-20 sized tube with a 24mm BT-50 tube. This, like all tubes, can be used for an actual airframe, but it's also the standard size for a D or E black powder motor. The tube, centering rings, and longer motor hook I bought at - a great source for parts. Even if you're not building your own designs or hacking a kit as I'm doing here, it's good to have spare parts - to replace something that gets crushed in shipping or happens to be missing from a kit!

When I built the Quest Quadrunner, this came in handy - it was missing two thrust rings! These go inside the motor tube to keep the motors from shooting out the top of the rocket! Fortunately, I'd purchased a twenty-pack of BT-20 thrust rings from

The blue rings came with the kit - but I needed four! The brown rings came from
Thankfully, many things in model rocketry are of a standard size.
Estes ships kits in a cellophane bag you have to cut open, but Quest uses sturdy cardboard boxes which you can close again. To keep the parts safe while I take my time building this kit, I put the parts in the box in which the Quadrunner came. Just tape the label card from the kit you're building onto the old box, and you won't forget what's inside!

I needed to cut the BT-50 to size - 3.75 inches long. When cutting a tube, it's a good idea to have something sturdy inside to support it, so you get a cleaner cut. I have a 4-inch long BT-50 coupler, so I slipped that inside.

Tube couplers are quite useful things. Not only can they be used to join tubes to make longer rockets and support tubes as you cut them, I have used them to join scrap tube to rockets while painting them, to avoid getting overspray  inside the tube. This turned out to be really important when building my two-stage rockets, because you join those temporarily together for flight - again, with a tube coupler - and having paint or primer inside makes the stages stick together. You can buy an assortment of tube couplers - and centering rings, tubes - all kinds of parts - on websites like and - another well-reputed rocketry supplier (but one I haven't used myself as yet). I like having a box of spare parts - then I can fix something, or spontaneously take on a project like this one.

One of the few Estes accessories I really like - the tube cutting guide. These come in a number of standard tube sizes, and they go onto the tube and lock in place. They help you cut tubes cleanly, or mark a line around the circumference.

The cut has been made - not perfect, but much cleaner than I'd have gotten doing it freehand.
I need to get better at this, I admit...
 Above, you see the Estes Tube Cutting Guide in action - or, pre- and post-action. I wish I had a set of these in metal - they're great, but it is easy to accidentally shave off a bit of the plastic with a sharp hobby knife, leaving a jagged edge for next time.

Because of this, I always use the blank side for cutting, and the side that says "Estes" and the BT-number for tracing lines around the tube.

Next, I mark a line for the motor hook to be inserted, 1/4 inch from the end. Since the tube and motor are both 3.75 inches long, this means the motor will overhang the rocket by 1/4 inch. You use your hobby knife to cut a tiny slot there to insert the motor hook, just like we did in Part 3 of the Skill Level 1 series, Building the Big Bertha.
A thrust ring for the 24mm motor tube also happens to be the same sized ring when you put an 18mm (BT-20) motor into a BT-50 rocket. You see how many of these tubes and rings have multiple applications? A rocket can be 24mm in diameter with a 18mm motor, or 18mm in diameter with a 13mm mini-motor. The tubes and rings serve multiple functions.
Thrust ring - again, from JonRocket.
 Just like in the standard kit, you insert the end of the motor hook into the slot. I've marked a line on the tube where it'll be OK to tape down the hook.

In many Estes kits, as in the Big Bertha, there is a Mylar ring which holds the hook in place. If you look at the fin tabs above, you'll see they have a little indentation to accommodate that Mylar ring.

Since I'm using my own tube, I don't have a ring of that size. But tape works quite well - and it's thinner, so I don't need to worry about cutting an indentation into the fin tabs when I trim those down.

 Once the motor hook was in place, I glued in the thrust ring just above, or forward, of it. As with all my low power rockets, I'm using wood glue, because it's nice and strong.

Put a ring of glue just inside the end of the tube where the hook was inserted, and place the ring inside. Press it down on a piece of wax paper on the table or cutting mat - this will make sure it goes in straight and evenly, and the wax paper prevents your getting glue on your work surface.
 Here's where I made a slight mistake, which we'll deal with later. In order to get a nice, tight fit, I marked on the motor tube where the fin tab should go - the idea was to get the centering rings right up to that line, so that the fins would be held in place not merely by the glue on the tab, but from the friction between the centering rings.
I cut tiny slots in the centering rings to leave space for that motor hook, then glued them in place. I was only going to use two, but if you look closely at the picture below, you'll see that I damaged the forward ring slightly - the one to the left of the blue tape in the picture.

To make up for that, and to add strength and make sure I got the motor tube well centered, I added a third ring. With a longer motor mount like this, and such narrow rings, it's probably not a bad idea. It's very common in high power rocketry - it adds strength to the whole mounting system. So I did it here.

This camera tends to exaggerate perspective a little, but if you roll the motor mount across the table and it goes straight, your centering rings are straight enough.

This kit will take me a while. I'm also starting my first mid power rocket - the Quest Big Dog. And I have a few longer, technical posts I'm working at for this blog. So this project won't be completed on this blog for a little bit. But I'll keep you updated, and of course I'll share the launch video with you!

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