Building the Janus II - Part 1 - Parts!

Man, I am beat! Long day at work... It was my plan today to start a post for rocket n00bs on launching your first rocket, with some basic information on rocket motors - or engines, if you like to call them that (both seem acceptable, but a lot of people will tell you that "motors" is technically more correct).

But since I'm writing this blog partly for people just getting into rocketry, I want to make sure that the information I'm giving you is correct, and so I need to do a little fact-checking, and also take a few pictures, and after the work day I had today, that's not gonna happen tonight.

But I did want to show you what I'm up to. I'm building the second edition of my Janus series of scratch-built rockets - the Janus II.

Janus II - a larger, 3-finned version of the Janus I. With one D and one E motor, this should go more than twice as high.

If you've been reading this blog, you know that the Janus I prototype had to be retired after one flight, due to some damage - partly inflicted on the flying field (centering rings), and partly inflicted at home (two broken booster fins). It's now sitting on a shelf with my other rockets, and I'm keeping it on the shelf as a display model.

Janus I before glorious flight and tragic fin damage

I was never really keen on the paint scheme, anyway - I was in a rush to finish it by our scheduled launch date, and it flew really well - I was pretty proud to have designed and built this rocket, and even though I went for a bit more complicated with my first one - two stages, and I'd never even built a two-stage kit before - it worked exactly as I'd hoped.

But I've decided to raise my ambitions a little further. You may have read my mentions of a recent rocketry book, Make: Rockets: Down-to-Earth Rocket Science by Mike Westerfield. This book is great, but I think, for me, the biggest thing it did was to make me feel like I could actually build my own rockets, without necessarily needing a kit. I think if I hadn't read this book, I'd still be a long way from designing my own.

In the book, Westerfield shows how to build a camera payload to go on one of his series of boosters with the name Ceres. Here's an OpenRocket image of the Ceres B booster with what he calls the ICU2 camera payload bay:

I decided to build this payload bay and add it to the Janus II. It's a payload bay that will fit onto any BT-60 rocket. The BT-60 is a standard size paper tube (the name "BT-60" is actually an Estes thing, but a lot of rocketry suppliers now use the BT-whatever to describe their body tube sizes, because they're understood by everyone) about 1.64 inches in diameter. The Big Bertha has this size tube, so this payload could go on top of that rocket - thought it might be a bit heavy for the Bertha.

Anyway, it's a nice, fat tube for bigger low power rockets - though not the biggest, certainly.


I was building the Ceres B booster, but I had a mishap when I tried to install my own homemade ejection baffle - this is a device that blocks burning propellant from the rocket motor ejection charge, so that you don't need to use recovery wadding (if you're a rocket n00b and don't know what I'm talking about, we'll cover it in the "Launching Your First Rocket" post I promise is coming soon - or you can read this post of mine for now). Also, according to the OpenRocket simulation, this rocket may be a bit overstable. Meaning it can turn easily into the wind, which can be a problem! (I'll have some video up on my Youtube channel soon of this very thing happening to my Big Bertha on a breezy launch day.)

I could have fixed this problem by either shortening the length of the airframe, or making the fins smaller, or maybe even adding cool-looking smaller fins up closer to the front (front fins are not a good idea, unless you've got good sized rear fins first - we'll talk about all this stability stuff in an upcoming post). But I had another idea...

Since the Janus rockets are built from BT-60 tubes, I decided, "Hey! Wouldn't a cool idea be to put a payload on top of the next Janus rocket?" I like the idea of taking what I've done and taking it further, so this is what I've decided to attempt next.

Janus II - three fins per stage instead of four, larger motors, an airframe two inches longer than Janus I,
and with Mike Westerfield's ICU2 camera payload bay. Check out his book -
it really is what gave me the confidence to start designing my own rockets.

The camera will be pointed downward, and hopefully will see through the fins - perhaps even see the first stage fall away.

Anyway, I recently got a huge box of stuff from JonRocket.com - I love this website! You can find most components you need for low power rockets, and it's all really reasonable - and the more you buy, the cheaper shipping is (flat rate for anything up to $40; free shipping for over $40). I assembled all the parts I'll need for Janus II with Camera Payload Bay. Here they are:

Design sheets - these give a picture of the rocket design, plus a list
of all components you will use and their sizes, as well as templates for
cutting things like fins and centering rings - even a nose cone template, if
you want to make your own - and have the know-how. Both OpenRocket
and RockSim - sold by Apogee Components - let you print these.

All the parts I will need to make the Janus II and the payload bay - plus some options

Here's a breakdown of the parts.

Payload Bay

BT-80 (2.6 inch diameter tube - we'll use a 3-inch length of this); balsa transition (used to connect a BT-60 and BT-80 tube - they come in many different sizes, but that's what we'll use in this case); 2.6-inch diameter rounded nose cone (from Estes); parachutes (this rocket will have two - one for the rocket body, and one for the payload, which will descend on its own chute - but I haven't decided what size I need just yet. I have several.); tiny video camera; screw eyes (for attaching the balsa transition to the parachute - not sure these are the right size. I'll have to do some reading)

This transition will be at the bottom or aft of the payload section, and is what will hold the
camera. It will be split in half, and a small area for the camera will be hollowed out with a
Dremel tool near the outside wall, with a hole for the lens to see through. This will be
pointed downward, toward the ground, and if I can get it to the right
angle, hopefully you'll see the fins in the video.

So tiny video camera!

The Rocket Itself

BT-60 for both the booster and sustainer (main rocket) body; BT-50 (perfect for a 24mm "D" or "E" motor tube. This will be cut to length - you can buy them ready cut, but this is cheaper, and you can make a whole rocket from one tube); 4 centering rings (two for each section of the rocket - you can cut your own, but I bought these at JonRocket.com - I'm not good at cutting my own - circles are tough!); thrust ring (also used as a centering ring for a BT-50 rocket with a BT-20 motor mount - that's for a standard A-C sized motor you see in most low power rockets); tube coupler (for joining the stages together); Kevlar shock cord (I might need to attach some elastic to one end to prevent the body tube ripping from the ejection - more on this when I post about building this rocket); 3/16 inch launch lugs (this rocket needs a bigger launch rod than your standard Skill Level 1 kit like the Big Bertha); parachute; 3/32 inch basswood (for fins - the Janus I had 1/8 inch fins. I wanted to make these a little thinner to reduce weight and to take less time sanding - because I will airfoil these, and I'll show you how); 1/8 inch basswood (we need to make a half-inch "standoff" for the launch lugs - because the payload bay is fatter than the body of the rocket)

So, that's it for now. I started building this rocket last night, and I have to say, construction goes pretty quickly on these once you have some rocket building experience. The part that takes the longest is the design phase - because you do need to put some thought in to it.

Now, for you rocket n00bs out there reading this - the idea of designing and building your own might seem daunting, I know. Two months ago, I never would have thought I could do this. But I intend to help walk you through the process of designing and building your first scratch build - something simple, that, as long as you feel comfortable putting together a few kits, you can do.

For now, though, I just wanted to show you one of my latest projects. I can't wait to show you the video.