Monday, March 16, 2015

Mid Power - Building the Quest Big Dog (Part 2)

Click here for Part 1

Back to building the Big Dog, from Quest Aerospace...

After assembling the motor mount, I next decided to mark fin lines and a launch lug line - though I'll be using mini or micro rail buttons for this rocket.

If you're new to rockets, here's what I mean. Launch lugs were the standard for all hobby rockets until not too long ago. But in the last - I don't know for sure - decade or so, a lot of rocketeers have switched over to rail buttons. Instead of a thin, wobbly launch rod, which can suffer from wire whip if it gets too long for its diameter, mid and high power rocketeers often use something called t-slot rail to keep a rocket upright on the launch pad.

One of the most common rails used is a square aluminum extrusion, used to build all kinds of modular industrial things, with a cross-section that looks like this:

Instead of launch lugs, two (or sometimes more for really big rockets) small buttons are attached to the rocket, and they slide down into the slots in the rail. The result is a really sturdy launch platform.

Launch button (bottom) fitting into a rail slot.
Image from
Sorry to digress. Back to the subject at hand.

I cut out the paper fin guide from the instructions, I carefully lined it up around the tube and taped it in place. There was a slight gap between the fourth fin line and the edge of the paper guide, which I figured was intentional. I marked each fin line with a little pencil dot.

As always, I decided which end I wanted to be the forward and which end the aft, and marked them so as not to
forget. I don't remember how I chose on this rocket, but it may have had to do with how the centering rings fit.

Estes fin guides have an additional line, the launch lug line, exactly between two of the fin lines. The Quest guide didn't have that. In order to get the launch lug line exactly spaced between two fins, I folded the guide over and matched up to of the fin lines, then put a pencil mark on the fold.

Then I attempted to match up the fin lines with the little pencil dots on the airframe, and that's when I noticed a flaw in the kit - the dots didn't line up with the lines. I can only have noticed this because I must have laid the guide down between the first and fourth fins. I double-checked the lines with the other dot, and they all matched up. Something was off here!

A few months ago, I bought a digital caliper, because it was inexpensive, and because I figured I'd use it for... well, something. In any case, I like having precise measuring tools.

I got out the caliper and measured the diameter of the body tube. The box claimed the airframe was 1.92 inches. The caliper told a different story - 1.97 inches!

I knew this thing would come in handy!

I opened the free RockSim file for the Big Dog, available from Apogee Components. I don't have RockSim, but OpenRocket will open those files. I saw that, according the the sim file, the rocket should have an outer diameter of 1.97 inches. That's not much of a difference, but if the paper fin guide was made with a smaller diameter in mind, you'd get fins that are ever so slightly unevenly placed - all too close together, except for the first and fourth fin, which would be slightly too far apart. You might not even notice the difference unless you looked closely, but I look closely. It would have bothered me.

So, I used OpenRocket to print off a new fin guide - with an additional advantage that this one would already have a launch lug line marked.

I taped this guide to the rocket, and everything matched up correctly, so I marked all the lines.

Finally, I grabbed a length of aluminum angle and extended all the lines.

Normally, I extend the launch lug line all the way down the body tube, so I can sight along it and verify that I have the lug on straight. I keep the fin lines rather short. But, due to probably a little sloppiness on my part, I've gotten a few fins on not quite perfectly straight when using the Guillotine fin jig lately. The Guillotine is a great tool, but like all tools, you must use it with care.

Just to be sure I'm getting the fins on true, I extended the fin lines most of the way down the body tube. That way, I can check the fins are on the same vertical line on both ends of the fin jig before gluing them down.

You can see that before I did all this, I marked the tube spirals with a pencil so I could see the lines clearly when I filled them in.

The spiral groove on this rocket is really tight, and I had trouble in spots keeping the pencil tip in them. In spots, I couldn't even feel the groove with my fingernail. It's possible I could get away with just priming the rocket with a good coat of filler primer, but I don't want to get to the end of the build and see grooves. So I filled them in.

I didn't take any pictures of this process, but I just blogged about how I do it.

I filled the grooves, let them dry, then lightly sanded.

Next, I installed the motor mount and began airfoiling the fins. I'll talk about that in the next build post on this rocket.

Click here for Part 3.

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  1. Here are a few tips about using the Guillotine Fin Jig. I have found it helpful to mark the fins with double lines, spaced according to the actual fin thickness. (That tip was given to me by Doug Sams). As you have already discovered, body tubes are not always exactly the same diameter as specified and the paper wraps that come with rocket kits do not always line up as intended. Nothing made by the hand of man is ever "perfect" and that includes component parts as well as fin jigs. Personally, I like to make my own paper wraps by directly wrapping the tube in hand. (I did an essay about my technique for Apogee awhile back)

    One of the problems with my jigs is the fact that the so-called "architectural aluminum" angle stock available to me is not always extruded perfectly "square". It is usually slightly less than a true 90 degrees. The costs involved with producing a perfectly milled 90 degree pair of rails would push the cost of this contraption out of the reach of most builders, myself included! (If I ever find a perfectly square section of aluminum angle, I'm saving it for myself)

    The best one can do is to carefully adjust the rails for fin thickness and then rotate the body tube gently with an unglued fin in place to register with the pair of fin lines. Once this step is done and visually checked, DON'T MOVE ANYTHING! Apply the glue to the root edge of the fin, set it in place and allow it the fully set before going on to the next fin.

    Another trick is the use of a good framing square, or better yet, plastic architectural drafting triangle to check the fins for radial alignment. This is done by setting your jig on a flat surface and visually checking to see if it is perpendicular by also placing the square or triangle against the fin and the flat surface.

    I hope this helps both you and your followers.

    1. Thanks for the tips, Ted! I've found that if I rush things, I make a mistake. And on more than one occasion, I've raced against the clock to get a new rocket ready for a scheduled launch day. These ones usually turn out messier than I like.

  2. I also use aluminum angle stock for marking alignment lines. Another good use for it is when you have to join two body tubes together; after gluing the two tubes together at the coupler, I'll roll the tubes across the top of a table a couple of times to get things worked out to straight, and then rest the tubes in the aluminum angle, gently holding them in place with blue painter's tape, as they dry. Straight as an arrow!

    1. That's not a bad tip! Getting coupled tubes straight can be a challenge sometimes, particularly with a shorter coupler.