Scratch - Cheaper Than a Girlfriend {Scratch} Original Design / Scratch Built

Contributed by Brett Keller

Manufacturer: Scratch
Style: Sport

Photos by John and Brett Keller

Project Overview

[Rocket Pic]The Cheaper Than a Girlfriend (C.T.A.G. for short, pronounced See-Tag) was my consuming obsession for about 2 months. It is 4 inches in diameter, 50 inches long, and 36.5 ounces in weight. It flies on Aerotech Single-Use G80-4T motors. All the details of the original ideas, building, and the first launch, are include in this article.

Research

I really had no idea what I was doing. I didn't go for a kit because of the cost. So, I turned to the only really good resource I had: the Internet. I would like to thank all those on the Rocketry Online Chat room for the countless discussions on the various facettes of building mid-power rockets. Very special thanks go out to Paul Gray, Nick Hills, and all those other internet rocketeers who gave me advice. Without them I probably wouldn't have ever gotten this rocket project off the ground.

Roots of a project

I like rockets. That is the simple driving statement behind this project. Some people ask me why I like rocketry. I usually ask them if they have been to a rocket launch of mine. The answer is usually no, but if it is yes, then I really can't understand them. I've always been interested in rocketry and space flight since my early school days, so this is really the most recent extension of that interest.

I had recently finished building and launching my Keller, a LOC/Precision Graduator. That rocket is wonderful, but it really wasn't that large in comparison to my low-power rockets. I already have several 2.6 inch diameter rockets, so it wasn't much bigger, just heavier and more powerful . I wanted a rocket that was really BIG and POWERFUL. I toyed around with various 3 and 4 inch designs. The 3 inch diameter would have gotten a much better altitude, but I went with the 4 inch diameter design because I didn't think the 3 inch would be a really big improvement on the 2.6 inch rockets I ahd been flying.

So now my goal was to see what the very largets rocket I could launch on motor a G motor was. The reason I kept it in the large model rocket range instead of getting a H or I motor is very simple. I am not certified to L1 and neither is my dad or anyone else we know. We don't have any large rocket launches near enough that we would drive to, and I didn't really want to jump up to HPR after only one mid-power launch. But, I decided to build this rocket extra strong, so that in the future I might be able to launch it on 29mm H reloads. As it was, I really didn't have enough experience to build the rocket I was building.

I discovered that the most powerful motor I could get is the Aerotech G80-4T. So, I designed my model with a 29mm motor mount and the 3.9 inch tubes. I knew that it would be very hard to get more than one length of Bt-3.9 up on a G, so I knew that 2 was out of the question.

So here I am with plans for a 3.9 inch diameter, 50 inch long rocket. Now where do I get the parts?

Parts Selection

[Rocket Pic]I have to say that the largest factor in picking out what parts to use was cost. This was followed fairly closely by weight. I really don't have a lot of money for a rocket like this, and it took me long enough to get what I ended up paying.

For the main parts I went with LOC/Precision. This was because their parts were cheaper and lighter than PML parts, and I already had experience with one of their kits. The nose cone was the single most expensive piece of the rocket, costing $14.50. The bosy tube I got was unslotted. this was because I wanted a 4-fin design, instead of the 3-slot tube they sold, and because the unslotted tube was a lot less. I also bought my shock cord from LOC/Precision (4 yards of SC-375), though I later found I could get the same cord for about 1/3 the price locally. I went ahead and bought and installed 15 feet of the 1/2 inch elastic cord, just in case I needed more than I thought. I used the other shock cord to beef up the Keller’s recovery system. The launch lugs also came from LOC. I used the 1/4 inch lugs becuase I knew I could get a 1/4 inch rod locally. I also got the motor mount tube and centering rings from LOC (more on them later).

Other than the parts above, I got all of the other parts some other way. Each part will be discussed more later when I tell about installing it.

Funding

I soon realized that along with larger rockets come MUCH larger costs. My LOC Graduator had been a Christmas gift, but I knew I would be funding the CTAG by myself. I bought as few parts as possible from rocket companies and scrounged around a lot around Searcy. My dad really helped a lot with finding different parts and on giving advice on different materials. Below is a list of where I got different parts, how much they cost, and the total project cost.

Rocket parts
Nose cone              LOC/Precision        14.50 3.9 inch Body tube     LOC/Precision        7.90 29mm motor mount tube  LOC/Precision        3.60 CR 3.9-1.14            LOC/Precision        2.25     "" " " "  "            LOC/Precision        2.25   Motor hooks            Estes                NA Motor block            around-the-house     NA Original Shock cord    LOC/Precision        3.60 Other shock cord       local Hancock Fabric 1.25 Shock cord mount       local Ace Hardware   0.35 Launch lugs            LOC/Precision        0.60 Plywood for fins       Hobby Lobby          6.50 30 minute Epoxy        local Ace Hardware   3.50           Bondo                  local Autozone       5.00 Primer                 local Wal-Mart       1.00 per can,3 cans Spray Paint            local Wal-Mart       1.00 per can,5 cans Shipping costs         NA                   8.00 Total Rocket cost---------------------61.55
Launch supplies

Launch rod             local Ace Hardware   2.25 G80-4T                 Aerotech             13.95               NCR igniter package    Hobby Lobby          4.50 Total Suplies cost for 1st launch-----20.70


Total Project cost (including first launch):
$82.25

As you can see, along with higher power rockets come much higher costs. My most expensive rocket before this was a LOC Graduator, which cost (a then extremely high!) $32.

Motor Mount Assembly and Slotting the Tubes

The parts finally came in from LOC/Precision and I was impressed! You don't realize how much larger 3.9 inch tubing is in comparison to 2.6 tubes until you see it! The nose cone was huge! Now it was time to get to work!

Of all the parts of a rocketry project, I probably enjoy the motor mount assembly and installment the most next to launching. I have no idea why this is, but it is. I started out the building by cutting the motor mount tube(MMT) in half with an X-Acto knife. it comes in 34 inch lengths, and I didn't need that much.

After this I took to Estes engine hooks and cut off the forward hook section on both. I epoxied these on opposite sides of the MMT and wrapped a few layers of masking tape around them, far enough up that it wouldn't get in the way of the rear centering ring (CR). I cut two small notches in the rear CR to allow for the motor hooks. Next I epoxied the centering rings to the motor mount tube, about 1/3 of an inch away from each end. After allowing it to dry I made several epoxy fillets around them and the motor hooks.

Now it was time to slot the tubes. I marked the 4 fin slots and the launch lug placement. I then very carefully cut the slots out with my trusty X-Acto. It wasn't the prettiest job ever, but it works great. if you buold your own mid-power rocket I highly recommend slotting the tubes yourself to save the money.

Shock Cord Mount Attachment

I had had some fairly bad experiences with LOC shock cord mounts, so I decided to go with a local part. I bought a 3/16 by 1 1/2 inch metal eye bolt, with two nuts, form Ace hardware for only 35 cents. To attach this I drilled a slightly smaller hole in the top CR and filled this with epoxy. I screwed in the bolt and put nuts on both ends (NOTE: You have to put one of the nuts on before you put the bolt in the hole!). I screwed everything on tight and put a layer of epoxy over it all. This made for a good firm attachment. I later learned that U-Bolts are much better but I had gone too far to change it, and I was positive the screw eye was stong enought.

I had hear a lot of bad stories about shock cords burning through, and I had had some experiences with smaller rockets too. I decided I needed some sort of non-flammeable cord to attach the shock cord to the shock cord mount. I finally found it: braided steel wire, commonly used for hanging paintings (my dad is an artist so we had some, but you can find some at any hardware store). I took about 3 feet of it and doubled it over. I knotted and twisted one end onto the shock cord mount on the other end I attached a large steel key ring. This I would later tie the shock cord to. Now that I had accomplished this, and I knew that I could replace the shock cord in the future (the steel wire extended 2 or 3 inches outside of the body tube), I was ready to install the motor mount assembly into the body tube.

Motor Mount Installment and Fins

[Rocket Pic]Now that I was ready to install the motor mount, I decided against it right then because I wanted to make sure we got the tab on the fins the right length. We (actually it was my dad) cut out the fins to the right size using a band saw and an electric Sabre saw. the fins were 3/32 plywood, which I later found out wasn't strong enough. After I was sure the tabs were the right length, I spread a layer of epoxy on the inside of the body tube, and some on the edges of the centering rings. I slid the motor mount assembly in, rotating it slowly, until I had the back centering ring even with the back of the body tube. I went ahead and epoxied the fins into the slots. I used plenty of epoxy and later I went back and put really good epoxy fillets on. This was all because of some bad experiences with my Graduator. I guess I really wanted that rocket to be strong, and I actually overdid the strengthening. I decided to reinforce the fin joints with fiberglass mesh. On this rocket it was really overly cautious, but I did it anyway.

[Rocket Pic]
Fiberglass

Among many other firsts, this was also my first experience with fiberglass. I did a lot of research, and finally found out that you can use fiberglass mesh (commonly used for tent screens, and we happened to have some on hand) as the fiberglass and imbed it in a layer of epoxy. I went out and bought some more epoxy and got to work. I imbedded a 3 inch by 5 inch strip into the body tube/fin joint and applied more epoxy on top of that. To hold it in place I put waxpaper on it and held a piece of wood onto it with clothespins (see photo). This system worked wondefully, although working around the bottom launch lug was frustratiing. For more info on fiberglassin', go to Rocketry Online's INFOCentral.

[Rocket Pic]
 

Parachute

When I initially designed the project, I had an umbrella chute in mind. I knew John Babb (a.k.a. Rocket Dawg) had a 3 foot diameter piece of umbrella fabric he had caniballized off a broken umbrella. he wasn’t using it in a rocket, so he let me have it to use in my CTAG, with the condition that if he ever need a parachute that size I would give it back and get another. For shroud lines I use 2 foot lengths of nylon cord (fairly thin), and sewed and knotted it on. I had to burn the ends of the nylon shrouds to keep 'em from unravelling, but othjer than that it worked great! I attached the 'chute to the shock cord about 6 inches from the nose cone with a series of knots. I had originally went with a 3/8 inch wide, 12 foot elastic shock cord, but I found that I could get a 1/2 inch, 15 foot cord for only $1.25 at a local fabric store. This, along with the shock cord mount attachment, which, no matter how hard I pulled, already wouldn't come out, made the recovery system very strong.

Bodywork

Bodywork is the pits. On the CTAG I had to use Bondo, so I ended up doing a lot of sanding. Bondo, which is commonly available at car repair shops, this is probably the hardest part of building a rocket physically, and it stinks. For about a 1 month period, here's what my Saturday's were like: Bondo, sand, breakfast, bondo, sand, sand, more Bondo, sand, lunch, Bondo, sand, sand, sand even more, supper, sand, bondo, sand, sand, sand, bondo, sand, go to bed, have nightmares about sanding, wake up (okay, so its a little exaggerated, but I did sand a WHOLE LOT!). I highly recommend wearing a dust-mask or something if you sand a lot on your rocket, all that dust will clog up your lungs and you mgiht not get to see your rocket launch! Overall, Bondo is a great project. The seemingly endless hours of sanding really paid off when I got the rocket painted and it had such a smooth finish.

Finishing

After all of the work I put into this rocket, mainly the long hours of fiberglassing and sanding, the painting sure seemed like a breeze. I had finally decided, after many days of thinking and drawing and coloring pics with colored pencils to see how different color schemes would look, I dumped all the rerally fancy ideas and decided to go with a classic, bad-to-the-bone look. A very glossy black main coat would be accented with a fire red stripe, about 1 1/2 inches wide, the length of the rocket. it would bve lined up with a fin so that it would continued and make one of the fins pure red. Also, a small, 1 inch wide red stripe went around the top of the body tube, right below the nose cone. This all combined with a very nice shiny reflective LOC/Precision decal supplied with the parts to make a very attractive rocket.

I used about 4 coats of equipment grey primer (3 11 oz. cans of spray paint) to help smooth out all the little places the Bondo wouldn't work well in. Then I painted the sections I wanted to be red with two layers of fire red Wal-Mart brand spray-paint (this stuff is wondeful, dries quick, and only costs $0.98 per can!). I let this dry overnight, then masked it off with newspaper and 1 inch wide strips of masking tape. Then I covered it all with 3 layers of gloss black Wal-Mart brand spray paint, with 4 going on the nose because of a mistake I made in the priming process. this meant I had as much as 6 or 7 layers of paint in areas! The paint on its own ended up weighing 1.3 ounces on its own. I added the decal, and I was done with the bodywork and finishing. (finally! I had spent about 2 weeks building, 1 week glassing, and 5 weeks sanding, along wioth a few days painting, for a total of over 2 months work! I probably had anywhere from 40 to 60 man-hours in on that rocket). It really is rewarding when you finally get to show off the end result (alomst) of your project to the people who have been hearing about it for months on end.

Motor Selection and Prep

Once I found that the rocket's weight after painting was 36.5 ounces, I had to make the big motor selection choice. Should I go with the G40-4, or the G80-4. I really wanted the G40 because it was a White lightning, but I had to go with the G80-4T because of its higher intial thrust.

Now came the waiting game. Because of conflicts, etc., I wouldn't be able to launch the CTAG for several weeks after I finished the rocket. During this time I finished up work on several smaller projetcs, work on my webpage, and start planning my next project, the Mission Impossible . (the name, just like that of the CTAG, was stolen, this time from HPR magazine) This would be a 4 inch diameter rocket capable of using 5 times the power of the CTAG! But that was for the future, and is probably destined to be a group project, due to the cost. I had more important, sooner events to think of. I was organizing the second official launch of the Searcy Youth Rocketry Club and working on my other rockets.

Countdown

Around this time I was getting nervous. I had been working on the rocket for 2 months, and I wanted everything to go perfectly. I ran countless simulations using different programs. Most of them told me that the CTAG would fly about 1000 feet up. Then I ran a stability check.....uh-oh... According to Astrocad the CTAG had a stability criteria of only 0. Owch. This didn't make much sense, considering there were several 4 inch diameter kits on the market like this that don't require any nose weight. However, I decided to stay on the safe side. i added nose weight until I got a 1.5 caliber stability rating. This required 11 ounces of nose weight, bringing the total weight to 47.5 ounces and the predicted altitude of 700 feet.

Now I was ready. It was time to put my chips on the table and go for broke. This was the biggest thing to hit our area rocket wise since who-knows-when. I had my launch field set up. i called the FAA and notified them of my intentions. I called my friends and then started going through my preflight checklist. I made sure all my ground support equipment was working.

Then the day of the launch dawned. After getting home from church, I started prepping rockets and going over the CTAG over and over.

Liftoff

I called some friends, made sure everyone was coming, and headed out to the launch fields. I launch at the public soccer fields south of Searcy. We have about 40 acres of cleared land with very few trees in sight.

People started showing up and the launch got under way. There were lots of rockets launched, mainly in the A-C range. We also launched 5 D engine rockets. My Keller had an interesting launch. Then it was time to launch the CTAG. I had been recommend to use one of the North Coast igniters instead of the Aerotech ones, so I did. The motor was held in place by a heavy friction fit, a thrust block made of masking tape on the aft of the casing, 2 Estes engine hooks, and several layers of masking tape wrapped around the end of the motor mount tube and masking tape thrust block. I put the rocket on the launch pad and hooked it up to our launch system. I stepped back to the ends of the 75 foot long ignition wires and showed one of my friends how to attach the other end of the wires to a car battery so that I could watch the rocket better. I looked around. Everyone was ready.

10, 9, 8, 7, 6, 5, 4, 3, 2, 1...

LIFTOFF!

The G80 let out a blast of flame and a loud thundering. The rocket leaped off the pad and into the air, leaving a light smoke trail. It reached apogee and coasted over. It was coming down when the nose cone seperated and the parachute came open. The rocket floated gently down on its chute for a landing about 75 yards away. Some of my friends and I raced out to get the rocket out of the tall grass. The rocket was in perfect condition. It was amazing! We folded the chute and shock cord so it would be easier to carry. I packed up my rockets and went home to review the ideo tape of the launch over and over.

Followup

All of the overengineering and research paid off. The Cheaper Than a Girlfriend had worked perfectly. Here I would like to thank all those who helped out.

Thanks Everyone!
Especially the following:
My dad, fo helping me with various aspects of the rocket, including welding the launch system and buying the motor for me since I'm underaged. I would also like to thank all the Rocketry Online chat room and discussion forum people. They were invaluable inresearching different areas of constrcution and launching. Another great aide was all the members of the IAR and GYRO for their advice. Also, special thanks to Nick Hills, Paul Gray, and John Babb. Without all of them, this project would have never happened.

After a rocket project like that and a perfect launch I was extremely happy. This led to dreaming of larger and more powerful rockets......hehe.....(Be watching for future project reports).

Note on Photos: The construction photos were all taken by me, and the launch pictures were taken by my dad and I. However, the launch photos, which were taken from still frames of the video tape. These didn't turn out, so i'll have to wait until my dad can get his launch photos developed and scanned. Until then, you'll just have to imagine the launch....

You can email me any questions or comments you have about the CTAG or rocketry at toprocket@hotmail.com

comment Post a Comment