General – Moto-Joe Blog

TEN THINGS I LEARNED IN MY FIRST YEAR OF ROCKETRY

By Dale Netherton

I got into 007-01 rocketry in November 1999 and started with an Estes silver comet. On my first anniversary, I launched on two H180 motors my pride and joy, a 4 inch diameter, 7 foot tall rocket named “Blinded by Science”. Only the fin can and motor mount assembly survived. I guess there was something prophetic about the name. At the launch it was commented that you should learn more from failed launches than from prefect ones. I made this list in the hope that others new to the hobby would learn from my efforts this year and be spared such spectacularly educational flights.

10 – Start small and work your way up the alphabet.

9 – Krylon. No substitute.

8 – Loc/Precision nose cones are not indestructible. 007-02

7 – An E-30 rocket motor makes a really cool sound as it travels through the motor tube into the rocket. (Securely mount your motor!)

6 – 9V Duracell batteries are made up of 6 really small batteries and you need to use a thin wire with a hook in it to get them out of a Loc/Precision nose cone.

5 – Even an Aerotech reloadable motor will blow-by if the O-ring is in on the wrong side of the delay element. Read the instructions, even if you have assembled that motor a zillion times.

4 – Speaking of Aerotech, don’t use Aerotech igniters in a cluster! Make your own or buy electric matches from Daveyfire or Oxral!

3 – There is no such thing as a stupid question! If something doesn’t look or sound right ask the LCO or any of the many long standing members.

2 – Don’t be afraid to make mistakes. People may learn something from your crash. The folks at Lockheed will testify to this.

1 – Get involved with a club. The members of NCR are some of the most knowledgeable and friendly people I have ever had the pleasure of being with. Launching with a club comes with a long list of benefits.

Improved Method to Attach Shock Cords to Balsa Nosecones

For years, shock cords have been attached to balsa wood nosecones using a screw eye. The screw eye was threaded into the balsa and the shock cord was knotted to the screw eye. Many improved upon this by removing the screw eye, injecting a small amount of glue into the hole, and replacing the screw eye. For elastic or rubber shock cords, this was a pretty good solution. With the rise of inelastic materials for shock cords like Nylon and Kevlar, the screw eye has lost its value. A stronger alternative is to lose the screw eye and glue the shock cord directly into the nosecone.

004-01 First, drill a hole about 3/16” diameter by 1/2” deep close to the center on the base of your balsa wood nosecone. Don’t worry too much about the hole being straight, on-center, or true to the axis of the nosecone. Tie a knot on the end of your Kevlar or Nylon shock cord about 3/16” across and cut off most of the loose end. Fill the about 3/4 of the hole with white or epoxy glue. Use a stick or piece of wire to push the knot into the glue and as deep in the hole as the knot will go. Fill the remaining hole with the same glue and wipe away any overflow. Place the nosecone in a cup or some sort of support that will keep it oriented tip down and support the shock cord so it is straight out of the bottom of the nosecone. When the glue has cured, it is ready to use.

AeroTech “Moonburners”

All solid fuel, high power motors, H through O impulse, commercially available to the public are of the same general design, varying only in the details.

motot-cutaway

Although other fuels are possible, ammonium perchlorate composite (AP or APCP) is the fuel (and oxidizer) of choice. The AP is cast in thin walled cardboard tubes, cut to length and drilled with a core hole. These are called the grains. The core hole can vary from a single hole concentric with the axis of the cylinder to star shaped or a slot along one side of the grain. (The core shapes for U.S. ICBMs are closely guarded secrets. I’ve been told by an Air Force missile technician that he can’t tell me the shape but it isn’t simple like ours.) The core shape and location has a lot of effect on the profile of the burn. Your motor has an off axis hole to somewhat inhibit the rapidity of the burn. During asse

mbly, the alignment of these holes is really important. The shape of the grain cross section during the burn is what gave it the name “moon burner”. Obviously the fuel burns from the inside out. This offset core burns slower because one side runs out of fuel before the other side does, leaving propellant on one side in the shape of a crescent moon. Less obviously, in most motors, the fuel also burns up and down (parallel to the axis of the motor) from the grain boundaries. To further slow your motor down, they have you glue the grains together with epoxy (it is in the instructions). medusa This is also very important. Your motor uses a Medusa nozzle, having 7 converging/diverging nozzles, instead of the usual 1, all coming from the one combustion chamber.

I don’t know if it mentions it in the instructions but you need to make sure ONE of these nozzles lines up with the core hole in your grains. Failure to do so can make it very difficult to insert the igniter (more about this later). I haven’t personally flown the M750 but have flown many moon burners with Medusa nozzles. Likely there is grain that has a tapered cut from the core hole to the center of the motor. Be sure that grain is on the bottom and with the tapered cut on the bottom. Failure to do this can result in vectored thrust and a somewhat less than optimal flight. The grains all get stacked within an ablative liner that protects the motor case from being destroyed during the burn. Having it be destroyed after the burn might be acceptable but bad things happen if it burns through during the burn. This whole assembly is held within an aluminum tube called the casing. The ends, fore and aft, of the tube are finished with closures. O-rings keep the hot gasses going where they should. All I can say is follow the instructions to the letter. Sometimes, it’s a good idea to have one person do the assembly step by step with at least one other person watching to make sure that everything goes according to instructions, checking off each step as it is done. It’s also a basic requirement to familiarize yourself with the assembly procedure prior to getting to launch site. IF at the end of the assembly you end with an extra O-ring or some other part, you should disassemble the motor and start over. A successful flight requires all of the motor components.

Your igniter should be a big one or two fired in parallel (remember you’ll have to get the igniters in through the nozzle hole). The heads of the igniters should be inserted ALL THE WAY to the top of the motor. Igniting the bottom of the motor impedes flow of gasses from the front of the motor and can change your thrust curve from one that will boost your bird to many thousands of feet to one with much lower average thrust per second but a much longer burn. Many people mount the igniter on the end of a thin stick that has been cut long enough to push the copper wired igniter all the way to the top. Once again, if you aren’t sure the igniter is all the way to the top, start over.

If you have multiple people doing the general flight preparations on site (motor assembly, loading ejection charges, setting up avionics and such) where time is a factor and conditions are not like being in a lab, having a check list, doing everything sequentially, with somebody in charge of keeping track of what is done is a really good idea. I’ve known too many good projects come to a less than optimal ending because everybody assumed, wrongly, that somebody else had done something essential.

One of our goals at Moto Joe is to be a technical resource for our hobby. Please feel free to send your questions keeping in mind, of course, that some things are difficult to explain from 2000 miles away.