Hybrid Rocket Motor

I really like rockets. So for a senior year engineering project in High School, I decided to make one. I enlisted the help of my fellow student, Sam Ward, and began work. We decided a hybrid rocket would be the safest design we could do. For our design, we are using compressed oxygen to fuel a burning acrylic (Poly[methyl methacrylate]).

This instructable covers how to make a hybrid rocket motor using acrylic and oxygen gas. It is very dangerous and should be tested with utmost caution.

This took us 1 school year to create, including research any multiple side projects and other difficulties.

Hybrid Rocket Motor by Cody Warren and Sam Ward in Rockets

Step 1: Creating Parts

What you’ll need:

· 1 1/4” Acrylic rod, 4 feet long

· 2” Steel rod, 1018 Mild/Low Carbon, 1 foot long

· 2”X 2” X 1” Aluminum block

· Caliper

Machines needed:

- Metal lathe

- Drill press

- Metal saw

- Milling machine

Note: All of the parts shown are the materials for static test

Cut or acquire a 1 foot section of acrylic rod. Put in a lathe, and center drill a ¼” (.25”) hole through. If a lathe is not available, a drill press with a proper setup should be able to hold the rod.

There is 2 ways go about nozzle creation: going through the math for a bell nozzle or creating a simple conical nozzle. Bell nozzles are one of the most effective nozzle types for lower atmosphere low speed rockets, but are difficult to design and create. This instructable will be covering the design and creation of a bell nozzle. · R-radius · e-exit · t-throat · Rt-radius of the throat · Re-radius of the exit Input a given number for Rt. If you put in .25 inches for the throat radius, you can take that number and apply it to 1.5Rt and .382Rt, which as seen in the figure are the curves near the throat. Following the .382Rt arc, a parabola is used for the rest of the progress. After inputting a parabola, the exit of the nozzle ended up having a .75 inch radius. You will need Re, or the radius of the exit, to find the expansion ratio, or ԑ. To find ԑ, the total area of the exit must be divided by the total area of the throat. To find the area, you us the equation. After I found the areas, the equation was 2.4/.196=12.25. The higher the expansion ratio, the more efficient the nozzle is. Lengthening the nozzle results in a higher expansion ratio, but this is counter balanced by the added weight of a larger nozzle. The actual length of the nozzle is up to the creator, but the nozzle I created ended with a total length of 2.45 inches. The image to the left is my CAD model of the design, which was created in Solidworks. The image to the left is a scale drawing, where every square on the paper is equal to about 1/8 of an inch. This is mainly the math of models for the nozzle geometry, not the additional parts to attach it to the static test or a rocket.

Mark on your steel rod where the nozzles ends will be, you will need to cut there later. Next, use a marker to make a line around 6’ from the end of the rod. After that is completed, use a lathe cutting bit to cut a straight line through the steel rod. Make sure the cut is straight but lining it up with a square to ensure it is a at a 90 degree angle to the rod. After this, use progressively larger drill bits to hollow out as much of the nozzles interior as you can and stay inside your calculator shape. To figure out how deep each hole needs to go, use either a CAD program to measure how far into the steel. After using multiple drill bits to hollow out the center to your satisfaction, use a boring bar, as shown in the picture, then create the desired curve as closely as possible. The curve created by the boring bar will still be rougher than desired, but you want to have to cut as little as possible after you are done with the boring bar. The images below show the progress form the progress with drill bits to the end product from the boring bar. Once the boring bar work is done, you must create a tool bit that fits your ideal curve. I used a ¾ inch chunk of tool steel and ground it down to my approximate curve. After using it with my rod in the lathe, the interior curve became smoother and overall more ready for a test.

First I took a large piece of aluminum block and cut it into halves then used a two jaw chuck to level the top of the pieces. After which I taped the four corner holes and the center hole to. After which I switched over two a ¾’’ tungsten carbide bit to tap the larger hole. By setting the mill to level and using calipers I was able to precisely tap the center holes to the correct depth. After which I used a smaller end mill to create the divot around the edge of the center tap this was later used for silicone O-rings.

Once the nozzle is completed, drill 4 holes in the flat end near the edge approximately 1 inch deep. These holes will then needed to be tapped so bolts can go throut them. We used ¼” bolts. After these are drilled, match them to holes on the separate block (see other guide) and puts the bolts in to secure the nozzle to the test rig.

To test the motor, we created a holder made of sheet metal cut and welded together to hold the test rig, a scale, and to hold on to a base. Shown here, is the vertical version of our test rig. Initially we made a horizontal one which did not allow for measureable results so we remade it to be vertical. The square at the bottom is the space where our bathroom scale slides in, with the test rig itself riding in the longer rectangular part in which is can slide along the metal guides, allowing for it to move so we can measure the force output. The 2 green metal bars on the bottom are made to have bolts run through them which go into the base that is used, which for this project was a large concrete block. After this setup has been created, bolt the setup down to a large heavy object. Make sure you apply pressure to your setup to ensure it is firmly locked to your anchor block.

The first part would be to actually attach the test rig to the base. After this, set up some kind of barrier to be between you and the test rig, in case of an explosion or fire. Also, attach the hose for the oxygen tank, but not the tank itself, and have a holder for the tank on the outside part of the barrier. Setup any cameras where you wish to have recorded, we had one recording the scale measurements and one recording the exhaust itself. The last thing before starting the test rig up is to attach the oxygen tank, but do not turn the valve to allow oxygen through.

The first step is to check that the frame is strongly attached. If it is, prepare by preparing a fire extinguisher or some other retardant in case of a fire. After this is ready, prepare some sort of igniter that can be pushed into the fuel rod through the nozzle. For my test, we took a tiny wooden stick, wrapped it in duct tape, dipped in in wax, and caught it on fire to ignite the place. Attach the oxygen tank, turn on the nozzle and stick this igniter inside the fuel rod to start the test. Use a camera to record the movement of the scale, so you would know the pressure created by the motors ignition.

Included on this step is pictures of the motor and its pieces at their completed stage.

Also, we have our first test fire of the rocket.