Introduction: Hybrid Rocket Engine
Rockets are awesome. This is an undeniable truth.
Many years ago, when I first discovered model rockets, they captivated me. Those little gunpowder engines had just enough thrust to make a painted cardboard tube soar hundreds of feet in the air.
In this instructable, we will build a slightly more complicated hybrid rocket engine, and re-ignite my fascination with machines that spit fire. Also, we will find a more creative way to destroy that pile of old math homework in the corner of my desk.
Before we get started, a little note on safety:
This project uses oxidizer (in this case, just oxygen) to make a cylindrical section of solid fuel burn very very quickly. This can be seriously dangerous and problematic if not handled correctly. It never hurts to do a little extra research before you start a project like this, and you can never have too many fire extinguishers nearby.
Proceed with extreme caution. If you have absolutely no idea what you're doing, then it's probably best not to try this at home.
And with that out of the way, let's get started!
Step 1: Materials and Tools
Materials: (note that all measurements are in imperial but the equivalent in metric works too)
- 12 in. steel pipe with at least a 1 in. diameter
- 1 in. steel end cap
- 1 in. steel coupling
- 1 in. to 1/2 in. steel bushing
- 1/2 in. to 3/8 in. hose barb
- 5 or more feet of 3/8 in. vinyl tubing (though a more heat-resistant tubing would be better)
- oxygen regulator valve (found one on Amazon here)
- 1.4 oz. oxygen tank
- *I used matches to ignite the rocket. This is a terrible idea. Use a proper Estes rocket launch controller
- and a few other things that I completely forgot about
- various pliers and wrenches
- saw horse (for the test stand)
- metal clamps (for the test stand)
- thread tape
- safety goggles
- fire extinguisher (though I hope you won't need it)
- a healthy fear of fire
- a small amount of patience
- and a camera (I want to see what you make!)
- a weekend to build and test
- allow a few extra hours for additional research
- a few minutes of safety prep
- a few seconds of glory!
Step 2: Science!
Before we build our model engine, let's review the science behind rocket engines.
If you're just here to see this thing spit fire, feel free to skip this step. If you want to learn about rockets, stay a while and watch the video.
Pretty much every rocket ever (with the exception of toys that use air or baking soda or whatever) use some sort of combustion reaction to produce hot gas, make it move really quickly, and create thrust. If you're familiar with the fire triangle, then you already know that combustion requires three things:
Usually, fire takes advantage of oxygen in the air and heat from an ignition source. In this case however, we are gonna give it a little boost. With an extra source of oxygen, the fuel will burn much faster. At a certain point, the fuel will burn fast enough to produce thrust and behave like a rocket.
For our design, I will start by using paper (old math homework) as fuel and a 1.4 oz oxygen bottle as the source of oxidizer. Held in a steel pipe with a modified end cap to act as a nozzle, this should safely produce a small amount of thrust.
Step 3: Assembly
The whole thing is assembled pretty easily from plumbing parts. It is important to keep some thread tape between each set of threads to help keep an airtight seal. This will prevent oxygen or hot exhaust from leaking out the sides.
To make the nozzle, I flattened out a spot on the top of the steel end cap and drilled a small hole. I should have just used a stepped drill bit, but instead I started small and just worked up to about a quarter-inch hole. Using a much larger drill bit (about 1/2 in.), I then tapered the sides of the hole to better resemble the shape of a rocket nozzle.
The nozzle will screw on to one end of the steel pipe, while adapters for the oxygen will attach to the other. You'll need to put fuel in the rocket before everything is sealed up, so I recommend waiting to attach the nozzle until after the fuel is inserted.
The above images in words:
- steel coupling screws onto steel pipe
- steel bushing screws into steel coupling
- brass bushing screws into steel bushing
- brass hose barb screws into brass bushing
- nozzle screws onto other end of steel pipe
- make sure everything has plenty of thread tape
The assembly really isn't very complicated. Once everything is screwed together tight, you're done! Time to move on to the fuel.
Step 4: Fuel
So you've probably figured out by now that a hybrid rocket engine requires two completely different kinds of fuel. The first kind will be the fuel itself, which can be literally anything that burns (wood, sawdust, wax, plastic, even meat). For this project, we'll start by using paper and work up to more exotic fuels. The second thing we need is oxidizer. In this case, we're going to be using pure oxygen gas. Nitrous oxide works just as well, but I don't have any laughing gas on hand, so we'll stick with the oxygen bottles you can buy in the hardware store.
Let's start by making a fuel grain out of paper. You'll need lots of scrap paper (I used a pile of old math homework), some tape or glue, and patience. (This could get a little tedious) I started by tightly wrapping a piece of paper around a metal rod about the width of a pencil. Tape that piece of paper in place so it doesn't unwind, then repeat. Keep adding paper until the diameter of the paper tube is an inch. The paper fuel tube should fit snugly into the steel combustion chamber. Make sure a small hole at least 1/4 in. wide runs through the middle of the paper tube. This hole will be critical in getting the engine started.
After you've done that, set up the oxygen tank. Screw the valve onto the top of the tank and attach the vinyl hose to the hose barb.
Step 5: FIRE!
Here comes the really fun part. But before we fire this thing up, it's important to review some basic safety precautions.
- make sure the area is cleared of flammable material. Keep the rocket at least 10 feet away from anything that could catch on fire
- make sure the engine is securely mounted to something that won't move. I clamped mine to a heavy-duty saw horse.
- keep a fire extinguisher at the ready
- I sprayed the whole area with a garden hose before starting the engine. This prevented any possible debris from starting fires in the grass
- wear eye protection
- do not ever put any part of your body in the path of the engine flare. Use metal pliers to ignite the engine, not your fingers.
Disclaimer: Rockets are really dangerous, so use extreme caution. I am not responsible for your actions, you are.
Alright, now we can get ready to light it.
Here's the initial procedure I followed, thought I do not recommend repeating this:
- make sure all hoses are connected and the oxygen valve is completely shut off
- grip a match at a 90 degree angle with a pair of pliers so that your hands are not in the path of the engine flare
- light the match and begin adding a small amount of oxygen (matches were a bad idea)
- insert the match into the nozzle and increase the flow of oxygen. the fuel should begin to catch fire
- increase the flow of oxygen and take a step back
- to stop the burn, shut off the oxygen completely and allow a few minutes for the engine to cool. submerse any unspent fuel in water to ensure no risk of fire.
Here's how it probably should be done:
- make sure all hoses are properly connected and the oxygen valve is completely shut off
- insert a model rocket engine igniter into the engine nozzle. Ensure that the igniter is wired to a proper launch controller
- begin to add a small amount of oxygen into the combustion chamber
- ignite the engine by pressing down on the launch button and the launch key on the controller
- increase the flow of oxygen until a consistent burn is reached
- immediately shut off the flow of oxygen if the engine begins to behave unexpectedly.
- after shutting off the engine's oxygen supply, give it a few minutes to cool completely. Submerse any remaining fuel in a bucket of water to prevent unexpected fires.
Step 6: Going Further
Just about anything can be used as the solid fuel for a hybrid rocket. Paraffin wax, PVC, and plastics are common and effective, but nearly anything that burns can be used as fuel.
If you have any crazy suggestions for interesting fuel sources, please let me know! I might test them out sometime in the future.
And if you've thought of anything practical (and sortof safe) to do with this rocket, leave me a suggestion in the comments. I might make it happen!
If you enjoyed this project, please vote for it in the Science contest and consider subscribing to me on YouTube. I don't post projects often, but I try to make them interesting and fun to watch.
If you'd like to learn more about hybrid rocket engines: