Hybrid Rocket Engine

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About: I'm a science and engineering enthusiast that loves to build cool and often impractical things. I started making things years ago, but have recently decided to start sharing the strange things I make with th...

Intro: 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
  • matches*
  • and a few other things that I completely forgot about

Tools:

  • 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!)
  • [these and other tools can be purchased on sale from Gearbest.com]

Time:

  • 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:

  1. heat
  2. fuel
  3. oxygen

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:

  1. steel coupling screws onto steel pipe
  2. steel bushing screws into steel coupling
  3. brass bushing screws into steel bushing
  4. brass hose barb screws into brass bushing
  5. nozzle screws onto other end of steel pipe
  6. make sure everything has plenty of thread tape

Got that?

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:

  1. make sure all hoses are connected and the oxygen valve is completely shut off
  2. 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
  3. light the match and begin adding a small amount of oxygen (matches were a bad idea)
  4. insert the match into the nozzle and increase the flow of oxygen. the fuel should begin to catch fire
  5. increase the flow of oxygen and take a step back
  6. 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:

  1. make sure all hoses are properly connected and the oxygen valve is completely shut off
  2. insert a model rocket engine igniter into the engine nozzle. Ensure that the igniter is wired to a proper launch controller
  3. begin to add a small amount of oxygen into the combustion chamber
  4. ignite the engine by pressing down on the launch button and the launch key on the controller
  5. increase the flow of oxygen until a consistent burn is reached
  6. immediately shut off the flow of oxygen if the engine begins to behave unexpectedly.
  7. 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:

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    66 Discussions

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    pat_pending

    1 year ago

    Nice Instructable mate. Agree with the comments about thrust but you mentioned this was a model of a hybrid engine and not something 'flight ready'. If you do want to make a flying rocket, the R-Candy solid fuel that Grant Thompson (A.K.A the king of random) makes works well and is pretty safe using the safety precautions you are taking already. Regarding the 'danger' of what you're doing: at school the teacher would use liquid oxygen and dip a biscuit in it then touch it with a long match/splint. This thing would explode with a wild bang! Guess things weren't dangerous in the 80s. It's a small wonder the human race survived at all :-) . Keep making. Stay safe

    1 reply
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    LabRatMattpat_pending

    Reply 1 year ago

    I've flown solid fuel rockets many times before, and I've seen Grant's video on the topic, but I find other forms of propulsion fascinating. Yes, oxygen is dangerous. I cannot legally obtain liquid oxygen though, so I won't be doing anything crazy like that.

    "Guess things weren't dangerous in the 80s" is probably the best single sentence I've read in this entire comments section.

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    Tecwyn Twmffat

    1 year ago

    Obviously safety is an issue here, but what happened with your math grades? Could I use my annual tax return as rocket fuel as an effective tax avoidance scheme?

    1 reply
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    LabRatMattTecwyn Twmffat

    Reply 1 year ago

    Hahaha, the math homework had already been graded, and my grades in math were rather high. My math teacher however, had a tendency to assign a series of long and redundant math problems for homework, giving me a special hatred for the long and redundant problems. I had long joked about hosting a burn party with classmates who shared my views on the assignments, but I decided to destroy the papers with a little more flare instead. 0

    I do not endorse any tax avoidance schemes, but that would certainly work as fuel.

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    bpark1000

    1 year ago

    You need to ignite the engine at the end where you inject the oxygen (could be done with electric igniter inserted all the way in). The "rocket's" success isn't verified unless you measure its thrust-time product.

    Be careful, the pipe could explode! You should have secondary containment.

    Be careful

    3 replies
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    LabRatMattbpark1000

    Reply 1 year ago

    Thanks for the information. It's highly unlikely that it will explode as it cannot build enough pressure to damage the steel, but thanks for the concern. I will re-evaluate that before testing any other types of fuels.

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    bpark1000

    Reply 1 year ago

    TATP is an explosive, not a rocket fuel grain. PLEASE DO NOT MAKE TATP! Your procedure omits the most important step, keeping the mix below 10 degrees C during the reaction. if you don't, you make a related compound that is spontaneously explosive!

    This stuff (even when properly made) is dangerous, even when ignited in the open! It is used by terrorists for bombs! Please don't make this stuff!

    Flash paper is made of nitrocellulose, not TATP. Nitrocellulose is energetically flammable, but not explosive unless confined.

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    LabRatMattbpark1000

    Reply 1 year ago

    Thanks for the info. I rockets are not the same as explosives. I will avoid that substance.

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    jdbwizzard

    1 year ago

    For a further safety concern you could use a remote rocket launch system to ignite an ematch. I build solid rocket engines and this is how a test fire them so I can be in a safe place while I light them. Good information though.

    1 reply
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    LabRatMattjdbwizzard

    Reply 1 year ago

    I'm thinking of building a much safer ignition system in the future. Using a match wasn't the best idea.

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    Tesla_Shock

    1 year ago

    Watch Grant Thompson's video on how to make liquid Oxygen and Nitrogen. Combine the two as Oxidant and Combustible, and you get the most efficient rocket that has been (publicly) created to date! If you're feeling a little dumped with liquid Nitrogen, try Alcohol ( 90% or higher is preferable) or Kerosene, like the Germans and Werner Van Braun in WW2. Just be prepared for your contraption to belch black smoke. If you have no idea who Van Braun is, go check him out! You'll be surprised. I'd leave liquid oxygen in the mix, unless you're in the states, because Oxidizers are hare to find. The most popular oxidizer variants to Liquid Oxygen are Hydrogen Peroxide, but for that you need 80% or more and is virtually impossible to find without having a lot of contacts in the chemical industry, or making it yourself, or Potassium Permanganate, which isn't really sold outside of the states (I lived in the states for a looooong time and did this stuff a lot but then I moved to the EU where EVERYTHING IS GOD DAMNED CONTROLLED FOR THE SLIGHTEST POSSIBILITY OF EXPLOSION! AAAARGH!) Anyway, good luck and hit me up if you have any other questions!

    2 replies
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    thormjTesla_Shock

    Reply 1 year ago

    Just be really careful with Tesla_Shock's answer... if you mix LOX with some things, it burns really well. If you mix LOX with other things (eg but not limited to, Alchohols), you have an pipe bomb instead of a rocket motor because the flame front is too fast to get out of the nozzle... and it can be unsafe/unstable for -hours-.

    Find a Tripoli / NAR club, look up and understand flame fronts and grain designs, and start understanding how a rocket nozzle works and why it is important. Unintended consequences can go from pipe-bombs (why you should use a remote igniter and have a safety shied around the test stand) to 300 feet horizontal launch, lighting a huge fire, and even to more oh-crap-moments.

    But a great launch is beautiful to behold. Best of luck and *be safe*!

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    LabRatMattTesla_Shock

    Reply 1 year ago

    I wrote a school paper on Van Braun a while back! I plan on keeping this project a little more simple, as I don't have enough chemistry experience to attempt anything with fancier oxidizers, but I know someone who does. I also live near the east coast of the US, so liquid oxygen is hard to come by. I might eventually get around to some more complex experiments but not until much later. Thanks for the info!

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    LabRatMattMichaelAtOz

    Reply 1 year ago

    thanks for the link! I will look into that for future tests

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    BudD3

    1 year ago

    Experiment.

    This is not a "rocket" engine. This is nothing more than a dangerous experiment.

    GWR1 said it all. He told it like it is.

    3 replies
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    PaulA23BudD3

    Reply 1 year ago

    Research. This is nothing more than someone researching the physics of what makes a rocket work, and is no more dangerous than a "real" model rocket. There isn't any "telling it like it is" because the OP has already REPEATEDLY noted the dangerous nature of the project, as well as the valuable lessons learned. Whatever happened to experimentation and (God forbid) helping, thereby increasing knowledge for all who stumble across the information?

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    LabRatMattBudD3

    Reply 1 year ago

    Yes, this project is dangerous. Fire always is. But this is still a fully functional model of a hybrid rocket engine not unlike those used on suborbital spacecraft (though suborbital spacecraft engines are not usually made from hardware store materials). There appears to be an argument brewing over the safety of this particular project and the nature of dangerous projects on Instructables. I took lots of off-camera precautions and did much research before I even started this project. I knew the risks and how to mitigate them, and I just hoped to share the project and my experiences with this community.

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    AlphaOmega1LabRatMatt

    Reply 1 year ago

    Nobody has to make your prototype hybrid rocket engine (which it clearly is). We live in an age (as mentioned above) where nobody wants to take responsibility for their own actions, yet happy to stand around waving their 'holier than thou' flags with no personal cost; yet it gives them (they assume) the moral high-ground. They are usually the ones that never achieve anything. Anyone can point after the event.

    I say top marks for your design and planning. You could easily have spent twice as long writing a RA, and yet nobody was hurt despite that! Joining a rocketry club is probably a good idea (although I could just as easily give many reasons for that not being a good idea - you'll be near other people with rockets for one!)

    I'm keen on extreme sports, and as a kid blew stuff up with my own explosives (chemistry is fun!), even made my own bullets using casings from a fun fair. Launched Greek fire balls from a home-made Roman catapult, blasted around on motorbikes (of course), taught myself to repel off of railway bridges, on and on! Why am I here in one piece all this time later? I think about and understand the dangers, and don't trust my life to other people.

    I have to post a letter in a moment, I don't have a RA, but either way, I could die getting to the post box - such is life.

    Live life, be safe and don't be put off.