Introduction: Hybrid Rocket Engine

Picture of 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

Picture of 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

Picture of 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!

Picture of 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:

Comments

pat_pending (author)2017-07-17

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

LabRatMatt (author)pat_pending2017-07-18

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.

Tecwyn Twmffat (author)2017-07-18

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?

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.

bpark1000 (author)2017-07-17

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

LabRatMatt (author)bpark10002017-07-17

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.

jdbwizzard (author)2017-07-17

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.

LabRatMatt (author)jdbwizzard2017-07-17

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

Tesla_Shock (author)2017-07-13

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!

thormj (author)Tesla_Shock2017-07-17

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*!

LabRatMatt (author)Tesla_Shock2017-07-13

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!

MichaelAtOz (author)2017-07-14

I'd suggest a blowback preventer/flashback arrester/whatEvaYaCallIt

https://en.wikipedia.org/w/index.php?title=Flashba...

Particularly if you go to more potent fuels.

LabRatMatt (author)MichaelAtOz2017-07-15

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

Michal Choma (author)2017-07-15

nice,

BudD3 (author)2017-07-13

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.

PaulA23 (author)BudD32017-07-15

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?

LabRatMatt (author)BudD32017-07-13

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.

AlphaOmega1 (author)LabRatMatt2017-07-14

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.

LabRatMatt (author)AlphaOmega12017-07-14

Thank you! Those extreme sports sound exciting, but a they sound very dangerous (even to the kid with a can of compressed oxygen and a match). Best of luck with those extreme sports, and try not to hurt yourself on the way to the post box!

CPUDOCTHE1. (author)2017-07-14

That seems more like burning paper in an oxygen rich environment to me than a rocket engine. When I was in highschool, in the shop, we found an old wooden spool of solder (about a 1 lb spool). We stuck it on the end of a lighted oxy/acetylene welding tip. As soon as the wood started burning, we turned off the acetylene and cranked up the oxygen. It made a cool flame. I didn't know it was an rocket engine. You could emulate the same thing for a lot less cost and time buy just drilling a hole through a piece of wood igniting the wood and blowing oxygen through the hole. Seems pretty safe to me as long as you don't catch your &*( on fire. We all know that oxygen does NOT blow up and ONLY supports combustion.

LabRatMatt (author)CPUDOCTHE1.2017-07-14

Very cool! This was more of a model than an actual engine, as it doesn't produce a significant amount of thrust, but the same concepts are applied to actual suborbital engines. I would like to test other types of fuels in this engine, which is why I didn't just poke a hole in a piece of wood.

doomsday55 (author)2017-07-14

My god the safety nazis are out in force over this, get over yourselves and appreciate what the guy has done as a first foray into experimental rocketry, If some of the pioneers of rocketry had took advice from some of you we would have gone backwards.Stop wrapping every on up in cotton wool and let them make mistakes that is how we progress in life, Well done for probing into the black arts of rocketry and good luck for the future.

LabRatMatt (author)doomsday552017-07-14

Thank you! Yes, it would appear that a large portion of these comments are concerning safety, which is perfectly valid in a project like this. I appreciate their concern and input, especially with a project like this, though I do think a few have exaggerated the dangers of this engine.

micheld51 (author)2017-07-14

The interesting part is that, in hybrid motors, as soon as the pressure in the combustion chamber rise above the pressure of the oxydiser tank, the flow of the oxydiser stops and the pressure in the combustion chamber drops. This is a sort of self regulation of the pressure... Could we say that if the combustion chamber is build strong enough to hold the same pressure as the oxydiser, it is safe?

I wonder what was the performance of this engine. Having a beautiful flame is one thing, having a thrust is another...

LabRatMatt (author)micheld512017-07-14

Interesting! The thrust produced is very small. This was more of a proof-of-concept model than an actual rocket.

Omarsibles (author)2017-07-14

With all tje yacking that is being posted, I figured this isn't gonna sound as bad as it would under different circumstances...

An argument can. E made that what you demonstrated there was nothing like a rocket. A "rocket" is defined as "a cylindrical projectile that can be propelled to a great height or distance by the combustion of its contents, used typically as a firework or signal".

While your "rocket" is in fact of cylindrecal shape, it was static, not propelled, it remained at the ssme height, wheyher it was lit or not, and it did not move to great heights or distance... it did not move at all!

What you demonstrated in that video was a "torch"! Actually, a "torch with potential to become a rocket".

Just kidding... Well done!

LabRatMatt (author)Omarsibles2017-07-14

Well thanks for clearing that up! Yeah this is a model of the engine not an actual flight-ready rocket.

tsturtevant (author)2017-07-13

This is dangerous. Wow you were lucky. Maybe you should remove this constructable to avoid a chance that others are not injured. I know you posted a disclaimer. But those are just words. Good luck

itsmescotty (author)tsturtevant2017-07-13

really??

Getting out of bed is dangerous, the bathroom floor might get wet after your shower and you could fall. Driving in your car = possible accident. Your phone could explode.

Come to think about it staying in bed could be dangerous, bedsores, lack of exercise means fat which is bad for your circulatory system leading to heart problems - the lists for both options are endless!

The leading cause of death in the world is living!

nearboston (author)itsmescotty2017-07-13

When I get out of bed there isn't an open canister of O2 and a lit macht sitting on the floor

itsmescotty (author)nearboston2017-07-13

Doesn't negate the dangers of getting out (or staying in) bed. Life is dangerous, it's how you chose to live it.
I'll bet you have a problem with commercial divers that place their lives in the hands of a person rubbing the dive station and the support equipment. Think about being 170m underwater knowing your life hinges on dozens (or more) variables. This simplistic 'rocket motor' would probly never fly because of the weight and an ancillary oxygen tank. Having been a missile technician in the navy and a deep air/mixed gas commercial diver I have some experience.

nearboston (author)itsmescotty2017-07-13

No. In fact i would rather enjoy seeing the support people rubbing all over th dive station and support equipment.

Being an adult and choosing a dangerous profession is a leeeeeetle bit different than having a highschool kid post bad instructions on how to hook up an O2 tank to a pipe and set a match to it.

But...rock on dude. I actually have a lot of respect for people who do what you do.

srilyk (author)nearboston2017-07-14

What's the difference between an adult making choices with dangerous things and a teenager?

Yes, teenage brains are not entirely formed up to calculate risk that well, and some of their decision making skills might not be entirely up to par, but I'd much rather have a kid trying things and actually *thinking* about safety than an adult who's just too rushed to bother.

The only difference between this kid now and in 10 years will just be more experience, and the more of it he gets earlier on the better he'll be.

LabRatMatt (author)nearboston2017-07-13

Very true

LabRatMatt (author)itsmescotty2017-07-13

Yes, life is dangerous. But it becomes considerably more dangerous when rockets are involved, so I understand the concerns in this comment thread.

LabRatMatt (author)tsturtevant2017-07-13

There appears to be a large argument brewing in the comments over the safety of this project. I have no intentions of removing the project from this site, though I appreciate your concern. I trust that anyone attempting to re-create an engine such as this will take adequate precautions or at the very least use common sense. I was not lucky. I spent hours researching the risks and mitigations before even beginning this project, and I took extra safety precautions off-camera.

GWR1 (author)2017-07-13

As someone with some experience in High Powered Rocketry if you are looking to make your own large rocket motor for actual use in a rocket you are better off using a solid propellant such as ammonium perchlorate or a sugar and stump remover motor. As far as hybrid motors go you guys are really lucky you didn't get hurt. If you went to any reputable rocket launch site and tried to test or launch this motor the way you show in this instrcutable you would get a long talk and a complete revamping of your procedures. You should NEVER be lighting a motor with a match whether it is solid, hybrid or liquid. The safety procedures around the handling of your oxidizer are also not sufficient. Finally your test stand was definitely not safe you are lucky that your motor did not produce very much thrust because it could have easily toppled your test stand for this reason most rocket motors are tested with the nozzle pointed towards the sky and a good amount of the motor buried in the ground. You should really look at the National Association of Rocketry website nar.org and the Tripoli Rocketry Association at tripoli.org for safety procedures and information on how to test, and Launch rockets safely. I hope you can find a NAR or Tripoli club near you so that you can learn more about High Power Rocketry and how to test and launch safely

Nyxius (author)GWR12017-07-13

Mistakes are the best form of education. Procedure does not prevent mistakes, it shifts blame. The goal of a good safety policy is to limit the damage of a mistake such that one can learn from it. I thought they did a great job with what they had available. For future lessons I would recommend that they understand tipping problems and how to estimate thrust. Safety practice is not good engineering, but good engineering practice is safe.

nearboston (author)Nyxius2017-07-13

This is the biggest crock of bull I have ever read!

"Mistakes are the best form of Education"?

"Procedure does not prevent mistakes it shifts blame"?

"The goal of a good safety policy is to limit the damage of a mistake such that one can learn from it"?

Lies, lies, and more lies!

You are more dangerous than the kid with a can of oxygen and a match!!

LabRatMatt (author)nearboston2017-07-13

Yes, I would somewhat agree with that. Mistakes (especially around rockets) are probably not the best form of education.

srilyk (author)LabRatMatt2017-07-14

Mistakes around rockets are usually called "explosions", and have an unfortunate tendency to be fatal. But, they *are* an effective form of education for those who are still around.

AlphaOmega1 (author)nearboston2017-07-14

Nyxius is correct, the point of a risk assessment is to understand the risks, and when it goes wrong, you hold it up to the coroner and say, look, we followed these procedures, I'm innocent!

When sitting on H&S committees and writing risk assessments, you should hold one word at the forefront, "Reasonable". Are you taking reasonable precautions? And one thought, "who will the coroner point at in the event of an accident?"

You can write all the guidelines you like, accidents will still happen, have a chat with NASA.

If you are playing with kit in a field on your own, the guidelines will be different from working with others in a formal club. Why? because people want to pass the blame.

Today, it's ALWAYS the fault of someone else. Stick you hand into the blades of a mincer, it'll be the fault of the manufacturer!

To LAB RAT MATT. Well done!

LabRatMatt (author)Nyxius2017-07-13

even I'm not sure if I completely agree with that. Mistakes are a very effective form of education, but I'd rather not make them with a rocket.

LabRatMatt (author)GWR12017-07-13

Thank you for your concern. I will check the NAR guildlines before I do anymore rocketry experiments. I would also appreciate it if you elaborated or explained why my handling of the oxidizer was insufficient and how it should be handled.

Tesla_Shock (author)GWR12017-07-13

GWR is right. Rocketry is really dangerous. I almost had my head taken off by a high powered rocket that fell as it was detonating. And that was the last time I ever tried a makeshift launching pad....

JohnS1107 (author)2017-07-13

You do realise without the whole in the end, this is a pipe bomb !!!

Not only that, if an overly nosey neighbor sees this and reports it to the police, you could end up on a terrerist watch list of some sort. Just sayin

CPUDOCTHE1. (author)JohnS11072017-07-14

It looks like a pipe WITHOUT the BOOM to me.

Nyxius (author)JohnS11072017-07-13

This cannot be passed off as a pipe bomb. A) there is no binary mixture of fuel and oxidizer, until it is running. B) there are watch lists for everything. If he's smart enough to build this, then he's already probably on 2 or 3. I don't even bother to count the ones I'm on.

AlphaOmega1 (author)Nyxius2017-07-14

Igniting it might be an issue too.

Phone taps can be fun, especially when the 'tapper' doesn't know that the 'tappee' knows.

LabRatMatt (author)JohnS11072017-07-13

I've done lots of research in preparation for this project. Things get very dangerous when there is no hole to relieve built up pressure, but I made sure the hole in the nozzle was larger than it needed to be to ensure no extra pressure was built up. Additionally, there are weak points in the joint between the oxygen hose and the combustion chamber. If pressure does begin to build, the hose will detach (and likely catch fire) but the engine will not explode.

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