Hybrid Rocket Engines

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Dec 22, 2016
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Hey guys! I'm currently performing a research project for one of my engineering classes on hybrid rocket engine design, particularly in regards to homemade designs. While researching, I've found examples and demonstrations of engines made out of acrylic or rubber but I was interested if anyone on here knew of any other materials being used as the solid propellant? I also found that most people used either compressed air, oxygen, or nitrogen as their oxidizer which I understand because those are the most suitable oxidizing gases due to availability and cost. However, if any of you have any knowledge of any others being used, feel free to leave a comment below. Lastly, I am also researching how normal people are making their engines safe to use and be around. I've found examples of people extending strings from the throttle so they can activate them from a distance, as well as having an emergency cut off switch that can end the engine at any given moment. If you have any examples of features others that may benefit safety, I would greatly appreciate it! Thanks!
Ok, lets get back to basics.

There is no such thing as a safe rocket motor. A rocket motor contains very energetic materials, which when combusted as per design, yield thrust. But there is no guarantee that it will work as per design.

I am one of the people who does some work on experimental hybrid rocket motors. My "little" test motor has a safe standoff distance of 1500', IIRC. If something goes seriously wrong you don't want to be nearby. Rocket motors are NOT forgiving of design or operational errors, and even when done right, still might CATO or do other bad things. Those videos you see of people starting motors with strings, or turning a valve on a tank by hand, are videos by people who really don't have a clue what they are doing. They are serious accidents waiting to happen. DO NOT EMULATE THEM!

By the way you asked your question, I'm going to assume you have no background in chemistry, probably not much yet in physics or engineering? I'm guessing you are a student. Please do not take this the wrong way, but you need a lot more knowledge and study, plus ideally some experience, before trying to design/make your own hybrid rocket motor.

Rocket motors are fascinating!

My recommendation if you are really interested is to join the NAR or TRIPOLI, and go through the certification process through level 2. At that point you can start to be able to fly research motors, provided you are an adult. IMHO, if you are not an adult, do not work on research motors. Research motors are ones that you engineer and produce, rather than purchasing commercially.

Now if you are simply researching what others have done, and are not going to work on producing a motor, then that is fine. But you will find that lots of the details you might want to get are not as available here as you might think. Rocket motor and propellant/oxidizer/fuel design is restricted to the research section of these forums. That subforum requires US citizenship and level 2 certification to get access. The reasons are (1) we don't want kids and adults killing or seriously injuring themselves or others, or destroying property, (2) we don't want the reputation of having helped someone do #1, and (3) there are ITAR and other regulations in the US concerning rocket technology.

For instance I've put enough info into a thread of mine in the research section for a dedicated person to essentially reproduce my first EX hybrid rocket motor. I spent quite a while designing before starting to machine the parts, and the machining took me longer than it should have. Others here have more experience than I, but again, they are not publishing their designs out in the open. It would be nice if ITAR didn't exist, etc, but that's not the case. Sorry.

Keep in mind that in the end, the only difference between idiot, amateur, and professional rocket motors, is the extent of the knowledge base and the size of the budget. There are some small professional motors. There are some decent sized amateur ones. And then there are the things the idiots do. Please don't be one of those!


PS - Generally a hybrid rocket motor does not use a solid propellant, but does use a solid fuel. A propellant has fuel and oxidizer both present in the mix. A fuel is simply something combustible. That's why hybrids are safer before you load the oxidizer. The motor can't go boom if the oxidizer is not present. The motor only contains a flammable solid. Most anything that will burn will work. Some things of course work much better than others.
AS far as propellant compositions, your best bet is the Hybrids 2017 thread (or 2016, 2015, etc....)
Anything more than that is pretty much only on the Research forum (You've heard this before if I remember correctly)

For safe activation; Remote GSE (Ground Support Equipment)
That's flow valves, Oxidizer fill operations, ignition, everything. A bit more involved than the good old solid rocket igniter.
One minor caveat: forum rules require evidence of a HP cert, not an L2 cert.

FLYING an EX at a TRA launch does, of course, require the L2.
For safety, distance equals time and that is your friend.

Last June a IREC team I mentor had an over-pressurization of their motor on the test stand. The casing is 4" diameter x 88" long and the nozzle was approximately 7 feet off of the ground. The stand is a 6" I-Beam anchored with ~2500 pounds of bricks. The casing was found 125 yards away. The mid bulkhead, after bouncing off the flame deflector was found 50 yards away, and it impacted a piece of sheet metal. It left quite the imprint on the sheet metal at that distance. Anything with mass traveled much further than I would have expected. Not to mention the large ensuing fireball because the motor is a tribrid and uses alcohol as the fuel.

The team was approximately 100 yards away, around the corner from the motor and was in a shipping container half dug into a hill that they turned into their command center. They were using remote video to monitor and watch the test. After putting out the fire, we took sprinkler flags and marked the location of all the pieces of the motor we could find. This really let them, and myself, see just how far things travel, and that we have minimum safe distances for a reason. Also, video lets you watch the test in more detail than you would be able to in person.

There is no safe rocket motor, there are only degrees of risk.

PS - Generally a hybrid rocket motor does not use a solid propellant, but does use a solid fuel. A propellant has fuel and oxidizer both present in the mix. A fuel is simply something combustible.

And nitrogen wouldn't be a very good choice as an oxidizer (unless you want a really really safe motor!).

(I know. The OP meant nitrous oxide.)

-- Roger
Thanks for the quick replies! I'm not planning on building anything myself it's strictly for research purposes but I understand the concern. Many of you guessed I'm still a student which is correct so I don't have the time or money to try anything this complex on my own. I sadly don't have HP certification so I don't have access to that subforum but it sounds really cool; I may try to get access one day. Thanks again!
L1 cost: TRA membership, local club membership, some mailing tubes, and whatever disposable motor your onsite vendor has in stock.