Micro-Hybrid / Mini-Hybrid Thread

Whenever I start a new project I like to create a thread for it just for posterity's sake; most of the information I'm able to gather from the internet comes from finding threads other people used to archive their trials and tribulations, so I have to pay back into the system whenever I learn things. Here I am going to document the hopefully non-disastrous attempts at using a mini-hybrid rocket motor.

This is an incredibly expensive and relatively low performance way of building and flying an E motor, but it is valuable and entertaining as a way to learn a little about hybrids (which I've never flown) and gives me a platform to do a small amount of propellant research (which I've never done).


The mini hybrid motor kit gets you the aluminum parts here, snap rings, piercer, and a graphite nozzle. Everything else is on you.

Here's my BOM so far:

Mini Hybrid lightweight motor hardware $94 (+$7.95 S&H) Micronitrorocketry.com
Propellant Mandrel: Precision Ground 3/8” Aluminum Rod 12” Length $15 Amazon
018 Silicone O-Ring 100 pack $8.16 Amazon
009 Buna-N O-Ring 100 pack $2.08 Amazon
3M Super 77 Multi-Purpose Spray Adhesive $5.82 Amazon
Scotch Postal Wrapping Paper 30”x30’ $5.49 Amazon
DuPont Krytox Grease 0.5oz Tube $14.99 Amazon
16g Whip Cream Chargers (15 ct) $34.49 Amazon

Please note that by shopping around you will likely get better prices on whip cream chargers and mandrels, I am simply someone who has made peace with his Amazon dependency and doesn't care anymore. The expensive as heck Krytox grease can also possibly be ignored by simply using a new o-ring each firing, but I've never used krytox so I figured why not check it out.

Mini vs Micro: As I understand it the primary difference between mini and micro motors is that mini uses the larger 16g nitrous chargers and micro uses the smaller 8g nitrous chargers. The guide notes that because the chargers are filled to approximately the same pressure, just at different volumes, each motor will produce about the same amount of thrust though the mini should burn twice as long.

Fuel Grains: I’m going with simple a paper grain since it’s super easy to start with, there are also some research recipes in the guide so be careful where you post details of the file. I rolled four 7-8" wide lengths of paper grains, which was pretty easy (if messy with spray glue) and should yield about 8 firings worth of fuel. Next time I will just use watered down wood glue (way cheaper and easier to control). After letting them dry for a day a cut them into ~3.1" lengths using a mitre hand saw, and sanded the faces smooth and flat with a power sander. This was overall super easy.

Pre-heater Grains: Everyone recommends using 18mm blue thunder reloads. I don't have any and they seem expensive, but I do have a bunch of 24mm reloads kicking around so I sacrificed one F39 grain and used an xacto knife to cut it into several pre-heater grains.

Burst discs: The Micronitro guide recommends using soda bottle plastic (PET) or ping pong balls and cutting correctly sized circles out with a punch. The website sells a punch for like $30, but I decided it would be easier and faster to cut some out on my laser cutter. I sliced the top off of a thin plastic water bottle and created some flat strips out of it, then cut out various sizes of circle to ensure I'd find one that made a good seal between the O-Ring and the piercer carrier. I ordered some flat sheet of PET from amazon, but it's thicker (0.020") than the guide recommends so we'll see how it works.


Assembly of a motor for firing is a little tricky:
Put a fuel grain in the bottom with an O-ring on either side, slide the nozzle in behind it, nozzle washer, and install the bottom snap ring. (Easy part)
Use a 3/4" dowel to tamp down the fuel grain and O-ring from the top side.
Assemble the preheater/piercer: APCP grain goes in the bottom side, top side gets a burst disc, O-ring, and the brass piercing nozzle. Those last three parts sit loose until they are compressed during final assembly, and they are very easy to shake out of place. Be careful!
Wrap some masking tape around the nitrous cartridge to ensure it fits snugly then slide it in carefully- too fast and you'll bump into the piercer and release your nitrous, but if you hold the motor horizontally then the piercer will fall out of place and you'll have to disassemble the whole thing and start over. Here is the assembly in question, halfway down the motor tube on top of the fuel grain:


Add the screw plate and install the snap ring on the top side.
Screw in the pusher screw from the top which slides the nitrous cartridge against the piercer.

Just before flight/fire prep you'll screw it in to puncture the cartridge, and nitrous pressure will be held back by the thin plastic burst disc and sealed by the upper (small) O-ring. Wear protective gear!

Assembled motor ready to fire:



So now that I've verified that my water bottle burst disc holds pressure effectively, I'm ready to test fire. I need to hit the hardware store today to get some pipe clamps so that I can hold the motor against something heavy as an improvised test stand. Video to follow.

Very informative, you are walking a well thought out path. As for the burst discs; I'm curious to see how well they burn through. For me, I would probably opt for the "punch" to create discs, as I do not have access to a laser cutter. Fuel research should be fun for this project... I'm thinking cored or poured candles could be interesting project.

The laser cutter isn't necessary I'm just a nerd for toys.

Even without the punch, the burst disc size is not dependent upon a lot of precision. It has to be:
-big enough to allow the 009 O-ring to form a sealing surface on it
-small enough to fit inside the piercer well

The guide describes it in a way that made me think it needed to be super precise, but after staring at the schematic for a while I emailed Lee Dexter from micronitro and he confirmed that as long as you cover those two bases it should work. Worst case is that you will not have a seal and your nitrous cartridge will vent to atmosphere during assembly, which is a little wasteful but not a big deal. It is therefore possible that you could just cut some by hand using curved scissors, a knife, or simply get a piece of brass tube with sharpened edges to act as a punch.

Here is a punch that I made for my burst disc.

On the fuel research:

The guide I keep referencing was emailed to me when I purchased the motor but is available for purchase separately. It's very well done and fairly complete, including appendices that have full guides for making fuel grains from kraft paper (like I did), HTPB, plastic tube/pipe, and spent Estes motor casings (paper). The thing I found fascinating (and what got me to buy this in the first place) was the multi-burn possibility of using Estes motors with -0 delays to provide initial thrust then ignite the pre-heater grain. This gets complicated, but the guide describes it well.

op:
Subscribed. I really want to see how these things work.

I have had the original Rene Caldera micro for years. I have made several grains and have used the C6-0 trick. All worked well, but without a stand, I couldn't detect any performance differences. The guy who machined it for me made a range of injector sizes centered around the recommended one. I never could detect any performance difference between them either. In theory they would change the burn rate vs. the total impulse.

The only 'trick' I would offer is to shim the nitrous chargers so they are very well centered in the motor. On the micro they are quite loose and, until I started shimming them with a sectioned piece of tube, I occasionally would get some leakage when the motor was primed. Now, I could detect the performance difference when this happened.

Also, the nozzles can be machined from materials that are cheaper than graphite. My guy made me aluminum ones from some stock he had and even one plastic one. They may wear faster than graphite but there was no detectable erosion after one firing on the plastic. Hmmm, maybe that's nylon....I can't remember.

Your mini looks like fun.

rstaff3 said:

The only 'trick' I would offer is to shim the nitrous chargers so they are very well centered in the motor. On the micro they are quite loose and, until I started shimming them with a sectioned piece of tube, I occasionally would get some leakage when the motor was primed. Now, I could detect the performance difference when this happened.

Click to expand...

I started putting 2-3 o-rings on top of the cartridge it helped stop the small leaks and helped center the cartridge.

I'm just using masking tape so far, about two wraps has made it pretty snug.

That will work!

Static test number 001 is now complete. Zero casualties, collateral damage minimal.

https://youtu.be/fhHgLEhtgfM

As expected the motor goes crazy hard for a fraction of a second followed by a long regressive burn.

From igniter lighting to burning nitrous was about 1.03s.
The huge blue/purple flame and thrust spike up front lasted about 0.17seconds (I assume this to be nitrous burning itself at the max pressure of my cartridge before the fuel grain has a chance to light but will have to research)
A steadily declining orange flame lasted for 1.4 more seconds after that, followed by 2.6 seconds of sputtering.

If measurable thrust is connected to visible exterior flame then I'd say the motor lasted about half a second less than it's predicted to. If it's not necessarily connected (like the weak end of the regressive burn can still be making a small amount of thrust for that half second without a big orange flame) then it probably performed as expected. I've read online that the fills from cartridge to cartridge are not necessarily consistent and you can expect variability depending on brand.

Later when I'm ready I'll dissect the motor and see if everything survived.

My "test stand" was just a few spare pieces of rail I bolted together and held on the ground with weights. I guess I need to get a load cell and build a legit stand now, since without data I really have no clue about what's going on.

Makeshift test stand:

A t-nut screwed in above the motor provided a thrust block, so all the pipe clamps had to do was keep it secured to the rail and centered radially in the groove.

Paper fuel grain post-firing:


O-rings in fine shape after firing:



Weight of all components after firing: dropped from pre-launch weight of 141.3g to burnout weight of 122.6g. That may not be a very good mass fraction considering there's no rocket attached.

This is VERY interesting. Thanks for posting!

Excellent work, that motor looked to burn real nice, congratulations!! Those 16g seem to be the ticket to a nice long burn motor, a hybrid characteristic I prefer in the larger motors. Keep up the good work, what is next?

Thanks guys. I ordered some stuff to make HTPB grains, so that will be the next project. Until that happens, I'm going to use the Art Applewhite stealth pyramid plans to make a tumble recovery flight test vehicle that I can launch.

The blue was your preheat grain.

I've contemplated buying one of these to play around with. Looks pretty cool.

Very cool, I got one of these a few months back to play with but I've been too busy to do anything with it. Do you have a picture of one of your cut-up F39 preheater grains? Also, what igniter are you using to start it?

If you have a research crowd nearby, they will undoubtedly have some leftover grains that they can give you. I got a 38mm grain and it has lasted a long time. I probably will never need another as I only fly mine one a year or so. I use a properly sized brass tube to cut the plugs.

patelldp said:

The blue was your preheat grain.

I've contemplated buying one of these to play around with. Looks pretty cool.

Click to expand...

That makes sense, but I didn't anticipate that such a small chunk of it would produce such a large flame. Is the remaining burn of APCP following burst disc failure magnified by the onrush of N2O? I understand that part of the reason for using APCP as a preheater is to raise the temperature of the nitrous to the point that it will disassociate and that it can burn as a monopropellant even if the fuel grain isn't there. Is it common to get such a spike during preheater burn when igniting hybrids? Would that part of the visible flame measure on a load cell as the high initial thrust?

cerving said:

Very cool, I got one of these a few months back to play with but I've been too busy to do anything with it. Do you have a picture of one of your cut-up F39 preheater grains? Also, what igniter are you using to start it?

Click to expand...

Thanks. I also use a brass tube (17/32 OD) to cut the grain, makes it really easy. For this first firing I just used one of the tiny motor lighters that came with that 24mm reload kit that I stole the grain from. The guide it comes with gives instructions for making your own, I bought the supplies but haven't gotten around to it.

rstaff3 said:

If you have a research crowd nearby, they will undoubtedly have some leftover grains that they can give you. I got a 38mm grain and it has lasted a long time. I probably will never need another as I only fly mine one a year or so. I use a properly sized brass tube to cut the plugs.

Click to expand...

I am looking for one around Tampa so I can learn more about motor making, but that would be a good bonus.

I also wanted to put in a happy plug for Lee Dexter/Micronitrorocketry who sold me this thing: he's responded to all of my emails asking questions quickly. When I was figuring out the burst disc manufacturing I asked if I could buy some and he mailed me a dozen for free just to get me started, with a nice note.

The reality of a lot of hobby businesses is that they are side projects for folks with other concerns, which means that the vendor's attentiveness to customers' concerns will wane and fizzle over time. I have no idea how long he's been selling these or how long he will, but he's doing great right now so if you have been thinking about one of these you should get in while the gettin's good.

smapdiage9 said:

I also wanted to put in a happy plug for Lee Dexter/Micronitrorocketry who sold me this thing: he's responded to all of my emails asking questions quickly. When I was figuring out the burst disc manufacturing I asked if I could buy some and he mailed me a dozen for free just to get me started, with a nice note.

The reality of a lot of hobby businesses is that they are side projects for folks with other concerns, which means that the vendor's attentiveness to customers' concerns will wane and fizzle over time. I have no idea how long he's been selling these or how long he will, but he's doing great right now so if you have been thinking about one of these you should get in while the gettin's good.

Click to expand...

+1 Lee is the best! I urge anyone who is interested in these motors to shoot him an email!

Great work James! It looks like you have made some very nice paper grains and the video of your first burn looks good and very clean! As patelldp pointed out above, the blue flame following ignition is the remnants of your preheater grain burning off as the nitrous starts to flow. The reason your burn time may appear to be shorter than normal is due to the size of your injector. For about the last 2 years I have been supplying the minis with an 0.025" injector as standard instead of the 0.020" injector used in the original static tests shown in the User's Guide (and as is still supplied with the micro). You should notice a boost in trust and a reduction in burn time.

Lee Dexter
Micro Nitro Rocketry

Thanks! That's good info about the injector, I was wondering why it was so much shorter.

I did complete assembly of my Amazon Prime Pyramid (not priority stealth pyramid!) for this motor's first flight, but got so caught up trying to recover my big rockets out of a swamp this month that I didn't get around to the test launch. Don't worry everyone, it'll fly soon!

Finally got to fly my mini hybrid this weekend, and everything was nominal. It was my first hybrid, my first research motor, and my first oddroc so pretty exciting! The high drag pyramid looked funny with so much of the motor sticking out of the top and while it leapt off the pad it was a slow, but rapidly spinning, ascent to what looked like 200-300'. It was definitely spinning on the way down but I am skeptical of what value the "helicopter recovery" offered above it simply falling since it went down almost as fast as it came up. Pretty loud for an E motor and the sputtering at the end of the burn made it unique, lots of people wanted to know about the motor which is always fun.

Notes for future use: Field assembly was very simple because I've assembled it a few times before, but it could go wrong easily if it's your first time so practice before bringing it out. Wind makes a difference when you're dealing with tiny parts! The screw is either a weird size (or just not metric) since neither of the allen sets I brought had a key that fit (between 4mm and 5mm) but I made do with a small set of vice grips. Flying a tiny rocket from the far pads because its EX means wide angle phone video is entirely useless so set up a camera near the pad next time. The 1/4" rod fit snugly, I will try to sand down my grooves a tiny bit next time.

Does anyone know what happened to Micro Nitro Rocketry? I was planning to buy one of these systems this fall/winter. I had bookmarked their website, but now it appears to be down.

He's on indefinite hiatus as he does other things. I was looking forward to picking one up as well. Theres a guy in the UK doing the same thing but shipping will be a pain.

Does anyone know if this is a legit alternative?

https://aeroconsystems.com/cart/rockets-and-rocket-motors/screamer-16g-hybrid-rocket-motor/

Aerocon will occasionally show things they don't have for sale. This might be one of them. I thought Screamer was unavailable as of a few years ago. Hopefully I'm wrong.

Gerald

I just called Aerocon. The S'Creamer isn't available either.

Does anyone have any other suggestions for a low-cost entry point into hybrids? I personally don't mind handling cylinders of N2O (I'm a chemist), so the whip cream canisters weren't the thing that attracted me to these systems. I was just thinking that I would get the most use out of something in the MPR/low-HPR range.