Hi

I'm not native English speaker so please forgive me my grammatical and vocabular mistakes

I've spent months on reading and studying rocketry and 3 months ago decided to give it a try. At first, it really seemed easy - take KNO3 and Sugar, mill it, pack it and fire. Well, the first attempt actually worked and my miniature rocket took off. But after that I failed to repeat it - and I could not understand whats wrong. Usually there was no thrust - lot of smoke but that was all. Then decided to add iron oxide but this resulted in CATO. Chemically it did not make any sense, because adding 1% of iron oxide could not make such a big difference in propellant burn speed. After that I realised that I have something completey wrong with my propellant making process.

* Important note - its my obsession that I'll make everything myself. The casing, the propellant, the electronics, nozzle, parachute, flight control systems etc ... Ofcourse, some electronics I need to buy but overally, I'm doing the engineering myself.

* The Goal - build a multistage rocket with altimeter, HD camera, parachute and microprocessor based data loggers able to reach at least 1km.

To make it more interesting, I'll post a Youtube link that shows the last successful static engine test. After that, a story how I got to that result.
Recorded with Iphone 8
Playback: 240fps
Recorded thrust: 16-18kg (from slow motion)

Safety.
I know that there are lot of people worried about safety and believe me, from that point of view, I know what I am doing.
* remote electric ignition
* safe blast zone surrounded with concrete walls
* big fire distinguisher
* Not a soul living around me (kilometers before any other people) - so also sound and bangs are not the issue
* Thorough studying of Nakkas publications, consultations with chemists and physics.

The propellant
I'm using well known KNO3/Sugar 65/35 ratio. Once I got it working, it's extremely powerful propellant for DIY rocket engines.
* You will never need Fe2O3 or other aluminium additives - I suggest not to use them because they even more fasten burn rate and will most probably end with detonation known also as CATO.

1) First I started making propellant by milling both components separately and mixing them together and packed into casing. This resulted in very poor results and each and every test gave different readings. I ditched this method. Most of the tests ended with CATO meaning pressure pike increased burn rate so much that it caused a detonation (explosion).
2) My second attempt was to dissolve both components in water and heating the water out. I thought that I was doing it correctly but the propellant did nothing else than lot of smoke. From beginning it seemed a bit strange as when adding pressure to cooled down propellant (with a thumb) it went back "liquid" - so clearly the water was still in the crystals or there on the molecular level.
3) Finally, after consulting with chemist who showed me proper heating process I managed to do it! Dissolved both components in 90C water and heated the mixture very carefully until it got really really thick, constantly stirring and watching temperature closely. After the water is mostly out, the mixture gets really thick and it's hard to stir it. If that one cools down, its "rock hard" and when taking a small piece of it and when igniting it, the small piece of the propellant itself "flies off" (really fast burn rate) - it's really cool effect

* make sure you have good KNO3 - make the propellant, burn it in the casing and look for residue. My casing are mostly clean, very very little residue left. If you have lot of residue, your KNO3 most probably is with bad quality or there is something completely wrong with the propellant mixture.

* I did lot of tests with Fe2O3 and for me this was too much. Burn speed fastened alwasys too much and I was not anymore in control - ended always with CATO. So at least for me, using any additives at this point is pointless. I dont need that extra punch, I need to decrease the current punch!

Casing
My decision from the beginning has been to use stainless steel casing. Why?
* it's way more safe than PVC
* It handles way more PSI than PVC
* Its much easier to construct lightweight stainless steel constructions that anything else
* I'm doing all the welding and other machinery stuff myself
* Stainless steel casing in explosion does not throw small metal fragments (sorry, my english - dont know the exact terminology, but you probably get what I mean). In case of explosion, weakest part is teared off, steel stretches and the weakest part is the nozzle or the cap.
* Paper and cardboard are not for me

From local hardware store I found excellent engine casing - a stainless steel pipe that has proper diameter(5cm) and length of 1m - which means I can cut it into several pieces. Total cost: 7 euros. Not so bad Grinded off the paint (otherwise you could not weld it), cutted it into 25cm pieces and prepped the casing for the nozzle and cap. For all the tests Im using at the moment the same dimensions - otherwise you really cant make any post test conclusions...
The pipe itself:


The Nozzle is also from stainless steel, well machined and strong alloy.


As both the casing and nozzle are from stainless steel it was fairly easy to weld them together. I know, the welding is hideous


Till that moment everything was really easy. The casing was there, the nozzle was there. Only things left were to pack the propellant to the casing and construct steel cap for the casing. And the cap proved to be complicated...

So after days of drawing and thinking I came up with the following Cap design (it's a paint drawing but you can get the idea):


* You can't really weld if the propellant is in the casing- so how do you close the cap? There's no thread! Glue? Nooo, no glue can withstand such PSI.
So what I tried:
1) My first test was to use bolts. I used M4 bolts. This test ended with CATO and the cap was just teared off, all the bolts bent. FAIL

2) My second test was to use M6 bolts. This test ended with CATO and the cap was again just teared off. The bolts were actually intackt but they were just teared off from the casing

Unfortunately I dont have the picture how it looked like but the whole cap with bolts were teared off from the main engine.

3) Then, after thinking about it, I realised that the problem is the THREAD of the bolt. When the PSI gets up, bolts THREAD acts as a knife and just cuts through the stainless steel. So this was a setback... I really did not know how to continue... After couple of days thinking I remembered an ingenious invention - rivets! They dont have thread and they fill the hole perfectly!

4) So this leaded to another test using 6mm aluminium rivets. Unfortunately this test ended again with CATO - the rivets were just cut off. THe cutting was so clean like someone had done it with very sharp knife. Threw away alu rivets and bought 4.8mm stainless steel rivets.

5) The last test, also the successful one, used double line 4.8mm stainless steel rivets.

As you can see I added two lines of rivets. And this made the work. The CAP withstand the PSI and the static test was successful as you can see from the video above.

Next steps

1) simulate current results also in OpenMotors.
Propellant length: 13.5cm
Propellant width: 4.8cm
Propellant core diameter: 0.8cm
Nozzle throat diameter: 1.2cm
Burn-time: will extract from slow motion video
Thrust: mechanical spring scale recorded 16-18kg of thrust (by slow motion).

I'll post the results as soon as I get them

2) PSI still seems too high. Will simulate different options and probably also try ligher KNO3 and Sugar mixture or two grains with different burn speeds (the last one probably is the best option) in effort to lower the PSI

3) Prep a third casing, nozzle and a cap for seventh test. This time also will add mechanical PSI measurement equipment to get even better understanding whats happening in the engine. This finally allows to actually compare numbers with OpenMotor software.

I do hope I can keep my blog here. I'm not trying to invent anything new, it's a hobby for me And hopefully, if everything goes well I can have the first launch test this winter somewhere in January and then already with altitude measurement equipment, HD cam, GPS tracker and proper parachute system.

Feel free to comment. I am more than happy if anyone suggest ingenious mechanical or chemical tricks/solutions.

Update 03.11.2019
run OpenMotor simulation and it seems to match pretty well with test. Attached img below.
Unfortunately this time I did not have mechanical PSI measurement system to verify OpenMotor results - will fix this for the next test.
Anyways, the results are here:

 

I'm surprised that PSI is so low. I expected it to be around 400-500 psi. If thats so, its a good news for me (or not) - this can explain why the cap of the casing did not fail this time.
I still believe I need to do additional test with mechanical PSI measuremnt system so I could verify OpenMotor results. If the PSI is correct then I can slowly start pushing it to find the limits of my new cap design.