Steel casing keeps exploding

[priitserk]

Hi

I'm using KNO3/Sugar 65/35 custom home made propellant.
Propellant is excellent, well made, using dissolve and heating process.

Steel casing:
25cm length, 5cm diameter (propellant 20cm with 5cm diameter, core 0.8cm )
Nozzle with 1.2cm diameter

Picture of the casing and the custom nozzle


The problem is that this design keeps exploding quite immediately.

Can it be that high pressure presses propellant directly into the nozzle, clogs it and CATO?
By OpenMotor the peak PSI should be 500 and steel casing can easily stand it.

My thesis is the only one I could think of... Any ideas? Has anyone experienced anything similar? I do ignite it not from the head but directly from the bottom core.
This is as far as I have got:


At this point I recorded 12kg thrust but in the next millisecond it exploded (probably huge PSI peak and only thing I could think of was that propellant clogged nozzle - casing itself usually flies 50-60meters after the explosion, so the PSI is huge) and shread off steel cap of the engine. Sounds possible or something else? I'm thinking on redesigning the casing and adding washer between nozzle and propellant that should help to avoid propellant clogging the nozzle.
Ive done like 6 tests already and same result - CATO. And it alwasy shreads off the cap of the casing.

Edited November 2, 2019 by priitserk

[priitserk]

another theory i just found is that there can be air bubbles in the grain. Need to think a bit on how to check it and if they are there then how to avoid them.

[patsroom]

I'm not much of a rocket person. But I will say that steel and even plastics are some of the last choices to make a rocket motor from. We are talking about the main casing material' the nozzle material should be OK. Paper tubing would be the first choice as if it CATOs then the harm cause is reduced. That being my two cents worth I will retire from this post...................Pat

[MadMat]

What is the ratio of nozzle diameter to combustion chamber diameter? From the outside view, It looks like your nozzle is rather big, but can't truly tell. How thick is this steel casing, 500 psi is pretty dam high considering a propane cylinder is usually at less than 200 psi. So you may just be over-stressing your casing. Most times the nozzle should be 1/3 the diameter of the ID of the combustion chamber. Maybe you need to go slightly larger than 1/3? When you say you ignite from bottom core am I to assume that your rocket is a cored grain as opposed to an end burner? If you are running a core try a run with an end burner configuration. If it still explodes, your hypothesis could be correct, but personally, I doubt it. As you mentioned, you could be experiencing is voids in your grain. How are you filling you engine? I ask this because with a welded top cap that makes me assume you are filling your engine through the nozzle. this would be VERY conducive to creating voids in your grain. Just some ideas

Edited November 3, 2019 by MadMat

[priitserk]

Got it solved. The cap was too weak. Doubled stainless steel rivets got the job done.

@Patsroom - stainless steel is excellent for rocket engine casing. Why? In case of CATO it does not explode into small pieces. Weakest part is just teared off. Paper tubing is not for me - no way to carry decent payload nor can it handle a bit bigger pressures.
No worries, Im using safety in highest standard. Remote ignition, blast zone and concrete walls between myself and the rocket.

I'll also make my thread where I'll provide more info on my tests.

 

First successful test after 6 failed ones (three months of reading, studing and testing)