Hobby rocket fail?

[FieryCreations]

I made some KNO3/table sugar rocket candy by melting and pouring into a small cardboard tube. While it didn't go far, it had a lot of thrust and wanted to. Pretty sure the giant random bit I drilled out the nozzle with was the cause.


This time I tried making TKORs screw in rockets with rammed powder. Unlike before, I dried the KNO3 first and used sorbitol in a 65/35 mix. I drilled the nozzle carefully with a 7/32 bit.

This rocket had almost no thrust. It didn't even wiggle on the launch stick...it just vented out smoke. Any idea what may have happened? I tried 3 home made motors that took forever and none of them did anything but smoke. I'm thinking something with the KNO3, because I tried lighting some homemade BP I made with it and it took forever to even light any sized mesh particles. Did heating the KNO3 do something to mess it up?? I checked the tube and it looks like 90% of the propellant did burn so wth?

[cmjlab]

How did you dry the KNO3?  In the oven at 175°F for an hour or 2 won't ruin it.  If you took a torch to it, I believe it thermally decomposes to some other potassium salt products, but I think that involves sustained temps above 350-400°f (a Google search will tell you the chemical properties of KNO3 such as boiling point, decomposition, etc )

[cmjlab]

Correction - my temps above should be listed in Celsius not Fahrenheit.  A quick search shows it decomposes at 400°C or 715°F

[FieryCreations]

I dried it at 300F for 30 minutes as he did in the video.

I think I found the problem. I did not grind or mill the KNO3. The first time I'm guessing because I melted it with the sugar and stirred it, the particle size didn't matter.

I made a small batch the way I did for the failed rocket motor and I couldn't even get it to light holding a flame on it. 

When you make BP or rocket candy is it a good idea to dry all components separately before mixing? Sulphur, sugar, charcoal, etc. ?

[cmjlab]

The charcoal and KN03 can certainly hold a decent amount of moisture.  It couldn't hurt to dry those.  I don't think sulfur really retains moisture, but it does melt at a relatively low temp so probably not a good idea there.  As for sugar, I have no idea how you dry that efficiently.  Maybe throw it in a drying box of you have one, or put in the sun on a low humidity day.

[FieryCreations]

I'm baffled. It wasn't because it was dry. I followed the tutorial exactly and once again it didn't leave the stick. It just sat there cranking out smoke. At least this time the ejection charge popped the parachute out at the end. 

This motor is 3/4" and says to use a 7/32 bit drilled all the way through the propellant. Does my fuel mix suck or is something else off? The 7/32 bit hole seems like it makes way too big of a hole for the ratio of propellant to hole. It looks much bigger than an Estes rocket nozzle.


We lit a small piece of Estes propellant and it went up all at once like good BP. My mixture burns waaaay slower. Estes looks poured while mine is powder. But so is the guy in the video's. What am I missing? 
 

 

[cmjlab]

I have no idea why you're results are different than TKOR's.  

Are you using Sorbitol or Confectioners sugar? (Your first post said you are using Sorbitol).  If using Sorbitol, you may need to find a formula specific to that vs. confectioners sugar.

Are you shaking the KNO3/Sugar to mix?  Often to get a good homogenous mix, screening or milling is recommended.  Even using a window screen in a frame to mix your KN03/Sugar propellant could make a difference.

You said earlier your KN03 was course.  Did you mill it down?  Particle size of the oxidizer will have a huge effect on how well it performs, which is why confectioners sugar is also a good way to go since it is already finely milled.

Hope you are able to figure out your issue, its often something simple we overlook!

[FieryCreations]

4 hours ago, cmjlab said:

I have no idea why you're results are different than TKOR's.  

Are you using Sorbitol or Confectioners sugar? (Your first post said you are using Sorbitol).  If using Sorbitol, you may need to find a formula specific to that vs. confectioners sugar.

Are you shaking the KNO3/Sugar to mix?  Often to get a good homogenous mix, screening or milling is recommended.  Even using a window screen in a frame to mix your KN03/Sugar propellant could make a difference.

You said earlier your KN03 was course.  Did you mill it down?  Particle size of the oxidizer will have a huge effect on how well it performs, which is why confectioners sugar is also a good way to go since it is already finely milled.

Hope you are able to figure out your issue, its often something simple we overlook!

Yeah I switched to Sorbitol. I did a tiny batch test and table sugar seemed more reactive, but not a ton.

I was mixing at 65/30. I dried my KNO3 and ball milled it to dust then mixed in the mortar ground sorbitol well. I didn't screen it though.

The first motor I tested was just on a stick and it seemed way more aggressive. For that I actually melted and poured the fuel. I think I'm going to see if I can use the same TKOR motor plan but pour it in instead of packing. Once it hardens I'll add the powdered delay mix and clay plug. 

Should powdered fuel be extremely different than melted/poured? That small piece of Estes engine was extreeeemely fast and aggressive compared to my mix. 

[cmjlab]

I would think that the melted stuff which is dissolved, then melted, would be much better incorporated, and would burn more vigorously, requiring a larger diameter nozzle.

Have you tried making a smaller core through the clay nozzle and into the propellant?  As in dropping down in size in 1/64" increments?

I can't think of any reasons other than those listed as to why it wouldn't work.  It's also been a few years since I've made a KNO3/Confectioners sugar rocket, I feel like I was using Iron Oxide (Rust) as a catalyst at the time too (1% if I recall correctly).

[cmjlab]

You got my interest enough that I went and read the following information to learn more about Sorbitol propellant.

Interestingly enough, both Sorbitol and KNO3 need to be finely milled prior to mixing, then milled together.  It also recommended putting the Sorbitol in a desiccator to remove extra moisture.

After the 2 have been milled together, the Sorbitol is melted (leaving the fine KN03 particles in suspension in the liquid Sorbitol) then casted into propellant grain or rocket motor.  

I believe that you will find the answer to your questions at this link. Hope it helps.

https://www.nakka-rocketry.net/sorb.html

[FieryCreations]

My first thought was the nozzle was too big, but after reading it sounds like going small can cause it to explode.

I made another batch of melted and it does burn way more vigorously again! Thanks!

[cmjlab]

One off the ways a nozzled b.p. rocket can be tweaked (aside from tuning the formula) is to change the nozzle size (unless of course you're using a pre-made spindle built for an already optimized formula). Most change the formula though, because they have an already predetermined size spindle they are using, so it's much easier to change the formula.

Short answer - yes, making the core too small can allow enough back pressure to build up in the core of your rocket and cause a CATO.  However if you aren't getting enough thrust, and can't change / don't want to change the formula, you XAN adjust the nozzle size smaller or bigger.

*Be careful if drilling into propellant that you are going slow, cleaning the flutes of your drill bit often, touching the bit to make sure it's not getting too hot, doing it away from any other flammable items in case it does ignite on you, etc.*

Edited December 16, 2023 by cmjlab

[FieryCreations]

2 hours ago, cmjlab said:

One off the ways a nozzled b.p. rocket can be tweaked (aside from tuning the formula) is to change the nozzle size (unless of course you're using a pre-made spindle built for an already optimized formula). Most change the formula though, because they have an already predetermined size spindle they are using, so it's much easier to change the formula.

Short answer - yes, making the core too small can allow enough back pressure to build up in the core of your rocket and cause a CATO.  However if you aren't getting enough thrust, and can't change / don't want to change the formula, you XAN adjust the nozzle size smaller or bigger.

*Be careful if drilling into propellant that you are going slow, cleaning the flutes of your drill bit often, touching the bit to make sure it's not getting too hot, doing it away from any other flammable items in case it does ignite on you, etc.*

Appreciate the last part about being safe. The drilling makes me nervous. 

Going to try this poured one. I think it will do better. When drilling the pressed powder the hole seemed too big because small pieces would come out of the sides of the propellant no matter how much I packed it in. From the first one I did as an experiment I remember the poured fuel being a cleaner material to drill. 


I'm excited to try building more than a janky trash rocket. I just want to get the kids Youtube build one working before moving on haha. 

I don't have an electric wok or double boiler with precise temp control. I do have an electric chemistry hotplate and temperature gun though. How hot can I get this without worrying it will ignite? I've been doing 250F but it's not very runny for pouring it. I see the melting point for KNO3 is really high, but no ignition temp.

I'm not using water after reading it's not that helpful and requires you do some special steps to drive it all off. I'm just melting the (sorbitol) in the mixture and stirring. 

[FieryCreations]

Or I may just give up on this design. Every PVC riser I buy has a different ID. New batch is too small for my packing rod. Got stuck and had to break one off. I should have found a better design before going through all this. 

[cmjlab]

 

Quick answer:  Keep the temp at Sorbitol's melting point 123°C / 254°F or slightly above, while ensuring it stays well below the melting point of KN03 (334°C / 634°F). ***Keep it below 265°F** if you see dark brown / black discoloration bubbling up OR dark smoke coming out of the bubbles, GET AWAY FROM IT, it will ignite at any point.***

Long Answer: 

Sorbitol started "...melting after 8 min at 91°C..." And was fully melted after 18 minutes at 123°C (~254°F)....".  [R. Nakka]

According to Nakka, ~254°F is the temp it the mixture will plateau at, until fully the Sorbitol is mostly melted, then the temperature should start to rise.  If allows to continue heating, it would eventually reach the Sorbitols boiling point, then eventually ignition point.  But, you will be turning it off and casting your propellant before that happens.

254°F is well below the melting point of KN03 (334°C / 634°F), which if you actually did melt the KNO3, then it could overcome the thermal loss and rise in temp to the actual ignition point.......  But that doesn't sound like a risk with a lab heater and a thermometer.

Edited December 16, 2023 by cmjlab

[cmjlab]

There's no KN03 ignition point listed because on its own it won't ignite, it will break down into bi-products. 

However, for this use, you are adding a fuel and making the KNO3 flammable at a lower temperature.  

***Do not let it start burning.  If you see smoke or black / dark brown discoloration turn it OFF, it's about to ignite****

[FieryCreations]

Hahaha ok all that time was worth it for how much this made me laugh. Now I now how Hamas feels. 

At least it got off the ground... Melting and pouring definitely increased the power but something is still off. Even if it didn't turn there's no way it would have went very high.

Edited December 17, 2023 by FieryCreations

[FieryCreations]

I have more questions. Ball milling the KNO3 and sorbitol would be far superior than mixing by hand from what I gather. But is that necessary if you're melting it anyway? Would melting it, drying it, and then pulverizing it to pack powder be any different than packing the well mixed powders without melting it first?

I have all the stuff, I just don't know what the optimal procedure would be. Definitely want to build some rocket core jigs. Luckily this one got pretty smashed so it's time for a better design. From the more professional looking Youtube vids it looks like no one uses powder and always casts it after melting. 

Edited December 17, 2023 by FieryCreations

[cmjlab]

Again, this is based on Nakata's write-up, but you are melting the Sorbitol, not the KNO3.  The KN03 particles are floating in the liquid Sorbitol.  So the smaller the particles of KN03, the better and more intimate the mixture will be.  

Ex. A baseball size of KNO3 can be floated in liquid Sorbitol, then cast into a large container to simulate a rocket propellant grain / motor.  It won't burn well, because there is less surface area exposed to mix with the fuel, so it has to burn through the outer layer of oxidizer, to allow the fuel to mix with the next layer, and so on....etc.

Ex. 2: A milled KNO3 / Sorbitol mixture that fits through -325 mesh screen, is already intimately mixed.  Now melt the Sorbitol and the fuel can get into more surface area of the oxidizer, and burn ALL at once, instead of layer by layer.  (Shitty example but you get the gist)

[cmjlab]

51 minutes ago, FieryCreations said:

I have more questions. Ball milling the KNO3 and sorbitol would be far superior than mixing by hand from what I gather. But is that necessary if you're melting it anyway? Would melting it, drying it, and then pulverizing it to pack powder be any different than packing the well mixed powders without melting it first?

I have all the stuff, I just don't know what the optimal procedure would be. Definitely want to build some rocket core jigs. Luckily this one got pretty smashed so it's time for a better design. From the more professional looking Youtube vids it looks like no one uses powder and always casts it after melting. 

The optimal procedure is probably the one described by Nakata - I believe he's considered an expert in amateur rocketry.  Also, if you read through old threads and look for people like Mumbles, David F, and others who are far more knowledgeable in the field, and some even more so in rocketry, you can find a lot of good info. 

I've shared everything I understand (and some that I stole from others, lol, like Nakata) but again, I don't make many rockets anymore. I messed with R-Candy rockets a bit, and some B.P., lots of whistle, and some strobe rockets, but I don't consider myself all that smart on rockets.  I'm more of a cylinder shell person..... :-).  

 

[FieryCreations]

13 minutes ago, cmjlab said:

The optimal procedure is probably the one described by Nakata - I believe he's considered an expert in amateur rocketry.  Also, if you read through old threads and look for people like Mumbles, David F, and others who are far more knowledgeable in the field, and some even more so in rocketry, you can find a lot of good info. 

I've shared everything I understand (and some that I stole from others, lol, like Nakata) but again, I don't make many rockets anymore. I messed with R-Candy rockets a bit, and some B.P., lots of whistle, and some strobe rockets, but I don't consider myself all that smart on rockets.  I'm more of a cylinder shell person..... :-).  

 

Thanks. I'm a little slow but I got ya. All of the above it is.

I can't wait to get into shells. Black match supplies and kraft paper showing up soon. Like everything else I learn about, I want to learn everything all at once.

Thanks for the help.