What waste products are formed from sugar rocket fuel?

[thescienceguy49494]

TL;DR Summary

whenever i burn potassium nitrate+sugar rocket fuel, it always leaves behind this big hunk of black stuff, and sometimes a greenish liquid as well. what is that?

Hello, forums!

So I recently have been getting into chemistry stuff. my latest endeavor is the "sugar rocket fuel" thing where you mix potassium nitrate (stump remover) and sugar (granulated) together to make something that when lit on fire, burns very quickly while releasing tons of hot gasses.

after some experimentation, I've found that a 50/50 ratio works best, at least for me, but there is one thing that I cannot figure out. every single time I burn some of the powdered fuel, it leaves behind this big hunk of black stuff that stays very hot for awhile, then eventually cools. usually there is as much of this stuff as there was of the powdered fuel, and sometimes, the reaction produces a greenish liquid that eventually cools to a greenish solid as well.

YouTube videos and online tutorials show similar things happening but never explain what exactly it is. after hours of frustrated googling having yielded nothing, I came here, because my curiosity is getting to me here. what is that stuff? thanks,

-John

 

[sysprog]

We (pre-teen kids) used to do it with sodium chlorate (general weed/plant killer) and sugar, but I think that the idea is not much different $-$ as far as I know, the black residue is mainly combusted (oxidated) salty hydrocarbon remnants . . .

 

[thescienceguy49494]

I thought sodium chlorate was, like, super dangerous and highly explosive? also I'm using stump remover that is supposedly "pure" (definitely not actually pure) potassium nitrate. in all the YouTube vids I've watched on this formula, sometimes it leaves behind nothing at all, or only a smudge of black, sometimes its just the green liquid, sometimes an assload of black crap. . . I know its probably carbon or ash or something non important but my ocd has other ideas

 

[hutchphd]

The optimal sugar rocket mixture from my extensive early teen rocket engineering was about 60/40 KNO3 to sucrose (probably by volume I had no good scale). These produced copious smoke but not a bunch of ash. Never did I see a green liquid. Much data available on youtube, some of it good
I think probably your rockets are fuel (sugar) rich, hence all the carbon crud. My most successful designs used powdered clay nozzles. Some flew well. You can get KNO3 on amazon. Sodium chlorate sounds like a bad idea. It is quite toxic.

 

[sysprog]

Sodium chlorate sounds like a bad idea. It is quite toxic.

Well, we were pre-teen kids, but I agree $-$ it's not just a specific weed killer; it's a very general-purpose plant killer, which means that it's highly toxic $-$ not so so bad if you get it on your hands, but you'd better wash them immediately $-$ you don't want to ingest it orally or get it in your eyes.

 

[thescienceguy49494]

yeah sodium chlorate sounded bad. the green stuff only occurs when I tried to use powdered sugar, which was more powerful but much harder to light, so I'm just using normal granulated sugar. even the 50/50 mix I'm using takes a minute or so of holding a lighter to it for it to go, but then it burns just as well as the ones on YouTube, but leaves behind a hump of steaming black crap. I will try 60/40 tomorrow. any ideas on how to make this easier to light? haven't tried putting this in a rocket yet but it needs to have a fuse. I don't want to be standing next to it holding a lighter to it for a minute and a half praying I don't die :)

 

[Borek]

Black residue is mostly a mixture of char and tar.

Ignition is probably a matter of temperature - the hotter the flame, the easier to ignite the mixture (I am not saying this is the only factor). We used a small battery and electric wire for ignition.

Powdered sugar tends to clump. Where I live it is sold with some preservatives added (from what I understand mostly SiO₂). None of the things present should give any greenish color - but that's assuming both sugar and nitrate to be pure. They can contain many impurities, some of them probably capable of giving green hue.

Stoichiometric mixture is is about 73/27 (nitrate/sucrose) - any composition with less nitrate is guaranteed to not being able to convert all carbon to carbon dioxide. But the optimal mixture has to be determined experimentally, as (at least in rocketry) it is not about full combustion, but about the highest speed and the highest volume of gases produced - and that depends on ton of factors.

For really, really good results you want both sugar and nitrate to be as fine as possible and mixed as evenly as possible. I suppose ball mill will work.

 

[hutchphd]

For really, really good results you want both sugar and nitrate to be as fine as possible and mixed as evenly as possible. I suppose ball mill will work.

Most folks use this stuff in "candy" form (which brings a host of other issues). Then only the granularity of the KNO3 matters as the sugar is melted. As I remember it was quite hygroscopic which was a problem in Midwestern US summers

Stoichiometric mixture is is about 73/27 (nitrate/sucrose)

Now I am trying to recall if I understood stoichiometry at age 12...actually I think I did, but would I have bothered?

 

[JcFrank]

My favorite is a 70/30 KN03 to sucrose (powdered) ratio by weight. 70% oxidizer will make a cleaner burn. I use a dedicated coffee grinder to grind JUST THE OXIDIZER to a fine powder. NEVER power grind or mill a fuel/oxidizer mix. The safest way I've found is to use a little water to partially dissolve the mixture, then to hand grind it in a mortar and pestle. Pour it on parchment paper on a cookie sheet and place it somewhere warm and dry to dehydrate. The result is a slightly sticky mold able product that is easy to pack into an engine.

 

[Tapemeasureman]

Rocket candy is very hygroscopic, and can pull that moisture out of the air very quickly. 70/30 is fine ratio, now throw a couple percent of FeO2 into the mix. If very dry, crunchy dry it leaves very little residue, and is VERY fast Very powerful.

If allowed any moisture into the mix its hard to light and burns very slow too slow to get enough lift to go anywhere. This includes right out of the pan you cooked it in I don't think you can cook it enough to get completely dry in the pan. The sugar starts to caramelize and ultimately turn black its power is going down hill.

Jiust cook it long enough for the visually noticeable steam to stop. Let it cool, break it up into small pieces and dry in the oven at 215 F for a little while, when you pull it out and its still soft but gets real hard as it dries. This stuff lights super easy, is way way more powerful , burns way way faster.

Note that a couple percent of Aluminum has a similar, but different effect, and that black and brown ferrous oxide have slightly different effects as opposed to red. The secondary drying made all the difference in the world to my rockets, that and adding a pinch of rust, so much so my nozzles had to be widened to avoid just exploding instead of taking off they burn twice as fast if not faster.