New to sugar rockets - what am I doing wrong?

[shniggysaurus]

answer is probably lots of stuff . . .

 

have any of you tried Grant Thompsons sugar rockets? They looked so easy I had to give them a try. I have tested three so far with the same results. I get a good initial burn with what seems like decent thrust and then an explosion and the end cap blows out. The nozzle is still intact though. Here are a couple of videos showing what happened. The third test shows a little better the time that it blows. I'm guessing the pressure is too great for my kitty litter end cap? If I added the delay grain would it help? Should I make the bore hole a little bigger?

 

First test.

Third test.

 

 

Thanks for helping a noob out.

[stix]

I'm glad you asked this question on this forum. There are lots of experienced people here willing to help, and I'm sure you will sort it out if you have the patience and are willing to learn. A much better prospect than those youtube videos.

 

That Grant Thompson video was discussed in this thread: http://www.amateurpyro.com/forums/topic/10324-substitute-for-fuse/

 

The thread wanders off track here and there, but there is some very good info. The upshot is that a lot of youtube videos on sugar rockets make it look all too easy. There are many variables involved.

 

Be aware that using PVC tubing as the motor casing can be dangerous as it shatters if the motor explodes. I watched your two tests, and it looks like a nice test facility you have, I also enjoyed the brick moving . Just make sure you keep well back.

 

Your guess about the pressure being too great is correct - the first point of failure being the end cap. One does wonder though what would have happened if the end cap didn't blow... CATO and shards of pvc flying in all directions... or huge amounts of thrust...?. Maybe the end cap is acting as a relief valve or sorts.

 

I would do two things: Secure the end cap better. Perhaps by using more kitty litter and also make sure the inside of the tube is roughened - either by course glass paper, grooves etc. But more importantly, drill the nozzle out a couple of millimeters (don't worry about the core). Then go from there.

 

Quite a few people get on this forum looking for "quick fix" solutions for making sugar rocket motors, those people soon leave and go on their own path. To what end, we'll never know. Sorry, if that sounds a bit dramatic. Look, sugar rockets aren't THAT hard to make successfully, but they also aren't THAT easy either - it takes a bit of perseverance.

 

If you really have the drive, you'll sort it out. Above all, stay safe.

 

Cheers.

[Oinikis]

Well, if the plug is not hard enough, you could mix some wax in it. I'm sure if you search the forums you'll find the technique. Essentially you grind the kitty litter into a powder, using a coffee grinder (Do not use one you use for food), and pour molten was on it and mix. But if you say they are intact, just outside, so this isn't necessary.

The other problem is that the plug doesn't have enough grip, so you have to recess it. Cut a small length of the dowel, and glue it to a plank, and ram your rocket while having the tube on that, and at the top, leave a gap too. When you ram, the tube bulged, and if you leave some spare tube ar the end, it will be narrower in respect to the part where the plug is, ultimately holding it in stronger.

As Stixy said, be careful, it a pressure vessel after all.

[MadMat]

Is it possible that heat is getting to the PVC casing, it softens, then... BOOM? Convolute wound (not spiral wound) cardboard tubes are the way to go. I recently had some problems with rockets blowing apart about 10 feet up from launch. The wall of the tubes was a bit on the thin side. I fixed the problem by upping the diameter of the nozzle.

[shniggysaurus]

wow. Thanks so much for the help. you guys make this forum amazing. I am really interested in getting in to making my own motors and looks like I have tons to learn. That other thread you posted was very useful. so before I buy some tooling and cooking the serious type of rcandy I'm going to try a couple of things here.

 

1. enlarge the nozzle a couple of mm

2. add the dry propellant in smaller increments and compact after each addition

3. roll some cardboard tubes to use instead of pvc

 

Thanks again for the help! I'm sure when I get more time I will be hitting you guys up on how to make legitimate rocket motors.

[MadMat]

Just a thought for making the grain. Try cooking your mixture in a pan with water. You would be surprised how little water will completely dissolve the KNO3 and sugar when it gets near boiling. You then boil off the water and pack the melted mixture into your tubes. One of the biggest advantages to doing it this way is that the KNO3 and sugar are intimately mixed without the need for grinding and milling. I have tried melting the mixture dry and with water... I have found that by using water you get longer pot life (more time to pack multiple tubes) and contrary to what I have read, the viscosity of the mixture seems lower than dry cooked Rcandy; this makes packing the tubes easier. Though its not the safest way of doing it, I cook my Rcandy in an old sauce pan on an electric hot plate. I've cooked a lot of batches without a single mishap.

[mikeee]

A Presto Fry Daddy works well for cooking sugar fuel, the adjustable temp control works well

for controlling the temps to melt the fuel and not burn it.

[stix]

1. enlarge the nozzle a couple of mm

2. add the dry propellant in smaller increments and compact after each addition

3. roll some cardboard tubes to use instead of pvc

 

That sounds like a good plan. Are you able to source cardboard tubes instead of rolling them? If you haven't already, I suggest you read this thread. http://www.amateurpyro.com/forums/topic/11162-thin-wall-tubes/?do=findComment&comment=149196

MadMat has been able to get good results using thin(ish) walled bought tubes.

 

If you really want to roll your own tubes, there's lots of info on this forum, one example is this one: http://www.amateurpyro.com/forums/topic/10063-tube-roller-4oz-1-and-3-or-more/

Although it starts off with a tube rolling machine, there is lots of info regarding hand rolling techniques and glues. (If I remember correctly).

 

A Presto Fry Daddy works well for cooking sugar fuel, the adjustable temp control works well

for controlling the temps to melt the fuel and not burn it.

 

Agree with mikeee on this. Pick up a used electric frypan/skillet from a 2nd hand store. Much easier to control and also gives you peace of mind.

Edited September 23, 2015 by stix

[douglas423]

It seems that too much pressure is forming and causing it to explode. Try making the nozzle hole a little bit bigger and see if that solves it.

[MadMat]

I just wanted to say that my electric hot plate has a thermostat of sorts on it. It's not graduated in a degree scale, but after using it a couple times I figured out to set it on max while boiling the water, then turn it down as the water is close to being all boiled off. Haven't burned a batch yet

Edited September 27, 2015 by MadMat

[Redrocketman]

Hi, newbie too. Having the exact problem with my rockets, based on G.Ts tutorial- following it religiously. Only difference I have added 1% red iron oxide - perhaps increasing burn rate too much?? Even dimpled inside of motor tube with soldering iron, which helped but still getting 40 - 60% plug blowouts. Will try going up to 8mm nozzle, coring grain 6mm. Having great success with recrystallisation technique in large rockets, ( un plugged) but chew through consumables like no tomorrow!! This is much more cost effective, but no luck holding pressed clay plugs in. Any advice would also be much appreciated!! Maybe not coring full length would help??

[stix]

I'd say the addition of red iron oxide is the likely culprit - it increases the burn rate as you already know. In my experience using rio in a cored/internal burner with a nozzle is unnecessary and likely to cato. Red iron oxide (or some other burn rate enhancer) is pretty much required in a cored nozzle-less motor because standard sugar fuel doesn't burn quick enough, as is the case with end burners with a nozzle.

 

There are some things you haven't been clear on. Are you using the pressed powder method for these smaller variety, or re-crystallisation? Also, what diameter and length is the grain/core. A good starting point is to have the nozzle throat diameter around 1/3rd the outer diameter of the grain.

 

Unless your nozzle is really small, my first move would be to leave all as is, and drop the rio. Then evaluate from there. When experimenting, It's always best to change only ONE thing at a time otherwise you won't really know where the issue is and how to remedy it.

[Redrocketman]

Gday Pyrotechnician, thanks 4 advice, I thought using burn rate enhancer in these smaller motors would be contentious!! Same brew I use in my large nozzle less cored rockets ( honestly insane thrust!! ). I forgot to mention this was an experimental mix with 1g sulphur per 100 g fuel. Just trying different things. With these smaller motors they are dry pressed, 2-3/4" length grain / 1" dia. Nozzle diameter 7/32 I think or 6.5mm in my language!! 3/4" pressed plugs both ends with inside of motor case thoroughly scored.

 

Like u said I think I've just made a bit of a "hot" mix & need to back off to a standard 65/35 kno3 / Su mix.

 

Last thing, will the addition of sulphur add to the chance of a blow out or over pressure? I added it under the assumption it would help with ignition??

 

Cheers

[Redrocketman]

Sorry, I mean stix!!!

[stix]

Forget about using Sulphur. I think your trying too many things at once, ie. too many variables. Go back to the basic 65/35 mix. It's not that hard to ignite - no need for ignition aides. Do some research on this forum about home-made e-matches.

 

I'm all for experimenting, but there has to be a goal. Experimenting for the sake of it is pretty much pointless, although I guess you can learn something from it. The point of experimenting for me is to improve on something. In the case of sugar rockets it would be using different types of sugars and simpler methods of casting.

 

Going by your specs, the nozzle does seem a bit on the small side. Also the motor grain is rather stumpy, ie. short and fat (Approx. 70mm long, 25mm wide with a 6.5mm core - if I've read your specs correctly). What this means is that you may not get enough thrust at the start for lift off, but huge amounts at the end. This could result in your rocket lifting off slowly then going sideways with ever increasing thrust. Not good from a safety aspect.

 

The Kn ratios are approx. 43 at the start, finishing around 168. The Kn ratio is the area of fuel burning compared to the area of the nozzle (basic math). If you don't know or understand then do some research and ask here.

 

Using PVC tubes for rocket motors is pretty much frowned upon - they can shatter. From what I've read, the embedded pieces in your flesh are hard to pick up on x-rays. If you must, then make sure you use a remote ignition and set up some sort of barricade to protect yourself and others.

 

Stay Safe - Cheers.

[Redrocketman]

No worries, sulphur gone. I totally agree, trying too much at once serves no real purpose. I am just new to dry pressing small motors, & have now found their limits it think!!

 

I understand ur safety concerns, but I have a secure steel test chamber set up to 20m lead to remote ignition system.

 

As said, all my core less large rockets ( 400mm X 45mm cardboard) are sensational running the hot mix ( recrystallised ) piercing the clouds in seconds!!, but I'll return to the " standard " 65/35 mix for these small nozzled rockets, & take the nozzle dia out 2mm & lengthen the chamber & let u know results.

 

Thanx again

[Emkata]

My first two tries end up with explosion too i used cartoon tube and gypsum plugs and i was packing the dry mix of Kno3 and Sugar like in the video but my third rocket was with plastic tube again gypsum plugs but this time i melted the sugar to caramel then i added fine powdered Kno3 and mixed it then cast it into the tube dont be scared if this mix will set off when heating it on the stove its very heat stable only flame can set it of maybe the explosion is due to wrong packing dry mix try casting it it may work and dont forget to drill a hole all the way to your upper plug.

 

Sorry for bad english.

[Connor]

Hi, I have been making sugar rocket engines for years but only done the powdered ones which have worked very well. Is there an advantage to cooking the fuel vs packing it dry? Which one will be more powerful? I have been making engines about the size of a AA battery and they go pretty high (500-800') could I get more height if I cooked the mixture?

[mabuse00]

Is there an advantage to cooking the fuel vs packing it dry? Which one will be more powerful? I have been making engines about the size of a AA battery and they go pretty high (500-800') could I get more height if I cooked the mixture?

 

If you master the art of casting properly, good consistency is possible without the need for heavy pressing equipment.

When bigger motors come into play you might want to cast them.

 

Casting is not about power.

[Connor]

Sounds good I will have to give it a try, when they are cast do you make them core burners or end? what do you use for nozzles?

 

Thanks!

[Arthur]

Beware of PVC tubing, It works for so many tries then one goes bang and fills you with shrapnel -which costs time and doctors money.

[stix]

Sounds good I will have to give it a try, when they are cast do you make them core burners or end? what do you use for nozzles?

 

Thanks!

You probably won't have much luck with an end burner with sugar fuel. You'll get a better result from making a core/bore into the fuel grain.

As far as nozzles go, the simplest way it to use a clay+wax mix.

 

In my view, melting sugar fuel creates a more powerful fuel than just pressing a powdered mix. It consolidates the fuel grain more. You need a lot of pressure to consolidate a powdered mix.

 

Have Fun. Stay Safe