Rocket Motors Gone Wrong

[loganater]

Hey everyone, before I say anything let me just tell you that during all of these "launches" I was in a rural area far away from anyone, and I took the proper safety precautions to make sure nobody could get hurt. I recognize that the line between rocket motors and bombs is very thin and I have been very careful to not endanger anything or anyone.

 

About 8 months ago I took an interest in rocketry and wanted to make my own rocket from the ground up. Starting with the motor, I decided that following Grant Thompson's sugar rocket tutorials would be a great and easy way to start. These motors use R-Candy as a propellant, packed clay for an end cap, and 3/4" PVC for a body. I closely followed the tutorial and the first test launch worked flawlessly. I made a couple more after those worked just as well. Then I launched my first homemade rocket and it worked perfectly! After that I decided to try adding in an ejection charge for a parachute, but then the rocket motor end cap gave out. After that, I went back to testing a couple motors with no ejection charge and got varying results, but most just giving out and doing the same thing. I got frustrated and gave it a rest for a little while since I got bored. Recently I have got back into it and started taking things a little more serious. I have made over 10 motors, doing different things for each of them. I scored the inside of the PVC to give the clay end cap a better grip, I packed the powders harder, and softer. Regardless, all of the motor's end caps gave out every time. Yesterday, I decided that clay would just not cut it. I purchased some PVC caps and PVC cement to cap the ends of the motors after I put the clay in. I drilled in a 7/32" hole in the clay and PVC cap to make the nozzle, and drilled all the way into the propellant to give the rocket a core (like I always have). When I test launched the motor, it went about 20 to 30 feet, then completely exploded, meaning I accidentally made a pipe bomb instead of a rocket motor. I used SCH 40 PVC which is rated for 480 PSI. Could someone please tell me how this could happen? I thought for sure that a 7/32" nozzle would allow the exhaust to escape quick enough not to surpass 480 PSI. I also don't know how it could have worked in the past with only clay, yet now the PVC can't even withstand the pressure.

 

P.S. I know using PVC for rocket motors is frowned upon, but I'm just trying to get the hang of things before I come up with my own designs.

Edited July 22, 2018 by loganater

[Baldor]

First suspect: Bad packing of the fuel. If there are cracks or voids in the grain, it will easily explode.

[Mumbles]

The clay/PVC joint is almost always going to be the weakest point, and be the point of failure. It's still possible for them to explode like your last test, but it's less common. A glued PVC joint is stronger than the pipe itself. When a rocket over-pressurizes the weakest point is the tubing. This causes a much more violet explosion as you just experienced. A malfunction that would normally just pop out the clay, makes the whole thing explode.

 

There are several causes including what Baldor just mentioned. Bigger or longer rockets can also have the tube walls begin to soften with longer burn times.

 

There are several advantages to paper tubes that I enjoy, but also several disadvantages. PVC is seemingly more common in amateur rocketry, but basically taboo in amateur pyrotechnics and pyrotechnic rockets.

[loganater]

Thanks for your help! Do you guys have any ideas on what I can do to fix this issue?

[TYRONEEZEKIEL]

Definitely pack it better. Smaller increments and consistent way of packing. Start with oversized clay nozzles and dial it in. Or you can go with a cored nozzleless rocket to avoid the whole nozzle failure issue.

[gdeputy]

If you're serious about building serious rockets, i strongly suggest checking out Tripoli Rocketry Association (www.tripoli.org) and finding the nearest Prefect/club to you. There are lots of folks doing sugar rockets, even a group working on a 'space shot' using a sugar motor. If you want to avoid re-inventing the wheel, there's a huge pool of talent and experience there. Not to mention there are organized launches with the appropriate FAA waivers, etc.

 

I personally cant help you much with sugar motors, i prefer Ammonium Perchlorate composite propellants, but there are lots of folks out there who do.

[stix]

As suggested, powdered sugar fuel really needs to be compressed well. Smaller increments will definitely help. If I could be bothered searching back my previous posts, more than a year ago, I did take a bit of a dimm view at Grant Thompson's method as there were a few things left out - namely the importance of packing and fuel density - it all looked too easy.

 

[EDIT] Also the shaking around in the tub business is a bit slack - how would you know how well mixed it is? All these variables (the more the worse) makes it harder to replicate what he did. If you got 50% success, then I reckon that's pretty good.

 

I'm not sure if hammer pressed powder would cause voids or cracks, but if your rammer isn't tight enough, then that could cause the flame to propagate up the fuel grain outside between the pvc wall - which is the weakest point of the fuel grain.

 

You could try to make some sort of external retainer which would allow you to apply more force. Even a few of those hose clamps would help a lot. The other thing you could do is to roughly work out the density during and after packing (a bit of math). That way, if you knew what the density was (with a good working design) you are in a good position to decide if a launch is worthwhile.

 

You could also look at melted fuel, ie. r-candy.

 

In any case, which ever way you go, if you do have a genuine interest, then I'm sure you'll work out what method suits best. There's lots of good advice on this forum to help you, so stick with it.

 

Cheers.

Edited July 24, 2018 by stix

[loganater]

Thanks! I think I’ll try making some r-candy motors

[stix]

With r-candy, you'll get better performance from the fuel, but the downside is it can be difficult to cast and can burn you easily - much like toffee can burn when stuck to your fingers.

 

There are various ways to make r-candy. The two most common are the melted method and the re-crystallized method. The easiest and first point to start is (IMO) the re-crystallized method because it doesn't require milling your KNO3.

 

Whichever way, just be careful. Not just on burning yourself making it, but lighting a small rolled strand can potentially "take-off" and hit you or someone else, or set fire to something.

 

Take precautions, do some simple small tests.

 

If you do that, then you'll see that pressing dry powder (unless you have a good pressing apparatus) is inefficient. I guarantee you will laugh your arse off when you realize how much better r-candy is.

 

Like I said, please be careful and ask questions here first.

Edited July 24, 2018 by stix

[Arthur]

PVC pipe has a pressure rating when cold but NOT when at rocket flame temperatures.

 

Be careful with airspace -someone controls it maybe civil or military. Join a rocketry club get their best advice and use their shoot sites and their airspace permissions. Some amateur rocketry goes to 100,000 feet and gets FAA clearance to use airspace.

[loganater]

There are no rocketry clubs in my area, what can I do?

 

 

Edit: There is one about 45 minutes away but they only do up to "C" sized rockets, which isn't really of interest to me

Edited July 25, 2018 by loganater

[gdeputy]

Yeah, that sounds like a NAR section, which wont support what you're looking into.

 

Where are you located?

[Arthur]

I suspect that with most rocket clubs you will be expected to start small and progress to bigger motors.

[nils]

I use plywood endcap and plywood nozzle. Much stronger and lighter than clay. I secure these by hot bending plastic pipe edges. Then I use fibreglass tape to make everything extremely strong.

Here is one example:

Here I also use steel T-nut inside nozzle hole to avoid erosion:

[loganater]

Alright, I ditched the dry powder and the clay since they didn’t seem to be working and came up with a new design. I made a reusable rocket casing where I can make the grains of fuel separately. I made the body out of PVC and the end cap and nozzle out of Durham’s water puddy. The nozzle is made out of a separate PVC adapter and can be screwed on and off. This way I can add the fuel grains in. I tested it yesterday and it worked really well! Thanks everyone for the help

[Arthur]

You still need a place and permission to fly your rocket. Few rocketry clubs are going to have new members who want straight into the high power/altitude range with no experience logged with clubs

[CrossOut]

Ever consider making bp rockets? You can get tooling and supplies pretty easily and I'd argue they're safer and more consistent than r candy.. not to mention we have a lot more experience with bp.. start with a 1lb rocket and move up from there.. they're pretty cheap to make as well.

[Zmuro]

Ever consider making bp rockets? You can get tooling and supplies pretty easily and I'd argue they're safer and more consistent than r candy.. not to mention we have a lot more experience with bp.. start with a 1lb rocket and move up from there.. they're pretty cheap to make as well.

That's exactly what I wanted to say. BP end burners are easy to make and quite reliable, but BP nozzleless rockets are my favorite.