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grwise1

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I'm not sure how strict this forum is regarding homemade rocket motors, but I just wanted take a second and share my experience with them.
I recently built a pvc engine that used homemade rocket candy (in Bates grains); which I launched a couple days ago. I used James Yawn's great simulation calculator to determine nozzle size, Throat dia, etc... Anyhow I have had past success in such ventures and love the challenge of building these things. However, to my dismay and surprise, my most recent launch ended in a violent explosion shortly after lift off. I did not have any sort of shield and am quite lucky I was not hit by any shrapnel. So all that to say, despite all the calculations in the world, things don't always go as planned, and homemade rocket motors can be quite dangerous. (It ended up that a conversion error lead to a miscalulated pressure estimate, which in reality was more than 3 times the rating of my pvc.)
I really hate being that "safety guy" but I just wanted to use my experience to help remind folks of the dangers of homemade motors, and all rockets in general.

Anyhow, here is the video

[video=youtube;Nj9hTAOeyJc]https://www.youtube.com/watch?v=Nj9hTAOeyJc[/video]
 
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cavecentral

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They are discussed in the research forum. Some do sugar, most is APCP. PVC is discouraged (aluminum is the preferred material), although if you are at a safe distance, the shrapnel from PVC will not reach you.
 

Zeus-cat

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Discussion of how to make motors is not allowed in the open forum. Therefore you should remove the second video.
 

ksaves2

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Yup, blew up. Need to peruse some more motor making resources and do find a launcher with a longer cable so you're at a safe distance. To parrot the above the flight video is fine but confine your discussion of recipes and techniques to the
research forum. You'll have to request to be admitted. Kurt
 

cwbullet

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PVC is not a good motor casing. It tends to fail and throw fragments. Thanks for removing the video on your motor making.
 

Rex R

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a few things caught my attention, in addition to longer wire on your controller...your pad should have a blast/jet deflector to reduce the chance of starting a fire under the pad(and melting it). think you also made the assumption that pvc fittings have the same pressure rating as the pipe...they do not typically fittings are rated 50 -75% of the pipes rating, especially threaded fittings.
Rex
 

dhbarr

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Also your lead was a bit short and you overrode the inline key with a small screwdriver. If your rocket had launched in fewer pieces, one or more of those corrugated cardboard fins very likely would have shredded, resulting in the motor careening about wildly.

I know it may seem like I'm piling on, but.... please do be careful!
 

Zeus-cat

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I remember a discussion about PVC as a motor casing and I thought every time you pressurized the PVC it decreased the strength of it. So if it was rated at 100 PSI initially and you stressed it close to that level, then the next time it should be considered to be only rated at 70 PSI. I don't remember the specifics, but it seemed that after just a few uses it was derated so much that you couldn't use it. And in my opinion you shouldn't use it at all as it does shatter easily.
 

rockets

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I'm not sure how strict this forum is regarding homemade rocket motors, but I just wanted take a second and share my experience with them.
I recently built a pvc engine that used homemade rocket candy (in Bates grains); which I launched a couple days ago. I used James Yawn's great simulation calculator to determine nozzle size, Throat dia, etc... Anyhow I have had past success in such ventures and love the challenge of building these things. However, to my dismay and surprise, my most recent launch ended in a violent explosion shortly after lift off. I did not have any sort of shield and am quite lucky I was not hit by any shrapnel. So all that to say, despite all the calculations in the world, things don't always go as planned, and homemade rocket motors can be quite dangerous. (It ended up that a conversion error lead to a miscalulated pressure estimate, which in reality was more than 3 times the rating of my pvc.)
I really hate being that "safety guy" but I just wanted to use my experience to help remind folks of the dangers of homemade motors, and all rockets in general.

Anyhow, here is the video

[video=youtube;Nj9hTAOeyJc]https://www.youtube.com/watch?v=Nj9hTAOeyJc[/video]
The motor overpressurized I bet, and the PVC wasn't strong enough to handle it.
Thanks,
 

grwise1

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Hey guys. Thanks for the constructive criticism.

I would like to stress that this was just a crude test of the motor, I was not attempting to perform a high performance flight. (Yes I do have a blast shield, it was removed for a reason. And, yes, I lost the inline key, and used a screw driver to override it. Is that really a cardinal sin? I call it improvising.)

Anyhow thanks for all you all's comments regarding the pvc. I honestly choose that because it was the easiest to work with, however I will probably look into using metal casings in the future.
 

grwise1

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I remember a discussion about PVC as a motor casing and I thought every time you pressurized the PVC it decreased the strength of it. So if it was rated at 100 PSI initially and you stressed it close to that level, then the next time it should be considered to be only rated at 70 PSI. I don't remember the specifics, but it seemed that after just a few uses it was derated so much that you couldn't use it. And in my opinion you shouldn't use it at all as it does shatter easily.
Hmm, interesting point. Yeah, I probably need to go to aluminum casings in the future...
 

Steve Shannon

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Tripoli has set the minimum safe distance for homemade rocket motors at 200 feet, meaning that all persons should be at least that distance before attempting to ignite such a motor. We also prohibit the use of PVC because it fails in what's called brittle fracturing, meaning it ends up with numerous sharp pieces.
There are stories that pvc shards cannot be seen in the human body in x-rays, but I've also read first hand accounts that people certainly can see pvc shards in the human body. I would strongly suggest you learn more than you know now and completely avoid having shards in your body in the first place.


Steve Shannon
 

dhbarr

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My point about the key wasn't that something other than the official key was used, but more that the improvised key appeared to be wedged "on", thus defeating one partial function as an additional safety.
 

rstaff3

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I have done a few PVC motors and with 200+ feet I seriously doubt that shrapnel is a real issue. But DO NOT get close to them! That being said, once you go aluminum, you'll never go back.
 

Nick@JET

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The responses have been relatively nice compared to others in the past lol. Good to see you man up and share honestly about your experience. I know there are some YouTube videos on some pretty dangerous engine building techniques, and some much better ones by some on this forum - and some of those even don't end well

Glad you were not hurt - I don't have one of these but can be had on eBay for cheap - and claim 200m which will get you to safe area.

ImageUploadedByRocketry Forum1501691629.726574.jpg

Also I think you need to be certified L1 or L2 to get access to the restricted area. But you can request to Administrator to get access to discuss rocket engine building
 

Steve Shannon

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I have done a few PVC motors and with 200+ feet I seriously doubt that shrapnel is a real issue. But DO NOT get close to them! That being said, once you go aluminum, you'll never go back.
My understanding is that it was tried at 100 feet and shrapnel landed close by so it was given a margin of safety. [emoji15]
 

rstaff3

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I just flew mine on the series of pads that was rated at one higher level than the motor. I have also heard from what appeared to be knowledgeable people that the PVC shards lose momentum fast. I have also seen the closure from a J350 during the Texas period bounce off an angled blast deflector and bounce up to the flight line. Not that is would have been able to hurt you in the last 3/4 of its travels.

However, while PVC sounds cheap, it is much nicer to have your motors fit regular rockets nicely. Plus, I hate cutting PVC. Plus there is no reason to buck the system when the system makes sense.
 

OverTheTop

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We had an issue at work when a PVC pressure vessel let go explosively (although all was contained). It was not used above its rated pressure, but the failure was a fatigue issue due to pressure cycling.
 

Sabrina

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Hi Grant, I enjoyed your video, thanks for sharing.

You might want to get a longer launch wire and some safety glasses. A burning chunk of propellant in the eye can mess-up your whole day. The blast deflector was already mentioned - it's a really good idea, along with a gallon or two of water, a shovel, maybe a friend or two to assist. I know you were not planning on starting a huge grass fire, but you absolutely must plan and be ready to put one out just in case.

I think its cool when young people like you (and me - I'm 14) are interested in science and weather and chemistry and making awesome things. Don't get discouraged by any of the criticism you might get posting in this forum. Instead, be inspired to learn and make new skills and new friends.

I launch rockets too.

Here's a picture from the rocket I launched last Sunday. :cool:

vlcsnap-2017-08-01-19h57m13s620.jpg
 

DavidMcCann

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When an Aluminum case fails-





This case also over pressurized. The top snap ring groove let go, the top of the case peeled back/broke away, the forward bulkhead blew out and the pressure hitting the inside of the rocket destroyed the booster. No bits made it to the flight line. the top half of the rocket recovered under parachute. Case was recovered with the nozzle and many parts of the booster.

Not my best day....but a 'safe' failure.
 

Bat-mite

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Hey guys. Thanks for the constructive criticism.

I would like to stress that this was just a crude test of the motor, I was not attempting to perform a high performance flight. (Yes I do have a blast shield, it was removed for a reason. And, yes, I lost the inline key, and used a screw driver to override it. Is that really a cardinal sin? I call it improvising.)

Anyhow thanks for all you all's comments regarding the pvc. I honestly choose that because it was the easiest to work with, however I will probably look into using metal casings in the future.
When you say "metal," please amend that to "aluminum." Steel casing = grenade = bad.
 

grwise1

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Hi Grant, I enjoyed your video, thanks for sharing.

You might want to get a longer launch wire and some safety glasses. A burning chunk of propellant in the eye can mess-up your whole day. The blast deflector was already mentioned - it's a really good idea, along with a gallon or two of water, a shovel, maybe a friend or two to assist. I know you were not planning on starting a huge grass fire, but you absolutely must plan and be ready to put one out just in case.

I think its cool when young people like you (and me - I'm 14) are interested in science and weather and chemistry and making awesome things. Don't get discouraged by any of the criticism you might get posting in this forum. Instead, be inspired to learn and make new skills and new friends.

I launch rockets too.

Here's a picture from the rocket I launched last Sunday. :cool:
Yeah, for sure.

Hey nice pic! I myself launched a homamade rocket with a GoPro a couple years ago. It's really sweet to get footage from up that high. Your image reminded me I need to do it more often.
 

grwise1

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Yeah, definately a best case outcome for such a scenario. Thanks for sharing.
BTW: Is that graphite you use for your nozzle? I've heard it's the best material, but also a pain to work with.
 

DavidMcCann

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Yeah, definately a best case outcome for such a scenario. Thanks for sharing.
BTW: Is that graphite you use for your nozzle? I've heard it's the best material, but also a pain to work with.
Yes, graphite on the nozzle. There are places to order sets- If you have access to a lathe, there's info in the EX section on how to make them.

https://www.rocketmotorparts.com
https://lokiresearch.com/secure/store.asp?groupid=5520031443530
https://binderdesign.com/store/page11.html


I missed a bunch, hopefully others will chime in
 

rstaff3

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I just used my Loki cases and designed around the available nozzle. My foray was short but I still have a case set for commercial reloads.
 

ksaves2

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The responses have been relatively nice compared to others in the past lol. Good to see you man up and share honestly about your experience. I know there are some YouTube videos on some pretty dangerous engine building techniques, and some much better ones by some on this forum - and some of those even don't end well

Glad you were not hurt - I don't have one of these but can be had on eBay for cheap - and claim 200m which will get you to safe area.

View attachment 324882

Also I think you need to be certified L1 or L2 to get access to the restricted area. But you can request to Administrator to get access to discuss rocket engine building
Forget those wireless thingies above unless you use it to energize a relay to a larger battery. You'll fry the relay in the receiver. Only good to pass 10 amps. Kurt
 

prfesser

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FWIW PVC pipe will show up on x-rays, but that's because the fillers are at least partially opaque. Way back when I was starting motor-making I tried a PVC-cased motor. It let go, and did produce a lot of sharp-edged bits that flew rather farther than I was comfortable with. I coulda sh*t through a screen door without hitting the wire. :eek: PVC is simply too unpredictable, especially when it comes to Research propellants.

Also FWIW inexpensive aluminum casing stock can be had from Texas Towers. It's 6063-T832 alloy which is slightly weaker (42000 kpsi) than 6061-T6 (45,000 kpsi) and their tubing is rather thin-walled, 0.058". Even so, a 29 mm casing from this stock should be able to handle up to roughly 700 psi; 38 mm, up to 500 psi.

Best regards,
Terry
 
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