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Final steps in making BP motors - seeking advice


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#1 hcb

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Posted 24 February 2020 - 05:58 PM

30+ years ago, I tried making BP rocket motors.  Didn't go too well.  Now, I have a son and want to show him this stuff and finally get that job done.  I learned a lot in 30 years.  I'm making progress.  I'd like advice on the final steps, though, please.

 

I have some BP made up.  I used home-made willow charcoal in a 75-15-10 charcoal/KNO3/sulfur.  I added 5% dextrin (home made) to some of the BP, but I also have some BP without the dextrin.  I have granulated some of the BP with dextrin and lit it.  It's darned fast but that's not a quantitative assessment.  It was all ball milled (3 ingredients together) and it not only burns fast but it also burns cleanly (for BP).  I have some motor tubes (new) and nozzle mix and I've made some tooling to make end burners.  I'm using 1/2" ID tubes and I formed a 1/8" nozzle.

 

Here's what I've gotten so far: I used a mallet and rammed a nozzle and about 12g of powder in 3g increments into a tube, taped a 3/32" aluminum TIG filler rod to the side and lit it. It flew (my first successful BP motor flight ever) but it was lethargic and laid over in flight and dove to the ground.  My second attempt was like the first attempt but I used about 8g of powder and I used a shop press and rammed it in stages and lit it upside down on a block.  It burned but not well.  Those were both with the dextrin BP mix.  My third attempt was with 75/15/10 and no dextrin, all rammed with the shop press.  As before, I taped a rod to the side and lit it.  It flew a little bit more energetically but still not like the commercial motors by any means.

 

None of the comps have been wetted.  This may be a problem.  I didn't have a reinforcing liner on the outside of the cardboard tube so I was limited in the force I was able to apply to the comp in the tube with the tooling (it started to bulge the tube).  I have built a sleeve for the tube but won't get to use it until tomorrow.

 

I've read somewhere that someone used mineral oil in their comp for a rocket motor, is that a thing?  Should I be using a different ratio for the BP?  Will wetting (and then drying) the fuel improve the performance?  Anything I'm missing here?  I'd like to make end-burners that function well because it would seem that higher flights would be achieved with longer burns (yes, longer burn at less force, so maybe it doesn't matter...but I'd still like to get an end-burner to work well).

 

Thank you.

 

--HC



#2 Boophoenix

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Posted 24 February 2020 - 08:45 PM

 I am by no means a rocket guy so follow other advice or choose my input wisely.
 

 Be very careful with those tig rods for sticks! Talk about a lawn dart, ouch! Grab ya some small dowel rods at the hardware store would be cheaper if you’re buy the rods and not hurt as much should one find a target upon return. Square dowels seem preferred, but I’ve used round my time or two. I think I cut 36” dowels in half for 18” if I am remembering correctly.

 

 To start with my first guess on the one that took a bad turn I might add a little more stick to it. It sounds off balance from the little I know about rockets. Or maybe it ran out of thrust. 

 

 Secondly 12 grams seems a wee light for an amount of propellant in a rocket even a 1/2”, but I haven’t made a rocket myself in years now. I could be thinking off here as 10 grams can do a fair bit of work on a baseball. 
 

 Dextrin is not needed in the comp. 

 

 75/15/10 seems a little hot for a willow fueled end burn rocket especially as a beginner, but that’s not the tail of your experiences so far. I believe I used 60/20/10 when I made mine rocket formula which is a much calmer formula.
 

 Lightly dampening the comp can greatly help you with dust and consolidation. A very recent conversation with a seasoned rocket fellow 2% moisture was a topic of the conversation. 
 

 Good luck! I’m sure someone with more valuable info and experience will step up soon.



#3 Bigmark

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Posted 24 February 2020 - 09:59 PM

How deep a core do you have on your end burn tooling. You may need to drill it a little deeper. But do so by hand not with a power drill. And definitely scrap the tig rods. And get you some 1/4" dowel rods. By drilling the core out should get you a little bit more thrust. When I first started I made end burners and quickly found by drilling the core a bit deeper they would hope up pretty quick. This was an end burner i drilled out.https://youtu.be/ZfccV3ApmU0

#4 Arthur

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Posted 25 February 2020 - 02:29 AM

The stick is for aerodynamic stability in flight, square sticks are well regarded, sticks of wood are safer than metal. and the balance point of the complete rocket should be just behind the motor, on the stick. 

 

An end burning rocket can be made more powerful by drilling a short core very gently, or making a tool for ramming with a short nipple,



#5 hcb

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Posted 25 February 2020 - 06:59 PM

Thank you for the replies.  I'm not worried about the aluminum rods: 1) we're in the country so it's not like we're gonna hit someone and 2) they're only 3/32" diameter which makes them flimsy.  I did find one thing, though, they seem to bend too easily, either from the heat of the propulsion gasses or by the impact of the motor on the ground.  Two test flights today ended with the rod in a banana shape.

 

Today I pressed two test motors, these were with the shop press and a pressure gauge (as seen on some article on Skylighter).  That gave me more pressure and consistency than hitting my tooling with a hammer.  One motor had BP mixed with 2% H2O, one motor was dry.  Both had 75/15/10 and no dextrin.  I'm in north central Texas and the humidity seems to stay at 50-100% so my BP probably has absorbed a fair amount of moisture (plus the KNO3 before it was used in the BP).  So, I put the 2% H2O motor into a food dehydrator to dry it out and put the dry motor in there just for good measure.  I dried the motors for 1:15.  I launched them both and, wow, they actually flew really well.  Still not Estes but way better than my first attempts.  Of course, having changed 2 variables at the same time, I have no idea which did what: was it the drier fuel mix or the heavier packing force?  I dunno.

 

The nozzle former I made is supposed to leave a 1/8" cavity at the end of the fuel grain but that's dependent on the nozzle mix being put in in the correct amount which I have to guess at.  I need to come up with a consistent amount of nozzle mix and weighing it out is garbage as my current scale for this reads in whole grams.  I'm gonna go to a volume measure and nail this down.  Then I can make that consistent.  Then I can mess with different amounts of core at the nozzle-end of the grain.  That's got me excited because I could core some out to get a little more "pop" off the pad to get stabilized then drop to a slower, but longer, steady acceleration.  Theoretically.  When I was a kid messing with this stuff, I hated the idea of a core-burner.  I was wound tight.  I'm still kinda wound tight.  I still want an end-burner.  But I'm not opposed to "cheating" a bit off the pad. 

 

I do need a better nozzle material, I think, though.  I'm using ball-milled cheap kitty litter powder and some super fine sand and a touch of canning wax.  It's eroding to different degrees on the motors, some more, some less.  I think we have an oilfield supply place around here where I can buy 50# bags of bentonite clay.  I bought some about 18 years ago and it was just silly-fine stuff.  More like water than powder.  It's used in drilling mud.  I might try some of that.  A book I had as a kid spoke of Durham's water putty.  I see there's a little info online about it.  I'll look around.  Any pointers on what/which material I should be using for nozzle material?

 

Thanks again for the responses.

 

--HC



#6 Mumbles

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Posted 25 February 2020 - 07:39 PM

What size tubes are you using. End burners tend to take off kind of gracefully. You might just not have enough propellant to get a good lift and burn duration. Typically about 5" tubes are used.
Just so you guys quit asking, here is the link to the old forum. http://www.xsorbit2....forum/index.cgi

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#7 Boophoenix

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Posted 25 February 2020 - 09:07 PM

 I think you’ll probably find that you will be hard pressed to duplicate Estes exactly. They’ve had many years experience and lots of research. I would assume their manufacturing process is way different than ours. I’d think to produce a mass quantity of rockets in a production environment like that they would be single pressed on the company at the least. That’s just a suspicion though. 
 

 Again I’m not a rocket guy, but did have the honor of a few back and fourths with Ed Brown before his passing. As I understand it he was with Estes for 45 years. He probably knew more about black powder than most of us combined. He was darned good about not giving away trade secrets I think. Although I never asked about any either. 

 
I’ll give you some food for thought. More pressure on the comp may give ya the idea of more comp to do the work. However at the same time all of our other experiences tell us more consolidation comes slower burn rate. How these things factor percentage wise into rockets are beyond my experience currently, but not my pondering. 
 

 Mumbles, I do believe 1/2 ID tubes are mentioned in the first post. 



#8 SharkWhisperer

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Posted 25 February 2020 - 09:57 PM

Thank you for the replies.  I'm not worried about the aluminum rods: 1) we're in the country so it's not like we're gonna hit someone and 2) they're only 3/32" diameter which makes them flimsy.  I did find one thing, though, they seem to bend too easily, either from the heat of the propulsion gasses or by the impact of the motor on the ground.  Two test flights today ended with the rod in a banana shape.

 

Today I pressed two test motors, these were with the shop press and a pressure gauge (as seen on some article on Skylighter).  That gave me more pressure and consistency than hitting my tooling with a hammer.  One motor had BP mixed with 2% H2O, one motor was dry.  Both had 75/15/10 and no dextrin.  I'm in north central Texas and the humidity seems to stay at 50-100% so my BP probably has absorbed a fair amount of moisture (plus the KNO3 before it was used in the BP).  So, I put the 2% H2O motor into a food dehydrator to dry it out and put the dry motor in there just for good measure.  I dried the motors for 1:15.  I launched them both and, wow, they actually flew really well.  Still not Estes but way better than my first attempts.  Of course, having changed 2 variables at the same time, I have no idea which did what: was it the drier fuel mix or the heavier packing force?  I dunno.

 

The nozzle former I made is supposed to leave a 1/8" cavity at the end of the fuel grain but that's dependent on the nozzle mix being put in in the correct amount which I have to guess at.  I need to come up with a consistent amount of nozzle mix and weighing it out is garbage as my current scale for this reads in whole grams.  I'm gonna go to a volume measure and nail this down.  Then I can make that consistent.  Then I can mess with different amounts of core at the nozzle-end of the grain.  That's got me excited because I could core some out to get a little more "pop" off the pad to get stabilized then drop to a slower, but longer, steady acceleration.  Theoretically.  When I was a kid messing with this stuff, I hated the idea of a core-burner.  I was wound tight.  I'm still kinda wound tight.  I still want an end-burner.  But I'm not opposed to "cheating" a bit off the pad. 

 

I do need a better nozzle material, I think, though.  I'm using ball-milled cheap kitty litter powder and some super fine sand and a touch of canning wax.  It's eroding to different degrees on the motors, some more, some less.  I think we have an oilfield supply place around here where I can buy 50# bags of bentonite clay.  I bought some about 18 years ago and it was just silly-fine stuff.  More like water than powder.  It's used in drilling mud.  I might try some of that.  A book I had as a kid spoke of Durham's water putty.  I see there's a little info online about it.  I'll look around.  Any pointers on what/which material I should be using for nozzle material?

 

Thanks again for the responses.

 

--HC

You're on your way. Cut loose the $10 and buy a digital scale that does 0.01 gram measurements!  It's necessary if you're making small batches. Or one that reads 0.1 grams through a higher weight range, same price. Pretty reasonable. And necessary!

 

Read this article if you haven't yet: https://www.skylight...-rocket-engines . It's not that challenging. Plus, you can put motors 5x the power of expensive stock Estes motors in your birds for 10-20% of the price. Sheesh, it ain't rocket science....oh my, well, yes it is. But the easy part. I send homemade motors up in BT50 tubes all the time, with great results. Desired delay. Ejection charge... And they come home often, too. You can do it, too.

 

I love fireworking, but also enjoy sending retrievable rockets a quarter mile upwards. Commercial motors are expensive. Touch base if you need further tips on optimizing your endburners...

 

SW

 

p.s. Please upload pix of your nozzles; before and after...  And as Mumbles queried, what diameter, and length, are your motors??? And nozzles? How much fuel? Remind me please of the precise fuel composition??? As an "end" burner, does it have a short core? Of course it does. How long and what's the diameter. All are important considerations.

 

Your kitty litter is probably fine (ground to powder in a coffee/spice grinder?) for an endburner, for smaller (1 pound) motors, without wax/oil addition. Show pix of supposed nozzle erosion, please, and don't mess with but one variable at a time to dial this in properly!

 

I would be worried about the aluminum rods!! Use wood please, for good reason.


Edited by SharkWhisperer, 26 February 2020 - 10:52 AM.


#9 Boophoenix

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Posted 25 February 2020 - 11:40 PM

 Just an option you don’t need an oil supply place. Pottery supply shop should have plenty of fine powdered bentonite if that’s what you want. It can be purchased by the pound too, but may cost more than the sack the oil place likely has. 

 

 My few times on rockets and my other items I just used kitty litter right outa the box. I have used the powdered bentonite as well from a pottery store. I had no noticeable variation in either that caught my attention. 
 

 I’ve always meant to look, but don’t go to the auto parts store often. One here use to have a cheap oil dry that was a pain in the ascot to get up of the floor if it ever got rolled in by a creeper wheel. I always kind of wondered if it wasn’t a bentonite or similar. 
 

 Hmm, may have been partially correct

https://hw.menardc.c...sorbentILMS.PDF



#10 SharkWhisperer

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Posted 26 February 2020 - 10:47 AM

The cheapest kitty litter is generally pure bentonite, and is the exact same stuff the fireworks companies will be happy to send you for $1/pound plus shipping. A 20-pound bag of the cheapest litter they have from Walmart (be sure to get unscented) will last you years. But, and this is a common mistake, do not pound/press nozzles with the granular stuff. It won't work well. Grind it to a fine powder in a coffee mill--no need or benefit from slow ball milling, and it'll make outstanding tough nozzles that are night-and-day better in performance than granular-based clay nozzles. Add whatever you wish (e.g. wax), but I've never had an issue with nozzle blow-out or slag accumulation with pure bentonite powder alone. I have paraffin for smoke bombs but never felt a need to enhance nozzles on up to G-force motors. I just rough up the insides of the tube at nozzle height for a little more bite and smack em good and tight. They're pretty durable.

 

Homemade tooling is simple to make and is super cheap if you don't have professional metal endburner tooling. Most commercial endburner tooling is similar to gerb tooking (slight size/diameter variation) and has a spindle approx an inch long; performance will fall off with shorter "cores".

 

I've also reloaded used motors, with the only inner nozzle cleaning being done with a metal test tube brush (that also pulls any leftover slag off the walls). It smooths out the inner nozzle surface. Not the same as a new nozzle, but often performs without noticeable differences. I've never tried a third reload, however, because I don't trust the tubes and they're cheap enough to use new (but free to reuse); maybe if you silicate coated the insides first if there wasn't excessive tube wall erosion, and it was cleaned up inside pretty well.


Edited by SharkWhisperer, 26 February 2020 - 10:52 AM.


#11 Arthur

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Posted 26 February 2020 - 12:58 PM

A wooden square stick tends to be better for guiding the rocket than a round wooden stick, the corners are actually useful aerodynamically and may help with flight dynamics. 



#12 SharkWhisperer

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Posted 26 February 2020 - 04:17 PM

Not to mention the likely cost and overall weight savings vs using metal rods (spears from aloft...).

 

Arthur, what's your view on rectangular vs square sticks (wooden) for guidance?



#13 hcb

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Posted 26 February 2020 - 06:12 PM

What size tubes are you using. End burners tend to take off kind of gracefully. You might just not have enough propellant to get a good lift and burn duration. Typically about 5" tubes are used.

 

I'm using 1/2" ID tubes currently.  They are 5" long.  Graceful is good to a point but I think stick-stabilized "rockets" are more forgiving of graceful starts than a "real" model rocket which is all kinds of top heavy and needs airflow to keep it pointing the direction you want it to go.

 

--HC



#14 hcb

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Posted 26 February 2020 - 06:15 PM

 I think you’ll probably find that you will be hard pressed to duplicate Estes exactly. They’ve had many years experience and lots of research. I would assume their manufacturing process is way different than ours. I’d think to produce a mass quantity of rockets in a production environment like that they would be single pressed on the company at the least. That’s just a suspicion though. 
 

 Again I’m not a rocket guy, but did have the honor of a few back and fourths with Ed Brown before his passing. As I understand it he was with Estes for 45 years. He probably knew more about black powder than most of us combined. He was darned good about not giving away trade secrets I think. Although I never asked about any either. 

 
I’ll give you some food for thought. More pressure on the comp may give ya the idea of more comp to do the work. However at the same time all of our other experiences tell us more consolidation comes slower burn rate. How these things factor percentage wise into rockets are beyond my experience currently, but not my pondering. 
 

 Mumbles, I do believe 1/2 ID tubes are mentioned in the first post. 

 

I totally agree that duplicating their products is likely too much of a challenge.  I do, however, wish to have an end burner "like" their's.  What's weird about the compression of the comp is that since I started pressing the comp with a shop press and a pressure indicator (and wrapping the tubes in a reinforcing sleeve), I get better burns/flights.  However, I simultaneously started drying the finished motors in the dehydrator, so I have changed two variables at once.

 

--HC



#15 hcb

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Posted 26 February 2020 - 06:31 PM

You're on your way. Cut loose the $10 and buy a digital scale that does 0.01 gram measurements!  It's necessary if you're making small batches. Or one that reads 0.1 grams through a higher weight range, same price. Pretty reasonable. And necessary!

 

Read this article if you haven't yet: https://www.skylight...-rocket-engines . It's not that challenging. Plus, you can put motors 5x the power of expensive stock Estes motors in your birds for 10-20% of the price. Sheesh, it ain't rocket science....oh my, well, yes it is. But the easy part. I send homemade motors up in BT50 tubes all the time, with great results. Desired delay. Ejection charge... And they come home often, too. You can do it, too.

 

I love fireworking, but also enjoy sending retrievable rockets a quarter mile upwards. Commercial motors are expensive. Touch base if you need further tips on optimizing your endburners...

 

SW

 

p.s. Please upload pix of your nozzles; before and after...  And as Mumbles queried, what diameter, and length, are your motors??? And nozzles? How much fuel? Remind me please of the precise fuel composition??? As an "end" burner, does it have a short core? Of course it does. How long and what's the diameter. All are important considerations.

 

Your kitty litter is probably fine (ground to powder in a coffee/spice grinder?) for an endburner, for smaller (1 pound) motors, without wax/oil addition. Show pix of supposed nozzle erosion, please, and don't mess with but one variable at a time to dial this in properly!

 

I would be worried about the aluminum rods!! Use wood please, for good reason.

 

It's not for being cheap that I don't have a tenth-gram scale.  I dabble in so many different things that I've got all kinds of equipment, and not all of it at one place.  I try to use what I have and thought that scale would work, before I knew I'd need to be measuring tenths of a gram.  I may get another scale but for now I think this will work.  I bought a set of cooking measuring spoons and did some work today and found that if I add the materials by volume, not weight, they are easy to measure consistently.  I also put a ring (engraved on the lathe) on the compression tool (ram) so I know where the nozzle cone ends and where the packing of the nozzle material needs to end and I got some consistent results. 

 

I have read that article a couple of times, actually.  :)  I appreciate the offer of advice: yes, please, anytime.  I'm going to post results from today after I address the replies since yesterday.

 

I'll try to take pictures tomorrow and post them tomorrow night (I don't get to live where my shop is).

 

Currently, I'm using 1/2" ID tubes x 5" length.  Comp is 75/15/10 KNO3-charcoal-sulfur, all ball milled together for 3 hours.  The charcoal is willow, I made myself, which was ball milled as it came out of the retort (paint can).  The potassium nitrate I'm using currently came from eBay or Amazon, same with the sulfur.  It's not flowers of sulfur or dusting/wettable sulfur.  The nozzle mix is the kitty litter powder (ball milled and screened) with a little fine sand and the canning wax.  I'm pressing the nozzle mix and the comp at about 3k PSI.

 

I got some wood dowels because the heat from the motor (or the impact with the Earth) is bending the aluminum rods.  The smallest wood dowel rod from Lowe's (1/16" I think) weighs as much as the aluminum rod.  The aluminum rods are tiny.

 

--HC



#16 hcb

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Posted 26 February 2020 - 06:35 PM

 Just an option you don’t need an oil supply place. Pottery supply shop should have plenty of fine powdered bentonite if that’s what you want. It can be purchased by the pound too, but may cost more than the sack the oil place likely has. 

 

 My few times on rockets and my other items I just used kitty litter right outa the box. I have used the powdered bentonite as well from a pottery store. I had no noticeable variation in either that caught my attention. 
 

 I’ve always meant to look, but don’t go to the auto parts store often. One here use to have a cheap oil dry that was a pain in the ascot to get up of the floor if it ever got rolled in by a creeper wheel. I always kind of wondered if it wasn’t a bentonite or similar. 
 

 Hmm, may have been partially correct

https://hw.menardc.c...sorbentILMS.PDF

 

Well, around here, it's easier to find an oil field supply store than a pottery store.  :)  The closest pottery shop, per Google Maps, is about an hour.  I bought a bag of the cheapest, non-scented kitty litter that Wally World had.  On the back, at the bottom, the producing company's name is: OilDri.  That would be the stuff you're referring to at the auto parts store.  And, yes, it's a special event trying to get it out of rough concrete after it's been run over by something rigid and heavy.  :)

 

--HC



#17 hcb

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Posted 26 February 2020 - 06:49 PM

Today I made two more motors.  First, I made new tooling for the nozzle formation and the initial entry into the fuel grain.  Then I carefully figured out how much nozzle mix to add to the tube, by volume, so that I could get the level of the nozzle where it needed to be on the tooling/former and so I could do it repeatably.  Considering that some amount of core would be necessary to pep up the start, I went from a nominal 1/8" intrusion into the fuel grain to 3/8" into the fuel grain.  Being that I had not had a way to do things with consistency before, sometimes the entry into the grain was non-existent and required that I drill through nozzle clay.  That's not like that anymore.  I changed the pressure gauge on my pressure indicator from 0-5k PSI to 0-1k PSI which works much better for the forces I'm applying.

 

Now I had a way to repeatably make my nozzles, add fuel in consistent increments, and measure how much force was applied to each step.  Score!

 

This evening, I fired both the motors.  And both took off quickly and then both blew after rising a few feet from the launch tube.  One entire assembly (basically a bottle rocket based on a 1/2" ID tube) disappeared.  The second unit, though, I found.  The wooden rod had snapped at the motor.  From watching the "flights", both appeared to blow the fuel and capping clay plug out the top of the motor, not blowing the nozzle out.  The recovered motor did, in fact, still have it's clay nozzle.  It was, however, damaged, but still there.  So, apparently, I need to shorten that core into the fuel grain.  The reason for making such a big jump up, from 1/8" nominal to 3/8" nominal, is that it's easier to cut that rod down than it is to cut it longer.  I'll mess with it tomorrow, shortening it about 1/8" of an inch.  The goal will be to get enough fuel burning in the beginning to get the rocket moving quickly but then back off the pressure before it blows the motor.  I may try adding some dextrin to the mix (I have a batch of the same BP with 5% dextrin in it) to maybe make the fuel grain stronger.  Since that may retard the burn rate, I may test this tooling with that mix before cutting the rod.  I'll have to ruminate on that.

 

--HC



#18 BetICouldMake1

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Posted 27 February 2020 - 09:11 AM

Today I made two more motors.  First, I made new tooling for the nozzle formation and the initial entry into the fuel grain.  Then I carefully figured out how much nozzle mix to add to the tube, by volume, so that I could get the level of the nozzle where it needed to be on the tooling/former and so I could do it repeatably.  Considering that some amount of core would be necessary to pep up the start, I went from a nominal 1/8" intrusion into the fuel grain to 3/8" into the fuel grain.  Being that I had not had a way to do things with consistency before, sometimes the entry into the grain was non-existent and required that I drill through nozzle clay.  That's not like that anymore.  I changed the pressure gauge on my pressure indicator from 0-5k PSI to 0-1k PSI which works much better for the forces I'm applying.

 

Now I had a way to repeatably make my nozzles, add fuel in consistent increments, and measure how much force was applied to each step.  Score!

 

This evening, I fired both the motors.  And both took off quickly and then both blew after rising a few feet from the launch tube.  One entire assembly (basically a bottle rocket based on a 1/2" ID tube) disappeared.  The second unit, though, I found.  The wooden rod had snapped at the motor.  From watching the "flights", both appeared to blow the fuel and capping clay plug out the top of the motor, not blowing the nozzle out.  The recovered motor did, in fact, still have it's clay nozzle.  It was, however, damaged, but still there.  So, apparently, I need to shorten that core into the fuel grain.  The reason for making such a big jump up, from 1/8" nominal to 3/8" nominal, is that it's easier to cut that rod down than it is to cut it longer.  I'll mess with it tomorrow, shortening it about 1/8" of an inch.  The goal will be to get enough fuel burning in the beginning to get the rocket moving quickly but then back off the pressure before it blows the motor.  I may try adding some dextrin to the mix (I have a batch of the same BP with 5% dextrin in it) to maybe make the fuel grain stronger.  Since that may retard the burn rate, I may test this tooling with that mix before cutting the rod.  I'll have to ruminate on that.

 

--HC

 

Question, when you say you are pressing to 3k psi do you mean 3k on the comp or 3k on the gauge? 

 

On the recovered motor was the tube damaged or did you just blow out the bulkhead? 

 

A couple suggestions/ideas: when pressing using 1-2% added moisture there is no need to dry the motors after. Many have found this level of moisture actually increases burn rate. When you dry your finished motors removing that moisture is more likely to cause issues by shrinking your fuel grain, your tube, or both. Ideally you want to press your motors with the fuel and tube at something close to an environmental equilibrium, meaning they will not be gaining or losing moisture after pressing and will therefore be as dimensionally stable as possible. If your fuel grain shrank or cracked from drying it could have cased the blow through problems you mentioned. 

 

There's no need to add a binder to your fuel. The compaction from pressing, especially with 1-2% water, will create an extremely durable grain. A binder will just slow things down, which is fine if you want to use it as a burn rate modifier, but on an end burner you should really be able to use your hottest bp without issue. 

 

Good luck with it all and keep good notes! As you mention, consistency is key when it comes to building rockets.


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#19 justvisiting

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Posted 27 February 2020 - 11:36 AM

When I did some tests with water-dampened propellant on 3lb rockets, I found that the water did not leave. I tracked the weight of the motors for almost a month, and they all weighed within a gram of the starting weight. They were nozzleless and nozzled, with 2 1/2% and 3% water, respectively. Sorry, no end burners. I also found that the pressing force could be lowered from 7500psi to 3400psi, and still produce a monolithic grain. The tubes were waxed. 

 

Dampening the propellant gives much better compaction, allowing more propellant to fit in the tube. The potassium nitrate acts like cement, and the grain is like stone. It's as dense as commercial black powder. I think end burners would benefit greatly from dampened propellant, small increments, and waxed tubes. I would maybe even go the extra step of lightly pucking and granulating the damp propellant, so that the rammer travel per increment is minimized.



#20 SharkWhisperer

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Posted 27 February 2020 - 06:30 PM

Be careful with your volumetric measurements of nitrate when making fuel--it's super hygroscopic and you will have difficulty in standardizing your fuel if one batch contains extra water in the initial mix versus the next. Especially if you're blowing endcaps (not tubes/nozzles???) on an endburner, which is weird. I dry out my KNO3 religiously in the oven (safe and quick insurance) before making large batches, or if it has sat around awhile (even with dessicant) because it will absorb atmospheric water. Then at least you will know if your oxidizer ratio is truly the same from batch to batch; a few % makes a difference. Especially when you're starting at CATO and dialing back to ideal.

 

But really amigo, a reasonable scale that measures 0.01 grams costs perhaps $10, and, for small batches, is much more helpful in repairing/optimizing fuels than relying on cooking spoon volumes!

 

Maybe press an extra gram or two of clay on your bulkhead and see if it makes your tubes blow instead. You're having coreburner issues with an endburner... Why change your nozzle (except for diameter) if it's not blowing out or undergoing crippling deformation? If cat litter alone works, and nozzles are not (yet) blowing out, eroding, or filling with slag, why change yet another variable? Maybe leave it (for now)!

 

Dextrin? Do I recall you were considering using dextrin? NO! Get your BP consistent. You're lifting a rocket not a shell. No dextrin needed. Adjust your BP ratios to adjust your BP burn rate, but solve your end cap blowouts first.

 

Weight bedamned, your aluminum rod tails scare the hell outta me, no matter how desolate your testing grounds are (i.e., worried that YOU might get speared, not a bystander). Glad they bent on impact because they have no role in 1/2" rocket guidance.

 

Keep at it, good luck, and please stay safe!


Edited by SharkWhisperer, 28 February 2020 - 09:21 AM.





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