Rcandy vs BP

[MadMat]

I have made a lot of successful rcandy rockets of various sizes, but every time I try to make a bp rocket I get cato's.

Yesterday, I tried making a bp rocket using the 1lb rocket propellant in our formulary and I essentially made a bomb on a stick (made a hell of a boom though!). My specifications were; rocket tube 7/8"ID x 6.5" long x 1/4" wall, core was 3/16" x 3.5", nozzle was 7/32". Does this sound too small?? Or, maybe I did something else wrong? I know the grain was rammed solid as I took apart the first one I produced to inspect it. Very solid grain, core was well formed, nozzle and header were solid and well seated. Maybe I needed a larger core/nozzle?

[lloyd]

For any BP rocket, a reasonably safe place to start is with a 33% nozzle. So, for 7/8", it would be about 0.291" at its smallest portion of the throat (if it's not just a straight hole).

 

That's an "L" lettered drill. 19/64" is the next fractional drill up from that, and 9/32" the next fractional drill smaller.

 

Yes... your nozzle seems too small, if it was good BP.

 

If you're making your own BP 'rough mix' fuel, you can probably go significantly smaller than that, but that's a good place to start.

 

Also, you say nothing of the pressure to which you pressed or rammed it, nor whether or not it's cored. Cored BP rockets are usually nozzle-less, while nozzled ones are end-burners -- quite unlike R-candy, which is almost always cored with a nearly full-length pintle.

 

LLoyd

Edited March 16, 2016 by lloyd

[MadMat]

For making the propellant I; (all my chems except the tail charcoal are 80 mesh or finer) mixed the KNO3, sulfur, and fuel charcoal (willow) and ball milled for 1 hour. I then added the tail charcoal (40 mesh pine) and the aluminum (-60, +200 flitter) to the ball mill and milled for 15 minutes. I test burned a small sample and it burned rather slowly, in fact, considerably slower than my usual rcandy. I imagine under the pressure developed in the rocket that burn rate sped up considerably though. BTW I have both letter and number drills. I also have a tapered reamer for making my nozzles, unfortunately not one as big as I will need. Thanks for the starting % on bp nozzles I will try that in the future.

Edited March 16, 2016 by MadMat

[lloyd]

'still didn't say anything of the pressing pressure on the composition, nor on coring or lack-of.

 

Lloyd

[MadMat]

the rocket was rammed by hand (I made my own tooling complete with core pin and clearance holed ram). But the resulting grain was very solid and hard. I guess I am going to have to make a larger core pin and ram for bp.

Edited March 16, 2016 by MadMat

[lloyd]

You said "core pin". Do you mean the grain is cored? I mentioned that above... BP nozzled motors are not normally cored. And cored ones are usually nozzle-less.

 

LLoyd

[MadMat]

Hmm.... I guess was following the way an Estes rocket engine was made. Though it's been a number of years since I saw one

[lloyd]

Most Estes motors have pretty short cores, some no more than a mere 'dimple'. Those that have long cores also have larger nozzles.

 

I don't know if Ed Brown inhabits this space, but if he does (having worked at Estes for decades), he'd be able to clarify that better than I.

 

LLoyd

[Mumbles]

I have made a lot of successful rcandy rockets of various sizes, but every time I try to make a bp rocket I get cato's.

Yesterday, I tried making a bp rocket using the 1lb rocket propellant in our formulary and I essentially made a bomb on a stick (made a hell of a boom though!). My specifications were; rocket tube 7/8"ID x 6.5" long x 1/4" wall, core was 3/16" x 3.5", nozzle was 7/32". Does this sound too small?? Or, maybe I did something else wrong? I know the grain was rammed solid as I took apart the first one I produced to inspect it. Very solid grain, core was well formed, nozzle and header were solid and well seated. Maybe I needed a larger core/nozzle?

 

He mentioned the core in the first post.

 

Madmat, a few things. Firstly, that is not a very common formula. The proportions look fine though. The formulary thing was kind of abandoned until we can figure out how to make it do what we want. Anything in there is a crapshoot at this point to be totally honest. I'd suggest against milling the composition once the aluminum and coarse charcoal though. You want to maintain the coarse charcoal, and most would advise against milling metals with oxidizers. I know of many people who save the metal containing fuel/delay for the area above the spindle as well. Secondly, which is more of a semantics thing, a 1lb rocket has a 3/4" ID. The "standard" proportions for a core burning black powder rocket are nominally 3/4" ID x 7.5" long with as thick of walls as needed. Typically the core will be about 7x the ID of the tube (5.25" long for 3/4" tube), and about 1/3 of the tube diameter (.25" for 3/4" tube).

 

The nozzleless cored rockets are a relatively new thing. Perhaps not new by invention, but new by popularity. They're normally made with as hot of BP as you can produce as well. Trying to use a formula designed for nozzled core burners in a nozzleless rocket will probably lead to somewhat disappointing results. That said, they tend to be a lot more forgiving in terms of manufacture.

[Nessalco]

What Lloyd said. The surface area exposed to burning paired to the nozzle size gives you some indication of how a motor will hold together. Your nozzle is too small for a cored motor. The larger the area of fuel exposed to burning, the larger the nozzle must be - and any core at all increases the burning area.

 

I ran the configuration you described through a program called BurnSim, which can simulate the function of rocket motors. I used a fuel description for standard BP, no binder, moderately hot. I got a chamber pressure of 716PSI, which is on the high end for a paper tube, but manageable with good quality tubes and processes. If you're running a hot BP the pressure will be higher.

 

This suggests a few things are possibly causing you problems:

 

- Insufficient loading pressure. If the grain is inadequately compressed, it will burn far more quickly, raising chamber pressure

 

- Inadequate tubes. You're going to need a good quality tube to hold that much pressure with 1/4" walls.

 

- Microfractures in the grain. When you are at the bleeding edge of holding together a motor, one tiny crack in the grain can send the motor over the edge.

 

So, some questions. What are you using for tubes (source)? Loading pressure? Are you waxing the tubes?

 

Estes motors have varying degrees of core, depending on the design requirements. For the most part they are short to very short.

 

Kevin

[lloyd]

Yep, Mumb... missed it completely, jumping right to the question, instead of the answer! <G>.

 

That said, that comp will probably perform at least well-enough to be a rocket, even without a core. He might have to shrink the nozzle some after his first bench-test *upside down*, like a gerb!

 

Lloyd

[lloyd]

BTW, Mumbles...

 

I have difficulty mentally separating 'product' from 'hobby devices'.

 

I think there was a period of time during the model rocketry craze when nozzle-less rockets became less popular with amateur fireworkers, but that wasn't the case with manufacturers. When I got into pyro back in the late 1960s, a nozzle-less BP rocket was a 'safe' product for manufacturers to issue, because the likelihood of a CATO was slim.

 

Now, because of the unpredictable flight of any free-flight rocket, most US manufacturers only make line-rockets, if any at all. But from my perspective, not only are they not new (which you mentioned), but they were very popular for a time, for commercial manufacturers.

 

Lloyd

[Mumbles]

Ah yeah, I was speaking from a hobbyist point of view. I didn't realize they went back that far even. My first encounter with them was from a guy going by DJ (real name Donald Josar) in the early 2000's. He was around rec.pyro and PML I think. Maybe he reinvented the wheel, but I've noticed an additional resurgence in the last few years on places like Passfire and Fireworking as well. Even Ben Smith and Caleb now carry tooling for them.

 

http://www.wichitabuggywhip.com/fireworks/rockets/nozzleless.html

[lloyd]

Yup, and they go like a bat out of hell!

 

Lloyd

[Seymour]

For a rocket with a nossle and a core, using BP chemicals my favourite fuel for a few years after I started was 60% KNO3, 30% Charcoal and 10% Sulfur.

 

The chemicals can be ball milled, or just screened together. This fuel is very forgiving and has a very rich tail of charcoal sparks

[MadMat]

About the core, when I test fired a small sample of the formula, it burned very slowly. I figured, for that slow of a burning grain, if I didn't use a core, I would get a FTL (failure to launch). I am out of town right now, but when I get back I will try another rocket with pretty much the same dimensions, just a lot bigger nozzle diameter. Mumbles, as far as ball milling the completed formula, it was for only 15 minutes and my tail charcoal was 40 mesh to begin with, so if the milling knocked it down to even 80 mesh I figured it would be just fine. And yeah I know mixing oxidizers and metals with a ball mill is a bad idea but since the KNO3 was already incorporated with the fuel charcoal, I figured it was only a slightly increased risk. and like I said only it was only 15 minutes ( I suppose if I make any more in the future I will just screen the mixture together). I know this cato was not a simple tube failure. I have used these same tubes for rcandy rockets and they have always held up fine, even with fairly small nozzles. This cato was an explosion. There wasn't a piece of the tube left bigger than 1/4 inch. I was kind of amazed when I did the clean up the next morning.

[MadMat]

Well I made another rocket with the same dimensions except, a much larger nozzle (.312") and a very short core (approx 5/8" long). It didn't explode and flew! I do realize my first assumption about needing a full core because the grain burned slowly was correct. The rocket took off just fine but the flight petered out at about 40' and fell back down even with the fuel burning. I believe it flew just fine for the duration the core lasted and then the thrust dropped off considerably. It did produce a bright, long lasting tail though. I think with a slightly smaller nozzle and a full core, I should have a very nice formula for a rocket.

[AlteredMatter]

In my opinion, CIA(precipitation) black powder works better in rockets. Due to it's slower burning characteristics, there is a much smaller chance of CATO. I've never had a rocket explode when I use that, it seems to burn the perfect speed, and using standard ball milled black powder I tried a few times - Boom... . I know that many rocket fuel formulas exist, but since sometimes you have to add charcoal to slow it down, why not just used precipitation black powder?

[Nessalco]

You need a fresh wax.

 

Kevin

[schroedinger]

CIA powder is expensive and inconsistent in power. Better use powder of shorter milling time. CIA powder is comparable to 1 h milling powder. Edited April 22, 2016 by schroedinger

[AlteredMatter]

There are wrong ways to do the CIA method, If you do it correctly, it works well for just about any type of firework - other than for a high energetic burst charge - which you could ball mill the CIA black powder and you'll end up with really strong black powder. I've used it for just about everything. If you make it correctly, it works quite well. How is that a high cost? Isopropyl alcohol really isn't that expensive. It's much safer too, since all the components are submerged in water. You can also make up to 1 Kilogram of precipitation black powder at a time. Precipitation black powder can be used without any great consequence. You don't need super fast black powder to make fireworks. So please tell me, how do I need a fresh wax?

[schroedinger]

Welt 1 kg of bp is used up in a blink. Many people here need at least 20 kg per year. That are costs summing up. For me i would double the costs of the bp, for others triple it or more and then you end up with a lower quality product.

Then we have the next step in precipation process, squezing out the water. Did you ever test how much kno3 is still dissolved? You waste quite a good ammount.

Ok you're right as bigger the shells becomme, you need less powerfull bp or you have to counter with booster or more bp. Both putting cost or dangers into the process.

Think about lift, 15-20g of ball milled bp take a 3" shell up to a nice height. CIA won't to that, you need a maron or at least 40g.

 

Then you say you can make 1kg at a time, that depends on your pot size, but with the right mill you can mill the same or more in a run.

 

What he meant with you need a fresh wax, is that you should wax your ro ket tubes. If your tube is of good quality you can use nit bp i a rocket w/o CATO.

[AlteredMatter]

Oh, I apologize. I thought he was insulting me by saying I needed a fresh wax . Personally, instead of wax though a sodium silicate solution I believe would work much better. Yes, you're right, some potassium nitrate is lost during the precipitation procedure. But it crystallizes out of solution, couldn't you just recycle the excess potassium nitrate for another batch? Now I agree, ball milling is much cheaper and it also depends in my opinion if the mill is very efficient. I would say that you'd need a pretty big ball mill to produce that much at a time - being half full of lead media and the right RPM, definitely.

[schroedinger]

No sodium silicate wont do a better job, it propably just makes the things worse. You should do some reading on waxing tubes.

[MadMat]

I've never waxed my tubes, but I'm actually a rcandy type guy. I just tried to make a bp rocket on a whim. Never really read the purpose or procedure for waxing the tubes.