What I have been doing (Novice endeavours with rockets)

[stix]

Not bad - good to see a difference, although not much.

 

I keep forgetting that you are doing internal/core burners for both. I'm thinking nozzled end burners and nozzleless core burners.

 

I've found a little water does improve the speed of bp, but of course this effect is short lived.

[Baldor]

While I wait for a delayed shipping of aluminium stock for making the nozzleless ramers, I'm starting to plan for the next step: The headers.

 

Planing to do 2" and 3", maybe I even try 4" (Really, 50, 75 and 100mm). I think I have the concept pretty clear, but still have some doubts.

 

Stars:

 

Seems I need 1/4" and 3/8" sizes (6 and 8 mm diameter) I will use only BP based streamers for starters, I don't want to start accumulating chemicals still.

 

Something I can´t found is the height of the stars. What is the diameter to height proportion on a typical pressed star? Also, when making a star plate, What is the proportion of pin length to finished length? (Compression ratio? :-) )

 

Breaking:

 

I read smaller shells need more powerful breaking. Will a bag of slow F do the trick? 3FG BP will be OK for filling the rest of the shell?

 

TLUD:

 

I want to have more control over the charcoal I use, and also, I'm tired of paying trough the nose for powdered charcoal that is not intended for pyro. So a tlud is also in the very near future. Would a typical metallic paper bin be up to the task, or are they too flimsy?

 

Next week, 19mm nozzleless core burner tests.

[Baldor]

TEchnicaly not a CATO, so let it call it a CAST (Catastrophic static test),

 

 

19mm nozzleless core burner, same BP as previous 15mm tests. It failed by the bulkhead, the tube is bent but whole. I found later the clay bulkhead a couple of meters away, seems in one piece, but i cant be sure as it was soaking in the rain.

 

Later I will post post mortem photos, and photos and description of the tooling. I have a lot of thinking to do, and maybe some machining. No problem since I'm out of black match and I have been waiting for two weeks to stop raining.

[stix]

Nice that you put a positive spin on it.

 

I love the slo mo - is the load cell still ok?

[Baldor]

You learn from the failures too. For me, this test is a good test. I collected a lot of data during the building process, and the nice guys in the discord chat helped a lot.

 

I don't know if the load cell survived. First I must fasten all the loose screws in the stand. :-)

[stix]

Failures are the best way to learn.

[Baldor]

Ok, some photos as promissed.

 

This is the tooling. 7mm bronze core, base and ramers aluminium, all finished to 400grit + scotvh brite.

 

 

You can see also the failed tube. It's a little bent, but no failures in the tube. What failed is the bulkhead. A lot of acordioning during the pressing, and clik,clik sound after every increment, I could sense the clicks in my hand. I should have stopped after the first increment, but this is what tests are for, so I keep going.

 

Spindle diameter is a little under 7mm, length about 135mm. (Notice I used a dual marked tape measure for the benefit of americans :-) )

 

Now the sleeve... I can't machine a 200mm interior in one pass, so I decided to split the sleeve in three parts. HDPE, 45mm OD, 25.4mm ID.

 

 

 

Notice the red X in one of them. I had trouble extracting the tube from this, needs a little remachining. I can´t notice any difference in width in the tube after pressing, and only a hair thin mark in the unions, visible but not noticeable at tact. Seems it can work. What the experts think?

 

Of course, increments... I used two reaspoons increments, what gave me about 1 ID increment. I used 1 ID with the 15mm rockets without problem, Well, in 19mm rockets is a problem.

 

After some conferencing with experts in the discord chat, the cause is obvious. Too big increments.

 

Some questions for the experts here:

 

Is there a too small spindle for nozzleless core burners? Is 7mm to small for a 1lb rocket?

 

What is a good material for making dummy rockets? I want to solve the acordioning problem, and don't want to use 60g of BP in every experiment. Bentonite? Charcoal? Flour maybe?

 

What do you think of the tooling? Something obviously wrong?

[stix]

My initial observation for the "CAST" as you nicely call it, was over-pressurization/constriction. My reasoning for that conclusion is because the failure occurred almost instantly! - the only other thing would cause that is if the grain was cracked or there was a cavity between the bulkhead or walls.

 

My calculations say your start Kn Ratio was around 77. That's very high for a bp motor. Most of my (sugar) motors of a similar size start at around 70 which is fine for a fuel that burns at around 1/2 a centimeter per second, which is much much slower bp burn rate.

 

Maybe your spindle needs to be shorter and more tapered? or ignited from the top?

 

I've only ever made a few bp motors and they were end burners (slightly ported) and they worked fine - except for the time the port was drilled too deep . However, my experience tells me that just by "looking" at your design, I would be expecting cato. Unless it was slow burning fuel.

 

btw. I do like the look of the tooling. Also I hope the CAST didn't ruin the software

Edited April 16, 2018 by stix

[Baldor]

It is nozzleles, stix. The problem was the bulkhead. Too thin, and a wrinkled tube.

[stix]

Yes Baldor, I do know this test was nozzleless. But in your design configuration, it may well have had a 7mm diam. nozzle. - no difference.

 

Imagine you do a test using an aluminium tube - you certainly won't get the wrinkles. You can also apply far more force to the bulkhead, and the powder with no chance of separation from the wall or bulkhead.

 

I can assure you that you will get a much BIGGER result!!!!! even with 4mm thick Al. I've witnessed it. When I say BIGGER, I don't mean in a good way.

 

Unless of course you are using slow burning bp?

 

What I'm saying is that I doubt you can contain a design like you have, without "CAST" unless you have a slower burning fuel, or a very strong casing. BP is pretty powerful.

 

Happy to be proven wrong .

Edited April 16, 2018 by stix

[Baldor]

It's the same BP that worked with the 15mm motors. I asked in the discord chat, and 1/3ID for nozzleless is OK with fast fuel. The problem is with the method, and maybe the sleeve. I'm out of black match, so there will be a few days until the next test, still raining, but seems this is the last day.

[stix]

Well 1/3ID "seems" absolutely ok, but it importantly depends on the length too. Your core length is way too long for your fast bp fuel to give nothing but cato.

 

The 15mm version was also able to be compacted more easily and therefore it will have the effect of slowing down the burn rate (with bp).

 

Scaling up a design takes a bit of thought. In melted sugar motors, the fuel can be made more consistent in density when poured into grains and therefore has the overall affect in that the burn rate "slows down" the bigger you go. In a PB pressed motor, the bigger you go, it's always going to be harder to press it to the same density and there will also be issues with "crimpling" of the sides, Therefore with BP motors, when you go bigger, the burn rate will generally increase because it's harder to compact the fuel to the same density.

 

This is my basic philosophy of "Scale Up" theory. Take it, or leave it.

 

It may be worthwhile doing some measurements to determine the density of your fuel (not hard to do), ie. 15mmID compared to the 19mm version. Density of the fuel is often overlooked, but is a big contributing factor to the burn rate.

 

And btw. if you asked about it in the discord chat, then why wasn't fuel/core length, density, Kn ratios etc. discussed?

Edited April 16, 2018 by stix

[Baldor]

Well, shorting an spindle is easy, lengthening it not so. So first I will solve the other problems, and if it still cato, will be time to shorten it. :-)

 

All the other wasn´t disused because it seemed to big increments was the primary suspect.

[justvisiting]

Baldor, you should be able to make excellent nozzleless BP rockets with that tooling. The click click click that you can hear and feel is the propellant grain sliding on the spindle as it rebounds from the pressing force. I use wax in the propellant or on the tube to have zero CATO. For long term storage, I may do both. Some folks mill the BP with mineral oil, but I prefer wax. I use 2% of the propellant weight, dissolved in naphtha (camp fuel). The propellant is then granulated through a screen. Using straight mill dust is the worst way to make black powder rockets, IMHO. Granulating your propellant with wax will give you reliability and allow you to use 1 ID increments.

 

EDIT: you can also rub some candle wax on the spindle to help with the clicking.

Edited April 16, 2018 by justvisiting

[Baldor]

I will wax the tubes. I waxed some tubes with my nozzled 15mm core burners, and noticed no difference, but with 19mm could be diferent.

 

I don't exactly use green BP from the mill. I wet it with a little bit of water, and granulate it trough a screen. Would pressing pucks (without binder), corning and screening give better results?

 

Took some time away of the rockets. I'm out of black match, just started raining when I wanted to make an small batch. Seems today is the last day of rain, so I'm making a pocket sized version of Calebs black match machine. Tomorrow should be finished.

[justvisiting]

I think pucking, corning, and screening is unnecessary and may be detrimental. Using wax helps with CATOs. But if the rocket was not going to CATO anyway- it still won't You didn't have a CATO though. You had a blowthrough. You can just use your milled propellant instead of clay for a bulkhead, for 25mm above the spindle. This will give maybe 2-3 second delay after the main burn.

[Mumbles]

Maybe I missed it, but what material are you using for the bulkheads? I know some people add a coarse, hard material to their clay mixtures to make it bite into the tube harder. Have you ever tried to cut into a dummy tube or spent tube to see how well the bulkhead is grabbing the wall?

[ollie366]

Not hijack the OP thread, but I'm not clear on why granulated BP performs better than milled BP when pressed. It would seem the packing density for pressed milled would be higher (slightly) than for pressed granulated due to the smaller particle size and less interstitial air. I've seen the comment many times that the best performance is achieved using granulated but don't quite get why. Can someone please enlighten me?

[Baldor]

Plain bentonite, Mumbles. I had no problem with the 15mm tubes, and my plan is to use only delay as the bulkhead in "production". I'm using the bentonite to protect the test stand. I think that the bulkhead was too thin. The bulkhead in spent 15mm tubes has been always intact.

 

Olie, if you use a press, i think the density will be the same. The usual fuel for rockets is milled, a little water added, and screened, no need to puck and corn. If you not use a press and just ram, and use hard grains, you have the risk of the grains not consolidating enough.

[Baldor]

Made 60m of black match with 150g of BP. A breeze with a pocket replica of Calebs machine.

 

The machine likes bigger batches, when more than half the capacity have been emptied, it will like a replenish.

 

Test in 24h. If it burns well, i think I have BM for this year.

[Baldor]

Seems I put a little too much CMC in the BP. The leftovers take forever to dry. BM seems to work fine, but I will try a larger batch with less MCM.

 

Finished a little tooling for waxing tubes brush style, and waxed a dozen of tubes in a moment. I previously tried the cone tool and didn't like the spillovers.

 

Grinded some charcoal of unknown origin for testing the pressing method. It doesn't consolidate well, so will try with bentonite.

 

Made an apprentice mistake and bent the 19mm tooling spindle. Still, seems the acordioning is under control. (Used a wood riser and all went wrong)

[Mumbles]

How much CMC did you add? It's more of a emulsifier, and not a dedicated binder. Black match can certainly take some time to dry if you effectively got the slurry to penetrate the string. It seems like it should be automatic, but 48-72 hr is not unrealistic for it to dry to the core depending on your environment.

[Baldor]

BM dried in 24 hours, but I'm not happy with the penetration in the cotton string. It works good enough, but could be better. It's hot and not very humid here now. I prepared the BP before knowing about CMC, so it had 4% dextrin, and I added a little more than 1% CMC with 100% water. Next batch will be 3% dextrin 1% CMC. What is taking forever to dry are the leftover. It forms a dry film, but the inside is the consistency of gelatin after a week of drying.

[justvisiting]

Shimizu recommends using a water bath to heat the mixture for making black match. Due to the higher solubility of potassium nitrate in hot water, it allows less water to be used. This allows for MUCH faster drying. I have been applying the same principle for granulating my screen-mixed black powders, with excellent results. Ned has a challenge going to make lift with a screened mixture using unmilled airfloat charcoal. I'll be using the hot water method to take the prize, unless somebody uses the same idea and beats me to the finish line

[Baldor]

I'm using a replica of Calebs machine to make the BM, so keeping it hot will be a problem. But I can try to use warm water instead of cold, and try to work fast.