Rocket fuel burn mechanics?

[NeighborJ]

I am trying to wrap my brain around how the shape of the burning surface area expands during the thrust phase of whistle motors. It is my intention to find the best spindle shape, which can provide the most total impulse.

In my mind I visualize a cone shaped spindle which can burn as a cone in all directions until the cone grows more obtuse and then burns as a delay half hemisphere. I have also thought of stepped spindles in which all but the smallest taper are coated with a burn inhibiting wax. The theory of this is to expose more surface area as the core expands in order to maintain optimum case pressure.

I love to experiment with these types of things and I consider the research and development of such things to be just as exciting as any other hobby.

[mikeee]

Good Whistle Fuels will quickly exceed the mechanical properties of "motor tubes" used in the "Pyro" hobby.

You would most likely need to step up to machined rocket motor tubes to test the limits on most whistle fuels.

A long narrow tapered spindle will get you super quick launches pushing the limits of standard pyro motor tubes.

Fusing all the way into the motor core will create a very fast increasing fuel burn rate which can cato easily.

You can move your fusing down the core to find a sweet spot, or fuse at the bottom edge if still a problem with catos.

With really hot whistle fuels I have used a few drops of super glue to coat the inner core when the fuel strength exceeds the motor tube strength.

This technique slows down the flame progression in the fuel core and allows the motor to launch without catoing.

The other variable is the fuel grain compression, how many pounds of force used while pressing the motors.

One batch of whistle fuel can turn out different each time based on production method, screening method, chemicals, solvents, catalysts etc.

Edited August 6, 2016 by mikeee

[NeighborJ]

I like the ca glue idea. And it has got me thinking, I may be able to create a cone shaped spindle with a long thin spindle on top. Then coat the thin core with CA, this may keep the burning cone extended for a longer time and with little change in burning surface area.

I've already reached the limits of what the case can handle so I'm looking for ways to increase the amount of useable fuel being burnt and lengthen the burn.

[DavidF]

NeighborJ, you might find this interesting. http://www.wichitabuggywhip.com/fireworks/rockets/erich/index.html

[NeighborJ]

Thanks DavidF this is exactly the info I was looking for, however it has no mention of a burn inhibitor coating on the lower steps. Would this even be beneficial to prevent ignition of these outer cores until they can be best utilized. It said he could lift 350g headers on a 4 oz motor that's quite impressive. The inhibitor may help with smaller shells but it mite just sit on the pad until the second stage kicks in with that big of a header.

[Maserface]

http://www.amateurpyro.com/forums/topic/10475-whistle-spindle-comparisons/

 

Give that a look

[NeighborJ]

Wow I'm surprised how little the data changed between the different spindles. The skinny spindle was the most powerful but burned out only a tenth of a second quicker then the slowest motor. I guess my dreams of extending the burn of these motors is not going to happen. Oh well, I know I'm not the first to test these but it's doing these experiments like this which allows me to understand what is truly going on inside the motors. and so it appears it works exactly the same as any other nozzleless motor, once the motor lights pressure in the core rises and accelerates the burn, which more than compensates for the quickly eroding core. These things are a runaway train.

[Maserface]

The graphs shown on creagans website are only speculative, and in my experience only a half truth. If I remember, I'll pull up those tests and post the impulses of each. I recall the stepped motor having a slight "bench" in the impulse, but at the end of the day, more propellant = more thrust (there are exceptions, though).

[NeighborJ]

I've been using cuoxy catalyzed sodium sali w/ 3% mineral oil. I wonder if the bench result is more pronounced with a slower fuel. I've even tried nozzle end burners to lengthen the burn but they Cato every time I try to make the grain longer than 1 1/2 in. I kept turning bigger nozzles but it seems they burn hotter and faster the longer it burns no matter what I try.

[Maserface]

I have some potassium benzoate whistle I could re-test those three spindles if you'd like. I had intended on this anyway but I haven't gotten around to it yet.

 

I suspect it will show the same end result, but we can see!

 

I think if you want to extend the burn time, you will need to go with bates grains or two or three stage rockets. I've also considered adding something to whistle to reduce the burn coefficient, maybe ammonium perchlorate or potassium nitrate.

[NeighborJ]

I'm not sure that you can avoid the progressive burn characteristics in the sali whistle motors. I think the only thing which saves these core burners from Cato is the fact that it runs out of fuel before the pressure builds too much. Like I said even the end burners exhibit the progressive burn and will pop after a second or two. I've seen whistle end burner videos with longer burn time and I believe it was benzo and on Caleb's tooling.

If I had a greater understanding of what causes this runaway reaction mabe a solution can be found.

Until then I believe the benzo will provide better results on the stepped spindles but not much. I'd love to have one of those acme test stands😖.

[Maserface]

I believe the oxidizer is responsible for the burn rate coefficient, potassium perchlorate compositions burn exponentially faster under pressure than they do at say atmospheric pressure. It's generally agreed on that sodium salicylate based whistles are just faster burning to begin with than the potassium and sodium benzoate whistles, and I agree although I haven't done definitive tests (yet).

 

there is usually a short spike of thrust at the beginning of the thrust phase in end burners, I think it's from the short nipple that some tooling creates in the propellant grain, and sometimes it's useful to get the motor moving. Does your end burner tooling make any sort of core in the grain?

[NeighborJ]

I thought the same thing, so I took extra care to ensure the convergent met just at the very top of the spindle. I also suspected the propellant may not even be burned the entire way to the wall of the motor so I increased the nozzle dia to 1/3. The two things that may be working against it is the catalyst and the fact that the throat ended up being about 4x longer then the dia. I'm trying to only change one variable at a time and each design takes time to machine and test.

[mikeee]

One cause for a cato is the flame progression on the outer edge between the tube wall and the fuel grain.

Dave F. has done much research into this problem and started waxing the inside of his tubes prior to pressing the motor.

The wax inhibits the flame path and provides a "seal" on the outer edge of the fuel grain.

This one method will eliminate 1/2 of the cato's most people have problems with.

This process also allows the fuel grain to properly compress and eliminates tube "buldges".

[NeighborJ]

I like the waxing idea it never occurred to me for use on end burners. I've used it on larger 4# + 6# bp motors to solve the problem of tube pull down.

My understanding of it's use on end burners differs from what you described. I don't think it really prevents tube buldges or aids compaction but would rather coat the outside of the fuel with wax and act as a burn inhibitor. This should help when or if the motor swells under combustion pressure and exposes the outside of the fuel grain to the flame it shouldn't progress beyond the flame front. At any rate I agree it is a good idea and I will test this today.

[OldMarine]

I had one rocket Cato last night at the party. It was one of two that used unwaxed tubes. Unfortunately it also had my first white stars on board. It was one Heckuva ground bloom!

[NeighborJ]

Yippeeee!!!

Success, I waxed the tubes but this alone was not enough. I tested it with my largest 4oz nozzle and it was an almost instant Cato, a few cuffs then another bomb on a stick. The Cato was different, instead of a case rupture it was a bulkhead blowout, thanks to the lubricating quality of the wax. So back to the drawing board I went.

This time I went overboard with the nozzle size and started with a 1/4" spindle, a 1-2 ratio, and it lifted slowly with no Cato. So I turned the spindle down a little more (to 7/32") and this time I had perfectly acceptable performance with a short 1" long grain. The final test was a 5 1/2" long motor (waxed) and it flew great, it disappeared into the sky and I am still awaiting reentry procedure from Houston.😄

While this test was a success, and I did manage to find a steady burn point for whistle, I'm not entirely sure how this can help extend burn time on nozzleless coreburn whistles. The two motors operate on two entirely different principals.

It seems that the short skinny core is operating at peak ability for these motors.

BTW I will continue to wax the tubes, even for endburners. And here's a pic of the new 4oz tooling and the 5 1/2 in motor.

[mikeee]

A forstner bit works good for reaming the wax from the top and bottom of a waxed motor tube to allow a nozzle mix or bulkhead to bite into the wall.

Several people have also used a plug in one end of the tube while waxing to keep the wax from impregnating that area of the tube.

Some people pour the hot wax and some people use a shotgun swab in the tube both of these methods can be used successfully.

When I started waxing my motor tubes I no longer had wrinkles or bulges in the pressed motor as the fuel grain tends to consolidate into one grain instead of each individual increment of fuel added to the motor. It also depends on the pressure used on the fuel grain, a good tube support and tubes solidly supported during the fuel grain pressing allows higher pressures (more fuel) (solid fuel grain). This is one variable that everyone has their own ideas on what is better, more reliable or more power, thrust etc. I press my whistle motors at twice the level as many other people do with their motors.

Edited August 7, 2016 by mikeee

[NeighborJ]

I don't have a guage on my press but I've tack welded a socket to the hand crank so I can use my torque wrench to gauge how much force I am applying. I do know that I have cut way back on the amount of force I apply because the comp would squeeze out around the rammer, also the smaller tubes would pull down inside the tube support and destroy the casing. I now press until I feel the comp just start to squeese around the rammer, then I set the torque to that. If I would get a wrinkle on a tube it's usually from a misalignment either from the ram or a crooked cut,I don't use that motor and the comp gets crushed out of the tube and recycled.i don't seem to have any consolidation issues with the proven motor designs, so when I have a Cato I know it's a design issue. I have burr bits for an end grinder to clean out the wax but I think I will try naphtha on a rag first that way I shouldn't weaken the tubes.

[Maserface]

For the curious, I have uploaded the impulses of the motors mentioned above

 

http://imgur.com/WzoQbxH

http://imgur.com/KDyP1wA

http://imgur.com/r0FhAa8

Edited August 10, 2016 by Maserface

[NeighborJ]

That graph is a lot easier to read than on the original post. I made copies of the Eric Hunkins 4oz and 1lb pusher motors. They are almost identical to your fatty mcgoo motors. The 1# motor is a stick shredder, even with two sticks. It needs a header otherwise it will be raining toothpicks. I'm almost scared to try the slim pickins version with the extra 10% of peak thrust.😬

I did try another design today which did not go well. It was a 4oz pusher design on top of an almost straight cylinder with a rubber plug installed in the shoulder between them. I expected it to work as a two stage motor, first burning the cylinder core then burning around the plug and into the pusher core. It catoed almost instantly and I can only assume the plug got shoved into the top of the motor or I cracked the grain installing it. I may try the CA glue on the entire upper chamber as Milkeee first suggested.

Edited August 8, 2016 by NeighborJ

[DavidF]

I don't think there's any one of us that knows everything about rockets, and I don't think there ever will be. Every examination of whistle rockets is incomplete in some way. I think it is because whistle rockets are a lot more complicated than many of us realize. A couple of things for you to consider NeighborJ: you have chosen the most powerful 'catalyst' and are using it with the most powerful fuel. I have sometimes wondered why nobody uses CuOxy with salicylate-based whistle. Maybe you are showing us why?

 

One time when I was having a problem making long winded screamers CATOing one after another, Mumbles said "Why would you want to use a catalyst to increase the burn rate if your rockets keep blowing up?" Hmmm. All I could think of was that I'm a young sheep and all I know is Baa Anyhow, this Eric fellow used to mock folks for using 'catalysts', saying they just liked the pretty colors. Maybe he had a point. He did say he doesn't use catalysts in his whistle propellant. That's a huge thing right there. Come to think of it, I can't think of anybody else that does not use a catalyst in whistle rockets.

 

I have been able to use SLD #2 whistle (iron oxide catalyst) all the way up a BP spindle in a 1# rocket with a waxed tube. I considered that quite a feat. I can't comment on the storability though. I did it to prove the value of waxing the tubes, but I have no real-life use for such a zippy rocket. Squeezing the most power out of a rocket is always taking it closer to CATO. To me, it's like having a Ferrari. Where do I park the damn thing? Where can I enjoy the power? How can I keep it safe? Or pay for the insurance? Alas, my Hyundai always starts, always gets me to the beer store, and never disappears. And, I don't have to wax it to make it work

[NeighborJ]

DavidF mabe I'm asking the wrong crowd. I love rockets, first and foremost. To me the fun of this hobby is not in the shells that go in the air, but the design and build of scratch made rockets. I understand that it is only part of this hobby but it is the part I love most. It's all about the journey,and not always the destination.

I make shells to but only because that's what people expect to see. I can make big shells but I don't think I could ever compete with you guys because the love is just not there. So I intend to explore any and all barriers or boundaries, testing the limits of every style rocket I can conceive. For me the rocket is highlight the burst and stars are the bells and whistles. Mabe I'll get bored with rockets someday and I'll concentrate more on that perfect burst, but I doubt it, there are too many kinds of rockets.

I don't think I was clear on my last post. When I said it was a stick shredder I meant that it was a sucess, it took off so fast that the double sticks I was using literally shredded and came back down in about ten pieces.

Edited August 8, 2016 by NeighborJ

[DavidF]

Wrong crowd, hail no! Look at all the things that have been shared with you already! Maybe my most important point was lost in my longish post. I'll make it just by itself:

 

 

I have never heard of any pyro ever using such a powerful catalyst with such a powerful fuel in their whistle propellant. Sure, tests were done with a stubby little spindle. But nobody else uses your propellant mix (that I know of).

 

 

Now, you have taken that rare mixture and used it as the basis for your experiments. It seems to me that failure- or at least lack of reliability- are very likely. Why not test your ideas and theories with something that has a better chance of working for you? After a few 'wins' you can then graduate to more powerful propellants until failures creep in.

 

I say what I say with no disrespect intended. I'm an adventurer too. I want to see you succeed.

[NeighborJ]

I certainly haven't posted all my exploits on this site. I've gotten this to work with bp, and I have reliable working whistle motors in each size and on this fuel. It's the characteristics of the fuel which I want to learn more of and I certainly don't mind failure. It's these failures and the info I profligate from you all which lead to success. I love it when info and application click together and turn on the lightbulb. I'm having more fun with this cowboy style rocket science then I ever did following blueprints and working out calculations.

Besides I'm out of work for the last month so I'd be going some other kind of crazy doing something else.