Coreburner tools tuned for nozzleless motors, bulkhead thickness

[mabuse00]

Some dealers offer tools that are specially designed to make nozzleless rockets. I like nozzleless rockets, their reliability is so much superiour...

 

Since there is no nozzle that has to cling to the tube, the collar height is reduced to make more volume available for fuel. OK, so much for that.

I wonder if apart from reducing the collar, they also increased the overall length of the spindle?

 

 

In my opinion the spindle should be as long as possible, to reduce the need for superhot fuels. Especially when whistlemix should not be used.

 

 

Problem:

All my tubes are 10 calibres long. Thus the spindle length is naturally limited.

 

 

What is you opinion about safe bulkhead thickness, what's the minimum thickness in your opinion?

Do you have other thoughts on how to get maximum thrust out of a 10x id tube?

Edited September 25, 2016 by mabuse00

[NeighborJ]

I can't speak for other tooling but I make my own. I usually don't increase the length too much, mabe up to 8x the dia, but I will make the spindle thinner. And I never use bulkheads on nozzleless, they have almost no case pressure at the end of a burn so I don't feel it is needed.

[dagabu]

The trend among the rocketeers at PGI had been to dispose with the clay bulkhead altogether and use a fuel for delay as well as the bulkhead. This makes perfect sense for our purposes at PGI but also has implications world wide to keep the spent rocket casings from coming down like a missile.

 

The clay also takes up space in the tube, space that could be used for brilliant tails, etc.

 

Safe bulkhead thickness for my rockets has always been 1-1/2 ID above the spindle as long as timing is not critical. I use x2 ID if I am want to delay further or drill back to dial in the timing. I know that immediately someone will post about drilling back into the delay and the use of Ti. Yes, this is problematic, I agree. the issue is that with a clay bulkhead, you still have to drill through the clay to get to the delay, right? Even the "new" bulkhead tools that press the passfire into the clay have to be cleaned out with a drill bit to remove the tiny bit of clay that gets compressed in the bottom of the passfire hole.

 

Im not saying to not use a clay bulkhead, to each his/her own, it its your preference, go for it! The issue in our little world at PGI is how to eliminate the potential danger of the rocket returning to earth and adding clay as a bulkhead or nozzle makes the use of a "stick buster" a necessity, removing all clay makes for a shallow glide angle and thus, less missile danger.

[ExplosiveCoek]

Agree with both replies above. I press whistle and BP rockets, and don't use a bulkhead either. 3/4 to 1.5 x ID is more than enough 'bulkhead' to keep your rockets from popping, even with high powered whistle rockets. I can press 22 mm ID rockets without any bulkhead that can reliable lift a 6'' round to proper height.

[mabuse00]

In fact I should have wrote delay

 

I'm already using fuel as bulkhead. I had some priming issues with passfire holes in the past. That's why I usually use fuel - to have a strong ignition to anything that follows.

 

So I'd like to correct my question:

How much fuel above the spindle is needed for blowout prevention?

 

Standart coreburner would be 7,5 x id.

With shorter collar 8x id.

 

More length can only be achieved in sacrificing "bulkhead" fuel.

 

 

I wonder if these special nozzleless tools do make sense. 7,5x id vs. maybe 8,2 to 8,4x id is not very much difference. About 10%...

 

Any opinion on the influence of thinner spindles?

A little more pressure maybe - much difference?

[NeighborJ]

The delay bulkhead can be thought of as fuel which burns just as fast as the fuel in the core. So if the tip of the spindle is mabe 5/16" from the wall, then you can expect the bulkhead fuel to burn at the same rate and the first 5/16" of fuel above the spindle will contribute to the thrust. After that is burned, the rest of your bulkhead is the delay and works to hold back the case pressure during thrust. This amount of delay/bulkhead is determined by the amount of delay or how well your fuel is consolidated. If my fuel is very well consolidated, then I can get away with as little as 1/8" of comp above the 5/16" of fuel already on top of the spindle, 7/16-1/2" absolute minimum with no delay. All these measurements are taken from my notes of a 1# spindle. I use a special, slow burning delay fuel above the bulkhead on motors with excessively long spindles to save on space.

The shape of my spindles change as well between nozzled and nozzleless. The nozzleless spindle is missing the strait choke at the bottom and is a straight taper from base to tip. I made a few extra drifts to prevent fuel from squeezing between the spindle and the drift.

The thinner spindles do increase thrust during the ignition of the motor and will get the motor moving much quicker than a normal spindle. This is useful on smaller motors which seem to handle the incredible pressure spike well, the larger motors can't always take it. It is important to remember that nozzleless motors achieve their peak thrust immediately and quickly die off. The nozzled motors build pressure and thrust all the way to burnout. So the advantage of the nless motor is it's ability to instantly lobb a shell to high speed so it can coast to apogee. It's much like a mortar. The draw back of the nozzled motor is the fact that a lot of thrust is wasted until the rocket builds enough pressure to start moving. This can be a problem on larger shells.

[mabuse00]

 

If my fuel is very well consolidated, then I can get away with as little as 1/8" of comp above the 5/16" of fuel already on top of the spindle, 7/16-1/2" absolute minimum with no delay

I would have never though that so little material will withstand the pressure. OK, nozzleless has much less pressure, but still a lot...

 

Thanks for the insight guys.

[dagabu]

Pretty much 1 ID is the minimum for a nozzleless from my experience. The higher the pressure, the faster the burn so in a nozzled rocket, the pressure may exceed the bulkhead depth before the pressure drops off.

[NeighborJ]

Dag, you've been making these motors a lot longer than I have and I value your experience. I have always understood the fuel in the core will burn equally in all directions away from the core. It may burn a little faster at the nozzle due to the abrasive nature of the high velocity gasses. But once the fire reaches the wall the pressure should drop quickly as the burning surface area is quickly reduced. By the time the tube wall at the tip of the spindle is exposed there is almost zero pressure in the case and the 1/8" bulkhead which is left has no problem holding back this pressure. If I have misunderstood the way these things burn, then I would be open to learn something new. I do use an extra 3/8" of slow burning delay above this comp for a three second delay, which does give the motor a full 1 dia of comp above the tip, but in theory it shouldn't be necessary for prevention of blowout.

[calebkessinger]

When in doubt ... Shorten the spindle and add whistle..

 

Clay is needed if your dealing with a very small slug of slow delay fuel. I mostly just use the fuel i'm pressing the rocket with so it's usually not that slow and I press about an inch of it so things are good. Unless you are pushing the limits and using super hot fuel they will stay together.

[dagabu]

Dag, you've been making these motors a lot longer than I have and I value your experience. I have always understood the fuel in the core will burn equally in all directions away from the core. It may burn a little faster at the nozzle due to the abrasive nature of the high velocity gasses. But once the fire reaches the wall the pressure should drop quickly as the burning surface area is quickly reduced. By the time the tube wall at the tip of the spindle is exposed there is almost zero pressure in the case and the 1/8" bulkhead which is left has no problem holding back this pressure. If I have misunderstood the way these things burn, then I would be open to learn something new. I do use an extra 3/8" of slow burning delay above this comp for a three second delay, which does give the motor a full 1 dia of comp above the tip, but in theory it shouldn't be necessary for prevention of blowout.

 

I suggest a static test of different spindles, nozzles and delays to discover where the delay need to be thicker or can be kept thinner. In a #1 nozzleless rocket, I can stay with 1 ID, nozzled (same fuel) I have to increase it by about 20%. I have not performed exhaustive testing to determine the exact line where that occurs but this thread may yet lead us there?

[Bensmith]

Hey Mabuse,

 

The nozzleless core burn set that I developed (pictured below) does in fact have a spindle that is a bit taller than a standard BP setup. This does create a bit of additional surface area inproves the thrust of the motor. Plus, when making strobes, the additional height gives you a bit more area around the spindle to press strobe fuel. The more of the spindle that can be covered with strobe fuel, the more pronounced the popping.

 

As others have mentioned, many rocketeers have gotten away from clay bulkheads. In the vast majority of motors, a clay bulkhead serves no real purpose and can really impair the ignition of a header. In face, this past weekend a fellow pyro ended up removing all headers from his motors due to ignition problems caused by a clay bulkhead that wouldn't pass fire properly. I typically run a plug of delay comp anywhere from 1-2X the ID of the motor for my bulkhead. Works great!

 

 

[DavidF]

mabuse00, most of the rockets I fly are nozzleless. I use regular BP tooling in the 1lb and 3lb sizes. With 1 to 1 1/2 tube IDs of propellant/delay/bulkhead, there is plenty of room above the spindle tip on a 7.5" or 10" long tube. I don't really see your reason for concern.

 

For facility, you can use a tube extender, a really short final drift ( I have a few of those), or Caleb's bulkhead-forming tool. I have 2 of those. I only use the bulkhead-forming tool if I am using something like 75-15-10 with 10% additional coarse charcoal for some tail. Higher charcoal comps are more crumbly and less dense, and I trust them less as bulkheads. Caleb's bulkhead tool has never caused me an ignition failure yet. A partial increment of BP is pressed in before the clay. So, when you stick black match into the perfectly formed hole, the end of the match is surrounded by BP. EZPZ, as Ned would say- and does say.

 

I do use fairly hot propellants in mine, but if you mill the charcoal very finely, you can have plenty of power from screen-mixed 75-15-10. Using slightly damp propellant (2% water) will get you a bit more power still. Using a bit of whistle mixed in, OR underneath the BP propellant will also give you a lot more pep.

 

It is said that nozzleless rockets are regressive-burning, and that nozzled rockets are progressive-burning. I'm no rocket scientist, but my thrust curves look pretty much the same for both kinds on the same tooling. The burn times may vary, but the shape of the curves are about the same otherwise. It is true that the 'aperture' in nozzleless rockets is instantly opening up, but at the same time, the surface area of burning material is greatly increasing. I have often wondered about this relationship. Maybe one of our more seasoned rocket guys could shed more light on it for me/us.

[Maserface]

mabuse00, most of the rockets I fly are nozzleless. I use regular BP tooling in the 1lb and 3lb sizes. With 1 to 1 1/2 tube IDs of propellant/delay/bulkhead, there is plenty of room above the spindle tip on a 7.5" or 10" long tube. I don't really see your reason for concern.

 

For facility, you can use a tube extender, a really short final drift ( I have a few of those), or Caleb's bulkhead-forming tool. I have 2 of those. I only use the bulkhead-forming tool if I am using something like 75-15-10 with 10% additional coarse charcoal for some tail. Higher charcoal comps are more crumbly and less dense, and I trust them less as bulkheads. Caleb's bulkhead tool has never caused me an ignition failure yet. A partial increment of BP is pressed in before the clay. So, when you stick black match into the perfectly formed hole, the end of the match is surrounded by BP. EZPZ, as Ned would say- and does say.

 

I do use fairly hot propellants in mine, but if you mill the charcoal very finely, you can have plenty of power from screen-mixed 75-15-10. Using slightly damp propellant (2% water) will get you a bit more power still. Using a bit of whistle mixed in, OR underneath the BP propellant will also give you a lot more pep.

 

It is said that nozzleless rockets are regressive-burning, and that nozzled rockets are progressive-burning. I'm no rocket scientist, but my thrust curves look pretty much the same for both kinds on the same tooling. The burn times may vary, but the shape of the curves are about the same otherwise. It is true that the 'aperture' in nozzleless rockets is instantly opening up, but at the same time, the surface area of burning material is greatly increasing. I have often wondered about this relationship. Maybe one of our more seasoned rocket guys could shed more light on it for me/us.

 

Progressive/regressive burn rates describe the burning surface area relative to the opening or nozzle. In both cases, the surface area burning is greatly increasing, but with nozzle-less motors, the opening is also greatly increasing, so then the ratio of burning surface area to opening (also called the Kn) is decreasing as the burn continues. The Kn in nozzle-less motors is the highest at the instant of ignition, gradually regressing until its done. The Kn in nozzled motors is the lowest at the instant of ignition, gradually progressing until its done (assuming the all of the propellant burns out at the same time, in reality there is a small dropoff at the end of the burn).

 

 

Edited September 28, 2016 by Maserface

[NeighborJ]

Mabuse, from your original post, you briefly mentioned the use of a slower fuel for use in your long spindled nozzleless motor. These parameters can be calculated but I believe you will need a much longer motor to produce the kind of thrust you desire.

You would need to mathematically calculate the max burning surface area of a nozzled motor at the moment of burnout, then use that area to create a nozzle area to fuel area ratio.

Take that ratio and then use it as the initial surface area to aperture ratio of a nozzleless motor. You will find it is not a practical design and is impossible with a standard length tube.

I find it easier to make fast fuel as opposed to nozzled motor fuel. It often takes adjusting to get performance comparable to previous batches. It is this reason why most of my motors are hot, nozzleless.

DavidF, I've tried to make use of a clay passfire bulkhead former and was plagued with catos. From reading your post it is clear to me that my error was in not adding the partial increment of bp under the clay. Without it my former was pushing the tip into the already consolidated delay fuel, causing cracks. Not to mention it often drove clay into the delay, causing a passfire blockage. I had given up on the whole idea but when i find time to play with it again I will try what you described.

[DavidF]

Maserface, I think we are on the same page. But, I guess the answer to this question leads to another question. At what point in the core are the motors ignited? I J-hook mine with visco. Could it be that my apparent progressive burning is due to igniting the core from the bottom? I've lit my nozzled motors all the same way too, J-hooked just high enough up the core to stay stuck and light it. I never thought to ignite my nozzleless propellant cores from the top and compare the curves to nozzled motors lit the same way. If I did it that way, I could see the difference between progressive/regressive burning from a different standpoint, as it applies to black powder rockets. What do you think about that?

 

NeighborJ, the Caleb bulkhead former is spring-loaded. Maybe the tool you used is one of the older styles with the hollow tip. I talked to Caleb about this, and he came up with an excellent tool, IMO. This is it:

http://www.woodysrocks.com/store/p59/Bulkhead_Formers_%28New_Style%29.html

This is what I use now if I want a clay bulkhead in a rocket. I like it, and it has never once failed so far.

[Maserface]

DavidF,

 

taken from the simulation program, "Burn Scheme: The propellant core burns at a uniform rate at every point along its surface. The top, circular area of the core burns both upward and laterally, forming a hemisphere. The sides of the core burn laterally toward the wall of the casing. The simulation ends when the entire length of the propellant core has burned to the wall. The burning surface of the propellant core is estimated using a formula for a trucated cone."

 

Can you elaborate on what you mean by "apparent progressive" burning?

[DavidF]

OK Maserface, I will try I italicized apparent so as to not contradict widely accepted information on the subject. I made the assumption that the experts are right, and it is I who is missing something. Ed Brown says the same thing you and your literature say.

 

BUT, when I look at my Acme impulse curves for nozzleless and nozzled black powder motors, I do not see what your first chart shows. I have never seen an impulse curve that starts high and regresses. I have only read about it. So, I look for a reason. The only reason I can think of is that we are seeing a rise as the whole core lights 'slowly' when lit from the bottom with J-hooked visco.

 

In my reality according to Acme, there is no such thing as regressive burning in coreburners. In every single instance with at least 50 tests, the thrust goes up, and then the thrust goes down. The shape of the impulse curves vary a bit, sure. But with a nozzled and nozzleless pair of motors made identically except for the nozzle, the thrust curves are pretty much the same shape- in my tests.

 

My test results seem to contradict the idea of regressive burning. But I'm not fool enough to think I discovered something the scientists missed I only mentioned top-lighting the core to try to reconcile the concept of regressive burning with what is before my eyes. I would never top-light the core on a hot BP motor though. That's just asking for problems.

 

I have to ask: does anybody have an Acme test result that shows regressive burning?

[Maserface]

Ok, that is what I kind of assumed you were thinking-

 

My charts dont show thrust/impulse, they graph the changing Kn, which again is the ratio of burning surface area, to opening/nozzle (simply put). Kn and pressure are somewhat relative, if you were to measure the internal pressure of your rocket motors, the graphs would look very similar to the ones I linked above.

 

In a nozzle-less motor, as the burning surface area increases, so does the thrust. The Kn (and/or internal pressure) though, is decreasing. In a nozzled motor, as the surface area increases, so does the thrust. The Kn (and/or internal pressure) in this case, is increasing.

 

I hope im not making this more complicated for you, haha, if I had a better understanding myself maybe I could explain it better. I am no expert.

[mabuse00]

 

a clay bulkhead serves no real purpose and can really impair the ignition of a header. In face, this past weekend a fellow pyro ended up removing all headers from his motors due to ignition problems caused by a clay bulkhead that wouldn't pass fire properly

That's what I exprienced a few years ago. I used a slower fuel above the spindle and a passfire hole without quickmatch lead through was not enough. Spoiled my NYE...

 

 

Concerning slower fuel - well, the point is, it would be convenient for me not to have to use ballmilled fuel.

In my experience bigger calibres are less dependent on extremely hot fuel. I look at 1lb and 3lb.

75-15-10 screenmixed - using airfloat charcoal as delivered and coffee grindered nitrate - would be very nice. Ball milling the nitrate alone - maybe.

I'd also like to use some kind of dust preventing binder, which will inevitably slow thew fuel down.

 

I know about the typical behaviour of nozzleless rockets.

From the point of aesthetics, hmm...

From the point of safely shooting something exactly where you want it to go, yes!

A perfect workhorse.

 

Thank you all for your interesting posts

Edited September 29, 2016 by mabuse00