Neutral and progressive burn from nozzleless motors

[NeighborJ]

One of the things I dislike about nozzleless motors is the fact that they all have a hopelessly regressive burn rate. I feel that I may have a motor design which can provide a neutral or even progressive burn.

The fundamental idea revolves around preserving additional surface area with an inhibiting layer which can burn away at the appropriate time to match the aperture size. This should result in a steady(high) case pressure until the moment of burnout.

Nozzleless motors have the highest case pressure at the moment of complete core ignition, then case pressure drops significantly despite an observable increase in thrust. I believe this inhibited layer placed on a large divergent cone of fuel can maintain a high case pressure and make more thrust.

I've selected a whistle rocket motor for testing and before I spend time or money on making this motor, I'd like to bounce these ideas off the members here. Below is a diagram of my proposed motor. The inhibiting layer will coat the entire divergent cone and will likely be made with superglue after the motor is pressed.

[lloyd]

"One of the things I dislike about nozzleless motors is the fact that they all have a hopelessly regressive burn rate. I feel that I may have a motor design which can provide a neutral or even progressive burn."

---------------

That's only true of the 'partial core' design you show. That isn't representative of all nozzle-less designs. A full-length conical core is approximately neutral, and certain specially-shaped ones can even be progressive.

 

I personally think it would be tricky and prone to failure to deliberately count on compromising inhibiting layers at certain points in the burn.

 

Keep in mind that, even with a regressive burn rate, you may be able to sustain constant acceleration, because although the thrust drops off with time, so does the weight.

 

Lloyd

Edited February 19, 2017 by lloyd

[NeighborJ]

That is simply not true Lloyd, even though a motor burns regressive, it will still continue to increase in thrust. The burn rate refers to case pressure and pays no mind to the thrust. A simple cone or dome design will provide a neutral burn but will also sacrifice thrust and to the point it may not even get off the ground.

This design is intended for optimized motors (dialed in) for maximum case pressure. I intend that the inhibiting layer will burn away as aperture burns away. The ratio of core length to aperture will be maintained at 5.35:1. There will be no need to dial in the inhibiting layer because if the aperture gets larger it will need to expose more fuel to do so.

[lloyd]

Soo.... let me be clear on this... you want a purely linear burn rate, regardless of the fact that the mass exhausted increases continually without the mods you suggest?

 

I only ask, because the only way "thrust can increase continuously" when the 'nozzle' aperture remains the same size is if the mass being thrown out is increasing continuously. The 'aperture' is the open end of the tube, not the size of the hole in the fuel grain.

 

This is the very definition of a "progressive burn", where more fuel per second is burnt as the burn progresses. And, if that's the case, how is that a "regressive burn"?

 

So... what am I missing? I must've missed something.

 

(And seriously, I'm NOT being sarcastic. I'd like to know what it is that is in your interpretation of both increasing mass exit AND 'regressive' burn rate. I'm not a rocket expert... and I get from your vehement rejection of my comment that you are.)

 

Lloyd

Edited February 19, 2017 by lloyd

[NeighborJ]

I am referring to the aperture as the choke point in the fuel grain at the bottom of the core Measurement "B",as it enlarges it will reach to size "C". This aperture point moves closer to the bottom of the motor as the fuel burns. I have taken into account how much longer to make this divergent cone by calculating how much fuel will be consumed above the spindle tip at each aperture size and set it to 5.35:1. This allows the case pressure to remain steady(neutral burn) and the thrust to steadily increase far beyond what a normal nozzleless(regressive) motor is capable of. Edited February 19, 2017 by NeighborJ

[DavidF]

It won't be the first or last time, but- I don't get it. Aside from making a more 'interesting' Acme result, what is the purpose? I've been told that my nozzleless motors are regressive burning. I've been told that nozzled BP motors are progressive burning. When I look at the thrust curves of either type, they look about the same. I always light from the bottom of the core with J-hooked visco. I'm trying to see what advantage there is to going down this path. In my thought experiment, the lower portion of the fuel grain is pressed with a much more dense, slower-burning propellant, so the core expands slower than it would otherwise.

[NeighborJ]

Last summer I did some experimenting on whistle rockets with a slow burning comp pressed around the base. Long story short it creates a progressive burn with an insane amount of thrust but they were touchy, often catoing at the end of the burn. I believe this design will be more reliable and easier to make. It's all about peak thrust, I see a nozzleless motor and I see wasted energy. The motors which worked last summer were moving at speeds in excess of the sound barrier. Thou this motor would be violent and fast burning, I believe it will be more gentle than a mortar launch and will lift larger shells than normal motors.

[lloyd]

You misconstrue the way a nozzle-less, cored grain burns. What you're calling "the aperture" moves progressively AWAY from the bottom of the tube.

 

Not only does the 'core' enlarge, but essentially all of the fuel grain below it is burnt-away during the process.

 

And unless the core is flush with or very close to the bottom end of the tube, regardless of the size of the 'nozzle' (even if it's the full i.d. of the tube), the end of the tube is the actual 'aperture'.

 

Lloyd

[NeighborJ]

On normal nozzleless motors I agree with you Lloyd. But this motor has an inhibited layer on that divergent shaped cone to prevent the aperture from moving up and away from the exit.

Both nozzles and nozzleless motors have wasted potential. Nozzles have low case pressure at the burn initialization, while Nless have low case pressure at the end of the burn. This is an attempt to benefit from peak optimization at both ends of the spectrum. I've seen the advantages of this kind of optimization and it is impressive.

[lloyd]

Well... let us know how it goes! I presume the only way you can determine the performance will be with a load cell test-stand, and to plot the thrust curves.

 

I have no good idea how you can interpret the actual shape of the burn front inside the grain, except by extrapolation from thrust figures.

 

If you don't have a test stand, Ed Brown sells copies of my load cell amplifier, and load cells themselves are available pretty inexpensively.

 

(oh... and there was a recent change to the schematic and artwork to make it work better over the entire range of possible applied excitation voltages on the load cells.)

 

Lloyd

Edited February 19, 2017 by lloyd

[NeighborJ]

No load cell is needed for testing fully optimized motors. The info gathered from a test stand would be inconclusive for determining if a burn is progressive or not. The only way to truly test this is with a pressure gauge. I'm not certain how that could be done but the point of CATO is certainly what I use. The initial lit core is optimized and if it should instantly CATO then I can surmise that the inhibitor layer on the bottom was not sufficient. If it catos near the end of the burn then I will know that the burn was progressive. However if there is no CATO then I will know that the burn was either neutral or regressive. At this point a steeper, longer inhibited divergent cone can be tested to draw the burn profile closer to neutral until it does CATO then back it off a bit. This is all no simple task and requires many design changes to determine what the motor is doing. All of my optimized motors are tested in this manner.

[lloyd]

What's the goal? Is it to send an effect up, or to try to get as high as you can?

 

I can tell you that folks are more entertained by effects that display at a height where they can appreciate them.

 

Lloyd

[NeighborJ]

The burn is shorter than a normal motor but with more thrust. The goal here would be to lift larger shells with smaller motors and using velocity to carry the shell to display height much like a mortar.

[lloyd]

Good luck. Keep us updated as to how it goes.

 

Lloyd

[DavidF]

I don't want this post to sound like an advertisement for my technique of waxing rocket tubes BUT sometimes I have to bring it up to make a point. I don't know what "hopelessly regressive" means numerically or empirically. So, for me, it's an undefined term.

 

I too have felt that power was being wasted with nozzleless motors. To increase thrust, I increased the power and burn rate to the point of CATO. This was quite vexing. Then, I overcame the physical limitations of the rocket making methods in common use. Yes, it was easily done by simply waxing the tubes. Suddenly, a black powder rocket could have far more power with no CATO. OK, so you are talking about using whistle. Tube waxing works with those too.

 

Adding a nozzle to a hot 75-15-10 BP nozzleless motor (which would be unheard of before tube-waxing) gave about a 15% increase in impulse. In my experiments, the objective was to lift a 6" ball shell with a 1 pound motor made on standard BP tooling, with no additives. Sure, it was a light shell, and it didn't go as high as I'd like- but I did it. It's on my Youtube channel.

 

I guess my point here is that rocket propellant formulas have been adjusted to have less impulse to prevent CATO. The underlying physical problems that caused CATO were never addressed fully. Until the physical problems were addressed, full power could never be achieved. "Dialing in" was what everybody had to do, and the propellant in most pyrotechnic rockets was weakened or slowed to make them work without blowing up.

 

So now, let's fast forward to this newer new idea, the subject of this thread. It's unclear to me whether the "hopelessly regressive" burning was simply a failure to extract maximum value from the chosen propellant. It's unclear to me what the performance was that was so unsatisfactory, in any quantifiable terms. It's unclear to me how much increase in impulse you expect to get. How much weight do you hope to lift, with what size rocket?

 

I would unabashedly say that I have at least doubled the power that could be gotten from a nozzleless rocket with my techniques. So, I just have to wonder if you are trying to find a more circuitous route to get to what can already be done, or trying to outdo what can already be done. I think I can say with some confidence that if you are not waxing the tubes, you are not getting the most from your propellant to begin with. I'd start there, and build on that if I were you.

 

I don't mean to be cheeky, but you asked for input

[Mumbles]

In other input cyanoacrylate, the active component of superglue, softens around 150C and will likely be of no consequence to a rocket propellant. Most of the more suitable things I can think of are inorganic. Perhaps waterglass or a slurry or more refractory components like graphite or metal oxides might provide a more satisfactory retardant.

[NeighborJ]

Certain motors get waxed and others don't need it. Optimized motors definitely get waxed. I am not unhappy with the performance of my normal motors but I just want to play around with different possibilities and see what is possible. I suppose I have a backwards approach to motor design because I'd rather adjust the spindle to a given fuel and building teqnique than adjust the fuel to the spindle. I rarely buy tooling because I can't adjust the spindles.

I'm picking on the nozzleless motor because I truly believe more power can be extracted as shown to me by my experiments. There is no satisfaction on this end until I know I've done everything possible to squeeze every last Newton of thrust out of a motor, even if it makes it unuseable as a pyrotechnic motor. I guess my amateur rocketry background is my true passion.

The motor I plan on starting with is 4# and plan on scaling up if it works. I can't see the benefit of using this design on the smaller motors because the fuel grain is to thin to make a big difference. 3# may be thick enough but if it works at all it will be on the thicker walled grains. I am hoping Styx will chime in because he is quite familiar with progressive and regressive burn profiles and their relationship to optimizing a rocket motor.

Now Mumbles that is the kind of idea that I was fishing for. I just threw the superglue idea out there as a possibility. I've used it before with good luck as an inhibitor but I'm sure there are some other options which may work better as you suggest.

Edited February 20, 2017 by NeighborJ

[DavidF]

I guess we all have our 'thing', right? For me, it is important to do my work with off the shelf tooling and the usual formulas. I'll admit that I have a hard time wrapping my head around the concept you've introduced here. I think in terms of pyrotechnic rockets only, but that's all I know about. It would be nice to have exact definitions of regressive and progressive burning laid out for us regular folks that may not have a crystal clear understanding of the terms.

[NeighborJ]

The burn profile can be broken down by the different types of motors but each have exceptions.

:endburners produce a neutral burn. They have a steady case pressure and a fixed oraffice so it's thrust curve is a strait horizontal line.

:nozzled coreburners have a progressive burn. They use a fixed oraffice but as the burn progresses the volume of hot gas passed thru it increases causing a rise in case pressure and thrust until the fuel is all spent.

:nozzleless motors are more tricky, the oraffice(nozzle,aperture) and the fuel surface area both increase but disproportionately. As the fuel burns it increases available surface area but the oraffice it is forced thru is increased beyond what would be a neutral burn. The pressure drops but it stays between a balance point of steadily decreasing pressure and a steadily increasing oraffice. So the result is a spike of pressure at initialization causing hi thrust and as the oraffice increases pressure drops but never enough to cause a loss of thrust. The thrust profile ends up similar to the nozzled motor but with a shorter burn time.

: my proposed motor. In theory this motor should attempt to maintain a steady hi case pressure for the duration of the burn despite the steadily increasing oraffice size. The initial thrust in a 4# motor should be comparable to a 1# motor but at the tail end of the burn it should be closer to the peak thrust of a 6# motor. The resulting thrust curve will be the shortest one yet but it should provide an increasing linear curve.

I'm not certain if I've covered this description well but it's hard for me to convey this theory.

[DavidF]

OK, so when you say "burn", you are referring to the amount of propellant burning at a given time. The increase or decrease in thrust is based upon that, is what you are saying? I'm thinking my experience with black powder motors on standard black powder tooling does not apply here. I know what a short or long burn time is for me. I don't know what a short or long burn time is for you. For me, a short burn time on a nozzleless BP motor is .2 second. A long burn time with the same kind of motor is .5 second or more. With whistle rockets my shortest burn time is .15 second. Maybe if I knew how much time, how much impulse, or what you expect to lift with what motor, I could understand better. Any attempts at increasing the burn time of any nozzleless motor I have made have predictably resulted in less impulse. Burn times can be adjusted with diluents or fuels, or by adjusting particle size of the chems that make up the BP propellant. The result (for me) has always been the same- less impulse.

Because black powder is so variable, I don't see how it could be considered as the propellant for what you are suggesting. Is that right? The burn times of whistle rockets are much less. But whistle rockets already lift much more than black powder rockets in apples to apples comparisons. So, I wonder what burn rate you consider to be 'hopelessly regressive'?

 

Maybe I will understand your thinking better if I pose this question: are you saying that your proposed nozzleless configuration is going to have more thrust than the hotter of your propellants would have if you simply used it in a nozzled rocket?

[NeighborJ]

The objective here is simple. The 4# motor starts with an exposed core of the hottest sali propellant possible with a core dimension of 1/2" by 2-1/2". It will burn and by the end of the burn the core dimension will be 1" by 5". This all will happen in a fraction of a second.

The burn speed of the propellant(any propellant) directly correlates with the case pressure. The core is already as long as it can possibly be without Cato the entire time the propellant is burning instead of only at initialization as normal nozzleless motors are.

Edited February 20, 2017 by NeighborJ

[wildcherryxoxo]

What's the point if you aren't going to collect any real data? It's one thing to claim supersonic rockets but entirely another to actually achieve it.

 

Looking forward to your "results" either way

[NeighborJ]

I tell you what Cherry, when I get this dialed in, you can come for a visit. We can measure out 1125ft and you can stand at the other end with a stopwatch while I launch it at you.

[dagabu]

So, I am WAY out of my element talking about the math it would take to postulate the correct result or even have an idea of HOW one would achieve your desired result but it occurred to me (forgive me if I am off on my thinking) that you could just use a slower burning comp for a "nozzle" and use varying mixtures of your comp for each increment without having to inhibit the grain?

[NeighborJ]

Absolutely Dave, that was what I did last summer and it worked awesome. The slower comp I used was an AP/benzo mix but what I didn't do was extend the spindle to account for the slower increment of fuel. Even without being used to its full potential those motors were insane.

I consider this new design to be a step up because it will burn more fuel in the same amount of time.