K getting way off course, I just want to know what was considered to be slow burn rate and if there was a value that the FAA rule uses to judge a class 1 rocket by. From what I have gatherd from here, and further reading on the subject is it just can be instant, like shot from a gun. An explosive charge to propel the rocket.
TA
well in your post, you need a compostition that can meet the atf method of testing, and not detonate at a rate considered to be an explosive. IMO humble opinion. Whilst the FAA does not have a propellant method of testing i am aware of And certainly not within FAR 101. part 14.
here is my final comment on "burn rates"
Burn Rate: R=(a*P^n) ; why it is not important, and why it is important. Why we legally appreciate it, and why we implement it in our models. More importantly, what you should not expect of burn rates.
The following is written completely as I see it. So with some forbearance, let it be known I am not educated in this, I am not an expert, and certainly not all knowing. Havent even read a book. The following I have taken completely from others, and are as I have come to understand them, and tie the ends together for my own intents and purposes.
Secondly, I have recently stumbled over myself, and stumbled onto myself in the due to an innate inability to effectively convey thought to expressed language; written and verbal. While it may appear here, please note I am merely sharing a record intended for my ends.(to try to convey my own thoughts to myself accurately) Enjoy the entertainment value.
Back to the important stuff! For me the most relevant history for burn rates exists in the legal battle between the BATFE, and hobby rocketry enthusiasts. The methods of defining any character(istic) must always be well defined. Tests, are how we ensure methods are true and correct. This is both true in engineering, and in legal doctrines. Thus, the Method, the test, must be held true, for reason to dictate that the results or affects are true and accurate. In this legal battle, there were no methods or tests. Just an enforcement and inclusion that was reasoned without methods or testing. Irrelevant to the why, essentially the premise of argument by the government was to appear to have reason. Go figure, it failed. What we as a hobby said was youre doing it wrong. With the science behind this material the proper method is this, and the results are this so our reason stands true; yours doesnt. What we said was: Burn rates in Ammonium Per chlorate Composite Propellant must be measured in a strand burner on a linear measurement with a constant surface area, and constant pressure. And by this test, the method is always true. So, the results dictate the material does not meet the government standard definition of an explosive. There are many other issues present, but here is the small part burn rate played in a much bigger legal issue.
Thats certainly not why burn rate is important for me, so here is why burn rate is important for me.
Burn rate is used to simulate, or measure the rate at which a propellant combusts and decomposes into a gas from a solid, producing gas to build pressure and create thrust. (I dont make liquid or hybrids APCP only)
In making motors burn rate is essential to making them successful, not producing to little thrust, or causing so much gas they rupture the pressure vessel. More to it, but maybe future writing
So irrelevant to everything else, my goal is to describe methods and tests to determine burn rates. Well, the hobby says a bomb strand burner is the accurate method. As in the equation R=(A*p^n) a bomb strand burner isolates the burning surface area, a is a constant rate, and P in a strand burner is also constant at a designated pressure; sea level ,300, 750 or 1200 psi. (Note: fictional numbers that give relevance). With the first test maintaining standardized atmospheric pressure gives you the value for A. The next test(s) with increased pressure, allows you to measure directly N since you can back calculate (A*P^n). So, a bomb strand burner gives precise and repeatable values for A and N to come up with R = burn rate which is used in modeling a solid rocket motor performance and or design. Depending on if its being built or used.
Heres the fun thing
Who has a bomb strand burner? 15.00 for an internet PhD, might get you called Dr., but not a strand burner.
So, the hobby has taken on a practical practice. This is not a method, because it will fail tests. Its practically useful, or has practicality.
Basically, the method is to make a neutral bates geometry, and measure the burn rate by the thrust curve of the motor on a load cell, compared to modeled surface area calculations. I agree there is merit to the method, but the truth is more of a fib. While you can characterize a propellant burn rate with decent A and N values through this method, they are not true and correct values. So the method is not true and correct. I have to admit, calculating is not a strong point and having a little idea, but not an expert on the Surface area modeling.(So you can say I like to half ass it. Practically anyway)
Heres why I believe its a fib:
First point is the analytical logical side. Strand burners are supposed to measure burn rates of materials, and were duty purposed as such. Rocket motors are duty purposed to generate thrust and impart motions. I would think the logic says each is probably capable of the others objective, but not quite recommended. Based on safety
..analyzing it says, inherently false results will be yielded in each case.
This isnt an all inclusive list, but let us look at why APCP rocket motors may be bad when used to calculate burning rates of propellant. (not the rate of the motor).
Burn rate is affected by , thermodynamics, pressure, composition, and partical velocity. (may not be true, butt
. Remember this is my account and reason, for what I see as reality here.)
Thermodynamics: why is it a part of burn rate?
It is a part of burn rate, in that the decomposition of the fuel, binder and oxidizer are all sensitive to temperature. Auto ignition tells you how much energy it takes in calories to go from ambient to combustion. If ambient is high, it takes little energy, if ambient is low, it takes more. Thermodynamics inside a burning motor, in my most humble opinion is a model at best, and an unknown in a test. But heres how it plays into the propellant burn rate when inside a motor. Imagine a surface burning and thermal energy is conducted through the surface into the underlying material probably similar to sleeping under a heating blanket versus on top of it. The energy imparted into the fuel, makes it burn faster, because it takes less combustion energy to decompose, increasing the burning temperature, that again, increases the burn rate! Thus, it is safe to say, that thermodynamic gradients cause varying burn rates across the burning surface inside a running rocket motor. Envision a bates grain, where the end and the core are burning; the corner of the grain, particularly the top corner is going to burn very fast, and re-shape itself to another shape, as it un-uniformly burns
probably a radius due to acceleration of the gas
but would be concave(ish) if uniform radiant heat soak was modeled. (I have read thesis statements that discuss this property specifically
I need to read them again.) You can see where the radiant heat soak of the burning surface would be more uniform in a strand burner than a motor. Propellants that conduct the heat easy are probably harder to model than propellants that are better at insulating heat soak in a Burn rate vs. surface area simulation. There are some others, and some overlap, so lets move on
.
Pressure: how does it affect burn rate?
As explained earlier, pressure alone causes increased burn rates. Its a variable in the formula after all. This is because pressure, reduces the calorie requirement to decompose, or change state of a molecule. Pressure also changes thermal conductivity (as above) and density of the environment creating its own calorie exchange. (has the lack of physics/chemistry collegic/highschool education reared its nasty head yet)
(note: propellant compositions, have varying pressure sensitivity, so you can say, a low sensitivity propellant is okay to test in a motor, a highly pressure sensitive composition like fine micron AP, should be tested where the pressure is controllable- aka, strand burner.)
Pressure, in an unstable motor, is the BEST example that comes to mind. In your mind, envision two surfaces held together, with burning surfaces, .125 apart. (Obviously tho, it grows per the experienced burn rate
)The pressure between these burning surfaces would be greater than the ambient pressure, compared to the core with surfaces, held at .25, .5, or 2.25 respectfully
In an unstable motor, the ends may burn stable because of the [more stabler] pressure, while the core searches for a happy zone of pressure and burning surface. Mass flux may have a role to play here? I understand the pressure also acts uniformly, but I refuse to discount the pressure gradient, caused by surfaces in varying distances of contact, causing variances in actual burn rates. Its safe to say due to ambient and proximity created pressure gradients, there are some portions of the motor that have higher burn rates than others.
Ignition of a rocket motor, does it cause random, unpredictable pressure gradients as it pressurizes the motor, and transmits un-uniform heat into the propellant? Maybe the method is not random and unpredictable for reliable motor ignition, but certainly so when lighting a motor for characterizing the propellant. Its often important with correctly starting a motor. Again, not all inclusive or maybe eve correct, but what I think about when I say, 13 grain, not 10.
Composition: why does it play into burn rates?
To clarify, I believe this was explained in the previous two items. As the composition is what we are after all trying to characterize. But I listed it as a factor because I found it relevant to my thinking. Much like multimodal propellant can be used to both increase, or decrease, and give stability to the burn. The composition directly plays, into the calorie requirements, combustibility, pressure and temperature sensitivity and more.
Structural composition, however, is not the same for a rocket motor, designed to take the loads of pressurization and or use into account as it is not an issue in a strand burner. Where a working rocket motor, may tear its propellant apart, and fail itself, a strand burner would be able to successfully measure a burn rate. Tight rigid structures burn slower than loosey goosey structures
.
Partical velocity: how it increases burn rates?
Well, back to the thesis, and some other documents, where models for erosive burning are concerned, state that: with increased particle acceleration across any burning surfaces reduces the distance of the flame front to the surface area, and increases both structural separation of the surface and the conductance of the thermal energy into the surface, rather than away from the surface as something that has an open flame front would do. Thus, the core does not have uniform burn rate, in long motors where there are fast moving particles (transonic) near the end of the core. VS. a short fat beer can motor where the particles on the end of the grain are faster than the core flow.(none of which are transonic till the nozzle.)
Since a rocket motor, uses a model for burning surface area, particle erosion, and un-uniform thermal gradients would cause your model to not be correct in evaluating the actual surface area constant. This tells me, the increased burn rate, would not necessarily be considered: erosive burning: in a manner of erosion, but is erosive burning, in that it increases the rate where it occurs, and would not be everywhere uniformly. Strand burning does not have this problem.
Particle velocity, also will crate pressure gradients, and shockwaves that cause variances in burn rates of actual rocket motors based on the flow of the particles and their attained velocity in general.
In conclusion, I have come to find that I have enjoyed my coke and rum, thinking of what issues affect A, and what affects N. Whilst thinking about profound things
like suing the gooberment and winning. And most of all... in a rocket motor you may experience every where between .022 and .035 A value in the burn rate, and .25 to .4 N value, (at the same &&&&& time)- to quote 2chains...
where your using an A of .03, and an N .25 based on the "MOTOR"
TRA winning
I understand I may mis-construe things, I may mis- speak, some things I just dont understand or am not aware of
But please, understand its of love for the awesome hobby, not of malice to whom I consider friends who know better
But do remember, when discussing burn rates. It has to do with the propellant and its classification as a propellant and proving its not an explosive
and to call it an explosive would be capricious application without method or reason.
. Not with a classifying a rocket motor.
But, in my case, making it work the way I want it to
Clay
Hence nozzles rockets .. .where I got my information from regarding "ideal choked flow" is directly from Rogers aeroscience website... he has a great set of technical pubs, that disagrees with that comment.
