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-   -   BP motor ejection pressure (http://www.oldrocketforum.com/showthread.php?t=13377)

Rich Holmes 01-10-2014 09:59 AM

BP motor ejection pressure
 
There probably are others, but here is an ejection charge calculator; you can use it to calculate how much powder you need to get a given pressure, or conversely how much pressure you get from a given amount of powder, depending on the volume of the airframe needing to be pressurized.

My question is, how can you do the latter for BP motor ejection charges? Presumably it would be similar, but I don't know how much of what kind of charge is used in the various BP motors; also, I don't know how differences in construction between BP and composite motors would come into play.

Anyone have information on this? I'm interested in working out some of the physics behind ejection and its implications for shock cords.

Jerry Irvine 01-10-2014 10:13 AM

Our 24mm and 29mm SU motors used a bit over 1/4g of 3F BP and reliably ejected 4" x 18" sections.

Jerry

Rich Holmes 01-13-2014 02:41 PM

So no one knows?

Jerry Irvine 01-13-2014 04:46 PM

You could make a fixture to test it with a pressure vs time trace with a known exit port size. Some of the HPR electronics accept a serial sensor so you could repurpose an otherwise rocket object to perform the test.

Then publish it in SR.

Model rocketry has been around a long time, but not every question has been answered.

Jerry

"Model rocketry is too diverse for the Estes catalog." - Jerry Irvine

http://www.google.com/search?ie=UTF...+pressure+tests

shockwaveriderz 01-13-2014 08:49 PM

Are you asking what size of black powder is used in Estes Black Powder rocket motors versus the BP used in Composite Ejection charges? Or are you also asking the amount used by the various BP motors, depending on their size, 13/18/24/29 millimeter.

From my taking Estes BP motors apart it looks to be 3F in size. I could be wrong. Most composite BP ejection charge powder I believe is 4F. The only difference in burn characteristics between 3F and 4F, or for that matter 2F etc is burn rate. Smaller particle BP(4F) has a faster burn rate than 3F. Pressed BP versus powder BP also burns at slower rates.

The gas constant of 4F BP versus any other size BP is independent of grain size.


Hope this helps.

Terry Dean

Jerry Irvine 01-13-2014 08:50 PM

Errortech uses 4F.

BP has a flat burning rate vs pressure curve! 4F has more surface but the burning rate is the same.

It may pressurize a vessel differently. That was the original question.

Tech Jerry

Rich Holmes 01-13-2014 09:34 PM

What I'd like to know basically is: How much pressure is exerted on a nose cone given the volume of the rocket (or rather the portion that gets pressurized) and the type of BP motor? Ideally for a variety of motor sizes. Knowing how that pressure varies with time would be even better, though that probably gets too complicated.

So for example, for HPR, I can go to http://www.rimworld.com/nassarocket...calc/index.html and put in 4" diameter, 10" length, 1 g 4F, hit "Calculate pressure", and it tells me 15.42 PSI.

(That page makes things a little too complicated with its table of values of C; really the relation is just

P = M / (0.0004 * D^2 * L)

so here M = 1 g, D = 4, L = 10 and you can just plug those in to get P.)

I'd like to be able to do the same for BP motors, but doing that requires knowing the quantity of charge in a given motor, and whether the constant 0.0004 is appropriate for BP motor charges too, or should be something different. (Though I'm not looking for any great precision, so maybe assuming I can use 0.0004 is good enough; then all I need is the sizes of the charges for various motors.)

Jerry Irvine 01-14-2014 05:04 AM

NAR S&T publishes motor propellant masses for all certified motors.

Rich Holmes 01-14-2014 05:32 AM

Quote:
Originally Posted by Jerry Irvine
NAR S&T publishes motor propellant masses for all certified motors.

Yes. But not as far as I can see ejection charge masses.

Jerry Irvine 01-14-2014 06:11 AM

Quote:
Originally Posted by Rich Holmes
Yes. But not as far as I can see ejection charge masses.
The initial mass minus the burnout mass minus the propellant mass = the delay and ejection mass. But you are correct. I would estimate most BP ejections to be under 1/4 gram. That is actually too big for most smaller diameter model rockets and results in recovery system failures.

If mini motor ejections are over 1/8 gram they are too large. They are also of low accuracy. I feel from personal experience they can vary in mass 30% or so.

USR used 1/4g 3F BP for C-F motors and that was suitable for rockets 1" - 2.7" diameter and certainly lengths up to 18".

Jerry

GregGleason 01-14-2014 02:28 PM

Another way is to approach the problem is how much force is required to eject the cone. Once you know the force required (regardless of pressure), you can "back solve" for the amount of BP required, given the length and the interior diameter are available.

Some cones may require less force to remove than others, per a given rocket.

A cone with a required force of 100 N (22.48 lb-force) with a 10" length and 4" ID would only need 0.07 grams of BP (about 2.15 psi). At least according to my calculator. The 4" ID has an area of 12.57 sq. in., so the force on that area would ~30 lb-force (2.15 psi * 12.57 sq. in.). Hope this all makes sense. Now that's just the force to remove it.

If you want your cone to be accelerated to a certain "muzzle velocity", then then more calcs are needed that also incorporate the nose cone's mass and perhaps include drag as well.

Hmmm .... it would be an interesting experiment to test in the "real world".

Greg

Rich Holmes 01-14-2014 02:59 PM

For a rocket built according to the usual "rule" that you should be able to pick it up by the nose cone without its coming loose, but should come loose if you shake it (not a rule that makes a lot of sense, I think, but it's what people usually go by) then the force required to push the nose cone out is only a little more than the weight of the rocket.

Needless to say, I don't believe an Estes C motor provides only about 5 ounces of ejection pressure in a typical 18mm rocket.

So I don't think that gives a very useful approach to estimating real world BP motor ejection pressures.

GregGleason 01-14-2014 03:19 PM

That's why I mentioned that it will get you to the amount of BP required to remove the nozzle at a minimum.

You can calculate the force required to accelerate a nose cone to a given velocity, if that is what you are going for. That's F=ma.

Greg

Rich Holmes 01-14-2014 03:24 PM

Actually I'm trying to go the other way and find the velocity, which is why I need the force.

GregGleason 01-14-2014 04:38 PM

As we know, "F" is a function of the amount of BP.

So, are you trying to determine the amount of BP Estes uses in their motors? If so, have you contacted them to see if they would divulge that information?

One of the things that determines acceleration is the rate of burn. A faster build up of "pop" will throw a nose cone father with the same mass of 4F BP as opposed to 2F BP. They will both make the same amount of gas products, but one will do so more rapidly than the other. That's my theory anyway. Lot's of BP will have variances to them, just to make things a bit more interesting*.

Greg

* Someone flew in our rocket club last month had his rocket destroyed because the BP he used underperformed (i.e., the BP didn't burn fast enough). The burn rate was so slow that separation never occurred and the rocket came in ballistic. The flyer contacted the vendor and found that his lot number was one that came from a "bad" lot.

shockwaveriderz 01-14-2014 07:55 PM

I went back and looked at some of my old Sport Magazine magazines and found a Jan/Feb 2007 issue with an article entitled, "Ejection Charge Measurements". IN it is a quote by ED Brown, formerly of 40 years at Estes Industries stating the ejection charge sizes as : 13mmm =0.4 grain, 18mm = 0.6 grain and 24mm = 1.0 grain, where 1 grain = .06479891 grams.

hth

Terry Dean

Jerry Irvine 01-14-2014 08:12 PM

Quote:
Originally Posted by shockwaveriderz
I went back and looked at some of my old Sport Magazine magazines and found a Jan/Feb 2007 issue with an article entitled, "Ejection Charge Measurements". IN it is a quote by ED Brown, formerly of 40 years at Estes Industries stating the ejection charge sizes as : 13mmm =0.4 grain, 18mm = 0.6 grain and 24mm = 1.0 grain, where 1 grain = .06479891 grams.

hth

Terry Dean
That means the accuracy is more like -0 +100%

13mm 0.4 grain 0.026g
18mm 0.6 grain 0.039g
24mm 1.0 grain 0.065g

Those all seem lower than I estimated by one order of magnitude. So I question the claim as a missed digit.

We all should measure a representative sample of disassembled motors and report the data.

Jerry

Rich Holmes 01-14-2014 08:53 PM

Quote:
Originally Posted by shockwaveriderz
I went back and looked at some of my old Sport Magazine magazines and found a Jan/Feb 2007 issue with an article entitled, "Ejection Charge Measurements". IN it is a quote by ED Brown, formerly of 40 years at Estes Industries stating the ejection charge sizes as : 13mmm =0.4 grain, 18mm = 0.6 grain and 24mm = 1.0 grain, where 1 grain = .06479891 grams.

hth

Terry Dean

Excellent!

Rich Holmes 01-14-2014 09:03 PM

Quote:
Originally Posted by Jerry Irvine
That means the accuracy is more like -0 +100%

13mm 0.4 grain 0.026g
18mm 0.6 grain 0.030g
24mm 1.0 grain 0.065g

Those all seem lower than I estimated by one order of magnitude. So I question the claim as a missed digit.

We all should measure a representative sample of disassembled motors and report the data.

Jerry

I get 0.039g for the conversion from 0.6 grains.

If I go again to http://www.rimworld.com/nassarocket...calc/index.html and blithely enter 0.039g for the ejection charge along with 1.6" and 15" for diameter and length (corresponding roughly to the pressurized volume of a Big Bertha), when I calculate the pressure I get 2.51 PSI. The nose cone area is just about 2 square inches so that's 5 lb of force on the nose cone. If it were a factor of 10 higher that'd be 50 lb force, and I'm pretty sure that would result in even more Estes rubber band failures than actually occur. 5 lb on the other hand I find believable. Note the above page's recommendation to aim for 8 to 15 PSI (albeit for larger rockets), and of course the Big Bertha has a larger volume than many 18 mm LPRs so I'd expect to see pressures on the low side there.

So, provisionally, I think I'm willing to believe those numbers. Unless someone knows better...

shockwaveriderz 01-15-2014 08:13 PM

Rich, I was informed that Ed Brown was misquoted and that he used grams instead of grains. So that would be 0.4g for 13mm, 0.6g for 18mm and 1.0g for 24mm. Sorry for the confusion as I was quoting from a private message board quote from a 6 year old magazine.

Hope this helps

Terry Dean

Rich Holmes 01-15-2014 10:09 PM

Well, that's annoying. That bumps the Big Bertha calculated pressure from 2.5 PSI to something like 40 PSI, which I have a very hard time believing; so then I'm thinking the 0.0004 factor used for composite motor ejection charges is inappropriate for black powder motors. At least I have the relative sizes of the charges for various size BP motors, but still no value for the pressure.

I'm hoping one of our local club members still has that issue and I can get a copy of the article from them. Maybe it'll have some more information. NARTS seems not to have it as an individual back issue, though they do have the whole year on CD.

FondyFlyer 01-15-2014 10:39 PM

Remember that in an estes type engine, you are not presurizing a closed vessel, but one with a substantial leak path ( the now open nozzle ). I would not use those calcs.

Rich Holmes 01-16-2014 05:21 AM

And you're also breaking the clay cap.

astronot 01-16-2014 08:30 AM

Quote:
Originally Posted by shockwaveriderz
Rich, I was informed that Ed Brown was misquoted and that he used grams instead of grains. So that would be 0.4g for 13mm, 0.6g for 18mm and 1.0g for 24mm. Sorry for the confusion as I was quoting from a private message board quote from a 6 year old magazine.

Hope this helps

Terry Dean


Rich Holmes I hope the above quote is true because you need to remember that BP grains are measured by volume, and not by weight. I'm pretty sure you are aware of that, but I just wanted to make sure you did in case this was leading to any confusion on your part. That makes a big difference when dealing with grains of BP, 1 grain of BP by volume will weigh less than 1 grain of BP by weight. So if going by weight then you have a substantial charge of BP for ejection.

But if the measurements a in grams then you have nothing to worry about unless the calculation you are using is assuming Grains by volume vs grains by weight, or grams.

That should be about as clear as mud by now,

David

FondyFlyer 01-16-2014 08:36 AM

Quote:
Originally Posted by Rich Holmes
And you're also breaking the clay cap.


This is also true. For years I have used 2psi as the pop-off pressure for my LP stuff. Seems to have worked well, and is easy to duplicate on the bench using compressed air and a decent regulator.

Jerry Irvine 01-16-2014 08:50 AM

Quote:
Originally Posted by shockwaveriderz
Rich, I was informed that Ed Brown was misquoted and that he used grams instead of grains. So that would be 0.4g for 13mm, 0.6g for 18mm and 1.0g for 24mm. Sorry for the confusion as I was quoting from a private message board quote from a 6 year old magazine.

Hope this helps

Terry Dean
I heard from Ed himself and have replied to him to verify that he meant grams not grains since I find it hard to believe either figure is accurate. Ed Brown reports he cannot remember what the values are but that some of the powder is captured in the clay during pressing, which makes the need for more than one might expect.

I suggest weighing actual charges to get a viable answer. The below figures seem FUBAR.

13mm 0.4 grain 0.026g
18mm 0.6 grain 0.039g
24mm 1.0 grain 0.065g

I used in my motor mfg:
13mm-18mm 1/4g
24mm-29mm 1/2g
38mm-54mm 3/4g

Those were dispensed by volume and were thus +0-15%, and retained with a paper cap.

I find it very difficult to believe an Estes 24mm motor has 1 gram of BP ejection. I would estimate it to be about 3/8g.

Jerry

Rich Holmes 01-16-2014 01:49 PM

Over on TRF Zebedee called my attention to this page http://info-central.org/?article=303 which has essentially the same calculator as I ilnked above, along with more text including this paragraph:

Quote:
Now, so you will have to think, given the ejection charge mass in a D12 motor is .85 grams, what pressure is generated inside an Estes Phoenix model, with a 2.4" diameter and 8" long chute compartment?

So, first, this is an assertion that a D12 has 0.85 g ejection charge, close to though a little smaller than the 1 g mentioned above. But who knows where that number came from.

Second, it implies the calculation is good for BP ejection. (The answer to the question posed is: 45.5 PSI.)

But above that bit is an explanation that makes the formula's origin clear; it's just based on the total quantity of gas produced when black powder burns, and it assumes all of that gas goes into pressurizing the parachute compartment. As said above, that assumption may not be good enough for BP motor ejection charges.

Jerry Irvine 01-16-2014 01:55 PM

That explains why Gnomes essentially explode on ejection.

If it is true the amount poured into a D12 is 1g and someone measured it as 0.85g net, that is at least consistent with the Ed Brown comment of some being embedded in the clay cap.

We are starting to get closer to believability. However this means Estes ejections are particularly heavy and mine were light by comparison despite tens of thousands of perfect flights and ejections.

Just to update the table accordingly:

13mm 0.4g
18mm 0.6g
24mm 1.0g

The amount of gas leak out the nozzle is trivial.

Wanna try a cool test? Epoxy over the ejection of a C6-7. Do one flight with electronics of a model with a traditional C6-7 then one with the epoxied over ejection with electronic deploy. The second one will go over 15% higher.

http://v-serv.com/usr/kits/stripper.htm

Jerry

Rich Holmes 01-24-2014 12:49 PM

I got a copy of the Larsons' article via Interlibrary Loan. Interesting, and directly relevant to what I'm thinking about.

They reported on a series of tests in which various Estes motors were ignited in a Quest Courier fitted with two pressure sensors, at either end of the pressurized volume, on a test stand. The pressure was recorded every 0.15 millisecond for 25 ms after the ejection charge fired.

I disagree with the Larsons’ interpretation of their data, which was driven by the aforementioned misinformation about the size of the ejection charges used in Estes motors.
The Larsons noted that the typical B motor maximum pressure they measured, 5 PSIG, agrees with the calculated maximum an 18 mm charge can produce (based on the quantity of gas one gets from fully burning the charge and the size of the pressurized volume), and on that basis assumed the measured maximum corresponds to the maximum capability of the charge. However, the Estes charges are in fact 15 times larger than the Larsons thought! In fact I believe if they had measured pressures in a sealed vessel, they would have seen much larger, much later pressure maxima, but in their experiment the nose cone ejection truncated the pressurization at a lower value. The average pressure maxima, and the average pressurization rate, both seem to vary little if at all between the various engines and the homemade canisters, which cover a factor of 10 in ejection charge mass. (The Larsons thought the canister charges were about 60% larger than the D/E charges, when in fact they were 10 times smaller.) My guess is the charge size to first order affects only the maximum pressure achievable, and not the pressurization rate, and then the nose cone ejection truncates the pressurization well before that maximum can occur, at least in the rocket they used.

Jerry Irvine 01-24-2014 01:18 PM

I like scientific posts.

Estes may put pacifiers in the ejection material itself like it does with delay. That would necessitate more mass. Their ejections are not insane.

ed.brown 01-24-2014 09:51 PM

It's been quite awhile since I last read the Larson's work; but if my memory is correct, their results showed pretty wide variations in pressure even with motors they had cleaned the ejection out of and replaced with precisely measured amounts of ejection. It's a very interesting and frustrating problem trying to come up with an amount of ejection that is suitable for all sizes of rockets that the motor might be used in.
Regards,
Ed

Rich Holmes 01-25-2014 06:46 AM

Thanks, I'd overlooked that point. To my eye the variations in those tests were smaller, but still a factor of 2 or so.


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