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.

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.

And you're also breaking the clay cap.

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

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.

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

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:


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.

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

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.

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.