Ye Olde Rocket Forum

Ye Olde Rocket Forum (http://www.oldrocketforum.com/index.php)
-   FreeForAll (http://www.oldrocketforum.com/forumdisplay.php?f=10)
-   -   BP motor ejection pressure (http://www.oldrocketforum.com/showthread.php?t=13377)

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


All times are GMT -5. The time now is 03:46 PM.

Powered by: vBulletin Version 3.0.7
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.