Shock! (Part 3)

Here is an archived TRF thread from January 2007 on shock cord lengths. A lot of nothing, most of it, including one person screeching about how useless it is to try to get any quantitative understanding; but the OP, CQBArms, did some test stand research (but didn’t supply any actual data), and Super Sabre contributed a GIF with a time breakdown of an ejection event — one that resulted in an “Estes dent”. And Rocketguy101 posted this interesting remark:

I got my Jan/Feb issue today…

There is an article by Janice and Harold Larson “Ejection Pressure Measurements”–they built a rig to hold a rocket, and put pressure taps in the body to measure the pressure of an ejection charge. Very interesting results!! As stated elsewhere in this thread–ejection charges in A-D motors vary greatly. They have a photo sequence of a “weak” and “strong” ejection–be interesting to compare CQB’s test rig results.

Well, how about that. The article’s called “Ejection Pressure Measurements”, not “Ejection Charge Measurements”, and it’s by Janice and Harold Larson, not Ed Brown — and going back to my source, I see the phrase “by Ed Brown” refers to a quote from the article, not the article itself. Well, that article’s clearly got more of interest in it than what I’ve heard so far. I’ve submitted a request to Interlibrary Loan; can’t hurt. If they can’t come up with it, I’ll go ahead and buy the CD.

But back to that GIF. Never mind the “Estes dent”, what’s happening right at the start — how fast is that separation?

It’s apparently an Estes Rubicon. No longer in production but the 2007 catalog says:

SPECS: Length: 22.9 in. (58.2 cm)
Dia.: 3.21 in. (81.5 mm)
Wt.: 7.3 oz. (206.9 grams)
Recommended Engines: D12-3, D12-5

It looks to be almost directly side-on in the GIF and if the ejection is too, it looks like the separation was about 2 body lengths (without nose cone, not that I’m being that precise) at 0.07 s and about 3 body lengths at 0.13 s (which, if taken at face value, means it was slowing down significantly, but I’m not sure the numbers are precise enough to say that). Those correspond to average separation speeds of 17 m/s and 13 m/s respectively.

Irritatingly enough, the Rubicon used a weird nose cone found nowhere else, as far as I know, and I didn’t know how much it weighed. So I found and downloaded a RockSim file according to which the nose cone weighs… 0 grams. It’s made of a custom material whose density is 0 g/cm^3. Nice stuff if you can get it. So then I found another sim file here that used polystyrene with a wall thickness of 0.5 cm (!) for the nose cone, but the weight was overridden to 56.7 g. With 42.5 g nose weight added to that. The Estes instructions say nothing about nose weight, so I’ll just take 57 g for the nose, 207 for the empty rocket, and 43 (22) g for the full (empty) motor. Body tube diameter is 8.09 cm, but the pressurized volume’s not a simple cylinder; there’s a stuffy tube. Well, I’ll call it the equivalent of an 18 cm long, 8.09 cm diameter tube, just for numbers I can easily stick in my spreadsheet.

Then putting in 1 g ejection charge, this calculator says ejection pressure is 34 PSI. With those numbers plugged in I get a separation rate of about 32 m/s.

Well, that agrees within a factor of 2 with the GIF, and given all the uncertainties in the weights, dimensions, and distances, and the aforementioned variability in ejection charge pressures, that’s a pretty good agreement. Still, suppose you wanted to force the separation rate to 15 m/s by lowering the pressure; what pressure would you have to use? Answer: 7.5 PSI. So if you believed everything except the pressure calculation, you’d conclude the pressure calculation overestimates by about a factor of 4. I don’t believe everything, though, so I’ll just say nothing’s an order of magnitude out of whack.





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