Opinions sought on Sp'Rocketry Article: DIY Altitude Chamber

Sp'Rocketry Readers, I'd like to get some feed back on an article that I've written for this month's issue of Sp'Rocketry. The article is titled "Build Your
Own Altitude Chamber." I decided to go a little deaper than most Sp'Rocketry Articles (or HPR and ER, for that matter) and delve into the basic physics that support the design. In the article, I discussed the basic gas-law physics for a piston-based altitude chamber, curve fit the non-linear standard altitude tables, and charted some options for different size chambers. I finished up the article with DIY construction/test/validation of the small chamber itself. There were quite a few equations in the article. So, I'd like to get some opinions on the structure of the article. Too much physics chaff (i.e., too deep)? Too boring? Please be honest; you're not gonna hurt my feelings.
I'm seriously considering doing a similar article on the Art and Science of Ejection Charges, where I'd lay out the basic gas-law physics using PV=nRT, ultimately establishing the Force vs Cross-sectional Area vs Mass of BP relationships, then discussing the importance of keying on ejection force instead of ejection pressure (especially for larger diameter airframes). I'll probably mention my old frend 'adiabatic compression' a time or two, also. I'll also offer up a spreadsheet that allows the user to compare ejection force vs ejection pressure vs mass of BP. Of course, the best analysis in the world doesn't substitute for a good ground test. So, would it be worth my time to wirte this article; really meaning, would you be interested in reading an article like this?
Thanks, Chuck
--
Chuck Pierce
President, Huntsville Area Rocketry Association
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On Sat, 22 May 2004 07:22:46 -0500, Chuck Pierce

I think it would be a great article. This type of information is normally way beyond what I "need" to know, but I always find it interesting. I think you did a good job explaining the physics in your altitude chamber article; it wasn't too difficult from someone without a heavy physics background (like me) to understand. Coupling the physics explanations with a real-world altitude chamber makes it even more effective.
I'd be very interested in other articles by you, or others, that cover the science of our hobby.
Mario Perdue NAR #22012 Sr. L2 for email drop the planet
http://roci.indyrockets.org "X-ray-Delta-One, this is Mission Control, two-one-five-six, transmission concluded."
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Chuck,
It didn't have too much math for me. But since I have an MSEE I might be biased. :-)
You had me excited with the curve fitting part for a minute but when I realized that you were using a square root to get from pressure to altitude I relaxed. I was looking for something better than a piecewise linear interpolation method for this. But a square root is just as bad from a computational standpoint as the standard equation.
Speaking of which, I have no clue why you bothered fitting a curve to that data table as good equations are readily available:
http://www.grc.nasa.gov/WWW/K-12/airplane/atmos.html
Chuck Pierce wrote:

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David W. Schultz
http://home.earthlink.net/~david.schultz
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David Schultz wrote:

Yeah...but that takes all the fun out of it :)
Doug
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Why take the easy way out??? I actually didn't take the time to look for a pre-existing curve; so, I reinvented the wheel, I guess. :-)
cp
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Chuck Pierce
President, Huntsville Area Rocketry Association
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Short answer: WRITE IT!
I'd love to see this type of article, if for no other reason than it is different from the run-of-the-mill type of stuff.
I would ask that you write it so that it's understandable at the 'smart kid' level, but put additional stuff in a sidebar article for those of us who do want to read it. That way, it can be used as a teaching tool for kids without being a problem.
David Erbas-White
Chuck Pierce wrote:

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Chuck, I loved it. I miss the tech stuff from the old MRm days. Even though I didn't understand the calculus then, that spurred me on to ask teachers what those equations meant, and pointed me towards getting into engineering. I wish SpRocketry would put a little more tech back into rocketry.
Of course, I am a geek...my sig says it all
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David Stribling
But it _is_ rocket science!
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wrote in message

Do it. I'll explain the math to Randy. ; )
Verna
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On Sat, 22 May 2004 07:22:46 -0500, Chuck Pierce

Might point out that the typical "'good ground test" does not model the separartion dynamics very well. In a real free body event, the rocket body is accelerated backwards, and the overall nc-body separation velocity is reduced. With the body fixed in a ground test, a greater velocity is imparted to the nc.
At least that's the way it works out in my mind - somebody might want to check me on this.
GC
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We sometimes put both parts on rollers or dowels to more closely approximate reality and to reduce damage.
Jerry
Tech poster!
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Jerry Irvine, Box 1242, Claremont, California 91711 USA
Opinion, the whole thing. <mail to: snipped-for-privacy@gte.net>
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Gary Crowell / VCP wrote:

In the air or on the ground, the nose cone will have the same velocity relative to the rocket body. In the air the cone will have less velocity, as seen by an observer on the ground. But because the rocket body will be moving in the opposite direction it will hit the end of the cord just as hard as it did on the ground. A ground test is valid.
-JT
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chuck: the more the merrier....I am re-reading the article again and again to soak it all in ....
shockie B)
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Chuck,
After reading this r.m.r. post I was anxiously awaiting my bulk mail delivery of SR which finally came yesterday. Back B.K. (Before Kids) I used to write tech filled articls for our club newsletter and Extreme Rocketry (the "Rocket Science" articles) so I was dying to see how someone else approached the subject.
I enjoyed your article and it seems we have similar styles: peppering the tech info with hopefully entertaining quips and suggestions to the reader that the equations and science really are not that difficult if they just read along with you. I always thought diagrams and figures were a must and you fulfilled that requirement. I started thinking about he old descriptions of "what is pressure" in physics text books where they show cartoons of air molecules hitting one another at different volumes but that may have been going a bit too far so I think you had a pretty good balance of showing enough to get your point across but not so much to be boring.
I never really got a lot of feedback on what I wrote except that Brent kept asking for more stuff for ER and once in a while someone would send in a letter saying they like the tech features. Good idea you asking here, I never thought of it.
How much time did the article take you? That has been my problem the past couple years in trying to find time to write these things (I still have several ideas, but no time!!!) I know I must have spent well over a hundred hours on the article about supersonic flight for ER. Between research, creating diagrams and equations and then writing and re-writing things 4 or 5 times it all adds up.
Here's some links to NIRA club news letters I had articles in to give you some samples of how someone else approaches tech writing for the masses.
http://www.nira-rocketry.org/LeadingEdge/JulAug99.pdf http://www.nira-rocketry.org/LeadingEdge/MayJune00.pdf http://www.nira-rocketry.org/LeadingEdge/JanFeb02.pdf
Looking forward to more articles from you... hopefully you have the time to devote to them.
Good Luck
nOrM
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Woops, The second link should have been:
http://www.nira-rocketry.org/LeadingEdge/MayJun00.pdf
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On Tue, 25 May 2004 10:54:12 GMT, "nOrM Dziedzic"

I laughed, I cried. It is the strange tale of a geeky engineer with perplexing skills trying to impress the women in his life with a Mason jar, aquarium tubing, and few odd bits, but the women would only roll their eyes in response. I found the writing style more of an annoyance, but good, bad, or ugly, I'd still like to see more technical content in Sport Rocketry.
Alan

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Chuck Pierce wrote:

Chuck, I thought your article was very well written; informative and entertaining. it was a great overview of the math too.
the women in my life are somewhat entertained by syringes and tubing and mason jars and mathy articles.
looking forward to more like that from you!!
Sport Rocketry is a great magazine, especially for the size of the distribution.
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Thanks for the feedback. I appreciate the positive and not-so-positive comments that I've received both in this thread and via email. I will plan on writing some more similar techy articles when I can scare up a little free time, which has come at a real premium these few weeks.
Best regards! Chuck
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Chuck Pierce
President, Huntsville Area Rocketry Association
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Chuck
Just finished the article and found it quite well done. A refreshing distraction of pure theory applied to practical use. I've already laid out my plans to "bash" your idea with a few "improvements."
The syringe as a vacuum source is novel, as I probably would have gone with something more like your ill-fated brake tool.
One thing that might have made your figures a bit more accurate would have been a more precise measure of the mason jar volume/ A simple graduated cylinder would have helped a bunch.
Barring that, I would have got as close to 20 degrees C as possible and weighed the jar full and empty. you could have then easily calculated the volume by using a basic presumption that 1g of water = 1 cubic centimeter (I think that is right, Any Chemists out there?)
A
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Faster, better, cheaper! A buddy of mine recently gave me a 140cc syringe (from an EMT heart attach packet, I guess). The 140cc syringe pulls a vacuum equivalent to roughly 4K.

Yep, I try to KISS it as much possible, though it's often hard not to go for kewlness points for complexity and accuracy.

I did that. I used a graduated cylinder to figure out that the 850cc Mason jar was really 950 cc to the lip. Though unplanned, I did actually run the test at very close to 70F. The ambient temp really won't matter much, though, since the process is practically isothermal. The biggest inaccuracy is not knowing the volume of the altimeter. Still, though, the real value in this small altitude chamber is getting in the ball park to get a warm fuzzy that your altimters are functioning properly.
Thanks for the feedback! Chuck
--
Chuck Pierce
NAR 78629, TRA 9308, Level 2
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Since I can't leave well enough alone....
How about this:
Wrap the altimeter in something that makes it completely waterproof. Immerse it in a graduated cylinder and measure the displacement.
For future publication and pure science, possibly write the mfg for some zorched altimeters (beyond repair an dunk them in the raw.
I know you wanted warm fuzzies, but engineers get those when we sometimes take things way too far...

out
with
have
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