Rapier motor chemistry, production, and testing
 

Hello All,

I came across this illustrated Minimakety.cz interview (see: http://minimakety.cz/rapier/other/I...07/index-en.php ) with Dr. Jan Zigmund, inventor of the Rapier www.rapier.cz low-thrust rocket motor that is a single-use replacement for the Jetex motors. (I found the link on the Jetex.org web site here: http://jetex.org/motors/motors-rapier.html ).

He provided interesting descriptions of his propellant chemistry choices (designed to cool the exhaust), his motor production methods, and the very simple static test equipment (a mechanical postal scale and a dictaphone to verbally record thrust/time figures) that he uses.

Enjoy!

Don't know about the dictaphone, but the postal scale was how GHS tested the first model rocket motors

Yes, but he would have to have been able to "out-gab an auctioneer" to verbally record the thrust/time data for any model rocket motor (with the possible exception of an FSI F7 motor). :-)

Oh my! You obviously never met G. Harry Stine - he could out-gab a room full of auctioneers! :chuckle:

Vern did the same. That is why the first version of the B14 (in newtons) was called the B16 (in *pounds*). Because it floored the fairly undamped postal scale he used for testing! It was only later when he built some real testing equipment that he found it was actually only three pounds!

I never heard him talk *that* fast. Out-talk them? yea, he probably could!



This guy, however...

A static test setup has been on my mind lately.

I did a little shopping for load cells which are metal shapes with strain guages attached and wired to produce voltages proportional to the strain, when biased properly.

However, load cells seem to start at over $125. Ouch.

I did find this:
NCR Load Cell
Which appears to be the load cell for a cash register installation. Any thoughts on whether it would be adaptable to an engine test stand? I won't be testing anything larger than an E, so I imagine that it's maximum load is high enough--probably in the neighborhood of 20 - 25 lbs, because heavy items cross the scales at grocery stores. However, I'm not sure about its damping and responsiveness.

I figure I could hook the thing up to one of the Freescale 9S12 Development Kit Boards and wire it to the built-in A-D converters. Then a little bit of code to sample and record the A-D port every ten milliseconds or some such. Then interface the thing to a PC, dump the memory, and import it to Excel for graph creation and printing.

I don' t know why I could not find this a couple of weeks ago when I looked. Today I found the Installation Manual and User's Guide for the 7870 scanner/scale at NCR. The Scale load cell capacity is up to 30 lbs and the resolution is .01lbs. However, it does not register weights less than .2 lbs. That may be a bit high, but I don't know if that's a limitation of the load cell or the entire unit. It seems like larger capacity load cells give up some sensitivity to tiny loads.

I guess I may just have to order one and experiment with it a bit.

Aerocon Systems lists a number of load cells, test stands and data acquisition systems on their website. Their biggest load cell is rated to 4000 lb./ft., and sells for $185. Their smallest l. c., rated at 20kg (44 lb./ft.), is $30. I know absolutely zip about any of this stuff, but I recently ran across it on the site, so I thought that I would mention it here.

MarkII

Besides Aerocon, if your patient, you can get them at a good price on eBay.

Besides the load cell your going to need an amplifier and an A to D converter. The converter is the easy part. You can get them here with sofware for $50 if you can live with 240 samples/sec. You have to pay more to get the high speed software. The amplifier is going to cost you around $100 unless you can build one yourself.