The Body Tube

 The Thrifty Rocketeer blog continues....

It's about time that this blog got back to it's mission...saving you money in this hobby.


There are lots of flavors in the hobby to keep everyone satisfied.

Some build scale model rockets... some build families of identical rockets, scaled up or down.. some build kits.... while other prefer scratch builds and experimental designs.

Recently, I've stumbled onto a series of old Estes kit plans that apparently taught rocketeers to build their own rockets from plans... or to supplement their kits by cutting their own fins according to the plan.

Many of these older rocket plans are hosted on various websites.  One is SpaceModeling.org    , where the webmaster makes the PDF files available for download, and just asks you for a donation to keep it going and to make it worth his while.

I have downloaded two so far, and will be sending him a little something shortly.
http://www.spacemodeling.org/jimz/est2038.htm 


But I was having trouble finding some of the parts that looked pretty standard, really.

Now, some rocket websites and vendors have slick pages and web pages that hawk only their own kits.  And some manufacturers have various categories of rockets...by skill... or design... or other sort-able theme.  The point is that most are all user friendly, and market to the general public, allowing you to navigate and shop with your eyes.

But there's one site that offers mainly supplies, and lots of them.  The system is organized and efficient, but it's not always intuitive or user-friendly to the general public.  You're shopping for PARTS... not glossy packaged kits...although they are available there also.  But there are LOTS and LOTS of parts on this site, and most of them don't have photos, but stock parts drawings.

Anyway, my point is that I was looking for a cheap BT-80 body tube 9" 15/32" long for this rocket.  I had most all the small parts, or could make and cut them from scrap I had accumulated.  I found by searching for BT-80...dozens of listings..most of which were BT-80 tubes of various lengths.  I found 8" and 9" tubes for about $4.39 each...but when it came to a 9.3" tube, the price shot up to $8.19 each.

I couldn't understand the sudden price jump.  So I wrote to the owner to ask why the sudden jump in almost double the price.  After all, I just wanted a cheap BT-80 body tube.

I wondered if maybe the material the craft tube was made of was somehow better or more expensive.  I couldn't figure out why 1/3" more of tube would cost so much.  My target was 9 15/32" length, though I could not figure out why it wasn't just 9.5" and be done with it.

I also looked at the quantities on hand.  There were almost 83 units of the 8" length available.  There were more than 86 of the 9" on hand.  But the 9.3" length had only 4 units available.  Could it be that the price depended on supply and demand??

It turns out the answer is even simpler than I had imagined.

I got a very nice e-mail back from the owner who patiently explained that the price for some parts was a function of the cost to produce it. That is, they cut their craft tubes to length, stocking most of the standard lengths, but willing to cut custom lengths if necessary.  Custom orders would reflect the time and effort to generate the number of parts that were being ordered....like tens or hundreds for assembling various kits for other vendors and rocket sellers.

But the answer for why the price would suddenly double is even simpler than that.  They cut their lengths from 18" craft tubes.  Therefore, a 9" tube is very easy to cut. They get two of them for every cut that they make on their machine.  However, a 9.3" produces one usable tube, and leaves about 8.7" scrap.  Now, that 8.7" length can then be cut into an 8" length or two 4" lengths, etc.  But there's still about .7" scrap left over due to the initial cut.  

That's why the price nearly doubles to get that extra fraction of a tube... you can only get one per 18" tube.

It's so simple and so obvious, now that he explained.  I felt almost foolish to not have seen it.

I appreciated his patient answer and placed my order for ONE 9" BT-80 tube, and a few other things like a PNC-80 nosecone, a BT-20 motor mount, some centering rings and a thrust ring stop.  I was glad to shell out for those lower priced parts, but drew the line at 9.3"....and the higher price.

After all, this IS the Thrifty Rocketeer blog...

**Footnote:  Since writing this, I have discovered that the cheapest PNC-80 nosecone is a bit taller than an ogive, and more pointed elliptical...and it's translucent in color.  That's good for me, cause if you push a glow stick into it, it will glow for night launches!  But the ogive that I originally wanted was quite expensive...until I discovered that it's the nosecone used on the Mega Mosquito model.  And low and behold, I've got three at my disposal...one white, one yellow and one lime green.  Now, I don't want to paint it bright red to match my Alien Probe rocket, so I'm going to settle for the translucent nose-cone and call it square when I can launch it in the daylight before the upcoming "night launch" for Halloween!


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