Model Forum / General / Rockets / February 2004

I could never understand the use of the term "Dog Barf" to describe
the stuff anyway...

Most samples of actual "Dog Barf" I've seen usually are watery,
multi-colored, containing large chunks of partially digested food,
bones, sticks, rocks, plastic bags, and the occasional tapeworm...

<g>

tah

--

Tod A. Hilty
Hilty Information Systems

Do not look in the direction of the flash...
Curl up in a ball as you hit the ground...

Please replace weinerboy dot org with adelphia dot net for reply.

Thanks for the clarification. This should be in the FAQ. I should use it
to pack mail orders :)

I have a huge supply of dog barf that I got for about $5.00 at a hardware
store. Bob has a point, it is possible to blow a hole through it if the
column is too short. Just what is too short?  I feel the column has to be at
least 2 BT dia in length. Thus barf is good for rockets up to 2--3 inches in
diam. After that the column gets stupidly long and weight becomes a factor.

I haven't had problems with P or A brand pistons.  But if you hadn't figure
out that phx is Phoenix AZ perhaps you might miss why that might be.  We
have zero humidity, well ok really around 15%.  We also don't have cold
weather, though it's rather chilly out there this morning (68ish).  Our club
launches with a miss-fire alley configuration, so the rocket is only out
there long enough to connect, check continuity, and walk to the card box at
the LCO table and back.  Usually I'm walking fast as he's already reading my
card.

Being as mine work, I'm not really qualified to tell you much about them
(failure modes).  Most of the comments I've read say to remove them, though
I'm guessing that's after a single failure, so they're probably not much
more qualified.  From what I understand, the gas pushes the piston which
pushes the chute and nose out.  So the nose still has to have the right fit,
but more importantly, the piston must have close tolerances to the body
tube.  Too close and it obviously isn't going to move, nor let the gases by.
Too loose and the gasses go around the piston, but is that a problem?
Issues with too close are swelling due to moisture (paper products),
contraction/expansion due to temperature and differing materials, buildup
from the ejection charge on the tubing.

So I push the piston in and out to see if it's rubbing on anything.  And
unlike most experiences I've read, I've never had to sand the piston to fit.
Then I'm cautious about bottoming it out.  On the Sumo, there's -just-
enough room for the piston and chute, though I believe that's because I used
a T-nut at the centering ring for the rail button.  Wrap of tape around the
motor to make sure the pressure goes another way (Mirage crash cause) and
off she goes.  Best I can do at this point is suggest a news group search on
'piston group:rec.models.rockets'  and/or with sumo in there.

Sorry can't be of more help regarding the piston, but as you said 'kit' and
not 'rocket' I'm assuming it's not built?  When I first get (or am deciding
to purchase) a rocket, I check out its review at:
www.rocketreviews.com
to see what others have experienced.  I know they copied an rmr post of mine
under one of the aerotech kit reviews which has generic comments for all the
kits.  Flip through those as almost all the construction is the same.  If my
comments are confusing, keep in mind they copied a block out of the rmr
post, so the witticism doesn't quite carry.

My Sumo (I keep typing suma4 for some reason) contacted a tree, ok really an
overgrown bush, and snapped a fin.  I'm in the process of grinding out the
old material and have a new fin to glue in.  Then I need to pink up the
color because it's too fuchsia.

Joel. phx

And when someone says pistons, I think motor pistons which have rings.  Has
anyone adapted go-kart piston rings to a rocket yet?