Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!sri-unix!billw
From: billw@unix.SRI.COM (William E. Westfield)
Newsgroups: sci.electronics
Subject: Re: 9-volt / rocket launcher?
Message-ID: <26051@sri-unix.SRI.COM>
Date: 3 Jan 89 06:24:20 GMT
References: <2557@ddsw1.MCS.COM> <6767@pucc.Princeton.EDU>
Reply-To: billw@unix.sri.com (William E. Westfield)
Organization: SRI, Menlo Park, CA.
Lines: 54


Mumble.  Freedom.  Right.  In a recent SF convention panel on censorship,
I did mention that I found it rather depressing that modern encycolpedias
no longer seem to include detailed formulas for various explosive and
pyrotechnic substances.  I guess I believe in evolution, but perhaps
population densities are getting to high to make this practical.  Sigh.

Anyway, the easiest thing I ever found to ignite electrically is steel
wool - 3 volts or so will do it (and it is cheap too!).  If you want to
get fancy, you can seperate individual strands and make a sort of lamp
filamant with a couple copper leads.  With a bigger battery, you can just
stick a couple of leads into a loosley packed lump of the stuff.

Now for the electronics lesson...

What you want to do is get a small piece of wire very hot.  To do this
you want the wire to dissipate a lot of power (in the form of heat, of
course).  The relevant equations are (Voltage = Current * Resitance) and
(Power = Current * Voltage)  Combine them and get (Power = R * Current ^2)
or (Power = (Voltage ^2)/R).  This first of these is more useful - You want
the part of the ignition system that is supposed to get hot to have a higher
resistance than the rest of the circuit - If you made the whole thing out of
nichrome, then the power available will be dissipated equally thoughout,
and no particular part will get hot enough to be interesting.  Then you
want the wire that is getting hot to have appropriate properties - nichrome
is widely used because of its electrical characteristics and because is
holds up well to repeated heatings and coolings (not particurally important
for an igniter).  Steel wool burns nicely.  Wire wrap wire melts annoyingly
if you happen to short out a polapulse battery with it :-)

The other problem is that there is a maximum current/power that any given
battery can supply.  This can be modled by assigning the battery an internal
resistance (although the actual limitaion may be elsewhere, eg the activity
of the chemical reaction involved).  If the maximum current a lithium coin
cell can provide in 30 mA, then its internal resistance is 100 ohms, and
it isn't going to make anything hot.

When I was first introduced to model rocketry, the recomended ignition
system was a car battery and a piece of nichrome wire.  Battery technolgy
has improved a lot since then - alkaline batteries, as well as Nicads and
"Polapulse" batteries can supply reasonable current, although I don't know
whether a 9V battery is good enough - the individual cells are smaller than
even a AAA sized battery (which raises interesting possibilities for all
you people interested in building small things of all sorts !).  A couple
of small Nicads would be a better idea (or a 9V nicad battery).

State of the art research type batteries have such high internal energy
that they make nice bombs all by themseleves.  Short them out an all the
heat gets dissipated internally into the solute, some of which are volatile
and flamable.  I believe that someone was actually killed when one of the
polymer batteries they were working on accidently shorted...


Bill Westfield