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Re: Re: Arcjet Motor

Gene Harlan wrote:
> Not knowing anything about rocket motors, is a burn really a "burn". I guess
> I am asking if there is a flame, is it ignited, and by what? Or is it
> a chemical reaction?

It depends.  In the case of the Ammonia ArcJet motor, the answer is NO.
This is described in the message you are quoting:

> -----Original Message-----
> From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
> Behalf Of i8cvs
> The ArcJect thruster is about 100 mN,giving an acceleration of order
> 0,0002 m/s^2 provided the ammonia is heated by electric arc, then
> the heated gas is expanded via a nozzle and thermal energy is
> converted into kinetic energy.

In the case of the liquid fuel motor, the answer is YES.  Monomethyl
Hydrazine (MMH) is mixed with Nitrogen Tetroxide (N2O4).  The MMH
burns (is oxidized) by the N2O4.  This is a chemical reaction,
similar to what happens when you light a match to gasoline.  In that
case, hydrocarbons (gasoline) are oxidized by atmospheric oxygen (O2)
to produce mostly carbon monoxide (CO), carbon dioxide (CO2), and
water (H2O).  The advantage of MMH + N2O4 is that they are
"hypergolic", which means that the simple act of mixing them together
causes spontaneous combustion to occur.  No spark, heat, or other
external ignition is needed.

But the word "burn" is used loosely in either case.  With the
ammonia arcjet, there will be heat involved during normal operation,
but the heat is provided by stored electrical energy (originally
produced by the photocells), and is used to accelerate the speed of
propellant expulsion as described above.  So heat is not a RESULT
of firing the motor, it is a method of increasing the output of
the motor.

Just for completeness' sake (some would say just because I am too
verbose when I compose email ;-), people also use the word "burn"
when they talk about recording a compact disc in a CD-R or CD-RW
drive.  I don't really know if the chemical reaction in the disc
is an oxidation/reduction reaction ("burning"), but it has gotten
that name at a minimum because a (relatively) high-power laser beam
is focused on the disc and produces local heating that causes a
chemical reaction that produces a change in optical characteristics
of the disc that can be detected (read back) with a low-power laser
beam later on.

John (KB0ZEV)
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