Have done a lot of work with Magnalium in APCP.
Now that I am retired from rocketry, I guess should not keep 20+ year old data
a secret?
1) Magnalium provides a lower ISP density than using either Al or Mg alone. It
is also more costly than Mg or Al powders.
Would not suggest using it, unless you need it to solve another challenge, or
it is cheaper for you to obtain.
2) Magnalium is more stable than Mg alone in presence of oxidizer, IE It does
not oxidize in APCP matrix very quickly. Had some shelf storage test samples
where MgAl particles showed almost no changes after 10 years. While the
uncoated Mg powder inside APCP grains had visible surface corrosion. By
comparison, samples using coated metal powders showed some minor surface
changes. NFPA regulations require APCP to have minimum 5 year shelf life, and
most uncoated metal powder passes (though not recommended as metal powder
corrosion in APCP matrix inhibits initial ignition).
3) Magnalium has lower combustion temperature than Mg (but higher than Al).
When using large levels of metal powder (>3-5%), MgAl alloy will provide a
lower chamber temperature and might reduce insulation requirements .vs. Mg.
Note-When using high levels of any metal, be aware that a motor needs
sufficient chamber volume and length to allow complete reaction of metal powder
into reacted molecules. Failure to have large enough space, will result in
metal slag collecting on nozzle and/or burning in plume (creating lower
combustion efficiency than expected).
4) Magnalium creates distinct “instability” to APCP combustion (.vs. use of
separate Mg and/or Al powders)
a- Most noticeable impact of Magnalium instability is change to acoustic
resonance during combustion, but can also be observed as pressure/thrust
“noise” depending on particle sizes. Do not have data handy, but my tests
with MgAl alloy powders generated a low frequency instability between
500-2500Hz. If test stand sensors react fast enough it is easy to spot a
potential MgAl alloy APCP motor test burn by uniform noise across entire thrust
curve.
After I learned how to control the acoustic effect created by MgAl alloy, I
used it to help create a louder “roar” in some of AMW color motors I developed
in 80’s. Furthermore, part of thunderous roar generated by my famous SkidMark
motors was due MgAl alloy powder instability. J
b- A secondary impact of Mg & Al in APCP is changes to exhaust plume color.
Again, I do not have data handy to quote exact color temperatures from
spectroscopy plots; but aluminum burns lower temp yellow white color, Magnesium
burns hot with a blue white color; while Mg & Al together burns cooler than Mg
with a more neutral white color. This property is extremely helpful when
adding metal needed for increased ISP, yet need to stop metal from washing out
color in special effects motor plume. Using a MgAl alloy makes most green
plumes a darker green, and exhibit less color washout at nozzle exit.
I am not a chemist/physicist , best I could figure out about instability was
this:
When Magnalium undergoes combustion inside APCP motor, it appears to have 2
phase transitions. First phase is conversion from solid to liquid, and if
chamber temperature is high enough - second phase is separation of Al and Mg
into plasma. Since Al and MG have different activation energies, specific
heat, and different melting points; it is the 2nd phase alloy breakdown that
creates instability. Based on some limited color temperature measurements on
nozzle less motors, I believe 2nd phase actually lowers chamber temperature in
a finite space when it happens, which creates pressure variations.
PS – Over the years since I first created AMW motors using MgAl, it’s use
diminished.
The new purveyors of my work have re-designed the retail motors to: be cheaper
(MgAl alloy is 2-3x cost of separate powders), have simplified mfg (similar but
not same instability can be created without MgAl), generate reduced color
density (Green is cool regardless of whether it is dark or light), and reduced
sound output (newer White Wolf motors are wimpy Chihuahua snarl compared to
original growling dogs that left my mixer).
Most difficult part of using Mg or MgAl (.vs. Al) is increase in chamber
temperatures, especially when using large amounts of metal (>3-5%). I can not
count how many times I have witnessed high metal APCP motors work first burn,
and then experience an overpressure casing failure the second burn. In past
folks did not realize how much heat the graphite nozzle transmits to Al casing
on commercial reload designs, and they cook casing during testing. If you
intend to build any motor with more 5% metal, be sure to fully insulate
graphite nozzle from casing, or use phenolic/ceramic nozzle material that
insulates combustion heat away from case.
Keep pointy end up!
Jim
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Vertical Limits
Sent: Wednesday, April 04, 2018 2:53 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Aluminium vs Magnalium
Hi list,
When making a fairly straight forward APCP with 70/8 AP/Al what would be the
benefit of exchanging the Al for Magnalium? I see some pretty big motors from
the CSXT and Qu8k using Magnalium. Or in general: what are the benefits of
adding magnesium to an all Al propellant?
For a short introduction:
After a intermezzo of about 15 years I rediscovered the arocket list and
subscribed again. Last year I changed from using sugar motors to APCP motors.
On <http://www.verticallimits.nl/> www.verticallimits.nl a recent static test
of a 10kNs APCP motor can be found. I will be flying this motor in 3 weeks’
time, all going well.
Regards,
VL