Phoenix
Page 39
Appendix B:About Rocket Motors
parachute or other recovery device. The
motors contain devices called a delay charge
and ejection charge to actuate the parachute at
the preset time. The length of the time delay is
indicated by another number in the motor des-
ignation.
Thus, an F80-
5 would have
a 5 second
delay time.
Phoenix is
radio con-
trolled, and
we just fly it
into a glide
whenever it
slows down.
No built in
delay time is
required. The motors are designated some-
thing like “F13-RC”, where the RC stands for
“radio control”. The lack of the delay and ejec-
tion charges is the reason you cannot use a
Phoenix motor in a conventional model rocket.
If you use a conventional model rocket motor
in Phoenix, the ejection charge will damage the
model.
W
E have included typical thrust-time
curves for some of the Phoenix motors.
The F13-RC is specifically designed for a low
cost per flight. It is also the reload kit to use for
initial flights. Its total impulse of 60 newton
seconds will launch a typical Phoenix to about
500 feet in altitude, about the same as you
would get from a typical hi-start launch. (Your
mileage may vary.) It has an initial thrust of 7
to 8 pounds. This gives an initial acceleration
of about 5 g’s to get the model up to flying
speed. The
thrust then
decreases to
about 2 pounds
(sustain thrust)
to give a nice
steady climb.
T
HE G12-RC is
designed for
high perfor-
mance flying
once your
Phoenix is
trimmed out,
and you have some experience flying it. It is
basically the same motor as the F13, but it has
a longer propellant charge to give a longer
burn time and more altitude. A typical Phoenix
will reach about a 1000 foot altitude on a G12.
The impulse on the G was selected to launch
Phoenix to about the highest altitude where
you can see it well enough to still fly it. Why
does the G12 have a lower thrust than the F13
even if it has more total impulse? Well, the both
the peak thrust and the sustain thrust are the
same on the two motors, but the G spends
much more time at the sustain level, so the
average thrust is lower.
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
Thrust (pounds)
Seconds
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
9
10
Thrust (pounds)
Seconds
F13-RC
G12-RC
