The most common problem users of RMS motors experience is ejection problems.
Usually, ejection problems can be attributed to user assembly error. This
article will address the three most common types of failures and some possible
causes.
First, let's discuss how the delay/ejection is supposed to work. When the
motor ignites, the bottom of the delay column also ignites. The delay column
burns from the bottom to the top and upon reaching the top, causes the black
powder ejection charge to fire. The delay column does not burn at a constant
rate; it burns faster when the motor is firing than it does after motor burnout.
Since the burn times of the three propellant types (Blue Thunder, White
Lightning and Black Jack) are different, a Medium delay for a Black Jack is NOT
a Medium delay for a White Lightning or Blue Thunder. DO NOT MIX DELAY COLUMNS
FROM DIFFERENT PROPELLANT TYPES! An I195 Black Jack Medium will be a 10 second
delay for the I195....it will be much, much longer on an I435 Blue Thunder
because the Blue Thunder burns much quicker. It is okay to exchange delays with
the same propellant type, i.e. an I284 Medium will also be a medium for an I211,
I161 and H123. They are all White Lightning's. It will NOT be a medium for a
Blue Thunder or a Black Jack motor.
Many rocketeers carry spare delay columns in their range box. They label them
short, medium or long. There is no such thing as a generic MEDIUM (or Short or
Long) delay! There is a White Lightning Medium, a Blue Thunder Medium and a
Black Jack Medium. Do not accept a delay from somebody unless you know the
propellant type the delay was expected to work with.
Motor Does Not Eject
There are only two ways a motor cannot eject at all....either the delay
column did not ignite or the black powder charge did not ignite. The most common
reason the delay column does not ignite is grease contaminating the bottom
surface of the delay column. How does this happen? Easy. You've just greased the
o-rings and threads. So, your thumb and forefinger (at least) are coated with
grease. Now, you use your greasy fingers to push the delay column into the delay
insulator. Result.....you just got grease on the bottom surface of the delay
column. The solution is easy; do NOT touch the bottom or top surface of the
delay column with greasy fingers.
Another failure mode is the delay column burns but does not ignite the black
powder. Again, this is usually caused by grease either on the top surface of the
delay column or grease in the touch hole. Grease on the top surface of the delay
column will extinguish the delay column without igniting the black powder. Also,
the black powder MUST be in contact with the delay column. The burning delay
column does not generate a lot of fire. Especially with 3F powder, it is
possible for the powder grains to block the touch hole without going through the
hole. Be sure the powder grains go through the touch hole and are in contact
with the delay element. Use a tooth pick or other non-metallic instrument to
insure the powder went through the hole. Also, if you don't fill the entire
cavity with black powder (which you won't with small diameter rockets), the
black powder can spill out of the touch hole if you turn the rocket over or even
lay it on it's side. If you don't fill the entire cavity with black powder, use
a piece of fireproof wadding on top of the powder (under the tape disc) to hold
the black powder IN the touch hole.
Finally, there is another failure mode that the user has no control over. If
the motor experiences a nozzle failure while firing, the sudden depressurization
and rapid temperature drop inside the motor case will frequently extinguish the
delay column, along with the unburned propellant slugs. In this case, barometric
or radio control deployment will be required to save the rocket (a good idea in
all cases).
Motor Ejects Early
There are only two ways that fire can reach the ejection charge; through the
delay column (the desired path) or through the o-ring(s) (very bad). First,
let's discuss the delay column route. Under normal circumstances, the delay
column burns from the bottom to the top. It must NOT burn from the outside
(around the circumference) inward. This would result in a very short delay.
However, when the motor is firing, the area between the delay column and the
delay insulator is bathed in fire. Yes, it's a tight fit, but the hot burning
gasses DO get between the delay column and the delay insulator. So, what keeps
the entire circumference of the delay column from igniting and burning inward?
The answer is the thin layer of waxed paper surrounding the delay column. For
the 54mm and 98mm motors, the instructions call for greasing the outside of the
delay column. This further prevents the circumference of the delay column from
igniting. For the 29mm and 38mm motors, the delay column fits very tight inside
the delay insulator. Sometimes, it's difficult to get the delay column inside
the insulator it's so tight. Sometimes, you have to squeeze the delay column and
use your fingernail to try to get it started. What can happen? If you tear the
paper coating just a little, the delay column will ignite where the paper was
torn and burn outside in when the hot gasses fill the gap between the insulator
and the element. This will cause a delay of 2 or 3 seconds rather than the
desired delay. MAKE SURE the white wax paper surrounding the delay element is
undamaged. MAKE SURE you don't accidentally peel back the paper while inserting
the element into the insulator. I push the delay element partially out the other
side of the insulator so I can inspect it and insure I didn't peel the paper
off.
Another reminder: do NOT mix delay columns from different propellant types. A
Blue Thunder Short delay (nominal 6 seconds) would be an extremely short delay
(probably around 2 seconds) with a Black Jack. This is because the much longer
burn time of the Black Jack keeps the case pressurized longer than the Blue
Thunder. Remember, the delay column burns faster when the case is under
pressure.
O-ring failure can also cause early ejection. It can also cause a blow-by.
O-ring failures are discussed in the next section (Blow-by).
Blow-By
A blow-by describes what happens when the forward closure does not contain
the motor thrust. In addition to coming out the nozzle, during a blow-by motor
thrust is also coming out the forward closure. This failure mode usually
destroys the forward closure and frequently destroys the rocket. The forward
closure does not make a very good nozzle. It melts instantly when thrust goes
through it. It is essential that thrust exit the motor ONLY through the nozzle.
Yet, there is a hole in the forward closure (the touch hole). What keeps the
motor thrust from exiting through the touch hole? The o-ring. The 29s and 38s
use one o-ring while the 54s and 98s use two (in addition to the backup ring).
As stated earlier, when the motor ignites the hot gasses fill the gap between
the delay column and the delay insulator. They also fill the gap between the
delay insulator and the forward closure. These hot gasses are blocked from
reaching the touch hole (and ejection charge) by the o-ring. Blow-bys are always
caused by o-ring problems.
Those of you who scuba dive may have first hand experience with o-ring
failures. Have you ever used a camera or underwater light while diving? They
both use o-rings to keep the water out. Have you ever had the unfortunate
experience of having a hair or a speck of sand on the o-ring? I have. The result
is flooding of your equipment. The o-ring cannot seal around a hair or speck of
sand, and as the pressure increases the o-ring lets water through. The o-ring
must be absolutely clean.
Same thing with rocket motors. ANY contamination on the o-ring will prevent
it from sealing and will allow the high pressure hot gasses to leak past the
o-ring. This will cause the ejection charge to fire instantly and may cause a
blow-by to develop as the o-ring degrades as the gasses leak past. Many
rocketeers assemble their motors in the field at a launch. They grease their
o-rings and then place them on a dirty table or accidentally drop them on the
ground. The greased o-ring easily picks up specks of dirt, grass, and other
contaminants which stick to the grease. It's very difficult to clean every
single speck of dirt off the o-ring, yet it is essential. Those of us with long
(and shedding) hair also have a problem with hair getting on the o-ring. Believe
me. ONE SPECK of dirt, or ONE HAIR on the o-ring can cause failure. You must
keep the o-rings absolutely clean.
Conclusion
In conclusion, this article does not claim to address every failure mode. In
particular, it does not address the most obvious failure mode to many users,
manufacturer error. Yes, the manufacturer makes mistakes. Occasionally, the
wrong delay element is included with the reload kit. However, the majority of
forward closure failures can be traced to user error. "I didn't do anything
wrong....it's Aerotech's fault" is usually the first thing you hear as a motor
fails and spews thrust from both ends of a burning rocket. Well.....maybe. There
might also be a tiny speck of dirt on the o-ring that the user didn't see.
Carefully following the reload instructions along with the suggestions in this
article will go a long way toward improving motor reliability.
