The Aerotech 98mm Reloadable Motor System (RMS) cases are unique amongst the
RMS line - they are machined differently than the smaller RMS cases. The stock
98mm aluminum pipes that arrive at the machinist vary in both inner diameter
(ID) and outer diameter (OD). In other words, neither the outside nor the inside
of the pipe are perfect circles. This variation also means that the wall
thickness of the pipe varies. This is not a problem as the thinnest part of the
pipe is well within the required tolerance for safety.
After the pipe is cut to the correct case length, it is externally machined
to a perfect circle. This is necessary to insure that the motor will fit
smoothly into the motor mount tube. Now, what about the inside of the case? It
is not necessary to machine the entire inside of the case. It is not a rifle
barrel...nothing passes through the case. The only thing that must fit smoothly
inside the case is the liner. The liner is sized to fit inside the case without
the need for machining, even at the point of smallest ID. However, the forward
and aft o-rings DO require a perfect circle to seal against. The o-rings are
sized to fit a circle of a certain radius. If the radius is a little too large,
the o-ring will still seal though it will be stretched. If the radius is too
small, however, there is excess o-ring material. This excess material will bunch
up and cause wrinkles in the o-ring, preventing it from sealing. In other words,
a 4 inch o-ring may still seal a 4 1/4 inch case (with a little stretching) but
it will NEVER seal a 3 3/4 inch case. You can stretch an o-ring a little, but
you cannot compress it without creating wrinkles. These wrinkles will prevent a
seal.
So, once the threads are cut into the case, the machinist mills a perfect
circle of the proper radius into the inside of the case just far enough to
accommodate the o-ring. It is pointless (and expensive) to machine into the case
any further than necessary. He only machines in as far as necessary, which is
just past where the o-ring will seal against the case. The reference for this
internal machining is the exterior of the case. Remember, the exterior has
already been machined to a perfect circle. The interior perfect circle is
created by machining the inside to create a constant wall thickness till just
past the point where the o-ring sits. At the point where machining stops, a
"lip" is left.
This lip is obvious about 1 1/2 inches inside the case. If the case was of
fairly constant thickness at that point, the lip will go all the way around. If
the case was thicker on one side than the other, the lip will be visible on one
side (where the excess material was cut away) but not visible on the other
(where there was no material to cut away). Many owners feel that there was some
sort of machining error when they look in their case and see this lip only going
half or three quarters of the way around. This is not true. Every case is
different because the wall thickness varies with each case, while still
remaining within tolerance. Whether the lip goes all the way around or not, it
doesn't make any difference. The lip is past the point where the o-ring seals
against the case. The owner needs to get used to the fact that the inside of his
case may not look symmetrical. That is okay.
Looking inside a 98mm case, the lip is clearly visible 1 3/8
inches from the end of the case.
The same case and the same side shown above, now rotated 180
degrees. Notice the lip fades out from the bottom, then completely disappears.
THIS IS OKAY.
A common misconception is that the o-ring rests on that lip, and if the lip
doesn't go all the way around, the o-ring will have nothing to rest against.
That is not the case. As the forward closure is tightened, the forward o-ring
slides into the groove on the forward closure. As the aft closure is tightened,
the o-ring slides into the groove on the nozzle insert. In neither case does it
"rest" on the machining lip. The lip does nothing to help the o-ring
seal....again, it is simply the point where internal machining stopped because
it was no longer necessary.
The most common question we get is: "Couldn't you guys buy stock aluminum
pipe of better quality? Couldn't you buy it with a more constant wall thickness
and better trueness?". The answer is, of course we could. How much more would
you be willing to pay? As the required tolerance ("trueness") increases, so does
the cost. We could buy "mil-spec" aluminum tubing. It would be much more
expensive and there would be no additional benefit, other than cosmetic. The
Aerotech reloads don't require "perfect" cases - they are designed to work with
cases of a lower tolerance, lower cost, and that are commonly available.
Similarly, the entire inside of the case could be machined. Again, it would
significantly raise the cost of the case while providing no additional benefit,
other than cosmetic. We think you would rather have a lower cost motor than pay
for cosmetic improvements that are hidden inside the motor.
So, fly your 98mm motors with confidence. They are powerful, reliable and
cost-effective.
