The final step in the assembly of a reloadable motor (not including
installing the ejection charge) is to tighten the aft closure. The instructions
remind you that it is normal to have a small gap between the aft closure and the
case. Just how small a gap is "a small gap" and is having any gap at all
something to worry about?
To answer the second question first, it is nothing to worry about. In fact,
it is positive confirmation that the o-rings are compressed. Let's think about
what is happening during the final tightening of the aft closure. The o-ring
first seats itself into it's groove on the nozzle insert. The resistance you
feel as you continue to tighten the closure is the o-ring being compressed and
expanding outward to seal against the case. This seal prevents the hot gasses
from escaping through the threads. Once the o-ring is compressed, it will do
it's job and further tightening is not necessary. DO NOT USE TOOLS TO TIGHTEN
THE CLOSURE. It is possible to damage the o-ring with extreme tightening
pressure. Even though the o-ring is greased (at least it should be) the closure
could tear the o-ring if excessive pressure is used. Remember, the closure is
turning but the o-ring is not. The grease keeps the o-ring slippery so the
closure doesn't grab and tear the o-ring as it turns and tightens against
it.
Now that you've got the o-ring tightly compressed, there may still be a gap
between the aft closure and the case. Customers have called and asked about
gasses escaping through this gap between the case and closure. That cannot
happen. If you felt the o-ring compress when you tightened the aft closure,
there will be no leakage. The limiting factor on how large the gap can be
between the case and closure is thread engagement. Any threads you can see in
the gap are obviously not contributing to holding the closure in place. Aerotech
does not publish a "minimum threads required to be engaged" value. However, on
the 98mm aft closure there is about a 1/8 inch separation between the top thread
and the knurled part of the aft closure. This means you could have a 1/8 inch
gap between the closure and the case and still have all the threads engaged.
This would perform perfectly.
WHAT YOU ARE AFTER IS O-RING COMPRESSION AND THREAD ENGAGEMENT. IF YOU HAVE
THESE TWO ITEMS, THE MOTOR WILL PERFORM. I also suspect that even if you see up
to 90 degrees of A thread in the gap, the motor will perform fine. For safety
reasons, there are considerably more threads than required to hold the closure
in place.
Why does the gap happen? The gap is caused by the liner being a fraction too
long. All liners are not exactly equal, and a liner that is a fraction too long
will function perfectly. However, imagine a liner that is too short. The
closures would tighten all the way to the case without compressing the o-rings.
Without compressed o-rings, gasses would escape through the threads and the
motor would fail. So, since a little long is okay and a little short is a motor
failure, all the liner tolerance is on the looooonnnnnng side.
Finally, this gap between the aft closure and the case is not always present.
It is rarely seen on 29mm and 38mm motors. It is seen more often on 54mm and
98mm motors. If the closure screws all the way in and butts up against the case
AND you felt the o-rings compress, great. If the closure easily screws all the
way in and you never felt any resistance from the o-ring compressing, you may
want to check your liner length. You may have one of the extremely rare cases
where the liner is too short. Do not fly a motor without compressed o-rings. It
WILL fail.
The liner lengths for the three 54mm cases are as follows:
54/852............6.950 inches
54/1280........10.280 inches
54/1706........13.610 inches
The liner lengths for the four 98mm cases are as follows:
98/2560............6.625 inches
98/5120..........12.688 inches
98/7680..........18.750 inches
98/10240........24.813 inches
