David's right. This is either user error or a reload issue, not a casing
defect. Contact AT either by email or phone. If it turns out to be a problem
with the reload they'll make good on it -- they have excellent warranty
Don't try to use the damaged case unless you want to launch your rocket in
several directions simultaneously. :)
First, I made a mistake, the engine was an H128, not H180. I know it
doesn't matter except the H128 burns a litte bit longer. I used the
"normal" about of grease I think. I did not notice the problem until
after I cleaned out the liner so I don't know what it looked like. I
used the standard igniter that came with the engine.
BTW, Dr. Rocket got back to me and for a small fee they will replace
Does the engine suppose to rattle when assemblied? I know some motors
do and it is acceptable. I think I remember this one did. If it is not
suppose to than I would suspect a liner defect. Either way I am sure
AT won't do anything now since the flight did return safely.
I believe I put it together properly using the right amount of grease,
etc. As I stated earlier I think it rattled a little when it was
together. I threw out the liner before I noticed the problem so I
can't tell for sure. I know AT is a great company but I don't think
they would do anything in this case.
Dr. Rocket got back to me and will replace the casing for a small fee.
I am mailing back the old one today.
Grains rattling is fine, as long as the o-rings are compressed. If
you put in the phenolic "washer" things and the o-rings, and could
feel the o-rings compressing as you closed the casing, then you are
A Bates-grain motor design, like most of the Aerotech motors, relies
on the ends of the propellant grains being exposed, in order to
provide a fairly even thrust curve. Since thrust depends on surface
area of propellant burning, the grain getting shorter as it burns from
both ends is supposed to compensate for the core getting larger as it
burns from the center. So, the grains must be free to move apart a
bit, in order for flame to propagate to their ends in addition to
their cores. Thus, they tend to rattle slightly in the small motors,
and sometimes quite a bit in the larger motors.
The motor's proper operation is due to the liner pushing, through the
discs at each end, in both directions to compress the o-rings at each
end. If the liner is cracked or otherwise damaged (or worse -- too
short), the o-rings won't provide the necessary seal, and you can get
flame and pressure escaping where they shouldn't do so. Similarly, a
cracked liner can allow flame to directly impinge upon the sides of
the casing, which can soften and bulge the casing, as you've seen....
Hope this helps,
- Rick "Motor geek" Dickinson
Nice response Rick. I think recently I heard about the J420's which
had a short liner causing problems. Once a motor is assemblied I would
think it is not easy to determine if the rattle is coming from the
grains themselves or the liner moving. Maybe if one was to measure the
liner length but who knows how long it suppose to be or who is going
to do that during a launch?
You don't get any rattling with AMW motors.
Also, you may notice that a J570 coughs near the end of the burn.
This is due to the propellant grain dropping down as they get shorter &
burning left over propellant.
It had a little resistance coming out but not usuall. I am use to it
because to make the NCR motor tubes to fit the AT RMS, I have to
remove one layer of paper in the NCR rockets which makes inserting and
removing an RMS a little bit of a tight fit.
Dunno if this is the same effect, but I've noticed on quite a few Pro38
flights that at the end of the burn, there is a distinct 'cough' (as you put
it) and a change in the smoke trail. I'm sure i've seen a Pro38 thrust curve
with a peak just before burnout, could this be the same sort of thing going
Weren't J570s also subject to an issue with grain compression under high
BTW I saw a J570 casing at the last launch I was at - makes the Pro38 6G
casing look a little on the short side! There are at least 2 J570s in
people's boxes over here, I hope I see one go!
UKRA 1345 L0
"Gravity assisted pieces of the rocket raining from the sky should be
avoided. It is also financially undesirable."
-Portland State Aerospace Society