Re: Dr Rocket Casing has a bulge


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
service.
Don't try to use the damaged case unless you want to launch your rocket in
several directions simultaneously. :)
Reply to
RayDunakin
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That thing's a paperweight now. Roy's on the money.
BTW, wattsamattu, Joe?! Couldn't come over and see me last weekend?
Reply to
Gene
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 the casing.
Joe C.
Reply to
Joe C
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.
Joe C.
Reply to
Joe C
I think the motor rattled a little when assemblied. For some reloads this seems to be OK. For an H128 I don't know. Sounds like a good topic for the FAQ section.
Joe C.
Reply to
Joe C
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.
Joe C.
Reply to
Joe C
I was there Thursday and Friday only. I saw Dennis and others, but not you. Were you hiding? I didn't see any signs and flashing search lights so I assume you weren't there. 8-)
Joe C.
Reply to
Joe C
Why for a fee? I thought they warranted these things?
Unless, of course, the fee is just a token shipping charge....
- Rick "Disappointed that they would charge" Dickinson
Reply to
Rick Dickinson
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 fine.
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
Reply to
Rick Dickinson
They charge $10. I assume for shipping.
Joe C.
Reply to
Joe C
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?
Joe C.
Reply to
Joe C
The grains may rattle, and that's ok. If the liner is too short, that would be a problem.
Reply to
RayDunakin
If it is warranty work it is overpriced.
If it is consumer repoonsibility, it is underpriced.
Sounds like a "compromise" to me.
Jerry
Reply to
Jerry Irvine
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.
JD
Reply to
JDcluster
quick question....just HOW did you get that case to come out?
Double check your forte's motor mount.
Reply to
tater schuld
It is normal for AT reloadable motors to rattle after being assembled. That is neither a good nor bad sign in this case.
-- David
Reply to
David
I remember I felt normal resistance when screwing in the aft closure so I think the liner was long enough.
Joe C.
Reply to
Joe C
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.
Joe C.
Reply to
Joe C
I was there Friday through Tuesday.
From Joe Cool?! Ya kiddin' me??!!
You didn't hear the sirens, JC? THEY were for me..... :-)>
Reply to
Gene
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 on?
Weren't J570s also subject to an issue with grain compression under high acceleration?
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!
-- Niall Oswald ========= UKRA 1345 L0 EARS 1151 MARS
"Gravity assisted pieces of the rocket raining from the sky should be avoided. It is also financially undesirable." -Portland State Aerospace Society
Reply to
Niall Oswald

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