J420 1/4" gap in grains

I put together a J420R 38mm 6grain AT RMS this evening getting ready for
tomorrow's launch.
While I have seen many engines have a gap in the grains to the end closure,
that allow them to move back and forth in the case, this gap was exactly
1/4", or 8/32".
The liner is sized 1/4" longer then the grains are long, and yes, the seal
disk is in place.
I have a single use Ellis J220 that even has grain movement back and forth
inside the sealed case, but is 1/4" normal ?
since everything is sealed, I can't see any reason a CATO would occur.
just wondering.
Reply to
almax
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I've assembled RMS motors before where there seemed to be definite "end play" of the propellant within the liner, and the firing was normal... I don't know about 1/4" but I think some endplay is a common condition that doesn't cause trouble. The more critical bit would be proper liner length to get the proper squeeze on the main o-rings. (I suspect the propellant slugs might be "toleranced" on the short side, to avoid an "interference fit" situation where the grains, rather than the o-rings + liner, were the first parts to come under compression as the closures were tightened. That could be as bad as a too-short liner...)
1/4 inch would be equivalent to 1 mm per grain short in a 6 grain assembly. I doubt AT's saw cutting system holds much tighter than a 1mm range, so the spec is probably something like +0.00mm/-1.00mm relative to the "nominal maximum" length that exactly fills the "nominal minimum length" liner. If all the grains were from a slightly short batch, the gap would be most noticeable in a motor with more grains.
(JMHO, YMMV, IANAAE, etc.)
-dave w
Reply to
David Weinshenker
Take it from an "AT critic". Their grain cutting tolerance is well under 0.5mm.
Their liner cutting tolerance sucks hard however :)
Reply to
Jerry Irvine
Since Bates grains burn on both ends anyway, I can't see any reason why a bit more play between them would affect burn rate one way or the other. You should be fine as long as all the seals compress properly. You don't want to blow a seal....
- Rick "Call me Marlin" Dickinson
Reply to
Rick Dickinson
I know that when Pete Davy gave an impromptu AT 'masterclass' at the last EARS launch (given that AT is not available over here, and all thats left is what people bought before), he did say that sometimes you'll hear the grains move after you've assembled the motor.
As other people have said, doesn't the bates grain burn on all surfaces?
-- Niall Oswald ========= UKRA 1345 L0 EARS 1151
Reply to
Niall Oswald
Both ends and the core. (The outside is supposed to be inhibited by the casting tube.)
Burning the ends shortens the grain, which offsets the increase of area as the core expands, so the thrust is more nearly constant during the run. A core-burning-only grain layout gives a much more progressive thrust curve, increasing from ignition toward burnout.
-dave w
Reply to
David Weinshenker
Yes, this is true... I have burned 240 loads in a 38mm/480 case with no problems. THE LINER HAS TO BE THE CORRECT SIZE FOR THE CASE though. Once the motor pressurizes(sp) I don't think it's very relavent how much space the propellant occupies in the case. I have used less propellant grains than what an alloted case size asks for in both 29mm and 38mm cases, and have never had any problems. Performance doesn't suffer either. (Save for the heavier bigger case size.) Call me a cynic, but all the different case sizes has more to do with selling cases or certification issues than anything else. (Please correct me if I am wrong.)
Oh, and pardon the bad spelling in my post... Daniel
Reply to
Daniel Franklin
About the only issue is that, if there is a *lot* of extra space to pressurize, you may have initial chuffing problems bringing the motor up to pressure, and you may lose a little bit of total impulse, as some of the former exhaust gasses get put to work pressurizing the larger volume before usable thrust starts.
For a worst-case scenario, think of using a single 38mm White Lightning grain in the J570 (38/1080) casing, which normally holds 9 grains. Instead of the expected G61 motor, you'd probably see a G30 or thereabouts, as much of the initial thrust spike is "used up" pressurizing that big long casing.
Come to think of it, I doubt I'd trust delays to get reliably lit from that far away.... That may be the *real* reason we haven't seen HPR motors with spacers to fit larger casings.
I'd still like to see certified HPR motors that share casings, like the 24/40 and 29/40-120 "hobby" casings do. Of course, with less volume to pressurize, the delay lighting and pressure buildup issues are probably minimal in those smaller sizes. I'd be willing to use plugged closures and electronic ejection, just to get the additional variety, in any case.
- Rick "Motor musings" Dickinson
Reply to
Rick Dickinson
The Syner-Jet reloadable motors used spacers which were positioned forward of the closure, so that there was no excess space to pressurize and no problem lighting the delay. It was a pretty nifty system.
Reply to
RayDunakin
I'm having trouble visualizing this.
If you have a spacer forward of the (aft, I assume) closure, is the spacer a hollow tube (similar to those used in the D9 load for the 24/40 motor)? If so, how does it ensure there is no excess space to pressurize?
If the spacer is solid (filling the empty space), then how does the flame ever reach the delay element?
About all I can imagine as a compromise is a solid spacer with a central hole the size of the motor core running through it lengthwise. I'd still worry about the flames lighting the delay train, especially since it'd be at the extreme end of a narrow tunnel.... Also, weight might be an issue in larger motors.
Perhaps a spool-shaped spacer, constructed like a motor tube with two centering rings, would eliminate both the pressurization and the weight concerns.
Interesting mental excercise, in any case.
- Rick "mental motor model manipulator" Dickinson
Reply to
Rick Dickinson
Now *that's* clever! Simply push the forward closure further into the forward end of the casing, and secure it with the same clip you usually use. Excellent case of "thinking outside the box", there!
- Rick "impressed" Dickinson
Reply to
Rick Dickinson
odd, my h242's do. i I think the I's we have used also did. there was a gap like the liner was too long. worked fine tho....
Reply to
tater schuld
As I said earlier, a lot of 'em seem to have a bit of axial play of the grains inside the liner - the critical thing is for the liner to be long enough to put the o-rings in compression by the time the closure flanges seat on the ends of the case.
-dave w
Reply to
David Weinshenker

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