Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
CTI 75mm Motor Assembly Question
#1
I am assembling my first CTI 75mm motor (L-910 in a 3G case).  The instructions say to leave out the last grain spacer o-ring if it extends more than 1/3 out of the liner tube.  In my case it's about 2/3.  So when I leave it out and assemble the motor there is a slight space for the grains to move.

Is this OK, or should I take the motor apart and put the extra o-ring in?
NAR 98479 L3
Reply
#2
(11-15-2015, 10:12 AM)dreago Wrote: I am assembling my first CTI 75mm motor (L-910 in a 3G case).  The instructions say to leave out the last grain spacer o-ring if it extends more than 1/3 out of the liner tube.  In my case it's about 2/3.  So when I leave it out and assemble the motor there is a slight space for the grains to move.

Is this OK, or should I take the motor apart and put the extra o-ring in?


I've only flown the 75 mm M2245, and the top grain sat down about .25 inch from the end of the liner after assembling.  I couldn't see where a grain spacer was doing anything on the top grain so I left it out.  It flew just fine.
Reply
#3
(11-15-2015, 10:12 AM)dreago Wrote: I am assembling my first CTI 75mm motor (L-910 in a 3G case).  The instructions say to leave out the last grain spacer o-ring if it extends more than 1/3 out of the liner tube.  In my case it's about 2/3.  So when I leave it out and assemble the motor there is a slight space for the grains to move.

Is this OK, or should I take the motor apart and put the extra o-ring in?

Your last spacer o-ring extends 2/3 of it's diameter past the end of the liner tube. OK to leave this o-ring out and you may notice a slight rattle of the grains when the motor is fully assembled.               
                                                   
Make note of what the CTI 75mm instructions say about the forward case closure being flush or slightly depressed when tightened and also check the instrcutions for the correct orientation of the nozzle holder that contains the 2 o-rings. These are both important issues that I review in the instructions every time I assemble.
Tom Cohen

MDRA...Maryland
MARS...Geneseo N.Y.
Reply
#4
(11-15-2015, 05:37 PM)Tom C Wrote:
(11-15-2015, 10:12 AM)dreago Wrote: I am assembling my first CTI 75mm motor (L-910 in a 3G case).  The instructions say to leave out the last grain spacer o-ring if it extends more than 1/3 out of the liner tube.  In my case it's about 2/3.  So when I leave it out and assemble the motor there is a slight space for the grains to move.

Is this OK, or should I take the motor apart and put the extra o-ring in?

Your last spacer o-ring extends 2/3 of it's diameter past the end of the liner tube. OK to leave this o-ring out and you may notice a slight rattle of the grains when the motor is fully assembled.               
                                                   
Make note of what the CTI 75mm instructions say about the forward case closure being flush or slightly depressed when tightened and also check the instrcutions for the correct orientation of the nozzle holder that contains the 2 o-rings. These are both important issues that I review in the instructions every time I assemble.

I second this. If the instructions call for it, do it. Make sure you have everything right. I just assembled my first 75mm motor last weekend (also an L910), but I did so under the watchful eye of Robert DeHate so I could ask him whenever I had a question. In my case, the top o-ring spacer sat flush with the liner so I left it in.

In my experience, a little bit of rattle with the grains is fine. However my experience was with a 54 4grain K160 that had about 1/8"-3/16" of travel. The burn was perfectly fine.

NAR# 98194
Level 1: CTI I-216, 3,043'
Level 2: CTI K-740, 5,999'

Personal altitude record: 12,400' CTI L395
2014 total impulse: 9,018.2 Ns (76% M)
2015 total impulse: 7,171.7 Ns (40% M)
2016 total impulse: 18,664.2 Ns (91% N)
2017 total impulse: 8,281.1 Ns (80% M)
Reply
#5
Thanks MarkH and Tom.  That's what I figured but it's very reassuring to get first hand info.

Tom, after reading your note, I took the nozzle out just to make SURE I didn't reverse the nozzle holder.  In all the fiddling around during assembly, I managed to put in on backwards.  I think you just saved me a motor and possibly a rocket. 

Don
NAR 98479 L3
Reply
#6
The 75mm CTI M2245 that Mark H. describes requires the grains to be bonded (glued) into the motor liner.

From CTI...Pro75® motors requiring grain bonding:  K2000, L3200, M1300, M1545, M1675, M2020, M2080, M2245, M2250, M3100, M3700  The following motors require bonding of the grains in the liner: K2000/L3200 Vmax™, M1300 Imax™ Dual Thrust, M2250 C-Star™, M3100/M3700 White Thunder™, M2020/M2245 Imax™, M2080 Skidmark™, M1545 Green3™, and M1675 Pink™. Bonding the grains in the liner prevents premature blowout of the grains under high acceleration loads or by high core mass flows.

Dreago is using a 75mm L910 where there is no gluing of the grains.

Unrelated to Dreago's concern...
Assembly instructions from CTI do not differ between the motors needing bonding and those not needing bonding. I will ask CTI about the use of the last spacer o-ring in bonded-glued 75 mm motors.
Tom Cohen

MDRA...Maryland
MARS...Geneseo N.Y.
Reply
#7
Posted by Tom C.
How is the last (most forward) o-ring spacer installed in a bonded (glued grains in liner) CTI motor?

CTI's response by Jeroen posted on TRF and copied here....

 The forward spacer o-ring can be left out. Once the grains are bonded in, it has no purpose. Load the grains from the nozzle end as per instructions; leaving out the o-ring spacer between the insulator plate and the grain. Once all grains are bonded in and the nozzle is seated properly, the grains can be pushed down from the top of the liner to ensure all grains are seated properly. Leave the liner/grain/nozzle assembly upright to cure.

Jeroen
Tom Cohen

MDRA...Maryland
MARS...Geneseo N.Y.
Reply
#8
You don't get more definitive than that!



Later!

--Coop
Reply


Forum Jump:


Users browsing this thread: 1 Guest(s)