Aerotech 38mm nozzle fitment question

[Tom Flint]

Hello hive mind, hope you can help me. I have an older Aerotech H242 load. The nozzle extension that protrudes from the motor is considerably smaller in diameter than the nozzle hole in the aft closure I have. I'd say there's about a 3/32" annular gap around the nozzle extension. I cannot find a listing on aerotech's website or in any instructions stating that there is a different aft closure with a different nozzle hole size for this motor. If I push on the nozzle hard it will move slightly within the annular gap I mentioned previously. This doesn't seem correct.

 

[CoyoteNumber2]

The nozzle hole used to be smaller on all 38mm aft closures until a new reload circa 2000 with a larger nozzle (I don't remember which) required the hole to be bigger. All 38mm aft closures have been manufactured with the larger hole since.

Seems to me that as long as the O-rings are properly compressed, the assembly is fine. After all, nothing will be side-loading the nozzle in flight.

 

[CoyoteNumber2]

 

[3stoogesrocketry]

The new larger closure was made for the new "Blue Thunder" propellent that was developed in the late 90's . The forward seal disk was also made for the hotter loads .

 

[Sandy H.]

I thought I had both types of rear closures, but it seems that I only have the larger type. I measured the opening and it was 0.824".

Pics attached are of an H73 nozzle in bag, J350 nozzle in bag and the rear closure. I note that both the H73 and J350 seems to have a recess that would center the nozzle in the rear closure, regardless of the rear nozzle actual size, since the H73 is much smaller than the J350.

At the end of the day, it might be worth an email to Aerotech to get the real facts about compatibility if you have a few days. Maybe post the question to the 'Aerotech Open Thread?'

Sandy.

 

[Rocketjunkie]

The smaller nozzles work just fine in the larger hole rear closure. All new hardware has the large hole and the smaller loads still have the smaller nozzles. You will *not* have any problems using the small nozzle in the large hole closure. Yes, there will be a gap but the phenolic is strong enough to withstand the pressure. The J350 was introduced at LDRS XV in 1996 at Orangeburg. This was the first reload to use the larger nozzle and the first to use the 38/720 case.

 

[Rocketjunkie]

The OD of the large part of both nozzles are the same (1"). If you look at the instructions http://www.aerotech-rocketry.com/cu...MS_Instructions/38mm/38_240-360j_in_20063.pdf you will see in the aft end a fiber washer goes over the rear of the propellant grain, then a 3/16" thick O-ring and then the nozzle presses into the O-ring. This holds the nozzle in position and the rear closure seals against the O-ring. There is also a recess in the closure that fits around the nozzle shoulder. ALL the 38 mm motors use this rear nozzle mount system.

 

[Tom Flint]

Thanks everybody! It worked fine.

 

[Rocketjunkie]

 

[jimzcatz]

Thanks everybody! It worked fine.

Of course it did. It is also NOT a motor modification to drill out your small hole closures to the larger size. I have done so on over a dozen,per Gary R.