Using CTI 75mm motor spacers question.

BryRocket

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Hello,

I have a Gen3 75mm 4g CTI case and 2 spacers. I intend to fly a 2 grain reload first time around. This probably seems like a dumb question but I couldn’t find the answer (or confirmation) in the instructions so I’m just looking to firm up what I believe to be true and get some peace of mind.

Anyway, the spacers just go into the case immediately forward the forward closure and then screw in the forward retaining ring correct?

Also, is there any other consideration when using the spacers?

I know the instructions mention building it up in a particular order, omitting the last fwd grain spacing O ring, etc. It then mentions observing said O ring when you place the fwd seal disk. If I follow the instructions and just add the fwd seal disk, fwd closure and then the spacers and then the retaining ring, I really won’t be able to observe that O ring situation well. Any advice there?

Thanks a ton, Y’all!


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jsdemar

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My understanding is that the Pro75 case should only be used with one casing spacer. A 4-grain case with a 3-grain load, for example.
See end of this document: https://www.pro38.com/pdfs/Pro75_notes_V1.8.pdf

If you can fit the top oring under the seal disk, you should try. It will compress. Add a light bead of thick grease at the edge of the liner ID inside the top and it will help hold the oring there. Then put the seal disk on top.

Fwd closure goes in next, then the case spacer, then the retaining ring.
 

OverTheTop

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Worth checking, but I believe it is just illuminating a difference between the 75mm and the smaller motors. For the smaller motors they can use one or two spacers, but the first one must specifically be an XL spacer (if using the 6GXL casing) with a different length. So to use two spacers in say a 29mm 6GXL casing you would use one XL spacer and one standard spacer, to take it back to a 4-grain setup. It seems the 75mm only needs one type of spacer, even for the 6GXL casing.
 

jsdemar

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The 6XL 75 has 6 longer grains with a total extra length equal to a standard 75 grain. As you noted, that's not the situation with the smaller diameter cases.
 

madmax

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2 spacers max in a standard case. I'm sure somewhere in the archives of this site, Jeroen from CTI states 2 spacers used is fine. This was a pretty popular question when they first came out with the spacers. I have flown an L1115 in the 6 grain case with no issues. That being said I wouldn't recommend it for the simple reason of doing so makes motor assembly quite the chore. 75mm grains are about 5" long. With 2 spacers that puts the forward closure, insulator and o rings 10" down. Don't know about you but I don't have 10" fingers. It was difficult to build up the forward end of the motor and not have something shift out of place.

Also just thought I would point out that the 75mm 6gxl case should be labeled a 7G. It is a full grain longer than the 6G.

Yes, the spacers are in front of the closure then the ring holds them in.

It's been 8 years since I flew it so I can't help with the last question.
 

karlbaum

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I interpreted that as saying only a 6XL case can have 2 spacers. Best to check with CTI or an authorized dealer.

Here is the footnote :
A 6GXL case can be spaced to a 6G case with a regular spacer. Up to two spacers can be used.

There are two parts to the note. First is that the 6GXL case uses a regular size spacer to bring it down to 6G. Second is that up to two spacers can be used in a case.

The two spacer rule is common with all CTI motors as well as Aerotech.
 

Steve Shannon

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I interpreted that as saying only a 6XL case can have 2 spacers. Best to check with CTI or an authorized dealer.

That’s how I interpret it also, John. The footnote was meant to address a specific exception. Otherwise the sentence about using two spacers should have been included in the paragraph above, instead of the pretty specific sentence saying that the spacers could be used to allow using a reload in the next size case. Otherwise they should have said something like “spacers may be used to adjust a case to use a reload up to two grain sizes smaller.”
The directions are not as clearly written as they should be.


Steve Shannon
 

BryRocket

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My understanding is that the Pro75 case should only be used with one casing spacer. A 4-grain case with a 3-grain load, for example.
See end of this document: https://www.pro38.com/pdfs/Pro75_notes_V1.8.pdf

If you can fit the top oring under the seal disk, you should try. It will compress. Add a light bead of thick grease at the edge of the liner ID inside the top and it will help hold the oring there. Then put the seal disk on top.

Fwd closure goes in next, then the case spacer, then the retaining ring.

Thank you! This is exactly the document I was looking for.


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jsdemar

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Also just thought I would point out that the 75mm 6gxl case should be labeled a 7G. It is a full grain longer than the 6G.

No, 6XL is a better name because it holds 6 extra-long grains. There is no such thing as a 7-grain reload for that case. For a good reason, if you look at mass flux and erosion.
 

BryRocket

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Man, thank you all for the quick responses! I really appreciate it. I also agree with Madmax that using 2 spacers could be a bit difficult to build. I really intend to fly 3 and 4 grain reloads most of the time but the field I have access to (until September-ish) can be troublesome much over 7K from what I understand so I was hoping to fly it a little lower with the 2G reload.

I was also worried about the observation of the most forward and last grain spacer O ring but now that I’m visualizing that, I guess you make the determination to use it vs discard it after seeing how well it fits in the liner and right before inserting the fwd seal disk and BEFORE you slide the case over the liner, so you actually view that before it disappears into the depths of the case.

So, buy a 2G case or just use the 2 spacers??[emoji848]


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jimzcatz

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Just FYI, sort of off topic. There are no XL spacers for 29mm. They are all the same. I believe pro 38 are the only ones that use smaller XL spacer.
 

BryRocket

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Just FYI, sort of off topic. There are no XL spacers for 29mm. They are all the same. I believe pro 38 are the only ones that use smaller XL spacer.

I’ve been sitting here reading on this as well. Looks like:

29mm uses 1 kind of spacer and you can space a 6XL to a 6g w one spacer and down to a 5g with another.

38mm uses two spacer sizes. You can size a 6XL to a 6g by using 1XL spacer AND a regular spacer. Can then make a 6XL case a 5g. By using 2 regular spacers and an XL spacer.

54mm uses two sizes as well but you only need the XL spacer to go from a 6XL to a 6g and then another regular spacer to go to a 5g.

75 and up use 1 type of spacer.


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jsdemar

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I’ve been sitting here reading on this as well. Looks like:

29mm uses 1 kind of spacer and you can space a 6XL to a 6g w one spacer and down to a 5g with another.

38mm uses two spacer sizes. You can size a 6XL to a 6g by using 1XL spacer AND a regular spacer. Can then make a 6XL case a 5g. By using 2 regular spacers and an XL spacer.

54mm uses two sizes as well but you only need the XL spacer to go from a 6XL to a 6g and then another regular spacer to go to a 5g.

75 and up use 1 type of spacer.

That is the way I interpret the CTI documentation also. The one exception is that I have flown the 38mm 6XL case with a 6-grain load with only the single, smaller 6XL spacer.
 
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Zebedee

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Here is the footnote :
A 6GXL case can be spaced to a 6G case with a regular spacer. Up to two spacers can be used.

There are two parts to the note. First is that the 6GXL case uses a regular size spacer to bring it down to 6G. Second is that up to two spacers can be used in a case.

The two spacer rule is common with all CTI motors as well as Aerotech.

Yep.
 

BryRocket

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Hello y’all,

I have assembled my 2 grain motor into the 4 grain case. I followed all the directions exactly until the part about inserting the forward closure. I inserted the fwd closure and then pressed it in by tightening the retaining ring. I then removed the retaining ring and inserted each of the two spacers. I then inserted and tightened the retaining ring. Earlier in the directions it said to tighten the aft closure all the way and then back it off a half turn. When inserting the fwd retaining ring the directions say that the fwd retaining ring should be flush with the case and mentions that it’s more important that the fwd retaining ring be flush than the aft retaining ring. I feel like my fwd retaining ring is at least 1-1.5 turns away from being flush and I have the aft ring about 1 turn away from being flush. I use the wrench so I feel I have it quite tight.

Any thoughts on this? Does it look like my two retaining rings are in enough? Would you back the fwd off and tighten the aft?

IMG_3797.jpg
IMG_3799.jpg IMG_3800.jpg
 
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BryRocket

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Also, seeing as the aft retaining ring is also the thrust ring, wouldn’t it be more important to have the aft retaining ring snugged up to the case? I can’t figure out why it matters all that much which one is flush/snugged up more, etc.
 

BryRocket

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FWIW,

I have decided that the instruction note that I was concerned with...

NOTE:it is best to have the forward closure retaining ring flush or slightly submerged and the nozzle retaining ring protruding by a half turn or so, than vice versa. There is more tolerance for o-ring location at the nozzle end. There will always be some minor variation in the length of internal components due to manufacturing tolerances.

...is perhaps not that big of a deal when using spacers.

I believe they mention the importance of having the fwd retaining ring flush or recessed and the increase tolerance at the nozzle end because when using a full case (4 grains in a 4 grain case, etc) the fwd closure is so forward and they want to make sure that its o-rings are aft of the case threads. Because my fwd closure sits about midway down the case when using 2 grains and spacers, I don't believe it is important. Also, I'm starting to think that the spacers are made on the longer side because there is more tolerance for a slightly longer spacer vs too short of one.

I've taken the motor apart a few times and am confident its put together correctly. I have the aft retaining/thrust ring snug to the case and then the fwd retaining ring is probably 1.5 turns from being flush but its in there quite well.

Please correct me if I’m wrong.

IMG_3815.jpg
IMG_3814.jpg
 
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cerving

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You want to back out the nozzle retainer 1/2 or one turn, then make your forward retaining ring flush. Then, tighten the nozzle retainer. You'll be surprised that after your flight, the nozzle retainer that you though was really tight will loosen up a lot... most likely due to o-ring erosion.
 

BryRocket

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You want to back out the nozzle retainer 1/2 or one turn, then make your forward retaining ring flush. Then, tighten the nozzle retainer. You'll be surprised that after your flight, the nozzle retainer that you though was really tight will loosen up a lot... most likely due to o-ring erosion.

Thanks Cris. I ended up doing close to that. The fwd ring got pretty close to flush after. I don’t think I’m regarded by myself or otherwise as a strong person but I got it as hand tight as I could with the CTI wrench. Feeling good abt it now.
 
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