No 4-grain motors with CTI 6XL casing and spacers?

[billdz]

I've been using a CTI Pro38 6XL casing, which with spacers allows the use of 4G to 6XL reloads. I just bought a Pro54 6XL and assumed it worked the same way, but apparently it does not. According to the instructions (https://pro38.com/pdfs/Pro54_Case_Spacer.pdf), only two spacers (one regular and one XL spacer) may be used with the Pro54 6XL case. If I understand this correctly, this means that only 6XL, 6, and 5 grain reloads may be used with this case. So I'll have to buy a separate case for my 4G 54mm reloads, or sell them? Why can't we use one XL and 2 regular spacers with a 4G 54mm reload, as we do with 4G 38mm reloads? I'm sure there's a reason but the instructions don't say.
Thanks,
Bill

 

[DavidMcCann]

As I've heard, theres a limit of 2 spacers with ALL cases. reason is that each spacer increases the the possible error in size/ spacing . if each one is cut to +/- .XXX", each added spacer increases the possible error. Apparently that error becomes unacceptable at 2 spacers.

 

[woferry]

4G in a 6GXL has never been OK. The spacer system lets you cover two ranges (which sometimes only takes 2 spacers, sometimes 3), so a 6GXL can do 6G and 5G, but not 4G. See the drawings on CTI's website, pro38.com (choose the size, go to "hardware", and find the spacer instructions), the second page generally covers the 6GXL cases/spacers.

Turns out the 29/38/54 instructions can all be found on one page, here. It shows that for 29mm there is no XL spacer, 6GXL is basically just 7G. For 38mm you use the XL spacer along with a standard spacer to go from 6GXL to 6G, and XL +2 standard spacers to get to 5G (but you can never use more than 2 standard spacers while following CTI's instructions). For 54mm you just use the XL spacer to go from 6GXL to 6G, and are allowed to use one standard spacer with the XL spacer to get to 5G.

 

[Titan II]

As I've heard, theres a limit of 2 spacers with ALL cases. reason is that each spacer increases the the possible error in size/ spacing . if each one is cut to +/- .XXX", each added spacer increases the possible error. Apparently that error becomes unacceptable at 2 spacers.

You can use three spacers (2 regular and 1 XL) in a 38mm 6XL case.

 

[billdz]

You can use three spacers (2 regular and 1 XL) in a 38mm 6XL case.

But not in a 54mm 6XL case.

 

[RainierWolfcastle]

You can use three spacers (2 regular and 1 XL) in a 38mm 6XL case.

But not in a 54mm 6XL case.

That still only gets you to a 5G load in either case, so any case with spacers can fly it's own load, one size down or two sizes down (6XL, 6 and 5).

To fly every size in 29-54 you need either:

-6XL, 6 and 3 grain cases
-6XL, 5 and 3 grain cases or
-6XL, 4 and 2.

First option would be common after getting a starter set and adding a 6XL.

 

[billdz]

That still only gets you to a 5G load in either case, so any case with spacers can fly it's own load, one size down or two sizes down (6XL, 6 and 5).

That's incorrect, you can fly a 4G with a 38mm 6XL case (2 regular spacers and XL spacer).

 

[DavidMcCann]

You can use three spacers (2 regular and 1 XL) in a 38mm 6XL case.

https://www.pro38.com/pdfs/Pro38_Case_Spacer.pdf

says up to two spacers.

There was a rule someplace on this, I'm having a hard time finding it for some reason.

 

[DavidMcCann]

NAR lists spacer limits here- https://www.nar.org/wp-content/uploads/2014/08/CertifiedReloadableMotorSpacers.pdf

 

[RainierWolfcastle]

That's incorrect, you can fly a 4G with a 38mm 6XL case (2 regular spacers and XL spacer).

No, you need a 38xl spacer plus a normal spacer to get to 6g, then another spacer to get to 5g. See the 38mm spacer instructions on the cti website


Sent from my SM-G950F using Tapatalk

 

[Eric]

That's incorrect, you can fly a 4G with a 38mm 6XL case (2 regular spacers and XL spacer).

That is incorrect. I flew A J270G for my level 2 cert. 5 grain motor. Used 2 regular and 1 XL spacer. A 38mm 6XL case can not legally fly a 4 grain motor.

 

[billdz]

No, you need a 38xl spacer plus a normal spacer to get to 6g, then another spacer to get to 5g. See the 38mm spacer instructions on the cti website

You're right, I stand corrected, thanks for pointing that out. Both 38mm and 54mm 6XL case can only do down to 5G, just different configurations to get there.
38mm 6XL for 5G = XL spacer plus two regular spacers
38mm 6XL for 6G = XL spacer plus one regular spacer
54mm 6XL for 5G = XL spacer plus one regular spacer
54mm 6XL for 6G = XL spacer alone (no regular spacer used)

 

[DavidMcCann]

You're right, I stand corrected, thanks for pointing that out. Both 38mm and 54mm 6XL case can only do down to 5G, just different configurations to get there.
38mm 6XL for 5G = XL spacer plus two regular spacers
38mm 6XL for 6G = XL spacer plus one regular spacer
54mm 6XL for 5G = XL spacer plus one regular spacer
54mm 6XL for 6G = XL spacer alone (no regular spacer used)

That would be three spacers, which isn't permitted. The CTI drawings show a 6XL set up that way, but the notes all clearly say only 2 spacers, and that regular may be used in 2G-6G cases, and that a 6XL may be spaced to a 6 by using a reg and an XL spacer. NAR documents back this up, and I know TRA has the rule, but I'm having trouble finding it in print.

 

[billdz]

NAR lists spacer limits here- https://www.nar.org/wp-content/uploads/2014/08/CertifiedReloadableMotorSpacers.pdf

The 38mm XL spacer does not count as a "spacer"? The NAR documents says maximum of 2 spacers in CTI. But CTI says OK to use one XL spacer and 2 regular spacers (total of 3) to fly a 5G. See https://www.pro38.com/pdfs/Pro38_Case_Spacer.pdf, picture on p.2.

 

[billdz]

That would be three spacers, which isn't permitted. The CTI drawings show a 6XL set up that way, but the notes all clearly say only 2 spacers, and that regular may be used in 2G-6G cases, and that a 6XL may be spaced to a 6 by using a reg and an XL spacer. NAR documents back this up, and I know TRA has the rule, but I'm having trouble finding it in print.

Oh, you sent this at the same moment I sent post #14, that's very interesting that the drawing is incorrect. So you can only fly 6XL and 6 with the 38mm 6XL case. I did not know that and have flown at least one 5G in a 6XL case.

 

[DavidMcCann]

Oh, you sent this at the same moment I sent post #14, that's very interesting that the drawing is incorrect. So you can only fly 6XL and 6 with the 38mm 6XL case.

Yea, it's weird... I'm guessing the line drawings were done prior to testing and the ruling that 2 was the limit. I've seen conflicting opinions from people as to why it was set at 2. With some people involved in testing saying it added too much weight and affected motor performance with all the dead weight metal. I don't buy that, as it's east to account for that in sims properly. I've seen others say it's tolerance stacking. That I do buy. Too many cuts and it becomes questionable. I'd say they should just make bigger spacers, but then you're basically at the point you should just get another case for the cost. In a 2014 thread on the topic Reese claimed in longer cases it was too hard to clean and inspect that far down a long case to assure you'd get a good o-ring seal. Krech made the claim the cases became too heavy and it affected thrust and performance.

 

[billdz]

On further review, if David is correct (I don't doubt him), it's not just the drawing that is incorrect. The instructions say:
**
1) Drop the regular spacer(s) into motor case with the rounded end towards the rounded end of the motor.
2) Drop an XL spacer into the motor case.
**
The "(s)" in step one suggests it is OK to use more than one regular spacer, and apparently it is not OK to do so.

 

[DavidMcCann]

On further review, if David is correct (I don't doubt him), it's not just the drawing that is incorrect. The instructions say:
**
1) Drop the regular spacer(s) into motor case with the rounded end towards the rounded end of the motor.
2) Drop an XL spacer into the motor case.
**
The "(s)" in step one suggests it is OK to use more than one regular spacer, and apparently it is not OK to do so.

Ahah, it does. I read the first page and didn't read the second close enough. I do recall several debates on this over the years... most coming down to "is the XL spacer, a spacer?" and "I'll fly what I want to" I've got no dog in the fight.... But find it an interesting topic of discussion on a purely rule-based discussion. In reality, I don't think using 3 spacers is going to blow anything up or spin a rocket out of control.

 

[woferry]

Of course the page 2 instructions also say:

(1) Drop the regular spacer(s) into motor case with the rounded end towards the rounded end of the motor.
(2) Drop an XL spacer into the motor case.

So it's pretty clear to me that they meant what they pictured, two standard plus one XL spacer, and the limit on the first page (the non-XL page) is referring to 2 standard spacers max. Otherwise they'd have made the XL spacer the length of the existing XL plus a standard, so that 5G-in-6GXL would have been a single XL spacer and a standard spacer, just like the Pro54 cases. This is just the cheaper solution as the XL spacer uses less material.

The standard spacers have a rounded side, so there's clearly going to be a bit more slop when you butt the two standard spacers together (flat side against a rounded side), but the XL spacer does not have a rounded side, it's flat on both ends, so you put a flat edge against a flat edge.

 

[Titan II]

Regarding the use of 54mm spacers. This is from the Tripoli website and something to think about....but only at a Tripoli research launch or other venue outside of Tripoli or NAR sanctions.

10. I want to ‘kit-bash’ some commercial motor parts to create a ‘custom’ motor. Is this allowed at a TRL?

Yes, this is considered a Research motor. However, this is not allowed at non- TRLs since the motor is not certified.

 

[DavidMcCann]

Of course the page 2 instructions also say:

(1) Drop the regular spacer(s) into motor case with the rounded end towards the rounded end of the motor.
(2) Drop an XL spacer into the motor case.

So it's pretty clear to me that they meant what they pictured, two standard plus one XL spacer, and the limit on the first page (the non-XL page) is referring to 2 standard spacers max. Otherwise they'd have made the XL spacer the length of the existing XL plus a standard, so that 5G-in-6GXL would have been a single XL spacer and a standard spacer, just like the Pro54 cases. This is just the cheaper solution as the XL spacer uses less material.

The standard spacers have a rounded side, so there's clearly going to be a bit more slop when you butt the two standard spacers together (flat side against a rounded side), but the XL spacer does not have a rounded side, it's flat on both ends, so you put a flat edge against a flat edge.

Of course, there is no such ambiguity on NAR's list, which clearly limits it at 2. CTI's directions don't override NAR or TRA, which have both only certified the use of 2 spacers. Unless an XL spacer, isn't a spacer? I believe AT got past this by making the XL spacer on 29mm RAS's a single longer spacer the length of a regular spacer and the XL distance, so you'd still only be using 2 actual spacers.

 

[Titan II]

Yea, it's weird... I'm guessing the line drawings were done prior to testing and the ruling that 2 was the limit. I've seen conflicting opinions from people as to why it was set at 2. With some people involved in testing saying it added too much weight and affected motor performance with all the dead weight metal. I don't buy that, as it's east to account for that in sims properly. I've seen others say it's tolerance stacking. That I do buy. Too many cuts and it becomes questionable. I'd say they should just make bigger spacers, but then you're basically at the point you should just get another case for the cost. In a 2014 thread on the topic Reese claimed in longer cases it was too hard to clean and inspect that far down a long case to assure you'd get a good o-ring seal. Krech made the claim the cases became too heavy and it affected thrust and performance.

Here is the thread.

https://www.rocketryforum.com/showthread.php?120547-cesaroni-spacer-purpose

The claims by Krech and Reese have no merit. The one by Krech was discussed by Cook. Reese also claimed that the "spacer pushes the forward closure down into the casing, requiring the forward o-ring to seal to the case wall wherever it stops. It can be difficult to inspect the middle section of very long casings to make sure the wall is clean enough for the o-ring to seal against". The CTI 38 forward closure does not have an O-ring that seals against the case.

 

[3stoogesrocketry]

I was always under the assumption ( was mentioned on this fourm ) that only 2 spacers could be used simply because TMT only tested motors with up to two spacers .
Manufacturing variance is a unexeptible answer to me as I am a machinist . If a CNC lathe cannot be accurate to withing +/- .005 ( and that is a huge ballpark ) then a new facility should be choosen. Also lets do some math. Say you want to fly a 4 grain in a 6xl case . Lets put all the tolorances at the max of +.005 . Thats +.015 , or 3 sheets of notebook paper . This is not enough to damage / affect the performance of the case . How many people have flown a brand A 98mm motor where the rear clouser has a 1/8 inch of threads sticking out still ? I know I have , and it states in the directions this is a possability .

Oh , and I have flown a K2045 in a 6xl case with zero issues .

Eric