Looking at CTI hardware, what is the limit on the number of spacers?

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CrazyOB

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I am looking at getting CTI 29mm hardware for my L1. The CTI 3 grain case Certification Special is a little tough to get from Apogee. If I get a 'free' 4 grain case can I still fly 1 grain motors in it?

Am I limited to the number of spacers like Aerotech?:sad:

Thanks in advance.
 
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You can only use 2 spacers in a case. For a 4 grain case you could use 0, for a 4 grain motor, 1 for a 3 grain motor, or 2 for a 2 grain motor, but you could NOT use 3 spacers and fly a 1 grain motor.

Dave A said:
Just curious but I wonder why that is limited to 2 spacers other than to sell more cases?
The case does not know the difference how many are in there.
Click to expand...

Agreed, but I don't know anyone other than the manufacturers who have the test equipment necessary to certify 'more than 2 spacers' for flight with more than the 'recommended' number of spacers legally (for a club launch) at anything other than a TRA experimental launch.

Until then, it's MAX 2 per case for me. YMMV. Motors are cheap, rockets are not.
 
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Thanks peoples. I'll have to look at the 2 grain reloads and see if they are of interest to me. Then I can order the four grain case, or wait until the 3 grain casing is back in stock.
 
Apogee lists the 29mm starter set as in stock, it includes the 3g case, end closure spacers and a delay drilling tool for $26 more than the bare 3g case. then get a 5g case for your cert special and you will have a range of options.
Remember with the 29mm cert special you still need to buy an aft enclosure and ideally a delay drill.
After you cert you can then buy a 6xl case which takes a normal spacer for 6g loads.
 
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Be careful about counting XL spacers. In some sizes, the XL spacer is to be added to a regular spacer and then counts as one spacer for the two pieces. In other sizes, the XL spacer is used on its own and counts as one spacer by itself.

In still other sizes, the XL grain is simply a seventh grain and no separate part is sold.
 
The 2 space limit has nothing to do with the manufacturers. It was mandated by the certification authorities: NAR S&T, TRA TMT and CAR MTC for safety reasons.

Motors are certified to provide a certain level of performance in a rocket. If you increase the weight of the motor but you do not add propellant, the Thrust to Weight ratio of the motor drops. For example with 38 mm CTI motor casings, the extra dead weight of the spacer and extra casing length is 50 grams per spacer or 100 grams for 2 spacers or 3.5 oz. For a 1G load, the loaded motor weight increases by 50% from approximately 200 grams in a 1G casing to 300 grams in the 3 G casing. As most rocket that would use a reloadable casing have a minimum weight equal to the loaded weight of a 1 G motor, so by limiting the maximum number of spacers to 2, a maximum weight increase of ~25% would be expected and that would reduce the apogee by a maximum of 20% if no delay adjustment is accounted for, and a typical recovery system should be able to handle the g-shock from a 2-3 second late deployment. The certification authorities were uncomfortable to allow a greater decrease in performance.

Bob Krech, NAR S&T
 
People should be able to account for the weight difference, no different than adding a payload.
 
bobkrech said:
The 2 space limit has nothing to do with the manufacturers. It was mandated by the certification authorities: NAR S&T, TRA TMT and CAR MTC for safety reasons.

Motors are certified to provide a certain level of performance in a rocket. If you increase the weight of the motor but you do not add propellant, the Thrust to Weight ratio of the motor drops. For example with 38 mm CTI motor casings, the extra dead weight of the spacer and extra casing length is 50 grams per spacer or 100 grams for 2 spacers or 3.5 oz. For a 1G load, the loaded motor weight increases by 50% from approximately 200 grams in a 1G casing to 300 grams in the 3 G casing. As most rocket that would use a reloadable casing have a minimum weight equal to the loaded weight of a 1 G motor, so by limiting the maximum number of spacers to 2, a maximum weight increase of ~25% would be expected and that would reduce the apogee by a maximum of 20% if no delay adjustment is accounted for, and a typical recovery system should be able to handle the g-shock from a 2-3 second late deployment. The certification authorities were uncomfortable to allow a greater decrease in performance.

Bob Krech, NAR S&T
Click to expand...

That totally makes sense. Even without the limit, I kind of figured there would be diminishing returns with too many spacers.
 
Given calculations for a new thrust to weight ratio, seems like (aside from regulatory standards) using 3 spacers is ehOK? Of course you might spend more money on spacers than just buying a new case =D?
 
Crooksz_Rocketz said:
Given calculations for a new thrust to weight ratio, seems like (aside from regulatory standards) using 3 spacers is ehOK? Of course you might spend more money on spacers than just buying a new case =D?
Click to expand...
You can't launch an uncertified motor/reload at a NAR or TRA launch, and the motor/reload is not certified if you use more than 2 spacers, so no, its not ok.

Bob
 
bobkrech said:
You can't launch an uncertified motor/reload at a NAR or TRA launch, and the motor/reload is not certified if you use more than 2 spacers, so no, its not ok.

Bob
Click to expand...

Rules aside, you can't seriously tell me there's a real reason you can't use 3 spacers in a 6 grain case to fire off a 3 grain motor..... the few oz in a say, 53 oz rocket are not a big deal.
 
From an engineering standpoint I can tell you there may be an issue with tolerance stack up. Basically nothing is perfect so each spacer and case has a limit to how far off the nominal dimension it can be without being rejected as a bad part. If you happen to get a reload and spacers on the short side and a case on the long side, or the other way around the maximum mismatch between reload/spacer length and case length is the sum of the tolerances, the more spacers the larger the potential mismatch. There is very likely a maximum mismatch tolerance beyond which the possibility of cato increases rapidly.
 
Precision tolerances was exactly my understanding. I believe it, too, seeing how every 2-3G motor I have feels perfect yet the 5-6G ones vary as the closure is screwed in.
 
CrazyOB said:
Thanks peoples. I'll have to look at the 2 grain reloads and see if they are of interest to me. Then I can order the four grain case, or wait until the 3 grain casing is back in stock.
Click to expand...

Why wait? Performance Hobbies has them in stock. Give Ken a shout.
 
If you're going to radical rocketeers this saturday, I'll be there with the big red trailer. I've got them in stock
 
bobkrech said:
The 2 space limit has nothing to do with the manufacturers. It was mandated by the certification authorities: NAR S&T, TRA TMT and CAR MTC for safety reasons.
Click to expand...
So, that's the ruling factor; all else is noise. But not worthless noise, so...

Motors are certified to provide a certain level of performance in a rocket. If you increase the weight of the motor but you do not add propellant, the Thrust to Weight ratio of the motor drops. For example with 38 mm CTI motor casings, the extra dead weight of the spacer and extra casing length is 50 grams per spacer or 100 grams for 2 spacers or 3.5 oz. For a 1G load, the loaded motor weight increases by 50% from approximately 200 grams in a 1G casing to 300 grams in the 3 G casing. As most rocket that would use a reloadable casing have a minimum weight equal to the loaded weight of a 1 G motor, so by limiting the maximum number of spacers to 2, a maximum weight increase of ~25% would be expected and that would reduce the apogee by a maximum of 20% if no delay adjustment is accounted for, and a typical recovery system should be able to handle the g-shock from a 2-3 second late deployment. The certification authorities were uncomfortable to allow a greater decrease in performance.
Click to expand...
That makes sense for a 1G load in a 3G case; it becomes relatively less important in longer cases. On the other hand, saying something like "Only one spacer in a 2 grain case (duh), up to two spacers for a 3 or 4 grain case, up to three spacers for a 5 or 6 grain case, and up to four spacers for longer cases" would add a level of complexity that I can well see wanting to avoid. Simple rules are (all else being equal) best as they are easiest to understand and follow.

Ravenex said:
From an engineering standpoint I can tell you there may be an issue with tolerance stack up. Basically nothing is perfect so each spacer and case has a limit to how far off the nominal dimension it can be without being rejected as a bad part. If you happen to get a reload and spacers on the short side and a case on the long side, or the other way around the maximum mismatch between reload/spacer length and case length is the sum of the tolerances, the more spacers the larger the potential mismatch. There is very likely a maximum mismatch tolerance beyond which the possibility of cato increases rapidly.
Click to expand...
Well... The same can be said of the tolerance stack up with more and more grains. Is there any reason to think the tolerance on spacer length is any worse than that on grain length? If not, then this doesn't really hold water.

There is another reason: unforeseen effects. It's easy to sit here at our keyboards and think "That oughta work OK," but there could easily be variables that none of us has considered. Just for example, does the free volume inside the case have any effect on the performance? I don't see why it would, but I sure can't prove it wouldn't.
 
jqavins said:
Well... The same can be said of the tolerance stack up with more and more grains. Is there any reason to think the tolerance on spacer length is any worse than that on grain length? If not, then this doesn't really hold water.
Click to expand...

I disagree, the spacer in the CTI system holder the forward closure against the liner, the liner to the nozzle, and the nozzle to the aft closure. Tolerance stack up here could crush the liner or allow the nozzle or forward closure to unseat in the liner. The grain length is irrelevant. Further the stack up is cumulative and all other relevant components should already be accounted for at their maximum values. It's not an issue of the lowest tolerance component but the sum of the tolerances of all components so every spacer adds up even if they are a tighter tolerance than other components.
 
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The total length of the stack
n
m
Σ

LGrain i
+
Σ

LSpacer i
i=1
i=1
So the total tolerance stackup is

n
m
Σ

εGrain
+
Σ

εSpacer
i=1
i=1
or nεGrain + mεSpacer.

Taking a six grain case, for example, with two spacers you get 4εGrain + 2εSpacer and with three spacers it's 3εGrain + 3εSpacer. Three spacers make for a worse total only if εSpacer is greater than εGrain.
 
It may have to do with pressure as well. A motor with too much space in it will require more propellant to build up pressure than one with more propellant, so it would reduce motor performance to unsafe level.
 
The spacers are outside the motor (but inside the case) - there's no difference in the "space" inside the motor.
 
If you pick up both the 3-grain and a 6-grain cases, and also pick up 2 spacers, you can fly anything from a 1 grain to a 6 grain. the other option is to buy a 1-grain case (they are relatively inexpensive), a 4 grain, and a 6xl grain. Then pick up 2 spacers plus the xl spacer (it adds an extra inch) and allows you to fly 5 and 6 grain motors in the 6xl case.
 
EeebeeE said:
If you pick up both the 3-grain and a 6-grain cases, and also pick up 2 spacers, you can fly anything from a 1 grain to a 6 grain. the other option is to buy a 1-grain case (they are relatively inexpensive), a 4 grain, and a 6xl grain. Then pick up 2 spacers plus the xl spacer (it adds an extra inch) and allows you to fly 5 and 6 grain motors in the 6xl case.
Click to expand...

I went with 3G, 5G, and 6XL. I found that the 6G options are much less compelling than the 5G and 6XL ones, and having that 5G case lets me fit 5G reloads in rockets that will not take a 6G case.
 
Have any of the motors actually been tested using said spacers?

JD

bobkrech said:
You can't launch an uncertified motor/reload at a NAR or TRA launch, and the motor/reload is not certified if you use more than 2 spacers, so no, its not ok.

Bob
Click to expand...
 

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