cesaroni spacer purpose

I receiver my first Cesaroni reload kit today, and was surprised at how little work is needed. There is practically no assembly at all.

The way the kit and liner are shaped made me wonder why the spacers are needed. Perhaps the reload starts melting and the spacer is needed to keep the grains at the base of the casing? It looks like one could try firing without a spacer, though that person will not be me!

watermelonman said:

I receiver my first Cesaroni reload kit today, and was surprised at how little work is needed. There is practically no assembly at all.

The way the kit and liner are shaped made me wonder why the spacers are needed. Perhaps the reload starts melting and the spacer is needed to keep the grains at the base of the casing? It looks like one could try firing without a spacer, though that person will not be me!

Click to expand...

If the spacer is left out, the forward closure will slide forward and unseal from the liner. Then you will have flames directly touching the metal case, causing rapid weakening and failure.

CarVac said:

If the spacer is left out, the forward closure will slide forward and unseal from the liner. Then you will have flames directly touching the metal case, causing rapid weakening and failure.

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In this case it is a 38mm without closures. I am sure something will still go bad, though! Or maybe you meant closure of the reload and not closure of the casing?

A closure is a closure, whether it came with the casing or the reload. If you don't keep the forward closure held securely in place with a spacer, using the reload in a longer case will allow the pressure in the liner to pop it out and will end your flight quickly and catastrophically. If the closure would stay in by itself, why would you need a casing at all?

I think you guys are talking about the delay/eject module, right? CTI doesn't call it a closure - hence the confusion.

"The thing at the top that keeps the fire from coming out the wrong end"?

The real answer to the original question is this, the spacers are NEEDED when you use a case 1-2 grains longer than your reload. If you think it will work without the spacers, make sure it is in a rocket that: A- you don't really want to see fly B- said rocket is not planning a trip back to your "hanger" and C- you have fire extinguishers and garbage cans near. The spacers are not for "show". If you are really that unsure about what you have, I would recommend hooking up with someone who can help you learn a few important things before you hurt yourself or burn down your fleet.

daveyfire said:

"The thing at the top that keeps the fire from coming out the wrong end"?

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That's funny! Gave me a good laugh.

watermelonman said:

In this case it is a 38mm without closures. I am sure something will still go bad, though! Or maybe you meant closure of the reload and not closure of the casing?

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Hi Watermelonman,

Don't be afraid to ask questions. It takes awhile to learn all of the terminology. I still don't know all of the terminology for AT motors. Here is a link to CTi 38 mm reload instructions. Maybe this will help us figure out exactly what you are referring to. The CTI cases are open, the forward "closure" is the delay /eject module. If you are not using motor ejection charges, this module will be plugged. There are actual CTI spacers that are used with smaller motors in longer casings. IE, you can use a 1-grain motor in a 3-grain casing with two spacers, or a 2-grain motor in a 3-grain casing with 1 spacer, or a 3-grain motor in a 3-grain casing with no spacers. This saves on the number of casings that you need to purchase. I hope this helps. Again don't be afraid to ask questions, and have fun launching your new reload.

forgot link: https://www.pro38.com/pdfs/Pro38Instns.pdf

eggplant said:

A closure is a closure, whether it came with the casing or the reload. If you don't keep the forward closure held securely in place with a spacer, using the reload in a longer case will allow the pressure in the liner to pop it out and will end your flight quickly and catastrophically. If the closure would stay in by itself, why would you need a casing at all?

Click to expand...

Well, for the sides of course. The top, on the other hand, looked like it might be of similar material to the Aerotech single use motors, in which case it could probably take a beating. Now that you mention it, it seems obvious that as the sides give way the top will need precise constraint.

GRIFFIN said:

The real answer to the original question is this, the spacers are NEEDED when you use a case 1-2 grains longer than your reload. If you think it will work without the spacers, make sure it is in a rocket that: A- you don't really want to see fly B- said rocket is not planning a trip back to your "hanger" and C- you have fire extinguishers and garbage cans near. The spacers are not for "show". If you are really that unsure about what you have, I would recommend hooking up with someone who can help you learn a few important things before you hurt yourself or burn down your fleet.

Click to expand...

To be clear I am not suggesting that someone run a short motor without spacers. I am an inquisitive type, and I wanted to know what would fail, and more importantly why.

4kids49 said:

Don't be afraid to ask questions. It takes awhile to learn all of the terminology. I still don't know all of the terminology for AT motors. Here is a link to CTi 38 mm reload instructions. Maybe this will help us figure out exactly what you are referring to. The CTI cases are open, the forward "closure" is the delay /eject module. If you are not using motor ejection charges, this module will be plugged. There are actual CTI spacers that are used with smaller motors in longer casings. IE, you can use a 1-grain motor in a 3-grain casing with two spacers, or a 2-grain motor in a 3-grain casing with 1 spacer, or a 3-grain motor in a 3-grain casing with no spacers. This saves on the number of casings that you need to purchase. I hope this helps. Again don't be afraid to ask questions, and have fun launching your new reload.

Click to expand...

Thanks a ton!

The delay/ejection module can be easily removed to adjust the delay time and then pushed back in to the top of the motor. Without the spacer, that module will be blown forward as soon as the motor lights followed by a big flame up the center of your rocket. The spacer is there to make sure the module has something solid to hold it in place.

Look at these pages to see how the spacers work. https://pro38.com/pdfs/Pro38_Case_Spacer.pdf

Spacers allow you to use shorter reloads in a longer casing. For example for the Pro 38 motors, if you purchase the 3G and 6G casings, you can use any 38 mm reload from 1G thru 3G in the 3G casing and 4G thru 6G in the 6G casing. The NAR/TRA/CAR rules limit the number of spacers that you can use in a casing to 2.

Bob

Quick question why can't you use more than 2 spacers?

If I am not mistaken, it is because they are not certified.

Primativerocketry said:

Quick question why can't you use more than 2 spacers?

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I think it's a combination of two factors:

• Adding more than two spacers starts to put a lot of inert weight (empty case + spacer) at the wrong end of the rocket. For 29mm, it's 27 grams of dead weight added per spacer. For 38mm, it's 50 grams. For 54mm, it's 120 grams. 76mm and 98mm spacers don't have their additional weight listed in the CTI product notes, but a Pro75 spacer weighs 227 grams, so it's safe to say you'd be adding a couple of pounds.
• 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.

daveyfire said:

I think it's a combination of two factors:

• Adding more than two spacers starts to put a lot of inert weight (empty case + spacer) at the wrong end of the rocket. For 29mm, it's 27 grams of dead weight added per spacer. For 38mm, it's 50 grams. For 54mm, it's 120 grams. 76mm and 98mm spacers don't have their additional weight listed in the CTI product notes, but a Pro75 spacer weighs 227 grams, so it's safe to say you'd be adding a couple of pounds.
• 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.

Click to expand...

It is also length tolerances stacking.

If you forget the spacer, here is what's going to happen:

[video=youtube;FaZxM1OtU28]https://www.youtube.com/watch?v=FaZxM1OtU28[/video]


-Alex

azzie said:

If you forget the spacer, here is what's going to happen:

...

-Alex

Click to expand...

Hah, great video. Glad someone else tried so I do not feel any need to do so!

Thx I understand now... Great vid I have witnessed people forgetting the entire case...

blackbrandt said:

If I am not mistaken, it is because they are not certified.

Click to expand...

daveyfire said:

I think it's a combination of two factors:

• Adding more than two spacers starts to put a lot of inert weight (empty case + spacer) at the wrong end of the rocket. For 29mm, it's 27 grams of dead weight added per spacer. For 38mm, it's 50 grams. For 54mm, it's 120 grams. 76mm and 98mm spacers don't have their additional weight listed in the CTI product notes, but a Pro75 spacer weighs 227 grams, so it's safe to say you'd be adding a couple of pounds.
• 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.

Click to expand...

The reason NAR/TRA/CAR do not permit the use of more than 2 spacers is that it alters the thrust to weight ratio of the motor sufficiently resulting in a slower acceleration off the pad and/or the CG/CP relationship which under the worst circumstances result in an unstable flight.

Bob

bobkrech said:

The reason NAR/TRA/CAR do not permit the use of more than 2 spacers is that it alters the thrust to weight ratio of the motor sufficiently resulting in a slower acceleration off the pad and/or the CG/CP relationship which under the worst circumstances result in an unstable flight.

Bob

Click to expand...

I don't understand that at all. The thrust curve will be the same. The weight of the rocket for the purposes of t/w ratio is always actual weight with the motor loaded. Yes, the rocket will be heavier...but you still have to weigh it as the RSO table and figure required thrust. I don't see how the weight of the motor hardware is any more significant than the weight of any other part of the rocket. I could also make it heavier by using an MD retainer adapter. Or adding a camera. Or whatever.

I think CarVac is correct there with length tolerance stacking.

It may also have something to do with the ejection charge being buried too deep in the hole ... ?

azzie said:

If you forget the spacer, here is what's going to happen:

[video=youtube;FaZxM1OtU28]https://www.youtube.com/watch?v=FaZxM1OtU28[/video]


-Alex

Click to expand...

When I first saw this video, I don't know if it was the lighting or me being blind, but I thought it was an animated rendition/simulation instead of a real rocket.

and to the original topic, if you take the reload of of it's packaging you will notices that it all separates really easily. I remember the first time I used a CTI reload and it all came apart when I pulled it out of the packaging... propellant grains rolling everywhere...

Bat-mite said:

It may also have something to do with the ejection charge being buried too deep in the hole ... ?

Click to expand...

I'm betting spacer tolerancing, like CarVac said. Ejection tube volume for a given grain stack length is essentially the same (lower with the spacers, actually, but in a trivial amount) - i.e.: if you eject charge is on the top of a 1 grain reload, there's just as much volume and distance between the charge and the recovery package whether you fly a 1G case or a 6XL/7G case. Weight balance means nothing; it's stable as assembled or it isn't. If you want a tight pack of the spacers against the top of the reload, though, there's a limit to the number of (-) tolerance spacers you can stack in a (+) tolerance case and still be able to make up the distance within the thread range of the closure (or reload in the case of Pro38s).