aerotech hobbyline reload gap

I have one of the 29/40-120 cases and I love the motor ranges I can buy for it. However in assembling smaller reloads, I thought the lack of full grain could produce a problem burning, much like this infamous video of a Cesaroni motor being fired with a spacer missing -
[video=youtube;FaZxM1OtU28]https://www.youtube.com/watch?v=FaZxM1OtU28[/video]

Well, since that does not happen all the time with 29/40-120 casings, why? Is the amount of propellant simply small enough to avoid this explosive failure, or is there something else at play?

Thanks!

There is a grain spacer in the smaller 29/40-120 reloads. It's the small paper tube which goes inside the liner with the grain. When the 29/40-120 is properly assembled, the forward closure and aft closure form tight seals against the O-rings and liners.

CTI spacers are different. They go fore of the fore closure. If this spacer is missing, the forward closure is not pressed into the liner. When the CTI comes up to pressure, it will blow the forward closure out of the liner, and lots of heat, fire and bad things go shooting up the rocket. I cannot say the ejection charge fired, but I see no reason why it wouldn't since the little paper cap could easily burn through.

Kevin

kruland said:

There is a grain spacer in the smaller 29/40-120 reloads. It's the small paper tube which goes inside the liner with the grain. When the 29/40-120 is properly assembled, the forward closure and aft closure form tight seals against the O-rings and liners.

CTI spacers are different. They go fore of the fore closure. If this spacer is missing, the forward closure is not pressed into the liner. When the CTI comes up to pressure, it will blow the forward closure out of the liner, and lots of heat, fire and bad things go shooting up the rocket. I cannot say the ejection charge fired, but I see no reason why it wouldn't since the little paper cap could easily burn through.

Kevin

Click to expand...

What he said!

Leaving a spacer out of a CTI would be similar to leaving the forward closure off of an Aerotech reload or the forward snap ring off a Loki reload.

There are some of the EX guys that fly large motors with similar configurations to the 29/40-120 with smaller loads. The grains do not fill the liner. Some of these guys just epoxy a 1/4" plywood plug in the liner above the grains. Some use a shorter liners and spacers above the forward closure (lile CTI) and others just epoxy the grains in place and make sure the igniter doesn't extend past the top grain when it's installed. It all works the same. The volume above the grain is pressured by the burning grain and doesn't really affect the performance of the motor much.