Static Test question

[13in33]

I dont think this belongs in the research forum, as I dont need that much information. Just asking if anyone had a problem like this before.
Hello, im currently having trouble with my static tests.
The motor seems to have 2 or 3 fast blasts after the ignition, before reaching a stable combustion. Its unable to burn correctly from the start.
Anyone has experienced this and knows what could be the cause?

 

[lakeroadster]

I dont think this belongs in the research forum, as I dont need that much information. Just asking if anyone had a problem like this before.
Hello, im currently having trouble with my static tests.
The motor seems to have 2 or 3 fast blasts after the ignition, before reaching a stable combustion. Its unable to burn correctly from the start.
Anyone has experienced this and knows what could be the cause?

What brand, size, type of motor?

 

[rocket_troy]

I dont think this belongs in the research forum, as I dont need that much information. Just asking if anyone had a problem like this before.
Hello, im currently having trouble with my static tests.
The motor seems to have 2 or 3 fast blasts after the ignition, before reaching a stable combustion. Its unable to burn correctly from the start.
Anyone has experienced this and knows what could be the cause?

Are you referring to chuffing?

Well, it's probably a safe bet that you are, so a brief explanation:

Fundamentally, chuffing is the result of having a propellant that’s too heterogeneous for the chamber dynamics for certain periods of the burn profile ie. There’s not enough steady state choking occurring at the throat to allow for steady state combustion to occur at the propellant surface. Think of solid composite propellant combustion as a feedback cycle involving a propellant surface, a diffusion zone, a mixing zone, a flame/reaction zone and thermal feedback to the surface. All those zones are gaseous, hence contain pneumatic “spring” energy from the chamber pressure. If there are fluctuations from either the nozzle being borderline on choked flow conditions or the normal shock in the divergent section is unstable, that pneumatic energy within those zones can expand and push the source of the thermal feedback further away from the surface, hence reducing thermal feedback to the surface and potentially extinguishing parts of the combustion. There’s generally enough residual heat and energy within some sections of the propellant surface to either maintain some combustion or re-ignite to begin the process again.

Chuffing can generally be suppressed by increasing the KN ratio or even providing a more homogeneous propellant mix.

TP

 

[cls]

Where do you put the igniter hot spot? Is it all the way in to the end of the core? Or somewhere in the middle?

 

[prfesser]

In a Bates grain configuration, most rocketnutz put the starter the whole way up to the forward end. I usually back it off slightly, to perhaps the middle of the top grain. It seems to reduce the burn-through of the liner, often seen at the forward end.