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JimJarvis50

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Does anyone have a qualitative feel for the ease of lighting the various CTI propellants?

Classic
Imax
Pink
Blue
Red
White

Thanks.

Jim
 
Are you talking about the propellants themselves, or the motors? As CTI include a BP pellet in the top of most (all) of their motors that helps it start quickly.
 
Are you talking about the propellants themselves, or the motors? As CTI include a BP pellet in the top of most (all) of their motors that helps it start quickly.

These would be in 75 mm size. I suspect they don't have a pellet (but then again, I haven't ever flown a 75mm CTI).

Jim
 
No, 75mm CTI motors don't use pellets, they use an igniter with pyrogen.

I've only flown White, C-Star, and White Thunder in 75mm. The White takes a few seconds to come up; in my recollection the C-Star and White Thunder were faster.
 
From memory, though I haven't flown any in a while (hoping to remedy that this summer...), I would rank them from easiest to hardest as:

Vmax
White Thunder
C-Star
Blue
Smoky
Red
Classic
White
Imax
Skid
Green

I haven't flown Mellow, so I can't rank that one. Pink is just a bunch of red grains with one blue grain, so it isn't a separate propellant type, similar to how the dual thrust motors are just mixed propellant types.
 
I have flown a 75mm 3Grain classic, and a 75mm 4G blue. From the videos there was about 3 second delay between the igniter firing and the motor coming up to pressure in the classic, and about 5 seconds (with a good chuff in the middle) for the blue.

Does anybody know why there's so much of a delay with these motors? Does CTI undersize the igniters?
 
Guys, thanks for your input. I decided to pose the question to CTI directly. I'll report back....

Jim
 
Does anybody know why there's so much of a delay with these motors? Does CTI undersize the igniters?
I think there's just a lot of surface area in the core of a larger motor and it takes a while for it all to ignite and pressure up. Aerotech White Lightning 75mm takes even longer with anything but maybe the largest enhanced initiators.
 
Would stringing a pyrodex pellet onto the igniter lead right below the head help things along?

Yes, it should help. You could also augment the igniter using a fast burning propellant attached to the igniter.
 
Three years ago, I lit a Gorilla moonburner at somewhere around 40K. It took 6 seconds to come up to pressure, even though the amount of energy I add at the top of the motor is significant. I recently found out about "the hub", and I have access to one, so I can consider replacing the Gorilla with any (?) of the corresponding CTI motors. All would result in more speed than the Gorilla, which isn't what I want, but if I could better control the timing of the motor coming up to pressure, there would be an offset to the speed increase. So, that's what I'm up to. Looking at the CTI options, I think the only one that makes sense is the blue motor, and so what I really want to confirm is that their blue propellant is fast, as is the case with other blue formulations.

Jim
 
Jim,

From my experience, CTI's blue isn't as fast as Loki/AT blue to come up to pressure, but still lights happily. The 75mm CTI blues that I've flown are cast on a mandrel and shipped undrilled, so the port surfaces are slick/smooth. A quick scuff to break the surface would probably help a lot with ignition speed.

On your Gorilla-powered sustainer, did you have anything plugging the nozzle (e.g., taped-over foam) to help regulate the ignition transient?

David
 
I have not had a CTi 75mm motor do anything but light within the first second or two, but, I don't use the CTi ignitors.

[Mods - a description of a questionably legal act has been deleted until further review]

I have not yet lit one in the air, but I would expect the same light time.

So far it has worked on C-Star (Slow), White and Red. I also light my research motors the same way.
 
Last edited by a moderator:
Jim,

From my experience, CTI's blue isn't as fast as Loki/AT blue to come up to pressure, but still lights happily. The 75mm CTI blues that I've flown are cast on a mandrel and shipped undrilled, so the port surfaces are slick/smooth. A quick scuff to break the surface would probably help a lot with ignition speed.

On your Gorilla-powered sustainer, did you have anything plugging the nozzle (e.g., taped-over foam) to help regulate the ignition transient?

David

This.

And a real igniter sized correctly for the core volume.
 
Jim,

From my experience, CTI's blue isn't as fast as Loki/AT blue to come up to pressure, but still lights happily. The 75mm CTI blues that I've flown are cast on a mandrel and shipped undrilled, so the port surfaces are slick/smooth. A quick scuff to break the surface would probably help a lot with ignition speed.

On your Gorilla-powered sustainer, did you have anything plugging the nozzle (e.g., taped-over foam) to help regulate the ignition transient?

David

Good ideas. I had a cap covering the back of the motor tube, but it was not air-tight. What's "taped-over foam" and "regulation of the ignition transient"?

Jim
 
This.

And a real igniter sized correctly for the core volume.

What's a "real" igniter, and how would someone calculate the correct igniter size for a moonburner at 50K in a rocket traveling at Mach 1.2?

I posed the question of the ease of ignition to CTI, and the reason for my interest, and they volunteered to give me some ideas for getting this done. Won't happen until next week. I'll report back.

Jim
 
You can review my igniter thread in the research forum for formula options and sizing.

The nozzle plug should hold pressure at least to 200 psi for 1 sec. Depending on the normal operating pressure of the motor, it could hold to higher pressure.

Positioning and retaining the igniter are limited for an unmodified commercial motor. If you can live with a modified fwd closure, let me know and I can point you to some options. Since it will see high g's from the booster, it needs some attention.
 
What's "taped-over foam"
Pink foam or squishy grey foam from the hardware store... cut a 1/2-1" thick disc, put a few wraps of foil tape over it for environmental protection, and wedge it in the nozzle :) It'll get out of the way quickly when the time is right.

and "regulation of the ignition transient"?
Don't let your igniter gases get sucked out of the nozzle before the motor's lit. Most ignition theories say you need to sustain somewhere between 50-200 psi in the motor before it'll stay lit. (Propellant doesn't achieve its "operational" flame structure until the chamber pressure passes tens of atmospheres.)

Basically, keep the igniter stuff inside long enough for it to do its job--that's what the foam plug is for--and you should be in business!
 
So in the vein of plugging the nozzle, why is it most people seem to vent their AT nozzle red caps, seems like leaving them intact would allow proper pressurization and the igniter is already fully burned before the pressure spikes enough to blow the cap off. Am I interpreting this correctly, that not venting CTI or AT nozzle caps should increase ignition reliability?
 
So in the vein of plugging the nozzle, why is it most people seem to vent their AT nozzle red caps, seems like leaving them intact would allow proper pressurization and the igniter is already fully burned before the pressure spikes enough to blow the cap off. Am I interpreting this correctly, that not venting CTI or AT nozzle caps should increase ignition reliability?

Venting the red cap allows pressure to build for a longer time without just popping the cap, spitting the igniter, and killing the pressure nearly instantly.
 
So in the vein of plugging the nozzle, why is it most people seem to vent their AT nozzle red caps, seems like leaving them intact would allow proper pressurization and the igniter is already fully burned before the pressure spikes enough to blow the cap off. Am I interpreting this correctly, that not venting CTI or AT nozzle caps should increase ignition reliability?

Dipped bridgewire igniters burn too long for the plug to do its job. As Steve mentioned, if you don't let them vent as they burn, they won't transfer enough energy to the motor before they get spit out. "Real" igniters (that John is alluding to) burn much faster, and transfer their energy in a much shorter amount of time, and can use a plugged nozzle, but require more precise design work. For Jim's specialized application, this makes sense. For general sport flying, it's not worth the effort--the standard technique works well without too much headache.
 
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