C6-0 burn through force vs ejection force of C6-3 or other s standard engines

I am sure this has been explained elsewhere but i am still unclear on the concept.

When a C6-0 or other booster engine burns almost all the propellant, the remaining propellant forward, which has been acting as a bulkhead, breaks through and the burning and expanding gases can now blow forward through a big hole as well s back through the nozzle. This forward projection of gas and burning propellent chunks has to have some force to it. In fact, this seems to be one of the concerns with two stages that the second stage might get "blown off" before ignition. The vent holes serve one or two purposes. First they allow the hot gases and particles to move forward to sustainer nozzle. Second (i think) they MAY relieve some of the pressure to prevent premature separation before sustainer ignition.

Delay engines have a delay charge followed by an ejection charge covered by a clay cap. The ejection charge i assume is also black powder. Is the ejection charge powder formulated differently to make the force GREATER than the C6-0 burn through? Or does the clay cap take more pressure to break?

BABAR said:

I am sure this has been explained elsewhere but i am still unclear on the concept.

When a C6-0 or other booster engine burns almost all the propellant, the remaining propellant forward, which has been acting as a bulkhead, breaks through and the burning and expanding gases can now blow forward through a big hole as well s back through the nozzle. This forward projection of gas and burning propellent chunks has to have some force to it. In fact, this seems to be one of the concerns with two stages that the second stage might get "blown off" before ignition. The vent holes serve one or two purposes. First they allow the hot gases and particles to move forward to sustainer nozzle. Second (i think) they MAY relieve some of the pressure to prevent premature separation before sustainer ignition.

Delay engines have a delay charge followed by an ejection charge covered by a clay cap. The ejection charge i assume is also black powder. Is the ejection charge powder formulated differently to make the force GREATER than the C6-0 burn through? Or does the clay cap take more pressure to break?

Click to expand...

Ejection charges are usually granular BP sorta like what you'd use in a muzzleloading gun. Because of the small particle size, it burns essentially instantly, very hot and very fast, and creates a huge volume of gas (relatively speaking) almost instantly, which of course bursts the clay cap and blows it out, pressurizing the tube and blowing the cone and laundry out. In addition, the slower-burning solid slug of time delay propellant (which is a slower burning formulation of BP pressed into a solid slug, which also makes it burn much slower (the propellant grain is a fast-burning BP much like the ejection charge, but being pressed into a solid slug makes it burn slower too, so it creates thrust and doesn't "explode" like gunpowder in a cartridge or muzzleloader). The slower-burning solid slug of time delay BP burns through at some point, probably near the center (due to the concave shape of the burning propellant face due to the dimple in the end of the grain where the ignitor goes). This in effect "touches off" the ejection charge-- the delay grain continues to burn until all the remaining BP is expended (which can cause the "hibachi effect" in some rockets/motors). The delay grain burns like a donut or a lifesaver, gradually getting thinner from the center outward and from the back side forward until it's all gone.

The same thing basically happens in a -0 motor as it does with the delay grain... only there's about 90-100 PSI of pressure behind the burning slug of propellant in a -0 motor at burnthrough... (there is virtually no pressure behind the delay grain, which is why it isn't producing thrust, only tracking smoke). When the disk of propellant gets too thin to contain the pressure anymore, it ruptures very suddenly and releases a jet of high pressure, extremely high temperature burning propellant gas, which shatters a good part of the remaining propellant disk (or all of it) and sends all that burning mess forward toward the upper stage motor. For tape together staging, it doesn't have far to travel, and the tape holds the casings together long enough for the upper motor to ignite (hopefully) and when it lights and comes up to pressure, it pressurizes the lower casing to about 100 psi and of course the tape joint cannot sustain that much pressure, and it bursts/burns through and the pressure spike blows the lower stage off. For gap-staged rockets, where there is a long tube or distance between the booster motor and sustainer motor, the vents give all that "cold air" in the tube a place to go... when the burning propellant disk burns through and ruptures, it sends a blast of flaming hot gases and burning propellant bits forward up the tube, which if there were no vents, would tend to compress the air trapped in the tube, which would tend to blow the stages apart prematurely. The shock wave should be moving up the tube at the speed of sound, but it still takes a bit for the hot gases and burning bits to make it up there and ignite the upper stage motor. The vents buy a little time by giving that trapped air somewhere to go, rather than building up pressure inside the tube. It also makes room for the hot expanding combustion gases coming up from the booster motor. Once these gases/bits ignite the upper stage, the upper stage motor starts cranking out hot pressurized gas CONTINUOUSLY, which pressurizes the tube enough (assuming the vents aren't TOO big) to blow the stages apart...

Make sense??

Later! OL JR

That was a good explanation!

When I was a kid, I did not pay much attention to the motor I should use. Instead, I used what I had. Occasionally, that meant using a booster where something with a delay should have been used. Although ejections were premature, they did happen.

I remembered this when I wanted a very short delay. I reasoned that the booster motors should have enough oomph to push the laundry.

The proof is in the pudding. The attached photo is my PEPP Aeroshell at ejection. It is not blowing up. What you are seeing is 12xC6-0s burning through at once. The chute was ejected (but did not have time to open) and the rocket was recovered without any damage.

I need to find a shorter delay (or a less windy day!)

So does the gunpowder-like ejection charge have MORE force than the propellant burn through off the booster?

JAL3 said:

What you are seeing is 12xC6-0s burning through at once. The chute was ejected (but did not have time to open) and the rocket was recovered without any damage.

I need to find a shorter delay (or a less windy day!)

Click to expand...

You're going to have a hard time finding a delay less than 0! However, if you replaced a couple of the C6-0's with B6-0's the B6's will have a delay of -1 relative to the C6's.

BABAR said:

So does the gunpowder-like ejection charge have MORE force than the propellant burn through off the booster?

Click to expand...

Don't know that it's ever been specifically measured (Officially) since using booster motor blow-thru as an "ejection charge substitute" isn't an "approved use from the motor manufacturer" (and thus at the very best considered a "gray area" of the safety code if not an outright violation of it).

My "off the cuff guesstimate" would be they are fairly "on par" with one another... the booster motor is a black powder disk blowing to bits (which subsequently burn producing gobs of hot expanding gases) with about 100 PSI of back pressure of hot expanding gas behind it, and the ejection charge is basically a small charge of granulated gunpowder (similar to that used as projectile propellant in guns) burning VERY quickly to produce massive amounts of hot expanding gas with virtually NO backpressure coming from the motor itself (and contributed to by the continuing slow production of gases from the delay grain until it burns out).

I'm sure there's a measureable difference, but I'm unsure as to which side it would come down on or the magnitude of that difference...

Later! OL JR

It is NOT a violation of the safety code to use a booster motor to eject a recovery system or activate a recovery system. Not a violation in any way. Zero grey area.

Booster motors have been used for years in such applications including the "CATO" kit from Estes.

The force of the entire ruptured surface and chunks of propellant burning is very large. It is probaly close to a normal ejection charge but with more of a jet of flame and more chunks flying forward (which burn out fast, but may require a bit more wadding).

shreadvector said:

It is NOT a violation of the safety code to use a booster motor to eject a recovery system or activate a recovery system. Not a violation in any way. Zero grey area.

Booster motors have been used for years in such applications including the "CATO" kit from Estes.

The force of the entire ruptured surface and chunks of propellant burning is very large. It is probaly close to a normal ejection charge but with more of a jet of flame and more chunks flying forward (which burn out fast, but may require a bit more wadding).

Click to expand...

Okay... learn something new everyday...

Figured I'd couch it in "gray area" language to prevent folks from having a coronary...

Later! OL JR