BP ejection charges at high altitude

[Johnsfolly]

Does anyone have experience with BP ejection charges at high altitude, over 30,000?

 

[tfish]

most recent thread.....https://www.rocketryforum.com/showthread.php?133135-High-altitude-recovery

more threads https://www.google.com/webhp?comple...+altitude+ejection+site:www.rocketryforum.com

Tony

 

[Johnsfolly]

Thanks for the references. I had heard rumours that BP was ineffective at high altitudes but couldn't find out why. I have 20 years experience constructing fireworks and couldn't understand why the same BP technology couldn't be made to work for ejection charges. I don't have any experimental evidence but from my fireworks experience I'm fairly confident that BP ejection charges will work well as long as the charges are well confined.

Fireworks shells are typically lifted by a black powder charge in a pipe mortar, a similar application to separating the payload tube from the airframe tube. The black powder is typically 2FA, much slower burning than 4Fg. Larger fireworks (>10") use even larger grains. They key factor is not the quickness of the burning of the BP but that most BP grains are ignited to enable a large volume of gas is released.

This is also the key factor in bursting the aerial shell when it is at altitude. The casing of the firework (usually made of pasted kraft paper) must be very rigid to confine the burning BP until the generated hot gases have ignited not only all the BP burst charge but each of the stars within the shell. Strong confinement is essential. As well, it is important to NOT pack the burst charge tightly. Space must be left for the hot gases to move through the burst charge into and around the stars. To achieve this, the burst charge is often BP coated onto rice husks.

I have flown to 23,000' using BP charges, centrifuge cups with three wraps of electrical tape (similar to "bombs" (about the size of a hockey puck) used in movie explosion scenes sending barrels high in the sky. I am planning a flight to 50,000' and plan to use BP ejection charges. I plan to make these of pasted wraps of kraft paper (probably three to five turns of 80 lb kraft then 3 to 5 wraps of electrical tape). These "shot" charges will be more short and squat rather than long and narrow for better transfer of combustion gases to all BP grains.

Comments?

 

[tfish]

One though comes to mind. Scattering seems to be one of the issues at High Altitude. Your method seems that is would really scatter it. I'd test it in a vacuum chamber before flying.

Tony

 

[NateLowrie]

I would read up on Jim Jarvis' experience with charges. He has been above 100k on more flights than just about anyone. I can say his current configuration of a fairly long brass tube works really well for make sure all the BP is burned.

That said, be sure to test in a vacuum chamber...

 

[ksaves2]

I would read up on Jim Jarvis' experience with charges. He has been above 100k on more flights than just about anyone. I can say his current configuration of a fairly long brass tube works really well for make sure all the BP is burned.

That said, be sure to test in a vacuum chamber...

Ummmm, That could be hard for some folks to do. I'd just replicate what has been found to work reliably and one will probably do well. Kurt

 

[bobkrech]

....This is also the key factor in bursting the aerial shell when it is at altitude. The casing of the firework (usually made of pasted kraft paper) must be very rigid to confine the burning BP until the generated hot gases have ignited not only all the BP burst charge but each of the stars within the shell. Strong confinement is essential. As well, it is important to NOT pack the burst charge tightly. Space must be left for the hot gases to move through the burst charge into and around the stars. To achieve this, the burst charge is often BP coated onto rice husks.......

Comments?

You answered your own question. The problem with using BP charges at high altitude is in the mind of the user who does not understand this.........

If all the BP grains ignite, they will all burn. If only some of the BP grains ignite, most of the BP grains will be scattered about without ignition.....

Confinement is the key!!!!

BP in a musket is confined by the barrel and the musket ball, so all the BP grains ignite and burnout within the barrel.

In battle reenactments where blank BP charges are uses, a wadding pad must be rammed down the barrel in place of a musket ball to retard BP grain motion until they all ignite...otherwise you'd shoot a lot of unburned BP grains....

At atmospheric pressure, a simple confinement device like a rubber glove finger or similar container is all that is required since the air surrounding the BP charge is sufficient to retard the hot gas expansion until all of the BP grains are exposed to the initial hot gas burst from the initial BP grains that ignited.

As altitude, simple confinement methods are not good enough because the reduced air density allows the BP grans to move away from the ignition source and for the hot gases to expand and cool faster than the ignition process required to ignite the grains furthest away from the initial ignition point.

The physical arrangement of the confinement system is less important than the act of confinement so they are several way to do it. Using a physical stronger confinement device that does not fail until all the BP grains ignite is the key to successful high altitude deployments. Taped centrifuge tubes, stiff surgical tubes, T-tubes or a container with a rupture disk will all confine the BP grains in hot gas until they have all ignited.

The most reliable way to accomplish this is to confine the BP charge in a hermetic sealed container. This is how NASA and DoD do it. This protects the BP from moisture and contaminants during storage so the BP will be as easy to ignite as it was on the day it was sealed in the igniter container. Most of these devices have a foil diaphragm that will rupture at several hundred psi and a foil catcher so the hot foil or metal particles will not damage downstream objects.

There is enough space between BP grains for hot gas passage unless the grains are deformed by compression. Some porous wadding to hold the BP against the ignition source is strongly recommended as it will also retard grain motion and enhance complete hot gas ignition.

YMMV but this is what works. All the time.

 

[JimJarvis50]

YMMV but this is what works. All the time.

Yep, what he said.

One of the nice things about the brass tube approach that I use, and the stiff tube approach that Tony is developing, is that the pressure you use them at doesn't really matter. For my charges, all of the BP burns all of the time. Therefore, if the charges work on the ground, they will work at altitude in the same way. This allows you to do ground testing without having to do it in a vacuum. If the containment is not sound, then the performance may vary with altitude, and the correct amount of BP to use is more of a guess.

Jim

 

[tfish]

Sort of an update as to what I've been working on. Jim's method works! One issue I found with it was how "energetic" it was. It even says so in Jim's write up. I'm just trying to remove some of the 'harshness' of the thing with what I'm doing. As I was thinking the ignition process through it dawned on me that one of the things that is helping scatter our BP is the placement of our ematch at the bottom of our charge holders. I opted to place my ematch on top of my charge holders. The rational is that anything trying to escape then must pass by the other burning matter. Also, my use of dog barf is to act as a shock absorber to lessen the harshness of the device. I feel my device is about half has harsh as Jim's system.
The black electrical tape is not used to seal in air to make it airtight. It is solely there to hold the match and dog barf in place. I've tested this system in a vacuum chamber to 27.5 hmg (for several minutes) with 100% of the BP consumed. I've test flown it once and the .5 gram charge in a 3" rocket was to efficient and I parted the Kevlar recovery cord....go figure.

a picture is worth a 1000 words.





Tony