Ejection Charge: Hot Chunks Of Fire or Just Hot Gases

[lakeroadster]

I'm in the throws of building a scratch build that rear deploys (2) chutes from side pods.

Yesterday I was wondering if the "hot ejection gases / bits" that are likely to remain in the main body tube will be a fire hazard?

Anybody here ever done a bench test to see just what the ejection charge amounts to (just gasses - or gas and blazing hot bit), or perhaps have built a rocket with similar configuration?

This one is designed for dual Aerotech 24mm F32 motors.

Thanks in advance for your thoughts / advice / comments?

 

[mbeels]

My rockets that have ejection charge baffles "rattle" when I shake them, so something is definitely in there, but it hasn't shown to be a fire hazard (so far). From your drawing, I'd guess that you'd be able to shake those bits out, by laying the rocket on its side, and shaking them down into one of the side pods.

 

[lakeroadster]

My rockets that have ejection charge baffles "rattle" when I shake them, so something is definitely in there, but it hasn't shown to be a fire hazard (so far). From your drawing, I'd guess that you'd be able to shake those bits out, by laying the rocket on its side, and shaking them down into one of the side pods.

What's the baffle made out of, something flammable?

 

[DaveW6DPS]

Most to the stuff that winds up in baffles is other material. If you have ever seen an ejection charge at night you should have seen a substantial number of sparks in the ejecta.

They can wind up in body tube and cause some burning.

 

[mbeels]

What's the baffle made out of, something flammable?

Mine are a hodgepodge mix of thin walled body tubes (like BT-50), cardstock centering rings, and sometimes plywood. In my Alien Space Probe the baffle is very close to the top of the motor mount tube (short rocket) so I added some aluminum foil to bear the brunt of the blast. I'm not sure if it was necessary, but easier to do before I needed it.

 

[heada]

If you duct the gases from the main tube to the side tubes, you should be fine.

 

[Stefan2k4]

Aren't most ejection charges black powder? If I recall my chemistry correctly, black powder produces a lot of solid products when it burns. So,it make sense that there are lots of either hot solids or hot molten liquids that quickly solidify on contacting surfaces and transfer their heat to those surfaces.

https://pyrodata.com/definitions/Black-Powder

The products of burning do not follow any simple equation. One study showed it produced: 55.91% solid products-- Potassium carbonate, Potassium sulfate, Potassium sulfide, Sulfur, Potassium nitrate, Potassium thiocyanate, Carbon, Ammonium carbonate--and 42.98% gaseous products--Carbon dioxide, Nitrogen, Carbon monoxide, Hydrogen sulfide, Hydrogen, Methane, Water.

 

[afadeev]

I'm in the throws of building a scratch build that rear deploys (2) chutes from side pods.

Cool rocket design!

Yesterday I was wondering if the "hot ejection gases / bits" that are likely to remain in the main body tube will be a fire hazard?
Anybody here ever done a bench test to see just what the ejection charge amounts to (just gasses - or gas and blazing hot bit), or perhaps have built a rocket with similar configuration?

Here is a slow-mo video of a BP motor (Estes D12-5) and ejection charge firing underwater.
There is a lot of distracting and cool imagery, but if you pay close attention, you will see pieces of black (hot) particles being ejected and bouncing around in the water. Best observed around 5:37-5:41 time stamps, after the smoke clears.
The ejection charge is far too quick and violent to be inspected frame-by-frame, but I would expect a similar mix of hot particles bouncing around:

Thanks in advance for your thoughts / advice / comments?

I would coat all body tube surfaces that will be exposed to hot gasses with thin CA.
This will provide a layer of reinforcement, and non-flammable barrier for hot particles to bounce of.

Also notice how violent ejection firing event is. That pressure wave will need to be controlled, routed to the side pods, then U-turned to push the chutes out.

I remember reading a build thread from someone who tried similar ducting modification with an Estes SR-71 kit. That person built SR-71 with dual motors in side-pods, ducting both ejection charges from the side-pods to the main airframe. The ducting did not withstand the ejection charge pressure, and blew open one of the wings. Model came back down ballistic, and was written off.

HTH

 

[Daddyisabar]

Also depends on the motor used. The old AT SU 18mm motors would keep burning a bit after ejection, giving rise to the hibachi effect.

 

[Daddyisabar]

Airplane rockets can have multiple motors and cramped recovery compartments. Can be mitigated with CA soak and well prepared dog barf sandwiches. How about leaving off an ejection charge on one of the motors. Tone down the amount of BP used.

Use a SU plugged or a red label motor with a nice expoxy plug. Just learn to live with and repair a few burned bits on the chute. Just depends on how far to the dark side you want to go.

 

[Daddyisabar]

Cool rocket design!



Here is a slow-mo video of a BP motor (Estes D12-5) and ejection charge firing underwater.
There is a lot of distracting and cool imagery, but if you pay close attention, you will see pieces of black (hot) particles being ejected and bouncing around in the water. Best observed around 5:37-5:41 time stamps, after the smoke clears.
The ejection charge is far too quick and violent to be inspected frame-by-frame, but I would expect a similar mix of hot particles bouncing around:





I would coat all body tube surfaces that will be exposed to hot gasses with thin CA.
This will provide a layer of reinforcement, and non-flammable barrier for hot particles to bounce of.

Also notice how violent ejection firing event is. That pressure wave will need to be controlled, routed to the side pods, then U-turned to push the chutes out.

I remember reading a build thread from someone who tried similar ducting modification with an Estes SR-71 kit. That person built SR-71 with dual motors in side-pods, ducting both ejection charges from the side-pods to the main airframe. The ducting did not withstand the ejection charge pressure, and blew open one of the wings. Model came back down ballistic, and was written off.

HTH

I have two three motor SR 71 conversions and have flown them many times with C6 0 plugged in the pods. Super easy build and I can't seem to CRASH them. The hardest part is keeping the nose from drag separation, gotta get the snugness just right every time .

 

[afadeev]

I have two three motor SR 71 conversions and have flown them many times with C6 0 plugged in the pods. Super easy build and I can't seem to CRASH them. The hardest part is keeping the nose from drag separation, gotta get the smugness just right every time .

Build thread?

 

[kuririn]

I have an old Estes Gemini DC. It is a single motor dual parachute and rear eject. Chutes are blown out from ducted booster side pods. I find that unless both pods "pop" at exactly the same time the relief of pressure and chute deployment in one pod will cause the other pod not to pop, or only partially eject. Not a big problem for this rocket cuz one chute will safely bring it down. Yours may need two.
As far as ejection gasses, yes they also contain flaming embers. The exposure of the wood and paper rocket parts is brief and gasses/embers are mostly ejected along with the laundry. Coating exposed surfaces with something like CA or plain wood glue or epoxy will protect them. Don't think that is really needed, but couldn't hurt.
I don't see a baffle in your drawings. You will still need wadding or a nomex protector for your chutes.
Happy flying!

 

[Daddyisabar]

Build thread?

I can't seem to post pics here anymore and I know there was one on the forum of it flying . Real easy. Just have to hack & sand the plastic nozzles to allow motors in the pods , friction fit. Add additional nose weight and that is it. The real sin and abomination comes with plugging the motor. Such horror not allowed on this family forum!

 

[BABAR]

Any chance you could just put a few small scraps of wadding material in forward of your motors? These should trap the fiery bits but still be small enough to blow out the ports.

 

[afadeev]

I can't seem to post pics here anymore and I know there was one on the forum of it flying . Real easy. Just have to hack & sand the plastic nozzles to allow motors in the pods , friction fit. Add additional nose weight and that is it. The real sin and abomination comes with plugging the motor. Such horror not allowed on this family forum!

I don't want to hijack this thread, but I had recently finished building a SR-71, and did proactively install 18mm motor mounts into the side pods, just in case.
My concern was the added weight in the rear of the rocket if I were to use them, and the impact on stability. How much more weight did you add to the nose to balance it out?

 

[mbeels]

Sounds like a good name for a rocket, "Hot Chunks of Fire".

 

[Daddyisabar]

I don't want to hijack this thread, but I had recently finished building an SR-71, and did proactively install 18mm motor mounts into the side pods, just in case.
My concern was the added weight in the rear of the rocket if I were to use them, and the impact on stability. How much more weight did you add to the nose to balance it out?

The way too do it is to load up a nice flying stock build and find the CG. Then load up your cluster and add weight to get the same spot.

 

[Initiator001]

I was not paying close attention when I saw the title of this thread.

At first I thought it was a discussion of eating Taco Bell food.

(I apologize for taking this thread off-tangent).

 

[lakeroadster]

Sounds like a good name for a rocket, "Hot Chunks of Fire".

Goodness Gracious Hot Chunks Of Fire!

 

[BABAR]

The way too do it is to load up a nice flying stock build and find the CG. Then load up your cluster and add weight to get the same spot.

That sounds rights, but adding the motors increases your mass which (for the SAME motor power ) means you are slower off the rod or rail. Assuming all clusters light you probably make up for the mass with your "trust in thrust" mantra. If the whole cluster doesn't light you now have a heavier underpowered model. From what read on Deuces Wild it flew okay on one mototr.

 

[mjennings]

I have a Gemini DC as well, after many flights the gasses burned through the tubes opposite the vents from the main body. I epoxied some small squares of thin Aluminum over the holes, and got many more flights. Now there is burn through under a fun that I'm trying to figure out how to repair. The dual rear deployment is tricky to pack usually takes a blow test or three to get that just right packing to get both chutes out.

Really cool design. If it's just got gasses or chunks of burning rocket love, there's the definite possibility of charring damage over many flights.

 

[lakeroadster]

....I would coat all body tube surfaces that will be exposed to hot gasses with thin CA.
This will provide a layer of reinforcement, and non-flammable barrier for hot particles to bounce of.

Also notice how violent ejection firing event is. That pressure wave will need to be controlled, routed to the side pods, then U-turned to push the chutes out....

I'm thinking perhaps a divider in the body tube might help "split" the pressure wave to help ensure both parachutes deploy? I'll fire up the CAD station and look at some options.

....How about leaving off an ejection charge on one of the motors.......

My plan in regard to motors is a F32-6T and a F32-8T.... a staggered ejection charge which also provides a "belt and suspenders" approach in the event one motor doesn't fire.

..... The exposure of the wood and paper rocket parts is brief and gasses/embers are mostly ejected along with the laundry. Coating exposed surfaces with something like CA or plain wood glue or epoxy will protect them. Don't think that is really needed, but couldn't hurt.
I don't see a baffle in your drawings. You will still need wadding or a nomex protector for your chutes.
Happy flying!

Any chance you could just put a few small scraps of wadding material in forward of your motors? These should trap the fiery bits but still be small enough to blow out the ports.

Wadding in the side pods to protect the chutes, sure. But I hesitate to put wadding in the main body tube as it may plug, or restrict, the openings going to the side pods?

 

[BABAR]

Wadding in the side pods to protect the chutes, sure. But I hesitate to put wadding in the main body tube as it may plug, or restrict, the openings going to the side pods?

I think you’d be okay with a few 1 inch shreds

 

[Daddyisabar]

Staggered ejection charges may give peace of mind but can be hell on shock cords.

 

[Daddyisabar]

I have never split a compartment. That would act as a stuffer tube for each side.

 

[lakeroadster]

Staggered ejection charges may give peace of mind but can be hell on shock cords.

How so? If they are staggered and the first ejection works, the laundry will be outside the tube.

 

[lakeroadster]

I revised the design by adding:

• An additional pressure portal on both sides and,
• An internal divider.

Also I coated the inside of the body tube and pods with thinned down Gorilla Glue.

 

[Daddyisabar]

How so? If they are staggered and the first ejection works, the laundry will be outside the tube.

The bottom of the shock cord and mount are are still in the tube and naked to the second charge.

 

[lakeroadster]

The bottom of the shock cord and mount are are still in the tube and naked to the second charge.

Got ya. Not so though on this design.