3 Stage Boosted Bertha

[A2R]

As I'm preparing to launch the 3-stage version in the spring, I was wondering if I should seal off the second stage from the third stage. With my current setup, sparks could potentially go past the second stage (between the 3 engines) and possibly pre-ignite the third stage. If I completely seal off the second stage (which I think I should do) from the third stage, do I need some sort of venting between the second and third stages? The gap should only be a couple of inches between stages, so I don't know if any sort of venting is necessary.

 

[BABAR]

Intriguing.

I want to make sure I have the nomenclature, correct.

I consider the first stage the booster,

in your case the second is the mid stage,

and the third stage is the sustainer.

If there is no gap between the second and third stages, does that mean that the sustainer motor “nests” into mid stage motor tube?

If this is the case, for a short 1-2 inch gap you probable don’t need a vent, although by habit I tend to put a vent hole in the lower stage tube just below the “nesting” point of the upper stage. Since I gap up to 50 plus inches I just automatically vent everything somehow.

Again, if upper stage motor nests into mid-stage, I don’t see any pathway for first stage to “accidentally” ignite third stage/sustainer

If you don’t “nest” the sustainer motor(s) into the mid stage, although theoretically it is POSSIBLE for first stage motor blow through to ignite sustainer, it’s REALLY unlikely.

Love to see pics, and hoping for at least three straight vertical trails!

 

[lakeroadster]

0.89 ounces of nose weight...

​

 

[A2R]

Intriguing.

I want to make sure I have the nomenclature, correct.

I consider the first stage the booster,

in your case the second is the mid stage,

and the third stage is the sustainer.

If there is no gap between the second and third stages, does that mean that the sustainer motor “nests” into mid stage motor tube?

If this is the case, for a short 1-2 inch gap you probable don’t need a vent, although by habit I tend to put a vent hole in the lower stage tube just below the “nesting” point of the upper stage. Since I gap up to 50 plus inches I just automatically vent everything somehow.

Again, if upper stage motor nests into mid-stage, I don’t see any pathway for first stage to “accidentally” ignite third stage/sustainer

If you don’t “nest” the sustainer motor(s) into the mid stage, although theoretically it is POSSIBLE for first stage motor blow through to ignite sustainer, it’s REALLY unlikely.

Love to see pics, and hoping for at least three straight vertical trails!

I have about a two inch gap between the 1st and 2nd stage and another 2 inches between the second and third. When I ran at as a two stage I painted some quick burst on the face of the sustainer stage to help ensure ignition.

 

[BABAR]

. When I ran at as a two stage I painted some quick burst on the face of the sustainer stage to help ensure ignition.

hmmm, that quick burst maaaaaaaay change the situation of first stage igniting sustainer.

Per Emma Kristal’s NARAM research


Successful standard black powder staging is not, as Stine wrote, due to flaming particles entering the sustainer (or in 3 or more stages, next) gap staged motor nozzle, but due to high energy photons. Which kinda makes sense, as the nozzle is a blind ending tube, and the harder you try to push more matter (gas, particles, what have you) into it, the more it’s just gonna compress and resist. Whereas photons have no trouble at all penetrating an air gap. So if you can get the photon source (hot flaming gas) CLOSE to the nozzle, some of the released photons will “shine” on the exposed propellant at apex of the nozzle and heat it up.

For gaps up to an unknown distance (I think two inches is probably fine unducted and unvented, I don’t know what the upper limit is) the flaming gas at burn through is probably concentrated enough that you don’t need a vent and it will still light for the intended motor to motor ignition. But on a three stage bird with no ducting or venting direct to the sustainer, even if the second stage “leaks” a bit and allows some gas through, I doubt it would be enough to photons to penetrate the air in the nozzle and light the propellant.

If however you paint the nozzle surface with flammable material, in my book all bets are off, even a small amount of hot gas might ignite the surface powder and ignite the motor.

I don’t really think you need the quick burst if you set up the motors right and keep the gap under 2 inches.

For black powder staging, particularly when you go to three stages, in my experience the most concerning thing is, unlike electronic staging, you have no altitude or tilt or velocity or whatever lockouts for the staging event. Once you light the lowest/first stage, the next two stages are going to light (or not) completely independent of the rocket’s attitude, altitude, and inertia. Winds that are not a problem for a standard single stage flight can turn a three stage sustainer into a cruise missile or a land shark. And it takes a lot of intestinal fortitude to finally finish a rocket, get a sunny day, get to the field, and decide the winds are just too high and you are NOT gonna fly that bird today and stick it back in the car unlit.

 

[SolarYellow]

0.89 ounces of nose weight...

View attachment 640392​

Huh. Suddenly, I think I know what to do with all those leftover sets of Baby Bertha fins.

 

[goose_in_co]

Huh. Suddenly, I think I know what to do with all those leftover sets of Baby Bertha fins.

That would be a good use for them, but the Baby Bertha fins are a little smaller than the Big Bertha fins.

 

[A2R]

hmmm, that quick burst maaaaaaaay change the situation of first stage igniting sustainer.

Per Emma Kristal’s NARAM research


Successful standard black powder staging is not, as Stine wrote, due to flaming particles entering the sustainer (or in 3 or more stages, next) gap staged motor nozzle, but due to high energy photons. Which kinda makes sense, as the nozzle is a blind ending tube, and the harder you try to push more matter (gas, particles, what have you) into it, the more it’s just gonna compress and resist. Whereas photons have no trouble at all penetrating an air gap. So if you can get the photon source (hot flaming gas) CLOSE to the nozzle, some of the released photons will “shine” on the exposed propellant at apex of the nozzle and heat it up.

For gaps up to an unknown distance (I think two inches is probably fine unducted and unvented, I don’t know what the upper limit is) the flaming gas at burn through is probably concentrated enough that you don’t need a vent and it will still light for the intended motor to motor ignition. But on a three stage bird with no ducting or venting direct to the sustainer, even if the second stage “leaks” a bit and allows some gas through, I doubt it would be enough to photons to penetrate the air in the nozzle and light the propellant.

If however you paint the nozzle surface with flammable material, in my book all bets are off, even a small amount of hot gas might ignite the surface powder and ignite the motor.

I don’t really think you need the quick burst if you set up the motors right and keep the gap under 2 inches.

For black powder staging, particularly when you go to three stages, in my experience the most concerning thing is, unlike electronic staging, you have no altitude or tilt or velocity or whatever lockouts for the staging event. Once you light the lowest/first stage, the next two stages are going to light (or not) completely independent of the rocket’s attitude, altitude, and inertia. Winds that are not a problem for a standard single stage flight can turn a three stage sustainer into a cruise missile or a land shark. And it takes a lot of intestinal fortitude to finally finish a rocket, get a sunny day, get to the field, and decide the winds are just too high and you are NOT gonna fly that bird today and stick it back in the car unlit.

I agree that you make some valid points. That was my first multi-stage/cluster rocket, and I just wanted all 6 motors to light and have a stage separation. I saw the quick burst as insurance. But after talking/typing out loud, I can see where it may not be beneficial and could have some drawbacks on a 3-stage setup. Also, last time, I was sure to keep both sets of engines in line with each other for whatever that was worth.
With regards to wind and weather in general, I have a number in my head (5 mph max) I basically got this from RockSim but I'm open to feedback.

 

[A2R]

That would be a good use for them, but the Baby Bertha fins are a little smaller than the Big Bertha fins.

This reminds me that I need to schedule some time with the laser cutter to make a stack of backup wings.

 

[BABAR]

With regards to wind and weather in general, I have a number in my head (5 mph max) I basically got this from RockSim but I'm open to feedback.

Funny, <= 5 mph was my SWAG as well!

 

[Alan15578]

I have about a two inch gap between the 1st and 2nd stage and another 2 inches between the second and third. When I ran at as a two stage I painted some quick burst on the face of the sustainer stage to help ensure ignition.

I always wanted to try something like that. Although I was thinking of a disk of flash paper secured with a Sure-Shot. Is quick burst the best material to paint on the nozzle of a BP motor to increase reliability of upper stage ignition?

I'm not sure I buy into that photon ignition explanation, but the way to test it would be to entirely block the path to the upper stage with a clean clear Pyrex glass slide, and test the upper stage ignition probability.

 

[BABAR]

I'm not sure I buy into that photon ignition explanation, but the way to test it would be to entirely block the path to the upper stage with a clean clear Pyrex glass slide, and test the upper stage ignition probability.

This may be like arguing how many angels can dance on the head of a pin, but so be it,

I think the plexiglass test would fail. I was thinking of an opposite test, a mesh screen that would allow gas to go through but not particles.

Logically I think it makes more sense than particle theory especially when you consider long gap staging.

I’ve done over 6 feet with a 24mm D motor on a test stand, although it failed in flight. I’ve don’t over 50 inches in flight, both successful.

Ducting and venting is essential in long gap staging whichever theory you want to use. You have to get a concentrated bonus of hot gas and/or particles from the booster to the sustainer nozzle.

There is no way to vent THROUGH the nozzle, it’s essentially a deep bowl with no outlet, so the best you can do is vent just below the nozzle out the side (or all sides, I leave a gap of about 1/8” between the upper end of the extended booster motor tube and the butt of the sustainer motor.

So all the gas and particles make it TO the nozzle, but the particles are going to go out the sides (could inertia drive a particle into the nozzle? Yes, but at 6 feet it would have to be well aimed, have enough mass to deviate from the flow stream which is going out the sides, and enough fire to light the powder. It just seems a bit unlikely that a flaming particle will follow the stream all the up the length of the extended motor mount tube and then suddenly deviate due to inertia fortuitously into a closed space.)

BUT, now imaging a ball or stream of flaming hot gas that reaches up to but NOT into the nozzle. This burning gas is emitting a sphere of outwardly streaming hot photons in all directions. While the jet stream affects the flow of the gas, it does NOT affect the directions of the photons emitted BY the gas. If I can get that ball to just below the nozzle, the concentration of photons is hopefully sufficient to light the powder, even if it is spread roughly spherically around the gas cloud.

Ducting and venting do two things in long gap staging.

First, you have a dead space of air temperature gas between the motors that, if allowed to compress, may blow the stages apart before the gas/particles arrive.

Second, the combination guides the gas/particle cloud to the opening of the nozzle.

 

[A2R]

I always wanted to try something like that. Although I was thinking of a disk of flash paper secured with a Sure-Shot. Is quick burst the best material to paint on the nozzle of a BP motor to increase reliability of upper stage ignition?

I'm not sure I buy into that photon ignition explanation, but the way to test it would be to entirely block the path to the upper stage with a clean clear Pyrex glass slide, and test the upper stage ignition probability.

I have no clue if it's the best material for us on a nozzle, but I used it, and all the second-stage motors lit. I'm hoping I can get this three-stage rocket to stay vertical and get all three stages to light on this next one. I will report back with my result, but I will probably have to wait about a month. The club launch is this weekend, but I can attend, and winds are supposed to be 15mph anyway.

 

[BABAR]

winds are supposed to be 15mph anyway.