# BP Staging Question...

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#### Mike Haberer

##### Well-Known Member
TRF Supporter
When staging BP engines, do the booster and sustained motors need to be abutted or can there be a stuffer tube between them. I ask because I am looking at a design where the booster is more than a booster engine in length (more like 6 inches, details as to why in the eventual build thread). The concern is igniting the sustainer, I can design it with a stuffer tube that is fire resistant and/or replaceable if it gets damaged over time.

##### Well-Known Member
Gap staging is easy. At 6 inches, no tube is needed. The heat and flame will get to the motor no problem.

#### Mike Haberer

##### Well-Known Member
TRF Supporter
I figured so, but wanted to verify. I'll make the MMT long enough to abut the sustained engine to save the @ end of the sustainer. I'll spray the inside of the MMT in front of the booster with 2000 degree black automotive paint.

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#### Zeus-cat

##### Well-Known Member
Do some research on gap staging. I have never done, but I have a design or two where it would be needed. You may want/need vent holes in the booster to make sure the upper stage has time to ignite instead of getting pushed off by pressure. Gap staging is a well known topic so you should find information that will help you decide what you need.

#### Mike Haberer

##### Well-Known Member
TRF Supporter
Do some research on gap staging. I have never done, but I have a design or two where it would be needed. You may want/need vent holes in the booster to make sure the upper stage has time to ignite instead of getting pushed off by pressure. Gap staging is a well known topic so you should find information that will help you decide what you need.
Thanks. By extending the MMT to the sustainer engine it won't be gap staged, it will be direct ignition like LPR staged rockets.

#### SharkWhisperer

##### Well-Known Member
Thanks. By extending the MMT to the sustainer engine it won't be gap staged, it will be direct ignition like LPR staged rockets.
Good idea to ground test that MMT assembly before installing and flying.

With BP to BP gap staging, I'd likely increase likelihood of second stage ignition with a little 4Fg/NC lacquer slurry on the receiving nozzle encasing some BP grains poured down the nozzle. Pretty much guaranteed fire transfer, and safer than relying on direct fire transfer that *usually* works over several inches with LPR/MPR, though many of us have seen failures.

#### teepot

##### Well-Known Member
TRF Supporter
There is a thread here somewhere that mentioned a gap of 4' or more. I built a two stage recently and also painted the booster inside and out with high temp paint.

#### Nytrunner

##### Pop lugs, not drugs
TRF Supporter
By extending the MMT to the sustainer engine it won't be gap staged,
Clarification: Unless the booster motor is directly abutting the sustainer motor, it is gap staged.

Even with the motor mount extending to the sustainer, you've got a slug of constrained air in between motors. That can get compressed by the ejection and pop the sustainer off before lighting.

Like Zeus said above, a vent hole is needed to allow that air out of the way as the ejection products move forward.

#### SharkWhisperer

##### Well-Known Member
Clarification: Unless the booster motor is directly abutting the sustainer motor, it is gap staged.

Even with the motor mount extending to the sustainer, you've got a slug of constrained air in between motors. That can get compressed by the ejection and pop the sustainer off before lighting.

Like Zeus said above, a vent hole is needed to allow that air out of the way as the ejection products move forward.
Ventilation is optional with a booster motor, though I almost always use vent holes (can fill them with hot glue or similar so they're smooth till heated and open up (maybe) if particularly anal--I dont. It's not an ejection charge, and carrys a fraction of the gas volume as an ejection charge, and it's vigorous but not "all' that vigorous of a flame. As another noted yesterday, a booster ignition "charge" is not anything except the propellent grain surface (with no clay endcap covering it in Estes BP motors) that continues burning. The relative pressures force flame forward, out the open tube moreso than the restricted nozzle end. Nothing high-tech. Propellent grain. End. No delay. No ejection. No endcap. Surprised Estes' dextrin-bound grains, built wide open to the environment, remain reliable after decades without major thermal cycling issues; probably humidity cycling with boosters could be problematic, too, but never read a verifiable report. Always with gap staging, I soup up my recipient motor nozzle with BP and cap it with BP/NC lacquer. Guaranteed flame transfer.

I mentioned using classic ejection motors and boosters for the same purpose in other threads, and how to accommodate them to guarantee reliable flame transfer from both, so perhaps poke through my recent posts for lengthy details that i don't plan to rewrite here. I specialize in "fire transfer", for various applications...

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#### JPalmer621

##### Well-Known Member
I fly a gap stage Saturn V. Stage 1 has 5 D12 motors. I ran the motor tube up to the stage 2 which only has 1 D12. I put 2 vent holes at the top of the tube to vent the preasure from popping the seconded stage. It is about 12 inches apart.

#### SharkWhisperer

##### Well-Known Member
Prfessr (Terry) indicates that he's successfully done direct flame transfer over 50"+ in a BT50, without bothering with a chimney/stuffer tube. Impressive.

But me? Would make an interesting ground test, but I'd have some serious convincing to overcome before I'd attempt that in flight. However, using any of many various "assisted" ignition/flame transfer approaches it'd be easy-peasy, even over that long distance (any distance, actually).

The flame-forward of a booster is absolutely not the high pressure achieved with a BP ejection charge. Venting is useful for evacuating cold gap air to be replaced by the booster propellant gasses, but it's pretty unlikely to develop anywhere close to the gas pressures from an ejection charge even without venting. Good insurance, though.

#### Dugway

##### Well-Known Member
The Apogee Flying Machine relies on zero delay motors to deploy streamers on both side boosters, so definitely strong enough that you want to vent the first stage for reliable staging.

#### BABAR

##### Builds Rockets for NASA
TRF Supporter
My max successful gap stage is 51 inches (done twice). I have done 72 inches on a test stand but was unsuccessfully in flight. I routinely and successfully do 18 and 36 inches.

IMO vents are necessary for All gap staging for two reasons.

1. You must have a free pathway for the white hot gas to flow to expose the nozzle opening. Because the nozzle is a “cul de sac”, you can’t get flow THROUGH the nozzle, you have to get hot photons close to the nozzle as they have no problem penetrating the “air dam” of the nozzle. Here’s a great NARAM presentation which appears to debunk Stine’s particle theory in his book.

2. Zero delay motor “blow through” (there is no clay cap, just a solid propellant slug) seems weaker than the typical standard Estes “shotgun” ejection charge, but it does have some pressure, and particularly for short gaps (and anything where the motors are not taped is essentially a short gap) IMO needs a vent to prevent the sustainer from “blowing off” before it ignites.

if you are going minimum diameter to minimum diameter, the vent needs to come out the side of the rocket. If you are going LESS than minimum diameter, you can put the holes in the stuffer tube or chimney or whatever you call it, these should be placed just below the base of the sustainer motor casing. but you will need holes somehere in the outer portion of the booster to let the gas out. HERE you can be creative, for cosmetic purposes you cam put holes in the BOOSTER centering rings and duct the gas out the rear, so no holes in the side of the rocket.

For me, biggest challenge in long gap staging is booster recovery, long boosters do not tumble well. They tend to return ballistic if no other means of recovery added. My favorite solution is a cluster booster with a small short delay motor “pod” on the side with a nose cone and chute. Almost ANY delay is short enough for BP powder, as the zero delay will burn and ignite sustainer well before even the shortest delay auxilliary motor. You WANT some delay, as you’d like the booster to slow a bit before deploying a chute. I usually find an A8-3 nice, it has a big nozzle, so I can easily get an E-match in there (I like E-matches for clusters, they are fairly reliable and they fit in As and Ds, Unfortunately, the cheap ones don’t fit into B and C motors, at least the ones I have don’t.) An unexpected benefit of the A motor, if the A auxilliary lights but the main doesn’t, the rocket barely makes it off the pad and the sustainer CANNOT light since main booster is source of sustainer ignition. I have seen this once, it is entertaining, a bit embarrassing, but very safe as the rocket has no power to go anywhere and just falls to the ground next to the pad.

While I generally do well with the new(well, not so new anymore) pyrogen free Estes starters for individual motors, I don’t have as much success with them for clusters.

hope you get two straight and vertical trails and a short recovery walk for booster and sustainer

#### SharkWhisperer

##### Well-Known Member
My max successful gap stage is 51 inches (done twice). I have done 72 inches on a test stand but was unsuccessfully in flight. I routinely and successfully do 18 and 36 inches.

IMO vents are necessary for All gap staging for two reasons.

1. You must have a free pathway for the white hot gas to flow to expose the nozzle opening. Because the nozzle is a “cul de sac”, you can’t get flow THROUGH the nozzle, you have to get hot photons close to the nozzle as they have no problem penetrating the “air dam” of the nozzle. Here’s a great NARAM presentation which appears to debunk Stine’s particle theory in his book.

2. Zero delay motor “blow through” (there is no clay cap, just a solid propellant slug) seems weaker than the typical standard Estes “shotgun” ejection charge, but it does have some pressure, and particularly for short gaps (and anything where the motors are not taped is essentially a short gap) IMO needs a vent to prevent the sustainer from “blowing off” before it ignites.

if you are going minimum diameter to minimum diameter, the vent needs to come out the side of the rocket. If you are going LESS than minimum diameter, you can put the holes in the stuffer tube or chimney or whatever you call it, these should be placed just below the base of the sustainer motor casing. but you will need holes somehere in the outer portion of the booster to let the gas out. HERE you can be creative, for cosmetic purposes you cam put holes in the BOOSTER centering rings and duct the gas out the rear, so no holes in the side of the rocket.

For me, biggest challenge in long gap staging is booster recovery, long boosters do not tumble well. They tend to return ballistic if no other means of recovery added. My favorite solution is a cluster booster with a small short delay motor “pod” on the side with a nose cone and chute. Almost ANY delay is short enough for BP powder, as the zero delay will burn and ignite sustainer well before even the shortest delay auxilliary motor. You WANT some delay, as you’d like the booster to slow a bit before deploying a chute. I usually find an A8-3 nice, it has a big nozzle, so I can easily get an E-match in there (I like E-matches for clusters, they are fairly reliable and they fit in As and Ds, Unfortunately, the cheap ones don’t fit into B and C motors, at least the ones I have don’t.) An unexpected benefit of the A motor, if the A auxilliary lights but the main doesn’t, the rocket barely makes it off the pad and the sustainer CANNOT light since main booster is source of sustainer ignition. I have seen this once, it is entertaining, a bit embarrassing, but very safe as the rocket has no power to go anywhere and just falls to the ground next to the pad.

While I generally do well with the new(well, not so new anymore) pyrogen free Estes starters for individual motors, I don’t have as much success with them for clusters.

hope you get two straight and vertical trails and a short recovery walk for booster and sustainer
Nice, thorough discussion. Tx.

#### Stefan2k4

##### Active Member
The flame-forward of a booster is absolutely not the high pressure achieved with a BP ejection charge. Venting is useful for evacuating cold gap air to be replaced by the booster propellant gasses, but it's pretty unlikely to develop anywhere close to the gas pressures from an ejection charge even without venting. Good insurance, though.
No doubt, it's likely true the booster motors don't develop as much pressure as a normal ejection charge, but they still develop a fairly substantial amount of pressure. Check out the Estes Cato that used a C6-0. The pressure developed when the C6-0 burnt through was enough to actuate a piston that pushed against a dowel and a larger piston in the upper body tube that pushed off the nose and pushed out a parachute. Then when the piston reached it's limit, it pushed the motor mount tube backward which separated the motor tube from the upper body tube and released the segmented panels with each fin attached thus simulating a cato. So, clearly there must be a significant amount of pressure to accomplish all of this. My guess is it's quite likely that as the propellant grain approaches the point of burn through and the front wall gets very thin, there's likely some point it cracks and exposes more surface area which increases burn rate and spikes the pressure somewhat.

#### caveduck

##### semi old rocketeer
Good discussion here. The need for venting was discovered a really long time ago and (I believe) first commercialized as the Centuri "Pass-Port" as seen in the Black Widow. I saw that NARAM report presented and it did a great job of showing that it's the column of hot plasma that does the igniting of the upper stage, not any kind of particles.

FAI Scale Altitude models are often gap-staged at distances of half a meter...they use what's called a "flash tube" with some BP stuck to the inside to enhance reliability. It works really well, or they would not do it at the world championships

#### SharkWhisperer

##### Well-Known Member
No doubt, it's likely true the booster motors don't develop as much pressure as a normal ejection charge, but they still develop a fairly substantial amount of pressure.

My guess is it's quite likely that as the propellant grain approaches the point of burn through and the front wall gets very thin, there's likely some point it cracks and exposes more surface area which increases burn rate and spikes the pressure somewhat.
Agreed that booster's put out a decent flame front. There's a recent D12-0 video burn on YT; unfortunately it's a vertical nose-down burn so you can see flame spread out the holes of its test mount, but cannot appreciate the length/shape of the actual boost burn flame front. While there's plenty of static motor tests clearly showing BP ejection charges going off in detail, seems nobody's done a side-by-side video comparing a BP booster and sustainer from the same motor class, which would be useful. Regarding burn rate/pressure in the booster, you think the burn-through will expose surface area and increase pressure. Sure, but not instantly. Any initial burn through-related increase in burning propellant mass would instantly encounter the lower airframe pressures (vs the high internal motor pressures). This would reciprocally slow burning further as pressure instantly dissipates, but might speed back up as repressurization of the now much larger airframe volume (or stuffer tube...regardless, it's low ambient pressure until the burn-through) occurs.

Besides videoing the actual flame fronts in ground testing, it'd be very useful to actually compare intra-BT pressure wave shapes, magnitudes, and timing in a body tube exposed to a sustainer burn-through vs a typical ejection charge. Would be very surprised if this hasn't been done already, probably many times, but I've not been able to locate any actual such data to date. Of course any BP motor manufacturer tests booster and ejector flame/pressure characteristics in depth. But seeing a visual burn comparison, as well as a pressure/time graph, would definitely be interesting. Ejection charges are by nature designed with a safety overpressure margin--a more violent ejection event can be mitigated by tube strengthening and a longer shock cord whereas a wimpy ejection could have you running for cover. Although beyond the scope of this specific thread, it is an easy task to enhance (or muzzle) flame generation & delivery--to anything desired actually--from a booster motor distal opening with some very simple ahem "research" modifications.

#### shockie

##### High Plains Drifter
Good discussion here. The need for venting was discovered a really long time ago and (I believe) first commercialized as the Centuri "Pass-Port" as seen in the Black Widow. I saw that NARAM report presented and it did a great job of showing that it's the column of hot plasma that does the igniting of the upper stage, not any kind of particles.

FAI Scale Altitude models are often gap-staged at distances of half a meter...they use what's called a "flash tube" with some BP stuck to the inside to enhance reliability. It works really well, or they would not do it at the world championships
Per that NARAM report.....this seems to indicate that there is particulate being blow out by booster engines and not just a flame front: check out around 1:36

AT 1:36 2 D12-0 are burning through and sparks are heading everywhere out the exhaust ports..... I'm trying to get the original footage so I can slow it and see it even better

#### SharkWhisperer

##### Well-Known Member
Per that NARAM report.....this seems to indicate that there is particulate being blow out by booster engines and not just a flame front: check out around 1:36

AT 1:36 2 D12-0 are burning through and sparks are heading everywhere out the exhaust ports..... I'm trying to get the original footage so I can slow it and see it even better
There's two video versions floating around on YT by the same guys. One a lengthy droning discussion that was linked to in the E12-0 video link (the two outer booster motors are D12-10s, not E's as the thread title suggested, but they're all 24 mm diameter. An E12-0 would be expected to behave identically to a D12-0. Another shorter video is out there that just shows the action scene.

But no matter--it's clear that a bunch of burning particulate is blowing out of booster BP motors--it's expected. All the "boost" is is the end burn of the BP propellent grain, which upon burn-through of course shoots burning propellant frags along the pressure gradient (i.e., out the front of the motor against lower pressure than the booster combustion chamber--briefly, at least). Hot gasses sure, but a whole lot of BP frags that would instead be ejected out the nozzle in a sustainer with a delay charge buffering the propellant from the ejection charge (which also spits hot particles of its own in addition to gas/flame...).

Yeah, I'd be wearing pretty thick goggles before I'd try to light a cigar using a BP booster motor that's passing fire forward! Even a baby A8-0!

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