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  #11  
Old 11-08-2013, 11:47 PM
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luke strawwalker luke strawwalker is offline
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Quote:
Originally Posted by blackshire
This is interesting. *If* the black powder is fuel-rich, the "excess" fuel could be burning with the oxygen in the volume of air inside the 'plenum' in the lower motor, above the booster propellant grain.


There wouldn't be any air in the space below the upper nozzle-- at least not a few hundredths of a second after the first motor burn-through... it would be consumed by the blow-through and ignition of the upper motor...

Once the upper motor ignites, it will simply repressurize the lower casing to a level required to create equal flow through both nozzles, proportional to their diameters...

Later! OL JR
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  #12  
Old 11-09-2013, 08:53 AM
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Originally Posted by Gus
Blackshire,

In test flying FAI altitude models I've had two instances where the 12" long 40 mm diameter booster (roughly BT-60 size) did not separate from the gap staged 6" long BT-20 sized sustainer. Both booster and sustainer were using Estes 13 mm motors. In each instance the sustainer sent its thrust exhaust down through the 12" long gap staging tube and out the spent A10-0 motor in the booster. Surprisingly, neither booster caught fire and, although thrust was greatly diminished by being sent through a "muffler", the rockets could still clearly be seen to be thrusting. Not something I'd intentionally do but very interesting to see and similar, I suspect, to what you'd see in a tandem.

Steve
Thank you--and Jerry--for posting your information here. The data and experience indicate that true tandem motors (with butt-joined or slightly overlapping casings) produce somewhat greater impulse than both motors alone; the burning inner wall of the lower (booster) motor's case probably adds to the mass flow, as Jerry noted. Regarding what's legal and what's not as per the Pink Book, it is totally irrelevant to me because I fly alone, in a moist green pasture. My only "rule" is a model rocket application of the Wiccan rede (in more modern English): "If it harms no one, do what you wish."
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  #13  
Old 11-09-2013, 09:07 AM
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Model rocketry is only one of many ways to comply with the existing law and operate propulsion devices. It is not "illegal" to make a tandem motor or even a firecracker to report rocket altitude. They simply have different compliance methods than the model rocket safety code and NFPA-1122.

A tandem can be operated entirely legally and safely. If NAR chose to bless them they could within existing NFPA-1122 since neither motor is "modified". Applying adhesive to a model rocket motor in the form of masking tape is not considered a modification either, but you are gluing something to the motor. When an FAI guy super glues the motor into the mount to prevent ejection, that tiny spot of glue allows it to be removed on command by the user.

Estes not honoring its warranty if you use the product in a manner other than they explicitly recommend, is simply them planting a flag on their own hill, not saying you cannot also have a hill. If you use an Estes motor in a Quest kit, they do not explicitly warranty it, but they sure like it because you used their main revenue generator (That's woosh generator to you).

So if you make tandem motors and use them in a USR Tandem Goodness, which is designed bespoke for tandems, they like your using up their motors, but in no way would honor a claim for a failed product if it (the already used and expended) happened to burn through. USR would get the replacement kit sale.

There is no rule against tandems, outside of the NAR suggesting its ~5000 members not use them, 40 years ago almost nobody even remembers. There are well over 200,000 model rocketeers. And, yes, if they were submitted for cert by a 3rd party, they would be tested before they were ruled on. The folks running NAR S&T are former MIT geeks. Inescapable folks.

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Last edited by Jerry Irvine : 11-10-2013 at 03:33 PM.
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  #14  
Old 11-09-2013, 09:28 AM
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Quote:
Originally Posted by luke strawwalker
There wouldn't be any air in the space below the upper nozzle-- at least not a few hundredths of a second after the first motor burn-through... it would be consumed by the blow-through and ignition of the upper motor...

Once the upper motor ignites, it will simply repressurize the lower casing to a level required to create equal flow through both nozzles, proportional to their diameters...

Later! OL JR
That air wouldn't just vanish; it would be pushed forward into the upper motor's nozzle (along with the hot gases and burning propellant particles from the ruptured remnant propellant "disc" of the lower booster motor); regardless of whether that air's oxygen supported combustion of fuel-rich black powder (if it *is* fuel-rich; I don't know if it is or not) or combustion of the cardboard inner casing wall (or both), it would serve to increase combustion and therefore the gas production, which would increase the mass flow out through the booster motor's nozzle. (Even such a small volume of *pure* oxygen [the oxygen content of air is about one-fifth of that volume] will make a glowing wood "splint" go WHOOSH! and create a large volume of gaseous combustion products, so even that small volume of entrapped air between the motors--the nitrogen and minor constituents as well as the oxygen--contributes to the mass flow as well as the combustion for a brief time.)
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  #15  
Old 11-09-2013, 10:19 AM
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Quote:
Originally Posted by blackshire
That air wouldn't just vanish; it would be pushed forward into the upper motor's nozzle (along with the hot gases and burning propellant particles from the ruptured remnant propellant "disc" of the lower booster motor); regardless of whether that air's oxygen supported combustion of fuel-rich black powder (if it *is* fuel-rich; I don't know if it is or not) or combustion of the cardboard inner casing wall (or both), it would serve to increase combustion and therefore the gas production, which would increase the mass flow out through the booster motor's nozzle. (Even such a small volume of *pure* oxygen [the oxygen content of air is about one-fifth of that volume] will make a glowing wood "splint" go WHOOSH! and create a large volume of gaseous combustion products, so even that small volume of entrapped air between the motors--the nitrogen and minor constituents as well as the oxygen--contributes to the mass flow as well as the combustion for a brief time.)


Maybe for a few hundredths of a second... no more...

You'd have to show me data to convince me otherwise...

The ablation of the lower casing inner wall I can see... this couple CC's of entrapped air trapped between the lower motor propellant and the upper nozzle contributing to additional power when it's being passed and mixed with burning BP at the precise time thrust drops in the lower motor due to burnthrough of the propellant disk and ignition of the upper motor, and repressurization of the lower case to equalize flow rates through both nozzles... negligible to non-detectable I'd say...

Later! OL JR
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  #16  
Old 11-09-2013, 05:06 PM
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The entrapped air is about 0.000001% of the deal and the eroded paper is about 10% of the deal.

The more interesting thing is the BP burn rate vs pressure does not change. So the burn time of the second motor is exactly the same (the motor is NOT modified!). But, BP ISP increases with pressure so some of the thrust increase comes from the increased ISP from the interstage pressure spike and the increased pressure by having "free fuel" from the eroding paper. Don't forget that even paper contains both fuel and oxidizer. The BP itself has an unconventional oxidizer in Sulfur.

And that interstage spike you get, try modeling that in Rocsim. The altitude advantage will shock you. You will need electronic ejection because no motor has a long enough installed delay.

Don't forget zinc-sulfur micrograin motors. ISP 50-60!

BP is 60-80.

APCP is 180-245.

Sea Level.

Tech Jerry
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  #17  
Old 11-09-2013, 08:21 PM
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I never said or implied that the entrapped air contributes a significant amount of impulse, just that it cannot be discounted, as its oxygen content supports some additional combustion and its mass contributes to the total mass flow, albeit briefly and in a small measure, before it is "replaced" in that volume by combustion products. But to move from academic to practical aspects of this configuration:

Since tandem motors work and can be used anywhere except at NAR-sanctioned events (which means at/in the majority of model rocket launches), and since you, Jerry, have already produced a tandem motor rocket kit, this is a propulsion option that deserves to be popularized. It would also add realism to scale models of rockets and missiles that use or used two-pulse solid propellant rocket motors; these include the Boeing AGM-69 SRAM (Short-Range Attack Missile, see: http://en.wikipedia.org/wiki/AGM-69_SRAM ), the SRAM's Soviet/Russian equivalent (the Raduga KH-15 "Kickback," see: http://en.wikipedia.org/wiki/Raduga_Kh-15 ), the "Sugar Shot To Space" amateur rocketry project's rocket (see: http://sugarshot.org/index.html ), and at least one Chinese Air-To-Air missile.
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  #18  
Old 11-10-2013, 09:11 AM
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Quote:
Originally Posted by Jerry Irvine
The folks running NAR S&T are former MIT geeks.



None of the current S & T staff attended MIT as a student. Jack is employed by MIT. We were all educated elsewhere but could easily be labeled geeks.

I personally don't have a problem with tandem motors but I suspect that if you tried it with the current Quest motors the first stage casing would burn through and damage the motor mount.
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  #19  
Old 11-10-2013, 10:26 AM
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Quote:
Originally Posted by blackshire
... as its oxygen content supports some additional combustion ...

Um. Black powder already contains an optimal amount of oxidizer; adding oxygen doesn't help matters, though one might argue that it would support burning out more of the casing.
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  #20  
Old 11-10-2013, 11:25 AM
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Quote:
Originally Posted by billspad
I personally don't have a problem with tandem motors but I suspect that if you tried it with the current Quest motors the first stage casing would burn through and damage the motor mount.
The controvercy was over whether proven combos of tandems, some insulated, some not, could be used in NAR contests. The suggestion was made to static test and "certify" certain combos so the true net total impulse was known so "hot" motors were not possible.

The "problem" arose because it was noted that two motors combined in a tandem or group had more power than the two combined in traditional mode.

About that same time an FSI E60 and FSI F100 combined (staged or clustered) were under 80ns (an F) for purposes of NAR competition, for example.

I believe the attached images are from the Trip Barber tandem report. I believe the initial and final mass data would be the most instructive.

Jerry

As a shout out to GH both the Frost report and the Barber report are repleat with references to B14's!
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Last edited by Jerry Irvine : 11-10-2013 at 02:59 PM.
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