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  #1  
Old 05-23-2013, 03:28 AM
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blackshire blackshire is offline
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Default "Ersatz" tandem motors?

Hello All,

The two-pulse rocket motors used in the "Sugar Shot to Space" rockets (see: http://sugarshot.org/index.html ) got me wondering about something that would be pertinent to flying scale models of these vehicles:

Tandem motors (for example, a 13 mm mini motor whose nozzle end is friction-fitted [or glued] into the front end of an 18 mm motor) are not permitted by the NAR, because they were ruled to constitute modification of motors. However, what is the status of "ersatz" tandem motors--that is, two motors that are positioned (in motor mounts) in a model rocket so that the upper motor thrusts through the lower motor? For example, in a "Sugar Shot" scale model, two motors (say, a C6-0 and an A8-5) could be mounted in line--as if they were in a two-stage model--but in a double-length motor mount tube instead of in two separating stage airframes. Also:

Flying scale models of the AGM-69 SRAM (Short-Range Attack Missile, see: http://en.wikipedia.org/wiki/AGM-69_SRAM and http://www.google.com/search?q=Shor...biw=792&bih=377 ) and its Russian analog, the Raduga KH-15 (see: http://www.globalsecurity.org/milit...ussia/as-16.htm and http://www.google.com/search?q=kh-1...biw=792&bih=377 ), which also use(d) two-pulse solid rocket motors, would also benefit from "ersatz" tandem motors, as they would enable realistic powered flight characteristics.

I hope this information will be helpful.
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Old 05-23-2013, 07:08 AM
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Shreadvector Shreadvector is offline
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Burn through of the lower casing was the main failure mode and safety concern, so your proposed configuration will still have the safety issues PLUS the added new issue of no seal whatsover between the two motors, so there i no way that the hot exhaust from the upper motor will stay in the lower motor and flow only out the lower nozzle. It will leak out the butt joint between the non attatched motors and produce a flaming mess.
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Old 05-23-2013, 08:38 AM
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Old 05-23-2013, 05:51 PM
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Quote:
Originally Posted by Shreadvector
Burn through of the lower casing was the main failure mode and safety concern, so your proposed configuration will still have the safety issues PLUS the added new issue of no seal whatsover between the two motors, so there i no way that the hot exhaust from the upper motor will stay in the lower motor and flow only out the lower nozzle. It will leak out the butt joint between the non attatched motors and produce a flaming mess.
Not if the junction between the motors was properly designed, although this would be easier to do with an 18 mm-to-13 mm (or a 24 mm-to-18 mm) motor combination. For example:

With an 18 mm/13 mm powered "Sugar Shot" scale model having, say, a BT-50 body tube, the rocket's body tube could "break" above the tandem motor mount, with a JT-50 tube coupler (stage coupler) glued into the top end of the lower tube section. The 18 mm lower motor would be secured in a standard motor mount that would be equipped with a motor clip and a BT-5/BT-20 centering ring, which would also serve as a thrust ring. The BT-5 upper motor mount tube would be glued to the BT-5/BT-20 centering ring. Also:

The 13 mm motor would be inserted into its motor mount through the *top* end of the BT-5 motor mount tube, with a "backward-mounted" 13 mm motor clip preventing the 13 mm motor from moving forward when it was thrusting. A spacer shim or ring inserted atop the 13 mm motor would ensure a tight butt joint between the two motors. In addition:

All of the interior tube surfaces at the junction plane of the motors, as well as the inside surfaces of both motor mount tubes, could be flame-proofed with "water glass" (a solution of sodium silicate, which is also used to flame-proof the rolled cardstock [and thin rolled 1/64" sheet balsa] exhaust tubes of Jetex model airplanes). Two wraps of masking tape applied to the 18 mm motor--one wrap at the top and one wrap at the nozzle end--would serve as gaskets. As well:

Such a model could be flown either with -only- the lower 18 mm motor (using a 13 mm motor spacer tube in the upper motor mount in order to further protect the inside walls of the upper motor mount tube from the lower motor's ejection charge) or with the tandem-mounted motors. For the latter configuration, a B6-0 to 1/2A3-4T combination should work well.
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Old 05-23-2013, 06:04 PM
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Quote:
Originally Posted by Jerry Irvine
Very interesting..! Do you sell the tandem motor tech report separately?
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Old 11-08-2013, 01:55 PM
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There were a few articles on Tandem Motors in Model Rocketry magazine. Someone with access to the Mrm index could look those up and post them so I could research the exact combinations that proved good and bad.

Jerry

cites:

Tandem Model Rocket Engines, Trip Barber: MR Feb 76, 3p. and NARAM-17 R&D, 66p. - Dean

Anyway, the entire thrust curve for the second motor is 10% higher thrust than the normal motor or the booster motor. I ran them on the MIT test stand in the late 1970's. - Shred

I'm more interested in understanding the 10%. If what you're saying is the second half of the curve looks like the first half except the thrust is 10% higher all the way across, that would indicate that more is happening than just the blow through propellant being used. I find it hard to believe that it's coming from just the first casing being burned but I also can't come up with another explanation. You don't find that interesting? - Spad

I found it interesting in the 1970's. It was explained. it is the additional mass of the inside of the lower casing being eroded. A normal long burning motor will have a small amount of casing erode and over time the lower end can get too thin and burn through. With a tandem motor, the exhaust from the upper motor is injected at high speed into the lower motor casing because there is a nozzle in the upper motor. This erodes the crap out of the lower casing. f=ma. casing has m. more m gives you more f. Yes, again, the thrust curve of the upper motor is identical to a normal curve but 10% higher thrust for the exact same time. - Shred

Doug Frost entered a R&D report in 1966 at NARAM-8 on the subject of what he called "Ganged" motors. It is reported that he worked 2 years on this project. Doug entered an updated and expanded version of this report at NARAM-12(1970). - Dean

if you used say a B6-2/B6-6 combo, you would have one of those pulse-BP motors, that FSI tried out once. - Dean

Tandem motors created a 10% boost in the complete thrust curve for only the upper motor - Shred

In tandem motors, with the booster blow-through chunks contained, there was a HUGE and very short spike in pressure and thrust followed by the upper motor ignition. This secondary pressure spike is very nasty and much higher than normal peak thrust. - Shred

I cannot speak to other manufacturers or Estes since I've retired, but while I was there, the "0" engines (another controversy) did have more propellant than the delay versions of the same engines. This is because operators adjust powder to reach target impulse figures during testing. It has been many years since I tested any tandem engines, but, if my memory is correct, the source of extra total impulse in my testing appeared to be due to the powder from the booster motor being contained and utilized and very little else. I do not remember seeing any appreciable increase in thrust level from the upper engine. - Ed Brown

the entire thrust curve for the second stage of a two stage tandem (C6-0 epoxied below a C6-5 or C6-7 with a BT-20 sleeve epoxied around the both of them) was 10% higher than the C6-0 or another C6-5 or C6-7 from the same package of motors used to construct the tandem. You had one curve and then the next one started after the booster blowthrough spike and looked the same but higher. - Shred

As others have said the practice was stopped because the manufactures said it using the motors in way they did not intend. I used tandems before this was brought out and never had a failure - Winings

Last edited by Jerry Irvine : 11-08-2013 at 02:51 PM.
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Old 11-08-2013, 04:37 PM
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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.
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Old 11-08-2013, 06:23 PM
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Jerry Irvine Jerry Irvine is offline
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Not enough oxygen mass there to matter. However it changes the stoich ratio of the upper propellant and also adds massflow from casing erosion. However the plenum comment is valid.

Tandems rock. I was pissed when they didn't get approved, and yes I agree with one poster it was a result of one "competitor" (Pink Book Lawyer) making issue to maintain a competitive advantage. Estes was ignoring it until then (when NAR made a ruling).

Curious about Wining's or Frost's credibility? I refer you to Model Rocketry magazine (repeatedly).

At NSL Lucerne I showed Trip a copy of the Tandem Goodness kit. He managed to keep a straight face. I would cheerfully send him a few!

The value of the Barber and Frost reports is determining which combos work and which need an insulation tube.

Tandem goodness can be built without the motor mount tubes installed in the rings to accept an insulated motor "group" directly.

When a Tandem catos, it happens at altitude safely, and the manufacturer is in no way liable. What members do with their property is their business. Heck, sign an online waiver for access.

Model rocketry is so safe we are splitting hairs now. Wild West needed back.

My takeaway from those many Googled references, was the BP Estes gets varies widely and they have to do live batch testing, hence Vern's valuable and cool test stand product. Everyone should buy one.

Tech Jerry
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Last edited by Jerry Irvine : 11-09-2013 at 03:04 PM.
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Old 11-08-2013, 07:33 PM
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Jerry Irvine Jerry Irvine is offline
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To mis-quote GH, if it's technically legal, and works most of the time, or is reasonable without personal or property damage, DO IT!!!!

See, I used the extra exclamation points and everything.

cites:

Heck, sign an online waiver for access.

___Tandem approved
___Power as a minor approved
___Igniters, Fuses, Ejections
___HPR Level 1 and 2 approved
___AMR level 3 and 4 approved
___Pro approved
___Ship anything MR anywhere

Last edited by Jerry Irvine : 11-08-2013 at 08:02 PM.
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Old 11-08-2013, 11:26 PM
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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
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