MClark
Well-Known Member
As you state the Nike is a REAL motor, it was designed to be a rocket case and is made of steel. Doesn't compare to irrigation pipe.
I still would not use a seamed tube.
I still would not use a seamed tube.
An "R"-powered rocket build
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Ez2cDave
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Hi, Jarrett !
I am currently designing a 6" diameter, about 11 ft long, all PML Phenolic tube, L3 rocket, doubled with coupler's, since the outer tube will not be glassed. I'm predicting an empty weight of 22 - 24 lbs. - This will fly on a "Baby M" and stay sub-Transonic ( under 900 ft/sec ), probably around 7000 - 7500 ft AGL.
I am not using all-thread, except in a couple of high-stress areas where the shock cords attach and in the electronics bay.
One thing that we BOTH have to watch out for is when we go to slide the outer tube over the coupler's, because the adhesive may "grab" the tube before it can be slid all the way into position. I plan to do mine in sections, inserting the coupler's, one at a time. The PML Phenolic coupler's come in 48" lengths and I will be cutting them into shorter lengths to provide less surface area to "grab", which would be a "disaster" ! I will not be using additional Centering Rings, since all-thread is being sparsely-used.
You mentioned using a 4"dia. airframe . . . Let me suggest that you increase that to a 5" / 5.5" / 6" diameter with a 75mm motor mount. Your final overall length will probably end up being 9 - 12 ft tall.
Dave F.
Ez2cDave
Well-Known Member
Mark,
Chuck never said it was "irrigation pipe", as I recall . . . The REAL Nike booster also produces over 87,000 lbs of thrust for 3.5 seconds, too. Chuck's motor will produce only about 7% of that.
Dave F.
This looks seamless https://www.onlinemetals.com/merchant.cfm?id=71&step=2⊤_cat=60 9" od 1/2" wall 6061 T6 extruded
Ez2cDave
Well-Known Member
Paul,
Yes, it does say "extruded". That particular tube is 6-61-T6 Aluminum / 9.00" OD / 8.00" ID / .500" Wall / 15.702 lbs/ft. / $1565.63
So, an 8ft length would weigh 125.6 lb. . . .
According to Chuck, his empty motor ( presumably with end closures ) weighs 90 lb.. He never mentioned its length, as I recall.
Dave F.
Rail Dawg
Always learning!!!
That’s a great link Paul. I’ll contact them about the seams.
To others cautious about the current casing having a seam trust me it’s being addressed.
Pressure-testing to 2000 psi and ensuring little heat transfers to the casing might make it viable.
I’ve got two of the best big motor builders in the country on this and won’t commit the casing to launch unless it passes all specs.
I do like the tube Paul posted. Extruded can be inferior to DOM due to variances in the ID.
An issue that can be worked around but it is a factor.
All your inputs are welcome of course. We look at everything.
Chuck C.
He said 7 1/2'
You need a wider angle lens for larger projects... 
Ez2cDave
Well-Known Member
Chuck,
When they pressure-tested the Nike M-5 boosters, they used 1575 psi for 3 minutes, according to the attached PDF document . . . An interesting read !
Dave F.
Ez2cDave
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Rocketjunkie
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Burnsim is theoretical. Everything is perfect, grains all burn out at exactly the same time. It does not take into account erosive burning or manufacturing variations. By the core/throat ratio, this motor will be quite erosive. There will be a tail off in thrust as the bottom grain burns out first and the initial thrust will be higher than the sim shows..
Ez2cDave
Well-Known Member
One of my major concerns with the "theoretical" Burnsim graph is that I have a nagging feeling that we only got to see theoretical data for 2/3, or less, of the total motor burn. It only showed a "curve", starting at 4000 lb-thrust / Peak Thrust / back down to 4000 lb-thrust, without showing all of the other data, down to Zero Thrust.
It's a lot like looking at data from a drag car on a Dynamometer, when analyzing the "Torque Curve", to establish optimum Shift Points, in that it disregards power production at higher RPM's once the Torque drops below the "target threshold". They call that "power under the curve", but it is not complete data for the entire Dyno pull.
I don't think it would have much of an influence on the absolute maximum airspeed of the rocket, since Thrust is declining, BUT I know it will keep the rocket above Mach for a longer period of time. The concern there is aerodynamic heating and performance well above the 42,000 ft. AGL, predicted by another Sim, possibly 50,000 - 55,000 ft. AGL ( We can't know for sure without the rest of the theoretical Time-Thrust Curve ).
Hopefully, the motor will pass pressure-testing and we will be able to see a complete, "non-theoretical", Time-Thrust Curve, from an actual test-firing.
Dave F.
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eggplant
L3 | NAR 93664, TRA 17791
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Though the motor obviously won't shut down that quickly, there is no reason to assume that motors must have a long trail off. The regressive thrust curves of many commercial motors are due to the erosive phenolic nozzles companies like Aerotech and CTI use, which have nozzle throats that constantly increase in diameter as the burn progresses. These motors typically feature progressive grain geometries, but the constantly increasing nozzle throat makes the overall motor regressive. Companies like Loki and most EX motor makers use graphite nozzles that do not erode. A constant nozzle throat diameter paired with a properly designed grain geometry can result in a very neutral profile as shown in the burnsim output. If you don't believe me, check out some real results:
The Loki L480 features a very neutral burn profile that returns to startup thrust and then shuts down in around a third of a second:
A 1G 98mm motor my team burned on the test stand shut down even faster (forgive our low-res load cell, it was sized for larger motors):
For this specific motor, though, there is no reason to believe that the thrust curve will be exactly as pictured. The high mass flux at startup indicates that there might be erosive burning near the ignition transient, meaning that the bottom grains will probably burn out before the upper ones. This would mean a longer shutdown period, but similar delivered impulse. Pat has been making motors much longer than I have, so I'm sure he's aware of these effects and prepared a burnsim model that he thinks will be useful. Until a test burn proves otherwise, that is likely a good approximation of the real motor.
The Loki L480 features a very neutral burn profile that returns to startup thrust and then shuts down in around a third of a second:
A 1G 98mm motor my team burned on the test stand shut down even faster (forgive our low-res load cell, it was sized for larger motors):
For this specific motor, though, there is no reason to believe that the thrust curve will be exactly as pictured. The high mass flux at startup indicates that there might be erosive burning near the ignition transient, meaning that the bottom grains will probably burn out before the upper ones. This would mean a longer shutdown period, but similar delivered impulse. Pat has been making motors much longer than I have, so I'm sure he's aware of these effects and prepared a burnsim model that he thinks will be useful. Until a test burn proves otherwise, that is likely a good approximation of the real motor.
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Ez2cDave
Well-Known Member
After reading that info you posted and seeing the data graphs, many of my concerns have been relieved. I feel considerably more confident and "comfortable" now. We have been wrestling with Fin planform changes to address potential flutter problems in the original design. Experimental motors are not in my repertoire, at least not yet . . . Now, if I could just pick the right Lottery numbers - LOL !
Thank you, very much !
Dave F.
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This is exactly why I was asking Chuck about the forthcoming test and how much data would be gathered
To eggplant’s post, no doubt that a one grain motor with a graphite nozzle can “turn off,” but the motor in question here is a multi grain beast that is several feet tall. In this configuration it is very likely that the grain closest to the nozzle will be consumed earlier than the grain furthest from the nozzle and this,graphite nozzle or not, will affect the thrust curve
Ez2cDave
Well-Known Member
I am absolutely certain of one thing . . .
Each Team Member has their own area of expertise and I, for one, am exceedingly grateful that the physical aspects of the motor are NOT in my "job description" !
We are the "Steely-Eyed Missile-Men", each in our own way . . . Adapt & Overcome !
Dave F.
Rail Dawg
Always learning!!!
The motor test stand to be used is one of the most sophisticated ones available to amateurs from what I understand.
Not only internal pressures but temperature of the outer casing along with other parameters.
Quite a bit of discussion going on about getting a new casing built to the necessary high standards. Looking at an 8” comparable to what we currently have.
Chuck C.
Ez2cDave
Well-Known Member
Chuck,
Unless the length of the case increases, the decrease in diameter will, likely, drop you out of the "R" impulse range . . .
In order to maintain the "status quo" of the current motor ( 5725.566 cubic inches, based on a 9" OD and length of 7.5 ft / 90" - since I don't know the ID of the case ), with an 8" OD case, the length would need to increase from 90" to 114" . . .
Of course, I believe those tubes come in 120" lengths. Using the full length of 120" in an 8" casing would increase volume by 5.35 over the current 9" casing volume.
I'm no "Motor Guru" but, it seems to me that chancing the Length / Diameter ratio might change the total "characteristics" of the motor and we would be, possibly, back to "square one" on Fin Planform / Specs.
Dave F.
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Rail Dawg
Always learning!!!
I hear you Dave. The current motor casing is 7.5” ID so I think we’re still in the “R” range with the new motor.
The fins as currently designed should work quite well.
Chuck C.
The fins as currently designed should work quite well.
Chuck C.
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There is a huge difference between hot rolled aluminum pipe and DOM/Extruded tube . As Mark mentioned , never use pipe , it will ruin your day . Also the Nike motors are 5/16 welded steel gas pipe. The motor "may" hold 1000psi that you are shooting for , cold . Once you heat it up , all bets are off .
Eric
Eric
Ez2cDave
Well-Known Member
Chuck,
OK . . . Somehow, I was under the impression that it was 9" dia. and 7.5 ft long.
Thanks,
Dave F.
Rail Dawg
Always learning!!!
Agreed and we’re pushing towards all new hardware to ensure all bases are covered.
Chuck C.
Ez2cDave
Well-Known Member
Rail Dawg
Always learning!!!
Looking to start fresh Dave.
Going to play it safe. Looking forward to the process of getting a good casing.
Chuck C.
8" Makes sense, you can modernize the design while you're at it
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Chuck,
If you are buying a new tube, 6061-t6 Aluminum tube is typically made to either ASTM-B221 or ASTM-B241, with B241 being more desirable as it is a pipe spec [and not just 'schedule' sizes]. The strength of the material is equivalent for either spec. You also can get a material certification that should include a mill test report that will tell you what the strength of your lot was. I have used B221 tube [structural] for higher pressure cases just fine, but I do hydro-test it first [I always hydro-test a new design]. If the tube you have is of uncertain parentage, you can just hydro-test it and be done. As long as it is the alloy you think it is, it really should be fine.
It is very straightforward to hydro-test a case if you want to do it yourself. If you are paying someone to hydro-test your tube, I would recommend that you have them test the completed case, as your closures most likely are the weak link. You just need a plug for where the nozzle goes. That said, just testing to an arbitrary pressure is not a great plan. You will want to at least predict what the yield strength of each closure design is [not just the strength of the tube] first, before selecting a proof pressure.
br/
Tony
If you are buying a new tube, 6061-t6 Aluminum tube is typically made to either ASTM-B221 or ASTM-B241, with B241 being more desirable as it is a pipe spec [and not just 'schedule' sizes]. The strength of the material is equivalent for either spec. You also can get a material certification that should include a mill test report that will tell you what the strength of your lot was. I have used B221 tube [structural] for higher pressure cases just fine, but I do hydro-test it first [I always hydro-test a new design]. If the tube you have is of uncertain parentage, you can just hydro-test it and be done. As long as it is the alloy you think it is, it really should be fine.
It is very straightforward to hydro-test a case if you want to do it yourself. If you are paying someone to hydro-test your tube, I would recommend that you have them test the completed case, as your closures most likely are the weak link. You just need a plug for where the nozzle goes. That said, just testing to an arbitrary pressure is not a great plan. You will want to at least predict what the yield strength of each closure design is [not just the strength of the tube] first, before selecting a proof pressure.
br/
Tony
Rail Dawg
Always learning!!!
Great info there Tony.
Looking forward to a motor casing with a bolted upper enclosure and thrust ring. It’s also going to have a nozzle carrier.
Heat transfer and hot gas going where it’s not supposed to is a big problem. Best to make a casing out of the right material with the right hardware.
Again good stuff! Thanks.
Chuck C.
Ggggg%%
From what I've been able to find... the terms " pipe" and " tube" refer to whether a tube is spec'ed to ID or OD with a " pipe" being held to an ID. ....ASTM B221 doesn't appear to be a true seamless tube as it's usually folded over a mandrel and through heat and pressure, welded into a tube as its pushed through a " porthole" die....ASTM B241 is an extruded seamless tube or pipe rated for pressure applications as is a ASTM B210 with the B210 being " drawn" I assume this is a post production step the equivalent of DOM....any of this could be wrong ... it seems like in the early days of the internet it was much easier to find tech information...these days the search engines are too busy marketing products
Rail Dawg
Always learning!!!
A pipe is made to carry something and a tube is structural. The main thing is the new casing will be seamless and exact in measurements. It’s good stuff you wrote.
Looking forward to getting the casing nailed down as I need to know it’s exact OD so I can continue to order parts for the build.
Chuck C.
Nytrunner
Pop lugs, not drugs
Very interesting distinction. Ironic that my greatest exposure to tubing has been to carry hydraulic fluid and propellants
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