3-way inline clustering safety

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neil_w

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I've been toying with the idea of doing a 3-way inline cluster, and just for added giggles canting the two outer motors to provide the most appealing smoke trails. The outboards would be more for show than anything else, so they'd be smaller than the primary in the center. Something like this:




Can anyone provide some guidance as to the advisability of doing this, particularly withe regard to safety if all three motors doing light?

The biggest risk would seem to be if the center and only one of the outboards doesn't light. But with the central motor being quite a bit more thrusty than the outboard, I don't know how much effect the outboard motor would have on the flight. Also, due to its canting, the outboard is pointed a bit more directly towards the CG (but not *exactly* towards it, probably), so it should apply less torque to the model in flight.

Is there an analytical way to assess this, or rules of thumb, or is it just gut instinct? Is this just a bad idea?

cluster.png
 

dpower

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It depends on the specific motors you want to use, and the weight / drag of the Rocket. Assuming you need the center motor for a good flight, using 3 motors inline will work just fine if you take care in ignition, using a 12v system, and Q2G2 igniters, or Estes Solar igniters (the old ones that have pyrogen). Test your igniters, and have good electrical connections between all 3, either twisting them together if they reach, soldering them with extra wire, or using a clip whip (though clip whips are less reliable due to 3x connections).

Canting the outboards toward the CG does help if one of the outboard doesn't light. My Deuces wild flies straight if only 1 motor lights.
 

BDB

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I think this is an awesome idea!

The Tres by Fliskits has three canted motors. I saw one take off last month when only two lit. The trajectory was definitely not vertical, but it wasn't dangerous either. Just made for a long recovery walk. You will just need to devise some sort of clip whip for the igniters (or buy one from Rocketarium or Apogee).

FWIW...I'm working on a 2-stage rocket right now with a canted 3x24mm cluster in the booster and a single 24mm in the sustainer. I bought the motor mount from Jim Flis. It was pretty easy to assemble and mount.

ImageUploadedByRocketry Forum1487376893.681764.jpg
 

rstaff3

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I have done that twice. So far so good. On mine, the outboards don't vent to pop the chute, so the middle motor not lighting would be the worst case. I've either used classic Q2G2s or quickmatch, which are petty reliable.

I can't say with any certainty what would happen with only one outboard. My personal experience with the Deuces Wild is mixed. I've had two outboard misfires on my several models. One went upward as it is touted to do, the other ejected on the ground. I forget what the wind was like on each.
 

EXPjawa

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Neil, along the line of what Dick is saying, I would suggest porting the outboard tubes into the core. That way, if the rocket does get off the pad on the outer motors only, it has a shot at deploying. Though if this is the project we were discussing earlier, I'd still suggest making that core motor 29mm due to weight. Two 18mm motors might not do much to lift it anyway...
 

neil_w

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Neil, along the line of what Dick is saying, I would suggest porting the outboard tubes into the core. That way, if the rocket does get off the pad on the outer motors only, it has a shot at deploying. Though if this is the project we were discussing earlier, I'd still suggest making that core motor 29mm due to weight. Two 18mm motors might not do much to lift it anyway...
It could be that project, or another; I had been already been thinking about doing something like this for a while. If it is that project, it'd probably be at a size where 29mm would be too big.

Porting the outboards into rocket is problematic because the smaller outboards are likely to fire their ejection charges too early (e.g.: an A8-5 and B6-6 would both fire before than a D12-5). My goal would be for the rocket to not really go very far without the central motor, so even if it just went up and then came down without deploying it wouldn't be a big deal. I have plenty of sim fiddling to do before I'd really be ready to go forward with this; even then I might or might not choose to do it. For now I just wanted to first get an idea of what I should look out for.
 

EXPjawa

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That's a good point I didn't consider. I do have a spreadsheet that I made up a while back that compares the relative burn and delay times of the normal Estes motors; you're right - that would be a real tricky thing to line up timing wise. I've only ever ported for like-sized motors in parallel tubes.

As a side note, it occurred to me that it would also be interesting to sort of quick & dirty stage the pods so they essentially burn longer. Have a motor tube that's recessed farther in, tape, say, a B6-0 and B6-7 together and friction fit the top motor in. Maybe there's enough motor tube that the booster motor isn't entirely hanging in the air inside the pod, but can burn out and drop off, and the second motor keeps going. Thereby you've lengthened your deployment delay, and added a sweet visual effect (if its still within sight).
 

neil_w

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As a side note, it occurred to me that it would also be interesting to sort of quick & dirty stage the pods so they essentially burn longer. Have a motor tube that's recessed farther in, tape, say, a B6-0 and B6-7 together and friction fit the top motor in. Maybe there's enough motor tube that the booster motor isn't entirely hanging in the air inside the pod, but can burn out and drop off, and the second motor keeps going. Thereby you've lengthened your deployment delay, and added a sweet visual effect (if its still within sight).
That's sounds entertaining but even riskier, plus Mr. Krushnic might have something to say if the sustainer motor were recessed too far (I think?).

I think where I am heading is to see if I can (a) get enough cant to direct the outboard thrust close to the CG, and (b) adjust the motor sizes so that the model won't fly much without the central motor lit. That combination should minimize the negative consequences of having less than all three motors light up.

Once I start fiddling with the sim we'll see where it goes (if anywhere).
 

Nytrunner

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That's sounds entertaining but even riskier, plus Mr. Krushnic might have something to say if the sustainer motor were recessed too far (I think?).

I think where I am heading is to see if I can (a) get enough cant to direct the outboard thrust close to the CG, and (b) adjust the motor sizes so that the model won't fly much without the central motor lit. That combination should minimize the negative consequences of having less than all three motors light up.

Once I start fiddling with the sim we'll see where it goes (if anywhere).
If you build the Aft first (with canted motors) you may be able to intentionally place the CG at their thrustline intersection when you design the front half.
 

neil_w

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If you build the Aft first (with canted motors) you may be able to intentionally place the CG at their thrustline intersection when you design the front half.
Possibly so, although I think I'd need somewhat more cant than shown in my little drawing. There'll be a good amount of weight in the rear with 3 motors, so the CG is likely to be reasonably far aft, certainly further than the engines are pointed to in the picture. Don't wanna have to put *too* much nose weight in there. But it is definitely something I can keep in mind while designing. If the boosters could be made to point directly at the CG (or close to it, at least) then I'd probably feel pretty comfortable launching it regardless of how many motors light.
 

aerostadt

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If the boosters could be made to point directly at the CG (or close to it, at least) then I'd probably feel pretty comfortable launching it regardless of how many motors light.
You've got it. That's what you want to do. The model wants to spin around the CG. If only one outboard motor ignites and it's line of thrust goes through the CG, that motor's thrust will have a zero moment-arm and will not be able to produce a torque about the CG. There will be a net side force in the horizontal direction, which hopefully is designed to be small, so that the rocket will not perfectly straight up, but the model will not turn turn somersaults.
 

BABAR

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How far off center are the outboards? If they are right next to the central main I am not convinced you need to cant them at all. You should be fine as long as two motors light, and as long as all ejection charges go to your laundry.
 

Micromeister

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Neil:
I've been building and flying in-line Clusters for decades both for sport and in Cluster Altitude competitions. Canted motors only help if they are canted toward the loaded CG.
I've stopped canting in-line cluster motors since about 1988 when in test if one of the outboard motors fails to ignite the model will do a very quick cartwheel into the ground to the side the motor did not light. regardless of the cant degree the initial thrust overpowers the canted motor offset.

One of my favorite in-line 3 motor clusters is Estes SR-71. in the beginning I used B6-0 or C6-0's in the outborads. One thing to remember is no matter how well you tape or try to retain the boosters in outboard sealed tubes, the casings WILL be ejected at burn-thru. This is perfectly Legal in sport flying and it's a lot less involved than trying to vent the boosters into the core or to the outside but does have a devastating down side if the core does not lgnite... check out photo #3 below:)

I've gone with venting the outboard motors into the core through 3/32" x 3/16" rectangluar brass tubing soldered into 4 tube units that equal the inside motor casing volumn. with this system -0 Booster motor are no longer used, eliminating the posibility of the core motor mis-fire allowing the rocket to fly great on the up side but having no possibility of ejecting a recovery systemon the downside, streamlines in Destroying the model.
With internal venting to the Core, the Core motor will have a shorter delay then the outboards. In the case of the SR-71, Deep Space Transport or another favorite The Geo-Sat HLV the core motor is usually a C6-5 with two C6-7's in the outboards. HO MAN what great LPR cluster flights!!
Below are a few photos of these in-line clustered Sport flying models and a view of the brass tube vent unit.

013Lp06d_3pic 3-C6 SR-71 Blackbird Flt(Manassas VA)_07-90.jpg


021a-sm_Geo-Sat HLV 3C_03-21-89.jpg


021b_Geo-Sat 3C cluster motor end_01-21-09.jpg


063-b1c-sm_Deep Space Transport 3C Lift-0ff 2pic_07-06-91.jpg


SR-71-e1-sm_FS 3-32 x 3-16th ducts 160dpi_01-07.jpg
 

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TangoJuliet

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John, that SR-71 photo is a beauty... Too bad it didn't survive. :( And your Geo-Sat HLV reminds me of a Ventris SRB I saw here on the forums with droppable boosters; a design I intend to replicate myself, but now that I've seen your HLV I'm thinking I won't be sticking with the Ventris color scheme. :wink:
 

rstaff3

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I canted the outboards on my three motor in-line clusters to get a slightly more impressive plume, not to keep it stable if one doesn't go. They have a very slight cant and, therefore, don't do a whole lot in that regards. I've also canted three motor non-inline clusters for that reason. Those look nice on boost!
 

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