Hence the joke and the
. Sorry, it’s not always as funny when written out.I built one last year and lost it on the first flight. 1/2A3-2T.
Cluster Thrust VS Single Large Motor
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. Sorry, it’s not always as funny when written out.I built one last year and lost it on the first flight. 1/2A3-2T.
MadRocketer
Determined to do it differently
If you don’t cluster, you can’t have fly away boosters. If you don’t have fly away boosters you can’t make really neat smoke trails. 
waltr
Well-Known Member
There is a case where I am considering a Cluster due to a single motor does not have enough speed off the rail.
This is for the booster of a composite 2-stage rocket. Main constraint is it needs to stay within the NAR & FAA Low Power rules, needs a 10 to 1 Thrust to Weight and lauch weight under 1.5kgm, this means average thrust of up to 147N. The one I did build is about 1.3kg but the highest thrust motor that can fly as a LPR is the G80. Max average thrust for LPR is 80N but 160N if a cluster. Therefore, two motors with average thrust of about 64N is perfect and there is are a number to chose from and still keep propellant weight under the 125gm FAA limit in F or G impulse classes.
This is for the booster of a composite 2-stage rocket. Main constraint is it needs to stay within the NAR & FAA Low Power rules, needs a 10 to 1 Thrust to Weight and lauch weight under 1.5kgm, this means average thrust of up to 147N. The one I did build is about 1.3kg but the highest thrust motor that can fly as a LPR is the G80. Max average thrust for LPR is 80N but 160N if a cluster. Therefore, two motors with average thrust of about 64N is perfect and there is are a number to chose from and still keep propellant weight under the 125gm FAA limit in F or G impulse classes.
Ya know your are in the LPR area of this forum, which has its own definitions of LPR. In MPR you could cluster up to four 30 gram, HAZMAT free, propellant F motors, although currently available choices will not get you to 320N.
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That might be my fault. I'm the original poster. Maybe I should have put this into "propulsion", but I was starting out with an 18mm cluster versus a single 24mm motor, so I felt this was OK at the time. But a lot of folks don't read the Rocketry Forum by, well, "forum"... They look at "What's new" and comment from there without knowing which subject area they are in.
waltr
Well-Known Member
Yes, the case I outlined requires keeping the rocket in the LP Class 1 under NAR, NFPA & FAA rules.
There is no legal MPR class, only CLASS 1 (LPR) and Class 2 (HPR).
In your follow up math you only talk about stage 1 propellant weight... don't forget to add stage 2.
As far as I know that 125gr limit is total at lift-off, of ALL propellant in the rocket. Not per stage.
Why 10:1? I think the requirement for thrust-to-weight ratio is only 3:1, so long as everything else checks out.
waltr
Well-Known Member
2-stage with delayed sustainer ignition works much better with 10:1 else the rocket can start turning before sustainer motor comes up to pressure.
3:1 is minimum and ok if no to very light wind. Many Club RCO's require 5:1.
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as in many things (Orange or Black, Honda or Toyota, three fins or four) there isn’t a right answer.
like you, I still think starting with the lowest diameter sustainer for maiden flight is a wise choice. Once you have “proven” the design, going up on motors is a trade-off which depends on what satisfies you personally.
you DO obviously need to account for the stability change a larger sustainer motor imparts, usually by adding nose weight. And this added mass of larger motor and more nose weight is gonna reduce the speed off the pad. Both problems are not at all insurmountable (well, if you can find the right motors, Estes still needs to release a C5-0 if they want to re-release the MIRV), just things that need to be taken into account.
i think an issue that is more challenging is the weathercocking issue. Without a big field and a tracker, especially Minimum diameter small fin birds (or large fin bird, although they don’t go as far) can cover a long distance LATERALLY while at the same time may be at a high altitude and had to track VISUALLY. Of course, the same thing can happen with a single stage with a large long burn motor, although tendency to weathercock is a bit less because you don’t need as much “finnage.”
as for boosters, my usual prioritit’s are to
1). get the stack up to speed before it leaves the rod or rail, and THEN
2). burn out as quickly as possible so it stages low.
Those are often diametrically opposed, and the first priority always wins out over the second.
since for low power usually doesn’t use trackers and relies on the Mark 1 Eyeball for tracking, ESPECIALLY for tumble recovery boosters which have no streamer or chute waggling in the breeze on descent and after touch down, I don’t personally like high altitude staging (again, High Power with electronic tracking is a whole different animal.)
if your are gonna go big, you have to hope for a very low wind day and be smart enough to put the rocket back in the car if you get there and the winds pick up. Sometimes I am not smart enough to do that ,(“ I came all the way out here and I am darn well gonna fly it!”)
all that said, I will admit that a max stack (what, Comanche 3 with D12-0 to C6-0 to C6-7 on a low wind day) is a beautiful thing to watch, and even better if you get all the pieces back!
Having seen my converted Hi-Flier XL go on an E12-0/E12-8 stack for its first two-stage flight, I have to agree. It might have been the most thrilling flight I’ve had since getting back into flying. Watching the thing claw skyward on the booster for that long burn, then stage beautifully and shoot off to apogee, I doubt I’ll ever forget that.
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OK, I've started *thinking* about making a swappable motor mount that I can use to swap between a cluster of motors and a single. But I can't seem to quite figure out how to make it work so that it's secure and reliable and also, easy to work with. I'm currently thinking two screws to hold it in place, but that doesn't seem like it's really going to take the stress of many flights.
Maybe I need to contract the services of @lakeroadster to design something for me, but here's the rough concept:
The QUAD RUNNER is a BT-70 tube, or pretty damn close to it. I want to have a single swappable 24mm mount *and* a cluster of 4 18mm motors. If that can't work (because 4 18mm motor nearly fills that entire BT-70 space, leaving little room for any swapping/mount hardware) then a cluster of 3, or even 2 is fine. I'll just rename it the Duo-Runner... I was originally intending to just shove in the 24mm I had bought for it and rename it Mono-Runner.
My current idea is to glue in two strips of basswood into the body tube and use that as the centering rings guiderail and lock position by epoxying in two nuts that would hold everything secure once the bolts are screwed into via the drilled holes in the flamy-end of the motor mount's plywood centering rings.
Maybe I need to contract the services of @lakeroadster to design something for me, but here's the rough concept:
The QUAD RUNNER is a BT-70 tube, or pretty damn close to it. I want to have a single swappable 24mm mount *and* a cluster of 4 18mm motors. If that can't work (because 4 18mm motor nearly fills that entire BT-70 space, leaving little room for any swapping/mount hardware) then a cluster of 3, or even 2 is fine. I'll just rename it the Duo-Runner... I was originally intending to just shove in the 24mm I had bought for it and rename it Mono-Runner.
My current idea is to glue in two strips of basswood into the body tube and use that as the centering rings guiderail and lock position by epoxying in two nuts that would hold everything secure once the bolts are screwed into via the drilled holes in the flamy-end of the motor mount's plywood centering rings.
lakeroadster
When in doubt... build hell-for-stout!
A Quad Runner... (1st time I've seen one of these)
I'm thinking it needs fixed, glued in, centering rings to add rigidity to the airframe in the fin can area....
I'm thinking it needs fixed, glued in, centering rings to add rigidity to the airframe in the fin can area....
heada
Well-Known Member
LOC uses removable plastic rivets. Build the motor mount in a coupler and secure the coupler with the rivets. Only drawback is surface mount fins. Check their instructions online.
lakeroadster
When in doubt... build hell-for-stout!
The coupler is a great idea... if it fits with a quad cluster... and from the looks of it, it won't.
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Outside the box thinking.
buy another rocket for $30 and build it with your single motor mount.
https://bellevillehobby.com/product/quest-quad-runner-model-rocket/
might be same amount of work and less money (and more successful) than trying to build a dual purpose motor mount.
heada
Well-Known Member
4x 18mm fits in the 1.9" tube because that's the base config. Doing that same 4x 18mm in a 1.9" coupler should be the same as without a coupler.
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A coupler *is* a great idea, and one I'm going to try another time. Changed my mind AGAIN, and I'm just going to build it with the 24mm Motor Mount as I originally intended in my original post on this thread. A single BP "E" is still going to be cheaper than 4 C6-5 engines, less costly to fly that way. I may reserve clustering for rockets that truly require it, like a model with 3 body tubes, like a semi-scale SLS or something. This rocket has been waiting 2 months already, I'm getting tired of it being unbuilt for want of a motor mount.
lakeroadster
When in doubt... build hell-for-stout!
Except for a given size a coupler tube is a smaller inside diameter than a body tube. In the photo, the (4) 18mm motors look tight in the body tube. Will they fit in a coupler?
The answer is no if the coupler is smaller than 1.78 inside diameter.
https://www.engineeringtoolbox.com/smaller-circles-in-larger-circle-d_1849.html
FYI: If the body tube is 1.9".... It's not a BT-70. A BT-70 is 2.217" o.d. x 2.175" i.d. (0.021 wall)
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And again, doing more rethinking...
Just went to this website:
https://www.rocketreviews.com/combine-motors.html
It allowed me to compare the thrustcurve of 4 C6 motors versus the single E12, although I had to tell it that the 4 C6's were stage one and the E12 was stage 2. WOW. the cluster of 4 C6 motors is actually double the "power" of an E12, as I was informed at the start of this thread. So I could theoretically fly an E30-4T or so, and get away with it.
Well, I'm still going to play it "low and slow" for the rest of this year, but, I can just make a second rocket next year with the clusters, as someone else here suggested. Kits are not that expensive. At least, not in the Mid-Power category. Once you start with the 4" diameter and up, then things get pricey.
Maybe I'll make my Shipping Tube rocket into a "double D (dual 24mm)" rather than a 29mm. 2D motors might be cheaper than an F, and no Hazmat fees...
Just went to this website:
https://www.rocketreviews.com/combine-motors.html
It allowed me to compare the thrustcurve of 4 C6 motors versus the single E12, although I had to tell it that the 4 C6's were stage one and the E12 was stage 2. WOW. the cluster of 4 C6 motors is actually double the "power" of an E12, as I was informed at the start of this thread. So I could theoretically fly an E30-4T or so, and get away with it.
Well, I'm still going to play it "low and slow" for the rest of this year, but, I can just make a second rocket next year with the clusters, as someone else here suggested. Kits are not that expensive. At least, not in the Mid-Power category. Once you start with the 4" diameter and up, then things get pricey.
Maybe I'll make my Shipping Tube rocket into a "double D (dual 24mm)" rather than a 29mm. 2D motors might be cheaper than an F, and no Hazmat fees...
heada
Well-Known Member
The E30 is an expensive motor (2-pack is ~$30). For nearly half the cost, look at the Q-Jet motors. D22 and E26 could be options (2-pack is ~$18).
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Roger that. I actually have the Q-Jet E26 in my range box right now, but (unfortunately), this isn't on the list of motors at the website. Flying my Big Daddy on that in 2 weeks. (See my Avatar, that's on a BP Estes E12, but I've since "graduated" to composite 24mm for that bird) Big Daddy loves more thrust, it seems the more you give it, the better it flies.
Very much so. I once saw one go on an Apogee E6. Arcing over under thrust is a pretty anxiety-inducing thing to witness.
lakeroadster
When in doubt... build hell-for-stout!
Yeah... but the E30 is freaking awesome. $6 upgrade... gets you
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You get what you pay for.Yeah... but the E30 is freaking awesome. $6 upgrade... gets you
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Yes, the E30-4T was very impressive. Almost sent my Big Daddy too far away to easily retrieve, but I got it back and it lives to fly again in 2 weeks! So excited!
I was going to suggest using a coupler too.
Eh, I'm not a fan of the E26 in the Big Daddy. Not enough kick at the start. Flies more straight on an E12/D12 or the D22 Q-Jet. Note the lack of thrust at the start for the E26 in the thrust curves.
That's also why I built a 4x24mm Big Daddy, so I can use E26s.
My stock HiFlier XL flies really well on a D12-7, I can't imagine what altitude you could get by staging E motors. But then my stock XL might get similar altitude on an E30, but not as much fun.
I've gotten more paranoid these days with long burn motors. I've seen a lot of rockets take a turn right off the rod and go horizontal for long distances under thrust. I'll probably be overly cautious with mine.
Why not just simulate it with OpenRocket or Rocksim? Assuming the Rocksim file is accurate from Apogee's website...
22.1m/s off rod w 4x C6-7 and 14.5m/s w/ E12-6
422m Apogee w/ the cluster and 326 w/ the 24mm motor
Note to the OpenRocket folks -- Apogee had 4 individual motors which I just switched to a 4 engine cluster. For some reason it forecasts a lower speed leaving the launch rod and lower apogee. Should be the same as above, no?
22.1m/s off rod w 4x C6-7 and 14.5m/s w/ E12-6
422m Apogee w/ the cluster and 326 w/ the 24mm motor
Note to the OpenRocket folks -- Apogee had 4 individual motors which I just switched to a 4 engine cluster. For some reason it forecasts a lower speed leaving the launch rod and lower apogee. Should be the same as above, no?
