9 Motor Cluster (with gimbaled center motor)

[Yukon@K-9 Rocket Tech]

Hi everyone! I have been developing a open source TVC system for model rocket. I then got a crazy Idea of clustering 9 motors. Of course I highly doubt I would attempt this anytime soon, But I was wondering if anyone ever clustered slow burn motors with normal motors. I made a rough CAD design to show my concept. So the 8 motors around are to produce tremendous thrust at the boost phase, and the ejection charge will eject the spent motors out. Because the center motor would be in theory a slow burn motor, It'll continue to burn for a few more seconds. Also the ejected spent motors would create a lot more space, so the TVC computer would detect this, and command more degrees of freedom of gimbaling. Of course there are many complex challenges and problems and of source, safety concerns with this, but I'm just curious what everyone think about this. Also by no means is the CAD a actual design, it a very rough "sketch" to visualize my concept.

P.S. Would make a great Falcon 9 model rocket! This is the gimbal system I made, It's open source https://www.thingiverse.com/thing:3657663

 

[jqavins]

Clustering different burn times is a nice idea I've heard of often; one (center) or more (outer) high thrust motors for speed off the rod and one or more long burners for altitude makes sense. What bother's me here is a whole lotta thrust from the bunch of short burns, with only one low thrust motor gimballed to provide stability. It seems like the normal manufacturing tolerance on thrust, causing imbalance among your eight outer motors, might very well overwhelm the control authority of the gimbal system.

 

[Zeus-cat]

What kind of motors are you thinking about? Six D motors makes the rocket high power based on propellant weight so you need FAA approvals to launch. Also, lighting 9 motors can be tricky. Look into the flash pan method (for BP only) or design an electrical system that can handle 9 igniters at once (not a trivial problem).

 

[Yukon@K-9 Rocket Tech]

What kind of motors are you thinking about? Six D motors makes the rocket high power based on propellant weight so you need FAA approvals to launch. Also, lighting 9 motors can be tricky. Look into the flash pan method (for BP only) or design an electrical system that can handle 9 igniters at once (not a trivial problem).

Yes I am aware of FAA regulations. I am testing various methods to light all 9 motors at once. Ideally I want to use a igniter, but the tricky part is how all igniter's response times change. As for motors I was thinking of a E12-4 cluster and a central F10-4 hot staged with 2

 

[Yukon@K-9 Rocket Tech]

Clustering different burn times is a nice idea I've heard of often; one (center) or more (outer) high thrust motors for speed off the rod and one or more long burners for altitude makes sense. What bother's me here is a whole lotta thrust from the bunch of short burns, with only one low thrust motor gimballed to provide stability. It seems like the normal manufacturing tolerance on thrust, causing imbalance among your eight outer motors, might very well overwhelm the control authority of the gimbal system.

Come to think of it, your absolutely right. It would be Ideal to have a higher thrust in the center for control authority. If all eight had higher thrust, they all would have to be perfect, and the gimbal won't do much help. I should go hunting for high thrust slow urn motors, a tricky combo, or work on hot staging

 

[jqavins]

When you need high thrust and a long burn, the result, unavoidably, is high total impulse. The center motor will have to be something like an Aerotech H45, Aerotech I59 or Cesaroni I55, or some such. OK, I'm only finger-in-the-breeze guessing that no G motor would have sufficient trust and satisfying burn time; I could be wrong, but I'm really skeptical.

 

[Yukon@K-9 Rocket Tech]

Yes I was thinking about the total impulse being high. It seems like I need to go with smaller motors, or get my HPR Level 1 cert soon. And also find a launch site, then file a FAA waiver. I did guess clustering would get complex. The initial plan as mentioned was to eject all 8 motors after burn completion, but 8 hot motors falling out of the sky is a horrible Idea, and falls away from so many safety codes, so I'll probably ditch that. Of course this was a fun Idea, and something I do want to try in the future, but I think it's better to stick with a single motor for a bit. I forgot that clustering easily gets you into the HPR category!

 

[jqavins]

OK, but there may be an alternative. Instead of going big on the central motor, why not go small on the outer ones. Go with 13 mm, let's say, ¼A3T outboards. They have a peak thrust of 5 N at about 140 ms, total impulse of 0.6 Ns, and average thrust of 2.4 N. (Gosh, I love ThrustCurve.org!) Eight of them would be equivalent to a really short burning B20, peaking at 40 N to get off the pad fast. Add an E9 central motor in the gimbal and you're probably good. That arrangement would easily fit in a BT-80 (and would fit snugly in an E-Rockets LT-225) and shouldn't be hard to build light. Don't eject the outer motors, as doing that safely adds size, weight, and complexity.

(Come to think of it, I don't know the weight of your gimbal system, so maybe going small isn't an option. Darn. But it'd be a pretty cool cluster in a regular fin stabilized rocket; now I may have to do it.)

 

[Yukon@K-9 Rocket Tech]

Thank you! Yes I think I planned way to big. going small and moving up is better than going big and failing hard. seeing the specs, downsizing the concept makes it look cool, but I think in that case it is much more better performance wise to use one single G motor or something. The point of clustering was to sort of "throttle" the model rocket. I think I'll build a small model for testing purposes to help me when I move up to high powered stuff. It'll look really cool though