SpaceX's Falcon9 reusable boosters with nine Merlin engines are not the only cluster motor rockets in town ;-)
Besides better components, powerful composite motors, robust ground support equipment (GSE) and excellent colour illustrated docs, Klima has a superb range of model rocket kits featuring clustered motors.
Estes, sadly, has none at the moment. In fact, we often upgrade some Estes kits with Klima motor mounts for experienced builders and flyers. For instance, Estes Doorknob.
Clustered kits aren't more difficult to build than single motor model rocket kits. You can use the same glue as usual; epoxy is preferred for bigger clusters and speed of assembly.
The Klima Auriga, Solar Flare and Proxima kits have easy-to-build fin assemblies (fin cans) that feature super accurate alignment. The Europa has a pre-slotted body tube and features through-the-wall construction with alignment slots in the motor mount. EZ PZ
With clustered motor mounts you have your choice of motors to suit your flying conditions and recovery area. You can even mix and match. 2 x A6-4s plus 2 x B4-4s work fine, for instance, when symmetrically loaded.
When clustering Kilima's 20Ns D9 motors we recommend, when possible, to combine with D9-Ps (plugged motors = no ejection charge) because you don't need all that puff all at once.
Klima also has a range of long burn motors (B2-0 or P, C2-0 or P, and D3-0 or P) that can be used with C6s and D9s with long ejection delays for great altitude and continuous smoke trails.
Of course, when you want to ignite more than one model rocket motor at a time, you need to have a reliable solution. The Estes ProSeries II controller can light clusters of up to four motors. But the Klima method will light any number of clustered motors using Igniter Sticks in each nozzle joined up with Tape Match.
And for real rocket scientists ;-) you can even STAGE clustered motors with Igniter Sticks and Tape Match. We've done that with the Klima Auriga (four motors > three motors) and Klima Europa (five motors > five motors). You can see a video of the 2-stage Auriga on the Model Rocket Shop FaceBook page.
And if you are into clusters big time, you'll burn a lot of motors. We have money-saving Team Packs of popular motors to help spread the load ;-)
Knock yourselves out ;-)