Marc Goldstein (Marc_G on the Rocketry Forum) has come up with a very smart method for interchangeable motor mounts.In the picture at the right, short half round dowels are glued directly opposite the root edges of the fins, inside the body tube.
Click to enlarge the picture and read the copy detail.
In the picture, the half dowel pieces are flush with the end of the body tube. In this earlier version, the half dowels sit below the lower centering ring of the engine mount with the mount is slid in, turned and "locked" in place.
Here's Marc's interchangeable motor mounts, 24mm on the left and 18mm on the right. Notice the the half circles cut out of both the top and bottom centering rings.
These half circles allow the mount to be slid in over the dowels.
You could also build cluster mounts the same way.
When the mount butts up against the forward centering ring (already glued in the main body just forward of the engine mount position) the mount is turned, locking it in place.
Marc's first engine mount builds are HERE
His new version using basswood strips instead of dowels is HERE
This mount reminds me a little of the Semroc SLS interchangeable mounts. With the Semroc mounts, short thru-the-wall fin tabs lock onto the centering rings of the engine mount.
With Marc's half dowel mount idea, you could use this in most any kit, maybe BT-55 and above. From an existing kit you could use the included surface mount fins.
I'm showing my ignorance I suppose, but why couldn't you have the rocket be built with the larger motor tube, and then have a sleeve that slides into the larger tube with the smaller motor when you want to use one? That way you wouldn't have to remove anything to change.
ReplyDeleteThe outer edge of the smaller motor mount would butt up against the engine ring of the larger mount and have one of it's own inside the smaller engine tube.
Seems easier.
Hi Mustang,
ReplyDeleteI can't speak for Marc and his design, but he also builds cluster mounts for this twist/lock system.
I use 20/50 mounts all the time for smaller models. Marc's system can be used for 18mm, 24mm, 2 engine and 3 engine clusters. You can't change out and use a cluster without replacing the entire engine mount. So, he's got one rocket and many engine possibilites.
I know from reading his posts that the cluster mounts are a little more complicated to design the twist lock, but they do work.
Still, think it's a system worth sharing. I always like seeing something new that inspires another build.
I used screw retention of the interchangeable engine mounts on my New Centurion: http://rocketry.gonnerman.org/newcenturion.html
ReplyDeleteUnfortunately, due to an overly tight fit the 1x24 mount was destroyed when I tried to remove it after its first flight, so now I just have 1x18 and 2x18 mounts for that rocket. But since I rarely fly 24mm engines it's not much of a loss.