Interchangeable Motor Mounts: Mini Build Thread

Hi folks,

I had a lot of interest in my interchangeable motor mount system so I'm posting such detail as I've got. This isn't really a full build thread but should be enough to let anyone reproduce what I've done and customize it to their needs. It's only one of several interchangeable mount systems I've seen posted here, but it's quick and easy.

I made this specifically for BT-60 based rockets. With adaptation it could be made to work for other tube sizes.

Originally I had thought to just use a 24 mm E mount that with adaptors could be made to work for 24 mm D, 18 mm and 20 mm (Quest D5) engines. But then I started getting into the idea of doing clusters and I wanted to be able to swap a cluster into one of my BT60 birds.

This thread will explore how I came by my current system and where I may go next. I'll be happy to answer questions but just know that this week I've got some work stuff going on and may not be on the board again until the weekend depending how things go.

Marc

The first step was building the BT-60 rockets. I built them normally except didn't put in any motor mount or trifold shock cord mount. For this project I build a pair of them, one "Der Orange Max" and one "Der Big Green Max."

Next I built a standard 18 mm mount using Estes parts, plus a triple 18 mm mount from Sunward I think it was. I noted that the Sunward mount was a little longer than the 18 mm because the wood end piece of the triple mount represents additional length beyond the engine tubes.

Here's a picture of these mount builds in progress. Ignore the dual 18 mount; it doesn't factor in (I'll explain later).



After building the 18 mount, I built it out to the same total length as the triple mount with a small ring of 1/32" aircraft ply from the hobby store. Basically, I made a centering ring out of the ply and stuck it to the end with wood glue. I also made another one of them and used it to stiffen the aft centering ring (this is shown in the picture while drying with clamps).

When the mounts were dried overnight, I made a thrust ring to be mounted in each of the two rockets. Since the mounts themselves would not be glued in, they needed a thrust ring to push against.

When I started the project I had no extra centering rings, but during the project I got my order from BRS so suddenly had lots of extra stuff to play with. So for one rocket I made a thrust ring out of 1/32cnd" ply, the other I simply used a fiber BT-50 to BT-60 centering ring. Both of them got a notch and a loop of kevlar cord for shock cord mounting.

In each case I pushed the rings most of the way in using the mounts, then I put in some glue just forward of the thrust ring, then pushed the ring all the way into place so that just a bit of the motor mount sticks out of the aft end of the BT-60. This let the glue lock the rings in place. Later I put Titebod Trim and Moulding Glue fillets on the fore end for strength, and a little bit of regular wood glue on the aft end of the ring as well.

Here's a picture looking into the back of each of the rockets.

You will note on the green rocket I've got three bits of semi-circular dowel have been mounted at the end of the body tube. I chose to mount them where the fins attach, for the added strength and rigidity. These pieces of dowel are about 1.5 cm long, sized such that when the mount is fully inserted, the aft side of the aft centering ring on the 18 mm mount buts against the fore end of the dowel strips. Actually I gave it a mm slack to make sure things would work out.

I found it was a bit annoying to cut the dowels so I used a thinner dowel without halfing it on the second rocket. I don't think I have a picture, sorry. I believe it was a 5/32cnd dowel but I might be wrong. Just as with the first rocket, I stuck it using double-glue technique with wood glue inside the tube adjacent to the fins.

Then I used a Dremel tool and cut notches in my two motor mounts (triple and single 18 mm), plus two more I had built in the meantime (a 24 mm and a 20 mm).

The tool had the small sanding drum on it; I just buzzed through the centering rings at the three appropriate spots. Didn't do a particularly neat job of it; next time will do better.

Here's a pic:



The key thing is to get the notches in the right spot and make them a little bigger than you think you will need to because you need some room for slop.

Now, the triple mount doesn't have an aft centering ring, so in its case I just glued on some more thin dowels in the appropriate spots so that the are not in the way upon insertion, but with a twist-lock action they engage with the dowels mounted in the body tube.

The main thing in all of this is that the aft centering rings on the mounts (or the aft edges on the bits of dowel for the cluster mount) all have to end at the same distance from the fore end of the mounts, so that they fit past the retaining dowels. The retaining dowels are critical for holding in the mount during ejection phase.

I haven't had any problems with either the cardboard reinforced with ply rings bending during ejection, or even just the black fiber rings (from BRS). They have all been strong enough to withstand the ejection pulse. I wouldn't bet on the plain Estes rings holding up though.

What's next?

Well, Geometry makes it difficult to use a dual 18 mm cluster in this system because finding spots for the triple slots is hard/impossible with a dual cluster. I have thoughts of using just a single dowel retainer that would work with almost any motor configuration, though I wonder if that is enough to reliably retain the mount during ejection, particularly with a big engine with strong ejection or a triple cluster. I may try this and see. With a triple mount you have almost no extra room within a BT-60.

I have flown the two Maxes a total of about 15 times with one or other of the mounts. The mounts performed very well in all cases and have been easy to swap on the fly in the field.

Sadly yesterday I reported the dramatic end of my Green Max. The crash also cracked the triple motor mount so I will need to build a new one.

Sorry for the rushed descriptions and for possibly not being around this week to answer any questions, but I'll pop by as soon as I can. I'd love to see others improve on what I've done and post how-tos so I can copy

Nice. I did something similar for my New Centurion, a BT-60 rocket built to resemble the old Centuri Centurion:



I used screws for engine mount retention.

More here:

https://rocketry.newcenturycomputers.net/newcenturion.html