Interchangeable Engine Mount

[Solomoriah]

I'm planning to build a semi-clone of the old Century Centurion. Yes, I know that Semroc makes a clone; I'm planning to buy the laser-cut fins from them. But I have this big pile of surplus BT-60 (well, several pieces anyway) so I'm going to get a similar BT-60 sized cone instead of going with the standard ST-16 tubing.

Anyway... I'd like to have an interchangeable engine mount, similar to the one in the Century Power System X-16 rocket. I don't know how that engine mount was secured, and that's my question... I'm looking for ideas for securing the engine mount in the rocket.

I've considered two options so far. First, and most obvious, is to use an engine hook. However, that would mean that the hook would be exposed on the outside of the body. I don't care for that at all.

The second alternative I've thought of is to drill small holes through the body tube and engine mount couplers, glue nuts on the inside and use screws to secure the mount. I think that two such screw mounts ought to be enough, especially if I use a short piece of coupler in the body tube as a thrust ring for the mount. That way, the screws would only have to hold out against the ejection charge. I don't think a couple of screw heads would be nearly as ugly on the outside as an engine hook held down with gauze or whatever.

I'm leery of installing metal screws and nuts in the rocket, for obvious safety reasons (CATO = shrapnel) but I suppose I might go with nylon screws if I can figure out where to get them.

I'm hoping to be able to switch between 18mm, 24mm, and 2x18mm mounts (if not for the cluster, I'd just put in a 24mm mount and use an adapter for 18mm).

As I said, though, I'm hoping for suggestions. Possibly there is an alternative I haven't thought of... or a better way to do one of the ones I have.

[barone]

I'm planning to build a semi-clone of the old Century Centurion. Yes, I know that Semroc makes a clone; I'm planning to buy the laser-cut fins from them. But I have this big pile of surplus BT-60 (well, several pieces anyway) so I'm going to get a similar BT-60 sized cone instead of going with the standard ST-16 tubing.

Anyway... I'd like to have an interchangeable engine mount, similar to the one in the Century Power System X-16 rocket. I don't know how that engine mount was secured, and that's my question... I'm looking for ideas for securing the engine mount in the rocket.

I've considered two options so far. First, and most obvious, is to use an engine hook. However, that would mean that the hook would be exposed on the outside of the body. I don't care for that at all.

The second alternative I've thought of is to drill small holes through the body tube and engine mount couplers, glue nuts on the inside and use screws to secure the mount. I think that two such screw mounts ought to be enough, especially if I use a short piece of coupler in the body tube as a thrust ring for the mount. That way, the screws would only have to hold out against the ejection charge. I don't think a couple of screw heads would be nearly as ugly on the outside as an engine hook held down with gauze or whatever.

I'm leery of installing metal screws and nuts in the rocket, for obvious safety reasons (CATO = shrapnel) but I suppose I might go with nylon screws if I can figure out where to get them.

I'm hoping to be able to switch between 18mm, 24mm, and 2x18mm mounts (if not for the cluster, I'd just put in a 24mm mount and use an adapter for 18mm).

As I said, though, I'm hoping for suggestions. Possibly there is an alternative I haven't thought of... or a better way to do one of the ones I have.Why not use the same method Semroc uses with the SLS Laser X? Basically, through the wall fins with the tabs acting as the stops and retainers.....motor mounts having slots to slide through the fin tabs and then twist the mounts to lock in place.

[tbzep]

They used an engine hook taped to the outside of the body tube to secure the engine mount. The engine mount was made with a body tube coupler with the centering rings at each end to make a nice package that would work well with the engine hook.

[Solomoriah]

Why not use the same method Semroc uses with the SLS Laser X? Basically, through the wall fins with the tabs acting as the stops and retainers.....motor mounts having slots to slide through the fin tabs and then twist the mounts to lock in place.
Hmm. I'll think about that one... sounds interesting. But I won't be able to use the laser-cut Centurion fins if I go that route.

They used an engine hook taped to the outside of the body tube to secure the engine mount. The engine mount was made with a body tube coupler with the centering rings at each end to make a nice package that would work well with the engine hook.
Ah. The tube coupler as part of the engine mount was a fairly standard Centuri thing in that era... I remember being shocked when I opened my first Baby Bertha kit and there wasn't a tube coupler to glue the centering rings to.

[tbzep]

Ah. The tube coupler as part of the engine mount was a fairly standard Centuri thing in that era... I remember being shocked when I opened my first Baby Bertha kit and there wasn't a tube coupler to glue the centering rings to.

Yep. It was very strong, but you had to be careful when installing to keep the glue from locking it down before you got it positioned. It was a little heavier than two centering rings alone, but you didn't have to fool with fillets, so the overall weight might have been pretty close with both methods.

[Solomoriah]

These days I use 50 min. epoxy when installing couplers... that whole freezing-up thing is really annoying. The last time I put a coupler in with yellow glue it froze 1/8" early; I filled the gap in the body, but I don't want to mess with that nonsense ever again...

I have a rear-ejection rocket in the works with a coupler as part of the engine mount. I used epoxy to glue in the "thrust ring" (3/4" of coupler) so it wouldn't freeze before I had it where I wanted it.

[Doug Sams]

As I said, though, I'm hoping for suggestions. Possibly there is an alternative I haven't thought of... or a better way to do one of the ones I have.Here's the scheme I use. I have deployed it in BT-60 and BT-70 tubes. The interchangeable motor mounts each include motor retention. A 4-40 or 2-56 all thread runs from the bulkhead inside the BT-60 or -70 and out just past the end of the tube. The mounts slide over the all thread and a nut holds them in place.

Here are the four mounts I have for BT-60.

http://home.flash.net/~samily/better-bertha/bt-60-mmas-2p.jpg



Here's an article on it. The bulkhead and all thread are pictured therein.
http://home.flash.net/~samily/better-bertha/BB-plans.pdf



These are the four mounts I made for BT-70.

http://home.flash.net/~samily/midget80/70mmas4-2-2b.jpg

The triple 24 has been used in a staged configuration forgoing positive motor retention since the center nut/washer would interfere with the booster motors. In that case, the sustainer motors were simply friction fit into the tubes.


Here's a good shot inside the BT-70 (itself encased in a BT-80). You can see the all thread and the bulkhead. BTW, I attached the kevlar/recovery harness on the other side of the bulkhead.

http://home.flash.net/~samily/midget80/aft-open2.jpg

HTH.

Doug

[Solomoriah]

Hey, that's pretty cool! I'll go read your page shortly... I'm curious what you used for that bulkhead. I don't want to make the rocket too heavy to fly on a single 18mm since the classic kit was a 1x18mm rocket.

[Doug Sams]

I'm curious what you used for that bulkhead. I don't want to make the rocket too heavy to fly on a single 18mm since the classic kit was a 1x18mm rocket.The ring was a 1/4" thick piece of birch or A/C ply. In the linked article, it was stepped, with half the thickness in a short piece of coupler and the other half out. The reason for the coupler was to ensure the ring didn't get cocked during insertion into the rocket. IOW, to keep it aligned on axis.

But on other birds which share these mounts, the ring is flush at the end of the coupler.

In any case, the thrust is transferred thru the ring/coupler as well as the ejection forces (from the all thread) and the jerk of the shock cord, so I'm hesistant to scrimp on it; it needs to be hearty.

Weight is certainly a concern, but to get the flexibility, there's going to be a price. Where I save on weight is using balsa ply for most of the pieces in the motor mounts. They are very light.

Doug

[tbzep]

Doug's system is great for mid and high power, but it's a bit heavy for a low power BT-60 based rocket. You can make a much lighter interchangeable mount for it if you want to. Your rocket should be able to fly on a single B6-4 if built correctly.

[Doug Sams]

Doug's system is great for mid and high power, but it's a bit heavy for a low power BT-60 based rocket. You can make a much lighter interchangeable mount for it if you want to. Your rocket should be able to fly on a single B6-4 if built correctly.Tim,

It's not that bad. The Yellow Bertha, with the BT-60 version, still flies on single B6-2's and C6-3's. With the ring being bored out and the all thread 2-56, it's not as heavy as it might seem.

Doug

[tbzep]

Tim,

It's not that bad. The Yellow Bertha, with the BT-60 version, still flies on single B6-2's and C6-3's. With the ring being bored out and the all thread 2-56, it's not as heavy as it might seem.

Doug

That's lighter than I thought, but I can't imagine how low the B6-2 flight would be considering Estes has shorter than advertised ejection charges on their A8-3 and B6-4 motors. :eek:

[JRThro]

I'd like to have an interchangeable engine mount, similar to the one in the Century Power System X-16 rocket. I don't know how that engine mount was secured, and that's my question... I'm looking for ideas for securing the engine mount in the rocket.

I've considered two options so far. First, and most obvious, is to use an engine hook. However, that would mean that the hook would be exposed on the outside of the body. I don't care for that at all.

(SNIP)

I'm hoping to be able to switch between 18mm, 24mm, and 2x18mm mounts (if not for the cluster, I'd just put in a 24mm mount and use an adapter for 18mm).

As I said, though, I'm hoping for suggestions. Possibly there is an alternative I haven't thought of... or a better way to do one of the ones I have.
Bump.

How did this turn out? What method did you end up using?

If you used a motor mount mounting tube (or something) that's big enough to hold your interchangeable mounts but is smaller than the BT-60 body, then you wouldn't have to mount the hook on the outside of the body, and you might be able to recess the hook slightly into the aft end so it wouldn't be visible from the outside at all.

[Solomoriah]

I couldn't do that, though. Using BT-60, there isn't enough room outside of a 2x18 cluster for the inner tube.

As I explained here:

http://forums.rocketshoppe.com/showthread.php?t=3515

"I used screws for engine-mount retention. I dislike using any metal, but I don't think two cheap screws are much of a safety hazard (certainly no more than the engine hook is). The wire hook shown is used to extract the engine mounts because they fit rather tightly; but I don't trust them as friction-fit only, hence the screws."

I have a more-or-less complete explanation on my site:

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

[Solomoriah]

Incidentally, I'm planning to buy a 20mm engine mount kit for the D5 engines when they become available, and add it to the mix. My non-Rocksim figures are about 900' on the D5-4 (assuming its delay is pretty close to actually being 4 seconds).

An 8 second ascent seems pretty cool for a single-engine bird...