Tower:

I have had a tower similar to this one before for 30mm. but refined it somewhat
for the 40mm version.

Refer to: http://www.spacemodeling.org/gcg/omnipad_tower/tower_assembly.jpg

The heart of the tower is the Base Cylinder, which holds the rails aligned.  The
Base cylinder has fillets that match the surface of the rails to hold them in
alignment, so that the only thing necessary to attach the rails to the cylinder
is to simply tape them using electrical tape or vinyl tape. It's not elegant,
but it works well.

The tricky part is fabricating the cylinder. Ideally it would be a tube that is
the perfect diameter to hold the rails just far apart for a loose slide fit of
the model without being too loose. I wasnt able to find such a perfect fit of
tube. What I used was BT-60, with some tweaking. Now of course BT-60 is not
stiff enough to hold up to such a task, and neither would the perfect tube for
that matter. The tube needs reinforcement inside. For the BT-60 that was
somewhat easy to do. first, I glued a piece of Totally Tubular (Jim Fackert) I
used some Estes BT-20 to BT-60 centering rings to help place a BT-20 inside. One
ring was slid about 1.5" inside, the other just barely inside so as to just
align the BT-20.  Thick CA was dribbled inside the end with the 1.5" recess to
help to seal off that end so nothing would leak.  Then, I got out some old
Alumilite that was too old to cast good parts with, but still useful for filling
bulk volume.  I mixed up enough to pour in and fill the 1.5" recess. This is why
the recess had to be sealed well, so Alumilite would not leak inside.  After
that cured rigidly, then the centering ring at the other end that held the BT-20
was removed. Then more Alumilite was mixed up to fill the rest of the void. The
strength was overkill, if I was doing it again with fresher good Alumilite I'd
add some filler to the middle mixes since only the last 1.5-2" or so at the ends
really need to be sturdy.

So, that produced a good sturdy stiff cylinder, but the BT-60 was a bit too
thick. I peeled off a lot of the outer layers of the BT-60, but the peeling did
not produce enough  of a reduction in diameter. So I ended up doing some sanding
with a sanding block at three places 120 degrees apart to get the desired
diameter. That was a crude way to do it, not ideal, but thats what I ended up
with. If I did it again, I might try making a very thick-walled fiberglass tube
on the aluminum mandrel, using some 1.5 oz cloth or many layers of 3/4 oz cloth,
to be sure the tube would be large enough in diameter and relatively sturdy to
be able to do the same thing as done with the BT-60 regarding filling it with
resin.

With the cylinder of the right diameter, the last thing was to give it fillets
to align the rails. That was done by using two special styrene discs with holes
at the exact spacing needed for the desired diameter.  I drew up a pattern in
MacDraw for the centerlines of the holes, and rubber cemented the pattern to
some styrene so the holes could be made.

With the two discs at either end of the rails, with the cylinder inside, the
rails were checked to be sure they were parallel to each other with no twist.
Five Minute epoxy was dabbed along the cylinder and the rails to produce the
beginnings of a fillet. I could only do two fillet sides at a time. After the
first six fillets were done, then I followed up with some more epoxy to make the
fillets larger.  After allowing a very long cure time, then it was time to
remove the rails from the cylinder. Well, that was a problem. I had used vinyl
tape so the epoxy would pop off, but the epoxy stuck a bit better than I
expected. I did finally get the rails off, but in the process broke parts of a
couple of fillets. Fortunately the fillets still were fine for aligning the
rails, so I just left them that way.  If I did it again Id apply a lot of mold
release on the rails where the fillets were created.

Once that was done, a test assembly by laying the rails in place and wrapping
with tape. Just fine, nice solid assembly, sturdy, and fast (no bolts!).

Thats not to say the tower was done, but that was the most important part.

The main rails are 40" of .505 fiberglass tubing. The upper rail extensions are
25", making for a total tower length of 65".

12" of .414 tubing are used for joiners. A little bit of mylar tape is used to
help get a bit snugger fit.

The top ring is always a problem. However, I already had one, an adjustable one
I used for an older 30mm tower made similar to this one. The tower top ring
dates back to a design I used for a top ring in 1978, made out of spruce and
using some telescoping brass square tubing with a good friction fit for
adjustment.  The spruce top ring itself is 6-sided, three bolts and wingnuts
hold the 3 pieces together, so the top ring can be disassembled for transport.
Also the brass/music wire spacer rods are removable too.  Thats not to say I
might not want to get hold of a better top ring for the WSMC, but this one
worked fine.

The tower is attached to the Omni-Pad by bolting one of the rails to the pad.
Due to the piston, some of the rail length is not used for guidance, so there is
no problem by attaching bolts to the side. Besides, the tower is more rigidly
attached that way than a bottom attachment. I did originally think of attaching
via the side of the cylinder, but attaching via the side of a rail was simplest.

To be sure that the tower rail will not rock side to side when bolted to the
pad, I made up an interface piece out of balsa. Some 1 wide 3/8 thick balsa
that has two 1/2 pieces of triangular balsa glued side by side, the triangles
oriented inwards so that there is a 90 degree V-notch as a result.

To help keep the rails from bending apart near the middle of the tower, one of
the two styrene discs used for the fillet process is also used. The disc is
located a few inches below the top of the piston head. The ring has a 1/2" hole
in it, which works fine with 11mm pistons.

Speaking of pistons, the tower was designed from the start with piston use in
mind. That is why the tower is so long, since the piston takes up so much
length.

The reason for the BT-20 cast inside of the base cylinder was to give a good
universal means for attaching a piston to the tower.   Any sort of attachment
can be made up and simply plugged inside the BT-20.

Now, there is one mistake I made, in retrospect. To get maximum use of the tower
length, I decided to interface the piston in such a way that the lower few
inches of the piston tube itself fit down inside. In theory, that was great. In
practice, it was a problem. The fiberglass tubing core of the piston assembly
was free to slide, too free to slide. When I set up a model in the tower, I
lowered it onto the ignitor protruding from the piston head, and then while
holding the model dead still, slide the piston tube up to fit over the bottom of
the engine. Well, what ended up happening was that due to fiction,  the core of
the piston also slid up with the piston tube, and sometimes crushed the ignitor.
What I need to do is to change the interface so that the core can be firmly
anchored so it cant accidentally slide up.

Refer to: http://www.spacemodeling.org/gcg/omnipad_tower/omnipad_tower.jpg

In the photo of the Omni-Pad and tower, you can see the whole tower. One view
shows the model with piston at the bottom position, and one view shows how much
tower guidance is left at the piston that the piston fully extends.

The way the tower sits relatively close to the ground, there is enough room to
be able to put the model into the tower without tilting it or using a chair. For
someone shorter, that could be a problem. The first line of defense would be to
mount the tower even closer to the ground, and indeed if I needed to I could
have made it sit almost touching the ground.

************ Omni-Pad

Every tower needs some way to hold it in place and adjust the launch angle. Some
like the Apogee/Medalist tower have their own legs. Most others use a separate
device to hold thew tower, such as a Tripod or pad.

Too often the camera tripods are not sturdy enough. Often they wobble in the
wind with a tower attached. In enough wind, the solution has been to add a sting
to act as a guy wire to prevent the tower from blowing around too much. Thats
an on the field  band-aid, not a really good solution. You cant easily change
the angle to counteract weathercocking since the guy wire isnt easy to adjust,
and if there are three towers all using the guy wire method in the same pad
area, eventually you know whats going to happen.

So, heres some info on the pad I use. I call it "Omni-Pad" since it's used for
holding all kinds of launch devices.  C-rails, R/C RBG towers, Pistons, Rods, and
conventional towers. I won't get into extreme detail, someone who wants to
make a close (or exact) copy can let me know and I'll photocopy a drawing I have
and take some more details pics where needed.

Again refer to: http://www.spacemodeling.org/gcg/omnipad_tower/omnipad_tower.jpg

Basic design criteria was that it be able to break down easily for transport,
and not require any tools to assemble.  The no-tools aspect is covered by use of
wing-nuts or thumb-nuts, and the occasional thumb-screw.  6/32 size used for commonality.

My first Omni-Pads broke down for transport OK, but they were made out of 1 x 2
wood, making them pretty heavy and requiring a 48" long shipping box.  The last
version I made up breaks down into parts no longer than 30" and I can fit one
inside a suitcase. It is made up of 3/4" square Aluminum tubing and 5/8" wood
dowels.  The Aluminum tubing is used for the main parts.  The 5/8" dowel is used
for extending the pad out for a larger footprint, for the front leg and outer
legs. The dowels simply slide inside of the aluminum tubing. A threaded insert
in the dowel and a hole in the aluminum tubing allows a thumb-screw to be used
to secure each dowel in place.

The pad needs two diagonal struts for support. The lateral (side) strut keeps
the pad from wobbling about the yaw axis. I use some U cross section aluminum
extrusion for that, but it could be anything suitable.

The most important diagonal strut is the one for elevation. The lower end of it
attaches to one of several 6/32 threaded rod at specific places along the
forward 5/8 dowel (The 6/32 rods are really just 6/32 screws with the heads cut
off, and CAed into the dowel).  They are located in places that result in 90,
85, 80, 70, 60, and 45 degree elevations. Of course since the Omni-Pad was
mostly meant for R/C use, the exact angle didnt matter much. But it has been a
bit of trouble when used for ballistic rockets.

For the flyoffs I changed that strut to one using a turnbuckle so the elevation
angle could be adjusted several degrees. The turnbuckle uses 3/8 threaded rod.
It was grafted to some fiberglass tubing to act as a joiner between the threaded
rod and dowel with a hole at each end. You can see in the OmniPad_Turnbuckle
photo what it looks like.

http://www.spacemodeling.org/gcg/omnipad_tower/omnipad_turnbuckle.jpg

The angle is adjustable backwards too. When theres a lot of wind, it is useful
to be able to tilt the tower AWAY from the wind so that as the model
weathercocks it will weathercock vertical, and hopefully not weathercock much
into the wind. Depends on some events as to how important that might be.

To help keep the pad from blowing over, the front leg is staked into the ground.
A very large screw eye is attached to the front end of it, so a large 3/8 nail
can be used to stake it down. Since I face the front end into the wind, it does
not have to be staked elsewhere. If the wind changes, the pad normally is just
pivoted around the staked front end, though in a tight space if need be the
front end gets re-staked.

Now, theres all kind of ways to make pads and towers and so forth. I dont
expect to see exact clones of the Omni-Pad or my tower. But there are some parts
that folks might want to consider doing with their pads and towers. I just hope
we dont see more towers doing a hula dance in the wind, or tied to an
inconvenient guy wire that is both in the way and does not allow for easy angle
adjustments.

- George G.