Mock 4
The Mock 4 is my fourth scratch-built model rocket, and third high-power bird.
I was midway through building the Big Ugly Bug II,
when the thought occured to me, "What if I lose this model? I won't get my level 1
certification, and I won't have another model to try again until I can build one.
Maybe I should build a backup..." Thus the idea of the Mock 4 was born.
Before I found the commercial nose cone for the 4" BUB2, I was planning on taking
an Aerotech plastic nose, filling it with two-part expanding foam, and then sanding
it down to the proper shape and size. Thankfully I didn't have to do that, but
buying the other nose left me with the Aerotech 4" nose, some 4" tubing, and some
29mm tubing. Throw in some centering rings, fins, a 'chute and a launch lug, and
I'd have my backup rocket. At least, that was the plan.
When I popped the nose into the tube and sat it on end, it just didn't look right.
It was, well, too short for a high-power rocket. It would serve nicely as a
payload section, however. So, I talked to Tim, the president of our club and also
head of Rocket Technologies, our
local high-power rocketry supplier (handy, that). I looked over several kits
he had in stock, but decided to go with another scratch-built of my own design.
I bought another length of slotted 4" cardboard tubing, three G10 fins intended for
a Tomahawk, a launch lug, parachute, and nomex pad. He didn't have any 4" to 29mm
centering rings in stock, so I got some 38mm ones and a one foot section of 38mm
motor mount tubing, plus a 4" tubing coupler.
As with all my scratch built designs, a bit of part tweaking was in order. I cut
a disk from 1/4" scrap plywood and epoxied it into the coupler to make the lower
bulkhead for the payload section. The fin slots needed to be lengthened to
accomodate the fins. The fins themselves had to have some tabs added so they would
reach the motor mount tube through the slots, and finally I cut the launch lug in
half to get a more secure mount on the launch rod (two lugs spaced two feet apart
is much better than one).
I install the motor mount and fins, and foamed the space between body tube and
motor mount. I made my own motor retention system out of T-nuts, long bolts,
and some chunks cut from aluminium bar stock.
Since the tube was so long I decided not to mount the shock cord to an eye-bolt in
the engine mount. Instead, I made the other classic mount, involving a short loop
of nylon rope, a bit of tape, and lots of epoxy. I installed the bulkhead, nose,
launch lugs, and strengthened the bulkhead lower surface and tube inner surface
with some CA adhesive. After some sanding to make them fit again, the big (69"
tall) rocket was ready for paint.
The name Mock 4 came to me when I noticed my rocket was similar in design to a
friend's LOC 4 kit. Differences are: my rocket has a payload section, is much
longer, has G10 fins mounted through-the-wall (the LOC has plywood fins glued
to the outer tube only), and has a 38mm motor mount compared to the LOC's 29mm.
I added a 24" parachute, about 20' of shock cord, a nomex pad instead of
wadding, and a 38 to 29mm adapter so I could fly the rocket with H and I motors.
I really like the way the rocket turned out. It started as just a backup rocket
for my level 1 certification attempt, in case the BUB2 got lost or broken, but
turned into a rocket that may also allow me to do my level 2 certification. At
just over 3 pounds, wRASP predicts a flight
to 850 feet on an Aerotech H128W (classic level 1 cert motor), or to 3040 feet
on Aerotech's J350W (level 2).
Update: I flew the Mock 4 for the first time on March 23rd, 2002. I had
just made my successful Level 1 certification flight on the BUB2, but the Mock 4
just begged to be flown. Since I hadn't tested my Dr. Rocket 29mm 240 casing
(I used the end closures from it and a 180 tube borrowed from a friend to do
the certification launch, since H128W's won't fit in the 240 tube), I figured
it was a good excuse. I bought an Aerotech H97J BlackJack reload kit and
assembled it in the 29/240 casing. With the 29 to 38mm adapter I installed
the motor in the Mock 4, and put it on the launch pad.
The Mock 4 flew very well with the H97J, even though I hadn't simulated it with
a motor that small. The BlackJack fuel load made a nice, black smoke trail,
and the rocket flew very straight (the BUB2 had weathercocked due to its
fin configuration and short body). The Mock 4 only suffered a paint chip on
the nosecone, so it would have made a very servicable backup rocket for the
Level 1 attempt had the BUB2 suffered a failure.
Update: The Mock 4's second flight was on April 20, 2002. This was the
first flight of my fully assembled Dr. Rocket 29/240 casing, using an Aerotech
H180W fuel load. The more powerful motor made for a beautiful, straight flight
of the '4. Unfortunatly it landed in mud over 6" deep, which I got to slog through
since our club member who owns an ATV hadn't arrived at that point. The rocket
required some hosing off to clean away the mud, but was otherwise undamaged.
I really like the results flying the '4 with the H180W motor.
Update: August 6, 2005 After flying successfully on an I motor last year,
I finally made my Level 2 certification attempt with the Mock 4. The motor was
an Aerotech J420R, loaded in a Dr. Rocket 38/720 case. Concerned about the
mass of the motor and what it would do to the CG, I put 1 pound of ballast (a
water bottle) in the payload section. I also stuffed a nylon case from a folding
chair on top to restrain an cushion it. The launch was spectacular, but the
rocket shredded just after burnout. The thinking is that the water bottle's
momentum carried it forward with enough force to rip the retaining screws out
of the nosecone and knock it off the rocket. The body then folded in half and
the rocket came apart, scattering debris over a large area. The speed of
the flight stripped every last fleck of paint off the nosecone! Some
images of the flight are
online. The parts that came back are probably in good enough shape that I
can rebuild the Mock 4, we'll see.