Early R/C rocket, no roll hold.
Basic R/C rocket parts.
Rocket frame with parts in place.
Photocell circuit board.
This is the circuit board, the chute-switch servo and the roll servo.
Centering-adjust pot, to left of CDS photocell, is accessible from outside.
Photocell pictorial wiring diagram.
Roll control surfaces inside fins, sun window.
THE FLORIDA MOSQUITO
If you've ever had an R/C model car, you know that it's fun and easy to
operate because control is intuitive. And flying R/C trainer planes that
have high wings, high dihedral, low wing loading and a slow glide are also
fun and easy to fly because of their natural stability. But a rocket only
has arrow-like, one-axis stability.
Four large rear fins with canard (front) control fins is a good
configuration for guided rocket control, if roll around the axis can be
neutralized. This way rocket control can be simple and intuitive.
The method we use to gain roll stability is simple and works well.
It requires launching only on a clear day with the sun low on the
horizon. A control circuit is used to sense the sun and generate
pulses sent to a model servo to correct any roll. Then the operator can
concentrate on guiding the rocket by the joystick on the transmitter.
The circuit that changes the CDS photocell resistance into different
pulse widths for the servo is a simple 555 timer that can be assembled
on an experimenter's circuit board. There are several boards to choose
from at Radio Shack. Other parts are .01, .22 and 10 mf capacitors, a
silicon diode (276-1122), a CdS cell (276-116), a 68k, 1/4 watt resistor,
and the proper connectors for your make of servo.
If you are firing the rocket vertically it is best to shoot when the
sun is low on the horizon during morning or evening. Set up the launch
pad so the sun shines onto half the photocell thru the window in the
body tube. As the rocket turns, the servo must move the control surfaces
so as to turn the rocket back to keep the sunlight on half the cell. If
light totally floods the cell, it should roll back to be half-covered.
If it is shaded entirely, it should roll to be half-lighted again. After
you are sure the roll neutralizer is working, check the X-Y controls to
see that they respond just like the joystick movement when you're looking
up at the rocket. If control surfaces move backwards, change the servo-
reversing switches on the transmitter. If X works in Y direction, change
the servo leads at the receiver, or move yourself around to where you are
oriented correctly with the rocket.
The R/C rocket we have uses Futaba S-33 Micro servos (.6 oz) and their
Micro R4H four-channel receiver with the 250 mAh batteries. This keeps
weight down and performance up with F or G motors. If you can find them,
100 MaH batteries are even lighter and sufficient due to the short flight
time. Two channels are for directional control and one for the chute-
ejection-switch servo. This leaves one channel free.
The roll-control circuit does not use a radio channel, just some battery
power. You could also have ignition by radio command. The model-rocket
crowd won't allow you on their sites with this feature. Or you could use
the other channel to ignite a trailing smoke charge.
A foot switch can be used for ignition, so the operator can be ready,
holding the transmitter in both hands at launch and looking up.
A complete parts list is difficult since there are so many ways to
build the rocket. Included are some model-plane hardware parts. If
you decide to use Estes BT-70 you will need two sections, two BNC-70's
(nose cone and boattail-engine holder), one JT-70 coupler, one 1/2" x
1/4" x 36" spruce piece for rails, a 12" piece of 1/4" x 1/4" spruce
for some cross beams, a sheet of 1/8" ply for disks and canard fins, 3/32"
ply for main fins, 1/16" music wire for control shafts, CA adhesive,
linkage hardware, four hinges, two control horns, balsa filler coat,
spray paint, 1/8-teaspoon measure, FFg black powder and Estes ignitors
for chute-ejection charges, wires to go from servo switch to chute-eject
charge, mini toggle on-off switch, coping saw, hobby knife and chisel,
hand drill & bits, sandpaper, 36" chute from 2-mil trash bag, parcel tape,
rubber shock cord, chute anchor strap, snap hooks, chute leader, chute
wadding and masking tape for the charge packet.
On the top disk is a cardboard or 1/16" ply stop disk with a
diameter the same as the O.D. of the BT-70. The 1/8" ply disks have a
diameter of the I.D. The stop disk is CA'ed to the top 1/8" disk and
rests on top of the main body tube in use. The chute compartment rests
against the stop disk from the other side, sliding over the JT-70
coupler which is CA'ed to the stop disk. The nose cone goes into the
upper BT-70 section. The motor compartment-boattail seats against the
lower disk on the main rails and against the main body tube
simultaneously when the inner frame is fully seated against the top of
the body tube. Cut the main body tube after fully contructing the
inner frame and the motor section. The motor thrust pushes equally
against the rails and the main body tube and the fixed rear fins.
To assemble the rocket, begin by charging the battery. Attach the
roll-control pushrods to the roll-servo horn. Attach pushrods from the
control-servo horns to the arms on the fin control shafts. Adjust these
so the control surfaces are zeroed when the transmitter joystick is
neutral and tighten the arm setscrews. Install the battery in its place,
turn on the transmitter and on the receiver. This will zero all the
servos. Be sure transmitter trim controls are zeroed and that the
"throttle" stick (chute-eject) is low. Now slide the inner frame into
the main body tube. Attach the roll-control pushrods to the control
horns on the two roll-control surfaces. Seat the inner frame and adjust
the roll control surfaces so they're zeroed. Install the boattail so it
just touches the inner frame rear disk while fully seated in the body tube.
Always check continuity of chute-charge ignitors with an ohmmeter.
Prepare a chute-ejection charge by soldering the a 6" length of the
speaker wires to the twin-ignitor leads. Put masking tape behind the
ignitor heads & put 1/8 tsp of FFg black powder onto the tape. Put more
masking tape over this charge so it forms a sealed packet. Be sure the
tape keeps the leads from shorting. Make up as many of these as you
will use at the launch. Join the ejection-charge wires to the rocket
leads using Dean's connectors.
Install an F (500' height) or G (1000') motor of moderate thrust and
long burn. The motor is now ready for prepping and launch. The
launch rod should be 1/4" diameter and 8 feet long. If it's aluminum,
polish with a soaped scouring pad. A target kite can be flown on a
windy day. Tethered balloons (use a bunch) are best on a calm day.
We found that a rocket can be seen at much greater distances after fuel
burnout if it has a flare, which is better than smoke. A bright, white
flare can be made by mixing 280 grains of barium nitrate, 110 grains of
aluminum powder and 16 grains of dextrine. Weigh using a powder scale
used for reloading bullets. The mixture is dampened with water to make a
thick paste and stirred thoroughly. It should be molded in a tapered
cardboard trough for added strength, over a bamboo skewer. It must dry for
about a week. The flare is 1/4" at the tip and 3/4" at the base so it
stays equally visible as it gets farther away. A 6" long flare burns for
22 seconds. The flare must be ignited before the launching with a
propane torch and this can cause some anxiety. It is best to have some
reliable helpers and a countdown check-list.
CdS = cadmium sulfide (photocell)
CA = cyanoacrylate ("super glue") adhesive
FFg = medium-grain black powder
Our original motor impulse recorder.
The springs are calibrated with brass pan-balance weights strung over the
pulley at the top. Although crude, this worked very well. So our calculated
altitudes matched the actual flights close enough.
Our homemade black powder motors.
These were 3/4" inside diameter endburners with 5 second burn times. They
werr GoexFFFFg and 5% clay powder rammed with a 3-lb hand sledge.
Our Super-8 (film) camera theodolite.
The angle of the camera shows in the same view as the rocket. This was used
back when we built the radio-control and the live-eye radio-control rockets.