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Starfire has any
electronics payload bay built-in at the top of the booster. It is designed
to carry a camcorder for recording onboard video. More information
on that can be found here. That
payload bay is also designed to carry the electronic timers needed for
igniting the outboard air start motors. To arm the timer system it is
necessary to have access to a series of switches. These switches are
used to power-up the timers once Starfire is on the pad and ready for
launch. The switches must also disconnect the safety shunts on the
igniters for the outboard motors as well as connect them to the timer.
The photos below will illustrate how the timer switches are mounted and
accessed prior to flight. |
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1/16" thick x 3/4" wide aluminum
bar stock material from the local hardware store was used to create a
metal mounting strip
to hold the timer arming switches. First the aluminum strip was cut to
approximately the proper length. |
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Next three square wood
dowels 1/2" x 1/2" were used to create a holder for the aluminum strip.
Two of the wood dowels are longer than the third. These two
were slotted along their length using the width of a table saw blade.
(1/8") The two were then epoxied together with the slots facing each
other and with the shorter dowel in between them. |
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This is a close-up of the
holder with the opposing slots that allow the metal strip to be inserted.
Click on the photo to get a closer look. |
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This photo shows bulkhead
B4 in the Starfire booster along with the three 5/16" threaded rods that
are part of the booster internal structure.
The switch holder was sized
to exactly fit between the two bulkheads B3 and B4 at the top of the
Starfire booster section. An opening was cut in bulkhead B4 that
allows the metal strip to pass through in order to be inserted into the
holder. The opening also accommodates the switches that will extend out
from the metal strip.
When Starfire is complete,
the switches will be mounted on this metal strip and it can be inserted or
removed from this holder from within the booster payload bay. |
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Front view of the holder and
the metal strip. |
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The metal strip was drilled
to allow up to seven switches to be mounted. For the first flights
only four were needed so that is what is shown here. Making the
metal strip easily removable allows the switches to be serviced or rewired
as needed for future flights. |
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Switches are mounted to the
metal strip. |
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The metal
strip was cut flush with the top of bulkhead B4. This was a test fit with
four switches in place to make sure all the required clearances were
adequate. The three
plastic tubes seen in this photo are for running igniter wires from the
electronics bay down to the outboard motors at the bottom of the booster.
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An aluminum cross bar was
added to cover over the top of the metal strip to hold the strip in place
during flight. Two small threaded brass inserts were installed on
either side of the opening to allow two #6-32 x 1/2" machine screws
to hold the cross bar in place. The rest of the opening was left
open to allow the wires from all the switches to pass through.
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Another view of the cross bar
and switch holder scheme. |
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Now that the switches have
been mounted, we need to drill holes in the booster body tube to allow
access to these switches when Starfire is on the launch pad. These
are rotary switches so we just need small holes for a 1/8" diameter
screwdriver to be inserted to activate the switches.
A laser level was setup on
a tripod and aimed at the center of the face of the first switch.
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Next the booster body tube
was slid over the booster being careful to not move the booster with
respect to the laser level. If you look closely at this photo you can see
the red laser dot of light that indicates where the hole for the first
switch should be drilled. A felt tip pen was used to mark this point
on the body tube. This process was repeated for the rest of the
switches to make sure each access hole would be accurately located. |
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After the access holes were
drilled, the body tubing was slid back onto the booster and a 1/8" screw
driver was used to check the alignment. Perfect!
At this point we are ready
to proceed with the rest of the booster construction process. Such
as installing the body tube, finishing the fin fillets and installing the
coupler tube at the top of the booster.
Details about that are here. |
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This photo shows the timer
electronics used in Starfire to ignite the outboard air start motors. Two
Missile Works
PET2 timers are mounted to a single prototyping pref board. The perf
board is itself mounted to a round piece of G10 that has holes and slots
in it to mount into the Starfire booster electronics payload bay. The pref
board also has connectors on it for the e-match leads from the motor
igniters. The whole assembly is prewired and then dropped into the
Starfire payload bay. |
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The mounting plate was
designed with large openings in the edges that allow it to easily pass by
the nuts and washers on the threaded rods in the electronics bay.
Once past these nuts and washers, the plate can be rotated and the
threaded rods enter the narrow slots in the plate. Nuts on the rods
are then tightened to secure the plate.
After this photo was taken,
additional openings were cut from this plate to accommodate the strip of
switches and the wiring harness. |
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This view shows the entire
timer assembly with six switches installed. Each timer has a power switch
and two channel switches. The channel switches are wired such that
they disconnect and shunt the igniter leads in the "off" position. In the
"on" position, they release the safety shunt and connect both leads of the
igniter to the appropriate timer output.
This assembly is easily
installed into Starfire by sliding the strip of switches into the holder
from the top of the payload bay. The timer PCA is then lowered onto
stand-offs on the retainer plate and nuts are added to hold it in place. |
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Back side of the timer
assembly. The red and black leads at the top are for the connections
to the battery pack. A 9.6V Ni-MH battery pack mounts in the
same electronics bay. It actually mounts on the same plate with the
camcorder not the timers. The battery pack is one sold for use in R/C
electric cars and is available at places like Radio Shack. It is a
set of eight AA cells welded together and provides 1.6AH. |
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On the side of the timer
assembly is a special connector that is used to start the timers.
The mating part to the connector is the black piece. It is normally
installed into the connector and has jumpers that make connection to the
"pull pin" circuit on the PET2 timers. A section of monofilament
fishing line is tied to the black part. The other end of the fishing
line will be tied to the launch pad. When Starfire lifts off the
pad, the fishing line will pull the black piece out of the connector.
This starts the timers. The fishing line also easily breaks as
Starfire climbs higher up the launch rail. |
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