· Rocket Launcher

June 2006
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Laser target-finding, self-reloading multi-shot rotary rocket launcher.

This side view of the rocket launcher reveals most of its key features. On top of all sits the feeder mechanism, which consists of a reservoir for up to five rockets and a feeding motor that delivers them into the launch bay.
The long black rails are the ones on which the rocket comes out. The trigger mechanism (behind the yellow liftarms) pulls back a hammer and loads two rubber bands. These pull the hammer forward, hitting the rocket and firing it.
The red plates are part of the tilting mechanism, which was disabled in the final build. The entire structure sits on a turntable allowing a pivoting action. The battery boxes act as a supporting base.

The close-up shot of the feeder mechanism shows its functioning. Rockets get inserted at the yellow arrows, and sit in a vertical slot (first two missiles). Below is a cross-shaped assembly that rotates, grabbing one rocket at a time (third rocket) and placing it in the launch bay (fourth rocket).
The device behind the feeder (just above the RCX) is an all-LEGO stepper motor. Read on...

The stepper motor (based on Robert Munafo's design) makes exactly one rotation with each impulse. The yellow bush gets pulled up by a rubber band. The motor gets turned on just enough time to make half a turn, then it's put in float mode, and the rubber band completes the turn. The red and green gears are 2:1 reduction stages. So, for each turn the motor makes, the blue cross-shaped assembly makes exactly 1/4th of a turn. The rockets fall between two of the cross's arms, so exactly one gets moved at a time. This ensures a precise movement without the need for control sensors.

The other main characteristic of the rocket launcher is its ability to locate targets autonomously. This is done using a custom-made laser sensor (original design by Philippe Hurbain), that returns an impulse when its laser light gets reflected directly back into its photocell. Using reflective targets and a rotating sensor, the target finding task is made easy.
A special thanks goes to Heiko P. for generously donating the laser sensor.

These are the targets. They are covered with retroreflecting tape. This tape has the property of (unlike normal mirrors) reflecting any incoming light directly back to the source, in this case, the laser sensor. You can see this in the first picture, where the camera's flash gets bounced back just the same. The second picture shows the laser sensor's light. The black panels are just to make the targets slightly larger and therefore easier to hit, as the aim is not 100% exact.

The target finding task is accomplished using the Technic turntable at the bottom of the structure. It has a rotation sensor made out of a polarity switch (which short-circuits with each 1/4 turn, making it look like a touch sensor closing) to limit the sweeping range.

The original idea was to make the whole robot capable of tilting the launcher, allowing it to shoot down targets at variable distances. However, this brought numerous problems:
First, the feeder+trigger+RCX was a very heavy mass to move. It had to be done slowly, and I devised a 960:1 reduction gearing (see first picture, the launcher is attached to the green bricks). But the axles started twisting and the structure coming apart despite the locking beams and liftarms, the gears no longer interlocking. At least, none of the gears lost teeth... :-)
The second problem was determining the distance to the target (the laser sensor was not suitable). The second picture shows the laser sensor mounted on a tilting module over the firing rails. It could rotate and detect at which angle it hit the target, and thus the inclination angle could be calculated. This would have made the program too large, the procedure too long and the robot even heavier.
In the end, the angle of attack was fixed and the optimum target distance laid at about 85 cm from the robot's base.

Here are some renderings of the model's MLCAD file (you can download it here). The NQC program file used in the launcher can be found here.

Last but not least, a video showing the rocket launcher in action. Click the link at the end of this page for a high-resolution version of the video (640x480).

[High-Res pictures available]

[High-Res video available (MP4, 11 MB)]

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