A number of years ago, I bought a remote-controlled (R/C) four-wheeled robot base but didn't end up doing anything exciting with it beyond assembling it and driving it around in the dirt a little bit. Eventually, my brother and I began discussing turning it into a mobile rocket launcher - we grew up obsessed with Estes Model Rockets and still had many of them and all the necessary equipment. Once I had some time to spare, I spent some quality time with some Futaba radio equipment, SolidWorks, a LaserCAMM, and some tools, and voila - I had a remote-controlled, four-wheeled, adjustable angle model rocket launcher.
Step 1: Establish Your Base
Unfortunately, I don't have any information on the base I used, so the photo will have to suffice. Inside the black metal shell, you'll find 4 12V-200rpm motors controlled by 2 motor controllers with servo leads. On the top of the body there are 4 threaded screw holes which we'll use to attach our platform base.
Also in this photo you can see the following, most of which we'll discuss later:
- On/off switch, charging port, and status LED on side of body
- Drive battery packs (white)
- R/C receiver
- R/C battery pack (red)
- R/C switch (hanging off rear of body)
In any case, because I already had the mobile base built (and because it wasn't my design), I won't talk any more about it. You can decide what kind of base you want, and then move on to plan out what the electronics and the launch platform will look like!
Step 2: Electronics
The R/C system (middle 1/3 of schematic) consists of the basics of any R/C project:
- a receiver (I used a Futaba 7-channel R138DP). The receiver is the "brains" of the entire system, giving control signals to everything else.
- a switch
- a battery pack that will ultimately power the launch platform servo.
The drive system (top 1/3 of schematic) is responsible for making the whole thing move around the world. The components are:
- a battery pack
- a switch
- two motor controllers with their input coming from the receiver and their output going to the motors
- four motors, two controlled by each motor controller
Launch Platform System
This "system" only consists of a single servo - the one that will control the position of the launch platform and therefore the rocket. The rest of this system is mechanical, and that's the focus of the next step. Two important notes here: first, make sure this servo is strong enough, or is rated at a high enough torque to handle the launchpad - my first attempt failed to lift the rocket out of cruise mode. And second, I used the throttle control stick on my radio to control this servo, which meant that due to its lack of springiness returning it back to the zero position, I could have the rocket stay at any angle between 0 and 90 degrees - very cool.
The ignition system (bottom 1/3 of schematic) includes:
- a battery pack appropriate for the required current for the ignitors
- an R/C switch - a circuit that only passes current when it receives a "HI" signal from the receiver - e.g. when I flip the switch on my radio. I bought an RCE200 switch from Robot Marketplace, and it works wonderfully.
- the ignitor - I used an Estes Model Rocket Ignitor, attached to the wires by small alligator clips
Again, this was sort of a brute force electronics solution, but I wanted to focus my attention on designing a sweet launch platform instead of designing too much custom circuitry or hacking the mobile base kit too much. The most notable way to streamline it would have been to consolidate battery packs and use voltage regulators and other components to achieve the correct levels. If you design a better or custom system, please share!
Alright, let's move on to the mechanical stuff!
Step 3: The Launch Mechanism
There are many ways to make the launch platform's angle variable - horizontal in cruise mode, vertical in launch mode, or anywhere in between (for a more exciting launch mode?). At first, I was thinking of attaching the servo directly to the blast shield itself, making it a simple hinge. However, after considering placement as well as required torque to actuate the hinge, I thought a four-bar linkage would be a better choice.
The linkage I decided on is geometrically simple - the driving link and the coupler are the same length, and the idle ground link is the appropriate length such that it makes a 45 degree angle with the base when the rocket is at 0 or 90 degrees. This turns out to be the length of the other links multiplied by the square root of 2, just like a 45-45-90 triangle. With these parameters, the rocket's position on the mobile base doesn't change much at all when moving from cruise to launch mode. Additionally, the driving bar in the linkage is vertical in launch mode, minimizing (or in theory eliminating) the torque on the servo from the weight of the rocket. Check out the photos for a clear visual of the explanation.
Making the Parts
I designed the parts using SolidWorks and cut the acrylic with a LaserCAMM laser cutter. The parts in the images below are numbered and identified below:
1. The base of the linkage that will attach to the top of the body. The four round holes at the corners are for screws, the large hole at the center near the rear is for wires to feed up from the body, the three pairs of square holes spaced closely are for the linkage attachment pieces, and the pair of square holes spaced farther apart is for the servo holder.
2. Linkage attachment pieces - insert feet into holes on base and attach linkage bars through hole. Note: there are 4 shown in the image, but you'll only need 3, since the servo is the 4th base bar.
3. Servo attachment bar - this is the only driving bar, since the other "driving" bar isn't driven by anything, and is just added for support. Note the widening in the center of the bar - this is so that it sits on the base in cruise mode, allowing the base to hold the weight of the rocket instead of the servo.
4. Support "driving" bar - on opposite side from the servo attachment bar.
5. Idle grounded bars - note the length difference.
6. Coupler bars - these attach to the driving bar at one end and the idle grounded bars at the other end. The launch platform fits into the notches.
7. Servo holder - servo fits snugly underneath.
8. Launch platform - serves as structure beneath the rocket and will hold the launch rod. The 4 holes at the corners are to attach the aluminum blast shield, the larger hole at the center near the top was for wires to come through (though I ended up not feeding them through), the hole at the very center is for the launch rod, and the 4 holes surrounding that are for the launch rod holders.
9. Launch rod holders - these extended behind the launch platform with washers as spacers to hold the launch rod in place.
Once all the parts are cut and ready to assemble, you fit them all together like a puzzle (the beauty of putting in the design effort at the beginning!), and you've got a platform that smoothly transitions between horizontal and vertical.
The Finishing Touches
Now that we have a great launch mechanism, we can add the blast shield and a set screw to lock the launch rod in place. I grabbed a piece of thin aluminum from Ace and cut it so that it perfectly covered the acrylic launch platform. This piece is not only critical to preserve the acrylic, but it also makes the project look a little more heavy-duty. Next, I super-glued a set screw on the underside of the launch platform in between two of the launch rod holders to lock the rod in place. The blast shield is easily visible in the photos; check out the rear-angle side view of the project to see the set screw (silver with gold nut).
A final note for radio customization: I have a Futaba 9CAP radio that's highly customizable using the built-in software, and I did some tweaking to the settings to get the responses I wanted. For example, I had to change the bounds of the throttle so that the stick all the way down meant the platform was in its horizontal position and vice versa for the vertical position.