Elementary
Pad Set-up
System
Controller
Motivation
There are innumerable launch controllers out there, from Estes' venerable Electron Beam to Pratt Hobbies' Sure Fire II, so why design another? In order of personal importance, there are:
Safety
While all commercially available launch systems that meet NAR guidelines are inherently safe, there are still some things about them that make me nervous. First, most are relay-based, connecting the power source to the igniter through the contacts of a relay. While I've only seen relay contacts weld once and it was discovered before the next unsuspecting rocketeer had a motor burning in his face, that one time was enough for me. Relays can fail, and their failure mode is design-dependent; in other words, they might fail with an open circuit . . . they might not. I wanted something that would always fail open, never in a short.Second, many have launch power right under the hands of the LCO. While I've never seen anybody get shocked at a launch, a car battery has nailed me in the past, and its something I won't soon forget. Proper design should keep the current present at the controller to safe levels.
Cost
I've seen three model rocket controllers ganged together to form a three pad launch system; very cheap, very safe, but not high power capable. Some commercial controllers extend this idea to HPR by ganging up to six individual launch controllers together in one box. Very cool but very expensive. Purpose-designed multi-pad controllers are generally cheaper, but being relay- based, aren't inexpensive either. The cheapest 12 pad controller around, Sure Fire II, is still over $200US, way beyond what I wanted to pay.
Convenience
Next to lugging the pads out and setting them up, the most painful part about setting up a multi-pad launch area is running the leads. There's little one can do about the individual pad leads, but there's no excuse for having multiple leads from the power box back to the controller. The lighter the wires, the easier it is to wind them up at the end of the day, and having launch power at the controller requires heavy gage stuff.And speaking of batteries, have you ever had the motorcycle battery in your launch system fall over in the back of your car? Yuck. Forget that battery acid is dangerous, how are you going to explain that huge stain in the trunk to your wife? There is no need for this, today's battery technology is better than those leaky old things.
Components
So here's what I came up with.
Power Box
The power box sits at or near the pads, and consists of two 12 VDC sealed lead acid batteries wired in parallel and twelve P-channel Darlington transistors. A relay acts as a power source select for them, connecting the positive terminal of the battery pair to their source pins when enabled. The unit selected is the MJ4031 in a TO-204AE (TO-3) case.
Control Panel
The control panel consists of two AA batteries wired in series to perform pad selection though a twelve-position rotary switch, a 7.2 volt Lithium battery for continuity checks and launch control, an arming key receptacle, a continuity/launch switch, and a single status indicator lamp. The continuity/launch switch acts as a power source selector, removing the 7.2 VDC continuity source and enabling the 12 VDC launch source at the power box. The pad selector switch can be any size, and could be replaced with a bank of toggle switches to permit simultaneous launch of more than one rocket (drag racing).
Features
I think these are the major strengths of my MPLS:
High Safety
- The low current of the continuity circuit (~9 mA) prevents accidental ignition, even of low current ignitors.
- The separate power sources for continuity and launch circuits and resultant isolation of launch power to the pad area prevent shock hazards.
- The use of solid state components for launch power prevents relay contact welding and an accidental launch.
Low Cost
- The minimal parts count and solid state design reduces the possibility of failures, and parts are cheap to replace.
- Cabling from the control panel to the power box is light gage and use low current connectors, reducing bulk and cost.
Convenience/Ease of Use
- The use of sealed lead acid cells prevents fluid leakage, and the controller batteries are inexpensive.
- The low parts count and solid state design reduces the size of each box; the control panel for the twelve-pad system shown fits in the palm of the hand.
- Control to pad box connection is though a single connector.
- The simple control panel allows quick training and prevents mistakes.
Power Management
- The power transistor's low resistance (~25% of the circuit) permits most of the voltage drop to be across the igniter, insuring ignition.
- The 560-ohm resistor limits the 7.2 volt continuity check circuit to ~9 mA, allowing the use of low-current igniters.
Improvements
There is a lot about this design I think could be improved, but the prototype was cheap and meets all my requirements. Still, as time, money and parts permit, I hope to make the following upgrades.
Drag Racing
Replacing the selector switch with a bank of individual ones would allow drag races, but I have a number of problems with this. The first, obviously, is cost; switches ain't cheap. The second is safety, what if the LCO forgets to turn them all off? The third is bulk; all those switches would at least double the size of the controller. I have no idea how I'll do this, but it needs to be done.
At-Pad Continuity Check
A selector switch, a push-button and either an LED (preferred) or buzzer in the power box would be all that is needed here.
At-Controller Igniter Verification
Igniters have ~½ ohm resistance. It would be nice if the LCO had some indication back at the controller that the igniter is likely to fire. A panel meter seems bulky, but three LEDs, a few resistors, transistors and some planning should be enough to tell if the line resistance is somewhere between ¼ and ¾ ohms. While this wouldn't work very well for clusters, it'd be OK for everything else.
TO-3 Replacement
Replacing the TO-3 packages with Darlingtons in DIP packages would reduce the size of the power box and lower replacement cost.
Usage
The unit has become our club's primary MPLS, and has successfully launched a J350 motor, so is L2-rated. That rocketeer did have to add some thermalite to his ignitor, as the system burnt it out too quickly, so some work nereds to be done. Still, the system came though its first launch unscathed and is ready to go again.
