April 22

Launch box

Ah, Fourth of July.  The joys of smokebombs and explosives.  The delight of shooting off rockets.  Who wouldn’t want their toddler to participate?  To see his shining face as there is a boom or a shower of sparks at the press of a button?

I planned to use a model rocketry ignition system to create a safe way for the youth mentioned above to participate.  Unfortunately I had given all of my model rocketry equipment away in an effort to increase usable space (and to get it in the hands of budding rocketeers).  Commercial options were available, but they were relatively expensive, and frankly lacked visual appeal.  Engineer’s audacity (not hubris, though) may have had something to do with it.

Initial research / Functional Specification
I started with a fairly clear idea of what I wanted and how I wanted it to work.  The launch box itself should look like something a government scientist or military agent would carry – aluminum box, black briefcase, field com box or similar.  It should have large functional switches and buttons (power, arm, and ignition), various LED indication (power, safe, ready, continuity, armed), and a removable safety key that disarms the launcher.  The last is a feature of every commercial hobby rocket launcher I have seen, and is doubly important when you are on the launchpad and there is an excitable toddler near the controls.

Familiar with the operation, I thought that surely there was little more to commercial launchers than a couple of switches allowing or breaking a dead short across the battery.  Turns out I was correct:

From Robot Room (link in references)

The continuity indicator was the part that had puzzled me a bit – was there some low-current path sensing that was implemented?  In a way, yes.  Hot-wire igniters, like the sort Estes rockets sells with their motors, have a blow and no-blow current level.  Referring to the previously mentioned electronic igniters (which apparently Estes no longer sells separately), which will be the assumed reference for igniters for the rest of this entry, due to their widespread availability and familiarity), the manufacturer’s sheet specifies a minimum of 500mA @6V to ignite them, and that current below 20mA is safe.  Briody’s widely-referenced paper on electronic rocket igniters indicates that 12A is quite likely for said igniters given a chance.

Launch Box HandCAD schematic
Launch Box HandCAD schematic

The schematic for this is fairly simple: several series switches which, when all closed, cause a dead short across the battery.  I wanted to be able to operate from a variety of sources and voltages, including 4xAA, lantern battery, C- and D- cell packs, and small to medium gel-cell lead-acid batteries.  Most of the way through parts gathering, I also added an audible option: warning beep when the box was LIVE (safety key inserted), and continuous tone when it was LIVE and ARMed.

S1 was a simple heavy-duty toggle switch.  The S2 safety is a 1/4″ TS jack – the 1/4″ T(R)S system has rather good voltage and current carrying capabilities.  The “key” is a TS plug with the TS shorted via heavy wire.  S3 is a self-illuminated pilot switch, and S4 is an old large push-button switch with a pneumatic delay system built into it.  A lovely bit of work, though you must actuate it in a specific way to get a reliable delay from it.

NOTE: while the buzzer control worked fine on a breadboard using fill-in LED indicators @6V, when assembled on a pad-per-hole board and wired into the 12V system, it smoked Q1.  A fast-approaching due date meant that I did not debug the cause, nor implement the buzzer.  Which is probably for the best, given its current use as equal parts functional launcher and fun toy to imagine launching rockets.

My box requirements were fulfilled with a thrift-store find that would make a 12-year-old girl clap with delight.  Below is the brief layout and build montage

I used a project box as the battery compartment, both to provide a rigid protective housing and to protect the rest of the box if the batteries had an unfortunate accident.  The case was a bit flimsy – the frame is aluminum, but the walls are pressed-paper board.  To reinforce it, a piece of polycarbonate was glued to the bottom, the switch plate and battery compartment were bonded to the sides, and the edge with the handle was reinforced with a long scrap piece of aluminum bar (instead of relying on two rivets into paper board).

Lay it all out, drill and glue and solder and glue and paint some more, and a quite successful launch box results, both for electronic ignition model rockets, and air cannon rockets, and for imaginative play, too.

References / further reading:
Robot Room: Estes beam schematic
Estes Igniters and their uses: Estes document on
Electrical Current Requirements of Model Rocket Igniters (pdf warning) by Robert Briody
National Association of Rocketry safety code

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Posted 2015-04-22 by JourneymanWizard in category "Projects and Tech

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