The Biggest Fan

 The Thrifty Rocketeer blog continues...

This is going to be a departure from my usual comments and tips directly related to rocketry .  But bear with me, there is a connection.

We've all gone to club launches where there are several launch pads.  Most of these are controlled through a panel launch controller, via a weak current that runs out to the launch pad through a standard electrical extension power cord.  But when it gets to the launch rod, it is typical for a jumper/adapter, or clip lead to be used.

This consists of a short power cord (lamp, toaster, fan or other power cord) with a male plug on one end to mate with the extension cord, and bare wires at the other end that typically have alligator clips or micro test clips that connect onto the igniter wires.  

Most of us haven't given this much thought, as we're focused on our rocket and the motor's igniter and getting a good connection that shows continuity.  Someone else (an equipment manager?) takes care of the wiring and set-up.  And when such a clip lead fails, they just "take care of it". It's not unusual for several spare clip leads to be on hand for just such a swap-out.  But where do they come from? 

One source for me over the years has been a standard box fan. 

 In our college town, most of the student rental houses do not have air conditioning.  In late spring and early fall, students sweat in their houses while studying and run out to buy a box fan or window fan at the nearest Wal-Mart, K-Mart, Ames, Lowes, Menards or similar outlet.  These fans are typically 20" square and 4-5 inches deep, made of white PVC plastic grills that screw into a formed sheet metal frame.  The electric motor in the center powers a PVC fan blade hub with anywhere from 3 to 5 blades, and is secured to two metal cross beams.  The speed control switch is typically on the top of the frame next to carrying handle.

But it is not at all unusual to find such a floor fan laying out at the curb at end of the year move-out, or in late summer as things cool down.   You can usually find them with busted grills, or cracked or missing fan blades... but they almost ALWAYS still have their power cord attached.

Now, standard safety practice for any appliance (toaster, fan, mixer, coffee pot, etc.) with an electrical short or unsafe condition is to have the power cord cut off... to signal anyone that the unit is not safe and should not be powered up.  But in this college town, after it has been unplugged, no one bothers to cut the cord off with scissors but just haul it to the curb for disposal.  You can usually find a half dozen of them when driving the streets of the town. 

I confess it is a guilty pleasure of mine to collect these fans and assess them for repair-ability. That is, can the parts from one be salvaged to replace those of another.... taking two junked fans and getting one working safe fan out of the deal.  The first step is, of course,  before anything else, plug the fan into a power socket and turn it on to assess if it runs at all.  If the fan is missing a blade, cracked, rattles, sparks or shows any other symptom, it should be immediately unplugged and disposed of.  As I have mentioned, if it's electrical or problems with the motor itself, cut the cord off close to the body of the fan (AFTER UN-PLUGGING IT) and salvage only the power cord.

 The process usually involves removing six screws from the grill work, wiping down the dirty fan blades and inspecting them, dusting off the motor, cleaning the grill with a disposable paint brush or Q-tips and sometimes just taking just the grill work or the plastic fan blades into the shower.  A quick rinse and inspection will tell you if there are any faults in the plastic and whether you should throw it away.  

Now, I admit that I have taken the plastic fan blade from one unit, and an intact plastic grill off one fan to add to a second damaged unit and complete it.  All that's needed is a little patience to get the six screws out, and to carefully pull the blade assembly straight up and off the shaft of the motor.  You'll note that the motor shaft has a flattened side that is pretty standard and will fit only one way into the opening in the center of the fan hub.  If the motor is clean and turns, it may be worth a drop of motor oil around the bearings and reassembly of the good, usable parts to complete the fan.

But again, I say, any fan that is missing a blade, has excessive cracked or missing grill work, or sparks, smolders or stalls should be thrown out with its power cord cut off.

Over the years, I've recovered a dozen such fans, and acquired perhaps a half dozen power cords, suitable for alligator clips to be added (soldered) onto the bare wire ends... and repurposed for rocketry launch clip leads.  To accomplish this, you'll need to be able to strip a quarter inch of insulation, tin the wires and solder an alligator clip onto each wire.   You'll have to get your own alligator or micro test clips from Radio Shack or other parts supplier to have on hand.

It has occurred to me that producing such a launch clip lead on the end of a male power plug could be considered a very dangerous device... so I urge you to keep close track of those completed plugs, and not let them fall into the hands of children who don't know any better.

It occurs to me that if you tie an overhand knot with the  separated/unzipped wires, the two clips will try to stay separate naturally.  But this is only a slight bit of encouragement.  The risk is still there.

Perhaps a better solution is to have two alligator clips with individual rubber jackets over them, to keep them insulated from each other.  Unfortunately, this also means that the rubber jackets will burn under the heat of a rocket motor launch, and is not very practical for our purposes.

The final suggestion that I have for you is to try cutting the insulated wires to unequal lengths before soldering on the alligator clips.  That is, one lead will be 3 to 4 inches longer than its sister lead, and the chance of the two clips touching and shorting out is vastly reduced.  Unfortunately, this also means that the contact point for the igniter wires is separated by this large gap also.  

You can probably bend the longer lead in a broad U to double back and reach the igniter wire.  It may take a few attempts before you get this right.

It's a balance between having two live alligator clips next to one another, vs. having them naturally keeping their distance until their connection is desired. 

 

Now I know there are some of you who are saying, "What's all the worry about this?  Why should we worry about it, when the current is so slight and we're careful when we connect our leads?"   The answer is, an ounce of prevention is worth a pound of cure.  The safety you can design into this simple jumper cable clip lead may protect someone down the road.  It certainly can't hurt to review your safety practices through the eyes of younger rocketeers who are just getting into the hobby.

So, I'm a great fan of repurposing power cords cut off from fans, toasters, mixers, can openers, but only when done safely.  And I hope I've given you something to think about the next time your review your equipment and prepare to replace those all important igniter clips.


Til next time, this has been the Thrifty Rocketeer saying "Stay safe!" 

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