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A rocket launcher is only as powerful as its battery and cables. You'll have fewer launches and more igniter failures if you only use four alkaline batteries and thin wire. On this page, we'll examine the battery, wiring, and connectors I selected for my custom launch controller.
Rocket engine igniters consume many amps of current to ignite. All batteries have a certain amount of internal resistance that constrains the amount of current that can be delivered. Even worse, the battery voltage drops as a circuit attempts to draw out higher and higher currents. Regardless of other factors, a battery’s size, chemistry, and charge level dictate the maximum power it can provide.
Consumer AA alkaline batteries have an internal resistance around 0.15 ohms at room temperature. Four batteries in series would have approximately 0.6 ohms (0.15 ohms × 4) of resistance.
The wire running from the batteries to the igniter has resistance. A 24 AWG size cable that is 17 feet long will have a total end-to-end (out and back) length of 34 feet. That is around 0.87 ohms of resistance.
If the igniter has 0.72 ohms of resistance and the batteries and wire have 1.47 ohms (0.6 + 0.87), then the total resistance is 2.19 ohms. Four fresh alkalines produce about 1.6 V × 4 = 6.4 V. According to Ohm’s law, in the absolute best case, the alkalines could deliver 6.4 V / 2.19 ohms = 2.92 amps. Not bad.
Unfortunately, over 2/3 the power is used in the wire and battery, since their combined resistance is 2/3 of the total. So, the igniter only sees 2.1 V across it -- best case. That means it is heating up with only 6.13 watts, rather than the total battery output of 18.688 watts.
In reality, the batteries are not going to be fully charged and the voltage is going to droop when trying to draw that much current.
Switching to NiMH AA batteries results in lower resistance (about 0.05 ohms) but also a lower starting voltage (1.33 V × 4 = 5.32 V). The current increases to 2.97 amps with the igniter seeing 2.13 V, with a slightly improved 6.34 watts of ignition power. Although this is only a slight improvement, you can recharge the NiMH batteries.
For the maximum number of rocket launches and the fewest number of igniter failures, you want to select:
Many rocketeers use motorcycle, lawnmower, or car batteries for their rocket launcher. These are usually lead-acid rechargeable batteries. The variety of VRLA (valve-regulated lead-acid) batteries include gel cells and absorbent glass mat (AGM).
I selected a sealed maintenance-free AGM motorcycle battery.
12V valve-regulated lead-acid VRLA absorbent glass mat AGM battery.
“Maintenance free” refers to the fact that other lead-acid batteries occasionally need water or other fluids added to top-off the electrolyte. “Sealed” usually goes hand-in-hand with maintenance-free. “Absorbent glass mat” (AGM) and gel cell batteries won’t spill electrolyte if they are punctured, split, or otherwise ruptured.
This particular battery is a Scorpion YTZ7S, which is 12 V, 6 Ah, with 130 cold-cranking amps (CCA or peak current). It is fairly small -- weighing a luggable 5.2 pounds. You'll want to pay attention to battery weight depending on the location of your launch site.
The capacity and peak current of this battery ensure I'll obtain many, many launches per charge. Be warned, the more powerful a battery is the more dangerous it is. Do not touch or operate such a battery under wet or damp conditions, including when your hands are sweaty. It could kill you.
The battery costs under $50 with shipping. You'll need a special charger if you don’t already have one designed for AGM batteries. This is a worthwhile investment if you’re going to launch a lot of rockets or plan on reusing the battery in a robot.
I’m normally opposed to lead-based products, however lead-acid batteries are one of the most successfully recycled products in the world. Additionally, you and the environment aren’t going to come into contact with the lead under normal operating circumstances.
This motorcycle battery came with fairly basic plain steel 10-32 nuts and bolts to attach cables. I swapped them out for stainless-steel, 3/8-inch length, Shear-Loc thumb screws with 5/8-inch diameter plastic knurled caps (K02B-1032-0.37S. McMaster-Carr #98704A520). That way, I wouldn’t have to touch the metal terminals and can unscrew the cables in a hurry without a tool, if necessary.
Hook and battery.
Attached to the cable is a crimp-on spade terminal with nylon insulation (McMaster-Carr #69145K176). I chose a spade terminal, as opposed to a ring or hook, to allow for fast disconnects.
It doesn’t matter if you have the world’s best battery if you pair it with wimpy skinny wires. The wire resistance will restrict the peak current and will dissipate power as heat.
Rocket launch wire: (top) Estes Electron Beam, (bottom) 12 AWG.
The thick cable has a red wire and a black wire. Although polarity isn’t important for most igniters, the red color makes it easier to see in the grass, which may prevent tripping.
The Estes wire resistance measured 0.9 ohms. In comparison, the 34 feet of 12 gauge wire was below what my meter could measure, but is expected to be around 0.05 ohms. (I have since figured out how to measure such low resistances.)
A 12 V AWG battery at full charge (say 13 V) that can deliver 130 amps would have an internal resistance of 0.1 ohms. Combine that with a 0.05 ohm cable for a total resistance of 0.15 ohms. The 0.72 ohm igniter would see 10.7 V out of 13 V, 14.87 amps, for 159 watts of power. That’s 2600% the power of the alkaline and wimpy wire.
All of these numbers assume best-case scenarios. Actual power output will be lower.
The igniter is usually connected to the launch controller using alligator clips. (Some special igniters have custom connectors.)
The problem with big thick cables is that their weight may pull the igniter out of the model rocket engine. So, I added a short length of 22 AWG wire from the end of the cable to the alligator clips. This shouldn’t add significant resistance.
Attaching alligator clip to igniter cable.
It was a pain to try to solder such a thin wire inside of such a thick wire. And, I imagine the impedance mismatch does all sorts of nasty inductance and wave bounceback tricks. Nevertheless, hold your nose and cover the end with heatshrink tubing.
The alligator clips were leftover from a pair of test cables. But, you can buy fancy stainless steel clips (Mueller Electric Co BU-60X) at DigiKey (#314-1036) for less than one dollar each. Theoretically the stainless steel will resist exhaust damage better than copper or plain steel.
Igniter in rocket on launchpad.
Don’t forget to bend the ends of the igniter into a U or hook shape to get double contact with the alligator teeth.
Banana jacks and banana plugs are popular for bench power supplies. I assumed they would be a good choice for the rocket launcher.
The first sign of trouble was that the wire wouldn’t fit through the hole in the insulating cover for the banana plug.
V groove holds banana cover while enlarging hole for lower gage wire.
To drill a larger hole, the banana cover was placed in the v-groove of a milling machine vise. After drilling, you must slide the cover onto the wire before soldering the wire onto the banana plug, because the cover needs to screw on from the wire side.
The next sign of trouble was that the 12 AWG wire was too big to fit into the hole of the metal banana plug itself, so I had to taper the tip of the wire a bit with trimmers.
Soldering wire onto banana plug.
My soldering iron did not have enough power to solder a thick copper wire to a piece of metal. The heat dissipates too quickly. It just doesn’t work. I had to step up to a soldering gun for this job.
I’m not satisfied with the big blob of solder connecting the wire to the plug. It seems like it is going to come loose. Additionally, I had to file it down a bit to screw on the banana cover insulation.
Banana plugs not fully inserted.
Unfortunately, I was not totally successful in getting the wire and solder to be completely flush. As such, the covers don’t screw on entirely. Although the plug still makes excellent conductive contact, I don’t like the exposed metal from a safety perspective.
What I’d really like are some fully sheathed banana plugs to avoid the potential for touching the metal ends when connecting or disconnecting the cables. But, that requires compatible jacks.
There are a bunch of banana jacks to choose from. I wanted jacks that accept sheathed banana plugs but also can have ring or spade terminals screwed to them.
Banana jack with insulated mount (left) and bare bolts (right).
Unfortunately, my launch controller’s metal enclosure means that the jacks need to have electrical insulation at the location of the panel mount. The jack on the left will have plastic touching the enclosure, whereas the jack on the right has exposed metal bolts in the center that could come in contract with the enclosure. That would cause a short circuit or potentially harm the operator.
Fortunately, the insulated panel mount jacks and plugs are available in a nice matching set of ten colors. I guess I'll take that under the circumstances.
Colorful banana plug and jack.
The jacks are E-Z Hook 9284-S (DigiKey 461-1216) and the plugs are E-Z Hook 9202-S (DigiKey 461-1213). Sadly, the 10 pairs will set you back $25. Ouch. Perhaps the color coding would be worth it in application where miswiring was hazardous or time consuming.
This part of the project would have gone better if I had used a plastic project enclosure and a slightly narrower cable. In fact, if I had to do it over again, I would have broken the rocket launcher into two parts:
Before we get to best part of the article (the launch day photos), let’s briefly look at the machining of the case...