current balancing - Mass Pads

Current Balancing Resistors – Mass Pad

A few years back I put together a mass pad for a youth group launch. The mass pad launches 5 rockets at once, one input split 5 ways.

Most of the time everything works well, with all 5 rockets launching almost simultaneously, but then there would be times where 1 or 2 rockets wouldn’t launch or would take considerably longer to launch. One thing I did notice when ALL of the rockets used the same type of igniter, things were more reliable.

The system is powered by a good size 12v battery, I don’t think that battery voltage or capability had much to do with the variability.

Theory, The lowest current igniters start to go first which causes:
1) A plasma forms on these igniters lowering the resistance (increasing the current through these igniters and causing voltage drop.
2) Once these rockets launch there is a reasonable chance that the clips will touch each other
3) The higher current igniters don’t fire until the lower current ones go
4) If there are clips touching from one of the other pads, there may be too large of a voltage drop and current draw on the shorted clips to allow the igniter to go.

So, here is what I am thinking, If I put a one Ohm resistor (high power one!) in line with each set of clips, I will have a voltage drop across all the clips but I will prevent any one set of clips from drawing too many amps even if one is shorted out. it will help balance the current.

Thoughts?

You will need to provide details as to how your mass pad is designed

the electrical for the pad is very simple, each rod has a set of clips, using an extension cord, 14ga wire, I cut off the receptacle side and soldered on clips. I 'plug in' the other side to a power strip, this is how i get all 5 to connect together. The 'plug' for the power strip is connected to the relay box. Very low resistance in the wires and power strip.

Ok, so by using modified 110 volt (I'm assuming your in USA/Canada) you have wired the pads up in parallel. Which is the right way but you must have enough kick in your 12v battery, I'm assuming this is a car battery and is properly charged. If the igniters are mixed then they will heat at different rates depending on their resistance, the effect of having multiple igniters on the same circuit will a negligible affect on this heating provided the battery is good. A good battery typically puts out 100 - 200 Amps you are only using say 5 tops. I'm assuming that your cords and all plugs are all good too if not the this will have a similar effect to having a partially flat battery. Other than that there is a question of what propellant is in your motors and what material in on the igniter. So yes if you want to make it fair make sure everyone has the same igniter, make sure your equipment is all good and perhaps consider doing the drag race earlier rather than latter on launch day so your battery is fully charged.

Agree with most of what SpaceManMat posted.
With these additions what you are doing is basicly a Cluster ignition using 5 leads. All the minute differences in Wire lead length, amount of solder on the clips, difference in Starter wire diameter, distance from the Battery to the starters, slight difference in propellant mixture, etc make a world of difference when trying to get all motors to ignite at the same instant. Instant Ignition really doesn't happen. Very close but not the exact moment. Each motor will get up to ignition temperature as fast as the current can flow to that individual set of clips & ignitor.
All these tiny resistance additions slow that process. We are only talking about 100ths or 1000ths of a second but in real ignition time that can be some pretty long ticks.
You did mention a Relay so we'll assume you have moved the Battery to the launcher side of the circuit. shortening the current travel distance considerably. So your only variables are in the clip lead lengths, amount of solder in each clip, type of igniter lead, Where on that igniter the clips are attached, and how clean the starter wire and clips are. Yep! all these tiny minor things make a difference.

An Estes igniter has a resistance of ~0.8 amps and an all-fire rating of about 2 amps. At 12 volts it will draw 12/0.8 = 15 amps and at 7.5x the all-fire current the igniter pyro ignition time is < 1 ms. 5 igniters will try to draw 75 amps so you will not weld the relay. A 7 AH PBac gell cell will source ~300 amps peak so with the relay you can get all the current you need to get instant ignition.

Use an automotive relay https://www.allelectronics.com/make-a-store/item/rly-351/12vdc-spdt-40a-automotive-relay/1.html and a commercial automotive relay socket https://www.allelectronics.com/make-a-store/item/srly-2/socket-for-automotive-relay/1.html in the high current section of your launcher.

The relay will deliver 150+ amps in a launcher application without welding the relay, and the socket uses 14 gauge wire in the high current section. Use 18-16 gauge zip cord lamp cord for the igniter clips and use wire nuts to make the connections. 14 gauge copper wire has a resistance of 0.0025 ohms per foot. 10' will have a resistance off 0.025 ohms and will pass 480 amps at 12 volts so 75 amps for 5 igniters is not an issue. 40' of 18 gauge zip cord has a resistance of 0.25 ohms so it will pass 48 amps at 12 volts so 15 amps per igniter is not an issue.

Bob

You're getting some good advice here but I wanted to touch on one point that hasn't been mentioned so far.

You state "Theory, The lowest current igniters start to go first" which is not a guaranteed truth. How much current any particular igniter style draws, and how quickly it fires, are not directly related. In other words you can find igniters that draw low current and are slow and you can find ones that draw high current and are fast. And, of course, vice versa.

In my opinion the only way to ensure similar ignition times in your scenario (which as was pointed out is really just a big cluster) is to use the same type of igniter across the board. Make sense? Good luck!