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Anton.bennettHHS

Anton.bennettHHS

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Hi, I am a High school student building a launch controller for a rocket club as part of my workshop project.

This is the first time I have practically worked with electronics, and to be honest I don't really know where to start.
I have discussed with some others and I think this is the battery I will be using (might be overkill?)

They want the controller to have an arming switch, keyswitch, momentary button, a check for continuity and led counter for how much battery/how many launches remains. the controller connects to the ignitors through alligator clips.

Any help on where to start or some example circuits I could look at?
 
Are you only using this for one rocket? If that's the case, the basic controller is pretty simple. The battery gauge and something to indicate launches remaining is the big fly in the ointment. You could just go with a 12V battery like this and not worry about it. It will launch 100+ rockets between charges.

Here's simple schematic for a single launcher.

Basic Controller.png

The resisters should be 470 Ohm for 12 V, and for a 7.4V system, about 260 ohms.
 
The basic launch controller is the easy part. However, if you require a smart battery status monitor that indicates battery voltage AND an estimate of how many launches are left before a recharge is necessary, then you will most likely require a small processor board with an LCD readout.

You could do this with an Arduino with LCD shield. With a knowledge of the battery’s discharge characteristic (from the battery’s data sheet) you could estimate how much charge is remaining in the battery by monitoring its voltage as it discharges. You could use a lookup table or a polynomial expression that describes the discharge curve. From this you could estimate how many discharges from each launch are available.

I’m sure others will come up with other methods to do this. I haven’t put much thought into the method above. It was just the first idea that popped into my head.
 
Voyager1 said:
The basic launch controller is the easy part. However, if you require a smart battery status monitor that indicates battery voltage AND an estimate of how many launches are left before a recharge is necessary, then you will most likely require a small processor board with an LCD readout.

You could do this with an Arduino with LCD shield. With a knowledge of the battery’s discharge characteristic (from the battery’s data sheet) you could estimate how much charge is remaining in the battery by monitoring its voltage as it discharges. You could use a lookup table or a polynomial expression that describes the discharge curve. From this you could estimate how many discharges from each launch are available.

I’m sure others will come up with other methods to do this. I haven’t put much thought into the method above. It was just the first idea that popped into my head.
Click to expand...
If you wanted to avoid processors, you could probably check that with a push button that closes a short through a suitable shunt with a suitable voltmeter across it. The shunt could even be an old incandescent automotive lamp (dunno... 20W?) that resembles the power requirements of a typical igniter or more to also provide an illuminated indication.

Saying that, I've probably put even less thought into it than you :p

TP
 
OverTheTop said:
A voltmeter (moving coil or digital) can be a good indication of battery capacity, particularly for lead-acid batteries. It is a simple solution but requires interpretation.
https://www.ebay.com/itm/125325241336
You can find them even cheaper than this.
Click to expand...
To add to this, a voltmeter is a bad indication of the charge state of a lithium battery due to its flat discharge curve. If you want to monitor lithium, you need to know it was fully charged, what its real capacity is, and how much energy you have cumulatively removed from it. Do that calculation and work out what's left. Which is a lot trickier. This is one of the reasons larger lithium batteries have full-blown battery management systems built in to monitor the battery status and protect the battery from doing anything silly with temperature monitoring, temperature over and under protection, over and under voltage protection, and current limiting.
With small model battery capacity this is not usually deemed necessary as the cost and weight penalties outweigh the smaller replacement cost of a small battery.
Your battery monitoring part of this project is going to be the tricky part. I'd talk to whoever is setting the task to narrow down the scope of that.
Troys suggestion of testing with a bulb is a good practical real-world test that is simple. If the bulb glows brightly when the fire button is used, you are good to fire a rocket igniter. If they're happy with that, that's the easy practical solution.
Norm
 
RocketScientistAustralia said:
To add to this, a voltmeter is a bad indication of the charge state of a lithium battery due to its flat discharge curve.
Click to expand...
I did allude to this in the comment about lead-acid batteries in my earlier post. Yes, Li batteries have a very flat curve and then fall off the cliff. In my experience they are also of finite life expectancy. I usually expect to replace them after about three years. NiCd or NiMH don't have the power density of lithium batteries, but life expectancy is fantastic in comparison, hence the use of them in my controller.
 
Is the "launches remaining" counter just a gee-whiz sort of feature? Or — what is driving that requirement? Assuming the typical Estes-type igniter (or Aerotech FirstFire micros as what come in Q-Jets) you should get literally hundreds of flights per charge of the battery you linked to. The duty cycle for launching a rocket is very short bursts of current (if it doesn't go in one second, something is wrong) and LiPolys, even those with modest "C" ratings can source a bunch of current.

That said, I'd consider a 3-cell of similar capacity rather than 2-cell battery . That will get you to 12.6V fully charged and, with appropriate wiring to carry the current, let you fire just about anything in the low- and mid-power rocket realm including most all composite igniters/initiators and small clusters.
 
OverTheTop said:
I did allude to this in the comment about lead-acid batteries in my earlier post. Yes, Li batteries have a very flat curve and then fall off the cliff. In my experience they are also of finite life expectancy. I usually expect to replace them after about three years. NiCd or NiMH don't have the power density of lithium batteries, but life expectancy is fantastic in comparison, hence the use of them in my controller.
Click to expand...
You've got to stop doing The Times cryptic crossword..... :)
 
Last edited:
Sooner Boomer said:
For LiPO batteries - $4

https://www.allelectronics.com/item/lbt-79/li-po-battery-level-meter/1.html
Click to expand...
This is simply a digital voltmeter with preset alarms. It will give you a warning when the battery level gets to a set point.

It will not tell you the capacity remaining in the battery which was a requirement of the OP.

These are useful as safety warning devices to land quickly on a model aircraft. From the voltage, you can say you have a fully charged battery and you can say you have a nearly flat battery. But not much else.

A battery capacity calculation for a LiPo is a much more complex calculation and you cannot easily use the voltage curve, it's just too flat in the middle to get any useful information from. So this is useful to tell you when you get to the last 20-30 % of charge as the curve starts to steepen then for a LiPo. From 100%-95% it's steep. But anything between 95% and 70-80% discharge is pretty much impossible to identify where you are reliably just using the voltage. In that region if you discharge the battery the voltage will drop a bit and rise again when you stop discharging as the electrochemical reaction stabilizes internally. The amount of that rise and fall could be over 50% of the flat discharge curve region. This is because you are effectively trying to interpret your discharge state from the self discharge curve and the load discharge curve and somewhere in between as the chemistry settles.


You can use the voltage curve of a lead acid battery as a general capacity indication due to the steeper discharge curve making it easier to interpret the state of charge remaining from it.
Norm
 
Last edited:
You can't figure out how much power/electricity is still left in a battery without fully discharging it. The best you can do is infer how much "juice" it has left. One of the common ways to do this is measure a battery, either at rest or under load and compare that to a known discharge curve.

Honestly, this is what I would do. Design your controller with a given battery and see how many launches it can do, but still have a little bit of "buffer." For example, let's say you get 53 launches before any noticeable delays in the time it takes to set off the igniter or otherwises show any signs of becoming low. Tell yourself that your given battery will provide 50 launches before it needs to be recharged. Then, design your controller with a simple electronic "counter" that notes how many launches have taken place on a single battery. Once you hit 50 launches, the controller signals it's time to recharge the battery.

Or, just keep things simple and tell yourself to recharge the battery after every X number of launches or X days of launching.
 
The other issue in identifying the remaining capacity is that the amount of energy you use each time is a very small number. You could waste more energy illuminating the LEDs for the length of time they are on than is required to fire an igniter. (Unless there is a short circuit fault).


You could eliminate the battery completely by using a solar panel to charge a firing capacitor.
A 5W 12v panel would be fine and a current-limited buck-boost regulator to get a fixed voltage into a bank of capacitors. You then only need to identify that you are good to fire by monitoring the voltage on the capacitor bank.
If there is insufficient sun,(which would only practically be if it was raining) you'll not be attempting to launch anyway.

Edit: I like my own idea so much, I've ordered 2 of the buck boost controller. https://www.ebay.com.au/itm/404150650516
 
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Seems like a 2200mah battery could deliver conservatively 3600 firings assuming an average igniter current of 2.2 amps over 1 second.
 
Thank you everyone for your help, it is all extremely useful.
an indicator of remaining launches/ battery amount is a requirement from my primary stakeholder, but looking at this a counter or low battery warning light look like good options.
With 3600 firings, as sghioto said, should I go for a smaller battery? This controller will only be used for 10-20 launches a quarter. Maybe something around This?
 
Yes, I would go with a 3S battery and design the controller for 12V. Then you can use the 3S Lipo or a 12V SLA. Many more options.

As for the stakeholder requirement for remaining launches/ battery amount, I would seriously question them about where those requirements come from, why they are included, and how much time and money they are willing to spend to include that requirement. That additional requirements adds a LOT to the cost and complexity of the controller with little to no increase in functionality or usability. A spare battery would be easier. It takes me back to my days as a government contractor where some fed with no idea of what they were really asking for would add requirements to a contract that would increase the cost by 10 - 100 thousands of dollars with no real value added. Where do you think $1000 hammers come from?
 
I am forever amazed at the ridiculous nature of of some people's "requirements." In this case: an "led counter for how much battery/how many launches remains." There are so many actual variables in the equation as to make it simply impossible to do more than estimate an answer to either part of the question.

Just to name a few more variables that have not been mentioned:
How long is the "fire button" engaged after ignition?
Was the bridge wire of the ignitor broken upon ignition or did the wiring melt thru and keep the circuit live longer?
If you are using a "light bulb" for an ignition tester, are you counting each test as an ignition as you should when determining battery amps used?
Did the alligator clips make contact with a metal blast-plate during the ignition sequence and use more amps than needed to just fire the igniter?
Did the positive igniter clip hit the ground while still putting out amps and discharge some into the ground?
Are you taking into account that different manufacturer's igniters use differing amounts of amps to fire a motor and that there are even variations in amps needed while using the same manufacturer's igniters?
Are you using brand new alligator clips for every launch? I ask because corrosion takes its own toll on getting amps to pass thru an igniter.
As a standard battery (non-LIPO) slowly discharges its voltage from a full charge down to eventually a minimum charge, it takes more energy/time holding down the "fire button" to fire igniters.
How old is the battery? How much of a charge can it actually hold? When was it last recharged? Was it a "fast charger" or a "slow trickle" charger? What is the amp/hour designation of the battery?

And these variables are just off the top of my head. There are so many more variables as to make the question just as valid as the guy who looks at your rocket and asks, "How high does it go?" Asking for an "led counter for how much battery/how many launches remains" is just that same kind of a question as far as its validity is concerned.

The best that can be said is that with a good volt-meter you can tell when your battery is getting low enough that it needs recharging. And as a manufacturer of commercial launch systems for individuals and clubs, I ALWAYS recommend using a battery that is going to hold much more of a charge than what you think you are going to need for your whole launch. There is little that's more frustrating to a flyer at a rocket launch than club batteries that are not charged enough to get the job done. Been there, done that, decided to fix the problem!

And the OP said that the launcher will only be used for 10-20 launches per quarter. When you say "10-20 launches" do you mean launching 10-20 single motor rockets per quarter? Or do you mean 10-20 launch events with multiple launches per event per quarter? Might make a difference....

Brad, the "Rocket Rev.," Wilson
 
Have a 999-0 countdown timer/display that cycles with every button push, at zero activate a relay that disables the circuit. 100% accuracy. With 2.6 factor of safety, I might add. Disguise the reset button by labeling it "push before charging".
 
Anton.bennettHHS said:
Thank you everyone for your help, it is all extremely useful.
an indicator of remaining launches/ battery amount is a requirement from my primary stakeholder, but looking at this a counter or low battery warning light look like good options.
With 3600 firings, as sghioto said, should I go for a smaller battery? This controller will only be used for 10-20 launches a quarter. Maybe something around This?
Click to expand...

Frankly, tell the stakeholder that if your going to only have 20 or less launches a quarter, you simply recharge the battery after each use and don't worry about it. A battery voltage meter is all you need, not an amount of ammo left counter.

Use the original battery you posted, but maybe use a 11 volt lipo version instead of 7 volt in my opinion.
 
There are tons of cheap lead acid gel cells available now for kids toys. $20 each, and a 12v 7ah battery will last all season; best of all, a voltage Is a good indicator of charge. I got two old ones for free, and even too degraded to use in the toy, they last for a season's launches without a recharge.
 
Grog6 said:
There are tons of cheap lead acid gel cells available now for kids toys. $20 each, and a 12v 7ah battery will last all season; best of all, a voltage Is a good indicator of charge. I got two old ones for free, and even too degraded to use in the toy, they last for a season's launches without a recharge.
Click to expand...
I was given a motorcycle battery with a bad cell that wouldn't start the motorcycle. I used that for launching LPR, including 3 & 4 motor cluster for 3 years before I put it on a charger. I think I probably could have gotten another year or two out of it, but why risk getting to the field and having it not work.
 
Rschub said:
Have a 999-0 countdown timer/display that cycles with every button push, at zero activate a relay that disables the circuit. 100% accuracy. With 2.6 factor of safety, I might add. Disguise the reset button by labeling it "push before charging".
Click to expand...
This seems pretty straight forward.
In terms of storage, I've seen that LiPo's need to be discharged down to a certain voltage per cell for storage. Since this will only be launching 10-20 rockets a quarter (After school program) would it be best to use a charger that has a storage capability?
My primary stakeholder wants the controller to have an internal battery compartment so using larger or heavier batteries instead may not work.
 

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