Need to Spec a Battery Pack for a Rocket Launcher

Thread Starter

phillipsoasis

Joined Aug 22, 2022
75
I have designed a rocket launcher and I need to spec a rechargeable battery pack for it. I know very little about batteries. I have the following data from field tests of the launcher using a FlashFish E300 portable battery pack. The launcher ramps up the current to the igniter in ~300 msec steps from zero to ~4 A. Basically, the launcher heats up the igniter until it ignites, which then ignites the rocket motor. I found that ramping up the current uses less current for a shorter amount of time than just dumping 4 A into the igniter at once. The quiescent current for the circuit is ~100 mA because the FlashFish needs this much current to keep it from automatically turning off.

Data from a typical launch:

Duration (sec)
Current (mA)
Voltage
0.314​
96.68​
15.337​
0.312​
306.152​
15.198​
0.312​
883.008​
14.996​
0.312​
1185.938​
14.844​
0.312​
696.094​
14.722​
0.312​
1401.855​
14.601​
0.312​
2107.617​
14.476​
0.312​
1421.191​
14.359​
0.313​
2313.867​
14.237​
0.312​
1359.961​
14.116​
0.312​
3667.383​
13.999​

If I need enough power for 40 launches in an 8 hour day before recharging the battery, what battery technology do I need and how should I spec the battery? I need the batteries to fit into a 6" x 8" x 1-2" enclosure. I am also interested in learning how you thought about this battery design.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
18,554
The first thing to know is the actual current and time to achieve 100% success for every launch. That will give you the current/time product in amp-hours . Then double that amount because battery performance will certainly be reduced as the battery runs down.
There are both "hot wire" igniters and "exploding bridge-wire igniters, which produce much less delay and a much more positive ignition.
To know the total energy required for each ignition you can plot the points and then integrate the area under the curve.
Since the battery voltage drops during the process, it becomes more complex. It is not at all likely that the battery voltage drop will remain constant for all 40 launches, as the initial charge energy is consumed.
What we are not given is the capacity of the battery pack used.

My point here is that it is unwise to provide the minimum of calculated capability.
 

WBahn

Joined Mar 31, 2012
30,067
Is the voltage in your table the voltage of the battery pack?

What voltage does your rocket launcher need in order to operate?

Your data is very erratic -- it goes up, then down, then up, then way down, then up.

In what way is this "typical"? Is this really what it's designed to do?

Your typical launch takes nearly four seconds to actually ignite the motor?

You might want to redesign your ignitor circuit.

Let's say that your launcher ramps the current from 0 mA to 4000 mA from 0 s to 4 s. If it hasn't launched in 4 seconds, it shuts down as a failed launch. That is an average current of 2 A over that 4 s, or 8 A·s of charge. Forty launches would then consume 320 A·s of charge. That's about 90 mAh of charge. Double that to give a nice margin and 200 mAh should be sufficient.

This is a very small capacity, so your main focus needs to be on identifying a battery that can deliver 4 A of current for a few seconds at a time. A few AA NiCd would do the trick handily.

That you don't need much capacity isn't too surprising given that the simple Estes ignition modules typically used four AA alkalines and would last for hundreds of launches with no problem. I don't recall how much current those usually took to ignite -- I measured it some 30+ years ago, but have no recollection of the numbers.

Here's a suggestion for your consideration.

See if a single 'AA' NiCd has sufficient voltage to ignite one of the igniters when placed direction across it. If not -- that's good, because it allows you to do the following:

Make your circuit so that it runs off a single NiCd (or more, but in parallel). Then, design your launch controller so that when the system is armed, it uses a boost converter to pump the voltage up to the level needed needed to drive current through the igniter, preferably in a true current-ramped fashion, with launch success and launch fail detection.

This makes it extremely difficult for the launcher to accidentally provide enough current to the igniter since there is insufficient voltage until it is armed.
 

Thread Starter

phillipsoasis

Joined Aug 22, 2022
75
Is the voltage in your table the voltage of the battery pack?
>>>Yes

What voltage does your rocket launcher need in order to operate?
>>>Do you mean the circuitry or just the igniter? The circuitry runs off of 5V and 3.3V. The igniter needs about 12V and around 23 A for ~200 ms to ignite. See charts below.

>>>This is the only data I have found to characterize igniters, but it is from 2000, so they probably have changed a lot since then. The first "standard" igniter was just nichrome wire, so it had to glow to igniter the motor.
Screenshot from 2024-02-29 15-32-12.png
>>>Modern igniters are more like the second graph because they have pyrogen glued to the tip of the wire.
Screenshot from 2024-02-29 15-32-26.png

Your data is very erratic -- it goes up, then down, then up, then way down, then up.

In what way is this "typical"? Is this really what it's designed to do?

>>>The basic circuit is a DAC controlling the gate of a MOSFET. My measurements most likely have some sampling error (ADC) as well as issues with the voltage sag as the current is ramped up to ignite the igniter. The 5V logic voltage comes from a buck converter, so it is stable, and the 3.3V logic level comes from the 5V and it is stable as well through a launch cycle.

Your typical launch takes nearly four seconds to actually ignite the motor?

>>>When I was testing early on I was getting a lot of voltage sag from the battery pack because I was drawing upwards of 6 A, and I read that is not a good thing/dangerous. So, I run the circuit in a ramp fashion, which uses less total current in each step and the battery voltage does not sag as much. It does take more time to heat the wire and ignite the pyrogen. I would like to reduce that time with a different battery configuration.

You might want to redesign your ignitor circuit.

Let's say that your launcher ramps the current from 0 mA to 4000 mA from 0 s to 4 s. If it hasn't launched in 4 seconds, it shuts down as a failed launch. That is an average current of 2 A over that 4 s, or 8 A·s of charge. Forty launches would then consume 320 A·s of charge. That's about 90 mAh of charge. Double that to give a nice margin and 200 mAh should be sufficient.

This is a very small capacity, so your main focus needs to be on identifying a battery that can deliver 4 A of current for a few seconds at a time. A few AA NiCd would do the trick handily.

That you don't need much capacity isn't too surprising given that the simple Estes ignition modules typically used four AA alkalines and would last for hundreds of launches with no problem. I don't recall how much current those usually took to ignite -- I measured it some 30+ years ago, but have no recollection of the numbers.

>>>From my recent experience 4 AA alkalines only last for a few launches. Nowadays, there is a 9 volt battery in launchers, but that barley lasts for 10 launches. The recommended method is to use a 12 V car battery for a day of launching. Most of the energy is lost heating the wires from the launcher to the launch stand (15 feet). My launcher uses a cell phone to communicate with the launcher at the launch stand. Igniter cable is ~ 12 inches.

Here's a suggestion for your consideration.

See if a single 'AA' NiCd has sufficient voltage to ignite one of the igniters when placed direction across it. If not -- that's good, because it allows you to do the following:

Make your circuit so that it runs off a single NiCd (or more, but in parallel). Then, design your launch controller so that when the system is armed, it uses a boost converter to pump the voltage up to the level needed needed to drive current through the igniter, preferably in a true current-ramped fashion, with launch success and launch fail detection.

This makes it extremely difficult for the launcher to accidentally provide enough current to the igniter since there is insufficient voltage until it is armed.
 

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Thread Starter

phillipsoasis

Joined Aug 22, 2022
75
The first thing to know is the actual current and time to achieve 100% success for every launch. That will give you the current/time product in amp-hours . Then double that amount because battery performance will certainly be reduced as the battery runs down.
There are both "hot wire" igniters and "exploding bridge-wire igniters, which produce much less delay and a much more positive ignition.
To know the total energy required for each ignition you can plot the points and then integrate the area under the curve.
Since the battery voltage drops during the process, it becomes more complex. It is not at all likely that the battery voltage drop will remain constant for all 40 launches, as the initial charge energy is consumed.
What we are not given is the capacity of the battery pack used.

My point here is that it is unwise to provide the minimum of calculated capability.
The battery pack is FLASHFISH 330W Portable Power Station, 81000mAh 300Wh Solar Generator with 110V AC/DC/USB/PD-Type-c/Car Port/SOS Light, Backup Battery Pack Power for CPAP Outdoor Adventure Camping Emergency
 

MisterBill2

Joined Jan 23, 2018
18,554
OK, so this does not appear to be the simple "ESTES" rocket launcher. Those are hot-wire devices and I am not sure where high current thru plasma would be involved, unless the flame from the engine exhaust is highly conductive. If that is the case, then possibly the power application could be terminated as soon as the presence of conduction in the plasma was sensed.
 

Thread Starter

phillipsoasis

Joined Aug 22, 2022
75
OK, so this does not appear to be the simple "ESTES" rocket launcher. Those are hot-wire devices and I am not sure where high current thru plasma would be involved, unless the flame from the engine exhaust is highly conductive. If that is the case, then possibly the power application could be terminated as soon as the presence of conduction in the plasma was sensed.
The plasma comes from the ignition of the pyrogen on the tip of the igniter. It is that plasma that ignites the rocket motor. I actually sense the loss of continuity in the igniter circuit when the igniter wire breaks to shut off the current if the timer has not yet expired. However, the wire does not always break.
 

MisterBill2

Joined Jan 23, 2018
18,554
OK, then these igniters are not the same as the older "ESTES" igniters. I have re-used those igniters quite a few times, by carefully inserting them into the engine and then packing in the material from a match head. That was my recourse during one group racket launching session when inexperienced scouts did not understand how to correctly insert the igniters. So we ran out of the igniters long before running out of engines. Tedious but effective.
 

Thread Starter

phillipsoasis

Joined Aug 22, 2022
75
OK, then these igniters are not the same as the older "ESTES" igniters. I have re-used those igniters quite a few times, by carefully inserting them into the engine and then packing in the material from a match head. That was my recourse during one group racket launching session when inexperienced scouts did not understand how to correctly insert the igniters. So we ran out of the igniters long before running out of engines. Tedious but effective.
I remember back in the day wrapping the igniter wire (nichrome) around a pencil tip to make a small coil to concentrate the heat output and make the ignition more reliable. No longer needed....the newer igniter are very reliable with the pyrogen coated tip, but can only be used once. No more cotton balls to hold in the igniter. They come with a plastic plug that holds them in for launch. Much much easier.
 

MisterBill2

Joined Jan 23, 2018
18,554
The igniters in this thread must be for a different class of rockets, it seems. It has been quite a few years since i worked with scouts on those rocket projects, and I have not followed the advances.
Now it is also clear that the ignition system in this thread is far more than just a hot wire.
I still wonder about the whole concept of ramping up the current, since based on the calculations presented it does not seem that the actual battery drain would ever be a whole lot.

Are these rockets for higher altitude weather sensing? Or is it possible that these "rockets" are more for some sort of military actions? That thought just came to me.
 

Thread Starter

phillipsoasis

Joined Aug 22, 2022
75
The igniters in this thread must be for a different class of rockets, it seems. It has been quite a few years since i worked with scouts on those rocket projects, and I have not followed the advances.
Now it is also clear that the ignition system in this thread is far more than just a hot wire.
I still wonder about the whole concept of ramping up the current, since based on the calculations presented it does not seem that the actual battery drain would ever be a whole lot.

Are these rockets for higher altitude weather sensing? Or is it possible that these "rockets" are more for some sort of military actions? That thought just came to me.
Trust me, I am not writing these messages from a desk at Area 51.....lol. These igniters are the basic igniters for simple model rockets - A-G motors from Estes. Quest has come up with a totally different propellant material and ignition system and very different igniters for their motors. But I don't like their motors because they produce a huge amount of black smoke compared to the small amount of white smoke from the Estes A-G motors.

The data I shared above is over 20 years old so not that reliable, but it was all I could find to design the launcher. While running tests (igniting lots of igniters) I found that if I tried to igniter the igniter with a single surge of current I needed over 6 A and the battery voltage would sag 40-50%. I understand that stressing Li-Ion batteries like that is not a good thing. I experimented a lot, and found that I could ramp up the current in stages and only need 2-3 A for ignition. My assumption is that I am pre-heating the igniter in stages so the igniter eventually gets hot enough to ignite the pyrogen with less total current than if I just "short the battery across the igniter".
 

WBahn

Joined Mar 31, 2012
30,067
OK, then these igniters are not the same as the older "ESTES" igniters. I have re-used those igniters quite a few times, by carefully inserting them into the engine and then packing in the material from a match head. That was my recourse during one group racket launching session when inexperienced scouts did not understand how to correctly insert the igniters. So we ran out of the igniters long before running out of engines. Tedious but effective.
The ignitors I'm familiar with (dating back to the 1975 time frame) are single-use consisting of two leads bridged by a small piece of nichrome (~1/8" long) that is coated with a bulb of pyrotechnic material consisting of saltpeter, carbon, cornstarch, and hide glue. We installed the ignitors all the way into the engine nozzle and then used a piece of recovery wadding (which was always on hand, but just about anything will work) to jam it up in there, using a pencil tip to push it in firmly. If you installed them correctly, they were extremely reliable, but kids often didn't take the time to do it right and so misfires were common during the first portion of a group rocket launch, but became rare by the end of the day.
 

MisterBill2

Joined Jan 23, 2018
18,554
There is another type of detonator/igniter that does not use a nichrome heater wire, and acts much faster. That is the "exploding bridge wire initiator, which is claimed to be much less prone to accidental activation. They function much more like a "fast-blow electrical fuse, with the bridge wire providing a cloud of metal vapor/plasma to ignite the charge. The current is a few amps but the time is only a few milliseconds, at most.
 

Thread Starter

phillipsoasis

Joined Aug 22, 2022
75
There is another type of detonator/igniter that does not use a nichrome heater wire, and acts much faster. That is the "exploding bridge wire initiator, which is claimed to be much less prone to accidental activation. They function much more like a "fast-blow electrical fuse, with the bridge wire providing a cloud of metal vapor/plasma to ignite the charge. The current is a few amps but the time is only a few milliseconds, at most.
The igniters I use are called StarTech.
 

Ylli

Joined Nov 13, 2015
1,088
The only reason I can see for ramping the current is to perhaps reduce net draw from the battery. Doing that over a simple on-off isn't going to save much. I like the idea of limiting the time the current flows in the case of a failed launch (with a shorted igniter) and also terminating the current when an open circuit is detected. Those StarTech igniters have been pretty much standard Estes or quite a while.
 

Thread Starter

phillipsoasis

Joined Aug 22, 2022
75
The only reason I can see for ramping the current is to perhaps reduce net draw from the battery. Doing that over a simple on-off isn't going to save much. I like the idea of limiting the time the current flows in the case of a failed launch (with a shorted igniter) and also terminating the current when an open circuit is detected. Those StarTech igniters have been pretty much standard Estes or quite a while.
It is more to reduce the instantaneous draw from the battery. The current pulses heat the wire until the pyrogen ignites.
 

MisterBill2

Joined Jan 23, 2018
18,554
The current surge from the battery could also be reduced by arranging to do the firing pulse from a charged super-capacitor.Several Farads charged to 15 volts It could also provide protection against a shorted circuit by having the battery disconnected duringthe ignition cycle. And it would also terminate the cycle when the charge was consumed.. Not he simplest scheme but certainly worth a bit of thought.
 

WBahn

Joined Mar 31, 2012
30,067
It is more to reduce the instantaneous draw from the battery. The current pulses heat the wire until the pyrogen ignites.
But to what end?

These igniters were designed to be used with the extremely simple launch controllers powered by four AA batteries that have been used for decades. Those batteries were more than sufficient for the 40 launches (I've gotten well over 100 launches on a set of batteries over the course of a multiday event) you are trying to achieve. Most of the battery energy went into lighting the flashlight bulb that provided the continuity test, as evidenced by the significant increase in number of launches you could get if you didn't leave the launch key in during the entire countdown, but instead inserted it for a fraction of a second to verify connectivity, removed the key, and then reinserted it as your countdown got to 1 just before you pressed the launch button.

An AA battery typically has about 2500 mAh of capacity and, since all four are in series in the Estes controller, that is the capacity for the set (just at 6 V instead of 1.5 V).

As near as I can tell, that FlashFish unit you mentioned has a battery capacity of something like 60,000 mAh to 81,000 mAh, so you should be able to get well over 1,000 launches on a charge. If you aren't even getting 40 launches from it, you have much bigger problems that are not going to be addressed by minor improvements in how much charge is used on a single launch.

What is the actual problem that you are experiencing?
 

Thread Starter

phillipsoasis

Joined Aug 22, 2022
75
But to what end?

These igniters were designed to be used with the extremely simple launch controllers powered by four AA batteries that have been used for decades. Those batteries were more than sufficient for the 40 launches (I've gotten well over 100 launches on a set of batteries over the course of a multiday event) you are trying to achieve. Most of the battery energy went into lighting the flashlight bulb that provided the continuity test, as evidenced by the significant increase in number of launches you could get if you didn't leave the launch key in during the entire countdown, but instead inserted it for a fraction of a second to verify connectivity, removed the key, and then reinserted it as your countdown got to 1 just before you pressed the launch button.

An AA battery typically has about 2500 mAh of capacity and, since all four are in series in the Estes controller, that is the capacity for the set (just at 6 V instead of 1.5 V).

As near as I can tell, that FlashFish unit you mentioned has a battery capacity of something like 60,000 mAh to 81,000 mAh, so you should be able to get well over 1,000 launches on a charge. If you aren't even getting 40 launches from it, you have much bigger problems that are not going to be addressed by minor improvements in how much charge is used on a single launch.

What is the actual problem that you are experiencing?
From my experience teaching rocketry and launching a lot of B6-4 motorswith StarTech igniters in the past 23 years, the AA batteries and 9 V batteries are only good for a few launches before they need to be replaced. Most of the energy is wasted in the 15 feet of wire from the launch table to the launcher/igniter. That is why I built my own launcher with the battery located at the launch stand and I control the launcher from my cell phone at the launch table.

I want to get rid of the FlashFish battery pack and just use 4 rechargeable batteries for the following reasons:
* The FlashFish requires the launcher to draw ~200 mA or it shuts down - looking to reduce parts count on the iginter board
* As I mentioned above, I have to ramp up the current from the FlashFish, so there is a noticeable delay from hitting the launch button and the rocket leaving the launch stand.
* 4 rechargeable batteries and a BMS will fit in the box I used for the igniter, so there is less stuff to carry and setup at the field

Ingiter testing in the 2000s indicate that a pyrogen coated nichrome wire needs about 12 A for ~200 msec to ignite. I am sure the StarTech igniters have evolved since then, but this is all the data that publicly exists that I can find..

I was looking for some battery recommendations.
 

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Ylli

Joined Nov 13, 2015
1,088
From a googling, I see the "All-Fire" current of the Estes igniters is listed as 2 amps. These being relatively inexpensive devices, that value is not guaranteed, but should be in the ballpark. But 2 amps will not necessarily burn the bridgewire. It takes about 6 amps to insure the bridgewire opens. After that, if the igniter is still in the exhaust stream, there will be a current due to the plasma bridging the wires.
 
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