[TriEmbed] Rocket derby help

Thu Feb 2 01:27:19 CST 2017

Craig,
I launched a few model rockets in my day also.  It is all about getting the igniter hot enough and that is all about pumping current through it.  Your enemy is resistance. Major sources of resistance are the wire, the igniter itself and the battery.

Let’s look at each one.

1) The wire.  You apparently measured the resistance of the wire and measured 1.2 ohms.  That seems to match with Kevin’s mention of CAT5 specs of 2.5 ohms per hundred feet.  Something to consider is that you probably measured one direction but remember the current has to return so it encounters an equivalent resistance on the way back.  Let’s call the total wire resistance 2.5 ohms.

2) The igniter is just a resistor in wire form.  That’s what makes it work. It’s resistance is so great that it gets hot when you pump current through it.  Think about the wires in  your toaster, same thing. The second entry in this discussion (http://www.rocketryforum.com/showthread.php?21958-Ignitor-specs) says that Estes igniters are 0.8 ohms. .  You can measure that directly with an appropriate ohmmeter but let’s assume that is true.

3) The battery itself has an effective internal resistance.  This is partially real resistance and partially due to chemical processescncook001 at yahoo.com but the effect is to put additional resistance in your circuit. That is why battery voltage drops at the terminals with heavy loads.  Different battery types have higher or lower internal resistance.  NiCads are much lower than alkaline batteries.  You can get much more instantaneous current out of a nicad than an alkaline or carbon cell of the same size.  If you use a battery with low internal resistance such as a lead-acid gel cel or a set of nicads, this shouldn’t be a problem.

Neglecting any effects of the switches and connectors (may or may not be valid but you indicate you checked them), we have a total resistance of 3.3 ohms.  According to the specs of the article Rodney linked to, we need at least 2 amps of current flowing through our igniter to make sure it lights.  Let’s assume this is true of our igniters for now.  We therefore need a minimum voltage of 3.3 ohms x 2 amps = 6.6 volts.  This indicates that a 6 volt battery is marginal at best and non functional as it ages and the voltage drops. Being an engineer and preferring to use at least a 2X margin, it appears that a 12 volt battery should work fine, even at the end of the long CAT5 cable.  Also, as an engineer and a subscriber to the KISS (keep it simple stupid) theory, I would avoid relays at all cost.  They don’t appear necessary.

There is still another option which is to use lower resistance wire.  That can be achieved by using larger wire or just using multiple wires in parallel in the CAT5 cable.  You have 4 pairs so you can wire 4 together and still have a conductor out and a conductor back. That would reduce your wire resistance to 1/4 so now you are looking at 0.63 ohms for the wire so 1.43 x 2 amps = 2.86 volts or conversely  a 6 volt battery can drive 6/1.43= 4.2 amps through the igniter.  That should be sufficient.

In conclusion, you can probably use a 12 volt battery or use multiple pairs in your CAT5 cable (or both) and get many reliable launches.

Paul Holzworth

> On Jan 30, 2017, at 10:59 PM, Rodney Radford via TriEmbed <triembed at triembed.org> wrote:
> 
> This is a nice writeup of testing of various igniters showing the current profile during a launch cycle. 
> 
> Skip forward to figure 11 and it shows a peak current of 12.39 amps with a 12v source:
> 
> https://publicmissiles.com/IgnitersWhitePaperbyG-Wiz.pdf <https://publicmissiles.com/IgnitersWhitePaperbyG-Wiz.pdf>
> 
> While this article shows the initial fire current is 2.0 amps, but it is 3x this value (6.0 amps) to burn the igniter through.
> 
> So if you really have 1.2 ohms between the battery and the launch box, the voltage drop on the wire will be too high to ignite the wire.
> 
> I agree with Tad's suggestion of putting the launch battery at the launch box, and then use a smaller battery in the control just to trigger relays, which then supply high current through shorter wires. This is how the high power guys do it that have much higher current requirements, and would also be useful even with low power clustering systems.
> 
> 
> 
> 
> On Mon, Jan 30, 2017 at 10:48 PM, Shane Trent via TriEmbed <triembed at triembed.org <mailto:triembed at triembed.org>> wrote:
> Estes reports that 90% of engine ignition problems are caused by the ignitor not touching the propellant. It seems they recommend a plug to hold the igniter. That is new to me, I haven't launched a rocket in 30 years!
> 
> http://www2.estesrockets.com/pdf/Model%20Rocket%20Engines%20&%20Igniters.pdf <http://www2.estesrockets.com/pdf/Model%20Rocket%20Engines%20&%20Igniters.pdf>
> 
> Shane
> 
> On Mon, Jan 30, 2017 at 10:39 PM Shane Trent <shanedtrent at gmail.com <mailto:shanedtrent at gmail.com>> wrote:
> Craig, 
> 
> I suggest changing to a 12V battery. A small SLA should fit the bill and offer plenty of launches per charge. 
> 
> Shane
> 
> On Mon, Jan 30, 2017 at 10:25 PM John Vaughters via TriEmbed <triembed at triembed.org <mailto:triembed at triembed.org>> wrote:
> Craig,
> 
> I do not know what it takes to light the fuse, but right off the bat, your currents seem tiny. Are you sure you got the scale right? Also, cat 5 cable does not transmit alot of current, but it may be enough. I just do not know how much current you need. I also do not have the specs on the igniter and what voltage/current is proper. I think you need to start with the specs for the igniter, look that up first. Then consider upping the voltage at the button box to get the proper voltage at the igniter. If it worked before then it may be as simple as verifying the specs of the igniter and the provided voltage, but be careful if it is truly only cat 5 cable. It is very low current cable.
> 
> Did I miss the email about the car race results and solution? If not, please let us know how it went.
> 
> Thanks,
> 
> John Vaughters
> 
> 
> On Monday, January 30, 2017 9:43 PM, Craig Cook via TriEmbed <triembed at triembed.org <mailto:triembed at triembed.org>> wrote:
> 
> 
> Now that Pinewood derby is over, onto my next challenge.
> 
> Last year I was launch master for our Cub Pack.  We have been having problems with our rocket launch system.  6V large square battery powers the system.  There is a rotary switch where you can select from position 1,2,3 or 4.  This box is connected via 45 feet of cat 5e wire to junction box (DB9 serial connectors attach from the launch box to cable, and another one to the junction box).  The junction box is split out to 4 sets of wire with alligator clips.  These clips attach to the rocket fuse (1,2,3 or 4).  The Cub presses a button on the launch box, energy is dumped into the fuse and the rocket should take off.
> 
> The last few years we have had problems with the fuse not getting enough energy to burn fast enough and launch the rocket, result is an unhappy Cub.
> 
> I cut off and re-soldered the alligator clips thinking that would solve the problem.  It didn't.  I checked and I have 0 resistance from the junction box to alligator clips, so I think my soldering was ok.
> 
> I measure 1.2 ohms of resistance from the launch box to the junction box going over the 45 feet of wire.
> 
> Battery has 6.5 Volts, at the junction box I measure 4.19 Volts.
> 
> Battery has 0.005 Amps, at the junction box I have 0.003 Amps.
> 
> Is there a way to store energy at the junction box, then have it released when a switch is pushed?  I think that is what a capacitor is for, but not sure.
> 
> 
> I was considering replacing the cat 5e wire and DB9 connectors with RJ45 ones.  Would that help?
> 
> No idea how old the batteries are, are the Amps too low?
> 
> Whoever made this setup knew what they were doing.  It has been well put together.
> 
> 
> Thanks
> 
> Craig
> 
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