1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How can I ignite these three rocket igniters?

  1. Jul 18, 2009 #1
    First off, let me say that what I am working on has taken me a few days to get to work and I just cant figure it out. It is really frustrating so any help is really appreciated.

    I have three igniters. They are connected to a circuit. Once a switch is turned to "on" the three igniters are supposed to ignite.

    This is the diagram

    I am using two nine volt batteries each charged at 9.50 volts. Now, if I remove the igniters from the diagram and test the voltage of each side of the wire, the voltage reads about 9.50 volts at all locations. Now, when I attach the igniters to these wires, and to a test ignition, only one of the igniters ignite. the batteries are connected in parallel, and the igniters are connected in parallel also.
  2. jcsd
  3. Jul 18, 2009 #2


    User Avatar
    Science Advisor

    I thought your batteries were to be in series, for a nominal 18 V.
  4. Jul 18, 2009 #3
    They were in series but then I removed them in parallel. This is because when I tested the circuit with the batteries in parallel only one igniter ignited.

    Also, as takled about in my previous post, the voltages near the igniters were rapidly jumping from one voltage to another. I found out this is because of the clips I was using. Apparantly, they cant be used in this sort of project.
  5. Jul 18, 2009 #4


    User Avatar
    Science Advisor

    Try using something other than 9 V batteries. Those small rectangular batteries have a very high internal resistance and cannot supply a large current--typically less than an amp, because they are comprised of 6 smaller 1.5 V cells (usually AAAA-size) in series. I would suggest a small 12 V lead-acid gel cell, such as used for alarm systems, UPS units and emergency lighting since these have a much smaller internal resistance and can supply several tens of amps easily.
  6. Jul 18, 2009 #5
    Ok now where can I buy this item? It needs to be inside of my rocket which means that it neeeds to be light.
  7. Jul 18, 2009 #6
    Ok. Now I need to buy this item from radio shack because that is the only hardware store in my area. The problem is that I cannot find this item that they sell.
  8. Jul 18, 2009 #7


    User Avatar
    Science Advisor

    I thought the battery was 200 feet away?
  9. Jul 18, 2009 #8
    No. That is a different circuit. i already figured out how to launch the rocket frmo 200 feet away using one car battery. Once the rocket is in the sky, another circuit will be turned to on which will ignite a different part of the rocket in the sky.

    That is what I am trying to figure out. I am trying to figure out how to launch these three igniters.
  10. Jul 18, 2009 #9


    User Avatar
    Science Advisor

    Ahh. In that case, use lithium cells. They are very light and can source a lot of current.
  11. Jul 18, 2009 #10
    Ok. I looked into these batteries. It appears that their voltages are about 3 volts. I have found that the igniters need atleast 9 volts to ignite. I have found this through trial and error. Do you think we will definitly be able to know what batteries my circuit needs if we had detailed information about the voltage and amperage required to ignite the igniters?

    It seems like getting those batteries will just lead to more trial and error.
  12. Jul 18, 2009 #11
    Also, I would suggest putting the ignighters in series. This would ensure that they all light and at the same time.

    Also, Unless I'm mistaken, I believe you can buy model rocket motors with an ejection charge that also lights the next stage.
  13. Jul 18, 2009 #12
    I understand that the ejection charge can do that but the ejection charge in this design is beign used for other applications. Therefore, I need to use a battery to ignite tthe followign stage.

    Why exactly does putting the batteries in series ensure that they all light at the same time?
  14. Jul 18, 2009 #13
    Not only the batteries, but put the ingighters in series. Actually, I'm rethinking that now.
  15. Jul 18, 2009 #14


    User Avatar
    Science Advisor

    No, don't do that. What will happen inevitably is that one will fire and fuse open first, leaving the rest on an open circuit and unable to fire. Like those annoying Christmas tree lights. One goes out and the entire string is dead.
  16. Jul 18, 2009 #15
    That's right. I see it now. Once again comes the glorious lesson of being wrong.

    You do want the batteries in series though. Double the voltage, double the curent.


    i = v/r
  17. Jul 18, 2009 #16


    User Avatar
    Science Advisor

    Almost. You do double the open-circuit voltage, but remember that you're also putting the batteries' internal resistances in series so your actual load voltage and current will be somewhat less than doubled because your load resistance in series with the battery internal resistance act as a voltage divider.
  18. Jul 18, 2009 #17
    Ok. Now you are saying that I should put these two 9 volt batteries in series? Also, after looking through my notes, I found that if I have two batteries charged at 10 volts in parallel in the circuit, then all igniters ignite simultaneously.

    Now, if I were to use these lithium cells, it appears that the voltage will be three volts which my understanding shows is just too low. The only problem is I cannot charge my batteries up to 10 volts and have them stay at that voltage.

    Any ideas?
  19. Jul 18, 2009 #18
    Bad advice. More likely is a parallel wiring results in a fuse close of one of the igniters pulling all the amps leaving not enough for the rest of the igniters. There is a reason the law required series wiring of the blasting caps in the mine. Setting off twenty shots at a time in the mine required the caps to be wired in series. One check for continuity ensures that all caps are made up. The blaster charged a large capacitor to ensure a large amperage flowed thru the series circuit igniting all the shots. Wiring in parallel would require the testing of each circuit for continuity. Series wiring ensures that all are in the circuit. If one is not made up, then none will go off. Parallel wiring may result in only one shot going off because the rest are not making a circuit. Not good when men will have to load out the shot material with maybe several live sticks of Tovex in the gob.

    Wire the blasting caps or igniters in series and use a car battery for your three rocket igniters. Test for continuity with a high impedance quality meter such as a Fluke 77 or find a galvanometer. Illegal to use the Fluke in the mine. Only the galvanometer.

    The following link mentions the advantages of series wiring and the disadvantages of parallel wiring of your igniters. The problems generally manifest on larger numbers of igniters but it helps to understand what is going on and get it right the first time.
    Last edited: Jul 18, 2009
  20. Jul 18, 2009 #19
    Ok. Now there is a problem I can see with this. Say that I use the igniters in series. If the circuit is turned to on, I can see the first igniter in the series to ignite first. Then, I can see this igniter becoming burnt out which will then ruin the continuity of the circuit. Thsi will then cause the other two igniters to not igntie. If this hapens, then I am in trouble. These three igniters need to ignite simultaneously. That is why I chose to use a parallel circuit.

    let me know what you think.
  21. Jul 18, 2009 #20
    Read the link. Much better and more authoritative than this old retired coal miner. Your picture of the chain of events in the series setup is eliminated by having enough amps available at your blaster. The one in the mine was pretty heavy and it took about 15 seconds for the capacitor to come up enough to light the ready light. For your rocket motors I think a 12 volt car battery will be enough. You're not trying to set off 20 shots at a time and some of those shots might have three or more sticks of Tovex. Just pretend that what you are doing can be so unforgiving that you can't take chances.

    How would you test your parallel circuit? Series is simple. Check the two wires you hook to your blaster or battery and in a series an open on any igniter will be revealed. Not so on a parallel arrangement. http://www.pyromate.com/Basics-of-Electrical-Firing.htm" [Broken] Also refresh the page as I have altered my previous post somewhat.
    Last edited by a moderator: May 4, 2017
  22. Jul 18, 2009 #21
    I know how to test the parallel circuit and check for continuity. When I am using a circuit with the igniters in parallel, the igniters just do not ignite simultaneously. I guess that havign the igniters in serieis would seem like a better idea because then they will all ignite at the same time, guaranteed, correct?

    Now, I have used an equation. I am usign three estes igniters each which have a resistance of 0.8 ohms

    V= 0.15 Amps x 2.7 ohms = 0.36 volts

    Clearly, 0.36 volts will not be enough. What is the problem?

    I also cannot use a car battery because thsi particyular circuit will be used in flight and a car battery will just be too heavy.
  23. Jul 19, 2009 #22
    OK. I've been looking around and have found a couple of other links. http://books.google.com/books?id=sd...vntDw&sa=X&oi=book_result&ct=result&resnum=1"


    If you can't test, since I understand the beast is already in the air, then you need to put a little shotgun shell powder in with the igniter or something else easily ignited by a brief heating to combustion temperature. I'd often considered building a 2 liter water first stage with a trigger detection of some type (accelerometer?) of the decrease of acceleration when the water ran out and so fire off the chemical engines. Never did it though. Have to consider how tuff those Estes igniters are and how reliable. I've had problems in single igniter launches before and found the igniter open electrically.

    To make a series set of igniters go off well you need enough voltage and enough amperage. 500 mfd 35 volt capacitors don't weigh much. Charge them to twenty four volts. Amperage is the same in all the igniters in series circuits. Parallel would be ok if you could guarantee each igniter had the same resistance and all were hooked up. The parallel arrangement uses more power which translates to unneeded weight in your case, I think. You could charge the capacitor(s) on the ground and they could be the ignition source. Don't know what you've got going though. At liftoff they could break an umbilical line and the craft could be running then on internal power provided by the capacitor.
    Last edited by a moderator: Apr 24, 2017
  24. Jul 19, 2009 #23
    I am looking to make this as simple as possible. I have a switch which will be turned to on by a seperate system. Now, this switch then needs to cause three rocket engines to ignite simultaneously. I also have 9 volt batteries to use as the capacitor.

    Now, working with what I have, do you beleive the batteries should be in serieis or parallel? Do you believe the igniters should be in seriies or parallel?

  25. Jul 19, 2009 #24
    The only thing I can guarantee is that when you make your first solo skydive that you will have this thought, "I hope all this stuff works." With the series, if there is no continuity you'll still have 3 good engines. With parallel, who knows.

    You need a single pulse with enough voltage and amps to set off that blue stuff on the igniters which will set off the engines, hopefully simultaneously. That is why I would use, and have used in single stage launches, a little bit of shotgun shell powder in each nozzle. Not the fff or ffff rifle powder. They be ok for an ejection charge but burns too fast for the ignition aid purpose. Half of a pencil eraser or so is what I used. Still, if there be an open, you're screwed.

    Get your calculator out and a formula and figure the numbers on a 35 volt 500 mfd DC aluminum electrolytic polarity sensitive capacitor charged to 24 volts. I'm just guessing it would work. Maybe two of them in parallel. Test this on the igniters alone in a little powder on the ground. You just need one good pulse. If you've any battery left after that you're carrying wasted weight. The capacitors are about a 25 cent piece around and about 2 inches long and light weight.
  26. Jul 19, 2009 #25
    Is your switch electronic? A mercury inertia switch? 9 volt batteries is what you want to use instead of a capacitor. Right? negitron has already mentioned the internal impedance of the 9 volt batteries. They're made of a bunch of little bitty single cells. They work by a chemical reaction which is a negative. A capacitor works by the storage of an electric field with almost no internal impedance. This is why a capacitor storage for an electric car could be charged very quickly and discharged very quickly instead of the slower amp draw and supply of the chemical batteries.

    Two 9 volt batteries in series is 18 volts. I don't know how many amps they will flow in a dead short. I think that the 24 volt charge on the capacitor will flow more amps quicker due to the little to no impedance.

    But if you are going to use the heavy batteries. Go for the batteries in series as well as the igniters. Nothing so far has indicated you can't put that capacitor across the positive and negative of your power supply to give it a good shot of amperage when your switch closes. I think your problem stems from the lack of a good high amperage pulse. Those 9 volters just don't get it for that application. The capacitors across the positive and negative (watch the polarity of electrolytics) will improve the high amp pulse considerably.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook