Model Rocket Launcher

[Kdog44]

I want to design/build a model rocket controller. The igniters used to activate the rocket engines have a resistance of 0.6904 ohms. The minimum current needed to fire the igniter is 2 A. I was going to use the following circuit (Shown Below). Note that I plan on charging the capacitor with a FET. Is there a better way of supplying a huge amount of current (2 A) to a resistive element then what I propose? The time it takes to ignite the Igniter with a current of 2 A is neglectable. Thanks for any help!

[Jope]

You can use a MOSFET.

[Refrigerator]

2A is not huge, your average NiCd rechargeable cell can do more than that for as short as it takes to kick off the igniter and for switching the current you can use almost any MOSFET no problem.

[IanB]

I want to design/build a model rocket controller. The igniters used to activate the rocket engines have a resistance of 0.6904 ohms. The minimum current needed to fire the igniter is 2 A. I was going to use the following circuit (Shown Below). Note that I plan on charging the capacitor with a FET. Is there a better way of supplying a huge amount of current (2 A) to a resistive element then what I propose? The time it takes to ignite the Igniter with a current of 2 A is neglectable. Thanks for any help!

A capacitor is not a good choice, a battery would be much better.

If we look at the voltage required, 0.69 ohms x 2 amps = 1.38 volts. This is slightly above what can be delivered by a single NiMH cell, so you would need two cells in series (which is not a big issue). A good cell like an Eneloop can supply 2 A without even breaking a sweat, but even a crap cell will have no trouble with a moderately low current like that.

Most likely, you don't even need a relay and a local power source. You could put the power source at the remote control station and just use a long wire. Just calculate the voltage drop of 2 A in your chosen wire and adjust the source voltage accordingly.

[Refrigerator]

I prefer relays unless you need to sync a lot of igniters.

Use MOSFET only when you know what you are doing. Without proper design-for-reliability engineering, a solid state detonator can be very dangerous, especially considering it powers a rocket.

Apparently, you do not know how to build a MOSFET based switch PROPERLY. Properly means fail safe, or even fail operational, depends on your requirements.

If the MOSFET fails short or misfires by floating voltage/ESD when you are wiring the detonator to the rocket engine, you can get yourself BADLY hurt.

I guess if he wants it to be completely fail-safe he could just connect a toggle switch ( of appropriate amperage ) in between the blast cap and the battery (+a long wire), no complicated electronics, no nothing.

[Kdog44]

I want to design/build a model rocket controller. The igniters used to activate the rocket engines have a resistance of 0.6904 ohms. The minimum current needed to fire the igniter is 2 A. I was going to use the following circuit (Shown Below). Note that I plan on charging the capacitor with a FET. Is there a better way of supplying a huge amount of current (2 A) to a resistive element then what I propose? The time it takes to ignite the Igniter with a current of 2 A is neglectable. Thanks for any help!

If we look at the voltage required, 0.69 ohms x 2 amps = 1.38 volts. This is slightly above what can be delivered by a single NiMH cell, so you would need two cells in series (which is not a big issue). A good cell like an Eneloop can supply 2 A without even breaking a sweat, but even a crap cell will have no trouble with a moderately low current like that.

Hmm that makes sense. Two seperate battery sources would be pretty easy to do ( one for the control system, and one for the igniter ).

I prefer relays unless you need to sync a lot of igniters.

Use MOSFET only when you know what you are doing. Without proper design-for-reliability engineering, a solid state detonator can be very dangerous, especially considering it powers a rocket.

Apparently, you do not know how to build a MOSFET based switch PROPERLY. Properly means fail safe, or even fail operational, depends on your requirements.

If the MOSFET fails short or misfires by floating voltage/ESD when you are wiring the detonator to the rocket engine, you can get yourself BADLY hurt.

I guess if he wants it to be completely fail-safe he could just connect a toggle switch ( of appropriate amperage ) in between the blast cap and the battery (+a long wire), no complicated electronics, no nothing.

That would work, and be economical. Thanks

[Ian.M]

Toggle switches can weld closed.  Use a knife switch as the contact position is visually obvious.

Anyone who wires the detonator end with the firing station end already hooked up is applying for a Darwin award.   Any competently designed multi-channel electronic firing controller will use a detachable terminal block or other effective mechanical lockout.  Any remote control firing system with local energy storage near the detonator needs a pin and fishing line style safety switch so it can be physically enabled after everyone has cleared the launch safety zone.

Come on guys, this isn't rocket science .... 

[IanB]

Come on guys, this isn't rocket science .... 

Actually, it is rocket science... 

[retrolefty]

I would think that any compenent iginitor circuit would establish and verify three different states..

1. Open circuit, such that the 'command console' would indicate a lack of continuity through the igniter say from a broken wire/open circuit with the igniter.
2. Low level current to indicate 'armed and dangerous' and prepared for firing.
3. Full 2 amp current indication indicating firing command.

Having full feedback control along with mechanical 'lock outs', I would think, be a safety requirement?

[IanB]

I would think that any compenent iginitor circuit would establish and verify three different states..

1. Open circuit, such that the 'command console' would indicate a lack of continuity through the igniter say from a broken wire/open circuit with the igniter.
2. Low level current to indicate 'armed and dangerous' and prepared for firing.
3. Full 2 amp current indication indicating firing command.

Having full feedback control along with mechanical 'lock outs', I would think, be a safety requirement?

I wouldn't think so. You would have "Lockout/Armed" (a key switch), where Lockout shorts the igniter ensuring zero volts across it and disconnects the firing button, Armed connects to the firing button. If you place the key switch in Armed and then press the firing button then power is applied to the igniter.

If you press the firing button and no ignition results this does not produce a hazard so there is no need to indicate ahead of time whether the firing button is likely to work. It either will work or it won't. You find out by pressing it. If it doesn't work you unplug the igniter cable and do fault finding.

That's my way of thinking, anyway.

[Nerull]

As someone who has been doing high power rockets for years, and leaving aside the eye-twitching induced by calling anything involved a 'detonator', a launch system should be designed with a lockout system such that an electrical failure in the launch control system cannot fire anything when not armed, and the system should never be armed unless it is safe to launch. There should be some indication - ideally audible - that the system IS armed, and continuity checking is a very common and useful but not essential feature. What IS essential is that any continuity check be properly designed for all possible ignition systems - especially ematches, or less common these days but still around - flashbulbs. These fire at very low currents and controllers like the ones Estes sells can set them off with the continuity check.

The safest solution is to buy a prebuilt launch control system, or at least use a proven design. Asking a bunch of people who have never seen a model rocket how they think it should work might provide some useful info, and a whole lot of useless info.

[richard.cs]

I have made a capacitor discharge one of these,  it had a few tens of millifarads charged to about 80V with a hand cranked generator. Switching was with mechanical contacts,  safety interlock shorted the output. It looked pretty but it wasn't amazingly practical. Did have enough volts to fire over very long lengths of lightweight cable though.

Generally it's not an approach I would recommend, firing directly with a small battery is easier. You say that the time is negligible but what would be useful to know is the required ignition energy, particularly if you want to do it with a capacitor. Mine stored around 0.5×80×80×0.02=64 Joules but that was very much overkill. 

[LabSpokane]

All you need is a battery and a switch. The prefab controllers used standard  alkaline batteries. I used a motorcycle battery when I built my own as a kid. I used a standard toggle switch as a master with a pushbutton to fire. I had zero issues with welded contacts. Just use a quality switch and it will be fine.

And for what it's worth, rocket igniters in this context are not "detonators."  It's just a piece of nichrome wire with essentially a match head. The risk is minimal if common sense is employed.

[edy]

Why not a simple circuit with 1 battery, 1 ignitor, 1 key-switch and 1 push button all in series? The key switch would have to be turned into "armed" or "on" position AND then you would need to press in the pushbutton to finally close the last part of the circuit to allow current to circulate and ignite the ignitor. Is that going to be unsafe? What is wrong with just that?

[helius]

It is especially useful to be able to test continuity of an igniter circuit. Having a lamp or a resistor+led that can be switched in to the loop is a good addition.

[LabSpokane]

Why not a simple circuit with 1 battery, 1 ignitor, 1 key-switch and 1 push button all in series? The key switch would have to be turned into "armed" or "on" position AND then you would need to press in the pushbutton to finally close the last part of the circuit to allow current to circulate and ignite the ignitor. Is that going to be unsafe? What is wrong with just that?

Nothing.  This is a simple project.  That's what I built close to four decades ago when I was in the fourth grade, on my own, from my Dad's scrap parts bin and Radio Shack.  The continuity light is nice when the alligator clips get fouled.  The more voltage the better.  One can get an 8AA battery holder off eBay for $2.50 and it even has its own master switch.  You'll need to up the gauge of the wires, though.  12V helps tremendously with more reliable ignition.  If you want to get fancy, parallel in an LED with maybe a 1K to 10K resistor after the master and across the fire button for a continuity check. 

None of this complex or dangerous if common sense is followed.  It's easy to test a few ignitors to make sure your continuity test won't set them off. 

[AG6QR]

Why not a simple circuit with 1 battery, 1 ignitor, 1 key-switch and 1 push button all in series? The key switch would have to be turned into "armed" or "on" position AND then you would need to press in the pushbutton to finally close the last part of the circuit to allow current to circulate and ignite the ignitor. Is that going to be unsafe? What is wrong with just that?

The key switch has to be of the type that only allows the key to be removed when the switch is open, but with that clarification, your suggestion is fine.  The idea about the key is that a person who has the key in his physical possession when he goes to the launch pad should be assured that the wires are not going to become hot.  A standard commercial key switch with a lock cylinder isn't always necessary -- you could achieve a similar effect by using something like a pair of banana jacks and a shorting bar, where the shorting bar serves as the "key" that the person carries with him to the launch pad.  Or a phone jack and a phone plug with tip and shield shorted together.  However you do it, there should only be one "key" anywhere in the gear you bring out on launch day.

[LabSpokane]

Why not a simple circuit with 1 battery, 1 ignitor, 1 key-switch and 1 push button all in series? The key switch would have to be turned into "armed" or "on" position AND then you would need to press in the pushbutton to finally close the last part of the circuit to allow current to circulate and ignite the ignitor. Is that going to be unsafe? What is wrong with just that?

The key switch has to be of the type that only allows the key to be removed when the switch is open,

Here's one for $11 on ebay: http://www.ebay.com/itm/DETAINED-OFF-ON-CHICAGO-KEY-WAY-GAMING-ACCESS-CONTROL-SWITCH-LOCK-/261968034502?pt=LH_DefaultDomain_0&hash=item3cfe82fac6

I'm sure there's something from China for under $3 as well. 

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For less than $3, just search for "motorcycle ignition switch":

http://www.ebay.com/sch/i.html?_odkw=key+switch+retained&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR2.TRC0.A0.H0.Xmotorcycle+ignition+switch.TRS0.G%7C1%7C0&_nkw=motorcycle+ignition+switch&_sacat=0