[RocketsNW] [OROC Members] (Set) 10-way Drag Race - Sheridan Oregon September20th @ Noo...

[Krausert, Robert]

Thanks Steve for helping to answer this question. Goal of the ematches is first to attempt lighting each motor as close together as possible, and second protecting the launch equipment. Certainly regarding the drag race, I hope it works out to be a fun time for all.

Cheers,
Robert

-----Original Message-----
From: members-bounces at oregonrocketry.org [mailto:members-bounces at oregonrocketry.org] On Behalf Of Steve Tarr
Sent: Thursday, August 21, 2008 8:38 AM
To: W. Raymond Stoner
Cc: members at oregonrocketry.org; rockets at rocketsnw.com
Subject: Re: [OROC Members] [RocketsNW] (Set) 10-way Drag Race - Sheridan Oregon September20th @ Noo...

Ray,
You're confusing the current that *would* flow if there was no
resistance in the launch system with the current *needed* to fire the
match.  I think Robert's saying that the "all fire" current for his
matches is 32mA each, so you only need 320mA total to fire 10 of them.
In that case, and assuming that each ematch is 1.5 ohms, the launch
system only has to maintain 0.048 volts across the match with 32mA
flowing in order to fire it.  As previous discussions on this list have
pointed out, ematches like this can be successfully fired with even the
little "button cell" batteries.

In practice, a lot more current flows.  The current is mostly limited by
the resistance of the launch system, especially the alligator clip
connections.  That may easily be as much as 3 ohms, perhaps as much as
10 ohms if the clips are dirty.  When you hook a 12 volt 10 ohm launch
system to a 0.5 ohm copperhead, the firing current is a little over 1
amp, just barely enough to get it lit.  But that's still plenty of
current to light an ematch.

When you hook the ematches in parallel, there's enough variation from
match to match and in the connecting wires/clips/whatever that the
current doesn't split evenly.  But it doesn't really matter since the
worst-case current will still be well over the 32mA needed to guarantee
that the match fires.

The real problem with hooking that many igniters in parallel to the OROC
launch system is that the relays are only rated at 10 or 20 amps, and
the whole system is fused at (usually) 30 amps.  Although you only
*need* 320mA to fire the ematches, the actual current will be much
higher.  If you're using copperheads or even standard igniters, it's
possible for the fuse to blow before all the igniters fire.  If you use
ematches (which light faster), the ematches will all light but the fuse
may still blow.  That's OK -- we do have plenty of spare fuses -- but
the other alternative is to use a "drag race" system with a higher
current relay and a much bigger fuse.  I don't know what Robert's using;
the old OROC drag race system used a car starter relay and perhaps
6-gauge wire to the battery.

-Steve

W. Raymond Stoner wrote:
> I'm confused, how do 10 ematches total to 320mA?  I'm not aware of any
> ematches that are rated at a 32mA "all fire" current.  The math just doesn't
> add up to me.
>
> I'm certainly not an ematch expert, but I know that ematches have a minimum
> fire current and an all fire current.  Most of the ematches that I've
> measured come in somewhere around 1.5 ohms resistance.  10 ematches in
> parallel equate to 0.15 ohms resistance, 13.2V from a fresh battery comes to
> 88 amps of current.  I can do the math in detail to show what I am talking
> about...
>
> Even if you wire the ematches up in series I still don't get the current
> down to 320mA.
>
> Help the confused.
>
> Ray
>
> -----Original Message-----
> From: rockets-bounces at rocketsnw.com
> [mailto:rockets-bounces at rocketsnw.com]On Behalf Of Robert Krausert
> Sent: Wednesday, August 20, 2008 7:04 PM
> To: Mfreptiles at aol.com; members at oregonrocketry.org;
> rockets at rocketsnw.com
> Subject: Re: [RocketsNW] (Set) 10-way Drag Race - Sheridan Oregon
> September20th @ Noo...
>
>
> I have special uncoated ematches, and I then dip them into pyrogen. Ignites
> the pyrogen, but leaving out the coating prevents the pop that may not
> ignite them. Every flight I done over the past two years has been with these
> matches.
>
> Cheers,
> Robert
>   ----- Original Message -----
>   From: Mfreptiles at aol.com
>   To: lawndart.robert at gmail.com ; members at oregonrocketry.org ;
> rockets at rocketsnw.com
>   Sent: Wednesday, August 20, 2008 6:52 PM
>   Subject: Re: [RocketsNW] (Set) 10-way Drag Race - Sheridan Oregon
> September 20th @ Noo...
>
>
>   In a message dated 8/20/2008 6:37:59 P.M. Pacific Daylight Time,
> lawndart.robert at gmail.com writes:
>     Use of ematches will only require 320 mA to light all 10 at the same
> time, very easy for the house system to complete. >>
>
>   I do have one question about this.  Will they be dipped e-matches or
> augmented in some way?  Normally, a plain commercial e-match will not light
> an APCP motor by itself.
>
>   Mike Fisher
>
>
>
>
>
>
>
>
>
>
> ----------------------------------------------------------------------------
> --
>   It's only a deal if it's where you want to go. Find your travel deal here.
>
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