hotwire ejection charges

The Rocketry Forum

Help Support The Rocketry Forum:

labounty8

Active Member
Joined
Jan 18, 2009
Messages
39
Reaction score
0
i am having trouble getting pyrodex rs to ignite in my newtons third hotwire
ejection canisters while doing ground test. i am using a perfectflite mawd altimiter. nichrome wire is burning but not setting off pyrodex. any help would be appreciated.
 

jdud

Well-Known Member
Joined
Jan 19, 2009
Messages
244
Reaction score
0
I use the hotwires with 4f black powder and a missileworks timer - no problem. I've never used pyrodex - are the granules of fine texture?
 

Marsman

Well-Known Member
Joined
Feb 25, 2009
Messages
167
Reaction score
0
The MAWD may not have enough current long enough to fire the ematch head.

When I use Pyrodex and an RRC2 mini with the Newton's 3rd cannisters, I use slightly more than I would for BP. Make sure that you use the included foam plugs and add a layer or two of electrical tape over the top of the flip top to keep to powder contained until it has burned enough to pressurize the tube.
 

labounty8

Active Member
Joined
Jan 18, 2009
Messages
39
Reaction score
0
it was firing the ematch head because it lost continuity after test,but didnt ignite pyrodex. on three different ejection canisters. i just hooked a battery directly to the fourth canister and it fired ok.
 

m85476585

Well-Known Member
Joined
Jan 18, 2009
Messages
2,664
Reaction score
2
I have found the Hotwire canisters to be a little fragile. Did you have continuity before you tried to fire it after loading it with powder?
 

labounty8

Active Member
Joined
Jan 18, 2009
Messages
39
Reaction score
0
yes i checked before, and after i loaded with powder
 

newtons3rd

Member
Joined
Feb 6, 2009
Messages
7
Reaction score
0
Very Odd....

We have tested with several BP substitutes will great results.

Only time that I have seen this in use is when the element was not covered with the BP.

Never had much of a problem with fragility since we changed the way we seal the ecan where the wire enters.

We tested a couple different flavors of pyrodex with good results, not sure what might be going on.

Mike
N3
 

labounty8

Active Member
Joined
Jan 18, 2009
Messages
39
Reaction score
0
the pyrodex i have has been sitting in the garage a year or two. so i think i will get a new bottle before i try it again.
 

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,352
Reaction score
39
i am having trouble getting pyrodex rs to ignite in my newtons third hotwire
ejection canisters while doing ground test. i am using a perfectflite mawd altimiter. nichrome wire is burning but not setting off pyrodex. any help would be appreciated.
You need to reread the instructions for both your altimeter and the ejection canisters.

The MAWD has a Capacitive Discharge pyrocircuit that does not supply enough current for a long enough time to operate the N3 hot wwire ejection canister. It is designed for e-matches only.

You need to use an altimeter with a DC pyrocircuit with a second pyro battery.

Bob
 

newtons3rd

Member
Joined
Feb 6, 2009
Messages
7
Reaction score
0
Sorry but have to disagree we have dozens of customers using the MAWD with our ecans with great results.
 

AHansom

Well-Known Member
Joined
Jan 22, 2009
Messages
883
Reaction score
114
Location
Santa Barbara, CA.
Sorry but have to disagree we have dozens of customers using the MAWD with our ecans with great results.
I use four ecans per flight using a MAWD as primary and R2CC as backup. I have used over 40 and never had a falure during flight or ground testing.

I tried using Pyrodex and found that if the igniter is centerd lenght wise in the canister it worked better. I guess the powder can burn both ways instead of from one end to the other.
 

Viperfixr

Born Again Rocketeer
Joined
Jan 19, 2009
Messages
1,474
Reaction score
67
I use the hotwires with 4f black powder and a missileworks timer - no problem.
+1

I used a MAWD and Newton's 3rd small ejection canisters for my L2 last February--no problems at all. I've flown them four times total without fault, using ffffg powder.
 

labounty8

Active Member
Joined
Jan 18, 2009
Messages
39
Reaction score
0
i think i figured out the problem. i made the pyrodex finer by smashing it with plastic spoons. had a sucessful test and launch.
 

jcsalem

Well-Known Member
Joined
Feb 26, 2009
Messages
451
Reaction score
0
Sorry Bob, but I have to agree with Mike and others.

I've successfully lit dozens of N3 canisters using an MAWD. Not a single failure attributable to the N3-MAWD combination. I love the simplicity of the N3 canisters.

Jim
 

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,352
Reaction score
39
Sorry but have to disagree we have dozens of customers using the MAWD with our ecans with great results.
I don't normally misread a spec, but I misread your wire size, and as a result, came to the wrong conclusion.

Most hot wire ignition systems use relatively thick resistance wire, and won't work with a CD pyro circuit because of high current demand and the heat capacity of the wire itself. Your canister uses 48 AWG which I misread as 38 AWG. (A 30 something AWG or thicker wire would have very low resistance and high heat capacity both of which would deplete a capacitor before the wire got hot.) The 48 AWG greatly increases the wire resistance which reduces the current draw, and greatly reduces the heat capacity of the wire, so the wire gets hot quickly and stays hot for 10's of milliseconds which is sufficient time to deflagrate Pyrodex(R) if it is in good contact with the hot wire.

BP has a low ignition threshold and burns very rapidly whereas Pryodex(R) burns significantly slower at atmospheric pressure than BP. Because of the high burn rate, almost any hot wire source will ignite BP and the ignition will become self sustaining and deflagrate without external pressure on the powder. When the ignition of unconfined Pyrodex(R) is attempted, the individual grains of Pyrodex(R) in direct contact with a heat source will ignite, but the hot gases are generated at a much slower rate than BP and thus will take a much longer time to ignite adjacent particles not directly in contact with heating source. Heat transfer rate is proportional to the gas pressure and mass flow, and as Pyrodex(R) burns slower than BP, the local gas pressure has to be commensurately higher to obtain the same burn rate as BP or the burning area much be proportionally larger.

This can be overcome by increasing the burn rate of the Pyrodex(R). There are several ways to accomplish this, all of which are possible in your canister because the hot wire is thin, has little heat capacity, and stays hot for a relatively long time (10's of milliseconds vs milliseconds).

0.) Because your wire is so thin, and has little heat capacity, it gets to the ignition temperature in less than a millisecond. Because the initial resistance is high, and gets higher as it gets hotter, the RC time constant for capacitor depletion is 10's of milliseconds not milliseconds.

1.) Your hot wire is long compared with a conventional bridge wire so many more grains are directly in contact with the hot wire than with a bridge wire. More contact area gives more burning area which increase the heat release rate.

2.) The foam prevents the Pyrodex(R) from moving away from the hot wire so the grains in contact with the hot wire get hotter and generate more hot gas.

3.) The canister cap forms a seal and allows pressure to build for a short period of time. The higher the pressure, the faster the burn.

All these factors add up so while it may take several milliseconds for the Pyrodex(R) to deflagrate versus the 100's of microsecond for BP to do the same, the high hot wire temperature, the large contact area, the foam confinement, and the sealed canister all promote smooth combustion of the Pyrodex(R) before the canister opens.

This means you did your homework when you designed the canister.

Bob
 

shoitsma_86

Member
Joined
Mar 1, 2009
Messages
12
Reaction score
0
I also had trouble using the N3rd hotwire canisters with Pyrodex at first. For consistent results, you have to pack the Pyrodex EXTREMELY tightly in the canister. Depending on how much you fill the canister, the foam piece provided may or may not be enough.

What I do with my canisters is fill them up with the desired amount of Pyrodex and then PACK dog barf in on top, cramming as much in as physically possible with the tip of a ballpoint pen. Then I close the cap and seal the cap down with two layers of electrical tape.

As was stated above, the reason this needs to be done is because of the slower burn rate of Pyrodex compared to black powder. If you use my method I guarantee you'll get more consistent results.
 

newtons3rd

Member
Joined
Feb 6, 2009
Messages
7
Reaction score
0
Bob;

WOW that was a nearly verbatim commentary of the discussions during the design phase of the HotWire products. And far more lucid and succinct than I was able to author, Mind if I quote you? :)

We had some great beta testers (trusted their rockets to a new product) that collected very good data sets and provided plenty of feedback. We made changes to the original design based on the great input we had from the flyer's, so my hats off to them.
 
Top