"Show me a man who wants to cluster model rocket motors and I'll show you a man with a poor understanding of the mathematics of reliability."

"Show me a man who wants to air-start a cluster of model rocket motors and I'll show you a man who likes watching rockets crash."

Still, you can't reason someone out of a position they didn't reason their way into. A couple of friends are doing clustering, both in the air and from the ground. As such I felt compelled to help minimise the INSANELY HUGE chance of failure.

In my experience, the principle failure mode of cluster ignitions is one igniter firing, then going short circuit and pulling so much current that the power source can't fire the other igniters. If you are doing this on a 9V battery then you're really asking for it... How much current do igniters draw? Several lifetimes ago I made a study of it.

I recently built a few launch controllers. As part of this I tested some FirstFire igniters using the same methodology. They pull about 12A for 100ms (or so) when you flick the switch. This current drops quicky as the bridgewire fuses and you're just pushing current through the plasma. But 12A will rape a 9V battery!

What's needed is a way to regulate the current to each igniter such that one igniter can't draw enough current to stop the others going off. I like Smart Fets They limit temperature, current and lots of other things...

The circuit is dead simple:


So's the board:



BTW, this was all done late last night about halfway into a bottle of red. The focus looked fine at the time....

The battery I'm using here is a 3 cell Lithium Polymer 900mAH jobbie. It's rated for a continuous discharge of about 12A, but it'll put out much more than that for a few hundred ms. 900mAH is way to big for this app, but smaller batteries can't put out as much current.
If it were me I'd go to a larger cell, but my brief was to keep the weight down, this pack weighs only slightly more than a 9V battery..

Anyhow... the test

PK

To answer your question, yes, that does look alright. Very nifty stuff.

Now I'm curious about what sort of rocket needs to worry about weight when 4 motors are used in an airstart config though.

Excellent work!! now I don't know anything about who you are making this for, except I heard down the grape vine it only needed to start 3 motors (Must have been the wine) So whoever this is for should have an even greater probability of successfully air starting (assuming it is for air ) the 3 motors that they were chasing cases for...

Alright... maybe I do know something, and should this all work correctly, then it will complete the 2nd of 3 phases for air starts. Perhaps something may travel to WA in the near future

All you interested QRS ppl can see this in action next Sunday.

Scoop may yet drink enough to make a 4 motor airstart seem like a good idea..

Hic..... .......Hic.......Ummmm 4 you say?? Hmmmmm......Hic....parshme teh red wines again.......Hic

Airstart ya say........hmmm sounds like a good project!

Now where did I put those 38mm tubes????

Update. It's been pointed out that the original circuit was a bit sensitive and could fire on the continuity checks of some electronics. I've updated the schematic it now needs about 30mA of drive to turn the FETS on.

PK

Got a phone call from a launch site about an hour ago. Judging by the excited tones and million mile an hour dialog, I'm guessing it worked

Two changes I will make to subsequent versions is to replace all the 470 ohm pull downs with a 50 ohm 2W resistor (apparently the timer still indicated failed continuity) and add an LED that comes on when it fires for testing purposes.

Worked perfectly more to the point

This will be my quick report on the event, need to download the pics and get some other peoples shots too which I will post soon enough.

For those who don't know, thanks to help of PK's electronic wisdom and Robert from G-Wiz goodies, I managed to successfully launch a scratch built 2 stage cluster which consisted of an I211 in the booster and 3 x G64 in the upper stage.

The booster contained a G-Wiz LCX which activated PK's smart fet board firing 3 First Fire Jrs. In the ground testing, this fired them with ease, however due to the layout of my booster avionics bay, the FFJ leads were a little short and I extended them by about 15cm with some excess igniter wire, this proved to be no issue at all. The thrust from the G motors also performed the separation from the booster.

A few seconds after separation, the LCX deployed due to either apogee or altitude (yet to be confirmed when I get the data) as I had both connected to the same ejection charge. The booster flew to ~800' and was recovered approx 100m from the pad.

The upper stage continued to ~3000' (also to be confirmed) with successful apogee deployment by a G-Wiz HCX. I purposely used the HCX in the upper stage so it would record all of the flight and not just the booster. A 40" chute was used and was a little big for the day as the wind carried it to approx 700m away. It did have a very gentle landing though.

Now that I have completed phase 2 of 3 for my clustering/staging/air starts, it is time to hit the drawing board for the final phase which should certainly be a crowd pleaser As for the current rocket, the upper stage shall be put in semi retirement having completed 2 of the 3 phases successfully. The booster however will receive some extra strengthening treatment and get ready for some hard flights in WA.

That sounds great kopius! I'm really looking forward to the photos/videos of this one.

Were you watching the booster after separation? I had my eye on it and thought it tumbled a fair way before the chute deployed. I was actually really impressed with the way it fell and deployed. Fantastic flight

Finally just put together a video with the testing of the smartfet board and also a few pics of components.

Gotta love Top Gun theme!!

http://www.ausrocketry.com/kopius/clustard2.wmv

That nose cone has been successfully used for a few purposes now and fingers crossed this next QRS launch I will have the GPS module running in it.

Very nice!

Very impressive kopius! That was one sweet flight. How was the sustainer secured to the booster? Did it just sit in the PVC pipe?

Awesome stuff Kopius.


Here's one I did earlier :

http://www.members.optusnet.com.au/~cst ... launch.mpg

Solid to airstart hybrid.

As per yours the staging electronics were in the booster.

CS

Was just lightly friction fitted. The ignition of the cluster separated the booster as per the nice black stain on the last photo.