Hi Everyone !
Thank you for all the supportive comments and ideas here. Nice to see an active and supportive forum. I am very familiar with the Apogee videos and some from John Coker. All very useful.
Here are some of my specific questions (all in context of dual deployment):
Are sheer pins really needed ? What type of sheer pins ?
Sheer pins only needed on the upper part of the dual deployment ?
What is a decent motor selection for the Madcow Torrent ? Apogee RocketSim indicates Aerotech H550ST - ?? s delay
What should the motor ejection charge delay be for the dual deployment assuming that the avionics will fire at apogee for the drogue chute ?
If the motor uses a delay charge - it will only deploy the drogue chute, not the main, right ?
What ejection charge power is typically used in the little ebay "cups" to eject the drogue / main chutes ?
How much powder should I use ? Is there a standard measure ?
Best way to test dual deployment ? Put it all together and use the Telemetrum to eject the charges while laying horizontally on the ground ?
Does anyone have a checklist for a dual deployment I can "borrow" ?
Thanks,
Mike
There's lots of ways to do these things, so keep that in mind as more people weigh in. These are my opinions...
I'd use shear pins on the main but not the drogue. 2 #2 nylon screws should be plenty. I have seen that people use brass plates to cut the shear pins more cleanly, but I don't have experience doing that since my dual deploys have all been fiberglass.
The motor selection will really depend on your all-up weight. If it's around the 63 oz/1700g for the stock kit, you'll want a minimum of 120-150N of initial thrust. In the Aerotech DMS line, I'd look at either the H100W (white flame/smoke) or the H219T (blue flame, low smoke). For a cert flight, I'd probably tend to the H219 for a faster kick off the pad, but that's how I usually roll. Regardless, simulate the rocket (including an all-up weight override) in OpenRocket or RockSim, put in a ~6 foot rail, and see what the speed off the rail is. You'll want ~45+ feet/s off the rail, more if it's windy. The H550ST won't overstress a well-built rocket, but it is a big kick in the pants.
The simulation will tell you what the delay should be.
If you pack the chutes the "normal" way, the motor ejection charge will only eject the drogue. That's enough to make the flight safe-ish. It's unlikely to harm anyone, though it's coming down fast enough it might break itself or something it lands on. If you're choosing where to put redundancy, put it on the drogue unless you plan to eject the main at apogee as mentioned above.
Normally you'd use FFFFg black powder for the ejection charge. For quantity, you'll need to do ground testing. Start with the amount in the motor that you're using, and adjust up and down as needed. You want the rocket to separate "with authority" so that the parachutes come out of the tube and the recovery harness is mostly or entirely extended. You don't need so much that the nose cone is hitting the end of the recovery harness hard.
My checklist is below. Note that there are very reasonable disagreements about when to test continuity with the ejection charges. I build the charges and then connect them and test continuity before I assemble the AV bay into the top and bottom of the rocket. Other people prefer to do that test with a bare ematch before building the charge. Regardless, you should have safety glasses/face shield on when you test continuity and there should be nobody in the immediate vicinity.
- Lay out parts
- Empty BP from motor
- Build ejection charges. Drogue ¼ tsp, main 3/8 tsp for primary charges
- Load motor into casing. Tape igniter to side of rocket.
- Connect flight electronics batteries
- Assemble AV bay with wires sticking out
- Attach drogue shock cord to fin can and AV bay
- Pack drogue and harness into fin can
- Fold and pack main and harness into upper airframe
- Connect GPS battery and install nose cone bulkhead
- Connect main harness to nose cone and AV bay
- Twist and tape ejection charges
- In a clear area, turn on flight electronics to check continuity. Turn off immediately
- Install AV bay in lower airframe
- Install upper airframe with retention screws
- Install motor and retention
- Take to RSO, put on the rail, raise rail to launch position
- Turn on Eggtimer Classic, arm Eggtimer Quantum
- Check continuity on both channels on both altimeters [insert beep codes]
- Insert igniter and test continuity
- LAUNCH!
- Recover, turn off altimeters
- Open nose cone and unplug GPS transmitter
