Solid state electromagnetic recovery system   

Introduction
This recovery system is entirely based on my previously developed high-voltage pin puller system described here.
In this system a pin was sucked into the core of an electromagnetic coil. This system was very successful and deployed fool-proof. The only disadvantage in my opinion is that there are still moving parts - the pin - that may be difficult to design and build.
This new system gets rid of all moving parts and is based on the functionality of the explosive bolt.
The explosive bolt is in principle a hollow bolt sealed with a screwable cover and filled with an explosive charge. A V-shaped notch is machined in the centre of the bolt. Upon ignition of the explosive charge, the bolt will fracture at the location of the notch and both parts of the bolt will fly away with considerable velocity. This makes this type of bolt very well suited for applications such as the release of a parachute hatch or nose cone. Explosive bolts have been used since the very first space flights.


This is a picture of one of four explosive bolts used to connect the launch escape tower to the Apollo 13 Command Module.

In this deployment system I do not use any explosives or pyro devices (safety !!).
The explosive bolts are simulated by reversing the current through an electromagnet. In the electromagnetic pin-puller I used a coil from a normal relay with the ferrite core removed. In this design I use the coil with the iron core completely as it is. Just pull the complete coil + core from any 12V relay.
Take a small permanent magnet and attach it to the core of the relay coil. It will be attached to this core very strongly - depending on the force of the magnet. Try to find some small "super magnets" as used in a cat collar.

The larger the magnetic attraction force, the bigger the repulsion force. When a high-voltage (flash) capacitor is discharged over the coil, a strong magnetic field is generated by the current flowing through the coil. When this magnetic field has the opposite polarity of the permanent magnet, it will be thrown off the ferromagnetic core of the coil. Two magnets with reverse polarity will push away from each other: attraction turns into repulsion. Voila, that is the whole theory behind this new recovery system.
Performing initial tests on this system, I was quite surprised that even a very powerful magnet as used eg in cats collars, was shot off about 1 meter from the coil when discharging even a small 10 MicroFarad capacitor charged to about 300 Volts.



A lot of theory on solenoids and magnetic force can be found on the internet, eg here and here.
Building this recovery system

  • 0. First you will need to convert a disposable flash camera into a high voltage charger as described here.
  • 1. Get 2 12V relays and disassemble them so that you are left with the coil + core only.
    Disassemble a relay
  • 2. Wrap some insulation tape around the coils to protect the wire windings. Do this immediately since the coil windings get damaged quickly when handling the coils.
  • 3. Glue the 2 coils into a wooden base, diameter = nose cone diameter. In my case this is about 10 cm, a little less than the inner diameter of the bottles I use. Attach some fixation point for the parachute at the top and a fixation point for the rocket at the bottom. In this case the base is connected to the rocket with a soda bottle top.
    Wooden nosecone base
  • 4. Connect the 2 coils in parallel to the circuitry described here.
    The schematic diagram is the same as for the pin puller. The only difference is that now there are 2 coils connected in parallel.
    Electronics Electronics Electronics
    For the inertial switch I used a simple single pole toggle switch with a weight attached to the handle.

    Before launch the handle is placed upward, the switch is open. After the launch shock the switch handle has moved downward because of the big acceleration forces and the switch is closed. The small timing capacitor starts charging from the big bulk flash capacitor and the potmeter in series with the 2M2 resistor form the timing chain. The delay can be adjusted between 2 - 10 seconds. This is enough for my rockets to reach apogee.
  • 5. Determine the polarity of the coils with respect to the 2 permanent magnets. Take one magnet and mark one side (top/bottom) with a pencil, marker or something. Attach the magnet to the coil core with the marked side. Fire the circuit. When the magnet is pushed off the coil, the polarity is OK. Otherwise reverse the 2 wires to the coil. Do the same test with the same magnet and the other coil. Now take the second magnet. Attach it to a coil and fire the circuit. When it is pushed off, mark the bottom side of this magnet. This way you can make sure that the polarity of the 2 magnets is the same. In other words, it doesn't matter which magnet you attach to which coil anymore. This is just to make sure that the nose cone can be placed either way on the 2 coils.
  • 6. Place the nose cone on the wooden base and carefully mark the position of the 2 coils. Glue the magnets in the nose cone so that they can be attached to the coils in the wooden base plate.
    Nose cone with magnets
  • 7. Add a PET skirt to the wooden electronics base. This will seal the gap between the wooden platform and the rocket body. I used 2 M3 screws that at the same time serve as the positive and negative connection terminals for my high voltage charger. Note that the electronic components on the PCB are at the bottom side of the nose cone base.
    The rocket on which the base is mounted here is a 2 x 1.5 liter Robinson coupled pressure vessel with 3 lexopan fins.
    Nose cone base + PET skirt Base on top of rocket
  • 8. When necessary, add some foam to the nose cone in order to help the compressed parachute to push off the nose cone. A PET "spring" could also be used here.

    Launching

  • 1.Wrap the chute in the nose cone.
  • 2.Attach the nose cone to the wooden base with the 2 magnets. Compress the parachute + foam gently. This will provide the necessary upward force to push the nose cone off once the magnets are detached when the on-board timer fires the deployment coils.
  • 3.Make sure the inertial switch is "up" = OFF.
  • 4.Charge the on-board flash capacitor with your HV charging unit until the LED from the original disposable camera lights up, indicating full charge. Take the necessary safety precautions or this will be a shocking experience! Use both hands to operate both push buttons !
  • 5.Check if the rocket is retained on the launcher by the U-pin or clark-gable system. This may sound ridiculous, but I have forgotten this step more than once.
  • 6.Pressurize the rocket.
  • 7.Launch and wait until the nose cone separates and the chute is deployed...
  • 8.Start walking and retrieve your rocket or better yet: send someone from the launch crew over to go and get it.
  • 9.Sit back and enjoy your favourite regional beer while your launch crew prepares everything for the next launch.



    First tests : success !!!Click here to see the results!


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