Magnetic Deployment Device

Content was originally published here under CHUTES  section in July 2009 and was transferred to its own page on June 13, 2011

My magnet powered deployment device

After experimenting and tinkering with different designs I am going to finally release what I believe is an original and reliable device that will forcibly separate the nose cone and eject the parachute. It will function when the rocket makes a nearly vertical flight and begins to decelerate, perhaps a little early for achieving maximum altitude but that may be adjustable by adding a short delay to the release mechanism. More importantly it will function as soon as the rocket turns horizontal or worse yet when it begins to nose dive. A magnet being attracted to another magnet is used to power the device as soon as the magnetic attraction is stronger than the force of gravity pulling on the weight of the magnet. The chance discovery of an unusual magnet gave me the idea for its use for the deployment device.

magnetic spheres
magnetic spheres

I spotted these in a clearance bin of toys at a small grocery store of all places, four pair for a dollar! They are called “Imaginetix“, the package lists the company as Toy Box Unlimited, Indianola, IA 50125, and of course they are made in China. There was also a version made in an oblong shape. They are very strong magnets and will jump to each other from several inches apart. That is what gave me the idea to power the chute release. If you can’t find these at a toy store or “dollar” store, these may also be sold as “rattlesnake eggs” at a chain of imported tool stores. I saw the ads for them but never went to find them in the store. If you hold them in your hand separated and toss them gently into the air they jump together and bounce many times while making a high pitched vibrating rattle sound.

They are 3/4 inch in diameter and fit perfectly loose inside 3/4 inch SCH-40 PVC pipe. They look and sound like acrylic plastic so I assumed that a magnet must be embedded inside the plastic ball. However, when I tried drilling a hole in one to attach a small metal rod for a mechanism I was considering, drilling barely made a dent! Alien technology? The entire ball seems to be a magnet. Maybe a diamond bit would work. Super glue attaching the rod failed when the ball made strong impacts with the other magnets. The idea of attaching a metal rod was abandoned for a different and better design.

How the magnet operates the deployment device

If you place a magnet on a table and then slowly lower another magnet over it, at some point the magnetic attraction will be stronger than gravity on it and it will jump up to the other magnet. Take note of the shortest distance between them before the jump. The magnets must be kept separated at least this distance in the deployment device. This is how they will be when the rocket is vertical. In my device the magnetic ball sits inside a PVC tube at the bottom and a pair of more ordinary rectangular magnets are fastened across the top of the tube. The magnetic ball is not attracted enough to jump when the tube is vertical but when you tilt the tube toward horizontal the ball will jump with a very forceful impact. It is much stronger than just the pull of the magnet alone. This impact will be used to operate the release mechanism.

After trying several mechanisms that could be triggered by this impact, I found that they were getting complicated and hard to set up especially while the rocket was mounted on the launcher. This also made them unpredictable when actually launched and fragile when they hit the ground in a nosedive. They were also getting heavy.  My current design which I will describe is also heavy but probably could be made lighter. My personal goal is not to set altitude records but rather to make a reliable device that will perform predictably over and over again making launching fun without the agony of watching your work smash into pieces on impact.

The  simplest method to use this magnetic impact that I’ve tried so far is to drill two small holes in the PVC tube 180 degrees apart about 3/4 inch below the fixed top magnets. The magnetic attraction is extremely strong at this distance yet leaves enough movement for our purpose. A piece of nylon fishing line is passed through these holes and the ends are tied to the catches that secure the nose cone in place. The line must be taut when the catches are holding down the nose cone or attached to whatever you are using to deploy the chute. The magnetic ball is placed in the bottom of the tube below the line and something must then be put on to keep the ball in the tube. I just drilled two small holes at the very bottom of the tube and tied string tightly between them. The PVC tube is securely mounted vertically within the rocket.  Either at deceleration or turning the tube horizontal the magnetic ball will jump toward the fixed magnets. When it does it will hit the fishing line across the tube and pull it along to the end of the tube to the other magnets or as far as the line can be pulled. The catches are pulled open and there goes your parachute! The two rectangular magnets built into the top of the tube have holes in their centers. Resetting is as simple as pushing the magnetic ball back to the bottom of the tube using a small wooden or plastic rod through the hole in the top magnets.

 Details of the Deployment Section

Here are photos and explanations of my prototype.

Top View

mech_sect_top_detail

  1.  Hold down catches are made from cable ties. Be sure to use ones that have a square edge on the head facing the tail. Beveled edges won’t hold. File them slightly if necessary. A small piece of cable tie is put through the head to make it easier to grip them when locking on the nose cone.
  2. Short pieces of vinyl blind are used as guides to align this section with the nose cone.
  3. Short green curved pieces of bottle sides are wedged in place to help keep the cable ties straight up and are very important in keeping the rocket sections aligned under the tension from the elastic that separates the nose cone. Otherwise the sections can distort and cause unexpected nose cone ejection.
  4. Nylon fishing line running from the cable tie catches through the PVC tube of the magnetic device described above.
  5. Small hole in the center of the 1/8th inch plywood disk. It aligns with the magnets in the PVC tube that is glued beneath the plywood. A small wood or plastic rod is pushed through here to reset the magnet.
  6. Elastic band that helps to hold the plywood disk in place. It is also used to attach the chute. A Velcro strap goes around the elastic band to attach the parachute’s ring.

Bottom View

mech_sect_bottom_detail

  1. PVC tube with magnetic ball visible inside.
  2. High density foam ring around PVC tube for cushioning against the pressurized bottle’s bottom. It’s cut from a pool toy ‘noodle’.
  3. Wide ring cut from the top of a sour cream container.  A little over an inch of the side is left besides the heavy rim. UPDATE: During testing this section tended to pull off of the bottle when the main chute opened. That left the rest of the rocket dangling below this section on a safety line connecting them. Either this section will have to be taped to the bottle, or the container ring will be eliminated in favor of a much longer piece of bottle than the 4 inches used here so that it can be forced directly over the bottle end.
  4. Heads from cable ties placed over the tails of the ones used for catches to keep them from being pulled out of place.
  5. The elastic band seen in the top view. It passes around the pieces of blind to help hold them in place.
  6. One inch rings cut from 4 inch PVC drain pipe. They are force fit inside the top edge of the bottle skin of this section. One ring has a very small part cut out of it to make it slightly smaller in diameter. It is wedged inside the other and offset a little to allow the plywood disk to be recessed inside them. They are pop riveted together to secure the cable tie catches in place.

Side View

mech_sect_side_detail

  1. Cable tie catch.
  2. Pop rivets holding PVC rings together with cable ties sandwiched between them. Notice the downward offset of the inner PVC ring.
  3. Velcro strap for attaching parachute to elastic band.
  4. Piece of vinyl blind used for alignment.
  5. The elastic band around  the piece of blind.
  6. The magnet tube.
  7. Foam ring around magnet tube.

The containers used in this project.

containers

On the left is an 8 ounce yogurt container. It was chosen for the main parachute’s container because its size fits within the rest of the device and its heavy taper keeps the chute from hanging up inside.

On the right is a 16 ounce sour cream container. They can be forced over the bottom end of 2 liter bottles and sections cut from the bottles can be pushed over their tapered sides. I’ve used these containers for temporary nose cones while testing and they survived low altitude crashes even onto concrete. UPDATE: The main reason this container was used is that it is easier and faster to attach to different bottles than forcing a tube cut from a bottle over another bottle. That was good while testing and carrying the rocket to the launch site. The drawback is that it does not hold on as well under stress and probably should not be used in your final model.

The Nose Cone

Bottom View

nose_bottom

The bottom four inches of a bottle’s straight section was used for the deployment module. The remaining top of the bottle was used for the nose cone.

A one inch section of 4 inch PVC is wedged into the top part of the bottle so it mates with the other module below it. The PVC fits well and makes the cut edges of bottle sturdy. The PVC ring has four notches in the lower edge for the black elastic band. Four small holes are drilled in the PVC for the ends of the elastic to pass through and be knotted on the inside.

A piece of smooth, thin material is hemmed on two opposite sides so that elastic can be passed through the hems. A neat hole is punched in the center of the material for a string to pass through. Two equal lengths of strong elastic are passed through the hems and the ends are knotted through the PVC. This creates a little trampoline inside the nose cone.

The orange string is attached to the bottle cap in the nose, passes through the hole in the trampoline and ties to the bottom of a yogurt cup. The string should be long enough for the yogurt cup to be away from the nose cone but not too far.

Top Viewnose_top

Attaching the nose cone chute

nose_with_chute

I’m using an 18 inch chute just for the nose cone. It probably isn’t necessary because it floats down fairly gently on its own, but it also serves as a drogue chute for the main one in case the main one should hang up inside the yogurt cup. Fold it in half, then in half again and wrap the lines around it. The rubber band for attaching it is passed over the yogurt cup to loosely fit around the orange string.

Putting the nose cone together

nose_chute_packed

Set the nose cone over the yogurt cup, carefully centering the trampoline. The folded chute should be evenly covered and as flat as possible.

Where the catches will hold the nose cone down, small holes must be made in the bottle top just above the PVC ring. You can see one at the very front in the picture above where the white ring shows through the green bottle. The catches actually hook over the top edge of the PVC ring.

Preparing For Launch

mech_section_chute

The deployment module would already have been installed on the rocket and the rocket would be on the launcher ready to be pressurized. Be sure to reset the magnet to the bottom of its tube after mounting to your launcher!

Attach the parachute to the top of the module.

I use heavy rubber bands looped through the parachute lines to absorb shock and fasten them to the rocket with Velcro bands.

mech_section_chute_ready

Roll up the chute and lines

Straighten out the chute and lines as usual. Roll it into a tight cylinder and neatly wrap the lines around it. Stand the rolled chute on end with the lines kept rolled around it. If a yogurt cup won’t fit easily over it you will have to redo it until it does. That is a 36 inch parachute in the picture and it fits. That may be larger than needed but this is a heavy device.

Installing the nose cone

mech_section_nose_attached

Pick up the nose cone assembly keeping it even inside and set its yogurt cup carefully over the main chute. Don’t let any lines slip out. The cup must rest evenly on the plywood top and be centered. If the cup bulges too much it may interfere with the lines that pull the catches open and it won’t let the nose cone lock down safely. If it does, pack the main chute tighter and smaller.

Line up the two holes in the side of the nose cone with the cable tie catches. Lower the nose cone until it fits flush with the top of the other module and the catches should snap out into place. Working with it a few times will make it easier. The catches may need adjustment to get it to work evenly.

The two modules should of course stay together when you let go. It should also not eject when shaken a little as in launching.

How it deploys the chute

When the catches release, the trampoline forces the yogurt cup out of the nose cone, propelling it away from the rest of the rocket. In indoor tests the nose cone is ejected sometimes almost two yards when the rocket is held horizontally but usually less than a foot when vertical. The main chute’s lines spiral out of the cup first as it pulls away, then the chute material unfurls already at the end of its lines. The drogue chute on the nose cone should also help it to pull away from the main rocket.

rocket_complete

This rocket was test launched September 9, 2009. It flew straight but at a low altitude. It is heavy with the new mechanism installed. The chutes deployed well and carried the rocket gently to the ground unharmed. The mechanism obviously ejected the nose cone when deceleration began. The nose cone continued some distance higher before its chute slowed it down. The main chute seemed to open before the rocket had a chance to turn over. I am satisfied that the mechanism will deploy the chute under most any condition but there needs to be a way to delay it slightly closer to apogee. And making it lighter would definitely be a plus.

Revision 1 : posted September 23, 2009

The deployment section was taken apart for modifications with the goals of :

  • Eliminating the container ring shown in the first photos by using a longer section of bottle to hold this section tightly on the rocket.
  • Making it lighter in weight for better performance and altitude by removing unnecessary materials where possible.
  • Modifying the PVC tube that holds the magnetic ball so that it will be possible to prevent early deployment caused by sudden deceleration and yet make sure it still forces deployment whenever the rocket turns horizontal or nose down.

The first goal was the easiest to achieve. The four inch section of bottle was replaced with a six inch section.

For the second goal, most of the inner PVC ring was removed where it was not needed either for support or strength. Holes were drilled in the 1/8 th inch plywood top to lighten it a tiny bit. More may be drilled out later. Some of the PVC tube for the magnet was drilled and cut out to modify it for the third goal and that lightened it a bit as well.

The third goal was possibly reached as well. Only testing will tell how reliable it is. Two slots, 180 degrees apart, were cut out of the lower part of the tube. Two shortened cable ties were fastened over the slots with their heads pointing inside the tube.  They move out of the way when the ball passes through the tube. However, when a loop of string is wrapped around the tube at the slotted part, they can’t move out of the way because the string forces them down. The ball is trapped at the bottom of the tube and can’t jump up when deceleration tries to make it jump. If somehow the string can be held under just a little tension until apogee, either by timing or another device, I believe this will work! It must also release tension on the string should the flight go wrong and start to dive. As long as the magnetic ball can move, the rest of the device should take care of itself should that happen.

Here are some photos taken after the first revision.

R1_bottom

R1_top

R1_ring_top

R1_ring_side1

R1_ring_side2

R1_ring_bottom

The pieces of vinyl mini blind are not in place in the above photos but they are still important. They are wedged in place between the PVC ring and the plywood disc with the black elastic band looped over their lower ends.

R1_tube_bottom

The black string with the tiny white beads passes through the tube and the ends are secured to the cable ties that serve as the catches for the nose cone.

R1_tube_side1

R1_tube_side2

The short cable ties are held on to the tube by a very heavy white rubber band and a cable tie. After testing they may be hot glued to the rest of the glue holding the tube to the plywood.

R1_tube_side1_tied

R1_tube_side2_tied

The two photos above show a red string around the slotted part of the tube. When a loop of string is held tight around this place, the cable tie heads are blocking the magnetic ball from rising in the tube. Here the ball is shown in position as it would be at launch.

Now on to finding a way to release tension on the string at the right time in a normal flight, as well as letting go in time in emergencies to prevent a crash from failed deployment.

This is the first thing I’m thinking about trying to time the release. Wrap a string twice around the tube and tie it tightly. Slide it over the slotted area so that it will be looser, forming one tight loop and another loose one. Attach a thin rubber band to the looser loop of string and run the other end of the band through a small hole in the outer skin of this module. Then place a small stick or something similar through that end of the band. The band and string should be slack enough now to not cause the tube to be blocked by the cable ties. If you wind the small stick like you would wind the prop on a toy rubber band powered airplane that should provide the tension needed to block the tube during deceleration. The wound-up stick would be released at launch and would hopefully unwind enough to release the tension in time. Maybe a little friction could be used to slow down the unwinding if necessary. Possibly a small fin could be attached to the stick so that the wind current would slow the unwinding down until the rocket lost most of its speed.

My second idea would be to use a small air flap to hold tension on the rubber band while the rocket is at full speed but that would not work if the rocket flew in a low arc and never really slowed down until impact. That would be really bad!

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