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Recovery Systems

The recovery system is going to save my rocket from going crash and smash into the ground, so it is worth designing properly. There are two main elements; the parachute and the release mechanism
The parachute must be:
  • big enough to slow the rocket's descent
  • small enough to pack inside a release mechanism
  • lightweight to reduce payload
  • strong to avoid breaking
The release system must be:
  • reliable, ejecting the parachute at the right moment to let it open before the rocket hits the ground.
  • lightweight.

Replacement Parachute module

posted 3 Jan 2017, 06:04 by Andy Lakin-Hall   [ updated 10 Jan 2017, 05:52 ]

In my last big crash my side-releasing parachute module was destroyed, so it's time to build a new one.

I had previously built a new internal mounting from corriflute, and experimented with arduino and a Wii Nunchuck to control the servo that will release the parachute.

The measurements for this inner mounting were taken from the previous module and it is basically a V-shape mounted between two circles of corriflute.

The V has three sections.

Inside the V is where the parachute is packed, along with a spring to eject it once the bay door is released.

One side of the V is where the servo is mounted. Roughly central, the servo arm in the lock position holds an elastic band that retains the parachute bay door closed. Once the servo arm moves to the open position, the elastic band is released and the bay door can pop open.

The other arm of the V is the mounting position for the electronics. For this module, I have decided to use a BBC micro:bit. This small microcontroller includes everything I need for a system to detect apogee and release the parachute. I've posted the code I created for this under my Other Projects section.

The module requires a shell so it can be mounted on a water rocket. I normally use 2 litre PET bottles, so my module needs to be compatible.

Here's the build process, starting with a bottle.

The bottle has been cleaned to remove all the glue from the label. The next thing to do is cut off the bottom. I have a jig to make sure the cut line is marked totally straight.

The cut end will be the top of the shell, and needs curling over to strengthen the resulting cylinder and help it retain its proper circular profile.

To curl the plastic, it needs gently heating over a frying pan whilst being held upright. To achieve this, I have a curling former made of cardboard. This holds the bottle correctly while it is heated and twisted to make the curl.

Here's a video of me doing that.

When the end is curled it looks like this.

 

That's the first part of the shell made. Next for the end spacer. This is made from a lid from a tub of drinking chocolate. I discovered these are very close to the internal diameter of the bottle.

The centre of the lid needs cutting out carefully with a compass cutter.

The ring now needs gluing to the inner mount with a bead of Evo Stick.
The glue needs about 10 minutes to cure before the parts are out together and then pushed into the shell. You can see how the spacer gives a couple of centimetres clearance at the top, which is enough for the door flap and fitting for a nose section to the rocket.

Next job is to mark out where the door needs to be cut. This needs to be as vide as the V, but just a little bit smaller that the length. The restricting factor is the door flap, which needs to cover the hole, but still leave space for the module to be mounted to the pressure vessel below and the nose section above.

Finally the door section is cut from a second bottle. The ridges in the plastic for the door are lined up with the ridges on the module, and then the flap is marked out and cut before being taped in position.

The module is now ready for finishing with the electronics.

The micro:bit needs a 3v supply, but the Servo needs 4.5v. 

I need to pinch the lead and connector from the 3v box, and fit it inside the 4.5v box.

The 3v micro:bit battery box comes apart easily.

Here the 3v lines for the micro:bit are soldered in place inside the 4.5v battery box.

Here the Servo is mounted on the parachute module internal support.

Velcro pad on the top of the module to hold the battery box in place.

The battery box is mounted to the top of the parachute module.

The micro:bit is held in place with Velcro. Leads are soldered to the tags for the Servo and the piezo speaker.

The inner module is fitted inside the outer shell. The elastic band retaining the door closed is locked in place by the Servo arm. When the Servo moves left, the elastic band is released, allowing the parachute door to pop open.


What's wrong with my Radio Control system?

posted 12 May 2013, 01:04 by Andy Lakin-Hall   [ updated 12 May 2013, 06:13 ]

The RC system is an essential part of the recovery system on LS08. Last time out it worked, but behaved erratically. Right now it won't work at all.

What's gone wrong?

The system has essentially three parts - the receiver, the servos and the battery. I suspect that the NiCad battery pack might be the cause, but I need to eliminate other options as well.

Servo Check

To test the servos away from the rest of the RC system, I am using a PICAXE microcontroller which I had previously configured to drive a servo.

PICAXE 10 microcontroller driving a servo.

This circuit was designed with a shake sensor. The idea being that the circuit would detect the movement of launch, then operate the servo after a defined delay to release the parachute. Unfortunately the shake sensor was far too sensitive, and the movement of the rocket as it was being pressurised and then slightly moved by the wind continually triggered the servo movements, ejecting the parachute before launch.

The microcontroller still works as programmed, and makes an effective servo tester. 

Connecting any standard servo to the adapter and powering on the circuit, any motion of the circuit board is sensed by the motion sensor, causing the servo of move back and forward.

I have used this circuit to check the small servo from the rocket recovery pod, and it operates correctly.
The problem with the RC receiver system is therefore elsewhere.

RC Receiver Checking

The radio control receiver I have is an Acoms 27MHz AM 2-channel unit. The instruction manual doesn't show the connections properly, but I have worked out how it should be. Servos connect to the bottom plugs with the yellow Signal lead left, Red positive power in the middle and black negative power to the right. The upper power plug has three pins, but the left plug is not connected.

Acoms 27MHz AM 2ch Receiver Connections

I have improvised a battery connection using a 3xAA pack and a cable from an old PC HDD indicator LED with a header connector attached. The little header connector fits straight in the BATT socket of the receiver. Ideally this would be a 3-pin header to ensure correct polarity but, as this is cobbled from my bits box, the 2-pin connector will do.

Here is the full setup:

Battery and servo connected to Acoms 27MHz AM Receiver

Running under battery power like this the servo operates just as it should, so the problem must be with the NiCad battery pack.

NiCad Battery Checking

The NiCad battery pack should be giving 4.8V, but on the multimeter it only reads 2.76. This is a bit of a problem.

Checking the voltage from my NiCad battery.

For some reason the pack is not charging properly, so next I need to check that it is connected properly. The polarity of the connection is shown below:


Connecting this to the charger which I used previously, I'd like to try leaving the battery pack on slow charge for 10 hours. Normally I'd do this overnight, but as I'm not sure about the thing getting hot and possibly causing a fire I will play it safe and just give it 2 hours under close supervision.

After 2 hours, the battery pack is still cool to touch. The voltage still reads about the same, so the thing isn't charging at all. I know the charger is working - the metre reads 10.4 volts from the charger. The charger does have an indicator LED, which is not illuminating, so all I can assume is my NiCad pack is done for.

Conclusion

So, the NiCad pack is the problem. I'm still not sure why it won't charge - I'll have to keep experimenting with that. In the meantime, the rest of the system works perfectly well, so I can try replacing the NiCad pack with the 3xAA battery. I'll need to compare the relative weight so I don't make it too heavy, but I should be up and ready to launch again soon.



Side Ejecting Recovery Pod

posted 21 Apr 2013, 14:08 by Andy Lakin-Hall   [ updated 21 Apr 2013, 14:09 ]

Finally I have had an opportunity to build a new recovery pod. This one based on this Mk2 Side Deployment Mechanism by Air Command.

Using the corriflute mount I built last year, I have mounted my micro servo and a switch to one side of this.

I have cut a section from a smooth PET bottle and fixed this to the corriflute to make the ejection plate.

Here is the top view, showing the mounting of the Ni-Cad battery pack and the RC Receiver unit inside. Both of these are held in place with Velcro pads so they can be removed if necessary. The purple lid from the Drinking Chocolate tub formats a great bulkhead section where these can be mounted.


The entire rocket is assembled as shown below.

The Possibilities of a Drinking Chocolate Tin

posted 8 Apr 2011, 02:06 by Andy Lakin-Hall   [ updated 8 Apr 2011, 04:29 ]


I have discovered that the cardboard container that drinking chocolate comes in is pretty close to the diameter of a 2 litre PET bottle, so I thought that this might be a useful source of payload pods for rockets.

 

Most particularly of interest at the moment is the construction of a second parachute release mechanism to accommodate the new 600mm parachute I built the other week. The tin is a rigid cardboard tube with a thin steel base to which things might be bolted. After digging out my vernier caliper I got the measurements...

Internal Diameter93.5mm
External Diameter95.5mm
Base plate internal diameter92.1mm 
Base plate external diameter    96.7mm 
Height130.9mm
Depth of base recess3.3mm 

Now my first plan is to use one of these tins to make a side opening release. I had a go at one of these before, but without much success. I found that accurately cutting the slippery PET was too tricky. I'm hoping that the cardboard will be easier and allow me to experiment and repeat accurate cuts. Here's hoping.

Construction

I've cut two circles out of a sheet of junk corriflute to fit exactly inside the tin, that's as close to 93.5mm as it is possible to cut this stuff. Then cut a rectangle to 112mm high and 140mm wide. I scored a line down the centre, and folded this to give a pair of joined panels 120mm x 70mm. I glued these to the circles top and bottom with contact adhesive.


Mini-Chute

posted 2 Apr 2011, 03:14 by Andy Lakin-Hall   [ updated 2 Apr 2011, 04:30 ]

With the extra ripstop I have left over I have decided to make an extra mini-chute. 
With my separating nose-cone design, the rocket returns to ground in two pieces. I have been using a small parachute which I bought from a model pyro-rocket supplier. It's just a circle of fabric, about 350mm in diameter. I'm going to make a hemispherical mini-chute of similar diameter, just because I can.

Here are the gore measurements using this useful javascript page from Arthur Dibble.

 
Point Height Half Gore Width (with seam)
0 5 74
1 19 74
2 32 73
3 46 72
4 60 70
5 74 68
6 87 66
7 101 64
8 115 61
9 129 57
10 142 54
11 156 50
12 170 45
13 184 41
14 197 36
15 211 31
16 225 26
17 239 21
18 252 16
19 266 10
20 280 5

Two new parachutes

posted 30 Mar 2011, 06:41 by Andy Lakin-Hall   [ updated 3 Apr 2011, 13:24 ]

The Nylon rip-stop material I got is really easy to work with, and there's heaps of it too, so I have decided to build two new parachutes - one at 400mm diameter and another at 600mm.
I already had both gore templates made up so building the 400mm parachute was really quick, only taking two afternoons to finish.
Here is a picture of the twelve 600mm gores cut out and ready for stitching, along with the cardboard template I used to mark them out.

Each gore has an extra 5mm around the edge to allow for stitching.
Here is a close-up showing the stitching around the edge and at the seam. I have stitched the 13mm White Seam Binding Tape to the edge to prevent any fraying. The rip-stop doesn't fray easily, but the binding will keep the outside edge from becoming tatty.
To strengthen the apex of the parachute I have made a little circle of ripstop and stitched the binding around the edge. This is stitched in place at the top.


I still have my spool of braided Dacron line from Bristol Kitestore. I need to cut six lengths of cord, which will make twelve shroud lines; one for each gore.

Once again, following Richard Nakka, the length of each of these six cords is given by 
L = 2 .25 * ( D + S ) where D = basic diameter of parachute, S = stitching length. 
The stitching length will be 80mm in this case, so each shroud line should be 2.25 * 608, which is 1368mm, so lets round that to 140cm, which will allow for a couple of centimetres leeway with my wonky stitching.


The shrouds are all stitched to the seams of the parachute at just one end. Then all six lines are threaded through a length of 5mm heatshrink tubing. Then the other end of each line is stitched to the opposite edge of the canopy.

The heatshrink tubing prevents wear and tear on the shroud lines and makes a neat little loop once a larger piece of 7mm heatshrink is slid over the top.

Here are my two new parachutes, ready for attaching to rockets.




Time for a new Parachute

posted 18 Mar 2011, 09:24 by Andy Lakin-Hall   [ updated 18 Mar 2011, 09:31 ]

I've discovered my vinyl tablecloth parachute is full of holes. Probably from landing in a gorse bush.
I have decided to make a new one based on the 400mm gore template, but this time using proper ripstop nylon.
After a quick google, I came across Fabrics-n-Stuff, who are now supplying me with 2metres of Lightweight Porcher Kite/Paragliding Ripstop Fabric Flo Yellow at £7.90 and a 50 metre Roll of 13mm White Seam Binding Tape for £2.49. 
Hopefully that will come next week sometime and I can get building straight away.
I still have lots of the braided cord from Bristol Kitestore, so I can make the shrouds, and I have some heat-shrink tubing in my soldering kit, so I won't need anything else except a ball-point sewing machine needle. It might be worth seeing if I can borrow a sewing machine that will do zig-zag stitch - mine only does straight lines.

400mm Parachute

posted 31 Mar 2010, 03:17 by Andy Lakin-Hall   [ updated 31 Mar 2010, 03:21 ]

I've spent some days building a new 400mm parachute.
The steps are documented on my Parachute Design pages.
It's made from plastic tablecloths, which fold up without creasing, and unfurl really well.
Next task is to mount it inside the release mechanism.

In-line Parachute Release

posted 20 Mar 2010, 13:23 by Andy Lakin-Hall   [ updated 9 Apr 2011, 06:37 ]

I came across an alternative design for an in-line parachute release mechanism from those clever blokes at Air Command, which looks more promising than the side release system I had been trying.

This system tucks the parachute beneath the nose cone, which is held in place by elastic bands on the horns of a servo. As my previous system worked (or didn't work) with an RC Servo, I had the expensive bits in hand already, so I though I'd give it a go.

I needed the top and body of a PET bottle to make the nose cone and servo chamber. I wrapped some card around the bottle to make a marking guide. This made sure my lines around the bottle would be drawn straight. The the top and bottom cuts were made.
To give a little strength to the nose cone and servo chamber, I needed to put a little curl on the plastic ends. I used the card as a curling guide and gently heated the ends on a hot non-stick frying pan, turning the plastic continuously.
This produced the right curl with a little gentle persuasion.

Next I cut three circles from corriflute. One to fit snugly inside the curled cylinder, the other two just smaller than the curl of the nose cone, so that the nose cone would be held in place, but not tightly. I used two smaller circles, as the single circle was not tall enough.
A servo was mounted to the bottom of the larger corriflute circle using foam card, so that the actuator horn could be mounted on the outside of the chamber, without fouling on the edges.

The nose cone had a cap glued in place, and a circle of PET fixed inside as a spring to eject the nosecone.
The glue on these was left to cure overnight.

Here is the inside of the pod, now complete with corriflute sides. You can see how the servo is mounted, along with the Battery and RC Receiver unit.

Here is the pod, mounted on a lifting body. The nose cone has been fitted, and I am using a ball of string as a fair substitute for a parachute.
The nose cone is held in place by two elastic bands - each wrapped tight to the wooden bar, and then loosely stretched to the horns of the servo.

Here is the unit in operation
(Yes I realise that's a ball of string. I didn't have the parachute finished when I made this video.)

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