Weather Balloon payload knots?

[steamfire]

Hello experienced knot tyers, I come to you with hat in hand, having failed at creating a satisfactory knot for a strange job.

Short version:  Several medium cubical packages need to be suspended centered and level on a vertical rope, and should sustain 1/2 hour of extreme tossing/jerking in all directions.

Long version:  These boxes are amateur science experiment payloads hanging below weather balloons.  I have launched nearly 30 flights of these, and never came up with a trustworthy way to attach the boxes without covering my attempted knots with yards of duct tape.  They weigh approximately 4lbs (1.8kg) and are usually 1ft x 1ft x 1ft (.3m x .3m x .3m).  They are usually made of styrofoam (expanded polystyrene foam), often common disposable food coolers are used.  Rope is usually about 1/4in synthetic cord of some sort, that has mroe than 100lbs working load strength.

The ride up is rather leisurely and calm, from ground level the balloons rise at about 1000 feet per minute (5m/s).  The balloons burst between 60,000 ft to 120,000 ft (18km to 36km) above sea level.  The thin air provides little resistance to the parachute above 30,000ft (9km), and while the educated guesser wouldn't likely suspect, the mangled (or missing) remains have shown that the packages are slammed, smashed, yanked, twisted, spun, and whipped severely on the way down.

So the challenge is this - to develop a rigging method to hold these boxes, preferrably with the following features:
-Boxes are level and centered on the vertical rope
-Have one entire side of the box removable until the last minute, when that too should be secured (to connect the batteries before launch)
-can be tied with synthetic rope
-vertical rope must proceed on downward below to support more of the same boxes on the line.
-Having the rigging not be formed from the main rope is actually preferrable (so you can attach the box quickly to a main rope upon arrival at the launch site)

For any takers of this challenge, I will test and fly suggestions that test well, and keep you updated and post photos of the results.  I plan to share and disseminate the successful strategy to all amateur balloonists - the attachment is the bane of most launches.  If one person comes up with the complete solution, we can present it to the ballooning community named after you.

Thanks,
Dan Bowen
Amateur Balloon Scientist
spiritofknoxville.com

[roo]

Look at this link, if you haven't already:

http://notableknotindex.webs.com/boxsling.html

Then, print out the page and turn the image upside down.  The upside down version would be the starting point for how you attach the second box to the topmost box.  A series of double-ended box slings would therefore need to be made, depending on how many boxes are used.

The sling can be made ahead of time, and just slipped over the box when things are ready to go.  Personally, I see nothing wrong with using tape to stabilize things if you deem it appropriate.

[DerekSmith]

Dan,

What a delightful challenge, thank you for bringing it to us.

May I start by presuming that all is not quite as simple as you make out:  You say that several boxes can be suspended in tandem, one from another and presumably each can weigh ca 4lb. so for a ten box string, the top box has a load of 36lb beneath it and that in a spinning snagging and particularly snapping motion, the shock force transmitted into any one boxe's rigging can be substantial (this is always worsened by 90degree corners and 90degree knot loadings).

The cord is presumably taken over the edges of the styro boxes without any load spreading, so would I be right in guessing that there can be significant load compression of the styro which in turn is going to lead to the line being too long and therefore slack after the styro has compressed.

How far away is each box from its neighbours?  Is it permitted for this distance to be reduced, if possible to nothing so the payload becomes a stick of boxes where whipping and therefore shock loads cannot happen or at least is greatly reduced?

Has your 1/4" cord ever broken or is it only knot / structure failure you are experiencing.

Presumably, if one box 'fails' this must not lead to loss of any of the other boxes.

At this stage, I have a feeling that an answer to your challenge is to be found by addressing the structure of the bindings rather than in any specific knot.

Finally, is weight an issue, or are we allowed to build in any 'elasticity' to absorb the shock loads before they do damage?

Derek

[DerekSmith]

OK,

While we are waiting for clarification, here is my first stab.

It works on a couple of principles: First we build in some controlled elasticity, and second we minimise whipping by creating a 'scientific stick'.

The knots are very simple, the only one you might have to learn is the four strand Matthew Walker Knot (MWK).  The best video I have seen for this was made by Phil the Rope here http://www.gr8-knots.com/Matthew_Walker.htm

Why the MWK?  Not because it is a very hansom knot (which it is), but because of its functionality:  It is extremely stable and strong and it is one of the best load distributing knots going.

You will also need to make up some harness loops and two loading loops.  You will need one harness loop between every box, and a load loop for the top and one for the bottom.

Harness loops can be made out of 3mm polyprop braid as they are not going to do much work - they are only going to keep the load cords centred on the face of the boxes.  To make up some harness loops, tape a 1/4" cane to the centre of each face of a box as a template so you have a box with four short canes sticking out of the top of it.  Take a length of 3mm, fold back 6" to make a bight and tie an overhand in the bight to make a loop about 1" long - put this over one of the canes and take the cord across the top of the box to the next cane.  Make a small loop about an inch from the cane and protruding past the cane sufficiently so that when you tie an overhand to make the second loop, it slips snugly over the cane.  Repeat this for the other two canes, then tie the two remaining ends together with a reef knot, then tape or glue the ends to lock them.  You should now have a square with four loops that just stick out past the sides of the box. Make up a number of these loops so you have one to go between every box.

Now you need a top harness and a bottom harness.  make these out of 1/4" cord almost identical to the harness hoops except extend the loops 2" beyond the side of the box.  These harnesses are going to take some tension, so if you can splice, then join the ends with a splice, or otherwise use a good knot like the Double Fisherman's then whip or tape the ends in place.

OK, now you are ready to make up the Instrument Stack.

Place a harness on the floor and put a box onto it so the loops stick out middle of each face.  Experience will tell you if it is necessary to strengthen the edges of this bottom box with some card corners.
Put a harness loop on top of the box - loops sticking out each face, another box, another harness etc. etc until you have built the whole stack, then put the top harness loop on the top.

Now take two lengths of 1/4" cord nearly three times the height of the stack.  Middle each cord and put the two loops together.  Fix them together with an elastic band making two loops about 3" long.  Now tie a 4 strand MWK.  The double loops will become your tie on for the balloon and the parachute.

Place the MWK in the centre of the top box with a length hanging down each face of the stack.  Pass each cord through the loop of the Top harness.  Pass one cord down one side, threading it through the harness loops as you go, then through the bottom harness loop and back up the stack, going through the harness loops again and through the top harness loop.  Lightly tie it in place with a half hitch around the two strands running down the side of the stack.

Repeat with threading the opposite side cord through the harness loops and back up and tie off.
Now thread the back cord in the same manner, leaving the fronts clear to have the batteries installed.
Install the batteries, close the faces and thread the front cord down and back up.

Once the four cords are threaded, it is time to tighten the assembly up.
Best done with two people, release the half hitches on the side cords and simultaneously pull them down to tighten them, ie. clamping the MWK against the top of the stack.  Tie each cord off with two half hitches made around both strands of cord just below the top harness loops.

Now tighten the front and back cords, and tie them off.  Make all four ends permanent by taping them to their pair of cords. 

As the front and back cords were tightened, if the harness sizes were correct, the cords should have been nipped in at every box/box junction by the tightened harness loops.  This is virtually all the work these between box harnesses will do, holding the long cord central to the faces.

The bottom harness is taking all the tension load and will take all the mass load, as well as any whip / shock load that reaches it.
The top harness is taking the 'holding tension of the tightened cords, but will take little more than this.
The four cords are tied off at the top of the stack, but travel down the stack, around and back up the stack to the MWK.  This long length of cord acts as the elastic shock absorber, expanding under load and feeding through the loops of the top harness and passing any shock waves in an attenuated form down to the bottom of the stack.  For this reason the 1/4" cord should be a dynamic cord rather than a static type, in order to allow the elasticity to be present.

Clip the top loops to the balloon and the chute and Bon Voyarge.

Any good for a first guess?
Derek

[Dan_Lehman]

In a few words and few implied devices,
I'm wondering if somehow the items could be caught
in some sort of bag?

Another tactic:  rig just the parachute support to run through eyes
on the sides/ends of the items and be tied to the bottom-most one,
so that when the parachute bears their load, they will like beads on
a string all come together (and afterwards not have any distance
in which to build momemtum for damaging impacts)!?

Third idea:  forget cordage as inter-device joints, and use a rigid
member (PVC pipe, light & strong enough -- w/some flex?), maybe
screwed into a flat plate that presses against the carried box, and
which might be itself secured by line or synthetic tape around the
box.  This would prevent individual wild movements & impacts.
-- a veritable flying shish kabob!  (It would also allow for fixing
inter-device spacial orientations.)  A prudent step though would
be to include some back-up uniter of cord, in case the rigid
material snapped for some reason -- figuring that a banged-up
box is better than a dropped-from-4miles-up one.

 - - - - - - - - - - - - -

How is the descent initiated?  -- balloon reaches upper limit and
bursts, or ... ?  How far does one have to chase the parachute?!

--dl*
====

[DerekSmith]

OK, a little experimentation and I have come up with a problem.  The styro boxes are useless as bearing surfaces as far as abrasion and point pressure goes.  The 1/4" cord can easily chafe through the styro with a sawing action, and under load around the edges it can chop a box in halves under even modest load.

I solved the problem easily enough by fitting compressed cardboard edges (the type used to protected boxes on pallets) to the top edges of the top box and the bottom edges of the bottom box.  Although these should be held in place by the tension in the harness, it might be wise to fix them in place with some super tack adhesive.  If you are having trouble getting hold of some of this edging (most trash bins behind shopping stores should have some in them) then try getting hold of a 2" cardboard tube with ca 1/8" wall thickness and cut it into 1/3rd segments, ca 8" long.  Glue two together to make a 1/4" thick corner, and when these have set, fix them to the top and bottom corners.

It is particularly important to protect the top corners because the elasticity designed into the system will allow the MWK to move up and away from the top of the stack when load is applied.  This will put considerable tension into the cord as it runs over the top corners and the dynamic nature of the cord will allow some cord to 'flow' in response to the load.  On the unprotected corner, this saws through the styro very easily.  The bottom corners do not experience sawing, but do have to take a lot of point loading, so card corners to spread the load are advisable.

I have tried making up a harness with 1" wide tape, but while it is easier to set up and adjust using click connectors and tension shackles, the rig is probably too rigid, it does not have the safety of the inbuilt shock absorbing capability of the MWK system, although it probably could be made lighter because the load spreading on the corners probably removes the need for corner reinforcement.  The disadvantage though is that you would have to do a lot of sewing to be sure you had good joint strengths.

At this point, the only worry I have with the MWK system is where the load lines go through the bottom harness loops, turn 180 and return to the top harness.  As tension changes, there is some movement through those loops, so there is a risk of chafing cord against loop.  If you decide to test this system, please pay particular attention to wear in these four junctions.  If you find any we might have to consider putting some protective metal or GRP eyes into them.

An unquantified worry relates to the poor conductivity of the very thin high altitude.  Each time the cord is asked to absorb the energy of a shake or snatch, that energy is converted into heat in the cord and we rely on conductivity into the very cold air to carry the heat away.  If the air is too thin and conductivity is too low, then the cord will heat up and weaken.  Would it be possible to thread some very fine thermocouples into the cord and record the temperatures involved so we can either build in that factor or discard it ?

Derek

Until you come back with some insoluble problems I think I will stick with the MWK as my preferred suggestion.

[SS369]

My two cents worth: Put the styro packages into individual square tarps and then use suitable knotting to affix the drawn around tarps to the main line. A terminal knot at the bottom, maybe some along the main line at pre-figured locations and varied length slings attached with Prusik knots could work.
The boxes may not be absolutely level, but they will survive. They could be adjusted so before the flight if the is duly paramount. But then I don't think they would stay level on the thrashing way down regardless of the attachments, sans enough weight to keep them so.

SS

[DerekSmith]

Just a question Dan,

Has anyone experimented with using two balloons?  A big one for the main lift and a smaller, less inflated one that will stay inflated when the main one bursts.  It will not be enough to keep the payload afloat, but will act as a far more effective anchor on the downward trip in the thin outer atmosphere where the chute has virtually no influence.

As the package descends, the gas will compress and the balloon will have less influence on the drag, but the chute will be biting more on the thickening atmosphere, taking over the task of stabilising and slowing the descent.

Doubtless cost is an issue.  Any other factors that preclude this approach ?

Derek

[DerekSmith]

Ho Hum, another change of mind.

Make the bottom harness out of 1" wide webbing.

Require:
Four 1" wide D rings,
one 1.5" wide D ring
four lengths of 1" wide flat braid webbing each the width of the container plus 12"
four lengths of webbing 9" long.
High strength cobblers thread.

Heat seal the ends of the webbing.  Sew the long pieces into a cross and thread on the large D ring across the junction (this will be the anchor cord connection).
Take a 1" D ring, slip it over one of the ends of the cross, and fold the webbing over 3" from the end and sew this down well to anchor the D ring.  Repeat for the other 3 ends.  These ends are going to take significant force, so consider having them professionally sewn (go to a rock climbers store).

Sew on the remaining four lengths diagonally across the four leads, these do not have to be strong, they are just to keep the four harness bands centred on the face of the cube.

The advantage of using webbing for the bottom harness is that it dispenses with the need for corner bracing (lighter) and builds in metal D rings to cut down chafing of the main cords when shock tension is applied to the load.

You may choose to dispense with the centre D ring as an anchor point and simply tie the anchor line directly to the crossed straps.

Derek

[roo]

Option 2
Sew up sleeves for the boxes out of nylon mesh with excess material on the top and bottom.

Material examples:
http://www.seattlefabrics.com/mesh.html

Then gather up the sleeve ends to use as passsive objects to hitch to with the following:
http://notableknotindex.webs.com/sailorhitches.html

The arrangement will look like this:
rope
sleeve
rope
sleeve
rope
sleeve
etc. as needed.

You will have to tie half the hitches immediately before take off, as they will seal off the boxes, unless you want to install zippers on your sleeves, in which case the sleeve ends can be looped off and attached to the rope with common loop knots.

[DerekSmith]

Hi Dan,

I had a read of your website - interesting stuff.

I noticed that you carry ballast up with the flight and dump this at night when you are no longer getting solar heating.  Logic would indicate that you are storing this ballast in the lower cubes.  If that is the case, could I suggest an alternative.

On the way up and in flight, it does not matter where in the stack the centre of gravity (COG) is, but on the way down it is a different matter.  As the cubes fall, they experience an 'effective' buoyancy caused by their velocity into the thin air, and working against this will be their mass.  So a cube packed with batteries and electronics will have more mass and therefore less effective buoyancy than an empty cube (that used to contain ballast).  As these two fall, the empty one will 'drag back' on the heavier one.  If the heaviest ones are at the bottom of the stack and the lightest at the top, then the stack will punch its way through the air in an orderly manner, but if the lighter ones are at the bottom (because they used to contain ballast) then they will drag upwards and distort the stack into a U shape which will dance all over the place.  So on the decent, can you arrange for the cubes to be arranged in order of mass with the heaviest at the bottom.

It does not matter quite as much if you work with a solid instrument stick (strapping the cubes together with the MWK system), but you still want the COG to be as low as possible during descent to effectively 'drag' the stack down, rather than have the COG high and trying to 'push' the stack through the air - the slightest perturbation and the stack will start to tumble (but perhaps you want this to slow the rate of descent?).

My second thought relates to the use of ballast.  Once you have dumped it on the first night, you have too much He on the second day and no more ballast for the second night (unless you only dropped half on the first night or your He is leaking).

Anyway, instead of ballast, how about 'Dynamic Lift' - fit a lightweight PV panel, a small plastic tyre inflater pump and a lightweight pressure cylinder (half a dozen large soda bottles).  During the heat of the day, use the solar PV generated power to pump He into the pressure bottles from the balloon, then at night, release it back into the balloon to maintain lift.  You can cycle this way until your He just leaks away, especially if you can use this buoyancy control intelligently to keep you in the jet stream.

Happy drifting.
Derek

[roo]

Hi Dan,

I had a read of your website - interesting stuff.

I noticed that you carry ballast up with the flight and dump this at night when you are no longer getting solar heating.  Logic would indicate that you are storing this ballast in the lower cubes.  If that is the case, could I suggest an alternative.
...snip

As much as I'd like to tear apart your long tangents built on unanswered questions, incomplete information, and a pile of flimsy assumptions, this isn't the right forum for it.

[DerekSmith]

Hi Dan,

I had a read of your website - interesting stuff.

I noticed that you carry ballast up with the flight and dump this at night when you are no longer getting solar heating.  Logic would indicate that you are storing this ballast in the lower cubes.  If that is the case, could I suggest an alternative.
...snip

As much as I'd like to tear apart your long tangents built on unanswered questions, incomplete information, and a pile of flimsy assumptions, this isn't the right forum for it.

Roo,  you are such a gentleman, thank you for being so considerate of my wanderings.  However, how could you tear apart anything built on flimsy assumptions other than to suggest that the assumptions are wrong, to which I would - in the absence of additional information - have to agree that you might probably be quite right.

-----------------------------

However, apart from applying whimsy and imagination to a problem we are still waiting for more information on, I have done some web searches and found a load more videos from this team with their equipment.

The following pictures taken from flights they called SNOX l and SNOX lll showed that my flimsy assumption about the ballast being at the bottom seems to be proven correct, although my reading of "several medium sized boxes" as maybe ca ten is way off beam, because in these flights it is just one box and the ballast.  Also, the balloon is only inflated with 290 cubic feet of Helium which has a lift of ca 20lb, the major part of which they need to give a fast ascent.

Our man Dan is on the left in the first picture and the red and white canister is a pressurised canister of alcohol - the ballast.







You can see from the images that Dan was not kidding when he said he went heavy on the tape, although the cord looks a little less than 1/4".

My comment about the ballast canister flying around and causing damage is highly probable and the cannister either needs fixing tightly to the main control box or integrating into it.  I also think that they might consider hanging the main box 'diamond like' from one corner and making this the 'normal' orientation for the cannister as this will remove much of the risk of the flat bottomed box 'fluttering' as it hits denser atmosphere.

Now you have a little more to go on Roo, what do you think ? ? ?  Because as far as I am concerned, this very much is the right forum for a discussion on how to 'bind up' this cargo, and if you don't consider the hazards of the journey, then any offered binding is going to be suspect and not doing our best for this poster.

Derek

[roo]

Now you have a little more to go on Roo, what do you think ? ? ?

They'll probably curse me forever more for this, but:
http://www.aviationforum.org/
http://www.rocketryforum.com/

You can ask all about parachute deployment at high altitudes.
http://www.stratofox.org/pics/csxt-booster-2004/reentry-booms.html