Questions. 3D printing materials; Parachute attachment and more...

[Rokas Filipsons]

Hello

There are a few questions I can't find the answers to.

1. Which 3D printing material is best suited for HAB platform parts? The parts will be on the outside of the platform, so they will not be protected from cold and other factors.

2. How is a parachute and a balloon attached? Consistent, parallel or somehow? Also, what size gaps should there be between the payload, the parachute, and the balloon?

3. How to equalize the internal pressure of a sealed payload with the external pressure? A small hole in the top of the box? Or maybe pressure equalization is not necessary?

4. Rotation reduction. In my project, the rotation of the platform needs to be minimized. I have found information about this, I will try to distribute the masses as far as possible from the center of mass. But I also found a video of how the platform is stabilized in laminar airflow. I know that wind currents are usually turbulent, so maybe anyone knows, or has tested, whether such stabilizers work in a turbulent flow? https://youtu.be/x9oTxGYRFF8

[Alan Jarvis]

Hi,

(1) Materials for HAB in general:

From my point of view as a materials engineer, it would depend on your requirements.

What are these parts required to do?

As in, what strength, toughness, ductility, density etc do you need?

The main issue is going to be the effect of low temperature on material properties.

Most plastic spec sheets give the Range of Temperatures they can be used in.

UV will degrade most plastics, but unless you are going to be doing a long flight, or else a lot of flights in total it's likely not going to be a big deal for you.

As in a hundred hours total exposure will probably be OK for anything you'd 3D print.

Given the extremely wide range of plastics available to 3D print with, you'll probably have to check each one that you consider using: it will be hard to generalise.

For example, ABS in any form is not likely going to work: it starts getting brittle at -20 C.

However, why do you want to use 3D printed parts?

Unless you're making some small part, for anything that's structural (as in it needs to be strong, flexible and tough) I think plastics are not the best material group to look at.

Balloons are a classic aerospace application, and that means you almost always want high strength to weight ratio materials.

Which are things like composites (fibreglass, carbon fibre/kevlar based etc etc).

Or for metals, titanium and some aluminium alloys.

But that's getting quite expensive, and do you NEED that?

Personally I'd look at the "natural" composits first: like wood...:-0

Look at bamboo.

It's REALLY strong, tough, flexible and cheap for its weight.

For something you can shape, try balsa wood.

If you look at what drones or radio control planes use, I think they don't use much 3D plastic, do they?

They don't need the low temperature criteria, but they do need good strength to weight ratios.

(2) I think you mean is the balloon etc attached in series or parallel, right?

And most rigs are in series.

From top to bottom: balloon, parachute, payload (c/w radar reflector).

(3) Pressure:

If you do some more research, you'll see that usually nothing is sealed, even cameras.

There's usually no need: unless you end up falling into water I guess.

And in that case, you probably cannot recover your package anyways.

It just adds weight to seal things.

And for cameras, it tends to trap moisture when initially seasled that later condenses and causes problems.

(4) I'm generalising here, and I'm new at this myself, but for many HAB's, especially for beginners, I think you tend to just live with whatever orientation you get.

So you end up using a camera with a big Field of View, or even a partial fish-eye to compensate.

Or several cameras.

In terms of the flow being turbulent, as in being "violent", remember that these are all "lighter than air", so unless you're in an extreme situation, at higher altitudes you are moving at rest relative to the air.

So I think you'll find that any horizontal flow RELATIVE to your payload is going to be quite low.

For instance, if you look at the really BIG ballons, they go up to hundreds of meters in diameter: there is NO way that THEY end up needing to live in a turbulent flow: they'd fall apart.

When you consider the Jet Stream, it's fast relative to the GROUND.

And planes experience turbulence, but they are heavier than air, and under power.

In addition, whether or not the flow is turbulent or not (as in laminar) is not really the issue.

A supersonic bullet is in turbulent flow, however until it gets close to Mach One, it tends to track very "smoothly" as long as it's axi-symmetrical, or streamlined.

If you calculate the Reynolds Number, almost any air flow you'll see will be turbulent.

As for the ascent, you can control that to some extent, but a common rate is 300m/min, which SOUNDS fast, but in fact is only 18km/h, which is not fast (a relatively slow bike ride...).

But it is a good question about trying to orient the package.

If it's just to keep it steady relative to the surrounding AIRFLOW, then fins, or a tail as that YouTube video should help.

But if you need to keep it steady relative to the GROUND, then that is trickier.

You'd need a sensor that told gave you a DIRECTION, and then a way to use that to POINT/ORIENT your payload.

You could add a sort-of a partial drone to your system: use a nav package with magnetic compass, and use that to give feedback to power one or two small propellors to keep you pointed.

Or  use a light sensor to detect the brightest direction (the sun).

And you might look at long duration launches that use solar panels: they almost certainly use a system.

Hope that helps,

Alan

[John Laidler]

Here's a photo of a payload I flew as we found it in a field after the flight. It is essentially a box made from 20mm thick sheets of expanded polystyrene in layers, one on top of the other with sections cut out of the middle of the inner sheets to make compartments. All stuck together with UHU POR adhesive. Thin braided fishing line was passed around it, two loops in total. The distance from the payload to the parachute was about 4m and the parachute to the balloon was 5m, all with the same braided line. All the knots were fixed with a drop of superglue. 

On the side of the payload is a Raspberry Pi camera secured with two zip ties. The whole of the payload is wrapped in Kapton tape which was also used to secure the battery compartment door just before launch. The interior is not sealed. There are two internal compartments, one for the batteries and one for the Pi Zero. Air could pass between them through the hole for the power supply connecting batteries to the Pi and then to atmosphere through the hole for the antenna, which was not sealed. 

John

On Saturday, 27 March 2021 at 21:59:22 UTC rokas.f...@gmail.com wrote:

[Rokas Filipsons]

Thanks for the reply.

Yes, this will be my first constructed stratospheric balloon, but I ask not unnecessarily as it is part of my undergraduate project.

(1) Primarily for 3D printing materials. I have not been able to find the lowest use temperature for any material, so I want to know which ones you use (if you use). Or how to search because I only find the highest operating temperature. The 3D printer will print a laser beam reflector holder and several mounts.

(3) Then how to attach the top of the box (lid) if it is not glued? Stick with adhesive tape? Since the attachment of the box to the parachute itself will be through the rope through the holes in the box, is it simply to press the lid by making a loop? How else do you attach the top of the box and the box itself (to the parachute)?

I am adding some images of my project:

1. The main view

2. View inside from the side. White parts will be printed with a 3D printer.

3. Top of the box. Holes in the rope are visible.

4.I tried to depict the rope loop I mentioned.

[Rokas Filipsons]

Thanks for the reply

Is there any methodology or rules for determining the distance between the load, the parachute and the balloon?

[John Laidler]

I haven't heard of 3D printed components being used on a HAB but I would be surprised if it hasn't been tried.  The stresses in flight are very low but when the balloon bursts there is a risk of the payload being hit by part of the balloon, particularly the neck which is the thickest and heaviest part, then of course there is the landing.  I can only suggest trying what tests you can manage, even just freezing a sample in a domestic freezer might tell you something.  You would want to design a simple test, say a beam supported at either end and then a weight added in the middle, comparing the deflection at ambient and cold temperatures and of course ideally, what force is needed to bring about failure.

With your design of box you could use the lines to secure the top. If the line was passed down through one hole looped around underneath brought up the other hole then passed across the top and down the first hole then round the bottom again and finally up to the top again.  While slack the top could be lifted up but I would still add some tape so it will stay on during the landing.

I'm not aware of any methodology for determining line lengths.  I had the balloon to parachute length longer than the parachute to payload and on reflection I think this should be the other way round so the remains of the balloon should be held away from the payload.  I was surprised by the distances used when I first read about them but when the balloon bursts it is very violent and bits of latex can strike the payload. The other thing is for photography you want the payload to be as still as possible and the oscillations of a long pendulum will be slower than a short one. The amplitude of the oscillations will be greater but I don't that matters. The payload will still spin but from what I've seen they often stop spinning after launch and stay pointing in the same direction for a while.  The payload and parachute  form a double pendulum which is a challenging maths problem you might want to include in your project!

John

[Steve]

My 10 pence worth:

In general the longer the distance between the balloon and the payload the longer it takes for the payload to swing back and forth (i.e. like a pendulum), however the longer the line the more separated the balloon and payload are and therefore more subject to differing wind speeds at different horizontal layers.  Another consideration is that there will be some turbulence directly underneath the balloon as it ascends.

Many folk use the rule of 1/3s when setting line length - that is 1/3 of the total line length above the parachute and 2/3 below (or visa versa) that way when the balloon pops the remnants swing down and hang about mid line parachute-payload (or hang under the payload) - so the remnants don't hit or get tangled up with the payload (covering the camera or twisting up with the antenna).

Parachutes run in-line with the payload-balloon line - we generally use special "balloon" parachutes with an extra attachment point at the apex.  Alternatively you can modify a model rocketry parachute or make your own.

Long lines between the parachute and payload tend to make the parachute-payload combination cone in descent.

Another issue with long lines is the danger if it all lands on a power line and dangles down (believe me this is not an unusual event - its happened to me a few times).

As for fins on the payload and the stabilized in laminar airflow - in general they just act as fins to pick up any turbulence and side guts and spin the payload. 

My advice is to make the payload as symmetrical about the C of G as possible - distribute mass as far from the center as possible.

I would not rely on any 3D printed plastic part for anything structural that might cause things to detach.  I generally attach safety lines on anything that might break off.

I've seen PLA 3D printed parts work fine on HABs - and in general plastics seem fine as long as they are not taking load - I have seen sizable nylon zip ties fail (due to the cold) when used structurally (parachute attachment).

I would ensure that any printed part does not contain a sealed void - sealed plastic models have been known to pop open.

My advice would be to use polystyrene foam as the box as this helps both thermally and any impact that might happen.

Not sure where your launching - but in the UK, EU and US there are rules for detachment force on the line between the payload and balloon - for light balloons this is generally about 230N (23Kg)

Loop the lines under the payload - Ive used your loop round idea in the past and its worked well - although it can be a pain if you have to get back into the payload.  

These days I tend to use double sided hook and loop to go right round the payload to hold the lid/door on. 

Its good to put some form of registration lip on the box lid - mainly to ensure a reasonably good seal.

    Steve

On 18/04/2021 09:36, Rokas Filipsons wrote:

Thanks for the reply

Is there any methodology or rules for determining the distance between the load, the parachute and the balloon?

2021 m. balandžio 12 d., pirmadienis 10:42:55 UTC+3 John Laidler rašė:

Here's a photo of a payload I flew as we found it in a field after the flight. It is essentially a box made from 20mm thick sheets of expanded polystyrene in layers, one on top of the other with sections cut out of the middle of the inner sheets to make compartments. All stuck together with UHU POR adhesive. Thin braided fishing line was passed around it, two loops in total. The distance from the payload to the parachute was about 4m and the parachute to the balloon was 5m, all with the same braided line. All the knots were fixed with a drop of superglue. 

On the side of the payload is a Raspberry Pi camera secured with two zip ties. The whole of the payload is wrapped in Kapton tape which was also used to secure the battery compartment door just before launch. The interior is not sealed. There are two internal compartments, one for the batteries and one for the Pi Zero. Air could pass between them through the hole for the power supply connecting batteries to the Pi and then to atmosphere through the hole for the antenna, which was not sealed. 

John

On Saturday, 27 March 2021 at 21:59:22 UTC rokas.f...@gmail.com wrote:

Hello

There are a few questions I can't find the answers to.

1. Which 3D printing material is best suited for HAB platform parts? The parts will be on the outside of the platform, so they will not be protected from cold and other factors.

2. How is a parachute and a balloon attached? Consistent, parallel or somehow? Also, what size gaps should there be between the payload, the parachute, and the balloon?

3. How to equalize the internal pressure of a sealed payload with the external pressure? A small hole in the top of the box? Or maybe pressure equalization is not necessary?

4. Rotation reduction. In my project, the rotation of the platform needs to be minimized. I have found information about this, I will try to distribute the masses as far as possible from the center of mass. But I also found a video of how the platform is stabilized in laminar airflow. I know that wind currents are usually turbulent, so maybe anyone knows, or has tested, whether such stabilizers work in a turbulent flow? https://youtu.be/x9oTxGYRFF8

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[j.r.h...@gmail.com]

I've used 3D printed parts in the past on a couple of balloon launches both internally and externally. The internal structure sits in a polystyrene box and the guidelines go through both the 3D printed structure and the payload box. I have used PLA as a payload arm for lightweight components I have wanted to take photos of and they have worked fine, tend to snap on impact.

Trying out a similar sun shield for the GoPro on a flight tomorrow that has been 3D printed. Wanted to simplify the geometry need to cut out of the polystyrene to get a clear field of view.

[Rokas Filipsons]

It’s nice to hear that the printed parts work. At the moment, I am only dealing with the question of which plastic to use - PLA or PETG. Printed parts also will be used inside and outside of the box.