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Feb 21, 2017, 10:00 PM
Registered User

Is it even possible?

Hello folks,
I am falling upon the mercy of your collective knowledge to help answer some questions about a lighter than air solution to a problem I have.
Forgive my ignorance of proper terminology. I will try not to say anything too dumb.

Is it possible to build an (envelope?) In the shape of a vertical cylinder, approximately 3-4ft in diameter by 15-20ft tall?

Lets say I took that "balloon" up to 10,000ft, inflated it, applied ballast weight to make it negatively buoyant to achieve a specific descent rate and dropped it. What effect could be expected due to air pressure increasing and temperature rising? Could a cylindrical envelope be engineered to account for these effects?

I am trying to develop a rapid prototype and proof of concept so any and all input would be fantastic! Hopefully you will consider this a fun thought exercise!
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Feb 22, 2017, 05:36 AM
Melbourne, Australia
Assuming you don't mind a little bulging at the ends, it is fairly easy to make a cylinder as described. Assuming you use helium, a cylinder of 1m in diameter and 5 meters in length will have a lift of around ~4 kg (at sea level) and if you used fabric of, say, 25 gsm it would have a weight of ~425g, so plenty of free lift. At 10,000 feet air pressure is ~70%, so still plenty of free lift.

... if you inflate it at 10,000 feet though, ballast it and drop it, you will need to include a pressure relief valve - otherwise it will increase in pressure and pop at some point, as the thin fabric will be unable to cope with the pressure :-).

So... not so much the geometry, but you will want a valve!
Feb 22, 2017, 10:55 AM
Go small or go home
ruzam's Avatar
If you fill it at 10,000 and make it negatively buoyant, it will descend. As it falls (floats) to the ground the atmospheric pressure will increase, and the envelope will collapse (by 30% if pegacat's numbers are correct). It will become more and more negatively bouyant as it falls.

By the time it hits the ground, the envelope will be acting more like a parachute and it may be falling pretty fast. Unless temperature causes expansion of the helium to make up for the pressure difference. That could probably be calculated if you know the 10,000 and ground temperature difference.

My guess is, temperature won't be enough to make up for the increasing pressure and you're safe to seal it and drop it. Actually, I think that's a safe bet, otherwise high atmosphere balloons wouldn't need so much extra expansion space.
Feb 22, 2017, 12:53 PM
Registered User
So as it descends, the pressure would squeeze the helium into the upper portion of the tubeand the bottom would start to fold in on itself? Would I be correct to assume that the increase in internal pressure would be offset by the increase in external pressure therefore causing no effective change in the psi load on the seams? Or is that the first of the dumb things I will say in this thread?
Last edited by Nerdbird; Feb 22, 2017 at 02:00 PM.
Feb 22, 2017, 03:44 PM
Go small or go home
ruzam's Avatar
Originally Posted by Nerdbird
So as it descends, the pressure would squeeze the helium into the upper portion of the tube and the bottom would start to fold in on itself?
I believe so. The helium would take up less volume as the external pressure increases, and being lighter than air it will naturally float to the top, leaving the bottom to just suck up into flat empty excess envelope. Unless your plan is to over fill the envelope at altitude so that at ground level pressure (and temperature) the envelope is still full?

Originally Posted by Nerdbird
Would I be correct to assume that the increase in internal pressure would be offset by the increase in external pressure therefore causing no effective change in the psi load on the seams?
FAIK. The psi load on the seams would become less as it descends.
Feb 22, 2017, 03:55 PM
Registered User
Nerdbird, I can't imagine it would not be possible. Unless you overly ballast I imagine it would eventually slow and then float at some altitude. If the point is for it to hit the ground then use a rock or something. Good luck and please post a video. Cap
Feb 22, 2017, 05:10 PM
Registered User
Exactly, right? The goal is to formulate the volume of helium in the cylinder required to achieve a predictable rate of descent, even if the rate varies. The formula would take temp/pressure at altitude and factor in temp/pressure lower to the ground.
So there would be LESS pressure on the seams as it descends??
I will post some drawings to illustrate some of the finer points. If I wanted to build a quick and dirty proof of concept, what would you gentlemen recommend as a material?
Feb 22, 2017, 05:35 PM
Registered User
PV=nRT? Seam pressure is not my forte... If possible make the seams extra strong, the luffing or flapping as it desends probably causes more stress than static pressures. I think theoretically highest pressure is when it is at max decelleration. I have no idea what I'm saying though. I meant post video of it when you drop it, if you can... I'm no blimper or ballooner, but maybe mylar? Cap
Feb 23, 2017, 02:43 AM
Melbourne, Australia
Dagnabbit... going down, not up! So ignore what I said about bursting :-)

So yes, it deflates. The temperature difference (rule of thumb - 2 degrees / 1000 feet) really won't make that much difference; maybe 8% over 10,000 feet? *But* your gas won't heat up quickly enough unless you're descending really slowly.

Figuring out *exactly* how much helium to use might be more difficult; if you actually tried to fill it at 10,000 feet you'd need to worry about the gas temperature dropping from expansion, but it would be more practical to partially fill it on the ground and use a separable 'lead' balloon to get it to altitude... has the advantage that you could get the ballasting correct with your feet on the ground :-)
Feb 23, 2017, 07:12 PM
Registered User
Pegacat, up or down, same difference, just a sign + or -, I make that mistake all the time. Nerdbird, You don't even need to use helium if you want to attempt to predict or test what an objects rate of desent is, you could use a feather or a stone as Galeleo did. I think you need to state your theorem so we might understand the root of your question. Cap
Feb 24, 2017, 06:48 PM
Registered User
What I am looking to do is hang the balloon from a scale at 10,000 ft and be able to calculate the starting weight for a given descent rate. Now if I understand this correctly, the increase in pressure will reduce the volume of helium causing faster descent. The calculations would also tell me at what altitude the payload would descend too quickly. This would be my maximum launch altitude without solving the fast descent rate issue.
Does that make any sense whatsoever?
Feb 25, 2017, 01:24 AM
Melbourne, Australia
Hmmm... the problem is the descent rate is actually tricky to calculate - to get that you need to know the drag, and your cylindrical vessel (if it isn't rigid) will change it's drag as it deflates (it will become larger as it becomes more flappy), and the air resistance will change as it becomes denser

On the flip side, I've found helium weather balloons appear to have a fairly constant rise rate; as they're elastic they keep a nice round shape, and as they expand the air gets thinner - it seems to even out, but I've never run the maths - it probably changes a bit with altitude, but the whole drag ~ velocity**2 thing means big changes aren't super likely...
Feb 25, 2017, 06:25 PM
Registered User
That seems to be good news since I would require a pretty slow and gentle descent rate, something under 10ft/sec.
A question of construction...what if I were to build the envelope out of a lightweight fabric or other material a bit more durable than mylar and just stuff it with smaller mylar balloons? The unlimited length of the tube shaped envelope means I could make up for the extra weight by increasing the length. Does that make any sense?
If one mylar balloon pops, the whole tube wont deflate. I would ideally like some sort of abbrasion protection.
Feb 26, 2017, 04:01 AM
Registered User
Nerdbird, is this just an exercise or are you actually going to build it and bring it up in an airplane and shove it out the door at 10000 ft? Not sure it would survive the first second or two. Are you going to have flight recorder on it? Do you plan to find the thing after landing? Putting balloons in a fabric sack seems reasonable, although it may be a bit lumpy. Cap
Feb 27, 2017, 03:19 AM
Registered User
While shoving it out the door of a plane at altitude sounds like hilarious fun, no. Without getting all proprietary-this and non-disclosure-that...imagine you had to inspect the elevator shafts of the worlds tallest buildings. Man on a rope? Too dangerous. Camera on a rope? Too whippy and unstable. Drone? Possible but has massive challenges in that environment. 360 degree camera balloon? Well that might just be crazy enough to work! The elevator shafts are relatively clear of potential obstruction and have almost no airflow. Drop from the top and get 360 degree 4k images of every nut bolt and guide all the way down. And before you ask the obvious question, lets also assume that the bottom floor is unreachable so I cant send the payload up.

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