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Comment by projectramo | Hacker News Reader

At first I thought the kicker was going to be: the gas is helium, it just floats.

And then I thought: why is that a joke? Could you save fuel keeping it aloft with a lighter than air gas and just use the fuel to move it around?

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I think you just invented the dirigible?

Ha, yes.

I guess another way to phrase the question would be: why don't they use dirigibles to solve extended air time problems?

Dirigibles require huge volumes, because the lifting power of gas is small. Those huge volumes make for a relatively weak, slow, and difficult-to-maneuver craft. A big reason why airships disappeared in favor of airplanes was that airships had a really tough time handling or avoiding bad weather because of these limitations.

I assume huge volumes are also easily detectible?

Seems reasonable. All else being equal, your radar cross section is proportional to your actual cross section. Plus it's probably easy to spot a blimp with your eyes.

I have a blurry recollection of seeing an aircraft design that was a combination of an airplane and a dirigible. It looked like a poofy manta ray, an inflated rigid lifting body or flying wing. It wasn't quite VTOL & needed a very short runway, but it also gained more maneuverability in flight. Maybe it was too much of a compromise though.

Perhaps you're thinking of the infamous 'Deltoid Pumpkinseed'[1][2] ?

[1] https://en.wikipedia.org/wiki/AEREON_26

[2] https://www.amazon.com/Deltoid-Pumpkin-Seed-John-McPhee/dp/0...

You mean this one?

http://mashable.com/2017/05/24/airlander-butt-plane-successf...

There's actually been more than one of these types of aircraft (known as Lighter Than Air - or LTA Aircraft), either as an idea or prototype throughout history.

Could it be this one [0], or its predecessor [1]?

[0] http://www.latimes.com/business/la-fi-lockheed-martin-aircra... [1] https://en.wikipedia.org/wiki/Lockheed_Martin_P-791

That type of vehicle is called a hybrid airship!

[1] https://en.wikipedia.org/wiki/Hybrid_airship

The lifting power of helium is small, as opposed to hydrogen. Also, helium has supply problems.

Is it not possible to have a modern airship with hydrogen as its primary lifting agent?

Hydrogen doesn't buy you that much additional lifting power.

Air is about 1.2kg/m^3 at STP, so that's the theoretical maximum you can lift with a cubic meter of gas bag. (And you can only get that value if you could somehow hold a vacuum, of course.)

Hydrogen is about 0.09kg/m^3 at STP, so you can lift about 1.1kg/m^3. Helium is about 0.18kg/m^3, so you can lift about 1kg/m^3. Helium is twice as dense, but that only costs you about 10% of your lifting power since both are so light compared to air.

I see no reason you couldn't use hydrogen, but I don't know if anyone would want to take the risk. (Specific flaws of the Hindenburg aside, hydrogen is explosive across a distressingly wide range of mixture ratios with air.) If you did use hydrogen, I think the main motivation would be avoiding the use of scarce helium, not the minor increase in lift capacity.

If it's an unmanned drone-blimp, the safety issue because somewhat less critical.

But still critical enough to make it impractical. How would you do maintenance on a hydrogen airship without risking the people doing the maintenance? And you'd never be able to fly over populated areas.

It is possible but we all know what happened to the Hindenburg. Using hydrogen is asking for a fire to happen.

https://en.m.wikipedia.org/wiki/Hindenburg_disaster

Far more helium airships have wrecked than hydrogen airships, even counting only before the Hindenburg.

Wrecked, yes, but they probably didn't burn like a torch with people jumping out of them to avoid burning alive.

What you saw was the engine-fuel burning on the ground; the hydrogen went straight up in flames and burnt no-one.

Keep in mind that the Hindenburg was supposed to be helium filled, but due to import restrictions they couldn't get enough.

Vacuum still has the best lifting power of them all..

We just need a material as light as a balloon but rigid enough to hold its shape and maintain a huge vacuum at 1 atmosphere outside pressure.

Perhaps graphene aerogel, with a nonpermeable graphene casing?

I'm not sure of what its behavior would be with all its air sucked out, though.

like a hair full of nothing..

I wondered why this wasn't applied to suitcases to make them lighter and then common sense kicked in regarding just how much actual helium would be required to lift even an empty suitcase...

It's not even an issue of how much helium is needed; no amount of helium will make a typical suitcase buoyant.

Filling it with helium at ambient pressure wouldn't produce nearly enough lift, and increasing the pressure would make it heavier, not lighter. What's needed is not mass, but volume.

I feel like you're being deliberately obtuse. I think jianshen meant how big of a helium balloon you'd need, not how much helium you'd have to put inside the suit case.

Making them a vacuum would help better.

They are used to solve extended air time. That's what a weather balloon is.

If what's bugging you is why lighter-than-air vehicles aren't used for commercial transportation, that's an entirely different issue.

weather balloons are typically not up for that long, 90 or so minutes on average. So I don't think that's solving the extended air time. In fact they are pretty much designed to go up and pop and come back down. Lighter than air in general can be used to solve extended air time, and we're back to dirigibles, not weather balloons.

Project Loon seems to be doing fine. They're using balloons to keep equipment aloft for months at a time.

sure, but I wouldn't exactly call that a weather balloon.

Well, I'm disappointed that the majority of their flights are this short. I thought there were some of them which were permanent atmospheric weather stations.

But short flight times don't really have to be the case, because it seems that they can fly for extended periods of time: one of NASA's balloons stayed up for 46 days [0]. The US air force also uses balloons as surveillance stations, though I don't know the actual flight time of these [1][2]. JLENS was supposed to stay up for 30 days at a time.

[0]: https://www.nasa.gov/centers/wallops/news/supertiger-record....

[1]: https://en.wikipedia.org/wiki/Tethered_Aerostat_Radar_System

[2]: https://en.wikipedia.org/wiki/JLENS

sure but these aren't really weather balloons. the last two are basically tethered dirigibles. the first one is the closest and it's more like what project loon is doing. Yes, it's true, balloons can stay up for long periods of time, but this is not how weather balloons are made or what they are designed for. Weather balloons are designed to pop at a specific pressure and fall. As the balloon rises, the volume of gas expands due to there being less atmospheric pressure. At a point the balloon pops and it falls back to earth. The loon and the balloon in your first link probably use a system that adjusts the pressure in the balloon by compressing it into a storage container. This allows it to "hold" an altitude and keep the balloon from ascending until it bursts. Also, at least in loon's case, it uses a different material than a standard weather balloon. This probably also plays a roll in it's expandability, resistance to long term UV and keeping itself from popping.

The Stanford Student Space Initiative's valbal is an interesting project where they add a lightweight set of ballast and a valve to a weather balloon so it can maintain an altitude. They're limited by power, the amount of ballast, and how much gas they can bleed but they can get flight times of days. https://stanfordssi.org/blog/ssi-52-breaks-world-record-agai...

That does bug me actually. I want a Jules Verne-esque air-cruise-ship tour round the world.

Jules Verne was an advocate of heavier-than-air flying machines though.

Ok well leave out the man himself and include instead the brass-covered super-geared world he created.

I want a steampunk blimp ok? There. You made me say it.

You want Castle Wulfenbach.

https://www.google.co.uk/amp/s/girlgeniusadventures.com/2013...

The U.S. military is way ahead of you, they've been used for extended surveillance in war zones since 2004: http://www.nytimes.com/2012/05/13/world/asia/in-afghanistan-... and even domestically: https://theintercept.com/2017/04/24/nsa-blimp-spied-in-the-u...

And before that, I believe surveillance blimps have been used in some form or another since the 19th century.

Not to mention outside Washington DC. :)

https://www.washingtonpost.com/news/the-switch/wp/2015/10/28...

Jesus, they shaped it like a giant bomb. No wonder everyone is uncomfortable. :-/

Helium is rare and expensive and becoming even more so as time goes on.

Hydrogen has this nasty habit of being extremely flammable.

Rigid or semi-rigid airships when scaled up are really difficult to moor and arrange ground handling for, which is why some of the recent prototypes that have been flown integrate skirts and reverse-hovercraft like suction apparatus that can hold them down on a flat field.

The empire state building was originally intended as a Zeppelin mooring location, but the plan was abandoned due to wind and inability to control a zeppelin to that level of precision.

Not sure what the flight-time is, but this is pretty cool (helium-filled RC craft that looks like an airliner and flies indoors):

https://www.youtube.com/watch?v=2Kk5EGpYuck

The article said that the lighter than air aircraft got blown around by the wind too much. They needed a drone that could loiter over a specific area for an extended period of time, presumably acting as a communication relay for disaster or war zones.

probably because the need is real, just not the "for natural disaster" reason they gave.

I assume you're only asking what address to send the summons to, huh?

Balloons work well if you're travelling with the wind. It's a lot of drag if you're fighting the wind.

But suppose you change altitude to find favorable winds? That's what Project Loon is doing: https://x.company/loon/

I thought of that the other day, wondering how well you could possibly control a hot air balloon (or helium) based drone. My guess is wind forces etc would be extremely difficult to deal with

With a (very) good weather model, it's possible to change altitude to pick up winds going roughly where you want to go. I don't know if anything does this autonomously - Google's Project Loon is the obvious candidate.

Lol, look up google's project Loon. They are trying to give everybody internet with balooons that stay in the air a long time. They steer by going up or down into different wind currents at different altitudes

Fill it with methane/natural gas, and you can use the fuel to keep it aloft, and to move it around.

Your balloon would have to be about twice as large to compensate for the higher density of methane (.656, according to google)

I also am not sure that would be much safer than using hydrogen. Methane ignites a bit harder and would initially burn a lot slower (hydrogen has a much higher upper explosive limit (https://en.wikipedia.org/wiki/Flammability_limit), so it would need to disperse more for the gas at the center of the balloon to ignite), but _any_ fire near the surface of the balloon eventually would be catastrophic, and putting fire extinguishers everywhere isn't an option because of their weight, in combination with the huge area of your balloon)

That was my thought, too. E.g. a zeppelin-like drone could fly forever.

Heat the air with direct solar to give it lift. Make it clear on the outside and have something black on the inside to absorb more heat. For bonus points, let it be deployable during flight...fly drone into position in plane mode, pop the blimp, inflating it with air from the forward travel inertia, heat it once with a small rocket motor or something, then let the sun keep it hot enough to float. When done, eject the balloon and fly back. I'm sure there are lots of details around why they don't do this.

There used to be (maybe still is?) a toy, sold by Edmund Scientific and others - that was basically a thin black and big dry cleaning bag. You'd basically take it outside on a sunny day, whip it around a bit to "fill" it with some air, tie it closed, and then let it heat up in the sun. Eventually, it would take shape, becoming buoyant, at which point you could let it go to float off (presumably tied down with a string if you wanted to keep it).

I don't know if it is still available or not, or whether anyone ever added remote controls to it...

Hmm - apparently it or something similar is still available - or you can try to build one yourself:

http://www.instructables.com/id/Make-a-Solar-Heated-Balloon/

Bonus points- make the Ballonhull in place from carbon harvested from the atmosphere..

Think about how much weight a party balloon with helium can support. Not a lot. It could definitely not carry a camera, for example.

Hot air balloons adjust altitude by heating the helium inside the balloon and letting it cool back down as appropriate. You would need a similar mechanism, but in much smaller format.

hot air balloons use hot air (which is less dense than cool air) not helium.

they are open by design, unlike dirigibles. hot air balloons increase altitude by adding more hot air via a burner, and decrease altitude by letting the air diffuse out through holes in the top of the balloon.

Whoops, yeah you are right. I blame lack of caffeine!

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