Info on hydrogen and (tethered) hydrogen aerostats

[Windy_Skies]

@Windy_Skies

One idea might be to fill the balloon with nitrogen instead of or in addition to air, but I think then the risk of asphyxiation might exceed the original risk of explosion. Maybe there is a balance.

https://www.reddit.com/r/AerospaceEngineering/comments/1g1dtt5/how_do_hybrid_airships_take_off_and_land/

[–]GrafZeppelin127 1 point 15 hours ago

They take off and land much like an airplane or tiltrotor like the Osprey does, it really depends on their load, their aerodynamic lift ratio, and how much vectored thrust they can generate.

The pictured airship is a Lockheed-Martin LMH-1, the design of which was sold and is now the Z1 from AT2 Aerospace. Under light loading conditions, it can take off with vectored thrust much like a helicopter. Under maximum load, it has a short takeoff and landing run.

Interestingly, airships—including hybrid airships—are usually designed, when taking off statically heavy, to be able to support very high “crab” angles relative to a runway, if they’re using one. That’s how Cold War-era Navy blimps operated from runways while up to 11% static heaviness, even in 40+ knot winds. These high crab angles are necessary, since their take-off roll is so much slower and shorter than a normal jet airliner. Ideally, they wouldn’t confine themselves to using a straight runway at all, but rather operate from a 360° takeoff and landing circle, simply pointing into the wind and taking off into whatever direction it’s coming from.

The ship itself is an airfoil, actually. It acts as a lifting body with extremely low wing loading.

Interestingly, an airship doesn’t even need to be lozenge-shaped or flattened like this in order to generate sufficient lift to provide for up to its own gross weight or more in purely aerodynamic lift. Airships are large enough that even though the normal cigar/teardrop shape isn’t particularly efficient as a wing, all it needs is a surprisingly small angle of attack to generate a lot of lift. Of course, that creates more drag and requires more engine power to prevent stalling, but it can be done.

The reason many hybrids are shaped like the one above is because, despite the fact that at zero angle of attack it is strictly worse than an elliptical cylinder-shape from a drag perspective (due to higher proportional skin drag and a larger frontal area per volume), such a shape can generate aerodynamic lift more efficiently at smaller angles of attack.

Oh that’s fascinating. How fast do these large bodies have to travel to offset their weight with aerodynamic lift?

Not particularly fast. To give a specific example, according to NASA parametric design studies from a few decades ago, the optimum cruising speed (assuming allowance for a 15-knot headwind) for a conventionally-shaped rigid hybrid airship with a gross weight of 400,000 pounds and a beta-factor of 0.5 (in other words, 50% aerostatic lift and 50% aerodynamic lift) would be a bit below 140 knots at zero range, i.e. not accounting for the productivity impact of increasing fuel burn and the higher fuel weight that would imply. If one assumes a range of 1,000 miles, the ideal would be 80-90 knots, and slowly decline from there with increasing range.

There are a huge number of different configurations that are most ideal for a given productivity goal or mission range or mission profile, however. There’s no clear winner when it comes to a rigid, nonrigid, or metalclad design, they all have points at which they are the optimal choice. Likewise, smaller airships benefit from having a higher percentage of their lift come from aerodynamic lift, whereas larger airships are more efficient in general and lose productivity and efficiency with any addition of aerodynamic lift; in other words, the ideal “small” airship is actually an airplane, if one can at all be used, and the most efficient/productive large airship (nearly as big or bigger than historical Zeppelins) actually loses efficiency and productivity with increasing inputs of aerodynamic lift, i.e. it’s best for them to have neutral buoyancy. The longer the range, the slower the optimum speed as well, assuming it’s burning heavier-than-air fuel and not using solar and fuel cells or something like that.

I assume they use helium in these for lift?

Naturally. An 8% difference in lift is a small price to pay for not having to engineer and certify a completely novel fire-suppression system for a hydrogen-filled envelope, which would likely entail a double hull of some sort inerted with a separate nonflammable gas like helium or nitrogen (difficult to achieve with a nonrigid or semirigid design like the one above).

Thanks not knowledgeable about this. Do they ever use heat like hot air balloons for these? I would imagine that doesn’t make sense fuel wise, but thought I would ask.

Superheat (buoyancy gained by heating the envelope above the ambient temperature) is, indeed, a proposal for some, and has been experimented with in the past, but generally the current zeitgeist is to move away from that kind of thing—which would entail capturing waste heat or installing burners—in favor of switching over to an electric drivetrain using hydrogen fuel cells.

The reason for this is simple: liquid hydrogen weighs roughly a third as much as an equivalent energy content of diesel fuel. And that’s accounting for the differences in fuel tanks, equipment, and energy generation mechanisms. That means tens of tons of freed-up lift that would otherwise go to carrying fuel, as well as having a convenient source of ballast or antiballast on demand (as hydrogen creates free water far in excess of its own mass, due to the higher atomic weight of the atmospheric oxygen that bonds to it. Hydrogen can also be safely released into smaller antiballast-ballonets ensconced within the larger helium hull or gas cells in order to create more lift).

https://www.reddit.com/user/GrafZeppelin127, like the username suggests, comments on things like this often.

https://www.reddit.com/r/solarpunk/comments/1g1n72f/this_is_what_i_picture_when_i_imagine_solarpunk/

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https://www.reddit.com/r/aviation/comments/1fzww9f/today_i_saw_a_blimp_on_the_ground_and_take_off/

Don’t underestimate the usefulness of being a giant windsock! I’ve heard pilots lauding blimps and Zeppelins like these as being the best damn wind indicators at the airport, super easy to see.

Plus, conducting landings in a potential 360° arc allowed Navy blimps back during the Cold War to operate reliably in terrible 40+ knot blizzards and thunderstorms that grounded all other planes and helicopters. Though obviously this modern Zeppelin NT wouldn’t fly in such conditions; it’s not like there’s much filming, sightseeing, or advertising potential in a blizzard.

Damn, 40+ knot blizzards! Dirigible have always been fascinating to me. Never had the chance to be on one though. Flown in many other aircraft, mostly military.

Mahalo for sharing your Zeppelin knowledge. Gotta read about the US Navy’s use of them during the Cold War.

Yep, those kinds of operations were conducted during Project Lincoln and Operation Whole Gale; you can sum those up as the Navy brass going “let’s deliberately fly these blimps into ice storms to prove their unreliability so that we can justify cannibalizing this program which we see as anachronistic to fund our own pet projects!”

It didn’t work, the airships beat the pants off the WV-2 airplanes they were set to compete against, demonstrating an 88% reliability rating in inclement weather, and they were better, more stable radar platforms to boot which cost between 1/2 and 1/3 to operate per aircraft. However, the brass still got the last laugh and denied the program funding and upgrades, before terminating it shortly after.

That’s rad. I’ve got some reading to do now. I’m still pissed the Navy replaced the P-3 with the ugly ass P-8.

[–]GrafZeppelin127 2 points 1 day ago

I’d recommend starting here (USN LTA: Cold War Era – Airship History) for a more general overview of the Cold War years. There’s more you can find in books like Sky Ships: a History of the Airship in the United States Navy.

https://www.reddit.com/r/AskReddit/comments/1fuc9d3/whats_a_fact_about_the_world_that_sounds_totally/lq1et7l/?context=3

Are you a airship knowledgeable person because I have a question… How the heck are they now just thrown around by winds and updraft and stuff? They seem so large and like a giant sail I never could make that work in my brain.

[–]GrafZeppelin127 2 points 9 days ago

A big part of that is power, which is essential for both resisting winds and also maintaining control in them. Airships have engines, and all other things being equal, the more powerful they are, the better they’re able to operate in bad weather.

As a general rule of thumb, airships can land and take off (the most wind-sensitive portion of their operation) in winds that are approximately half their top speed. Not coincidentally, that means that an airship’s weather-handling ability is determined largely by how fast it is, not how big it is, though small airships have more drag per unit volume than large ones and thus are disadvantaged.

It’s possible to overcome this disadvantage with enough power, though. The fastest and most weather-capable airships of all time were the Navy’s Cold War-era ZPG-3W blimps, which were only about half the length of the largest historical airships (thus about 1/4 the drag and 1/8 the volume), but had proportionally twice as much power. They had a top speed of 82 knots, and could operate reliably even in gales, blizzards, and thunderstorms that grounded all other planes and helicopters. This was due to their ability to fly for days on end, unlike other aircraft which run out of fuel within a few hours, and also their ability to land cautiously at low speeds without fear of stalling like a plane or unbalancing and toppling over like a helicopter.

https://www.reddit.com/r/AskEngineers/comments/1fu4la7/how_many_drone_sized_airships_would_it_take_to/

This is impractical not just because of the square-cube law, the impracticality of coordination, and inefficiencies from redundancy, as others point out, but also due to the related issue of lift-to-drag ratios.

Any airship regardless of its design or construction is going to have proportionally greater drag and less lift at small sizes rather than large ones. At the size of a small blimp, the lift-to-drag ratio is about 3-5, similar to a helicopter. An airliner has a ratio of about 15-20. Large airships could have ratios of upwards of 30-50. This effect is exponential.