u/GrafZeppelin127

These little vignettes have a certain quirky charm, don’t they? The main classes Goodyear wanted to implement were Deluxe, Pullman, and Economy—with 112-288 passengers depending on the balance between the three classes.

Ultimately, with World War II upending the world and the dawning of the Jet Age, Goodyear President Paul Litchfield never managed to drum up enough interest to make the American passenger rigid airship a reality.

These little vignettes have a certain quirky charm, don’t they? The main classes Goodyear wanted to implement were Deluxe, Pullman, and Economy—with 112-288 passengers depending on the balance between the three classes.

From the NASA/Department of Commerce Feasibility Study of Modern Airships.

Hybrid powertrains often have more compromises than benefits in many applications. Airships, however, may be one of those edge cases where a hybrid powertrain offers the best of both worlds, due to the unique demands and capabilities of the platform.

Hybrid Air Vehicles, Flying Whales, LTA Research, Kelluu, and other modern airship companies all see the writing on the wall: hydrogen has about three times the gravimetric energy density of the next-most-powerful chemical fuel source, excluding nuclear reactions. That on its own can multiply the endurance, range, payload, and productivity of airships tremendously.

For instance, the Hindenburg carried 21 tons of passengers and cargo, and 67 tons of diesel and oil. Reverse those two figures, and the ship is making more than three times as much revenue for a given trip while also being more energy-efficient, and those performance and revenue increases could translate to a proportionally even more drastic increase in profits, given the mostly fixed nature of the other operating expenses such as labor, insurance, amortization, and so on. NASA engineer Dr. Mark Ardema calculated that significantly reducing structural or fuel weight can increase airship productivity by 200-500% depending on design and mission.

Hydrogen fuel is therefore essential for economical airship operations, particularly when starting out with low production numbers and high R&D costs. There are two primary means of using hydrogen to supply power for a distributed network of electric thrusters: fuel cells and turbogenerators. Fuel cells are heavier (balance-of-plant being about 1 kW/kg) but 50-60% efficient, and small aviation-grade turbogenerators are far lighter (up to about 8 kW/kg) but only about 25-30% efficient.

Thus, there is a point where the weight saved by using turbogenerators is paid back and starts going into the negatives because of the higher fuel use. In other words, turbogenerators are generally superior over short distances, and fuel cells are better for long distances. Both can be used to charge a small but high-voltage bank of batteries to smooth out imbalances between power delivery and supply, or supply the airship’s grid directly. Even with good power and discharge rates, though, batteries are simply far too heavy to store more than a few minutes’ worth of peak power demand themselves, and need a near-constant supply of energy to keep them charged.

As former Flying Whales CEO Sébastien Bougon pointed out, the design and certification of a new large airship model would cost at minimum about a billion dollars. Since that’s such a vast barrier to entry, ideally you’d want the first transport airship on the market to be a generalist that is just as capable over short distances as it is over long distances, rather than being over-optimized for one or the other.

There is a large speed component to this calculation as well. According to Boeing, over short distances of 300 nautical miles (such as LA–SF), a neutrally buoyant rigid airship with 100 tons of payload will have an optimal cruising speed of 145 knots. Over transcontinental distances of up to 3,000 nautical miles, that optimum drops to 82 knots, and over intercontinental distances of 5,000 nautical miles, it drops to 63 knots.

Ideally a generalist airship should have enough reserve power to meet any optimal speed curve between its minimum and maximum range, but as a matter of practicality, linear increases in speed require exponential increases in power. To use the USS Macon as an example, the ship would require about 400 kW to fly at 35 knots, 3,340 kW to fly at 70 knots, and 6,420 kW to fly at 90 knots. In other words, doubling speed from 35 to 70 knots requires a roughly eightfold increase in power.

LTA Research and Hybrid Air Vehicles both posit efficient loiter speeds of 20 knots and long-distance cruising speeds of 40-55 knots, and both their planned ships have an intended top speed of 70 knots. Perhaps it is not a coincidence that 70 knots is considered by Zeppelin to be the bare minimum required speed for reliable scheduled service in the North Atlantic, since the weather tends to be fiercer over oceans than over land. 40 knots and 70 knots also happen to be the respective cruising speed and top speed of the highly capable ZPG-2W Navy blimps that performed admirably in severe weather tests like Project Lincoln and Operation Whole Gale. Those ZPG-2Ws had an 88% availability rate, and the proposed successor maritime patrol airship (MPA) would have preferred to increase that to 95% by employing a 90-knot top speed and VTOL capabilities, but the consensus seems to be that 70 knots is a good enough starting point for top speed.

So, putting aside the higher speeds that would be better suited to all-weather roles and short-range operations, let’s assume a 50 knot cruising speed is a decent enough compromise between the mutually exclusive demands of longer range and higher speed. 70 knots is likewise the minimum top speed necessary for safety margin to avoid or contend against headwinds and inclement weather. For a generic midsized airship a bit smaller than the Macon, it would need about 1 megawatt (1,000 kW) of power to cruise at 50 knots, and 3 MW to reach 70 knots.

Since the vast majority of its time would be spent at cruising speed with only intermittent uses of its top speed to deal with weather and headwinds, such a ship would gain almost all of the efficiency benefits of a purely fuel cell powertrain if it had one megawatt of fuel cells on board for 0-50 knot operations. The additional two megawatts of reserve power capacity for 51-70 knot speeds would just be sitting there unused most of the time anyway, so one might as well opt for the vastly lighter option and have those spare two megawatts consist of turbogenerators. That would also dovetail nicely with the longer maintenance intervals of fuel cells vs. turbogenerators, which one must consider in light of the thousands of annual flight hours necessary for economical operations.

It’s also worth considering that many turbogenerators are fuel-flexible, which opens up the possibility of using liquid fuels as ballast to remain in trim. The fuel cells could consume hydrogen kept in lightweight gaseous fuel ballonets safely ensconced inside helium gas cells, and the loss of buoyancy as the gaseous hydrogen is consumed could be compensated by burning cheaper, easier-to-use liquid fuels in a turbogenerator. That would additionally avoid the logistical headache of dealing with liquid hydrogen refueling infrastructure and cryogenic storage, and the exceedingly heavy pressure tanks needed by compressed hydrogen, so long as the fuel gas cells could be designed to sufficiently avoid contamination. Some kinds of fuel cells demand very high purity hydrogen, other kinds are much more tolerant of impurities, and a few are even fuel-flexible as well, but that last type is mostly still under development.

In the long term, as fuel-flexible, high-efficiency, high-temperature fuel cells improve in the lab and reach certification, the advantages of turbogenerators will erode and probably vanish someday. Until such time, though, the near-term practical merits of a hybrid approach definitely deserves consideration.

Flight testing continues for LTA Research's proof-of-concept airship. Here's a clip of her upon completing a 5 hour flight on 9/1 at Moffett Field, CA. The next day she made her longest flight to date, 10 hours.

The 1929 British airship R100 introduced several highly innovative design features seen in later airships, not least of which being the fact that it placed its passenger spaces inside the hull rather than in an external gondola to increase living space while reducing drag.

Over the decades, there have been several examinations of steam as a potential lift gas, ranging from the Germans’ experimental HeiDAS and HeiDAS UH balloons in the 2000s, to the USA Department of Commerce’s seminal 1970s Feasibility Study of Modern Airships, to NASA’s 1980s examinations of LTA vehicle concepts. Only a few snippets of these efforts are shown here.

Presently, there exists a severe gap between extremely slow but incredibly inexpensive hot air airships, and helium airships which are far more capable but about 10 times as expensive to buy, and even more expensive to operate. For most applications that would demand only a few hundred flight hours per year, which is to say the vast majority of general aviation applications, the functional inability of helium airships to be deflated and packed away when not in use makes them prohibitive due to ground crew, masting, and hangaring costs. Even switching to hydrogen wouldn’t change this dynamic, since hydrogen airships were seldom deflated either, as every inflation and deflation event carries fire risks and still costs quite a bit of money.

Current thermal airships do have easy storage capability, and are able to be packed away into a trailer after every flight in just an hour or two, but their dependence on hot air cripples their performance and renders them incapable of accomplishing the vast majority of uses that a helium airship of similar mass might be put to. Some thermal airships have gone for decades with only a few dozen hours of total flight time, which makes them very inexpensive relative to helium airship storage costs, but extremely expensive when it comes to effective lifetime costs per flight-hour.

The use of steam might be the key to unlocking similar speed, payload, and endurance to helium airships, but at a fraction of the cost—only marginally more than using hot air. But since greater speed, endurance, and payload could yield much higher utilization and greater productivity, that implies a steam airship has the potential to vastly outperform a hot air airship in terms of generating profits, and likewise outperform a helium airship on costs by a huge margin. If a steam airship could command the same revenues of a helium airship but at a tenth to a fifth the capital and operating cost, that would be a great boon to advertisers, recreational flyers, scientists, border patrols, and airship companies that don’t want to rely on aging helium blimps like American Blimp Corporation A-60R for flight training their pilots—such as the ones LTA Research and Flying Whales are forced to use, for lack of viable alternatives.

Since the HeiDAS team, Festo, and UltraMagic have already come up with suitable insulation materials to contain steam, and since steam balloons’ excellent performance relative to hot air balloons has already been confirmed by empirical data (~2.5x greater lift at 150°C than a normal hot air balloon of similar volume) collected from experimental subscale models, the remaining issues to resolve are to come up with a full-scale flying prototype to suss out any unforeseen problems, discover the best commercial or bespoke generator to use to produce steam onsite for safe and rapid inflation, design an easily-deflated hull with fully pneumatic or collapsible nose cones and tail fins that are capable of withstanding helium airship speeds, develop procedures for the safe venting of steam at the end of flight, and then proceed to certification and commercialization.

That all sounds like a lot, and it is, but the potential upside of breaking helium’s monopoly on long-endurance airship operations would make such a venture far more likely to result in wider proliferation of airships and trained airship pilots than merely designing the next generation of conventional helium blimp, such as China’s recently-certified Xiangyun-class AS700 and AS700D—since no matter how commendably advanced the propulsion, thrust vectoring, and avionics gets for these new airships relative to the older, smaller A-60R, they still have the same storage problems as all other helium blimps.

A thermal airship even capable of the modest feat of matching but not exceeding the performance of the A-60R would be immensely impactful for the airship industry. 600 mile range, 15-hour endurance, 30 knot cruise, and 50 knot top speed doesn’t sound like much on paper, but if it can come from an airship that costs as much as a Cessna and can be stored in a trailer or garage instead of chased around by a full-time cross-country ground crew? That would be very consequential indeed.