Dirigible

Dirigible (pronounced dir-i-juh-buhl or dih-rij-uh-buhl)

(1) An airship.

(2) A machine designed for or capable of being directed, controlled, or steered.

1583:  From the Latin dīrigere (to set straight), present active infinitive of dīrigō (steer, to direct) where it existed as an adjective with the meaning in the literal sense.  The use to describe airships dates from 1885, a direct borrowing from the French balloon dirigeable (steerable balloon), from the adjective dirigeable (capable of being directed or guided), a practice English would repeat with words like aileron and fuselage, reflecting the early French lead in aviation.  Dirigible is a noun & adjective, indirigibility & dirigibility are nouns and indirigible is an adjective; the noun plural is dirigibles. 

The Hindenburg, 1937

Frontal schematic of Hindenburg (LZ 129).

Built in the mid-1930s, Nazi Germany’s two Hindenburg-class dirigibles were hydrogen-filled, passenger-carrying rigid airships, the family named after Generalfeldmarschall Paul von Hindenburg (1847–1934; Reichspräsident (1925-1934) of the German Weimar Republic 1918-1933). They were the last of their type and the largest ever to fly, longer than three of the original Boeing 747s and only slightly shorter than RMS Titanic.  The first commissioned was Hindenburg (LZ 129) which undertook its maiden flight in March 1936 and powered by four Daimler-Benz DB602 V16 diesel engines rated at 890 kW (1190 horsepower), under neutral atmospheric conditions it typically cruised at 130 km/h (80 mph).  That made for leisurely transatlantic crossings (albeit much faster than ocean liners) but it could carry 72 passengers in some comfort as well as several tonnes for freight over a maximum range of 14,000 km (8700 miles).  By contrast, the Junkers Ju 52 (a contemporary airliner), although able to cruise faster at 210 km/h (130 mph) rarely carried more than 18 passengers with range restricted to a hardly intercontinental 1000 km (625 miles).  With a Berlin-New York ticket priced at the equivalent of around US$8000 in 2024 values, the big dirigible was the Concorde of its era and although obviously not supersonic, it was more luxurious and unlike the noisy, vibrating Ju 52, passengers enjoyed the comfort of small cabins, washrooms, a bar, a dining room and several lounges.  Although it may seem surprising given the craft's fiery demise, located within the body of the craft (surrounded by all that combustible hydrogen) was a smoking room.  To enter or leave the smoking room, passengers had to pass through an air-lock designed to ensure the flammable gas never came into contact with a naked flame, the barman's most important duty to check each of his customers as they went to leave, ensuring no forgetful smokers left with cigarettes still lit.

Hindenburg's interior was decorated in the then fashionable art deco "ocean liner" style: dining room (left), bar with a waiting bottle of Benedictine liqueur (centre) and lounge with world map annotated showing symbols illustrating the routes flown by the dirigible (right).

The safety record of the big dirigibles in the previous two decades had been patchy with a half-dozen crashes involving French, US and British craft with a death toll of some 250 souls.  Despite this, the Hindenburg operated a regular and profitable transatlantic passenger service for over a year, flying to both South and North America.  However, the designers were forced to use highly flammable hydrogen rather than inert helium because the US, the only country able to supply the necessary volume of the latter, refused on grounds of national security.  On 6 May 1937, while attempting to dock at Lakehurst Naval Air Station, New Jersey, the Hindenburg was destroyed by fire with the loss of 36 lives and although a variety of hypotheses have been suggested, the cause of ignition remains unknown.  The disaster shattered public confidence and marked an abrupt end of the passenger airship era, the Hindenburg’s sister-ship, the Graf Zeppelin never operated on a regular passenger service and scrapped in 1940.  Hitler's last word on the matter of the accident was "it was an act of God".  The combination of the Hindenburg disaster and the rapid improvement in the range and capacity of post-war airliners meant there was no return of the big passenger dirigibles, even among those manufacturers with access to ample supplies of helium.  Despite that, the things never went away and for decades there has been interest by both the military and commercial concerns in using them for a variety of purposes including as camera platforms, carriers of scientific instruments and survey vehicles for mining exploration and oceanographic research.  Dirigible the word however endured less well.  Preferred use now is the generic airship or, sometimes inaccurately given it’s technically different from the rigid or semi-rigid dirigible, the pleasing blimp.

Last moments of the Hindenburg, Lakehurst Naval Air Station, New Jersey, 6 May 1937.

One novel (although an adaptation of a concept in use for almost a century) use which remains at the proof of concept stage is as a lift vehicle for the LOHAN (Low Orbit Helium Assisted Navigator) and although there was no direct connection to Lindsay Lohan (not noted for her interest in aerospace matters), the project team cheerfully did admit the acronym was constructed as a publicity tool, blatantly trading on her notoriety (the name dating from 2012).  The idea is that because the cost of delivering a rocket-powered spaceplane into the stratosphere is so significant, if the travel from the ground to the critical altitude at which the rocket engine will ignite can be handled instead by a helium-filled dirigible, the economics of such devices become more compelling.  The concept is for the dirigible to carry a large number of rockets to an altitude of 65,000 feet (20,000 m) at which point they’re released, allowing their journey to continue to the planned altitude of some 80,000 feet (25,000 m), the dirigible returning to ground to be re-used for subsequent launches.  The initial testing plan was to use a large helium balloon to carry as small scale (3D-printed) rocket but despite years of negotiation, the developers were never able to secure a launch licence from US aviation authorities.

The LOHAN spaceplane.

Interestingly, the substantially amateur team found the greatest engineering challenge was not the lift device or the design of the spaceplane but ensuring the rocket reliably would fire as required, the thin atmosphere at 20,000 m containing only some 5% of the oxygen typically found as sea level, the solution being an exotic igniter-mix, a kind of super spark-plug which works when 12 miles (20 km) high.  Without the prototype able to be tested, it’s not clear if things are in abeyance.