Airships date back to the 19th century, had a peak use in the first Weltkrieg and died off rather quickly. In 1938, however, a naturally occurring endemic gas called "sturbane" was found to have extreme buoyancy around atmospheric air and even in very low pressure environments once subjected to a chemical process. It was extracted, worked and commercialised as "liftgas". Airships operating by liftgas can carry far heavier loads than a helium or hydrogen variant while capable of extremely high altitude flight. In 1941 a Ferozen engineer invented a method for mechanical altitude control that revolutionised airship design. Airships, instead of becoming obsolete, had become a neat if niche addition to Oceanykan air forces even into the jet fighter era. Even some foreign nations, such as Germany or the USSR, developed liftgas-based reconnaissance systems.
The dawn of airspace trade theory made regular air travel a lot more important and omnipresent, while dwarfing airships. Disillusioned and losing revenue, the airship manufacturing companies began designing and constructing even better designs. These new generation vessels were capable of continued travel at up to an unthinkable 20km above sea level. After electric heating, oxygen compressors, turbocharger design advancements and the development of effective navigation techniques (based in dead reckoning and astronavigation) were implemented, the first of these marvellous machines was launched. It crashed and all crew on board died. On the second attempt all crew died as the pressure suits were unfit for such high altitudes. But on the third attempt, a spring of 1957, the entire landmass of Australia was successfully crossed at a constant 18km altitude in six days.
Stratospheric trade theory is very similar to airspace trade theory. To avoid the filthy peasants, dirty robberies and bloody brawls in ground and sea, air travel becomes a profitable way to practice commerce. However, airspace trade theory has freed itself from the plague that is air piracy. This creates a fourth space where only liftgas-powered airships can operate. Rockets and the most modern of aircraft may reach it too, but the vast majority of fighters and transport craft are unable to do so. Granted, almost as soon as liftgas airships became available, air piracy expanded into the stratosphere, but the risk is much smaller..