I discuss some of the problems with making medieval balloons, and point out that it would be almost impossible to build diribible airships in medieval times without extraterrestrial or time traveler technology. But I do suggest a few possible uses for non dirigible balloons in a medieval society.
You say about methane:
True, it may be flammable, but it cannot leak out of paper or silk, easy-to-produce hull materials in medieval levels. So this idea got into my mind.
Are paper and silk easy to produce materials at medieval levels of technology?
Part One: Paper.
In ancient times, the paper equivlents used the the Mediterranian region were papyrus from Egypt and parchment, prepared animal skins, which was better but more expensive than papyrus. And I think that in early 20th century airships the gasbags were made of many, many pieces of animal skin sewn together.
Paper was invented in China. Cai Lun (c. 50-62 AD to 121 AD), an imperial eunuch official, is tradionally credited with perfecting papermaking.
He is traditionally regarded as the inventor of paper and the modern papermaking process. Although early forms of paper had existed since the 3rd century BCE, he occupies a pivotal place in the history of paper due to his addition of pulp via tree bark and hemp ends, which resulted in the large-scale manufacture and worldwide spread of paper.
Cai's improvements to paper-making are considered to have had an enormous impact on human history, and of those who created China's Four Great Inventions—the compass, gunpowder, papermaking and printing—Cai is the only inventor whose name is known. Although in China he is revered in ancestor worship, deified as the god of papermaking, and appears in Chinese folklore, he is mostly unknown outside of East Asia. His hometown in Leiyang remains an active center of paper production.
Paper was used in Central Asia by the 8th century but its origin is not clear. According to the 11th century Persian historian, Al-Thaʽālibī, Chinese prisoners captured at the Battle of Talas in 751 introduced paper manufacturing to Samarkand. However there are no contemporary Arab sources for this battle. A Chinese prisoner, Du Huan, who later returned to China reported weavers, painters, goldsmiths, and silversmiths among the prisoners taken, but no papermakers. According to Al-Nadim, a writer in Baghdad during the 10th century, Chinese craftsmen made paper in Khorasan:
Then there is the Khurasani paper made of flax, which some say appeared in the days of the Umayyads, while others say it was during the Abbasid regime. Some say that it was an ancient product and others say that it is recent. It is stated that craftsmen from China made it in Khurasan in the form of Chinese paper.
The oldest known paper document in Europe is the Mozarab Missal of Silos from the 11th century, probably using paper made in the Islamic part of the Iberian Peninsula. They used hemp and linen rags as a source of fiber. The first recorded paper mill in the Iberian Peninsula was in Xàtiva in 1056. Papermaking reached Europe as early as 1085 in Toledo and was firmly established in Xàtiva, Spain by 1150. During the 13th century mills were established in Amalfi, Fabriano, and Treviso, Italy, and other Italian towns by 1340. Papermaking then spread further northwards, with evidence of paper being made in Troyes, France by 1348, in Holland sometime around 1340–1350, and in Nuremberg, Germany by 1390 in a mill set up by Ulman Stromer. This was just about the time when the woodcut printmaking technique was transferred from fabric to paper in the old master print and popular prints. There was a paper mill in Switzerland by 1432 and the first mill in England was set up by John Tate around 1490 near Hertford, but the first commercially successful paper mill in Britain did not occur before 1588 when John Spilman set up a mill near Dartford in Kent. During this time, paper making spread to Austria by 1469, to Poland by 1491, to Russia by 1576, to the Netherlands by 1586, to Denmark by 1596, and to Sweden by 1612.
I am not certain whether the "medieval" in the question refers to medieval China, medieval India, medieval Islamic society, or medieval Europe, but all those societies had paper making before the end of the middle ages.
And I wonder how cheap or expensive paper was in those medieval societies and how large the usual sheets of paper were, and how large they could have been made with medieval technology.
Even a tiny size hot air balloon would require an air bag at least 100 by 100 feet or meters, and thus a total area of 10,000 square feet or meters. And I doubt whether an airbag that large could have been made in a single piece of paper.
And I have to wonder if medieval glue or medieval thread could hold together a paper airbag made of of tens, hundreds, or thousands of pieces. And of course if someone is considered physiclaly weak the cliche is to say "he couldn't fight his way out of a paper bag". I can imagine airship disasters caused by giant paper airbags ripping apart from the stress of supporting the weight of the gondola.
Part two: Silk.
Several kinds of wild silk, produced by caterpillars other than the mulberry silkworm, have been known and spun in China, South Asia, and Europe since ancient times, e.g. the production of Eri silk in Assam, India. However, the scale of production was always far smaller than for cultivated silks. There are several reasons for this: first, they differ from the domesticated varieties in colour and texture and are therefore less uniform; second, cocoons gathered in the wild have usually had the pupa emerge from them before being discovered so the silk thread that makes up the cocoon has been torn into shorter lengths; and third, many wild cocoons are covered in a mineral layer that prevents attempts to reel from them long strands of silk.4 Thus, the only way to obtain silk suitable for spinning into textiles in areas where commercial silks are not cultivated was by tedious and labor-intensive carding.
Silk use in fabric was first developed in ancient China.12 The earliest evidence for silk is the presence of the silk protein fibroin in soil samples from two tombs at the neolithic site Jiahu in Henan, which date back about 8,500 years.14 The earliest surviving example of silk fabric dates from about 3630 BC, and was used as the wrapping for the body of a child at a Yangshao culture site in Qingtaicun near Xingyang, Henan.12
Legend gives credit for developing silk to a Chinese empress, Leizu (Hsi-Ling-Shih, Lei-Tzu). Silks were originally reserved for the Emperors of China for their own use and gifts to others, but spread gradually through Chinese culture and trade both geographically and socially, and then to many regions of Asia. Because of its texture and lustre, silk rapidly became a popular luxury fabric in the many areas accessible to Chinese merchants. Silk was in great demand, and became a staple of pre-industrial international trade. Silk was also used as a surface for writing, especially during the Warring States period (475-221 BCE). The fabric was light, it survived the damp climate of the Yangtze region, absorbed ink well, and provided a white background for the text. In July 2007, archaeologists discovered intricately woven and dyed silk textiles in a tomb in Jiangxi province, dated to the Eastern Zhou dynasty roughly 2,500 years ago. Although historians have suspected a long history of a formative textile industry in ancient China, this find of silk textiles employing "complicated techniques" of weaving and dyeing provides direct evidence for silks dating before the Mawangdui-discovery and other silks dating to the Han dynasty (202 BC – 220 AD).
Silk is described in a chapter of the Fan Shengzhi shu from the Western Han (202 BC – 9 AD). There is a surviving calendar for silk production in an Eastern Han (25–220 AD) document. The two other known works on silk from the Han period are lost.8 The first evidence of the long distance silk trade is the finding of silk in the hair of an Egyptian mummy of the 21st dynasty, c.1070 BC. The silk trade reached as far as the Indian subcontinent, the Middle East, Europe, and North Africa. This trade was so extensive that the major set of trade routes between Europe and Asia came to be known as the Silk Road.
The Emperors of China strove to keep knowledge of sericulture secret to maintain the Chinese monopoly. Nonetheless, sericulture reached Korea with technological aid from China around 200 BC, the ancient Kingdom of Khotan by AD 50, and India by AD 140.
In the Odyssey, 19.233, when Odysseus, while pretending to be someone else, is questioned by Penelope about her husband's clothing, he says that he wore a shirt "gleaming like the skin of a dried onion" (varies with translations, literal translation here) which could refer to the lustrous quality of silk fabric. Aristotle wrote of Coa vestis, a wild silk textile from Kos. Sea silk from certain large sea shells was also valued. The Roman Empire knew of and traded in silk, and Chinese silk was the most highly priced luxury good imported by them. During the reign of emperor Tiberius, sumptuary laws were passed that forbade men from wearing silk garments, but these proved ineffectual. The Historia Augusta mentions that the third-century emperor Elagabalus was the first Roman to wear garments of pure silk, whereas it had been customary to wear fabrics of silk/cotton or silk/linen blends. Despite the popularity of silk, the secret of silk-making only reached Europe around AD 550, via the Byzantine Empire. Contemporary accounts state that monks working for the emperor Justinian I smuggled silkworm eggs to Constantinople from China inside hollow canes. All top-quality looms and weavers were located inside the Great Palace complex in Constantinople, and the cloth produced was used in imperial robes or in diplomacy, as gifts to foreign dignitaries. The remainder was sold at very high prices.
Italy was the most important producer of silk during the Medieval age. The first center to introduce silk production to Italy was the city of Catanzaro during the 11th century in the region of Calabria. The silk of Catanzaro supplied almost all of Europe and was sold in a large market fair in the port of Reggio Calabria, to Spanish, Venetian, Genovese, and Dutch merchants. Catanzaro became the lace capital of the world with a large silkworm breeding facility that produced all the laces and linens used in the Vatican. The city was world-famous for its fine fabrication of silks, velvets, damasks, and brocades.
Another notable center was the Italian city-state of Lucca which largely financed itself through silk-production and silk-trading, beginning in the 12th century. Other Italian cities involved in silk production were Genoa, Venice, and Florence. The Piedmont area of Northern Italy became a major silk producing area when water-powered silk throwing machines were developed.
The Silk Exchange in Valencia from the 15th century—where previously in 1348 also perxal (percale) was traded as some kind of silk—illustrates the power and wealth of one of the great Mediterranean mercantile cities.
So by the late middle ages silk making had spread from China to large parts of Asia, northern Africa, and to southern Europe. And I have wonder how much silk was produced and how expensive it was, since only a minority of people in those areas wore silk garments.
Part Thrre: Paper and/or silk for balloons?
At the time of their invention hot air balloons were usually made of silk, which is wonderfully suited to the purpose. Unfortunately, it is also prohibitively expensive and extremely difficult to repair. Modern hot air balloon fabrics have replaced silk with lighter and stronger options. Most hot air balloons are made of nylon now. Tyvek is sometimes used with experimental balloons, but its flammability makes it unsuitable for most purposes.
On November 21, 1783 the first free flight carrying a human occurred in Paris, France in a hot air balloon made of paper and silk made by the Montgolfier brothers.
The first public demonstartion of a hot air balloon was by the Montgolfier brothers in 1783.
To make a public demonstration and to claim its invention the brothers constructed a globe-shaped balloon of sackcloth tightened with three thin layers of paper inside. The envelope could contain nearly 790 m³ (28,000 cubic feet) of air and weighed 225 kg (500 lb). It was constructed of four pieces (the dome and three lateral bands) and held together by 1,800 buttons. A reinforcing fish net of cord covered the outside of the envelope.
On 4 June 1783, they flew the balloon at Annonay in front of a group of dignitaries from the états ″particuliers″. The flight covered 2 km (1.2 mi), lasted 10 minutes, and had an estimated altitude of 1,600–2,000 m (5,200–6,600 ft). Word of their success quickly reached Paris. Étienne went to the capital to make further demonstrations and to solidify the brothers' claim to the invention of flight. Joseph, given his unkempt appearance and shyness, remained with the family. Étienne was the epitome of sober virtues ... modest in clothes and manner...9
So some early balloons combined paper and silk, probably in an attempt to combine the good qualities of each material.
Parth Four: A Hydrogen filled Medieval Balloon?
I note that the first hydrogen filled balloon was launched shortly afterwards, on August 27, 1783.
Charles conceived the idea that hydrogen would be a suitable lifting agent for balloons having studied the work of Robert Boyle's Boyle's Law which was published 100 years earlier in 1662, and of his contemporaries Henry Cavendish, Joseph Black and Tiberius Cavallo.3 He designed the craft and then worked in conjunction with the Robert brothers, Anne-Jean and Nicolas-Louis, to build it in their workshop at the Place des Victoires in Paris.4 The brothers invented the methodology for the lightweight, airtight gas bag by dissolving rubber in a solution of turpentine and varnished the sheets of silk that were stitched together to make the main envelope. They used alternate strips of red and white silk, but the discolouration of the varnishing/rubberising process left a red and yellow result.3
So as early as 1783 balloons of silk, without paper, were built, using a coating on the silk in an attempt to make it airtight.
Charles and the Robert brothers bullt the first attempt at a dirigible balloon which flew on July 15, 1784, but the propulsion method didn't work.
How was the hydrogen for the first hydrogen balloon produced?
It was filled with hydrogen that had been made by pouring nearly a quarter of a tonne of sulphuric acid onto a half a tonne of scrap iron.9 The hydrogen gas was fed into the balloon via lead pipes; but as it was not passed through cold water, great difficulty was experienced in filling the balloon completely (the gas was hot when produced, but as it cooled in the balloon, it contracted).
I note that 18th century chemistry had only comparatively recently evolved from medieval alchemy.
Sulfuric acid was called "oil of vitriol" by medieval European alchemists because it was prepared by roasting "green vitriol" (iron(II) sulfate) in an iron retort. The first vague allusions to it appear in the works of Vincent of Beauvais, in the Compositum de Compositis ascribed to Saint Albertus Magnus, and in pseudo-Geber's Summa perfectionis (all thirteenth century AD).
In the seventeenth century, the German-Dutch chemist Johann Glauber prepared sulfuric acid by burning sulfur together with saltpeter (potassium nitrate, KNO3), in the presence of steam. As saltpeter decomposes, it oxidizes the sulfur to SO3, which combines with water to produce sulfuric acid. In 1736, Joshua Ward, a London pharmacist, used this method to begin the first large-scale production of sulfuric acid.
So it seems to me that a medieval society with a more advanced alchemy/chemstry might be able to produce sulpheric acid and hydrogen for hydrogen balloons.
Part Five: Dirigibles?
A dirigible is a dirigible balloon. Dirigible means capable of being steered, guided, or directed. As I wrote above, Charles and the Robert brothers attempted to built the first dirigible balloon in 1784, following the proposals of Jean-Baptist Meusnier.
Meusnier is sometimes portrayed as the inventor of the dirigible, because of an uncompleted project he conceived in 1784, not long after the first balloon flights of the Montgolfiers, and presented to the French Academy of Sciences. This concerned an elliptical balloon (ballonet) 84 metres long, with a capacity of 1,700 cubic metres, powered by three propellers driven by 80 men. The basket, in the form of a boat, was suspended from the canopy on a system of three ropes.
In 1852, sixty six years after Charles and the Robert brothers 'oar powered' dirigible, Henri Giffard's design for the first successful powered airship was inspired by Meusnier's ideas.5
On 24 September 1852, Giffard made the first powered and controlled flight travelling 27 km from Paris to Élancourt.3 The wind was too strong to allow him to make way against it, so he was unable to return to the start.3 However, he was able to make turns and circles, proving that a powered airship could be steered and controlled.4
So Gifford's flight showed that a dirigible powered by a steam engine was possible.
But it was not until internal compustion engines could produce more power than steam engines while having less weight than steam engines, that either airplanes or dirigibles becme practical. The first practical dirigible was probably the first Zeppelin in 1900.
So I find it a little hard to believe in medieval airships using hot air, hydrogen, methane, or any other lifting gas, whithout a strong low weight power source such as would not be developed until centuries after the middle ages.
Part Six: Uses For (Possibly Methane Filled) Balloons in the Middle Ages.
If powered flight would be a very improbable use for balloons, methane filled or otherwise, without a power source which would be very improbable to develop in the middle ages, what could balloons be used for?
user535733 suggested tethered aero wagons pulled by oxen on the ground.
tethered balloons were used for military observation purposes in the US Civil War for 1861-1865 and as early as the French Revolutionary War.
The first military use of observation balloons was by the French Aerostatic Corps during the French Revolutionary Wars, the very first time during the Battle of Fleurus (1794).1 The oldest preserved observation balloon, L'Intrépide, is on display in a Vienna museum. They were also used by both sides during the American Civil War (1861–65)2 and continued in use during the Franco-Prussian War (1870–71).3 Balloons were first deployed by the British Army's Royal Engineers during the expeditions to Bechuanaland in 1884 and Suakin in 1885.4 They were also deployed during the Second Boer War (1899–1902), where they were used in artillery observation at the Battle of Magersfontein and during the Siege of Ladysmith.5 In South America, a reconnaissance balloon was deployed by Brazil in July 1867 during the Paraguayan War.9
- Other military uses?
Johann Muller von Konigsburg, or Regiomontanus (1436-1475), was a German scholar. Legend says he made mechanical flying animals, an eagle which flew to great a visiting emperor, and a tiny fly exhibited indoors. This has inspired the famous comment about the wonders of minituarization: "Who admireth not the fly of Regiomontanus more than his eagle".
I have read a suggestion that his flying eagle could have simply been a kite. And that reminds that I read something about some sort of aerial devices, some shaped like eagls (one or two headed?) used by German armies in the early modern era. Those flying devices might have been kites or powered by gunpowder rockets, like the somewhat later rockets of Mysore and Congreave.
What military purpose would those aerial devices have? If they were propelled by gunpowder rockets they could strike someone and explode. And if they simply flew above an army they might intimidate the opposing army.
And you may remember the Macy's Thanksgiving Day Parades in NYC with giant balloons shaped like all kinds of fictional characters carried along the streets. So possibly in a medieval society vast balloons shaped like dragons or other monsters could be made and floated above and army to scare members of opposing armies.
A gas holder or gasholder, also known as a gasometer, is a large container in which natural gas or town gas is stored near atmospheric pressure at ambient temperatures. The volume of the container follows the quantity of stored gas, with pressure coming from the weight of a movable cap. Typical volumes for large gas holders are about 50,000 cubic metres (1,800,000 cu ft), with 60-metre (200 ft) diameter structures.
I note that methane is a major component of natural gas which is stored in gas holders.
Many gas holders have a gas tank surrounded by a frame work of girders. Such a framework could enclose a balloon full of methane, for example.
I have seen pictures of balloons with all sorts of custom made shapes,including buildings. And of course a building shaped balloon would have to be tied down to keep from floating away instead of exerting weight upon its foundations.
Here is a link https://en.wikipedia.org/wiki/USS_Los_Angeles_(ZR-3)#/media/File:Zr3nearvertical.jpg to a photo of the 658 foot 4 inch long airship USS Los Angeles tied to a 200foot tall mooring tower on August 25, 1927. In this terrifying photo, a sudden gus tof wind has pushed the Los Angeles almost vertical.
Suppose that the outer girder framework of a gasometer was built as tall as an airship was long, and a vertical airship was built and inflated inside the framework and tied to the framework. If the weight of the framework and the lift of the airshop inside were almost identical, the frame with an attached airship inside might possibly be constructed to heights of hundreds or thousands of feet or meters without becoming too heavy to to stand.
An air-supported (or air-inflated) structure is any building that derives its structural integrity from the use of internal pressurized air to inflate a pliable material (i.e. structural fabric) envelope, so that air is the main support of the structure, and where access is via airlocks.
And obviously a pressurized building couldn't have been made in medieval times.
But people could build a vast hall with walls and no roof. And if they could build a large balloon with the dimensions of the hall, they could float it over the walls and attach it to the walls as a roof over the hall. Thus large balloons could be built to look like vast domed roofs when seen from the outside.
So hypothetical medieval balloons could have had hypothetical uses in medieval architecture.
- One Way Transportation?
If there is a place with winds with fairly steady direction and speed, balloons could be launched from that place in the direction of the prevailing winds. Presumably when they reached open fields or lakes near the destination community they could be deflated to land, and folded up for eventual transport back to where they came from, while the passengers have much shorter trip to their destination than if went htewhole way on land.
Possibly the place of launching could be some sort of observation point, and messages sent by balloons would usually reach headquarters much sooner messages sent on horseback.
Even though it would be highly difficult to make powered dirigible airships in medieval times without technology from aliens or time travelers, other uses for balloons can be imagined.