The brothers subsequently experimented with an elongated elliptical shape for the hydrogen envelope in a balloon they attempted to power and steer by means of oars and umbrellas. In September 1784 the brothers flew 186 km from Paris to Beuvry, the world's first flight of more than 100 km.
The Robert brothers were skilled engineers with a workshop at the Place des Victoires in Paris, who worked with professor Jacques Charles to build the first usable hydrogen balloon in 1783. Charles conceived the idea that hydrogen would be a suitable lifting agent for balloons because, as a chemist, he had studied the work of his contemporaries Henry Cavendish, Joseph Black and Tiberius Cavallo.
Jacques Charles designed the hydrogen balloon and the Robert brothers invented the methodology for constructing the lightweight, airtight gas bag. They dissolved 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.
Jacques Charles and the Robert brothers launched their balloon, the world's first hydrogen-filled balloon, on August 27, 1783, from the Champ-de-Mars (now the site of the Eiffel Tower); Benjamin Franklin was among the crowd of onlookers. The balloon was comparatively small, a 35 cubic-metre sphere of rubberised silk, and only capable of lifting about 9 kg. It was filled with hydrogen that had been made by pouring nearly a quarter of a tonne of sulphuric acid onto half a tonne of scrap iron. The hydrogen gas was fed into the envelope through 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). Daily progress bulletins were issued on the inflation; and the crowd was so great that on the 26th the balloon was moved secretly by night to the Champ-de-Mars, a distance of 4 kilometres.
The balloon flew northwards for 45 minutes, pursued by chasers on horseback, and landed 21 kilometres away in the village of Gonesse where the reportedly terrified local peasants attacked it with pitchforks or knives and destroyed it. The project was funded by a subscription organised by Barthélemy Faujas de Saint-Fond.
At 13:45 on December 1, 1783, Professor Jacques Charles (after whom a hydrogen balloon came to be called a Charlière ) and the Robert brothers launched a new manned balloon from the Jardin des Tuileries in Paris, amid vast crowds and excitement. The balloon was held on ropes and led to its final launch place by four of the leading noblemen in France, the Marechal de Richelieu, Marshal de Biron, the Bailiff of Suffren, and the Duke of Chaulnes. Jacques Charles was accompanied by Nicolas-Louis Robert as co-pilot of the 380-cubic-metre, hydrogen-filled, balloon. The envelope was fitted with a hydrogen release valve and was covered with a net from which the basket was suspended. Sand ballast was used to control altitude. They ascended to a height of about 1,800 feet (550 m) and landed at sunset in Nesles-la-Vallée after a 2-hour, 5-minute flight covering 36 km. The chasers on horseback, who were led by the Duc de Chartres, held down the craft while both Charles and Nicolas-Louis alighted.
Jacques Charles then decided to ascend again, but alone this time because the balloon had lost some of its hydrogen. The balloon ascended rapidly to an altitude of approximately 3,000 metres, rising into the sunlight again, so that Charles then saw a second sunset. He began suffering from aching pain in his ears so he 'valved' to release gas, and descended to land gently about 3 km away at Tour du Lay. Unlike the Robert brothers, Charles never flew again.