Whereas the engines in propeller-powered aircraft generally achieve their maximum efficiency at much lower speeds and altitudes, jet engines and aircraft achieve maximum efficiency (see specific impulse) at speeds close to or even well above the speed of sound. Jet aircraft generally cruise at faster than about M 0.8 (609mph, 981km/h or 273m/s) at altitudes around 10,000–15,000 metres (33,000–49,000ft) or more.
Frank Whittle, an English inventor and RAF officer, developed the concept of the jet engine in 1928,Hans von Ohain in Germany, a decade later developed the concept independently at the end of the 1930s. He wrote in February 1936 to Ernst Heinkel, telling him of the design and its possibilities. However, it can be argued that the English engineer A. A. Griffith, who published a paper in July 1926 on compressors and turbines, also deserves credit.
Yemen (i/ˈjɛmən/; Arabic:اليَمَنal-Yaman), officially known as the Republic of Yemen (الجمهورية اليمنيةal-Jumhūrīyah al-Yamanīyah), is an Arab country in Western Asia, occupying the southwestern to the southern end of the Arabian Peninsula. Yemen is the second largest country in the peninsula, occupying 527,970km2 (203,850sqmi). The coastline stretches for about 2,000km (1,200mi). It is bordered by Saudi Arabia to the north, the Red Sea to the west, the Gulf of Aden and Arabian Sea to the south, and Oman to the east. Although Yemen's constitutionally stated capital is the city of Sana'a, the city has been under rebel control since February 2015. Because of this, Yemen's capital has been temporarily relocated to the port city of Aden, on the southern coast. Yemen's territory includes more than 200 islands; the largest of these is Socotra.
Yemen was the home of the Sabaeans (biblical Sheba), a trading state that flourished for over a thousand years and probably also included parts of modern-day Ethiopia and Eritrea. In 275 AD, the region came under the rule of the later Jewish influenced Himyarite Kingdom. Christianity arrived in the 4th century AD whereas Judaism and local paganism were already established. Islam spread quickly in the 7th century and Yemenite troops were crucial in the expansion of the early Islamic conquests. Administration of Yemen has long been notoriously difficult. Several dynasties emerged from the 9th to 16th century, the Rasulid being the strongest and most prosperous. The country was divided between the Ottoman and British empires in the early 20th century. The ZaydiMutawakkilite Kingdom of Yemen was established after World War I in North Yemen before the creation of Yemen Arab Republic in 1962. South Yemen remained a British protectorate until 1967. The two Yemeni states united to form the modern republic of Yemen in 1990.
The Yemen Arab Republic (YAR; Arabic:الجمهورية العربية اليمنيةal-Jumhūrīyah al-‘Arabīyah al-Yamanīyah), also known as North Yemen or Yemen (Sana'a), was a country from 1962 to 1990 in the northwestern part of what is now Yemen. Its capital was at Sana'a. It united with the People's Democratic Republic of Yemen, commonly known as South Yemen, on May 22, 1990, to form the current Republic of Yemen.