Combustion

Part 13 - Rockets 3

Rocket engineers realized that a vehicle with enough velocity to orbit the Earth had enough kinetic energy to vaporize itself on re-entering the Earth's atmosphere. (A satellite in low Earth orbit, at an altitude of 200 to 2000 km, has a velocity of up to 28,000 km/h (17,000 mph) which means it has a specific kinetic energy of up to 30 MJ/kg). 

And yet, they knew that some meteorites reached the ground without being destroyed by friction with the Earth's atmosphere.

In 1951, H. Julian Allen and A. J. Eggers, of the U.S. National Advisory Committee for Aeronautics (NACA), discovered that a blunt, high drag shape, dissipated about 99% of the energy into the air, rather than into the vehicle. This meant that a satellite could be safely returned to the ground without the aid of massive amounts fuel for retro rockets.

Air molecules moving over the wings an aircraft provide the lifting force but also a drag force that is overcome by the engines. Exactly how the air affects an aircraft depends upon the ratio of the speed of the aircraft to the speed of sound through the air. This is known as the Mach number in honour of Ernst Mach, a physicist who studied shockwaves created by supersonic bullets late in the 19th century. 

A speed of Mach 1 means that an aircraft is flying at the speed of sound and a spacecraft, re-entering the earth's atmosphere, is travelling very much faster, at a hypersonic speed close to Mach 25. Low earth orbit re-entry speeds are typically near 17,500 mph. Many miles above the earth's surface, the air density is very low but the temperature of the airflow is so high that the chemical bonds of the diatomic molecules of the air are broken. This produces an electrically charged plasma around the re-entry vehicle while its lower surface generates strong shock waves.

U.S. research was abruptly energized on October 4, 1957, when the Soviet Union launched the satellite Sputnik 1 into Earth orbit. Less than a month later, the Soviets launched a satellite carrying a dog named Laika who survived in space for seven days before being anaethetized as the oxygen supply ran out.

The blunt body theory was initially treated as a military secret (as a nuclear weapon in orbit was useless if it was unable to survive re-entry) but it was eventually published in 1958 and was used to design heat shields for the re-entry capsules of the Mercury, Gemini, Apollo, and Soyuz space capsules. An ablative heat shield made of ceramic material slowly burned away in the high temperature plasma flow, behind the bow shock wave. The change of phase of the ceramic shield, from solid to liquid to gas, and the convection of the flow away from the spacecraft protecting the astronauts from the heat of re-entry.

After a few failures, the United States launched an Explorer 1 satellite, on a Jupiter-C rocket, into space in February 1958 and later that year, the National Aeronautics and Space Administration (NASA) was established as a civilian agency with the goal of peaceful exploration of space for the benefit of all people.

On October 7, 1959, the Soviet probe Luna 3 took the first photographs of the, previously unseen, far side of the Moon.

However, Sputnik had proven that the Soviet R-7 rocket was capable of reaching any point in the USA carrying a nuclear weapon. It was potentially the first intercontinental ballistic missile or ICBM. 

Governments realized that there was no feasible defence against rocket propelled nuclear weapons, once they were launched. Conventional long range bombers were no longer the only way to deliver nuclear weapons. The Soviet R-7 and American Atlas, and Titan rockets were quickly modified to carry nuclear warheads and the Cold War became more intense.

On April 12, 1961, the Vostok-K rocket launched Yuri Gagarin into the vacuum of space. He was the first man to orbited once around the planet (in 108 minutes) and safely return to the ground in the descent module Vostok I. On May 5, 1961, a Mercury-Redstone 3 launched Alan Shepard into a suborbital flight mission to become the first American and the second person to travel into space.