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A space capsule is an often manned spacecraft which has a simple shape for the main section, without any wings or other features to create lift during atmospheric reentry. Capsules have been used in most of the manned space programs to date, including the world's first manned spacecraft Vostok and Mercury, as well as in later Soviet Voskhod, Soyuz, Zond/L1, L3, TKS, US Gemini, Apollo Command Module, Chinese Shenzhou and US, Russian and Indian manned spacecraft currently being developed. A capsule is the specified form for the Orion Multi-Purpose Crew Vehicle.
A manned space capsule must have everything necessary for everyday life, including air, water and food. The space capsule must also protect astronauts from the cold and radiation of space. A capsule must be well insulated and have a system that controls the inside temperature and environment. It also must have a way that the astronauts would not be knocked around during launch or reentry. Additionally, since the inside will be weightless, there must be a way for the astronauts to stay in their seats during the flight. For this each seat has a system of straps and buckles. One of the most important things that a space capsule must have is a way to communicate with people back on Earth, or mission control.
Space capsules have typically been smaller than 5 meters (16 feet) in diameter, although there is no engineering limit to larger sizes. As the capsule is both volumetrically efficient and structurally strong, it is typically possible to construct small capsules of performance comparable in all but lift-to-drag ratio to a lifting body or delta wing form for less cost. This has been especially pronounced in the case of the Soyuz manned spacecraft. Most space capsules have used an ablative heat shield for reentry and been non-reusable. The Orion Multi-Purpose Crew Vehicle appears likely, as of December 2005, to be a ten-times reusable capsule with a replaceable heat shield. There is no limit, save for lack of engineering experience, on using high-temperature ceramic tiles or ultra-high temperature ceramic sheets on space capsules.
Materials for the space capsule are designed in different ways, like the Apollo Command Module’s aluminum honeycomb structure. Aluminum is very light, and the structure gives the space capsule extra strength. The early space craft had a coating of glass embedded with synthetic resin and put in very high temperatures. Carbon fiber, reinforced plastics and ceramic are new materials that are constantly being made better for use in space exploration.
Space capsules are well-suited to high-temperature and dynamic loading reentries. Whereas delta-wing gliders such as the Space Shuttle can reenter from Low Earth Orbit and lifting bodies are capable of entry from as far away as the Moon, it is rare to find designs for reentry vehicles from Mars that are not capsules. The current RKK Energia design for the Kliper, being capable of flights to Mars, is an exception.
Engineers building a space capsule must take forces such as gravity and drag into consideration. The space capsule must be strong enough to slow down quickly, must endure extremely high or low temperatures, and must survive the landing. When the space capsule comes close to a planet’s or moon’s surface, it has to slow down at a very exact rate. If it slows down too quickly, everything in the capsule will be crushed. If it doesn’t slow down quickly enough, it will crash into the surface and be destroyed. There are additional requirements for atmospheric reentry. If the angle of attack is too shallow, the capsule may skip off the surface of the atmosphere. If the angle of attack is too steep, the deceleration forces may be too high or the heat of reentry may exceed the tolerances of the heat shield.
Capsules reenter aft-end first with the occupants lying down, as this is the optimum position for the human body to withstand the decelerative g-force. The aft end is formed in a rounded shape (blunt body), as this forms a shock wave that doesn't touch the capsule, and the heat is deflected away rather than melting the vehicle.
The Apollo Command Module reentered with the center of mass offset from the center line; this caused the capsule to assume an angled attitude through the air, providing a sideways lift to be used for directional control. Rotational thrusters were used to steer the capsule under either automatic or manual control by changing the lift vector.
At lower altitudes and speeds parachutes are used to slow the capsule down by making more drag.
The space capsules also have to be able to withstand the impact when they reach the Earth’s surface. All US manned capsules (Mercury, Gemini, Apollo) would land on water; the Soviet/Russian Soyuz and Chinese Shenzhou (and planned US, Russian, Indian) manned capsules uses small rockets to touch down on land. In the lighter gravity of Mars, airbags were sufficient to land some of the robotic missions safely.
Drag is the space capsule’s resistance to it being pushed though air. Air is a mixture of different molecules, including nitrogen, oxygen and carbon dioxide. Anything falling through air hits these molecules and therefore slows down. The amount of drag on a capsule depends on many things, including the density of the air, and the shape, mass, diameter and roughness of the capsule. The speed of a space craft highly depends on the combined effect of the two forces — gravity, which can speed up a rocket, and drag, which will slow down the rocket. Space capsules entering Earth’s atmosphere will be considerably slowed because our atmosphere is so thick.
When the space capsule comes through the atmosphere the capsule compresses the air in front of it which heats up to very high temperatures (contrary to popular belief friction is not significant).
A good example for this is a shooting star. A shooting star, which is usually tiny, creates so much heat coming through the atmosphere that the air around the meteorite glows white hot. So when a huge object like a space capsule comes through, even more heat is created.
As the space capsule slows down, the compression of the air molecules hitting the capsules surface creates a lot of heat. The surface of a capsule can get to 1480 °C (2700 F) as it descends through the Earth’s atmosphere. All this heat has to be directed away. Space capsules are typically coated with a material that melts and then vaporizes ("ablation"). It may seem counterproductive, but the vaporization takes heat away from the capsule. This keeps the reentry heat from getting inside the capsule. Capsules see a more intense heating regime than spaceplanes and ceramics such as used on the Space Shuttle are usually less suitable, and all capsules have used ablation.
In practice, capsules do create a significant and useful amount of lift. This lift is used to control the trajectory of the capsule, allowing reduced g-forces on the crew, as well as reducing the peak heat transfer into the capsule. The longer the vehicle spends at high altitude, the thinner the air is and the less heat is conducted. For example, the Apollo CM had a lift to drag ratio of about 0.35. In the absence of any lift the Apollo capsule would have been subjected to about 20g deceleration (8g for low-Earth-orbiting spacecraft), but by using lift the trajectory was kept to around 4g.
In 1958, the United States announced Project Mercury, with the goal of sending men into low Earth orbit, and selected a group of seven astronauts to pilot them. The single-man capsule was designed and patented by a team led by Maxime Faget. The McDonnell Aircraft company won the contract to build Mercury. The cone-shaped capsule was 6 feet (1.8 m) in diameter with a blunt-body heat shield at the base. Total capsule length was 10.8 feet (3.3 m), including a retrorocket pack strapped over the heat shield. The heaviest capsule had a gross weight of 3,000 pounds (1,400 kg), and was capable of just over one day of orbital flight. A 24.1-foot (7.3 m)-long launch escape tower was connected to the nose of the capsule at launch, containing a solid-fueled rocket capable of carrying the capsule away from the launch vehicle in case of an emergency. Capsules for suborbital flights used a radiative heat shield made of beryllium and were launched with the Mercury-Redstone Launch Vehicle. Orbital capsules used an ablative heat shield made of fiberglass embedded in an aluminum honeycomb matrix, and were launched with the Atlas LV-3B.
Meanwhile, the Soviet Union pursed its spaceflight program in complete secrecy after announcing their intent to launch an unmanned satellite at the International Geophysical Year conference in 1955, not announcing space flights until after their successful completion. After sending the first animal, a dog named Laika, into orbit on Sputnik 2 on November 3, 1957, their Vostok program succeeded in launching the first man (the Soviets referred to him as a cosmonaut), Yuri Gargarin, into a single orbit and recovering him safely on April 12, 1961. The US launched its first Mercury astronaut Alan Shepard on a suborbital flight almost a month later, on May 5. The Soviets launched a second Vostok on a one-day flight on August 6, before the US finally orbited the first American, John Glenn, on February 20, 1962. The United States launched a total of two manned suborbital Mercury capsules and four manned orbital capsules, with the longest flight, Mercury-Atlas 9, making 22 orbits and lasting 32 and one-half hours.
Since the Soviets had plenty of lift capability from launch vehicles derived from their R-7 Semyorka intercontinental ballistic missile, the Vostok craft was much heavier than Mercury. Details of the Vostok design were not publicly disclosed until four years after Gagarin's flight, and the identity of rocket and spacecraft designer Sergei Korolev (the Soviet counterpart to Faget and Wernher von Braun) was not publicly disclosed until after his death in 1966. The capsule was a sphere completely covered in ablative heat shield material, 2.3 meters (7.5 ft) in diameter, weighing 2,460 kilograms (5,420 lb). This shape required covering the capsule with a nose cone to maintain a low-drag profile for launch. The interior cabin was a cylinder approximately 1 meter (3.3 ft) (about the size of a telephone booth), nearly perpendicular to the capsule's longitudinal axis. The cosmonaut sat in an ejection seat with his own parachute for escape during a launch emergency. The capsule had its own parachute for landing on the ground. Although official Soviet sources (including Gagarin himself, who was forced to lie by his superiors) stated that Gagarin had landed inside his capsule (thus qualifying for the first manned spaceflight according to International Aeronautical Federation (IAF) rules), it was later revealed that, like subsequent Vostok cosmonauts, Gagarin had ejected and landed separately from the capsule. The capsule was serviced by an aft-facing conical equipment module 2.25 meters (7.4 ft) long by 2.43 meters (8.0 ft), weighing 2,270 kilograms (5,000 lb) containing nitrogen and oxygen breathing gasses, batteries, fuel, attitude control thrusters, and the retrorocket. It could support flights as long as ten days. The Soviets launched a total of six successful manned Vostok capsules, the last two pairs in cocurrent flights. The longest flight was just short of five days, on Vostok 5 in June 14–19, 1963.
After reaching the one-day potential of the Mercury capsule on the Mercury-Atlas 9 flight, NASA decided to discontinue further Mercury flights and upgrade the spacecraft to extend its endurance, to recover the lead taken by the Soviets. McDonnell Aircraft started plans for an upgraded design based on Mercury, but with a more modular design moving support systems outside the capsule. A larger size also allowed carrying two astronauts. McDonnell called their new design Mercury Mark II. Meanwhile, NASA had kicked off the three-man Apollo program to land men on the Moon, and decided a two-man, extended spacecraft would help develop the spaceflight capability needed for Apollo. They chose McDonnell as the contractor for what became Project Gemini without competitive bidding, based on McDonnell's Project Mercury performance.
The Gemini capsule was an enlarged version of the Mercury, with a 7.5-foot (2.3 m)-diameter heat shield. Propulsion, electrical power, attitude control and retrorockets were placed in an external equipment module (unknowingly, similar to the Vostok design) which adapted the capsule to the 10-foot (3.0 m)-diameter Gemini-Titan launch vehicle. Thrusters were added for translation control as well as attitude control, and the forward thrusters allowed the spacecraft to change its orbital inclination and altitude. Batteries were replaced with hydrogen-oxygen fuel cells for long-duration power generation. The spacecraft could stay in orbit for up to two weeks. The capsule had two hatch doors that could be opened and closed in flight, allowing extra-vehicular activity (EVA, known as "space-walking"). The avionics equipment inside the capsule did not require ambient air cooling, allowing the cabin to be depressurized during EVA. The launch escape system used astronaut ejection seats instead of an external escape rocket tower. The complete spacecraft weighed 7,100 to 8,350 pounds (3,220 to 3,790 kg).
Before the US could launch the first manned Gemini mission Gemini 3 on March 23, 1965, the Soviets announced the flight of Voskhod 1 with a three-man crew on October 12, 1964. The Soviet press boasted of a "shirt-sleeve" atmosphere for the cosmonauts. Then, the flight of the two-man Voskhod 2 was announced on March 18, 1965, with cosmonaut Alexey Leonov performing the first EVA, before the US could perform its first EVA by astronaut Ed White in Gemini 4 on June 3, 1965. However, no further flights of Voskhod were made. Over the course of ten manned Gemini flights in 1965 and 1966, the US gained the lead in spaceflight capability over the Soviets, by demonstrating flights of up to two weeks, rendezvous and docking, and that the rigors of EVA could be overcome to perform useful work outside a spacecraft. The Soviets made no manned flights during this period.
When details of the Voskhod design were later revealed, it was simply a modification of the one-man Vostok rather than a new design, with a three-man rectangular cabin replacing the cylinder, and larger retrorockets. The Voskhod 1 crew could not wear space suits, because the spacecraft taxed the limits of the launch vehicle payload capability. The complete Voskhod spacecraft weighed 5,682 kilograms (12,527 lb). On Voskhod 2, one cosmonaut seat was replaced with a cumbersome, inflatable airlock (which gave Leonov much trouble reentering the capsule) necessitated by the inability of the capsule avionics to avoid overheating in the vacuum of space. The design proved so troublesome, that the Soviets discontinued the program after the two flights.
Unknown at the time, the Communist People's Republic of China had planned in 1967 to enter the Space Race with a manned spacecraft called Shuguang, copied from the US Gemini design. These plans were cancelled in 1972 due to financial and political problems.
The Apollo spacecraft was first conceived in 1960 as a three-man craft to follow Project Mercury, with an open-ended mission. It could be used to ferry astronauts to an Earth-orbiting space station, or for flights around or orbiting the Moon, and possibly landing on it. NASA solicited feasibility study designs from several companies in 1960 and 1961, while Faget and the Space Task Group worked on their own design using a conical/blunt-body capsule (Command Module) supported by a cylindrical Service Module providing electrical power and propulsion. NASA reviewed the entrants' designs in May 1961, but when President John F. Kennedy proposed a national effort to land a man on the Moon during the 1960s, NASA decided to reject the feasibility studies and proceed with Faget's design, focused on the lunar landing mission. The contract to build Apollo was awarded to North American Aviation.
The Apollo Command/Service Module (CSM) was originally designed to take three men directly to the surface of the Moon, atop a large landing stage with legs. The Command Module sized out at 12 feet 10 inches (3.91 m) in diameter, by 11 feet 1.5 inches (3.39 m) long. The Service Module was 13 feet (4.0 m) long, with a total vehicle length of 36 feet 2.5 inches (11.04 m) including the engine bell. The hypergolic propellant service propulsion engine was sized at 20,500 pounds-force (91,000 N) to lift the CSM off the lunar surface and send it back to Earth. This required a single-launch vehicle much larger than the Saturn V, or else multiple Saturn V launches to assemble it in Earth orbit before sending it to the Moon.
Early on, it was decided to use the lunar orbit rendezvous method, using a smaller Lunar Excursion Module (LEM) to ferry two of the men between lunar orbit and the surface. The reduction in mass allowed the lunar mission to be launched with a single Saturn V. Since significant development work had started on the design, it was decided to continue with the existing design as Block I, while a Block II version capable of rendezvous with the LEM would be developed in parallel. Besides addition of a docking tunnel and probe, Block II would employ equipment improvements based on lessons learned from the Block I design. Block I would be used for unmanned test flights and a limited number of Earth orbit manned flights. Though the service propulsion engine was now bigger than required, its design was not changed since significant development was already in progress; however, the propellant tanks were downsized slightly to reflect the modified fuel requirement. Based on astronaut preference, the Block II CM would replace the two-piece plug door hatch cover, chosen to avoid an accidental hatch opening such as had happened on Gus Grissom's Mercury-Redstone 4 flight, with a one-piece, outward-opening hatch to make egress easier at the end of the mission.
The Mercury-Gemini practice of using a prelaunch atmosphere of 16.7 pounds per square inch (1,150 mbar) pure oxygen proved to be disastrous in combination with the plug-door hatch design. While participating in a pre-launch test on the pad on January 27, 1967 in preparation for the first manned launch in February, the entire crew of Apollo 1—Grissom, Edward H. White, and Roger Chaffee—were killed in a fire that swept through the cabin. The plug door made it impossible for the astronauts to escape or be removed before their deaths. An investigation revealed the fire was probably started by a spark from a frayed wire, and fed by combustible materials that should not have been in the cabin. The manned flight program was delayed while design changes were made to the Block II spacecraft to replace the pure oxygen pre-launch atmosphere with an air-like nitrogen/oxygen mixture, eliminate combustible materials from the cabin and the astronauts' space suits, and seal all electrical wiring and corrosive coolant lines.
The Block II spacecraft weighed 63,500 pounds (28,800 kg) fully fueled, and was used in four manned Earth and lunar orbital test flights, and seven manned lunar landing missions. A modified version of the spacecraft was also used to ferry three crews to the Skylab space station, and the Apollo-Soyuz Test Project mission which docked with a Soviet Soyuz spacecraft. The Apollo spacecraft was retired after 1974.
In 1963, Korolev first proposed the three-man Soyuz spacecraft for use in Earth orbit assembly of a lunar exploration mission. He was pressured by Soviet premier Nikita Khrushchev to postpone development of Soyuz to work on Voskhod, and later allowed to develop Soyuz for space station and lunar exploration missions. He employed a small, lightweight bell-shaped reentry capsule, with an orbital crew module attached to its nose, containing the bulk of the mission living space. The service module would use two panels of electric solar cells for power generation, and contained a propulsion system engine. The 7K-OK model designed for Earth orbit used a 2,810-kilogram (6,190 lb) reentry module measuring 2.17 meters (7.1 ft) in diameter by 2.24 meters (7.3 ft) long, with an interior volume of 4.00 cubic meters (141 cu ft). The 1,100-kilogram (2,400 lb) spheroidal orbital module measured 2.25 meters (7.4 ft) in diameter by 3.45 meters (11.3 ft) long with a docking probe, with an interior volume of 5.00 cubic meters (177 cu ft). The total spacecraft mass was 6,560 kilograms (14,460 lb).
Ten of these craft flew manned after Korolev's death, from 1967 to 1971. The first (Soyuz 1) and last (Soyuz 11) resulted in the first in-space fatalities. Korolev had developed a 9,850-kilogram (21,720 lb) 7K-LOK variant for use in the lunar mission, but this was never flown manned.
The Russians continued to develop and fly the Soyuz to this day.
The PRC developed its Shenzhou spacecraft in the 1990s based on the same concept (orbital, reentry and service modules) as Soyuz. Its first unmanned test flight was in 1999, and the first manned flight in October 2003 carried Yang Liwei for 14 Earth orbits.