The atmosphere surrounding the Earth and supporting life here makes spaceflight harder than it would be if it did not exist. It is said, only half-jokingly, that getting to orbit is like getting “halfway to anywhere” because of the energy necessary to reach it. Generally overlooked, however, is just how difficult it also is to come home from orbit. All of the energy expended to get to orbit has to be dissipated on the way back to Earth, in the form of extreme heating. In addition to the aerodynamic concerns with high-speed flight, there are serious thermodynamic issues with a 17,500 mph plunge through the atmosphere.
The technology needed to survive reentry matured rather quickly in the 1950s. The warheads developed during the Cold War for ballistic missiles led directly to the capsules that first allowed humans to venture into space. The Mercury capsule, accordingly, required a design that allowed a longer, lower-g reentry drove the development of several new technologies. Despite this, the technologies for ballistic reentry paved the way for human flight into space and reentry therefrom.
While most proposals for satellites between 1946 and 1957 avoided the difficult problem of reentry, early it became obvious that the heating of reentry had to be understood and mitigated for human spaceflight to proceed. Three approaches dominated thinking.
- A heat sink concept that sought to move quickly from space through the upper atmosphere. Superheating proved a serious problem, however, and materials to protect the spacecraft a major concern.
- Circulating a fluid through the spacecraft’s skin to soak up the heat of reentry; championed by Wernher von Braun to support a grandiose vision of astronauts returning from wheeled space stations aboard huge spaceplanes.
- A blunt-body concept, a major breakthrough, which shaped the course of spaceflight research and provided the basis for all successful reentry vehicles.
All human spaceflight projects actually flown by the United States prior to the Space Shuttle employed a blunt-body reentry design with an ablative shield to dissipate the heat generated by atmospheric friction. This approach has also been used in reconnaissance, warhead, and scientific reentry successfully from the 1950s to the present. Additionally, the question of what materials to use to protect the spacecraft during blunt-body reentry led to research on metallic, ceramic, and ablative heat shields. All researchers soon agreed that ablative technology offered the greatest chance of success.
All of the American efforts until the Space Shuttle, and the Soviet and Chinese human capsules, used from one to three parachutes for return to Earth. For the Americans, the capsules landed in the ocean and were recovered by ship. Both the Soviet/Russian and Chinese spacecraft have always been recovered on land, which presented the crew with a harder landing than would be the case in the sea but obviated the need for naval deployments to recover the capsule and crew. For Project Gemini NASA toyed with the possibility of using a paraglider being developed at Langley Research Center for “dry” landings instead of a “splashdown” in water and recovery by the Navy. The engineers never did get the paraglider to work properly and eventually dropped it from the program in favor of a parachute system like the one used for Mercury.
The U.S. also used parachutes to return film canisters from the nation’s first reconnaissance satellite, CORONA, flown between 1960 and 1972. This program employed satellites with cameras and film launched into near-polar orbits to provide frequent coverage of the USSR. After the film was exposed, it was wound onto reels in a special reentry capsule that separated from the spacecraft at about 160 km altitude and then at 20,000 m jettisoned its heat shield and deployed a parachute. Air Force planes flying over the Pacific then snagged the parachute and capsule, returning the film for processing and analysis.
By the latter 1960s NASA officials had made the decision to abandon capsules with blunt-body ablative recovery systems that relied on parachutes. Instead Space Shuttle, which still had a blunt-body configuration, used a new ceramic tile and Reinforced Carbon-Carbon for its thermal protection. Parachutes were also jettisoned in favor of a delta-wing aerodynamic concept that allowed runway landings. Despite many challenges, and the loss of one vehicle and crew due to a failure with the thermal protection system in 2003, this approach worked since first flown in 1981 through the end of the Space Shuttle program in 2011. Overall, NASA flew 135 Space Shuttle missions over the course of its career.
The Soviet Union also built a space shuttle, the Buran, which flew only one mission without a crew in 1988. Like its American counterpart, Buran landed with delta wings on a runway. With the demise of the Soviet Union, however, the Soviet Ministry of Defense realized that this system was expensive and without a firm rationale. In 1993 NPO Energia’s head, Yuri Semenov, publicly announced the end of the project.
As the twenty-first century has progressed, the preferred method for returning to Earth remains ablative heat shields for the dissipation of excess heat and speed and parachutes for soft landing. Generally, this has worked effectively but the Soviet Union did lose one mission, Vladimir Komarov’s Soyuz 1 on April 24, 1967, when his reentry system failed. The Russians likewise used the ablative heat shield and parachute approach for recovery of three lunar sample return missions, Luna 16 which successfully returned 101 grams of lunar soil in 1970, Luna 20 which returned 30 grams in 1974, and Luna 24 which returned 170.1 grams in 1976.