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Inflatable structures in space
Although they are often perceived as a relic from the 18th century, inflatable balloons actually helped launch the Space Age. And in the following decades of space exploration, inflatable devices have played numerous, albeit episodic roles, mostly in the shadow of traditional "hard-body" spacecraft.
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Above: A concept of the Russian inflatable module for the International Space Station, ISS.
At the dawn of the Space Age
In 1951, six years before Sputnik, British researchers Kenneth Gatland, Alan Dixon and Anthony Kunesch made a presentation of a hypothetical space launch vehicle at the Second International Conference on Astronautics in London. Although the document focused on the rocket, it also described its payload as a metallic paper balloon. According to the authors, after its release into orbit, the satellite would inflate to enable its use as a target for optical and radar observations. Illustrations accompanying the proposal were probably the first depictions of an inflatable satellite in space. (786)
Five years later, in January 1956, William J. O'Sullivan, an American engineer from Langley Research Center, proposed a 20-inch inflatable satellite, which would be used to determine density of the upper atmosphere. The inflatable spacecraft was described and depicted in a patent filed by O'Sullivan on Aug. 20, 1959. This work would become the basis for several "inflatable" projects at the beginning of the Space Age.
Far Side rocket
Perhaps the most exotic use of inflatable technology for space exploration happened in 1957, when a 60-meter balloon lofted a 893-kilogram rocket dubbed Far Side to an altitude of 30.5 kilometers. From there, the four-stage rocket fired upward, piercing the top of the balloon and on its way to delivering 2.3 kilograms of cosmic ray sensors and other scientific instruments to an incredible altitude of 6,440 kilometers above the Earth's surface.
A 30-inch balloon
Today, only space historians remember that an aluminized plastic balloon filled with nitrogen gas was among the first spacecraft proposed for launch during the International Geophysical Year, which opened the Space Age in 1957. On July 26, 1958, around two months after the launch of the third Soviet satellite, NACA (a precursor to NASA) adopted a 20-inch balloon proposal as the third payload for its Vanguard satellite project. In the course of development, mass savings enabled to increase the diameter of the shiny reflective ball to 30 inches, making it large enough to be just visible from the ground with optical telescopes. The balloon was launched along with another payload on April 13, 1959, however its Vanguard SLV-5 rocket failed to reach orbit.
A similar balloon was then used in three suborbital flights of the Nike-Cajun rocket under the Hi-Ball project of the Lincoln Laboratory at the Massachusetts Institute of Technology, MIT, aimed to test long-range radar. The balloon failed to inflate during the first launch on April 7, 1958, but worked by the book during the next attempt on May 21. The balloon was designed to inflate at an altitude of about 95 miles and then ascend to 150 miles into space. The sphere was tracked by the radar and captured on film. There was another attempt to repeat the experiment on October 7, but it, again, resulted in the rocket failure. (787)
A much larger inflatable satellite was developed jointly by NASA and by US Army engineers. The Beacon satellite was designed to test orbit-degrading atmospheric drag in the near-Earth space, but, the project acquired a political significance in the wake of the Soviet Sputnik. After the launch of the second Soviet satellite in November 1957, the Beacon was squeezed into the hectic flight manifest of the Vanguard rocket, but after its spectacular failures, the satellite had to wait for newer boosters. (787)
In April 1958, a model of Beacon even became a prop during contentious hearings at the House Select Committee on Aeronautics and Space Exploration.
After four suborbital launches of the Nike Cajun rocket from the Wallops Island, Beacon-1 was fired into orbit on October 23, 1958, on a Jupiter-C rocket. The satellite was expected to inflate to a diameter of 12 feet, likely becoming the first US satellite easily discernable from the ground. Unfortunately, due to a premature separation from its upper stages, Beacon-1 never made it into orbit.
The second Beacon satellite was launched on Aug. 15, 1959, on the Juno-2 rocket. Yet again, the rocket failed to reach orbital speed, however the telemetry from the mission indicated that the payload had been released on its suborbital trajectory and successfully inflated.
In 1960, NASA made two attempts to orbit an aluminized 30.5-meter inflatable satellite called Echo, which was designed to serve as a reflector for microwave signals, echoing across intercontinental distances in early experiments with satellite communications. The first launch attempt on May 13 failed, but a second satellite -- Echo 1A -- entered orbit successfully on August 12.
On Jan. 25, 1964, NASA followed with a 41.1-meter Echo-2 satellite. However the idea of passive communications reflectors turned out to be still-born, replaced by the "active" transponders of traditional communications satellites.
When NASA embarked on the development of its second-generation manned spacecraft called Gemini, engineers considered an inflatable delta wing as an alternative to the primary landing method of splashing down into the ocean with traditional parachutes. The inflatable paraglider promised to enable a controlled landing of a two-seat capsule on land, however pressures of the space race left no time to resolve technical challenges of the new system. Still, inflatable air bags were used on the Command Module of the next-generation Apollo spacecraft to ensure its vertical position following water landing.
The USSR pioneered the use of inflatable structures in manned space flight with a flexible airlock that was launched aboard the Voskhod-2 spacecraft in 1965. During that historic mission, this unorthodox design enabled the world's first spacewalk by Soviet cosmonaut Aleksei Leonov.
In 1966, after many failures, the Soviet Luna-9 unmanned probe achieved the world's first soft landing on the Moon. As it transpired, the flower-like E-6 lander of the Luna-9 mission used inflatable air bags to soften the impact onto the lunar surface. This method would play a lasting role in planetary exploration both in the USSR and in the United States and remains the most significant application of inflatable technology for space exploration to date.
Soviet scientists went much further with inflatables, placing instrument-carrying balloons on a pair of Vega spacecraft heading to Venus in 1984. Following their flyby of the planet, Vega probes dropped their reentry capsules, which in turn, released a pair of traditional landers and inflatable balloons to float in the misty atmosphere over the alien world.
The inflatable landing system first proven in the Luna-9 mission in 1966, was re-incarnated in the post-Soviet Mars-96 project. A pair of two small landers carried onboard a single main spacecraft were to attempt to land on the surface of the Red Planet. Shortly before impacting the surface, a pair of airbags would be inflated around each lander to soften the impact. The spacecraft would bounce as high as 70 meters into the thin air before they would finally come to rest. Lines holding the two bags would then be cut and the lander would free fall one meter to the ground.
In addition to traditional surface landers, Mars-96 spacecraft also carried a pair of the so-called penetrators. These needle-shaped vehicles were designed to strike the surface of the planet at high speed and penetrate 4-6 meters into the soil. After braking in the Martian atmosphere with the help of an inflatable heat shield, the penetrators were expected to strike the surface of Mars at a speed of around 60-80 meters per second.
Unfortunately, Mars-96 was stranded in the Earth orbit and neither the inflatable bags of its landers, nor the inflatable heat shields on its penetrators had a chance to prove themselves.
In that same launch window of 1996, which saw the demise of Mars-96, NASA launched the Mars Pathfinder mission, which also relied on inflatable bags to complete its soft landing. The spacecraft completed a flawless trip to Mars and the inflatable airbag system successfully delivered a lander and a small rover on the surface of the Red Planet in July 1997.
Mars Exploration Rover
Seven years later, NASA relied on the tried and tested inflatable cushioning system again, delivering a pair of highly successful Mars Exploration Rovers, a.k.a. Spirit and Opportunity, onto the surface of the Red Planet.
Back in Russia, many space projects faced a budget crunch in the wake of severe economic problems of the post-Soviet period. However the European Space Agency, ESA, deemed the Russian inflatable heat shield system from the Mars-96 project as being worthy of further development, possibly as a low-cost method of returning cargo from the International Space Station, ISS.
ESA co-funded the Inflatable Reentry and Descent Technology, IRDT, project together with the European Commission and Daimler Aerospace, DASA. The International Science and Technology Center, a Moscow-based inter-government organization dedicated to non-proliferation, awarded the contract to build the spacecraft to NPO Lavochkin in Moscow. DASA built a sensor package for the project.
A pair of IRDT devices were launched on Feb. 8, 2000, on the same Soyuz rocket from Baikonur. The smaller device was expected to return an experimental package from orbit, while the much larger second device was attached to the Fregat upper stage. To protect the one-ton Fregat during its fiery reentry, the IRDT shield was to inflate from a one-meter compact package up to 12-16 meters in diameter shortly before the reentry.
After a six-hour orbital flight both IRDT devices successfully inflated and reentered, however an apparent failure of radio beacons on both payloads coupled with bad weather at the landing site in southern Russia hampered search efforts for several days. In the end, only a small device was recovered.
The IRDT shield found a new application in the Cosmos-1 solar sailing project. It was privately funded by the US-based Cosmos Studios via the Planetary Society and developed at NPO Lavochkin's Babakin Center in Khimki during the first decade of the 21st century. If successful, it could be the world's first vehicle to use solar sail to propel itself in space.
Launched on a converted ballistic missile, Cosmos-1 would reach an apogee of its ballistic trajectory at an altitude of about 400 kilometers. At that time, it was expected to deploy a pair of fan-like sections of the solar sail, which would be held in place by an inflatable frame. The spacecraft was then expected to reenter the atmosphere and land at the Kura impact range in the Kamchatka Peninsula with the help of the inflatable heat shield. The sail itself would burn up on reentry.
The first attempt to launch the device in 2001 was unsuccessful due to a launch vehicle failure. On July 12, 2002, a second inflatable device was launched from a Navy submarine, however a Volna ballistic missile failed once again.
On October 7, 2005, another Volna rocket carrying the Demonstrator D-2R inflatable braking device, NTU, flew what appeared to be a normal flight from the Barents Sea toward the Kura impact range in the Kamchatka Peninsula. However, initial efforts to locate the landing craft at the impact site were unsuccessful. The telemetry analysis showed that the inflatable device separated from the rocket and was spin-stabilized. Its navigation, video-monitoring and autonomous radio-telemetry systems were activated. The telemetry transmission from the spacecraft was received at the Kura impact range and the reentry device was released and inflated some 356 seconds after the launch and an altitude of 238 kilometers.
The spacecraft entered the discernible atmosphere at an altitude of 100 kilometers and soon after that its transmissions were interrupted by a layer of plasma, as it would be expected during a normal reentry. Following the dissipation of plasma, radio contact was restored, but it continued for only 25 seconds. No further data was received from the craft and no debris was ever found at the expected landing site. Preliminary information indicated that the spacecraft may have overflown Kamchatka, falling in the Pacific Ocean.
After so many failures, most of which did not involve the inflatable systems themselves, the heat shield developed during Mars-96, IRDT and Cosmos-1 projects has remained "on the shelf" for possible future flight opportunities. During the 2000s, Finnish scientists hoped to deliver a mini-weather station on the surface of Mars with the help of the same inflatable device. However the spacecraft designed to hitchhike to Mars onboard the ill-fated Phobos-Grunt mission was grounded around 2008. Instead, multiple landers of the same design would be deployed during the Mars-NET mission, which was conceived to study internal structure of the planet and the weather on Mars in unprecedented detail.
According to the mission scenario, Mars-NET's "mother ship" would approach the Red Planet and drop a cluster of small landers, which would spread all over the planet for a simultaneous global studies on the surface. These landers would use inflatable landing systems. The actual responsibility for the development of landing vehicles was to be delegated to the Finnish Meteorological Institute, FMI, under a contract with the Russian government within a scheme to cover the Soviet debt to Finland.
Read (and see) much more about this and many other space projects in Russia
Page author: Anatoly Zak; Last update: August 18, 2016
Page editor: Alain Chabot; Edits: August 15, 2013, August 10, 2016
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A launch vehicle with an inflatable satellite proposed by British engineers in 1951. Credit: J. W. Wood / British Interplanetary Society
The Far Side rocket (top) taking off from a balloon (bottom). Credit: NASA
The inflatable Echo satellite during ground testing. Credit: NASA
The inflatable landing system for the Gemini spacecraft. Copyright © 2011 Anatoly Zak
The inflatable airlock from the Voskhod-2 mission. Copyright © 2009 Anatoly Zak
Soviet Luna landers used inflatable air bags to achieve the world's first soft landing on the Moon. Credit: RKK Energia
The Command Module of the Apollo spacecraft had inflatable air bags at the top of the capsule designed to keep the vehicle upright after splashdown. Copyright © 2011 Anatoly Zak
In 1985, the USSR released a pair of inflatable balloons in the atmosphere of Venus. Copyright © 2011 Anatoly Zak
Mars Pathfinder successfully reached the Red Planet with a help of inflatable air bags. Copyright © 2011 Anatoly Zak
The Cosmos-1 solar-sailing spacecraft during pre-launch processing. Credit: NPO Lavochkin
A lander with an inflatable heat-shield for the Mars-NET mission appeared in Le Bourget in 2005. Copyright © 2005 Anatoly Zak