The Pod People [Inflatable Reentry Vehicles]
By James Oberg
Air & Space Magazine
November 2003, pp. 58-63
They’re coming to bring you safely back to Earth. Maybe.
In 1964, most viewers of televised space “shots,” as they were called then, knew what it took to protect a spacecraft from the fire of reentry. It took big, heavy shields bolted to strong metal pressurized vessels. One of the most nerve-racking moments of the early space program had been the final minutes of John Glenn’s 1962 Mercury flight, when the world waited to learn whether his shield had remained attached to the Friendship 7 capsule during the violent reentry.
Two years later, on June 10, 1964, almost no one was watching as another, much lighter vehicle entered the atmosphere with no one on board. But in engineering terms it was nearly as daring as the Mercury flights had been. Launched on a sounding rocket to an altitude of 96 miles over White Sands, New Mexico, the craft dove back toward Earth at a speed of more than 5,000 mph—without a heavy heat shield. Its thermal protection had been sprayed on. Even stranger, the craft was inflated like a balloon in a Thanksgiving Day parade.
The contraption was called IMP, for Inflatable Micrometeoroid Paraglider. It was developed by a team at NASA’s Langley Research Center in Virginia, led by a young chief engineer named Bill Kinard. Now 71, Kinard is still at Langley, now an emeritus engineer. His paraglider, though, has been all but forgotten.
Now engineers on both sides of the Atlantic hope to see a distant descendant of IMP fly. A European-Russian team has built and tested, not altogether successfully, an inflatable reentry vehicle, and has scheduled another test for next spring. So far, their dish-shaped spacecraft appears capable of protecting an instrument capsule on the return to Earth. Designers have big plans for the invention, which has been dubbed Inflatable Reentry and Descent Technology. First, it would return cargo from the International Space Station. Eventually—if formidable technical and even psychological hurdles can be jumped—it could serve as a personal escape pod for astronauts forced to bail out from orbit.
With IRDT, the idea of inflatable spacecraft is experiencing, if not a renaissance, at least a curious second look after decades of false starts and periodic bursts of hot air. “Technically, inflatables are feasible,” says retired NASA futurist Joe Loftus, who once headed the advanced planning office at the Johnson Space Center in Houston. “The question is: What is it that will make them desirable?”
Back in 1962, Bill Kinard thought he knew the answer. His goal was simple: Get something large, light, and cheap up above the atmosphere for just a few minutes, then return it to Earth safely. Scientists wanted to refine their models of meteorite and micrometeorite density in space so engineers would know how much protection to install on space vehicles. “We were interested in exposing large areas in space” to see how much bombardment they could withstand, Kinard says.
Rigid metal structures were too heavy and expensive, so Kinard’s team turned to unconventional approaches. Langley had already come up with the inflatable Echo balloon, which had an aluminum-coated Mylar surface that in 1960 was used to bounce radio signals back to the ground. Another group at the center was looking at using a modified Rogallo wing (the famous prototype of the hang glider) as an alternative to parachutes for gently landing a Mercury or Gemini capsule. To reduce weight, the engineers replaced the wing’s metal struts with long cylinders made of tough fabric, which were inflated with gas until they became rigid.
Kinard’s team liked that idea, and started working on a paraglider that would be inflated with compressed nitrogen and would have a large surface area that could be covered with meteoroid detectors, like Space Age flypaper. After launch, the paraglider would separate from its carrier rocket, inflate, spend about five minutes getting pelted by meteoroids, then fly back through the atmosphere for a desert landing at the Army’s White Sands missile range in New Mexico.
Early tests were encouraging. “I was amazed that a paraglider with a flexible canopy could fly so well at supersonic speeds,” Kinard recalls. “We did wind tunnel tests, and the [wing] fabric was like a piece of metal—no flutter, perfectly stable.” Flying at up to Mach 8 would generate heat from friction with atmospheric molecules, so the glider got a thin coating of silicone over its flexible Fiberglas fabric.
The actual flight of the IMP, on an Aerobee-150 sounding rocket in 1964, had mixed results. The paraglider inflated properly but turned upside down. Then the nose-cone-jettison system failed to sever all the connecting wires, so the glider began its descent through the atmosphere dragging an anchor. Amazingly, it righted itself and flew briefly, until the aerodynamic pressure and heat got too much. At that point, one wing boom collapsed, probably from the stress of dragging the nose cone, and the IMP dropped to the desert like a wounded duck. It took weeks to find all the meteoroid collection panels that were torn off the wing as it fell.
So much for IMP. Scientists soon found another way to get micrometeoroid detectors into space – on test flights of Saturn boosters. But the idea of inflatable reentry vehicles had caught the imagination of engineers, including several outside NASA, who started musing about light spacecraft that could make the return trip all the way from Earth orbit. “We did nothing intentional to inspire it,” Kinard says, “but we got lots of attention.”
One of those engineers was Robert Brodsky, who at the time was in charge of bringing in work to Aerojet-General's Space General division, which had built the Aerobee rocket. Brodsky had designed the electronics and other systems for IMP, while Goodrich had built the structure. Even before IMP’s test flight, both companies started pushing the concept of inflatable reentry with their customers, primarily the Department of Defense. In the early 1960s, the Pentagon was flirting with developing its own astronaut program and was interested in proposals for a small, storable craft that could return a person from orbit in a hurry.
“I got the idea that we could alter the IMP design sufficiently to turn it into a space lifeboat,” Brodsky says. He envisioned it as an option for dire emergencies, like a life raft on an oceanliner.
“There were only two reactions,” Brodsky recalls. “Initially, sheer incredulity. Then—seeing the challenge—great enthusiasm.” In 1962 the Air Force materials lab gave his company $250,000 to look into the concept; the funding later grew to $1 million. “In those days, that was a lot of money,” he says.
The project lead was Jesse (“Bud”) Keville, a 37-year-old engineer who set to work designing the lifeboat and building and testing components. Space General called it Project FIRST, for “Fabrication of Inflatable Reentry Structures for Test.” Keville’s team kept the basic IMP paraglider with its three inflatable struts, and placed a prone astronaut in the center strut. The lifeboat weighed a mere 850 pounds. Stowed on a spacecraft, it could fit in a three-foot by 10-foot cylinder; inflated, it was 23 feet long, with a wingspan of 28 feet. The engineers even came up with a deluxe three-person version weighing 1,500 pounds, and a six-person model that weighed a ton.
The inflatable wing spars were made of nickel-chromium alloy mesh. For thermal protection, the mesh was laminated with silicone rubber impregnated with finely ground ceramic. A vehicle returning from orbit would experience more heating than IMP had during its suborbital flight, and vacuum chamber tests showed that this material could handle temperatures up to 2,000 degrees Fahrenheit. The wing material, says Keville, “resembled a lightweight burlap.” At first he had trouble finding a textile company that could handle the tricky job of weaving metal yarn, but he finally found one called Prodesco, in Perkasie, Pennsylvania. Keville spent weeks in the small town, which had “only one general store and a Quaker church.”
The FIRST lifeboat was designed to be folded up in a small container on the outside of a spacecraft. An astronaut abandoning ship would enter the pod through a small hatch leading to the outside. After inflating the paraglider with nitrogen fed through a hose or from gas bottles, the escapee would fire solid rockets in the central spar to deorbit the craft. The fall from orbit (400,000 feet) down to 120,000 feet would take half an hour, with attitude control jets used for maneuvering. Once it became aerodynamic in the lower atmosphere, the paraglider could be steered by changing the pressure within the inflatable spars to achieve a kind of wing-warping. The landing would take place anywhere within a footprint 450 miles wide and 1,400 miles long.
Five years of research convinced the FIRST engineers that the concept was feasible. Unfortunately, by the late 1960s it was no longer wanted. Neither Apollo nor Skylab, NASA’s first space station, were in the market for a bailout system, and the Department of Defense was already starting to back away from plans for its own station. Inflatable lifeboats had become the answer to a question no one was asking.
From the beginning, Brodsky had had grander things in mind for FIRST, based on futuristic schemes that Wernher von Braun and others were espousing at the time. “[FIRST] was begun to meet an apparent need for a wheel-like rotating space station,” he says. “It was terminated when it was apparent that we were too early. The idea of a [large] manned space station was no longer in vogue.”
That hadn’t stopped other engineers from exploring similar bailout concepts, though. Other companies had learned of the FIRST project, and throughout the 1960s they came up with various ways to improve on it. General Electric produced probably the most famous concept, called MOOSE (the acronym originally stood for Man Out Of Space Easiest, but the name was later changed to the more sober Manned Orbital Operations Safety Equipment). Instead of inflating the structure with gas, MOOSE engineers used fast-setting polyurethane foam to hold a conical shape; the hardware which fit in a suitcase-like container and weighed 200 pounds, was even tested on a spacesuited volunteer.
Douglas Aircraft had a similar concept: Paracone, an inflatable shuttlecock-shaped structure made of Teflon-coated Rene-41 alloy fabric. The reentry vehicle was slowed to about 25 mph at impact, so no parachute was required, although the returning astronaut experienced about 10 Gs of acceleration.
The inflatable designs often were met with the same reaction from potential customers and other outsiders. “They were skeptical,” says Robert Kendall, who worked on Paracone at Douglas in the 1960s. He tried to overcome the doubts by pointing to inflatable structures used by the U.S. Navy: “I mentioned common, everyday inflatable structure applications such as truck tires, bags around payloads, vessel side protective balloons.”
It was still a tough sell, and remains so, even though the idea has resurfaced more than once since then. In Russia, engineers at the Babakin Space Center in Moscow turned to inflatables in the 1980s to produce lightweight vehicles for descending to Mars. The Mars 96 mission included an inflatable aerobraking system to slow down two small atmospheric-entry probes meant to study the Martian weather. But the spacecraft went off course immediately after its launch in 1996, crashing back to Earth over the Andes Mountains.
It was that project that led to current interest in inflatable reentry vehicles. While the Mars work was under way in Russia in the early 1990s, officials at the European Space Agency were eyeing a future need to return samples, film, tape, equipment, and other material from the proposed International Space Station. With cargo estimates running as high as a ton a year, ESA managers realized that if they used NASA’s space shuttle, they faced shipping charges exceeding $20 million annually.
Then Babakin, in partnership with the German aerospace company Astrium, knocked on the door with a concept for a low-cost ISS Download System, based on their Mars craft, that could return several hundred pounds at a time. In principle, it was similar to the Russian Raduga capsule, which had been used to bring material back from the Mir space station. But this system would be much lighter and cheaper.
ESA was intrigued enough to give the companies almost $2 million for the Inflatable Reentry Descent Technology program, which aims to prove the ability to bring back payloads from space inside an inflatable vehicle. First, the team built a probe called Demonstrator to carry a sensor package weighing about 44 pounds. Engineers at Babakin designed a wastebasket-size cylinder to house the instruments and payload and surrounded them with a pair of inflatable shields coated with ablating material. The first “cascade,” as the shields were called, was eight feet in diameter, and would slow the vehicle during the first phase of reentry. A second, 14-foot cascade would open during final descent to soften the landing.
In February 2000, a test of the shielded probe came close to success. The Demonstrator hardware flew on the first test of the Fregat, a new upper stage for Russia’s Soyuz rocket. Attached to the Fregat, the Demonstrator made five orbits at an altitude of 375 miles, then separated from the upper stage and inflated its first cascade. Tracked by Russian air defense radars, the vehicle descended as planned, enduring a maximum of 15 g’s, and landed near Orenburg, about 30 miles past the aim point. The good news was that temperatures inside the capsule had remained normal. The bad news was that the second cascade, designed to cushion the final impact, never opened. The Demonstrator hit the ground at 200 feet per second, essentially a freefall.
For the next test, in August 2001, the project bought a piggyback commercial launch on a converted Russian missile that also carried a solar sail experiment for the Planetary Society, a U.S. space advocacy group. Both payloads failed to separate from the third stage, and neither had a chance to deploy. The inflatable reentry vehicle didn’t even get an official name, and designers worked on making the separation mechanism more reliable.
The next test, called Demonstrator 2, was launched on the same three-stage Volna booster from a Russian submarine, with a planned landing zone in eastern Siberia's Kamchatka Peninsula. On July 12, 2002, the Volna rose from the Barents Sea and headed east into the predawn sky. Russian space officials immediately declared the launch a complete success and publicly confirmed the landing.
But as days passed without an actual recovery, Russian launch officials were forced to fess up: The 540-pound probe was lost. Accident investigators later determined that Demonstrator 2 had detached from its rocket too early, at around the time Volna’s second stage separated. “Because of the uncontrolled detachment,” their report concluded, “no conclusions can be drawn on its further behavior and on any performance aspects.”
That leaves the IRDT hardware still unvalidated after three tests. Project engineers have made a few fixes to their design and plan to try again next spring. So far, they’ve seen nothing that tells them the basic concept won’t work. Astrium (now part of European aerospace giant EADS) and Babakin have even formed a joint stock company, Return and Rescue Space Systems, to produce and sell inflatable reentry systems. But after the loss of Demonstrator 2, Helmut Hoffman, head of the company's Russian office, told the Russian magazine ITOGI: “The question of the project’s future financing will depend on the test results.”
ESA officials have a wide range of applications and spinoffs in mind for the new vehicle, if it can be made to work. Besides returning cargo from the space station, it could be used in emergencies for jumping out of skyscrapers. And Dieter Kassing, ESA’s IRDT program manager, has raised even more interesting possibilities: Talking to reporters following the failed 2002 test, he said, “I can imagine that this technology also could possibly be used for some sort of emergency and rescue mission for humans” in space.
In recent years, the notion of jumping from an orbiting spacecraft has been discussed only in the context of extreme sports. Rick Tumlinson, president of the California-based Space Frontier Foundation, calls it “orbital surfing.” Bevin McKinney, an engineer who worked on the privately funded (and now-abandoned) Rotary Rocket concept in the 1990s, has also tinkered with designs for an orbital escape system. His proposal involves a large parachute-shaped aerobrake made of ceramic fibers, plus an inflated heat shield in front of the astronaut’s body. “The trick is to come down very slowly from high altitude,” he told a reporter for a skydiving newsletter in 2001.
Among those pushing for a space bailout capability is Robert Kendall, Jr., son of the Douglas engineer who worked on Paracone in the 1960s. Together with his father, who retired in 1976, Kendall Jr. has patented several designs for inflatable air-drop systems. Department of Defense contracts on unrelated technologies kept the father-son team busy in the 1990s, but they continued to publish scientific papers advocating the use of inflatables for orbital reentry. Several years ago, says Kendall Sr., “We submitted a proposal [to the Air Force]to recover satellites, astronauts, microgravity experiments, and reusable launch vehicle components.” They even proposed a test flight to return a 360-pound instrument-equipped mannequin from orbit to a designated site on the ground. No one was interested enough to fund it.
Some aficionados of personal bailout systems still hold out hope that NASA will one day look in their direction. But so far, the agency’s plans for a rescue vehicle from the space station appear not to include one-person lifeboats.
FIRST designer Robert Brodsky still talks up the idea to pretty much anyone who will listen. He accuses NASA of being close-minded on the subject of inflatable reentry vehicles. “Lack of a mission need stopped the program in the ’60s,” he says. “But ‘not invented here’ in NASA is stopping it these days.” Several years ago, he proposed that the agency update the old FIRST concept as a crew return vehicle, or CRV, for NASA’s space station. “They patted me gently on the head, because their idea of a space lifeboat is radically and very expensively different from mine,” he says.
John Muratore, who until recently managed NASA’s CRV program at the Johnson Space Center, denies that the agency turned a blind eye to inflatables. “It’s an interesting technology, and we have looked at it,” he says. The problem, he says, is that “you chase a weight curve. You add weight to [withstand] the dynamic pressure, so the heating goes up, and the vehicle gets bigger and heavier.” His team studied a dozen designs and did a deep literature search. They heard from the inflatable backers. They just weren’t convinced.
All of which leaves Brodsky, Kendall, and the other pioneers of inflatable concepts where they’ve always been. Despite the technology’s interesting past and its promise for the future, it still doesn’t have a great present.
Stephan Walther, IRDT project scientist at Return and Rescue Space Systems' office in Bremen, Germany, hopes that will begin to change with next year’s test of the IRDT, and that a string of successes will eventually persuade ESA to pay for a full-up cargo system. As for inflatable lifeboats, “I’m personally convinced it’s possible,” he says, but adds that it will take a lot more research and engineering development before people are willing, literally, to climb on board.