Nearly 12 months since it embarked on its long-awaited Exploration Flight Test (EFT)-1 mission—which accomplished the farthest distance ever attained from the Home Planet by a human-capable vehicle, since the end of the Apollo era—NASA’s Orion Program presently stands on the threshold of its next major challenge: the unpiloted Exploration Mission (EM)-1, atop the maiden voyage of the mammoth Space Launch System (SLS) booster, no sooner than November 2018. In anticipation of this feat, which will see Orion delivered Beyond Low-Earth Orbit (BLEO) and onto a week-long voyage to circumnavigate the Moon, NASA announced last Thursday that the “Back Shell” element of the spacecraft’s critical heat shield is receiving enhancements to withstand the harsh temperature and velocity conditions expected during an atmospheric re-entry from lunar distance. Recent manufacturing work on the pressure vessel of the Orion Crew Module (CM) has also required ingenious solutions on the part of the NASA and Lockheed Martin engineering workforces.
Like the Apollo Command Module, which preceded it by almost five decades, the Orion Crew Module (CM) is conical in shape and protected during its return to Earth by a sophisticated Thermal Protection System (TPS). At its base is the 16.5-foot-diameter (5-meter) ablative base shield, which will bear the brunt of atmospheric heating and endure temperatures as high as 2,200 degrees Celsius (4,000 degrees Fahrenheit). This system is principally made from Avcoat, an Apollo-era material, consisting of silica fibers with an epoxy-novalic resin, filled in a fiberglass-phenolic honeycomb and manufactured directly onto Orion’s heat shield substructure, then installed as a complete unit onto the CM. Additionally, the conical walls of the CM (known as the “Back Shell”) are coated by an extensive “grid” of 970 black tiles.
As outlined in a previous article by AmericaSpace’s Mike Killian, these tiles owe their ancestry to the Space Shuttle Program and are outwardly similar in appearance to the systems which protected the bellies of the returning orbiters as they hurtled back to Earth on 134 occasions between April 1981 and July 2011. “The shuttles hit the atmosphere on re-entry at around 17,000 mph (27,360 km/h),” the article explained. “When Orion returns … it will hit the atmosphere at 20,000 mph, bringing hotter re-entry temperatures to go with its faster velocity. So even though the hottest the Space Shuttle tiles got was about 1,260 degrees Celsius (2,300 degrees Fahrenheit), the Orion Back Shell could get up to 1,730 degrees Celsius (3,150 degrees Fahrenheit), despite being in a cooler area of the vehicle compared to its ablative heat shield.”
Although Orion’s TPS was exhaustively tested during last year’s EFT-1 re-entry—in which the spacecraft descended from a peak apogee of 3,609 miles (5,808 km), about 15 times higher than the orbit of the International Space Station (ISS), and entered the atmosphere at a blistering 20,454 mph (32,917 km/h)—the next mission will place the system under significantly greater duress. Scheduled to fly no sooner than November 2018, EM-1 will be in space for more than three weeks, as opposed to EFT-1’s 4.5 hours, and can be expected to return to Earth under correspondingly more challenging conditions. “Orion’s Thermal Protection System is essential to successful future missions,” said John Kowal, NASA’s TPS lead for the Orion Program. “As we move toward building the system for EM-1, we’ve been able to take advantage of what we learned from building and flying Orion to refine our processes going forward.”
Unlike its predecessor, the EM-1 spacecraft will also subject its heat shield to the harsh radiation and temperature extremes of “cislunar space,” marking the first occasion that a human-capable vehicle has accomplished the journey beyond Geosynchronous Earth Orbit (GEO) and reached as far as the Moon, since the return of Apollo 17 in December 1972. In spite of the experience gained from EFT-1, it is expected that the EM-1 spacecraft will experience a faster return from lunar velocity, as high as 24,545 mph (39,501 km/h), equivalent to about 36,000 feet per second, as opposed to the 30,000 feet per second of its predecessor. “While the speed difference may seem subtle, the heating the vehicle sees increases exponentially as the speed increases,” NASA explained. “The work engineering teams across the country are doing prepares Orion’s heat shield to perform re-entry during any of missions planned near the Moon or in high lunar orbit in the coming years.”
In addition to the protection already afforded to the Back Shell by its TPS tiles, future Orion CMs will also benefit from a silver, metallic-based thermal-control coating. Similar to what is also used on the base heat shield, this will reduce heat loss when the spacecraft is pointed into deep space and experiencing its coldest conditions, as well as serving to limit the high temperatures endured by the CM when it faces the Sun. All told, the coating is expected to ensure that the Back Shell maintains a temperature range, prior to re-entry, from a minimum of -101 degrees Celsius (-150 degrees Fahrenheit) to a maximum of 278 degrees Celsius (550 degrees Fahrenheit). This will help to protect it against electrical surface charges in space and throughout the re-entry phase. “You’re trying to hit this sweet spot,” said Kowal, “because when you’re looking at the Sun, you don’t want to get too hot, and then when you’re not looking at the Sun, and instead at darkness, you don’t want to lose all the heat that the spacecraft generates.”
Since EFT-1, a number of advancements have been made to enhance Orion’s TPS hardware, including new techniques to streamline the labor-and-time-intensive manufacturing process and reduce the mass of the heat shield substructure. As described in a previous AmericaSpace article, NASA decided earlier this year to incorporate six “compression pads”—fabricated from an innovative, 3D-woven material of quartz fibers—for installation at the interface between the CM and Orion’s cylindrical Service Module (SM), where they will function in tandem with the primary Avcoat ablator. Elsewhere, the 5,000-pound (2,270-kg) base heat shield itself has been subjected to milling, core-sampling and laser-scanning at NASA’s Marshall Space Flight Center (MSFC) in Huntsville, Ala., and, in June, was delivered to NASA’s Langley Research Center (LaRC) in Hampton, Va., to undergo preparations for future water-impact testing.
More recently, in September, it was announced that the learning curve involved in building EFT-1’s base heat shield as a single, “monolithic” element had led to a decision to assemble the EM-1 hardware in 180 “blocks,” thereby easing the labor-intensive manufacturing process. Analysis and flight testing demonstrated that the strength of the Avcoat/honeycomb structure was below expectations, potentially enduring colder temperatures in space and increased heating during re-entry, thereby necessitating greater strength. It is expected that the design update will provide cost savings and shorten the current manufacturing time for the base heat shield by about 60 days. Also in September, prime contractor Lockheed Martin laid down the first welds for the EM-1 CM and Aerojet Rocketdyne completed a major subsystems review of its Reaction Control System (RCS) and jettison motor hardware for the mission.
Additional advances have been made in the manufacturing of the CM as a whole. Orion’s pressure vessel consists of seven large aluminum segments, three of which are known as “Cone Panels,” and it is the latter which have presented a unique challenge to NASA and Lockheed Martin since the first welds were laid down in September. When the initial test version of the spacecraft’s substructure was constructed, it employed 12 welds to bind together the seams of six cone panels and six longerons on the CM. These welds added significant weight, but through what NASA described as “diligent analysis and iterative design evaluations,” engineers were able to reduce the number of cone panels and welds. However, it was noted last week that “considerable resourcefulness and skill” was required on the part of the Orion engineering team to reduce the number of these cone panels.
During the manufacturing process, they had a tendency to flatten-out, or relax, more than was expected. “When you form something from a six-inch-thick (15.2 cm) metal plate, what holds it in shape is the metal itself,” explained Jim Bray, Lockheed Martin’s Orion CM director. “When you need it to bend even more, but start to remove metal from it, and make it thinner, it starts to lose its shape.” Since this relaxation of the panels posed a threat to the manufacturing process, engineers standardized the technical steps necessary to fabricate them and divided the task among three different expert machining companies. “The revised plan enabled unprecedented collaboration across the industry while working on all three panels in parallel, ultimately saving time,” NASA explained. Specific conditions, ranging from the temperature and humidity in the room to the rotational speed of the weld-head and its speed of movement, were extensively redefined in order to achieve success. All told, the TPS for the EM-1 mission is expected to be about 1,200 pounds (540 kg) lighter than that of EFT-1, whilst the CM itself is expected to benefit from a 20-percent weight reduction, equivalent to about 4,000 pounds (1,800 kg).
All of this work has contributed to Orion’s smooth passage through Key Decision Point (KDP)-C in September and through its multi-month Critical Design Review (CDR)—a process of evaluating the maturity of Orion’s myriad systems and subsystems, which is expected to conclude in early 2016—as efforts enter high gear ahead of EM-1. This will feature major participation from the European Space Agency (ESA), which is building the Orion Service Module (SM) for the mission and is expected to complete its own CDR and present to the NASA Agency Program Management Council, next spring.
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