A Reliable Lifeboat, Ambulance for the International Space Station
by Donald Georgi with Eric Darcy
With technologies that blaze a trail for future human spacecraft, NASA’s X-38 project is developing — at an unprecedented low cost — a prototype rescue vehicle to provide astronauts on the International Space Station an immediate return home in an emergency.
The vehicle is an innovative combination of a shape first tested in the 1970s and today’s latest aerospace technology. The X-38 already is flying in the actual conditions in which it must perform. Since 1997, increasingly complex, unpiloted atmospheric test flights of the X-38 have been under way at the Dryden Flight Research Center in California. An unpiloted X-38 space test vehicle, now under construction at the Johnson Space Center in Houston, will fly aboard the Space Shuttle in 2002 and descend to a landing independently. The X-38 is designed to fit the unique needs of a space station ‘lifeboat’ — long-term, maintenance-free reliability that is always in “turn-key” condition, ready to provide the entire
Station crew a quick, safe trip home under any circumstance. In addition to contributions from commercial companies and NASA centers coast-to- coast, international space agencies are participating with the United States in the X-38’s development. Contributions to the X-38 are being made by Germany, Belgium, Italy, the Netherlands, France, Spain, Sweden and Switzerland and 22 companies throughout Europe.
Pushing the Edge: Something New, Something Old
The X-38 couples a proven shape,taken largely from a 1970’s Air Force project called the X-24A, with dozens of new technologies — the world’s largest parafoil parachute; the first all- electric spacecraft controls; flight software developed in a quarter of the time required for past spacecraft; laser-initiated explosive mechanisms for deploying parachutes; and global positioning system-based navigation.
The crew rescue vehicle on the International Space Station will have to be capable of a maintenance-free reliability in orbit never before achieved by a human spacecraft — an ability to remain attached to the station for up to three years, all the while ready to depart in under three minutes, if needed. After leaving the station, it must be capable of returning a crew home in less than five hours, regardless of bad weather at some landing sites or the station’s position when it departs. With medical equipment aboard, the emergency spacecraft will be both a ‘space ambulance’ and a ‘space lifeboat.’ Capable of holding up to seven crew members, the rescue craft must have as high a passenger capacity as the space station.
The X-38 turns to the latest technology to meet these demands. Electrically powered spacecraft controls — rather than maintenance-intensive hydraulic systems more commonly used by today’s aircraft and the Space Shuttle — drastically reduce the X-38’s complexity and risks. By using a parafoil for its final descent, the X-38 does not need a long runway at the landing site, opening up many more options around the world as potential sites for a crew’s emergency trip home. Laser-fired explosives eliminate a risk from stray electromagnetic interference that could inadvertently cause a malfunction during the years a rescue vehicle must spend in space.
X-38 Technology: Expanding the Envelope of Spacecraft Design
• Electromechanical Actuators: Small electric motors that weigh only 10 pounds are powerful enough to move with six tons of force in a fraction of a second; they replace complicated conventional hydraulic systems to power the X-38’s flaps and rudders. Hydraulic systems account for up to 25 percent of the annual maintenance on commercial aircraft, and the electrical actuators on the X-38 serve as a forerunner for a technology that has the potential to make flight simpler and safer not only in space but also on Earth.
• Laser-Initiated Pyrotechnics: Never before used on a human spacecraft, the explosive charges that deploy the X-38’s parachutes are fired using a system of fiber optics and lasers. Using light instead of electricity simplifies the system and reduces the potential for electromagnetic interference during the extended stays the X-38 will experience in orbit.
• Landing Skids: Rather than temperature-sensitive tires, the X-38 uses simple skids as landing gear, eliminating the need to watch inflation pressures, brakes, or other complex mechanisms during the years it spends in space.
• Navigation: The X-38 uses a compact Global Positioning System and electronics technology for its primary navigation system; such navigation technology has never before been used as the primary navigation equipment on a human spacecraft, thus replacing the complex mechanical navigation platforms used as the primary system aboard the Space Shuttle. The GPS navigation system designed for the X-38 already has been flight-tested as a payload aboard the Space Shuttle.
• Lifting Body: The X-38’s special lifting body shape — a shape that creates lift so the craft can fly even though it has no wings — is a modified version of a shape tested by the Air Force in the late 1960s and 1970s. The Air Force’s previous testing has reduced the costs associated with the X-38. The lifting body shape gives the X-38 the capability to fly to a landing site during its descent, increasing the number of possible landing sites. Two movable fins and body flaps provide steering for the spacecraft as it descends into the atmosphere.
• Parafoil: A 7,500 square-foot parafoil, the world’s largest, allows the X-38 to have great flexibility to get a crew back to Earth quickly with dozens of potential landing sites around the world, eliminating the need for a miles-long runway to accommodate high-speed landings similar to the Space Shuttle. Using the parafoil to glide to its final descent , the X- 38 touches down at under 40 miles per hour and skids to a stop in only 150 feet. The giant X-38 parafoil, almost one and a half times as large as the wings of a 747 jumbo jet, may be a
technology that finds other uses, including future spacecaft and uses on Earth that require precise landings such as airdrops of humanitarian aid.
• Life Support: For reliability, the X-38’s life support system uses proven, simple technologies: Lithium batteries already used on many Shuttle-deployed satellites provide electricity. Active cooling of the cabin and electronics are provided by a sublimator technology first used on the Apollo lunar lander. Carbon dioxide is scrubbed from the cabin air using lithium hydroxide canisters that have been used virtually problem-free on all human spacecraft. The fire extinguishing system uses technology commonly found on advanced fighter aircraft. And, the communications system is identical to technologies used on most NASA satellites. As a custom-built rescue craft, the X-38 can provide a normal sea-level pressure atmosphere for seven crew members for at least nine hours, twice as long as is required for a worst-case return to Earth.
• Crew Cabin: The station “lifeboat” will hold a crew of seven — the entire crew of the station, ensuring no one is left behind in an emergency — and be capable of returning to Earth automatically. The crew will be able to take over manual control of some functions such as selecting a landing site and steering the parafoil during fmal descent. The crew will land in a supine position and be subjected to minimal force to protect members that may be sick, injured or deconditioned from long exposure to weightlessness. The crew can monitor the operation of an X-38 rescue vehicle and manually take over using color display screens and controls. The cabin is windowless; exterior views are provided to the crew by television cameras.
• Thermal Protection System: The X-38 is protected from the almost 3,000 degrees Fahrenheit experienced during entry into the atmosphere by the same thermal tiles and blankets that protect the Space Shuttle. But, underneath the insulation, the outer skin of the X-38 uses lightweight, superstrong composite materials for the first time. The use of a composite material reduces the amount of flex in the spacecraft’s skin and thus simplifies the way tiles are attached, allowing larger tiles to be used.
• Deorbit Propulsion Module: The only portion of the X-38 that is not reusable, the deorbit module, provides the thrust and orientation control required to begin the rescue craft’s descent. Designed for lightweight reliability, the module is built with composite materials, uses a single propellant and has its own set of batteries. To provide adequate backup capability, eight thrusters, each capable of producing 100 pounds of thrust, are fired for about 10 minutes to begin the X-38’s descent. If any thrusters fail, the others can be fired longer and maintain a safe trip home for the crew. In addition, eight smaller thrusters, capable of 25 pounds of thrust each, provide orientation control during the deorbit firing. After the engine firings are completed, the module is jettisoned and burns up in the atmosphere.
Taking Flight: Testing That Reduces Risks and Costs
An Unprecedented Efficiency —
The X-38 project is developing a prototype rescue spacecraft for less than a tenth of the cost of past estimates for such a vehicle. Development of the X-38 through the flight of an unpiloted space vehicle in 2002 is estimated to cost about $150 million. Previous estimates for the development of station rescue have ranged as $2 billion.
The estimated cost of the entire crew return vehicle project, from development through the construction of four operational spacecraft, ground simulators, spare parts, landing site support facilities and control center capabilities is less than $1 billion, less than half of the cost to manufacture a single Space Shuttle orbiter. To keep costs low, the X-38’s innovative, high-tech development approach uses computerized design, automated fabrication and computerized, laser inspection of many components for the space test vehicle now under construction at the Johnson Space Center in Houston. Rather than seeking early commercial bids on the spacecraft’s design, in-depth development and testing of the X-38 is being done largely “in-house” by NASA’s civil servants. The unusual approach allows NASA personnel to gain a superior understanding of the design, costs, tests, and risks associated with the spacecraft before seeking commercial bids.
Put to the Test — At the U.S. Army’s Yuma Proving Ground in Arizona, the X-38 team successfully tested the largest parafoil ever produced, 7,500 square feet, in February 2000. Flight tests that increase in complexity and altitude will continue with two more X- 38 atmospheric test vehicles, leading up to the first X-38 flight in space in 2005. The X-38 space test vehicle is already under construction at the Johnson Space Center. Large-scale X-38 atmospheric flight tests have been under way since 1997 and will continue, increasing in complexity and altitude each time, through 2005.
A National and International Partnership — The X-38 draws on talent and expertise coast to coast in the United States and throughout Europe. Led by NASA’s Johnson Space Center in Houston, NASA facilities include: flight testing at the Dryden Flight Research Center, CA; development of the Deorbit Propulsion System at the Marshall Space Flight Center in Huntsville, AL; tile manufacturing and launch processing at the Kennedy Space Center, FL; communications equipment from the Goddard Space Flight Center, MD; wind tunnel testing at the Langley Research Center, Hampton, VA; aerothermal analysis by the Ames Research Center, CA; and electromechanical actuator consultation from the Lewis Research Center, OH. In addition, the U.S. Army provides testing support at the Yuma Proving Ground, AZ; the U.S. Air Force has provided in-flight simulation support; and Sandia National Laboratories has provided parachute systems expertise. Companies with major roles in the project include: Scaled Composites, Inc., of Mojave, CA, construction of the atmospheric test vehicle aeroshells; Aerojet Gencorp of Sacramento, CA, construction of the space test vehicle’s Deorbit Propulsion Module; Honeywell Space Systems, Houston, development of the flight control software; and Pioneer Aerospace, Inc., of Columbia, MS, fabrication of the parafoil. In addition, the German Space Agency and the European Space Agency are contributing to the project, involving eight countries and 22 companies throughout Europe.
Lithium-manganese dioxide Primaries Provide Initial Lifeboat Power
Maximum reliability and performance with minimal weight are the requirements for manned space operations. To meet these goals, the X-38 designers selected a primary Lithium-manganese dioxide battery for the Descent Power System (DPS) of the De-orbit stage. Because the De-orbit Propulsion stage with its batteries will be jettisoned in space, primary Lithium chemistry was chosen because of its high energy density and long shelf life. Space savings and reduced complexity were other reasons for the selection in not having to accommodate recharging hardware. As the De-orbit module separates from the CRV, spacecraft power will be transferred to rechargeable batteries. (These batteries will be discussed in later issues.) After selecting primary Lithium chemistry, the specific choice of Lithium-manganese dioxide was made for its long shelf life and a 20% increase in energy density (Wh/l) over Lithium-sulfur oxide (Li/SO2) cells, which have been used on previous Shuttle payloads. The basis of the battery design was the very successful ASTRO-SPAS modular battery design developed by Friemann & Wolf (FriWo) in Duisberg,Germany. Their array consisted of 144 Li/SO2 cells rated at 27.5 Ah each in a 12 parallel string of 12 cells in series arrangement. For the X-38, the cell design was replaced with FriWo’s 33 Ah Li/Mn02 cell (P/N M62). A capacity gauge circuit and a wax packet heat sink were added. This hermetically sealed, cylindrically wound cell achieves 245 Wh/kg and 487 Wh/l and is almost entirely made by automated production lines at FriWo.
First Phase Power
When the lifeboat is needed to return to earth, the crew will enter the spacecraft from the Space Station and initiate automatic flight control. The power for initially operating all the lifeboat functions will be supplied by the primary Lithium-manganese dioxide battery located in the propulsion section. While these batteries are referred to as the DPS batteries, they only carry that name because of their location. It might be better to consider these batteries as the ‘first stage’ batteries. The departure of the batteries with the DPS assembly allows the choice of primary batteries. Similarly, the next ‘battery stage’ gains its name from its location in the crew cabin. Just as Apollo used multistage rockets to get to the moon, the X-38 uses multiple battery stages to get to the earth.
The nominal time for re-entry is only one hour, but an on-orbit loiter time of seven hours is required for deciding when and where to begin the re-entry. During this loiter peoriod, the maximum continuous average current draw will be 50 Amps for each battery if one of the four batteries failed completely. To increase the heat conduction to the wax heat sink, conductive carbon fiber core materials within the wax are glued to the battery module, contacting the battery base to enhance the flow of heat from the battery to the wax. These fibers have 90 percent porosity and increase the thermal conductivity by a factor of 50. When considering the combination of battery and heat sink, there is no net flow of heat from this assembly, allowing heat flow of the combination of battery and heat sink to be deemed an adiabatic process.
The nominal average current per battery is 37.5 Amps. The DPS batteries operate at 32 Volts, while the next stage In-cabin batteries operate at 28 V. which helps to keep the rechargeable In-cabin batteries from charging the primary DPS batteries during the battery transition. Each battery module is rated at 350 Ah (50 Ah. for 7 hours). It can handle peaks of 100 Amps for less than 3 seconds and 74 Amps for 1 minute.
While the DPS battery design window of 7 hours seems a bit long, there may be an extended need for on-orbit power if primary landing sites have weather problems. As the Lifeboat loiters for up to three orbits, ground reselection of other landing sites is made and information is uplinked to the Lifeboat. The total power available will allow the failure of one complete battery module and still meet the total energy requirements of 29.4 kWh.
Once re-entry requirements are defined, DPS power is needed to control orientation of the vehicle and initiate re-entry. The rocket fuel is hydrazine which is a liquid monopropellant (no oxidizer is needed). Smaller thrusters will be used while loitering on-orbit and align the vehicle for the de-orbit burn. Current spikes of up to 200 Amps are expected with nominal 150 Amp nominal spikes during the de-orbit burn. Loitering and reentry are one-time operations, so that with the jettisoning of the DPS module, the DPS batteries leave, also. In the transition just before jettison, the In-cabin Nickel-metal hydride 28 Volt batteries are brought on the bus to provide a smooth transition of spacecraft power.
Where to send the heat?
Similar to a bank robber trying to get rid of the bag of money when surrounded by police, battery heat in spacecraft is difficult to remove. If not quickly removed, battery temperatures can climb to unsafe levels that can result in cell venting and/or thermal runaway. On earth, heat conduction to the air is easily used to remove battery heat, but in the vacuum of space, heat must be conducted to other parts of the craft or radiated away with mechanisms, which require space and unwanted weight. The unique DPS batteries contain their own heat sink, employing a phase change material (PCM) consisting of wax stored in a carbon fiber core body. This wax has a melting temperature between 42-44 0C. As the batteries conduct heat to the wax, the temperature of the wax increases to the melting point. Just as with melting ice, the physical property of latent heat of fusion requires large amounts of heat to be absorbed to melt the wax to liquid at the same temperature. The heat absorbed by the wax per degree C is 124 times greater than the heat absorbed before or after the melting temperature. To facilitate heat flow within the block of wax, carbon fibers embedded in the wax increase heat to the wax by a factor of 50 over the rate for pure wax. Because the fibers of this 90% porous fibercore are 6-8 microns in diameter, they trap the wax and the necessary voids by capillary forces inside the fibercore. This feature accommodates the 15% volume expansion in going from solid to liquid wax without stressing the container that seals the wax. Previous PCM heat sink technology had to depend on very strong containers to prevent leakage that occurs when the majority of the void volume aggregates into one large bubble. This technology developed by Energy Sciences Laboratories, Inc. (ELSI), in San Diego, CA makes possible PCM heat sinks with specific latent heats exceeding 150 J/g.
State of charge measuring
Since the astronauts are betting their lives on the lifeboat power, the DPS batteries require a known capacity. In the X-38, one set of batteries will be ready for a three-year service period. To know the battery status, each module is equipped with a capacity gauging circuit, which accounts for the combination of self-discharge with temperature and the counting of Amp hours of discharge current. The gauge uses a 1.5 mOhm shunt inside the battery that was developed by Summit Products Corp. in Minden, NV. The status will be monitored for about 15 seconds during each checkout to maintain confidence in its readiness. Data on initial energy stored will be taken from ground based discharge performance of sister cells in the production lot.
The DPS battery was successfully qualified in testing which was completed in April of 2001. The lot of 1500 Li/MnO2 cells was certified with a number of performance and safety tests. During battery qualification testing, the battery passed shock and vibration tests. Performance testing in a thermal vacuum showed that the battery would discharge for greater than seven hours at 50 Amps under adiabatic conditions. Testing started at 30 degrees C and ran for seven hours and 20 minutes until cell temperatures reached 80 degrees C. The PCM composite heat sink stabilized battery temperatures at between 42 and 50 degrees C for 3.5 hours.
Like the many pioneering structure and control features of the rescue vehicle, this DPS battery module contributes three new features which are ‘firsts’ for-manned spacecraft. It is the largest (12 kWh) lithium battery module. It has the largest (3600 kJ) PCM heat sink, and it is the largest lithium battery employing a self-contained heat sink.
More to come
With such an elegant approach to system power, one would not expect it to end. Here the NASA team is not disappointed because investigations are still being conducted for developing an even more compact method of heat sinking using the vaporization of water. At sea level where water boils at 100 0C, but in outer space, reduced pressures can be made available by venting, so water at 8% of atmospheric pressure boils at 42 oC, the same temperature as that of the present wax heat sink. But, the latent heat of vaporization phase change of water is ten times greater than that of the melting wax, making it potentially a better heat sink. In a closed container, the vaporizing water would normally build pressure and raise the vaporizing temperature, but with a relief valve venting to space, the appropriate pressure can be maintained. The system has a conductive wick to assure that all the coolant remains in good thermal contact with the heat surface in microgravity, and a recuperator channels the expended vapor back through the heat transfer surface to better use the vapor. Operated as a Small Business Innovative Research grant to ELSI, phase 1 has been completed, and the second phase has been underway since January 2002. The vaporizing heat sink could reduce the 14.5 kg of melting wax with 1.5 kg of vaporizing water and reduce the overall heat sink mass from 27 to 10 kg. This means lower launch weight, lower launch cost and provision for additional space in the Crew Return Vehicle.
The batteries of the X-38 employ known chemistries and utilize them in creative ways to produce a better and less expensive vehicle. Just as the other innovations of the program will lead to new flight vehicle design, the battery design will provide other designers with creative ways to employ battery energy storage.