Testing a Satellite's Potential

Lincoln Laboratory's Environmental Test Lab helps MIT students realize their vision

Six seniors in MIT's Aeronautics and Astronautics Department hover over a 6 × 6–foot platform, working quietly and quickly around a mounted satellite. They are in Lincoln Laboratory's Environmental Test Laboratory (ETL) making modifications to a satellite that they and 27 other students have been developing over the past twelve months. They move quickly because they have a three-hour window before they catch the next shuttle back to campus for other classes. The next day another set of six or so students will continue where these students leave off.  

Victoria Stolyar, one of the MIT seniors, carefully adds super glue to a 12-inch piece of 80-pound test string to make it stiff enough to push through the holes in one of two solar panels that will power the satellite. The panels, which will deliver about 300 watts of energy, are necessary to provide power for the ion engines. The string Stolyar is threading through the panel's holes will hold the solar panels in position during the satellite’s launch. Once the satellite is in orbit, a command to a cutting mechanism will sever these cords and release the solar panels so they can extend to their full twelve-foot on-orbit length. The typical student-built satellite has solar cells attached to the body, but the MIT students have built the first student satellite that has deployable solar arrays. The solar cells used in the solar arrays were donated to the project by Loral, but students assembled the cells into the arrays.

The cord is proving difficult to thread through the panels and their alignment pieces and then tie to the attachment mechanism. Professor David Miller glances over at John Keesee, part-time Lincoln Laboratory technical staff member and liaison on the project, and comments, "We’re going to have to work on a solution for this during the summer."

Students perform a vibratin test on their satellite Left to right: Graduate student and Air Force Lieutenant Joseph Robinson and seniors Arthur Huang, Ryan McLinko, Victoria Stolyar, and Virginia Quaney perform vibration testing on satellite components at Lincoln Laboratory's Environmental Test Laboratory.

MIT undergraduate students are building this satellite as part of a three-semester Space Systems Product Development course designed to give students real-world experience building and testing hardware. The curriculum involves a CDIO approach: conceive, design, implement, and operate. This phased approach to the project takes students through the all the stages of the development process, giving them the experience of managing a full life-cycle project on an aerospace product.

"The three-semester program allows us to develop these students," says Keesee. "In their first three years in class, they learn to think about problems and solve problem sets, and in this senior year, a program such as this allows them to think about a problem, develop an approach, conceive a solution, build and test their design, develop preliminary design reviews and critical design reviews, and work out problems during testing phases. Essentially this program allows them to apply a rationale to the education they received. Most people don't get this experience until a few years at work. Once they make this jump, then the Laboratory serves as a mentor."

John Keesee and students John Keesee looks on as the seniors Frances Gonzalez and Emily Grosse complete the installation of solar panel restraints on the satellite before they begin x-axis vibration tests.

The 33-member team is divided into six subteams: systems; structure and thermal dynamics; avionics and communication; attitude determination and control combined with guidance, navigation, and control; propulsion and power; and orbits and operations. Two Air Force lieutenants who are graduate students are part of the design, build, and test phases and also serve as mentors to the students. The students are currently in the third semester of the project.

"We had no idea what to expect at the onset of this project and there was such a steep learning curve, interpreting the data, using our hands, and building hardware. It has been a great experience,"says Frances Gonzalez.

"This third and last semester," says Keesee, "is the culmination of everything they have worked on for the year. They are now performing final tests on what they built to see if it is successful." Keesee adds that "the students work as a team, make decisions and test to see what works and what is hard to implement. They can only learn this in a working laboratory. Their engineering judgment is developed here."

The students' satellite must fit within a 50 × 50 × 60–centimeter box (20 × 20 × 23.6 in) and weigh less than 50 kilograms (110 lb). The basic structure has four aluminum radiation panels that support all of the subsystems and are mounted to the main engine propellant tank in a way that brings to mind the Star Wars X-wing fighter. The satellite is controlled by two microprocessors, and one student developed a way to reprogram them while the satellite is in orbit, should that be necessary. Attitude determination is accomplished using a combination of a sun sensor, magnetometers, and an inertial measurement unit. Attitude control is actuated using a reaction wheel, small nitrogen thrusters, and gimballing the main engine. Navigation is accomplished on board using a GPS receiver, supplemented by ground tracking. Ground communication will use an onboard 2.4 gigahertz receiver/transmitter.

The main engine used for the maneuverability demonstration is a diverging cusped-field Hall-effect thruster that accelerates the propellant (xenon) by using an electromagnetic field. This engine uses permanent magnets to create the magnetic field and thereby has a significantly smaller mass than other electric thrusters. It will demonstrate the ability to provide 2000 m/s of orbit change energy. This particular satellite was sized to be capable of operating from geosynchronous transfer orbit to the moon.

"Our engine requires significantly more power to operate than is usually found in microsatellites," says Keesee. "In this level of power on orbit, we needed solar arrays with more area than could be obtained by mounting the solar cells on the body of the spacecraft, so this led us to a requirement for larger, deployed solar arrays. Because the engine is continuously firing when the satellite is exposed to the sun, the thrust vector of the engine is fixed by the orbit, so this further required the solar arrays to be articulated so that they could track the sun wherever the spacecraft was in its orbital arc. The articulated solar arrays, instead of the fixed, body-mounted solar cells found on other student spacecraft, added significant complexity to the design team and challenged the student fabrication capabilities. Testing at Lincoln Laboratory pointed out several weaknesses in the design that have been and are continuing to be corrected."

Because the spacecraft is severely weight limited, the students developed an innovative design using the propellant tank as a structural member of the spacecraft. Testing confirmed the suitability of the structure to withstand the loads during launch.

The design of a spacecraft thermal control system was challenging. The thermal vacuum testing accomplished at the ETL, a 6000-square-foot high-bay facility used for demonstrations of novel ground-based, airborne, and sea-based systems, allowed the team to define and validate the analytical models used to predict performance in space. This validation requires a thermal vacuum chamber because convection and conduction of thermal energy in room-temperature air conditions would completely hide the radiation cooling data. The ETL thermal vacuum chamber, which simulated the space environment, may later be used to verify the performance of the flight spacecraft. The chamber is evacuated to a pressure of one millionth of an atmosphere and the walls are cooled with liquid nitrogen to a temperature of 80 K (–316°F). Thermocouples and thermistors mounted on the spacecraft structural model recorded temperatures while the heating associated with various subsystems was turned on and off.

Prof. miller and Lt Richmond verify thermal sensor performanceProfessor David Miller from the Aeronautics and Astronautics Department works with graduate student and Air Force Lieutenant John Richmond as he verifies thermal sensor performance before starting the next phase of thermal vacuum testing.

"It's been very useful to use Lincoln Laboratory's facilities. Being able to test our work confirms what we are doing is correct," says Stephanie Couch.

Lincoln Laboratory's ETL provided a vibration test apparatus that was used to simulate the vibration and acoustic loads the satellite will experience during launch. By shaking the satellite in a carefully controlled set of frequencies and amplitudes in all three axes, the team created the loads and accelerations (exceeding 20 g's) found in the launch environment. ETL technicians mounted 22 accelerometers on the spacecraft at critical locations to validate the structural model and ensure the survival of the spacecraft.

Fuzhou Hu, a student who works on the avionics and helps the structures team performing vibration tests, says, "I think it's exciting because Lincoln Laboratory has the vibration table and vacuum chamber to allow us to work all phases of the project. These tests are critical to the project and help us understand more than just theory."

John Richmond, one of the Air Force lieutenants, adds, "Lincoln provides us the facilities to test our work. There is no other way to add this dimension to this project and class experience. The Laboratory staff has been extremely helpful with the setup of tests and teaching us about satellites."

The Laboratory staff members working with the students include Jon Kadish, from Lincoln Laboratory's Space Systems Analysis Group, who served as the mentor to the structure/thermal subsystem team during the initial design. Al Mason, who works in the Fabrication Engineering Group, operated the vibration test equipment in the ETL, and Jon Howell, of the same group, ran the thermal vacuum system for that series of tests.

Keesee works a little more than half his time in the Laboratory's Space Systems Analysis Group. On campus, Keesee is a senior lecturer and teaches several courses with Professor Miller at the graduate and undergraduate levels, generally in the areas of space vehicle or aero vehicle design. Keesee serves as the mentor to the MIT satellite team who will be responsible for the continued development of the vehicle after most of the current team graduates this June.

"These kinds of programs build bridges between Lincoln and the Institute. Laboratory personnel are involved in the course on campus and work closely with the students here at Lincoln on the implementation, testing, and operation phases," says Keesee.

"Lincoln Laboratory resources make this course an authentic experience for the students. Young engineers learn best by doing; hand-on experiences early in their careers lead to much better engineers and much better engineering," says Miller. "This program is also a chance to return to our roots in aerospace."

The program originated some years ago with a request from the Air Force Space and Missile Center (SMC). The Laboratory's leadership in the Aerospace Division and a professor of aeronautics and astronautics and engineering systems at MIT were approached about ways to improve the technical capabilities of the SMC workforce—less than one-third of the SMC staff had technical backgrounds. Kyle Yang, a Laboratory technical staff member who worked at SMC, collaborated with Miller and Keesee to develop a proposal for training officers. Today, Air Force officers Richmond and Joe Robinson work as mentors on the satellite team.

Recently the members of the Space Systems Product Development course provided a detailed acceptance review at MIT, defining the current status of the vehicle development. This acceptance review is essentially a "sell-off" of the vehicle and the students' cumulative work at this point in the development. In attendance was the next student team awaiting their chance to get on the shuttle from Cambridge to Lincoln Laboratory's Environmental Test Laboratory next fall.

Posted June 2009

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