Activities conducted using spacecraft have become a ubiquitous part of our society. From monitoring hurricanes to measuring the ozone layer to a host of defense missions, U.S. government agencies today rely heavily on satellites and aerospace technology to conduct their day-to-day activities. But how are satellites conceived, designed, produced and deployed?
NASA describes a spacecraft as “a vehicle or device designed for travel or operation outside the Earth’s atmosphere.” A satellite is described as “a type of spacecraft that orbits the Earth, the moon or another celestial body.” In this series, we will use the terms spacecraft and satellite interchangeably.
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Before a new spacecraft can be launched, its main structural, electronic and propulsion components must be attached to the satellite structure, connected to each other electrically, and then tested as an integrated system, a process called integration and test (I&T). In the very competitive aerospace industry, it’s no surprise that the planning behind these steps begins long before the space vehicle has even been designed. After all, you can’t afford to design a spacecraft that can’t be assembled, integrated and tested in a straightforward and cost-effective manner.
Burning Down Risk
“Typically, a few members of the integration and test team come on board a program very early to support the initial requirements development and flow-down process. We’re there to identify and help reduce or ‘burn-down’ risk for the program,” says Marty Sterling, a director of engineering for integration and test for Northrop Grumman. “We also work with the design folks on issues such as accessibility and testability, and help them by laying out notional test schedules which we will mature as the design matures.”
Once a company wins a spacecraft production contract, she adds, the I&T process begins to gain momentum. Her team begins working closely with the systems engineering and space vehicle engineering teams, helping them write theI&T plans and recommending small design changes to flight hardware that will allow I&T to proceed more smoothly.
“In those early days, we’ll also be designing the electrical ground support equipment used to test the space vehicle, and the mechanical ground support equipment used to support the structural build of the satellite and any ground testing prior to launch,” says Sterling.
Satellite production doesn’t begin in earnest, she clarifies, until a program has successfully passed its critical design review (CDR), a point at which most design details have been finalized and the customer has given their approval for production to begin. But the actual space vehicle I&T process doesn’t start until all of the satellite’s structural and electronic components have been fabricated, assembled and individually tested, a process that can take a year or more.
Assembling the Pieces
The procurement of the major components that make up a satellite is typically handled by a joint manufacturing and space vehicle team. This team acquires major satellite support subsystems such as propulsion, electrical power, and command and data handling, and fabricates the satellite bus, i.e. the physical structure that houses these subsystems and the mission payloads. The team is also responsible for the fabrication of hinges, gears, and gimbals used in movable or deployable subsystems; antenna and solar array structures; and radiators, which provide thermal control for different zones of the spacecraft.
Spacecraft boxes or units are fabricated, assembled and tested either by outside vendors or by manufacturing organizations within the spacecraft contractor’s company.
“We start fabricating circuit boards as soon as possible after CDR,” explains Ken Weber, communications payload deputy for a current Northrop Grumman satellite program. “Each board is populated with components, tested at the board level, then inserted into a mechanical frame to create what we call a slice, or plugged into a backplane that holds multiple circuit cards. The slices – the backplane and its plug cards – are then bolted together or enclosed in a housing and then tested as a single unit.”
For boxes manufactured in-house, Weber adds, unit-level testing is done under the supervision of the engineer responsible for the design of that unit. For units produced by outside vendors, the spacecraft contractor will typically send a team to the vendor’s site to inspect each flight unit, review its test data and confirm its readiness to be delivered to the contractor’s I&T facility.
The same is generally true of a satellite’s electrical power subsystem (EPS), which comprises of solar arrays, batteries and electronic units that generate, store, process and distribute power within the spacecraft.
“It can take about 18 to 24 months to fabricate, assemble and test the electrical boxes, both functionally and environmentally,” says Tommy Vo, the EPS manager for a current Northrop Grumman satellite program. “Assembly and test of a satellite’s solar arrays can take upward of 54 months.”
Vo’s team tests EPS boxes and solar arrays vigorously to ensure that each one meets its specification — a process called unit verification — before delivering them to the integration and test team.
A satellite’s propulsion system, which includes propellant tanks, thrusters, valves, heaters and precision metallic fuel lines, is treated differently from other subsystems, partly because of its critical role in satellite operations. As such, it is assembled and integrated with the bus by a team of propulsion specialists before the bus is delivered to the I&T team.
“The propulsion assembly team provides special expertise in handling, installing and welding the system together,” explains Arne Graffer, a senior satellite propulsion specialist with Northrop Grumman. “This work includes alignment of thrusters, electrical and functional checkouts, and proof and leak testing of the completed system. We have to demonstrate that the system will perform reliably under all flight conditions.”
Growing the Satellite
Once the propulsion system has been installed in the satellite bus, the integrated structure is delivered officially to the integration and test team.
“Typically, the bus is delivered to us as a main structure and a series of panels that form the outer ‘walls’ of the bus,” explains Sterling. “We begin by installing bus electronics into the bus structure, and attaching payload electronics onto these individual panels.” This process includes installing all the cabling required to interconnect the satellite’s electronics, she adds.
And then comes the moment of truth.
“We start the integration process by flowing voltage through one of the cables to make sure we get the expected signal out the other end,” says Sterling. “If it all looks good, we know it’s okay to mate that cable to the next box. Then we check the signal coming out of that box to make sure it’s what we expect.”
This validation process continues, she adds, until all the bus electronic units and wire harness cables have been tested and mated. Sterling’s team next performs a series of functional checks on the integrated system, still at ambient temperature, to make sure that all of the bus electronics units are communicating and interacting with each other as expected. The integration process is then expanded to include auxiliary payloads such as sensors and other mission-specific electronics.
Sterling’s team conducts this satellite checkout process with the aid of ground support test equipment. The test equipment functions, in effect, like a “ground station” sending and receiving data to and from the satellite. This communication also helps verify, therefore, the ability of the satellite bus and mission payloads to talk to the “Earth.”
The integration and test team also installs a satellite’s mechanical systems, such as its solar arrays, antennas, radiators and launch vehicle separation system, and then tests the ability of these systems to deploy properly. To ensure their proper operation on orbit, the team aligns these systems with a precision of .002″ or about half the thickness of a standard sheet of paper.
Simulating Launch and Deployment
Once a satellite is fully assembled, and its electrical systems have been proven functional at ambient temperature, the I&T team begins a series of rigorous environmental stress tests. Collectively, these tests are designed to prove (1) that the satellite can survive the extreme acoustic and vibration environment of launch, (2) that it can sustain the explosive shock associated with separation from the launch vehicle, and (3) that once on orbit, its electronic subsystems can operate successfully in the extreme temperature and radiation environments of space.
During vibration testing, the satellite is placed on a large shaker table and shaken for several minutes at frequencies expected during launch. For acoustic testing, the satellite is placed in a large chamber, then exposed to high-intensity sound waves that simulate the acoustic environment of launch. Shock testing involves exploding the ordnance that’s used on orbit to release the mechanical pins that hold deployable devices in their stowed position.
During acoustic, vibration and shock testing, Sterling noted, the satellite’s electronic systems are all placed in their launch configuration. For many of the satellite’s systems, such as mission payloads, that means the electronics are switched off completely.
Testing the Extremes of Space
Another key phase of environmental testing within integration and test is called thermal vacuum (thermal vac) testing. For this testing, the entire satellite is placed in a special chamber that can be pumped down to near-vacuum conditions. The chamber also includes high-performance heating and cooling equipment. As the I&T team “exercises” the satellite’s electronics functionally, the temperature inside the chamber is cycled repeatedly — typically six or seven times — between extremely hot and cold temperatures (+180 to -200 degrees C) over the course of several days.
In conjunction with thermal vac testing — either before or after — the I&T team also conducts electromagnetic interference/electromagnetic compatibility (EMI/EMC) testing to ensure that no devices on the satellite are emitting significant amounts of electromagnetic energy. Such emissions could interfere with the proper operation of the satellite bus or its mission payloads.
When all of the functional and environmental tests are complete, the I&T team puts the satellite into its shipping configuration with all mechanical appendages stowed, tests it one last time for electrical “aliveness” and then packs and ships the satellite by truck or cargo plane to the launch site.
But the I&T team’s work doesn’t end when the satellite leaves the factory.
At the launch site, explains Sterling, the I&T team unpacks the satellite and performs post-delivery health checks on its bus electronics and payloads to verify that the transportation process didn’t harm them. Then her team works closely with the launch vehicle team to integrate the satellite to the launch vehicle in preparation for launch.
“I think it’s safe to say that the I&T process never really ends until the launch vehicle clears the tower,” she advises.
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