A recent conversation with engineers at NASA’s Johnson Space Center in Houston revealed that there’s often “a world of difference” between the standards for circuit protection components for a typical Earthbound electronics engineering application, and those intended for use in spacecraft. However, for both environments, engineers have essentially the same goals: safeguarding life and protecting technology. Carlton Faller, a NASA electrical, electronic, and electromechanical (EEE) parts engineer, and Paul Delaune, Deputy Branch Chief - Command and Data Handling, shared some insights on the differing requirements associated with creating circuit designs for use in low-Earth orbit (LEO) and beyond.

Faller was a NASA contractor for Lockheed Martin prior to joining NASA as an employee in 2004. “We have a wide variety of specifications that govern the selection of micro-circuits, resistors, fuses, switches, relays, capacitors, wiring — all the component parts that go into the design of a circuit board or piece of electronics,” he said. “The requirements for each component are dependent upon the criticality of the application, the specific environment in which it will operate, and other factors relating to ensuring parts are appropriate for the application.”

Figure 1. NASA engineers prepare the service module of the Orion Multi-purpose Crew Vehicle for installation of the fairings designed to protect it during launch. Orion is scheduled for a first test launch this December.
In contrast with designers of terrestrial products, NASA designers and contractors are far less concerned about conformance with commercial standards from IEEE, UL, IEC, and other standards bodies, and more with compliance to guidelines set out in internally developed documents for each program. Faller noted, “For example, Document SSP 30312 is the International Space Station’s EEE parts control plan. That governs the rating of all parts and includes a section that addresses the proper rating and de-rating of fuses and other circuit protection devices. It says, ‘If your nominal current is X amps, then you must use a circuit protection device rated at somewhat greater than X amps. And then you must use a wire that is able to withstand the maximum current that the circuit protection device can pass without tripping.’”

Parts control plans for the various NASA development programs, including the Orion Multi-purpose Crew Vehicle NASA will launch later this year (Figure 1), are more likely to reference military specifications than commercial ones. For example, MIL-PRF-23419 covers instrument-type fuses designed to protect electrical, electronic, and communication equipment on DC and AC circuits up to 400 Hz. However, in addition to military specifications, these parts control plans include guidelines based on the agency’s experience with various types of components. Faller explained, “We’ve got some use limitations on certain part types based on their construction. This is historical knowledge that says, ‘In a vacuum, you can only use a certain type of fuse that’s rated lower than a specific current, because otherwise it tends to act funny.’ But that’s not an industry standard so much as in-house ‘tribal knowledge’ that we’ve developed over the decades.”

Paul Delaune manages a team of NASA engineers involved in electronic design and system management work. His branch focuses on spaceflight processors, networks, and instrumentation, including command and data handling systems for complete manned spacecraft, all the way down to small, independent data acquisition systems. He noted, “Often, we’re developing equipment that will experience significant vibration and thermal stresses. Carlton mentioned how we have to de-rate fuses — we often have to end up de-rating fuses right at 50 percent because fuses are designed to open when they get too hot due to the level of current flowing through them. On Earth, heat rises. In space, it doesn’t. All that heat from current flow gets concentrated right there, so fuses can blow at half the regular amperage rating.”

Figure 2. Encapsulated fuses prevent heat and sparks from being exposed to explosive gases or dust in the environment.
Delaune said, “Every function on a spacecraft has a different level of criticality assigned to it, which affects the way our electronics are designed. We classify items according to the worst-case effect of a failure on the vehicle, crew, and mission. A Criticality 1 dysfunction, such as a failure of the onboard computers, would result in the loss of a life or a vehicle. A Criticality 2 function loss would mean the loss of a mission. All other function losses are classified as Criticality 3. What that means is that we have a lot of older technology associated with Criticality 1 equipment because its performance has been proven, and the reliability testing required to meet the Criticality 1 standards is very rigorous.”

He continued, “If we’re designing a circuit for the control system of a spacecraft, we have to make sure it can continue to operate in the high-vibration environment it will encounter during launch, as well as under the G forces involved. Equipment that won’t be used until the spacecraft reaches orbit is typically soft-stowed, so it doesn’t experience the same level of vibration.”

Hazardous Work

Figure 3. Crewmembers working in microgravity on the International Space Station are constantly surrounded by instrumentation, which must be durable enough to withstand a 120-pound kick load.
Even the air the crew breathes has an impact on circuit protection and other electronic component choices. On the International Space Station, people and equipment are operating in an Earth-like oxygen/nitrogen environment. But when they go outside the station for an EVA (extravehicular activity), they need to transition gradually to pure oxygen because EVA suits are pressurized using pure oxygen at about 8 psi. The night before crewmembers go EVA, they must sleep in an airlock, breathing pure oxygen in order to rid themselves of the nitrogen in their blood to avoid the risk of “the bends,” which can occur with rapid depressurization from an atmosphere containing nitrogen or another inert gas. However, it’s crucial to choose components and materials that can be used safely in this potentially hazardous environment because of pure oxygen’s high flammability.

Obviously, spacecraft or EVA suits are far from the only hazardous working environments. On Earth, virtually every industry related to energy or basic materials production has potentially hazardous workplaces because gases, petroleum products, and airborne dusts tend, by their very nature, to be explosive if sources of sparks or excess heat are present. UL 913 establishes the standard for “Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III, Division 1, Hazardous (Classified) Locations.” Intrinsically safe apparatus are designed to prevent them from becoming sources of ignition of explosive atmospheres through a combination of energy limitation (i.e., limiting spark energy) and temperature limitation (i.e., limiting the surface temperature of the apparatus). Designing intrinsically safe apparatus often requires the use of an encapsulated fuse to limit the current under sustained abnormal conditions, to ensure that the circuit will open without generating a spark capable of causing ignition. Fuses such as PICO® 259-UL913 Series and PICO 305 Series fuses from Littelfuse (Figure 2) are encapsulated, which limits the energy and temperature generated during fuse operation. Encapsulation also prevents dust from entering the fuse body, eliminating the risk of explosion due to combustible dusts or ignitable fibers.

In space, the atmosphere itself can become conductive, which can lead to arcing problems due to corona discharges. These can occur, for example, following launch when a spacecraft passes through a critical low-pressure region at the furthest reaches of the Earth’s atmosphere, where air pressure falls to less than 1 psi. Although air is not particularly conductive on the Earth’s surface, in this low-pressure region, the atmosphere becomes significantly more conductive. Higher voltage circuits are especially susceptible to corona discharges. Corona deteriorates insulation near the discharge, reduces distribution efficiency, and disassociates some gases, producing noxious gases and odors. Within an electric power system, corona generates spurious high-frequency voltages that produce interference in communication links and malfunctions in sensitive circuits. The current pulses it produces in the circuits of high-voltage equipment can even be large enough to simulate, distort, or mask signals used in electrical communication, control, and measurements.

Unlike most electronic equipment designed for use on Earth, hardware in areas the crew can access must be able to withstand both depressurization events and the force of a 120-pound kick. Because they are weightless in space, crewmembers need to be able to launch themselves off of any interior surface in order to maneuver through the spacecraft (Figure 3). However, extreme care in component selection is necessary. For example, one misplaced kick would easily shatter a glass-bodied component, and the floating fragments could end up in a crewmember’s eyes or lungs.

The level of radiation encountered during spaceflight is another area where onboard instrumentation differs significantly from ordinary electronics. The Earth’s atmosphere protects both people and equipment on the ground from the majority of the Sun’s radiation, but on the space station, equipment is exposed to both protons and heavy ions from galactic cosmic rays and solar flares. The Van Allen radiation belts, which extend high above the atmosphere, are made up of high-energy particles trapped by Earth’s magnetic field. Radiation levels across the belts are affected by solar activity that causes energy and particles to flow into near-Earth space. During active periods, radiation levels can increase dramatically, creating hazardous space weather conditions with the potential to harm orbiting spacecraft and endanger humans in space.

“It takes an incredible amount of mass to shield something from radiation effectively. Instead, we try to design robust systems that can withstand the inevitable upsets and reset themselves or clear out any bad data that they may have gotten,” said Faller. “We validate prototypes of those systems using a proton cyclotron at the University of Indiana and simply run them in a proton beam to see how they respond. For critical hardware, we perform heavy ion testing, where we characterize each part individually to determine what sorts of anomalies that part might see, so our designers can design parts for each of those cases or select a different part that doesn’t have as many problems.”

Delaune added, “We’ve found that if you don’t shield equipment correctly, you can actually cause worse problems than if you hadn’t shielded it at all. If a particle strikes a piece of aluminum shielding, it slows the particle down, which can spew out other ions. A really high-energy particle will pass right through the part and won’t deposit any energy, but if you slow it down with some shielding, it might stop the particle and deposit a lot of energy in an IC, causing a single event upset or a latchup in the device.”

As NASA focuses on preparing for the first test flights of the Orion vehicle, one thing is certain to remain the same — a commitment to safeguarding the lives of all crewmembers by ensuring that the electronic equipment upon which their safety depends operates at peak performance. Although there are obvious differences in standards and approach between space-bound and Earth-bound applications, engineers at Littelfuse are just as committed as NASA engineers to protecting technology and protecting life.

This article was written by Tim Patel, Global Standards Manager in the Electronics Business Unit at Littelfuse, Inc. (Chicago, IL). For more information, Click Here .