Engineers claim FOD is a four-letter word, but there’s nothing funny about foreign object debris and its potential for disaster.
The incident occurred at around 10:25 a.m. on October 5, when the crew of the SpaceX platform was preparing the Crew Dragon. Endurance For launch on top of the Falcon 9 rocket. Four Crew-5 astronauts already inside the capsule and with the hatch closed saw a single human hair in a careful eyelid seal. The hair was designated FOD — the engineering term for foreign object debris — prompting the pad crew to take action.
The countdown clock had just passed T-90 minutes, so time was of the essence. The pillow staff quietly reopened Endurance‘s hatch and removed the offending string. They did another check, thoroughly cleaned the seal area again, and closed the hatch for a second and final time. A subsequent pressure check confirmed a tight seal.
The whole thing took only a few minutes and did not affect the launch. Falcon 9 blastoff was scheduled to take place at noon, Crew-5 astronauts Nicole Aunapu Mann, Josh Cassada, Koichi Wakata and Anna Kikina—successfully arrived at the International Space Station the next day.
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It’s obvious and completely understandable that the SpaceX pad team takes time to remove a feather before launch. In the aerospace industry, FOD is defined as any object that does not belong to a specific location, whether that location is a hatch seal, engine, cockpit, or runway. Misplaced debris can damage equipment, facilitate suboptimal performance of systems, and cause outright malfunctions.
This is a problem in many industries, but for the aerospace industry, it’s one that comes with a price tag of $4 billion annually. according to Boeing is managed by NASA FOD program At Kennedy Space Center, the goal is to “minimize the possibility of damage to or loss of aircraft or injury to personnel due to lost items within aircraft components resulting in the preservation of national resources.”
Speaking to me on a video call, Tom Simon, deputy spacecraft manager at NASA’s Johnson Space Center, said, “We’ve all been trained from day one to watch out for FOD when dealing with flight systems.” Foreign objects such as pencils, paper clips, screwdrivers, hair and dust may “seem small” but can result in “a seal slowly leaking from the sea,” among other things. “When we build systems, we take it seriously,” he said.
As an engineer, John Posey, lead engineer for NASA’s Crew Dragon, told me during the same call that FOD is “ingrained in your system.” FOD is “considered the biggest risk in training programs” because it carries the potential to “shoot down missiles and aircraft,” he said.
Simon and Posey could not speak to the specific policies and protocols of SpaceX, but they were not surprised by the pad crew’s actions in removing human hair. FOD on sealing surfaces is a serious concern. When working with a sealing surface and having to maintain a tight seal, “you don’t want something to be pressed against it,” Posey said. “Something like hair—depending on its size and direction—can result in a leak.”
Posey said the sudden appearance of FOD for time-sensitive situations, such as the final closure of a capsule hatch, should be included in the timeline and process, in addition to having a contingency plan in place should this scenario arise. Pad crews “have to go back, take the thing out, re-inspect and even clean the seal and then get on with what you’re trying to do,” Posey said.
It’s not just hatches that are prone to FOD risk. Launch operators implement processes to reduce the risk of FOD, such as using covers or shields when work such as cutting or sawing must be done near the spacecraft. And, of course, the operators themselves must be clean. Propulsion systems, where propellants and oxidizers are pumped through high-pressure systems, can be affected by FOD, said Posey, who worked on the Space Shuttle in its final days and spent “thousands of hours on the floor working on propulsion systems and technicians to make sure everything was OK.”
As Simon explains, the degree of cleanliness required often depends on the project or mission itself. Each system needs its own control plan, Posey said, with engineers defining acceptable limits and deciding what to inspect.
Cleanroom protocols for unmanned satellite launches are minimal “down to the point of handwashing and wearing gloves.” Crewed missions are a different story. “Not only do the avionics have to work with the crews, but you don’t want stuff flying all over the place,” Simon said. Once in orbit, microgravity can suddenly cause FOD to float around, including hair and dust. Posey said filtration systems are designed to deal with that sort of thing, “but you still want to prevent hassles,” such as requiring covers over hatch seals, among other measures. And “even the covers should be cleaned and checked for leaks,” he said.
Posey offered some sage advice: “Always make sure you open the system in a clean room, only do what you need to, and do a check before you close it.” And “if you see something that doesn’t look right, go in and investigate” because it’s a “necessary burden.” A second set of eyes wouldn’t hurt, I added. “FOD will find a way to get into your system,” Posey said, hence the term “Smart FOD.” I related an incident in which a boot or shoe cover was suddenly discovered in a Shottle compartment. “It just slipped off somebody’s foot, and that kind of thing can be funny in retrospect,” Posey said, but boots, tapes or anything else that doesn’t belong can be a ignition hazard.
Measures to prevent FOD from entering components or complex systems begin in the cleanroom, and each cleanroom has its own cleanliness requirements depending on the project. Posey said cleanrooms are “specially certified and controlled to a certain degree of cleanliness depending on what’s in there,” and items typically must be approved before being allowed into the cleanroom.
Lockers are available to store loose items; tape and sticky floor pads can provide room must-haves; and the tethers can catch anything accidentally dropped. The overalls, known to engineers as “bunny suits,” cover the arms and legs and usually include a hairnet hood. Beards are covered with beard nets and shoes get boots.
“Once you’re ready and everything’s ready, you go through the double doors,” Posey said, the first door “closes behind you and then you go into the clean room.” In the middle rooms of some double doors, I explained, “the air blows all over you, sucking in any dust or debris.” Staff will collect any EEM found and investigate where it came from and whether any additional controls are needed. Clean rooms are “never clean enough,” Posey said.
These measures are an additional but necessary headache. The good news is that FOD detection improves over time. Cameras are now routinely used to observe almost every corner of the ramp, while X-ray and CT scans can look inside objects and create 3D images of the part’s interior. With these tools, engineers can “see assembly problems” and “detect FOD that might not otherwise be found,” Posey said. Especially in an era when components can be reused, there is an improved importance for the private sector to sniff out FOD.
The human hair found inside the hatch seal may or may not cause problems during Crew-5’s flight, but it doesn’t matter. What matters is safety and the elimination of anything that could put human life at risk. Engineers will continue to look for FOD regardless of the inconvenience it may cause.
More: To remember Enterprise: A test craft that never flew into space.
