NASA evaluating MMOD strike damage on ISS radiator panel

by Chris Bergin

International Space Station (ISS) managers are currently evaluating what appear to be MicroMeteoroid and Orbital Debris (MMOD) impact to a panel on the Potential P4 Photovoltaic Radiator (PVR). While the situation is being monitored, data suggests there isn’t a leak from the system as a result of the 12 inch long puncture to the cover sheet.

MMOD Strike:

The hardware in question is one of the key elements of the heat rejection systems utilized by the orbital outpost.

The four 1650-pound PVRs consist of seven 6 ft. x 11 ft. panels, deployed by an electric motor driven “scissor” mechanism. The PVRs also internally flow liquid ammonia coolant and are capable of rejecting at least nine kilowatts of excess heat.

The launch of the first PVRs was conducted by Space Shuttle Endeavour during her STS-97 mission in November 2000, with their successful deployments occurring on December 3 and 5, 2000, and February 14, 2001.

It was via the regular assignment of taking photography of the ISS that resulted in observations of the damage that was been caused by what appears to be a MMOD strike.

“Potential P4 Photovoltaic Radiator (PVR) Micrometeoroid/Orbital Debris (MMOD) strike: During a recent review of downlinked imagery, the Image Science and Analysis Group noticed an anomalous feature on the third panel of the P4 PVR,” noted L2 ISS Status this week.

2014-07-04 19_51_41-L2 Level ISS On-Orbit Status Report Notes (July 2014)“Initial observations based on this imagery (acquired by L2), and subsequent imagery obtained using external cameras, suggest that the damage may be from an MMOD strike. ”

So far, the ISS does not appear to be suffering any ill effects of the damage, while no leaking from the panels has been observed at this time.

“A preliminary analysis indicates that visible damage to the cover sheet may be up to 12 inches in length,” added the L2 status. “At this time no indications of a leak have been observed, but teams will continue to monitor for impacts.”

MMOD strikes on space hardware are not uncommon and continue to be a major risk factor all spacecraft have to deal with.

Although exposed windows are built to withstand such impacts, a strike to Window 2 on the Cupola – back in 2012 – was one example of how even tiny strikes are noticeable.

“Cupola Window MMOD Strike. The crew reported an MMOD strike on Cupola window 2 and downlinked photos (full set available on L2) of it,” noted L2 level ISS Status (LINK). “The shutter for window 2 is closed now, as required by flight rules, until the MMOD strike has been evaluated.”

While the flight rules are naturally cautious, in order to ensure a large amount of margin for crew safety, the window was soon cleared by the subsequent evaluations.

It is understood that this was the first MMOD strike to be suffered by a Cupola window. No strikes have been noted since.

Per the resulting contingency evaluations, it was noted that in the event of the damage being more serious, on-orbit replacement of an entire window is an inbuilt design feature.

Such a replacement would require an EVA to fit an external pressure cover to allow for the changeout, with a pressure cover requiring a flight up to the ISS – likely aboard a SpaceX Dragon spacecraft.

The window is made up of four panes – an inner scratch pane to protect the pressure pane from accidental damage, two pressure panes 25mm thick to maintain cabin pressure, and finally an outer debris pane.  The debris panes can be replaced individually, via EVAs.

Recent MMOD related articles:

Such MMOD strikes were observed on a large amount of Space Shuttle missions, especially late into the mission when the orbiter had undocked from the protection of the ISS and was preparing to head home.

Such strikes – however minor, were always taken seriously, given they are the third biggest threat to losing a vehicle during a mission – second only to launch and re-entry.

Extensive evaluations took place ahead of each shuttle mission, not least Atlantis’ STS-125 flight into space, a mission that provided one final servicing mission to the Hubble Space Telescope.

This mission held the largest risk to the orbiter, given it was the only post Return To Flight mission that did not have the “safe haven” option of docking with the ISS, ahead of a rescue orbiter being flown to pick up the crew.

STS-125 would also see an increase in the concentration of MMOD, due to the region of Low Earth Orbit (LEO) Atlantis flew in for the majority of her mission.

The Program Requirements Control Board (PRCB) results – which took into account satellite breakups and a variety of other components evaluated to be in Atlantis’ orbital neighborhood – showed that the overall risk of a Loss Of Crew and Vehicle (LOC/V) scenario due to MMOD impact(s) to the Thermal Protect System (TPS) was 1 in 185, with an error factor of 1.35 based on MMOD distribution, velocity, and density uncertainties.

As noted, most of the risk was associated with impacts on the TPS, as opposed to the windows. In the end, Atlantis completed her mission successfully and without any MMOD related concerns.

Examples of Mission Management Team (MMT) evaluations into window damage during a mission can be seen via STS-126′s flight, where then MMT Chair LeRoy Cain discussed the Mission Evaluation Room (MER) report into an impact on Endeavour’s Window 6 (Pilot Window).

“MMOD Window 6 Impact – crew reported 0.5” diameter to the PLT right window to the thermal pane, 1/32” depth,” noted the MER report. “Based on analysis the window should have no issues for peak heating M=8.0, but could form cracks but remain intact during peak loading M=0.7. Pilot visibility should not be hindered by this location if it cracks.

“No concern for orbit thermal cycles. Dynamic loading on the pane takes into account for the window frame thermal response.”

Clarifying the impact showed no signs of risking a LOV/C during re-entry, it was noted that only a hole through more than just the thermal plane of the window would raise concerns “we might have to not come home,” which would have resulted in calling Discovery to provide a LON (Launch On Need) rescue.

Endeavour landed safely with no propagation of the MMOD damage during her reentry.

Both Endeavour and Atlantis would also receive a flesh wound from MMOD strikes to their radiator panels, a danger in its own rights, given the potential of an impact to one of the Freon-22 coolant plumbing inside the radiator panel.

Such an impact could cause an entire coolant loop to be shut down and be declared as ‘failed’. This would have forced an immediate landing on the earliest US landing opportunity.

An example of the risk was seen during STS-109, when a small piece of debris was lodged in Columbia’s coolant loop 2 and restricted the flow of Freon-22 in that coolant loop. The amount of Freon-22 in the coolant loop was slightly below the flight rule red-limit, but after exhaustive analysis by the engineers on the ground, they decided to press on with the mission.

Usually spotted during post flight processing, Endeavour’s radiator damage showed as a “bullet hole” on her left hand aft radiator number 4. Engineers repaired the area during her Orbiter Processing Facility (OPF) flow.

Atlantis’ damage during STS-115 was the second largest debris strike in Shuttle history, causing a .108 inch diameter hole at entry point, with one-half of an inch thick damage to the honeycomb structure.

Interestingly, United Space Alliance (USA) engineers discovered a piece of circuit board embedded into the radiator panel during repairs, showing the MMOD strike originated from a piece of another vehicle, likely an Upper Stage from an expendable vehicle or a satellite.

(Images: L2 Content (all MMOD content), and NASA)

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