A forensic level investigation into the condition of the Thermal Protection System (TPS) on the Exploration Flight Test -1 (EFT-1) Orion has evaluated the level of MicroMeteoroid and Orbital Debris (MMOD) damage on the vehicle. Orion’s MMOD impacts were evaluated by the Hypervelocity Impact Technology (HVIT) Group, showing the vehicle suffered more MMOD damage than the computation models had predicted.
MMOD is the unseen threat that all spacecraft need to be protected from, not least because they are impossible to track and thus avoid.
This is in contrast to the larger debris threats that the ISS can dodge via a Debris Avoidance Maneuver (DAM).
Such 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.
Although spacecraft are designed with a level of protection from such impacts, MMOD was the third biggest threat to losing an orbiter during her mission – second only to launch and re-entry.
The most serious event was 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. While the option for an immediate return to Earth was discussed, mission managers opted to press on with what was a successful flight.
Atlantis and Endeavour both suffered “bullet hole” impacts to their radiators, with Atlantis’ damage was sustained when she was hit by a tiny piece of circuit board on orbit – likely from a destroyed satellite. The damage held no mission impact and was only noticed once she had returned home and was in post-flight processing inside her Orbiter Processing Facility (OPF)
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 major MMOD concerns.
EFT-1 Orion’s maiden flight raised the bar on the altitude, rising high above the ISS’ orbit and beyond that of Hubble. The first orbit had an apogee altitude of 890 km and a perigee of 200 km. The second orbit reached an apogee of 5,808 km.
Last year’s launch atop United Launch Alliance’s Delta IV-Heavy marked the beginning of a flawless mission that tested numerous critical systems, allowing for a huge collection of data that will fed into the Critical Design Review (CDR) for Orion.
The data is already being fed into the next Orion – for Exploration Mission -1 (EM-1) – which is already undergoing construction at the Michoud Assembly Facility (MAF).
After being successfully recovered in the Pacific Ocean off of San Diego. NASA civil servants and contractors conducted an extensive evaluation of the vehicle’s condition at the Naval Base San Diego Mole Pier.
Visually, Orion appeared to be in great shape, with no obvious damage to the Thermal Protection System (TPS).
These inspections were carried out by the Johnson Space Center’s HVIT Group as well as Lockheed Martin personnel from Denver.
“The Orion capsule was recovered in good shape, but the forward bay cover (jettisoned during the main parachute deployment) sank before it could be recovered,” noted the HVIT outline.
Further inspections were conducted once Orion had arrived at the Launch Abort System Facility at the Kennedy Space Center.
“Capsule areas that were examined during the inspection campaign include the back shell thermal protection system tiles, back shell thermal barriers, reaction control system thruster nozzles, base heat shield acreage, docking hatch thermal protection system blankets, docking hatch window, and crew module windows,” added the notes.
The inspections included an initial screening for defects and anomalies, examination for distinctive hypervelocity impact (HVI) features, location and a detailed investigation to discern details of impact source.
“The first two activities are performed in the field while the documentation task can include both field and lab work. We have completed the field work and are now involved in the lab work. Specifically, we are now conducting non-destructive evaluation (NDE) of the suspected MMOD impact features using X-ray computed tomography (CT) and optical microscopy.
“After the NDE work, a scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM/EDX) will be used to characterize the elemental composition of HVI impactor residue.”
Not unlike the latter days of the Space Shuttle Program (SSP), the inspections provided an initial list of ROIs (Regions Of Interest) on the TPS – although for Shuttle those results were gained via the on-0rbit inspections and evaluated on the ground during the mission by the DAT (Damage Assessment Team).
For EFT-1, the initial surveys and characterizations produced 25 ROIs on backshell thermal protection system tiles. An internal imagery review and additional inspections reduced the ROI list down to six features that were designated as possible MMOD.
“Five of the six tiles containing the potential MMOD damage were removed and sent to the HVIT group at JSC for additional analysis with higher magnification microscopes and 2D/3D surface profilers,” added the HVIT.
“Four crew module windows and the hatch window were also examined for MMOD damage features. A total of 42 small window features were found; 27 on the crew module windows and 15 on the hatch window. It is very possible these small features were due to a non-MMOD source, but the investigation into the cause of these damages is ongoing.”
These initial evaluations have since been compared to the pre-flight predictions, showing the value of the test flight over computational models.
“An as-flown analysis was performed by Lockheed Martin against the final trajectory using the Bumper 3 code and the ORDEM 3 orbital debris environment. Preliminary results indicate that more damage was observed than predicted by the as-flown assessment.”
The team’s work is still ongoing, with a similar assessment performed for window damage based on internal fracture diameter and ORDEM 3 predicted 0.2 features of 0.3 mm and greater diameter. That assessment work is still underway.
Meanwhile, the EFT-1 heat shield is currently undergoing Avcoat extraction at the Marshall Space Flight Center (MSFC). NASA thermal protection system and structures personnel are on site to witness the machining and inspect the material.
(Images: Via L2 SLS and Orion sections – including exclusive – on board, pre-media event – photos (several hundred MBs) and several hundred GBs of Orion documentation since VSE onwards).
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