The Mission Management Team (MMT) have initially cleared a Micrometeoroid Orbital Debris (MMOD) strike to Endeavour\u2019s window 6 (Pilot\u2019s window), with full re-entry rationale expected. Meanwhile, Shuttle manager John Shannon praised Kennedy Space Center (KSC) engineers for an \u201cunbelievable\u201d vehicle.<\/p>\n
Impacts from MMOD are a usual \u2013 if not undesirable \u2013 event during missions, with several recent flights suffering from impacts to the orbiter\u2019s windows on the flight deck.<\/p>\n
MMOD is the third biggest threat to losing a vehicle during a mission \u2013 second only to launch and re-entry. Mitigation of MMOD strikes \u2013 such as when docked to the International Space Station (ISS) \u2013 and the tracking of space debris, provides some protection for the orbiters, though small strikes are commonplace.<\/p>\n
Several strikes on the orbiter\u2019s windows have been observed throughout the shuttle program \u2013 the last on STS-123 \u2013 and at least two MMOD strikes have also been suffered on the radiator panels since Return To Flight.<\/p>\n
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One such strike was later found to be a small slice of circuit board from an expended stage of another launch vehicle, which managed to punch a reasonably large hole through one of Atlantis\u2019 radiators, thankfully missing a Freon-21 coolant plumbing in the panel, which would have caused an emergency to be called, and the immediate order to land.<\/p>\n
It was previously believed that a similar event occurred on STS-109 in Columbia\u2019s Freon Coolant Loop (FCL1), which restricted the flow of Freon-21 in that loop. However, in a correction from the first publication of this article, the FCL1 orifice was blocked via an internal piece of debris, rather than MMOD.<\/p>\n
More recently, a MMOD strike was found on Endeavour during the STS-118 mission, again missing a Freon-21 coolant loop.<\/p>\n
Impacts on the windows are more commonplace \u2013 or at least are observed shortly after they impact due to their visibility \u2013 with the latest strike spotted by the STS-126 crew, who sent photographs down to the ground for evaluation.<\/p>\n
NASA mission patches (adsbygoogle = window.adsbygoogle || []).push({});<\/p>\n In another sign of how NASA extensivily works a problem, the Mission Management Team (MMT) immediately discussed the strike at their morning meeting.<\/p>\n \u201cMMOD Window 6 Impact \u2013 crew reported 0.5\u201d diameter to the PLT right window to the thermal pane, 1\/32\u201d depth,\u201d noted the Mission Evaluation Room (MER) \u201cFunny\u201d report.<\/p>\n \u201cBased 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.\u201d<\/p>\n \u201cNo concern for orbit thermal cycles. Dynamic loading on the pane takes into account for the window frame thermal response. Adding a day for the mission does not effect this window,\u201d noted one manager at the MMT, according to MMT minutes on L2.<\/p>\n \u201cIf you had a problem that you were concerned about what could you do? Different entry designs have different heat loads,\u201d asked MMT chair Leroy Cain.<\/p>\n \u201cThe thermal pane can handle defects like this in peak heating, but the concern would be pressure down low,\u201d responded Steve Stich from the Orbiter Project, \u201cbut not much you could do different for the pressure. Can\u2019t say if it will crack or not.\u201d<\/p>\n Clarifying the impact is not risking safety, Mr Cain asked for confirmation that the hole would not be risking a LOV\/C (Loss Of Vehicle\/Crew) during re-entry, which one manager responded that it would have to be a hole through more than just the thermal plane of the window where \u201cwe might have to not come home,\u201d which referenced calling LON (Launch On Need) rescue with Discovery.<\/p>\n In response to Mr Cain asking about flight history with similar and greater strikes to the windows, the response was noted that similar size has not been an issue, though there was uncertainty in crew measurement, but that on previous missions they have not seen flaw growth for this size of frame.<\/p>\n \u201cWe run the conservative analysis that comes out with negative margin, then run mission specific with a pressure profile that shows no effect, low density cracking, high density cracking or window blows out at ~M=1.0 which still wouldn\u2019t be an issue,\u201d added JSC Engineer\u2019s Steve Altemus. \u201cNot a concern for orbit or entry for a defect of this size.\u201d<\/p>\n Other orbiter engineering groups and the Crew Office added comments ranging from \u201cthere was some sensitivity in the analysis when we talked this at STS-123 case and maybe run this case and see what we have,\u201d to \u201ctrying to give heads up and no issue with this defect from orbiter perspective for emergency D\/O (Deorbit),\u201d which is typical for an engineering discussion.<\/p>\n The initial removal of the impact from a re-entry standpoint will be finalized later this week, with a meeting of the Orbiter Project Office (OPO) on Wednesday, before their findings are sent to the MMT.<\/p>\n \u201cWill talk it again at OPO on Wednesday and could bring back more details to MMT, but don\u2019t think story will change,\u201d added an Orbiter manager. \u201cWill not be catastrophic and any failure would occur late.\u201d<\/p>\n Backing up the opening thought process on the window damage has been an associated presentation \u2013 acquired by L2 \u2013 that also shows the damage should be ok to re-enter with, even if the window does crack during re-entry.<\/p>\n \u201cSTS-126 crew reported a micrometeoroid orbit debris (MMOD) impact on thermal window 6,\u201d opened the presentation. \u201cEstimated diameter of defect is 0.5\u201d (Measurements on orbit are not accurate through three pane system).<\/p>\n \u201cEvaluation of STS-126 Window 6 Damage: Damage diameter is estimated @ 0.5 inch. Damage depth is unknown, but it is estimated based on the crew estimated diameter. Research collected over the life of the program and from ISS, resulted in the development of a \u201crule of thumb\u201d for estimating MMOD depth<\/p>\n \u201c10:1 is on the conservative end of the scale and was chosen to estimate the depth of MMOD based on a reported diameter. Estimated depth is used in a fracture mechanics assessment for margin generation. It is not absolute, but provides an appropriately conservative estimate of glass strength when combined with typical fracture mechanics methods applied to glass.<\/p>\n \u201cApplying the 10:1 dia\/depth ratio, the estimated damage depth is 0.050\u201d. Using the descent certification load case (time consistent loads), side window #6 descent capability is 0.0029\u201d (Mach 0.8, \u2206p = 3.18, negative margin with FS = 1.4).\u201d<\/p>\n \u201cWindow Failure Mode: A ding in the glass will fail by propagating cracks outward from the ding. Low pressure\/stress loads that cause propagation result in a less dense cracking system,\u201d added the presentation. \u201cHigh pressure\/stress loads result in dense cracking systems.<\/p>\n \u201cFrom experience (proof testing of a window in a frame), the failed pieces will not be released from the window frame, even with a dense cracking system (high stress\/pressure) failure. Proof test failure occurred at approximately 7-8 psi (delta pressure).<\/p>\n \u201cGlass stayed intact under this load (restrained by the frame). Glass resisted engineers\u2019 attempts to \u201cpush it out\u201d of the frame. Highest burst load expected during descent is 3.18 psi. Load is not sufficient to push the glass pieces out of the frame.\u201d<\/p>\n Other bases were covered, such as via \u201ctrajectory consistent load cases\u201d, which also backed up the forward plan to give acceptance rationale to re-enter without any concern over the window failing.<\/p>\n \u201cAcceptance Rationale: The highest potential for window failure will occur at peak descent loads Mach 0.70 (past peak heating; in excess of Mach 8.0). Delta pressure across the pane is 2.04 psid,\u201d added the presentation. \u201cAt this low pressure, flaw propagation may manifest itself in the form of cracks radiating out from the defect site.<\/p>\n \u201cBased on previous testing, if a crack should occur, it is highly likely the windowpane will remain intact and retained, and peripheral view across the window will not be detrimentally affected.<\/p>\n \u201cAccepted Risk, MMOD criteria: no hole through the pane. Recommendation: Nominal Entry.\u201d<\/p>\n While all issues gain such wide-ranging discussion, this particular MMOD strike has proven to be one of only a few issues with Endeavour as she heads towards undocking on Friday.<\/p>\n With no TPS (Thermal Protection System) issues, and only a couple of minor engineering notes being listed by MER, Shuttle manager John Shannon noted his pride with the engineering team that processed Endeavour at the Kennedy Space Center (KSC), a team tasked with five launches in 2009.<\/p>\n \u201cThe Cape has put together an unbelievable vehicle. We are able to concentrate on the mission and supporting the ISS team. Entire organization is really pulling together, and you can see over the next year we will have a string of really good flights,\u201d noted Mr Shannon on the latest Shuttle Standup\/Integration report.<\/p>\n \u201cWe just have to keep it going, keep vigilant like we always have been. Keep doing that good work.\u201d<\/p>\n L2 members<\/em><\/em>: All documentation \u2013 from which the above article has quoted snippets \u2013 is available in full in the related L2 sections, now over 4000 gbs in size.<\/em><\/em><\/p>\n","protected":false},"excerpt":{"rendered":" The Mission Management Team (MMT) have initially cleared a Micrometeoroid Orbital Debris (MMOD) strike to Endeavour\u2019s window 6 (Pilot\u2019s window), with full re-entry rationale expected. Meanwhile, Shuttle manager John Shannon praised Kennedy Space Center (KSC) engineers for an \u201cunbelievable\u201d vehicle. Impacts from MMOD are a usual \u2013 if not undesirable \u2013 event during missions, with […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":"","_links_to":"","_links_to_target":""},"categories":[2],"tags":[8970,9197,9219],"class_list":["post-41252","post","type-post","status-publish","format-standard","hentry","category-news","tag-mmod","tag-mmt","tag-sts-126"],"acf":[],"_links":{"self":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/41252"}],"collection":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/comments?post=41252"}],"version-history":[{"count":0,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/41252\/revisions"}],"wp:attachment":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media?parent=41252"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/categories?post=41252"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/tags?post=41252"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}See Also<\/h4>\n
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In the end Mr Cain concluded that \u201cI am okay with the heads up and not all the analysis\/charts are presented. Wanted to make sure no one had concern for this window impact. We will wait for final answer.\u201d<\/p>\n![\"\"](\"https:\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2008\/11\/a311.jpg\")
As with all evaluation presentations, the worst case scenario is shown. For the case of an orbiter window, even far worse damage can be sustained by the three panel window system, and even then the window would not blow out.<\/p>\n