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<\/em><\/font>L2, for expanded documents and insider information used in our articles. To join L2, click the advert. <\/p>\nThe following is collated from four NASA documents \u2013 published in full on L2.<\/em><\/font><\/font><\/p>\n
While the ECO issue has been well documented, with a solution likely found by the replacement of the sensors in Discovery\u2019s tank (ET-119), the other issue \u2013 which nearly caused a late scrub during STS-114\u2019s countdown \u2013 is less publicised. Thursday\u2019s PRCB meeting discussed the troubleshooting plan (with an article to follow on their findings).<\/p>\n
The Marshall Space Flight Center (MSFC) designed duplex screen in the LH2 diffuser was blamed as the cause for nearly double the normal amount of cycles through the LH2 pressurization relief valve (hydrogen vent valve), breaching Launch Commit Criteria (LCC) \u2013 on both tanking tests, involving ET-120. The new design diffuser will never be used again. <\/font><\/p>\n
\u2018The diffusers will only use the single-shute-wire weave screen \u2013 the duplex screen will never be used again,\u2019 said a Michoud Assembly Facility (MAF) source. \u2018The problem with (the duplex screen) is that it almost over-diffused the GHe (Helium) entering the tank.\u2019 <\/font><\/p>\n The diffuser, which is used to inject helium into the tank, a process that aids the correct pressure and temperature of the ET prior to launch, needs to be working within parameters for a number of reasons. Notably with STS-121, the effects on the tank\u2019s foam will be crucial, especially if a scrub forces another \u2018cryo cycle\u2019 \u2013 which would not aid the prospects of reducing foam liberation on ascent due to the stresses involved in filling the tank full of supercold propellant.<\/p>\n MSFC, unhappy that their new diffuser was being blamed, asked for another tanking test with ET-121. That was rejected, despite an MSFC protest that there was something seriously wrong with the count \u2013 which was noted again after the launch of Discovery last July, with a total of 11 cycles observed, while the LCC is set at 12 cycles. Breaching LCC equates to a scrub being called. <\/font><\/p>\n Another problem NASA is concerned about is a leak from the tank \u2013 relating to the prepress. NASA deem small leaks in this scenario as \u2018acceptable risk\u2019 for the shuttle and crew, given the lack of an \u2018ignition source.\u2019 This is because the leak would be at the opposite end of the shuttle\u2019s engines (the major ignition source) \u2013 with only static electricity and lightning a potential danger.<\/p>\n <\/font><\/font><\/em>However, how to treat an observed leak is a topic that has varying opinions within NASA ranks. <\/font><\/p>\n \u2018STS-114\u2019s higher than expected LH2 prepress cycle count exposed sensitivity to ground prepress system performance (LCC ET-04),\u2019 noted a NASA document presented to last week\u2019s PRCB. \u2018Leak detection and preventing an inadvertent scrub are in conflict \u00e2\u20ac\u201c protection needs to be balanced.\u2019<\/font><\/p>\n Space tourism guides (adsbygoogle = window.adsbygoogle || []).push({});<\/p>\n For STS-121, 11 cycles won\u2019t cause a scrub, with the note that: \u2018for STS-121,10.5 cycles are expected (nominal \u2013 no variation), but variance stack-up could result in 13.3 cycles (LCC violation occurs at 14 cycles),\u2019 which shows NASA has moved the allowable amount of cycles before a scrub to be 14, as opposed to 11. <\/font><\/p>\n The calculation NASA has to balance is how many cycles are needed, how much pressure is going into the tank and if there\u2019s a leak affecting both readings. <\/font><\/p>\n \u2018This is why the cycle count is so crucial, because originally the LCC was developed to protect the tank against leakage,\u2019 added the MAF source. \u2018With the over-diffusion of helium (on ET-120 and ET-121), we weren\u2019t getting the pressure increase from each 0.5-sec burst, so extra cycles were feed into the tank.\u2019 <\/font><\/p>\n Those \u2018bursts\u2019 or \u2018pulses\u2019 are also the subject of a difference of opinion, based on ensuring there is enough \u2013 and not too much \u2013 pressure going into the tank. <\/font><\/p>\n \u2018SICB\/PSIG recommendation \u2013 accept current valve timing (0.476 seconds) with decreased LCC margin. Dissenting opinion \u2013 return valve timing to MLP-1&3 historical performance (0.484 to 0.539 seconds).\u2019<\/font><\/p>\n As far as what is deemed as a leak, the range between safe and unsafe is classed: \u2018GH2 leak of 0.005 lbm\/sec is flammable, but will not cause structural damage in flight. GH2 leak of 0.065 lbm\/sec is flammable and can cause structural damage in flight. The transition point from safe to unsafe is unknown.\u2019<\/p>\n \u2018Predicted pulse count for STS-121 is 10.5. If LCC limit is approached, but not broken, T-0 will take place with a leak (or performance dispersion equal to a leak) of roughly 888,000 SCIM (or 3,500 times specification).<\/font><\/p>\n \u2018One pulse is equal to roughly 250,000 SCIM (or 1,000 times specification). Unable to determine if a leak detected on the ground will remain constant in flight.\u2019<\/p>\n While most opinions on the risks associated with a leak remain within \u2018acceptable risk (catastrophic\/remote)\u2019 \u2013 NASA Propulsion Systems Engineering & Integration Office disagree. \u2018Presidential Commission Report identified leakage concern, they noted in their presentation, \u2018GH2 leakage greater than 0.005 lb\/s has potential for ignition. Hazard \u2013 Catastrophic failure due to structural overheating as a result of undetected GH2 leakage in the presence of an ignition source.\u2019 <\/font><\/p>\n <\/p>\n PSE&I recommend that \u2018Valve timing (should) be retested ~7 days prior to launch.\u2019 It is not clear if this was rejected, due to the \u2018dissenting\u2019 opinion of USA, ET Project and SCIM.<\/p>\n The real test will come in the actual countdown for the July 1 launch of Discovery, with a possible scrub due to any problem coming before T-43 seconds, from which point any error is \u2013 according to the documents \u2013 undetectable via the current procedures in at the Kennedy Space Center\u2019s Launch Control Center.<\/p>\n **NASASpaceflight.com Job Opportunities**<\/font> <\/font><\/em><\/p>\n <\/u><\/p>\n <\/font><\/p>\n","protected":false},"excerpt":{"rendered":" With most of the media attention focusing on how the External Tank performs during ascent on STS-121, NASA is putting into place plans to counter two major problems that occurred ahead of last year\u2019s STS-114. A series of documents show NASA is concerned about the LH2 vent value registering too many cycles during pre-press \u2013 […]<\/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":[],"class_list":["post-42033","post","type-post","status-publish","format-standard","hentry","category-news"],"acf":[],"_links":{"self":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/42033"}],"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=42033"}],"version-history":[{"count":0,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/42033\/revisions"}],"wp:attachment":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media?parent=42033"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/categories?post=42033"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/tags?post=42033"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"http:\/\/logo.cafepress.com\/3\/4811501.1268853.jpg\"){kind=logo}
<\/font>However, following the rollback and replacement of ET-120 with ET-121 \u2013 due to the ECO troubles with the former, again the cycle count was high, so much so sources insist they were one more cycle away from scrubbing the launch (now confirmed in documentation) \u2013 which eventually proceeded as planned. ET-121 used the old style diffuser. <\/font><\/p>\n<\/font>STS-121 Mission T-Shirt. Use discount code \u2018nasaspaceflight\u2019 when ordering from Countdown Creations & receive 5% off!<\/p>\n
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