The Marshall Space Flight Center (MSFC) designed duplex screen in the External Tank (ET) LH2 diffuser will no longer be used in any of the remaining ETs on future launches. <\/p>\n
The new diffuser screen causing 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 dual-screen version was the cause of unusual behaviour \u2013 to a point that incorrect readings or actions resulted. This nearly led to too much over-diffusing of helium entering the ET. It has not been revealed just how dangerous this was to the ET and\/or the Shuttle stack. <\/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 While problems with the ECO (Engine Cut Off) sensors ultimately led to the rollback of STS-114 \u2013 with ET-120 being replaced with what was to be Atlantis\u2019 ET (ET-121), set to fly the then-scheduled STS-121, MSFC requested a third tanking test to try and confirm ET-120\u2019s diffuser was at fault. <\/font><\/p>\n With ET-121 using the old style diffuser, NASA managers decided to proceed to launch without another tanking test, as other readings from the ET stayed within LCC parameters. <\/font><\/p>\n However, after one launch scrub \u2013 due to another problem with the ECO sensors \u2013 Discovery proceeded along the countdown on July 26 \u2013 with the ECO sensors remaining nominal. <\/font><\/p>\n What wasn\u2019t revealed, until later to this site \u2013 <\/font>http:\/\/www.nasaspaceflight.com\/content\/?id=3308<\/font><\/font><\/u> \u2013 was that ET-121 happened to be one cycle away from a launch scrub, cycling a total of 11 times, with the LCC set at 12 cycles.<\/font><\/p>\n Spaceflight history books (adsbygoogle = window.adsbygoogle || []).push({});<\/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 source. \u2018With the over-diffusion of helium, we weren\u2019t getting the pressure increase from each 0.5-sec burst \u2013 so, the incorrect screen type messed that up.\u2019 <\/font><\/p>\n What now appears to be the case was a conflict between the two screen types and the re-written requirement. Thus an incorrect procedure was to blame for the near over-cycle on ET-121, while the duplex screen was at fault on the over-cycling with ET-120.<\/p>\n \u2018The screen was actually correct in the spec,\u2019 the source noted. \u2018By the spec, the screen requirement was by pore count (i.e. number of \u2018holes\u2019 per inch), and you get an acceptable pore count with both simplex and duplex screens.<\/p>\n \u2018The requirement was re-written to clarify the screen type and wire count as well. <\/font><\/p>\n \u2018No chance that the incorrect screen will be used again.\u2019 <\/font><\/p>\n Ironically, the switch from ET-120 to ET-121 has been seen by some as more than unfortunate, given the resulting PAL ramp foam liberation from a previously re-worked area of the tank. However, it could have been worse, with ET-120 being used as a test specimen and showing hairline fractures along its own PAL ramp \u2013 believed to be the result the loading and unloading (twice) of super cold propellants during its two tanking tests. <\/font><\/p>\n All ETs will now fly without a PAL ramp, starting with STS-121, currently estimated to launch in May.<\/font><\/p>\n To follow up-to-the-minute news on Discovery\u2019s preparations for STS-121, click here: The Marshall Space Flight Center (MSFC) designed duplex screen in the External Tank (ET) LH2 diffuser will no longer be used in any of the remaining ETs on future launches. The new diffuser screen causing nearly double the normal amount of cycles through the LH2 pressurization relief valve (hydrogen vent valve), breaching Launch Commit Criteria […]<\/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-42188","post","type-post","status-publish","format-standard","hentry","category-news"],"acf":[],"_links":{"self":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/42188"}],"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=42188"}],"version-history":[{"count":0,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/42188\/revisions"}],"wp:attachment":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media?parent=42188"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/categories?post=42188"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/tags?post=42188"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
\nThe diffuser is used to inject helium into the tank, a process that aids the correct pressure and temperature of the ET prior to launch. <\/font> <\/p>\n
<\/em><\/font>http:\/\/forum.nasaspaceflight.com\/forums\/forum-view.asp?fid=2<\/em><\/font><\/p>\n","protected":false},"excerpt":{"rendered":"