Astrobotic PM-1 mission, CLPS mission with NASA and commercial payloads |
Astrobotic PM-1 mission, CLPS mission with NASA and commercial payloads |
Jan 14 2024, 05:58 PM
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#46
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Senior Member Group: Members Posts: 2547 Joined: 13-September 05 Member No.: 497 |
Without a schematic of the propulsion system, it's really difficult to intuit what may be happening. Having done a little digging, apparently even the ACS thrusters are biprop (MMH doesn't work as a monopropellant, the spacecraft I'm familiar with that used dual-mode monoprop/biprop used pure hydrazine.)
Normally the large pressure gradient between the helium tank and the rest of the system would prevent any migration back into the helium tank, but if the pressure is equalized then all bets are off. Systems that aren't so mass-constrained have one-way check valves to keep migration from happening anyway (see Figure 1 in https://www.researchgate.net/publication/23...ion_Performance ) but it seems that Peregrine cut out anything that wasn't absolutely required. -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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Jan 14 2024, 06:09 PM
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#47
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Member Group: Members Posts: 613 Joined: 23-February 07 From: Occasionally in Columbia, MD Member No.: 1764 |
This seems odd to me, since usually such a valve would open just once and then a downstream regulator would maintain system pressure at the desired safe level. But it sounds like they tried to avoid needing a regulator by just burping the valve open briefly (in hindsight maybe not such a good idea.) Cassini operated this way. I mean, it wasnt designed to operate this way (see my Haynes Cassini-Huygens Owners Workshop Manual) - it had a regulator, but somehow the regulator stuck open with a high leak rate - not enough to pop the propellant tank right away, thankfully, so the operators were able to close the upstream latch valve (which turned out to have a rather lower leak rate than specc'd, fortunately) and burped it later to maintain the ullage pressure as the fuel depleted. Sounds like Peregrine jumped to that approach - higher risk, but simpler/lighter Biprops are hard to get right - a lot of failures/anomalies attributable to this piece of the system - Mars Observer, Akatsuki, Cassini, JUNO..... |
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Jan 15 2024, 12:30 PM
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#48
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Member Group: Members Posts: 127 Joined: 3-September 12 From: Almeria, SE Spain Member No.: 6632 |
Latest update, very detailed and interesting:
https://www.astrobotic.com/update-17-for-pe...ne-mission-one/ The probe will burn up deliberately in Earth's atmosphere on Thursday evening (UTC) over the Great Barrier Reef. https://twitter.com/tony873004/status/1746288506509684988 Thorsten |
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Jan 19 2024, 08:57 AM
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#49
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Member Group: Members Posts: 127 Joined: 3-September 12 From: Almeria, SE Spain Member No.: 6632 |
"Peregrine appears to have reentered [Jan 18] around 2059 UTC as predicted"
https://twitter.com/planet4589/status/1748110238664802317 Thorsten |
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Jan 20 2024, 08:07 PM
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#50
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Senior Member Group: Members Posts: 2113 Joined: 13-February 10 From: Ontario Member No.: 5221 |
Some poignant video footage has been released, separation of the lander from the upper stage, and the Iris rover spinning its wheels in the vacuum. The CMU students can take heart in their rover working to the moon and back, even if they couldn't land.
https://www.astrobotic.com/final-update-for...ne-mission-one/ |
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Jan 29 2024, 04:58 AM
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#51
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Newbie Group: Members Posts: 1 Joined: 17-January 24 Member No.: 9314 |
Cassini operated this way. I mean, it wasnt designed to operate this way (see my Haynes Cassini-Huygens Owners Workshop Manual) - it had a regulator, but somehow the regulator stuck open with a high leak rate - not enough to pop the propellant tank right away, thankfully, so the operators were able to close the upstream latch valve (which turned out to have a rather lower leak rate than specc'd, fortunately) and burped it later to maintain the ullage pressure as the fuel depleted. Sounds like Peregrine jumped to that approach - higher risk, but simpler/lighter Biprops are hard to get right - a lot of failures/anomalies attributable to this piece of the system - Mars Observer, Akatsuki, Cassini, JUNO..... Isn’t it usual to use a pyrotechnic valve to isolate the He tank? Solenoid valves present the risk of valve bounce during periods of very high vibration during the launch; the valve stem lifts off the seat. The pyrovalve provides a hermetic seal until the charge is ignited, so it ensures zero leakage during the launch phase and is only fired when that is complete. I know that the thrusters on the mission have no previous flight heritage; I’m wondering whether other fluid-flow components in the propulsion system also lack heritage. Ed |
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Jan 29 2024, 06:54 AM
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#52
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Senior Member Group: Members Posts: 2547 Joined: 13-September 05 Member No.: 497 |
Isn’t it usual to use a pyrotechnic valve to isolate the He tank? Solenoid valves present the risk of valve bounce during periods of very high vibration during the launch; the valve stem lifts off the seat. The pyrovalve provides a hermetic seal until the charge is ignited, so it ensures zero leakage during the launch phase and is only fired when that is complete. Of course. The Peregrine leak only started after the Triton Space pyrovalves were fired. They apparently worked fine and the issue was downstream. QUOTE I know that the thrusters on the mission have no previous flight heritage; I’m wondering whether other fluid-flow components in the propulsion system also lack heritage. Possibly not, but heritage only goes so far. The components on Cassini and Juno had tons of heritage, but that didn't keep them from misbehaving. -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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