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#21
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Oberg: "The real significance of the ISS thruster test failure"
Dale wrote: I thought the Soyuz' shelf life was more an issue with its batteries, not its thrusters (?)... They're still somewhat closed mouthed about all this and the specifics of what decays over time. I'd be seriously concerned about what condition the propellant valves on the motors would be in after all this time and given the corrosive nature of the propellants, as well as all the hot/cold exposures they've had over the years as the station passes through Earth's shadow. If they had never been fired before it would be one thing, but these motors have been fired and shut down six years ago. I wouldn't try to start them, I'd be concerned something bad might happen; that module is basically a souped-up Mir core module, and Mir wasn't the most reliable thing ever made. Pat |
#22
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Oberg: "The real significance of the ISS thruster test failure"
Jeff Findley wrote: I thought the issue with Soyuz limited in orbit lifetime was due it's use of hydrogen peroxide for the descent module's attitude control system. I believe that the issue there is that hydrogen peroxide degrades over time due as it slowly turns back into oxygen and water. http://www.nasa.gov/mission_pages/st...ft_detail.html I'd heard it was degradation of the propellant feed system and the propellant tankage itself under the corrosive effects of the propellants. Batteries? H2O2? Propellants? Maybe the whole thing just pretty much goes to pot after a set amount of time. If you think about it, having the whole thing pretty much break down simultaneously would be a very economical way to design it; you wouldn't have any part be overbuilt for its intended mission, and that concept has Russian written all over it. Until we started using them as lifeboats we could not get any specifics of what a Soyuz's lifetime on-orbit even was, and just had to estimate it from how long they'd kept one up. That was even the case back in the Mir days, decades after it was introduced into service. What gets me is how the Russians have suddenly decided to rev up the engines to "see if they work"....there's got to be more to that story. Pat |
#23
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Oberg: "The real significance of the ISS thruster test failure"
Jeff Findley wrote: I'm not sure I understand this since the tanks on ISS can feed the thrusters on a docked Progress (certainly one docked at the aft port, not sure about other ports). It seems like this would be more useful for a collision avoidance maneuver when a Progress isn't docked, or for some reason the Progress engines fail to fire (from what I've read, you can still fire the ISS thrusters even when a Progress is docked to the aft port). We discussed this a couple of years back, when the loss of Columbia ended the regular Shuttle reboosts. The Zvezda module apparently has left-over fuel still onboard that could give a significant boost to the ISS' altitude. Pat |
#24
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Oberg: "The real significance of the ISS thruster test failure"
"Jeff Findley" wrote in
: "Pat Flannery" wrote in message ... Those are hypergolicly fueled engines using N2O4 and UDMH, and they don't trust the Soyuz, whose motors use the same propellants, after six months. I thought the issue with Soyuz limited in orbit lifetime was due it's use of hydrogen peroxide for the descent module's attitude control system. I believe that the issue there is that hydrogen peroxide degrades over time due as it slowly turns back into oxygen and water. http://www.nasa.gov/mission_pages/st...nts/soyuz/spac ecraft_detail.html That's what I thought as well, and it turns out to be true up to the Soyuz TM. I've recently learned that the TMA uses an improved grade of peroxide that increases the lifetime of the descent propulsion system from 180 to 270 days. However, Soyuz as a whole is limited to 200 days due to certification of other systems. I don't know which systems in particular are the limiting factors. -- JRF Reply-to address spam-proofed - to reply by E-mail, check "Organization" (I am not assimilated) and think one step ahead of IBM. |
#25
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Oberg: "The real significance of the ISS thruster test failure"
Pat Flannery wrote in
: Jeff Findley wrote: I'm not sure I understand this since the tanks on ISS can feed the thrusters on a docked Progress (certainly one docked at the aft port, not sure about other ports). It seems like this would be more useful for a collision avoidance maneuver when a Progress isn't docked, or for some reason the Progress engines fail to fire (from what I've read, you can still fire the ISS thrusters even when a Progress is docked to the aft port). We discussed this a couple of years back, when the loss of Columbia ended the regular Shuttle reboosts. The Zvezda module apparently has left-over fuel still onboard that could give a significant boost to the ISS' altitude. Right. And as others have stated, Zvezda can interconnect to a docked Progress on either port to reboost the station even if Zvezda's own engines are inop. (And even if there is no Progress docked, don't forget that only one Zvezda engine failed; they can still do single-engine burns if they must.) -- JRF Reply-to address spam-proofed - to reply by E-mail, check "Organization" (I am not assimilated) and think one step ahead of IBM. |
#26
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Oberg: "The real significance of the ISS thruster test failure"
Pat Flannery wrote in
: John Doe wrote: I don't think you can fault anyone for this at this point in time. We have to wait to see how they react in the event they cannot fix the thruster. As to why they are keen to test them: http://www.hq.nasa.gov/osf/station/images/issalt.gif Without the Shuttle reboosts they are slowly but surely getting lower despite the Progress reboosts. That plot is not as alarming as it might appear. ISS is now flying through solar minimum, so it can fly at a lower altitude while maintaining the same orbital lifetime. And as you pointed out yourself, Zvezda has plenty of propellant reserves that could be interconnected to Progress to boost ISS all the way back up to its maximum altitude in a contingency. Right now the Russians and NASA are taking advantage of solar minimum to optimize Progress and shuttle payload. -- JRF Reply-to address spam-proofed - to reply by E-mail, check "Organization" (I am not assimilated) and think one step ahead of IBM. |
#27
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Oberg: "The real significance of the ISS thruster test failure"
Jorge R. Frank wrote: That's what I thought as well, and it turns out to be true up to the Soyuz TM. I've recently learned that the TMA uses an improved grade of peroxide that increases the lifetime of the descent propulsion system from 180 to 270 days. It would be fun to know how they pulled that off- did they chill the H2O2, or use a stabilizing additive of some sort? Pat |
#28
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Oberg: "The real significance of the ISS thruster test failure"
Jorge R. Frank wrote: Right. And as others have stated, Zvezda can interconnect to a docked Progress on either port to reboost the station even if Zvezda's own engines are inop. (And even if there is no Progress docked, don't forget that only one Zvezda engine failed; they can still do single-engine burns if they must.) Looking at my Intermountain Railways model of the ISS, it looks like Zvezda has a total of three open docking ports; the one facing toward the rear of the station flanked by the two engines they are trying to use, and two mounted to the "docking sphere" that are at ninety degrees to the main axis of the station (up and down). How's the Progress supposed to get its center of thrust to pass through the ISS' center of mass from either of those two locations? Pat |
#29
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Oberg: "The real significance of the ISS thruster test failure"
Pat Flannery wrote in
: How's the Progress supposed to get its center of thrust to pass through the ISS' center of mass from either of those two locations? It doesn't; the resulting torque would be controlled by the RCS thrusters on Zvezda. Inefficient, but tolerable. The forward of the two nadir locations is very close to the ISS center of mass for the current config anyway. The loads from the Progress reboost engine are likewise tolerable even in this off-axis state; remember that this reboost engine is smaller than even a single shuttle primary RCS thruster. -- JRF Reply-to address spam-proofed - to reply by E-mail, check "Organization" (I am not assimilated) and think one step ahead of IBM. |
#30
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Oberg: "The real significance of the ISS thruster test failure"
Jorge R. Frank wrote: That plot is not as alarming as it might appear. ISS is now flying through solar minimum, so it can fly at a lower altitude while maintaining the same orbital lifetime. And as you pointed out yourself, Zvezda has plenty of propellant reserves that could be interconnected to Progress to boost ISS all the way back up to its maximum altitude in a contingency. Right now the Russians and NASA are taking advantage of solar minimum to optimize Progress and shuttle payload. Thought problem: Something doesn't go right on the next Shuttle flight and it gets grounded again. The Progress reboosts are helping, but how long can they keep kicking it up on there own once the Zvezda's fuel is expended? Do they have to wait around for the new ESA Jules Verne module to raise its orbit in lieu of the Shuttle if that occurs? There is one ironic aspect to all this- if the ISS really had been completed to the point it was supposed to be today on the original schedule, it would have been a lot more massive than it presently is, and those Progress reboosts would be having far less effect than they presently are. Having it lag so far behind schedule has kept it light enough for the Progress' to keep its orbit up despite the two groundings of the Shuttle. Pat |
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