SpaceX Falcon 9/Dragon CRS-2 SpX-2 MISSION GENERAL DISCUSSION

[Robotbeat]

redundancy probably did contribute to this non-LOM glitch.

Unknown without root cause and system schematics. Sure would be nice to see up-to-date diagram how the RCS is plumbed. Is there anything known for sure about Dragon RCS system, on top of what you can deduce from published photos? Wiki speaks of double redundancy, but reading more carefully it seems to refer to individual thrusters, might mean only thruster specific valves, not the pressurization system. *waving arms wildly* 

Simply having more Valves should mean a greater risk of an event like we had. That is what I'm basing it off of.

[Space Pete]

BTW, Pete - those are 657x488, not 1080p. Still look good, though. - thanks for posting.

Yep, I should clarify that - while the video I took those screengrabs from was 1080p fullscreen, those specific shots only occupied a portion of that screen - so although they come from a 1080p video, they won't be 1080p shots.

[ugordan]

640x480 was probably the native resolution of those shots, anyway. Onboard SD quality feed so likely NTSC resolution at best.

[Lurker Steve]

... although it is obviously inconvenient if a particular day is nominated for rendezvous and the plans for activities on board the station need to be rescheduled at the last minute.

But that's life. And not unusual or especially challenging for the ISS team, I'd imagine.

I assume it's just 1 less day to unload the supplies and pack the Dragon with down mass. I assume Dragon must leave on it's scheduled date so the next Visiting Vehicle can use that port. I assume there isn't really enough cargo on board, where a schedule compressed by a day or two becomes an issue.

[Norm38]

Simply having more Valves should mean a greater risk of an event like we had. That is what I'm basing it off of.

There's component level reliability and system level reliability.  The DFMEA sorts out the tradeoffs.  The goal is the most reliable overall system.

Say a valve has a 1% chance of failure on a given flight.  If there is only a single valve, then there is a 1% chance of LOM per flight.
If there are two valves, then yes the chance of valve component failure has doubled.  But the risk of system failure, requiring both valves fail, is 1% times 1%, for a 0.01% chance of LOM per flight.

Of course that is for random failures, not systemic issues.  But even there, a higher number of independent subsystems will increase reliability by giving a greater chance that at least one subsystem will still work.  As in this case, where 1 of 4 thrusters did come online.  If the propulsion system had been simplified, yes fewer components would have failed, but that may have left Dragon without any working thrusters at all.  A higher risk of component failure is acceptable if the system failure risk is lower.

[Robotbeat]

Simply having more Valves should mean a greater risk of an event like we had. That is what I'm basing it off of.

There's component level reliability and system level reliability.  The DFMEA sorts out the tradeoffs.  The goal is the most reliable overall system.

Say a valve has a 1% chance of failure on a given flight.  If there is only a single valve, then there is a 1% chance of LOM per flight.
If there are two valves, then yes the chance of valve component failure has doubled.  But the risk of system failure, requiring both valves fail, is 1% times 1%, for a 0.01% chance of LOM per flight.

Of course that is for random failures, not systemic issues.  But even there, a higher number of independent subsystems will increase reliability by giving a greater chance that at least one subsystem will still work.  As in this case, where 1 of 4 thrusters did come online.  If the propulsion system had been simplified, yes fewer components would have failed, but that may have left Dragon without any working thrusters at all.  A higher risk of component failure is acceptable if the system failure risk is lower.

That is more or less what I was trying to say. Especially that we should judge overall mission success as the primary criteria.

AnOther advantage of these more redundant system is that you get into volume production for the components sooner so you can find (without LOM) and fix latent design problems within the first few flights that normally would only show up in after a couple dozen flights. Ie because you are producing and operating so many units, your per unit reliability would go up (after finding design flaws), making your aggregate system-level reliability yet higher. This is something I deal ith in my part-time gig with designing/integrating data storage systems... If you have just a single hard-drive and have a failure, it's hard to know if it's just bad luck or if it's a bad batch of drives. If you have a bad batch and are integrating them into a RAID 6, first of all you can survive a much higher failure rate (without data loss) and you can tell very quickly (especially if you look at pre-failure data) if you have a bad batch of drives.

And like you mentioned with independent systems, you can reduce system failures by using unlike redundancy. For instance, the Shuttle had a bunch of redundant flight computers of the same type but also another of a different type running a different kind of flight software in parallel, so even if you have a major software flaw that takes out every other computer, you still can switch to the failsafe computer and not worry about it having the same problem. This does increase development cost by quite a bit, but it can save your bacon even in almost the worst case scenario.

Like you said, we don't know why one pod (allowing partial attitude control... and if it had 5 thrusters, perhaps even fully but crappy attitude control allowing also deorbiting) still worked fine... If it was because of unlike redundancy in that pod and its propulsion subsystem, then good on SpaceX. I actually don't think that is why, though. Does anyone know if they used a different system for the pod that avoided the problem?

[Joffan]

... although it is obviously inconvenient if a particular day is nominated for rendezvous and the plans for activities on board the station need to be rescheduled at the last minute.

But that's life. And not unusual or especially challenging for the ISS team, I'd imagine.

I assume it's just 1 less day to unload the supplies and pack the Dragon with down mass. I assume Dragon must leave on it's scheduled date so the next Visiting Vehicle can use that port. I assume there isn't really enough cargo on board, where a schedule compressed by a day or two becomes an issue.

CRS-2 Dragon definitely has on-board an experiment that runs for close to the full length of its visit and goes back down on the return trip. So potentially a day late rendezvous will mean a day late departure also.

What I was actually thinking of was today's activities for the crew. Those expecting to work on capture and berthing will have to reschedule other tasks into the day. But I don't suppose they're short of other tasks to switch to.

[Robotbeat]

Even still, The early rendezvous was a bonus. And the experiment may presumably still be powered... The launch occurred on time, so the time in microgravity will be the same.

[LegendCJS]

Well, with the little up as they are carrying this time this shouldn't be too hard, shouldn't it?

This statement both belongs in the general discussion thread, and conflicts with the pre-launch press conference saying that the "up" cargo load they are carrying this mission is about the maximum for the F9 v1.0.

[Robotbeat]

Well, with the little up as they are carrying this time this shouldn't be too hard, shouldn't it?

This statement both belongs in the general discussion thread, and conflicts with the pre-launch press conference saying that the "up" cargo load they are carrying this mission is about the maximum for the F9 v1.0.

Serious citation needed...

[Robotbeat]

Well, with the little up as they are carrying this time this shouldn't be too hard, shouldn't it?

This statement both belongs in the general discussion thread, and conflicts with the pre-launch press conference saying that the "up" cargo load they are carrying this mission is about the maximum for the F9 v1.0.

Serious citation needed...

Oh, c'mon... he said "pre-launch press conference" - how difficult is it for you to find this yourself?

http://forum.nasaspaceflight.com/index.php?topic=30700.msg1018922#msg1018922

Appreciated. Sorry, I was lazy.

[manboy]

... although it is obviously inconvenient if a particular day is nominated for rendezvous and the plans for activities on board the station need to be rescheduled at the last minute.

But that's life. And not unusual or especially challenging for the ISS team, I'd imagine.

I assume it's just 1 less day to unload the supplies and pack the Dragon with down mass. I assume Dragon must leave on it's scheduled date so the next Visiting Vehicle can use that port. I assume there isn't really enough cargo on board, where a schedule compressed by a day or two becomes an issue.

CRS-2 Dragon definitely has on-board an experiment that runs for close to the full length of its visit and goes back down on the return trip. So potentially a day late rendezvous will mean a day late departure also.

Which experiment?

[QuantumG]

Ya know, I don't know if we're maxed out up or not. We're probably close, on this particular flight, to being max mass.

Well, that was useful and certainly the definitive statement you made it out to be.

[Robotbeat]

...it's pretty irrelevant, except for winning Internet arguments. What matters for customers is v1.1 performance, which is much more of a question mark.

[vt_hokie]

This stuff is normal. As another poster said, if anything, abnormally FEW problems.... Chill! Enjoy the ride and stop worrying so much. (I admit, I was worried right after launch up till about 3 PM eastern but they fixed it)

Agreed.  The only thing I would add however is that to me it's a reminder that we still need to work on getting this low Earth orbit stuff down before we seriously talk about sending humans to Mars!

[Robotbeat]

This stuff is normal. As another poster said, if anything, abnormally FEW problems.... Chill! Enjoy the ride and stop worrying so much. (I admit, I was worried right after launch up till about 3 PM eastern but they fixed it)

Agreed.  The only thing I would add however is that to me it's a reminder that we still need to work on getting this low Earth orbit stuff down before we seriously talk about sending humans to Mars!

America hadn't even sent anyone to orbit before Kennedy said we're going to go to the Moon; neither had we ever sent a probe of any kind to the surface of the Moon.

[joek]

Ya know, I don't know if we're maxed out up or not. We're probably close, on this particular flight, to being max mass.

Well, that was useful and certainly the definitive statement you made it out to be.

I never claimed it was "definitive". LegendCJS's original statement was "about the maximum for the F9 v1.0" and if the president of the company says "probably close", that is good enough for me.

Note also Suffredini's follow-up to Shotwell which, best I can make out (?)...
Shotwell: I don't know if we're maxed out up or not. We're probably close, on this particular flight, to being max mass.
Suffredini: That's an understatement.
edit: based on johnmoe's post below, it's more likely:
Suffredini: That's what I understand.

[Joffan]

CRS-2 Dragon definitely has on-board an experiment that runs for close to the full length of its visit and goes back down on the return trip. So potentially a day late rendezvous will mean a day late departure also.

Which experiment?

You made me go and look...

Certainly this one, at least: http://www.nasa.gov/mission_pages/station/research/experiments/22.html

[johnmoe]

Shotwell: I don't know if we're maxed out up or not. We're probably close, on this particular flight, to being max mass.
Suffredini: That's an understatement.

I'm pretty sure he said "That's what I understand."

[Lar]

Ya know, I don't know if we're maxed out up or not. We're probably close, on this particular flight, to being max mass.

Well, that was useful and certainly the definitive statement you made it out to be.

I never claimed it was "definitive". LegendCJS's original statement was "about the maximum for the F9 v1.0" and if the president of the company says "probably close", that is good enough for me.

Note also Suffredini's follow-up to Shotwell which, best I can make out (?)...
Shotwell: I don't know if we're maxed out up or not. We're probably close, on this particular flight, to being max mass.
Suffredini: That's an understatement.

I like Suff, he's funny. Wonder if he's miffed about the mass or just trying to keep everyone honest.

So far, they are what, a bit behind on the 20 metric tonnes total, right? Or am I misremembering the mass on CRS-1 ?