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#240 by ChileVerde on 17 Nov, 2011 21:07
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I find it strange that Fobos-Grunt is supposedly in a safe mode, which has locked it into solar inertial.
Could you explain the solar inertial bit? (I've gotten to the end of what I understand about spacecraft operations.)
It occurred to me that the results that Ted Molczan came up with --QuoteIt is evident that in addition to drag, some unknown force raised the perigee at the rate of +0.633 km/d, but lowered
the apogee at nearly the identical rate.
-- has a fairly simple possible explanation. That is, there's a force, maybe a gas leak, acting on PG that has its direction more or less fixed in inertial space and has a component that acts in the direction of the spacecraft's velocity at apogee and against it at perigee:
Apogee
---> Force
-----------> Velocity
Perigee
---> Force
<----------- Velocity
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#241 by Jim on 17 Nov, 2011 21:15
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I find it strange that Fobos-Grunt is supposedly in a safe mode, which has locked it into solar inertial.
Could you explain the solar inertial bit?
Always pointing at the sun -
#242 by alk3997 on 17 Nov, 2011 21:15
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Solar inertial - pointed at the Sun (or through the Earth when the Sun is behind the Earth).
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#243 by John Duncan on 17 Nov, 2011 22:06
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Perhaps the computer is in an endless loop, waiting for the results of the first burn? Who knows but it is a sad thing.
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#244 by rweede on 17 Nov, 2011 22:21
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It occurred to me that the results that Ted Molczan came up with --QuoteIt is evident that in addition to drag, some unknown force raised the perigee at the rate of +0.633 km/d, but lowered
the apogee at nearly the identical rate.
-- has a fairly simple possible explanation. That is, there's a force, maybe a gas leak, acting on PG that has its direction more or less fixed in inertial space and has a component that acts in the direction of the spacecraft's velocity at apogee and against it at perigee:
The velocity vectors at perigee and apogee are at right angles to each other, so the force would have to act in different directions. -
#245 by ChileVerde on 17 Nov, 2011 22:25
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Solar inertial - pointed at the Sun (or through the Earth when the Sun is behind the Earth).
OK, so is the idea that, for short times, the sun stays in about the same direction from earth in inertial space, pointing at the sun gives a temporary (to within a degree per day) inertial equivalent? (Leaves aside rotation around the earth-sun axis.) -
#246 by Michael J on 17 Nov, 2011 22:25
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Some more information on the background of Phobos Grunt: http://www.russianspaceweb.com/phobos_grunt_origin.html#yh1
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#247 by Jim on 17 Nov, 2011 22:30
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Solar inertial - pointed at the Sun (or through the Earth when the Sun is behind the Earth).
OK, so is the idea that, for short times, the sun stays in about the same direction from earth in inertial space, pointing at the sun gives a temporary (to within a degree per day) inertial equivalent? (Leaves aside rotation around the earth-sun axis.)
Dont understand your question. The vehicle is always point at the sun no matter where it is as it orbits earth -
#248 by zorg2 on 17 Nov, 2011 23:30
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There was quite shocking information from the insiders on NK forum: The X-band communication station PA was seriosly underpowered : instead of 30000W klystron they purchased the second-hand 750W travelling wave tube, which could hardly provide the acceptable bandwidth on Mars orbit.
If true, it means, they didn't believe in success of the mission from the beginning.
http://translate.google.com/translate?hl=en&sl=auto&tl=en&u=http%3A%2F%2Fwww.novosti-kosmonavtiki.ru%2FphpBB2%2Fviewtopic.php%3Ft%3D10631%26postdays%3D0%26postorder%3Dasc%26start%3D7780QuoteI want to throw in my 5 cents to the topic. Только что общался с руководством фирмы занимавшейся поставкой УМ X-band для антенн в Медвежьих Озерах и Уссурийске по программе ФГ. Just talked to the leadership of the firm engaged in supplying the PA X-band antenna at Bear Lakes and Ussuriisk Program FY.
Изначально планировалось поставить 30кВт клистронный УМ. It was originally planned to put 30kW klystron PA. Но когда дошло до дела, оказалось что в предназначенном для УМ помещении сгнил пол и под тяжелым УМ с водяным охлаждением он просто рухнет. But when it came down to it, was that the room designated for the PA and rotted floor under heavy UM cooled it just collapses.
Выход был найден парадоксальный, 30кВт клистрон не ставить, а ограничиться second hand TWTA 750 Вт производства Xicom. The solution was found, paradoxical 30kW klystron is not set, and to limit second hand TWTA output 750W Xicom.
Стоить ли говорить что этой мощности явно недостаточно чтобы обеспечить надежный аплинк с аппаратом на марсианской орбите. Worth to say that this power is not enough to provide reliable uplink with the device on the Martian orbit.
Если это правда, то аппарат был заранее и сознательно списан в утиль If this is true, then the device was deliberately written off in advance and the scrap -
#249 by kevin-rf on 17 Nov, 2011 23:42
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The link does not work for me...
Also, do you speak ruissian because that google chicken scratch might mean that it's output is limited to 750W and not it is a 750W... It is that bad.
Try this link for the post: http://www.novosti-kosmonavtiki.ru/phpBB2/viewtopic.php?t=10631&start=7780
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#250 by olasek on 17 Nov, 2011 23:49
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Quote from: rweede link=topic=27289.msg830589#msg830589
The velocity vectors at perigee and apogee are at right angles to each other, so the force would have to act in different directions.
Simply not true. Velocity vectors at perogee and apogee are in fact parallel. -
#251 by ChileVerde on 17 Nov, 2011 23:58
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The vehicle is always point at the sun no matter where it is as it orbits earth
Yes. And the sun, for short periods of time, is always at about the same direction in inertial space relative to the earth. Think of the right ascension and declination of the sun as seen from earth for short times. -
#252 by alk3997 on 18 Nov, 2011 00:42
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Perhaps the computer is in an endless loop, waiting for the results of the first burn? Who knows but it is a sad thing.
A few people mentioned this idea in the previous thread. I would be surprised (maybe even shocked) if this occurred with a modern real-time operating system. Basically the operating system runs the tasks (including the tasks needed for a burn). It does this by allocating time by priority (asynchronous) or by time slices (synchronous). But the 'let's do a burn' program is never in charge of the processor, the operating system is.
So, let's say the burn was hung waiting. Then the other tasks the operating system has scheduled would continue to function. So, if telemetry was a running program, that program would continue to run as long as it had a high enough priority in an asynchronous scheme. In a synchronous scheme, time would be allocated to running that process based on how it was scheduled.
So, most modern real time systems will protect themselves from an application program that gets into an infinite loop. Even Windows XP and 7 do that and they aren't real time operating systems by any stretch of the imagination.
So, I doubt the infinite loop theory but I can't say for sure since I don't know what Fobos-Grunt used for an O/S (or computer).
Andy -
#253 by bernse on 18 Nov, 2011 02:09
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I don't know if this has been copied here already, but:
That is one of the more bizarre things I've read in a while. I don't know what's weirder, this or the conspiracy theory from earlier that the probe MUST BE ABLE to communicate because it has so many radios.
http://www.russianspaceweb.com/phobos_grunt_launch.html
On the night from November 15 to November 16, a ground station in Ussuriisk in the Russian Far East made a desperate and, apparently, completely hopeless attempt to communicate with the spacecraft, as if it had been in the Earth escape trajectory following a nominal burn of the MDU propulsion unit. Unnamed officials reportedly insisted on this "communication session," after reading on the Internet that some observers in Brazil had seen what looked like a firing of the probe's main engine. Obviously, nothing came out of this effort, except for more comments on the web about gross incompetence of the mission management.
This whole thing is just so sad. -
#254 by hop on 18 Nov, 2011 02:14
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Anatoly Zak has posted an update here http://www.russianspaceweb.com/phobos_grunt_scenario.html which I believe covers in human translated English much of the same information found in the NK posts I linked earlier.
According to this information, PG should have been transmitting telemetry periodically (40 minutes every 2 hours) if it was in proper safe mode, and of course should also respond directly if asked.
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#255 by just-nick on 18 Nov, 2011 03:32
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Perhaps the computer is in an endless loop, waiting for the results of the first burn? Who knows but it is a sad thing.
So, I doubt the infinite loop theory but I can't say for sure since I don't know what Fobos-Grunt used for an O/S (or computer).
I agree it may not be the most likely, but I don't think we can exclude some sort of loop, even if it isn't the prime mover in this failure. It needn't be a loop in the "10 PRINT "HELLO" 20 GOTO 10" mold, it could be a more complex behavior emerging from otherwise well intentioned code. What follows is rampant speculation and imagination:
Suppose PG initiates its TMI burn not based on time but on inertial position. As it nears that position, some element of the RTOS (or whatever is running the show, perhaps a single monolithic program) says "time to call the pre-TMI routine" and starts settling burns, pressurization sequences, etc. Then at some subsequent time (perhaps based on time, position, or other factors) the "TMI burn routine" is called. It identifies a problem, aborts itself, and hands control back over to the executive (or whatever).
If nothing comes along and pulls the pre-TMI routine or whatever its trigger is out of the queue, then that routine cane fire up again and repeat this process in an other orbit or whatever.
Again, this is pure speculation, a cheerful work of science fiction if you like, but somewhat informed by lots of reading of various spaceflight computer systems and a bit of mucking around with control routines for a positron collider back in my youth. There is pretty good documentation on the AGC and the Centaur's guidance computer on NTRS if you want to read about the development of these systems...just fascinating stuff.
Anyway, my point is that well intentioned code run by a competent executive can still produce complex failures, fail to clean up after itself, etc. particularly if it is (A) cobbled together and (B) poorly tested.
Cheers,
--N -
#256 by kevin-rf on 18 Nov, 2011 03:44
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Anatoly Zak has posted an update here http://www.russianspaceweb.com/phobos_grunt_scenario.html which I believe covers in human translated English much of the same information found in the NK posts I linked earlier.
According to this information, PG should have been transmitting telemetry periodically (40 minutes every 2 hours) if it was in proper safe mode, and of course should also respond directly if asked.
It also gives a nice answer about what happens when it enters the earths shadow. It was actually programmed to handle that event and keeps the bead accurate by use of the gyroscope. A separate system that appears to be working. I was wondering about that one... and it would explain why it can keep from tumbling while ignoring the ground! -
#257 by alk3997 on 18 Nov, 2011 04:01
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The big question I have after reading the latest Zak article is what happens after 8 days?
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#258 by Steven Pietrobon on 18 Nov, 2011 05:40
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Comga contacted me by email and helped to explain what is happening.
I'm working this out by myself for the first time so of course I may have missed out something. Thinking about it some more, Comga is absolutely right. We wait until the orbital plane of PG precesses such that the orbit relative to the ecliptic is in the right position (looking at the Earth with the Sun to the left and Earth in front, that is in the same direction as Earth's rotation around the Sun, PG's orbital plane needs to be edge on). When PG reaches this, it fires its engine to send it on a hyperbolic trajectory. The asymptotic trajectory will then be parallel to the Earth velocity vector.
So this means we only need to wait 30 days for the next firing opportunity. Actually, if the 6° per day is relative to the stars and as we want PG in the right plane relative to the Sun, this means the sun-relative precession is 6+360/365.25 = 7° per day. This means we need to wait 180°/7 = 25.7 days, which would be around 4 December.
Attached is diagram showing what is happening. I've shown Earth as if it was in the middle of northern hemisphere Winter, which is not correct and so the angles of the PG planes will be different to that shown, but the conclusion is still the same. That is, the inclination of PG's plane (as long as its edge on) does not affect the exit velocity, which remains the same. This also means that PG does have a TMI opportunity around 4 December.
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#259 by sdsds on 18 Nov, 2011 08:44
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I stared at your drawing and pondered what you wrote. The tersest way I can express the conclusion is, "We wait for times when the Earth's velocity vector is coplanar with the spacecraft's orbit." That condition will be met every ~180 degrees of precession, rather than every ~360 degrees, so another departure opportunity will occur in ~30 days.
It's true this won't matter if they can't uplink commands to the spacecraft. But if this second departure opportunity is real, it might explain why mission controllers didn't immediately request communications assistance from e.g. DSN. They have some time yet to establish contact using their own X-band antennae, perhaps after increasing their slew rates to enable tracking of F-G in LEO.
I doubt the claims that they have X-band transmitters that are too powerful, or not powerful enough. If its antenna could slew fast enough, wouldn't a ground station that works for Spektr-R work for F-G as well?