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#200 by simonbp on 17 Nov, 2011 06:08
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Here you go:
Here's a plot of that data with a simple least-squares linear regression fit to all the points except the first (which is an obvious outlier). The error bars are one standard deviation, and the fit is actually pretty good.
Thus, I'm still not believing the "actively thrusting" story. Some slow tumbling (and thus time-variable drag) would be perfectly sufficient to explain the variation.
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#201 by Comga on 17 Nov, 2011 06:23
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5 km descent in 8 days extrapolates to 20 km in 32 days. Then its 20 km in 16 days, then 8, then 4, then 2, ... 150 km and reentry around Jan 8, plus or minus a bunch, all by rule of thumb estimates.
So sad -
#202 by hop on 17 Nov, 2011 06:23
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Here's a plot of that data with a simple least-squares linear regression fit to all the points except the first (which is an obvious outlier). The error bars are one standard deviation, and the fit is actually pretty good.
Why are you plotting mean altitude ? The apogee is clearly and indisputably going down, the oddity is in the perigee.QuoteSome slow tumbling (and thus time-variable drag) would be perfectly sufficient to explain the variation.
http://digilander.libero.it/SATrack/Phobos.html?q=phobos#LNEW
Producing that by tumbling would be... impressive. -
#203 by Comga on 17 Nov, 2011 06:33
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Here's a plot of that data with a simple least-squares linear regression fit to all the points except the first (which is an obvious outlier). The error bars are one standard deviation, and the fit is actually pretty good.
Why are you plotting mean altitude ? The apogee is clearly and indisputably going down, the oddity is in the perigee.QuoteSome slow tumbling (and thus time-variable drag) would be perfectly sufficient to explain the variation.
http://digilander.libero.it/SATrack/Phobos.html?q=phobos#LNEW
Producing that by tumbling would be... impressive.
But it is said to be not tumbling but oriented to the Sun. A possibility is that some venting or anti-solar-torque ACS thrusting is prograde at apogee, increasing perigee, and retrograde at perigee, decreasing apogee. However, the decreased mean radius would be due to drag.
Variations in the residual atmospheric density can explain some of the variability in the rate of descent, but probably not the slight apparent increase.
What is the basis for those error bars? -
#204 by Steven Pietrobon on 17 Nov, 2011 09:10
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If the orbit passes through the point opposite the departure direction, the same burn is required when the S/C hits the tangent circle to escape with the requisite excess velocity . Whether the S/C is heading north or south shouldn't make a difference.
Do others concur?
I do not. It does make a difference whether you are heading north or south as you want your velocity vector to be approximately pointing in the same direction as the Earth's rotation around the Sun at TMI.
On launch day on 8 November, PG was heading over South America at night in a north-east direction. If the 6 degree per day recession is correct, then in 30 days time (on 8 December) PG will again be heading north-east, but on the day side of the Earth. This means the velocity vector is pointing backwards, which is no good, unless you want to go to Venus. On the dark side of Earth, PG will be heading south east which is the right direction.
However, the angle of PG to the ecliptic plane will have changed. On November 8, the angle between PG and the ecliptic plane is the inclination of PG to Earth (51.6º) minus the inclination of the Earth (23.4º), that is 51.6-23.4 = 28.2º. On 8 December the inclination of PG to the ecliptic is the inclination of PG to Earth (51.6º) plus the inclination of the Earth (23.4º), that is 51.6+23.4 = 79.8º. The higher this angle the more delta-V is required to reach Mars. The higher inclination of PG on 8 December, plus being very near the end of the window may mean that PG may not have enough propellant to reach and then orbit Mars.
If the Russians can get in contact and control of PG before it re-enters and if its too late to send PG to Mars, a possible rescue plan could have PG fire its engine to put it into say a 350 by 600 km orbit. There PG will stay for two years until the next window arrives around November 2013. The period of the orbit being designed so that PG is in the right position for TMI in two years time. This will rely on the on-board batteries being able to sustain two years worth of charging and discharging, which they probably were not designed to do. Going for a higher apogee will mean less charging cycles, but PG will be passing through the Van Allen radiation belts. Using the Moon and the Lagrange points it might be possible to have PG hang around the Earth-Moon system and then dive to 300 km perigee for the kick to Mars in two years time.
So I think its possible that PG can be saved, but before this can happen contact needs to be made, which unfortunately has not happened yet. -
#205 by scpc on 17 Nov, 2011 09:15
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A fault in the second-stage control systems seems to be the immediate cause. But journalist Anatoly Zak, the founding editor of russianspaceweb.com, says that the deeper problems are institutional. A culture of lax testing meant that some problems lay undetected, whereas known problems were ignored at the higher levels, he says. “The project was doomed from the beginning.”
http://www.nature.com/news/russia-gets-the-red-planet-blues-1.9361 -
#206 by Michael J on 17 Nov, 2011 11:38
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Seems China has thrown in the towel and considers its orbiter lost.
http://www.spacedaily.com/reports/Yinghuo_Was_Worth_It_999.html -
#207 by ChileVerde on 17 Nov, 2011 12:32
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Ted Molczan has been studying the evolution of the PG orbit using a high-fidelity propagator:
http://www.satobs.org/seesat/Nov-2011/0188.htmlQuoteIn an effort to determine whether the perigee rise could be explained by gravitational perturbations not modeled by SGP4 (the orbital model of TLEs), I [TM] compared the actual orbital elements against those yielded by propagating the earliest reliable TLE data using the STOAG (Semi-analytic Theory of mOtion under Air drag and Gravity) software. I propagated the epoch 11313.39819696 TLE through day 11319, varying the area to mass ratio, A/m, by trial and error to match the actual rate of decay of the semi-major axis.
He concludes,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.
This is all somewhat in media res, so the picture could well change as more information becomes available.
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#208 by kevin-rf on 17 Nov, 2011 13:31
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Is there a reason everyone is equating not tumbling to sun oriented? Unless someone has imaged it at high res. there is no way to prove that is sun oriented one way or another. There are other effects, like gravity gradient stabilization that can keep it from tumbling.
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#209 by alk3997 on 17 Nov, 2011 13:49
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A fault in the second-stage control systems seems to be the immediate cause. But journalist Anatoly Zak, the founding editor of russianspaceweb.com, says that the deeper problems are institutional. A culture of lax testing meant that some problems lay undetected, whereas known problems were ignored at the higher levels, he says. “The project was doomed from the beginning.”
http://www.nature.com/news/russia-gets-the-red-planet-blues-1.9361
That story was written two days ago and the author was just reporting rumor as fact with the "second-stage control system" theory. I don't know even know what the second-stage would have to do with anything or was it a two-stage control system? Meaningless words (at least that part of the story)...
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#210 by alk3997 on 17 Nov, 2011 13:51
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Is there a reason everyone is equating not tumbling to sun oriented? Unless someone has imaged it at high res. there is no way to prove that is sun oriented one way or another. There are other effects, like gravity gradient stabilization that can keep it from tumbling.
No, it's not shaped right for sustained gravity gradient. There is still torque on the vehicle from the exo-atmosphere at that altitude that would perturb the gravity gradient attitude, if even it was shaped right.
Take a look at Seasat as an example of a gravity gradient shape and cg. Even then there were oscillations in the gravity gradient attitude due to the Earth not being a perfect sphere with uniform mass.
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#211 by Ben the Space Brit on 17 Nov, 2011 14:00
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Ted Molczan has been studying the evolution of the PG orbit using a high-fidelity propagator:
http://www.satobs.org/seesat/Nov-2011/0188.html
(snip)
He concludes,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.
This is all somewhat in media res, so the picture could well change as more information becomes available.
That's too regular to be something random or uncontrolled. I'm still betting on this being RCS activity. The IAU is probably trying to work out why the sun is moving so quickly (as it thinks it is on a transfer orbit, not LEO). it may be compensating with lots of turns and burns, trying to fix a stability problem that isn't actually there. -
#212 by plutogno on 17 Nov, 2011 14:06
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No, it's not shaped right for sustained gravity gradient.
why do you say so? it has 75 per cent of its mass on one end (the MDU + fuel tank), a longeish truss and the rest of the mass at the other end (the lander). to me it looks like a gravity gradient-stabilized satellite! -
#213 by kevin-rf on 17 Nov, 2011 14:38
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There is still torque on the vehicle from the exo-atmosphere at that altitude that would perturb the gravity gradient attitude, if even it was shaped right.
Good point, but if the solar arrays have deployed, it will make a good dart. All the unused fuel mass forward and panels acting as feathers dragging the rest of the vehicle back. -
#214 by alk3997 on 17 Nov, 2011 14:41
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No, it's not shaped right for sustained gravity gradient.
why do you say so? it has 75 per cent of its mass on one end (the MDU + fuel tank), a longeish truss and the rest of the mass at the other end (the lander). to me it looks like a gravity gradient-stabilized satellite!
Not very long and not very big. Physics wants long - again see Seasat and even then it wasn't fully stable. TSS, when deployed, was an example of "long enough" for stable gravity gradient - but of course there were other issues with that project.
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#215 by simonbp on 17 Nov, 2011 14:43
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Why are you plotting mean altitude ? The apogee is clearly and indisputably going down, the oddity is in the perigee.
The energy of the orbit is directly proportional to the semimajor axis, and vice-versa. Drag or rocket thrusts change the kinetic energy of the vehicle, and thus the total energy of the orbit. Other forces, e.g. solar torques, precession due to oblateness, etc, would not change the total energy, but rather work to change the eccentricity and orientation of the orbit. Thus, plotting mean altitude provides a measurement that controls for all plausible forces that are not drag or thrust.
And if it is a time-variable drag effect, you would expect the largest variation in the periapse, as that is when the vehicle is moving fastest and through the thickest air. -
#216 by Comga on 17 Nov, 2011 15:05
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If the orbit passes through the point opposite the departure direction, the same burn is required when the S/C hits the tangent circle to escape with the requisite excess velocity . Whether the S/C is heading north or south shouldn't make a difference.
Do others concur?
I do not. It does make a difference whether you are heading north or south as you want your velocity vector to be approximately pointing in the same direction as the Earth's rotation around the Sun at TMI.
We will have to remain in disagreement.If the Russians can get in contact and control of PG before it re-enters and if its too late to send PG to Mars, a possible rescue plan could have PG fire its engine to put it into say a 350 by 600 km orbit.
That assumes that the spacecraft could split its burns into multiples. Plus waiting for 26 months in an orbit with a 300 km perigee would lose a lot of energy.So I think its possible that PG can be saved, but before this can happen contact needs to be made, which unfortunately has not happened yet.
We can agree on the last part of that statement, if not the first. -
#217 by ringshot on 17 Nov, 2011 15:24
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Is the TDRSS constellation capable of communicating with Phobos-Grunt?
It could be a solution for the contact time issue that they keep mentioning... we could even trade emergency TDRSS time for a Soyuz ride or two!
I suspect that the systems are simply incompatible though.
G'Day...Ron
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#218 by rdale on 17 Nov, 2011 15:43
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Already been asked on page 47
Not compatible.
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#219 by simonbp on 17 Nov, 2011 16:20
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If the Russians can get in contact and control of PG before it re-enters and if its too late to send PG to Mars, a possible rescue plan could have PG fire its engine to put it into say a 350 by 600 km orbit.
That assumes that the spacecraft could split its burns into multiples. Plus waiting for 26 months in an orbit with a 300 km perigee would lose a lot of energy.
Yeah, you don't want an elliptical (or low) long-term parking orbit. Parking it at or close to GEO would be best, but I doubt PG had the dV margin for that even before the orbit began to decay...
Not compatible.