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Saturday, July 16, 2011

Launch Video Notes

A few points that interested me while watching the live transmission were

1. All 6 strap-ons in 1st stage are not lit and seperated simultaneously. There are two sets .. 4 Ground lit ones ignite about T+ 0.4 seconds. The 2 airlit ones ignite at T+25. 1st set of strap-ons seperate at T+70, 2nd set seperates at T+92 secs.

However the most interesting part was during last few maneuvers depicted in these screenshots.

In the end the last kink was interesting. In case of normal Polar Satellite launch the satellite is just released when the launcher is parallel to surface of earth at equator but here the requirement was that the satellite should be released at a very critically decided angle so that the spacecraft velocity vector and earth's gravitational field vector make the spacecraft to attain the highly elliptical initial orbit of 284kms X 21000kms. Therefore the spacecraft is first raised to about 300 kms and then turned groundwards. This not only brings it to the required injection angle wrt earth surface but also increases velocity very rapidly to the required injection velocity of 9.5+ kms/sec required for achieving the desired elliptical orbit.

Added instants at the seperartion of Ground-lit strap-ons (T+89 sec) and 2nd stage ignition ( T + 103 sec).

I have added cursors ( Yellow horizontal lines ) at the following instances :

GL Strap-on separation @ 69.9 sec
AL Strap-on separation @ 92 sec
PS1 separation @ 112.4 sec and PS2 Ignition about .5 sec after that

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Anonymous said...


i wanted to post my question in BR, but i stopped posting there due to my nature of work and rudeness of some oldies@BR. This has reference to the PSLVC17 plots. At 60 secs after launch, the plots show deviations in time Vs velocity. The deviations are quite significant. and later i didnt see the blip continuously and there are breakages in the time vs altitude plot. there were some disturbances during stage ignitions as well. after the launch, one of the directors said they had some anxious moments. can you pleae explain on this?

s^3 said...

Hello anonymous,

The graphs in video start at 88 T+88 seconds so I assume that your time tagging of 60 sec is appox. There is no major event at T+60 but there is one at T+70 viz separation of 4 ground lit strap ons.This sudden decrease in huge weight must have given a jolt to the dynamic vehicle and ( this is MY guess ) the measuring instrumentation must have been introduced with some offset and this seems to have introduced ( apparent not real ) errors in indication of both altitude and velocity. I am saying this because immediately after the 1st stage instrumentation stops after 1st stage seperation @ 101 sec, the measured and actual plots of velocity and altitude as transmitted by 2nd stage instrumentation match even before it is ignited ( the small horizontal portion of constant velocity viz the coast ).
This my guess from the graphs. Post launch comments by scintists are not there in the video so I will search if it is available in some other video. I am sure the comments by experts will throw some more light in this matter.

Anonymous said...

Dear sir,

thanks for the reply. please see the youtube video of PSLVc17 as per the link given 2.25 of youtube time display, the plot is shown. The time stamp shown on the graph 88.8 sec. In this , the top plot (time Vs vel?)plot the difference in the predicted velocity??? and actual is glaringly different. But later the rocket caught up with the predicted trajectories. This type of behaviour i haven't seen in the PSLV launches held so far!!!!


venkat said...


i tried uploading that particular plot showing the deviation in the altitude and velocity.please give your comments. if the sudden acceleration is due to loss of weight due to strap on booster getting separated,why this event caused deviation?
i am curious to know. thanks.

s^3 said...

The Graphs cover a very short duration of time hence the scale is exaggrated making the small deviations visible which are less that .2 Kms. These deviations are not visible in a larger time scaled graph ( covering 3rd and 4th stage displays )
@ Venkat
The sudden increase in deviations start at the time spot where strap-ons separate. As explained in my earlier comment in response to Anon's 1st comment I feel that this increase is due to sudden loss in weight. Again, this is MY personal view.

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