MOM test fires liquid motor for orbit insertion on Wednesday

A sigh of relief today as the liquid fueled motor was successfully test fired for 4 seconds after having last been used 9 months ago.

The 440 Newton Liquid Apogee Motor (LAM) of India’s Mars Orbiter Spacecraft, last fired on December 01, 2013, was successfully fired for a duration of 3.968 seconds at 1430 hrs IST today (September 22, 2014). This operation of the spacecraft’s main liquid engine was also used for the spacecraft’s trajectory correction and changed its velocity by 2.18 metre/second. With this successful test firing, Mars Orbiter Insertion (MOI) operation of the spacecraft is scheduled to be performed on the morning of September 24, 2014 at 07:17:32 hrs IST by firing the LAM along with eight smaller liquid engines for a duration of about 24 minutes.

The MOM is now in (or just entering) Mars’ sphere of influence and is effectively falling freely towards Mars. The orbit insertion procedure is described here.

Mars Orbit Insertion for MAVEN and MOM

graphic: indiaspaceactivity

On 24th 0127 GMT, even as it is falling with speed of 4 kms/sec, the craft is reoriented so that the thrusters face forward  ( for the next orbit maneuver of retro thrust to reduce the speed and to direct it in the required orbit ). The re-orientation is completed @ 0147 GMT. MOM would by this time be falling with a speed of  4.48Km/sec.

After the disturbances due to reorientation have died, i.e. 5 seconds later,  the Retro action starts with the firing of  LAM using 8 thrusters ..  the fall speed by now is 4.68 km/s. Even as the thrusters start their effort to reduce the speed of MOM, the fallingspeed continues to increase due to the gathered momentum upto 5.89 km/s and thenspeed  starts falling. The LAM firing also directs the craft in the desired direction so that it achieves the required orbit around Mars. The LAM operation ends @0216GMT.

As shown in figure there the MOM speed would increase from 3.18 Kms/sec to 5.89Kms/sec @  0216GMT.

Now notice that MOM approached Mars parellel to  equator unlike MAVEN which approached over North pole.  The reason for these two approaches is that the final orbits of MAVEN and MOM are inclined @ 75 deg and @ 19 deg respectively. This is evident from the ‘ cross roads ‘ seen between MOM and MAVEN release in the combination diagram shown above.

In the above flow of sequence we did not have time to  mention a couple of more chilling facts: @ nearly the same time when LAM operation starts MOM would go behind Mars as seen from Earth. Secondly , even if it was visible, whatever happens at MOM will be known to Earth only after 12 minutes due to the distance between Earth and MOM. So all this (and next described operations ) are being done under the unsupervised pre-loaded command sequences. So the success indicates how well are we in anticipating the situation through modelling.

A detailed look at dynamics of acceleration during this critical operation is shown in this graphic. (numbers are in kms/sec/sec ) corrosponding to sky blue curve. (X axis is time axis from 0110GMT to 0310GMT of 24 Sep. )
Violet curve is the speed normalized to fit in this graph with that of acceleration. Actual speed variation shown corresponds to 3.88 km/sec at left to 5.89km/s at peak near center which falls to 4 km/s at lsft edge.

We have also marked the events of reorientation (between red dots) and retro rocket firing (between Green dots ) on acceleration curve.

Notice that the LAM retro action started at first green dot on left ( 0152 GMT ) ..  it has tried to reduce the speed but the fall is more powerful, so MOM speed continues to increase despite retro action. This continues and for sometime eventhough the acceleration has started reducing ( blue line coming down ), the gathered momentum  makes the craft to increase its speed for sometime. Finally however the thrusters succeed in reducing the speed ( reversal near center of violet line ) ..  the thrusters are also turned off at that moment.(green dot near the center of graph ). Now the total operation of MOI is over.

Incidentally, the point where speed reversal has taken space forms the lowest point near the surface of MOM prior to first perigee which would occur after the craft completes one revolution.

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