Charlotte Beskow update on ATV-5’s important task for today’s spacewalk:
Today on the International Space Station Barry Wilmore and Reid Wiseman will change out a sequential shunt unit during a spacewalk (EVA28). NASA TV will show it a bit later, once it gets underway . It is extra nice for the ATV team since Barry Wilmore worked on the ATV project during its development phase a few years ago.
Normally the Space Station flies in what we call XVV where the velocity is along the X axis (ie the axis that runs right along the Station’s body) from ATV-5 in the back to Node 2 in the front and with the truss with the massive Solar Arrays sticking out on both sides. For this spacewalk, the International Space Station will change to fly sideways, in a –YVV, ie the truss will be in line with the ground path. To v isualise it you can walk forward with arms outstretched, then turn 90 degrees to your right while still moving in the same direction so your left hand leads. ATV-5 make this attitude change happen and for that reason we all got up very early this morning as operations started around 5:00 GMT.
It will be a long day because flight controllers need several hours to prepare the spacewalk, the spacewalk itself will take around 6-7 hours and then we will return to a nominal configuration, flying along the X axis. This means that we will finish around 20:00 GMT this evening.
In principle it is simple. We activate ATV-5’s propulsion system, the Space Stations uses it to change its attitude to fly sideways then ATV-5 goes back to DORMANT mode and waits for the spacewalk to conclude. Afterwards we will reactivate the propulsion system, move the Station back to XVV as it usually does and go back to DORMANT mode again to conclude the day.
Reality is a bit more complex but very interesting. Many hours of discussions between Houston, Moscow and us here in Toulouse go into preparing the choreography to ensure that the actions are taken at the right time by the right control center:
- 05:31 we start the activation of the ATV propulsion system by updating the configuration of the ATV-ISS joint power set up (we need more power from the Space Station when we are in so called active mode). We then reprime the propulsion system.
- 06:20 repriming is completed and all the relevant plans are enabled on board ATV
- 06:30 we are ready to feather our solar arrays. This means that we protect them by turning them so that the solar cells face into space. They will remain fixed in this position for some time.
- 06:47 ATV-5 goes to a mode which allows attitude control of the Space Station. The Station can now command ATV-5’s thrusters.
- 07:05 ca. The attitude change has starts. We can “see” it via our displays which show the Euler angles of the Station. You need to use your brain to visualise it in your mind.
- 07:57: Moscow reports that the ISS attitude change is complete. We are ready for the next step. Since ATV-5 is in a configuration which is not optimal from a power point of view as its solar arrays are not facing the Sun we will stop all non-essential equipment such as the lights in ATV and the cabin fan.
- 08:09 ATV-CC flight controlles switch OFF the lights and stop the fan. We can see it on our screens thanks to the communication links
Power specialists will now maintain a watchful eye on the power situation by looking at the battery’s depth of discharge and other parameters. In parallel we also receive power from the Station to compensate for the “exotic”attitude. In about one hour we will go back to DORMANT mode and start rotating two of our solar arrays.
…. More to follow…