Saturday 21 December 2019

The stars did not align well for Starliner, it seems

click map to enlarge

Yesterday's Boeing CST-100 Starliner Orbital Flight Test was a true nailbiter. This blogpost briefly reitterates what happened, and what could have happened had they not been able to eventually raise the orbit.

Launched atop an Atlas V rocket, this uncrewed inaugural test flight of the new Boeing Starliner crew transport vehicle should have gone on its way to a docking at the ISS today, followed by undocking and landing at the White Sands Missile Range a week from now. The map above which I prepared pre-launch from information in the Starliner Press Kit and Starliner Notebook, shows what should have been the launch track and some keypoints on that track. As we now know, it went wrong at one of these keypoints.

Launch was at 11:36:43 UT. The Atlas V and Centaur upper stage performed fine, the Centaur inserting the Starliner in a 76 x 191 km suborbital trajectory some 12 minutes after launch. Three minutes later, the Starliner separated from the Centaur.

Next, 31 minutes after launch near 12:08 UT, it should have fired its own thrusters, in order to raise perigee and in this way circularize the orbit, becoming truely orbital.

And that went wrong.

Due to a misfunctioning Mission Elapsed Time clock, the Starliner's orbit insertion burn did not go as planned. Initially, an "attitude problem" was reported as well. The next half hour or so was nailbiting, as Boeing and NASA were not quite coming forward with information, apart from the ambiguous comment that the Starliner had "stabilized" its orbit (which is extremely ambiguous wording).

Those of us who know about orbits, realised that if no orbit insertion burn took place, the Starliner would continue on a suborbital trajectory, and reenter with or shortly after the Centaur upper stage (see also the end of this post, where I modelled this). The Centaur reentry was expected to occur south of Australia at about 12:30-12:35 UT (see the map in top of this post), and as the clock approached that time, it became really nailbiting: was the Starliner crew module still on orbit, or breaking up and burning up over the Indian Ocean?

Eventually, it became clear that, many minutes after the original burn time, Boeing did manage to do a burn that raised perigee from 76 to 180 km.

As an interesting sidenote: during the post-launch press conference, Boeing's Jim Chilton seemed to suggest (at 7:25 in below video) that following the timer anomaly, they tried to uplink new commands, but were faced with delays caused by the relay satellite(s) used (TDRS). It also transpired that on a crewed flight, the crew itself would have intervened in this stage:

Orbital data released by CSpOC provided the first unambiguous information to the world about the whereabouts of Starliner. A pre-burn orbit appeared first, showing a 76 x 191 suborbital orbit. As this was pre-burn, this still did not say much about Starliner's state. But shortly after that, a 186 x 221 km orbit was published, somewhat later followed by a new 180 x 221 km orbit. These showed that Starliner had reached a safe orbit around the earth.

The diagram below shows the altitudes of apogee and perigee of the orbit published so far (21 december 12 UT): currently it is in a 241 x 265 km orbit.

click diagram to enlarge

The amount of fuel spent in the emergency manoeuvres after the planned burn did not occur, was thus that it was no longer feasible to reach and dock to the ISS. Over the night, a new burn or series of burns therefore raised the orbit to 241 x 265 km, 58.4 degree inclined, lining it up for a landing at White Sands Missile Range on Sunday 22 December.

The current orbit (epoch 19355.3601887) results in a landing opportunity at White Sands between 12:45-12:55 UT on Sunday 22 December, approaching the range from over the eastern Pacific, as can be seen in the map I prepared below:

click map to enlarge
This is based on the current (epoch 19355.3601887) orbit. If new orbit adjustments happen, the projected time of landing might change slightly (e.g. a lowering of the orbit would make the Starliner speed ahead a bit, resulting in a slightly earlier landing time).

[UPDATE 21 Dec 22:15 UT: NASA has announced that the landing will be around 12:57 UT]

What if the orbit raise had failed completely?

Starting from the first, pre-boost orbit released, 76 x 191 km, I used GMAT to model what would have happened. I find that the Starliner, had it continued in that orbit, would have reentered over Polynesia around 12:50 UT,  about 1h 15m after launch, with its first revolution still uncompleted:

click map to enlarge

1 comment:

Ryan said...

"it became really nailbiting: was the Starliner crew module still on orbit, or breaking up and burning up over the Indian Ocean?"

Is it certain it would have burned up? I would have thought it would need to be able to reenter successfully if the engines on the Starliner failed.