Was SpaceX's launch of Formosat-5 more vertical than normal for any particular reason?

Question

Reading both the abstract and the plain language summary of the recent paper published in the journal Space Weather Gigantic Circular Shock Acoustic Waves in the Ionosphere Triggered by the Launch of FORMOSAT‐5 Satellite I am confused about what it exactly that was vertical and when. The abstract of the paper says it is the attitude of the rocket during orbital insertion that is vertical!

"...and was due to the unique, nearly vertical attitude of the rocket during orbit insertion."

I'm having difficulty how the Falcon-9 2nd stage could or would have a vertical attitude during orbital insertion!

The unusually large Shock Acoustic Wave (SAW) in total electron content (TEC) in the ionosphere is attributed to unusual verticality of either the trajectory or the attitude of this particular launch, depending on which section below you read.

FORMOSAT-5 (2017-049A) was launched from Vandenberg's Space Launch Complex 4-East, and deployed by the Falcon-9 into a nearly circular ~720 km, 98.3° Sun-synchronous orbit 24-Aug-2017.

Question: Was there a scientific goal or reason for this being so vertical? Did the low payload mass simply provide an extra degree of flexibility to make this option possible, or is this a direct and perhaps necessary consequence of the low payload mass?

Citation:

• Chou, M.‐Y., Shen, M.‐H., Lin, C. C. H., Yue, J., Chen, C.‐H., Liu, J.‐Y., & Lin, J.‐T. (2018). Gigantic circular shock acoustic waves in the ionosphere triggered by the launch of FORMOSAT‐5 satellite. Space Weather, 16, 172–184. https://doi.org/10.1002/2017SW001738

See also:

• Ars Technica: SpaceX launch last year punched huge, temporary hole in the ionosphere
• Engaget: SpaceX rocket carved giant hole in the ionosphere

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Abstract

The launch of SpaceX Falcon 9 rocket delivered Taiwan's FORMOSAT‐5 satellite to orbit from Vandenberg Air Force Base in California at 18:51:00 UT on 24 August 2017. To facilitate the delivery of FORMOSAT‐5 to its mission orbit altitude of ~720 km, the Falcon 9 made a steep initial ascent. During the launch, the supersonic rocket induced gigantic circular shock acoustic waves (SAWs) in total electron content (TEC) over the western United States beginning approximately 5 min after the liftoff. The circular SAWs emanated outward with ~20 min duration, horizontal phase velocities of ~629–726 m/s, horizontal wavelengths of ~390–450 km, and period of ~10.28 ± 1 min. This is the largest rocket‐induced circular SAWs on record, extending approximately 114–128°W in longitude and 26–39°N in latitude (~1,500 km in diameter), and was due to the unique, nearly vertical attitude of the rocket during orbit insertion. The rocket‐exhaust plume subsequently created a large‐scale ionospheric plasma hole (~900 km in diameter) with 10–70% TEC depletions in comparison with the reference days. While the circular SAWs, with a relatively small amplitude of TEC fluctuations, likely did not introduce range errors into the Global Navigation Satellite Systems navigation and positioning system, the subsequent ionospheric plasma hole, on the other hand, could have caused spatial gradients in the ionospheric plasma potentially leading to a range error of ~1 m.

Plain Language Summary

On 24 August 2017, a SpaceX Falcon 9 rocket departed from Vandenberg Air Force Base in California, carrying Taiwan's FORMOSAT‐5 Earth observation satellite into orbit. The lightly weighted solo payload enables the rocket to fly a lofted trajectory for direct insertion at the mission altitude of 720 km. This unique nearly vertical trajectory is different from the usual satellite launches that the rockets fly over horizontal trajectory and insert satellites at 200 km altitude followed by orbit maneuvers to its mission altitudes. Consequently, the rocket launch generated a gigantic circular shock wave in the ionosphere covering a wide area four times greater than California. It is followed by ionospheric hole (plasma depletions) due to rapid chemical reactions of rocket exhaust plumes and ionospheric plasma. The ionospheric hole causing large spatial gradients could lead to ~1 m range errors into GPS navigation and positioning system. Understanding how the rocket launches affect our upper atmosphere and space environment is important as these anthropogenic space weather events are expected to increase at an enormous rate in the near future. (emphasis added)

Answer 1

FORMOSAT-5 was deployed directly to a 720 km circular orbit, with only a single burn. In order to do a circular orbit so high, one has to have a more vertical ascent then would be typical. Basically, one has to be burning a significant amount of time near the apogee, which has to be 720 km in this instance. For a lower perigee insertion orbit, say 200-300 km, one has achieved this closeness to apogee very early on, if you want to be in a 700 km circular orbit with a single burn, that closeness takes a while to achieve.

Put another way, an effective vertical gain of 720 km is required. One can best get that by a vertical ascent initially, to avoid gravity and atmospheric drag.

For a very low orbit, one tends to start burning horizontally as low as possible, as it provides the most efficiency. Thus, by the time it reaches the ionosphere, which starts at 50 km, it is burning more horizontally then vertically. For the use case of a circular orbit with a single burn, one has to delay putting on the horizontal movement until one starts to approach the desired altitude, which prefers the more vertical ascent.

Of course, the final orbital insertion in the end will be horizontal, for either use case. The difference is no doubt the time period in between. The altitude at stage separation for Formosat-5 was about 92 km, for Iridium, which uses a 2 second stage burn to achieve a similar orbit, the altitude was about 60 km at stage separation. The fact that the first stage burned in the Ionosphere far more is likely the cause of the effect.

One can always use a lower energy solution if one desires, but in general reducing the number of second stage burns is desired, because it reduces the risk of rocket failure. As they found an orbital solution that could be done with a single second stage burn, they no doubt took it.