SpaceX Falcon 9 first stage in the final moments of its flight during the launch of the Iridium-1 mission earlier this month.
Carrying a total of ten Iridium-NEXT satellites, Iridium-1 starts the process of replacing the legacy Iridium satellite phone network. These satellites have much longer design lifespans, higher bandwidth, and can handle far more traffic than their predecessors… and the lack of the large polished aluminum transmitter plane arrays will finally eliminate the infamous “Iridium Flares” that have plagued stellar photography and observations for the last 20 years.
Roaring aloft from Vandenburg AFB’s SLC-4 at 9:54am PST, the Falcon 9 headed south out over the Pacific Ocean with the ten satellite of the Iridium-1 mission safely tucked away in the fairing atop the second stage.
The first stage carried the second stage and payload through the thickest part of the atmosphere and added about 1/7th of the downrange velocity needed to reach orbit before detaching and flipping end-for-end and firing three of its nine engines for several seconds to reduce its downrange velocity and put the stage on a rough course for the targeted LZ, the Autonomous Spaceport Drone Ship (ASDS) “Of Course I Still Love You”.
This was followed by the entry burn, a unique aspect of the Falcon 9 mission profile. Due to the high speeds of entry and the lack of a high-performance thermal protection system, SpaceX has adopted what the industry calls “hypersonic retropropulsion” to protect the first stage as it reenters the atmosphere. This slows the stage from 6 times the speed of sound to about 2.5 times, and provides a much finer trajectory correction to help put the stage on track for recovery.
While all this excitement is happening with the first stage, the second stage has been pushing to orbit, going from about 1km/s to, at this point, around 5.5km/s. (For those of you who don’t know metric, that’s REALLY freaking fast.) The fairing which has protected the ten satellites of the payload has been ejected, as it no longer needed to keep even the most tenuous wisps of atmosphere off the satellites.
Back at the first stage, the four sets of grid fins at the top of the booster have steered the stage to a pre-determined point in space above the ASDS, and the centermost engine lights for a fourth and final time. With a maximum thrust of nearly 100 tons, the engine provides an incredible thrust to weight ratio at this point in the flight. As a result, the engine is throttled back to around 70%, which is still a staggering 70 TONS of thrust… which is still more than the stage weighs!
Careful management of that thrust allows the onboard computer to put the stage at a dead stop right as it touches down on the ASDS, and the stage has successfully been returned to Earth.
High above the Pacific Ocean, the second stage completes most of the push to orbit and shuts down its engine, entering a 30 minute coast phase before a final burn to enter the target orbit, followed by the dispersal of the satellites.
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