SpaceX Falcon 9 grounded by 'sledgehammer' winds

Inclement high-altitude conditions keep lifter on pad

SpaceX was once again thwarted in its mission to get the SES-9 satellite off the ground from Cape Canaveral yesterday, this time due to "extreme high altitude wind shear", according to Elon Musk.

Lest there be any doubt of the possible effect of such wind shear on the Falcon 9 lifter, Musk elaborated: "Hits like a sledgehammer when going up supersonic."

The SES-9 mission is proving most bothersome for SpaceX. A first launch attempt last week was scuppered by inclement weather. A second pop ended in abort with with 101 seconds left on the countdown clock. Then, on Sunday, the Falcon 9's onboard computers shut down the rocket's nine Merlin engines just after ignition due to a a "low thrust alarm".

Sunday's scrub was caused in part by a tugboat which strayed into the downrange Atlantic Ocean safety zone. The incursion caused a 35-minute launch delay, prompting a rise in the Falcon 9's liquid oxygen (LOX) temperature.

The first Falcon 9 FT launch in December 2015. Pic: SpaceX

The SES-9 Falcon 9 is an enhanced "Full Thrust" model, which first flew - and successfully returned for a vertical landing - in December last year. Its nine first-stage Merlin engines burn kerosene (RP-1) and super-chilled (−207°C) LOX, the latter being a denser alternative to standard LOX, although it's tricky to store and maintain in its super-chilled state.

The Falcon 9 FT's RP-1 is also cooled to around -7°C to increase its density. The result of chilling both fuel components is the ability to cram more propellant mass into the fuel tanks to drive the lifter's performance-enhanced Merlins.

SpaceX is now eyeing a possible fifth SES-9 launch attempt on Friday, LOX temperature, rogue tugboats and sledgehammer wind shear permitting. ®