SpaceX has scrubbed the first attempted launch of Falcon Heavy Flight 2 due to high upper-level winds deemed too much of a risk to mission success. Thursday, April 11th’s recycled launch window is identical to Wednesday’s, stretching from 6:35pm-8:31pm ET (22:35-00:31 UTC).
According to SpaceX, both the Falcon Heavy Block 5 rocket and its Arabsat 6A satellite payload are in good health and would have been ready to launch on April 10th if the weather had been slightly more cooperative. While seemingly innocuous, something as basic as wind currents can risk the partial or total failure of rockets even as large as Falcon Heavy, The intricacy and stress-optimized nature of Falcon Heavy’s three interlinked boosters make the rocket particularly susceptible to off-nominal aerodynamic stresses, constantly performing a sort of balancing act to keep those boosters flying in a sort of formation at extreme speeds.
While rockets are also susceptible to extreme ground-level wind conditions, upper-level wind violations are far more common, particularly for SpaceX’s Falcon family. Falcon 9 and Heavy are uniquely at the mercy of these conditions due to a fineness ratio (height vs. width) unprecedented at their level of performance. In fact, Falcon boosters are so long, skinny, and mass-optimized that SpaceX actually pressurizes them with nitrogen during extended/unsupported periods in a horizontal orientation. In simpler terms, held near its extremities, Falcon first stages can actually damage themselves under their own weight by bending somewhere in the middle.
Past a certain point, this bending is highly undesirable. The walls of Falcon 9’s lithium-aluminum alloy propellant tanks – also doubling as part of the rocket’s load-bearing structure – are approximately 0.2 in (5mm) thick, around 40% thinner than an iPhone X. Weighing approximately 25,000 kg (55,000 lb) empty yet 550,000 kg (1,210,000 lb) when full of fuel, Falcon 9 is thus a bit like a rolled sheet of printer paper balancing under a textbook while accelerating at several Gs.
The problem is that Earth’s atmosphere is heavily stratified: there are many different horizontal layers of air that can end up moving in very different directions at very different speeds. Imagine for a second that you’re sprinting along a sidewalk but each sidewalk section is actually a treadmill moving slightly left or right. Running along at full speed, you abruptly hit several sections that are rapidly moving, say, right. Bad times are had. For Falcon Heavy (or 9), running into high upper-level winds is much less exaggerated but still ends up having the same effect: the rocket, like the runner, loses control authority and ends up a fair bit more sideways than intended. When rockets find themselves tilting more than a few degrees off of their vector, they start to bend and flex a lot. When rockets more than minutely bend and flex, they have a tendency to crumple and warp to the point that they will literally break into pieces a bit like a partially cooked spaghetti noodle.
Those same forces act on Falcon Heavy quite a bit differently than they do on a single Falcon 9, but the principle remains the same: sideways booster = bad news. The wind wins this round but SpaceX will be ready for another attempt today. Likely a strategic choice, Wednesday’s launch attempt was called off just shy of 20 minutes before propellant loading began, dramatically simplifying the process of turning Falcon Heavy around for another launch attempt.
Tune in later today for another exciting day of Falcon Heavy photos and prelaunch operations as photographers – including Teslarati’s Pauline Acalin and Tom Cross – return to Pad 39A to replace camera batteries and generally ogle an elegant rocket.
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