On Wednesday, December 9th, SpaceX Starship prototype SN8 finally took to the skies in what was to be a very mixed ascent to around 12.5 km altitude and return to Earth.
The much anticipated flight of the prototype vehicle, weighing approximately 672 tonnes with its partial fuel load, was far more successful than SpaceX had anticipated, even if the vehicle was lost in what SpaceX euphemistically calls a “rapid unplanned disassembly” or RUD.
The first attempt at a launch of the 50m tall vehicle was made on Tuesday, December 8th; but this was scrubbed after a pre-flight engine issue caused an automatic shut-down on all three Raptor motors. The second launch attempt, in the morning of Wednesday, December 9th, was aborted just 2 minutes and 6 seconds before engine ignition when a light aircraft strayed into the no-fly zone around the SpaceX facilities in Boca Chica, Texas.
However, at 16:00 CST (22:00 GMT) that day, the countdown resumed, and at 16:45:26 p.m. CST (22:45:56 GMT), the three Raptor engines on the vehicle ignited and ran up to around 80% thrust, lifting prototype SN8 into the air.
The entire flight was live streamed by SpaceX, with the initial ascent proceeding as anticipated. At 1 minute and 40 seconds into the flight, one of the Raptor engines shut down and gimballed itself away from the remaining two operating motors. 94 second later, a second of the Raptors did the same. At the time, some pundits commenting on the flight speculated the shut-down indicated something was amiss.
In actual fact, both engine shut-downs were planned. As the vehicle was flying with around 1/2 its normal fuel load, and getting lighter at the rate of 2.2 tonnes every second, the engines were shut down to reduce SN8’s thrust-to-weight ratio, naturally reducing its rate of ascent.
Even so, SN8 continued upwards under the thrust of the one remaining Raptor – Number 42, the latest and most modern Raptor engine evolution, with the vehicle’s reaction control system (RCS) firing thrusters around its hull in order to stay upright, until it reached a point where it was effectively hovering.
What happened next was one of the two most incredible sights witnessed in the testing of a space vehicle: as SN8 started to drop vertically backwards, Raptor 42 gimballed to direct its thrust at an angle, working with the RCS system to tip the entire vehicle over until it was falling more-or-less horizontally. At this point, the fore and aft flaps came into their own, working in tandem to hold the vehicle steady, much like a skydiver uses their arms and legs to maintain stability.
This skydive / bellyflop (as some unkindly refer to it) is how a Starship will make a return from orbit. Dropping into the atmosphere with the fore and aft flaps folded back against the hull to minimise their exposure to the fictional heat of atmospheric entry, an operational starship will be protected by heat shield tiles along its underside, after which the flaps fold out, acting as air brakes to slow the vehicle’s velocity as well as keeping it stable.
Dropping back through the atmosphere for almost two minutes, SN8 then completed the second most incredible sight seen in the testing of a spacecraft when, six minutes after launch, two of the Raptor motors re-ignited, using fuel from two small “header” tanks. These, coupled with the vehicle’s RCS tipped SN8 back to an upright position just 200 metres above ground.
The idea had been for the vehicle to then descend tail-first over the landing pad, deploy its landing feet and touch-down. However, it was at this point things went wrong. With just tens of metres to go, one of the two operating engines shut down. For several seconds, the remaining engine fought to maintain vehicle stability, its exhaust plume turning bright green. Seconds later, its landing legs having failed to deploy, SN8 slammed into the landing pad and exploded in the RUD SpaceX thought might occur at some point in the flight.
Initial analysis of data from the flight suggests that the header tanks suffered a pressurisation issue that prevented them pushing sufficient fuel into the two Raptor engines, causing one to shut down completely. The green plume from the second motor is thought to be one of two things: either that a) as the motor was so starved of fuel, it started consuming itself, material inside its turbopumps turning the exhaust green; or that b) as one engine shut-down unexpectedly, the second started gimballing wildly to try an maintain the vehicle’s orientation, and in doing so, smashed its engine bell into the other motor, exposing its copper cooling circuits, which caught fire and turned the exhaust plume green.
Continue reading “Space Sunday: the flight of SN8 and a round-up”
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