Space Sunday: the flight of SN8 and a round-up

Starship prototype SN8 drops horizontally towards the ground after a flight to 12.5 km altitude, its stability maintained by the fore-and-aft wing flaps. Credit: SpaceX

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. However, 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.

The moment of ignition caught by ground cameras (l) and camera on the hull of the vehicle (top r), and in the engine bay (bottom r). Credit: SpaceX

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.

The first of the Raptor engines shuts down – a planned part of the flight – as SN8 burns through its partial fuel load, so as to reduce its thrust-to-weight ratio. SpaceX

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.

The moment of tip-over: SN8’s Raptor 42, assisted by the vehicle’s RCS thrusters, starts to tip the vehicle over into an horizontal orientation. Credit: SpaceX

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.

SN8 in its skydive mode (l) with exterior cameras (r) showing the forward (top) and aft (bottom) flaps in action. Credit: SpaceX

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.

The unusual green exhaust plume of the single remaining Raptor motor is clearly visible as SN8 almost overshoots the landing pad, and the failed deployment of the landing legs is visible in the image of vehicle. Three second later, the vehicle hit the landing pad and exploded. Credit: SpaceX

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.

However, despite the total loss of the vehicle, SpaceX reported that SN8 achieved more than 95% of its mission goals, providing company with a wealth of data that will be put to use in improving future test flights and help move things ever closer to the first orbital launch of a Starship / Super Heavy booster combination.

The remains of SN8 on the landing pad, the nose cone and forward flaps the largest part of the wreckage, filmed by Mary, aka BocaChicaGal.

As such, SpaceX regard the flight as a success. So much so that on Thursday, December 10th, even while recovery crews were sifting through the remains of SN8 at the landing pad, the company announced it planned to move prototype SN9 from the fabrication High Bay building to the launch stand, possibly as early as Monday, December 14th.

Unfortunately, these plans ground to a halt on Friday, December 11th, when a support stand under SN9 collapsed,  toppling the vehicle sideways to impact the wall of the High bay building. At the time of writing, the extent of the damage done to SN9 (and the high bay building) is unclear, as is what will be required to return the vehicle to an upright position, or the repairs that might be required to get the vehicle back to being ready for transfer to the launch platform, or whether SpaceX will simply move to SN10, which is also currently under construction.

A Quick Round-up of Missions and Launches

Virgin Galactic Sub-Orbital Abort

Earlier in 2020, Virgin Galactic shifted the focus of their operations from their test facilities at the Mojave Air and Space Port in California to Spaceport USA in New Mexico, where they intend to offer fare-paying passengers the opportunity to experience “zero-gravity” in sub-orbital flights about their space planes.

Since then, the lead vehicle in their fleet, the VSS Unity, has been completing a round of final drop-tests from its carrier aircraft intended to lead up to the vehicle’s third sub-orbital test flight (the first two having taken place from Mojave Air and Space Port in 2018 and 2019), in readiness for operational flights to start in 2021.

On Saturday, December 12th, 2020, the company attempted to complete that sub-orbital flight, with VSS Unity being carried aloft by its WhiteKnightTwo carrier / launcher VMS Eve, which climbed to 15,000 metres before releasing the space plane. What should have happened is that once it had dropped clear of the carrier aircraft, the flight crew of pilot C.J. Sturckow and co-pilot Dave Mackay, should have ignited the vehicle’s rocket motor,allowing it to climb to an altitude of 80 km and then glide back to an unpowered landing.

Unfortunately, the motor refused to fire, leaving the crew with no alternative but to abort the flight and make a safe return to Earth. It’s not clear what caused the motor to fail, but Virgin Galactic are convinced they’ll be able to turn around VSS Unity, outfit it with a new motor and make a further attempt without overly disrupting their plans.

ESA’s Space RIDER: Construction Deal

The European Space Agency has finalised a €167 million ($200 million) contract with Thales Alenia Space Italy and Avio to deliver Space RIDER, a robotic space plane, which should make its flight flight in mid-2023.

The Space RIDER vehicle shown in cutaway, showing the open payload bay, forward parasail deployment system and after avionics. Credit: ESA

As I’ve previously reported, Space RIDER is one of the most expensive projects ESA has engaged upon, with €14 billion already having been committed to the project. It is intended to carry 800 Kg to orbit in what is effectively a two-stage vehicle: a lifting body craft that carries the payload in an enclosed cargo bay, and a thrust / power service module. It is designed to be launched atop Europe’s Vega-C vehicle.

Missions with the vehicle will last for around two months, allowing Space RIDER to offer a wide range of orbital opportunities and medium-term space-based experiments, whilst also allowing it to return to a suitable landing facility anywhere along its orbital track. At the end of the mission, the service module is jettisoned, allowing the lifting body to enter the atmosphere, using its shape to naturally survive re-entry and then glide, slow itself to a speed where a parasail can be deployed, allowing the craft to glide back to a horizontal landing.

The Space RIDER concept. Credit: ESA

The programme is directly supported by ten ESA member states.

NASA Announces First Artemis Astronauts and Face More SLS Problems

NASA has selected the first 18 American astronauts who will participate in the Artemis programme to return to the Moon. The selection comprises nine men and nine women – one of whom, the agency is determined, will be the first woman to set foot on the Moon, probably in the first crewed flight there, while two of the selected astronauts, Vic Glover and Kate Rubins, are currently on the International Space Station.

While the first selection are all US astronauts, NASA has stressed that international participation and crew slots for international crew members will be available in the future.

The first Artemis crewed landing on the Moon is supposedly due to take place in 2024. However, it is extremely unlikely that date will be met. For one thing, the programme requires an expenditure of some US $3.3 billion in fiscal year 2021 to develop a crucial part of the mission – the lunar landing vehicle (referred to as HLS – the Human landing System). However, it is unlikely the agency will receive more than US $1 billion it can put to HLS.

Five of the 18-strong selection of US astronauts NASA says will participate in the Artemis programme to return humans to the Moon. (L to r): Jessica Meir, Joseph Acaba, Jessica Watkins, Matthew Dominick and Anne McClain Credit: NASA TV

Furthermore, the Space Launch System (SLS), NASA’s new super rocket and workhorse for Artemis, is facing further slippage in its first test flight.

A “wet dress rehearsal” of the core stage of the first SLS booster was due to take place on December 7th. This would have seen the core stage fully fuelled and run through a countdown that stops just short of engine ignition. Unfortunately, the test – a precursor to the “green run” test of the stage that involves firing all for of the stage’s main engines for the eight minutes they’ll run during a launch – had to be aborted due to an integration issue between the core stage and the fuelling systems that prevented the fuels maintaining their required low temperatures during the loading process.

While not a flaw in the core stage design, the problem means that NASA needs to formulate a new process to ensure the stage’s fuels are loaded at the correct temperatures while on the test stand – thus delaying both the “wet dress rehearsal” and the green run test, both of which could impact the agency’s ability to launch the first SLS vehicle on an uncrewed flight at the end of 2021.

NASA and Boeing Set Starliner Test Flight Test

At the end of 2019, Boeing’s CST-100 Starliner undertook its first uncrewed test flight (see: Space Sunday: Starliner’s first orbital flight), which had been intended to check out the system from launch to  docking with the International space Station and then a return to Earth. The flight had been intended to clear the way for the vehicle to commence crewed flights to the ISS, working alongside SpaceX’s Crew Dragon vehicle.

Unfortunately, due to a software issues, the vehicle was unable to make its rendezvous with the space station, although the rest of the mission goals were achieved.

Following a set of reviews on what went wrong with the first test flight, Boeing has been working with NASA an launch provider United Launch Alliance to make changes to the vehicle and its support systems to ensure similar issues do not occur again.

As a result of this work, NASA and Boeing have announced the additional uncrewed flight of the system, which will attempt to complete those aspects of the first flight that could not be undertaken during the first test flight. It will take place on March 29th, 2021, again using a ULA Atlas 5 rocket that will lift-off from Canaveral Air Force station. The flight will be made at Boeing’s own expense, but if successful, will clear the way for the first crewed flight of CST-100, which is likely to take place in the second half of 2021.

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