SpaceX Starship saved by ‘burst disk’ after Raptor static fire ends badly

For the second time, a Starship Raptor engine test has caused a secondary fire that severed some of SpaceX's control over the rocket. (NASASpaceflight - bocachicalgal)

Around 7:15 pm local time, SpaceX Starship prototype SN8 fired up one or several Raptor engines for the third time ever, catching onlookers – only expected a dress rehearsal – by surprise. An hour later, CEO Elon Musk revealed that SpaceX had effectively lost control of the rocket.

Unfortunately for SpaceX, this is not the first incident in which a fire led to a loss of Starship control. Back in May 2020, Starship serial number 4 (SN4) – the first full-scale prototype to have a Raptor installed – completed its third successful static fire test. Moments later, a vent line adjacent to the engine section caught fire, burning for a minute or two. Ultimately, likely due to destroyed wiring or plumbing, SpaceX seemingly lost control of SN4 and had to wait a full two days for cryogenic propellant to boil off and evaporate before teams could approach the rocket to inspect, repair, and regain control.

Now, more likely than not, Starship SN8 has suffered a similar – but not identical – failure, cutting some level of control. Elon Musk took to Twitter about an hour after the rocket’s third Raptor static fire, revealing that SpaceX had lose control of Starship’s pneumatics, referring to hydraulic systems needed to operate most of the rocket’s valves. For SN8, that meant nothing but bad news.

As cryogenic liquids (and all things in general) warm up, they expand, taking up more volume. To counteract that never-ending process of cryogenic propellant warming up, boiling, and turning to gas, fresh propellant is almost continually loaded while warmer gas is vented, thus maintaining safe tank pressures. If the ability to vent those gases is lost, the ability to maintain safe pressures goes with it.

As Musk noted above, Starship SN8 thankfully – and unexpectedly – had one or several burst disks installed, referring to single-use mechanical valves designed to open (i.e. burst) above a specific pressure. SN8’s nosecone burst disk did just that, bursting to create an outlet for the pressure building inside the rocket and thus preventing the small nose-based liquid oxygen (LOx) tank from exploding.

A torrent of molten metal pours from Starship SN8’s engine section after a seemingly successful static fire. (NASASpaceflight – bocachicagal)

Unfortunately, the precursor to Starship losing control is a much less positive story. According to Musk, one of the Raptor engines SN8 ignited may have suffered a significant failure, melting one or more critical engine components. It’s unclear how exactly a seemingly contained engine failure evolved into a total loss of Starship hydraulics but it’s safe to say that redundancy will be added and updated designs will be implemented to ensure that a similar failure doesn’t reoccur.

Notably, both unofficial LabPadre and NASASpaceflight.com livestreams clearly showed Starship quite literally dripping molten metal for more than two minutes after the static fire. Whatever the cause of that extremely hot fire, anything that can continuously melt metal for minutes will have almost assuredly ravaged Starship SN8’s aft and the Raptor engines installed therein. It’s nothing short of miraculous that SN8’s main LOx tank wasn’t breached, as well.

Some 80 minutes after ignition and the resultant Raptor fire, Starship SN8’s nosecone ‘burst disk’ thankfully worked as designed, relieving building pressure and avoiding a far more destructive failure. (NASASpaceflight – bocachicagal)

Ultimately, SN8 will likely need extensive repairs – and one, two, or even three replacement engines – before it can safely restart testing and proceed towards its 15 km (~9.5 mi) launch debut. Additionally, SpaceX’s lack of valve control likely means that the company will have to wait at least 24+ hours before workers can safely return to the launch pad and begin those inspections and repairs.

Update: The roadblock was removed around 11pm local time and SpaceX workers appear to have already returned to the pad, signifying that Starship SN8 has been fully detanked and is safe to approach. Inspections and repairs will likely begin as soon as possible.

Eric Ralph: I write about space, among other things.
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