A full-scale Starship rocket has passed a critical test for the first time ever, strongly suggesting that the next-generation launch vehicle could be much closer to orbital readiness than most would imagine.
To be clear, a huge amount of work remains before Starship can be deemed anywhere close to its first orbital flight tests, not the least of which is the fabrication and assembly of the first massive Super Heavy booster(s). However, after Starship SN4’s latest successful May 9th test, it’s hard to see any apparent showstoppers that can’t be handled with a combination of fairly routine testing and iterative progress, as well as time and money. There is certainly room for improvement throughout the program but SpaceX has effectively demonstrated that the biggest practical concerns about its approach to Starship are moot.
Captured live on May 9th and 10th by local resident and photographer Mary (bocachicagal) with the help of NASASpaceflight.com, SpaceX worked for about two days to reconfigure its fourth full-scale Starship prototype after two successful Raptor engine static fires and prepare it for a different kind of test. That work mainly involved removing said Raptor and replacing it with a hydraulic ram stand used to simulate the thrust of 1-3 engines without actually needing to perform a static fire test, further allowing SpaceX to simulate much longer engine operations than its spartan test pad could survive. Around 9pm CDT on May 9th (02:00 UTC, May 10), Starship SN4’s latest trial began.
Known as a cryogenic pressure and load test, it differed from a prior “cryo proof test” completed on April 26th, in which Starship was fully loaded with liquid nitrogen (more than twice as cold as dry ice), pressurized to a bit less than 5 bar (~70 psi), and stressed with hydraulic rams. About a week later, after installing a Raptor engine on a full-scale Starship prototype for the first time ever, Starship SN4 fired up said engine on May 5th – another historic first for the next-generation launch vehicle. 30 hours later, SpaceX performed another wet dress rehearsal (WDR) with liquid methane and oxygen and fired up Starship’s Raptor engine again.
After about 48 hours of reconfiguration, SpaceX moved on to a much more serious cryogenic test. As noted by CEO Elon Musk, the 4.9 bar the rocket previously reached was accepted as enough to perform a Raptor static fire test and possibly enough for a low-stress, low-altitude flight test to ~150m (500 ft). For orbital flight, however, Starship needs to withstand a minimum of 6 bar (~90 psi), while 8.5 bar (125 psi) is preferable to give the rocket the 1.4x safety factor optimal for human spaceflight.
This time, SpaceX – having successfully gathered data from two static fire tests and several wet dress rehearsals – was ready to risk Starship SN4 and pressurized it all the way to 7.5 bar (~110 psi). While ~12% shy of minimum human spaceflight standards, Starship SN4 successfully reached and maintained 7.5 bar while the ship stressed with hydraulic rams to simulate the thrust of three Raptor engines, all of which it survived fully intact. What 7.5 bar does offer, however, is a 1.25x safety factor – on the higher end of aerospace industry standards for uncrewed orbital spaceflight (i.e. cargo/satellite launches).
Ready for orbit?
Technically, this means that – pending much additional testing and verification with different serial prototypes and (likely) higher pressures – Starship’s stainless steel structure is effectively qualified for uncrewed orbital launches. Of course, reality is much more complex. To actually perform and survive orbital flights, SpaceX will first need to build and similarly qualify the first Super Heavy boosters and ensure that those unprecedentedly large rockets can survive and sustain ~20-30 Raptor engines firing simultaneously.
Aside from Super Heavy, it’s unknown if SpaceX has begun testing Raptor engines at the durations they will need to burn to booster Starships into orbit (TBD; likely 5-10 minutes of continuous operation). Along those lines, SpaceX also needs to build, test, and qualify Raptor’s vacuum-optimized sibling to complement the sea level version’s smaller, less-efficient nozzle. Still, Musk has already revealed that RaptorVac could be a matter of weeks from its first static fire and rocket engine development – while incredibly challenging – is more of a known quantity for SpaceX.
Perhaps the most important unknown is whether SpaceX’s recent May 2020 WDRs and static fires have used autogenous pressurization, a more efficient method of pressurizing rockets by using hot gas generated by their own engines. It’s extremely likely that SpaceX has been autogenously pressurizing Starship SN4 for its recent tests, but if that weren’t the case, it would be a big source of schedule uncertainty without significant redesign work.
Ultimately, SpaceX appears to have proven that orbital-class rockets can be built cheaply out of commodified steel in extraordinarily spartan production facilities. Many, many challenges remain but the biggest uncertainty and hurdle facing SpaceX’s Starship program and ambitions is well on its way to being fully put to rest.
