SpaceX Starship ignites Raptor engine for the first time ahead of hop tests

Starhopper vents aggressively during a March 30th test. (NASASpaceflight - bocachicagal)

SpaceX CEO Elon Musk says that the company’s first full-scale Starship prototype has completed an inaugural hop test in South Texas, igniting its lone Raptor engine and straining against a trio of tethers attached to its legs.

While relatively minor in the scope of SpaceX’s next-generation rocket program, Starhopper’s successful tethered hop now paves the way towards untethered testing in which the terminally suborbital testbed could spend several minutes aloft and reach altitudes as high as 5 km (3 mi). Aside from Starhopper itself, this perhaps marks an even more significant milestone for Raptor, completing the engine’s first successful test-fire as part of an integrated flight vehicle.

Starhopper’s first successful Raptor ignition comes after the better part of two weeks of concerted testing of the integrated prototype, beginning around March 18/19. That testing included 5+ wet dress rehearsals (WDRs) that involved loading the vehicle with a significant quantity of liquid methane and oxygen propellant, verifying the performance of avionics and plumbing, and ultimately attempting to ignite Raptor.

Ironically, less than 24 hours before Starhopper’s successful ignition, SpaceX CEO Elon Musk had noted that the rough prototype, its ground systems, or both were suffering from issues caused by ice formation in propellant valves. Reading between the lines, it’s likely that the issues involved valves on both Starhopper and its ground support equipment ‘sticking’ (i.e. failing to actuate) when commanded. While not usually a particularly large risk for the overall health of the vehicle and pad, uncooperative valves will almost invariably throw a wrench in the gears of attempted rocket operations, particularly when dealing with cryogenic propellants like those used by Starhopper.

As the supercool methane and oxygen inevitably begin to warm after leaving the propellant plant and entering Starhopper’s tanks, a fraction of the liquid will gradually transition into gas and expand, requiring constant venting of the tanks to prevent overpressure events that could damage or destroy the rocket. Falcon 9 and Heavy exhibit this same behavior, as do most other liquid-fueled rockets. This helps to explain the massive venting seen throughout Starhopper’s half-dozen or so WDR tests, as well as large but routine fireballs as excess methane gas was burned off as part of the process of vehicle and pad pressure regulation.

One of Starhopper’s three tethers, April 2nd. (NASASpaceflight – bocachicagal)
Workers labor beneath Starhopper in the days leading up to the vehicle’s first hop test and Raptor ignition. (NASASpaceflight – bocachicagal)

According to Musk, “all systems [were] green” during Starhopper’s most critical test yet. If Raptor and its prototype host are still in good health after their integrated three-second ignition test, SpaceX could attempt several more static fires and tethered hops over the next few days, mirroring the extremely rapid test series observed in February with the first completed Raptor engine.

If all proceeds nominally, it’s possible that SpaceX could begin untethered hop tests in the near future. Regardless, this marks an excellent step forward for the company’s next-generation Starship/Super Heavy spaceship and launch vehicle – all data gathered in this phase will help to optimize and improve the final design of the first orbital vehicles.

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SpaceX Starship ignites Raptor engine for the first time ahead of hop tests
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