SpaceX wants to unleash Starhopper but longer Raptor test fires come first

SpaceX has reportedly fired a Raptor engine for more than 40 seconds, a necessary step along the path to untethered Starhopper test flights. (SpaceX & NASASpaceflight/bocachicagal)

According to SpaceX CEO Elon Musk, the next round of Starhopper activity will focus on removing the spacecraft prototype’s tethers and performing far more substantial hop tests.

Longer tests demand that SpaceX begins expanding the known performance envelope of its full-scale Raptor engine. Towards that end, longer-duration tests would need to be done at the company’s McGregor, TX development facilities to reduce risk, tests that Musk confirmed are already well underway. A recent Raptor static fire reportedly lasted no less than 40 seconds, more than enough time for a single-engine Starhopper to significantly expand both the maximum altitude and velocity of future hop tests. In support of the upcoming Starhopper test campaign, significant construction work is also ongoing at SpaceX’s Boca Chica test and development facilities.

Starhopper awaits its return to hop testing, April 24th. (NASASpaceflight – bocachicagal)

Unleashing the Hopper

During the months of March and April, SpaceX’s South Texas team effectively completed Starhopper and put the prototype through its first real tests. The process began with tank proof tests in which Starhopper’s tanks were filled with liquid nitrogen – relatively neutral and unreactive – to safely identify and repair any leaks, while also subjecting the vehicle to cryogenic temperatures. The proof testing also put the newly installed ground systems (GSE) and vehicle-pad connection hardware through their paces before moving to Starhopper’s nominal liquid oxygen and liquid methane propellant.

Following at least half a dozen or so wet dress rehearsals (WDRs) that saw Starhopper loaded with LOx and methane, SpaceX technicians analyzed the health of the prototype and soon began live tests with a Raptor engine installed. Designed to produce no less than 2000 kN (450,000 lbf, 205 mT) of thrust at full throttle, Raptor offers more than twice the max thrust of the latest variant of the Merlin 1D engine that powers Falcon 9 and Heavy (941 kN or 212,000 lbf). In other words, a single Raptor should be more than enough to lift Starhopper off the ground 150+ tons of propellant aboard.

In the above tweets, Musk indicates that Starhopper’s next step will be untethered hop tests powered by one Raptor engine. This is further supported by confirmation that Raptor completed its longest static fire test yet on April 27th.

After several unsuccessful test attempts, Starhopper completed two static fires (<10s combined) and hopped – tethered – a handful of feet off the ground on April 3rd and 5th, three weeks after Raptor was first installed. Days later, the lone Raptor engine was removed from Starhopper and shipped back to SpaceX’s Hawthorne, CA factory or McGregor, TX testing facilities for post-test analysis and inspection. In short, SpaceX used Starhopper as a sort of ad hoc test stand for the second serial Raptor (SN02) produced, completing two major acceptance tests simultaneously.

A handful of concise tweets published by Musk in the last few days of April implicitly confirmed that the next steps for Starhopper involved untethered flights off its South Texas pad, once again powered by a single Raptor engine. As both the prospective altitudes and flight times rise for future Starhopper tests, so do the risks posed to SpaceX’s adjacent facilities and the prototype itself. To minimize those risks and progress the Raptor program as a whole, SpaceX has been extensively testing the third serial Raptor (SN03) at its McGregor facilities. Instead of a rushed test regime similar to the one that almost completely destroyed Raptor SN01 less than two weeks after testing began, SN03 is participating in a more cautious and systematic series of tests.

Technicians work inside Starhopper’s LOx tank on April 22nd. (NASASpaceflight – bocachicagal)
While almost impossible to visualize the changes from a flat perspective, several dozen trucks of dirt and concrete were delivered to Starhopper’s pad last week. (NASASpaceflight – bocachicagal)

Confirmed by Elon Musk, this included significantly increasing the length of Raptor SN03’s latest static fires, culminating in an April 27th test that lasted ~40 seconds. Above all else, long test fires are necessary to demonstrate that Raptor can reliably operate for dozens of seconds at a time, given that any failure leading to a loss of thrust could cause Starhopper – basically a controlled explosive device – to fall out of the sky. The famous Musk/SpaceX ethos of moving fast and breaking things does not preclude a pragmatic attitude towards the destruction of facilities and prototypes that could take months and millions of dollars to rebuild.

The ETA of future hop tests is unclear. For the time being, it appears that SpaceX’s South Texas facilities will be caught up in construction work for at least another week. Whether or not Raptor SN03 is next in line for installation on Starhopper, SpaceX will likely put it through several more long-duration static fires before moving ahead with untethered hop tests. All things considered, the rough Starship prototype is unlikely to restart powered testing for another two or so weeks. Stay tuned!

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SpaceX wants to unleash Starhopper but longer Raptor test fires come first
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