Late at night on July 22nd, SpaceX’s South Texas team of technicians and engineers were busy testing a small but critical component of Starhopper, a testbed and low-fidelity Starship prototype meant to attempt its first untethered flight test as early as July 24th.

Monday evening’s testing centered around Starhopper’s cold gas nitrogen thrusters, multi-nozzle assemblies that appear to have quite literally been taken off of flight-proven Falcon 9 boosters. For Starhopper, they will act in a similar – albeit significantly reduced – fashion, serving to control the giant steel prototype’s attitude and augment its lone Raptor engine’s own thrust vectoring (i.e. steering) capability.

Although SpaceX has never released official numbers for the thrust of the cold gas thrusters used on Falcon 9 boosters and upper stages, it’s safe to say from their performance that the low-efficiency nitrogen thrusters produce roughly 5 kN (~1100 lbf) of thrust, perhaps up to 10+ kN. For an almost empty Falcon 9 booster, this translates to extremely rapid (sub-10s) flip maneuvers during return-to-launch-site (RTLS) landings.

At the same time, Falcon boosters have two sizes of cold-gas thrusters, with much larger high-performance (>10 kN) pods – located on the larger of the booster’s two raceways – focused on settling the rocket’s propellant after recovery-related coast periods. A duo of smaller 3-axis pods situated on the outside of the interstage serve as true attitude control system (ACS) thrusters, precisely pointing, flipping, and orienting boosters during vacuum operations and partially augmenting grid fin control authority during the late stages of landings. Despite their much smaller size, they still pack an impressive punch and are famous for almost saving tipping Falcon boosters during early (failed) landing attempts.

Starhopper, meanwhile, is dramatically larger than the Falcon 9 and Heavy boosters its tacked-on ACS thruster pods were designed for. It’s hard to know for sure but safe estimates peg the testbed’s dry mass somewhere around 50-75 metric tons (110,000-165,000 lb) thanks to the thick steel it was constructed out of. In other words, Starhopper likely weighs at least twice as much as an empty Falcon 9 booster (~25 metric tons).

To alleviate this mismatch, SpaceX arrived at a hilariously simple and cheap solution: install double the number of grave-robbed Falcon 9 thruster pods on Starhopper and voila! It was that duo of thruster pod pairs that were tested on July 22nd, visibly producing four distinct jets of pressurized nitrogen gas. Whenever Starhopper gets to hopping, those ACS thrusters should help the rocket precisely control its rotation, attitude, and – to a lesser extent – translation, hopefully helping to ensure a successful inaugural hover and divert test.

Scheduled to occur no earlier than Wednesday, July 24th, SpaceX plans to deconflict Cargo Dragon’s CRS-18 launch and Starhopper’s hover test, meaning that they will not happen simultaneously. In the ~70%-likely event that bad Florida weather delays CRS-18 to Thursday, July 25th, the road before Starhopper will be clear for an attempted hover on the 24th. Additionally, also reported first by NASASpaceflight.com, the test is expected to involve a divert, meaning that Starhopper will lift off, hover roughly 20m (65 ft) off the ground, and then carefully travel a few hundred feet East to a recently-constructed concrete pad for a soft landing.

Note they will want to deconflict with CRS-18, so if that launch is still on (dodgy weather) then perhaps hours before, or after launch? OR, *personal wish!!* go from CRS-18 webcast and then pad cameras at Boca Chica on the SpaceX webcast! ?➡️?— Chris B – NSF (@NASASpaceflight) July 23, 2019

This divert was tacitly confirmed by the arrival of a robotic transport mechanism, already used once before to move Starhopper from its build site to the launch pad. If the divert goes as planned, the transport equipment will be used to return Starhopper to its spartan launch mount and ground support equipment (GSE) umbilicals.

If Starhopper survives and Raptor SN06 performs nominally, it’s all but certain that the testbed rocket will be put through a series of increasingly ambitious test flights over the coming months – at least before SpaceX’s first higher-fidelity “Mk 1” Starship prototypes begin their own flight tests. According to CEO Elon Musk, those Starship test hops and flights could begin as few as 2-3 months from now – September or October 2019.

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