SpaceX’s team of South Texas engineers and technicians have dived into a program of critical pre-hop tests of the first assembled Starship prototype, a partial-fidelity vehicle – known as (Star)Hopper – meant to soon perform low-altitude, low-velocity hop tests powered by Raptor.

Despite a lack of official information is known and SpaceX’s general silence – even to local residents – about Starhopper’s testing, some barebones insight can be derived from what has and hasn’t been done or seen over the past seven days of testing, as well as five apparent wet dress rehearsals (WDRs). To verify the operational integrity of Starhopper and iron out best practices for what is effectively a one-off mobile test stand for Raptor, these WDRs (and one more active test) have seen the unusual prototype filled with some amount of liquid oxygen and methane propellant, taken to flight (hop?) pressures, and generally monitored closely to gather valuable telemetry and judge Starhopper’s condition and hop-readiness. Aside from Hopper, these tests also serve as a shakedown for complex pad and support facilities sprung up from a dirt pile in barely three months.

Fueling the beast

Starhopper’s five (ish) wet dress rehearsal tests have demonstrated an intriguing level of caution relative to the last few months of BFR program development. Depending on how much propellant SpaceX has been filled the vehicle with and how much of that propellant they are able to recycle after each attempt, each dress rehearsal could cost upwards of six figures (USD), while also putting the unusual steel structure through multiple stress cycles.

No official info has been provided beyond a brief indication that SpaceX means to static-fire Starhopper before transitioning to tethered hops, meaning that it’s quite difficult to determine what exactly the testing plan and schedule are. In other words, these ~5 WDR tests could have been the plan all along, or each test could be producing data that has lead launch engineers to scrub Raptor ignition attempts nominally planned at the end of each rehearsal. For an entirely new and unfamiliar design like Starhopper, it seems likely that at least one or two WDRs were planned before any attempt to static fire the hopper’s lone Raptor, although it could also be the case that – much like most SpaceX static fire attempts – the WDR was simply built in as a precursor to ignition, barring off-nominal telemetry.

The third and most visibly active test yet (above) occurred on March 25th and saw Starhopper briefly vent a cloud of gas from Raptor, with some viewers guessing that a Raptor preburner (partial ignition) test had been observed. It’s unclear whether this Raptor (SN02, the second produced) completed acceptance testing in McGregor, Texas on the way from California to Boca Chica. If not, then the caution on display in these WDR tests (i.e. no visible Raptor ignitions) could also be a side-effect of extremely young age of the full-scale engine test program, known (publicly) to have only completed a couple dozen seconds of hot-fire testing in February.

The fidelity of Starhopper relative to its orbit-facing successors is also unclear. If the prototype’s structures, avionics, and plumbing are actually more indicative of the finished product than they appear, it’s possible that SpaceX tendency towards accepting the destruction of test hardware is in a bit more of a cautious state than usual, with a total loss of vehicle amounting to a significant technical setback and schedule delay. Based on the vehicle’s appearance and the apparent decision to entirely set aside the idea of installing a new fairing on Starhopper, it seems far more plausible that the prototype is more of a glorified mobile test stand for Raptor engines and Starship avionics (software) than anything else.

If Starhopper really can’t function as something more than a marginally mobile test stand for Raptor(s), then the value of actually hopping the craft could be quite minimal, perhaps offering useful data on Raptor’s control loop and behavior during flight operations. Still, CEO Elon Musk has stated several times that SpaceX has gotten good enough at the actual task of landing rockets vertically that it’s effectively a known quantity for Raptor and BFR, whereas the exotic atmospheric operations planned for Starship are the main uncertainty for successful recoveries.

Simultaneously, SpaceX is building the first orbital-class Starship prototype just a few thousand feet away from Starhopper’s new roost, utilizing stainless steel sheets almost three times thinner than the quarter-inch-thick steel the first prototype was built out of. It’s likely that Starhopper’s career will thus end up being rather short, given that the completion of the first near-final Starship would further minimize the low-fidelity hopper’s utility. If it’s actually meant to reach orbit, the newest Starship prototype will require the tripod fins and canard wings shown in SpaceX’s latest renders in order to safely land for future test flights, while Starhopper appears to be far too heavy and simplistic to warrant the expensive and time-consuming task of outfitting it with aerodynamic control surfaces and a new nose cone capable of surviving the associated forces.

While additional testing may be done on Friday, March 29th, it appears that the next attempts for the first static fire (and hop tests) will begin next week (likely Monday) – SpaceX is unlikely to test on weekends due to the potential disruption it could cause for beach-going locals.

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