Amid a flurry of new construction at SpaceX’s Boca Chica facilities, technicians have begun to install thruster pods on Starhopper in anticipation of the prototype’s first untethered flights.
According to CEO Elon Musk, Starhopper’s “untethered hover tests” will begin with just one Raptor engine installed, potentially allowing hops to restart within the next few weeks. SpaceX is currently testing Raptor SN03 (and possibly SN02) a few hundred miles north in McGregor, Texas, just a few hours’ drive south once the engine is deemed flight-ready. Meanwhile, Starhopper itself needs a considerable amount of new hardware before it can begin Raptor-powered flight testing.
A Falcon Raptor-powered Starship
Purely from a visible perspective, the most important component Starhopper is missing is a way to control its attitude and remain stable while under Raptor power, particularly critical for hovering. Enter the aptly-named attitude control system (ACS), essentially a pod of omnidirectional thrusters. SpaceX already happens to have its own extremely mature ACS proven over nearly two dozen successful Falcon 9 and Heavy booster landings, as well as every Falcon upper stage that has ever flown. SpaceX’s ACS is based on powerful nitrogen gas thrusters, known for their white puffs during Falcon 9 booster recovery and landing operations.
On May 6th and 7th, SpaceX began to install what looked like Falcon ACS pods on Starhopper. Curiously, of the two pairs of thrusters now installed, half appear to be taken directly off of older mothballed Falcon 9 boosters, while the other two seem to have been acquired from a Falcon 9 Block 5 rocket. The latter pods may very well have come from Falcon 9 B1050, the booster that unintentionally landed in the Atlantic Ocean last December.
Based on the asymmetric location of the first two pod groups, Starhopper’s ACS will probably use a tripod layout. Additionally, the reason for the thruster pairs – versus Falcon 9’s single pods – is likely simple: Starhopper is far heavier than a Falcon booster. To get the same level of control authority, SpaceX is thus pairing pods together to double the functional strength of Starhopper’s ACS.
This leads smoothly to the installation of two (likely soon to be three) new composite-overwrapped pressure vessels (COPVs). Starhopper already has two COPVs installed on the outside of its upper tank dome, now effectively confirmed to be helium containers needed to pressurize the vehicle’s methane and oxygen tanks. Based on the fact that Starhopper’s new ACS pods appear to have come straight from Falcon boosters, it’s safe to say that the 2 (or 3) new COPVs will supply the hopper’s thrusters with gaseous nitrogen.
The Ugly Starshipling
In general, this is just the latest chapter in the book of the oddity that is Starhopper. With helium tank pressurization and nitrogen ACS thrusters taken straight from Falcon 9, a major facet of SpaceX’s Mars architecture is entirely missing from the prototype. Known as autogenous pressurization, BFR was meant to use gasified versions of its onboard liquid oxygen and methane to pressurize its propellant tanks. In a similar vein, BFR was expected to integrated the same propellant into its ACS. Simply put, helium is simply out of the question if SpaceX wants to realize its reusable Mars transport architecture. Mars does have a minute quantity of nitrogen available in its already very thin atmosphere, but extracting hundreds or thousands of kilograms is impractical in the near-term, particularly if the first Starship have to carry all of their extraction equipment from Earth.
Although Musk has seemingly confirmed that Starship and Super Heavy will use ACS thrusters more akin to the Falcon family’s cold nitrogen gas pods, he did also confirm that autogenous pressurization would be a part of even the earliest iterations of the rocket. The move from carbon fiber to steel tanks likely made a major difference, as carbon composites have extremely limited heat resistance.
Without autogenous pressurization and propellant tanks closer to the thickness of orbit-capable Starships, Starhopper is really more of a mobile test stand for Raptor than anything else. The ungainly vehicle also offers SpaceX engineers an opportunity to test Starship/Super Heavy avionics in flight conditions, particularly with respect to controlling a real Raptor engine on the fly.
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