It’s now clear that SpaceX is preparing to return to the roots of its Starship program with the latest in a series of one-off ‘test tanks’ meant to qualify upgrades to the rocket’s design and production.
Likely known as Starship SN7.2, the hardware will be the first standalone tank built and tested by SpaceX since SN7.1 was successfully pressurized to failure in a process known as burst testing in September 2020. Following in the footsteps of SN7.1, a simple test tank fully focused on qualifying a change in the steel alloy used to build Starships, SN7.2 was somewhat more complex, swapping one of two smooth forward domes with a thrust dome and adding a ‘skirt’ section.
Built out of the same steel alloy as SN7.1, SN7.2 went through similar testing but included the use of a hydraulic ram designed to simulate the thrust of one, two, or three Raptors on the ‘thrust puck’ those engines would otherwise attach to. Starship test tank SN7.2 appears to be quite similar to SN7.1 – but with one or two crucial differences.
The first difference, as noted above, is a reduction in the thickness of the steel rings that make up the outer walls and structure of SN7.2’s barrel-like tank section. SpaceX is believed to have reduced that skin thickness by 25% (4mm to 3mm) in an apparent effort to begin a weight reduction process necessary for Starships to eventually achieve their optimal payload goal of ~150 metric tons (~330,000 lb) to low Earth orbit.
From some angles, SN7.2’s steel rings do appear slightly flimsier or more liable to warp from the heat of welding than other test tanks in the SN7 range, but the differences are rather subtle. Regardless, a reduction from 4mm to 3mm steel rings could likely cut 5-10% from an orbit-capable Starship’s empty weight. When every gram of Starship mass reduction translates directly into an extra gram of payload, it’s safe to say that SpaceX is just getting started.
It’s unclear if a successful SN7.2 test campaign will result in similar reductions to the steel that makes up Starship tank domes and noses. SN7.2’s forward and thrust domes appear to be more or less identical to almost all prior Starship prototype hardware.
Aside from thinner steel skin, it’s also possible that SpaceX will attempt to hit two birds with one stone and test a second unproven change on SN7.2 – namely an upgraded ‘thrust puck’ design. That new puck design first appeared on a November 2020 shipment from SpaceX’s Hawthorne, CA headquarters. Referring to the cone-like structure Starship’s three central Raptor engines attach to and are fed propellant through, the new design simplifies plumbing complexity by allowing Starship’s fuel and fuel header tanks to attach directly to and feed methane through the puck.
It’s unclear which thrust puck design SN7.2 has settled on, though SpaceX’s decision to make SN7.2 an engine section test tank arguably points towards the new puck. Regardless, SpaceX will almost certainly install a skirt section – two reinforced rings – underneath SN7.2 once the tank is welded together, giving it the hold-down clamps needed to secure it to a launch mount while simulating Raptor thrust.