Amidst the growing buzz centered around the imminent second launch of Falcon Heavy, SpaceX’s South Texas team has continued to work on Starhopper and the first orbital Starship prototype. wrapping up the first major tests of the former and making new progress on the latter’s aeroshell.

For unknown reasons, SpaceX technicians uninstalled Starhopper’s Raptor – the second full-scale engine ever built – shortly after the vehicle’s first true hop test and proceeded to package it up for shipment elsewhere, likely McGregor’s test facilities or the Hawthorne factory. Simultaneously, the third completed Raptor (SN03) was recently installed in McGregor according to photos and observations published by NASASpaceflight.com, preparing to continue to the engineering verification tests that began in February. Once those tests are complete and the engine design is modified to account for the lessons learned with Raptor SN01, SpaceX’s next step will be to begin ramping Raptor production in preparation for multi-engine Starhopper testing and – eventually – the completion of the first orbit-capable Starship prototype.

Needless to say, SpaceX is juggling a lot of interconnected projects in an effort to speed its Starship/Super Heavy (formerly BFR) development program, none of which are being discussed by the company in more than a cursory manner. What follows is thus meant to be an informed but speculative estimate of what is currently going on and what is next for BFR.

Starhopper slips the surly bonds

Over the course of the last two weeks, SpaceX has been almost continuously testing the first integrated Starship prototype, a partial-fidelity vehicle known as Starhopper. The testing primarily involved almost a dozen wet dress rehearsals (WDRs) in which the rocket was filled with some quantity of liquid oxygen and methane propellant and helium for pressurization as engineers and technicians worked through several bugs preventing Raptor from safely operating. According to CEO Elon Musk, some form of ice – potentially methane, oxygen, or even water – was forming in or around parts known as “prevalves”, likely referring to valves involved in the process of supply rocket engines with the right amount of fuel and oxidizer.

Less than 24 hours later, those valve issues were apparently solved as Starhopper’s Raptor ignited for the first time in a spectacular nighttime fireball. 48 hours after that first ignition, SpaceX once again fueled Starhopper and ignited its Raptor engine, lifting a spectacular handful of feet into the air before reaching the end of its very short tethers. According to Musk, the first Raptor ignition was completed with “all systems green”. After the second test, no additional comments were made. Less than three days later, SpaceX technicians uninstalled Starhopper’s Raptor (SN02) and shipped it somewhere offsite, indicating that it may have suffered a fault similar to the one that caused relatively minor damage to Raptor SN01 at the end of its February test campaign. Regardless, it appears that this development will keep Starhopper grounded for the indefinite future barring the imminent shipment of Raptor SN04 or the completion of SN01’s refurbishment.

The Raptor pack grows

Starhopper’s unplanned grounding ties in to the current whereabouts of SpaceX’s ever-growing collection of full-scale Raptor engines, now up to three articles with several additional engines in various stages of completion. According to photos – included in the article below – taken by a member of NASASpaceflight’s L2 forums, Raptor SN03 has been delivered to SpaceX’s McGregor, TX development facilities and installed on the same horizontal test stand that hosted Raptor SN01 and its subscale precursors. Roughly two months after SpaceX first installed and began static-firing Raptor SN01, the arrival of SN03 points to the imminent restart of the engine’s critical early-life test campaign.

While the exact strategy behind SpaceX’s Raptor and BFR propulsion development programs are unclear, a rough outline can be estimated from the company’s earlier Merlin engine development and general best-practices in the well-documented history of rocket propulsion. A huge amount of hot-fire testing is traditionally done with new rocket engines to work out inevitable bugs and optimize engineering as modeling, component-level tests, and subscale prototypes begin to – often imperfectly – mesh with physical reality. It’s quite possible that SpaceX is treating the >1200 seconds it static fired subscale Raptor as the bulk of that development process, with the engine as it is today representing something that the company is extremely confident in.

Regardless, the somewhat buggy behavior exhibited by the integrated Raptor and Starhopper indicate the obvious: both are fairly immature hardware still in some form of development, be it the late (Raptor) or the very earliest stages (Starhopper). By performing even more testing and continuing to optimize and gain familiarity with the hardware at hand, the fairly predictable process of development will arrive at more or less finished products.

Starship’s first orbital prototype

Last but not least, work continues on what will hopefully become the first orbit-capable Starship prototype, built in full-scale out of sheets of stainless steel that are far thinner than the metal used to construct Starhopper. This, too, is a normal process of development – as progress is made, prototypes will gradually lose an emergency cushion of performance margins, a bit like a sculptor starting with a solid block of marble and whittling it down to a work of art. Starhopper is that marble block, with inelegant, rough angles and far more material bulk than truly necessary.

As seen above, the orbital prototype – just the second in a presumably unfinished series – is already dramatically more refined. Instead of the first facade-like nose cone built for Starhopper, Starship’s nose section is being built out of smoothly tapered stainless steel panels that appear identical to those used to assemble the rocket’s growing aeroshell and tankage. As of now, there are five publicly visible Starship sections in various forms of fabrication, followed by a half-dozen or so tank dome segments waiting to be welded together as finished bulkheads.

Intriguingly, the only quasi-public official render of SpaceX’s steel Starship features visible sections very similar to those seen on the orbital prototype’s welded hull. They aren’t all visible in the render, but those that are are a distinct match to the aspect ratio of the welded sections visible in South Texas.

Extrapolating from this observation, Starship, as rendered, is comprised of approximately 16 large cylinder sections and 4-8 tapered nose sections. Based on the real orbital prototype, each large section is 9m in diameter and ~2.5m tall. Assuming Starship is 55 meters (180 ft) tall, this would translate into 22 2.5m sections, a nearly perfect fit with what is shown in the official render. Back in South Texas, SpaceX has 6 tapered sections and 7 cylinder sections in work, meaning that they would reach around 32.5m (~105 ft) – about 60% of a Starship hull – if stacked today.

If we assume that SpaceX follows Falcon procedures to build the seven-Raptor thrust structure separately (~2 sections) and excludes most of the cargo bay (~2-3 sections) on the first orbit-capable Starship, those ~13 in-work sections could be just a tapered nose cone away from the prototype’s full aeroshell. Time will tell…

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