Formed in October 2019, Stoke Space secured its first significant round of funding – $9.1 million – less than three years ago. At that time, CEO and co-founder Andy Lapsa says that the startup had just five employees, no permanent workspace, and a “barren field” for a test site. Within 18 months, Stoke Space had turned that empty field into an impressive test facility, conducted numerous component tests, and assembled its first full-scale rocket engine – an exotic UFO-like device unlike any seen before.

It also raised another $65 million – enough funding to begin earnestly developing a potentially revolutionary rocket capable of launching more than 1.65 tons (~3600 lb) into orbit for less than half a million dollars. To realize that extremely ambitious goal, Stoke Space has taken the even more ambitious step of attempting to make the first rocket it develops fully reusable. Simultaneously, the company has incorporated several exotic technologies into that rocket, recently culminating in a surprise announcement that it will attempt to develop one of the most difficult types of engines to power that rocket’s booster stage.

The update that's rolling out to the fleet makes full use of the front and rear steering travel to minimize turning circle. In this case a reduction of 1.6 feet just over the air

— Wes (@wmorrill3) April 16, 2024

Full-flow staged combustion

At the end of an extended interview and tour with YouTuber Tim Dodd (The Everyday Astronaut), CEO Andy Lapsa revealed that Stoke Space has decided to build a full-flow staged combustion (FFSC) engine for the first stage of its reusable rocket. FFSC is the most efficient type of combustion cycle available for a chemical bipropellant rocket engine, but it’s also the most difficult to develop.

A full-flow engine attempts to squeeze every possible ounce of performance out of the propellant it consumes. The most powerful and efficient chemical rocket engines must consume huge volumes of propellant in a short amount of time without destroying the launch vehicle they’re attached to. To create pressure and spin the pumps that are needed to feed that propellant into their main combustion chamber, engines often burn a small amount of propellant in a separate gas generator or preburner. Gas-generator engines vent that exhaust overboard, reducing efficiency but making for a much simpler design. Staged-combustion engines use preburners to create gas that pumps liquid propellant, and that exhaust gas is eventually injected into the main combustion chamber.

Full-flow staged combustion sets itself apart by having two separate pumps and preburners for oxidizer and fuel. Unlike simpler variants of staged combustion, FFSC engines turn all of their propellant into gas before injecting it into the combustion chamber. That hot gas increases the heat of combustion and the pressure inside the combustion chamber, ensuring that virtually all of the propellant that flows through the engine is combusted and turned into thrust as efficiently as possible. FFSC is exceptionally difficult because of the extra-high temperatures and pressures it requires, as well as the need for an oxygen-rich preburner and pump. In a high-pressure, hot-oxygen environment, virtually anything imaginable – including most metals – will spontaneously combust.

Only complex custom-designed alloys can survive those conditions. SpaceX’s Raptor, the only FFSC engine that has ever flown, is especially difficult because it’s meant to be highly reusable. To be successful, Raptor will have to survive those conditions dozens or even hundreds of times in a row with little to no maintenance in between.

The first booster engine Stoke Space ever attempts to build will be a reusable full-flow staged combustion engine powered by liquid methane and liquid oxygen – essentially a smaller version of SpaceX’s Raptor. Stoke’s booster is otherwise familiar and features deployable landing legs like SpaceX’s Falcon boosters. Lapsa says it will likely also have grid fins.

Reusing the upper stage

In some ways, the upper stage of Stoke’s first rocket is even more ambitious. Powered by hydrogen and oxygen propellant, Stoke has designed a conical capsule-like upper stage with an integral fairing. The upper stage’s propulsion is exotic and unique. A large pump will feed propellant to up to 30 combustion chambers distributed around the rim of its heat shield. The exhaust coming from those 30 chambers will expand and partially push against the upper stage’s equally exotic metallic, liquid-cooled heat shield. That expansion against the heat shield improves the efficiency of the upper stage and means that its engine will technically be an aerospike.

Stoke has already begun testing a full-scale version of the upper stage’s UFO-like rocket engine with 15 combustion chambers. Since testing began in the second half of 2022, Stoke has completed dozens of static fires. Everyday Astronaut’s tour also revealed that the startup has made significant progress fabricating and assembling its first full-scale upper stage prototype – tanks, nosecone, heat shield, engine, and all.

Reminiscent of SpaceX’s Grasshopper and Starhopper campaigns, Stoke plans to conduct hop tests with that prototype if it makes it through qualification testing. On February 7th, Stoke also revealed that it’s begun testing a crucial component of its full-flow booster engine. All told, Stoke Space is making progress at a remarkable pace and continues to tackle the hardest problems. The startup has also avoided widely publicizing any specific deadlines, instead choosing to let hardware and tangible results speak for themselves. Only time will tell if that approach pays off, but Stoke is off to an exceptionally impressive start in an industry full of impressive rocket startups.

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Starship – and Raptor, by extension – has yet to reach orbit and is likely years away from scratching the surface of the established success and reliability of the Falcon upper stage and MVac. But compared to MVac, Raptor is more complex, more efficient, more than twice as powerful, experiences far more stress, and is three times younger.

And Raptor 2 isn’t the first version of the engine. Before SpaceX shipped its first Raptor 2 prototype, it manufactured 100 Raptor 1 engines between the start of full-scale testing in February 2018 and July 2021. By late 2021 or early 2022, when Raptor 2 took over, the total number of Raptor 1 engines produced likely reached somewhere between 125 and 150 – impressive but pale in comparison to SpaceX’s Raptor 2 ambitions.

From the start, Raptor 2’s purpose was to make future Raptors easier, faster, and cheaper to manufacture. The ultimate goal is to eventually reduce the cost of Raptor 2 production to $1000 per ton of thrust, or $230,000 at Raptor 2’s current target of 230 tons (~510,000 lbf) of thrust. As of mid-2019, Musk reported that each early Raptor 1 prototype cost “more” than $2 million for what would turn out to be 185 tons of thrust (~$11,000 per ton). It’s not clear if that ever appreciably changed.

In response, SpaceX strived to make Raptor 2 simpler wherever possible, removing a large part of the maze of primary, secondary, and tertiary plumbing. In 2022, CEO Elon Musk confirmed that SpaceX had even removed a complex torch igniter system for Raptor 2’s main combustion chamber. All that simplification made Raptor 2 much easier to build in theory, and SpaceX’s production figures have more than confirmed that theory. Despite those simplifications, SpaceX was also able to boost Raptor 2’s thrust by 25% by sacrificing just 1% of Raptor 1’s efficiency.

Beginning with its first delivery in February 2018, SpaceX produced the first 100 Raptor 1 engines in about 36 months. In the first 11 to 12 months of Raptor 2 production, SpaceX has delivered 200 engines. That translates to at least six times the average throughput, but the true figure is even higher. In June 2019, Musk stated that SpaceX was “aiming [to build a Raptor] engine every 12 hours by end of year.” As is usually the case, that progress took far longer to realize. But in October 2022, a senior NASA Artemis Program official revealed that SpaceX recently achieved sustained production of one Raptor 2 engine per day for a full week.

Such a high rate – likely making Raptor one of the fastest-produced orbital-class rocket engines in history – is required because SpaceX’s next-generation Starship rocket needs a huge amount of engines. The Starship upper stage currently requires three sea-level-optimized Raptors and three vacuum-optimized Raptors, and SpaceX has plans to increase that to nine engines total. Starship’s Super Heavy booster is powered by 33 sea-level Raptors.

Orbital-class versions of Starship and Super Heavy have never flown, let alone demonstrated successful recovery or reuse, so SpaceX has to operate under the assumption that every orbital test flight will consume 39 Raptors. Even after the reuse of Super Heavy boosters or Starships becomes viable, taking significant strain off of Raptor demand, SpaceX wants to manufacture a fleet of hundreds or even thousands of Starships and a similarly massive number of boosters. To outfit that massive fleet, SpaceX would have to mass-produce orbital-class Raptor engines at a scale that’s never been attempted.

But it will likely be years – if not a decade or longer – before SpaceX is in a position to attempt to create that mega-fleet. If the Raptor 2 engines SpaceX is already building are modestly reliable and reusable, and it doesn’t take more than 5-10 orbital test flights to begin reusing Starships and Super Heavy boosters, a production rate of one engine per day is arguably good enough to support the next few years of realistic engine demand.

SpaceX’s first orbital Starship launch attempt could occur as early as December 2022, although Q1 2023 is more likely. SpaceX currently has permission for up to five orbital Starship launches per year out of its Starbase, Texas facilities and will likely try to take full advantage of that with several back-to-back test flights in a period of 6-12 months.

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The first functional Raptor engine delivery in around half a year and the first Raptor V2 delivery ever appeared to arrive at Starbase on March 30th. About a month and a half prior, SpaceX brought an early Raptor V2 prototype damaged during testing to serve as a backdrop for CEO Elon Musk’s February 10th Starship presentation, marking the first time the public was allowed to see or photograph the engine up close.

Less than three months later, Raptor V2 engines that passed proof testing without damaging or destroying themselves have begun to rapidly pile up inside one of Starbase’s three main production tents.

Though Raptor V2 has plenty in common with its Raptor V1 and V1.5 predecessors and, for the most part, looks very similar, Musk has repeatedly stated that the engine represents a major evolution from past Raptors. Most importantly, Raptor V2 was designed to significantly cut production cost and time. To achieve that, almost every major component was either fully redesigned, tweaked, or refined in some way to make Raptor simpler and more compact.

One example is the decision to slash the number of flanges (mechanical joints) in the engine’s plumbing by replacing them with welds. Making plumbing more monolithic could remove dozens of parts, seals, and potential leak points and significantly speed up manufacturing at the cost of making it harder – if not impossible – for SpaceX to inspect and replace certain pipes or pipe sections in a modular manner.

That process was repeated throughout each Raptor system, resulting in an engine that looks more streamlined than earlier variants. As a result of its more refined design and improvements to other critical components, Musk says that even though Raptor V2 now costs about half as much to build as V1.5, it’s also “much more…reliable.”

Despite significantly improving Raptor’s reliability, simplicity, and cost, SpaceX also managed to boost its maximum thrust by almost 25%. Raptor V2 engines now “routinely” operate at record-breaking main combustion chamber pressures of 300+ bar (~4400 psi) and are able to produce up to 230 tons (~510,000 lbf) of thrust at sea level. The older Raptor V1.5 engines that flew on Starships SN8-SN11 and SN15 and were installed on Super Heavy Booster 4 and Ship 20 were designed to produce around 185 tons (~410,000 lbf) at 250 bar (~3600 psi).

Following the premature retirement of Super Heavy Booster 4 (B4), which was meant to help send Starship S20 to space on the rocket’s first orbital launch attempt, that orbital launch debut is now guaranteed to use a different booster and ship powered by Raptor V2 engines. Ship 24 is a strong candidate for the mission’s Starship, while it remains to be seen if SpaceX will fully repair and attempt to proceed with Booster 7 or if Booster 8 – which is almost complete – will take point.

Either way, the pair will need at least 39 qualified Raptor V2 engines to begin integrated testing, pass several major static fire milestones, and prepare for flight. Since SpaceX appeared to kick off Raptor V2 deliveries to Starbase on March 30th, a photo shared by Musk on April 26th revealed that the company has managed to deliver at least 18 of the upgraded engines in the last four weeks. At least one more engine was also delivered on April 28th.

That means that SpaceX already has enough engines to begin static fire tests with a full cluster of 13 central Raptors on Super Heavy B7 or B8. By the time Ship 24 is fully assembled, Booster 7 is repaired, or Booster 8 is completed, there’s a good chance that SpaceX will have all the engines it needs to fully outfit a Starship and Super Heavy pair – not quite by the end of April, as Musk predicted, but not far off.

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In mid-December, Musk revealed even more ambitious plans to upgrade Starship by stretching its propellant tanks and adding another three Raptor engines, potentially boosting the ship’s maximum thrust by 50% and substantially improving payload performance. These latest details are focused on an upgraded version of the Raptor engine and on additional changes to Starship’s structural design and assembly process.

According to Musk, as SpaceX continues to ramp up ground testing of the upgraded engine variant, “Raptor 2 now operates routinely at 300 bar main chamber pressure.” For context, on February 10th, 2019, just days after SpaceX began testing the first full-scale Raptor prototype ever completed, the engine briefly reached a main combustion chamber pressure just shy of 269 bar (3900 psi). That narrowly beat records set by Russia’s RD-270 and RD-180 engines, the latter of which is used on ULA’s Atlas V.

It took 18 months before Musk revealed clear proof that at least one Raptor prototype sustained such high chamber pressures over a minute or more of steady-state operations. The same engine peaked at an impressive 330 bar (~4800 psi), briefly producing 225 tons (~500,000 lb) of thrust and soundly beating out Russia’s never flown RD-701 engine, which crested 290-300 bar in testing. Another ~18 months after that milestone, Raptor isn’t quite operational in the sense of supporting orbital-class launches but the engine isn’t far from its first and has since supported dozens of Starship static fires and seven flight tests – five of which occurred in a period of just six months.

Already, despite the fact that Raptor 1 or 1.5 engines have yet to even attempt an orbital-class launch, SpaceX has almost entirely moved on to a new and improved variant known as Raptor 2. According to Musk, all Raptor ground testing at the company’s McGregor, Texas development campus is now focused on the new hardware, which reportedly features much cleaner plumbing and wiring. The biggest change to Raptor 2, though, is an almost 25% increase in maximum nominal thrust over Raptor 1/1.5 – from around 185 to 230 tons (408,000-507,000 lbf). That’s partially enabled by widening the ‘throat’ of Raptor’s nozzle, which sacrifices a small amount of efficiency for more power density. However, Raptor 2 also contains design improvements throughout to enable sustained, reliable operation at chamber pressures up to 300 bar – 10% higher than Raptor 1.5.

On October 24th, Musk subtly live-tweeted one of the first Raptor 2 static fires, revealing that the engine reached a chamber pressure of 321 bar (~4650 psi) and briefly produced around 245 tons (~540,000 lbf) of thrust before destroying itself. Now, a little over two months later, Musk says that Raptor 2 prototypes are routinely operating at 300 bar without major issues, meaning that they can ignite and safely shut down after burning for several minutes at those pressures. In theory, given that 300 bar is Raptor 2’s targeted chamber pressure at max thrust, that means that the engine is now “routinely” operating at the level SpaceX wants and needs to take Starship to the next level.

It’s likely that one or several months of work remain before SpaceX can begin qualifying the first Raptor 2 engines (or, more importantly, hypothetical Raptor 2 Vacuum or Boost variants) for the first Starship or Super Heavy prototypes designed for the new engine. Nonetheless, the rapid progress SpaceX has made in the first few months of Raptor 2 testing is extremely encouraging.

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The news – first broken by SpaceExplored – comes about a week after CNBC reported that Musk was “shaking up” SpaceX’s leadership by effectively firing its vice president of propulsion due to “a lack of progress” in the development of Starship’s Raptor engine. Now, apparently after taking his first good look ‘under the hood’ in a while, Musk says that “the Raptor production crisis is much worse than it seemed a few weeks ago.” Worse, the CEO has implied that if it “can’t get enough reliable Raptors made [by the end of 2022]…[SpaceX will] face a genuine risk of bankruptcy.”

The email raises both skepticism and several major questions.

First and foremost, can there be any truth to Musk’s claim that SpaceX could go bankrupt because of an unspecified “Raptor production crisis [and disaster]?” Put simply, not really. Musk’s argument is simple enough. According to his estimations, the first-generation (V1) Starlink satellite internet constellation is “financially weak by itself,” which has led SpaceX to develop a much larger, more advanced second-generation (V2) Starlink satellite and constellation that the company’s existing “Falcon [rockets have] neither the [payload] volume nor mass to orbit” to launch. To efficiently launch the Starlink V2 constellation, then, Musk says SpaceX needs Starship to be operational.

Up to that point, nothing in Musk’s email implies that a “Raptor production crisis” could pose any serious harm to SpaceX beyond annoying delays. More than two years ago, Musk believed that Raptor V1.0 already cost less than $1M to produce. As of 2021, SpaceX (again per Musk) is completing an average of one Raptor engine every two days and currently has 35 functional engines installed on Starship and Super Heavy booster prototypes in Boca Chica, Texas. Already, at a rate of one engine every 48 hours, SpaceX’s Raptor production capabilities are theoretically strong enough to fully outfit a significant Starship fleet.

Both stages of Starship are designed to be rapidly and fully reusable and absolutely need to be to efficiently and rapidly launch SpaceX’s Starlink V2 constellation. In theory, a production capacity of ~180 Raptors per year should allow SpaceX to outfit a fleet of three Super Heavies (99 engines) and 13 Starships (72 engines). Even if Super Heavy booster reuse is initially no faster than Falcon (~1 launch per month) and Starship reuse is no faster than Dragon (~3 launches per year), that fleet would be able to launch at least 36 times per years. Even if SpaceX’s former propulsion executives somehow pulled the wool over Musk’s eyes, tricking him into seeing engines that just weren’t there and hiding hundreds of millions of dollars in secret cost overruns from the company’s own accountants, an annual run rate of 100 Raptor engines at a cost of $5 million each would still be able to power a fleet of six reusable ships and two boosters capable of ~20 launches per year.

Musk says that SpaceX will only face the risk of bankruptcy if it “cannot achieve a Starship flight rate of at least once every two weeks next year” – equivalent to 26 launches annually. Again, being deceived for years would be a terrible look but nothing described above appears to have any chance of bankrupting SpaceX. However, the CEO also says that SpaceX “is spooling up” one or several factories to produce “several million” Starlink user terminals (dishes) per year in a process that “will consume massive capital [and assumes] that [Starlink V2 satellites] will be on orbit to handle the bandwidth demand.” He even goes as far as to say that those millions of terminals “will be useless otherwise.”

Once again, while what he describes is an undeniable hurdle for SpaceX, the company is making a choice to “consume massive capital” to “spool up” Starlink dish factories before the constellation capacity needed to take advantage of those dishes has been secured. SpaceX doesn’t need to make such a massive investment so quickly when it could instead split that money with Starship, ensure that Starship and Raptor and Starlink V2.0 satellites are ready or close to ready for routine launches, and then invest heavily in dish production.

For example, just this month, SpaceX raised almost $350M from investors that have a practically bottomless appetite for SpaceX investments. Combined, by the end of the year, SpaceX will have likely raised more than $2.3B in 2021 alone. Valued at more than $100 billion, the company could – as a last resort – feasibly raise double-digit billions in one fell swoop with an IPO. Put simply, the only way SpaceX could ever go bankrupt in the near term would be by consciously letting itself drown in a sea of life preservers.

This is not to say that SpaceX doesn’t have numerous massive challenges ahead of it, nor is it to say that its fundraising potential is truly limitless. Investors could eventually become disillusioned. It’s entirely possible that it will take SpaceX years longer than Musk expects to begin routine Starlink V2.0 launches with Starship. Environmental approvals alone could easily preclude more than five orbital Starship launches in 2022 and potentially prevent regular (i.e. biweekly) launches well into 2023. But the fact of the matter is that unless Elon Musk is telegraphing signs that the rest of the company’s finances are a house of cards, the odds of SpaceX actually going bankrupt anytime soon are vanishingly small. In reality, he’s likely just attempting to (for better or worse) instill some amount of fear and panic in SpaceX employees to encourage them to work more hours and take fewer days off.

Update: Musk has tweeted a brief public comment confirming that he believes bankruptcy is actually an unlikely – but not impossible – outcome for SpaceX.

If a severe global recession were to dry up capital availability / liquidity while SpaceX was losing billions on Starlink & Starship, then bankruptcy, while still unlikely, is not impossible.

GM & Chrysler went BK last recession.

“Only the paranoid survive.” – Grove

— Elon Musk (@elonmusk) November 30, 2021

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This time around, though, there’s reason to believe that the preparations SpaceX is making aren’t a false start and could culminate in one or several record-breaking Starship static fires as early as next week.

SpaceX installed Raptors on Starship S20 for the first time in early August, outfitting the 50m (165 ft) tall prototype with a full six engines as part of a fit test that ultimately saw it installed on top of a Super Heavy booster. Ship 20 only spent an hour on top of Booster 4, though, and was quickly returned to Starbase build site for Raptor removal and final outfitting. Ship 20 was then rolled back to the launch site and installed on one of two suborbital launch mounts and test stands in mid-August, where it has sat ever since.

Between September 4th and 10th, SpaceX then appeared to install three sea-level-optimized Raptor Center (RC) engines and one Raptor Vacuum (RVac) engine on Starship S20 before the prototype had completed any proof testing. Whether that set of installs was a fit test or an aborted attempt at full installation, SpaceX seemingly paused at three or four Raptors and ultimately removed the lone RVac and one or more of S20’s sea-level engines. Another sea-level Raptor was (re)installed on September 15th.

After a frenetic month of back and forth with no obvious rhyme or reason, all of Ship 20’s Raptors were removed and a series of hydraulic rams used to simulate engine thrust – removed, unused, back in August – were reinstalled. Starship then completed pneumatic and cryogenic proof tests in the last few days of September.

After another ten or so days of unusual downtime, SpaceX began reinstalling Raptors on Ship 20 – one sea-level and one vacuum – around October 10th for a static fire test campaign that began about a week later. Finally, on October 21st, SpaceX fired up the orbital-class prototype for the first time, also completing the first test of a Raptor Vacuum engine installed on a Starship. Barely an hour later, Starship S20 performed a second test, simultaneously firing up both RVac and RC engine in another first.

Barely a full day after that successful back-to-back static fire test, SpaceX rolled two more sea-level Raptors to the suborbital pad and installed them on Ship 20. Another unusual week of downtime later and, on October 28th, SpaceX has rolled two more Raptor Vacuum engines from the build site to the launch pad and staged them beside Starship. Once installed, Starship S20 will, for the second time, be fully outfitted with six Raptors. Having already fired up two of those engines without needing either replaced, though, there’s a decent chance that all six will actually be used before Ship 20’s next bout of engine removal/installation deja vu.

SpaceX has never fired more than three engines at a time on a Starship prototype or at its suborbital test site, so a number of firsts potentially lay before Ship 20 as it nears a second round of static fire testing. There is some uncertainty as to whether the suborbital test stands can actually handle the stress from static fires with more than three Raptors, but if they can, then S20 will likely be the first prototype to ignite more 4+ engines and could become the first Starship to fire all six engines at once.

SpaceX currently has one possible test window scheduled from 10am to 6pm CDT on Monday, November 1st, though it could be another week or more before Starship S20’s next static fire attempt if past trends continue.

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It remains to be seen exactly how many engines will be installed on Ship 20 or how many will be ignited during its first static fire test but barring the delivery of more Raptors, signs currently point to an initial test of two engines – one sea-level-optimized Raptor Center (RC) and one Raptor Vacuum with a much larger nozzle. Whenever Ship 20 does fire up those engines, it will be the first static fire of a RVac engine installed on a Starship and the first simultaneous, side-by-side static fire of two different Raptor variants. Since publishing time, SpaceX has cancelled a Tuesday road closure, pushing Starship S20’s first static fire attempt to no earlier than (NET) Wednesday evening.

For the third time in two months, SpaceX has begun installing Raptor engines on its first orbital-class Starship prototype – hopefully for good.

In no uncertain terms, Starship 20’s (S20) path to what could be its last Raptor installations has been about as windy and mysterious as they come. Starship 20 (S20) left the Starbase factory floor for the first time in early August – all six Raptors installed in another program first – for a brief fit check and photo op. After spending about an hour installed on top of Super Heavy Booster 4 (B4), Ship 20 was removed and returned to the build site, where teams removed all six engines and finished wiring and plumbing the vehicle.

Days before the ship’s long-anticipated trip to Starbase’s suborbital launch site for qualification testing, the mount SpaceX prepared for the process quickly had hydraulic rams – used to safely simulate Raptor thrust – were abruptly removed. Starship S20 was then installed on the Pad B mount, where SpaceX proceeded to reinstall six Raptors. Weeks later, after slow heat shield repairs neared completion, SpaceX again removed Ship 20’s Raptors and reinstalled the hydraulic rams it had removed – unused – the month prior. Finally, on September 30th, some seven weeks after the prototype arrived at the suborbital launch site, SpaceX put Starship S20 through its first major test – a lengthy ‘cryoproof’.

Now, ten days after completing a seemingly flawless cryoproof test on its first try, SpaceX has once again trucked multiple Raptors – at least one sea level and one vacuum engine – from the Starbase build site to Starship S20’s suborbital test stand. From the outside looking in, it’s hard not to view the contradictory path S20 took to its first tests – and is still taking to its first static fire(s) – as an unusually visible sign of some kind of internal tug of war or major communication failure between different SpaceX groups or executives.

It’s impossible to determine anything specific beyond the apparent fact that several of the steps taken from Ship 20’s first factory departure to its first cryoproof and static fire tests could have probably been deleted entirely with no harm done and many dozens of hours of work saved. At the end of the day, Starship S20 completed cryoproof testing without issue on the first try and is now seemingly on track to begin its first static fire test campaign later this month.

At the moment, SpaceX has three possible static fire test windows scheduled from 5pm to midnight CDT on Tuesday, Wednesday, and Thursday (Oct 12-14). A similar Monday window was canceled days ago on October 7th, suggesting that more cancellations are probably on the horizon. For now, there’s a chance that Starship S20 – with anywhere from two to all six Raptor engines installed – will fire up for the first time before next weekend. It’s hard to say how exactly SpaceX will proceed. It’s not inconceivable that SpaceX will install all six engines and gradually ramp up to a full six-engine static fire over several tests.

Given that SpaceX has already static fired three Raptor Center (RC) engines on multiple Starship and Super Heavy prototypes, odds are good that Starship S20’s test campaign will be similar – beginning with a three-Raptor static fire, in other words. SpaceX could then add one, two, or all three Raptor Vacuum engines into the fray for one or more additional tests with 4-6 engines total. It’s also possible that suborbital launch mount and pad limitations will prevent more than three engines from firing at once, in which case SpaceX would presumably perform two separate tests of Ship 20’s Raptor Center and Raptor Vacuum engines.

Given that two Raptor variants have never been static fired simultaneously on the same vehicle, it’s hard to imagine that SpaceX won’t also want to perform one or several combined static fires with Raptor Vacuum and Raptor Center engines on Ship 20.

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Known as Starship 20 or S20, the 50m (~165 ft) tall steel rocket prototype has been stationed at one of SpaceX’s two suborbital testing pads since August 13th. No testing has been done, though, and a small army of SpaceX technicians and engineers have instead spent the last three or so weeks effectively turning a collection of steel tanks, tubes, and parts into a functional rocket. While it’s unclear why SpaceX chose to do that outfitting work at an unsheltered launch pad, new activity suggests that it may be almost complete.

Exactly one month ago, SpaceX stacked Starship S20 on top of Super Heavy Booster 4 (B4) on August 6th, briefly creating the largest rocket in history and completing a fit test that was admittedly just as much a photo op. Ship 20 was rapidly destacked and returned to SpaceX’s Starbase factory, where all six of its Raptor engines were removed. About a week later, Ship 20 returned to the pad and has remained installed on Suborbital Pad B ever since.

At the time, the implication was that SpaceX had removed Ship 20’s engines to allow the prototype to complete cryogenic proof testing with hydraulic thrust simulators. However, despite having carefully modified Pad B over several weeks for that exact purpose, those modifications were rapidly removed before Ship 20’s second rollout. Precluding a proof test with thrust simulation, the next logical conclusion was that SpaceX would still perform a cryogenic proof test before reinstalling Ship 20’s Raptors and moving on to a static fire campaign.

Now, even that appears to have been p1recluded. Instead, as if Ship 20 were the second or third or fourth in a series of prototypes, SpaceX rolled three center Raptors to Pad B on September 5th and began installing the engines on Starship on the 6th. It’s hard to say anything with confidence given how chaotically Starship S20’s to-be-determined qualification testing has changed in the last several weeks but, with plenty of uncertainty, Raptor installation implies that the vehicle will perform its first ambient pressure and cryogenic proof tests with engines installed.

It remains to be seen if Ship 20’s three vacuum-optimized Raptor engines will also be installed over the next few days (seemingly the logical assumption) or if SpaceX will instead complete proof tests and center Raptor static fire testing before finally moving into new territory. SpaceX has never static fired more than three Raptors at once and certainly never tested multiple Raptor Vacuum (RVac) engines in close proximity – let alone all six simultaneously.

Meanwhile, much of the focus of the last few weeks appears to have been on finishing Ship 20 plumbing and avionics wire runs, though it’s hard to say exactly what has been done. What is extremely visible and easy to follow, though, is the process of finishing the first orbital-class Starship heat shield and repairing a few hundred tiles broken during its pathfinder installation. SpaceX has installed 500-1000+ tiles on flown Starship prototypes like SN15 but the company has never come close to the ~15,000 needed to cover the entire windward side of the world’s largest rocket upper stage.

SpaceX has undertaken that process for the first time over the last six or so weeks and unsurprisingly seen a number of successes and failures. At some point along the way, a significant fraction of the ceramic, dinner-plate-sized tiles SpaceX technicians installed chipped, broke, shattered, or ran into other fitment issues. Over the last month or so, a great deal of progress has been made fixing those problem tiles and SpaceX has also more or less completed tile installation on the angular ‘aerocovers’ that protect Starship’s flap mechanisms – requiring dozens of custom tiles with complex shapes and curves.

As of September 6th, Starship S20’s heat shield appears to be around 95% complete and the installation of Raptor engines implies that the rocket’s plumbing, avionics, and tankage are also nearly finished. In other words, after many weeks of work, SpaceX’s first orbital-class Starship prototype could be ready to kick off cryoproof and static fire testing just a week or so (and maybe less) from now. Stay tuned for updates!

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After accepting delivery of five new Raptor engines earlier the same day, Super Heavy Booster 4’s (B4) first Raptor rolled out of one of SpaceX’s Starship factory tents around 6pm CDT to form what would become a line of engines awaiting installation. One by one, Raptors were rolled out of that tent and by 5am CDT, an incredible 25 engines had been spotted and installed on Super Heavy. Come shift handoff around 6am, 27 Raptors had reportedly been installed in 12 hours. The two remaining engines likely joined them an hour or two later, marking 29 high-performance rocket engines installed in just 12-14 hours.

Historically, only one other feat of rocketry comes close. In the late 1960s and early 1970s, the Soviet Union was able to install its Saturn V-class “N1” Moon rocket’s 30 engines in just two days. Its NK-15 engines were about the same size as Raptor, approximately 80% as powerful, and collectively formed the most powerful rocket booster ever flown – though N1 never managed a fully successful launch before the program was canceled.

Of course, three of N1’s four test flights ultimately failed as a result (direct or indirect) of challenges with the first stage and its 30 NK-15 engines, marring their ~48-hour installations with the possibility that a less rushed procedure might have prevented issues that later doomed the rocket. The same is somewhat true for Super Heavy Booster 4, given that it’s unclear when the rocket will actually fly. B4 will also need to complete static fire testing – likely making it the most powerful rocket booster ever fired – before its current design and rapid Raptor installation can be considered an unqualified success.

Regardless, given that it was the first time SpaceX has ever attempted to install more than three Raptor engines on a Starship or Super Heavy prototype, managing 29 Raptors in half a day bodes very well for the engine’s ease of installation. Notably, with 29 engines installed, Super Heavy Booster 4 also technically has more thrust potential than any other rocket booster in history. With around 185 tons of thrust each at sea level, a 29-engine static fire could produce almost 5400 tons (~11.9M lbf) of thrust, beating out the Soviet N1 rocket’s ~4630 tons (~10.2M lbf).

SpaceX could reportedly roll Super Heavy Booster 4 to Starship’s nascent orbital launch pad as early as Tuesday, August 3rd, where the ‘launch table’ the rocket might rest on was just installed mere days ago.

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SpaceX began developing Raptor behind the scenes as far back as 2012 and 2013, when a small team successfully tested a full-scale Raptor preburner – a small but important subcomponent – at NASA’s Stennis Space Center (SSC) facilities. Three years later, in September 2016, CEO Elon Musk revealed the first integrated static fire of a Raptor prototype – though it would later become clear that that prototype was a subscale engine about the same size as Falcon 9’s Merlin 1D.

After two and a half years of subscale testing that helped SpaceX refine startup and shutdown sequences and the general operation of what quickly became the world’s most thoroughly tested full-flow staged combustion engine, SpaceX graduated to full-scale testing. Designed to produce about twice the thrust (~200 tons/440,000 lbf) of its subscale predecessors, the first full-scale Raptor engine shipped to SpaceX’s McGregor, Texas test facilities and completed its first static fire days later on February 3rd, 2019.

Notably, the very first full-scale Raptor prototype (SN1) not only survived its first test but lived long enough to complete several more, ultimately reaching SpaceX’s minimum thrust target four days after its first static fire. A vibration issue would soon require several months of troubleshooting and iterative build-test-fail cycles but Raptor was ultimately ready to support its first brief Starhopper hop tests in July and August.

Approximately 15 months after Raptor’s first flight, Starship prototype SN8 successfully lifted off with three engines, one of which performed a near-flawless four-minute burn to apogee. Eventually, six months after SN8’s successful ascent but failed landing, Starship SN15 successfully landed, demonstrating Raptor’s ability to reignite mid-flight. Since SN15’s May 2021 success, SpaceX appears to have completed anywhere from 20 to 35+ new Raptors as part of a dramatic acceleration in production to meet the needs of at least two imminent orbital Starship test flights – both of which will need approximately 35 engines each.

For additional information on Booster 3's engine placement. Refer to this diagram below!

Massive thanks to @StarshipGazer for providing super high resolution and detailed pictures which allowed me to figure these positions out. pic.twitter.com/j1s5qHoGJ2

— Artzius (@artzius) July 21, 2021

Per its label, RB16 – now better known as the 100th Raptor engine overall – is the 16th Raptor Boost engine built by SpaceX. “Boost” refers to the particular variant – in this case, a Raptor engine specifically designed for an outer ring of 20 engines on each Super Heavy booster. Unlike Raptor Center (RC) engines, the outer ring of Raptor Boost engines are fixed in place against the rocket’s skirt and aren’t designed to vector their thrust (i.e. gimbal). According to Musk, all sea level-optimized Raptor engines will ultimately produce approximately 230 tons (~510,000 lbf) of thrust.

Relative to almost any other large-scale engine development program in the last half-century, Raptor’s 29-month 100-engine milestone is an extraordinary achievement. The closest comparable engine is Blue Origin’s BE-4, which is expected to produce up to ~240 tons (~540,000 lbf) of thrust, uses an efficient (albeit slightly less so) combustion cycle, and relies on the same methane and oxygen propellant. Full-scale BE-4 testing began 16 months before Raptor in October 2017 and Blue Origin has reportedly only built and tested nine prototypes in the almost four years since. According to Musk, as of May 2021, SpaceX is now building more than a dozen Raptors – including prototypes and flight engines – every month.

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