Starship to be upgraded to increase LEO tonnage

 From Teslarati

SpaceX CEO Elon Musk says that future Starships – or at least certain Starship variants – are being upgraded with 50% more Raptor engines and stretched propellant tanks.

On December 17th, the CEO revealed the plans, confirming a tweet published three months prior stating that Starship was “begging for an extra 3 engines.” Musk was likely referring to the fact that a 9-engine Starship – combined with upcoming 33-engine Super Heavy boosters – would create a rocket with 42 engines, a number made famous as “the answer to the ultimate question of life [and everything]” in Hitchhiker’s Guide to the Galaxy – both of which the CEO vocally enjoys. As ever, it’s thus almost impossible to tell jokes from serious, consequential plans – as is the case with Starship.

Nonetheless, origination aside, adding another three Raptor engines to Starship – boosting the count from six to nine – and stretching its tanks could be a substantial upgrade.

According to amateur modelers, who are generally able to estimate rocket performance with enough information about its structures, shape, and engines; an optimal nine-engine Starship’s tanks would be stretched about 25% to store an additional 300 tons (650,000 lb) of cryogenic liquid oxygen and methane (LOx/LCH4). That upgraded Starship would have a liftoff mass close to 1600 tons (3.5M lb) and stand about 55 meters (~180 ft) tall – 10% taller than current ships.

At stage separation, close to vacuum, a stretched Starship with three sea-level-optimized Raptors (RCs) and six vacuum-optimized Raptors (RVacs) should produce at least 2000 tons (4.4M lbf) of thrust – and possibly more than 2250 tons (~5M lbf) depending on engine performance. At that upper level of thrust, Starship – an upper stage – would be just 10% less powerful than the first stage of Falcon Heavy, the most powerful operational rocket in the world.

Regardless of its thrust, dimensions, or weight, what matters most is how a stretched, nine-engine Starship would impact that overall rocket’s launch performance. If unofficial modelers are to be believed, the results are significant: compared to a ‘normal’ Starship with a six-engine upper stage and 33-engine booster, the stretched ship could theoretically boost the amount of payload the rocket can launch to low Earth orbit (LEO) from about 150 tons to 220 tons or more (330,000 to 485,000+ lb) – an almost 50% improvement. In fact, per another recent comment from Musk indicating that Starship – unlike almost all other rockets – won’t temporarily throttle down on ascent, the total payload performance could be a bit less than 230 tons (~500,000 lb) – more than 50% greater than a shorter six-engine Starship.

If those estimates are accurate, upgrading Starship with nine Raptors and stretching its tanks is a no-brainer. It might slow development and make all nine-engine ships cost a substantial fraction more but a 50% improvement in payload performance would significantly improve the efficiency of Starship’s more ambitious Moon and Mars launch profiles, which require numerous orbital refuelings.

In effect, a 50% payload increase would allow SpaceX to complete most refueling tasks more efficiently, quickly, and cheaply. Even if the upgrade plans mean that all Starships will be stretched and carry nine Raptors, fully refueling the new Starship variant in LEO could require 7 tanker launches instead of 8-10. If SpaceX doesn’t mind maintaining multiple distinct Starship variants, which appears to be the case, then ships that are exclusively dependent on refueling (Moon and Mars landers in particular) could stay at their current size, with ~1200 tons (~2.6M lb) of propellant storage and six Raptors. A fleet of upgraded Starships could thus refuel their smaller siblings with just 5-6 tanker launches.

However, there’s a good chance that the extra mass required to stretch Starship ~5.5m (~4 tons or ~9000 lb) is minor enough that SpaceX will instead stretch all Starship variants. In fact, for variants like NASA’s HLS Moon lander and future Marsbound Starships, which depend entirely on refueling to reach their destinations, stretched tanks and more propellant storage could increase the amount of payload they could send to the Moon, Mars, and other high-energy destinations by quite a bit. Ultimately, it will be fascinating to hear more details from SpaceX and Musk on how exactly the upgraded Starship design might benefit those operations in the coming weeks and months.

Rocket payload seems to me to rise at the square of size.  A small increase in power and size produces a disproportionate increase in payload, and (since both Starship and its Super Heavy booster will be fully re-usable) a disproportionate decrease in the cost of lifting one kilogram to orbit.  Observe that the increase in fuel needed is small (25% for Starship, 10% for Super Heavy/Starship combined) relative to the 50% rise in payload.  And fuel will represent most of the cost of launching to LEO (low Earth orbit), since both booster and Starship will be re-used.  I haven't estimated how much this improvement will cut the cost of a ticket to Mars, but the decrease should be substantial.  It will also dramatically cut the cost of launching the Starlink internet satellite constellation.

Other aerospace developers spend decades designing and building new craft, and only when the new craft are in production, do they start on improvements and refinements.  SpaceX hasn't even launched either of its mammoth spacecraft to orbit, and yet it is already moving towards much lower per kilogram cost launches.  

A Starship with 3 vacuum engines being loaded onto the Super Heavy booster.
Source: Teslarati