Thursday, May 23, 2024

Starship Faces Performance Shortfall for Lunar Missions

NASAStarship Faces Performance Shortfall for Lunar Missions

In this artist’s impression, one of the Artemis 3 astronauts deploys the Lunar Environmental Monitoring Station (LEMS) while her crewmate collects rock samples. Their Starship Human Landing System is in the background. Credit: NASA.

Perhaps the most frequent topic of conversation for the lunar exploration community is when American astronauts will return to the Moon.  NASA originally aimed to land the first woman and next man on the Moon by 2024, but that date was pushed back repeatedly to September of 2026 at the earliest.  The largest source of uncertainty in the schedule is SpaceX’s ambitious Starship Human Landing System (HLS), which NASA selected to ferry astronauts between the Orion crew vehicle and the lunar south pole.  While the HLS team is making progress with the development of Starship, Elon Musk recently disclosed a serious issue with the current iteration of the vehicle.  Starship is facing a 50% underperformance in terms of the payload which it can deliver to orbit.  If this issue is not rectified, it could have grave implications for Starship’s ability to complete a lunar mission.

Starship ascends above the nearby Boca Chica wildlife refuge during its third Integrated Flight Test (IFT-3). Credit: Mike Killian/AmericaSpace.

NASA understood that it was accepting a degree of risk when it selected Starship as the solitary lunar lander design for Artemis 3 and 4.  In the HLS Source Selection Statement, Kathy Lueders, the former leader of the defunct Human Exploration and Operations Mission Directorate, wrote, “I acknowledge the immense complexity and heightened risk associated with the very high number of events required to execute the front end of SpaceX’s mission, and this complexity largely translates into increased risk of operational schedule delays” [1].  However, the only alternatives were an overweight Dynetics lander and a Blue Origin lander which would need a substantial redesign to achieve NASA’s long-term goal of landing four astronauts on the Moon.  In this context, Starship had significant upside, as it far exceeded the HLS performance requirements.  SpaceX claimed that it could deliver 100 tons of cargo to the lunar surface, which would reduce the operating costs of the Artemis Base Camp.  In addition, SpaceX’s $2.9 billion bid was half the price of its competitors’ landers, since the company could offset Starship’s development cost by also utilizing it to launch satellites.

This rendering shows NASA and SpaceX’s revised design for the Starship Human Landing System, which builds upon lessons learned over the past three years. Credit: SpaceX.

Over the past year, SpaceX and NASA have made noteworthy progress with the development of HLS Starship.  After two failed test flights in April and November of 2023, Starship successfully reached orbital velocity on March 14th.  It became the most powerful rocket to ever reach this milestone.  The flight wasn’t perfect; the reaction control system’s thrusters were unable to control Starship’s attitude in orbit, and it burned up during reentry (although not before capturing spectacular video footage).  The three test flights demonstrate that SpaceX is learning dozens of lessons from each mission and gradually improving the rocket’s reliability.  In February, NASA and SpaceX completed over 200 tests of the docking system which will mate HLS Starship to Orion in lunar orbit.  Leaked renderings, which AmericaSpace analyzed last December, demonstrate that the HLS team is factoring all of these test results into a more robust and capable design for the lander.  However, its ability to reach the Moon in the first place was cast into doubt by an announcement which took place two weeks ago.

SpaceX CEO Elon Musk delivers a comprehensive update on the Starship program on April 4th, 2024. The white object behind him is the base of the HLS Starship mockup. Credit: SpaceX.

On April 4th, SpaceX CEO Elon Musk provided a public update on the Starship program from his company’s facility in Boca Chica, Texas [2].  This was not a novel event; Musk gives an in-depth presentation on Starship on a roughly annual basis.  He provided ample discussion on his visionary goal of establishing a colony on Mars.  Spaceflight media outlets offered glowing commentary on his claimed goals for Starship’s marginal launch cost and payload to orbit.  In reality, the most important announcement was arguably contained within one sentence: 

“Currently, Flight 3 would be around 40-50 tons to orbit.”

To understand the significance of this statement, one only needs to review prior statements about Starship’s performance.  Ever since Musk’s 2017 presentation, Starship’s estimated payload capacity has ranged between 100 and 150 tons to Low Earth Orbit (LEO).  SpaceX’s official Starship Payload Users Guide clearly states that “At the baseline reusable design, Starship can deliver over 100 metric tons to LEO” [3].  For the past six years, Starship’s diameter, height, and propellant mixture have remained constant.  The most straightforward interpretation of Musk’s comment is that the rocket is suffering from a 50% underperformance.

This NASA figure provides a detailed overview of Konstantin Tsiolkovsky’s Ideal Rocket Equation. Credit: NASA Glenn Research Center.

The potential causes of Starship’s performance shortfall can be understood using Russian physicist Konstantin Tsiolkovsky’s famous Rocket Equation [4].  This formula is complex, but in essence, it relates a rocket’s payload capacity to its mass and its efficiency (specific impulse).  Tsiolkovsky illustrated why launching a payload into orbit is an unforgiving challenge.  If the mass of a rocket’s upper stage grows, this must come at the expense of its payload since both elements are placed into orbit together.  An increase in the mass of the first stage has a less draconian impact, but it still will cut into the payload capacity. 

Several Raptor engine failures were experienced during 2023’s first integrated Starship/Super Heavy test flight. To mitigate these issues, SpaceX added additional shielding to the engines. Credit: SpaceX.

This is likely what happened to Starship.  To mitigate the risk that one exploding Raptor engine might cause a cascade of failures, SpaceX installed extra shielding around each of the 33 motors on the Super Heavy booster.  In addition, it installed a steel “hot staging” ring between the booster and the ship, which allows the latter to ignite its engines while the two stages are still attached.  It is worth noting that this component was supposed to increase the performance of the vehicle by 50%; SpaceX has not disclosed whether those gains were realized.  Other additions to the vehicle included components which mitigated the propellant leaks which partially contributed to the failure of the first test flight.  Each additional gram of mass ate into Starship’s payload capacity.

Following Musk’s disclosure of Starship’s current payload capacity, the SLS is still the world’s most powerful operational rocket in terms of payload delivered to orbit. Credit: Mike Killian/AmericaSpace.

To be fair, 40-50 tons is still an impressive payload mass.  Among operational rockets, it is only exceeded by NASA’s Space Launch System (SLS) and SpaceX’s own Falcon Heavy.  With the current design of Starship, SpaceX should be able to deploy the next generation of satellites in its Starlink constellation.  However, from a national perspective, Starship’s most important purpose is not launching satellites, nor is it colonizing Mars.  The Artemis program is arguably the most ambitious and important human spaceflight program since Apollo, and NASA is entirely reliant upon Starship if it wants to land its astronauts on the Moon by the end of the decade.

Starship and its most important near-term destination, the Moon, are seen prior to IFT-3. Credit: Mike Killian/AmericaSpace.

The success or failure of the Human Landing System program will be decided by Starship’s payload capacity. Due to its high dry (unfueled) mass, Starship HLS cannot reach the Moon without first refueling in LEO.  To complete the Artemis 3 mission, SpaceX must therefore implement orbital refueling on an unprecedented scale.  Even on Earth, loading cryogenic propellants into a launch vehicle is no easy feat; if anything, this will be more difficult in space.  Prior to every Artemis mission, a flotilla of reusable Starship tankers will transfer liquid oxygen and liquid methane to an orbiting propellant depot.  The lunar lander will then launch, receive a full load of fuel and oxidizer from the depot, and continue onwards to the Moon.

The probability of success for a multi-flight refueling campaign can be calculated with compound probabilities. Credit: Mike Griffin.

The number of tanker flights which will be required to complete Artemis 3 is hotly debated.  Estimates range from four [5] to nineteen [6] launches of propellant per lunar landing.  Former NASA Administrator Mike Griffin recently noted that the probability of mission success is directly correlated with the number of launches in each refueling campaign [7].  For instance, it is reasonable to assume that each individual Starship launch, plus the subsequent propellant transfer operation, will have a 98% probability of success once the procedure is refined.  If five tanker flights are required, the mission as a whole will succeed in 90% of scenarios.  In contrast, if twenty launches are needed, that probability drops to just 67%. 

Starship lifts off from Boca Chica to begin the IFT-3 mission. Credit: Mike Killian/AmericaSpace.

The precise number of tanker flights depends on several variables, including the Starship launch rate and the rate at which cryogenic propellant boils off to space while the depot is in orbit.  However, no parameter is more important than the vehicle’s payload capacity.  If Starship’s payload mass grows, the number of tanker flights required to complete an Artemis mission will decrease.  Conversely, a reduction in payload capacity will increase the number of propellant launches.

The current iteration of Starship can store 1,200 tons of liquid methane and liquid oxygen in its propellant tanks.  Recent renderings suggest that the lunar lander will be slightly taller, with a propellant load of approximately 1,500 tons.  If each tanker can deliver 100 tons of fuel to orbit as advertised, then it will take 15 flights to complete an Artemis mission.  This number is large, but given SpaceX’s demonstrated ability to scale up to a high cadence of missions, it is not insurmountable in medium- to long-term timeframes.  

Starship dwarfs its human admirers prior to the IFT-2 mission last November. Credit: Mike Killian/AmericaSpace.

However, if SpaceX is only able to launch 50 tons of propellant to orbit inside each Starship tanker, then it will need to launch the world’s largest rocket a staggering 30 times to refuel a single lunar lander.  Two additional launches will be required to place the Starship HLS and the propellant depot into orbit.  To make matters worse, this hypothetical manifest does not take boiloff into account.  Even if NASA and SpaceX achieve their stated goal of a 6-day turnaround between Starship launches, it will take over half a year to stage all of the propellant in orbit.  Several additional flights might be required to replace the oxygen and methane which are lost during this time period.

Even for talented organizations such as SpaceX and NASA, executing a coordinated campaign of 32+ flights seems costly and unsustainable at best, and infeasible at worst.  To extrapolate Griffin’s calculation, it would only have a 52% probability of success even if attempted.  Musk’s statement that Starship can only place 40-50 tons into orbit leaves little room for vacillation.  If Starship’s payload capacity does not increase, it is likely a showstopper for the Artemis program.

The segments of SpaceX’s second Starship launch tower await shipment from a construction facility near Cape Canaveral to the launch site at Boca Chica, Texas. Credit: Jeff Seibert/AmericaSpace.

With all this being said, supporters of lunar exploration should not despair.  Iterative development is a key tenant of SpaceX’s organizational philosophy, and the company’s history demonstrates that the capabilities of its vehicles often improve over time.  Its workhorse Falcon 9 booster has undergone five major block upgrades.  The original Falcon 9 was 157 feet tall, and it could deliver 10.4 tons of cargo to LEO.  The current Block 5 booster features upgraded engines, a reinforced thrust structure, stretched fuel tanks, and landing legs to enable reusability.  Over the course of a decade, Falcon 9’s height increased by 73 feet, while its payload grew by a factor of two.

SpaceX intends to stretch Starship’s fuel tanks and increase the thrust of its engines to increase its payload. Credit: SpaceX.

SpaceX intends to follow a similar model to move a lunar mission back into the realm of feasibility.  During the same address, Musk stated that his company is already designing an upgraded “Starship 2.”  Both stages of the rocket be slightly longer than their existing counterparts.  The ship will be stretched by six feet (1.8 meters); coincidentally, that would make its height equal to that of the latest design for Starship HLS.  Starship 2 will also feature the improved Raptor 3 engine and a new hot staging ring to reduce damage to the booster during stage separation.  Musk said that it should be able to place 100 tons into orbit, which would restore Starship’s intended payload capacity.

33 Raptor engines power IFT-2 uphill on 18 November 2023. Credit: Mike Killian/AmericaSpace.

Starship 2 will eventually be supplemented or replaced by the ungainly-looking Starship 3.  This launcher’s ship and booster will both be substantially stretched.  The complete stack will be a staggering 492 feet (150 meters) tall.  The Raptor engines will receive yet another performance upgrade.  According to Musk, Starship 3 will have a payload capacity of 200 tons to orbit.  This capability would enable it to refuel a Starship HLS with only eight tanker launches.  

It is worth noting that Starship 2 and Starship 3 are not immune to the current design’s issues with parasitic mass.  If the vehicles need to be reinforced to rectify issues discovered during test flights, they may not achieve their performance targets.

The performance of the Raptor engine has increased over time, while its complexity has decreased. Raptor 2 is used on the current iteration of Starship. Credit: SpaceX.

To lift these behemoths off the launch pad, SpaceX will also need to install a more powerful engine on the Super Heavy booster.  The Raptor 3 will produce 22% more thrust than the Raptor 2 engine which propelled the first three flights of Starship.  “Raptor 3 looks very simple,” said Musk.  “A lot of the complexity is hidden, because we have integral cooling channels in many parts of the engine that don’t exist in Raptor 2.  In order to not have a heat shield, it has to be very resilient.”  Raptor 3 entered testing last May.

SpaceX’s roadmap for increasingly capable Starship variants hinges on the success of the new engine.  This might explain why the Government Accountability Office (GAO) listed the development of Raptor as one of Starship HLS’ two largest technical hurdles, alongside orbital refueling, last November [8].  “In a February 2023 interview, HLS officials said that if the Raptor engine operates below performance levels needed to meet mission requirements, thereby delaying engine certification, then it is possible that the new main engine for the Human Landing System will not be ready to support the planned mission in December 2025.”  (After the report was issued, Artemis 3’s launch date was delayed by nine months.)

Starship heads towards orbit during IFT-3. Credit: Mike Killian/AmericaSpace.

It is currently unclear when Starship 2 and 3 will enter service.  After IFT-3, three more examples of the original Starship design remain in SpaceX’s inventory [9].  The improved variants will likely debut after the existing prototypes fly.  Media outlets such as NASASpaceflight [10] and Lab Padre [11] monitor SpaceX’s Boca Chica production facility around the clock.  As of this writing, their cameras have not conclusively identified any Starship 2 parts.  Therefore, the first upgraded ship will likely not fly until late this year or early next year.  Starship 3 likely lies farther in the future, as the performance metrics which Musk presented indicate that it will require yet another engine upgrade beyond Raptor 3.  SpaceX did not respond to a request for comment on when Starship 2 and 3 will enter service.

A cloud of debris expands away from Starship at high speed following the failure of the concrete beneath its launch pad during IFT-1, one year ago today. IFT-1 was the first mission in what has been a troubled, yet promising, development campaign. Credit: Mike Killian/AmericaSpace.

NASA still maintains that Artemis astronauts will land on the Moon by the end of 2026.  However, in January, Musk acknowledged that this milestone might not happen for up to five years [12].  While it is not necessarily fatal for the program, Starship’s performance shortfall provides additional evidence that the Artemis schedule will likely continue to slip.  Musk’s prediction that the next crewed lunar landing will not happen until the end of the decade may be more accurate than not.

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