On the one hand, NASA is developing its SLS rocket, the most powerful ever built by the agency, responsible for sending astronauts back to the Moon. On the other hand, SpaceX is developing a fully reusable launcher/vessel combo that is also responsible for sending humans into space. But concretely, how do these structures differ?
Once in service, SpaceX's launch system will consist of two stages:the launch vehicle known as the Super Heavy, and the Starship.
The Super Heavy will be powered by 28 Raptor engines, burning a combination of liquid methane and oxygen. As a reminder, the basic principle of a liquid fuel rocket engine is that two propellants – a fuel (methane) and an oxidizer (liquid oxygen) – are brought together in a combustion chamber to be ignited. The flame produces hot gas under high pressure which is expelled at high speed to produce thrust.
This Super Heavy rocket booster will provide vacuum thrust in the range of 3,500 kN, or about 357 tons (about twice as much as Apollo-era rockets ).
At the top of the launcher will be the Starship, itself powered by six other Raptor engines.
The Starship is designed to operate both in the vacuum of space and in the atmospheres of Earth and Mars, using small movable wings to glide toward a desired landing area. Upon approaching said area, the craft flips into a vertical position and uses its engines to perform a powered descent and touchdown. The Starship will also have enough thrust to lift off the surface of Mars or the Moon, overcome their gravity, and return to Earth.
At 118m in size (63m for the launcher, 55m for the ship), the Super Heavy/Starship combo will be a 4,400 ton juggernaut. Both structures will be fully reusable and the entire system is designed to lift over 100 tonnes of payload. This ship will also offer compartments for up to one hundred crew members, as well as additional fuel tanks in the event of refueling in space, which will be essential for long-duration human interplanetary spaceflight.
These structures are maturing very quickly. While the flights of the SN8 and SN9 prototypes are still fresh in memory, their successor, the SN10, is also preparing to take off. The first tests of the Super Heavy, for their part, will also begin this year.
The Space Launch System (SLS) is a launch vehicle developed by NASA since 2011 with the aim of propelling humans to the Moon. At 98 meters high, it will be slightly shorter than the 110-meter Saturn V rockets (Apollo era). On the other hand, this new launcher will be more powerful, producing 15% more thrust during takeoff and climb.
The central stage of the SLS will offer more than 3.3 million liters of liquid hydrogen and oxygen (the equivalent of an Olympic swimming pool and a half ). It will be powered by four RS-25 engines, which are upgraded versions of the US Space Shuttle's SSME engines. Their main difference with SpaceX's Raptors is that they burn liquid hydrogen instead of methane, therefore.
This booster will provide vacuum thrust in the range of 2227 kN, or approximately 232 tons.
The rocket's center stage will also be accompanied by two solid propellant boosters that will provide 80% of the total liftoff thrust. The second stage will be derived from the Delta IV second stage, and will be powered by a single RL-10 liquid propellant rocket motor (smaller and lighter than the RS-25 and already used by ATLAS and DELTA rockets). This one, again, will burn a mixture of liquid hydrogen and liquid oxygen.
Finally, at the very top will be the Orion spacecraft, responsible for transporting crews to the Moon. This capsule will also have its own engine and fuel supply, as well as secondary propulsion systems for the return to Earth.
The SLS is also designed to lift cargo weighing up to 120 tons, which is more than the Super Heavy/Starship. On the other hand, it is not expected that the central stage nor the booster rockets will be reusable with NASA (instead of landing, they will fall back into the ocean). In other words, there is therefore a higher cost with the SLS system, both in terms of materials and the environment.
Finally, much of the technology used with the SLS is inherited from previous missions, reducing research and development time. However, this does not necessarily guarantee the success of a project. And the SLS is the perfect example.
In mid-January, NASA indeed missed the static firing of its launcher. A failure recorded on one of the four main engines - supposed to burn for about eight minutes - unfortunately forced the agency to stop this test after 67 seconds. NASA is scheduled to perform a second engine test at the end of February.
Ultimately, this is a competition between an agency backed by years of testing and experience, but still limited by a fluctuating budget and administrative policy changes, and a relatively new company in the game, but which has already launched more than a hundred Falcon 9 rockets with a 98% success rate, and which has a dedicated long-term cash flow .
So which of SpaceX or NASA is likely to send humans back into space first? On the NASA side, the Artemis 2 mission, which provides for the return of American astronauts to our satellite, is scheduled for August 2023. However, it is a safe bet that this schedule, considered far too ambitious, will soon be relaxed, and therefore delayed.
For its part, SpaceX has communicated its intention to offer the first flights around the Moon as early as 2023. Elon Musk has also said that a crewed Mars mission could take place as soon as 2026. Here again, the timetable is considered very ambitious, but if there is a company capable of surprising us, it is SpaceX.
Finally, it is necessary to clarify one thing. NASA and SpaceX are not competitors. If SpaceX is where it is today, you can thank NASA for that. The US agency is indeed the company's biggest client and biggest supporter. Let's keep that in mind.
