1 - Introduction
Starship is a super-heavy launch system currently being developed by SpaceX. The system is designed to carry both crew and cargo to Low Earth Orbit, to the Moon, and to Mars. The design is completely reusable. Starship comprises two elements: the Starship spacecraft and the Super Heavy booster. Both vehicles employ chemical propulsion provided by the methane-fuelled Raptor engines.
2 - Starship Spacecraft
The Starship Spacecraft is one of the two major components of the launch system. It is a cylindrical vehicle 9 m in diameter and 50 m tall, with a conical top. Most of the volume is occupied by the Methane and Oxygen tanks, the former being on top of the latter employing a common bulkhead to reduce mass. Two smaller tanks, called header tanks, hold a small reserve of propellant, ensuring pipes can always access some propellants when lighting the engines. Thus, ignition is achieved even when the ship is not vertical or is in microgravity.
At the very bottom sits the engine bay, containing three sea-level Raptors and three vacuum Raptors; the two engine sets allow the Starship to maneuver in deep space and to land propulsively on Earth. To provide protection from reentry heating, the windward side needs special measures; as such, it is covered in insulating ceramic tiles; these are, where possible, hexagonal in shape and modular. Such measures should greatly simplify maintenance, a lesson from the Space Shuttle program. To provide control during atmospheric flight, two pairs of fins are installed on the forward and aft ends of the ship. They are also coated in ceramic tiles, albeit in custom shapes to conform to their particular geometry.
During reentry, Starship acts unlike any other spacecraft. It descends belly-first, using the four fins to provide control; this configuration is very similar to a skydiver, who controls their attitude using their arms and legs. This unconventional entry profile helps create the maximum amount of drag, something that will be critical in the thin Martian atmosphere. At the last possible moment, the Raptor engines are lit, and the vertical attitude is recovered using fins and engine gimbal. Starship can then land propulsively in a vertical orientation. This is a maneuver where the header tanks are crucial since propellants would settle on the tank walls during horizontal flight. During landing, the engine section swings significantly from left to right, but the payload section at the front moves relatively little.
The vehicles that SpaceX is manufacturing now comprise all of these major elements but mostly need more equipment to carry out a useful mission. Currently, at least four versions are planned.
- The first is a crewed version.
- A cargo version is also planned, capable of delivering large payloads to Earth orbit.
- A tanker version will augment the capabilities of both previous versions, and will be able to refuel other Starships in orbit.
- Lastly, a version specialized for landing on the Moon is in the works as part of the Artemis program. It is speculated that it will be equipped with top-mounted engines to avoid kicking up lunar dust and with a lift to allow astronauts to reach the surface.
3 - Super Heavy Booster
All Starship versions will be lifted most of the way to orbit by the Super Heavy booster. Like the Starship spacecraft, it is 9 m in diameter and uses methane and oxygen as propellants. However, it is considerably bigger, being 69 m tall, and capable of holding 3400 tons of fuel. Super Heavy will be powered by 33 Raptor engines: 3 in the center, 10 in an inner ring, and 20 in an outer ring. The first two sets are normal sea-level Raptors, while the remaining 20 are gimbal-less Raptor Boost. The number of engines has changed in the past and may do so in the future.
The booster will, of course, be reusable and will land propulsively. Like on Falcon 9, a set of grid fins will control the descent through the atmosphere. To simplify the design, the four diamond-shaped control surfaces aren’t deployed only during descent but are fixed throughout the flight. Instead of having a set of landing legs, the booster will be caught by mechanical arms on the launch tower. If this feat can be achieved, it might help simplify the reuse logistics. Like the Starship spacecraft, Super Heavy uses header tanks to ensure propellant flow during landing.
4 - Raptor engines
The beating heart of the Starship system is SpaceX’s Raptor engine. Its first test took place on September 25, 2016 in McGregor, Texas, and since then the company has constantly upgraded it. The engine is meant to provide around 2 MN (200 tons) of thrust, with reasonably high efficiency. The propellant combination of choice is liquid methane and liquid oxygen, often shortened to methalox. This solution is a good compromise between engine performance and propellant density. Liquid hydrogen, which would offer better performance, is much less dense and would require much bigger fuel tanks. As such, methalox was preferred.
Every rocket engine needs considerable amounts of energy to get the propellants from the tanks to the combustion chamber. To do so, small amounts of propellant are burnt to spin turbines, which in turn power the pumps. The exact way in which this is done greatly influences the final performance of the engine. The Raptor engine employs the Full-Flow Staged Combustion Cycle. This means both the fuel and the oxidizer flow into two gas generators. Here, they are burnt with just a small portion of oxidizer or fuel, thus leaving most of each propellant unburnt. Then the fuel-rich and oxidizer-rich mixtures first spin the turbines, then flow into the combustion chamber. Even if this requires much more plumbing, it offers advantages in performance and heat management.
Starship uses three slightly different models of the Raptor engine. First, there is the standard Raptor engine, with a nozzle optimized for operations at sea level. Then there is the Raptor Vacuum, or RVac, with a much larger nozzle optimized to work in a vacuum. Lastly, there is the Raptor Boost, or RBoost, which is optimized for sea level but lacks a gimbal to provide attitude control.
All engines currently produced are the Raptor 2 version, which is an evolved version of the original Raptor. It features great simplifications in plumbing and electronics, making it easier and cheaper to build and operate. Reducing complexity also eliminates possible failure points.
5 - Starbase
Many things set Starship apart from other launch systems. One of these is the place it is being developed in. Instead of development taking place behind closed doors, the several experimental vehicles have been assembled almost in plain sight.
The bulk of this development effort currently takes place at SpaceX’s Starbase facility, situated on the Texan coast next to the Boca Chica village, 3km North of the Mexican border. The facility is split in two: a production site and a launch facility. The production site, as the name suggests, supports the assembly of Starship spacecrafts and Super Heavy boosters from components manufactured elsewhere. The process starts in three big adjacent tents, 115 m long and 45 m wide. Here, steel ring sections are manufactured, Raptor engines are inspected and serviced, and other major components are assembled.
Stacking of the ring sections happens in purpose-built buildings that can contain the very tall finished vehicles. These are the Low Bay (North East of the tents), the Mid Bay, the High Bay, and the Mega Bay (South West of the tents). The latter three are the most used, while the Low Bay can only partially support the assembly of a vehicle. For example, some nosecones are built in the Low Bay.
The production facility also includes several storage areas. One of these is the Rocket Garden, where some old retired vehicles are being preserved. In the vicinity of the production site, SpaceX has also installed a solar farm and tracking equipment.
Testing of all the vehicles takes place at the launch facility 3.5 km down the road. On the western part of the site, there are the two suborbital pads, which were built for flight tests up to 10-15 km in altitude. They are currently used for static fire testing of Starship spacecrafts. The landing pad for past flights used to be in the middle of today’s complex, but it has since been destroyed and built over. The eastern part of the facility houses the orbital pad. Its two major elements are the launch mount, where the Super Heavy rocket sits, and the launch tower.
The latter is used to provide propellant, power, and data to the rocket, and to carry out stacking operations. In order to do so, the tower is fitted with two 30 m long mechanical arms, informally known as “chopsticks”. They can lift a Starship or Super Heavy and place it into position to form the finished vehicle. So far the facility has been used for stacking tests and to static fire Super Heavy. Of course, in the future, the pad is intended to be used for orbital flights. The propellants to support all of these operations are stored in several big tanks just north of the pad.
Credits: RGV Arial Photography
Starbase is not the only major site where Starship development is taking place. SpaceX is currently building a second launch tower at the Kennedy Space Center, to be used for future orbital launches. The structure is located inside the perimeter of LC-39A, which they use for both crewed and uncrewed Falcon 9 launches. So far the construction of the proper launch tower and of the launch mount are both taking place.
SpaceX used to operate another site in Florida, in Cocoa Beach, 15 km west of Port Canaveral. The site saw the construction of the Mark 2 prototype in 2019 and was closed in mid-2020. This was very early in Starship development, even before the current Serial Number designation (SN) was adopted. All development has since taken place in Texas.
6 - All the flights so far
Starhopper is one of the most iconic prototypes, despite having very little in common with the current breed of vehicles. Little more than a tank with a single Raptor engine, it flew a short 150 m hop on 27 August 2019. Nonetheless, it represented a crucial milestone for the Raptor, and also helped test software and validate the unconventional assembly environment. Today, it sits at the launch site in Starbase, where it witnesses the activities of the vehicles that have come after it.
Starhopper 150m test.Credits: Miguel Roberts/The Brownsville Herald
SN5 and SN6 were the following two prototypes to take off for short hops. These were full-sized tanks, althoughl acking a nosecone. This part was replaced by a small mass simulator. Under the power of a single Raptor engine, the ships performed one 150 m hop each, in August and September 2020 respectively. Both vehicles have since been scrapped.
The first high-altitude test flight took place on December 9 2020, when SN8, fitted with a nosecone and three Raptors, flew to 15 km. It then descended horizontally and flipped to a vertical attitude to perform the landing burn.
Unfortunately, one of the engines suffered a malfunction right after ignition, and the vehicle crashed on the concrete landing pad. SN9 attempted a similar flight on February 2nd 2021. She crashed due to the oxygen pre-burner of one of the engines failing during the landing attempt.
One month later, SN10 flew to 10 km, and managed to land in one piece, although not in perfect shape. The prototype hit the ground at an excessive 10 m/s, significantly damaging the landing legs and the engine skirt. It exploded a few minutes after the touchdown.
SN11 attempted to repeat and improve upon that near success on 30 March. However, it was most likely destroied via FTS after reigniting the engines for the landing attempt, while invisible due to heavy fog.
SN10 High Altitude Flight Test- Credits: SpaceX
Starship prototypes SN9 , SN10 and Starhopper at the pad.. Credits: SpaceX
The last time a Starship vehicle has flown so far was on May 5, 2021. On that date, SN15 managed to reach 10 km, descend horizontally, flip back to a vertical orientation, and land propulsively.
The intact vehicle has since been preserved in the Rocket Garden at the Production Site. No other atmospheric flights have since been attempted, and attention has shifted toward an orbital flight test involving the complete system.
7 - Other vehicles
Many other prototypes have been very relevant in the past. The most important of these is probably SN20, which was the first one to be stacked atop a Super Heavy booster. She also performed a static fire, and now resides in the Rocket Garden.
SN15 and SN22 are the other two ships there, the latter being a never-used vehicle. Booster 4 was the first booster to participate in a stacking test, together with SN20, and now resides in the Rocket Garden. The last vehicle there is B8, a test article that has only traveled to the launch site.
SpaceX Rocket Garden. Credits: Deven Perez via Twitter
8 - Future Plans
While current activities often take place in plain sight, details about future plans are much harder to come by. Even the purpose of prototypes being built is often unclear for a long time, and we have seen how SpaceX’s plans can change rapidly. However, it’s pretty clear the next major milestone currently being worked on is an orbital flight test. This would test both the reentry of a Starship spacecraft and the flight of a Super Heavy booster. SN24 and B7 may be the vehicles under testing at the Launch Facility right now, but there is no guarantee they will be the ones used to attempt the test.
After that, the schedule is even foggier. All we know is SpaceX has been awarded a NASA contract to develop and supply the crewed lunar lander for the Artemis Program. This will require the creation of a dedicated Starship variant, to be supported by the tanker one. We can speculate that this process will require many flights, especially for the latter version. Nonetheless, time has shown that no prediction can really be trusted in this context. All we can do is keep watching the developments at Starbase and at the Kennedy Space Center.
Mars Colony. Credits: Deven Perez via Twitter