Reusable rockets - The path to Mars and beyond

The space shuttle was launched with two solid rocket boosters and an external tank in the middle. After reaching a certain altitude, the rocket boosters at both sides of the spacecraft and the external tank would separate from the orbiter. The external tank would burn up in the atmosphere, whereas the two solid rocket boosters would fall back down to Earth and land in the ocean, allowing them to be retrieved and refurbished for later use. The reusable orbiter would land like an aeroplane on a runway after the completion of the mission.

Figure 1: Parts of the space shuttle. Nasa.gov. https://www.nasa.gov/images/content/107907main_STS_parts.gif.

Reusable spacecraft are space vehicles that can be reused with only minor refurbishments required before they embark on their next flight. This is unlike the expendable launch systems, where spacecraft would only be used once before being discarded. The main reason why reusable rockets are as desired as they are is because they reduce the costs of spaceflight by a massive amount. In the words of Elon Musk: “If one can figure out how to effectively reuse rockets just like aeroplanes, the cost of access to space will be reduced by as much as a factor of a hundred.³” This will significantly remove barriers to space and make it much more accessible, clearly demonstrating how the development of reusable spacecraft is integral to the advancement of space exploration.

The space shuttle program was a testament to humanity’s will to explore the universe. Sadly, the program was terminated in 2011, 40 years after its historic first crewed flight. However, the quest for the perfect reusable rocket was far from ending there. Companies such as SpaceX, Blue Origin, and Virgin Galactic are currently continuing the legacy of the space shuttle program by developing their very own reusable spacecraft.

SpaceX currently has the Falcon 9 in operation, a partially reusable rocket with 145 launches under its belt.⁴ It is designed to launch satellites, and eventually crewed spacecraft, into orbit. SpaceX also manufactures and uses the Falcon Heavy, which is the world’s most powerful operational rocket. The Falcon Heavy is a heavy-lift derivative of Falcon 9, combining a strengthened central core with two Falcon 9 first stages as the side boosters.⁵ It is essentially a more powerful version of the Falcon 9 rocket.

Figure 2: Falcon 9. https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2020/11/copernicus_sentinel-6_lifts_off_on_a_spacex_falcon_9_rocket/22340698-1-eng-GB/Copernicus_Sentinel-6_lifts_off_on_a_SpaceX_Falcon_9_rocket_pillars.jpg.

Starship is made up of two stages: the Super Heavy Booster, and the Starship spacecraft itself. Although both stages launch together, the Super Heavy Booster detaches itself from the Starship spacecraft in space. This is achieved by gimballing the booster’s engines, and due to the conservation of angular momentum, the latches between the booster and the spacecraft release, causing the booster to drift away from the spacecraft.⁶ The booster will then return to Earth, with its engines firing when nearing the landing site. The booster aims to land in Starbase, its launch site, allowing it to be ‘caught’ by the mechanical arms of a launch tower, to be prepared for its next flight.

Figure 4: Super Heavy Booster. https://cdn.arstechnica.net/wp-content/uploads/2021/07/ElonMusk_SuperHeavy_CROPPED.jpeg.

The Super Heavy Booster, standing 70m tall, is the first stage and will contain around 30 Raptor engines. Each Raptor engine has more than double the thrust of a Merlin engine, which is used in the Falcon 9.⁷ During descent, four unevenly spaced grid fins, attached to the side of the booster, aid in the booster’s descent and subsequent capture by the launch tower.

Figure 5: Grid Fins. https://image.cnbcfm.com/api/v1/image/106921361-1628017541849-E74epYyWQAIA027-orig_-_Copy.jpg?v=1628020267&w=720&h=405.

Meanwhile, the Starship spacecraft, the second stage, will house 6 Raptor engines, of which 3 will operate in the atmosphere, and the remaining 3 in space.⁸ The spacecraft stands 50m tall and has a dry mass of 85 tons.⁷

During its descent, the spacecraft will travel at approximately 28,000 km/h⁷, which means that it needs a reentry technique that will slow it down as much as possible. To do this, SpaceX has designed Starship to enter the atmosphere belly-first to generate as much air resistance as possible.⁸ However, in order to accomplish this, the spacecraft needs to somehow stabilise itself in that position, and this is done through four flaps that aid in the spacecraft’s orientation. During this descent, kinetic energy will be transformed into massive amounts of heat, so stainless steel has to be used for Starship to survive reentry.⁹ After it has slowed down significantly, the spacecraft performs a complicated manoeuvre involving its flaps and thrusters to align the spacecraft vertically before landing.⁷

Figure 6: Starship. https://upload.wikimedia.org/wikipedia/commons/thumb/e/e8/Starship_SN9_Open_Rear_Flap.jpg/1200px-Starship_SN9_Open_Rear_Flap.jpg.

Needless to say, the potential uses for Starship is limitless! As a matter of fact, Starship has already been included in talks regarding military missions.¹⁰ There have also been talks of using Starship to bring heavy manufacturing equipment to space to be used in the extraction of materials from nearby planets or moons.¹¹ Moreover, Starship could also be used for space tourism, and even replace aeroplanes for local planetary flights.

Elon Musk himself has proposed for Starship to be the vehicle of choice for the arrival of humans on Mars, as part of his goal of making humanity interplanetary, and eventually, interstellar. As Elon Musk has stated: “You want to wake up in the morning and think the future is going to be great - and that’s what being a spacefaring civilization is all about. It’s about believing in the future and thinking that the future will be better than the past. And I can’t think of anything more exciting than going out there and being among the stars.” ¹²