How Would SpaceX Land Its Starship?

 

Back in 2013, SpaceX made a huge announcement that they were going to try land their Falcon 9 rocket back on Earth. To multiply the complexity of this idea manifolds, they were going to try landing it on a floating drone ship in the middle of the ocean. Strange thing like this had never been done before and at the time, it seemed like an impossible mission but if it was accomplished successfully, it would be a massive step forward for rocket reusability. Thus, SpaceX started experimenting with various test vehicles to learn how to land a rocket. Few year later after going through many failures and blasts, SpaceX finally made the impossible, possible. Five years on from that historic first landing, the iconic sight of a Falcon 9 landing has become normal as they do it on a regular basis. Gaining experience from so many launches they had done in past five years, they had perfected their design and this had helped them save 1.4 billion dollars in last five years! Every time they reuse a Falcon 9, they save on the cost of building a brand new booster. As of now, SpaceX is the only company that has landed and reused an orbital rocket booster with other organisations like Blue Origin follow their path.

 Well, they are now focusing their efforts on Starship, the next evolution of crazy space ideas. Those of us who has followed the Starship program have been treated with some incredible visuals. The recent SN15 test flight was one of the craziest things SpaceX have ever done, after a series of failures or partial success, finally they were able to land Starship back to Earth. It has shown us that their ambitious plans for Starship are more realistic than what many of us thought. After all of the insane engineering things that SpaceX has done over the last five years, it seemed like things can't get any crazier. Until Elon bhaiya revealed their new plan to land Super Heavy back on Earth. But there is a twist in the plot, they aren't actually planning to land Super Heavy back on Earth but to catch it in the air itself as it makes its final approach to the landing mount.

 So, we have learnt how SpaceX are going to catch the Super Heavy booster. But in order to know the reason behind it, we need to understand the overall purpose to be served by Starship. Starship consists of two stages: the booster stage called ‘Super Heavy’ which is about 70 meters tall and the 2nd stage which is just called Starship which is about 50 meters tall. Super Heavy will take the 2nd stage out of the thickest parts of the atmosphere and Starship will do the rest of the job to get into orbit. After the  separation, Super Heavy will come back to Earth and land, just like the Falcon 9. The main purpose to be served by Starship is to travel to the Moon and Mars. In order to achieve this, SpaceX will use a huge number of of Starships to refuel in space. Apart from of this, SpaceX also wants to utilise Starship for Earth to Earth passenger travel. This means Starship will need to perform multiple take offs and landings a day with a turn around time on par with or maybe more than the airliners. Previously, SpaceX had planned to land Super Heavy directly onto the launch mount, making it instantly ready for its next flight. Although this idea of landing has many draw backs, it will require a level of precision much greater than Falcon 9 landings. This will require at least six extremely large landing legs with enough capability to absorb the shock while landing. But this is where the problems come in. 

For the Falcon 9 landings, the booster needs to be transported back to a refurbishment hangar where the boosters are inspected for the defects and renewed. Fixing of landing gear/legs are one of the most important part of their refurbishment process but this whole process takes an enormous amount of time which isn’t an option for Super Heavy. So coming up with an idea of removing the legs from the boosters will not only simply the turn around time but also will save a significant amount of weight. In rocket science,there is a saying - that every kilogram of mass saved can be used to carry more payloads into the orbit or the same payload to the higher orbits. With these 6 legs removed , SpaceX can reduce the mass by 10% of the total weight. So with no legs, SpaceX’s only option left is to catch the booster. So, they moved to the idea of landing Super Heavy on a landing pad and using a giant crane to lift it onto the launch mount. This will still require a great amount of precision since it the booster will need to hit its marks almost perfectly. 

 Apart from disadvantages, there are few advantages with Super Heavy compared to the Falcon 9. One of the big advantages Super Heavy has over the Falcon 9 is its ability to hover for a longer time. Falcon 9 is too powerful to hower even with one engine that's why it performs side burn  which involves the firing of the engine at a very exact time, the moment it touches the ground to bring its velocity to 0. As Super Heavy has more engines than the Falcon 9, it can bring down its thrust-to-weight ratio to 1 by turning off the other engines (it may have a total of 42 raptor engines). This will allow it to hover over the landing pad and give it extra time to adjust its position. They will try to catch the booster on its final approach with the help of a giant arm attached to the launch tower. After Super Heavy comes on land, it will thread the needle between these catching arms until they retract inwards and lock into place. The core point about this idea is that the catcher will hook onto the grid fins which will need to take the load of the entire booster. You may think, this seems like an huge amount of force to exert on the grid fins. Well, as since they are already designed to handle a large amount of drag upon re-entry, they are already strong enough in this direction. Though this method will handle most of it but it will still require shock absorption to handle the last bit of energy and bring the booster to a standstill. On the Falcon 9, the landing legs consist of telescopic tubes which are filled with compressed helium gas. As the legs fold in on themselves, the compressed gas exerts outwards force on it and acts as a shock absorber but in the case of Super Heavy main thing is that the shock absorption will be built into the catcher and not the rocket. This will reduce the amount of stress on the rocket, taking away any need for major refurbishment and thus can be quickly re-used after small inspections. SpaceX is likely to use such system of air suspension but on a much larger scale. Another idea could be to use a cable catching system, similar to the ones used on aircraft carriers which uses multiple steel ropes laid out on the runway which are designed to get caught by the tail hook of an aircraft. After the hook connects itself with the wires, the energy is absorbed by hydraulic damping systems beneath the deck, quickly bringing it to a halt. Removing all of this from the rocket and transferring it onto the landing arm will allow Super Heavy to be as light as possible,without putting too much stress on the rocket. Do you think weight saving is only part of the puzzle? Well, the main bonus here is the ability to quickly re-launch the booster. If it can be immediately lowered onto the launch mount after a landing, the booster can be secured, de-tanked and inspected for its next flight. After all, the de-tanking process is especially important for Starship since it will still contain excess methane after it lands. Methane gas isn’t allowed to be released into the atmosphere so, SpaceX plans to pump the leftovers back into the tank farm for it to be re-condensed and used for the next flight. So although we might have to wait a few more years for SpaceX to attempt this crazy idea, it’s inspiring to see them taking on these incredible engineering challenges.