This is a commonly asked question about SpaceX Booster Recovery. Why would SpaceX waste fuel igniting the engines and trying to land vertically rather than using parachutes like the Space Shuttle SRB’s did? There are a number of reasons a propulsively landing system is ideal, but they all tie back to SpaceX’s goal of rapid reusability.
As it happens, SpaceX intended to recover their first rocket, the Falcon 1, using a parachute system. The system would have used a smaller drogue shoot followed by a much larger main parachute. The Falcon 1 would have landed in water to be recovered much like the Space Shuttle’s Solid Rocket Boosters. This parachute system was never demonstrated during Falcon 1’s short life.
The Falcon 1 was only 21 meters tall with a mass of 28,000kg. The current Falcon 9 stands at 70 meters tall and weighs in at 550,000kg. (These are the heights and weights of the full rocket, the booster only makes up a portion of that). With the Falcon 9 being so much larger, a dramatically larger parachute would be required as well. As it happens, SpaceX attempted a parachute recovery for the first two Falcon 9 V1.0 launches but they were unsuccessful as the boosters did not survive reentry.
A different method for surviving reentry would need to be designed, and a more complex deployment system would need to be implemented so the Falcon 9 booster could survive a parachute descent. The mass of the parachute would also impact the amount of payload the Falcon 9 could deliver to orbit. The weight of the complete parachute system would offset the benefit from removing the fuel needed for propulsive landing. Propulsive landing primarily makes use of the merlin engines which are already in place for launch, and could not be removed even if SpaceX had opted for a parachute landing system.
Various Points of Failure
Liquid-fueled engines, like the Falcon 9’s Merlin engines, require a great deal of precision. Late last year, we saw an abort on SpaceX’s GPS III-SV04 mission due to a small amount of masking lacquer blocking a valve in the engine. A parachute-supported splashdown would cause the engines to be flooded with corrosive saltwater. The boosters would require more inspections and cleanings; this would likely have a significant impact on the life of the booster. Additional required cleaning would also decrease SpaceX’s launch cadence. SpaceX has been working to decrease the time between launches as much as possible. They currently have a record turnaround time of 27 days between two launches of the same booster, and time is money to SpaceX. In 2018, SpaceX had one low-speed booster splashdown. During the CRS-16 mission, booster B1050 had a hydraulic issue with the grid fins. The booster had a soft splashdown in water just off the coast of Cape Canaveral, but SpaceX never reused it.
Parachutes also introduce higher turnaround times and points of failure. Parachutes are easily damaged and must be repacked by hand. The heavy parachute deployment system must work perfectly each time. During Boeing’s pad abort test of Starliner in 2019, a missing pin caused one of the parachutes not to deploy. One final nail in the coffin for parachutes is the lack of control. A booster under parachute would be subject to the wind as it floats down, and SpaceX would not be able to land at a specific location.
What about the Fairings?
SpaceX does recover its fairings using a parachute. The fairings are very well suited for a parachute-based recovery due to their low mass (estimated at around 1,000kg each), and high surface area; which creates a very low terminal velocity. This allows each individual fairing to have a lightweight steerable parafoil. In order to get around the accuracy problem, and keep the fairings out of the ocean, SpaceX has been using the boats GO Ms. Chief and GO Ms. Tree to catch the fairings as they are falling. Elon confirmed that both the fairings and the ships run autopilot software to allow the two to meet at the right location for a catch.
Problems with fairing recovery
SpaceX has had some problems as they attempt to catch the fairings. Some have missed the net and been damaged. Even when SpaceX isn’t able to catch the fairings, they attempt to scoop them from the water to be reused. The corrosion from saltwater is not nearly as large a problem for the relatively simple fairings as it is for the complex liquid-fueled boosters. Recently, we saw the ship Shelia Bordelon arrive in Port Canaveral and practice scooping fairings. With the construction being done on GO Ms. Chief and GO Ms. Tree this may be a sign that SpaceX will opt to recover fairings from the water rather than attempting to catch them in the future.
Why did parachutes work for the Space Shuttle?
The Space Shuttle’s Solid Rocket Boosters (SRBs) are very different from SpaceX’s Falcon 9 boosters. The SRBs are essentially just large metal tubes filled with propellant, with a nozzle on one end. They could not be controlled to nearly the same degree that modern liquid-fueled engines can. Being basically a metal tube, there are minimal parts that would be affected by the corrosive saltwater. The Space Shuttle’s SRBs also did not reach nearly the velocity or altitude that Falcon 9 Boosters reach. Nonetheless, the SRBs would impact the water at 51mph. In order to refurbish the boosters, they were dismantled and had to undergo an extensive washing process. This process costs almost as much as building new SRBs. If you would like to read more on NASA’s booster refurbishment, you can do so here.
What about Rocket Lab?
In their “Return to Sender” mission last year, Rocket Lab tested their new parachute recovery system. Rocket Lab recovered the Electron booster from the ocean, but this booster will not fly again. This first test was to gather data on how the vehicle performed on reentry. For future missions, where they plan to reuse the booster, they will be catching the booster from a helicopter. With just a 12 meter tall first stage, the Electron vehicle is minuscule in comparison to the Falcon 9. Even fully loaded at liftoff, the Electron only weighs 12.5 tons. Implementing a similar parachute catch method with the heavier Falcon 9 would require a great deal more specialized equipment.
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