Evolution of Rocket Recovery (So Far...)

Ethan Wong

November 8th, 2024

Over a month ago, Starship’s booster was caught in the air by its Mechazilla mechanism. This method is often referred to using “chopsticks,” to catch a rocket, and was executed without any major errors on the first try during a Starship launch back in October this year.

To do this, the Super Heavy Booster utilized cold gas thrusters to alter its position, occasionally refiring some of its engines to help align itself upright and regain stability. Using the Starship live stream of this incredible “landing,” we can see that the booster was not perfectly perpendicular to the ground, and only adjusted itself in the final seconds; however, this was intentional. One of the deterrents in bringing this plan to life was a destroyed launch pad, so the Starship team angled the booster so it wouldn’t hit the launch tower if the process had failed. But since it looked like a successful run, they maneuvered the booster upright and became the talk all around the space (and twitter) community.  

Walter Isaacson posted about the origins of Musk’s Mechazilla method on Twitter, where he showcased a book years ago which discussed landing methods for the Starship booster, similar to the Falcon 9. Musk wondered if the Starship could be caught mid-air using the launch tower rather than adding additional weight with landing gear like the Falcon boosters, and proposed his idea. While originally doubted, the idea eventually sprang into action with the addition of “chopstick arms” (the launch pad already had arms, making this addition easy for SpaceX).  

Today, we’ve begun to observe more rockets utilizing unique recovery methods for their expensive boosters. For instance, SpaceX began landing their Falcon 9 boosters with landing legs, and Blue Origin started working on a self-landing booster after. And reusable rocket boosters aren’t a new idea. Werhner Von Braun released his design of the “Ferry Rocket” in the March edition of the 1952 Collier’s Magazine in his section titled “Man Will Conquer Space Soon.” This proposed reusable booster had two stages equipped with parachutes, while the final third stage was essentially a spaceplane with two sets of tapered wings, mirroring a Space Shuttle. While Braun intended for the spaceplane to carry cargo and crew with 5 engines, landing on the ground rather than in the ocean, he designed the rest of the rocket with over 50 engines–more than Starship, the most powerful rocket today.

The main reason for the increased development in reusable rocket boosters stems down to economic productivity. Space companies can’t throw away these expensive boosters, but also need some way to safely capture, clean and refurbish them for launch. Spending and time/effort is decreased by eliminating the need to rebuild new boosters. As of now, it is much more productive to have self-landing rocket boosters, however, as they indirectly help both space companies and the environment. 

In the 20th century during the age of Space Shuttles and solid rocket boosters (SRBs), NASA had already made these boosters reusable, yet encountered nuisances. In  Dr. R. R. Colwell, and Dr. A. Zachary’s “Study to Determine the Aquatic Biological Effects on the Solid Rocket Booster (SRB),” they highlight the inevitable colonization of microorganisms and other biotic life on the surfaces or SRB and parachutes in the oceans–a process called marine fouling. This problem led to increased time and money to clean and refurbish the equipment. And the nylon parachute’s material lost its tensile strength over time from such deterioration and post-cleaning. The team even assessed the cleaning process of the SRBs and parachutes, finding that power washing with water was partially ineffective, failing to completely remove microorganisms or parts of the “biofouling layer.” They did find more success in cleaning the parachutes with 2% sodium lauryl sulfate detergent, yet realized that this method would cause damage to the materials over time with repeated cleaning. 

NASA researchers did propose a solution involving a self-landing SRB. The design was known as the Liquid Fly Back Booster (LFBB) and was proposed in the 1990s for the Space Shuttle. These boosters would ideally be capable of self-flying and landings. And while regular SRBs required disassembly and cleaning after being transported from the ocean, LRBBs just needed to undergo minor touch-ups after landing themselves. Economically, this was clearly the winner. The LRBB also increased safety for Space Shuttle travel. It could function with the loss of one or more engines, given it had throttle capabilities. Additionally, it used a liquid propulsion system, allowing it to have easy shutdown in comparison to solid propulsion, which couldn’t be turned off. So why didn’t NASA make the switch? The LRBB’s complex designs and new challenges would be a greater hassle than revising the SRB–mainly for safety–which was NASA’s main concern after Shuttle failures such as Challenger.

The development of reusable boosters through self-landing, which has become more popular today, has paid huge dividends toward reducing aerospace’s impact on the environment. For instance, in a research study called “Toxic splash: Russian rocket stages dropped in Arctic waters raise health, environmental and legal concerns” by Michael Byers and Cameron Byers gave a short glimpse into one of the most harmful outcomes (excluding dramatic scenarios and stuff) that could come from rocket descent into ocean waters. The duo looked at the damages of UDMH propellant through Russia’s “Rocket” rocket back in 2017. As the stages fell into the oceans, they still had around 10% excess fuel which was intended as a safety measure for the rocket stages to make trajectory corrections to avoid landing in a populated area. However, when these stages landed in the waters, their extra UDMH led to massive problems.

UDMH propellant is extremely toxic. One popular destination mentioned in the study for these rockets is the Barents Sea, which is home to an abundance of cod fish and other marine life. While landing such rockets could greatly lessen damages to marine life and the waters from UDMH, the overall pollution of UDMH into the atmosphere speaks more toward cleaner propellants, and many space programs have started steering toward other options. For instance, China’s Long March rocket which utilized UDMH for most of their rockets, most recently launched their Long March 5 and 6 rockets with nontoxic propellants: combinations of liquid oxygen (LOX), with either liquid hydrogen or RP-1 kerosene. The propellant exposure is bad for both scientists and engineers manufacturing and preparing the rockets for launch, as well as marine life and nature that might be affected if propellant is released into the waters after descent. As for Starship, its boosters are failed with methane and liquid oxygen (LOX), which are definitely safer than UDMH for marine life and the oceans. However, the rocket booster’s material does still create debris and scraps, replicating the same waste–plastics and metals–from manufacturing. When taking this into consideration, self-landing boosters make more sense as well.

But regardless of if these (now-a-day self-landing) reusable rocket boosters are developed with the intention of saving money, protecting the environment, or both, it’ll be cool to see what new innovation succeeds next.