Credit: SpaceX
Watching a rocket launch is fascinating and nerve-racking. The lift-off and the powerful engines defying gravity give us a glimpse of how difficult a rocket launch can be. But as it disappears in the sky, we turn off the transmission and keep going with our own challenges.
Some of us might follow up on all the mission's activities, more when these are novel or are happening for the first time ever. Then, a few days, weeks, or months later, we learn about the crew's return and remember that someone was out in space. This last phase of spaceflight is what I want to talk about today. A phase that might not seem spectacular but that has its own challenges. Unable to see each step makes it difficult to appreciate its dangers and how critical it is to get things just right for the astronauts to land safely at the preselected location. In this case, I will use the recent return of the Axiom-3 mission as an example of how all this works.
Credit: SpaceX
The return story begins after the crew loads the Dragon capsule with all the experiments and hardware they are bringing back, if any -- the Axiom-3 mission brought back about 550 lbs. Soon after or days later, if there is a delay, astronauts put on their space suits and check the procedures for the undocking and landing parameters. From this point forward, the Dragon crew maintains close communication with the ground support teams, which follow every step of the journey and help the crew execute any activity that requires human intervention. In this case, and since the USA portion of the ISS is managed by NASA, the Johnson Space Center in Houston, TX, and SpaceX Mission Control in Hawthorne, CA, follow closely the undocking event.
Once the hatches of the Dragon capsule and ISS close, the capsule starts to depressurize. The process takes about one hour, after which the Dragon capsule autonomously undocks. But before this can happen, the Dragon crew and ground support confirm it is safe to detach at the scheduled time. In this case, the time is selected carefully to ensure communication throughout the return trip and for the capsule to safely reach the landing site.
Credit: NASA/SpaceX
Undocking starts when SpaceX gives the go call, the astronauts lower their space suit visors, and the Dragon capsule autonomously performs the undocking procedures. The first step retracts the ISS umbilical, which gives power to the Dragon capsule while docked to the ISS. The second step releases the 12 hooks holding the hard-docking between Dragon and ISS.
At this point, the ISS and the capsule Dragon start separation. Soon after, and to broaden this separation, Dragon executes two of its four departure burns. The first lasts just 16 seconds, and the second, 3.5 min later, lasts about 21 seconds. These burns move the Dragon capsule out of the "Keep Out Sphere" and the "Approach Ellipsoid." Both of these are imaginary ellipses that indicate safety zones for the ISS. The first is about 200 m in radius, while the second has dimensions 4 km x 2 km x 2 km; both are centered in the ISS. Any spacecraft visiting the ISS can only cross these boundaries after completing the safety procedures of NASA. These procedures are defined to keep the ISS safe from any accident during a docking or undocking maneuver.
Credit: R. Diaz
Dragon does all these maneuvers autonomously, so after crossing the "Keep Out Sphere," the astronauts are cleared to remove their suits, which they will not put on again until they start with the deorbit phase. The last two burns put the capsule in the correct orientation to follow the planned trajectory. After the capsule clears the "Approach Ellipsoid," and because this was not a NASA mission, NASA Johnson Space Center hands over mission follow-up to Space X and Axiom Space, which keeps in touch with the crew throughout the remaining hours of the flight. The duration of the phase between leaving the "Approach Ellipsoid" and starting to deorbit depends on many factors, including the location of the ISS at the time of undocking and the selected landing site.
Credit: SpaceX
During the last hours of flight -- which for Axiom-3 were 40 hours -- the Dragon capsule continues moving into position for reentry, executing several downhill phasing burn firings of the Dragon's Draco thrusters and adjusting the spacecraft's orbit. Throughout the time and before initiating the deorbiting phase, the SpaceX and Axiom-3 ground crew monitor the capsule systems and are available to answer any questions the astronauts might have; other than that, the Dragon crew can then relax for the rest of the trip. About one hour before splashdown, the astronauts and the ground crew of SpaceX reestablish formal communications and together review and confirm each of the remaining steps that will bring the crew safely back home. At this point, the astronauts have their space suits on and sit in their positions. The ground crew continues monitoring all the systems in the Dragon capsule, the weather at the landing site, and coordinating with the recovery crew that will retrieve the capsule after the splashdown.
Credit: SpaceX
Several autonomous tasks continue while the crew travels to the reentry location, like isolating the thermal control system fluid loops from the radiator to keep the internal temperature of Dragon and the crew's spacesuits comfortable during reentry. Another task is claw separation -- the first step in the trunk separation and jettison. The trunk is the cylindrical-unpressurized part of the spacecraft with solar arrays giving power to the Dragon capsule after leaving the ISS, and the claw connects the trunk to the capsule, delivering power, telemetry, and fluids. Once the trunk separates, it falls and burns in reentry to Earth. From this point forward, the Dragon capsule runs entirely on battery power. The separation also reveals the bottom section of the Dragon capsule, where the heat shield is located.
The next step uses the Dragon's forward Draco thrusters located in the cone section of the dragon. These perform a deorbit burn and put the capsule in the precise trajectory to the splashdown zone, in this case, off the coast of Daytona, FL. Once completed and not needed anymore, the cone protecting the forward bulkhead, closes and locks in preparation for reentry.
The deorbit phase lasts about 9 minutes when the vehicle orients to enter the atmosphere and starts slowing down from orbital velocity, approximately 17,500 mph. The friction of the spacecraft pushing through the air at high speed builds up a layer of super-hot plasma around the vehicle. This plasma reaches about 3,500 Fahrenheit in temperature, mainly felt by the heating shield at the bottom of the capsule. The hot plasma phase lasts about 7 minutes, resulting in a loss of communication with the crew, sometimes referred to as LOS, or Loss Of Signal. The communication resumes once the plasma has dissipated. Nitrox, a mixed breathable gas also used in scuba diving, keeps the temperature inside the cabin and the space suits at a pleasant level.
When SpaceX reestablishes communication with the Dragon capsule, its speed is about 350 mph. Because the Axiom-3 landing was during the daytime, this is when the ground crew started seeing the capsule appearing in the sky. Its location becomes evident once the parachutes deploy -- about two minutes later -- and slow down Dragon to 15 mph in preparation for a soft splashdown on the Ocean. The first set of parachutes is a pair called the drogue parachutes. These are small parachutes designed to stabilize the Dragon capsule and begin to slow it down. Less than a minute later, when the capsule is at about 6,500 ft of altitude and traveling at about 119 mph, another four parachutes deploy. These are the main parachutes, which are orange and white and further slow down the vehicle over the following 3 minutes.
Credit: SpaceX
Credit: SpaceX
Credit: SpaceX
The splashdown is at a predefined location, where the recovery teams are pre-positioned to retrieve the dragon capsule immediately -- in this case, the recovery vessel Shannon came to pick up the Axiom-3 crew. The recovery process takes about 30 minutes, starting with a ground crew reaching the Dragon capsule on board two fast boats. Upon arrival, they perform some ordinance checks, ensuring the vehicle is safe to approach. After capturing the capsule, and while awaiting Shannon to arrive, they climb onto the Dragon capsule and start rigging operations to prepare it to be lifted onto the vessel. They also ensure that the area and the vehicle are safe to continue hoisting operations. Simultaneously, other SpaceX ground crew also recovers the parachutes. Inside the capsule, the commander and pilot continue monitoring the vehicle and stay in contact with the ground crew. This is when the crew might also have their first Private Medical Conference or PMC with SpaceX Medical.
Credit: SpaceX
Once ready to lift Dragon, the hydraulic lift of Shannon goes down and slowly starts bringing closer the capsule to lift it onto the deck of the recovery vessel or Nest. After more safety checks, the capsule moves towards the egress platform. The recovery crew then opens the side hatch and installs protective tools to ensure the crew can safely egress the vehicle. The recovery crew then enters the capsule and helps each astronaut exit the Dragon capsule, where the medical team will be ready to help them and do the first medical check. After it is complete, the astronauts catch a helicopter flight back to shore, where they will transfer to a waiting aircraft ready to take them to Houston.
Credit: SpaceX
Some relevant information:
Credit: SpaceX Artist concept
The heating shield at the bottom of the capsule is the primary heat shield made of Pica 3.0, which stands for Phenolic Impregnated Carbon Ablator first generation. NASA developed Pica but later partnered with SpaceX and developed Pica X, the second-generation product used on all Dragon 1 CRS missions. With Pica X, SpaceX successfully resupplied the space station on 20 missions. Pica 3.0 has enhanced structural and thermal properties that optimize the heat while driving down cost and mass. This was developed specifically for use on Dragon 2 crew and cargo. The composition of the remainder of the crew Dragon capsule is primarily SpaceX's proprietary ablative material. This is another class of thermal protection that is lighter weight when compared to Pica. It protects the underlying composite structure during reentry, ensuring the structural capabilities are maintained while the crew Dragon experiences temperatures well over 3,000 Fahrenheit during peak reentry conditions. The characteristics of the TPS or Thermal Protection Systems, coupled with the EAS or ECLS environmental control and life support system in the pressurized interior, ensure that the Axiom-3 crew stays comfortable from reentry to splashdown.
The SpaceX ground team, in headquarters in Hawthorne, CA, monitors Dragon 24/7 and is trekking all of its systems to help make sure that the crew has a safe trip back home. This team, known as Mission Control, monitors the progress throughout the journey. On the console or headset are six key positions keeping track of the health of the vehicle and crew. The Mission Director, responsible for Dragon operations, is also in charge of the room. The person who talks to the astronauts is the Crew Operations and Resources Engineer, also called the Core. The other positions are focused on vehicle systems like navigation and control avionics software, propulsion, life support, and communications with ground segments.
Credit: SpaceX
SpaceX prepares to support several landing sites off the coast of Florida. These are either in the Gulf of Mexico or the Atlantic Ocean. Spreading the supported sites across multiple locations maximizes the return opportunities for this mission as it lowers the chance of waving off landing due to bad weather. Since Dragon could splashdown on either side of the Florida Panhandle, SpaceX has two identical and fully equipped recovery vessels ready to support the crew, Megan in the Gulf of Mexico and Shannon in the Atlantic Ocean -- in the case of the Axiom-3 mission and after considering many different variables, including what landing sites had favorable weather, SpaceX teams selected a primary landing site off the coast of Daytona, FL. Several weather-related items play into the splash-down and recovery operations. Some of the more obvious factors that impact operations are rain or the chance of lightning in the recovery zone, both for the safety of the crew inside the capsule and the recovery teams on the water. SpaceX also looks for wind speeds to be less than 15 ft per second or about 10 mph and relatively calm seas so that we can safely execute recovery operations, which includes landing a helicopter on the recovery ship to fly our crew back to shore.
SpaceX also closely coordinates with the United States Coast Guard to establish a safety zone to ensure Public Safety and the safety of those involved in the recovery operations and the crew on board the returning spacecraft. Multiple notices are issued to the mariners in advance and during recovery operations and Coast Guard Patrol boats are deployed to discourage boaters from entering the splash-down zones. Recovering spacecraft from the water is a very hazardous operation. Civilian watercrafts entering the area increase the risk to the astronauts in the capsule, the teams working to recover them from the water, and themselves.