SpaceX’s debut Cargo Dragon 2 docks to Station

by Tobias Corbett

SpaceX launched their 21st contracted Cargo resupply flight to the International Space Station (ISS), marking the first flight of Cargo Dragon 2, the company’s new and upgraded un-crewed cargo spacecraft.

The Commercial Resupply Services-21 (CRS-21) mission lifted off from Launch Complex-39A (LC-39A) at the Kennedy Space Center on 6 December 2020 at 16:17:08 UTC (11:17:08 EST) to start an approximately 30 day mission to the ISS to resupply the seven member Expedition 64 crew.

The automated docking occurred at 13:40 EST (18:40 UTC) to Node-2/Harmony’s space-facing zenith docking port.

The docking

After receiving a “go” for Approach Initiation, the first Cargo Dragon 2 will perform the burn 75 km distance from the Station.  The 90 second maneuver will change Cargo Dragon’s velocity by 0.79 m/s and align its orbit to intercept Waypoint Zero, 400 meters below the Station, at 12:29 EST (17:29 UTC).

The craft aligned itself onto the docking axis at 13:07 EST (18:07 UTC) at a range of 220 m while continuing to approach the outpost.

Cargo Dragon 2 will then arrive 20 m above the docking port at its final hold point, where it will remain as teams confirm that all personnel, Houston (NASA mission control), Hawthorne (SpaceX mission control), and the Station are all “go” for docking.

Station configuration prior to CRS-21. (Credit: NASA)

The Russian crew and Russian mission control near Moscow will also be monitoring Cargo Dragon 2’s automated approach — just as NASA monitors automated arrivals and departures of Russian Progress and Soyuz crafts.

The craft had to hold at the 20 m point until 13:36 EST (18:36 UTC) due to a predicted communications blockage on orbit during the original docking time.

Contact and Capture occurred at 13:40 EST (18:40 UTC) and will mark the first automated docking of a U.S. cargo vehicle to the International Space Station and the first time International Docking Adapter #3 is used on the Station.

IDA #2 is currently occupied by SpaceX’s Crew Dragon Resilience for the Crew-1 mission and previously hosted the Demo-2 and Demo-1 Crew Dragon missions as well.

Once Contact and Capture is achieved, the two vehicles will be allowed to let relative motions dampen before a hard dock occurs — a process that takes approximately 13 minutes.

Launch preparations

The Falcon 9 first stage booster that lofted the spacecraft into space was B1058, which had successfully flown three prior flights including the launch of SpaceX Demo-2 earlier this year (the first crewed test flight of SpaceX’s Crew Dragon spacecraft), ANASIS-II for South Korea, and an October Starlink mission.

B1058 and Dragon capsule C208, the first purpose-built Cargo Dragon 2 capsule, were rolled out to LC-39A on 2 December via SpaceX’s horizontal crawler-transporter vehicle, following which the rocket was moved upright and into launch position.

 

On 4 December, the rocket performed a routine static fire test, certifying it for flight.

Although the mission had remained on track for a launch attempt on 5 December, weather conditions in the recovery zone forced a scrub hours before liftoff. The 6 December attempt had more favorable weather conditions for both recovery and launch.

Unlike prior cargo resupply missions, the new Cargo Dragon 2 carried too much mass to permit a Return To Launch Site (RTLS) landing of the Falcon 9 first stage.  Instead, the first stage — like Crew Dragon, from which Cargo Dragon is now derived — made use of a drone ship in the Atlantic for landing and recovery.

Cargo Dragon to Cargo Dragon 2:

In December 2008, NASA awarded SpaceX a $1.6 billion contract for 12 resupply flights to the ISS under the Commercial Resupply Services (CRS) program. The contracts called for commercial companies to develop and operate un-crewed resupply spacecraft capable of delivering supplies like food and scientific equipment to the ISS.

SpaceX’s initial contract was extended to 20 flights in 2015, which kept the first version of Cargo Dragon flying until early 2020 when CRS-20 launched to resupply the Expedition 62 crew, marking the end of SpaceX’s original CRS contract.

Under the first round of CRS contracts, SpaceX developed the Cargo Dragon spacecraft, which was launched aboard their Falcon 9 rocket. Following two successful demonstration flights, SpaceX began operational resupply flights to the ISS with the launch of CRS-1 in October 2012.

Dragon capsule C208, the Cargo Dragon spacecraft to be used on CRS-21 during processing at SpaceX’s HQ in Hawthorne. A photo of its predecessor can be seen in the background. (Credit: SpaceX)

During the bidding process for CRS2 contracts, SpaceX submitted an upgraded Cargo Dragon design, known as Cargo Dragon 2, which was based of their Crew Dragon spacecraft.

As SpaceX’s first CRS2 flight, CRS-21 will mark Cargo Dragon 2’s first flight.

Unlike the first Dragon design, Crago Dragon 2 ditches the deployable solar “wings” and instead uses a trunk half covered in solar panels to provide power, like Crew Dragon.

The spacecraft also features a much smoother and cleaner exterior capsule design and a different docking mechanism, switching from the Common Berthing Mechanism (CBM) used on Cargo Dragon to the International Docking System Standard (IDSS) used on Crew Dragon and Boeing’s CST-100 Starliner spacecraft.

The switch from CBM to IDSS means that Cargo Dragon 2 will not use the berthing ports its predecessor used and instead will use one of two pressurized mating adapter (PMA) docking ports located on the ISS’s Harmony module. It also means Cargo Dragon 2 will not “berth” to the ISS, wherein Canadarm2 (also known as the Space Station Remote Manipulator System, or SSRMS) would grab hold of the spacecraft and manually attach (or berth) it to the Station. This process was used by the original Cargo Dragon and is currently utilized by Northrop Grumman’s Cygnus spacecraft.

Externally, Cargo Dragon 2 differs from its crewed counterpart, lacking windows and SuperDragon abort systems. Most differences between Crew Dragon and Crgo Dragon 2 are derived from the fact that Cargo Dragon is not required to have launch escape capability.

Crew Dragon is fitted with eight SpaceX-developed SuperDraco engines, located in four, two engine clusters around the outside of the capsule, which are there to pull the capsule and its crew to safety away from a Falcon 9 in the event of a catastrophic failure during fueling or launch. Since Cargo Dragon does not carry crew, the spacecraft does not have to carry those systems; therefore the SuperDracos have been removed from the Cargo Dragon capsule — a mass savings that allows for additional cargo to carried to Station instead.

CRS-21, Cargo Dragon 2’s first mission to the Station, seen on LC-39A on 4 December. The Falcon 9 booster will make its fourth flight with this mission. (Credit: Stephen Marr for NSF/L2)

Cargo Dragon 2 is still capable of surviving a Falcon 9 breakup during flight even without the escape system and would parachute to the Atlantic for recovery if a launch mishap were to occur and the timing of the mishap during flight allowed — as its predecessor was capable of doing after the in-flight loss of CRS-7 in June 2015.

For additional mass savings, SpaceX has also removed two of the four fins located on Dragon’s trunk, the only aerodynamic purpose of which is to keep the capsule stable in the event of a launch abort; therefore they were not needed for Cargo Dragon. Despite this, two were kept on the trunk — as they are half covered in solar panels, meaning removing them would result in a reduction of the electrical generation capability of the spacecraft.

Cargo Dragon 2 also lacks all of the life support and onboard control systems present on Crew Dragon that are needed for humans. Instead, it carries minimal support systems to ensure conditions are kept acceptable for hatch opening on the Station and ISS Crew ingress to the vehicle.

Payload rundown:

Aboard CRS-21 is 2,914 kilograms of pressurized and unpressurised cargo, broken down into ISS crew supplies, scientific experiments, spacewalk equipment, and other vehicle hardware. Included in that are five scientific experiments sponsored by NASA and their international partner agencies being flown to the Station in order to be carried out under the unique microgravity environment the orbital laboratory provides.

NASA’s BRazing of Aluminum alloys IN space (BRAINS), a physical science experiment, is one of those individual investigations being carried uphill by Cargo Dragon. The purpose of BRAINS is to study the effect of microgravity on “capillary flow, interface reactions, and bubble formation” during the solidification of brazing metals.

Brazing is a process that involves superheating certain metals in order to melt them and then solidifying them in a manner which allows them to be joined without the use of any other adhesive. How this could occur in microgravity is consequential to future space exploration as it is believed that brazing could be a process used to build future space habitats and other in space hardware.

Bishop mission patch. (Credit: NanoRacks)

CRS-21 will also act as a follow up to the European Space Agency’s (ESA’s) BioRock experiment that studied the interactions between microbes and rocks in microgravity, possibly affecting the future of biomining on Earth and in space.  BioRock launched on SpaceX’s CRS-18 resupply flight in July 2018.

Now, ESA’s BioAsteroid investigation is planned to act as a follow up. The experiment is relatively similar to its predecessor, investigating the interactions between rocks and microbes in space in order to inform later biomining efforts. It is also possible that the results of BioAsteriod will be useful for utilizing extraterrestrial regolith for life support purposes.

Also included within the scientific payload are NASA’s Cardinal Heart, Space Tango-Human Brain Organoids, and HemoCue experiments — which all go towards investigating several biological issues relating to how microgravity effects the human heart and brain, as well testing a new way to quickly and accurately count white blood cells — which are part of the body’s the immune system that protect against both infectious disease and foreign invaders.

Likewise, packed away in Dragon’s unpressurised “trunk” is Bishop, a new commercially developed airlock designed to deploy microsatellites from the ISS. Bishop is developed by NanoRacks LLC, an American company based in Houston, Texas, and has been involved in the space industry since its creation in 2009 — developing in-space hardware and other systems for private and public use.

Following Dragon’s arrival, Bishop will be remotely removed from the truck by the Space Station Remote Manipulator System (SSRMS; also known as Canadarm 2) and moved to the Tranquility (Node 3) module near the middle of the Station, where it will be located for the foreseeable future.

Rendering of Bishop with two external payloads connected to the outside. (Credit: Mack Crawford for NanoRacks)

Bishop is designed to deploy cubesats from the U.S. segment of the ISS (which included Europe, Japan, and Canada), without needing a spacewalk or the Kibo airlock module, located on the Japanese laboratory module. The airlock was originally intended to launch on SpaceX CRS-19 last December, although its launch was pushed back to CRS-21.

From its location on Tranquility, Bishop will be accessed by astronauts from inside of the ISS from a single entrance point on the airlock. Unlike traditional airlocks, Bishop does not have any hatches or exits for the payloads once loaded; instead, Canadarm2 will grab the airlock and remove the entire assembly was Tranquility before payloads are jettisoned from the same access point astronauts previous used to load the deployable elements once Canadarm2 has the airlock in the correct “deployment” position.

The airlock can also be taken by Canadarm2 to the Mobile Base System and then driven out along the Station’s truss to deploy payloads farther from the Station’s core modules for clearance concerns.

Although mainly intended for use in deploying small satellites, NanoRacks also advertises some of Bishop’s other uses, including being able to support spacewalks, being capable of easily getting new tools other other equipment outside the Station for use by astronauts already outside, or as a platform for scientific experiments and external payloads that would be exposed to the vacuum of space.

Lead image: CRS-21 Canadarm2 operations. Mack Crawford (NSF/L2)

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