NASA Prepares for MBSU Replacement on Thursday; SpaceX Eyes Friday for CRS-17 Dragon Launch

SpaceX stands primed for its fifth launch of the year when the CRS-17 Dragon cargo mission heads to the International Space Station (ISS) in the small hours of Friday morning (3 May). Liftoff of the Upgraded Falcon 9 is scheduled to occur from historic Space Launch Complex (SLC)-40 at Cape Canaveral Air Force Station, Fla., at 3:11 a.m. EDT.

Launch was routinely postponed from 26 to 30 April, then an additional 24 hours to 1 May, due in part to a delayed Static Fire Test of the nine Merlin 1D+ first-stage engines. It was subsequently delayed a further two days following an issue with the station’s Electrical Power System (EPS), discovered early Monday, 29 April.

The most recent gremlin to hit CRS-17 reared its head in the form of an issue with MBSU-3, one of the station’s four Main Bus Switching Units. Situated on the central S-0 truss—atop the U.S. Destiny lab—the box-like MBSUs act as distribution hubs for the eight electricity-generating power channels on the sprawling outpost. MBSU-3 is one of the “original” units, delivered by the crew of shuttle mission STS-110 with the S-0 truss itself, way back in April 2002. It is responsible for handling electrical loads from two channels on the starboard side of the ISS and, according to NASA’s Rob Navias in comments provided to AmericaSpace, the 220-pound (100-kilogram) MBSU-3 has never previously exhibited any trouble in 17 years of unblemished operations.

Due to their nature as an integral component within the EPS, repairing or replacing MBSUs are critically important and have previously been identified by NASA as one of its “Big 12” Extravehicular Activity (EVA) R&R tasks. These tasks center upon dealing with malfunctioning Orbital Replacement Unit (ORU) components which threaten to place the ISS into a “zero-fault-tolerant” state and are broadly categorized to cover critical elements of the station’s electrical power system—including External Multiplexer-Demultiplexers (Ext MDMs), Battery Charge/Discharge Units (BDCUs), Sequential Shunt Units (SSUs), Direct Current Switching Units (DCSUs), Direct Current Converter Units (DCCUs) and the MBSUs—together with critical parts of its thermal control system.

There are two spare MBSUs on station, both located on External Stowage Platform (ESP)-2, which resides on the port side of the Destiny lab. One of them will be robotically detached Thursday to replace the malfunctioning MBSU-3, which itself will be relocated and temp-stowed on ESP-2. 

According to, one of the key effects of the MBSU-3 issue which pertains to CRS-17 was a loss of redundancy in Canadarm2, which will serve a critical role during the robotic grappling and installation of Dragon onto the Earth-facing (or “nadir”) port of the station’s Harmony node. Another impact was upon the Alpha Magnetic Spectrometer (AMS), although NASA advised AmericaSpace that the instrument’s science-gathering capability was down for a “negligible” period of time.

Assuming an on-time launch Friday, the CRS-17 Dragon will arrive in the station’s vicinity early Sunday, for robotic capture by Canada’s David Saint-Jacques and NASA astronaut Nick Hague, who will be located in the multi-windowed cupola. Coming only two weeks after the launch and arrival of Northrop Grumman’s NG-11 Cygnus cargo ship to the adjacent Unity node, the berthing of CRS-17 promises to be a busy time for the Expedition 59 crew, commanded by Russian cosmonaut Oleg Kononenko, which includes two female astronauts and Saint-Jacques as Canada’s third long-duration ISS resident. This is not the first time that “dual berthed ops” have occurred on station, with the visiting vehicles of both Commercial Resupply Services (CRS) partners, SpaceX and Northrop Grumman, simultaneously in residence. That accolade belongs to the OA-5 Cygnus and CRS-8 Dragon, which arrived in late March and mid-April in 2016. However, the dual berthed ops with NG-11 and CRS-17 in the coming days represents the first occasion that two ships from the two CRS1 providers have arrived in such a short period of time.

“It will definitely be a large workload for the crew, but teams on the ground always make sure there is a plan to keep it manageable before these flights get scheduled,” NASA’s Dan Huot told AmericaSpace. “Cargo ops (both loading and unloading) will happen in tandem with both vehicles.” He added that the entire U.S. Operational Segment (USOS) crew—Canada’s Saint-Jacques and NASA’s Anne McClain, Nick Hague and Christina Koch—“will have cargo loading/unloading tasks on their timeline”.

Aboard Dragon’s pressurized cargo module for CRS-17 will be approximately 3,250 pounds (1,477 kg) of equipment, experiments and supplies for the Expedition 59 and successive crews. This figure includes 1,590 pounds (698 kg) of science hardware, 760 pounds (345 kg) of crew supplies, 754 pounds (342 kg) of vehicle equipment, 165 pounds (75 kg) of computer resources and smaller quantities of Extravehicular Activity (EVA) tools and parts and materials for the station’s Russian Operational Segment (ROS). External hardware in Dragon’s unpressurized “trunk” totals 2,130 pounds (965 kg) and is visibly dominated by NASA’s next-generation Orbiting Carbon Observatory (OCO-3), provided by the Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

Destined for robotic installation on Site 3 of the Exposed Facility (EF) of Japan’s Kibo lab, OCO-3 is expected to spend three years measuring carbon dioxide abundances in the atmosphere, with an accuracy capable of highlighting geographical distributions of sources and sinks on a regional scale. This will contribute greatly to the burgeoning corpus of data about the global carbon cycle and the effect of natural and human processes upon greenhouse gas abundances and distribution.

The first OCO spacecraft was launched atop a Taurus booster back in February 2009, but the failure of its payload fairing to properly separate resulted in the loss of the satellite, an eventuality which has been the subject of a recent NASA investigation into damning malpractice on the part of suppliers. A replacement spacecraft, OCO-2, was launched via Delta II in July 2014, with hardware for the fabrication of OCO-3 requisitioned from OCO-2 spares. It carries three parallel, high-resolution spectrometers for simultaneous observation of carbon dioxide and molecular oxygen absorption of sunlight reflected from Earth’s surface at near-infrared wavelengths. This provides OCO investigators with the spectral “fingerprints” of these absorption profiles as part of efforts to determine the numbers of molecules between the upper atmosphere and the surface.

Spares from OCO-2 were utilized to develop OCO-3 for long-duration emplacement upon the ISS exterior. Although similar to its predecessor, it also benefits from the addition of a two-axis pointing mirror for the targeting of cities and other areas on an order of 62 x 62 miles (100 x 100 km) for “snapshot mode” regional mapping. Additionally, OCO-3 includes a 330-foot-resolution (100-meter) context camera and the instrument will be kept cool by means of an on-board cryocooler, which holds the optics at -120 degrees Celsius (-184 Fahrenheit). It is confidently expected that OCO-3 data may be combined with other Earth-resources detectors currently at work on the ISS exterior, notably the Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) and Global Ecosystem Dynamics Investigation (GEDI), which were launched last June and December aboard a pair of SpaceX Dragons.

Yet OCO-3’s evolution has not been a smooth and faultless process. The project was green-lighted for development in December 2015, but was not included in the President’s Proposed Budget for FY2018 when it was released in February 2017. A year later, OCO-3 funding was restored with the Enacted Budget for FY2018, prompting newly-appointed NASA Administrator Jim Bridenstine to emphatically remark that “it’s not been cut…in fact, it’s going to be on-orbit very, very soon”. Thermal vacuum chamber testing of the instrument was completed in May 2018.

The transfer of OCO-3 from the trunk of Dragon over to the port side of the station’s Kibo facility will be conducted by means of Canadarm2. “Robotic ops are scheduled to start the day after berthing,” said Mr. Huot, indicating that first motion for OCO-3 can be expected to occur early next week, assuming an on-time launch Friday and on-time berthing Sunday. “Entirely ground-commanded robotics operation,” Mr. Huot told AmericaSpace. “Will be ten days total of operations, eight days of transferring cargo to/from Dragon trunk, one day for setup and one day for cleanup.”

Also in the trunk is the Space Test Program-Houston-6 (STP-H6), an X-ray communications testbed for space-based demonstration of new technologies for generating beams of modulated X-rays. Future potential applications include more efficient communications for deep-space missions or with hypersonic vehicles where plasma “sheaths” tend to disrupt traditional radio links.

CRS-17 forms part of an extension component to SpaceX’s CRS1 contract with NASA, signed back in December 2008. This initially called for 12 Dragon flights to the ISS, ferrying up to 44,000 pounds (20,000 kg) of payloads and supplies, but three additional missions were secured in early 2015, followed by five more in early 2016. These missions—which will see the CRS1 first round close-out with the CRS-20 Dragon flight—will bridge the gap before the second-round CRS2 contract gets underway next year.

Coming up in July 2019, the CRS-18 Dragon will ferry the second International Docking Adapter (IDA-3) for Commercial Crew operations, whilst CRS-19 in December will deliver Japan’s Hyperspectral Imager Suite (HISUI) remote-sensing instrument and CRS-20 in spring 2020 will bring the European Space Agency’s (ESA) Bartolomeo commercial payloads anchoring platform uphill for installation onto the forward side of the Columbus lab. When the CRS2 contracted missions commence next year, SpaceX is expected to stage at least six more cargo flights with its upgraded Dragon 2 vehicle through 2024.

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