ISS Utilization: Cygnus COTS/CRS (Commercial Orbital Transportation Services/Cargo Resupply Service)
The Cygnus spacecraft is a commercial unmanned resupply spacecraft, developed by OSC (Orbital Sciences Corporation) and TAS (Thales Alenia Space) as part of NASA's COTS (Commercial Orbital Transportation Services) demonstration program. It is designed to transport supplies to the ISS (International Space Station) after the retirement of the Space Shuttle program. 1) 2) 3) 4)
In addition to the COTS development and demonstration program, Orbital will utilize the Cygnus to perform the ISS resupply flights under the CRS (Commercial Resupply Service) contract. This NASA contract authorizes eight missions between 2011 and 2015 carrying approximately 20,000 kg of cargo to the ISS as well as disposal of ISS waste.
Table 1: Cygnus System Facilitates Exploration Goals 5)
Background: Since August 2000, unmanned ISS resupply missions have been regularly flown by Russian Progress spacecraft, as well as by two flights of the European ATV (Automated Transfer Vehicle) and two flights of the Japanese HTV (H-II Transfer Vehicle). With the Cygnus spacecraft, NASA seeks to increase its partnerships with domestic commercial aviation and aeronautics industry - to reduce reliance on its international partners during the anticipated five-year gap between the 2011 retirement of its aging Space Shuttles and the first operational flights of their successor, the Orion Crew Exploration Vehicle. 6) 7)
In 2008, NASA awarded two contracts — one to Orbital Sciences Corp. of Dulles, VA, and one to SpaceX (Space Exploration Technologies of Hawthorne, CA, — for commercial cargo resupply services to the International Space Station. NASA's COTS program is investing financial and technical resources to stimulate efforts within the private sector to develop and demonstrate safe, reliable, and cost-effective space transportation capabilities. 8) 9)
NASA initiatives like COTS and the agency's Commercial Crew Program are helping develop a robust U.S. commercial space transportation industry with the goal of achieving safe, reliable and cost-effective transportation to and from the space station and low Earth orbit. In addition to cargo flights, NASA's commercial space partners are making progress toward a launch of astronauts from U.S. soil in the next 5 years.
Figure 1: Illustration of the standard Cygnus cargo resupply vehicle (image credit: Orbital) 10)
Cygnus CRS vehicle:
The Cygnus system is a low-risk design incorporating elements drawn from Orbital and its partners’ existing, flight-proven spacecraft technologies. The Cygnus vehicle is comprised of two major modules:
1) SM (Service Module):
- Provides all utility services to the cargo modules
- Manages the autonomous rendezvous to the ISS
- Provides required resources to allow the mission to be successfully completed
- Structural interface to the launch vehicle and cargo modules
- Integrated at Orbital’s Dulles facility
- Power generation: 2 fixed solar wing arrays; power output = 3.5 kW (sun-pointed)
- Propellant: dual mode.
2) PCM (Pressurized Cargo Module)
- Supports NASA cargo requiring a pressurized environment
- Built by TAS-I (Thales Alenia Space-Italia) on the heritage of the MPLM (Multi-Purpose Logistics Module). - Note: The MPLM was developed by TAS through a previous program for the ISS on behalf of ASI (Italian Space Agency) for NASA; the other TAS-built modules with flights to the ISS were ATVs (Automated Transfer Vehicle) for ESA. 11)
The Cygnus spacecraft is of LEOStar/GEOStar bus heritage with a power generation of 3.5 kW DC power when sun pointed (2 fixed wing solar arrays with ZTJ Gallium Arsenide cells of Emcore). Propellant: Dual mode N2H4/NTO or N2H4.
The Cygnus PCM provides a pressurized volume for internal cargo delivery to the ISS.
- Cygnus is berthed to ISS Node 2 Nadir Port utilizing the Canadarm2
- Up to 2000 kg cargo can be accommodated by the Standard PCM
- Up to 2700 kg cargo can be accommodated by the Enhanced PCM
- Pressurized volume: 18.7 m3 /27 m3 (for Standard/Enhanced PCM versions)
• Standardized active and passive cargo accommodations available within PCM
- Passive cargo: CTB (Cargo Transfer Bags), M-bags, MDL (Mid-Deck Lockers)
- Active cargo: Two single MDL or one Double MDL sized payloads
- Disposal cargo (up to 1200 kg) reloaded into Cygnus prior to unberthing.
Table 2: Key parameters of the PCM (Pressurized Cargo Module)
Figure 2: Standard configuration of Cygnus (image credit: Orbital)
Figure 3: Enhanced configuration of Cygnus (image credit: Orbital)
SM (Service Module): Located in the aft section of the spacecraft, the Cygnus Service Module provides power generation & storage, vehicle control, propulsion, guidance and the Grapple Fixture for the Station's robotic arm. The SM is based on Orbital's GEOStar satellite bus and uses elements of NASA's Dawn spacecraft that was manufactured by Orbital. 12)
The SM contains the Main Propulsion and ACS (Attitude Control Subsystem) of the spacecraft. Cygnus features IHI BT-4 thrusters for orbit adjustment maneuvers. BT-4 was developed by IHI Aerospace, Japan and has a dry mass of 4 kg and a length of 0.65 m. The engine provides 450 N of thrust using Monomethylhydrazine fuel and Nitrogen Tetroxide Oxidizer. The propellants are stored in spherical tanks that are pressurized with Helium. The ACS of Cygnus is used for re-orientation and small rendezvous burns.
The SM is also equipped with the Guidance, Navigation and Control system of the vehicle as well as communications equipment to communicate with ground stations, ISS and the TDRSS (Tracking and Data Relay Satellite System).
Navigation system: Cygnus is equipped with Star Trackers and absolute GPS system to determine its position in orbit during free flight. During Rendezvous with the International Space Station, Cygnus switches to relative GPS to determine its position relative to the ISS. When beginning proximity operations, Cygnus starts using its proximity navigation system.
Cygnus uses a TriDAR (Triangulation and LIDAR Automated Rendezvous and Docking) system developed by Neptec Design Group of Kanata, Ontario, Canada with funding from CSA and NASA. TriDAR is a rendezvous navigation system that does not rely on any reference markers positioned on its target. Instead, TriDAR uses a laser-based 3D sensor and thermal imagers to collect 3D data of its target that is compared by software with the known shape of the target spacecraft. This enables TriDAR to calculate relative position, range and relative velocity. The computer algorithm is capable of calculating the 6 Degree Of Freedom (6DOF) relative pose in real time using a MILD (More Information Less Data) approach. TriDAR operates at distances ranging from 0.5 m to over 2000 m without sacrificing speed or precision at either end of the range. 13)
TriDAR's 3D sensor combines auto-synchronous laser triangulation technology with LIDAR (Light Detection and Ranging) in a single package to provide tracking data at short and long range. The laser triangulation system is based on the LCS (Laser Camera System) used on the Space Shuttle's Orbiter Boom Sensor System that was used to perform inspections of the vehicle's heat shield in orbit. TriDAR provides the functionality of two 3D scanners by multiplexing the two active subsystem’s optical paths. The thermal imager is used to extend the reach of the system beyond the operational range of LIDAR.
TriDAR was tested in space on Space Shuttle Missions STS-128 (August 2009), STS-131 (April 2010) and the final Shuttle Flight, STS-135 (July 2011). During STS-135, TriDAR started tracking the ISS from 34 km all the way through docking, and during undocking, the system provided impressive imagery of the ISS, providing 3D and thermal imagery of the Station as part of the last Shuttle-based flyaround of ISS.
Figure 4: Two Cygnus service modules in the manufacturing facility of OSC (image credit: OSC)
Figure 5: Artist's rendition of a Cygnus spacecraft approach to the ISS (image credit: Orbital)
On Aug.24, 2011, the Cygnus PCM spacecraft arrived at the Wallops Flight Facility, Wallops Island, VA. 14) During the next several months, Orbital's engineering team will integrate the pressurized module with the Cygnus service module that includes the spacecraft's avionics, propulsion and power systems.
The mission partners are (Ref. 2):
• OSC (Orbital Sciences Corporation) prime contractor: Engineering and development, Cygnus Service Module, mission and cargo operations
• TAS (Thales Alenia Space): PCM (Pressurized Cargo Module)
• MELCO (Mitsubishi Electric Corporation): Proximity location system
• Draper Laboratory: Guidance, navigation and fault tolerant computer support
• Odyssey Space Research: Visiting vehicle requirements support
• JAMSS America, Inc.: Operations support
• Vivace: Systems engineering support.
Launch: The Cygnus-1 spacecraft, also known as COTS-1 Demo (Commercial Orbital Transportation Services) to the ISS, was launched on September 18, 2013 from MARS (Mid-Atlantic Regional Spaceport) at Wallops Island, VA, USA. This logistics/test flight mission involved the maiden flight of Cygnus and the final test flight of the OSC Antares-110 launch vehicle. 15) 16) 17) 18) 19)
The history making launch marks the first time that a spacecraft launched from Virginia to the ISS. Cygnus separated from the rocket's second stage about 10 minutes after blast-off to reach Earth's orbit, marking the success of the launch. It later deployed both of its solar arrays to supply power to the spacecraft.
Figure 6: Schematic view of the Cygnus' flight profile (image credit: ESA) 20)
Figure 7: Artist's rendition of the Cygnus spacecraft docked to Node 2 of the ISS (image credit: Orbital)
Orbit: Near-circular orbit, altitude: 350-460 km to ISS, inclination =51.62°-51.68° (±0.1°).
Over the next several days, Cygnus will perform a series of maneuvers to test and prove its systems, ensuring it can safely enter the so-called "keep-out sphere" of the space station, a 200 m radius surrounding the complex. Mission controllers at Orbital will guide Cygnus to the vicinity of the ISS on Sept. 22 with berthing at the Node 2, the CBM (Common Berthing Mechanism).
The Cygnus-1 is expected to deliver ~590 kg of cargo to the ISS and dispose of about 1,000 kg through destructive reentry.
• On Oct. 23, 2013, Orbital's Cygnus spacecraft reentered Earth's atmosphere, marking the end of the highly successful cargo resupply demonstration mission Orbital conducted with its NASA partner. The end of the mission also marked the end of the five-year COTS (Commercial Orbital Transportation Services) joint Orbital/NASA development program. 21)
The Cygnus cargo logistics spacecraft reentered Earth's atmosphere over the Pacific Ocean east of New Zealand at approximately 7:15 hours (UTC). 22)
• On Oct. 22, 2013, the cargo resupply demonstration mission by OSC (Orbital Sciences Corp.) drew to a close as the Expedition 37 crew members aboard the International Space Station detached and released OSC's Cygnus spacecraft from the orbiting laboratory (11:31 GMT). - Cygnus had been attached to the space station's Harmony module (Node 2) for 23 days. 23) 24)
Figure 8: Photo of the Cygnus spacecraft as it was being released from Canadarm2 by the station astronauts (image credit: NASA)
- Prior to its departure from the ISS, Cygnus was loaded with items no longer needed aboard the station. Astronauts Karen Nyberg of NASA and Luca Parmitano of ESA detached the spacecraft using the station's robotic arm and released Cygnus. Orbital Sciences engineers now will conduct a series of planned burns and maneuvers to move Cygnus toward a destructive reentry in Earth's atmosphere on Oct. 23, 2013.
• The maiden flight of Cygnus and its 11-day journey to the station included a number of tests designed to demonstrate the spacecraft's ability to navigate, maneuver, lock on to the station and abort its approach. Following these demonstrations NASA cleared the spacecraft to approach the station on Sept. 29, 2013. Cygnus had been scheduled for a rendezvous with the space station on Sept. 22, but because of a data format mismatch, the first rendezvous attempt was postponed. Orbital updated and tested a software patch to resolve the issue (Ref. 23).
• On Oct. 7, 2013, the Expedition 37 crew aboard the ISS had completed unloading the cargo from the Cygnus PCM (Pressurized Cargo Module) and loaded the first layer of waste for disposal.
• On Sept. 30, 2013, the Expedition 37 crew opened the hatch between the station and Cygnus.
• On Sept. 29, 2013, the Cygnus spacecraft was grappled by the ISS's robotic arm, referred to as SSMRS (Space Station Remote Manipulator System), after a flawless approach and in-orbit demonstration sequence. Cygnus was berthed to the Harmony node where the Expedition 37 crew completed the installation.
• On Sept. 23,2013, Orbital and NASA together decided to postpone the approach, rendezvous, grapple and berthing operations of the Cygnus cargo spacecraft with the ISS until after the upcoming Soyuz crew operations are complete. The Soyuz crew is due to arrive at the ISS on Sept. 25. The earliest possible data for the next Cygnus approach and rendezvous with the ISS would be Sept. 28.
• Sept. 22, 2013: Following a discovery of a data format discrepancy between an on-board ISS navigation system and a similar system on Cygnus, today's rendezvous with the station was postponed. Hence, NASA and Orbital developed a detailed plan for a second rendezvous attempt (Ref. 24).
• Sept. 20, 2013: Before Cygnus can rendezvous and berth with the ISS, the requirements call for several thruster firings to raise its orbit and catch up with the ISS. Cygnus must also perform 10 maneuvers to demonstrate the safety capabilities of Cygnus. Once each demonstration maneuver is complete, Orbital will send a data package to NASA for review to verify that the demonstration has met its objectives (Ref. 23).
Small satellite ride shares on Cygnus:
CRS (Commercial Resupply Service) missions to the ISS (International Space Station) have the potential to provide a robust and repetitive platform for small satellite rideshares (e.g., CubeSats, nanosatellites). Cygnus enables hosting and rideshare opportunities at a regular interval, thus filling the void left after the retirement of the Space Shuttle. 25)
Rideshare concept of operations: The notional CRS concept of operations consists of five operational phases:
- ILOPS (Integrated Launch Operations)
- POPS (Phasing Operations)
- JOPS (Joint Operations)
- BOPS (Berthing Operations), and
- DROPS (Departure and Reentry Operations).
The progression of these phases is shown in Figure 9. For deployable rideshares, there are two options for deployment: deploying prior to ISS berthing and deploying after ISS berthing. Deploying prior to ISS berthing offers the advantage of potentially avoiding ISS safety regulations for deployables. However, this option directly interferes with established flight plans for the primary resupply mission.
Deployment would need to occur between POPS and JOPS such that a safe distance is ensured between the ISS and the small satellite. This is a feasible option, but the advantages may not be worth the intrusion of the CRS flight plans, since a failed deployment may put the primary mission at risk. Deploying after ISS berthing requires adherence to ISS safety regulations, but it is significantly less intrusive to the established flight plan. Deployment during DROPS (i.e. after resupply to the ISS is successfully completed) is simpler to incorporate, since the sole function of the vehicle during this phase is to depart station proximity and enter a controlled, destructive reentry.
Two flight operations phases have been proposed for rideshare accommodation on Cygnus. The DOPS (Deployable Operations Phase) covers operation of the Cygnus vehicle after ISS berthing for deployable small satellite rideshares. DOPS consists of three segments in which the vehicle departs station (departure), descends to the rideshare’s desired orbit (descent), and deploys the rideshare (deployment). The second segment requires a descent to deployment altitude, rather than allowing any ascent for ISS safety reasons. Ascending would require Cygnus to cross ISS altitude twice for ascent and descent, thereby presenting a potential collision hazard with the station. Deployment of a small satellite above (and near) ISS altitude may also present a hazard to the station through the satellite’s natural orbit decay. The entire DOPS duration adds approximately 24-36 hours to a notional CRS mission, depending on the desired orbit. During this time, the TT&C links are maintained through visible USN ground stations, subject to availability. NASA’s TDRSS is also used during other flight operations for CRS missions, but is likely not available for DOPS.
Figure 9: Notional CRS concept of operations (image credit: Orbital)
The second proposed operations phase, which is called the HOPS (Hosting Operations Phase), covers operation of the Cygnus vehicle after ISS berthing for non-deployable small satellite technology demonstrations. HOPS is similar to DOPS, however, the deployment segment is replaced by a pre-hosting and a hosting segment. Pre-hosting covers the steps in transitioning from a nominal descent, such as powering down non-essential components and charging the batteries to full capacity. The hosting segment is when the small satellite technology performs its intended functions while its performance is monitored. The hosting segment can potentially last one to two years, depending on the fuel margin available following the CRS mission, and communication is maintained via available USN stations. Figure 3 illustrates the flow of a FOP (Flight Operation Plan) for a notional CRS mission and for a mission with a deployable small satellite rideshare. The FOP shown can also be applied to non-deployables by replacing the deployment segment with pre-hosting and hosting segments.
Figure 10: Flight Operation Plan for notional CRS mission and for mission with deployable small satellites rideshare (image credit: Orbital)
CRS missions such as the Cygnus spacecraft and the Antares launch vehicle provide the potential for a robust and repetitive platform for small satellite rideshares. The rideshare capability can accommodate numerous small satellite configurations and a mass upwards to 250 kg. Rideshares with a CRS mission have excess power, especially for missions occurring after visiting the ISS. Several variables determine the spacecraft lifetime, including the pointing control accuracy, vehicle orientation during the mission, and the CRS mission operations and cargo. In general, missions up to two years are feasible.
Space-to-ground communications between the CRS spacecraft and the ground networks for TT&C use S-band. The uplink data rates are approximately 2 kbit/s (PCM/PSK/PM) and the downlink rate is 3 Mbit/s using QPSK modulation. Cygnus utilizes the USN (Universal Space Network) which is networked with the MCC (Mission Control Center) in Dulles, Virginia. The MCC-D coordinates with the NASA Mission Operations center and with JSC and Mission Control Houston during a nominal CRS mission, but during rideshare operations the architecture would likely become exclusively USN and MCC-D. MCC-D provides a dedicated operations center capable of managing the deployment of a rideshare or supporting the operation of a hosted technology demonstration on Cygnus.
Cygnus has 2 GB of memory for on-orbit storage. Data throughput to the ground to one USN station can average 0.6 GB per day, with margin available for rideshare data in addition to Cygnus’s telemetry generation. The actual margin available is dependent on which, if any, of the redundant systems are kept active during rideshare operations. For the case of a deployable rideshare, there is ample margin available for deployment telemetry if needed.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).