On the International Space Station machines break, people need food to eat and water to drink – so how do we keep our astronauts fed, provide them with water and service the machines?
The collective effort of the fleet of robotic resupply ships takes care of these issues for us.
The International Space Station is our prime model for learning how to build spacecraft that can survive in space for long periods of time, as well as learning what the effects of a zero-g environment on humans are, how to combat the negative effects of that, as well as providing a unique platform for scientific study that is not available down on the surface of Earth. However, the keyword here is learning. We are still learning the technology needed to make long term spaceflight without resupply viable for missions into deep space, and until we have those technologies figured to an incredibly reliable standard, we need to fall back on our fleet of robotic resupply ships to keep the ISS going.
Since the days of Salyut 6, Salyut 7 and the MIR space station, when the Russians moved from sending up one-piece space stations that had all their supplies on board at launch, sending their resupply cargo ship, the Progress, to space stations to support long duration missions has been commonplace.
[dropcap][/dropcap]The Progress is a Russian robotic expendable freighter spacecraft; designed to take supplies up to a space station, dock and allow astronauts to open her up and take the cargo out and then re-fill the craft with rubbish and other items that need disposing of. When the Progress leaves the space station it and the rubbish it carries is destroyed as it burns up on re-entering the atmosphere.
The design for Progress was taken from the Soyuz manned spacecraft, and is launched using the Soyuz-U launcher from the Baikonur Cosmodrome in Kazakhstan. The design has been updated over the years with lots of small modifications, also allowing for testing of systems that later went into the updated crew transportation Soyuz TMA-M spacecraft.
The Progress variation in use is the Progress-М 11F615A60 that can carry up to a total of 2,600 kg of mass to the ISS. This consists of a maximum of 1,540 kg of liquid mass and up to a maximum of 1,500 kg of dry mass, however it cannot utilize both these maximum mass values on one flight.
Progress vehicles can visit the station for up to six months and there are normally four Progress resupply missions per year. Visiting Progress spacecraft at the station are also capable of providing orbital boost burns when needed to adjust the orbit of the ISS.
[dropcap][/dropcap]In March 2008 the European Space Agency launched the first of their own robotic resupply vehicles. Named the Automated Transfer Vehicle, it is another unmanned robotic resupply vehicle that is not designed to be reusable. Three ATV’s have flown to the ISS with the fourth being launched on June 7th 2013. The ESA gives each ATV a name, Jules Verne, Johannes Kepler and Edoardo Amaldi have all been flown to ISS and have completed their missions and departed. The fourth ATV has been named Albert Einstein.
The ATV is designed to complement the Russian Progress craft, being able to carry around three times as much payload to the ISS while also being able to provide orbital boosting burns when attached to station like Progress. The ATV can stay docked to the ISS for up to six months time before needing to depart.
The ATV can carry a payload of 7,667 kg to ISS. The payload can consist of 1,500 kg to 5,500 kg of dry cargo, up to 840 kg of water, up to 100 kg of gas (nitrogen, oxygen, air – but only carries two types of gas per flight) and up to 4,700 kg of propellant for refuelling the station and use for orbital manoeuvre burns.
On 2 April 2012, the ESA announced that the ATV program would end after the fifth ATV is launched in 2014. In January 2013 the ESA and NASA announced new plans to adapt the ATV to act as the service module of the new Orion Multi Purpose Crew Vehicle (MPCV), building on the ATV to meet the needs of NASA and Orion.
[dropcap][/dropcap]The third robotic resupply ship to come online for ISS operations was the H-II Transfer Vehicle (HTV), also called Kounotori, operated by JAXA or Japan Aerospace Exploration Agency. The HTV is another expendable resupply ship, designed to support the Japanese Kibō laboratory (Japanese Experiment Module) on the ISS, while also being able to bring cargo and supplies for the rest of station as well.
Like the ESA ATV, the HTV can carry more than twice the cargo of a Progress vehicle, being able to carry 6,000 kg total mass as payload to the ISS. The vehicle is outfitted with the ability to take 1,500 kg of payload in an unpressurised payload bay, allowing direct transfer of cargo to the external experiment rack on the Kibō lab.
The HTV is modular setup, being made of interchangeable components that can replace the unpressurised bay for more pressurised cargo capacity. This is also the test bed for JAXA’s future manned space flight efforts. The next step is adding a return section to the HTV that would allow the HTV to return 1,600 kg of cargo from the ISS to earth. The first launch of a HTV with return capability is scheduled for 2017. In 2022, JAXA expects to be able to launch humans into space on a modified HTV system that would also allow for 400 kg of cargo payload to be taken as well.
[dropcap][/dropcap]Since the space shuttle was retired NASA has had no way to provide logistics for the ISS outside of getting their cargo put on other resupply ships. To fix this problem NASA developed the COTS or Commercial Orbital Transportation Services program to help commercial companies pick up the slack that the shuttle left behind. The contracts from this program for cargo missions to the ISS currently belong to two commercial US companies: Space Exploration Technologies Corporation (SpaceX) and Orbital Sciences Corporation.
SpaceX are currently working on fulfilling their contract with NASA for a minimum of twelve resupply flights of their Dragon cargo ship to the ISS. The Dragon has visited the station three times currently, once for the COTS demonstration mission required by NASA and twice under the CRS contract for resupply missions, and is currently berthed at station for its second operational stay.
Dragon has a total payload capability of 6,620 kg and splits this evenly between pressurized cargo and unpressurized cargo. The first time the unpressurized cargo trunk section was utilized was on the Dragon CRS-2 mission when some new external fixtures for the ISS were taken up to be installed on station to provide access to the stations radiators for astronauts on EVA, should those components ever need repair. In 2015, the extended trunk will be used to transport a Bigelow Aerospace Expandable Activity Module to the ISS for testing the viability of using inflatable module sections.
Dragon is currently the only spacecraft that can return sizable cargo to Earth, making it the most valuable of the resupply craft for the scientific community around the ISS. With Dragon, they can get experiment samples safely back from the ISS for the first time since the shuttle was retired. Dragon has the ability to return 2,500 kg of mass back to earth in the pressurized cabin, the external trunk is detached prior to re-entry and burns up in the atmosphere facilitating 2,600 kg of unpressurised waste disposal.
For scientific sample returns, Dragon is equipped with freezers in the pressurized section for the transportation of samples back to Earth. While the freezers had electrical issues due to water intrusion on CRS-1, CRS-2 has been retrofitted to reduce the risk of failure of the electrical systems of the freezers and from CRS-3 onward SpaceX have re-designed the system so the electrical system is fully shielded from any potential water intrusion. On the first mission the freezers were used to keep some ice cream cold while on the way up to the ISS.
SpaceX is currently working on the second generation of Dragon spacecraft. The upgraded Dragon will have a crew variant as well as propulsive landing capabilities, allowing the Dragon to land at SpaceX’s processing facility, removing the time needed to recover Dragon from the ocean after a splashdown allowing the Dragon to be launched many times in (relatively) rapid succession.
[dropcap][/dropcap]Orbital Sciences Corporation are currently in the final stages of testing their resupply craft, the Cygnus, and their rocket to take the Cygnus to ISS, the Antares.
Cygnus is designed to carry 2,000 kg of payload to the ISS in its standard configuration, 2,700 kg in its extended configuration and will be disposed of via destructive re-entry while carrying up to 1,200 kg of waste from the ISS.
The Cygnus is built around two parts, the Pressurized Cargo Module and a Service module based on the Orbital StarBus. Original plans to be able to interchange the PCM with other types of cargo module have since been dropped.
Cygnus is scheduled to make its maiden COTS evaluation flight in July 2013, with the maiden flight of the Antares in April. If successful, on the COTS demo missions Orbital will begin their CRS contract to take 20,000 kg to ISS over eight resupply missions.
Image Credit: Progress, ATV, HTV photos are from NASA
Image Credit: Dragon photo is from SpaceX
Image Credit: Cygnus artists impression from Orbital Sciences