An orbital propellant depot is a cache of propellant that is placed on an orbit about the Earth or another body to allow spacecraft to be fueled in space. Launching a spacecraft separately from some of its propellant enables missions with more massive payloads. It also facilitates life extension for satellites that have nearly reached end-of-life by consuming nearly all of their orbital maneuvering fuel. The spacecraft would conduct a space rendezvous with the depot, or vice versa, and then transfer propellant to be used for subsequent orbital maneuvers. An in-space fuel depot (alternative name) is not necessarily located near or at a space station.
Potential users of in-orbit refuelling and storage facilities include space agencies, defense ministries and communications satellite or other commercial companies.
While larger propellant depots are likely to be placed in low Earth orbit (LEO) and either on the way to the Moon at Earth-Moon Lagrange point 1 (EML-1) or behind the Moon at EML-2, Intelsat has recently contracted for an initial demonstration mission to refuel several satellites in geosynchronous orbit, beginning in 2015. Placing a depot in Mars orbit has also been suggested.[1]
Large upper stage rocket engines generally use cryogenic fuels like liquid hydrogen and liquid oxygen (LOX) that suffer from a problem called "boil off". The boil off from only a few days delay can result in the vehicle carrying insufficient fuel, potentially resulting in a mission abort. Cryogenics depots must therefore protect cryogenic propellants with sun shields and refrigeration equipment.[2]
Non-cryogenic, earth-storable liquid rocket propellants including RP-1 (kerosene), hydrazine and nitrogen tetroxide (NTO), and mildly cryogenic, space-storable propellants like liquid methane, can be kept in liquid form and do not suffer from excessive boil off. Additionally, gaseous or supercritical propellants such as those used by ion thrusters include xenon, argon,[3][4] and bismuth.[5]
Ex-NASA administrator Mike Griffin commented at the 52nd AAS Annual Meeting in Houston, November 2005, that "...at a conservatively low government price of $10,000/kg in LEO, 250 MT of fuel for two missions per year is worth $2.5 B, at government rates."[6]
One often overlooked aspect of propellant depots is that the total mass to orbit required for a mission can actually increase because of the need to launch more propellant tanks and boil-off mitigation hardware. This increase in mission mass must be offset by a large reduction in the cost of launching existing vehicles in order for the concept to be economically feasible. Generally the concept includes increasing launch rates for existing designs instead of developing new large monolithic rockets. Whether or not the cost model for propellant depots is sound remains to be demonstrated and is the key metric for the concept's success. The cost of large launch vehicles is so high that a depot able to hold the propellent lifted by two or more medium sized launch vehicles may be profitable.
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Propellant depots were proposed as part of the Space Transportation System (along with nuclear "tugs" to take payloads from LEO to other destinations) in the mid-1960s.[7]
In October 2009, the Air Force and United Launch Alliance (ULA) performed an experimental on-orbit demonstration on a modified Centaur upper stage on the DMSP-18 launch to improve "understanding of propellant settling and slosh, pressure control, RL10 chilldown and RL10 two-phase shutdown operations. "The light weight of DMSP-18 allowed 12,000 pounds (5,400 kg) of remaining LO2 and LH2 propellant, 28% of Centaur’s capacity," for the on-orbit demonstrations. The post-spacecraft mission extension ran 2.4 hours before executing the deorbit burn.[8]
ULA is also currently planning additional in-space laboratory experiments to further develop cryogenic fluid management technologies using the Centaur upper stage after primary payload separation. Named CRYOTE, or CRYogenic Orbital TEstbed, it will be a testbed for demonstrating a number of technologies needed for cryogenic propellant depots, with several small-scale demonstrations planned for 2012-2014.[9] As of August 2011[update], ULA says this mission could launch as soon as 2012 if funded.[10] The ULA CRYOTE small-scale demonstrations are intended to lead to a ULA large-scale cryo-sat flagship technology demonstration in 2015.[9]
The Future In-Space Operations (FISO) Working Group, a consortium of participants from NASA, industry and academia, discussed propellant depot concepts and plans on several occasions in 2010,[11] with presentations of optimal depot locations for human space exploration beyond low-Earth orbit,[12] a proposed simpler (single vehicle) first-generation propellant depot[9] and six important propellant-depot-related technologies for reusable cislunar transportation.[13]
NASA also has plans to mature techniques for enabling and enhancing space flights that use propellant depots in the "CRYOGENIC Propellant STorage And Transfer (CRYOSTAT) Mission". The CRYOSTAT vehicle is expected to be launched to LEO in 2015.[14]
The CRYOSTAT architecture comprises technologies in the following categories:[14]
The "Simple Depot" mission is currently proposed as the first PTSD mission, with launch as early as 2015, on an Atlas V 551. It will utilize the "used" (nearly-emptied) Centaur upper stage LH2 tank for long-term storage of LO2 while LH2 will be stored in the Simple Depot LH2 module, which is launched with only ambient-temperature gaseous Helium in it. The SD LH2 tank will be 3 metres (9.8 ft) diameter and 16 metres (52 ft) long, 110 cubic metres (3,900 cu ft) in volume, and can store 5 mT of LH2. "At a useful mixture ratio (MR) of 6:1 this quantity of LH2 can be paired with 25.7 mT of LO2, allowing for 0.7 mT of LH2 to be used for vapor cooling, for a total useful propellant mass of 30 mT. ... the described depot will have a boil-off rate of approaching 0.1 percent per day, consisting entirely of hydrogen."[15]
In September 2010, ULA released a Depot-Based Space Transportation Architecture concept to propose propellant depots that could be used as way-stations for other spacecraft to stop and refuel—either in low Earth orbit (LEO) for beyond-LEO missions, or at Lagrangian point L2 for interplanetary missions—at the AIAA Space 2010 conference. The concept proposes that waste gaseous hydrogen—an inevitable byproduct of long-term liquid hydrogen storage in the radiative heat environment of space—would be usable as a monopropellant in a solar-thermal propulsion system. The waste hydrogen would be productively utilized for both orbital stationkeeping and attitude control, as well as providing limited propellant and thrust to use for orbital maneuvers to better rendezvous with other spacecraft that would be inbound to receive fuel from the depot.[16] As part of the Depot-Based Space Transportation Architecture, ULA has proposed the Advanced Common Evolved Stage (ACES) upper stage rocket. ACES hardware is designed from the start to as an in-space propellant depot that could be used as way-stations for other rockets to stop and refuel on the way to beyond-LEO or interplanetary missions, and to provide the high-energy technical capacity for the cleanup of space debris.[16]
In August 2011, NASA made a significant contractual commitment to the development of propellant depot technology by funding four aerospace companies to "define demonstration missions that would validate the concept of storing cryogenic propellants in space to reduce the need for large launch vehicles for deep-space exploration."[17] These study contracts for storing/transferring cryogenic propellants and cryogenic depots were signed with Analytical Mechanics Associates, Boeing, Lockheed Martin and Ball Aerospace. Each company will receive US$600,000 under the contract.[17]
Because a large portion of a rocket is propellant at time of launch, proponents point out several advantages of using a propellant depot architecture. Spacecraft could be launched unfueled and thus require less structural mass.[18] An on-orbit market for refueling may be created where competition to deliver propellant for the cheapest price takes place, and it may also enable an economy of scale by permitting existing rockets to fly more often to refuel the depot.[18] If used in conjunction with a mining facility on the moon, water or propellant could be exported back to the depot, further reducing the cost of propellant.[19] Further, a depot architecture is cheaper and is more flexible than a government developed heavy lift rocket such as SLS.[18][20][21]
Transfer of liquid propellants in microgravity is complicated by the uncertain distribution of liquid and gasses within a tank. Propellant settling at an in-space depot is thus more challenging than in even a slight gravity field. ULA plans to use the DMSP-18 mission to flight-test centrifugal propellant settling as a cryogenic fuel management technique that might be used in future propellant depots.[22] The proposed Simple Depot PTSD mission utilizes several techniques to achieve adequate settling for propellant transfer.[15]
In the absence of gravity, propellant transfer is somewhat more difficult, since liquids can float away from the inlet.
As part of the Orbital Express mission in 2007, hydrazine propellant was successfully transferred between two single-purpose designed technology demonstration spacecraft. The Boeing servicing spacecraft ASTRO transferred propellant to the Ball Aerospace serviceable client spacecraft NEXTSat. Since no crew were present on either spacecraft, this was reported as the first autonomous spacecraft-to-spacecraft fluid transfer.[23]
After propellant has been transferred to a customer the depot's tanks will need refilling. Organizing the construction and launch of the tanker rockets bearing the new fuel is the responsibility of the propellant depot's operator. Since space agencies like NASA hope to be purchasers rather than owners, possible operators include the aerospace company that constructed the depot, manufactures of the rockets, a specialist space depot company or an oil/chemical company that refines the propellant. By using several tanker rockets the tankers can be smaller than the depot and larger than the spacecraft they are intended to resupply. Short range chemical propulsion tugs belonging to the depot may be used to simplify docking tanker rockets and large vehicles like Mars Transfer Vehicles.
Transfers of propellant between the LEO depot, reachable by rockets from Earth, and the deep space ones such as the Lagrange Points and Phobos depots can be performed using Solar electric propulsion (SEP) tugs.[24]
Two missions are currently under development or proposed to support propellant depot refilling. In addition to refueling and servicing geostationary communications satellites with the fuel that is initially launched with the MDA Space Infrastructure Servicing vehicle, the SIS vehicle is being designed to have the ability to orbitally maneuver to rendezvous with a replacement fuel canister after transferring the 2000 kg of fuel in the launch load, enabling further refueling of additional satellites after the initial multi-satellite servicing mission is complete.[25] The proposed Simple Depot cryogenic PTSD mission utilizes "remote berthing arm and docking and fluid transfer ports" both for propellant transfer to other vehicles, as well as for refilling the depot up to the full 30 tonne propellant capacity.[15]
S.T. Demetriades[26] proposed a method for refilling by collecting atmospheric gases. Moving in low Earth orbit, at an altitude of around 120 km, Demetriades' proposed depot extracts air from the fringes of the atmosphere, compresses and cools it, and extracts liquid oxygen. The remaining nitrogen is used as propellant for a nuclear-powered magnetohydrodynamic engine, which maintains the orbit, compensating for atmospheric drag.[26] This system was called “PROFAC” (PROpulsive Fluid ACcumulator).[27] There are, however, safety concerns with placing a nuclear reactor in low Earth orbit.
Demetriades' proposal was further refined by Christopher Jones and others[28] In this proposal, multiple collection vehicles accumulate propellent gases at around 120 km altitude, later transferring them to a higher orbit. However, Jones' proposal does require a network of orbital power-beaming satellites, to avoid placing nuclear reactors in orbit.
Asteroids can also be processed to provide liquid oxygen.[29]
Propellant depots in LEO are of little use for transfer between two low earth orbits when the depot is in a different orbital plane than the target orbit. The delta-v to make the necessary plane change is typically extremely high. On the other hand depots are typically proposed for exploration missions, where this restriction does not apply. Like all forms of low earth orbit rendezvous this still restricts departure windows. By contrast, launching directly from the ground without orbital refueling offers daily launch opportunities though it requires larger and more expensive launchers.[30]
The restrictions on departure windows arise because low earth orbits are susceptible to significant perturbations; even over short periods they are subject to nodal regression and, less importantly, precession of perigee. Equatorial depots are more stable but also more difficult to reach.[30]
For a propellant depot to effectively store cryogenic fluids, boil-off caused by heating from solar and other sources must be mitigated, eliminated,[22] or used for economic purposes.[16] For non-cryogenic propellants, boil-off is not a significant design problem.
United Launch Alliance (ULA) has proposed a cryogenic depot which would use a conical sun shield to protect the cold propellants from solar and Earth radiation. The open end of the cone allows residual heat to radiate to the cold of deep space, while the closed cone layers attenuates the radiative heat from the Sun and Earth.[31]
Approaches to the design of low-earth orbit (LEO) propellant depots are discussed in the 2009 Augustine report to NASA, which "examined the [then] current concepts for in-space refueling."[32] The report determined there are essentially two approaches to refuelling a spacecraft in LEO,
"The [Augustine report] found both of these concepts feasible with current technology, but in need of significant further engineering development and in-space demonstration." The report concluded that, with "some development investment, long-term life-cycle savings may be obtained."[32]
As of March 2010[update], a small-scale refueling demonstration project for reaction control system (RCS) fluids is under development. Canada-based MDA Corporation announced in early 2010 that they were designing a single spacecraft that would refuel other spacecraft in orbit as a satellite-servicing demonstration. "The business model, which is still evolving, could ask customers to pay per kilogram of fuel successfully added to their satellite, with the per-kilogram price being a function of the additional revenue the operator can expect to generate from the spacecraft’s extended operational life."[33]
The plan is that the fuel-depot vehicle would maneuver to an operational communications satellite, dock at the target satellite’s apogee-kick motor, remove a small part of the target spacecraft’s thermal protection blanket, connect to a fuel-pressure line and deliver the propellant. "MDA officials estimate the docking maneuver would take the communications satellite out of service for about 20 minutes."[33]
As of March 2011[update], MDA has secured a major customer for the initial demonstration project. Intelsat has agreed to purchase one-half of the 2,000 kilograms (4,400 lb) propellant payload that the MDA spacecraft would carry into geostationary orbit. Such a purchase would add somewhere between two and four years of additional service life for up to five Intelsat satellites, assuming 200 kg of fuel is delivered to each one.[34] As of March 2010[update], the spacecraft could be ready to begin refueling communication satellites by 2015.[35]
Competitive design alternatives to in-space RCS fuel transfer exist. The ViviSat Mission Extension Vehicle illustrates one alternative approach that would connect to the target satellite similarly to MDA SIS, via the kick motor, but will not transfer fuel. Rather, the Mission Extension Vehicle will use "its own thrusters to supply attitude control for the target."[36] ViviSat believes their approach is more simple and can operate at lower cost than MDA, while having the technical ability to dock with a greater number (90 percent) of the approximately 450 geostationary satellites in orbit.[36]