Propellant depot
Encyclopedia
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 fuelled 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 maneuver
s. 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.
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.
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 thruster
s include xenon
, argon
, and bismuth
.
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."
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.
(along with nuclear "tugs" to take payloads from LEO to other destinations) in the mid-1960s.
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 12000 lbs 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.
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.
, ULA says this mission could launch as soon as 2012 if funded.
The ULA CRYOTE small-scale demonstrations are intended to lead to a ULA large-scale cryo-sat flagship technology demonstration in 2015.
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,
with presentations of optimal depot locations for human space exploration beyond low-Earth orbit,
a proposed simpler (single vehicle) first-generation propellant depot
and six important propellant-depot-related technologies for reusable cislunar transportation.
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.
The CRYOSTAT architecture comprises technologies in the following categories:
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 (52.5 ft) long, 110 cubic metres (3,884.6 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."
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
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.
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
.
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."
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 under the contract.
-18 mission to flight-test centrifugal propellant settling as a cryogenic fuel management technique that might be used in future propellant depots. The proposed Simple Depot PTSD mission utilizes several techniques to achieve adequate settling for propellant transfer.
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.
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.
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 of fuel in the launch load, enabling further refueling of additional satellites after the initial multi-satellite servicing mission is complete.
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.
S.T. Demetriades 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
. This system was called “PROFAC” (PROpulsive Fluid ACcumulator
). There are, however, safety concerns with placing a nuclear reactor in low Earth orbit.
Demetriades' proposal was further refined by Christopher Jones and others 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.
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.
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.
(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.
to NASA
, which "examined the [then] current concepts for in-space refueling." 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."
(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."
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."
, MDA has secured a major customer for the initial demonstration project. Intelsat
has agreed to purchase one-half of the 2000 kilograms (4,409.2 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.
, the spacecraft could be ready to begin refueling communication satellites by 2015.
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."
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.
Propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...
that is placed on an orbit about the Earth or another body to allow spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
to be fuelled 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
Space rendezvous
A space rendezvous is an orbital maneuver during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance . Rendezvous requires a precise match of the orbital velocities of the two spacecraft, allowing them to remain at a constant...
with the depot, or vice versa, and then transfer propellant to be used for subsequent orbital maneuver
Orbital maneuver
In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft.For spacecraft far from Earth—for example those in orbits around the Sun—an orbital maneuver is called a deep-space maneuver .-delta-v:...
s. An in-space fuel depot (alternative name) is not necessarily located near or at a space station
Space station
A space station is a spacecraft capable of supporting a crew which is designed to remain in space for an extended period of time, and to which other spacecraft can dock. A space station is distinguished from other spacecraft used for human spaceflight by its lack of major propulsion or landing...
.
Potential users of in-orbit refuelling and storage facilities include space agencies, defense ministries and communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
or other commercial
Private spaceflight
Private spaceflight is flight above Earth altitude conducted by and paid for by an entity other than a government. In the early decades of the Space Age, the government space agencies of the Soviet Union and United States pioneered space technology augmented by collaboration with affiliated design...
companies.
While larger propellant depots are likely to be placed in low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
(LEO) and either on the way to the Moon at Earth-Moon Lagrange point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
1 (EML-1) or behind the Moon at EML-2, Intelsat
Intelsat
Intelsat, Ltd. is a communications satellite services provider.Originally formed as International Telecommunications Satellite Organization , it was—from 1964 to 2001—an intergovernmental consortium owning and managing a constellation of communications satellites providing international broadcast...
has recently contracted for an initial demonstration mission
Space Infrastructure Servicing
Space Infrastructure Servicing is a spacecraft being developed by Canadian aerospace firm MacDonald, Dettwiler and Associates to operate as a small-scale in-space refueling depot for communication satellites in geosynchronous orbit...
to refuel several satellites in geosynchronous orbit
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
, beginning in 2015. Placing a depot in Mars orbit has also been suggested.
Large upper stage rocket engines generally use cryogenic fuels like liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...
and liquid oxygen
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...
(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.
Non-cryogenic, earth-storable liquid rocket propellants
Liquid rocket propellants
The highest specific impulse chemical rockets use liquid propellants. This type of propellant has a long history going back to the first rockets and is still in use, for example in the Space Shuttle and Ariane 5.-Early development:...
including RP-1
RP-1
RP-1 is a highly refined form of kerosene outwardly similar to jet fuel, used as a rocket fuel. Although having a lower specific impulse than liquid hydrogen , RP-1 is cheaper, can be stored at room temperature, is far less of an explosive hazard and is far denser...
(kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
), hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
and nitrogen tetroxide (NTO), and mildly cryogenic, space-storable propellants like liquid methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, 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 thruster
Ion thruster
An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...
s include xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
, argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
, and bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
.
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."
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.
History and plans
Propellant depots were proposed as part of the Space Transportation SystemSpace Transportation System
The Space Transportation System is another name for the NASA Space Shuttle and Space Shuttle program. However, the name originates from, and can describe a more elaborate set of spacefaring hardware in the 1970s, although this meaning is obscure...
(along with nuclear "tugs" to take payloads from LEO to other destinations) in the mid-1960s.
In October 2009, the Air Force and United Launch Alliance
United Launch Alliance
United Launch Alliance is a joint venture of Lockheed Martin and Boeing. ULA was formed in December 2006 by combining the teams at these companies which provide spacecraft launch services to the government of the United States. U.S...
(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
Slosh dynamics
In fluid dynamics, slosh refers to the movement of liquid inside another object . Strictly speaking, the liquid must have a free surface to constitute a slosh dynamics problem, where the dynamics of the liquid can interact with the container to alter the system dynamics significantly...
, pressure control, RL10 chilldown and RL10 two-phase shutdown operations. "The light weight of DMSP-18 allowed 12000 lbs 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.
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.
, ULA says this mission could launch as soon as 2012 if funded.
The ULA CRYOTE small-scale demonstrations are intended to lead to a ULA large-scale cryo-sat flagship technology demonstration in 2015.
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,
with presentations of optimal depot locations for human space exploration beyond low-Earth orbit,
a proposed simpler (single vehicle) first-generation propellant depot
and six important propellant-depot-related technologies for reusable cislunar transportation.
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.
The CRYOSTAT architecture comprises technologies in the following categories:
- Storage of Cryogenic Propellants
- Cryogenic Fluid Transfer
- Instrumentation
- Automated Rendezvous and Docking (AR&D)
- Cryogenic Based Propulsion
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
Centaur
In Greek mythology, a centaur or hippocentaur is a member of a composite race of creatures, part human and part horse...
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 (52.5 ft) long, 110 cubic metres (3,884.6 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."
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
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
(LEO) for beyond-LEO missions, or at Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
for interplanetary missions—at the AIAA Space 2010 conference. The concept proposes that waste gaseous hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
—an inevitable byproduct of long-term liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...
storage in the radiative heat environment of space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
—would be usable as a monopropellant
Monopropellant rocket
A monopropellant rocket is a rocket that uses a single chemical as its propellant.-Chemical-reaction monopropellant rockets:...
in a solar-thermal propulsion system
Solar thermal rocket
Solar thermal propulsion is a form of spacecraft propulsion that makes use of solar power to directly heat reaction mass, and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. A solar thermal rocket only has to carry the means of capturing solar...
. The waste hydrogen would be productively utilized for both orbital stationkeeping
Orbital stationkeeping
In astrodynamics orbital station-keeping is a term used to describe the orbital maneuvers made by thruster burns that are needed to keep a spacecraft in a particular assigned orbit.For many Earth satellites the effects of the non-Keplerian forces, i.e...
and attitude control, as well as providing limited propellant and thrust to use for orbital maneuvers to better rendezvous
Space rendezvous
A space rendezvous is an orbital maneuver during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance . Rendezvous requires a precise match of the orbital velocities of the two spacecraft, allowing them to remain at a constant...
with other spacecraft that would be inbound to receive fuel from the depot.
As part of the Depot-Based Space Transportation Architecture, ULA has proposed the Advanced Common Evolved Stage
Advanced Common Evolved Stage
The Advanced Common Evolved Stage, or ACES, is a proposed upper stage rocket for use on space launch vehicles. The design concept is from the U.S. company United Launch Alliance . ACES is intended to boost satellite payloads to geosynchronous orbit or, in the case of an interplanetary space...
(ACES) upper stage rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
. 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
Space debris
Space debris, also known as orbital debris, space junk, and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to erosion, explosion...
.
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."
These study contracts for storing/transferring cryogenic propellants and cryogenic depots were signed with Analytical Mechanics Associates, Boeing, Lockheed Martin
Lockheed Martin
Lockheed Martin is an American global aerospace, defense, security, and advanced technology company with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta in March 1995. It is headquartered in Bethesda, Maryland, in the Washington Metropolitan Area....
and Ball Aerospace
Ball Aerospace
Ball Aerospace & Technologies Corp. is a manufacturer of spacecraft, components, and instruments for national defense, civil space and commercial space applications. The company is a wholly owned subsidiary of Ball Corp...
. Each company will receive under the contract.
Advantages
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. 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. 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. Further, a depot architecture is cheaper and is more flexible than a government developed heavy lift rocket such as SLS.Propellant settling
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 DMSPDefense Meteorological Satellite Program
The Defense Meteorological Satellite Program monitors meteorological, oceanographic, and solar-terrestrial physics for the United States Department of Defense. The program is now run by the National Oceanic and Atmospheric Administration. The mission of the satellites was revealed in March 1973...
-18 mission to flight-test centrifugal propellant settling as a cryogenic fuel management technique that might be used in future propellant depots. The proposed Simple Depot PTSD mission utilizes several techniques to achieve adequate settling for propellant transfer.
Propellant transfer
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
Orbital Express
thumb|Orbital Express: ASTRO and NEXTSatOrbital Express was a space mission managed by the United States Defense Advanced Research Projects Agency and a team led by engineers at NASA's Marshall Space Flight Center...
mission in 2007, hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
propellant was successfully transferred between two single-purpose designed technology demonstration spacecraft. The Boeing
Boeing
The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...
servicing spacecraft ASTRO
ASTRO
Autonomous Space Transport Robotic Operations , is an American technology demonstration satellite which was operated as part of the Orbital Express programme. It was used to demonstrate autonomous servicing and refuelling operations in orbit, performing tests on the NEXTSat satellite which was...
transferred propellant to the Ball Aerospace
Ball Aerospace
Ball Aerospace & Technologies Corp. is a manufacturer of spacecraft, components, and instruments for national defense, civil space and commercial space applications. The company is a wholly owned subsidiary of Ball Corp...
serviceable client spacecraft NEXTSat
NEXTSat
NEXTSat, or Next Generation Satellite and Commodities Spacecraft is an American technology demonstration satellite which was operated as part of the Orbital Express programme. It was used as a target spacecraft for a demonstration of autonomous servicing and refuelling operations performed by the...
. Since no crew were present on either spacecraft, this was reported as the first autonomous spacecraft-to-spacecraft fluid transfer.
Refilling
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.
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
Space Infrastructure Servicing
Space Infrastructure Servicing is a spacecraft being developed by Canadian aerospace firm MacDonald, Dettwiler and Associates to operate as a small-scale in-space refueling depot for communication satellites in geosynchronous orbit...
vehicle, the SIS vehicle is being designed to have the ability to orbitally maneuver
Orbital maneuver
In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft.For spacecraft far from Earth—for example those in orbits around the Sun—an orbital maneuver is called a deep-space maneuver .-delta-v:...
to rendezvous
Space rendezvous
A space rendezvous is an orbital maneuver during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance . Rendezvous requires a precise match of the orbital velocities of the two spacecraft, allowing them to remain at a constant...
with a replacement fuel canister after transferring the of fuel in the launch load, enabling further refueling of additional satellites after the initial multi-satellite servicing mission is complete.
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.
S.T. Demetriades proposed a method for refilling by collecting atmospheric gases. Moving in low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
, 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
Magnetohydrodynamics
Magnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes...
engine, which maintains the orbit, compensating for atmospheric drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
. This system was called “PROFAC” (PROpulsive Fluid ACcumulator
Propulsive Fluid Accumulator
A Propulsive Fluid Accumulator is an artificial Earth satellite which collects and stores oxygen and other atmospheric gases for in-situ refuelling of high-thrust rockets. This eliminates the need to lift oxidizer to orbit and therefore brings significant cost benefits.- Propulsive Fluid...
). There are, however, safety concerns with placing a nuclear reactor in low Earth orbit.
Demetriades' proposal was further refined by Christopher Jones and others 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.
Orbital planes and launch windows
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-vDelta-v
In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....
to make the necessary plane change
Orbital inclination change
Orbital inclination change is an orbital maneuver aimed at changing the inclination of an orbiting body's orbit. This maneuver is also known as an orbital plane change as the plane of the orbit is tipped. This maneuver requires a change in the orbital velocity vector at the orbital nodes Orbital...
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.
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
Nodal regression
Nodal precession is the precession of an orbital plane around the rotation axis of an astronomical body such as the Earth. This precession is due to the non spherical nature of a spinning body which creates a non spherical gravitational field....
and, less importantly, precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
of perigee. Equatorial depots are more stable but also more difficult to reach.
Boil-off mitigation
For a propellant depot to effectively store cryogenic fluids, boil-off caused by heating from solar and other sources must be mitigated, eliminated, or used for economic purposes. For non-cryogenic propellants, boil-off is not a significant design problem.Sun shields
United Launch AllianceUnited Launch Alliance
United Launch Alliance is a joint venture of Lockheed Martin and Boeing. ULA was formed in December 2006 by combining the teams at these companies which provide spacecraft launch services to the government of the United States. U.S...
(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.
Feasibility of propellant depots
Approaches to the design of low-earth orbit (LEO) propellant depots are discussed in the 2009 Augustine reportReview of United States Human Space Flight Plans Committee
The Review of United States Human Space Flight Plans Committee was a group reviewing the human spaceflight plans of the United States...
to NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
, which "examined the [then] current concepts for in-space refueling." The report determined there are essentially two approaches to refuelling a spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
in LEO,
- "a single tanker performs a rendezvous and docking with [a spacecraft] on orbit, transfers fuel and separates, much like an airborne tanker refuels an aircraft."
- "many tankers rendezvous and transfer fuel to an in-space depot. Then at a later time, a spacecraft docks with the depot, fuels, and departs Earth orbit."
"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."
In-space refueling demonstration project
, a small-scale refueling demonstration project for reaction control systemReaction control system
A reaction control system is a subsystem of a spacecraft whose purpose is attitude control and steering by the use of thrusters. An RCS system is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow...
(RCS) fluids is under development. Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
-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."
The plan is that the fuel-depot vehicle would maneuver
Orbital maneuver
In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft.For spacecraft far from Earth—for example those in orbits around the Sun—an orbital maneuver is called a deep-space maneuver .-delta-v:...
to an operational communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
, dock at the target satellite’s apogee-kick motor
Apogee kick motor
An apogee kick motor refers to a rocket motor that is regularly employed on artificial satellites destined for a geostationary orbit. As the vast majority of geostationary satellite launches are carried out from spaceports at a significant distance away from Earth's equator, the carrier rocket...
, 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."
, MDA has secured a major customer for the initial demonstration project. Intelsat
Intelsat
Intelsat, Ltd. is a communications satellite services provider.Originally formed as International Telecommunications Satellite Organization , it was—from 1964 to 2001—an intergovernmental consortium owning and managing a constellation of communications satellites providing international broadcast...
has agreed to purchase one-half of the 2000 kilograms (4,409.2 lb) propellant payload that the MDA spacecraft would carry into geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
. 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.
, the spacecraft could be ready to begin refueling communication satellites by 2015.
Competitive
Competition
Competition is a contest between individuals, groups, animals, etc. for territory, a niche, or a location of resources. It arises whenever two and only two strive for a goal which cannot be shared. Competition occurs naturally between living organisms which co-exist in the same environment. For...
design alternatives to in-space RCS
Reaction control system
A reaction control system is a subsystem of a spacecraft whose purpose is attitude control and steering by the use of thrusters. An RCS system is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow...
fuel transfer exist. The ViviSat Mission Extension Vehicle
Mission Extension Vehicle
The Mission Extension Vehicle is a spacecraft concept proposed by ViviSat, a 50/50 joint venture of aerospace firms U.S. Space and ATK, to operate as a small-scale in-space satellite-refueling spacecraft.-Technical capabilities and competition:...
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."
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
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
satellites in orbit.
See also
- Progress (spacecraft)
- Automated Transfer VehicleAutomated Transfer VehicleThe Automated Transfer Vehicle or ATV is an expendable, unmanned resupply spacecraft developed by the European Space Agency . ATVs are designed to supply the International Space Station with propellant, water, air, payload and experiments...
- Liquid rocket propellantsLiquid rocket propellantsThe highest specific impulse chemical rockets use liquid propellants. This type of propellant has a long history going back to the first rockets and is still in use, for example in the Space Shuttle and Ariane 5.-Early development:...
- Robotic Refueling Mission, a 2012/2013 set of NASA experiments on the International Space Station
- Asteroid miningAsteroid miningAsteroid mining refers to the possibility of exploiting raw materials from asteroids and planetoids in space, including near-Earth objects. Minerals and volatiles could be mined from an asteroid or spent comet to provide space construction material , to extract water and oxygen to sustain the lives...
- Propulsive Fluid AccumulatorPropulsive Fluid AccumulatorA Propulsive Fluid Accumulator is an artificial Earth satellite which collects and stores oxygen and other atmospheric gases for in-situ refuelling of high-thrust rockets. This eliminates the need to lift oxidizer to orbit and therefore brings significant cost benefits.- Propulsive Fluid...
, satellite that gathers oxygen and other gasses to supply the depot - Flexible pathFlexible pathThe flexible path is a set of destinations for further manned space exploration in the inner Solar System proposed in the Review of United States Human Space Flight Plans Committee of 2009 and was envisioned as alternative to the Moon-first and Mars-first approaches.In 2010 president Barack Obama...
option of the Review of United States Human Space Flight Plans CommitteeReview of United States Human Space Flight Plans CommitteeThe Review of United States Human Space Flight Plans Committee was a group reviewing the human spaceflight plans of the United States... - In-situ resource utilizationIn-Situ Resource UtilizationIn space exploration, in-situ resource utilization describes the proposed use of resources found or manufactured on other astronomical objects to further the goals of a space mission....
External links
- A Backgrounder for On-Orbit Satellite Servicing, March 2011
- Presentation of Boeing's proposed LEO Propellant Depot, 2007
- Animation of a Boeing depot launch and refuel operation (1 min)
- Plan to mine water on the moon using depots by Bill Stone of the Shackleton Energy CompanyShackleton Energy CompanyShackleton Energy Company was formed in 2007 in Del Valle, Texas with the explicit goal to prepare the equipment and technologies necessary for mining the Moon. Shackleton is a subsidiary of Stone Aerospace.-Approach:...
at TED 2011 (7min) - Augustine Commission meeting in Cocoa Beach, 2009 Advantages of a depot architecture by Jeff GreasonJeff GreasonJeff Greason is a founder of XCOR Aerospace, the Commercial Spaceflight Federation , was the team lead at Rotary Rocket for engine development, and previously worked at Intel....
(25 min) - Cislunar Space, The Next Frontier: enabling exploration by exporting moon water to depots Dr. Paul SpudisPaul SpudisPaul D. Spudis is an American geologist and lunar scientist.In 1976 he earned a B.S. in geology at the Arizona State University. Following his graduation he performed an internship at the Jet Propulsion Laboratory, during the Mars landing of that year. The following year he went to Brown University...
during ISDC 2011 (25 min) - A Settlement Strategy for NASA: the flexible path to Mars using depots and ISRU Jeff GreasonJeff GreasonJeff Greason is a founder of XCOR Aerospace, the Commercial Spaceflight Federation , was the team lead at Rotary Rocket for engine development, and previously worked at Intel....
at ISDC 2011 (42 min.)