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Orbital propellant depot

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NASA concept for a propellant depot from 1971

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 a initial demonstration mission to refuel several satellites in geosynchronous orbit, beginning in 2015. Placing a depot in Mars orbit has also been suggested. [1]

Rockets using cryogenic fuels like liquid hydrogen and liquid oxygen (LOX) 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. Since they are not mass critical, depots can protect their cryogenic propellants with sun shields and refrigeration equipment.[2]

Propellants used by ion thrusters include xenon and argon[3] and bismuth.[4] The upper stage of the SpaceX Falcon 9 chemical rockets use the Liquid rocket propellants LOX and the RP-1 version of kerosene. Other chemicals are used for in-space manoeuvring.[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]

History and Plans

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]

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, [10] with presentations of optimal depot locations for human space exploration beyond low-Earth orbit,[11] a proposed simpler (single vehicle) first-generation propellant depot[9] and six important propellant-depot-related technologies for reusable cislunar transportation.[12]

The ULA CRYOTE small-scale demonstrations are intended to lead to a ULA large-scale cryo-sat flagship technology demonstration in 2015.[9]

NASA also has plans to mature techniques for enabling and enhancing space flights that use propellant depots in the "Cryogenic Propellant Transfer and Storage Demonstration Mission" (PTSD). The PTSD vehicle is expected to be launched to LEO in 2015.[13]

The PTSD architecture comprises technologies in the following categories:[13][full citation needed]

  • 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 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."[14]

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.[15]

Engineering design issues

Another NASA concept from 1970

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,[16] or used for economic purposes.[15]

Sun shields

United Launch Alliance (ULA) has proposed a depot which would use a conical sun shield to protect the cryogenic propellant from solar and Earth radiation. The open end of the cone allows residual heat to radiate to deep space.[17]

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 DMSP-18 mission to flight-test centrifugal propellant settling as a cryogenic fuel management technique that might be used in future propellant depots.[16] The proposed Simple Depot PTSD mission utilizes several techniques to achieve adequate settling for propellant transfer.[14]

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 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.[18]

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.[19]

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.[20] 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.[14]

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-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.[citation needed] 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.[21]

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.[21]

Feasibility of propellant depots

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."[22] The report determined there are essentially two approaches to refuelling a spacecraft 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."[22]

In-space refueling demonstration project

Main article: MDA Space Infrastructure Servicing vehicle

As of March 2010, 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."[23]

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."[23]

As of March 2011, 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.[24] As of March 2010, the spacecraft could be ready to begin refueling communication satellites by 2015.[25]

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."[26] 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.[26]

See also

References

  1. ^ Jon Goff; et al. (2009). "Realistic Near-Term Propellant Depots" (PDF). American Institute of Aeronautics and Astronautics. {{cite web}}: Explicit use of et al. in: |author= (help) page 13
  2. ^ Jon Goff; et al. (2009). "Realistic Near-Term Propellant Depots" (PDF). American Institute of Aeronautics and Astronautics. {{cite web}}: Explicit use of et al. in: |author= (help) page 10
  3. ^ "Ad Astra website describing VASIMR technology".
  4. ^ Dean R. Massey, Lyon B. King and Jason M. Makela. "Development of a Direct Evaporation Bismuth Hall Thruster" (PDF). AIAA 2008-4520.
  5. ^ http://www.spacex.com/Falcon9UsersGuide_2009.pdf Falcon 9 User Guide
  6. ^ http://www.nasa.gov/pdf/138033main_griffin_aas1.pdf Remarks For AIAA Space 2005 Conference & Exhibition
  7. ^ Dewar, James. "To The End Of The Solar System: The Story Of The Nuclear Rocket". Apogee, 2003
  8. ^ Successful Flight Demonstration Conducted by the Air Force and United Launch Alliance Will Enhance Space Transportation: DMSP-18, United Launch Alliance, October 2009, accessed 2011-01-10.
  9. ^ a b c Propellant Depots Made Simple, Bernard Kutter, United Launch Alliance, FISO Colloquium, 2010-11-10, accessed 2011-01-10.
  10. ^ Future In-Space Operations (FISO) Working Group presentations, FISO, 2011-01-07, accessed 2011-01-10.
  11. ^ Potential Propellant Depot Locations Supporting Beyond-LEO Human Exploration, Daniel R. Adamo, FISO Colloquium, 2010-10-13, accessed 2011-08-22.
  12. ^ Top 10 Technologies for Reusable Cislunar Transportation, Dallas Bienhoff, Boeing, FISO Colloquium, 2010-12-01, accessed 2011-01-10.
  13. ^ a b http://nspires.nasaprs.com/external/viewrepositorydocument/cmdocumentid=230993/PTSD%20WhitePaper.pdf Cryogenic Propellant Transfer and Storage Demonstration Mission Point-of-Departure
  14. ^ a b c Bergin, Chris (2011-08-10). "NASA interest in an interplanetary highway supported by Propellant Depots". NASA Spaceflight. Retrieved 2011-08-11. a LO2/LH2 PTSD (Propellant Transfer and Storage Demonstration) mission by 2015. ... it would be launched on an Atlas 551 ... [which] would provide ~12 mT of Centaur residuals (combined LH2 and LO2) in a 28.5 degrees by 200 nm circular LEO.
  15. ^ a b Zegler, Frank (2010-09-02). "Evolving to a Depot-Based Space Transportation Architecture" (PDF). AIAA SPACE 2010 Conference & Exposition. AIAA. p. 3. Retrieved 2011-01-25. the waste hydrogen that has boiled off happens to be the best known propellant (as a monopropellant in a basic solar-thermal propulsion system) for this task. A practical depot must evolve hydrogen at a minimum rate that matches the station keeping demands. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  16. ^ a b Jon Goff; et al. (2009). "Realistic Near-Term Propellant Depots" (PDF). American Institute of Aeronautics and Astronautics. {{cite web}}: Explicit use of et al. in: |author= (help)
  17. ^ Bernard F. Kutter; et al. (2008). "A Practical, Affordable Cryogenic Propellant Depot Based on ULA's Flight Experience" (PDF). AIAA. {{cite web}}: Explicit use of et al. in: |author= (help)
  18. ^ "Boeing Orbital Express Conducts First Autonomous Spacecraft-to-Spacecraft Fluid and Component Transfer". Boeing. April 17, 2007.
  19. ^ "Human Lunar Exploration Mission Architectures, page 22" (PDF). NASA. March 1–2, 2004.
  20. ^ de Selding, Peter B. (2011-03-18). "Intelsat Signs Up for MDA's Satellite Refueling Service". Space News. Retrieved 2011-03-20. more than 40 different types of fueling systems ... SIS will be carrying enough tools to open 75 percent of the fueling systems aboard satellites now in geostationary orbit. ... the SIS spacecraft is designed to operate for seven years in orbit but that it is likely to be able to operate far longer than that. Key to the business model is MDA's ability to launch replacement fuel canisters that would be grappled by SIS and used to refuel dozens of satellites over a period of years. These canisters would be much lighter than the SIS vehicle and thus much less expensive to launch.
  21. ^ a b http://www.thespaceshow.com/detail.asp?q=1420
  22. ^ a b HSF Final Report: Seeking a Human Spaceflight Program Worthy of a Great Nation, October 2009, Review of U.S. Human Spaceflight Plans Committee, p. 65-66.
  23. ^ a b de Selding, Peter B. (2010-03-03). "MDA Designing In-orbit Servicing Spacecraft". Space News. Retrieved 2011-03-14. the refueling vehicle would dock at the target satellite's apogee-kick motor, peel off a section of the craft'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. ... The servicing robot would have an in-orbit life of about five years, and would carry enough fuel to perform 10 or 11 satellite-refueling or orbital-cleanup missions.
  24. ^ de Selding, Peter B. (2011-03-14). "Intelsat Signs Up for Satellite Refueling Service". Space News. Retrieved 2011-03-15. if the MDA spacecraft performs as planned, Intelsat will be paying a total of some $200 million to MDA. This assumes that four or five satellites are given around 200 kilograms each of fuel. ... The maiden flight of the vehicle would be on an International Launch Services Proton rocket, industry officials said. One official said the MDA spacecraft, including its 2,000 kilograms of refueling propellant, is likely to weigh around 6,000 kilograms at launch.
  25. ^ "Intelsat Picks MacDonald, Dettwiler and Associates Ltd. for Satellite Servicing". press release. CNW Group. Retrieved 2011-03-15. MDA plans to launch its Space Infrastructure Servicing ("SIS") vehicle into near geosynchronous orbit, where it will service commercial and government satellites in need of additional fuel, re-positioning or other maintenance. ... MDA and Intelsat will work together to finalize specifications and other requirements over the next six months before both parties authorize the build phase of the program. The first refueling mission is to be available 3.5 years following the commencement of the build phase. ... The services provided by MDA to Intelsat under this agreement are valued at more than US$280 million.
  26. ^ a b Morring, Frank, Jr. (2011-03-22). "An End To Space Trash?". Aviation Week. Retrieved 2011-03-21. ViviSat, a new 50-50 joint venture of U.S. Space and ATK, is marketing a satellite-refueling spacecraft that connects to a target spacecraft using the same probe-in-the-kick-motor approach as MDA, but does not transfer its fuel. Instead, the vehicle becomes a new fuel tank, using its own thrusters to supply attitude control for the target. ... [the ViviSat] concept is not as far along as MDA.{{cite news}}: CS1 maint: multiple names: authors list (link)
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