Suborbital Reusable Vehicles: A 10-Year Forecast of Market Demand

A new 102-page study “Suborbital Reusable Vehicles: A 10-Year Forecast of Market Demand” is now available in the NSS website Space Transportation section as a 10 MB PDF file.

Suborbital reusable vehicles (SRVs) are creating a new spaceflight industry. SRVs are commercially developed reusable space vehicles that may carry humans or cargo. The companies developing these vehicles typically target high flight rates and relatively low costs. SRVs capable of carrying humans are in development and planned for operations in the next few years. SRVs that carry cargo are operational now, with more planned.

This study forecasts 10-year demand for SRVs. The goal of this study is to provide information for government and industry decision makers on the emerging SRV market by analyzing dynamics, trends, and areas of uncertainty in eight distinct markets SRVs could address. This study was jointly funded by the Federal Aviation Administration Office of Commercial Space Transportation (FAA/AST) and Space Florida, and conducted by The Tauri Group.

Eleven SRVs are currently in active planning, development, or operation, by six companies. The payload capacity of these SRVs ranges from tens of kilograms to hundreds, with the largest currently planned vehicle capacity at about 700 kilograms. A number of SRVs can carry humans, with current designs for one to six passengers, in addition to one or two crew members in some cases. Some will also launch very small satellites.

The study concludes that demand for suborbital flights is sustained and appears sufficient to support multiple providers. Total baseline demand over 10 years exceeds $600 million in SRV flight revenue, supporting daily flight activity. The baseline reflects predictable demand based on current trends and consumer interest. In the growth scenario, reflecting increased marketing, demonstrated research successes, increasing awareness, and greater consumer uptake, multiple flights per day generate $1.6 billion in revenue over 10 years. In a constrained scenario, where consumer and enterprise spending drop relative to today’s trends, multiple weekly flights generate about $300 million over 10 years. Further potential could be realized through price reductions and unpredictable achievements such as major research discoveries, the identification of new commercial applications, the emergence of global brand value, and new government (especially military) uses for SRVs.

More Videos from the International Space Development Conference

The following presentations from the 2012 NSS International Space Development Conference in Washington, DC, are now available on the NSS website.

Jim Muncy Jim Muncy, Government Relations Consultant at XCOR. 64 minute video.
Alexandra Hall Alexandra Hall, Senior Director, Google Lunar X Prize. 44 minute video.
Rick Tumlinson Rick Tumlinson, Founder of Earthlight Institute. 52 minute video.
NSS Awards NSS 25th Anniversary Awards Presentations. 2 videos, 2 hours total.
Paul E. Damphousse Conference Wrap-Up: Paul E. Damphousse, NSS Executive Director. 52 minute video.

More Videos from the International Space Development Conference

The following presentations from the 2012 NSS International Space Development Conference in Washington, DC, are now available on the NSS website.

Michael Lopez-Alegria Michael Lopez-Alegria, President of the Commercial Spaceflight Federation and former NASA astronaut and International Space Station commander. Saturday Luncheon Keynote Address. 73 minute video.
Doug McCuistion Doug McCuistion, Director of NASA’s Mars Exploration Program. 58 minute video.
Mike Simpson International Space Sustainability Panel: Sarah Factor, Office of the Deputy Assistant Secretary of Defense for Space Policy; Philippe Hazane, CNES Representative and Space Attache, French Embassy; Ade Abiodun, Former Chairman of UN COPUOS; Lynn Cline, NASA Deputy Associate Administrator for Human Exploration and Operations, Retired. Chaired by Mike Simpson, Secure World Foundation. 81 minute video.

More Videos from the International Space Development Conference

The following presentations from the 2012 NSS International Space Development Conference in Washington, DC, are now available on the NSS website.

Jeff Greason Jeff Greason: The 20 Year Plan? Saturday Dinner Keynote Address. Greason is president of XCOR Aerospace and was a member of the President’s Human Space Flight Review Committee (Augustine Committee) in 2009. 54 minute video.
Hugh Downs NSS 25th Anniversary Governors Gala at the Smithsonian National Air and Space Museum featuring Master of Ceremonites Hugh Downs (Chairman of the NSS Board of Governors), Senator John Glenn, Commander Scott Carpenter, and Mark Sirangelo (Chairman, Sierra Nevada Space Systems). 70 minute video.

Videos from the International Space Development Conference Online

Videos of presentations from the 2012 NSS International Space Development Conference in Washington, DC, are now available on the NSS website.

Charles F. Bolden Charles F. Bolden, Administrator of NASA. Opening Keynote Address. Prior to becoming NASA Administrator, Bolden was a Shuttle astronaut who flew four missions, including the deployment of the Hubble Space Telescope. 60 minute video.
SpaceX Dragon SpaceX Update. The historic docking of the SpaceX Dragon spacecraft with the International Space Station occured during the ISDC on May 25th. This brief update by SpaceX as it was happening is accompanied by an announcement from NSS Director Jay Wittner that a portion of the remains of the late former NSS Chairman of the Executive Committee, Chris Pancratz, was aboard the Falcon 9 as it launched the Dragon. 10 minute video.
Eric Anderson Eric Anderson is Co-Chairman and Co-Founder of Planetary Resources, Inc., a private asteroid mining venture. He is also Chairman and Co-Founder of Space Adventures, and has sold nearly half a billion dollars in spaceflight missions, including all of the self-funded private citizens to have visited the International Space Station. Eric is a member of the NSS Board of Governors. 60 minute video.
Mark Sirangelo Mark N. Sirangelo, Corporate Vice President, Sierra Nevada Corporation (SNC) and Chairman of SNC Space Systems, whose products range from spacecraft actuators that power the Mars rovers, to hybrid rocket technologies that powered the first commercial astronaut to space, to Dream Chaser, a winged and piloted orbital commercial spacecraft. 39 minute video.
Steve Cook Steve Cook is Director of Space Technologies at Dynetics, which has been involved in both NASA and commercial space ventures, including the NASA Space Launch System, Stratolaunch Air Launch System, and the Google Lunar XPrize. 32 minute video.
Art Dula Art Dula, CEO and founder of Excalibur Almaz, a private spaceflight company, makes major announcements of what his company has been doing and plans to do (including private human cis-lunar flights). Mr. Dula is a member of the NSS Board of Governors. 57 minute video.

Mirrors in Space for Low-Cost Terrestrial Solar Electric Power at Night

The latest addition to the NSS Space Solar Power Library is a paper by Lewis M. Fraas of JX Crystals Inc, Issaquah, WA, on “Mirrors in Space for Low-Cost Terrestrial Solar Electric Power at Night.” An abstract is below; the link goes to a PDF of the full paper.

Abstract: A constellation of 18 mirror satellites is proposed in a polar sun synchronous dawn to dusk orbit at an altitude of approximately 1000 km above the earth. Each mirror satellite contains a multitude of 2 axis tracking mirror segments that collectively direct a sun beam down at a target solar electric field site delivering a solar intensity to that terrestrial site equivalent to the normal daylight sun intensity extending the sunlight hours at that site at dawn and at dusk each day. Each mirror satellite in the constellation has a diameter of approximately 10 km and each terrestrial solar electric field site has a similar diameter and can produce approximately 5 GW per terrestrial site. Assuming that in 10 years, there will be approximately 40 terrestrial solar electric field sites evenly distributed in sunny locations near cities around the world, this system can produce more affordable solar electric power during the day and further into the morning and evening hours. The typical operating hours or power plant capacity factor for a terrestrial solar electric power site can thus be extended by about 30%. Assuming a launch cost of $400/kg as was assumed in a recent NASA Space Power Satellite study for future launch costs, the mirror constellation pay back time will be less than 1 year.

KISS My Asteroid

Here is the Introduction to “Asteroid Retrieval Feasibility Study” by the Keck Institute for Space Studies (KISS) at the Jet Propulsion Laboratory, released April 2, 2012, and available on the National Space Society asteroid page under the “See also” section.

Illustration of an asteroid retrieval spacecraft in the
process of capturing a 7-m, 500-ton asteroid.
(Image Credit: Rick Sternbach / KISS)

The idea to exploit the natural resources of asteroids is older than the space program. Konstantin Tsiolkovskii included in The Exploration of Cosmic Space by Means of Reaction Motors, published in 1903, the “exploitation of asteroids” as one of his fourteen points for the conquest of space. More recently this idea was detailed in John Lewis’ book Mining the Sky, and it has long been a major theme of science fiction stories. The difference today is that the technology necessary to make this a reality is just now becoming available. To test the validity of this assertion, NASA sponsored a small study in 2010 to investigate the feasibility of identifying, robotically capturing, and returning to the International Space Station (ISS), an entire small near-Earth asteroid (NEA) – approximately 2-m diameter with a mass of order 10,000 kg – by 2025. This NASA study concluded that while challenging there were no fundamental show-stoppers that would make such a mission impossible. It was clear from this study that one of the most challenging aspects of the mission was the identification and characterization of target NEAs suitable for capture and return.

In 2011 the Keck Institute for Space Studies (KISS) sponsored a more in-depth investigation of the feasibility of returning an entire NEA to the vicinity of the Earth. The KISS study focused on returning an asteroid to a high lunar orbit instead of a low-Earth orbit. This would have several advantages. Chief among these is that it would be easier from a propulsion standpoint to return an asteroid to a high lunar orbit rather than take it down much deeper into the Earth’s gravity well. Therefore, larger, heavier asteroids could be retrieved. Since larger asteroids are easier to discover and characterize this helps to mitigate one of the key feasibility issues, i.e., identifying target asteroids for return. The KISS study eventually settled on the evaluation of the feasibility of retrieving a 7-m diameter asteroid with a mass of order 500,000 kg. To put this in perspective, the Apollo program returned 382 kg of moon rocks in six missions. The OSIRIS-REx mission [6] proposes to return at least 60 grams of surface material from a NEA by 2023. The Asteroid Capture and Return (ACR) mission, that is the focus of this KISS study, seeks return a 500,000-kg asteroid to a high lunar orbit by the year 2025.

The KISS study enlisted the expertise of people from around the nation including representatives from most of the NASA centers (ARC, GRC, GSFC, JPL, JSC, and LaRC), several universities (Caltech, Carnegie Mellon, Harvard, Naval Postgraduate School, UCLA, UCSC, and USC), as well as several private organizations (Arkyd Astronautics, Inc., The Planetary Society, B612 Foundation, and Florida Institute for Human and Machine Cognition). The people listed below participated in the KISS study and developed the contents of this report. The study was conducted over a six-month period beginning with a four-day workshop in September 2011 followed by a two-day workshop in February 2012, and concluding with the submission of this report in April 2012.

NSS Space Settlement Journal

The NSS Space Settlement Journal has commenced publication this month, beginning with two new papers:

A Contemporary Analysis of the O’Neill – Glaser Model for Space-based Solar Power and Habitat Construction by Peter A. Curreri, NASA Marshall Space Flight Center, and Michael K. Detweiler, Amadeus Consulting

Abstract: Solar Power Satellites, SPS, is a technology that promises unlimited energy free from chemical pollution and green house gas emissions. First expounded by Peter Glaser in 1969, the economic viability was in doubt primarily due to Earth launch costs. Concurrently Gerard O’Neill demonstrated that using 1970’s technologies, SPS could be economically viable if space based materials and labor were utilized, but only after large investments in in-space infrastructure. More recently the O’Neill – Glaser model was reevaluated finding that optimization of space worker habitat size results in substantially improved economics. This paper compares the optimized O’Neill – Glaser economic model with that of Earth launched SPS for the classical electrical power scenario and for the more ambitious scenario to arrest global climate change. The conclusion is that for the energy levels necessary to mitigate global increases in CO2, Earth launched SPS are not economically viable (even with more advanced technologies and more optimistic decreases in launch costs), however with this increase in energy demand the space derived SPS become even more economically compelling and in addition enhance human survival probabilities by enabling a substantial human population to live in space.

The Space Grid: Sun-synchronous orbiting SBSP Satellites with Equatorial orbiting Reflector Satellites for Earth and Space Energy by Royce Jones

Abstract: The development of an economically viable space-based solar power (SBSP) system is critical to the Earth’s future and for future space development. PowerSat technology is also critical to supporting sustainable private and government space ventures, including space lift, space exploration and space infrastructure development. Such a system would greatly expand the need for space lift capability from small reusable launch vehicles for SBSP satellite maintenance to large expendable launch vehicles for deploying GW class SBSP satellites into orbit. The technology needed for SBSP is also needed for in-space solar electric transportation systems needed for space colonization as the technology is the same. The hope has been that gradual improvement in photovoltaic or other technologies such as thermal systems would solve the mass to orbit problem for SBSP systems. However, this in itself does not appear sufficient to make SBSP economically viable. This paper presents a new architectural option for SBSP using a Sun -synchronous orbit (SS-O), wireless power transmission (WPT) and a space power relay (SPR). This new concept is called The Space Grid. The Space Grid relies on the use of two separate satellite constellations. The power satellite (PowerSat) constellation is placed in SS-O dusk to dawn orbit at 800km and has access to constant sunlight and is used to produce the power. The Equatorial reflector satellite (ReflectorSat) constellation is in a 4,000km equatorial orbit and is used to distribute the power to the rectenna on the Earth’s surface. The power is produced by the PowerSats in SS-O and beamed to the ReflectorSats in equatorial orbit and then bounced to the rectenna on the ground. This combination allows for the production and distribution of power to the Earth’s surface without the problems normally associated with non-Geostationary (GEO) PowerSat concepts and without having to place the PowerSats in GEO. The Space Grid reduces the mass of a PowerSat transmitter by approximately 67% by moving it closer then past GEO concepts and allows for higher power levels and therefore much smaller (60%) and less costly rectenna on the ground and reduces the minimum size from 5GW to only 2GW allowing quicker deployment of space energy to solve the Earth’s energy problems. WPT transmission could be microwave or laser but for this paper microwave will be used for easier comparison with past concepts.

The NSS Space Settlement Journal is an online, high-quality, peer-reviewed journal. NASA Liaison for the Journal is Simon “Pete” Worden, NASA Ames Research Center. Editors of the Journal are:

Al Globus, San Jose State University, Editor in Chief
Fred Becker, National Space Society
Anita Gale, International Space Settlement Design Competition
Peter Garretson, National Space Society
Mark Hopkins, National Space Society
John Lewis, University of Arizona
Scott Pace, George Washington University
Joseph Palaia, 4Frontiers Corporation

See Call for Papers if you are interested in contributing to the Journal.

International Space Solar Power Symposium Videos Now Online

Videos from the 2011 International Space Solar Power Symposium are the latest addition to the Space Solar Power Library on the National Space Society website.

Space Solar Power: The First International Assessment of Opportunities, Issues, and Potential Pathways Forward
John C. Mankins (Artemis Innovation) – 41 minutes

Space Solar Power: Near-Term Demonstrations and Applications (We Can Get There From Here)
Seth Potter (Boeing Company) – 49 minutes

Space Solar Power: Advantages of Space Solar Power
Darel Preble (Space Solar Power Institute) – 15 minutes

Space Solar Power: The Space Power Grid
Video produced by Georgia Tech School of Engineering – 5 minutes

Space Solar Power: Naval Research Laboratory Sandwich Panel R&D Status Update
Paul Jaffe (Naval Research Laboratory) – 65 minutes

Space Solar Power: Two Cell High Efficiency Rainbow Concentrator, the Space Alternative for Clean Energy
Thomas C. Taylor (Exploration Partners LLC) – 18 minutes

Space Solar Power: Asteroid Capture for Space Solar Power
Stephen D. Covey (Former Director of Research & Development for Applied Innovation) – 27 minutes

Space Solar Power: Towards an Early Profitable PowerSat
Al Globus (San Jose State University) – 24 minutes

Space Solar Power: A US-India Power Exchange Towards a Space Power Grid
Naryanan Komerath (Georgia Institute of Technology) – 23 minutes

Space Solar Power: Studies of Space Solar Power Systems (SSPS) in Japan
Tatsuhito Fujita (JAXA) – 18 minutes

Space Solar Power: WPT Experiments to Enable Future Solar Power Satellites
Nobuyuki Kaya (Kobe University, Japan) – 25 minutes

Orbital Propellant Depots: Building the Interplanetary Highway

by John K. Strickland, Jr.

August 5, 2011, was a highly significant date in the history of the space program. On that date, NASA announced contracts awarded to four aerospace companies to define demonstration missions to test the capabilities of cryogenic propellant depots. The contracts effectively created a design competition which would lead to the selection of a company to build an actual prototype depot. Cryogenic propellant depots are a critical component of our entire future space exploration and development effort.

A cryogenic propellant depot can rightly be called “a gas station in space.” If there were no gas stations along the interstate, you could only go about 150-200 miles from home by car before you would have to return. Just as gas stations extend the range of your car, orbital propellant depots would extend the range of your rockets.

For rockets and space vehicles, propellants usually take up 2/3 or more of their total mass. If a rocket puts a payload into an orbit from where the payload is intended to go further, such as to the Moon, that payload has to contain another smaller rocket and a final payload. Once in orbit, the first rocket’s upper stage is usually “out of gas” and since there are no gas stations in orbit right now, it becomes very expensive, useless space junk. If the “final” payload is expected to land on the Moon or Mars, it again has to have its own landing rocket and an even smaller final payload. This can be referred to as the Russian Nested Doll (Matryoshka) style architecture, with smaller rockets inside (or on top) of larger rockets. (A Russian Doll is a metaphor for an object with one or a series of similar objects inside it.) This is just the way things have had to be done since the start of the space age, since we have never had propellant depots in space. If a stage could be re-fueled in orbit, it could then be reused more than once.

When payloads are launched with cryogenic propellants, the propellants have to be loaded at the last minute since they start boiling off right away. A propellant depot does several very important things:

  1. It allows propellant to be loaded via docking ports from tanker rockets or propellant delivery capsules.
  2. It keeps the propellant from boiling away using multiple methods.
  3. It allows the propellants to be off-loaded into empty space vehicles and rocket stages just before a mission starts, while in micro or “zero” gravity in orbit, using the same or similar docking ports.
  4. It protects the fuel store with micrometeorite and space debris bumpers and maintains orbital stability and attitude, especially by aiming the solar shield towards the sun, and reports its condition and fuel status to the ground.

The typical propellant depot would contain liquid hydrogen and liquid oxygen, which respectively are about 430 and 300 degrees below zero Fahrenheit. To keep the propellant cold so it does not boil away, a depot can use four methods:

  1. Multi-layered super-insulation around the tank to reduce heat conduction.
  2. Structural isolation of the tank (like a thermos bottle) to reduce points where heat can be conducted through the structure into the tank. The depot will already be in a vacuum which is “free insulation.”
  3. A multi-layered sun-shield to reduce radiated heat from the Sun and Earth.
  4. An active cryo-cooler powered by solar panels to keep the fuel cold or to re-condense it as fast as it boils off.
Cryogenic propellant depot with single sunshade. Image credit: United Launch Alliance, B. Kutter, 2008.
Cryogenic propellant depot with single sunshade. Image credit: United Launch Alliance, B. Kutter, 2008.

NASA will probably select one of the four company designs to create a prototype depot. The designs will surely incorporate at least three of the four methods listed above. A prototype depot design may be based on an existing upper stage with its tanks. The initial prototype, which would cost between 200 and 300 million dollars, might not use the cryo-cooler part if it can demonstrate high efficiency with the static parts of its cooling system. Some companies have sophisticated and detailed designs which have existed for several years. One design might be able to keep boil-off losses to 1/10 of 1 percent a day or only 3 percent a month. It would probably hold at least 30 tons of propellants. Current cryo-coolers need to be scaled up from ones designed to keep space telescope instruments cold. For long duration propellant storage, a large cryo-cooler would definitely be needed, but it would not use a huge amount of power. The Spitzer space telescope maintained an insulated supply of liquid helium for its instruments without a cryo-cooler for 66 months.

A very significant result of having propellant depots in LEO is that it allows a spacecraft to be launched “dry” without fuel. This means that the total mass of the vehicle can be reduced by as much as 2/3 (the fuel), thus allowing a much larger spacecraft to be launched, or the same size spacecraft on a smaller rocket.

The same scenario can be used with depots placed at various destinations, such as geosynchronous orbit (GEO), L1, L2, or lunar or Mars orbit, where a depot would supply high performance cryogenic propellants to reach the Lunar or Martian surface. Depots can be sent to any destination orbit, since it is actually easier to keep the propellant cold away from the warm Earth’s infra-red radiation. The non-insulated or minimally insulated fuel tanks used by current spacecraft would lose all of their propellant during a trip to Mars, so only low performance (non-cryogenic) propellants have been used there to date. The depot can thus be considered as a component of a reusable in-space transport system that can do more than the sum of its parts.

Placing depots in remote destinations is the same as building gas stations in the mountains or the desert, far from the city. It allows you to conduct a full range of operations in the area of the depot with its secure propellant supply. If propellants can be created at the destinations from local materials, a local depot would provide a place to store them until they are needed. Since a depot can be a source of propulsion, energy, oxygen, and water, crew refuges may also be attached to remote depots as backup survival sites.

The cryogenic propellant depot is not only a critical part of the road beyond Low Earth Orbit (LEO), but would also be a boon for conducting routine operations in LEO. With the retirement of the Shuttle, the capacity to deliver large payloads to specific locations in LEO has been temporarily lost. Locations such as the space station or a propellant depot are usually thousands of miles from where a booster first enters orbit, and a booster, unlike the Shuttle, cannot move payloads around once in orbit.

Propellant depots offer an elegant solution to this problem because they could supply fuel to a reusable, insulated LEO space tug. The tug would remove the need for large payloads to carry their own propulsion systems to maneuver to locations such as the space station, the propellant depot, or other destinations. A space tug could also be used to rescue astronauts if they became stranded in an orbit away from the space station. A space tug was originally to be part of the space station program but was never funded. Such a space tug could be based on existing upper stages or upgrades of existing space station cargo delivery vehicles.

A re-usable LEO to GEO space tug can remove the need for the expendable upper stages currently needed to reach GEO. Such a tug would not be possible without an orbital propellant depot where it can refuel.

A low or Zero Boil-Off (ZBO) depot can be considered to be a propellant accumulator, since multiple small tanker vehicles can add propellant until the supply is sufficient for large missions. The value of a propellant accumulator can be illustrated by its potential use had it been included in the now-cancelled Constellation architecture. In that architecture, an Ares V would have been launched with a full lunar stack similar to that carried by a Saturn V, minus the crew capsule. NASA would have then had about two weeks to launch a crew to the orbiting lunar stack on an Ares I before enough of the hydrogen fuel boiled off to turn the multi-billion dollar stack into orbiting space junk and wasting the entire Ares V launch. Even a medium-sized cryo-cooler docked with the lunar stack in orbit would have been sufficient to keep the fuel cold. With a depot, a lunar stack could be launched dry (meaning a much lighter stack is needed, thus requiring a smaller booster, or alternatively a larger stack with the same size booster). The stack would rendezvous with the depot, load the dry stage that takes the stack to the Moon with propellant, and the stack would then leave Low Earth Orbit.

Using depots to re-fuel upper stages would mean that Heavy Lift Vehicles (HLVs) can be smaller than they would otherwise need to be. For example, a Falcon Heavy class HLV (50 metric tons) would be able to launch a dry payload equivalent to what an SLS class HLV (125 metric tons) would be able to launch wet, at a small fraction of the cost.

The new commitment of NASA money to propellant depot design lends more credibility to the concept. There are no fundamental barriers to building depots — it is an idea which has existed on paper for decades but for which no development money has been available. The longtime lack of funding acted to reduce the credibility of the idea despite its inherent value and technical feasibility. In addition, the business-as-usual focus on expendable launchers fostered a general lack of interest. Funding for depots was suggested by the second Augustine Commission in 2009 and is supported by Dr. Robert Braun, NASA Chief Technologist, as part of NASA’s advanced technologies initiative.

The four companies receiving the NASA contracts are Analytical Mechanics Associates Inc., Hampton, Va.; Ball Aerospace & Technologies, Boulder, Colo.; The Boeing Co., Huntington Beach, Calif.; and Lockheed Martin Space Systems Co., Littleton, Colo. Each contract is worth up to $600,000.

References/Links

This article as a permanent web page on the NSS website

Evolving to a Depot-Based Space Transportation Architecture (PDF paper 2010)

A Practical, Affordable Cryogenic Propellant Depot Based on ULA’s Flight Experience (PDF paper 2008)

Realistic Near-Term Propellant Depots: Implementation of a Critical Spacefaring Capability (PDF paper 2009)

LEO Propellant Depot: A Commercial Opportunity? (PDF slides 2007)

The Case for Orbital Propellant Depots (slides 2008)

Space Gas Station Would Blast Huge Payloads to the Moon (Popular Mechanics article 2009)

On-Orbit Propellant Resupply Options for Mars Exploration Architectures (PDF paper 2006)

NASA interest in an interplanetary highway supported by Propellant Depots (NASAspaceflight.com 2011)

Cryogenic Propellant Depots Design Concepts and Risk Reduction Activities (PDF slides 2011)