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.


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)

New Book: Psychology of Space Exploration

A new NASA publication is now available for free download from the NSS website: Psychology of Space Exploration — Contemporary Research in Historical Perspective, edited by Douglas Vakoch. The 267-page book is NASA Special Publication SP-4411 (2011).

Publishers description:

As we stand poised on the verge of a new era of spaceflight, we must rethink every element, including the human dimension. This book explores some of the contributions of psychology to yesterday’s great space race, today’s orbiter and International Space Station missions, and tomorrow’s journeys beyond Earth’s orbit. Early missions into space were typically brief, and crews were small, often drawn from a single nation. As an intensely competitive space race has given way to international cooperation over the decades, the challenges of communicating across cultural boundaries and dealing with interpersonal conflicts have become increasingly important, requiring different coping skills and sensibilities from “the right stuff” of early astronauts.

As astronauts travel to asteroids or establish a permanent colony on the Moon, with the eventual goal of reaching Mars, the duration of expeditions will increase markedly, as will the psychosocial stresses. Away from their home planet for extended times, future spacefarers will need to be increasingly self-sufficient and autonomous while they simultaneously deal with the complexities of heterogeneous, multicultural crews. Psychology of Space Exploration: Contemporary Research in Historical Perspective provides an analysis of these and other challenges facing future space explorers while at the same time presenting new empirical research on topics ranging from simulation studies of commercial spaceflights to the psychological benefits of viewing Earth from space.

In addition to examining contemporary psychological research, each essay also explicitly addresses the history of the psychology of space exploration. Leading contributors to the field place the latest theories and empirical findings in historical context by examining changes in space missions over the past half century, as well as reviewing developments in psychological science during the same period. The essays are innovative in their approaches and conclusions, providing novel insights for behavioral researchers and historians alike.

SpaceWorks Commercial Releases Analysis of Its First Revenue Space Solar Power Satellite Concept

The latest addition to the NSS Space Solar Power Library is a new paper released by SpaceWorks Commercial entitled “Operational Demonstration of Space Solar Power (SSP): Economic Analysis of a First Revenue Satellite (FRS)” [PDF], presented at the recent 28th International Symposium on Space Technology and Science (ISTS) in Japan.

The FRS would be a mid-power (1-20 MW of delivered power) space-to-ground demonstrator of SSP. The purpose would be two-fold: prove the end-to-end technical capability and then demonstrate operations over multiple years. The system would be turned over to commercial operators for public/private service.

Space Solar Power with Small, Operational Single-Launch Satellites

The latest addition to the NSS Space Solar Power Library is a paper by Al Globus which he presented at the 2011 International Space Development Conference: Towards an Early Profitable Power Satellite Part II. Globus investigated technologies and designs that could deliver small, operational single-launch power satellites for niche markets. A reasonably sized R&D program could jump start the project into a vigorous space solar power industry. Indeed, the necessary precursor work may be significantly easier than the paper suggests, with an almost 31% efficiency under space conditions. And if you missed it, here is Part 1 of the paper from 2010.

ISDC 2011 Video Presentation – Paul Spudis

Paul Spudis Dr. Paul Spudis: Cislunar Space – The Next Frontier. Dr. Spudis, of the Lunar and Planetary Institute in Huston, was presented the 2011 NSS Space Pioneer Award for Scientific and Engineering Paper for his paper on “Mission and Implementation of an Affordable Lunar Return.” 32 minute video (slides and paper included).

New International Law Textbook Discusses Lunar Real Estate

A new international law textbook contains an article on “Space Settlements, Property Rights, and International Law: Could a Lunar Settlement Claim the Lunar Real Estate it Needs to Survive?” by Alan Wasser and Douglas Jobes. Wasser, a former CEO of the National Space Society, argues in favor of “Land Claims Recognition” to help fund lunar settlements.

If and when the Moon and Mars are settled in the future through other incentives, the nations of Earth will eventually have to recognize these settlements’ authority over their own land. But to create an incentive now, governments would need to commit to recognizing that ownership in advance, rather than long after the fact.

Land claims recognition legislation would commit the Earth’s nations, in advance, to allowing a true private Lunar settlement to claim and sell (to people back on Earth) a reasonable amount of Lunar real estate in the area around the base, thus giving the founders of the Moon settlement a way to earn back the investment they made to establish the settlement.

The 42-page article was originally published in the Journal of Air Law and Commerce, Vol. 73, No. 1, 2008. The full article in PDF format is available on the NSS website as part of the NSS Lunar Bases and Settlement Library (“Additional Papers” section).

The textbook, International Law: Contemporary Issues and Future Developments, edited by Sanford R. Silverburg, was published in March 2011 by Westview Press.

New Coalition for Space Exploration PSA Video

The Coalition for Space Exploration, of which the National Space Society is a member, has produced another in its series of short public service announcement videos intended to provide some answers to the question “Why spend money on space when we have so many problems here on Earth?”

The new video is called “Think Outside the Circle” and can be viewed on the NSS website by clicking on the image below.

Think Outside the Box

NSS Board Members Speak Their Minds

Several NSS Board members were “caught” on camera at the recent International Space Development Conference, speaking about how they got interested in NSS, the importance of space, and related topics. These short videos have been added to the Board biography pages on the NSS website. Board members featured are current Directors Anita GaleMark Hopkins, Kirby Ikin, Jeffrey Liss, Joe Redfield, Stan Rosen, and Jay Wittner; Governors Lon Levin and Frederick I. Orway III; and former Director Richard Godwin.

Defending Planet Earth: National Research Council Final Report

The latest addition to the NSS website Planetary Defense Library is Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies: Final Report (June 2010), by the National Research Council. The 132 page book is available for free download or for purchase in hard copy.

Abstract: The United States spends approximately $4 million each year searching for near-Earth objects (NEOs). The objective is to detect those that may collide with Earth. The majority of this funding supports the operation of several observatories that scan the sky searching for NEOs. This, however, is insufficient in detecting the majority of NEOs that may present a tangible threat to humanity. A significantly smaller amount of funding supports ways to protect the Earth from such a potential collision or “mitigation.” In 2005, a Congressional mandate called for NASA to detect 90 percent of NEOs with diameters of 140 meters or greater by 2020. Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies identifies the need for detection of objects as small as 30 to 50 meters as these can be highly destructive. The book explores four main types of mitigation including civil defense, “slow push” or “pull” methods, kinetic impactors and nuclear explosions. It also asserts that responding effectively to hazards posed by NEOs requires national and international cooperation. Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies is a useful guide for scientists, astronomers, policy makers and engineers.

National Space Society Announces "NSS in Second Life" Machinima Contest Winner

The National Space Society announces that the winning video in the 2010 NSS in Second Life Machinima Contest is “Dreamer’s Journey” by Rocksea Renegade. Second Life is an online 3D virtual world where participants can interact as avatars. Machinima is an art form consisting of computer animation of such virtual worlds. Entries were required to have an outer space theme, with 50% of the video taking place in the National Space Society Second Life simulation. Musician Craig Lyons graciously permitted his music to be used in the videos.

Anyone may view the winning video plus the 10 other videos entered into this contest on the NSS website. Participation in NSS in Second Life requires registration and installation of free viewing software.