Orbital propellant depot
An orbital propellant depot is a cache of propellant that is placed in orbit around Earth or another body to allow spacecraft or the transfer stage of the spacecraft to be fueled in space. It is one of the types of space resource depots that have been proposed for enabling infrastructure-based space exploration. Many depot concepts exist depending on the type of fuel to be supplied, location, or type of depot which may also include a propellant tanker that delivers a single load to a spacecraft at a specified orbital location and then departs. In-space fuel depots are not necessarily located near or at a space station.
Potential users of in-orbit refueling and storage facilities include space agencies, defense ministries and communications satellite or other commercial companies.
Satellite servicing depots would extend the lifetime of satellites that have nearly consumed their orbital maneuvering fuel and are likely placed in a geosynchronous orbit. The spacecraft would conduct a space rendezvous with the depot, or vice versa, and then transfer propellant to be used for subsequent orbital maneuvers. In 2011, Intelsat showed interest in an initial demonstration mission to refuel several satellites in geosynchronous orbit, but all plans have been since scrapped.
A low Earth orbit depot's primary function would be to provide propellant to a transfer stage headed to the Moon, Mars, or possibly a geosynchronous orbit. Since all or a fraction of the transfer stage propellant can be off-loaded, the separately launched spacecraft with payload and/or crew could have a larger mass or use a smaller launch vehicle. With a LEO depot or tanker fill, the size of the launch vehicle can be reduced and the flight rate increased—or, with a newer mission architecture where the beyond-Earth-orbit spacecraft also serves as the second stage, can facilitate much larger payloads—which may reduce the total launch costs since the fixed costs are spread over more flights and fixed costs are usually lower with smaller launch vehicles. A depot could also be placed at Earth-Moon Lagrange point 1 or behind the Moon at EML-2 to reduce costs to travel to the Moon or Mars. Placing a depot in Mars orbit has also been suggested.
In 2024, on Starship’s third integrated flight, intravehicular propellant transfer in orbit was demonstrated, an intervehicle propellant transfer demonstration mission is planned for 2025, as this capability is critical for landing a crew on the Moon with the Starship HLS vehicle.
LEO depot fuels
For rockets and space vehicles, propellants usually take up 2/3 or more of their total mass.Large upper-stage rocket engines generally use a cryogenic fuel like liquid hydrogen and liquid oxygen as an oxidizer because of the large specific impulse possible, but must carefully consider a problem called "boil off", or the evaporation of the cryogenic propellant. The boil off from only a few days of delay may not allow sufficient fuel for higher orbit injection, potentially resulting in a mission abort. Lunar or Mars missions will require weeks to months to accumulate tens of thousands to hundreds of thousands of kilograms of propellant, so additional equipment may be required on the transfer stage or the depot to mitigate boiloff.
Non-cryogenic, earth-storable liquid rocket propellants including RP-1, hydrazine and nitrogen tetroxide, and mildly cryogenic, space-storable propellants like liquid methane and liquid oxygen, can be kept in liquid form with less boiloff than the cryogenic fuels, but also have lower specific impulse. Additionally, gaseous or supercritical propellants such as those used by ion thrusters include xenon, argon, and bismuth.
Propellant launch costs
Ex-NASA administrator Mike Griffin commented at the 52nd AAS Annual Meeting in Houston, Texas, November 2005, that "at a conservatively low government price of $10,000 per kg in LEO, 250 of fuel for two missions per year is worth $2.5 billion, at government rates".Cryogenic depot architectures and types
In the depot-centric architecture, the depot is filled by tankers, and then the propellant is transferred to an upper stage prior to orbit insertion, similar to a gas station filled by tankers for automobiles. By using a depot, the launch vehicle size can be reduced and the flight rate increased. Since the accumulation of propellant may take many weeks to months, careful consideration must be given to boiloff mitigation.In simple terms, a passive cryogenic depot is a transfer stage with stretched propellant tanks, additional insulation, and a sun shield. In one concept, hydrogen boiloff is also redirected to reduce or eliminate liquid oxygen boiloff and then used for attitude control, power, or reboots. An active cryogenic depot is a passive depot with additional power and refrigeration equipment/cryocoolers to reduce or eliminate propellant boiloff. Other active cryogenic depot concepts include electrically powered attitude control equipment to conserve fuel for the end payload.
Heavy lift versus depot-centric architectures
In the heavy lift architecture, propellant, which can be two-thirds or more of the total mission mass, is accumulated in fewer launches and possibly shorter time frame than the depot centric architecture. Typically the transfer stage is filled directly and no depot is included in the architecture. For cryogenic vehicles and cryogenic depots, additional boiloff mitigation equipment is typically included on the transfer stage, reducing payload fraction and requiring more propellant for the same payload unless the mitigation hardware is expended.Heavy Lift is compared with using Commercial Launch and Propellant Depots in this power point by Dr. Alan Wilhite given at FISO Telecon.
Feasibility of propellant depots
Both theoretical studies and funded development projects that are currently underway aim to provide insight into the feasibility of propellant depots. Studies have shown that a depot-centric architecture with smaller launch vehicles could be less expensive than a heavy-lift architecture over a 20-year time frame. The cost of large launch vehicles is so high that a depot able to hold the propellant lifted by two or more medium-sized launch vehicles may be cost effective and support more payload mass on beyond-Earth orbit trajectories.In a 2010 NASA study, an additional flight of an Ares V heavy launch vehicle was required to stage a US government Mars reference mission due to 70 tons of boiloff, assuming 0.1% boiloff/day for hydrolox propellant. The study identified the need to decrease the design boiloff rate by an order of magnitude or more.
Approaches to the design of low Earth orbit propellant depots were also discussed in the 2009 Augustine report to NASA, which "examined the current concepts for in-space refueling". The report determined there are essentially two approaches to refueling a spacecraft in LEO:
- Propellant tanker delivery. In this approach, a single tanker performs a rendezvous and docking with an on-orbit spacecraft. The tanker then transfers propellant and departs. This approach is "much like an airborne tanker refuels an aircraft".
- In-space depot. An alternative approach is for many tankers to rendezvous and transfer propellant to an orbital depot. Then, at a later time, a spacecraft may dock with the depot and receive a propellant load before departing Earth orbit.
In 2010 United Launch Alliance proposed their Advanced Cryogenic Evolved Stage tanker, a concept that dates to work by Boeing in 2006, sized to transport up to of propellant—in early design, a first flight was proposed for no earlier than 2023, with initial usage as a propellant tanker potentially beginning in the mid-2020s. ACES was not funded, but some of the ideas were used in the Centaur stage of the Vulcan Centaur rocket.
Beyond theoretical studies, since at least 2017, SpaceX has undertaken funded development of an interplanetary set of technologies. While the interplanetary mission architecture consists of a combination of several elements that are considered by SpaceX to be key to making long-duration beyond Earth orbit spaceflights possible by reducing the cost per ton delivered to Mars by multiple orders of magnitude over what NASA approaches have achieved, refilling of propellants in orbit is one of the four key elements. In a novel mission architecture, the SpaceX design intends to enable the long-journey spacecraft to expend almost all of its propellant load during the launch to low Earth orbit while it serves as the second stage of the SpaceX Starship, and then after refilling on orbit by multiple Starship tankers, provide the large amount of energy required to put the spacecraft onto an interplanetary trajectory. The Starship tanker is designed to transport approximately of propellant to low Earth orbit. In April 2021, NASA selected the SpaceX Lunar Starship with in-orbit refueling for their initial lunar human landing system.
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, or the depot tanker itself could serve as the second-stage on launch when it is reusable. An on-orbit market for refueling may be created where competition to deliver propellant for the lowest 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. An exploration program based on a depot architecture could be less expensive and more capable, not needing a specific rocket or a heavy lift such as the SLS to support multiple destinations such as the Moon, Lagrange points, asteroids, and Mars.NASA studies in 2011 showed lower cost and faster alternatives than the Heavy Lift Launch System and listed the following advantages:
- Tens of billions of dollars of cost savings to fit the budget profile
- Allowed first NEA/Lunar mission by 2024 using conservative budgets
- Launch every few months rather than once every 12–18 months
- Allows multiple competitors for propellant delivery
- Reduced critical path mission complexity