Propellant Depot - History and Plans

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.

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.

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. As of August 2011, 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 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."

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. 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 US$600,000 under the contract.