NASA and Commercial industry combine to outline FTD Propellant Depot plan

by Chris Bergin

A collaboration between experts at numerous NASA centers and commercial companies have created a plan for an “in-space LO2/LH2 PTSD (Propellant Transfer and Storage Demonstration) mission, to affordably support a 2015 demonstration and follow-on missions”, highlighting an exploration architecture built around existing vehicles and Propellant Depots.

PTSD Flagship Technology Demonstration:

Answering a Request For Information (RFI) in June, a broad range of NASA, other US government, academic and industrial participation resulted in a roadmap to enable a flagship demonstration mission of propellant storage and transfer ability in 2015.

Such a mission would build on the United Launch Alliance (ULA) exploration master plan, which removes the need for a Heavy Lift Launch Vehicle (HLV), instead combining the use of current EELV (Evolved Expendable Launch Vehicle) vehicles – such as Atlas V or Delta IV – with an on orbit ability to refuel in space via fuel depots.

See also: ULA claim gap reducing solution via EELV exploration master plan

“We as a group strongly believe that the use of orbital propellant transfer and storage (Depots) provides a breakthrough in space transportation enabling truly affordable, sustainable and flexible exploration to destinations beyond low Earth orbit (LEO),” noted the executive summary of the presentation – acquired by L2.

“We also believe that the most successful approach to a 2015 propellant storage and transfer flagship demonstration will build upon the foundation of the decades of cryogenic propellant experience developed and currently being used in our nation’s EELV fleet.”

Outlining a “simple, robust propellant transfer and storage mission”, the authors claim the architecture supports near term robotic and crewed missions to geostationary orbit (GEO), the Earth-Moon (EM) and Sun-Earth (SE) Lagrange points and the Moon, prior to evolving as technology and demand require, “efficiently supporting every destination in the Flexible Path including crewed missions to Mars.”

Strategically placed at one or more Lagrange points, such depots would remove the need for vehicles to launch with all the propellant required to complete a mission – one of the primary reasons for very large launch vehicles, due to the mass of the propellant they have to launch with.

“Propellant depots offer near-term ability to support demanding space missions without the expense of developing very large rockets to support each new mission. They also enable reuse of in-space stages and provide a market large enough to encourage access to orbit innovation,” noted the presentation.

Although the political refinements to the FY2011 budget proposal are ongoing, supporters of a flexible path exploration architecture note that propellent depots can save up to 57 percent of the launch mass being reserved for fuel, when undertaking high energy missions.

“With the proposed FY2011 budget, NASA intends to pursue a flexible exploration path that will take America to Lagrange points, near Earth objects (NEO’s), lunar and Mars orbit and lunar and eventually the Martian surface. All of these are high energy missions that require an energetic, high performance stage to initiate the journey from LEO,” added the presentation. “Use of LO2/LH2, vs. LO2/Methane, can reduce the initial mass in LEO by up to 57 percent.”

Options remain open on which propellents could be used, although all large-scale robotic and crewed beyond LEO missions propose the use of high efficiency LO2/LH2 propulsion for the majority of their propellant needs in LEO – as noted by the executive summary, and large sections of the presentation.

“To enable propellant storage and transfer to support such missions this decade, it is critical that the 2015 mission include LH2. The proposed flagship mission design takes full advantage of America’s decades of orbital LO2/LH2 CFM (Cryogenic Fluid Management) flight experience.”

Commonality with NASA’s own “Flexible Path” approach to exploration – a large presentation created after the Augustine Committee’s review into Human Space Flight – can also be found as part of the foundation of the PTSD FTD conclusions, which utilizes Orion with an EELV Upper Stage, such as the Atlas Centaur or the Delta Cryogenic Second Stage (DCSS).

Also see’s Flexible Path Review:
Part 1: Battle of the Heavy Lift Launchers – Monster 200mt vehicle noted
Part 2: Manned mission to construct huge GEO and deep space telescopes proposed
Part 3: NASA Flexible Path Evaluation of 2025 human mission to visit an asteriod
Part 4: Taking Aim on Phobos – NASA outline Flexible Path precursor to Man on Mars

“The crewed missions that NASA might conduct this decade are GEO, Lagrange points, lunar and NEOs. NASA will likely rely on Orion’s propulsion module, with storable propellants, for the return propulsion. Our recommendation is that NASA’s depot flagship mission focus on the storage, transfer and handling of LO2/LH2.

“Following the 12 month demonstration mission, a forward looking depot demonstration design would allow an Orion mission to a Lagrange point as early as 2016 using an existing EELV upper stage as the Earth Departure Stage (EDS).”

Historical and current experience are also cited as a positive in enabling a low risk approach towards the propellant depot FTD, with numerous Cryostats already in use or under development for scientific satellites, demonstrating the ability to efficiently store cryogens for long durations.

“America’s existing CFM capability is based on decades of experience storing LH2/LO2 in launch vehicle upper stages. This historic flight experience suggests that with proper design, a propellant depot can efficiently store large quantities of cryogenic liquid, including LH2, for years.

“Settled operations significantly simplify all aspects of orbital CFM enabling maximum use of existing, mature CFM techniques. Beyond greater technical maturity, settled CFM has been flight proven to actually enhance the cryo system operation by reducing liquid heating (much of the heating is absorbed by the ullage), reducing ullage mass (warmer ullage), reducing liquid residuals and improves reliability (avoids the requirement for pump circulation systems).”

Keeping the Cost Low:

The presentation also claims a simple LO2/LH2 depot – derived from existing hardware and CFM experience – can demonstrate all of NASA’s PTSD objectives within a $500m budget in under five years.

“Several groups have independently determined that LO2 and LH2 can be stored efficiently in a compact, light weight, affordable depot design. The simplicity of these depot concepts could support NASA’s 2015 PTSD demonstration mission. The depot could be launched on an Atlas 551 and stay within the proposed budget and schedule. The key to such a depot design is segregating the LO2, LH2 and warm equipment module.

Transverse spinning of the entire depot at ~1 degree per second would also provides a settled fluid environment that simplifies the cryo-fluid storage and handling.

“With effective design, analysis indicates that total system boil-off can be held to 0.01 percent per day, which is below the station keeping requirement. The use of propellant boiloff for station keeping was successfully used on the Saturn S-IVB stage (ullage motor) and hydrogen boil-off is currently used by the Delta IV Cryogenic Second Stage (DCSS) for settling.”

The aforementioned simplicity of the design – along with current technology of the EELV upper stage – are noted as key drivers for keeping the cost and schedule risk low.

“The LH2 module includes a large tank with minimal penetrations. This tank is connected to the mission module by six low conductivity composite struts. These struts, fluid transfer plumbing and wiring are vapor cooled to minimize heat reaching the LH2,” the presentation continued. “The entire tank is encapsulated in a robust, IMLI (insulation) blanket incorporating broad area cooling and MMOD protection.

“The LH2 module is launched empty and filled on orbit with Centaur residual LH2. Launching the LH2 module empty allows the module and adapter to be designed for orbital requirements and not ground and ascent environments. The LH2 module requires no foam insulation and the payload adapter structure can be very light weight and thermally efficient. Based on Centaur mass properties this LH2 module should weigh ~1 mT and have a 4.5 mT LH2 storage capacity.

“To minimize cost and schedule, the LH2 module can be derived from a Centaur tank. This allows hardware to be procured and outfitted in 2010/11 with large scale thermal vacuum testing starting as early as 2011. Near term experience with the system provides risk mitigation that is critical to a successful 2015 demonstration mission.

“Between the Centaur and the LH2 module resides the mission module. This module includes the solar panels, fluid controls, avionics and if desired, an orbital transfer vehicle (OTV) interface and remote berthing arm. The mission module could be derived from NASA’s planned automated rendezvous and docking (AR&D) demonstration vehicle.

“Alternatively the mission module could consist of a standard Atlas payload adapter containing avionics derived from existing spacecraft such as LCROSS, Orbital Express or XSS-11. The LO2 is stored in Centaur’s large LH2 tank. Storage of the LO2 in Centaur’s LH2 tank simplifies long duration, minimum boil-off storage.

“The Centaur is shrouded with IMLI (Insulation) to minimize heating. Hydrogen vapor cooling tubes are routed along key points on the upper stage to eliminate LO2 boil-off. The high sensible and latent heat of hydrogen makes hydrogen the ideal fluid for vapor cooling.”

For the PTSD mission, the LH2 and LO2 are provided as residuals from Centaur. Launching the PTSD on as Atlas 551 provides ~12 mT of combined LH2/LO2 residual if the PTSD weight is optimized, the presentation noted.

“Once on orbit the 2 mT of residual LH2 is transferred from Centaur’s LH2 tank to the thermally efficient LH2 module. Centaur’s LH2 tank is then vented to vacuum, fully evacuating the residual hydrogen gas. Following the LH2 tank ‘safing’, the ~10 mT of residual LO2 is transferred from Centaur’s LO2 tank to Centaur’s LH2 tank for long duration storage. This transfer eases the LO2 storage because Centaur’s LH2 tank is already very thermally efficient.”

Efficient system design, fluid in-flow into a tank, vapor cooling, integrated multi-layer insulation (IMLI) (incorporating micro-meteoroid and orbital debris (MMOD) protection and broad area cooling), transfer tube connection, subcooling and combined system operation are the critical technologies that must be demonstrated.

The Integrated MLI that provides enhanced thermal and MMOD protection is an innovative new technology where polymer substructures are integrated with radiation barriers to provide improved high performance cryogenic thermal insulation systems. IMLI is noted as having a significantly enhanced structural integrity and performance compared to standard cryogenic MLI.

“Standard MLI blankets have a surprisingly high level of protection from the impact of MMOD. The incoming particle is broken apart by the initial impact, and the resulting debris cloud is further retarded and broken up as it progresses through each layer. IMLI enhances standard MLI MMOD tolerance by combining increased and controlled spacing between layers and slightly thicker layers than standard MLI blankets.

“The inclusion of a thicker broad area cooling (BAC) layer further increases MMOD protection of IMLI. Increased spacing enhances the protective capability of the IMLI by providing distance over which a debris cloud spreads, thereby dissipating the energy of the cloud before impact onto the next layer.”

Reducing Boil-Off:

As intimated in the executive summary, the technology required to reduce the amount of LH2 boil-off from the depot is one of the key elements in the forward planning of such an architecture.

Options such as Vapor Cooled Shields (VCS) – using tank boil off (passive) or cryocoolers (active) – can be used to provide cooling. VCS systems have been flown by Ball Aerospace – who were one of the commercial companies involved in the presentation – on LH2 PRSA tanks.

“To incorporate passive cooling technologies, a combination of VCS and in-line para-to-ortho conversion can minimize boil-off losses from LH2 storage tanks. As a passive technique, VCS using cold evaporated hydrogen gas significantly reduces the amount of heat leakage into the LH2 storage tank over long mission durations. Para-to-ortho hydrogen conversion is an endothermic process which adsorbs heat.

“The VCS with a para-to-ortho converter can generate additional cooling and create a refrigeration effect to reduce boil-off of LH2. An integrated VCS, with a parato-ortho hydrogen converter, must be analyzed, designed and ground tested in order to demonstrate its advantage for the cryogenic flagship propellant depot.

“Transition metal catalyst studies with supported iron, nickel, copper, etc., will need to be conducted to optimize the converter performance in terms of percent para-H2 conversion at a given hydrogen feed rate or an equivalent gas hourly space velocity (GHSV). As the design GHSV of the reactor catalyst system increases, the overall size and mass of the para-to-ortho converter can be reduced.”

Broad Area Cooling (BAC) – investigated by NASA between 2004 and 2006 under the In-Space Cryogenic Propellant Depot (ISCPD) Project to substantially reduce or entirely eliminate boil-off losses with a minimal increase of total system mass – might also be integrated with cryogenic propellant storage tanks.

Also, other options, such as subcooling propellent below their boiling point at atmospheric pressure prior to launch, and enhanced analysis tools incorporated in cryogenic propellant utilization (PU) capability – which predicts the thermodynamic state of an on orbit cryogenic upper stage requires a high degree of fidelity – are also examined in the presentation.

Technology Development and Demonstration:

Ground testing would be a key element to a successful demonstration mission, while also reducing the risk involved. The importance of ground testing the new technologies received a large overview in the proposed path outlined in the presentation.

“Relying on numerous new technologies on a single flagship mission is a recipe for failure. A combination of ground component development and system tests in parallel with low cost rideshare demonstration should be pursued concurrently with the flagship depot development.

“Ground testing can demonstrate the technology/system functionality while also providing data to anchor analysis tools enabling better support of the flagship mission. The technology development plan should encompass a continuous process beyond the 2015 flagship mission encouraging continuous developing and allowing new technologies to be incorporated as they mature and offer mission benefits.”

Should NASA’s forward path include Propellent Depots, several existing facilities would be put to good use, such as the Cryo-Fluid testing of Centaur tanks at the ULA facility – which can provide invaluable data on the actual performance of proposed thermal protection schemes.

“Many features of the advanced thermal protection system required to allow long-term, large scale cryogenic storage can be demonstrated on the ground. IMLI, vapor cooling, BAC, light weight tank structures can be effectively demonstrated on the ground.

“This ground testing is directly applicable to orbital depots that rely on settling to handle the fluid-gas interface and is useful for zero-g cryogenic storage.

“While much has been learned from the multi-purpose hydrogen test bed testing conducted at MSFC (Marshall Space Flight Center), it is crucial that further ground testing utilize structures that are representative of flight systems, not 0.5 inch thick ‘boilplate’ aluminum tanks where substantial wall conduction influences the results.

While ULA are listed as offering NASA the loan of a Centaur tank, NASA’s Plum Brook B2 facility classed as “perfect” for large scale integrated testing.

“NASA (GRC, KSC, MSFC, JSC, Ames and GSFC) and ULA are collaborating to pursue such testing at in NASA’s B2 vacuum chamber. ULA has offered to loan NASA a Centaur tank – which would allows for ground based system testing to begin as early as 2011, whilst arriving with its long flight history to boot.

“Centaur’s 194 flights provide a vast database, both settled and zero-g, with which to anchor ground test data. Centaur’s low thermal structural mass and low conductivity are critical for long duration flight cryo-storage tanks. Equally important it is possible to duplicate thermal protection enhancements tested at Plum Brook on upcoming flights thanks to the Atlas’s ability to encapsulate Centaur in the payload fairing during assent,” the presentation continued.

“NASA’s Spacecraft Propulsion Research Facility (B2) is the world’s only facility capable of testing full-scale upper-stage launch vehicles and rocket engines under simulated high-altitude conditions. The engine or vehicle can be exposed for indefinite periods to low ambient pressures, low-background temperatures, and dynamic solar heating, simulating the environment the hardware will encounter during orbital or interplanetary travel.”

Near Term Demonstrations:

Orbital demonstrations are also noted, with a specific reference to the CRYogenic Orbital TEst (CRYOTE) – which offers a near term, low cost method to demonstrate in-space propellant depot CFM technologies.

Several NASA, DoD, and commercial missions with launch dates beginning in 2012 have been identified as candidates, and its compact geometry and minimally intrusive, light weight design make it compatible with numerous upcoming Atlas missions.

“The CRYogenic Orbital TEst (CRYOTE) concept utilizes a very creative rideshare implementation to allow demonstration of cryo-fluid transfer, long duration storage and a host of CFM technologies while minimizing the impact to the launch vehicle. CRYOTE rides inside the Atlas V payload adapter, similar to how LCROSS’s hydrazine tank was mounted.

“Following payload delivery, residual LH2 (or LO2) is transferred from Centaur into the CRYOTE tank. The act of transferring the LH2 from Centaur requires chilldown of the transfer lines and storage tank while demonstrating the ability to fill a receiving vessel.

“Inside of CRYOTE’s tank a multitude of CFM technologies can be demonstrated including propellant management, long term storage, mass gauging and liquid acquisition. CRYOTE offers an end to end, sub-scale demonstration of all propellant depot CFM technologies. Targeted missions are those with at least 1,000 lbs of mission performance margin.”

Playing to the current FY2011 budget proposal – and indeed several elements of the proposed changes via at least the Senate bill – the presentation notes the CRYOTE project includes the involvement of nearly every NASA center, academia and several companies.

“CRYOTE provides a successful example of how we all benefit from collaboration. In addition to orbital rideshare opportunities, some of the emerging reusable suborbital vehicles, such as those under development by Masten Space Systems, Armadillo Aerospace, XCOR, and others can provide platforms for short-duration (3-5min) microgravity CFM experiments.

“While microgravity durations are shorter, those technologies that can be tested in that brief of a window can benefit from much shorter design-build-test iteration cycles. Also, depending on the size of the suborbital vehicle, there can be significant commonality with the orbital CRYOTE system.”

The use of propellant depots in a future architecture do hold a good level of support throughout the future path proposals. One of the more recent mentions came via the expansive SD (Shuttle Derived) HLV (Heavy Lift Launch Vehicle) assessment presentation, when it referenced a joint role of working with commercial and international vehicles in a Beyond Earth Orbit (BEO) architecture.

By way of providing benefits to Lunar and Deep Space missions, the 726 page presentation (L2) – which was the final assessment of the SD HLV, prior to the very recent restart of assessments to note additional information on the In-line SD HLV – noted the addition of propellant deports would significantly improve the overall exploration architecture.

Their approach notably differs by way of the depot being launched by a single HLV to L1, for it then to be refuelled using commercial vehicles, reducing the mass required to launch from Earth’s surface on a Lunar or deep space mission via ‘dry’ – and potentially reusable – landers.

A HLV – of any kind – is not listed in any current ULA or commercial documentation, with experts claiming such a vehicle isn’t required under the EELV and propellant depot architecture.

Related Articles