NASA interest in an interplanetary highway supported by Propellant Depots

by Chris Bergin

NASA’s Human Architecture Team (HAT) is actively working on a roadmap towards evolvable demonstrations of Propellant Depots – with a potential goal of setting up an “interplanetary highway” to enable low cost exploration. With proposals being sought, industry sources point to a small, 30 metric ton capacity, Centaur derived depot as an initial leading candidate.

Propellant Depots:

Based around a solution to one of the central problems for Launch Vehicles and Spacecraft, propellant depots are a highly favored approach to removing the need to launch with all the fuel required to complete an entire mission – in turn allowing Launch Vehicles to lift more hardware into space.

They are – for lack of a better phrase – gas stations for spacecraft, a helpful tool for the new phase of exploration, which requires spacecraft to utilize a large amount of fuel to adventure out of Low Earth Orbit (LEO) – and return back home.

The potential ability to refuel cryogenic propulsion stages on-orbit would provide an innovative paradigm shift for space transportation, supporting NASA’s Exploration program as well as deep space robotic, national security and commercial missions.

Refueling enables large Beyond Earth Orbit (BEO) missions without relying “solely” on super Heavy Lift Vehicles (HLVs), from early Lagrange point missions to near Earth objects (NEO), the lunar surface and eventually Mars. Earth-to-orbit launch could also be optimized to provide competitive, cost-effective solutions that allow sustained exploration.

NASA interest in Propellant Depots is no secret, as much as the subject never seems to gain enough momentum via NASA’s public comments. However, internally – especially in recent months – NASA teams have been openly pressing forward with planning for at least a demonstration of the technology.

Indeed, it was this time last year when documentation noted “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),” as NASA teams discussed the viability of a LO2/LH2 PTSD (Propellant Transfer and Storage Demonstration) mission by 2015.

With the United Launch Alliance (ULA) also basing their exploration “master plan” around the use of their Atlas and Delta launch vehicles with a Propellant Depot architecture, progress then appeared to slow down to a snails pace by the latter half of 2010.

However, Propellant Depots are back, and with a bang, seemingly coinciding with NASA’s reorganization of their exploration based departments, as “Technology Development Activity” notes from the Johnson Space Center (JSC) made no effort to hide the interest of supporting the technology as a compliment – as opposed to alternative – to their Space Launch System (SLS) efforts.

“Innovative tasks and advanced development work opportunities were presented to the HQ Engineering Management Board. Looking at ESMD and SOMD guidance to propose a management and evaluation structure to select projects based on affordability and progress toward exploration in addition to SLS and MPCV (Orion) to get us out of LEO,” noted TDA notes (L2).

TDA – who cover a number of projects, including the proposed Power-Beaming Demonstration with the International Space Station (ISS) – worked a budget activity back in the Spring, prior to a planning effort which resulted in a presentation at NASA HQ.

“Working with the Commercial team and the HAT team on an evolutionary plan for propellant depots. Putting together a story on propellant depots, and what an evolutionary strategy for depots might be,” added the notes. “The team continues to develop a strategy for a propellant depot as an alternative for the future.”

Propellant Depots could prove to be a viable passenger on the SLS cargo missions, at least in the next decade, but the requirement to at least demonstrate the “gas stations” means an existing vehicle – such as a Delta IV or Atlas V – is the obvious route to take, one which would enable a sooner – rather than later – approach to setting up the opening salvo of what may become an interplanetary highway.

“Looking at the potential for use a propellant depot in concert with existing launch vehicles, and a strategy for implementation. To support that, anyone with issues or concerns with depot are invited to attend and share them,” notes continued over recent weeks. “Continuing to tighten up the story on propellant depots. Starting to look at a transfer vehicle and how that fits into the interplanetary highway concept.”

With the launch vehicle providing the ride uphill, placing the depots in their selected spots in space would likely be tasked to a tug vehicle, with references on the TDA notes referencing an Orbital Transfer Vehicle (OTV) – potentially a version of ESA’s Automated Transfer Vehicle (ATV).

“Continuing to develop a strategy for using propellant depots. A good concept was put together for some demonstrations that can be evolved. Will have a first look at an orbital transfer vehicle (OTV) concept on a reusable type OTV. Looking at some top priorities for the agency in terms of developing an interplanetary highway.”

For the interim, the Depot Team is reporting back to the Human Architecture Team (HAT) on the studies being evaluated with depots, whilst comparing them to the in-house mission designs under evaluation.

The “Simple Depot”:

With NASA’s intentions now public, via the selection of four companies to develop concepts for storing and transferring cryogenic propellants in space, several proposal reports to help define a mission concept to demonstrate the “cryogenic fluid management technologies, capabilities and infrastructure required for sustainable, affordable human presence in space”, are expected in the not too distant future.

In what is being noted as one of the leading concepts, the Simple Depot is a small, 30 metric ton capacity, Centaur derived depot, would allow exploration possibilities for Orion and other spacecraft, without the need for the additional “mission fuel” to be carried by the launch vehicle.

“By refueling the DCSS (Delta Cryogenic Second Stage) upper stage following launch of Orion on Delta IV heavy lift vehicle (as the example cites – as much as Orion is only currently set to launch on a test mission via this EELV), a 30 mT depot can support near-term missions of Orion to the Earth Moon Lagrange points or lunar fly-by missions,” notes an expansive 2011 presentation on the “Simple Depot” concept (L2).

“The same depot concept lends itself to much larger capacity depots using larger diameter tanks, upper stages and payload fairings. These larger depots can enable missions to NEO, the Lunar surface and Mars.

This concept includes two additional basic tenets incorporated into the design to allow for simplified development, reduce development costs and ensure mission success, namely taking advantage of existing experience and being built using hardware that is common to the rest of space transportation.

“The proposed Simple Depot concept satisfies all of these design principles. Its design employs settled propellant management and predominantly existing flight qualified hardware,” added the presentation.

“The design consists of a large LH2 tank connected by a warm mission module to the LO2 tank. This depot concept can be launched on a single Atlas mission requiring no on-orbit assembly allowing for complete system ground check out.”

The Simple Depot LH2 module is composed of a large tank with minimal penetrations – an important factor for storing cryogens. For the “small” 30 mT depot, the LH2 tank is a modified Centaur tank, as used as the main element of the upper stage of the Atlas V launch vehicle.

Commonality means the module is built on the same tooling, using the same procedures as construction of the Centaur.

“The LH2 module is launched with the LH2 tank filled with ambient temperature helium, not LH2. This allows the LH2 and mission modules to be designed primarily for orbital requirements not ground and ascent environments. With these substantially reduced requirements the skin gauge can be reduced from today’s 0.020” for Centaur’s to 0.013”,” the presentation noted.

“This is the same gauge as used on early Centaurs. This thinner tank wall allows the tank to be very light weight, (~500 kg). Made of corrosion resistant stainless steel, the thin tank walls reduce the conduction of energy to the liquid and results in a very low thermal mass that must be quenched when the tank is filled or when slosh waves splash warm walls.”

The LH2 tank is connected to the mission module by low conductivity Ball Aerospace heritage cryogenic composite struts. Keeping the entire LH2 module lightweight minimizes the required cross section of these struts.

This is critical to minimizing the structural heat transfer from the warm mission module to the very cold LH2 module. The struts can also be vapor cooled to further reduce conductive heat leakage into the LH2 tank.

The entire LH2 tank is encapsulated in a robust, Ball Aerospace IMLI blanket that incorporates radiation barriers, both vapor and active broad area cooling (BAC) as well as MMOD protection.

“The described LH2 tank is 3m in diameter by 16m long limited by the existing Atlas payload fairing. The tank is 110 m3 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.

“Accounting for the tank weight, plumbing, instrumentation and thermal protection the LH2 module is anticipated to weigh <2 mT. Based on analysis the described depot will have a boil-off rate of approaching 0.1 percent per day, consisting entirely of hydrogen.”

To conserve volume, allowing for a useful sized depot to be fully integrated on the ground and emplaced on-orbit in a single launch, the LO2 is stored in the upper stage’s propellant tank. As such, this requires a thermally efficient upper stage that can be completely encapsulated with MLI.

The presentation notes that the DCSS design encapsulates the LO2 tank in the inter-stage allowing the tank to be wrapped in MLI. The equipment shelf, RL10 engine, feedlines and inter tank struts all attach directly to the tank, however, resulting in thermal shorts.

While the DCSS LH2 tank sports fewer attachments, it is exposed to atmosphere during ascent preventing application of standard MLI without development of an application-specific aero fairing.

Atlas V fully encapsulates the Centaur inside the 5.4 m payload fairing and is currently flown with either a single or a 4-layer MLI blanket. However, Centaur’s LO2 tank aft bulkhead serves as the equipment shelf with the RL10 engine, feedlines, helium bottles, hydrazine bottles, pneumatics panel and reaction control system loop mounted directly to the bulkhead.

This results in substantial tank heating. Centaur’s LH2 tank however is very thermally efficient, especially if there is not a substantial thermal gradient across the common bulkhead.

“For these reasons the proposed Simple Depot would be launched on an Atlas and use Centaur’s LH2 tank to store the LO2,” notes the conclusions. “Centaur’s LH2 tank is also relatively large, with a volume of 47 m3 capable of containing 54 mT of LO2.”

It was, however, noted that several modifications – such as new valves and plumbing – would be required on the Centaur.

While the Simple Depot is so light that it could be launched on an Atlas 501, it would be launched on an Atlas 551 – the configuration which recently launched the Juno spacecraft. This vehicle would provide ~12 mT of Centaur residuals (combined LH2 and LO2) in a 28.5 degrees by 200 nm circular LEO.

Once safely delivered to orbit the LH2 module must be chilled prior to transfer of Centaur residual LH2. Centaur’s cold hydrogen ullage gas is vented through the LH2 mission module tank to chill the tank. This chilldown process has been demonstrated on past Centaur flights to chill the feedlines and RL10 pump housing.

“Once the LH2 module is chilled the transfer of Centaur’s ~2mT of residual LH2 can commence. This is conducted in a settled environment. The LH2 transfer is pressure fed. LH2 will enter the LH2 module tank subcooled, quenching the GH2 vapor and sucking in additional LH2,” adds the presentation.

“This “zero-vent fill” transfer process is indifferent to the liquid-gas interface. This zero-vent fill process has been demonstrated to be very effective, attaining nearly 100 percent fill.

“Following completion of the LH2 transfer, Centaur’s LH2 tank is vented to vacuum, fully evacuating the residual hydrogen gas. Following the Centaur LH2 tank “safing”, the ~10 mT of residual LO2 is transferred from the LO2 tank to the LH2 tank, the LO2 module tank, using the same transfer process. Once Centaur’s LO2 tank is completely drained the tank is locked up trapping the residual helium and GO2.

“This residual gas must be kept at a higher pressure than Centaur’s LH2 tank (LO2 module) to avoid reversing Centaur’s common bulkhead.”

The brains of the depot is located between the Centaur LO2 module and the LH2 module – known as the mission module. This module includes the flight computer, solar panels, batteries, fluid controls, avionics, remote berthing arm and docking and fluid transfer ports.

Other important elements of the depot are also noted, such as the sun shield, which can be used to shadow objects that must be kept very cold – such as a propellant depot.

“The James Web Space Telescope (JWST) uses an open cavity planer sun shield to ensure that the entire mirror/instrument assembly is maintained at a low temps”, the presentation continued.

“Propellant depots in free space, such as at a Lagrange point, can use this same shielding concept to provide a very cold environment where cryogenic, even LH2, storage is readily achieved.

“For small sun shields it may be possible to erect the sun shield prior to launch. However in most cases the shield will have to be deployed once on-orbit. The JWST uses a mechanical boom to deploy the sun shield. Alternatively a pneumatic boom, inflated with waste GH2, can be used to deploy and support the sun shield.”

For visiting spacecraft, the Autonomous Rendezvous and Docking (AR&D) capability is referenced, citing how the Russians have a proven capability, while the US is making strides, as seen with the Defense Advanced Research Projects Agency’s (DARPA) Orbital Express mission.

The ISS resupply ship fleet, namely Progress, ATV and HTV are also mentioned – although the future US spacecraft raise the hopes they will have the sufficient ability to utilize Propellant Depots.

“Robust AR&D development continues with, NASA’s Orion crew capsule, along with NASA’s two commercial orbital transportation services (COTS) program winners (SpaceX and Orbital Sciences Corporation). Results from these on-going programs will ensure that AR&D is widely available to support the servicing and use of propellant depots.”

With a 30 mT LO2/LH2 capacity, the described Centaur derived cryogenic propellant Simple Depot can provide near term operational use supporting large scale robotic missions and even crewed Earth Moon Lagrange point and lunar flyby

By making efficient use of the entire Atlas 5m payload fairing volume for the LH2 module the existing Atlas can launch a depot with 70 mT of combined LO2/LH2 capacity. With ULA’s proposed larger Advanced Common Evolved Stage (ACES) the depot capacity in a single EELV launch increases to 120 mT or even 200 mT with a 6.5m PLF.

Interestingly, while some class Propellant Depots as an alternative to HLV’s such as the SLS, the presentation notes the same concept can be applied to future heavy lifters, in order to allow launch of even larger capacity depots.

Also discussed are the relevant requirements a depot would need, such as tanker missions, to top up the depot in-situ. This is required due to the natural boil off of the propellant, although there would be flexibility, with refueling tanker missions launched with propellant mass sized to the selected launch vehicle.

In summary, the presentation adds that Propellant Depots can enhance the mission capability of exploration architectures regardless of the use of small reusable rockets, larger EELV class rockets or much larger heavy lift vehicles, while future replacement depots can sport improved technology, as an interplanetary highway is constructed in space.

(Images: L2 Content, ULA, Ball Aerospace)

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