Near-Term Propellant Depots: Implementation of a Critical Spacefaring Technology

1. Near-Term Propellant Depots: Implementation of a Critical Spacefaring Technology Jonathan Goff, Masten Space Systems Bernard Kutter and Frank Zegler, ULA Dallas Bienhoff and Frank Chandler, Boeing Jeffrey Marchetta, University of Memphis Presented by Jonathan Goff at AIAA SPACE 2009 Pasadena, CA September 17, 2009

2. What are Propellant Depots • Facilities in space that can receive, store, and transfer propellants and other fluids to visiting vehicles. • Can be located in LEO, at Lagrange Points, around other planetary bodies or at any other points of interest • Can be supplied from earth, offworld sources, and maybe even from planetary atmospheres • Can handle different sorts of fluids ranging from LOX/LH2 cryogenics to “space storables” to hypergols • Can range in size from a Falcon-1 launched single-use fuel tank with a docking adapter to massive, ISS-sized transportation nodes.

3. Historical Solutions to the Propellant Logistics Problem • Rocket-powered spaceflight isn’t the only historical example of logistically challenging transportation. • Similar Historical Analogies • Antarctic Exploration • Food/Fuel Caches • Steam-powered Navies • Naval Coaling Stations and Colliers • Steam-powered railroads • Coaling and watering stations • Long-range jet powered military planes and helicopters • Mid-air refuelling • The historical solution to this problem has always been to cache propellants along the way. • Early visionaries of the Space Age, including von Braun, recognized this reality as well. Propellant Depots are the Solution to Space Transportation Logistics Challenges Most In-line with Historical Precedent

4. Propellant Depot Questions • Key Questions about Propellant Depots • Are they technologically feasible at this time? • How would you go about doing depots? • What’s the best way to use them in a space transportation architecture? • What sort of missions/capabilities to depots enable? • How do they compare economically versus other options? • How do you handle the logistics of running a depot? } This Paper

5. Overview • Prop Depot Technologies • m-gravity Cryo Fluid Management • Thermal Control • Rendezvous and Propellant Transfer • Depot Concepts • Depot Technology Maturation Tools • Conclusions/Future Work

6. Propellant Depot Technologies m-gravity Cryo Fluid Management • While mg fluid handling is feasible, and sometimes desirable, settled handling is much higher TRL • There are many settling options, including: inertial (propulsive), tether-based, and electromagnetic • Inertial settling is highest TRL, with decades of operational experience (Saturn SIV-B, Centaur, DIV-US, Ariane-V, etc) • Fluid handling options interact with other depot design decisions • ED-tether based systems can use tether for reboost and settling. • Inertially settled depots can use boiloff from passive thermal control systems for settling and stationkeeping.

7. Propellant Depot Technologies Thermal Control • Passive versus Active, Zero Boiloff (ZBO) Thermal Control • ZBO propellant storage is technologically feasible, and greatly simplified by settling propellants • With settled propellants, active cooling is a lot closer to terrestrial experience than in mg conditions. • Passive systems can tend to be a lot simpler and more reliable than active cooling systems • Good passive thermal control is important even if active cooling is used • Lowers the amount of heat that has to be actively rejected • Acts as a backup in case of problems with active cooling • Interesting Observation #1: Boiled propellants can be reused for stationkeeping propulsive purposes, meaning that for LEO depots, ZBO might not be necessary. • Interesting Observation #2: Many of the features that make LH2 a headache for long-term storage make it useful in multi-fluid depots—LH2 is a wonderful “heat sponge” • Interesting Observation #3: Due to stationkeeping demands and the challenging thermal environment LEO depots push you towards a “use it or lose it”, high throughput mode of operations. • Interesting Observation #3a: Depots at L-points are more suited for long-term storage and less frequent use.

8. Propellant Depot TechnologiesRendezvous and Transfer • Recent research into orbital servicing (such as Orbital Express, XSS-11, FREND, etc) has significantly advanced the TRL of needed propellant transfer technologies • Efficient and safe depot operations require extremely reliable prox-ops and transfer • Need to minimize possibility of damage to depot or tanker from rendezvous/transfer operations • Berthing using robotic arms or Boom Rendezvous may be preferable to traditional docking • Many options for how to handle propellant delivery • Progress/ATV/COTS-like tanker spacecraft • Integrated-stage tanker spacecraft • “Dumb” tankers plus tugs • Personal Preference: Tugs for prox-ops plus dumb tankers (based on or integrated with the delivery stage) with standardized docking/propellant transfer interfaces • The most expensive bits get reused multiple times • They don’t have to be launched every time • Minimizes the amount of engineering an particular launch provider needs to provide propellant delivery services • Maximizes competition in propellant launch

9. Near-Term Depot Concepts • Single-Use “Pre-Depot” • Simple, typically 2-launch architecture • Enables unmanned and limited manned exploration missions using existing and near-term EELVs • Single-Launch Single-Fluid “Simple Depot” • Typically LOX-only, simpler than multi-fluid depots • Much larger depot capacity than the single-use pre-depot • Single-Launch “Dual-Fluid” Depot • LH2 tank is built integral to LV fairing, upper stage LH2 tank is converted to depot LOX tank after deployment • Can be based on existing or stretched versions of existing upper stages, or can be based on future upper stages like ACES or Raptor. • Doesn’t require orbital assembly to provide large propellant capacity (75-115mT of LOX/LH2) • Sufficient capacity to enable manned exploration without requiring HLVs • Self-deployable throughout the inner solar system • With a depot at L1/L2 as well as LEO, manned ESAS-class exploration feasible with existing upper stages (with mission kits) • Multi-Launch Modular Depots • Largest propellant capacity (200+ mT feasible) • Integral robotic arm for easier berthing • Can be combined with the above Dual-Fluid concept to reach 450+ mT capacities • Can be built up modularly

10. Depot Technology Maturation Tools • Low-cost, iterative technology maturation and demonstration testbeds reduce the cost and risk of reducing depots to practice • They enable demonstrating the few first-generation depot technologies that still need demonstration • They allow other promising options to be evaluated • CRYOTE (CRYogenic Orbital TEstbed) allows long-duration experiments in the space environment • Integrated with the ESPA ring for use with EELVs • Large experiment volume makes results easier to scale than previous CFM experiments • Relatively frequent flight opportunities as a secondary payload • Suborbital testbeds (CRYOSOTE) flown on reusable suborbital vehicles enable short duration, but very low-cost experiments • 2 - 5+ min mg time per flight • Flight costs <$50k • Rapid reflight capabilities • Large payload fairing compared to sounding rockets (60+in diameter) • Proof-of-concept experiments for various subsystems • Pre-fly orbital experiments for debugging before committing to expensive orbital missions

11. Conclusions/Future Work • There are several approaches to depots that are both useful for manned space transportation beyond LEO, while also being near-term feasible. • Recent development work on autonomous rendezvous and docking, orbital servicing and propellant transfer, orbital CFM testbeds, and suborbital RLVs lower the technological hurdles for implementing depots • Avenues for Future Investigation: • A lot of these recent concepts drastically change the picture for how depots would be used in space transportation, which suggests further research into how best to integrate these concepts • More investigation is needed to evaluate which approaches to tanker design and prox-ops are best, and how the economics of a multi-launch depot-centric architecture compares with alternatives