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Deimos base
An earlier concept for a human base on the Martian moon of Demios. A version of this, with facilities placed below the surface, could address many of the radiation hazards associated with human Mars exploration. (credit: Lockheed Martin)

Destination Deimos (part 2)

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Synergy of radiation protection, multi-use propellant, and advanced propulsion

Due to its high hydrogen content, water is an excellent radiation shielding material. Because significant quantities are required to provide RP5, depending on the surface area of the crew compartment, increased efficiencies can be realized if shielding materials can be multi-use.

Because it is utilized for crew sustenance and hygiene, radiation protection, and propellant, liquid water will account for almost 54 percent of the gross mass of our envisioned spacecraft prior to interplanetary departure.

The largest mass component of any foreseeable interplanetary vehicle—or any rocket, for that matter—is propellant. Water is a storable liquid over a wide range of easily maintained temperatures and can be used as a viable propellant with nuclear thermal propulsion (NTP), a technology that has been eagerly anticipated for decades and has a 40-year developmental pedigree that includes static test firings of evolving prototypes of increasing power, duration, and complexity. In essence, NTP consists of a rocket engine that is a solid core of controlled fissionable material. As the nuclear core heats up, liquid water is used to cool the core. At 2600°C, water flashes into steam but also begins to dissociate into hydrogen and oxygen atoms that exit the engine nozzle at high speed generating thrust.

The Space Shuttle Main Engines, using liquid hydrogen and oxygen, are the most efficient chemical propulsion system currently attainable with a specific impulse (Isp), a measure of rocket engine efficiency, of approximately 465 seconds. If NTP core temperatures could be maintained well in excess of 3000°C, a formidable but ultimately solvable materials challenge, the vast majority of water molecules dissociate directly into hydrogen and oxygen atoms instead of steam resulting in Isp approaching 900 seconds, essentially providing twice the “kick” of chemical propellant.

Once a mission architecture encompassing the synergistic elements described above is embraced, a reference interplanetary transport spacecraft begins to take shape. Liquid water can be transported and stored over long intervals in space with ease, especially compared to the exotic technical and power burdens necessary to produce and maintain cryogenic propellant or the toxicity hazards associated with hypergolic propellants. Water will arguably be the most common volatile found on asteroids or minor moons and, therefore, easily integrated into any usable ISRU scenario.

Because it is utilized for crew sustenance and hygiene, radiation protection, and propellant, liquid water will account for almost 54 percent of the gross mass of our envisioned spacecraft prior to interplanetary departure. We therefore propose the first human interplanetary transport spacecraft be christened Aquarius in deference to the Zodiacal Water-Bearer of that name.

Age of Aquarius

Three nuclear thermal engines with a cumulative mass of 41,700 kilograms (not counting propellant and water tank structural mass) achieving an Isp of 900 seconds, and generating a total thrust of 333,617 newtons and exhaust speed of 8.826 kilometers per second, will propel Aquarius and its crew of three on round-trip missions to Deimos. The spacecraft will use short-way (Type 1) transits lasting 200–240 days in accord with conjunction mission profiles. Note that first-generation “Peewee” NTP engines, with core sizes of 50 by 132 centimeters, developed by the NERVA project the late 1960s and early 1970s, generated 89,000 newtons of thrust with an Isp of 825 seconds. Engines envisioned for Aquarius require only a 25 percent increase in output per engine.

Approximately 500 days will be spent at Deimos dedicated to telerobotic Mars exploration and Aquarius resupply from pre-emplaced or ISRU resources before an Earth return transit of 200 to 240 days duration can begin. The three engines, utilizing “bi-modal” reactor operations, will power all electrical loads aboard Aquarius during cruise periods.

Open loop environmental control and life support systems (ECLSS) maximize simplicity and reliability avoiding operational risks of complex reclamation and recycling technologies. Almost 99 kilograms of consumables per day are dumped or vented overboard, making the vehicle approximately 20,000 kilograms lighter at Mars capture, decreasing the amount of propellant required for Deimos rendezvous.

The mass of Aquarius just prior to interplanetary departure is 357 tons, approximately 90 percent of the current International Space Station mass.

A cylindrical habitat module, 4.6 meters in diameter by 12.2 meters in length, will provide 67.58 cubic meters per crewmember, 3.38 times the NASA standard for “optimal” per capita habitable volume for missions longer than six months. RP5 sheielding (51.5 grams per square centimeter) for the entire hab will be accomplished by a special shielding “jacket” with two components. The hab structure and strategic consumables placement will provide 14.5 grams per square centimeter, and a sleeve surrounding the structure, consisting of 87,053 kilograms of multi-use water propellant, will provide the remaining 37 grams per square centimeter. At Deimos, RP100 during crew loiter periods between transits is assumed. Because the water radiation shield can be consumed as propellant as Mars is approached, radiation protection will dip below 51.5 grams per square centimeter for less than ten hours prior to Deimos arrival.

A docking/airlock/centrifuge (DAC) module, four meters in radius and two meters in length with a mass of 26,461 kilograms (including a 1,700-kilogram three-meter short-arm centrifuge designed to give the crew intermittent exposure to a gravity gradient of 1.0g at the heart and 2.5g at the feet), will be located at the forward end of the hab. Although pressurized, crew access will be limited to less than two hours per day because its volume is shielded from radiation only by DAC structure.

Synergy of spacecraft design, assembly, and reusability

Even with pre-emplacement of resources required for Deimos stay and Earth return, the mass of Aquarius just prior to interplanetary departure is 357 tons, approximately 90 percent of the current International Space Station mass. Attempting the same mission with chemical propellant (Isp of 450 seconds) would increase departure mass to 832 tons, or 210 percent of the current ISS mass.

Rather than the notional sci-fi version of a pencil-shaped interplanetary spacecraft with the crew quarters in the forward “nose” of the vehicle and the engines in the rear, Aquarius is designed to provide maximal radiation protection by surrounding the crew with spacecraft infrastructure, tankage, plumbing, consumables, propellant, and dedicated propellant shielding. Ideally, it could assume a more spherical shape with engines protruding from aft compartments.

Aquarius build-up and assembly takes place over several months in a special two-day 7,700-by-113,300-kilometer Earth elliptical parking orbit (EEPO) to reduce human and electronic exposure to particle radiation trapped in the geomagnetic field, and to take advantage of upper stage cryogen propellant in each of seven assembly boosters to propel Aquarius part way out of Earth’s gravity well during buildup. Each assembly booster is designed to place 130 tons into LEO. From there an upper stage will deliver a 50,000-kilogram payload using cryogenic propellant as Aquarius is assembled and supplied in EEPO.

The final delivery to Aquarius would consist of the crew, their gear, and personal items ferried by a much smaller launch vehicle. After appropriate checkout of Aquarius systems, a small retrograde burn of 0.242 kilometers per second at EEPO apogee will lower the perigee height from 7,725 to a mere 400 kilomeyters. At perigee, the combination of a prograde 1.116 kilometer per second trans-Mars injection (TMI) burn and the Oberth “slingshot” effect will propel Aquarius out of Earth’s gravity well and into an interplanetary trajectory, intercepting Mars 203 days later.

Since Aquarius can burn part of its radiation shield at mission end, an entirely new scenario resulting in reusability becomes possible.

At closest approach to Mars, a retrograde Mars orbital insertion (MOI) burn of 1.586 kilometers per second places Aquarius on an intercept trajectory to Deimos. A final prograde burn of 0.751 kilometers per second enables Aquarius to catch up to Deimos to implement final approach and rendezvous. The total delta-V expended during the 204-day transit from EEPO to Deimos is 3.695 kilometers per second.

Upon Deimos arrival, transfer of crew and cargo to the RP100 subsurface habitat is via a pressurized docking interface. During a 497-day stay time dedicated to extensive tele-robotic exploration of the surface of Mars, and perhaps both moons as well, the same interface would allow the transfer of pre-emplaced water and other consumables from the Deimos infrastructure to Aquarius prior to the return to Earth.

On Mission Day 700, a fully resupplied Aquarius performs a departure retrograde burn of 0.652 kilometers per second, initiating a Mars fly-by six and a half hours later. At closest approach, on the daylight side of the red planet, a prograde 1.553 kilometers per second trans Earth injection (TEI) burn puts Aquarius on an intercept path to Earth 237 days later.

During this cruise phase of the mission profile, Aquarius’ water radiation shield is in full RP5.1 (52.7 grams per square centimeter) protection mode. It is at precisely this point in the mission profile, as Earth looms ever larger at the end of the return transit, the synergy of multi-use propellant, enhanced radiation shielding, and advanced propulsion demonstrates its greatest advantage. In standard mission architectures, the crew would now be forced to transfer to a dedicated smaller vehicle and perform an Apollo-style direct entry. Slamming into the Earth’s atmosphere at speeds in excess of 11.9 kilometers per second after 933 days of almost continuous exposure to microgravity would put the crew at significant risk.

But since Aquarius can burn part of its radiation shield at mission end—that is, use the water in its radiation shield as propellant—an entirely new scenario resulting in reusability, avoidance of direct entry, and complete resupply with only 52 percent of its initial assembly mass now becomes routine on a repetitive basis.

At closest Earth approach on Mission Day 933, rather than ending its first and only mission by becoming just another piece of space junk, Aquarius ignites its nuclear engines once again for a retrograde 0.935-kilometer-per-second trans lunar injection (TLI) burn, utilizing a portion of its radiation shield as propellant, setting its on a path to a special parking orbit. Almost 70 hours later, it performs a retrograde 0.378-kilometer-per-second lunar orbit insertion (LOI) burn. About 7.5 hours after that, Aquarius executes a final prograde burn of 0.309 kilometers per second to gently enter the most stable orbit known in cis-lunar space, a selenocentric distant retrograde orbit (SDRO), one that takes the least amount of energy to maintain over extended periods of time.

Far from being fully depleted, its radiation protection level decreases from RP5.1 to 4.2, 3.8, and 3.5 at TLI, LOI, and SDRO insertion respectively, the last still providing an impressive 35.7 grams per square centimeter shielding equivalent.

By demonstrating practical round trips to Deimos, Aquarius could transport human explorers to more than 1,100 known near-Earth asteroids (NEAs) whose accessibility is occasionally greater.

In SDRO, Aquarius will rendezvous with previously established resident infrastructure providing full RP5 protection and equipment for a 30-day intensive exercise crew rehabilitation program before the crew returns to Earth in a separate vehicle. Aquarius will be resupplied, undergo required maintenance, and no doubt receive much needed TLC from its tenders for 574 days until the next Earth departure season for Mars. With a full consumables load prior to beginning its next mission, Aquarius can provide RP7.8 (80.4 grams per square centimeter) equivalent radiation shielding.

Our mission architecture avoids lugging a 10-ton Earth entry vehicle all the way to Deimos and back, then using it 933 days into the flight. It also eliminates the expense and complexity of rebuilding an Aquarius replacement from scratch for each subsequent mission.

The capacity to burn its radiation shield as propellant in contingency situations provides yet another synergistic advantage: expanded abort capability. Given the amount of water onboard, Aquarius could initiate return to its point of origin (Deimos or Earth) up to two weeks after departure.

Approximately 19 months after first entering SDRO, a fully resupplied and crewed Aquarius will execute three small burns leading to a trans Earth insertion trajectory, from which a prograde TMI burn of 0.891 kilometers per second, assisted by the Oberth effect, will put it on a path to Deimos for the second time.

The Aquarius concept is only one potential solution to practical interplanetary human spaceflight. Innovative synergistic mission architectures can bring into the realm of feasibility missions and destinations otherwise beyond the cusp of current capabilities. In this case the combination of precursor missions, resource pre-emplacement, advanced propulsion, enhanced radiation protection, multi-use propellant, intermittent artificial gravity, stable parking and departure orbits, and heavy-lift Earth launches combine to create a new class of reusable human interplanetary transport spacecraft, one pre-adapted to maximally benefit from the discovery of extractable water on any small body in the solar system. By demonstrating practical round trips to Deimos, Aquarius could transport human explorers to more than 1,100 known near-Earth asteroids (NEAs) whose accessibility is occasionally greater.

A more technical description of the Aquarius concept can be downloaded here. (free registration required)