Attributes of a reusable interplanetary human spaceflight transport are proposed and applied to example transits between the Earth/Moon system and Deimos, the outer moon of Mars. Because the transport is 54% water by mass at an interplanetary departure, it is christened Aquarius. In addition to supporting crew hydration/hygiene, water aboard Aquarius serves as propellant and as enhanced crew habitat radiation shielding during interplanetary transit. Key infrastructure and technology supporting Aquarius operations include pre-emplaced consumables and subsurface habitat at Deimos with crew radiation shielding equivalent to sea level on Earth, resupply in a selenocentric distant retrograde orbit, and nuclear thermal propulsion.
Advancing in-space nuclear thermal propulsion (NTP) technology to the point where fission reactor core temperatures exceeding 3000° C can be achieved during major translational maneuvers (burns). Under these conditions, water molecules pumped into the core will disassociate into hydrogen and oxygen atoms, and specific impulse ISP near 1000 s could be achieved. This level of efficiency, twice that attainable with chemical propulsion, dramatically reduces total mass for an interplanetary transport of specified payload mass.
When high propulsive efficiency is achieved with water as propellant, the practicality of interplanetary human spaceflight is enhanced in multiple respects.
1.liquid water is easily stored for months or years without exotic thermal conditioning burdens imposed by cryogens or toxicity hazards associated with hypergols.
2. liquid water stored about the crew habitat to support arrival propulsion requirements at an interplanetary destination also serves as an effective radiation shield during interplanetary transit.
3. water is arguably the most common volatile to be found on small bodies such as asteroids and minor moons throughout our solar system, leading to the promise of in-situ resource utilization (ISRU). With ISRU producing
water for propulsion, radiation shielding, and hydration/hygiene near an interplanetary destination, mass to be transported there from Earth in support of crew return is virtually eliminated.
Is technical risk of the presumably higher 3000° C nuclear reactor core temperature necessary to “burn” water
propellant and achieve this ISP (875-1000 ISP) a good trade against the potentially greater difficulties
of refining, storing, and transporting liquid hydrogen, particularly in an ISRU context
High efficiency propellent is created as water is heated to its 3000° C atomic disassociation temperature, and the system to accomplish this aboard Aquarius is assumed to be a nuclear reactor. This efficiency, together with return consumables pre-emplacement, reduces fully resupplied total Aquarius mass before an interplanetary departure to 357 t, about 90% of the assembled International Space Station’s mass in March 2014.
Aquarius uses dedicated water mass and propellant residuals following interplanetary departure to shield much of her onboard habitable volume from radiation. Until interplanetary arrival propellant consumption begins, she provides her crew radiation shielding protection equivalent to at least 5% of that offered by Earth’s atmosphere at sea level.
Because a large quantity of water mass must accompany Aquarius to shield her crew until an interplanetary arrival, burning that mass upon Earth return to permit her reuse is a logical consequence. Utilizing lunar orbit to garage and resupply Aquarius between roundtrips to Deimos has been demonstrated as a viable reuse strategy. Furthermore, flight profiles akin to those documented by this paper may be mandatory for crew survival if direct atmospheric entry poses unacceptable stress risks following an interplanetary roundtrip.
Aquarius is assembled in an elliptical Earth orbit with 2-day period and perigee above the inner Van Allen radiation belt. This orbit permits payload delivery about a day after launch and is thus able to capture the total energy deliverable by a cryogenic upper stage with great efficiency because long-term cryogenic storage is avoided. Assuming 130 t IMLEO per launch, Aquarius can be assembled and readied for her maiden interplanetary transit with an estimated 7.14 launches. Thanks to her reusability, even complete resupply of propellant and crew consumables amounts to 185 t, 52% of Aquarius fully loaded total mass.
An abundance of water aboard Aquarius opens up potentially useful abort options. Even following an interplanetary departure with relatively large propellant consumption, a viable return to the departure planet is demonstrated for an abort initiated nearly two weeks after departure.