A1.0 Introduction
NASA Special Publication 6107 details the work of the Exploration Study Team through the spring of 1994* . As described in that report, the primary role of the Reference Mission is two-fold. First, it is used to form a template by which subsequent exploration strategies may be evaluated for consideration as alternate or complementary approaches to the human exploration of Mars. Second, the Reference Mission is intended to stimulate additional thought and development in the exploration community and beyond.
In serving these two purposes, several components of the original Reference Mission (referred to as Version 1.0) have been modified to that which is presented in its current form Mars Reference Mission Version 3.0. The changes are manifested at the strategic, mission, and system levels of development, and augment or improve upon prior work done by NASA’s Exploration Study Team. To facilitate and document the ongoing work of the Exploration Team, this addendum will outline the current strategy (as of this addendum’s publication date) as well as provide a description of the current systems. Section two of this Addendum provides a brief overview of the changes to the reference approach which are strategic in nature, that is changes which cross many systems and elements. Section three provides a description of improvements to many of the individual systems and elements. Lastly, section four discusses several revolutionary mission approaches and technical options, currently under consideration by the exploration community, which can provide significant improvements in the mission architecture and mass estimates.
The original Reference Mission, compiled in the 1993-94 time frame, has been reviewed and improved in many facets of its design. Modifications to that strategy have been made to create a mission offering less risk, lower cost, and better technical approach than previous mission designs. This section will discuss the strategic modifications which have been made to the original Reference Mission, namely alteration of the launch strategy to greatly reduce the required size of the launch vehicle and revision of a mission strategy leading to the elimination of the initial habitat flight.
Perhaps the biggest assumption of the original Reference Mission centered on the launch system; specifically, a large, yet-to- be-developed launch vehicle was required to place the mission elements into low Earth orbit (LEO). The launch manifest for the mission elements is shown in Figure A2-1. As can be seen, a 200-metric- ton launch vehicle would be required to achieve a human mission in four launches. This scenario consists of three launches for the first trans-Mars injection (TMI) opportunity, followed by three launches at each subsequent opportunity. The first human mission consists of three cargo launches in the first injection opportunity followed by one piloted launch in the following opportunity, each manifested with the specific equipment as shown in the figure.
To graphically illustrate how each of the four launches are conducted to support the first human mission, Figure A2-2 is provided. During the first mission opportunity in 2011, the three cargo vehicles are launched on a nearly Hohmann transfers from Earth to Mars. Reference Mission Version 1.0 was designed such that the Earth Return Vehicle (ERV-1), containing the return habitat, enters a parking orbit about Mars by utilizing an aerocapture maneuver upon arrival at Mars. The other two cargo elements, Cargo-1 and Hab-1, perform an aerocapture followed by aeroentry and landing, delivering the dry ascent vehicle and crew surface habitat to the Martian surface. These components are followed 26 months later (at the next injection opportunity in 2014) by a second surface habitat, Hab-2, piloted by a crew of six. The crew performs an aerocapture followed by aeroentry and landing to the surface in close proximity to the previously deployed surface assets (Cargo-1 and Hab-1). After completion of the 500-day surface mission, the crew ascends to Mars orbit and rendezvous in Mars orbit with the pre-deployed return vehicle (ERV-1).
It was recognized that development of the large 200-metric ton launch vehicle posed a significant technology and development challenge to the mission strategy. Design of the large launcher raises several cost issues (development, new launch facilities, etc.), and the physical size of the launch vehicle is itself a potential limitation to implementing Version 1.0 of the Reference Mission. The requirement of a heavy lift booster was driven primarily by the initial mass to Low Earth Orbit (IMLEO); therefore, an effort was initiated in the fall of 1996 to reduce the required mass and volume of each launch. These efforts were undertaken
Figure A2-1 Cargo and Piloted Vehicles for Reference Mission Version 1.0.
Figure A2-2 Reference Mission Version 1.0 Mission Sequence
while balancing the need to minimize the number of launches to reduce ground launch costs and limit added operational complexity due to Low Earth Orbit (LEO) rendezvous and docking. In order to reduce the size of the launch vehicle, a critical examination of the payloads, in terms of their physical size and mass, was conducted. The goal of this modification was to remanifest the payload elements onto two smaller (80 metric ton class) launch vehicles rather than the single large vehicle.
Reducing the physical size of the launch elements is important from many aspects of the launch vehicle design, including reducing the mass of the systems and reducing the aerodynamic loads on the payload shroud. The geometry of the large (10 m diameter) aeroshell for the large launch vehicle, used for both the Mars lander and the surface habitat modules, is given in Figure A2-3. Of particular note is the unused volume between the lander / habitat and the aeroshell.
Figure A2-3 Mars Surface Lander and Habitat Aeroshells for Version 1.0.
A proposed solution to this excess volume is shown in Figure A2-4. In this design, the habitat structure is integrated with the Mars entry aeroshell and launch shroud. In addition to reducing the structural mass of the element, the integrated design serves several functions beyond those which were proposed in Version 1.0 of the Reference Mission. Specifically, the integrated habitat / pressure hull with a thermal protection system (TPS):
serves as both an Earth ascent shroud and Mars entry aeroshell
eliminates the need for on-orbit assembly / verification of the aeroshell
allows for stowage in an 80-metric-ton-class launch vehicle.
Figure A2-4 Habitat Repackaging Strategy.
During the outbound and return inter-planetary journeys, Reference Mission Version 1.0 allows for 90 m3 of pressurized volume per crew member. As can be seen in Figure A2-5, this value is consistent with data from previous space missions. It is desirable to maintain this living quality for the crew despite any subsequent changes which may occur to the original Reference Mission.
The second step in changing the launch strategy focused on reducing the system masses in order to reduce the mass delivery requirements for the launch system. The payload masses were critically examined, and any duplications were eliminated. In addition, studies were undertaken to scrub the system masses to achieve the required weight savings. The goal of this work was to reduce each payload delivery flight to accommodate the approximate volume and weight limitations of two 80-metric-ton launchers. These mass reductions are discussed in further detail in Section 3.0 of this Addendum.
Figure A2-5 Historical Space Habitat Pressurized Volume.
A2.1.3 Modified Launch Strategy
Reduction of the payload delivery flights’ mass and volume enables the opportunity to utilize a smaller launch vehicle. This repackaging allows the mission to change from a launch vehicle requiring a 200 metric-ton launch to two individual launches of magnitudes within the envelope of launch systems which can be evolved from current capabilities. This design, delivering the interplanetary propulsion system and cargo into Earth orbit separately, would require one rendezvous and docking operation prior to each outbound journey to Mars. While doubling the number of launches, this strategy eliminates the high costs of developing the large 200 metric-ton launch vehicle of Version 1.0.
While reviewing the original mission strategy, the initial habitat lander (Hab-1) was identified as a launch component which could potentially be eliminated. During the Spring of 1997 a team of engineers at the Johnson Space Center (JSC) investigated a concept of utilizing inflatable structures (known as the TransHab) instead of traditional hard aluminum structures for habitation systems (see Section 3 for more details of this concept). Results of this study demonstrated significant subsystem mass savings for the TransHab concept. Given the significant volume per unit mass increase provided by the inflatable TransHab concept, the attention of the Exploration Team returned to the launch packaging outlined in the original Reference focusing on techniques of augmenting the surface living volume.
As noted earlier, a sufficient level of pressurized living volume is critical for crew health maintenance. A TransHab-derived inflatable structure would provide such augmentation, arriving on the Mars surface in the Cargo-1 flight two years before the crew. Pre-plumbed and ready for integration into the life support of the Piloted Crew Lander, the inflatable structure would simply need to be installed by the crew upon arrival, as depicted in Figure A2-6.
The mass of the inflatable module (estimated at 3.1 metric-tons without crew accommodations or life support) could be substituted for the mass of the pressurized rover
Figure A2-6 Mars Surface Inflatable Habitat Concept.
(5 metric-tons) originally manifested on the Cargo-1 flight. The pressurized rover, deferred to the second Cargo delivery flight, would arrive a few months after the crew and would still be available for the majority of the mission. In essence, the redundancy of the pressurized rover (for the first Mars crew) has been traded for the elimination of an entire Mars-bound habitat flight.
A2.2.2 Redundancy Considerations
The concern that system redundancy would be reduced with the elimination of Hab-1 was mitigated by the redundancy already built into the Reference Mission. For example, several levels of redundancy are present in the mission architecture to address failure of the regenerative lift support system of a habitation module. Four levels of this redundancy are outlined below.
First level backup - In-Situ Resource Utilization processes generate enough water and oxygen for the entire surface mission to run "open loop."
Second level backup - The Ascent Vehicle / ISRU plant on Cargo-2 of the subsequent mission, arriving to the surface a few months after the crew, could be used to supply life support rather than for propellant production.
Third level backup - The surface could be abandoned for the orbiting Earth Return Vehicle, which has a sufficient food cache to last until the next trans-Earth injection window.
Fourth level backup - The Earth Return Vehicle (ERV-2) of the subsequent mission, arriving a few months after the crew, would provide an additional refuge for the crew if necessary.
The strategic modifications to the Reference Mission described in this section have significantly reduced many of the barriers faced during the formulation of the original approach. The combination of repackaging the mission elements into smaller launch vehicles along with elimination of the initial habitat lander has allowed significant reduction in launch vehicle size, from 200-metric tons down to 80-metric tons, while only introducing two additional flights to the overall launch manifest.
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