National Aeronautics and Space Administration
May 29, 1992
The present document provides a collective assessment of the Synthesis Group architectures. NASA has now completed its analysis of these architectures; four documents have been printed, each discussing in detail an individual architecture. The purpose of the current document is to look across all the architectures and to capture the knowledge gained from our analysis activity. This document (1) summarizes key features of the Synthesis Group architectures that are attractive, (2) identifies features for which we would offer alternative approaches, and (3) suggests recommendations to guide the next, near-term steps for SEI .
The Synthesis Group specified several important features common among the four architectures. As NASA examined each of the architectures in detail, we concluded these particular features enhance the quality of each architecture. Specifically, features strengthening the architectures are:
America at the Threshold presents four potential SEI architectures. (An architecture is defined as "a set of objectives ordered to achieve an overall capability and the sequential series of missions - including specific technical activities - to implement those objectives."1) Each is distinguished in terms of its approach, scope, emphasis, purpose, and balance between activities on the Moon and Mars.
1. MARS EXPLORATION: This set of missions returns humans to the Moon by 2005, but the primary objective is Mars science and exploration. The lunar capability is established to serve as a testbed of systems, equipment, operations, and experiments for later trips to Mars.Shortly after the release of America at the Threshold, NASA began an agency- wide effort to assess the technical and strategic details required to actually implement each of the four architectures. The Exploration Programs Office (ExPO) led this architecture analysis activity, coordinating and integrating the efforts of the participating NASA centers. ExPO has published an analysis document for each of the four Synthesis Group architectures (see the insert below).
2. SCIENCE EMPHASIS FOR THE MOON AND MARS: This architecture envisions "balanced" scientific return and exploration activities at both locations. A total of 13 human missions to the Moon and Mars are slated between 2003 and 2020.
3. MOON TO STAY AND MARS EXPLORATION: This set of missions establishes a human presence on the Moon in 2004, and, by 2007, 18 crew members live and work on one-year assignments. In addition to founding a lunar base, the crews conduct preparatory work for Mars journeys that begin in 2014.
4. SPACE RESOURCE UTILIZATION: These missions concentrate on turning the resources of space into usable, exploration-support products, with the potential toward export. The architecture promotes self-sufficiency with plans to convert Moon and Mars resources into materials for habitation and propellants for transportation.
By analyzing these architectures individually, NASA has developed an understanding of the implications of pursuing those particular themes and approaches. The current document, however, provides a collective view of the architectures, resulting in the recognition of important, cross-cutting ideas.
There are four Architecture Analysis documents -- one for each of the Synthesis Group architectures. These Architecture Analysis documents have the same format. Section 1 of each documents is a standard introduction. Section 2 outlines the architecture objectives, the key milestones and accomplishments, the technology/advanced development and human support strategies, and the end-to- end mission description. Section 3 provides detailed descriptions of the various systems defined for the architecture implementation, the reasoning behind the selections of the systems employed, and an overview of how the particular system is operated during the various phases of development within an architecture. Section 4 of each document lists possible issues as submitted by members of the NASA community; these issues are printed without screening or evaluation.
The documents are:
ANALYSIS OF THE SYNTHESIS GROUP'S MARS EXPLORATION ARCHITECTURE, ExPO Document XE-91-001, October, 1991.The sequence of events leading up to the NASA analysis of the Synthesis Group architectures is shown in Figure 2. More importantly, the figure also points to the next step in defining the SEI. Given the excellent template of possible architectures defined by the Synthesis Group, the need to study multiple, grand architectures has diminished. The current ExPO course of action focuses on specific, near-term missions, including robotic, precursor missions and the first human outpost on the Moon. Exploration program planning places these near-term missions in the context of the broader exploration agenda captured by the architectures. Input to that planning comes from this document (in the recommended features) and the other four architecture analysis documents. ExPO is applying the knowledge gained from this Synthesis Group architecture analysis activity to shape the precursor missions and the first lunar outpost. The Synthesis Group architectures will remain as guidelines for our long-term objectives while we define the early steps.
ANALYSIS OF THE SYNTHESIS GROUP'S SCIENCE EMPHASIS FOR THE MOON AND MARS ARCHITECTURE, ExPO Document XE-92-003, March, 1992.
ANALYSIS OF THE SYNTHESIS GROUP'S MOON TO STAY AND MARS EXPLORATION ARCHITECTURE, ExPO Document XE-92-001, January, 1992.
ANALYSIS OF THE SYNTHESIS GROUP'S SPACE RESOURCE UTILIZATION ARCHITECTURE, ExPO Document XE-92-002, February, 1992.
The analysis process began with understanding the Synthesis Group objectives for the architecture under consideration. These objectives were specified most notably in terms of three themes -- exploration and science, human presence, and space resource development. The NASA analysis team then translated the objectives into mission strategies for every operational capability in the architecture. (For example, with the SPACE RESOURCE UTILIZATION architecture, NASA charted the development of space resources from the demonstration phase to the local-usage phase to the export phase.)
Also dictating the strategic flow of the architecture were the groundrules stated in the Synthesis Group report. Groundrules were interpreted as strategic or implementation constraints, either specific to the architecture or common across all architectures. Specific constraints included the mission milestone schedule plus the key accomplishments to be met at each milestone. Whereas the specific constraints tended to influence the strategy, the common constraints impacted the architecture implementation. The following list specifies some of the most significant common constraints:
In addition to the higher-level architecture objectives, the Synthesis Group architecture descriptions contained a number of specific system-level recommendations. These recommendations were followed, whenever possible, while conducting the architecture analysis in order to be true to the Synthesis Group's intentions. (For example, the Science Emphasis for the Moon and Mars architecture specified a 4-meter optical telescope on the Moon by 2007. This science instrument was included in the manifest with the appropriate surface accommodations and operations analysis.) Thus, the resulting architecture analyses presented only one of many possible implementations for each of the Synthesis Group architectures.
This general approach of providing key decision points will afford the greatest flexibility (1) to redirect the initiative due to unexpected events or changes in available funding , and (2) to introduce new systems or hardware that measurably improve the pre-existing infrastructure.
It is the sum of the key decisions that formulate an architectural path. The four Synthesis Group architectures are each an assumed pathway through these decision points. Each of the architectures is designed to capture a practical range of scope and achievement within its own boundaries. While the actual SEI architecture that eventually will be implemented is unknowable, its scope and scale will be compared back to the Synthesis architectures.
The advantages of a "turn key" habitat design include: (1) a high degree of confidence in the habitat integrity due to predeployment and verification; (2) a majority of crew time devoted to conducting the mission rather than to preparation of the outpost; and (3) a less complex piloted lander (otherwise the crew would have to rely on the lander as a backup habitat for the entire duration of the surface mission). Robotic, precursor missions to the Moon and Mars in support of human missions
The Synthesis Group subscribed to the basic philosophy of preceding human landings with robotic, precursor missions. Specific implementations for the Moon and Mars include Site Reconnaissance Orbiters (SRO) and Rovers. The SRO's obtain high-resolution, global images of the planet. The objectives of the SRO are to identify areas of scientific interest, to locate potentially useful raw material concentrations, and to identify sites suitably safe for landing. Specific landing sites are then selected for further investigation by rovers. The objectives of the rover missions are to certify a safe human landing site and to collect, analyze, and verify materials identified remotely. The Mars rover missions are also responsible for determining possible toxic agents in the environment that might be damaging to crew health. As needed on the Moon or Mars, the rovers may emplace infrastructure, such as navigation aids, for subsequent human missions.
Moon and Mars represent different power system challenges. For daytime stays on the Moon, lightweight, easily deployed photovoltaic panels are the minimum mass option over nuclear or energy storage.The initial nuclear unit development is for the lunar outpost where deployment and safe, reliable operations are validated. For Mars, nuclear power is recommended over photovoltaics as part of a technology investment strategy. Furthermore, the performance of photovoltaics on Mars may be reduced due to dust storms and a solar flux less than half that at Earth. Nuclear surface power systems are capable of providing Mars base power to a megawatt level with reliable, long lifetimes.
For one full-day stay (28 Earth days) on the lunar surface, photovoltaic power systems with energy storage and nuclear systems are the prime candidates. For a 25 kW habitat load, nuclear systems will weigh one- fifth as much and save 8,000 kg on the lunar surface. Continuous base power that can increase to 1 MW will weigh about 12,500 kg using nuclear power, versus 330,000 kg using photovoltaics with energy storage.4
[T]he Synthesis Group finds that America's ability to return to the Moon and begin the exploration of Mars depends on two fundamental technologies: the restoration of a heavy lift launch capability and the redevelopment of a nuclear propulsion capability.5
Relatively small Earth-to-orbit capability requires more complex orbital operations to assemble the many pieces into lunar or martian transportation vehicles. While on-orbit operations cannot be fully eliminated, the use of heavy lift launch vehicles allows: (1) simplifying the complexity of those on- orbit operations towards rendezvous and docking maneuvers, and (2) reducing the launch-rate operational constraints at the launch site. For interplanetary missions, which require departing from Earth during precise "windows," simpler launch and orbital operations increase the likelihood of making those windows.
With its higher specific impulse, nuclear systems promise high performance with significant savings in propellant mass as compared to chemical systems. Since launch rates are heavily dependent upon the initial mass to low-Earth- orbit, the nuclear systems with their commensurate smaller propellant needs involve lower overall launch demands.
Furthermore, for a given mass of propellant, any technology -- such as nuclear propulsion -- providing improved efficiency enables the reduction of trip times between Earth and Mars. And, as the Synthesis Group Report states: "Biomedical and psychological concerns relative to the effects of prolonged zero gravity, space radiation, and confinement during Mars missions are strong incentives to reduce transit times."6
Another possible thematic distinction between architectures is the relative focus between lunar and martian activities. The four Synthesis Group architectures, however, all present a similar set of Mars missions.
The NASA team concurs that the fundamental theme of any architecture may be expressed as a relative mixture of:
By our definition, a concern addresses a groundrule imposed on all the architectures by the Synthesis Group. It has major repercussions throughout the architecture. The key, high-leverage concerns involve:
While it is appropriate for the Synthesis Group to establish a size for planning purposes, it is too early to commit to a specific number. Any increase in crew size above the minimum could significantly expand the scale of systems and infrastructure (and ultimately, cost), and therefore such an increase must be founded on either requirements to conduct the mission, human factors considerations, and/or operational efficiency goals. The crew size should be variable based on the content and timing of surface activities for that particular architecture. For example, the MARS EXPLORATION architecture maintains an austere, minimum approach, while the SPACE RESOURCE UTILIZATION architecture stresses using the Moon as a platform for in situ resource application with the intent toward base self sufficiency; these two diverse objectives could result in two crew sizes.
Addressing NASA's first human return missions to the Moon, it appears to be desirable to deliver large, integrated payloads to the lunar surface in order to minimize the construction required for an initial outpost. Our preliminary studies indicate this requires on the order of 200 to 220 metric tonnes in low Earth orbit. Using a dual-launch scenario, two 100 mt class vehicles must be processed and launched from KSC within a short period of time. This will add the penalty of on-orbit power, cryogenic boiloff, and system lifetime extensions to the first element launched. In addition, the trans-lunar injection window must be closely synchronized with the second launch; a launch delay may force a one-month-long mission delay and may result in loss of the initial payload. Finally, an autonomous rendezvous and docking system is required. A single launch of a 200 mt class HLLV directly to the Moon greatly simplifies the orbit operations of this type of mission. Preliminary analysis has shown that such a vehicle can be processed within existing KSC facilities, and, when matched with the piloted mission requirements, can reduce demands on launch operations.
The actual payload-to-orbit capability should be an output of analysis taking into account the surface payload requirements, operational constraints, and programmatics. Deriving the most effective approach to launch vehicle size will necessitate an evaluation based on life cycle cost, schedule availability, ground and on-orbit operations, and mission flight rates.
One of the key considerations in determining a viable launch vehicle size is the impact of launch rates. As previously mentioned, the Synthesis Group architectures follow a split mission strategy for both the Moon and Mars (i.e., piloted and cargo missions launched separately). The cargo launch frequency is dependent upon the amount of infrastructure required on the Moon and Mars to implement the goals of the architecture. The specific implementations, as developed by NASA, result in launch rates for the Moon of 2 to 10 per year. The MOON TO STAY AND MARS EXPLORATION architecture poses the most demanding lunar launch schedule, with launches peaking at 10 per year. With respect to the Mars missions, the SCIENCE EMPHASIS FOR THE MOON AND MARS architecture maintains the heaviest schedule with approximately 8 launches every other year (each Mars opportunity), peaking at 10.
The main concern regarding these launch rates involves ground processing feasibility. The Synthesis Group made the implied assumption that there would be a launch facility with access as needed. Current Kennedy Space Center facilities are not sufficient for handling up to 10 launches per year of a 150 mt HLLV or 8 launches per year of a 250 mt HLLV. During the timeframe of these architectures, post 2003, KSC facilities will be accommodating STS and possibly NLS missions plus any additional expendable-class launch vehicles. Ground processing requirements for the Space Exploration Initiative place exceeding demands on KSC's current capabilities, including integrating large payloads, processing and storing nuclear reactor systems, and maintaining large amounts of propellant. The complexity of ground operations for launch processing increases tremendously as the architectures evolve through the lunar and martian phases.
The total amount of time spent on the Moon prior to the 600-day-surface-stay Mars mission varies from architecture to architecture. Three of the four architectures (MOON TO STAY AND MARS EXPLORATION excluded) do not provide lunar surface stays anywhere near to a 600 day comparable duration. (For example, the MARS EXPLORATION architecture has a total amount of only approximately 300 days spent on both the Moon and Mars prior to the 600- day-surface-stay Mars mission.)
Are we implying that at least 600 days must be spent on the Moon prior to the 600-day-surface-stay Mars mission? No, not necessarily. We do not know today what will be required in order to ensure confidence that long-duration surface stays are feasible. Our concern is that the needed experience to prepare for such a 600-day surface stay could prove to be quite significant.
One of our concerns resides with the fact that the lunar and martian environments are so dissimilar. Hardware such as landers, habitats, and rovers may claim functional similarity, yet the characteristics of the environments demand very different system designs (particularly the thermal control and power systems). The one-sixth Earth gravity plus the lunar dust problem also pose some operational impediments for using the Moon to simulate Mars crew operations.
Another concern regarding the complete Mars dress rehearsal on the Moon is whether such a strategy is cost effective. Many of the features of the dress rehearsal appear "costly" at first glance, including duplication of surface systems at a simulation site for full-up verification, the launch and delivery of the Mars transfer habitat to lunar orbit, and the aggressive schedule required to fly Mars hardware six years prior to the actual Mars departure.
Alternatives to the complete Mars dress rehearsal exist. With respect to the concern over environment disparity, information from the robotic, precursor missions about Mars could be used to enhance fidelity of simulations conducted in Earth-based test facilities for certain aspects of the Mars mission. Furthermore, the Moon should be continually used throughout the outpost development as a testbed for operations concept development, subsystems development, and life sciences research for later Mars missions.
In terms of cost effectiveness, the orbital portion of the rehearsal could be performed in low-Earth orbit. Low-Earth orbit facilities could be utilized early to provide valuable life sciences research and zero-gravity systems verification for piloted Mars missions. Furthermore, a life sciences "dress rehearsal" could be conducted from low-Earth orbit in conjunction with the lunar outpost to simulate the zero-g/partial-g/zero-g profile of a Mars mission. (Later, when the actual Mars transit habitat has been constructed, it could be tested in low Earth orbit for a duration similar to the Mars transit period.) As previously recommended, integrating the testbed strategy throughout the lunar scenario and testing Mars equipment on Earth -- if that is the most practical location -- may reduce the cost of building a separate simulation site on the Moon. Such an approach could also relieve an aggressive Mars-hardware production schedule required to satisfy a full-up dress rehearsal of the first piloted Mars mission.
The concern over this mission strategy involves engineering design considerations. The orbiting crew member requirement most significantly impacts the design of the transportation crew module. Throughout the lunar mission phase, the crew module's standard mission includes transporting six people for a total of approximately six days. A severe mass penalty would be incurred in order to sustain one crew member, whether the duration is 14 days or 40 days. The additional mass to the crew module would be designed in systems such as power, thermal control, and life support. A rather costly alternative would be to build a separate crew module for the orbiting crew member flight, i.e., an additional vehicle to the one meeting the standard requirement of six people for six days.
The Synthesis Group rationale for this mission approach follows:
For the first two piloted missions to the Moon, one crew member remains in orbit to perform inflight experiments and to monitor the orbiting vehicle while the other five descend to the surface. All six go to the lunar surface after sufficient confidence is gained that the orbiting vehicle remains in an acceptable status while unattended.12
Since these formulated architectures describe a return to the Moon, not an initial visit, we are confident that automated systems on-board the orbiting vehicle are sufficient. A crew member in orbit is not required for rendezvous and docking procedures between the lander and the space transportation vehicle. (Indeed, if a contingency piloted rendezvous was needed, it could be flown by one of the ascending crew.) Automated orbital activities would be required after the Initial Operational Capability; therefore, they should be designed into the mission from the beginning. To ensure confidence and reduce the risk of failure, the automated procedures could be verified in Earth orbit prior to the return-to-the-Moon mission.
Several submitted issues focused upon specific implementation choices. The submitters believed that another implementation approach would be more effective. An analysis is needed to compare the proposed alternative with the nominal Synthesis Group approach. Examples of this category of studies are:
The eventual SEI architecture should initially strive to provide a realistic blend of themes. In order to maintain multiple themes, each one must be of a reasonable scope and scale so as not to overwhelm the others nor break the proverbial bank.
We cannot know today which theme (if any) will eventually emerge as the dominant focus for Moon and Mars exploration. (For example, if lunar resources prove to be readily extractable and abundant, then mission activities may be directed to capitalize on this viable capability.) We must strategically plan our program to insure that no theme is prematurely emphasized until its "payoffs" are proven. Thus, we leave open options for the decision- makers of the future.
Although the four Synthesis Group architectures each achieve very distinct long-term capabilities, their initial missions are reasonably similar. It is therefore possible to focus upon these near-term missions without knowing a priori the specific long-range architectural growth. Clearly, the value in knowing the potential long-range growth of an architecture is to insure that early decisions do not preclude evolutionary paths. However, even if the long-range growth of an architecture is not known, prudent design of the early missions can insure accommodation of future possibilities.
Given the similarity of the near-term missions among the architectures, we recommend that SEI activities now shift their major (though certainly not exclusive) emphasis to planning for lunar precursor, martian precursor, and first lunar outpost missions.
The NASA team believes the "campsite" concept is the most reasonable way to return initially to the Moon.
Parenthetically, NASA's Exploration Programs Office has already accepted this recommendation and is currently studying lunar campsite options. These studies have already begun to provide new details and new decisions. For example, the human return missions to the Moon will be designed to stay for 45 Earth-days in a campsite mode. (Initial sorties using only the piloted vehicle could stay for shorter periods on the lunar surface and might precede the 45-day-stay missions.) Unlike the SCIENCE EMPHASIS FOR THE MOON AND MARS architecture, the habitat will be reusable; therefore, the option exists to return to the campsite. Hence, evolution options to the initial campsite include establishing alternative campsites or enhancing the campsite for longer duration missions (see Figure 3).
We are not in a position today to know an exact crew size due to unclear, specific tasks and workloads. The point, however, is not to arbitrarily accept six as the proper crew size for the initial campsite missions.
Our preliminary belief -- based on a desire to reduce complexity of the lunar missions, of on-orbit operations, and of ground facilities -- is that the HLLV vehicle will be at the upper end of the range recommended by the Synthesis Group. Arriving at the most effective launch vehicle size will necessitate studies of:
The eventual SEI architecture to emerge will most probably not be any of those currently documented. It will, however, be an architecture to emerge out of the philosophy of the Synthesis Group . . . . a flexible, responsive series of accomplishments evolving anew at each key decision point. This waypoint philosophy may be one of the greatest legacies of the Synthesis Group.