 |
 |
||
 |
|
||
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
||||||||
 |
Projects: Aim for MarsStepping into the FutureA Workshop in Memory of the Columbia SevenOn April 29-30, 2003, The Planetary Society, the Association of Space Explorers, and the American Astronautical Society held a workshop at the George Washington University's Space Policy Institute about the future of human space transportation. The following was presented as a background paper to the workshop. Concepts for Human Space Transporation to Low-Earth Orbitby John Garvey
|
|||||||||||||
| Table 1. Launch Architecture Requirements for Deep Space Exploration by Humans |
|---|
| Must support human exploration of Mars |
| Provides Earth-to-LEO and LEO-to-Earth transportation |
| Capable of being implemented within 15 to 20 years |
| Crew safety is a paramount consideration |
| Distinct transportation modes / systems for humans and cargo |
| Consider international assets, services, systems and technologies where feasible |
| Attempt to maximize compatibility with existing NASA and DOD strategic plans and future capabilities |
Also important, but to a lesser degree, is the potential of such systems to support other, non-planetary exploration agendas, such as those of on-orbit commercial activities, remote sensing and a variety of astrophysical observations from the Sun-Earth L2 point.
Architecture Trades
Given this set of top-level requirements and constraints, the remaining trade space should be familiar to workshop participants. However, the fact that these issues are still the center of debate and the focus of billions of dollars in annual research funding, five decades into the Space Age, highlights the technical and political difficulties in achieving an acceptable, balanced set of solutions.
Figures 1 and 2 represent a categorization of these issues and are intended to serve as references for workshop discussions on priorities and preferences. At a first order, trade options are classified as either architectural or vehicle design in nature. Those trades that can be considered already resolved per the working assumptions of the conference are identified with thick borders and bold print. These should receive minimal attention during the workshop. By contrast, there are a number for which preliminary recommendations are provided here (thin solid borders), but are open to workshop discussion. The same applies to the remaining subset of topics for which no preference is indicated (dashed borders).
LEO Operations
The first obvious question that needs to be answered in defining the launch architecture is WHAT kind of on-orbit operations are to be supported. As noted in Figure 1, these range from none (the entire mission is accomplished using a single launch, such as was done on Apollo/Saturn), to full-up on-orbit assembly, integration and checkout. Decisions regarding deep space propulsion technologies, aerobraking at Mars and whether or not to employ in situ resource utilization technology have sensitivities on the order of a magnitude for such variables as LEO payload delivery quantities, fairing volumes and launch rates.
It is also worth noting that continued investments in LEO facilities and technologies that build upon experiences from ISS could also be expected to have synergism with Sun-Earth L2-based astronomical activities such as those addressed in Reference 1.
Inclination
After determining what LEO ops need to be supported, the next major launch architecture question is WHERE. The selection of a nominal inclination influences a number of key architecture features, ranging from launch sites to emergency crew return strategies and cross range requirements during both launch and return. As demonstrated with the International Space Station (ISS), the selection of a nominal LEO inclination is ultimately a compromise between technical priorities and terrestrial politics. The importance of baselining such an inclination is directly related to the level of on-orbit operations that will be required.
For optimizing both interplanetary trajectory transfer insertion and launch logistics, near-equatorial staging orbits are recommended. From this perspective, Korou and Alcantara rank very favorably as primary launch sites. However, it is doubtful that Washington-based sponsors will find it acceptable to cede billions of dollars in launch operations to these offshore locations. Furthermore, Russian launch capabilities are anticipated to again play a major role.
Against these conflicting requirements, a 28.3° -inclined orbit accessible from Cape Kennedy / Cape Canaveral represents a realistic compromise. Inclusion of Proton for heavy lift services and Soyuz for crew transportation might best be achieved by investing in new launch complexes located at more favorable latitudes. Should the workshop accept this position, one near-term opportunity for action is to endorse and lobby for ESA's proposed development of just such a Soyuz launch complex at Korou, with additional infrastructure to support eventual crewed missions.
Payload Mass to LEO
For crew transportation services to and from LEO, the issue again reduces to whether it necessary to develop an advanced Shuttle-replacement that promises enhanced safety and reduced operating costs compared to either the Shuttle and/or ELV-based systems like Soyuz and NASA's proposed Orbital Space Plane (OSP). Space Launch Initiative (SLI) studies confirmed once again that the Shuttle-replacement option requires an investment of tens of billions of dollars over several decades, with no guarantee of operational success. By contrast, OSP, complemented by Soyuz, Shuttle when available and China's Shenzhou system, is achievable this decade and is compatible with existing ELV capabilities. Furthermore, it should be possible to exploit potential synergism between these systems and Mars entry and landing craft.
The decision is less clear for cargo transportation, primarily because of the correlation between propellant delivery requirements and candidate deep space propulsion technologies (Reference 2). Conventional chemical propulsion (CP) will necessitate the delivery of an amount on the order of one million kilograms of propellant (presumably LH2/LOX), along with on-orbit cryogenic storage and transfer capabilities. With existing ELV's that can deliver approximately 25 thousand kg to LEO, this requirement translates into a total of forty launches per expedition.
Under this scenario, a new, dedicated "big-dumb booster" becomes a much more viable consideration. For comparison, a Saturn V-class vehicle would reduce the total number of flights needed to approximately ten. Lower launch costs for the relatively inexpensive propellant payload can be achieved by relaxing requirements for reliability and mission flexibility, along with the adoption of block procurement strategies. High reliability launchers would still be employed for launching vehicle hardware elements.
The alternative is to invest heavily in the deep space propulsion technologies discussed in Reference 2. These include solar and nuclear electric propulsion, nuclear thermal propulsion and solar sails, along with Martian aerobraking and in situ propellant production capabilities. The common trait is that they all have the potential to reduce requirements for LEO propellant deliveries. Besides facilitating human Mars exploration, such an approach also benefits other planetary exploration initiatives. Project Prometheus reflects one formal manifestation of this strategy and should be supported in upcoming budget negotiations.
Launch Rate
Grossly unrealistic projections for launch rates and payload requirements were among the biggest factors that contributed to Shuttle design compromises that ultimately have rendered it unable to operate as safely, efficiently and cost-effectively as originally intended. The annual launch rate for exploration-related crew transportation is equally dependent on assumptions regarding the level of LEO-based operations. Large-scale on-orbit integration and checkout over extended periods will require more frequent support than an approach in which only the actual exploration team is needed. At this early stage, the launch rates already achieved for ISS crew support prior to Columbia are probably as good as any (i.e. roughly five to eight missions per year).
By contrast, cargo transportation launch rates could vary by a factor of ten or more. To support the all-CP, ELV-based option noted previously, launch rates on the order of twelve to twenty launches per year for several years could be required.
Lead Time
The significance of lead times for human exploration launch support and strategies are more programmatic than technical. Current government launch planning cycles now are approximately on the order of five to ten years. By contrast, competitive pressures have driven commercial services to one-year cycle times. The extensive global overcapacity that now exists among launch providers creates significant leverage for procurements over the medium future. Furthermore, government advocates who are struggling to find justification for continued support of national programs that are bleeding hundreds of millions of dollars annually in this depressed market can be expected to become allies of any such iniiative that stimulates demand. Therefore, early adoption of a launch manifest, while maybe not necessary from a procurement perspective, would be helpful politically.
Injection Accuracy and LEO Rendezvous & Docking Operations
Injection accuracy requirements, along with rendezvous & docking operations, are several areas with opportunities for achieving economies of scale by investing in new on-orbit capabilities. Specifically, the implementation of an orbital maneuvering vehicle (OMV) would enable the relaxation of LEO insertion requirements and/or expand operational windows. For comparison, today's Progress and the soon-to-be operational Automated Transfer Vehicle (ATV) each incorporate sophisticated avionics subsystems for terminal rendezvous with the ISS that are utilized once and then disposed of. Transferring such avionics and rendezvous & docking functions to a reusable OMV would reduce launch system costs, while also improving reliability. In addition, a comparable OMV system could also be implemented on the ISS and in other deep space operations.
Regulatory Issues
Any international approach to launch support for human space exploration must address U.S. government export constraints. At this time, the Iran Non-Proliferation Act and International Traffic in Arms Regulations (ITAR) rank as two of the primary programmatic constraints to NASA as it struggles to re-establish Shuttle and ISS operations. As with ISS, the most straightforward way to address ITAR is to allocate launch services to the same international organizations and countries that provide the payloads. Technology discussions can then focus on interfaces.
In addition, as done on Sea Launch, payload encapsulation and other such design approaches can on a limited basis serve as acceptable techniques for minimizing technology export.
Financial Figure(s) of Merit (FOM)
Traditionally, the most common metric for assessing launch costs has been dollar-per-kilogram of payload delivered to a desired orbit. However, up-front non-recurring costs are frequently not included in such metrics, in part because government support is a key component and exact estimates are difficult to calculate. For a human exploration initiative, this non-recurring cost should be given as much priority, if not more, as the marginal operational costs. Workshop participants will recall that outsized estimates on the order of hundreds of billions of dollars were among the primary reasons for the demise of the first President Bush's proposed Space Exploration Initiative (SEI). An upfront investment of $20 to 40 billion to achieve operational cost reductions of 25 or even 50% constitute a hard argument to sell, particularly with existing, widespread excess launch capacity and looming federal budget deficits.
Vehicle Trades
Even the precursory survey of architecture trade options given above demonstrates that architecture definition is where the emphasis should be in the near-term. For launch vehicle trades, the workshop assumption to separate crew from cargo transportation to LEO enables the focus to shift to a) reusable versus expendable crew transportation systems and b) existing expendable launch vehicle systems versus new launchers for cargo.
For crew systems, the path of least technical risk and upfront non-recurring investment entails expendable capsules, updated with modern avionics. A slightly more ambitious goal that is still well within the realm of existing technology would be to incorporate a replaceable / refurbishable ablative heat shield that would enable reuse of the basic crew vehicle after relatively minimal (compared to Shuttle) servicing. Just such an capability was demonstrated with the unmanned prototype Gemini 2 spacecraft, which first flew in January, 1965 and then again in November 1966. Advocacy for inclusion of this option within the OSP System Requirements Review scheduled for later this year is another "next step" opportunity for the workshop coalition.
For the cargo launch option, a decision on whether to advocate development of a big dumb booster depends primarily on whether chemical propulsion is employed for the deep space trajectory maneuvers. One other potential motivation for large cargo boosters that received attention during SEI was whether it was necessary to launch large, already-assembled aerobrake structures that would be employed both for orbit capture at Mars and then on the return to Earth. Such aerobrakes could conceivably be assembled on-orbit, or might be dropped from consideration altogether, hence the uncertainty here on whether they constitute a real launch system design driver.
Observations and Recommendations
Within the context of launch services to low Earth orbit, the fundamental decision confronting advocates for human space exploration is straightforward - should we wait for new, advanced launch technologies and services that promise to make the task easier, or do we make do with existing systems that are based on expensive, decades-old propulsion technology.
The history of advanced launch vehicle development since Shuttle makes it difficult to recommend the first option. The path to planets has been blocked by seemingly endless paper studies and aborted X-vehicle projects that never made it to the launch pad. Furthermore, as addressed in the companion workshop papers, very real opportunities exist in other segments for enabling technologies that can provide order-of-magnitude improvements over the traditional all-chemical transportation architectures that were first proposed in the 1950s.
To keep things simple, the following summary points are put forward for workshop debate:
1. Crew transportation to and from LEO should utilize capsule-type vehicles launched on ELV's. NASA's proposed OSP is an obvious candidate, along with Soyuz launched from a new pad in Korou. To the extent Shuttle is available, it can complement these operations, but it should not be allowed to become a critical path factor as it is with the ISS.
2. Exploit the existing fleet of under-utilized launch systems. A new, expendable big dumb booster designed specifically for propellant delivery to LEO should only be considered if an all-chemical propulsion architecture is adopted.
3. The best way to facilitate launch operations is to invest in on-orbit operations, in-space propulsion and in situ resource production technologies that reduce payload-to-LEO delivery requirements
Just as important as defining a preferred set of architecture features is the identification of next steps for moving forward after the workshop. The following are consistent with the points made previously in this paper:
a. support ESA / Russian efforts to deploy Soyuz launch vehicle
from Korou. Push for the capability to accommodate the Soyuz crew
delivery system
b. push for the OSP initiative to proceed forward
c. support Project Prometheus and advocate expansion of investments
in comparable exploration technologies, including on-orbit operations,
solar sails, aerobraking and in situ propellant production
Finally, if only one lesson is to be learned from the loss of Columbia and its crew, it is that diversity in launch services is absolutely essential if we are to continue forward with human exploration of the solar system.
References
1. Huntress, Wesley, et. al., draft input for the International Academy of Astronautics Study "The Next Steps in Exploring Space," 11 April 2003 [PDF]
2. Stetson, Douglas, "Transportation Concepts for Human Space Exploration Beyond Low-Earth Orbit," Stepping into the Future workshop paper, 29-30 April 2003.
3. "Stepping into the Future - International Considerations," Stepping into the Future workshop paper, 29-30 April 2003.
[ what is this? ]