Aerospace generated this concept design for a two-stage-to-orbit reusable launch vehicle as part of a series of vehicle evaluation studies.

Future Launch Systems

Robert Hickman and Joseph Adams

Fast, cheap, and reliable space launch capability would be a tremendous asset to defense, civil, and commercial organizations alike. Aerospace is helping to ensure that all options are given proper consideration—because the decisions made today will profoundly affect the launch community for many years to come.

In the 46 years since Sputnik, the space age has seen progressive improvements in launch systems and corresponding enhancements in the services provided by space assets. Today's launch fleet routinely deploys sophisticated spacecraft for navigation, communication, meteorology, intelligence, surveillance, reconnaissance, and space exploration.

Though impressive, today's launch fleet is not without limitations. Launch costs and preparation times limit space applications to a handful of high-value services. A revolution in new space applications is possible, but would require a new generation of launch systems to reduce cost and preparation times. The Department of Defense and NASA have expressed interest in such "transformational" capability; but before pursuing such a system, three major interrelated questions must be answered.

First, what capabilities are envisioned for the system? The goals of the defense, civil, and commercial space sectors are different, and the degree to which common solutions can be developed will determine whether separate or joint programs are pursued. Second, what sort of system should be designed? The choice between an expendable and reusable system, for example, will depend on whether design techniques and manufacturing technologies can be improved enough to make reusable systems affordable. Third, what development strategy should be employed? The combination of risk tolerance, available budget, and timeframe of need will dictate whether developers seek radical advancements through aggressive technology projects or accept a safer, more incremental approach.

Aerospace has been seeking answers to all these questions with the goal of charting a course to provide the greatest benefit for all stakeholders. Beginning in 1996, for example, Aerospace's Future Spacelift Requirements Study evaluated near- and long-term national space mission needs, traditional and emerging markets, and the technology needed to address such markets. That landmark study helped shape the debate about what sort of systems should be developed—a debate that continues today.

System Capabilities

The needs and priorities of the Defense Department do not always match those of NASA or a global telecommunications firm. Cargo mass, crew, orbit, and launch frequency requirements can vary considerably from sector to sector. Moreover, while all might agree on the need for high performance and reliability with low cost and risk, they might have widely different notions about what these concepts mean.

Defense Perspective

Defense launch systems are in the midst of a major transition. The heritage launch systems that served the nation's needs for decades are now being retired and replaced by a new generation of launch vehicle families under the Air Force Evolved Expendable Launch Vehicle (EELV) program.

These vehicles are adequate to support today's mission manifest of national security satellites; however, the Air Force has identified a need to launch tactical space missions that support war fighters in real time. These missions would allow global strike capability, rapid augmentation of satellite constellations, rapid replacement of compromised space assets, deployment of specialized space vehicles for combat support, and wartime protection of American space assets. The Air Force is clearly considering that future military engagements may require the launch of large numbers of payloads, each weighing less than 6800 kilograms, in just a few days.

Prosecuting a war in this manner would be impossible without launch responsiveness. Aerospace is assisting the Air Force Space Command in analyzing ways to achieve such "operationally responsive spacelift." At this point, efforts are still focused on formulating requirements, operational concepts, and design options.

Civil Perspective

In the course of more than 20 years, the space shuttle has launched more than a million kilograms of cargo and sent more than 300 people into space. After the start of operations, however, it became increasingly clear that the shuttle was difficult to operate, maintain, and upgrade. Also, the differing orbiter configurations made each flight preparation a painstaking ordeal.

In 1997, NASA commissioned a study of a second-generation single-stage fully reusable system to replace the shuttle. The result was the experimental X-33 space plane. The prototype—along with its full-scale version, VentureStar—sparked considerable interest within the commercial sector; but the X-33 couldn't overcome all the technological challenges associated with its design. Meanwhile, the projected market for broadband communications satellites collapsed, deflating commercial interest and hastening the cancellation of not only the X-33 but the companion X-34 program as well.

As a result, NASA undertook an analysis of future systems that culminated in the so-called Integrated Space Transportation Plan. A major element of this plan was the Space Launch Initiative, a strategy to develop the architectural elements and associated technology for the shuttle replacement. Aerospace supported this initiative through requirements analysis and risk management techniques.

In the fall of 2002, NASA revamped its Integrated Space Transportation Plan by delineating three main building blocks. The first involved the Shuttle Life Extension Program, which considered how to keep the system flying until 2020. The second was the Orbital Space Plane, a vehicle that would initially serve as an escape craft for astronauts on the International Space Station. This would be followed a few years later by a crew transfer vessel launched atop an EELV. The third element was to develop promising technologies identified through the Space Launch Initiative.

With this backdrop, the space shuttle Columbia flew its 28th and final mission, launching on January 16, 2003, and breaking up 16 days later on its return to Earth. A new plan announced in early 2004 calls for a return to shuttle flights (until the International Space Station is completed) and development of a space vehicle capable of carrying a crew to the moon and beyond.

Commercial Perspective

The traditional commercial launch market is focused principally on lofting communications spacecraft into Earth orbit. The global market for launch of these payloads, in terms of mass, is many orders of magnitude lower than for any other transport industry. Whereas the worldwide commercial launch market is less than half a million kilograms per year, commercial U.S. aircraft transport more than 1000 times that mass each day.

A methodology developed at Aerospace to explore launch costs suggests that the low flight rate required to support traditional communications spacecraft is not large enough, by itself, to justify large economic investments needed to achieve dramatically lower launch costs.

Nontraditional ventures may provide opportunities for profitable space launch businesses. Aerospace studies suggest that if launch cost could be reduced to between $50 and $225 per kilogram to orbit, a variety of nontraditional markets would open, thus providing an environment that would foster a viable growth industry. Examples include suborbital and orbital human transport, fast global freight and package delivery, space manufacture, and space solar power.

Recently, the commercial sector has witnessed an emergence of space launch development entrepreneurs. Partly spurred on by the X Prize competition, a number of entrepreneurs are investing commercial capital to develop suborbital and orbital space transportation systems (see sidebar, The X Prize). Their success or failure could have long-term repercussions on the commercial launch sector, hastening or delaying the introduction of a rapid-response space launch service.

System Design

ICBMs can launch in large numbers on short notice. To do so, they must be preprocessed and stored in a nearly launch-ready configuration. This methodology could also support the needs for responsive spacelift, but would require a massive launch and storage infrastructure and advanced production of the expendable launch vehicles. For reusable launch vehicles to be feasible, processing timelines must be shortened to less than four days; longer timelines will drive fleet size and processing facility requirements to unaffordable levels.

Determining how best to provide these capabilities requires an evaluation of each vehicle option's cost and technical risks. Aerospace has been applying considerable resources to do just that.

Expendable Vehicles

Expendable launch vehicles could probably support responsive tactical space needs, just as ICBMs do, but the cost would be prohibitive. Current launch costs range from $11,000 to $22,000 per kilogram of payload to low Earth orbit. The significant efforts of the EELV program have achieved moderate cost reductions, particularly for the heavy-lift vehicles, which use the same production line as the medium-lift versions. This commonality effectively provides the heavy-lift rocket with production rate advantages over the Titan IV and also permits the costs of engineering and logistics to be spread over a larger number of vehicles.

Still, further significant decreases in expendable launch vehicle cost are not anticipated. Some industry analysts suggest that technological breakthroughs will reduce cost, but typically, key aspects of such technologies are not well understood, which makes them risky. Air-launched systems are also often identified as solutions; but although air-based launchers can support all-azimuth launches, they do not impart a significant velocity increment, and so do not substantially reduce the amount or cost of the expendable hardware.

Conventional Reusable Vehicles

Reusable launch vehicles are commonly proposed as responsive and inexpensive alternatives to expendable rockets. Analogies to aircraft systems suggest that reusing flight hardware should substantially reduce cost.

According to Aerospace analyses, reusable launch vehicles that have been optimized for minimum dry mass have staging velocities (that is, the velocity at which the second stage deploys) roughly between Mach 10.5 and 11.5. In this case, the orbiter will be about half the dry mass of the booster. The mass of the reusable launch vehicle will grow steadily as the staging velocity deviates from this range. For example, if the staging velocity grows higher, the booster must be bigger to generate more thrust; if the staging velocity is lower, the upper stage will have to make up the difference to reach orbit. This is the problem faced by single-stage reusable launch vehicles. Single-stage vehicles are not practical without significant advancements in materials and propulsion technologies; however, two-stage vehicles are undeniably feasible, given the state of existing technologies.

This graph depicts the relationship between dry weight and ideal separation velocity as it applies to two-stage-to-orbit reusable vehicles. An ideal separation velocity of approximately 2750 meters per second is roughly equivalent to Mach 4, after gravity and drag losses are accounted for. Below this separation speed, boosters can readily be designed to glide back to the launch base. Above this speed, jet propulsion is needed to fly the boosters home. Note that as one approaches the extremes of the curves, the weight relationship becomes very sensitive, which illustrates the difficulties in achieving single-stage-to-orbit vehicles. The design region for smallest (weight-optimized) vehicles is near the center of the curves.

A disadvantage of reusable launch vehicles is their relatively high initial costs. The combined cost of development, facilities, and fleet procurement will reach well into the billions of dollars, even for small fleets. For this reason, it may be impractical to develop completely separate reusable launch vehicle designs for defense, commercial, and civil communities. Rather, it will probably be more affordable to pursue modular development approaches to support the broad community. For example, derivatives of boosters and orbiters could be used in various configurations to support various payload classes. While the derivatives would not be identical to the original vehicles, they would possess common systems and components, thus reducing development and production costs. This commonality would also reduce the operational costs of logistics and sustaining engineering, which are major recurring costs.

Understanding the operability of such a system is crucial, as responsiveness will be the key defining characteristic of the next-generation launch system. Aerospace developed the Operability Design Model to estimate the maintenance and turnaround operations of future reusable launch vehicles. Using this tool, Aerospace determined that a new vehicle could improve operations one to two orders of magnitude compared with the space shuttle simply by implementing improved system designs, process improvements, and cutting-edge technologies.

Even with the industry's best operability analysis tools, experts agree that such estimates carry significant uncertainty. Credible estimates of turnaround time for the next reusable launch vehicle range from 2 to 10 days. This uncertainty is a problem for the Air Force, because it will affect how many vehicles and facilities are needed to accommodate a surge in demand (for example, during wartime). This affects cost sufficiently that the difference between a 2-day and 10-day turnaround may determine the ultimate choice between expendable or reusable launch vehicles.

Estimates of reusable launch vehicle production cost are also uncertain because the only actual data point is the space shuttle. The per-kilogram cost to build each orbiter was twice that of the Air Force's most expensive aircraft, the B-2 bomber. Were this to hold true for the next reusable launch vehicle, production costs would severely limit its affordability. There are, however, rational arguments suggesting the cost will be lower. For example, the shuttle was the first of its kind, and was never optimized to control production cost. The orbiters have life-support systems, and must be built to safeguard the lives of the crew. The shuttle features distributed, rather than modular, subsystems. The shuttle program did not have access to the latest materials and production technologies. All of these problems can be corrected or minimized by using modern designs, technologies, and production techniques. Nonetheless, a factor-of-two uncertainty in production cost greatly affects the decision on expendable versus reusable launch vehicles.

Air-Breathing Reusable Vehicles

The appeal of air-breathing vehicles is that they get their oxidizer from the atmosphere, rather than carry it with them. Thus, they might, at least in theory, be smaller and less expensive than conventional rockets. Still, some fundamental issues need to be addressed.

For example, air-breathing rockets must sustain combustion at hypersonic speeds while producing positive thrust. This has not been demonstrated, although projections of potential hypersonic performance have been made using computational fluid dynamics models; however, these models must be calibrated with test or flight data to be credible, and wind tunnels cannot produce conditions to simulate hypersonic combustion beyond a fraction of a second.

The thermal environment presents another problem. The hypersonic combustion process generates extreme heat. Extended hypersonic flight within the atmosphere can generate thermal and aerodynamic loading many times greater than that of equivalent conventional rockets. Thus, successful development of hypersonic air-breathing rockets will require highly advanced high-temperature technologies for engines and reusable structural thermal protection.

A further limitation is that runways can support aircraft weighing no more than about 635,000 kilograms. This places a ceiling on the gross weight of air-breathing reusable launch vehicles, all of which must take off horizontally. Relatively small changes in hypersonic performance predictions could cause this runway limit to be exceeded.

Sometimes, the argument is advanced that because air-breathing rockets operate from runways rather than launchpads, their recurring operations costs and timelines will be closer to aircraft costs and timelines; however, good operability stems from several factors, including component accessibility, operating margins, and component design life. To enable robust turnaround, designers must allocate sufficient dry mass and vehicle volume to allow robust subsystems (which are heavier than less robust ones). Whether or not a combination of weight growth and runway limitations would force compromises in operability and affordability remains an open question.

Thus, when one considers the theoretical nature of performance predictions, the advanced technological requirements, and the challenges for operability, it is clear that air-breathing concepts should be considered high risk well into the future.

Hybrid Vehicles

In its technical leadership role in the Air Force's Operationally Responsive Spacelift effort, Aerospace has also conducted analyses of hybrid reusable-expendable vehicles. These combine reusable boosters with expendable upper stages. The analysis suggests that such vehicles inherit an interesting combination of benefits from both elements.

Assuming optimal staging, at about Mach 7, hybrids expend about 35 percent of the hardware a comparable expendable rocket would expend. Thus, their recurring production costs are much lower. Also, the mass of the reusable booster stage for a hybrid is about 45 percent that of a fully reusable launch vehicle. Thus, development and production costs are significantly less. For these reasons, even relatively low launch rates could economically justify their development.

The hybrid vehicle also carries less risk than a fully reusable launch vehicle—primarily because it does not employ a reusable orbiter. Reusable orbiters present a difficult technical challenge, as they must survive on-orbit operations and reentry through Earth's atmosphere without significant damage. The reusable booster experiences a much less severe environment, resulting in fewer technical challenges and less risk.

Development Strategy

While many development strategies have been considered over the years, the Air Force and NASA both favor an evolutionary approach, focusing on incremental enhancements in capability. Both agencies also agree that ground and flight tests of a demonstration vehicle are critical—to reduce uncertainties regarding achievable production cost and responsiveness, to supply information needed to crystallize a decision on an objective system, and to provide an affordable flight test bed to demonstrate design features and technologies needed to achieve various future technical objectives.

This is an example of a notional spacelift architecture, designed by Aerospace to support a broad range of payloads, based on derivatives of only two vehicle elements.

The hybrid is considered a relatively low-risk first step toward an operationally responsive spacelift capability, one with clear advantages over expendable and reusable launch vehicles. The performance of this hybrid will have far-reaching implications. According to Aerospace analyses, if the cost and responsiveness of the reusable booster turn out to be on the low end of predictions, then the Air Force and NASA might be better off pursuing a fully reusable launch vehicle. If instead middle to high-end predictions are demonstrated, then the Air Force would probably prefer the hybrid configuration.

Clearly, no first step in an evolutionary process can satisfy all the objectives of defense, civil, and commercial sectors. But the evolutionary approach establishes a low-risk process for building upon successes, ultimately supporting most or all spacelift needs. Once a substantial portion of nonrecurring reusable launch vehicle development costs are absorbed, then the recurring costs of operating commercial reusable launch vehicles could be significantly lower than for modern expendable launch vehicle systems. Thus, development of a reusable launch vehicle system by NASA or DOD would offer opportunities to spin off commercial variants.

Future Technology

Properly focused technology development offers significant potential to increase system performance and reduce recurring cost. In fact, operability analyses performed by Aerospace identified the following technologies as particularly valuable for improving cost and operability in reusable launch vehicles: long-life components, nontoxic reaction-control systems, rugged thermal-protection systems, long-life propulsion systems, and autonomous health monitoring (see sidebar, Modeling Operability). All of these must be designed, of course, for quick turnaround—measured in hours rather than days or months. The only significant technology hurdle in this regard is the thermal-protection system. The proposed reusable-expendable hybrid demonstrator greatly mitigates this impediment.

The present technology base is adequate to achieve significant improvements in reusable launch vehicle responsiveness (compared with the shuttle). But by implementing an evolutionary development approach, even this capability can be incrementally enhanced via technology insertion at block upgrades. This will require that launch vehicles be designed to facilitate technology upgrades, employing modular systems (as in typical aircraft) rather than distributed systems (as in the shuttle). These are different methods of doing business for the government and for the spacelift industry. Implementation will be thorny—but in the end, it will determine how well risk is controlled and to what extent operability-enabling design features will be incorporated in the next generation of spacelift systems.

Acknowledgements

The authors thank Jay Penn, John Skratt, Glenn Law, and John Mayberry for their contributions to this article.

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