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Acfa Astronautica Vol. 47, Nos. 2-9. pp. 265-274, 2000 0 2000 International Astronautical Federation. Published by Elsevier Science Ltd

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A SPACE TRANSPORTATION ARCHITECTURE FOR THE FUTURE

Douglas Stanley STAS Study Manager

Orbital Sciences Corporation Dulles. VA USA

ABSTRACT

This paper summarizes the results of recent studies by Orbital to significantly reduce NASA’s future launch costs and improve crew safety through the implementation of a low-risk, evolutionary space transportation architecture. These studies were performed as a part of NASA’s Space Transportation Architecture Studies (STAS). The STA studies were commissioned by NASA Headquarters to advise NASA and the Executive Office of the President on the future direction of U.S space transportation. A large number of vehicles and architecture approaches were examined and evaluated. Orbital’s recommended architecture includes a small, multifunctional vehicle, referred to as a Space Taxirw, which would serve as: an emergency crew return vehicle for the International Space Station (ISS), a two-way human space transportation system, a small cargo delivery and return vehicle, and a passenger module for a future Reusable Launch Vehicle (RLV). The Space Taxi would initially be launched on a heavy-lift Evolved Expendable Launch Vehicle (EELV), currently under development by U.S. industry and the U.S. Air Force. Together with a small cargo carrier located behind the Space Taxi, this combination of vehicles would be used to meet future ISS servicing requirements. Later, a two-stage, commercially developed RLV would replace the EELV in launching the Space Taxi system at a significantly lower cost. This RLV system will provide NASA and other customers with unprecedented reductions in cost and improvements in reliability, safety, and performance. 0 2000 International Astronautical Federation. Published by Elsevier Science Ltd. All rights reserved.

INTRODUCTION

Over the past several years NAS.4 and the U.S. commercial industry have been actively developing new technologies and system approaches to dramatically reduce the cost of access to space through the introduction of Reusable Launch Vehicles (RLVs). Through the innovative X-34 flight vehicle and recently completed SpaceTransportation Architecture Studies (STAS), Orbital is leading the way in the development and application of RLV technologies. The STA studies were commissioned by NASA Headquarters to adviseNASA and the Executive Office of the President on the future direction of U.S space transportation. A large number of vehicles and architecture approaches were examined and evaluated. This paper summarizes Orbital’s approach to significantly reducing NASA’s future launch costs and improving crew safety through the implementation of a low-risk, evolutionary space transportation architecture.

1) Meeting NASA’s launch and human rating requirements

2) Providing significant cost savings

3) Providing resiliency against uncertain events

4) Minimizing government ownership

5) Maximizing industry’s role

6) Encouraging competition

7) Benefiting U.S. military and economic security

The time period during which the architecture would be developed and operated was assumed to be from 2000 to 2020. Because of the goals of reducing costs by minimizing government ownership and maximizing industry’s role, an extensive study of commercial launch requirements was also included, and the preferred architecture was designed to satisfy NASA, military, and commercial requirements.

STUDY OBJECTIVES STUDY APPROACH

The purpose of the STA studies was to advise NASA and the U.S. government on a preferred architecture approach to augment and eventually replace the current Space Shuttle, while satisfying a number of objectives:

In order to accomplish this multifaceted study effort. Orbital performed a wide variety of analyses, including:

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1) Evaluating key government and industry mission databases and developing a reference mission model and excursions.

2) Examining a wide variety of old and new space transportation concepts and developing a very large number of candidate architectures using combinations of these elements.

3) Creating a detailed Master Investment Model for examining the economic payoff of these architecture approaches to the government and potential equity investors.

3)

5)

6)

Evaluating the various architectures against criteria and sub-criteria of interest to NASA.

Selecting and analyzing in detail a preferred architecture that best satisfies these criteria.

Developing implementation roadmaps for the preferred architecture, including technology requirements. milestones, transition opportunities. and off-ramps.

7) Examining a wide variety of regulatory and policy

STUDY RESULTS

The STA studies were quite extensive in their scope and depth. They included detailed mission and cost analyses, system definitions, development plans, technology roadmaps, and policy and regulatory recommendations. A summary of all of these results would be significantly beyond the scope of this paper. Instead, the results that are discussed below will focus on the definition of a new Space TaxiT” Transportation System and related findings and recommendations. Orbital’s Space Taxi transportation system consists of a small Space Taxi vehicle, an adapter/launch escape system, cargo carriers located behind the Space Taxi, and a two-stage reusable launch vehicle (RLV) that will eventually be used to launch these elements.

Space Taxi Confiauration

System Summary

issues and potential government financial incentives to enable the implementation of the preferred Orbital’s proposed Space Taxi configuration, shown in

architecture approach. Figure 1, provides a low-risk architecture solution to satisfying h’ASA’s future human spaceflight needs with significant reductions in cost and order-of-magnitude

Weight Empty (Dry) Crew and Payload

I 22.2’

II I

Seven Crew 48 Mid-Deck Locker Equivalents

~tgure 1. apace mu KeJereitce corgtgurarrof~

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improvements in crew safety and reliability. The reference Space Taxi vehicle configuration is about 28 ft long, with a body width of less than 15 ft. It is a lifting body shape with canted tins and a small vertical stabilizer. When the canted fins are folded, the Space Taxi can fit in the Space Shuttle payload bay. The empty weight of the vehicle is 20,000 lb, and the total gross launch weight is approximately 28,500 lb. A 76-inch diameter pressurized cabin provides room for seven crew members, 48 Shuttle mid-deck locker equivalents (MLEs), and two extravehicular activity (EVA) suits, simultaneously. The Space Taxi is designed to perform the International Space Station (ISS) crew rotation mission currently performed by the Space Shuttle. Normal access to the cabin is through a cabin roof hatch, sized to mate with the ISS Pressurized Mating Adapters (PMAs). A larger access hatch at the aft end of the cabin can be used to berth the Space Taxi to Common Berthing Mechanism (CBM) ports on the ISS, allowing the loading of International Standard Payload Racks (ISPR) for return to Earth. The aft hatch also provides a second emergency crew egress exit in the event of a water landing.

As shown in Figure 2, the reference Space Taxi would initially be launched on top of a new Evolved Expendable Launch Vehicle (EELV). Later it would be launched on top of Orbital’s two-stage Reusable Launch Vehicle (RLV), which is described in more detail below. A key feature of the Space Taxi launch configuration is the use

EELV RLV

Fqwe 2. Space Tnv~ Launch Options.

of a Launch Escape System (LES) composed of solid rocket motors attached to the Space Taxi aft launch adapter, During a successful mission, these solid rocket motors would nominally be jettisoned from their location on the adapter cone after second stage ignition. If required for abort, the solid rockets have been sized to separate the Space Taxi from the launch vehicle with an acceleration of 8 g’s to allow intact recovery of the Space Taxi vehicle from a catastrophic booster failure at any time during ascent. A set of optional sustainer motors can be used to provide a higher probability of reaching a runway for safe landing at the launch site or downrange. In the event that a runway can not be reached, the Space Taxi is designed for a safe water landing with provisions for Apollo-style parachutes and flotation devices as shown in Figure 3.’ Use of this launch escape system allows Orbital’s Space Taxi to provide an order-of-magnitude improvement in crew safety and meet all of NASA’s human-rating requirements.

The Space Taxi configuration and approach has a tremendous amount of heritage in a series of detailed studies that was performed by NASAin the early-go’s of a vehicle referred to as the HL-20.‘.* In fact, an entire issue of the AIAA Journal of Spacecraft and Rockers was dedicated to this HL-20 vehicle.’ The aerodynamic shape of the I-IL-20 was based on a Russian-designed vehicle known as the BOR-4.’ This vehicle flew numerous sub- orbital trajectories from Russia in the 80’s and was recovered in the Indian Ocean. It was used to test thermal protection systems and control system approaches for the Russian Buran Orbiter vehicle. The aerodynamic shape and aerothermodynamic characteristics have been refined over three decades. The reference Space Taxi version has a trimmed subsonic lift-to-drag ratio (L/D) of 3.8 and a trimmed hypersonic LID of greater than 1.3, thus providing 1,100 nmi. of re-entry crossrange capability,

The Space Taxi uses a metallic pressure vessel to carry crew and supplies. All other primary and secondary structures are made of graphite composites. Durable ceramic thermal protection system (TN) tiles and blankets are bonded to the composite panels. The nose, canted-fin leading edges, and aerosurfaces are made of a high- temperature carbon silicon-carbide hot structure with insulation. The major subsystems are located outside the pressurized cabin and are accessible through panels on the upper body surface. The orbital propulsion system is a storable, non-toxic hydrogen peroxiddjP-4 system. The guidance, navigation, and control system of the Space Taxi was designed to operate in an autonomous mode to allow the safe return of the ISS crew in an emergency or to allow the Space Taxi to be operated as a cargo-only delivery and return vehicle.

Flgure 3. Space Tarr Aborr Approach.

.4s shown in Figure 3. the Space Tax] vehicle IS normally operated to and from the ISS as part of a system that mcludes a cargo-carrying “trailer”. attached to the rear, and actually towed by the Space Taxi. This “trailer” can

Flgure J. On-Orbrr Conjiguratron ofSpace Tau/PLK

be configured in two ways: as a Payload Logistics Vehicle IPLVt or as an Unpressurized Payload Logistics Vehicle (IIPLV). The personnel and cargo carrying capacities of the Space Taxi, PLV, and UPLV have been sized to make full use of the delivery capability of the heavy-lift EELV to the vicinity of the ISS. Therefore, the Space Taxi propulsion system only has to perform ISS approach, rendezvous, and later, de-orbit of the SpaceTaxi and PLV combmation.

As shown in Figure 3, the PLV is a pressurized cylinder capable ofreacting launch loads and moments. Its interior is sized to accommodate 12 ISP Racks, which are loaded and removed through a forward Common Berthing Mechanism (CBM) hatch. A low-cost, expendable array of reaction control jet modules can be located around a ring at the aft end of both the PLV and the UPLV, if required. This array would be controlled, however, by the avionics of the Space Taxi vehicle. The UPLV, shown in Figure 5, while still reacting launch loads and moments,

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Flgure 5. On-Orbrt Conjrguratron of Space TaxrUPLV

provides a bay for carrying an unpressurized logistics carrier (UPLC). At least six flights of the Space Taxi and cargo carriers are required to satisfy the annual servicing requirements of the 1%. Four of these flights would be crewed, while the remaining flights would be uncrewed.

System Advantaaes

The Space Taxi was specifically designed to be multi- functional and to provide a wide range of mission flexibility. The reference Space Taxi would serve as an ISS Crew Return Vehicle (CRV), a two-way human transportation system, a small cargo return vehicle and an RLV passenger module. Because the Space Taxi is designed to serve as a CRV, the true cost to NASA of the SpaceTaxi is only the marginal cost over what the Agency would have paid for CRV development and implementation. When a Space Taxi brings a fresh crew to the ISS every three months, the deconditioned crew members return in the Space Taxi that was left at the ISS

three months earlier. Hence, similar to the Soyuz approach, there is always a Space Taxi docked to the ISS to serve as a CRV. Because of its small size and potential to be launched on different boosters, Orbital’s Space Taxi approach provides a very high degree of operational flexibility. Future commercial variants of the Space Taxi might be used for satellite servicing and repair, satellite re-boost, space transfer, in-space construction, and even space salvage. Orbital does not believe that a significant enough commercial market exists in these areas to justify industry investment in the development of a Space Taxi; however, if such market opportunities do arise, a reduction in operating costs for the baseline government missions might be realizable.

Orbital’s reference Space Taxi system would provide an order of magnitude improvement in crew safety because of its use of a Launch Escape System (LES) that would allow intact recovery of the Space Taxi vehicle from a catastrophic booster failure at any time during ascent. The probability of crew survival on a single SpaceTaxi mission could be greater than .9995. The Space Taxi system provides an autonomous flight control capability that does not require crew members on cargo transfer flights. This separation of crew and cargo reduces the number of annual ctewed flights to four, thereby further reducing the probability of crew loss during the life of the program. The SpaceTaxi configuration also provides a low-g entry for injured, sick, ordeconditioned crew members, coupled with a smooth runway landing with direct pilot visibility.

The technologies employed by the Space Taxi system are summarized inTable 1, together with back-up technology approaches for contingency planning. These technologies are highly consistent with the timing and content of NASA’s on-going X-38/CRV technology program, currently being implemented by NASA’s Office of Space

Tub/e 1. Space Taxi Technology Requirements.

Flight. The only major technology requirement of the Space Taxi that is not included in the existing X-38KRV program is the advanced development of an integrated propulsion system that uses JP fuel and hydrogen peroxide. This system is currently under advanced development as a part of ajoint NASA/DOD technology program.‘ Other elements. such as the adapter/LES and PLV, use predominately state-of-the-art technologies. Hence. there is very little risk to the architecture implementation caused by technology availability for the Space Taxi-related

elements.

Two-Staae Reusable Launch Vehicle IRLV) Confiauration

System Summary

As a part of these STA studies Orbital developed a two-

stage RLV concept, which could provide unprecedented reductions in cost and improvements in reliability, safety, and performance. As shown in Figure 6. developing two identical vehicles to operate together in a two-stage-to- orbit (TSTO) configuration provides a large reduction in development cost and risk over an SST0 approach because of the much smaller size and weight of the vehicle that must be developed. This RLV is specifically designed to be multi-functional and to provide the widest possible

range of mission flexibility. As shown in Figure 7, development of a single vehicle could provide the flexibility of launching almost any weight payload. In addition, since the payloads are mounted externally, a wide range of payload volumes can be accommodated. Two vehicles launched together would provide the same capability as a heavy-lift EELV, a single vehicle could provide a significant sub-orbital trajectory capability; a single vehicle, augmented by varying numbers of solid rockets or liquid stages, could be used to launch small and medium-class payloads: and a single vehicle could also be used as a booster for a heavy-lift core stage to provide the capability of launching very large payloads. This flexibility is very important 10 its commercial vlabihty because the development cost can be amortized over a significant number of flights of payloads from \ar~ous weight classes. Because of the potential cost savings that il would provide to a range of customers, this RLV would be commercially developed, owned and operated

A smgle RLV is about 115 feet long and 22 feet in dIPmeter. with a gross weight of approximately 850,000 Ibs. As pictured in Figure 8, the configuration is a circular cross-section winged vehicle with wing-tip fin aerodynamic controllers, elevons and an aft body flap. It has integral aluminum-lithium propellant tanks, utilizing liquid oxygen and hydrogen propellants, with simple

Figure 6. SST0 Versus Blmese Approach.

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Figure 7. Orbital’s RlVAppmach.

cylindrical cross sections and elliptical end domes. With the exception of the reusable cryogenic propellant tanks, all other primary and secondary structures are made of graphite composites. Durable ceramic thermal protection system (TPS) tiles and blankets are bonded to the composire panels or to the external cryogenic insulation on the integral propellant tanks. The nose, wing leading edges, tip-fin controllers and aerosurfaces are made of a high-temperature carbon silicon-carbide hot structure with insulation. Each vehicle has three RS-2100 liquid hydrogen-fueled engines. These are highly operable, full- flow staged combustion cycle engines currently under advanced development by Boeing’s Rocketdyne Division and the US military.

Figure 8. Orbital’s RLV Conjgurarion.

The vehicle aerodynamic configuration and design approach has a significant heritage in a number of RLV design studies, including NASA’s 1993 Access-to-Space Study. It has a hypersonic lift-to-drag ratio of greater than 1.3. and the TPS is sized to provide greater than 1,100 nmi of re-entry cross-range capability. This configuration was extensively studied as an approach for a single-stage- to-orbit (SSTO) vehicle in the early 90’s and was used by NASA to demonstrate the feasibility of SSTO. These studies largely contributed to the timing and approach of NASA’s current Reusable Launch Vehicle Program, including the X-33 and X-34.

System Advantaaes

As mentioned above, Figure 6 indicates that developing two identical vehicles to operate together in a two-stage- to-orbit (TSTO) configuration provides a large reduction in development cost and risk over an SST0 approach because of the much smaller size and weight of the vehicle that must be developed. Internal studies by both Orbital and NASA have shown that this “bimese” approach will lead to a lower life cycle cost because of the lower amortization requirements for the development cost, despite the higher production and slightly higher operations cost per flight relative to an SSTO. The

operations costs between the two approaches are not expected to be significantly different. Although two vehicles must be processed and integrated with the two- stage approach, the vehicles are smaller, with less TPS to process, and use similar (smaller) facilities and ground support equipment, Orbital’s external payload approach greatly reduces the size and weight of the vehicle that must be developed and operated by allowing a very efficient propellant packaging (7 75 percent), while only recovering the high-valued items (engines, TPS. tanks, avionics, etc.). Because of its smaller size and TSTO approach, the reference approach has dramatically lower development risk than a similar SSTO. It is very difficult to design a feasible SST0 configuration with weight growth margins greater than 15 percent, and even then, the vehicle is always at the very edge of its performance margins. The weight savings gained through this approach can be used to build in more robust weight and

performance margins or to allow the use of lower-risk technologies (e.g. metallic, rather than composite cryogenic propellant tanks) to lower costs.

The circular cross-section winged vehicle configuration approach has also been shown to have a number of advantages over lifting body vehicles. It is lighter in weight andeasier to manufacture because of the simplicity of the structural and tank configuration. This light weight leads to lower development costs through smaller size and increased margins. It has also been demonscrated to be easier to trim and control during entry because of the flexibility that the wings provide in compensating for a far-aft center of gravity (c.g.). Furthermore, durtng atmospheric reentry, there is no shift in c,g. caused by payload-in and payload-out conditions because the payload is not returned.

Fgure 9. Space Tam%LVMmron Profile

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Space Taxi Mission Scenario

The Space Taxi could potentially be launched on multiple boosters, As shown in Figure 2, the reference Space Taxi would initially be launched on top of the new heavy-lift Delta IV EELV. Later it would be launched on top of Orbital’s two-stage Reusable Launch Vehicle (RLV). Once the two-stage RLV is in operation, the proven EELV would provide an alternate access capability for the Space Taxi if the RLV were not available for an extended period of time due to a catastrophic failure. Before the RLV becomes operational, the Space Taxi could be designed to be launched on an Atlas V or on an Ariane V if the Delta IV were not available for an extended period of time.

The nominal mission profile for launching the Space Taxi on this two-stage RLV is shown in Figure 9. The mission sequence for launching the Space Taxi on a heavy-lift EELV is similar. The Space Taxi is launched with two RLVs in a parallel bum configuration with cross-feeding of propellant. In this configuration, propellant from the booster is cross-fed to the orbiter, which carries the Space Taxi system on top. When the booster’s propellant is exhausted (at Mach 3.4 and 100,000 ft.), it separates and glides back unpowered to a runway landing at the launch site. The orbiter then completes the ascent trajectory using its own propellant and inserts into a low-Earth orbit. The 5. l-meter Delta IV second stage then separates and begins a bum to raise the Space Taxi system to an ISS orbit. Meanwhile, the orbiter RLV jettisons the mating collar between itself and the second stage, performs a de-orbit bum, re-enters, and glides to a horizontal landing at the launch site. The combined Space Taxi. adapter, and PLV approach the ISS and dock, using the top batch of the Space Taxi for the docking interface. The ISS manipulator arm is then used to detach the PLV and adapter from the Space Taxi and attach them to a pressurized port for unloading. At this point, the SpaceTaxi can begin serving in the position of a CRV for the next three months.

ISS operations include egress of crew from the arriving Space Taxi, unloading cargo from the arriving SpaceTaxi and the PLV, loading returnable cargo into the returning Space Taxi, loading disposable pressurized trash into the PLV, and mating the PLV to the returning Space Taxi. Return of the Space Taxi with the deconditioned crew consists of undocking and departing from the ISS, de- orbit, release of the PLV and adapter to bum up in the atmosphere, and re-entry, resulting in a controlled glide to the landing area. Actual touchdown proceeds much as the Shuttle lands today. When the vehicle is crewed, final guidance and landing can be performed under control of

the pilot. For an uncrewed flight, primary guidance is autonomous, with provisions for emergency override if ground controllers detect a problem.

STUDY FINDINGS

As noted above, the STA studies were quite extensive in their scope and depth. They included detailed mission and cost analyses, system definitions, development plans, technology roadmaps, and policy and regulatory recommendations. Hence, a large number of findings and recommendations were generated. A summary of all of these findings is beyond the scope of this paper. Instead. this section summarizes the key technical findings and recommendations related to the development and implementation of the proposed Space TaxiT\’ Transportation System.

1) Upon full implementation of the Space Taxi Transportation System and retirement of the Space Shuttle, NASA would save over $1 billion per year in launch costs under very conservative assumptions. These savings would represent a Net Present Value of $10-15 billion to NASA in current dollars over the period of 2000 to 2020 if NASA were to invest in the development of this architecture approach.

2) NASA would recover its investment in the Space Taxi Transportation System within two to three years after full system implementation and retirement of the Space Shuttle, and the required annual investment would not exceed NASA’s current budget wedge for space transportation development.

3) Implementation of the Space Taxi Transportation System would provide an order-of-magnitude improvement in crew safety because of its use of a Launch Escape System that would allow intact recovery of the Space Taxi vehicle in the event of a catastrophic booster failure at any time during ascent.

4) Because the Space Taxi is designed to serve as a Crew Return Vehicle (CRV) for the International Space Station, the true cost to NASA of the Space Taxi is only the marginal cost over what the Agency would have paid for CRV development and implementation.

5) The Space Taxi Transportation System relies on existing or evolutionary technologies, rather than revolutionary technology advances. This greatly reduces schedule and cost risk.

6) Because of its small size and potential to be launched on different boosters, the Space Taxi provides a high degree of operational flexibility. Future variants of the Space Taxi might be used for human exploration, satellite servicing and repair, satellite re-boost, space transfer, in-space construction, and even space salvage.

FUTURE WORK

NASA has recently initiated follow-on studies to evaluate 2) the potential of this architecture to meet future satellite servicing and human exploration requirements. In addition, these studies will identify and prioritize near- 3) term technology development initiatives to improve human safety, reduce life cycle cost and reduce schedule and cost risk. Results of these studies, which will last for several months, will be presented in future papers.

1)

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Stone, H. et al, “Special Section: HL-20 Personnel Launch System,” AIAA Journal of Spacecraft and

Rockets 30.521-634 (1993).

Clark. P.. The Soviet Manned Space Program, New

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Lewis, T.. “Interim Design Review for the Upper Stage Flight Experiment Program,” Orbital Sciences Corporation, Contract NAW98-010 (1998)