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D A Heald & T L Kessler, , "Single Stage to Orbit Vertical Takeoff and Landing Concept Technology Challenges", IAF-91-205.
Also downloadable from stage to orbit vertical takeoff and landing concept technology challenges.shtml

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Single Stage to Orbit Vertical Takeoff and Landing Concept Technology Challenges
Daniel A Heald


Thomas L Kessler


General Dynamics has developed a vertical takeoff and landing ( VTOL) concept for a single stage to orbit ( SSTO) under contract to the Strategic Defense Initiative Organization (SDIO). This paper briefly describes the configuration and its basic operations. Two key advanced technology areas are then discussed: high-performance rocket propulsion employing a plug nozzle arrangement and Integrated Health Management to facilitate very rapid turnaround between flights more like an aircraft than today's rockets. Our concept combines NASP and ALS/NLS technologies in an innovative way.


Single Stage to Orbit ( SSTO) has been a dream for 40 years. The fact that no boosters or drop tanks are expended means that a fully reusable SSTO vehicle could operate frequently at low recurring cost, like an airplane. Development of high-performance hydrogen-oxygen propulsion, pioneered by the General Dynamics Centaur, has encouraged the possibility of SSTO. The current development of lightweight, high-temperature materials for NASP may finally make the SSTO dream possible.

In June 1991 General Dynamics, McDonnell-Douglas, Rockwell and Boeing completed SSTO studies and technology demonstrations for the Strategic Defense Initiative Organization (SDIO). This customer established basic requirements as shown in Table 1. Intact abort and rapid turnaround suggest aircraft-type operations, radically different from historical launch vehicle procedures. Mission and payload guidelines included delivery and/or retrieval to Low Earth Orbit in the 10,000 pound range and compatibility with manned flights such as Space Station crew rotation. Rocket propulsion was given as the prime mover since the NASP program is already developing an air-breathing SSTO.

Table 1. Key SSTO system requirements.

  1. Intact abort any time during flight
    (Save the payload and/or crew)

  2. Rapid, low-cost turnaround
    (Low operating costs using only 350 man-days)

  3. Medium payloads deployed and/or retrieved
    (10,000 lb to low Earth orbit)

  4. Manned and/or unmanned operation
    (Automated flight plus inherent reliability and safety for crews)

  5. Rocket propulsion as the prime mover
    (Avoid dependency on NASP air-breathing technology)

Think unique aircraft operations, not unique space vehicle launches

If these goals could be achieved, access to space would be much more reliable and affordable. Early in the next century, SSTO might replace Atlas and Delta for medium payloads. Low-cost crew transportation to Spact Station Freedom would be available. Quick reaction deployment of military satellites would be possible in a crisis situation. As SSTO reliability and cost-effectiveness improved, new customers might appear, including commercial users such as global overnight package delivery. This paper describes our recommended SSTO concept: a vertical takeoff and vertical landing ( VTOL) configuration. The focus is on two key technologies: high-performance Aerospike propulsion and Integrated Health Management (IHM). We believe vehicle technology will soon enable SSTO performance, but tumaround in a week with a small crew will require extensive development in automation including I.H. M. This amounts to a change in culture in the launch vehicle business.

VTOL Concept

Figure 1 shows the General Dynamics selected systems concept and its circle of flight and ground operations. Vertical launch and ascent are similar to today's launch vehicles, except that inland bases such as White Sands are envisioned. The vehicle reenters on its base, as did the Apollo Command Module. Final deceleration and steering through crosswinds is done propulsively to land on a 600 ft. concrete circle.

Figure 1. General Dynamics concept - a system solution.

This main propulsion system obviously must be very versatile to be effective during launch and landing. Throttled retrothrust was used in the Apollo program for landing the Lunar Excursion Module on the Moon. Our trade studies indicated this is lighter than wings and wheeled landing gear. Note that the SSTO requirements did not include once-around capability or a particular crossrange. Wings provide flexibility during landing but restrict ascent operations due to shear wind loads.

After landing, the vehicle is safed and towed to the Turnaround Hangar, where maintenance, servicing, and the payload installation occurs. Extensive automation including IHM is required for rapid ground tumaround. The same smart sensors enable quick engine shutdown or abort before a catastrophe occurs during flight and provide data to print out an unscheduled maintenance list after landing.

Our selected configuration and its basic operation capabilities are shown on Figure 2. It is a conical shape with a blunt base, which is both the main engine and the reentry surface. The forebody half angle is 20 degrees. The engine consists of 12 modules arranged around a 40-ft circle with a center expansion surface truncated at 20% of complete isotropic expansion.

Figure 2. Vertical takeoff and landing SSTO.

This SSTO has extreme flexibility in payload packaging, with a wide array of payload sizes, lending itself well to a wide range of missions, both manned and unmanned. The reusable payload module is designed to accommodate STS-compatible payload attachment fittings. For manned missions a separable crew escape module, similar to Apollo, is used for added crew safety, but the core vehicle and payload module are identical to the unmanned version. Larger volume payloads may be accommodated through the use of expendable payload fairings. For crew ferry missions, the vehicle is intended for compatibility with the NASA Personnel Launch System (PLS) or Assured Crew Return Vehicle (ACRV). Because it is a base-entry VTOL the payload is always maintained in an upright orientation, a requirement for many of today's payloads. As a reusable vehicle. it can retrieve and return payloads as well as deliver them. Since the payload does not receive direct aerothermal impingement during reentry, the front end configuration my vary from one flight to another. The concept provides for return to the launch site if mechanical problems occur, thereby saving valuable payloads.

The required avionic functions must be performed in a reliable manner to ensure high probability of mission success and must fully support operational improvement for a vehicle tumaround in seven days with 350 man-days. Our operational vehicle avionics system, the Multi-Path Redundant Avionics Suite (MPRAS) will have dual fault tolerance and will take advantage of key operability features such as real-time, onboard Integrated Health Management. This system shares common technology with the F-22 Advanced Tactical Fighter (ATF) and the RAH-66 Light Helicopter (LH).

The MPRAS architecture is specifically designed for operability. The electronic packaging, which consists of common module cards and standard enclosures, have fully Line Replaceable Unit (LRU) and Built-In-Test (BIT) capability. The architecture significantly reduces avionic logistical requirements since only a small number of standard electronic modules fulfill all the subsystem requirements (see Figure 3).

Figure 3. SSTO avionics have high performance and reliability.

Another key operability feature, Integrated Health Management (IHM), is present throughout the operational vehicle infrastructure. The vehicle avionics system will provide the capability for detection of abnormal performance, prediction of impending failures, and selection of an alleviating stratagem. Ground health management will be used extensively in test/operations control to verify the operational status of all critical vehicle hardware and ground support equipment associated with assembly, prelaunch, post-landing and turnaround.

Our concept has a gross liftoff weight of 1.3 million pounds and an expected life in operation of 500 flights with major engine overhaul every 50 or 100.

Rather than optimize systems for very low weight, operations has heavily influenced the design of the SSTO vehicle as shown in Figure 4. IHM eliminates the need for many manual prelaunch system checks. A leg opening through the aft end of the vehicle provides direct access from the ground to about 80% of the components. Access panels around the low-temperature region of the vehicle allow easy access to the modular propulsion system. All thermal protection system panels are designed to allow for simple removal and replacement. There is a simple standardized interface point between the vehicle and the payload canister. Most payload operations occur off line. A flight-ready, pre-canisterized payload arrives at the turnaround facility for integration with the vehicle. During payload changeout, there is access to the avionics and RCS systems located in the forward part of the vehicle. The propellant tanks are constructed of low-risk, low-maintenance aluminium-lithium. The vehicle is powered by 1-2 removable reliable expander cycle engine modules. The use of differential throttling eliminates the need for actuators and hydraulics for the engines and complex flexible feedlines. The propellant feed system uses welded fittings and joints to reduce leaks, and isolation valves are used to simplify removal and replacement tasks. Liquid hydrogen and liquid oxygen are used for the ACS/RCS systems to eliminate the toxic propellants which also eliminates any need to evacuate the vehicle area during tumaround. The landing gear consists of six robust pneumatic landing legs. Special design attention is focused or. precluding hydrogen leakage in the reusable tank- and feedlines.

Figure 4. SSTO vehicle designed for operability.
Critical Technologies

Since SSTO has never been achieved, some advanced technologies must be applied to make any concept work. Table 2 lists the highest risk areas that need to be demonstrated. They are listed in order of criticality. Propulsion and IHM head our list and will be discussed further. Table 2. Critical technologies for SSTO

RequirementAdvanced technologyPrecedent

High-performance propulsion: vac Isp>467 secAerospike engine1972 Rocketdyne tests
Very rapid turnaround: 350 man-daysIntegrated Health Management and Automated Mission PlanningALS program
Reentry heating and stabilityHypersonics CFD with wind tunnel testsNASP program
Accurate vertical landing: 600ft radius in 30 knot windsFlight control software and deep throttled engineLEM lunar landings
Very light, hot structureAdvanced materials such as TMM and Gr/epNASP program
Smart redundant flight controlAdvanced avionics (AGN&C and MPRAS)ALS program, F-22
Very fast, efficient prototyping: scale flight in 2 yearsLean team managementYF-16,"Have Blue"

Our configuration is completely different from the usual sharp reentry shapes, so special wind tunnel tests and computational fluid dynamics (CFD) analyses are required, as shown in Figure 5. Shock impingement and hypersonic aerodynamic stability must be checked over the range of mach numbers and may lead to configuration and material refinements and body flaps for control.

Figure 5. Our balistic range tests clarify SSTO flow features.

Software will certainly be a pacing item on any new program, particulaily one with extensive health monitoring and rapid mission planning software requirements. General Dynamics is ready. to meet the challenges of SSTO software development with proven computer-aided software engineering (CASE) tools already in use on our ALS / NLS and Atlas programs. Modern CASE tools. such as Software Through Pictures ( M), allow much more rapid requirements generation and provide enhanced traceability without much of the administrative overhead associated with a standard MIL-2167A compliant program. Recent advances in object-oriented coding and associated autocode generators, such as Matrix X, allow rapid prototyping of software elements and full stepwise debugging at every level of the avionics architecture. from the individual card through integrated systems test. General Dynamics has linked these CASE tools in a Rapid Integrated Prototyping (RIP) environment that promises to cut software development time by a factor of 3-5 with enhanced software reliability.

Light weight and high propulsion performance both enable a single stage to reach orbit. The higher the Isp, the more current materials can be used. On the other hand. if a lower Isp is available, say from a conventional bell nozzle engine, then very advanced materials are necessary to achieve extremely low weight. Light weight also applies to the main engine, which is nearly 25% of the vehicle dry weight. Our philosophy was to strive for high engine performance, use more conventional materials such as aluminum-lithium, and later phase in advanced materials such as titanium metal matrix and graphite epoxy tanks when proven on the NASP program.

Another key technology involves management. The SSTO program plan aims at early prototype flights to allow early assessment of feasibility. This will require a very close-knit team practicing concurrent engineering very efficiently. It will necessitate unusually streamlined procurement, fabrication, inspection, and checkout policies and procedures. Such "lean team" "skunkworks" have been mainly used on classified aerospace programs. In 1972 General Dynamics started the very successful F-16 program by producing two Y-prototypes in less than two years for $37M.

Integrated Health Management

Integrated health management (IHM) is a systematic approach that provides an automated means of verifying the operational status of all critical hardware associated with vehicle assembly, launch, and support operations. IHM is capable of detecting abnormal performance and impending failures, and it functions as a diagnostic system for identifying and correcting a suspect component. The principal elements of IHM shown in Figure 6 are: 1) automated data analysis, 2) automatic checkout, and 3) ground support systems and airbome controls. The automated data analysis element concentrates on algorithms and display techniques for test and checkout validation. The automatic checkout element focuses on smart built-in test techniques that can be used to reduce overall test and checkout timelines. The ground support systems and airborne controls element concentrates on the actual hardware that will host the IHM system.

Figure 6. Integrated Health Management is an interdisciplinary technology.

IHM is the key to high flight safety and rapid, low-cost turnaround. As shown in Figure 7, the philosophy is to check key components throughout their life to detect impending failure before it happens and replace that component. For instance, rocket engine pump failure can be catastrophic, so its health should be monitored and shutdown should be triggered by data outside an acceptable range. Pump bearings are a wear item, which has been troublesome on the SSME. Pump bearing vibrations can be monitored to check for trends away from normal signature. This is standard practice on electrical utility generators.

Figure 7. Pervasive IHM supports both ground and flight operations.

Two propulsion items deserve special mention. Hydrogen leaks have delayed the Shuttle many times. Sensors that detect leaks have not been very reliable and often cannot determine the source of the leak. There is no experience with fully reusable cryogenic tanks. So there is the double challenge of designing leak-tight systems with long life plus a sensor system that will locate a leak if it occurs.

Propulsion system reliability can be significantly improved with engine-out capability. The philosophy is to design to make the mission even if one engine is shut off. Signs of trouble would be detected by the IHM system fast enough to shut down the engine before a catastrophic failure occurs. Both Shuttle and Saturn have successfully flown with engine-out. The challenge is to design the engine from the beginning for many reuses and for dependable ULM function.

Smart fault checking frees up people for management / judgment tasks rather than problem identification. IHM is the key to avoiding a "standing army" of ground support personnel doing manual checkouts between flights.

Much work remains even to predesign a complete IHM system from sensor to real-time data analysis. What are all the catastrophic failure modes? How many sensors would it take to monitor these areas? Should there be three of each sensor, with voting logic to eliminate a faulty -sensor? How can artificial intelligence / expert svstems technology benefit the system? What will this IHM system weigh and cost? Will its complexity degrade mission reliability?

Aerospike Engine

SSTO performance requires both lighter weight and higher propulsion Isp than existing engines. As shown in Figure 8. with current Shuttle SSME Isp of 455 sec (430 avg), SSTO would have to have a propellant mass fraction > 0.90, which necessitates using all NASP technology materials still in the laboratory phase. such as silicone carbide titanium. Lower Isp also forces reduced margins in structural robustness. But if the vacuum Isp reaches 470 (455 avg). then current materials such as titanium, aluminum-lithium. and carbon carbon will suffice. Therefore. the Aerospike-type engine is attactive because its high geometric expansion ratio, 366 in our concept, promises vacuum Isp = 470 sec.

Figure 8. Basic SSTO propulsion challenge.

During the Phase I SSTO program. General Dynamics Rockwell, Aerojet, and Rocketdyne cooperated on a 20-inch propulsion flow test that confirmed our analyses.

Conventional "bell" nozzle engines require a low expansion ratio for boost phase through the atmosphere and a high expansion ratio for vacuum performance. Both are needed on an SSTO, which suggests an extendible nozzle. This concept has a large area ratio nozzle retracted during the low-altitude portion of flight and deployed at near-vacuum conditions. This concept has been used successfully on some solid rocket motor systems. However, this performance gain is weighted against the complexity of the deployment mechanism, the storage problem of the large nozzle, increased weight and a longer landing gear.

An idea that arose in the early 1960s was the plug nozzle, which had all the features of altitude compensation without the need for the mechanisms and parts. During the 1970s Rocketdyne performed over 300 hot firings on Aerospike-type engines. One circular H/O design, rated at 250,000 pounds thrust, produced a vacuum Isp = 460 seconds. Our SSTO engine concept should be higher because of its higher chamber pressure and larger expansion ratio. The original annular combustion chamber encountered problems in maintaining accurate throat gap; so the proposed SSTO concept employs approximately 120 individual thrusters, an idea recommended by Aerojet. This introduces several additional losses due to increased wetted surface, round to rectangular cross sections, and flow interaction between adjacent plumes. These losses tend to lower vacuum Isp. But still, the Aerospike engine is only two-thirds as long as the corresponding bell nozzle. Our baseline Aerospike engine concept is shown in Figure 9. In addition to high performance, the Aerospike-type engine offers other significant advantages: thrust vector control by throttling segments without hydraulic actuators: engine-out (actually a "module" or 1/12 of the circle) capability, deep throttling for descent/landing, and lighter weight than a bell nozzle of the same expansion ratio.

Figure 9. Baseline main propulsion system characteristics.

In spite of old test data, new analyses, and wind tunnel tests, there are still uncertainties in flight performance, cooling, and rectangular to circular thruster section efficiencies. There are design, manufacturing. weight, and cost questions.

Concluding Comments

The VTOL SSTO is a promising but radical concept with very high risk. The two most challenging and potentially most rewarding technologies are Integrated Health Management and the Aerospike engine. We recommend continuing work in these areas not only for SSTO but also for other applications such as Lunar Lander.

Presentation Viewgraphs
D A Heald & T L Kessler, , "Single Stage to Orbit Vertical Takeoff and Landing Concept Technology Challenges", IAF-91-205.
Also downloadable from stage to orbit vertical takeoff and landing concept technology challenges.shtml

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