|A Look Back at Ames’ Contributions to the Shuttle||
April 12 marks a historic milestone in the human exploration of space. It is the 45th anniversary of the flight of cosmonaut Yuri Gagarin, the first human to orbit the Earth. It also is the 25th anniversary of the fight of STS-1, the first orbital flight of the Space Transportation System, or space shuttle. This truly remarkable achievement was the result of work by thousands of individuals at NASA Headquarters, NASA field centers, major portions of the aerospace industry and academia. |
Image left: Figure 1. STS-1 launched from KSC on April 12, 1981, with Commander John Young and pilot Robert Crippen.
Research at Ames has played a key role in the evolution of the shuttle program from the very beginning. The shape of the orbiter has its roots in the “lifting body” research pioneered by “Sy” Syvertson, Ames’ fourth director, and Al Eggers. Once its 1- to 2-week orbital mission is complete, the shuttle executes a de-orbit burn, which slows it for its descent into the atmosphere. Initial entry occurs at about Mach 25, or 25 times the speed of sound in air. During the high-speed portion of the entry, the vehicle holds a high angle of attack. It executes a “blunt body entry” maneuver pioneered by Ames’ second director, H. Julian “Harvey” Allen for the Mercury/Gemini/Apollo programs. After a long and fiery entry, the vehicle continues to dissipate energy through a series of S-turns. It then goes into subsonic flight and lands, unpowered, either at Dryden Flight Research Center or, as is most common today, at Kennedy Space Center (KSC). Astronaut pilots say the shuttle glides like a “falling brick,” so being able to land unpowered is quite an achievement.
This article describes some of Ames’ major contributions to the early development of the space shuttle and mentions a few of the many Ames employees whose contributions were crucial to the vehicle’s development. These include contributions to the shuttle ascent aerodynamics/aerothermodynamics (a combination of aerodynamics and thermal effects), the thermal protection system (TPS) that prevents the orbiter from burning up during reentry, low-speed approach and landing technology and simulator research. The center’s facilities that enabled these contributions also are briefly described.
Ames has supported space shuttle development for close to 30 years, beginning with the formation in the 1970s of a Shuttle Project Office, led by Victor Stevens and his deputy, Bob Nysmith. They managed projects at Ames at the request of the program’s lead center, Johnson Space Center. Hans Mark, Ames’ third director, played a key role in defining and directing Ames’ involvement in the shuttle program. Various directorates at Ames provided staff and facilities to execute projects.
Aerodynamics of the Orbiter/Boeing 747 Ferry Configuration.
Image right: Figure 2. 14-Foot wind tunnel model of space shuttle orbiter and 747 used to understand aerodynamics of mated vehicles.
One of Ames’ first tasks was to understand the aerodynamics of the specially modified Boeing 747 used to ferry the orbiter from Dryden to KSC. The aerodynamics of the mated vehicles and the interference of flows between the vehicles had to be well understood prior to committing to design and flight. Understanding the separation process of the 747 and the orbiter was another requirement. (Figure 2). Testing in Ames’ 14-foot wind tunnel was a major contribution to the successful flight test of the 747/full-scale orbiter model Enterprise.
Ames made a huge effort to develop the aerodynamics and aerothermodynamics for the shuttle. Victor Peterson, former deputy director of Ames, has stated that over 50 percent of the wind tunnel testing conducted for the shuttle was done at Ames. Ames’ contribution to these wind tunnel tests is a heritage of which we can all be very proud.
Nearly all the aerodynamic studies at Ames used the center’s extraordinary collection of wind tunnels, including the 40- by 80-foot wind tunnel, 12-foot pressure wind tunnel, the 2-foot, 11-foot and 14- foot transonic wind tunnels, the 6-by-6 foot, 8-by-7-foot and 9-by-7-foot supersonic wind tunnels, and the 3.5-foot hypersonic wind tunnel.
Image left: Figure 3. Schlieren photograph of shuttle vehicle/exhaust plume interactions from 9–foot by 7-foot wind tunnel test.
More than 10,000 hours of wind tunnel testing took place even before the award of the shuttle design and construction contract in 1972. More than 25,000 hours of wind tunnel testing occurred after this. Key contributors to the subsonic - supersonic elements of the activity included Richard (Pete) Peterson, Jake Drake, Dan Petroff, Jim Monford, Jack Bronson, Len Roberts and Jack Boyd.
Testing for the ascent stack (the orbiter, external tank and solid rocket boosters) aerodynamics and exhaust plume interactions was carried out in the 9-foot by 7-foot supersonic section of Ames’ Unitary Plan wind tunnel. (Figure 3). These tests helped engineers ensure that the aft portions of the vehicle were properly designed, and that they would safely function during ascent.
Other specialized aspects of Ames’ wind tunnels were very helpful in the shuttle’s development. Figure 4 shows multiple exposures of a special rig in the center’s 14- foot tunnel that was used to study the aerodynamics of an abort maneuver implemented at transonic mach numbers. This rig also was used in the study of the mated/separating configurations between the Enterprise and the 747 carrier aircraft.
Image right: Figure 4. Multiple-exposure photograph showing test positions of shuttle abort maneuver in the 14-foot.tunnel.
One of the most heavily used tunnels for shuttle testing was the 3.5-foot hypersonic wind tunnel, which was capable of simulating flight at Mach 5, 7 and 10. This facility provided about 47 percent of the total hours of wind tunnel testing at Ames. Many personnel were involved in this work, including Joe Marvin, Mike Horstman, Marvin Kussoy, Bill Lockman and Tom Polek.
Image left: Figure 5. Shuttle ascent stack in the 3.5-foot hypersonic wind tunnel.
Figure 5 shows a 1.5 percent ascent stack configuration in the 3.5-foot hypersonic wind tunnel test section. This model was tested at Mach 5. Another configuration tested in the 3.5-foot tunnel was secured to the sting by its tail, so the effects of protruding main engines and the orbital maneuvering system could be assessed. These studies led to the understanding of many different complex phenomena, including dynamics of shock-shock interactions caused from the proximity of the elements of the stack configurations, and the effects of split body flap deployments and turbulent flows.
Entry Aerodynamics and Aerothermodynamics
Before the space shuttle, most entry vehicles were relatively simple, blunt shapes with no aerodynamic control surfaces. The shuttle was to become the first airplane-like entry vehicle with movable control surfaces.
The 3.5-foot hypersonic wind tunnel contributed equally to both ascent and entry aerodynamics and entry aerothermodynamics. Figure 6 shows a shadowgraph of the side view of the orbiter at Mach 7. The fine lines enveloping the side view outline the front of a bow shock layer that forms over the vehicle. At higher Mach numbers, the bow wave is highly swept as shown in the figure, and the gases in this wave are shock-heated to very high temperatures. These shock-heated gases create an environment that would melt the surface of the vehicle were it made of materials such as aluminum or composites found in modern aircraft. Data and analyses from Ames’ wind tunnel simulations later were used to refine methods for estimating the heating over the full-scale shuttle.
Image top right: Figure 6. Shadowgraph of flow about the shuttle orbiter at Mach 7 showing the bow shock wave.
The entry aero/aerothermodynamics of the shuttle were performed before the advent of modern 3-dimensional real-gas computational fluid dynamics, a later accomplishment led by Ames. In the 1970s, personnel including John Howe, Chul Park, Dave Stewart, John Rakich and Mike Green, working under the leadership of Dean Chapman, Vic Peterson and Howard Larson, used clever, approximate analytical tools, experimental results and engineering judgment to model the aerodynamic forces, heating rates and heating loads to understand the shuttle entry flow environment. This knowledge was required for the development of the shuttle TPS, another area of key contribution by Ames.
Thermal Protection System Contributions.
The shuttle’s thermal protection system prevents the vehicle from burning up from the searing heat of hot gases that exist within a bow shock layer that envelops the vehicle as it re-enters Earth’s atmosphere. These gases reach temperatures as high as 25,000 degrees F, and heat the surface of the vehicle to as much as 3,000 degrees F. The vehicle enters the atmosphere at an angle of attack of about 40 degrees. Figure 7 depicts the elements of the thermal protection system developed or invented by Ames. Key participants in this research include Howard Goldstein, Dan Leiser, Marnel Smith and Dave Stewart.
Image left: Figure 7. Ames’ contributions to the space shuttle thermal protection system.
In the early 1970s, Ames and JSC evaluated a large number of candidate TPS materials for the space shuttle orbiter in their arc jet facilities. Among these new types of heat shield materials was the LI-900 silica tile system developed by Robert Beaseley and his team at Lockheed Missiles and Space Company, Sunnyvale, and several other conceptually similar systems developed by other companies. In order to understand why the various tile materials performed as they did in arc jet testing, Ames began a tile analysis research program, which rapidly turned into a tile development program. When the LI-900 tile system was chosen as the baseline in 1973, Ames had already begun to make significant contributions to the rapidly improving technology.
Ames showed in that same year how the purity of the silica fibers used in the tiles controlled their temperature capability and lifetime. In 1975, Ames invented the black borosilicate glass coating called Reaction Cured Glass (RCG) that was adopted by LMSC and the shuttle program in 1977 and that now covers two-thirds of the orbiters’ surface. This coating provides a thermally stable high-emmitance surface for the tiles, which serves to radiate away heat and allows the tiles to be manufactured to the demanding tolerance required. The coating covers the tile, which is made by bonding pure silica high temperature-resistant fibers. The finished tile substrate is similar in appearance and density to Styrofoam, but its thermal properties are such that the surface can be glowing white hot at over 2,300 degrees F and the back face of the tile never exceeds 250 degrees F, only a few inches below the surface. These remarkable heat-resistant tiles enable the space shuttle orbiter, which is essentially an aluminum airplane, to fly at hypersonic speeds.
In 1974, Ames invented the tile now known as LI-2200, which is stronger than LI-900 and contains silicon carbide to provide improved temperature capability. Adopted in 1978, this new tile replaced about 10 percent of the baseline LI-900 tile system on the first orbiter, Columbia, when a critical tile strength problem was encountered. Later, in 1977, Ames invented a new class of tiles called Fibrous Refractory Composite Insulation (FRCI 12). In 1980 it replaced about 10 percent of the earlier LI-2200 and LI- 900, providing a more durable TPS and saving about 500 pounds of the overall TPS weight.
Hot gas flow between the tiles during atmospheric entry was considered a serious problem during orbiter development. In response, Ames developed a gap filler, which consists of a ceramic cloth impregnated with a silicone polymer that was adopted as a solution to the gap heating for Columbia. The Ames gap filler was so successful that it was adopted as a permanent solution to the gap flow problems on all the orbiters. In excess of 10,000 are now used on each vehicle.
On the leeward side of the orbiter, gases are much cooler during entry. At first a low temperature reusable surface insulation (LRSI) tile developed by LMSC was used. Ames (with Johns Manville) developed a flexible silica blanket insulation called Advanced Flexible Reusable Surface Insulation (AFRSI) that replaced most of the LRSI on the last four orbiters (Challenger, Atlantis, Discovery and Endeavour) and was retrofitted to Columbia.
Arc jet Facilities Simulate Entry Heating.
Ames has a long heritage in the development of arc jets, tracing to the earliest days of NASA. These facilities are used to simulate the entry heating that occurs for locations on the body where the flow is brought to rest (the stagnation point, typically on the nose cap, wing leading edges and on the acreage of the vehicle).
Image right: Figure 8. Stagnation point test.
Simulations have to run from a few minutes to tens of minutes to understand the TPS materials’ response to the hot gas flow environment. To support shuttle development, Dean Chapman and others led the effort to up upgrade Ames’ capability. Ames’ facilities group, including Howard Stein, Warren Winnovich and Frank Centolanzi, implemented the upgrades. Ames’ 60 megawatt Interaction Heating Facility was brought on line in the mid-1970s. Highpressure air passes through the constricted arc heater (invented by Ames), where a “standing lightning bolt “ is created and about 50 percent of this energy is deposited as heat into the flowing gas.
Image left: Figure 9. “Missing tile” heating test.
The heated gases are expanded through either conical nozzles for stagnation point and wing leading edge testing (Figure 8), or through semi-elliptical nozzles for acreage tests. Ames’ capability of being able to test a 2-foot by 2-foot section of the acreage tile field in conditions duplicating aeroconvective heating and reacting boundary layer chemistry during simulated entry conditions was a critical element in the development of the shuttle TPS. Figure 9 is a photograph of the “missing tile” test run to understand the effects that would occur should a tile be lost prior to entry.
Low-Speed Descent Aerodynamics
Early shuttle concepts had orbiters that would have exhibited less than ideal aerodynamic characteristics upon return to Earth. This could have lead to poor handling qualities, especially during approach and landing. Personnel at Ames with expertise in guidance and control tackled the challenge of developing concepts that might compensate for deficient aerodynamics and ensure adequate handling qualities.
Still glowing red hot from its highspeed entry, the orbiter slows and descends into the supersonic/transonic/subsonic regime of its return. Here again, Ames’ wind tunnels played a key role in defining shuttle aerodynamics and design of the orbiter. The 2-foot transonic wind tunnel, with its capability up to Mach 1.4, was used to study potentially troublesome panel flutter problems. The 12-foot pressurized wind tunnel was used to investigate the orbiter’s low-speed handling characteristics.
Ames’ efforts demonstrated that unpowered landings could be made at speeds of at least 200 knots without significant problems. The 12-foot wind tunnel was used to define the aerodynamics of a specially modified Gulfstream 2 (G2) business jet with direct-lift flaps and side force generators. This vehicle was used for flight tests and astronaut training. Ames’ Convair CV 990 and the G2 aircraft were used to prove that the orbiter did not need a subsonic engine for fly-around landing capability, an important finding that avoided having to pay the weight penalty of hauling a landing engine, its fuel and supporting subsystem to orbit and back. The Gulfstream, now known as the STA (Shuttle Training Aircraft), is used to this day by pilot astronauts for in-flight proficiency training.
Finally, an awesome 36 percent scale model of the orbiter, 44 feet long, was fabricated and tested in Ames’ 40- by 80- foot wind tunnel. Figure 10 shows the model, then painted yellow, in the test section with a person in view to give the scale. This model and the 40- by 80- wind tunnel could create Reynolds numbers slightly higher than the 12-foot pressurized wind tunnel. An important purpose of the 40- by 80-foot testing was to identify the influence of the TPS on the orbiters’ lowspeed aerodynamics. This model still exists, painted with the striking black underbelly and white top. It is proudly displayed in front of the Ames Visitor Center, near the 40- by 80- where it was so intensely tested.
Approach/Landing Systems Development: FSAA
Landing simulation research for the shuttle orbiter began in the very early 1970s, using the Flight Simulator for Advanced Aircraft (FSAA). The large motion envelope of the FSAA provided many of the vital cockpit accelerations that enabled pilot astronauts to experience a truer “feel” of the g-forces of the orbiter during approach and landing. These simulations were conducted for that portion of the shuttle’s flight from supersonic (following re-entry) to approach and landing.
Image left: Figure 11. Kenneth White in the Space Shuttle Vehicle Simulator (1970).
For many years, prior to first flight, all the pilot astronauts who would eventually fly the orbiter spent many hours in the FSAA, identifying handling qualities that needed improvement, and control system shortcomings. In this process, the pilots gained invaluable training in the skills needed to successfully land the orbiter. It was in the FSAA that investigations were conducted that determined the need for the Heads-Up-Display (HUD), and its alphanumeric symbology that became the primary guidance system for orbiter landing. Figure 11 shows a very early (1970) photograph taken in the simulator when the shuttle work was just starting. Depicted is pilot Kenneth White in the Space Shuttle Vehicle Simulation Cockpit.
A pilot-induced oscillation (PIO) problem arose on the first approach and landing test program flight in July 1977, with pilots Fred Haise and Gordon Fullerton. A PIO is a longitudinal “porpoising” that worsens due to pilot over-control. It is generally not a piloting technique problem so much as a control system problem. On this first flight, as the oscillation began to diverge dangerously close to the ground, Haise had enough confidence and simulator training to simply let go of the controls and allow the oscillation
to damp itself out.
Following that, a major investigation was conducted in the FSAA to re-evaluate the control systems gains, in order to minimize the possibility of future PIO problems. In addition, work was conducted for several years in the simulator to investigate the terminal area energy management concepts designed by engineers at JSC. Development support for the space shuttle, prior to the first flight, also included approach/landing control system and handling qualities, heads-up display concept, speed brake scheduling, astronaut training, flight techniques for failure recovery, and landings of the shuttle from atop the 747 carrier aircraft,.
Vertical Motion Simulator
In 1980, Ames’ new Vertical Motion Simulator (VMS) began operation. It wasn’t long before the VMS earned a reputation as the best simulator anywhere for the continuation of engineering design and shuttle pilot training. Landing systems and flight rules are done on the VMS with astronaut crews and JSC engineers. Ames’ SimLab and VMS have supported the shuttle program on a continuing and scheduled basis ever since.
Work Supporting the Shuttle After the First Launch Ames has continued to make major contributions to the shuttle program over the two decades following the flight of STS-1. This includes work in the area of aero/aerothermodynamics, where very significant, benchmarking CFD calculations were accomplished for the shuttle ascent stack configurations and for orbiter re-entry. CFD was a key contributor to the redesign of the space shuttle main engine. In the area of TPS, a second-generation material called Toughened Unipiece Fibrous Insulation (TUFI) has been adopted and used to eliminate problems in regions of the orbiter where debris impact has proven to be an issue, especially on the aft heat shield and on the body flaps.
Image right: Figure 12. “Streak” photograph of the simulator showing how the piloted cabin moves to give the “feel” of flight and landing.
In piloted flight simulation, a very close working relationship developed between the orbiter engineering design people from JSC, the astronauts and Ames’ SimLab. Virtually every pilot astronaut cycled through the VMS sim. Every day, from one to four of the astronauts’ T-38s would park on the ramp beside the SimLab building, and the pilots would come in early and work late. More time was provided for commanders and pilots who had a near-term flight on the schedule. Besides looking at future design improvements in the flight control systems, the pilots would encounter every conceivable failure mode the JSC engineers could imagine. This training proved invaluable in preparing shuttle commanders and pilots to deal with a wide array of possible landing failures. In addition to crew training, the VMS has supported redesign of the brakes, nose wheel steering and Multifunction Electronic Display System (MEDS); engineering development of the drag parachute; flight control automation for the Extended Duration Orbiter; and “return to flight” studies after the Challenger accident.
Today, work continues on the shuttle in the areas of aero/aerothermodynamics, TPS, VMS support and cockpit upgrades.
Image top left: Figure 13. Successful landing of Columbia at Dryden Flight Research Center.
Space shuttle Columbia landed at Dryden Flight Research Center on April 14, 1981. The crew consisted of commander John Young and pilot Robert Crippen. The mission duration of 2 days, 6 hours, 20 minutes and 53 seconds included 36 orbits of the Earth. This first, brief mission proved the capability of the world’s first and only reusable space vehicle, and the world’s most reliable and versatile launch system.
Ames played a critical role in making the outstanding success of the space shuttle “happen,” especially in the areas of aero/aerothermodynamics, thermal protection systems and piloted flight simulation areas. It is one element of the center’s heritage that should be a source of pride to everyone at Ames. Figure 12. “Streak” photograph of the simulator showing how the piloted cabin moves to give the “feel” of flight and landing.
By Jim Arnold and Ann Sullivan, with contributions from Howard Goldstein, Tom Alderete and Jack Boyd. The article also contains information from the May 1, 1981 issue of the NASA Ames Astrogram.