How the Shuttle Orbiter Lost Its Jet Engines

The McDonnell Douglas design with a large flyback booster

As design work by various aerospace companies began on the Space Shuttle program in the late 1960s, it was a given that the Orbiter would have its own jet engines. Having its own air breathing engines offered three advantages- they would allow atmospheric flight testing much like any other aircraft was tested and pilots could practice landings in the run up to an orbital mission. The engines also facilitated ferry flights, repositioning the Orbiter amongst various facilities (landing, launch, overhaul, etc.). Having its own jet engine propulsion also gave the Orbiter cross range capability upon return from orbit. Some designers envisioned the Orbiter rendezvousing with a tanker for additional jet fuel. But in the ascent and in orbit, jet engines and fuel for those engines was dead weight that subtracted from potential payload. Even if designers went with an Orbiter design that was unpowered on its landing, the 1970 and 1971 design studies prominently featured a fully reusable two stage Space Shuttle with a big flyback booster that would have to have its own jet engines. Some of the designs for the flyback booster were massive with a need for as many as twelve jet engines. Soon the design of the flyback booster itself began to take on technical challenges that rivaled that of the Orbiter design itself. The weight of up to twelve jet engines and the necessary jet fuel cut into the payload of liquid hydrogen and liquid oxygen for the booster's rocket engines. Many of the flyback booster designs would need approximately 150,000 lbs of jet fuel (for comparison, a Boeing 777-200ER has a fuel capacity of roughly 300,000 lbs). Consideration was then given to using liquid hydrogen as fuel for the jet engines which would cut out the need for jet fuel tanks. In June 1970, NASA issued contracts to GE to study the feasibility of using liquid hydrogen in the F101 engine being developed for the B-1 bomber. Pratt and Whitney also got a similar contract to study the use of liquid hydrogen fuel in the F401 engine, the planned naval derivative of the USAF's F100 engine planned for the F-15 Eagle. Both companies showed that liquid hydrogen fueled jet engines saved about 2500 lbs of weight per jet engine compared to conventionally-fueled jet engines. The weight savings was modest at best. 

A typical high-key to low-key unpowered approach to landing

At the same time these studies were going on on how to save weight with Orbiter and flyback booster-mounted jet engines, with NASA there was a group at the Flight Research Center at Edwards AFB where unpowered landings were routine for many high speed research aircraft going back to the X-1 (the X-15 program being the most recent one at the time) and the graduates of the co-located Aerospace Research Pilot School had as a requirement that students demonstrate proficiency in unpowered landings using the school's Lockheed F-104 Starfighters which were throttled down to idle for the practice sessions. Even more demanding were the unpowered landings made by the lifting body program aircraft that lacked wings and derived their lift from their tubby fuselage designs. Regardless of what sort of aircraft was used, USAF test pilots and the NASA-FRC pilots used what was called "energy management" where they traded altitude for airspeed on the descent and used turns to bleed off speed in preparation for final approach. The first step in unpowered landings was the arrival at the "high key" which was high above the touchdown point. From the high key, a gradual 180 degree turn was made that allowed speed reduction and descent to the "low key" which was usually abeam the touchdown point. From the low key, the turn continued allowing more speed to bleed off and the descent to continue until lined up for final approach. If at any point the speed was excessive, speed brakes or gentle S-turns could be used to get down to the necessary airspeed. The lifting body pilots found that on final approach, diving at the runway touchdown point 15 degrees or more improved their accuracy as the speed improved the stability and the speedbrakes could be used to moderate the speed build up on final approach. An assessment by one of the experienced lifting body pilots in September 1970 showed that in 30 landings on a 10,000 foot runway from altitudes as high as 90,000 feet and speeds as high as Mach 2, the dispersion of the landing points was only 250 feet. 

However, the astronaut office in Houston at the Manned Spaceflight Center headed by Deke Slayton felt that unpowered landings for the Orbiter were too risky. Slayton was concerned that the test pilots were more proficient at unpowered landings than his astronauts would be, especially if they were returning from a 7-10 day orbital mission. The astronauts' views carried considerable weight for good reason and it took the USAF to swing the design work in favor of unpowered landings. 

I had posted previously that the Space Shuttle program's development phase was taking place during a period of budget austerity. One of the keys to navigating the budgetary climate of the day was to be sure to secure as much political support as possible since Congress determined the program budget. But in 1970 the program had some close calls, narrowly avoiding funding cuts in both the House and Senate. The Air Force offered to lend its support as it saw opportunity in the Shuttle program to launch heavy reconnaissance satellites. But NASA had baselined the Orbiter design at the time with a 25,000 lb payload to orbit. The USAF wanted to put its heavy reconnaissance satellites into polar orbit and the Orbiter needed a payload capacity of 40,000 lbs. That much payload weight into polar orbit (and unable to take advantage of the Earth's rotation for additional boost) was equal to a 65,000 lb payload launched for the Kennedy Space Center. NASA informed the USAF that the payload had to be baselined at 25,000 lbs due to the weight of the jet engines and their fuel. But it was apparent from the Congressional battles that NASA needed a strong ally like the USAF, so the jet engines were dropped from the Orbiter design and that allowed the payload capacity to orbit to meet the USAF requirements. 

The idea of onboard jet engines didn't end, though. NASA shifted towards the idea of removable kit that could be used for flight testing, ferry flights, and for return from orbit if the payload wasn't maxed out. This also coincided with the 1971-1972 time frame when the flyback booster was dropped as too much of a technical risk and the Space Shuttle began to look more like its final design- an Orbiter with an external tank and solid rocket boosters in what was called the TAOS configuration- Thrust Assisted Orbiter Shuttle. The significant weight savings by going to a TAOS configuration also helped cut development risk as there was a considerable amount of experience already with solid rocket boosters and large external tank structures to hold cryogenic fuels. 

The test pilots at NASA-FRC persisted in their opinion that jet engines were completely unnecessary in the Orbiter design. They had their long experience of over 10,000 unpowered landings since the X-1 program as their proof, but the astronauts insisted that the Orbiter was a much bigger aircraft than many of the X-planes. Another round of tests then were held by NASA-FRC, this time using their B-52 Stratofortress carrier aircraft. Set up in a high drag configuration with the engines at idle, pilots successfully and accurately landed the B-52. NASA-FRC then got some lifting body pilots who had never flown anything as big as the B-52 and had them fly the bomber through a simulated unpowered landing using energy management. They were able to land successfully and when the same pilots were asked to land the B-52 using a conventional powered low angle approach, none of them were able to do so. The test pilots the FRC even brought into two United Airlines pilots to fly the B-52 in simulated unpowered landings and they had no issue doing so, reporting that such landings were much easier than conventional landings. The test pilots then followed up the B-52 tests with the same tests using NASA's Convair 990 which could simulate the Orbiter aerodynamics on landing. 

The final iteration of a jet-engine powered Shuttle Orbiter (from the Dennis Jenkins book)

NASA finally got agreement to go to exclusively unpowered landings on return from orbit for the Shuttle Orbiter, but the jet engines still didn't go away. At the time of Rockwell's award in 1972, the Orbiter design featured two engines that deployed from the payload bay and two more engines that could be mounted on struts. Less than six months later, the Orbiter design dropped the internally mounted jet engines completely and they were to be mounted as a kit on the flat underside when needed for flight testing and ferry missions. It finally took the ferry range to kill the engines completely from the Orbiter design. The Orbiter was similar in size to a Douglas DC-9 but had twice the weight. It had a lot of drag since it wasn't optimized for atmospheric flight and the delta wing was highly loaded. With five jet engines mounted in pods on the underside and tank of jet fuel in the payload bay, the Orbiter had a ferry range of only 500 miles. With Space Shuttle sites across the nation and contingency fields overseas, a 500 mile range was simply unacceptable. NASA looked at aerial refueling during ferry, but this added complexity to a design that was already experiencing cost overruns. In February 1974, NASA deleted the jet engine requirement completely. As a result, both for flight testing and ferry flights, the Orbiter would need a carrier aircraft, but fortunately that was a lot more straightforward a development process!

The Buran analog with its four AL-31 jet engine nacelles

Interestingly in the Russian Buran Shuttle program, there was an aerodynamic test analog designed OK-GLI that made 25 atmospheric test flights with four Lyulka/Saturn AL-31 jet engines mounted in nacelles in the aft fuselage. A fuel tank sat in the payload bay. The AL-31 is the jet engine that is used on the Sukhoi Su-27 Flanker. Nine taxi tests and 25 test flights were made using the Buran analog from December 1984 to December 1989. The engines were used to takeoff and then were throttled back on the descent to landing. All of the flight testing took place at the Baikonur Cosmodrome. The operational Buran, however, would not have jet engines at all and the Antonov An-225 Myria was developed as the carrier aircraft to ferry the Buran orbiter. 

Source: Development of the Space Shuttle 1972-1981: History of the Space Shuttle, Volume Two by T.A. Heppenheimer. Smithsonian Institution Press, 2002, pp85-92. Space Shuttle: The History of the National Space Transportation System- The First 100 Missions by Dennis Jenkins. Specialty Press, 2008, pp187-192. Photos: NASA, Wikipedia, Dennis Jenkins. 

Rockwell Builds the Shuttle by Farming Out the Work

The Grumman Shuttle Orbiter design

On 26 July 1972, NASA announced that Rockwell International had been selected as the prime contractor for the Space Shuttle (specifically the Shuttle Orbiter) after an intense competition with Lockheed, Grumman, and McDonnell Douglas. Each contractor proposal also had to detail management of the complex program as well as its technical aspects and the lengthy proposals then went to a specially convened selection board at NASA which evaluated each submission. The top two proposals belonged to Rockwell and Grumman and showcased the effect that a good management proposal could have in winning the competition. From a technical standpoint, NASA scored the Grumman proposal the best, with Rockwell's orbiter design coming in second. Rockwell's submission, however, impressed the NASA selection board with its management system. With the cost overruns on several military programs like the Lockheed C-5 Galaxy on everyone's mind, Rockwell's management proposal stressed cost controls for what was to be the biggest aviation contract in years. With a technical design not much more inferior that the Grumman design, Rockwell was awarded the contract. 

In the industry slump as Vietnam was winding down, the Orbiter contract was a very big prize for any firm that could clinch the award. At the time of the selection, Rockwell had 6,200 employees in their Space Division and with the award, plans were in place to hire as many as 16,000 by 1975. Priority would given to anyone who had worked on the Apollo program. Despite the buoyant mood at Rockwell, things were considerably more glum at the losing contenders, Grumman, Lockheed, and McDonnell Douglas. Grumman had been a mainstay of the US space program from its early days, best known for its work on the Apollo Lunar Module. Company officials made plans for Grumman to be out of the space business by December 1972 along with the attendant layoffs. 

McDonnell Douglas (via McDonnell) had built the Mercury and Gemini spacecraft and was in the midst of winding down its work as the prime contractor for Skylab. The company was also suffering from a downturn in the world commercial aviation market that was affecting most greatly its Douglas DC-9 program. 11,000 layoffs were planned at McDonnell Douglas by 1973.

While Lockheed didn't have as prominent a role in the US manned spaceflight program in the 1960s as McDonnell Douglas or Grumman, their expertise in high speed flight as well as thermal protection systems was unparalleled in the industry at the time. 

Rockwell, however, recognized two realities that came with winning the Shuttle Orbiter contract. The first one was the limitations of its in-house expertise. Quite simply, Rockwell would need other aerospace companies for their skills and expertise to bring the Orbiter to fruition. The second reality was a bit more prosaic but nonetheless vital. Keep in mind that in the early 1970s there was an atmosphere of budget austerity and NASA was no less exempt from financial realities than any other government agency at the time. Subcontracting work on the Orbiter to other companies in effect would spread the footprint of the endeavor across the districts of multiple Congressional representatives who would be routinely voting on NASA's budgetary allocations for the Space Shuttle program. Subcontracts were a common way as well in the industry of building goodwill- by farming out work to competitors and keeping them active and in business, today's winner might one day become tomorrow's loser on a another program and could hope for subcontract work from a rival. 

Rockwell planned to subcontract at least 53% of the work on the Shuttle Orbiter and this had NASA's blessing as a means of preserving the American industrial base for spaceflight. Just weeks after winning the contract as the prime, Rockwell was already conducting seminars across the nation for potential subcontractors. By March 1973 Rockwell began selecting subcontractors for the program with NASA's approval. Grumman would work on the delta wing, McDonnell Douglas got the OMS (Orbital Maneuvering System), Fairchild Republic got the vertical fin, and the Convair Division of General Dynamics got the mid-fuselage/payload bay. Rockwell would be responsible for the nose and crew compartment as well as the aft fuselage that would house the three Rocketdyne SSME (Space Shuttle Main Engine) packages. Lockheed would get the External Tank while Thiokol got the contract for the SRB (Solid Rocket Booster). 

By the summer of 1975, 34,000 workers across 47 states and a broad host of companies across the American aerospace industry were working on the Space Shuttle program. The peak would be in 1977 with 47,000 workers. The post-Vietnam slump, the Space Shuttle program was very much the crown jewel of the US aviation industry. 

Source: Development of the Space Shuttle 1972-1981: History of the Space Shuttle, Volume Two by T.A. Heppenheimer. Smithsonian Institution Press, 2002. Illustration: Aerospace Projects Review

The 747 Shuttle Carrier Aircraft (SCA)

The Boeing 747 was one of two choices for the SCA

During the design and development of the Shuttle Orbiter, air breathing jet engines were a part of the design for some time until cost and technical considerations in early 1974 led to their deletion from the concept. No longer able to "self-ferry", NASA now faced the problem of how to get the Orbiter from the remote landing sites to the launch locations. At the time NASA Langley had a study ongoing on a large aircraft design called VIRTUS that would have carried the Orbiter under the center wing flanked by twin fuselages and a twin boom tail with power coming from four Pratt & Whitney JT9D turbofans. Design work had proceeded on VIRTUS as far as wind tunnel tests with a 1/34 scale model, but sheer size, long development time and costs involved for an aircraft that would be built in very small numbers resulted in end of the VIRTUS project. At the time, Jack Conroy, the developer of the Super Guppy concept that NASA was using to transport rocket stages, had suggested using a jumbo-class aircraft to carry the Orbiter on its back. Proposals were issued to the industry and Lockheed offered up a twin-fuselage aircraft based on the C-5 Galaxy with the Orbiter suspended underneath a new center wing section- but, like the earlier VIRTUS program, it was eliminated from consideration due to cost, development time and that the design was so wide, no runway available could accommodate the design. Boeing offered a modified version of the 747 that carried the Orbiter on its back that presented a much lower risk approach. Boeing at the time even suggested that the large external tanks could be carried on the back of a 747, but wind tunnel studies showed the idea to be less practical than initially thought. Lockheed had subsequently reworked its design to a simple modification of a C-5 Galaxy to carry the Orbiter on its back much like Boeing's proposal. 

By mid-1974 Boeing's 747-based proposal and Lockheed's simpler C-5 Galaxy-based proposal were the only serious contenders to become the new Shuttle Carrier Aircraft (SCA). On 24 April 1974 NASA selected the C-5 Galaxy proposal from Lockheed based on it having the least acquisition expense and a C-5 Galaxy would need less structural modification than a Boeing 747. Accordingly, NASA approached the United States Air Force with the proposal and a request to make three to five C-5s available. The USAF was very receptive to the idea and the Lockheed proposal only added 400 to 600 pounds of modifications to the Galaxy without adversely affecting its cargo carrying capacity when not being used to transport the Orbiter. An arrangement was set up whereby NASA would pay for the modifications and then lease the modified C-5s as needed from the Military Airlift Command. One Galaxy was agreed to be bailed to NASA full-time for development into the SCA and for use in the atmospheric flight tests with the planned first Orbiter (which would become the Shuttle Enterprise). Despite some lingering concerns about the effects of the Orbiter's wake on the C-5's T-tail, both NASA and the USAF had worked out an acceptable arrangement for both parties.

N905NA conducted the atmospheric landing tests with Enterprise

The downturn in the American economy in the early 1970s led numerous US airlines to release their 747 aircraft which were too large for the market at the time. As a result, the acquisition cost of the Boeing 747 as the SCA dropped much lower than that of the Lockheed proposal. With the ready availability of low-time 747s on the market, NASA abandoned plans for using the Galaxy as it was decided it was much easier in the end to have complete control of the SCA than to have to compromise with military priorities for use of the C-5 Galaxy. On 18 July 1974 NASA purchased a used Boeing 747-123 (N9668, msn 20107) from American Airlines. It was the 86th 747 off the production line at Everett and was delivered to American on 29 October 1970. By the time of the NASA purchase, it had only logged 8,999 flight hours and 2,985 cycles flying primarily transcontinental services between New York JFK and LAX. NASA re-registered the aircraft as N905NA. Before modification into the SCA configuration, N905NA was used for in-house studies with NASA Ames on wake vortices. Following conclusion of the wake vortex research program, Boeing initiated the $30 million conversion program on N905NA on 2 August 1976. 

Not often seen is the sense of humor NASA has with the SCA

Boeing installed new bulkheads to strengthen the fuselage with skin reinforcement at critical stress areas. The horizontal stabilizer structure was also beefed up along with the addition of telemetry and transponder test equipment, fittings for the Orbiter support struts, and the installation of a 747-200 rudder actuator system. Boeing also developed a set of removable modifications for the SCA- the first one was a telescopic forward support assembly that was used only during the atmospheric flight tests with the Shuttle Enterprise. This support would hold the Enterprise at a six-degree angle of attack to facilitate release during the flight tests. A fixed assembly was also developed for use during SCA ferry missions that held the Orbiter at a three-degree angle of attack, which induced less drag during the ferry flights. The aft support assemblies (there were two) were common to both the atmospheric flight tests and ferry flights and finally 10 foot by 20 foot vertical endplates were added to the end of the horizontal stabilizer to provide additional stability when carrying the Orbiter- in practice, though, NASA never removed the endplates. 

The 747's trim system was also modified to allow a greater range of trim in pitch to counteract the downwash off the Orbiter's wing on to the horizontal stabilizer. Most of the main deck interior was stripped out, but some seats were retained for support personnel during the ferry flights. The JT9D engines were also converted to allow a thrust increase from 43,500 pounds to 46,950 pounds of thrust. The current JT9D engines on the current incarnation of the 747 SCA are rated at 50,000 pounds of thrust. Since each Orbiter has a different empty weight, an adjustable ballast system using standard cargo containers in the forward underfuselage cargo compartment had to be developed to maintain the center of gravity. On 14 January 1977 Boeing finished the modification work and after a period of flight testing, it was delivered to NASA. 

Concerns about flight crew safety during the atmospheric flight tests with the Shuttle Enterprise led NASA to incorporate an escape system on N905NA since the flight crew of the Enterprise had ejection seats. The escape system was based on what was used on the 747 prototype during Boeing's 1969 flight tests- in the event of an emergency, a handle was pulled that blew out thirty fuselage windows to facilitate rapid decompression of the aircraft. Three seconds later an emergency egress hatch on the lower forward fuselage was blown out with extendable spoiler being deployed. The crew would make their way back from the flight deck to the middle of the upper deck lounge area where a hole leading to a 16 foot escape slide would lead them out the blown hatch and clear of the aircraft. Testing showed the flight crew could bail out of the 747 within 11 seconds. The atmospheric flight tests will be the subject of a future blog post, so stay tuned. 

N905NA has three upper deck windows, N911NA has five

In 1988 NASA acquired a second 747 to act as a back up to N905NA. Part of this was driven by the recommendations following the Challenger accident that a significant portion of Shuttle flights would still be landing at Edwards AFB. The first 747-100SR was purchased from Japan Air Lines where it had flown as JA8117, msn 20781. Boeing purchased the aircraft from JAL on behalf of NASA and conducted the necessary modification work to bring it up to SCA standards with the new tail number N911NA. On 20 November 1990 it was delivered to NASA and in 1995-1996 both of the 747 SCAs were repainted in NASA's new colors. 

During a ferry mission the SCA' smaximum speed it 250 KIAS (Mach 0.6) at an altitude of 13,000-15,000 feet with a range of approximately 1,150 miles. Without the Orbiter, the SCA cruises at 24,000-26,000 feet with a range of 6,300 miles. During ferry flights the usual crew is two pilots and two flight engineers, but only one flight engineer is needed on non-ferry flights. At one point NASA looked at inflight refueling of the SCA as the equipment was readily available as it was installed on a handful of the USAF's 747s- the E-4 airborne command posts and the two VC-25A presidential transport aircraft. Proximity flight tests were even carried out with N905NA and a KC-135 tanker minus the Orbiter, but the discovery of cracks at the base of N905NA led to the termination of the studies as it was feared that wake turbulence from the tanker may have been possible. Plans were in motion to fly the proximity tests with an Orbiter, but the costs involved and wake turbulence concerns led to the quiet abandonment of the idea. 

Source: Space Shuttle: The History of the National Space Transportation System- The First 100 Missions by Dennis R. Jenkins. Specialty Press, 2001, p195-202.

The Chrysler SERV: Thinking Out of the Box for Space Travel

Chrysler SERV ascends on its aerospike engine

By the late 1960s, the groundwork was being laid down that would eventually evolve in the Shuttle Transportation System that today is in the twilight of its career. The NASA-led studies that involved the major aerospace contractors of the day was divided into "phases" and at each phase candidate contractors had to demonstrate their concepts to the Manned Spacecraft Center (MSC) at the Johnson Space Center outside of Houston. Apollo was run out of the NASA headquarters in Washington, but the technical reach of a reusable spacecraft meant that NASA wanted the program leadership to be at a field center run by engineers- and at the time, only Houston and the Marshall Space Flight Center (MSFC) in Huntsville, Alabama, had the technical expertise for such an undertaking. Not wanting to put its eggs in one basket, though, NASA established on 6 July 1970 the Alternate Space Shuttle Concepts (ASCC) study to evaluate alternative concepts and proposals to what was already under development in cooperation with the aerospace industry and the MSC in Houston. Given that the MSC had it hands full, program leadership of the ASCC was assigned to the MSFC in Huntsville. Over 29 configurations were studied and millions in funding were provided. A joint submission by Grumman/Boeing was evaluated, along with one from Lockheed and one from Chrysler, which at the time had a thriving space division that had been in business since 1962 building the first stages for the Saturn I rocket and Saturn IB rocket. Chrysler's ASSC proposal was the recipient of a $1.9 million study contract for what has to be one of the most unorthodox if not outright unique space shuttle concepts ever taken seriously by NASA. 

Schematic diagram of the Chrysler SERV

Chrysler's design was called the SERV- Single-stage Earth-orbital Reusable Vehicle- and it looked like nothing else under study at the time. It was a large conical vehicle that looked like a supersized Apollo command module. It was 65 feet high and 90 feet in diameter with a central payload bay 23 feet wide and 60 feet long. Liquid hydrogen and liquid oxygen tanks then surrounded the payload bay to fill the rest of the volume of the SERV. Its propulsion engine was highly innovative and developed with assistance from Rocketdyne- the SERV had a 12-module liquid oxygen/liquid hydrogen aerospike engine integral to the base of the SERV that was 87.4 feet in diameter and just over 8 feet long. The engine developed an astounding 5.4 million pounds of thrust (by comparison, each Space Shuttle Main Engine develops about 400,000 lbs of thrust at launch). Each of the 12 modules were interconnected and each had a set of turbine driven fuel pumps that could run as high as 120% to compensate for the failure of any pair of pumps in the 12 modules. The engines designed were so powerful, that the SERV's aerospike had to be throttled back to 20% just before reaching "max-Q"- the point of highest aerodynamic stress to prevent overstressing the SERV during its ascent to orbit. A series of doors could close over the aerospike modules to protect them during re-entry. 

SERV with the MURP spaceplane. Note the doors for the jet engines.

At launch, the SERV weighed in at approximately 4.5 million lbs and different modules could be attached to the top of the SERV- the most studied were an external extension to the payload bay and the other was what was called the MURP- Manned Upper Reusable Payload, which was a small orbiter design with flick-out wings on return to Earth. The MURP could carry up to ten astronauts. Launches would have taken place from large concrete pads as the SERV had its own landing legs to support its weight. To return to the Earth, the SERV would re-enter the same way as the Apollo command module did, with the blunt end first and protected by thermal tiles. But instead of a water landing, at an altitude of 25,000 feet, a set of intakes and exhaust ports opened and four banks of seven jet engines (that's right, twenty eight engines!) powered by JP-4 fuel would start up and provide deceleration and maneuver capability that would bring the SERV back home for a soft landing on its own landing legs. The planned landing site for the SERV would have been on the skid strip at Cape Canaveral adjacent to the Kennedy Space Center. 

Chrysler's space division would have built the SERV at the Michoud facility that today has been responsible for the Space Shuttle's external tank. A specially-modified transport ship would then take the completed SERV to the Kennedy Space Center. Chrysler estimated that each SERV would cost $350 million each. However, by the time the ASCC studies were winding down, the Chrysler SERV got little attention as the design submissions from Lockheed and Grumman/Boeing were decidedly much more "conventional" than the SERV- but the SERV is a fascinating exercise in aerospace development when preconceived notions are cast aside and an innovative solution is found to meet an exacting set of requirements! 

Source: Space Shuttle: The History of the National Space Transportation System- The First 100 Missions by Dennis R. Jenkins. Specialty Press, 2001, p123-125.

The Shuttle Training Aircraft

Because of the unique and challenging flying qualities of the Space Shuttle Orbiter when it returns to Earth as a very heavy glider, NASA has had four Gulfstream II jets that have been modified to train Shuttle commanders and pilots in the complex task of bringing the Orbiter to a smooth landing after reentry. To accomplish this unique task, the Shuttle Training Aircraft (STA) has a number of modifications that set it apart from a standard GII business jet. The most important of these modifications is of course, internal, with the Advanced Digital Avionics System (ADAS) that replaces some of the seats (leaving nine seats) on the right side of the forward cabin. The ADAS takes in variables such as the weight of the Orbiter on return to Earth, runway and direction/elevation of the field in question and then moves the flight control surfaces and throttles of the STA to simulate how the Orbiter would respond to control inputs from its pilots.

The trainee pilot sits in the left side of the STA cockpit which has all the necessary instrumentation and heads-up display (HUD) as used on the Shuttle as well as the Orbiter's left hand stick controller. The front and side cockpit windows on the left side of the STA cockpit are also partially masked to give the pilot flying the simulation the same angular field of view as if he or she were on the Orbiter's flight deck.

The right side of the cockpit is occupied by the instructor who also has a HUD but is otherwise has stock GII instrumentation. The nosewheel steering has been relocated from the left side to the right, and there is a button that the instructor can press to exit the simulation and return the STA to the standard flight characteristics of the Gulfstream jet.

Externally, to make the Gulfstream fly like the Orbiter, the wings have been modified with three flying surfaces instead of the just two (flaps and aileron) used on the standard GII. The third surface is inboard section of the flaps and is a direct lift flap that can deflect up 30 degrees (this kills some of the wing lift and better simulates the aerodynamics of the Orbiter) and down 20 degrees as a standard flap. The ADAS also moves the flaperon (what was once the flap) and the aileron to faithfully replicate the flight characteristics of the Orbiter. The direct lift flaps are fast acting and also work in concert with the engines. Unlike a standard GII which has clamshell bucket reversers on the engines, the STA has cascade reversers installed that can be used in flight and if they fail, they automatically stow. The wings have also been structurally strengthened and the vortex generators just behind the leading edge are nearly full span to help the airflow stay attached to the wings during the extreme maneuvering performed by the STA. On the stock GII, the vortex generators are only outboard of the wing fence.

Each STA sortie consists of 10 simulated Orbiter approaches, starting from 35,000 feet. The speed is then set at 250 kts (the speed limit of the main landing gear extension) and the main landing gear is extended to create more drag for the steep approach of approximately 20-30 degrees. The ADAS also activates the thrust reversers as needed to maintain the fidelity of the simulation. The descent is flown at 300 kts at 20 degrees which translates to approximately 12,000 feet per minute descent rate. At this point, the pilots are literally hanging forward in their harnesses and only the ground fills the cockpit view. At final approach, the STA is at 250 kts and the instructor lowers the nose gear just in case the trainee pilot inadvertantly lands the STA. In the Orbiter, the point of touchdown corresponds with the STA still about 20 feet off the ground. At that point touchdown is considered to have been made the instructor exits the simulation, takes control and takes the STA back up to altitude for another simulation run.

Most STA flights take place at White Sands, New Mexico, with the aircraft based at El Paso International Airport. Three runways are marked out in the dry lakebed to simulate the runways at Edwards AFB, Kennedy Space Center, and the trans-oceanic abort landing sites at Istres AB, France, Zaragoza AB and Moron AB, both in Spain. Trainee pilots start off with a foundation of 20 STA flights and at this point become competent enough to assigned to an Orbiter crew. Once assigned to a crew, both the pilot and mission commander will fly the STA once a month. Nine months out from launch the STA flights are made every other week and then three months out the STA flights are made weekly. Previous pilots and mission commanders who have already flown to space start their every other week ramp up in the STA at six months out from launch. At three months from launch, the some of the weekly STA flights are made at Edwards AFB and at the Kennedy Space Center as well. Extra flights can also be requested by the trainee pilots.

Two weeks before launch two STAs are flown to the Kennedy Space Center and daily STA training flights are made, one of which is done in the full spacesuit for added realism. By the time a first time Shuttle mission commander blasts off, they will have made approximately 1,000 practice approaches in the STA. First time Shuttle pilots will have made a minimum of 500 STA approaches. With the Shuttle program winding down, no decisions have yet been made on the future of the STA aircraft.

Source: Air International, July 2010, Vol. 79, No.1. "NASA's Unique Approach- Space Shuttle Landing" by Dino Carrara, p82-91.

Lockheed's Contribution to the Space Shuttle Program

In the 1960s as preliminary design work began on what would become the Space Shuttle, thoughts turned to ways of protecting the vehicle on its fiery re-entry into the atmosphere. The thermal protection issue was closely intertwined with what structural material to use for the spacecraft. One school of thought was based on what was used on the X-15 program- a "hot structure" that was tolerant to high temperatures. Unfortunately this meant the use of expensive and possibly rare alloys. Since the X-15 was a smaller vehicle than what was planned for what would become the Shuttle, a hot structure made more sense. But a vehicle that was much bigger in size presented cost and materials challenges with the hot structure approach. Most of the design groups involved in the early planning preferred to use a conventional-alloy structure protected by a series of metallic shingles at first, but the weight of a reusable metallic thermal protection system was daunting, to say the least.

Lockheed had the necessary breakthrough that would allow the Shuttle to have a conventional-alloy structure but have a lightweight thermal protection system. Since the late 1950s Lockheed had been working on what it called a "reusable surface insulator" (RSI) made of ceramic compounds. By 1960 the company had applied for a patent on a material made of ceramic fibers. In 1962 Lockheed had created a ceramic electro-magnetically transparent radome 32 inches in diameter for the Apollo program, but the design changed and Lockheed's radome was never used in flight. However, the experience proved valuable for Lockheed which developed a special insulator that was made of a fibrous structure filled with the spaces filled with Plexiglass compound. They called it "Lockheat" and unlike the heatshields used on spacecraft of the day which were ablative (they charred to provide protection), Lockheat's material was called evaporative- the materials slowly evaporated, creating a protective gas layer. Different types of fibers were tested such as aluminum, silica, and born. Eventually by 1965 the company's Lockheat LI-1500 settled on silica fibers. It was light at only 15 lbs per cubic foot and was completely reusuable, being able to survive repeated heating past 2,500 degrees Fahrenheit. The Air Force flew a test sample in 1968 on a re-entry test vehicle and it passed with flying colors. LI-1500 is considered the progenitor of the Space Shuttle's current thermal protection tiles.

Lockheed continued to develop the silica-based RSI but they couldn't overcome the fact that it was very brittle. It couldn't cover an entire spacecraft, it had to be applied in small tiles calculated to be about 6 x 6 inches in size. With a small gap around each tile, the vehicle structure could deform slightly without compromising the integrity of thermal protection. Since the structure under tile even the small size of 36 square inches could still deform and crack it, the tile was bonded to a felt pad which in turn was bonded to the vehicle structure. A room-temperature vulcanizing adhesive was developed to affix the pad to the tile and the tile and pad to the spacecraft.

The silica-RSI tiles, though, weren't enough protection for the nose and wing leading edges for the proposed Shuttle Orbiter as those areas got even hotter than the tiles could tolerate. To solve this problem, the prime contractor of the Shuttle, North American Rockwell, turned to LTV (Ling-Temco-Vought) which had developed a reinforced carbon-carbon (RCC) material that was to be used on the cancelled X-20 Dyna Soar program. The RCC could easily heat up to 2,700 degrees F yet keep the spacecraft structure well below the 350 degrees F limit for aluminum. The black tiles were designed to protect up to 2,300 degrees F and white tiles were used to protect areas up to 1,200 degrees F. Those areas of the Orbiter that did not heat up past 750 degrees F were protected by a special blanket material called flexible reusable surface insulation, or FRSI.

As work continued on the Shuttle Orbiters in the mid-1970s, analysis found that stiff spots in the felt pads caused by gaps between the adhesive and the tiles and/or Orbiter structure could weaken the integrity of the tiles. NASA decided on what was called a "densification" process to strengthen the felt pads and a special silica slurry was created by DuPont that was applied to the back of the tile, baked for 2 hours, then waterproofed before being bonded to the felt pad.

But there was a problem. Columbia was nearing completion when the stiff spots in the felt pads were discovered and Columbia was due to be flown from Palmdale, California, to the Kennedy Space Center. Feeling that moving the Shuttle to the KSC was symbolic and important for program morale, Columbia was moved to KSC with 6,000 tiles missing and 24,000 already-installed tiles to be removed and modified. Not all the tiles had to be removed as it turned out, but each one already installed had to be tested for weakness. For 20 months, three shifts a day, six days per week, each tile was tested, removed and modified if necessary. The remaining 6,000 tiles still had to applied. This was March 1979 and by the end of November 1980 the tedious work was completed and further preparations could continue for Columbia's maiden flight on 12 April 1981.

Source: To Reach the High Frontier- A History of US Launch Vehicles, edited by Roger D. Launius and Dennis Jenkins. The University Press of Kentucky, 2002, p382-388.