In the late 1960s, as design work by various aerospace companies began on the Space Shuttle program, opinions were divided over whether or not the Space Shuttle Orbiter should have its own jet engines. While some of Orbiter’s designers believed jet engines offered significant advantages, a group of NASA engineers, concerned by the significant weight that jet engines and their fuel subtracted from the Orbiter’s potential payload capacity, wanted to pursue an Orbiter design that included unpowered landing. In the end, the issue was decided by NASA’s need to provide increased payload capacity to support the United States Air Force’s payload needs. NASA dropped the jet engine requirement from the Orbiter’s design and the Air Force helped NASA get its Space Shuttle budget through Congress.
Initially, the Orbiter’s designers believed that giving the Orbiter its own air-breathing engines offered three advantages: it would allow atmospheric flight testing and pilots could practice landings in the run up to an orbital mission, the engines would facilitate ferry flights, easing the repositioning of the Orbiter between various facilities, and it gave the Orbiter cross-range capability upon return from orbit. Some designers even envisioned the Orbiter rendezvousing with tankers to take on additional jet fuel.
As late as 1970 and 1971 when designers went with an Orbiter design that was unpowered on its landing, design studies still prominently featured a fully reusable two stage Space Shuttle with a big flyback booster that had its own jet engines. Some of the designs for the flyback booster were so big that they required as many as twelve jet engines. Soon, the design of the flyback booster began to take on technical challenges that rivaled those of the Orbiter 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 needed approximately 150,000 lbs of jet fuel (by comparison, a Boeing 777-200ER has a fuel capacity of roughly 300,000 lbs).
Consideration was given to using liquid hydrogen as fuel for the jet engines, cutting 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 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 for the F-15 Eagle. Both companies showed that liquid hydrogen-fueled jet engines saved about 2,500 lbs of weight per engine, compared to conventionally-fueled jet engines. The weight savings were modest at best.
At the same time that these studies were underway, a group of NASA researchers at the Flight Research Center at Edwards Air Force Base were studying unpowered landings. Graduates of the co-located Aerospace Research Pilot School were required to demonstrate proficiency in unpowered landings using the school’s Lockheed F-104 Starfighters, which were throttled down to idle for the practice sessions. And even more demanding were unpowered landings being tested by lifting body program aircraft, which lacked wings and derived their lift from their tubby fuselage designs.
Regardless of what sort of aircraft was used, United States Air Force test pilots and the NASA-FRC pilots used a technique called “energy management” in unpowered landings, 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,” usually abeam the touchdown point. From the low key, the turn continued, allowing more speed to bleed off and the descent to continue until the aircraft was 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.
But 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.
The Space Shuttle program’s development phase took 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 because Congress determined the program’s budget. 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 with a 25,000 lb payload to orbit. If the Air Force wanted to put its heavy reconnaissance satellites into polar orbit, the Orbiter needed a payload capacity of 40,000 lbs. To put 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 launch for the Kennedy Space Center. NASA told the Air Force that the payload had to be baselined at 25,000 lbs due to the weight of the jet engines and their fuel. But because NASA needed the Air Force in its budgetary battles with Congress, it dropped the jet engines from the Orbiter design, allowing payload capacity to orbit to meet the Air Force’s requirements.
But the idea of onboard jet engines didn’t die there. NASA shifted towards a plan of building a removable kit that could be used for flight testing, ferry flights, and for return from orbit if the payload wasn’t maxed out. This coincided with a period from 1971-1972 when the flyback booster was dropped because it was believed to be too much of a technical risk and the Space Shuttle began take on elements of its final design – an Orbiter with an external tank and solid rocket boosters – called the TAOS configuration: Thrust Assisted Orbiter Shuttle. The significant weight savings created by moving to a TAOS configuration helped cut development risk as there was a considerable amount of experience with solid rocket boosters and large external tank structures that held cryogenic fuels.
Test pilots at NASA-FRC persisted in their opinion that jet engines were completely unnecessary in the Orbiter design. Though they had racked up more than 10,000 unpowered landings beginning with the X-1 program, the astronauts insisted that the Orbiter was a much bigger aircraft than many of the X-planes. So NASA-FRC designed another round of tests 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 had lifting body pilots who had never flown anything as big as the B-52 fly the bomber through a simulated unpowered landing using energy management.
They were able to land successfully. 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 FRC even brought in two United Airlines pilots to fly the B-52 in simulated unpowered landings. They had no issue doing so, reporting that such landings were much easier than conventional landings. The test pilots followed up the B-52 tests with the same tests using NASA’s Convair 990, which could simulate the Orbiter aerodynamics on landing.
Ferrying the Orbiter
Although NASA finally got agreement for exclusively unpowered landings on return from orbit for the Shuttle Orbiter, plans for jet engines didn’t entirely go away. At the time of Rockwell’s award in 1972, the Orbiter’s design featured two engines that deployed from the payload bay and two additional 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 instead to be mounted as a kit on the flat underside when needed for flight testing and ferry missions.
It was only when ferry range became an issue that jet engines were finally dropped altogether from the Orbiter design. The Orbiter was similar in size to a Douglas DC-9 but had twice the weight. It also had a lot of drag, as 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 a tank of jet fuel in the payload bay, the Orbiter would have a ferry range of only 500 miles. With Space Shuttle sites across the nation and contingency fields overseas, a 500 mile range was unworkable. While NASA looked at aerial refueling during ferry, 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 require a carrier aircraft. Fortunately, that development process was considerably more straightforward.
Interestingly, the Russian Buran Shuttle program tested an aerodynamic test analog designed OK-GLI, which 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. From December 1984 to December 1989, nine taxi tests and 25 test flights were made using the Buran analog. The engines were used to takeoff and were then 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 and the Antonov An-225 Mryia was developed as the carrier aircraft to ferry the Buran orbiter.