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|DYNA –SOAR X-20: A LOOK AT HARDWARE AND TECHNOLOGY|
by Terry Smith
The cancellation of the X-20 DynaSoar project did not close out all T work on X-20 related research. Several different components and design features were tested well into the mid-1960s. Continued research into refractive metals and high speed lifting re-entry was carried out in ground test facilities and with the use of small scale research vehicles such as ASSET and the, Start Programs including Prime.
This article will attempt to show some of the more important aspects which led to research for the X-20 program. These ranged in 1963, from advanced concepts to the actual building of hardware.
The one Dyna-Soar objective that did not change throughout the constant redirection and bureaucratic mishandling of the program was Us.- to demonstrate a piloted vehicle capable of controlled lifting maneuverable re-entry. Some secondary objectives were to test the vehicle in hypersonic flight for extended periods and for pinpoint landings at predesignated locations.
Mission plans being developed at the time of cancellation describe an early test flight consisting of one orbit around the earth. Launch would take place from Cape Canaveral on a Titan III C boosting the X-20 to a velocity of 24,470 feet per second and an altitude of 320,000 feet at the end of a short bum of the Transtage. The X-20 vehicle with Transition and Transtage still attached would coast to a maximum altitude of 480,000 feet before beginning re-entry over the western Pacific. The Transtage and Transition sections would be jettisoned at the beginning of entry interface. Landing would occur at Edwards Air Force Base at a mission elapsed time of 107 minutes. The highest heat levels were expected to occur at speeds between 17,000 and 24,000 feet per second. In order to withstand these high levels of heat and structural loads, the internal and external structure of the X-20 would be constructed of exotic metals consisting of Rene'41 steel, molybdenum, and columbiu1n.
During the early competition for the Dyna-Soar contract, two different design philosophies were put forward to handle the problem of re-entry heating. The "Bell Aircraft" design for the Dyna-Soar used a systern of active cooling. This would use a network of tubes filled with a circulating liquid that were located in the leading edges and nose sections to cool the re-entry heat. The Boeing designed Dyna-Soar, on the other hand, would use a system of passive radiation cooled thermal protection. The Air Force, in picking Boeing's approach, caused major advances to be made in the development of new metals, ceramics, and high temperature insulations. New methods of manufactoring and testing would have to be invented to reach a level -where success would be assured for the program.
The environment the Dyna-Soar was expected to operate in exceeded all levels of aerodynamic knowledge and capability of available flight vehicle materials technology in 1958.
The final design of Dyna-Soar was the result of over 14,000 hours of wind tunnel tests, which included 1,800 hours of subsonic, 2,700 hours of supersonic, and 8,500 hours hypersonic. This research used at one time or another all of the available wind tunnel and shock tunnels in the U.S. The wing platform ended up as a pure delta with a sweep of 70 degrees. This gave a L/D ratio of 1.5 and a hypersonic lift coefficient of 0.6 with an expected 1,500 nautical mile cross range ability. The radiation cooled structure was designed to last through four flights and to carry a payload of 1,000 pounds contained in a payload compartment of 75 cubic feet.
The X-20 was designed to be a statically stable glider in the normal range of re-entry and subsonic glide conditions. In achieving this goal the design of the flight control system would be simplified. The design of the basic wing section on the 1960 S-20 configuration would use a double wedge upper surface and flat under surface. This would have provided good hypersonic flight characteristics and an ease of manufacture. This design would have required the addition of flip out fins for low speed flight, so the upper surface of the wing was modified to result in continued excellent hypersonic characteristics and improved low speed handling. This modification, however, resulted in problems at transonic speeds and increase in clevon hinge movement at low supersonic speeds. Wind tunnel studies showed that the addition of an aft body ramp would correct these problems. This would give the X-20 its distinctive hump back. ne above description is just one example of the many design challenges that the designers of Dyna-Soar overcame.
The internal structure of the X-20 differed greatly from conventional aircraft design of the period. This consisted of a truss framework with fixed and pinned joints in square and triangular elements that looked similar to bridge construction. This truss framework was made of Rene'41 steel, a 11superalloy" that could resist temperatures of up to 1,800 degrees F. A program was developed to expand the information base on Rene'41 steel that brought about new techniques on the manufacture, welding and extruding of this high strength material.
The internal truss structure would have been covered by a series of Rene'41 panels working together to become the load bearing airframe. Each Rene'41 panel would have been corrugated to add stiffness to the structure but would also allow expansion during re-entry heating. These panels also formed the inside layer of the X-20 heat shield. Covering the Rene'41 panels would have been a silica-fiber insulation called Q-felt or Dyna-Quartz. This would have protected the lower panels from heat transfer from the outer columbium. skin panels. Special attention was given to heat leakage at the expansion joints, access panels, the landing gear doors, and hinge area at the elevon control surfaces. Careful use of the Q-felt insulation and proper panel gap dimension control would allow adequate control of these problems.
The outer layer of the heat shield would have been made up of sections of D-36 columbium. These would have been attached to the underlying Rene-41 panels using a stand-off clip design. Although the D-36 columbium would have less strength at high temperatures than the molybdenum chosen for the leading edges, it could be machined and welded, properties needed in the construction of sections of the main airframe.
A major problem facing the X-20 project team in regards to the refractory metals used in the heat shield was oxidation. These special alloys, after exposure to high heat loads, would begin to oxidize and break down which could have led to structural failure. The answer to this problem was the development of an oxidation resistant silicide coating. A fluidized bed technique was used to coat both D-36 columbium and the TZM molybdenum. This process was specially developed to meet the production needs of Dyna-Soar. A final coating of Synar- I silicon carbide applied over the silicide coating would give Dyna-Soar its distinctive black color. These coatings would have had to be replaced after each flight. Tests conducted on a four panel heat shield with simulations of five re-entries showed that the coating repair could have been completed at post landing checkout.
'Me two parts of the heat shielding that would receive the highest heat levels would be the wing leading edge and the nosecap. Leading edge components were made up of TZM molybdenum, a half-titanium, halfmolybdenum alloy with small amounts of zirconium added. Both single and double shell designs were tested to the equivalent of four boost and re-entry cycles. These tests proved the capability of the design and also showed multiple use could be achieved. Later in the program, vehicle requirements and limits on steps and gaps in leading edge sections led designers to a simpler but heavier structural concept. Ths involved the use of a single milled TZM molybdenum shell attached to the truss framework by machined D-36 columbium fittings.
The design of the Dyna-Soar nosecap led to two independent design programs. Both ended in successful completion of the respective tested designs. A design by LingTenco-Vought, the one chosen as the flight article, consisted of a structural siliconized graphite shell overlaid with zirconia tiles that 'were held in place by zirconia pins. In case of cracks in the structure, the pins and tiles were: held in place by platinumrhodium wire. A back-up design by Boeing would have used a single-piece structure composed of zirconia reinforced with platinum-rhodium wire. During the molding process, shaped tiles were cast in the outside surface to allow thermal expansion and to control possible cracks from spreading. In the case of both nosecap designs, attachment to the glider truss structure would be accomplished by use of a forged TZM molybdenum ring that used a clamping action. This ring was attached to the Rene'41 truss by specially developed molybdenum rivets, nuts, and bolts.
Because scale model testing would not give satisfactory results for these ceramic components, full-size nosecaps were built and tested under simulated flight conditions. Using plasma jet, ramjet, and rocket exhaust, and placing the nosecapg into those environments, proved that both designs were safe for flight.
Dyna-Soar's cockpit was also an area that required new design concepts. The cockpit glazing would have been the largest carried on a manned spacecraft up to that time and would have required special methods of placement within the airframe. This would allow for expansion and contraction of the areas around the windows while maintaining air pressure within the pilot's compartment. With temperatures expected to reach near 2,000 degrees F. in the cockpit area, a special heat resistant shield would have been carried over the forward three windows. This would be constructed out of the same D-36 columbium used as the outer heat shield. The single side windows would have remained uncovered during re-entry as they would not have been subjected to these high heating rates. After the high heat phase of re-entry had passed, the heat shield covering the front windows would have been jettisoned to allow the pilot good forward vision
for landing. In the event that this heat shield did not jettison as planned, tests were carried out with a modified Douglas F51) with a Dyna-Soar window arrangement. It was proven that a pilot could still land the X-N with side window vision only, if the nood should arise.
The crew compartment would have been a welded aluminum structure pressurized with a mixed gas atmosphere of oxygen-nitrogen at 7.5 psi. A rocket propelled ejection seat for use by the pilot during the subsonic portions of boost and landing phases of the flight, was also adjustable to different positions for boost, on orbit, and Te-entry conditions.
Pilot control of X-20 would have been effected by standard rudder pedals and a new development at the time, a side arm flight controller. This would not only have controlled the flight.surfaces, but would have' also been used to control the on-orbit reaction control system,
The X-20 pilot would have faced an instrument panel similar to many research aircraft of its time with one notable exception known as the EMDI-Energy Management Display Indicator. This instrument, developed by General Precision, Inc., would have allowed the pilot of Dyna-Soar to stay within the thermal and structural limit of the vehicle. The display would have been a four inch cathode ray tube with transparent overlays that moved along with the forward flight of the X-20.
The EMDI would also have displayed information that would have allowed the X-20 pilot to pick from different landing sites along the precomputed footprint transparencies. Use of this instrument display avoided the requirement that the X-20 carry a large onboard computer. The X-20 consultant pilot group "flew" the EMDI in the flight simulator and reported favorable results.
Dyna-Soar would have used an inertial guidance unit provided by Minneapolis Honeywell. This unit was an adapted version of the system used for the Atlas Centaur. Twenty-four test flights were conducted aboard a McDonnell NF-101 B at the Eglin Gulf Test Range. Testing proved successful and several flight ready units were ready before the December 10, 1963 cancellation. These units were later flown aboard NASA's X-15 research aircraft with good results. Also included aboard X-20 was a three axis Stability Augmentation System (SAS). This system was designed with the Air Force design philosophies of pilot in control of the vehicle. An auto pilot was provided on Dyna-Soar and would have used an onboard adaptive-gain computer.
The idea of pilot in control of all flight regimes led to a supplemental contract to Boeing for the study of Dyna-Soar pilots' ability to control the Titan booster during the boost phase. Called Pilot in the Booster Loop (PIBOL), this study encompassed nearly 100 flights made in a six degree-of freedom fixed base simulator. Simulations were also run on the Johnsville Centrifuge and these showed no major effects on pilot performance even under the boost and acceleration environment. Final conclusions reached by this study pointed out the X-20 pilot could fly the boost phase of the mission with aid from the SAS.
The onboard power for Dyna-Soar would have been provided by two Auxiliary Power Units (APUs) designed by the Sundstrand Corporation. These two units were part of an integral power generation and cooling system that used cryogenic oxygen and hydrogen to power the APUs and help cool onboard instruments. The flight surfaces would have been powered by the generators, as well.
A cooling system designed by the Garrett Corporation would use hydrogen to extract heat from the cockpit and equipment bay. Redundant cooling loops would be used to transfer heat from the electrical generators and APUs to a hydrogen glycol-water heat exchanger.
Another development in the effort to keep the pilot cockpit and equipment bay cool was the invention of the water-wall. This heatsink was a gel mixture of 95% water and 5% cyanogum 41 jelling agent distributed in a series of wicks. These wicks would provide proper distribution of the water-gel during boost, on orbit, and re-entry phases. The water wall would effectively isolate the pilot cockpit from the thermal effects on the outer skin. The development of the water wall was considered one of the major accomplishments of the Dyna-Soar program. 'Me water panels would be used on either a radian ' t cooled system as with Dyna-Soar, or ablative system as with Mercury-Gemini.
One area of research that produced new technologies that survived the cancellation of Dyna-Soar was in the field of communications during the critical time of re-entry. 'Me Dyna-Soar's long re-entry flight path would have placed thespaceplane in an extended blackout of communications. The development of superhigh-frequency antennas using new construction techniques and advance materials produced flush mounted antennas of extremely light weight. Ground and air testing of the, systems showed that they could have been brought to flight ready status.
Dyna-Soar's landing gear would have been a three point skid arrangement. Conventional rubber tires on aluminum or steel rims could not be considered because reentry heat would be too high in the landing gear bays. The nose and two main gear struts would have been constructed of Inconel, as it was felt this material best fit the design criteria of resistance to high temperature and strength properties. Developed by Goodyear, the main gear skis resembled stiff wire brushes and were constructed of Rene'41 wire bristles wound around a series of longitudinal rods. Looking like an old fashioned kitchen dishpan, the Bendix designed nose skid was a one-piece Rene'41 forging. Tests were conducted on concrete and asphalt runways with good results although initial landings would be on the dry lakebed at Edwards Air Force Base. The design of the main skis provided a high degree of fiction allowing short skid--out distances of 4,500-8,000 feet which eliminated the need for brakes.
The Dyna-Soar would have been boosted into orbit aboard a Titan III C which was originally designed as a purpose built booster for the Dyna-Soar program. Titan III C would consist of a strengthened Titan If core with the addition of two five-segment, one hundred-twenty-inch solid propellant boosters. Considered the third stage, the Transtage would inject the Dyna-Soar into a very precise orbit. On orbit control would have been provided by a Bell Aerosystems designed system of redundant pairs of hydrogen peroxide jets. These were similar to those carried by the X-15 and Mercury Spacecraft.
The X-20 would also carry an emergency escape motor located in the transition section which could provide emergency escape during most of the Titan III C's boost phase. The Thiohol designed XM92 was a solid propellant four nozzle design which would produce 40,000 pounds for 13.4 seconds. This escape motor would also have been used to propel the X-20 to supersonic speeds during the later stages of the Air Launch program.
Although Dyna-Soar's pilot cockpit was to be heated and pressurized, the Air Force contracted with the David Clark Company who worked with USAF-ASD to develop a new spacesuit. A major improvement in this design was the elimination of the neck ring. This allowed the head to move within the helmet giving improved mobility and field of vision. It also resisted ballooning when pressurized to 5 psi pressu#. After program cancellation, NASA took over the contract and flew a modified version of the suit on Gemini seven.
After cancellation @f the Dyna-Soar program, a perception evolved among the press and the public that the X-20 had somehow failed because the t I technology could not be developed. As can@ be seen, there was no lack of technical base for the stated aims of the program. Dyna-Soar suffered from a problem that would come to be commonplace in connection with such programs as Space Shuttle and Space Station. Endless redirection of program objectives and lack of vision on the part of bureaucrats cut short a program which, if followed through to completion, would have expanded a technology base already advanced by research done to bring the X-20 to flight status. The X-20 materials research program was one of very few research bases available to designers of the Space Shuttle. If the Dyna-Soar had flown a full range test program, the heat shield protection system of the Shuttle would probably have been a radiation cooled structure with quicker turnaround times. With the cancellation of Dyna-Soar, the United States lost a chance to build a technology that could have led to a tiny reusable Space Transportation System. 0
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