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Navaho SSM-A-2

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Navaho SSM-A-2
Credit: © Mark Wade

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AKA: MX-770;NA-704;Navaho I;XSSM-A-2. Status: Cancelled 1950. Payload: 1,350 kg (2,970 lb). Thrust: 333.00 kN (74,861 lbf). Gross mass: 23,000 kg (50,000 lb). Height: 20.00 m (65.00 ft). Diameter: 1.70 m (5.50 ft). Span: 6.30 m (20.60 ft). Apogee: 25 km (15 mi).

On Halloween 1945, the US Army Air Force invited 17 aircraft systems manufacturers to submit proposals for design studies of long-range missiles of a variety of ranges and types, based on German technology. North American Aviation decided to propose American development of the A4b or A9, a winged, boost-glide version of the German V-2, against the Army Air Force mid-range supersonic missile requirement. On 24 March 1946 NAA received letter contract W33-038-ac-1491 for this missile, designated MX-770, with a range of up to 800 km. The payload requirement was increased from the 900 kg of the A9 to 1360 kg three months later.

By the end of 1946, two government-furnished V-2 engines arrived. North American settled on a three-phase research and development program. Phase 1 consisted of a study of the German A4b and A9 boost-glide missiles; Phase 2 consisted of study of replacing the rocket engines of the design with supersonic ramjets in order to extend the range; Phase 3 would determine the size and type of the booster needed to boost the reengined A9 to ramjet ignition speed.

As North American engineers tore down and reassembled the V-2 engines, it was clear that this Model 39 18-chamber engine was an engineering kludge, a prototype that was rushed into production because the ultimate planned engine was unavailable. By the end of the war the Germans had been testing a more advanced Model 39a single-chamber engine. So in the spring of 1947, it was decided to proceed immediately to design, construction and test of a new engine, the NA-704 Mark III, based on the German Model 39a. To assist in this, Bollay's team was free to draw on the expertise of the V-2 designers themselves, now working for the US Army. Konrad Dannenberg in particular had been intimately involved with the 'shower head' injector plate that was essential for the single-chamber motor. Dieter Huzel, a close associate of von Braun, was hired by North American as a full-time employee in order to better coordinate work with the German team. In September 1947, preliminary design of the Mark III began, aiming at the thrust of the V-2 engine but with a weight reduction of 15 percent.

The North American engineers invented a Kinetic Double-Integrating Accelerometer (KDIA) design for the navigation system that allowed an inertial platform to also measure distance traveled. This breakthrough would allow long-distance unpiloted high-accuracy navigation. North American aerodynamicists discovered that the A9 swept-wing design was unstable at transonic speeds. The company selected an aft-wing with forward canard layout for the MX-770. So by June 1947 the navigation concept, the propulsion, and the aerodynamic layout of the missile had all been fundamentally altered from the A9 baseline. North American had spent $3.9 million on the project. Development of specific components - the XN-1 navigation system, the Mark III engine - was initiated. Construction of the launch site for the Nativ test vehicle was begun at Holloman AFB in New Mexico.

In February 1948 the Air Force ordered a complete redesign of the MX-770 missile to increase its range from 800 km to 1600 km. This was necessary purely for political purposes - when the US Air Force was split from the Army the year before, it had been agreed that the Army would handle missiles under 1600 km range, and the Air Force over that. The MX-770 was now an Air Force missile, and therefore had to have a range over 1600 km. To achieve this, North American decided they would indeed have to change the missile from the A9 pure rocket boost-glide vehicle to one that would use ramjet engines for sustained Mach 3 cruise. This approach had also been studied by the Germans for advanced versions of the A9. The rocket engine was now used for initial boost and to get the vehicle up to the supersonic speed necessary for ramjet ignition. But even with the more efficient ramjets, the entire vehicle would have to grow by 33% to meet the new range requirement. Accordingly the design thrust for the Mark III engine increased from the 249 kN of the V-2 to 333 kN.

In February 1948 the missile was designated the XSSM-A-2. This design had reached 100% drawing release. It consisted of an integrated rocket booster, two ramjets for cruise, and XN-1 inertial navigation with a final dive on the target. But the inertial navigator drifted 1.6 km in accuracy for each hour of flight, which meant the missile could not meet the USAF 800 m CEP requirement.

Meanwhile the XSSM-A-2 was redefined by the customer as only the first step of a revised three-phase program for a family of vehicles using a rocket booster and ramjet cruise. The ultimate objective was an intercontinental missile.

By late November of 1949, the first version of the Mark III engine began testing at the new Santa Susana facility. Attempts for longer pressure-fed engine runs in December exhibited surges in combustion-chamber pressure (known as "hard starts") that were powerful enough to blow up the engine. By March 1950, a simplified and redesigned engine first topped its rated level of 333 kN for four and a half seconds. During May and June, full-thrust runs, exceeding a minute in duration, went well. Meanwhile, Wright had completed design of the ramjets in December 1949 and begun fabrication.

In April 1950, with the first three XSSM-A-2 airframes completed, the Air Force canceled flight test of that 1600-km range version of the Navaho. North American was instructed instead to proceed with development of the 10,200-km range version of the missile using the same aerodynamics, engines, and navigation systems already in development.

Nevertheless, development of the engine for the now-canceled XSSM-A-2 continued. Late in March 1950, the first complete engine, turbopumps included, was assembled. In August, this engine, designated XLR-43-NA-1 by the government, fired successfully for a full minute at 12.3 percent of rated thrust. Late in October, the first full-thrust firing reached 310 kN for less than five seconds. However now a new problem emerged - rough combustion during the build up to full thrust. However the North American engineers found a solution, and by March 1951 the problem of unstable combustion was under control.

In just three years of development, the North American team had delivered an engine that weighed less than half as much as the V-2's model 39 (668 kg versus 1126 kg), while delivering 34 percent more thrust. They had formed the corporate technology base for further American development of rocket engines.

By this time the Army had directed von Braun's team to develop at utmost speed an 800-km range pure ballistic missile. With minor modifications, the Mark III would fit this requirement. So although the Mark III never went into production for the Navaho or the Air Force, its Army derivative boosted various versions of the Redstone tactical missile. And it would be this engine on the Redstone that would place the first American satellite in orbit and boost the first American into space.

Extremely little information exists on the XSSM-A-2 version of the Navaho - just two drawings. Even though airframes were reportedly fabricated, no photographs have been found of them. The program was incredibly advanced and secret at the time. The values given here are estimates based on the thrust of the engine, and dimensions scaled from a comparative drawing.

in: 1959 dollars. Flyaway Unit Cost 1985$: 5.000 million. Maximum range: 1,600 km (900 mi). Boost Propulsion: Liquid rocket, Lox/Alcohol. Cruise Thrust: 16.000 kN (3,596 lbf). Cruise engine: XRJ47-W-1. Maximum speed: 3,100 kph (1,900 mph). Total Number Built: 3.

• MX-770 Three-Phase plan proposed. - . Nation: USA. Program: Navaho.

  Phase 1 consisted of a study of the German A4b and A9 boost-glide missiles; Phase 2 consisted of study of replacing the rocket engines of the design with supersonic ramjets in order to extend the range; Phase 3 would determine the size and type of the booster needed to boost the reengined A9 to ramjet ignition speed. At this time North American has a staff of 43 working at its Technical Research Laboratory in Los Angeles, including 12 PhD's and 18 ME's.

• Nuclear engines ruled out for MX-770 - . Nation: USA. Program: Navaho. North American completed its study of the possible use of nuclear-powered engines on the missile, and decided to use conventional propellants..

• Solid-fuel ramjets for MX-770 - . Nation: USA. Program: Navaho. North American studied solid-fuel ramjets as a possible propulsion source for the MX-770. It was decided to use a liquid-propellant ramjet..

• MX-770 design firms up, but requirements change - . Nation: USA. Program: Navaho.

  North American's Kinetic Double-Integrating Accelerometer (KDIA) design allowed an inertial platform to also measure distance traveled. This breakthrough would allow long-distance unpiloted high-accuracy navigation. North American also discovered that the A9 swept-wing design was unstable at transonic speeds. The company selected an aft-wing with forward canard layout for the MX-770.

• MX-770 navigation system - . Nation: USA. Program: Navaho. Full design and development of the XN-1 pure inertial navigation system for the MX-770 is begun by North American..

• MX-770 project staff reaches 500. - . Nation: USA. Program: Navaho.

  North American began construction of a wind tunnel at its new rocket test facility in the Santa Susanna mountains, in the northwest San Fernando Valley, north of Los Angeles. The Technical Research Center was renamed the Astrophysics Laboratory. The aerodynamics and control systems work on the MX-770 had already been fed back into North American's swept-wing F-86 Sabre fighter design.

• MX-770 redesignated XSSM-A-2 - . Nation: USA. Program: Navaho.

  The design had reached 100% drawing release. It consisted of an integrated rocket booster, two ramjets for cruise, and XN-1 inertial navigation with a final dive on the target. But the inertial navigator drifted 1.6 km in accuracy for each hour of flight, which meant the missile could not meet the USAF 800 m CEP requirement.

• XSSM-A-2 stellar-inertial navigation - . Nation: USA. Program: Navaho. Preliminary design review of North American's XN-2 navigation platform, which coupled the XN-1 inertial system with a star tracker to ensure continued accuracy even in long flights..

• Navaho engine tests - . Nation: USA. Program: Navaho. North American tests a 135 kN subscale version of the Navaho's booster engine as an aid to developing the injector plate..

• Navaho ramjet design complete - . Nation: USA. Program: Navaho. Wright completes design of the Navaho's supersonic ramjets, the largest in the United States..

• Navaho XLR-43 engine test. - . Nation: USA. Program: Navaho. First full-thrust test of 75,000 pound liquid rocket engine for the Navaho (XLR43-NA-1) conducted by North American at Santa Susana, Calif..

• Navaho engine full-thrust test - . Nation: USA. Program: Navaho. The XLR-43 engine achieves a thrust of 333 kN for a few seconds..