Navaho SSM-A-2 Credit - © Mark Wade Media Gallery

The first version of the Navaho developed in 1946-1950 was a Mach-3 ramjet vehicle with an integral rocket booster. Completion of the vehicle was cancelled in 1950 but the engine was used to power the Redstone ballistic missile.

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.

Manufacturer: North American. Apogee: 25 km (15 mi). Liftoff Thrust: 333.000 kN (74,861 lbf). Total Mass: 23,000 kg (50,000 lb). Core Diameter: 1.70 m (5.50 ft). Total Length: 20.00 m (65.00 ft). Span: 6.30 m (20.60 ft). Standard warhead mass: 1,350 kg (2,970 lb). Maximum range: 1,600 km (900 mi). Boost Propulsion: Liquid rocket, Lox/Alcohol. Boost engine: XLR-43-NA-1. Cruise Propulsion: Ramjet. Cruise engine: XRJ47-W-1. Cruise Thrust: 16.000 kN (3,596 lbf). Guidance: Inertial. Maximum speed: 3,100 kph (1,900 mph). Ceiling: 20,000 m (65,000 ft). Total Number Built: 3. Flyaway Unit Cost $: 5.000 million. in: 1959 unit dollars. Cost comments: Development cost through program cancellation for all Navaho versions.

1945 October 31 - Launch Vehicle: Matador, Mace, Rascal, Snark, Atlas A, Navaho SSM-A-2.

• US Strategic Missile Programs Begin Nation: USA. Program: Navaho. Request For Proposals were issued to 17 contractors by the US Army Air Force for studies of a 10-year R&D program of four missile types. The missiles were to be air-, rail-, road-, or ship- transportable, and fit in three range categories: 280 to 800 km; 800 to 2400 km; and 2400 to 8000 km. Minimum speed was 970 kph, requiring turbojet, ramjet, or rocket propulsion.References: 4460.

• Army Air Force awards nine one-year missile study contracts. Nation: USA. Program: Navaho. The MX-770 contract for an 800-km range boost-glide missile derived from the German A9 concept went to North American; this would evolve into the Navaho triple-sonic intercontinental cruise missile. Martin received a contract for development of the MX-771, a subsonic ground-launched cruise missile with an 800-km range; it would evolve into the Matador and Mace missiles. Curtiss-Wright and Republic received contracts for the MX-772 and MX-773 surface-to-surface missiles; they never advanced beyond the initial study stage. Convair received the contract for long-range rocket-powered missiles; this evolved into the Atlas ICBM. Northrtop received the MX-775 contract for a 5000-km range cruise missile; this eventually flew as the Snark. Bell receives a contract to develop the MX-776, a 160-km range rocket-powered supersonic missile to be launched from B-29 bombers. This would evolve into the Rascal. McDonnell received a study contract for the MX-777 air-to-surface missile; this evolved into the anti-submarine 'hydrobomb' concept and was eventually transferred to the Navy. Goodyear received contracts for the MX-778 and MX-779 air-to-surface missiles; these never advanced beyond the preliminary study stage. Concurrently, the USAAF had the GARPA surface-to-air missile project underway, which would evolve into Bomarc; the USA Army the Corporal and Hermes (later Redstone) surface-to-surface missiles and the Nike and Hermes A1 surface-to-air missiles; and the Navy a range of missile technology development projects (Regulus, Bat, Kingfisher, Little Joe, Lark, Bumblebee, Gorgon, Dove).References: 221.

• MX-770 strategic missile study contract awarded Nation: USA. Program: Navaho. North American received W33-038-ac-1491, a $500,000 letter contract, for study of the MX-770, an 800 km range, supersonic guided missile with a 900 kg nuclear warhead. References: 44.

• MX-770 fixed price contract finalized. Nation: USA. Program: Navaho. The payload requirement was increased to 1360 kg. North American was also to be provided with captured V-1 or American-built JB-1 Loon cruise missiles, Wasserfall or American Hermes C-2 missiles, and captured V-2 missiles. The company was also to produce as part of its development program an Americanised subscale V-2, the Nativ.References: 44.

• 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.References: 221.

• 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. References: 221.

• 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. References: 221.

• 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.References: 221.

• 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. References: 221.

• MX-770 Phase 2 Nation: USA. Program: Navaho. Component development for the MX-770 missile begins. North American had spent $3.9 million to date on the project. Construction of the launch site for the Nativ test vehicle is begun at Holloman AFB in New Mexico. References: 221.

• MX-770 redirection Nation: USA. Program: Navaho. The concentration on fhe 800-km range missile was to be abandoned. Prototype missiles were now to be produced in three phases. Phase I would produce a missile with a range of 280 to 800 km; Phase II, one of 800 to 2400 km; and Phase III one of 2400 to 8000 km.References: 44.

• US Army Air Corps assigned control of surface-to-surface strategic missiles Nation: USA. Program: Navaho. References: 4460.

• U.S. Army Air Corps becomes U.S. Air Force Nation: USA. Program: Navaho. The Air Force was now a separate service from the US Army. The agreement was made that the Air Force would only handle missiles with ranges over 1600 km. So the range requirement for the MX-770 (later the Navaho) was increased to 1600 km, while carrying a 1350-kg payload with an 800 m CEP, and it became an Air Force missile. The 800-km MX-771 (later Matador) became an Army missile. The MX-775 Snark already had an intercontinental range requirement, and became an Air Force missile.References: 4460.

• 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.References: 221.

• 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.References: 221.

• USAF wishes to concentrate on longer-range Navaho. Nation: USA. Program: Navaho. The Navaho was redefined by the customer in a revised three-phase program using a rocket booster and ramjet cruise. Track, air, and vertical pad launch were to be studied. The first phase would produce a missile with a range of 1600 km while carrying a 1350 kg warhead. Phase two would produce a missile that could carry a 1350 kg warhead to a 3200 to 4800 km range. Phase 3 would be the intercontinental version, carrying a 4500 kg nuclear warhead to an 8000 km range.References: 221.

• 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. References: 221.

• Advanced Navaho concept Nation: USA. Program: Navaho. North American began study of an advanced version of the Navaho, which would use a separate, jettisonable rocket booster. This would allow the cruise stage to be ignited at near-cruise velocity, and to be filled with only ramjet fuel, which would vastly extend the range. In this same month design of the XN-1 inertial navigator is completed.References: 221.

• North American Aerophysics Laboratory moved to Downey. Nation: USA. Program: Navaho. Together with the company's Electromechanical Division, the expanding group was moved into an ex-Consolidated Vultee bomber factory east of Los Angeles. It was here that the Navaho, and later Hound Dog missiles, the Apollo command module, and the Space Shuttle would be built.References: 221.

• Cold War intensifies - US missile programs given higher priority Nation: USA. Program: Navaho. Reacting to Russia's explosion of an atomic bomb, and the Communist victory in China's civil war, the US military begins increasing funding to the low-priority missile development programs begun in 1946. The Martin MX-771 tactical cruise missile is reinstated; additional funds are plowed into the Rascal and Snark programs; and the Corporal tactical missile is modified to carry a nuclear warhead. The Navaho aerodynamic design is frozen so that fabrication of the XSSM-A-2 flight articles can begin.References: 221.

• 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. References: 221.

• Navaho ramjet design complete Nation: USA. Program: Navaho. Wright completes design of the Navaho's supersonic ramjets, the largest in the United States. References: 221.

• 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. References: 17.

• Navaho reoriented Nation: USA. Program: Navaho. The XSSM-A-2 1600-km range version of the Navaho is canceled. The three airframes completed are abandoned. The USAF instructs North American to proceed instead with development of a 10,200-km range version of the missile using the same aerodynamics, engines, and navigation systems already in development. This is to deliver a 3150-kg nuclear payload, and is to be achieved by making a separable booster stage with two engines deliver a ramjet-only cruise stage to ignition velocity.References: 221.

• Navaho engine full-thrust test Nation: USA. Program: Navaho. The XLR-43 engine achieves a thrust of 333 kN for a few seconds. References: 221.

• LR43 run at 430 kN thrust Nation: USA. Program: Navaho. In preparation for development of the 540 kN LR71, the LR43 engine is run for six seconds at 430 kN thrust, compared to its 333 kN rated thrust. Preliminary design of the LR71's new lightweight tubular combustion chamber was begun. These brazed tubes would carry fuel to cool the chamber more evenly than the simpler double-walled German design of the LR43.References: 221.