|Type||Prototype intercontinental ballistic missile|
|Place of origin||United States|
|Mass||1,205 pounds (547 kg) empty, 4,090 pounds (1,860 kg) full,|
|Length||31.5 feet (9.6 m)|
|Width||6 feet 10 inches (2.08 m)|
|Diameter||30 inches (760 mm)|
|Engine||Four XLR35-RM-1 engines|
2,000 pounds-force (8.9 kN) each
|Propellant||Liquid oxygen as oxidizer|
Ethanol as fuel
|8,000 miles (13,000 km)|
The RTV-A-2 Hiroc (high-altitude rocket) was the United States' first attempt at an intercontinental ballistic missile (ICBM). The project was initially under the name MX-774, however the project was canceled in 1947, and leftover funds were used to build and launch three missiles, which were renamed to RTV-A-2 Hiroc. The design included several innovations; the gimballed engine provided guidance control, the internal gas pressure was used to support the airframe and the nose cap was separable. All of these concepts were later used on the Atlas missile.
The Hiroc missiles were 31.5 feet (9.6 m) long, had a fin span (the maximum width of the rocket, accounting for the fins) of 6 feet 10 inches (2.08 m), a diameter of 30 inches (760 mm), an empty weight of 1,205 pounds (547 kg) and a gross liftoff weight (GLOW) of 4,090 pounds (1,860 kg). Range was 8,000 miles (13,000 km) with a 500-pound (230 kg) payload.
The missile's propulsion system consisted of four XLR35-RM-1 engines, built by Reaction Motors Inc., which produced 2,000 lbf (8.9 kN) of thrust each and could independently swivel up to ten degrees either direction. The gimballing motion was used to control the flight path of the missile, replacing the system of the V-2 missile that used moving fins placed within a fixed engine. The gimbal system adds complexity to the engine mounting but preserves more of the energy of the rocket exhaust during maneuvering. The engines had a specific impulse of 210 s at sea level.
The Hiroc missile used liquid oxygen as its oxidizer, and alcohol for its fuel. The Hiroc missile did not have separate tanks for its fuel and oxidizer, which were instead contained in one tank separated by a bulkhead. The frame of the rocket was supported by the pressure of the gas inside the tank, which would be either propellant when used, or inert gasses when stored. Having gas pressure provide rigidity to the structure reduced the empty weight by requiring less metallic components for structural reinforcement, but made the missile very fragile because it required continuous pressurization.
The nose cone, which contained the ordnance that would impact the target, would separate from the rocket booster. This made the rocket lighter as only the nose cone and its ordnance had to be able to survive re-entry, rather than the rocket. The unique innovations of the Hiroc missiles, such as the gimballed engines, and the internally supported frame, would go on to inspire the Atlas rockets. Several things changed however, such as the aluminum used for the missiles skin of the Hiroc, was switched out for stainless steel in the Atlas, because stainless steel had a higher melting point. The engines of the Atlas missiles were also much stronger, generating a total of 150,000 lbf (670 kN) of thrust, compared to the Hiroc's total of 8,000 lbf (36 kN) thrust.
In April 1946, Convair received a 1.9 million dollar contract from the US Government to build and test ten MX-774 rocket. This was part of a large number of missile projects being developed by the US Army at that time, which included both ballistic missiles like Hiroc, and a variety of long-range cruise missiles as well. The original design of the Hiroc called for a missile that could deliver a 5,000 pounds (2,300 kg) payload 5,000 miles (8,000 km), and which had an accuracy that allowed it to deliver it to within 5,000 feet (1,500 m) of the target. The development was headed by Karel Bossart, who would go on to contribute to the creation of the Atlas rockets. The MX-774 was based directly upon the design of the V2 Rocket, with several noticeable changes, such as an integrated propellant tank, swivelling engines, pressurized body, and detachable nose cap.
As part of the Truman Doctrine, the USAAF's missile budget was cut in half from $29 to $13 million in what became known as "the black Christmas of 1946". Many of the projects were canceled outright, but MX-774 instead continued with reduced funding. The project was eventually canceled in June 1947 as the Army concentrated their efforts on cruise missiles, which were more promising at that time.
However, Convair arranged to use the remaining contract funding to launch three of the rockets, which were renamed RTV-A-2 Hiroc. The tests took place at White Sands Proving Grounds. The three tests took place on 13 July 1947, 27 September, and 2 December. These tests validated the concept of using gimballed engines for propulsion and guidance.
During the test on 13 July, the Hiroc reached a maximum height of 6,200 feet (1,900 m), but lost thrust after 12.6 seconds and hit the ground at 48.5 seconds, 415 feet (126 m) from the launch pad. Due to a mistake in packing, the payload recovery parachute failed to open; a camera and a few other instruments survived, so the test was deemed a partial success.
During the test on 27 September, the Hiroc reached an altitude of 24 miles (39 km) at 48 seconds and a maximum velocity of 2,350 feet per second (720 m/s). The parachute failed again, this time due to a battery problem; the Hiroc began to freefall before its oxygen tank exploded at 20,000 feet (6,100 m). This caused it to break up, but a camera and some instruments survived.
During the test on 2 December, the Hiroc reached a maximum height of 30 miles (48 km) and reached a maximum velocity of 2,653 feet per second (809 m/s). The parachute failed to open yet again, this time due to the nose cone damaging it after being ejected, while the Hiroc was at an altitude of 121,000 feet (37,000 m) and moving at a speed of 1,500 feet per second (460 m/s). The camera was recovered, although it was partly damaged.
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