Boeing Ground-to-Air Pilotless Aircraft

SAM-A-1 GAPA
Final check of Model 601 prior to launching at Holloman Air Force Base, c. 1949.
Place of origin	USA
Service history
In service	1946-1950
Used by	United States Air Force
Production history
Designed	1946
Manufacturer	Boeing
Specifications (Model 603)
Weight	2,000 pounds (910 kg) w/o booster
Length	21 ft (6.4 m) w/o booster
Diameter	10 in (250 mm)
Engine	Sustainer: Ramjet Booster: Solid-fueled rocket
Wingspan	9 ft (2.7 m)
Operational range	31 mi (50 km)
Flight ceiling	59,000 ft (18,000 m)
Speed	1,500 mph (Mach 2.0)
Guidance system	Midcourse: Beam riding Terminal: Active radar homing
Launch platform	Rail Launcher

External image

Alamogorda launch site map (p. 18) & "GAPA Model 601" on launcher (p. 20)

Boeing's Ground-to-Air Pilotless Aircraft (GAPA) was a short-range anti-aircraft missile (SAM) developed in the late 1940s by the US Army Air Force, and then the US Air Force after 1948. It was given the reference number SAM-A-1, the first Surface-to-Air Missile (SAM) in the 1947 tri-service designation system. By 1950 over 100 test rockets had been launched using a variety of configurations and power plants, with one launch in 1949 setting the altitude record for a ramjet powered vehicle at 59,000 ft (18,000 m).

GAPA faced strong competition from the US Army's Nike missile system, and was eventually cancelled in favour of Nike for deployment. The GAPA work was later re-used by the Boeing and Project Wizard team at the Michigan Aeronautical Research Center to develop a much longer-ranged missile, the CIM-10 Bomarc. Bomarc would end up competing with the Army's Hercules missile, and was deployed only in small numbers.

The inherent inaccuracy of anti-aircraft artillery means that when shells reach their targets they are randomly distributed in space. This distribution is much larger than the lethal radius of the shells, so the chance that any one shell will successfully hit the target is very small. Successful anti-aircraft gunnery therefore requires as many rounds to be fired as possible, increasing the chances that one of the rounds will get a "hit". German gunners estimated that an average of 2,800 shells were required to down a single Boeing B-17.

Flying faster means that the aircraft passes through the range of a gun more rapidly, reducing the number of rounds a particular gun can fire at that aircraft. Flying at higher altitudes has a similar effect, as it requires larger shells to reach those altitudes, and this typically results in slower firing rates for a variety of practical reasons. Aircraft using jet engines basically double the speed and altitude of conventional designs, so limiting the number of shells that the chance of hitting the bomber essentially dropped to zero. As early as 1942, German flak commanders were keenly aware of the problem, and expecting to face jet bombers, they began a missile development program to supplant their guns.