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The Federal Thunderbolt siren was a electro-mechanical and rotating-directional civil defense siren, created and produced by Federal Signal Corporation (FS). Federal began manufacturing the Thunderbolt in 1952, and ceasing production in 1990. A lot of cities in the United States bought the Thunderbolt for air-raid use and severe weather. The last thunderbolts were sold to somewhere in Michigan, only to be replaced sometime in 2009.

Pre-Production
The Thunderbolt went through many different changes throughout it's prototypical phase. These prototypes included such units as ones with rounded horns, rounded blower and rotator enclosures, even non-rotational ones. The rounded rotator and blower unit is the unit that appears on very early Federal advertisements. One unit turned up in photos from a siren study in Jefferson City, Missouri. Another unit was supposedly destroyed in the Trinity nuclear test in the early 50s. A person claimed to have found the remains of a rounded Thunderbolt rotator panel on a former test site, however no evidence ever surfaced to back this. It could be implied that the poster lied or was mistaken on his find. One other prototype unit turned up in photos that appears to have an oblong rotator. This unit was tested in Minneapolis, MN in early 1952. To this day, it is presumed that none of the prototype Thunderbolts survived into the 21st century.

Rotator
Originally the Thunderbolt used a motor and an Abart gear reducer driven by pulley to drive the large spur gear which makes the projector rotate. The pulleys utilized 3 different belt slots to change the RPM in which the siren rotates, from 2, 4, and 8 RPM. In the 80s Federal wanted to make the Thunderbolt more maintenance free, doing so by turning the motor and gear reducer to face each other and attached to make a direct drive system which rotated at 4 RPM. This rotator was used only on the B-series models.

Blowers
All Thunderbolts run on what is known as a "blower" to push air to the top of the siren to increase sound output. The type of blower was changed during production; starting in 1952, Thunderbolts used a 6M, then switched to the 5M blower around 1958, later to the 4M in 1965. This was used up until 1980, when Thunderbolts started to have the 4M roots blower mounted vertically, which was used all the way up to the end of its production in 1990. The units consisted of a motor and a Sutorbilt roots blower unit that was driven by belt pulleys attached to the blower and motor. The blower then took in air and pushed it into the pipes. They also had a "relief valve" that was mounted right outside of the roots blower, this acted as a "jam protection" system that would open if something was stuck in the standpipe. If something was stuck inside, air pressure would build up and would lift the relief valve open and let the air escape before the blower either, perforated the standpipe, blew off the chopper in the siren, or possibly cause the blower to literally explode.

The rough time-frame for each blower size is as follows: 1952-1958: 6M (A1 series) 1959-1964: 5M (Early A2) 1965-1981: 4M (Late A2) 1982-1990: C series (Same blower size as the late A2 units, just the actual blower was turned vertically)

With these different blowers, there were other differences as well. The covers for instance over production changed in size, and with the 4M hitting the scene in 1965, vents were added. All of them too had provisions on top of the frame to be bolted together as well as threaded rod on the rear of the frame to further secure them. When the C Series blowers were rolled out, this changed to the covers bolting down on the sides of the frame, and the rod replaced with hinges, as well as a handle riveted to the top of the blower cover.

Projectors
A notable feature of some Thunderbolts is their projector, or "horn". The earliest Thunderbolts had metal cross support bars, now referred to as "Jailbars", throughout the horn making a 3x3 grid in the front, 2x2 in the middle, and 2x2 in the back. This acted like reinforcements. Since the Thunderbolt was new at the time, Federal wanted to make sure the horn did not warp or bend as this would ruin the sound output. After 1952, Federal started testing to see if there really was any benefit from having these "supports" in the projector, this caused some models to have them, and some to lack them. After 1954, it became obvious that the thick metal panels used for the projector were strong enough to not compromise the projector itself. Federal decided to drop the reinforced projectors and continue making the models with nothing more than a mesh screen used to keep various animals, insects, and debris out so the rotor wouldn't lock up leading to a burnt motor. Approximately 50 of the reinforced projectors still exist to this day.

Custom Uses
Some Thunderbolts were adapted for other purposes as well. For instance, two units were adapted for train horns, by means of removing the rotator, and adding a custom flat projector as opposed to the regular Thunderbolt horn. It is believed that they were supercharged by pneumatics provided from the locomotive itself. This setup was two Union Pacific Centennial engines; numbers 6918 and 6924 specifically. The setups were installed in roughly Mid-1979, and were gone by around 1986. None of the two units are believed to still exist to this day. UP 6924 was scrapped in 1986, likely taking it's 1003 siren with it.