Thunderbolt

	Federal Signal Thunderbolt
Company	Federal Signal Corporation
Produced	1952-1990 (Supposedly a single year 1992 run)
Type	Rotational Electromechanical Rotational Electromechanical/Gasoline-Driven (Thunderbolt 2000)
Sound output	125-130 dB @ 100 ft
Frequency	60 Hz
Horsepower	2 hp
Voltage	120/240/480 V 1 or 3 ph
Blower horsepower	7.5 hp
Succeeded by	Federal Signal 2001 Federal Signal 508-128
Documentation	Manual

The Thunderbolt was a long-running series of supercharged rotating sirens manufactured by Federal Signal, and over its nearly 40-year production run it gained a reputation as one of the most popular civil defense sirens ever produced, sporting nearly unrivaled sound output when it was released, and continuing to be an incredibly popular siren until its discontinuation.

It is perhaps the most well-known of any American civil defense sirens, even with non-enthusiasts unfamiliar with sirens. In 1990, the Thunderbolt series would finally be discontinued after a 38-year production run, as Federal's then-new 2001-SRN outperformed it without the need of a blower, and also boasted a DC motor which allowed it to include battery backup. The last Thunderbolts to be manufactured were some C-Series Thunderbolt 1003s that were sent to Michigan, though they have since been replaced.

Despite the Thunderbolts being discontinued, elements of its design still remain in Federal's 2001 series and the 508-128 siren, with the 2001-SRN making use of the Thunderbolt's rotator, and even today the collector ring tube from the Thunderbolt is still used in Federal's sirens, 30 years later. Many Thunderbolts are still in service today, and while some have since been replaced by newer DC-capable sirens, many still continue to warn the public of danger.

History and design

Thunderbolt 1000 & 1000T

The Thunderbolt began production in 1952 as a supercharged electromechanical rotating siren. While most sirens are run on their own using air from the intake, the Thunderbolt was unique at the time for making use of a supercharger in the form of a belt-driven Roots blower, which was changed many times throughout the siren's production. This would feed compressed air into the chopper, massively increasing the siren's sound output. This idea would be copied by ACA in 1968, with the introduction of their supercharged Hurricane 130.

The siren has a unique design, with a very small tube holding the motor and chopper, with a large, exponential horn bolted to it, with two supports attached below helping to keep the horn in place. The siren's chopper is reused from Federal's Model 2, reusing the chopper and 2 hp motor of a Model 2-240, albeit without the legs. Rather than using a typical stator, the Thunderbolt makes use of a chopper cap with a single port opening where the horn attaches to. focusing all of the siren's output through that port. The siren's horn and chopper assembly are rotated using a belt-driven rotator inside of the box underneath the chopper tube, with collector rings inside the rotator allowing the siren to rotate 360 degrees. The rotator is adjustable, with 3 different speeds: 2 rpm, 4 rpm, and 8 rpm. The vast majority of Thunderbolts are set to 2 rpm, with 4 rpm also being common while 8 rpm is much less common. The Thunderbolt came in single and three phase variants, the Thunderbolt 1000B and 1000A respectively. This applies to all 3 models of Thunderbolt.

The Thunderbolt originally came in exclusively 5-port single tone, reusing the chopper from the Model 2. In 1954, the Thunderbolt 1000T was introduced, originally using the 4/5 port dual tone rotor from a Model 2T, though by 1959 5/6 port dual tone became the standard, with 4/5 being discontinued shortly after. Single-tone Thunderbolts became known as the Thunderbolt 1000. The Thunderbolt 1000T became the standard model of Thunderbolt in 1954, with the Thunderbolt 1000 becoming optional. The Thunderbolt 1000T was intended for civil defense purposes, while the Thunderbolt 1000 was switched to being used for other purposes, though it could still be used as a civil defense siren if necessary. The Thunderbolt 1000 was rated at 126 dB at 100 ft, while the 1000T and 1003 were rated at 124 dB. However, the decibel rating of these sirens varies depending on the blower used, with the Thunderbolt 1000 with a 5M blower rumored to have reached 130 dB at 100 ft. Typically, the higher air pressure created by the blower, the louder the siren will be.

Thunderbolt 1003

Around the same time that 5/6 port 1000Ts became standard in 1959, Federal Signal introduced a three-signal model known as the Thunderbolt 1003. This moved the horn slightly forwards and added two solenoids at the base of the horn, which would block one of the two ports in the horn, or both if desired, allowing for the siren to create a "hi-lo" or "pulse" signal. These were mainly meant for use in fire stations where the hi-lo signal would be used to separately warn of fire calls, since using the attack signal could cause confusion and panic. The Thunderbolt 1003 effectively replaced the Thunderbolt 1000 for this purpose. The rotator box on the 1003 was enlarged, to make room for the additional collector rings that were needed to run the solenoids. Also included with the 1003 would be a specialized RCM3 controller, which allowed for individual control of the solenoids in addition to the chopper, rotator, and blower. The return springs on the solenoids are prone to failure, with many units suffering from one or both solenoids failing to operate due to the springs wearing out.

Design changes

Pre-production and "Jailbar" Thunderbolts

The Thunderbolt went through several design changes throughout its life, with the prototype units featuring rounded rotator and blower boxes, and the horn facing slightly downwards by default. Another prototype came with a pressure release valve on top. These would be dropped when the Thunderbolt began production. Early production Thunderbolts had three sets of steel "jailbar" reinforcements inside of the horn, which was added by Federal as they feared the horn would warp and ruin the siren's sound output. Federal then experimented with horns that lacked these reinforcements, and by 1954 these jailbars would be discontinued entirely, as the horn proved more than strong enough. Instead, a mesh screen was added inside the horn to keep debris out and prevent the motor from burning up. Roughly 50 of these "jailbar" Thunderbolts exist today, some still in service nearly 70 years after their installation.

B and C-Series

In the early 1980s, Federal introduced the "B-Series" Thunderbolts. In an attempt to make the Thunderbolt more maintenance-free, the rotator was redesigned. Instead of using a motor to drive a belt connected to a gear reducer to drive the rotator, the motor was turned 90 degrees to face the gear reducer and was coupled directly to the gear reducer. These Thunderbolts could only rotate at 4 rpm, with the speed no longer being adjustable. These changes required the rotator box to be enlarged and offset from the siren's horn instead of being directly in the middle underneath. These changes also necessitated the use of external wiring to the chopper motor. The chopper motor itself was placed inside of a cylindrical cover.

The B-Series Thunderbolts did not last long in production, as they ironically were less reliable than the standard Thunderbolts due to problems with the C-face chopper motor coming loose from its mounting plate and becoming out of alignment, causing the chopper to get stuck on the stator and burning up the motor. By 1982, the B-Series was replaced by the C-Series, which returned to the previous belt-driven rotator and the previous unenclosed universal motor, alongside a vertically mounted blower. These C-Series Thunderbolts often suffered from poorer build quality compared to the A-Series models, which often led to their early replacement. The C-Series was the last version of the Thunderbolt made before its discontinuation in 1990. Supposedly, there was a single-year run in 1992 where Thunderbolts were manufactured.

Blowers

The Thunderbolt series of sirens are supercharged with what is known as a Roots blower. A Roots blower has two sets of lobes inside of a steel container which compress and supercharge the air flowing into it as they spin against one another, which is then piped up into the siren itself, massively increasing the siren's volume and range. These blowers were produced by Sutorbilt. The blowers took in air from above through a dedicated intake, and the lobes were driven through three belts by a 7.5 hp electric motor, which could be single or three phase. The siren's main pipe included a pressure release valve beside the blower which allowed relief if the standpipe happened to be clogged with debris. This helped prevent the air pressure from blowing holes in the standpipe or destroying the chopper, and even preventing the blower from potentially exploding in the event of a blockage. A Thunderbolt without a blower is effectively a Model 2 with a horn attached with worse airflow, and as such Thunderbolts are pathetically quiet and high pitched without them.

Blowers are only designed to spin one way, and if wired backwards (possible on 3 phase only), the blower will run in reverse, sucking air from the siren's horn and out of the blower. This results in the siren becoming very high pitched, and the blower becomes significantly louder, often nearly drowning out the siren itself because of this. This also causes the siren to have a near nonexistent winddown, as the air pressure causes the chopper to quickly come to a stop when the motor shuts off. This also causes a slow, extended windup as the chopper fights the airflow. This is known as "reverse wiring". This can only occur to three-phase units.

The blower, and the box it was contained in, went through several design changes throughout the years.

Blower models

6M (Series A1)

The earliest of the blowers used in the Thunderbolt series, the 6M was included with every Thunderbolt siren from 1952 until 1958. These blowers had a 6-inch diameter pipe with a sound pressure of 300 CFM and uses a much larger pulley driven at a slower speed than other blowers, which results in a pulsating sound for both the blower itself and the siren. This is especially noticeable on single tone units. Due to their age, 6M blowers are less common than the other blower models. Due to the lower sound pressure caused by the larger pipe, these tend to be slightly quieter than other blowers, though the pulsating effect helps mitigate this by being more attention grabbing. These would be replaced by the 5M in 1958. Single tone units are rated at roughly 129 dB on a 6M blower, while dual tone units are rated at 128 dB.

5M (Early Series A2)

The 5M was introduced in 1959 at the beginning of the A2 series and is generally regarded as the loudest and punchiest of the blowers, as well as the shortest lived. These blowers made use of a much smaller pulley with a 3-diameter pipe, which allowed a pressure of 350 CFM. Because of this change, Thunderbolts running on 5M blowers tend to be very punchy due to the additional air pressure, especially the Thunderbolt 1000. These blowers lack the pulsating effect of the 6M, instead having a low-pitched hum with a bit of a hiss. The increase in air pressure allows the siren to boast a rating of 130 dB on single tone units, and 129 dB on dual tone units. The 5M would be produced from 1959 until 1964, when it would be replaced by the 4M.

4M (Late Series A2)

The 4M began production in 1965, replacing the 5M. The 4M was created in an attempt to make the blower cheaper to produce, at the expense of sound output. These used a 1.5-inch diameter pipe, alongside a slightly smaller blower, which resulted in an air pressure of 250 CFM. These blowers are the most common blowers for Thunderbolts to use, and can be visually identified from other blowers as the 4M blower introduced a vent on the rear of the blower box. However, some early 4M blower boxes lack these vents. These blowers have a loud, high pitched hum, and run at a faster speed than the other blowers due to the smaller pulley. These blowers are far less punchy than the 5M and 6M, and instead have more of a "buzzy" sound to them. They're still very loud however, with single tone units being rated at 127 dB at 100 ft, while dual tone units are rated at 126 dB. These would end production in 1981, with the C-Series being introduced in 1982.

4M C-Series

The C-Series blower was introduced in 1982, when the C-Series Thunderbolts began production. These were the last blowers the Thunderbolt used before its discontinuation in 1990. The C-Series blower is essentially a 4M blower flipped onto its side into a vertical position. The pulley sizes were changed again, with the pulley being slightly larger on the blower compared to the motor. This causes the blower to run at a slower speed. Instead of the pipe being located below the blower, the pipe is mounted to the side of the blower, which causes the standpipe to be higher up on the blower box compared to earlier Thunderbolt models. The pressure release valve was also significantly enlarged. While this blower was mainly used by C-Series Thunderbolts, B-Series Thunderbolts also made use of these. Supposedly, there was a single-year run in 1992 where Thunderbolts were manufactured. These had a slightly shortened blower frame/cover. No known 1992 models are known to exist, and the information may be inaccurate.

Other variants

Thunderbolt 2000

The Federal Enterprises Thunderbolt 2000 was an early variant of Federal's popular Thunderbolt series and was a gas-powered variant of the siren. Rather than using an electric motor, the siren's A1 6M blower was powered by an Onan twin-cylinder gasoline engine. This engine also served as a generator for the siren, allowing it to operate independently of external power sources, which was important for areas where grid power was either unreliable, or would be in danger of being lost in an attack.

While it was a good idea in concept, it was never popular due to the increased maintenance the engine required over a basic electric motor. Grid power was also widely available by the time the siren had released. It had a very short production run, from 1952 until roughly 1954. They were mainly installed around California and Arizona. This idea would later be reintroduced in the form of the System 7000 (see below), which was a basic electric Thunderbolt on top of a shipping container containing the controls and a generator and could also operate independently of grid power. This idea would be used in several other sirens, including the Chrysler Air Raid Siren and the BN52/BN54 Mobil Directo, although this would fall out of favor by the mid 1950s.

The 2000 was only offered in 5 port single tone, and ran on an early 6M blower. Production of the 2000 was halted very early on in its production, as the idea never quite took off. They are not known to have been produced beyond the mid 1950s. The 2000 was discontinued just a couple of years after it first began production, and no dual tone units are known to have been produced.

Due to the increased maintenance required as well as the impracticality of operating gasoline powered sirens today, only a few units remain, none operating on their engines. Two of the surviving units were converted to electric motors for the blower, with one being in Phoenix, Arizona at Fire Station 22, and the other located at a school in Beebe, Arkansas, replacing a Whelen Vortex R4 in 2019 that was struck and fried by lightning. It is the last active Thunderbolt 2000, although it is essentially now a Thunderbolt 1000. There is only a single known Thunderbolt 2000 with its original engine intact, which is located in Windsor Hills, California. It was converted to propane at some point and is inactive and dilapidated like most of California's former civil defense siren systems.

System 7000

A self-contained version of the Thunderbolt series, with a similar concept to the Thunderbolt 2000. The blower and controls were enclosed inside a large enclosure which also housed a large gasoline-driven AC generator which would be used to power the siren. These were introduced sometime in the 1980s for use in areas where reliable power was unavailable, such as war zones or remote areas. The siren would be driven by standard mainline power when available, switching to the generator when power failed. The enclosure is weather, corrosion and heat resistant, with cooling fans allowing it to operate even in hot, hostile environments. It is also tamper-resistant to prevent vandalism or unauthorized activations, and includes a security system that activates an alarm in the event of somebody breaking in.

The System 7000 came with Thunderbolt 1000, 1000T and 1003 heads, known as the System 7012, 7022, and 7026 respectively. Most known units were System 7026 units in service in Kuwait during the Gulf War and have all since been replaced by Federal Signal Modulator electronic sirens, some of which are mounted on top of the old enclosures. One System 7026 unit is confirmed to still exist, on display at the Kuwait National Office of Civil Defense.

Thunderbolt 1003 Locomotive Horn

Some Thunderbolts were adapted for other purposes as well. For instance, two Thunderbolt 1003 units were adapted for use as train horns, by means of removing the rotator, and adding a flatter, wider 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 placed atop two EMD DDA40X locomotives operated by the Union Pacific Railroad with DDA40X units numbers 6918 and 6924 had 1003s installed atop their cabs. Another unit, EMD SDP35 number 1400, was also equipped with one of these. The setups were installed in roughly mid-1979, and were gone by around 1986. These Thunderbolts were paired with standard locomotive air horns, and were used only for emergency purposes such as clearing a visible obstruction on the tracks. It is speculated that both solenoids are shut by default, and are both opened when the siren is activated.

Examples

Thunderbolt 1000AT test in Memphis, Tennessee.