The physics of Thunderbird 4

Theo de Klerk

Continuing the countdown on the five International Rescue crafts, this time the yellow submarine Thunderbird 4 is next to be subjected to an examination to see how realistic it really is. Deceptively simple as it may look, it turns out to be the least likely of all Thunderbirds to function in reality. A lot of gullibility is needed to see Thunderbird 4 in operation as we came to admire it.

Thunderbird 4 side
Side elevation
Thunderbird 4 plan
Thunderbird 4 reverse plan
Reverse plan
Thunderbird 4 front
Front Elevation
Thunderbird 4 rear
Rear elevation

Updated technical specifications

Launch: dropping Pod 4

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Mighty Atom drop — stock footage

Let’s start at the beginning. Thunderbird 2 transports the submarine in its usual Pod 4 to the danger zone before Gordon gets his bit of the action. The heavy transporter aircrafts can hover above the water to an almost standstill using its four VTOL thrust engines before it drops the pod. This drop should be gentle to avoid any damage to the pod as well as to its rescue machines and people inside. We will ignore the fact that Thunderbird 2 seems to switch off its VTOL thrusters when it needs it most to stay airborne but a launch covered in smoke doesn’t make for a nice picture. Thunderbird 4 always travels in Pod 4. In the episode Attack of the Alligators! Jeff Tracy says 'Take Pod 6' but when Thunderbird 2 arrives at the alligator swamp you can clearly see the number '4' on the pod door. There is sense in this: the launch of Thunderbird 4 requires a special ramp and it seems logical to adapt a single pod, Pod 4, to meet these requirements rather than all of the pods. Applying some scaling factors, the drop of the pod by Thunderbird 2 is over a height of about 16.3 metres (judging by stock footage of Mighty Atom) — the better part of a five story building height.

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Man from M.I.5 launch footage

This drop takes about 1.8 seconds. During this time it accelerates downwards due to the gravitational pull of Earth and reaches a touchdown speed of 64km/h. When the pod touches the water surface, its descent is stopped by the water. The longer it takes for the water to convert the drop into a floating pod, the better for everything inside it. Watching some stock footage shows this slowdown takes about 1.2 seconds. That corresponds with a sudden brake that is three times as high as for car inflatable air bags to be activated. It is safe to say that everything and everyone inside the pod is badly shaken up if not strapped tight and cushioned for the sudden brake. Every rescue seems to start with a splitting headache for Gordon. This undesirable situation must have occurred to the special effects people also. In The Man From M.I 5 they filmed a much better way of releasing the pod. Thunderbird 2 lowers itself almost to water level and gradually releases the pod.

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Mysterious pod insertion in Atlantic Inferno

Once Thunderbird 4 has left its pod, It makes sense for Thunderbird 2 to re-insert the now empty Pod 4 into its fuselage to re-establish the aircraft’s aerodynamic design. Otherwise the big empty hole in its middle causes all sorts of aerodynamic and structural trouble for the aircraft: it causes increased strain on the sides and air will not flow smoothly over the hull but cause turbulence in the hole. Only in Atlantic Inferno, Thunderbird 2 mysteriously has picked up the pod as Virgil is wise to do.

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Frank Bellamy’s Blazing Danger and Operation Earthquake

Frank Bellamy attempted to address the pod issue in two of his comics showing 'progressive insights' in the operation. In Blazing Danger Thunderbird 2 lowers the pod using grabs that also pull it back in. In Operation Earthquake the pod is retracted while Thunderbird 2 is already moving on.

Launch — into the water

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Launch at sea (Atlantic Inferno)

Thunderbird 4 can move from its pod into the water in different ways. The usual way is sliding off an inclined ramp. Together with its thruster engines it builds up speed to dive into the water and proceed to the danger zone.

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Launch from land (Desperate Intruder)
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Launch from land (Attack of the Alligators!)

The same mechanism can be used to launch from land if the pod is standing conveniently near the water. The inclination of the ramp will then be much less because of the greater distance to the water that must be covered. In both cases the Thunderbird 4 thrusters provide most of the speed gain. There is a bit of a scaling issue with the launch as Thunderbird 4's size seems to change relative to the pod size in the various launch sequences. It is unclear how Thunderbird 4 returns into its pod after the rescue. If the pod was reinserted into Thunderbird 2, the pod must again be dropped and the hatch opened to allow Thunderbird 4 back in. It seems to make sense for Thunderbird 4 to enter the pod face-forward using the power of its thrusters and rotate back into launch position once inside. With a width of just over 9 metres and a pod width of 17 metres this is surely possible.

Weight of Thunderbird 4: Archimedes' Law

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Thunderbird 4 floats on water

The main purpose for Thunderbird 4 is to operate under water. This is by no means a small feat. From experience, everyone knows a heavy object will sink immediately while a light object will float.

What determines 'light' and 'heavy' and where does Thunderbird 4 fit in here? It all relates to what the Greek Archimedes had already found out. An object will float only when the water it displaces when pushed under, weighs more than the object itself. A heavy and light object of the same shape will sink and float respectively. Both displace the same amount of water (being of equal shape) but that amount weighs less than the weight of the heavy object, so it sinks. And the water weighs more than the lighter object, so it floats.

water line
water pushes up against the small part of TB4 submerged
the entire weight of TB4 pushes the submarine into the water
Thunderbird 4 front elevation
Thunderbird 4 floats as it balances its weight with the water push upwards

Only a very small part of Thunderbird 4 is actually under water. Using some scaling factors, it appears that only about .32m of its height is under water. The water displaced by Thunderbird 4 volume weighs 98,000N. That force is enough to push Thunderbird 4 upward by the water as much as the Earth gravity pulls it down. Therefore the weight of Thunderbird 4 also equals 98,000N (or a mass of 9.800kg) — about as much as about ten medium sized cars. Thunderbird 4 is very lightweight, highly buoyant and will therefore not go under by its own weight.

Being a submarine: float, dive, rise, level out

To fully submerge, Thunderbird 4’s entire volume of about 60m must be pressed under water. The displaced water produces an upward force of 600,000N and somehow Thunderbird 4 must be able to press itself down with a force of 502,000N as the water pushes it up with 600,000N while the submarine’s own weight does not exceed 98,000N. Thunderbird 4 resembles a polystyrene ball or pumped up air filled soccer ball that floats on water. You can push it down by using force but as soon as you remove the force the ball will pop up again. The only way to submerge without the need of heavy engines that push it down is by making Thunderbird 4 heavier. So heavy that its weight exceeds the upward push of the water and therefore it starts to sink (or 'dive' which is less fatalistic). There is a simple way to accomplish this which is very cost effective as well as fuel effective. It’s a method employed by all submarines: use water tanks. Fill them up (water enough in the sea) and Thunderbirds 4 gets heavier. Empty them (push the water back into the sea using pumps that do consume fuel) and it becomes lighter to either stay level or rise. To stay underwater on an even keel the water tanks (known as ballast tanks) must contain enough water to let the entire Thunderbird 4 weigh 600,000N. As shown earlier, for this purpose 502,000N must be added as additional weight by water intake. That equals a tank volume of 50.2m: 84% of Thunderbird 4’s space. On screen it remains unclear where these tanks are located within Thunderbird 4. To rise it must lose some weight, to dive it must gain some weight. This is achieved by either removing water from the tanks or adding some to it. For this purpose small tanks are used (known as trim tanks) that can be (partly) filled or emptied at will through valves and pumps. The big ballast tank keeps Thunderbird 4 at a steady level, the trim tanks allow it to rise or dive.

Thunderbird 4 side elevation
Thunderbird 4 floating on surface
Thunderbird 4 side elevation
Thunderbird 4 submerged diving fast
Thunderbird 4 side elevation
Thunderbird 4 submerged level
Thunderbird 4 side elevation
Thunderbird 4 submerged moving slowly upwards
Thunderbird 4 side elevation
Thunderbird 4 submerged moving slowly downwards
Thunderbird 4 side elevation
Thunderbird 4 submerged rising fast

Pod 4 size and weight

Pod 4 has the same size as all other five pods of Thunderbird 2. Applying some scaling factors again, the pod size must be approximately 17.1 meters wide, 29.8 meters long and 14.9 meters high. When Pod 4 floats in the water, about 1/7th of its height is submerged. The water volume displaced therefore equals 1085m which weighs 10,850,000N. This is just the empty pod without the special rescue machines that are transported in the pod. The calculated weight is about 12 times more than the often-quoted specified pod weight of 907,000N. It is therefore also a more substantial part of the total weight of Thunderbird 2 that carries the pod during flight.


Thunderbird 4 front elevation
up/down steering vanes
up/down steering vanes
left/right steering vanes
turbo-engines for surface sailing
Nacelle vanes used for steering

For Thunderbird 4 to move quickly, turbo engines are used. Ordinary boats may use big screws or propellers that push against the water. To reach the claimed speed of 160 knots (77m/s) by Thunderbird 4 three turbo-engines are installed inside the nacelles. One nacelle is placed on each side and one nacelle is placed on top of Thunderbird 4. The engines can work in both directions. This allows Thunderbird 4 to move forwards or backwards depending on whether the water is pressed out in forward or backward direction with great force. It depends entirely on the design of the engines how efficient its power converts it into motional energy. The rudder of Thunderbird 4 is cleverly built right in the exhaust water stream: each nacelle has such a set of rudders or steering vanes. The vertical vanes in the top nacelle will cause a left or right movement when they are rotated; the horizontal vanes in the side nacelles produce a thrust upwards or downwards. To be effective, these vanes must almost be at the outside of the nacelle and not partly covered by it which seems to be the case with Thunderbird 4. To sail at the surface as well as to launch from Pod 4 or from Tracy Island, two powerful thrust engines are mounted at the bottom at both sides of Thunderbird 4.

Thunderbird 4’s maximum speed of 77m/s is reached within 10 seconds. To deliver this energy in this short period, the engine must have a power of 17.8MW. It is about the energy produced by 230 medium sized cars. This energy is only the energy to get moving. Once moving, the water resistance must be overcome and additional energy is needed to do this. Having a rather blunt square front size of 4m, Thunderbird 4 by no means is hydrodynamic in shape the way Stingray is. At a speed of 77m/s the drag force requires a total energy consumption of 475MW. That can be delivered by 10 litres of petrol burned each second. Just as it is very unclear where the voluminous water ballast tanks are located inside the small Thunderbird 4, it is equally unclear where the fuel tanks are found to allow Thunderbird 4 to perform its rescue mission that may last for hours. A third mysterious factor is the type of engine used by the submarine. Traditional diesel-engines and other combustion engines require lots of fuel and oxygen to work properly — not stuff that is available under water for extended periods of time. Atomic (or chemical) engines are out of the question for the same reason. Nuclear reaction engines seem more likely as they can provide lots of energy from small amounts of nuclear fuel. The nuclear engine must be an ordinary fission reactor as there is not enough room to accommodate a fusion reactor like in Thunderbird 5. Protection of the aquanauts against nuclear radiation is another mystery as well as what is done with the produced nuclear waste. Thunderbird 4 shares the same unexplained issues that the other four Thunderbirds share: where is the engine, what type is the engine, where are the fuel tanks and what type of fuel is used? The machines are crammed full with imaginative rescue gear leaving no room for the basic requirements for propulsion. Unless they are all moving through forces by wires of course.

Critical depth

One aspect important to all submarines is the depth it can reach. The lower you go, the higher the pressure of the water layers above you. All these layers are pressing on the hull. A bit more at the bottom than on the top producing the upward force by the water. The large forces on all sides will try to squeeze Thunderbird 4 into a smaller volume — even squash it. Try a small inflated balloon and submerge it in a swimmingpool and see what happens! The hull of a submarine is built to withstand this pressure, but it can only do so upto a certain point. Going any deeper and the pressure wins and the submarine will be crushed. The shape of a submerged object is an important factor to withstand pressure. The ideal shape is spherical or at least cigar-shaped. Round forms distribute pressure forces most evenly along the hull. This allows for deeper dives before the hull is in danger of being crushed. Thunderbird 4 is far from cigar-shaped and is in immediate danger of not coping with the water pressure. Rescue attempts are estimated to occur mostly in the 0-500 metre depth region. Apparently Thunderbird 4 has a hull of a special alloy that can withstand the water pressure at this level. At 500 metres it already is 50 times more than the atmospheric pressure at sea level. And 500m is nothing compared to that of the Mariana Trench in the Pacific Ocean (10km).

International Rescue must admit defeat at these depths — a claim it can operate at 1000m depth is highly questionable.


Thunderbird 4 is divided into compartments that each can be sealed to prevent the submarine from flooding if something goes wrong in one compartment. This includes the ballast tank that is divided in many compartments: one compartment leaking does not jeopardise the mission by making Thunderbird 4 very heavy with only one way to go: down. The hull apparently can withstand the pressures of the not-so-deep. It allows for the inside of the cockpit to keep the air pressure at approximately the level of open atmosphere. This way Gordon can function without a pressure suit.

Defence mechanisms

Thunderbird 4 engines produce noise that gives away the typical submarine presence under water. It does not have any stealth technology to keep it hidden from prying eyes. However, the engine mounting inside the nacelles greatly reduces the noise compared to ordinary submarines with exterior propellers. With Thunderbird 2 mostly nearby, Thunderbird 4 does not have extensive armoury aboard. This would reduce its flexibility, speed and increase its weight (that by itself would not be bad in this case). For 'ordinary' protection it can fire torpedoes from the front rescue equipment area.

Operational capabilities

Thunderbird 4 needs to be employed in rescues, imposing conditions of high manoeuvrability (lightweight), high speed (powerful engines) and good sight for the aquanaut (cockpit). Television monitors can be useful, but nothing equals three-dimensional 180° visual sight. A powerful light trough at the front of Thunderbird 4 gives Gordon perfect vision. In muddy water the light trough can also emit different (longer, into infra-red and radio) wavelengths of light that have better penetration ranges and the image of which can be shown on the multi-function display monitor in the cockpit. The parabolic trough functions not only as reflector to beam the light but also works as receiving telescope for radio waves and infra-red waves. The display monitor is essential when the engines of Thunderbird 4 are set in reverse motion. Small cameras in the rear hull of Thunderbird 4 allow Gordon to see exactly where he is going when moving backwards. The cockpit allows for panoramic 180° views allowing Gordon maximum view on the rescue area.

Proposed updated cutaway

With the knowledge gained in this research, the original 1966 Thunderbirds Annual cutaway needs updating [changes are set in bold italic font] to reflect reality

Thunderbird 4 cutaway drawing
Move the pointer over the numbers in the
drawings to access the legend
Tap a circle to access the legend
tap anywhere else to cancel
steering vanes in side nacelle control diving and surfacing
vanes in top nacelle work as rudder
twin atomic generators provide motive power for main drive; the smaller 'mini-pile' at the rear works auxiliaries and superheats air for the port and starboard jet engines (inside top nacelle) turbo engines for forward or reverse movement; present in top and side nacelles
small jet engines, port and starboard, normally watertight sealed, are used for surface cruising pumps for pressurised air into or from the ballast tank
battery of rocket tubes used to launch Thunderbird 4 from its launch ramp in Pod 4 or Tracy Island
main turbo drive
one of six drive turbos that are used when submerged; these nacelles can be sealed, pumped dry and used as flotation chambers in an emergency
intake for surface jets – retracts when not in use turbo engine connection with nuclear power supply
grille systems prevent intake of anything that might damage turbines steering vanes if Thunderbird 4 is in reverse motion; otherwise fixed in neutral position to allow maximum water intake
auxiliary controls mounted on the rear bulkhead
telescopic 'Autolock' provides underwater exit
mechanism controlling position of lighting trough
lighting trough
cable gear for tilting trough to required angle
'electronic bar' light source set in parabolic reflector trough
lighting trough in raised position
solenoid ram loads missiles from
conveyor into
firing tube; missiles are mainly used for underwater demolition
hydraulic ram in extended position
central ram carrying interchangeable tool heads
port ram works independently of starboard if required
Physics 9th ed – Cutnell, Johnson
Thunderbirds Annual 1966
International Rescue (Haynes) – Graham Bleathman, Sam Denham
Thunderbird 4
Ship stability for Masters and Mates, 6th ed – Bryan Barrass, D. Derrett (Butterworth-Heinemann, 2006)
text ©2014 Theo de Klerk
article originally appeared in fab #79