The physics of Thunderbird 1

Theo de Klerk

Thunderbird 1 is designed as a rocket that can also be used as an aircraft. Its powerful tail engines allow a vertical take off reaching supersonic speeds of up to 19 times faster than the speed of sound. Its design is aerodynamic apart from its cross-shaped tail section that would cause quite a bit of air resistance – both supersonic as well as subsonic. At lower speeds its expanded wings provide additional lift the way normal aircraft stay aloft but with some off-the-cuff calculations their area is about half of what is needed. Like all Thunderbird machines it is difficult to determine where the engine fuel as well as the engines are located. The claimed atomic fusion reactor type engine sounds very futuristic but a fusion reactor inside Thunderbird 1 is unlikely with today's technology. The claimed choice of variable cycle gas turbines is the sort of flexible engine suitable for subsonic and supersonic flight as was proven by Concorde. When its rockets are used during launch it remains unexplained where the flames and heat are diverted to — in reality the launch bay and most of Tracy Villa would be severely damaged. All in all, Thunderbird 1 is very much a rocket based on technology of the sixties. Derek Meddings did his homework. Since then, experiments and new knowledge have replaced much of the then popular choices. If any reincarnation of Thunderbirds would feel the need to "update" the series, Thunderbird 1 is a likely target. But do we want it "improved" and "updated"?

Data on Thunderbird 1

Thunderbird 1 side
Side elevation
Thunderbird 1 front
Front elevation
Thunderbird 1 plan
Thunderbird 1 rear
Rear elevation
Thunderbird 1 elevations

Most facts and values on Thunderbird 1 revealed in the series or specification sheets in TV21 and its annuals are all in the same ballpark. Its height is given as 35m (115 feet). Using this allows us to derive the other measurements from the profile view picture given of the aircraft.


Length: 35m
Diameter base red nose cone: 3.18m
Height red nose cone: 5.05m
Diameter middle section: 4.87m
Height middle section: 19.65m
Diameter blue tail section: 7.11m
Height blue tail section: 4.87m
Width tail rocket section: 10.86m
Height rocket section: 4.12m
Wingspan tip-to-tip: 20.21m (fully open)
Wingspan from fuselage to-tip: 8.05m
Wing width: 2.99m (at tip) 7.49m (at connection to fuselage)
Wing length along fuselage: 14.03m (vertical from start to tip wing)
Wing area: 42.49m2 (one wing)


Mass: 126 900 kg (40 tons)


Maximum speed: 24,135km/h (= 15,000mph) = 6,704m/s = Mach 19.7)
Maximum altitude: 150,000ft (= 45.6km)
Action Range: assumed unlimited.

Launch and landing

From and into the hangar

Thunderbird 1 launch
launch of Thunderbird 1 from its launch bay
NASA rocket launch NASA rocket launch NASA rocket launch
a NASA launch using flame deflector tunnels
Thunderbird 1 landing Thunderbird 1 landing Thunderbird 1 landing Thunderbird 1 landing
landing of Thunderbird 1 (Danger At Ocean Deep)

Thunderbird 1 is capable of a rocket-like launch from its secret hangar hidden underneath the Tracy Villa swimming pool and gain speed fast. On return it is capable of landing vertically on its launch pad — not all that different from the way Thunderbird 3 does it. It seems likely that in real life the swimming pool and much of the Villa would turn to smouldering ashes if the Thunderbird 1 rocket launches the way it is seen to do.

The launch bay itself doesn't seem to have any flame deflector tunnels to redirect and remove the launch exhausts. NASA experienced problems with the launch of Space Shuttle STS-128 in 2008 and its flame deflector tunnel. Just as a swimming pool is coated with a protective layer before it is soaked, the NASA flame trench is sprayed with Fondue Fyre, a fire-resistant concrete, to shield it from fire and smoke. These flame tunnels deflect the flames from the rocket's main engines and boosters. The concrete trench used to deflect the flames was damaged as a result of epoxy carbonation and corrosion of steel anchors. Another concern is the damage from the pressure of soundwaves on the hull of the rocket and the payload. Especially in Thunderbird 1's underground hangar the sound cannot go anywhere.

During operation

Thunderbird 1 landing
Thunderbird 1 landing using VTOL
Harrier jet hovering
Harrier jet hovering
Harrier jet VTOL air stream analysis
analysis of air streams from its four VTOL rockets

When Thunderbird 1 arrives at or departs from the rescue scene, it always uses its VTOL (Vertical Take Off and Landing) rocket mounted in the middle of its fuselage. It is supposed to be a variable cycle gas turbine that is either used for take-off or landing or to hover above a rescue spot without landing.

A single VTOL rocket is like putting Thunderbird 1 on the point of a needle: the vessel is balanced precariously. The rocket must work from the centre of mass of the aircraft. Then it can raise or lower the aircraft as desired — all its mass can be considered concentrated at the point where the VTOL rocket applies its force. If the force is off-center, the rocket produces a torque which makes the aircraft spin around the centre of its mass so it will start to move in a somewhat circular trajectory: It will go nose down or tail down or sideways — anything but stable. This off-balance situation also applies to the three nacelles of Thunderbird 3. If one fails or produces less thrust than the other two, the rocket starts divert from its intended path. That one VTOL rocket must produce lift that equals or exceeds the entire weight of the aircraft: all 1,269,000N of it. Unlike Thunderbird 2 this weight is not shared amongst 4 VTOL rockets.

The technology of VTOL rockets is well known since the fifties and sixties and NATO Navy Harrier jets are one of the aircraft that can take off and land vertically (although they have 4 VTOL rockets). Another one is the Soviet Navy Yakovlev Yak-38. Just like the blades of a helicopter, the exhausts of a VTOL rocket cause lots of air streams that, according to Newton's Third Law, provide lift by pushing against the air underneath the aircraft. The pilot increases the push at take off and reduces it for a soft landing. The last Harrier was taken out off service in 2011. The successor of the YAK-38 was the YAK-141 but it was never taken into production. Currently, the only vertical take-off aircraft are helicopters.

A problem with VTOL is an increased weight during take off. This process is similar to the feeling in one's stomach when standing in an elevator that accelerates upwards until it reaches its final speed. The faster Thunderbird 1 wants to take off, the heavier it feels and the more power the VTOL rocket must deliver until the aircraft is in a high enough position to start moving horizontally using its tail engines.


Thunderbird 1 in flight Thunderbird 1 in flight
Thunderbird 1 changes to horizontal flight (as do the background clouds)

Most of the flying mechanics of normal flight by airfoil (wing), lift versus weight and air resistance (drag) and engine thrust have been discussed in the chapter on Thunderbird 2. There we also discussed the differences between flying at subsonic (below Mach 1 or the speed of sound) and supersonic (above Mach 1) speeds.

The pilot can (and must) change from vertical flight following launch to horizontal flight to the danger zone. He can fly at supersonic speeds (with retracted wings) or extend the wings and reduce speed to subsonic values using the lift provided by the extended wings. The stock footage used in the episodes fools the viewer as it is the camera that tilts rather than the aircraft: the background sky also goes into horizontal flight.

Supersonic flight

Thunderbird 1 shockwave
The shockwave caused at a speed Mach 3.9

During supersonic flight, Thunderbird 1 will not use its wings as they provide no lift and cause a lot of drag. The problem with any aircraft flying faster than the speed of sound is that the aircraft is "suddenly there". The sound produced by the aircraft moves slower than the aircraft itself. This causes a shockwave where all soundwaves that were produced at an earlier time, encounter each other. The energy of each wave front is piled up in a shockwave. The wave has the shape of a cone that is smaller when the flight speed is higher.

When the shockwave touches an object (or sweeps over the ground below) an enormous "bang" is heard when the energy of the shockwave is released. It can have sufficient power to destroy windows. As the same energy is concentrated in rings around the cone, the smaller the ring (the closer to the tip of the aircraft) the more severe and concentrated the impact on the ground or objects is. At high speed Thunderbird 1 should stay high in the air (since its distance to the surface determines the radius of the ring).

Thunderbird 1 has a problem with its tail section. This cross shaped engine block must not be troubled by shockwave energy release. To keep the block inside the cone (and never touch its circumference) Thunderbird 1 must travel at less than Mach 3.5 (or 3.5 x 340 m/s). Slower speeds mean wider cones (which disappear when the speed becomes subsonic). During supersonic flight, the tail engines must provide all forward thrust as the wings are retracted and would provide drag instead of lift when extended.

Subsonic flight: variable sweep wings ("swing wings")

Thunderbird 1 folding wings Thunderbird 1 folding wings Thunderbird 1 folding wings
Thunderbird 1 extends its wings during flight
Thunderbird 1 wing diagram
Thunderbird 1 swing wings

Variable sweep wings allow an aircraft to change its geometric shape to change its wing lift. This is most applicable to aircraft that fly at both high and low speeds. It is a technology that dates back to the sixties but since then new materials and increased knowledge of aerodynamics have allowed designs that can be as efficient without variable wings. Computer assisted and controlled flying also relaxed demands on pilots to react to stability issues.

It remains a mystery on where the landing legs are hidden inside the wings during flight and can be extended on landing. Especially since the wings are rather thin to allow for near supersonic speed.

Variable-sweep wings provide many advantages, particularly in regard to take-off distance and load-carrying ability that both depend on the amount of lift the wings can provide. The configuration also imposes a considerable penalty in weight and complexity due to the retractable wing mechanism and fuselage strengthening which makes it a less favourable choice if other options are available – which is the case in the 21st century.

Thunderbird 1 can fly straight upwards at rocket speeds. In these situations wings will produce mostly drag, and therefore Thunderbird 1 can retract them. Once flying horizontally, and especially at subsonic speed, Thunderbird 1 needs lift that is obtained from airfoils or wings. It can then extend its wings to their maximum width for maximum lift.

How realistic is the current size of the wings? The weight of Thunderbird 1 is quoted to be 1,269,000N. This weight must be lifted (L) by the wings to enable the aircraft to fly horizontally (this implies an "angle of attack" of 0°). We take an approximation of its lift coefficient (CL) to be 0.4. For airfoils to provide lift, the speed of the aircraft must be subsonic (less than 340m/s). Let's assume Thunderbird 1 flies at a speed (v) of 300m/s at an altitude of 10km (where the air density (ρ) is 0.41kg/m3).

Using these figures in the lift formula Lpv2ACL you can calculate that the wing area (A) must be approximately 172m2. The two wings of Thunderbird 1 have an area of 42.5m2 each. To provide the required lift for the aircraft's weight at the given speed and altitude, the wings will need to be enlarged by a factor of 2 (= 172/85).

Coincidentally, Thunderbird 1's wing area is undersized by almost the same factor as Thunderbird 2's wings (1.9 times). There may be a problem enlarging Thunderbird 1's wing area because when completely retracted into the fuselage they may not really fit. Making them fixed transforms Thunderbird 1 into a normal aircraft where the wings aren't really helping during its rocket launches and supersonic speeds.

Flying Altitude

Thunderbird 1 can fly at a maximum altitude of 45.6km which is in the upper part of the stratosphere layer of the atmosphere (stretching between 13-50km) where the ozone layer is found as well. The bottom layer (troposphere) reaches to about 13km and contains 80% of the total mass of the atmosphere. Weather and airstream jets are located in this bottom layer, as is commercial flight. The stratosphere is the next layer and contains about 10% of the atmospheric mass.

The usual supersonic cruising height of Thunderbird 1 will also be in the higher troposphere. It will encounter less resistance from thinner air layers making forward movement easier, especially when flying as a rocket. The shape of Thunderbird 1 is reasonably adapted to supersonic flight. It has a pointed nose cone and thin flat wings. The engine block at its tail however, is anything but supersonically shaped. By flying at speeds less than Mach 3.5 the tail section stays within the shockwave cone but it still remains a cause of drag that slows the aircraft down. This situation is also encountered at the tail fin of Thunderbird 2. Flying at subsonic speeds, the air drag remains (albeit reduced compared to supersonic).


Thunderbird 1 engines
Thunderbird 1 main engines
Thunderbird 1 booster rockets Thunderbird 1 booster rockets
Booster rockets (Man From MI.5)

Being a reconnaissance aircraft, Thunderbird 1 is sleek and fast and uses several engines for propulsion – each for a specific task or as failover in case one doesn't work. According to its technical data, Thunderbird 1 has several engines, all "atomic fusion powered". Not all of them are always used.

Most engines are of the variable cycle engine (VCE) type. This is an aircraft jet engine that is designed to operate efficiently under mixed flight conditions, such as subsonic, transonic and supersonic. The Concorde supersonic aircraft used this type of engines. The thrust these engines can deliver depends on the flow rate of air throughput (which is engine construction and fuel dependent) and the (positive) difference between the speed with which the air is blown out and the (slower) speed of the aircraft. The requirement for moderate afterburning (reheat) makes the engines rather noisy. The air intake is controlled by valves that can open or close. During take-off and approach, the engine behaves much like a normal civil turbo with an acceptable noise level (i.e. low specific thrust). However, for supersonic cruise, the valve variable inlet guide vanes and auxiliary intake are closed to minimize bypass flow and increase specific thrust.

Two of the outside booster rockets and the high sustaining rocket in the center are often seen during or shortly after launch. The central rocket is not used for landing.

The action range of the aircraft is said to be unlimited. This is rather doubtful as even Thunderbird 1 must be refuelled once in a while. Assuming however that it can cross half the world (and return home), it must be both very fuel efficient and have lightweight engines.

As with all Thunderbirds machines, it is unclear where the fuel tanks large enough to last an entire rescue operation at the other side of the world are located. "Atomic" (or "nuclear") sounds nice but this type of engine must deliver sufficient power to suck air in and force it out during supersonic travel. The size of Thunderbird 1 seems too small to allow for such an atomic reactor.

Thunderbird 1 cutaway drawing
Ram jet thrust pipes [see below]
Engine housing [see below]
Cooling fins
Atomic pile in sandwich shielding
Rocket propellant and pumps
Rear pitch-and-yaw jets centered in air intakes [at high speed
and in thin air normal control surfaces are inoperative]
Turbojet fuel tanks
Central services duct
Folding wing slot of girder section
gives great strength to fuselage
Centrally placed VTOL rocket with fuel
Folding wing, contains landing leg
Auxiliary motors and batteries
Braced wing hinge member and
hydraulic ram controlling wing
Life support systems, including oxygen
bottles and air recycling equipment
Pressure bulkhead
Air recycle main duct
Motor-driven circular plate in bulkhead, supporting pilot seat
Entry/exit hatch with folding ladder
for use when landed horizontally
Pilot area [see below]
Refrigerated hull
Instrument and check computers
Retractable destructor cannon [normally used
for demolition of dangerous wreckage, &c.]
Forward pitch-and-yaw jets
Probes and sensors inside shockwave heat cone
Control panel, all systems are automated where possible
to simplify the pilot's tremendous task at high velocity;
the system's check lights are at the top, at centre the
multipurpose TV screen on which can be projected route
maps, touch- down viewing and normal communication
Thrust and flight controls, mounted on the arms of the chair
Swing seat, alters position to keep the pilot upright
during change from vertical to horizontal flight
Ramjet intake from four outer ports
Heat exchanger; molten metal circulated from
A-pile [see above] passes heat to rammed air
which exhausts at ramjet thrust pipes at left
Ramjet thrust pipe
Ramjet thrust pipe
Central column separating engines from fuselage
One of four inner front ports intakes air which
passes to compressors and heat exchanger
One of four inner front ports intakes air which
passes to compressors and heat exchanger
Pipes serving heat exchangers
Heat exchanger for turbojet
Turbojet turbines
Centrally mounted high performance sustainer rocket
Jet exhaust ports
Fuel line
Booster rocket; one of four used for take-off
Exhaust ports of booster rockets
Exhaust ports of booster rockets
Cooling fins conduct excess heat from motors
in thin upper atmosphere [see above]
Cooling fins conduct excess heat from motors
in thin upper atmosphere [see above]
Cooling fins conduct excess heat from motors
in thin upper atmosphere [see above]
Folding wings — diagram showing
folded wing position in black
Aircraft Flight – A Description of the Physical Principles, 4th ed – R. Barnard, D. Philpott (Pearson, 2010)
Flight Physics – Essentials of Aeronautical Disciplines and Technology with Historical Notes. E. Torenbeek, et. al., (Springer, 2009) (translated from Dutch original Technical University of Delft Press Aëronautiek: Grondslagen en Techniek van het Vliegen 2002)
Understanding Flight – David F. Anderson, Scott Eberhardt (McGraw-Hill, 2001)
Introduction to Flight, 5th Ed. – John D. Anderson Jr. (McGraw-Hill, 2005)
Thunderbirds – Agents' Technical Manual – Graham Bleathman, Sam Denham (Haynes, 2012)