Reverse Plan
Plan
Front elevation
Rear elevation
Thunderbird 3 elevations
Not many measurements are known about Thunderbird 3 apart from its height. We’ve taken this height (85m) and scaled all other sizes to it using the profile shots provided.
Height: 86m (= 287ft.)
Nacelle span: 24m (in proportion to height: radius 15m, diameter 30m)
Diameter: 6.9m (in proportion to height rear: 13.5m, ring 9m, main top part 7.5m)
Mass: 570,992 kg (562 tons)
Emergency acceleration: 10g (1g = Earth gravitational pull, 9.81 m/s2)
Maximum sustained acceleration: 6g
Steering through pitch and yaw rockets: 12 in middle ring, 20 in the nose and 24 at rear for altitude (orientation) adjustment
Thunderbird 3 is stored in its hangar that is shielded and disguised by the Round House (whose interior or other functions are never shown in either one of the episodes). Just as the swimming pool hides Thunderbird 1's hangar, it seems very unlikely the house would escape being damaged during launch — it probably ends up being completely burned to a crisp. There is also no flame trench to speak of to divert the exhaust flames and fumes during launch. They seem to vanish into thin air. There is mention of a vent in the Thunderbirds Annual 1967 but that seems totally undersized to do its job. Quite unlike how real rockets are launched as the illustration from NASA below shows.
Watching Thunderbird 3 take off on stock footage used in The Uninvited, it seems to raise itself 9.3 metres within the first three seconds after launch. This can be deduced applying the scale factors to the on-screen image during those 3 seconds.
High school physics state that a distance s, travelled at a constant acceleration a during a period of time t, equates to s = ½at2. Filling in the details this becomes 9.3 = ½a32. Some simple math allows you to determine the acceleration of Thunderbird 3 to be a = 2 * 9.3 / 9 = 2.1m/s2.
The Earth pulls downwards with a force causing an acceleration of 9.8 m/s2 (better known as "1g"). To have an acceleration of 2.1 m/s2 in the opposite direction, the total acceleration of Thunderbird 3 must be 9.8 + 2.1 = 11.9 m/s2 (= 1.2g).
Any value above 1g is sufficient to leave Earth. With another basic high school formula that states that the force required to give a mass m an acceleration a must be equal to F = m ⋅ a. Given the acceleration and mass of Thunderbird 3, the three nacelles each produce a force of over 220kN. This seems a rather modest value for rushing into space quickly but it gets the job done at a leisurely speed.
The old Saturn V rocket that propelled Apollo capsules into orbit had an initial thrust of 34,000kN which is more in line with the technical data for Thunderbird 3 (written well before the Saturn V rockets were launched) that claim a thrust of about 20,000kN.
This thrust would equal an acceleration of Thunderbird 3 of 35m/s2 (3.5g). Such an acceleration requires an astronaut to be strapped in and lie flat on his back, rather than sitting upright in a chair the way Scott and Alan are shown to do during season 1 episodes.
It is not often shown how Thunderbird 3 makes its re-entry in the Earth’s atmosphere at the completion of a rescue or visit to Thunderbird 5. Only the final stage where its rockets fire and the spaceship makes a slow descent through the round house is shown. It appears it will land precisely on the three rocket cradles in the launch bay.
In principle, re-entry is nothing else than launch in reverse. At launch it tries to get to the rescue zone as quick as it can. It is full thrust (maximum acceleration) upwards against the gravitational pull back.
On re-entry there is no hurry to get back so a different approach can be taken. Remember Newton’s law on motion: an object moves with a constant speed (including zero, when it stands still) if there is no net force applied to it. When Thunderbird 3 approaches Earth, gravity will pull it towards Earth — it will do all the work to get the rocket back on the ground. Without resistance, this force will increase the speed and Thunderbird 3 will be in "free fall" down to Earth. The crew and the floor they stand on fall at the same rate: they would be weightless during the fall.
If the descent starts with zero speed somewhere high up in space then, due to the pull of Earth’s gravity, the rocket ship will land (or more likely crash) with a speed of 11.2km/s (the escape velocity of Earth) on the ground.
Thunderbird 3 comes down in one piece, with the availability of all its rockets and presumably sufficient fuel to use them. The astronauts will turn the spaceship around in such a way that its tail is pointing to Earth — just as it would be positioned during launch escaping from Earth.
At a height of 1,000km, the gravitational pull of Earth is about 0.7 of what it is at ground level: its strength is 0.7g. If Thunderbird 3 fires its rockets and produces a thrust of 0.7g in the opposite direction (upwards), the two forces cancel each other out and effectively there is no force on Thunderbird 3. It will simply hang motionless at a height of 1,000km.
Reducing the thrust a little bit (like down to 0.5g), results in a small win for gravity (0.7g – 0.5g = 0.2g), causing an acceleration downwards: the rocket moves down. Due to this downward acceleration, the speed increases each second (using some high school physics: v = a ⋅ t or velocity equals the product of acceleration and time, with an acceleration of 0.2g the speed becomes v = 0.2g ⋅ t). During this increasing descent speed the astronauts inside will feel somewhat lighter (as they would in an elevator that starts its descent).
At some comfortable speed, the output of the thrust rockets is increased to match the gravitational pull at that height. This ensures all forces cancel each other out and once more Thunderbird 3 descends gracefully at a fixed speed while the astronauts regain their "normal" weight.
While Thunderbird 3 approaches the Earth’s surface the gravitational pull becomes stronger and the rockets must compensate for this to keep the descent speed steady. In reality, the thrusters will not have to match gravity completely: air friction starts playing a role in the lower parts of the atmosphere. It heats up the rear of Thunderbird 3 while at the same time slowing the rocket ship down. Thunderbird 3's rockets therefore only have to provide the upward thrust that will match the gravitational pull minus the air friction.
The speed of descent is much lower than it would be under free fall. Therefore, Thunderbird 3 is heated up by air friction much less than a re-entering Apollo capsule that comes in under free fall with no brakes. The heating of the air ionizes the air molecules and the now electrically charged particles inhibit radio communication. This potentially happens at the bottom side of Thunderbird 3. Therefore all downward facing parts of Thunderbird 3 (the round bluish-grey tail of the rocket as well as the bottom of the nacelle rockets) must be made of heat-resistant material. Thunderbird 3's top is aerodynamically much better suited to lift-off than its bottom is to landing.
Unlike the Apollo capsules whose communication is completely blocked during descent, the only hotspot (and it's not even that hot) for Thunderbird 3 is its rear end — communication from the top of the rocket is mostly unaffected so Alan can stay in touch with Thunderbird 5 as well as Tracy Island while he makes his descent.
When the Round House comes into sight, Alan increases the thrust of Thunderbird 3's rockets as if lifting off again. In doing so he reduces its speed of descent to almost zero to allow him to precisely manœuvre the rocket through the Round House and into its three rocket-cradles. At this slow speed, the air friction plays no important role anymore although the hot rocket exhaust fumes still cause communication problems at the rear end. No doubt computer guidance is used for the final descent, using sensors and computers inside the Round House and the landing system of Thunderbird 3 itself. This is documented in the Thunderbirds Annual 1967 description of the launch bay. This must be an automated procedure as the astronauts have little vision on what’s happening at the tail end of the rocket.
Once in space, Thunderbird 3 does not land or take off anymore. It can approach other spaceships, align with them and move away from them. This is all done using its regular booster rockets and correction/stabilizing rockets. Being in the vacuum of space, none of the rockets make any noise — either at ignition or during use.
The human body can only endure accelerated motion to some extent. On Earth we experience gravity which causes an acceleration of 9.8m/s2, better known as "1g". If you jump out of an airplane, your speed increases (you accelerate) by 9.8m/s every second while you race to the Earth surface. Anything that compensates for the Earth’s gravity by an opposing larger acceleration away from Earth will be able to leave the planet. To do this at ground level, a minimum speed of 11.8m/s is required — the escape velocity of our planet.
From experiments conducted during jet fighter pilot training it turns out that a body moving upwards (head first) can endure a maximum acceleration of 5g. You feel five times as heavy at that moment and have difficulty moving your hands. The blood is drained from your head and sinks into your legs and you’re on the verge of passing out before dying. Going downwards, an acceleration of 6g is just about acceptable. All the blood is going to your head and you feel as if you stand are standing on your head. The best position to withstand any acceleration is by lying on your back, facing the direction in which you move. You can then survive just about 15g before passing out.
The acceptance levels Alan Tracy and his co-pilots of Thunderbird 3 can endure in different directions of movement are illustrated below. Alan’s position during take off in Thunderbirds Are GO! and the second season of the tv series is far preferable to the upright position he had in season one. Perhaps new insights in space travel during the Gemini project (around 1966) lead to this. Initially, Alan would experience a 5g acceleration sitting upright and he would pass out.
When traveling at emergency thrust of 10g or sustainable thrust at 6g, the astronauts must lie down if they want to survive the acceleration. Once Thunderbird 3 has reached a cruising speed and stops accelerating, the engines are shut off and the speed stabilizes to the value it has attained at that moment. The fact that Thunderbird 3 is seen to speed through space without any of its rockets blasting, is therefore pretty accurate.
Traveling at constant speed does imply that there is no thrust force and everything becomes weightless and starts floating about if not attached to the wall or floor somehow. The books shown in Figure 4 will certainly start floating around. It’s debatable whether bookshelves with books are a good idea in a space rocket.
Because Thunderbird 3 rushes through the dense Earth atmosphere as well as the vacuum of space, the hull contains many vents to allow the pressure to equalize on both sides of the hull. Only the living compartments of Thunderbird 3's astronauts remain pressurized.
Rockets in space have an easier time than submarines under water. For the latter, every 10 metres that is added to its diving depth results in additional layers of water above it that cause an extra pressure of 1 atmosphere. Rockets in space travel through a constant near vacuum and the pressure difference between the cold exterior of deep space and the warm pressurized interior of the living quarters remains around 1 atmosphere. Also, fuel tank hulls need to withstand an extra atmosphere in pressure to keep the fuel in the tank and keep it from exploding. The fuel must remain pressurized to push it through the fuel pipes. Without the pressure the fuel in the tanks would float around weightlessly.
Thunderbird 3 mostly adheres to the ideal of a streamlined, bullet shaped vessel allowing air to pass along its hull while speeding from launch bay to the vacuum of space. The parts that cause the most friction are the white docking ring and the top of the nacelle rockets. When these are shaped to allow the air to flow past without bumping into the surfaces, the aerodynamic design would be a lot better.
The air friction can be approximated with F = ½C ⋅ p ⋅ A ⋅ v2 where C is a constant determined by the shape of Thunderbird 3 (smaller if more aerodynamic) and A the area of the spacecraft seen from the direction in which it travels with speed v. After all, while moving forward the rocket must push aside all air that is in its path and this is proportional to this area. The air density will diminish with height. Therefore, during travel, the air friction p and the speed v of the spacecraft will change most while C and A remain the same. Just after launch the air density will be highest while the speed is lowest. But (the square of) the speed increases much faster than the density decreases. Once in space, the shape of Thunderbird 3 has little or no bearing on its functioning or travelling speed.
Instead of referring to the altitude of a rocket ship – as we do with regard to airplanes – we will look at the distance Thunderbird 3 can travel from Earth. At larger distances, the "height" above the Earth’s surface becomes more and more insignificant (one would have to subtract the Earth radius of 6,400km from the total distance).
During launch Thunderbird 3 burns fuel to accelerates continuously. Accelerated motion makes sure the least amount of time is needed to arrive at a rescue scene. But once the escape velocity of Earth is reached (11.2km/s), Thunderbird 3's astronauts may decide to switch off the engines. The speed obtained will be kept almost indefinitely according to Newton’s First Law which states that any object will keep moving at a constant speed provided no external force is applied to it.
External forces can work against your speed (e.g. solar wind or Earth's gravitational pull may slow you down) or increase your speed (engine thrust or another planet's gravitational pull).
The spaceship can glide to its destination without any fuel consumption — a trick often employed by satellites sent to remote planets and to places where no human has gone before. Spaceships have the advantage here of traveling through a vacuum with no resistance where airplanes constantly need to overcome air drag.
To see how long it takes for Thunderbird 3 to get somewhere, we modelled its outbound journey using data on acceleration of the spacecraft in the knowledge that the opposing force of Earth’s gravity reduces with the increase in distance from the planet.The results are shown in the diagram below. It shows both suggested accelerations of 1.2g (line indicated with "thrust 1") seen on the television screen or the more likely 3.5g (line "thrust 2") during travel to a distance equal to the lunar orbit (384,400km).
The results show that at an acceleration of 3.5g the braking influence of Earth’s gravity can be ignored after 33 minutes (2,000 seconds) and at an acceleration of 1.2g this takes 66 minutes (4,000 seconds), after which time Thunderbird 3 speeds along at a constant full thrust (at 1.2g = 11.9 m/s2 or 3.5g = 34.4 m/s2). The rocket is then at a distance of 143,000km from Earth. The next graph shows the same information but displays the distance from Earth in relation to the time travelled. Since most rescues are relatively close to Earth, the graph is useful for most space rescues.
Some relevant travel data that can be deducted from this model (ignoring influences of other heavenly bodies) is summarized in the table below. The altitude is measured from the Earth’s surface. For calculations the radius of the Earth (6,371km) must be added.
| target | altitude | travel time 1.2g | travel time 3.5g |
|---|---|---|---|
| Earth – exosphere | 1,000km | 15 minutes | 5 minutes |
| Earth – Thunderbird 5 | 7,000km | 33 minutes | 12 minutes |
| Earth – Moon | 384,400km | 2.5 hours | 80 minutes |
| Earth – Sun | 150,000,000km | 1.9 days | 1.1 days |
| travel data for Thunderbird 3 | |||
Some important data for most of Thunderbird 3’s rescues shown in the series is summarized in the table below.
| missions | altitude | travel time 1.2g | travel time 3.5g |
|---|---|---|---|
| Low Earth Orbit (LEO) (Alpha-2-0 in
Cry Wolf,
Space Observatory 3 in Impostors, KL-A in Ricochet) |
300-600km | 12 minutes | 3 minutes |
| geostationary orbits | 36,000km | 60 minutes | 27 minutes |
| lunar orbit | 384,400km | 150 minutes | 81 minutes |
| Sun Probe (vicinity of the Sun) | less than 150,000,000km | less than 1.9 days | less than 1.1 days |
| Thunderbird 3 missions | |||
Thunderbird 3 will normally move in a forward direction except when landing. For precise manœuvres in space Thunderbird 3 uses small pitch (up, down) and yaw (= left, right) rockets positioned around the rocket. Through short duration blasts, using fuel in the process, these are capable of changing Thunderbird 3’s orientation in space. One situation where this is needed is during docking with Thunderbird 5.
Thunderbird 3 must have reduced its speed relative to Thunderbird 5 to almost nil and the adjustments to align and dock with the space monitor are done through small forces on the rocket, provided by the pitch and yaw rockets. The episodes are accurate in not showing any blasts from the main rockets during the procedure.
Another, much more economical way that allows for more subtle changes than the pitch and yaw rockets is the flywheel (or reaction wheel) inside the rocket. By changing the orientation of the flywheel, the spacecraft does the exact opposite, rotating around its centre of mass. Together they can stabilize the spaceship. Any deviation in the spaceship's attitude is compensated for due to the Law of preservation of angular momentum (the amount of rotational movement). Changing the flywheel's orientation on purpose makes Thunderbird 3 change its position slightly. It allows Thunderbird 3 to rotate around its centre of mass. You may have seen a flywheel effect when someone sits in a swivel chair holding a rotating (bike)wheel. If this person re-orients the wheel (turns it sideways or upside down) the result will be that the chair starts to turn in the opposite direction.
An advantage of using a flywheel is that it is operated without using fuel for rockets. The pitch and yaw rockets still have their unique purpose: they can move the spaceship sideways (move the centre of mass of Thunderbird 3), something the flywheel cannot do as it tries to keep the centre of mass in place. Both means of course correction are therefore vital.
The space scenes that can be seen in a few television episodes and one of the feature films betray the fact that they were filmed in Earth's atmosphere as the hot gases from the rocket's exhausts rise upwards rather than follow a straight path in the direction they were expelled — Meddings later thought to solve this problem by turning both model and camera upside down so the exhaust plume would seem to flow downwards which is equally incorrect.
The thrust of Thunderbird 3’s rockets produces an artificial gravity in a way similar to the feeling of increased weight when an elevator starts to move upwards. An upwards acceleration of 1.2g makes you feel like weighing 1.2 times your normal weight.
Upon arrival at the rescue scene, Thunderbird 3 comes to a standstill in relation to the satellite or rocket in trouble. While its speed reduces, the floor is moving slower than the astronauts on board. Due to inertia, They continue their movement in a forward direction which feels as if they are thrown forward and they are likely to bump their heads into the ceiling – if nothing else.
It seems reasonable to assume that Brains thought about this and came up with some solution to keep the astronaut’s feet on the floor: using velcro shoes or magnetic forces or otherwise. All other objects however will be thrown forward (or upward) if not securely fixed to wall or floor. Like those books on the bookshelves? Who keeps putting them back? Better to remove them completely. Once the spaceship has come to a complete standstill or reached a constant speed there is no acceleration anymore and any feeling of weight is gone. Everything inside Thunderbird 3 floats around if not fixed to wall or floor. Please note that "airless" (vacuum) and "weightless" are not synonymous terms. Any vacuum sealed pack of coffee proves this: it is not weightless.
Thunderbird 3 has 3 chemical rockets for launch, landing, emergency boost and orbit change. It also supposedly has 3 ion-drive particle accelerators for use in deep space.
At launch, the rocket thrust is specified to be 19.8 x 106N (4.5M pounds) although watching the launch on screen would suggest an acceleration of 1.2g with a thrust of 6.7 ⋅ 106N.
The range of Thunderbird 3 is claimed to be unlimited although I think it would need to refuel at certain times. With space rescues mostly within the lunar orbit it seems reasonable to expect that the fuel tanks allow "unlimited" operations at those distances.
The power for the engines is supposed to be atomic fusion which seems to contradict thrust through chemical rockets.
It is important to realise that forward thrust of any machine or rocket is entirely dependent on Newton’s Third Law of dynamics. Every object moves because it pushes on something (be it the road surface or exhaust gases) and this substance pushes back with the same force in the opposite direction. It is this force that makes an object move.
Throwing out engine exhaust gases in a backward direction results in an equal force forward on Thunderbird 3 by the exhaust gases. This way rockets fly through the near vacuum of space.
The type of exhaust gas is in fact irrelevant for the motion. Anything thrown out at sufficient speed and with sufficient mass will do. If Thunderbird 3 had been spitting out sufficient coffee beans at the right speed, they would provide the forward force for the spacecraft. It so happens that kerosene is rather well suited for the job. It is relatively compact, can be heated easily and in doing so expands enormously, making it easy to be ejected at high speed hence providing pushback on Thunderbird 3.
To launch itself, Thunderbird 3 uses ordinary propellants in chemical rockets. With an almost constant mass of 570 x 103kg and an acceleration of 1.0g, the total force (or thrust) generated by Thunderbird 3's engines must be around 6 million newton. That’s quite small compared to the launch thrust of Sun Probe, the rocket in the episode of the same name. This amounted to 20 million (british) pounds, or 88 million N while the first stage of the Saturn V rocket blasts off with 7.6 million pounds (33 million N). Brains clearly devised a lightweight and economic space rocket!
Thunderbird 3's fuel consumption must be much more efficient than the technology of the old Saturn V rockets. Not only is Thunderbird 3 a slim rocket, but it also returns in one piece and does not shed any rocket stages once they are empty and become just ballast.
According to early technical designs of Thunderbird 3, the living quarters have a double hull for safety and additional rocket fuel was stored between the walls. Not only is this a dangerous location if for some reason the fuel explodes but when Thunderbird 3 is not accelerating and moving at a constant speed (including no speed — against what reference do you measure this?) there is no artificial gravity and the fuel would float around freely in the tanks and would not be likely to move to the engines unless forced to by pumps or pistons. These seem available in the big tanks but not in any other part of the rocket.
Once in outer space, with no air friction to overcome and a much reduced gravitational pull, other types of engines can be used that are much more economic to run but have less power and therefore provide smaller accelerations.
Ion rockets use Newton’s Third Law the same way as the chemical rockets do. The number of ions that are ejected determines the amount of mass ejected. Together with their speed, they determine the amount of pushback Thunderbird 3 receives. This method of propulsion delivers hardly any force at all. The total mass represented by the ejected ions is insignificant (a factor of 1022 less) compared to the mass ejected by chemical rockets. It works fine for satellites on a journey to the outer solar system that are in no hurry to get there but not for rescue craft for which time is at a premium.
Ion engines first ionize gas by stripping electrons from the atom. Next they accelerate the positively charged ions to a negatively charged plate. This has holes in it so most ions overshoot by passing through these holes. To avoid the rocket to become charged itself, the stripped electrons are re-joined with the ions just before they exit the engine nozzle. The inert gas Xenon is often used as propellant — which gives a blue light when the ions recombine with their electrons. With the obtained high speed the ions leave the nozzle and by Newton’s Third Law the rocket moves in the opposite direction to maintain the total momentum of the system.
The location of the ion rockets in Thunderbird 3 is correct: the three side arms would provide the electric field through which the ions are accelerated from the top to the bottom. Inside the main chemical rocket nozzles, electrons would recombine with the ions to become speeding atoms that leave Thunderbird 3 through the nozzles. And Newton’s Third Law provides the forward movement (reaction) of the rocket in response to the backward force of the exiting atoms.
To have three thrust engines in the three nacelles is not the most stable of configurations. ("Nacelle" means "small boat" and comes from the French language from which many aeronautical terms stem as the French explored flying in the early years. "Fuselage" is another, meaning "spindle shaped"). The centre of mass of the vessel obviously lies somewhere along its central axis connecting its top and tail. The three nacelles are positioned symmetrically around the hull and axis and when all engines deliver the same thrust, Thunderbird 3 will speed straight forward. If the thrust of one of the engines deviates from the other two, or if one or more of the engines fail, the ship will experience a net force outside the centre of mass, causing it to move along a curved trajectory instead of going straight on.
A publicity shot of Thunderbird 3 using only two of its engines depicts a situation that will eventually be disastrous for the spaceship. Look at the drawing in Figure 15 where only one engine works. It provides thrust on one side of the spaceship. The line along which this thrust force works does not run through Thunderbird 3's centre of mass. This centre will be on the axis of symmetry and somewhat down the middle since most of the mass is concentrated in the lower part of the rocket. Because the line of force of the rocket is at a distance from the centre of mass, this distance works as a lever by which the force starts to tilt the rocket and move it sideways (to the right). It keeps doing this, so the rocket moves in a circular orbit.
For this reason a more symmetric set of thrust rockets is preferable. When each of the three rockets is balanced by a similar one on the other side of the rocket, a more stable configuration is obtained. When one rocket fails, the spaceship computer can automatically switch off the opposite rocket too to maintain a straight course. And Alan will be notified of course. The remaining rockets provide a balanced forward thrust on all sides of the spaceship. Having twice the amount of engines, the available thrust also doubles. This makes the spaceship faster (but Alan may not be able to endure the higher acceleration). A set of rockets arranged in a circle around the circumference of the main fuselage would be even more stable, in light of the fact that there cannot be a main thruster rocket in the middle since the astronauts board the ship that way.
