Gravity Wheel

The main areas where people reside and sleep on board the Gen1 Enterprise will have 1g artificial gravity by using a gravity wheel. Inside this gravity wheel (a centrifuge), the downward force on a person is comparable to the force of gravity on Earth. The idea of a rotating wheel to emulate Earth’s gravity has been around since the start of the 20th century. One version, as seen below, was envisioned by Wernher Von Braun. The movie 2001: A Space Odyssey of course also featured a large and visually spectacular gravity wheel.

The gravity wheel in the Enterprise rotates within the closed saucer hull. This is accomplished by magnetically suspending the wheel as it rotates inside a sealed donut-shaped cavity that is formed along the interior perimeter of the saucer hull. The wheel touches no other parts of the ship as it continuously rotates at a constant rate. Click on the video above to see it rotate.

Image credit - NASA

Image credit – NASA

The shape of the Enterprise is a nice fit for accommodating this gravity wheel. Since the saucer section is large in diameter, this allows the wheel to rotate slowly and not make people in the wheel feel ill from motion sickness. The outside wheel diameter will be around 1480 feet. Research has shown that this diameter gives a gravity environment that people will be comfortable with.

The saucer hull is also not very thick when compared to its large diameter. The ratio of the diameter to hull thickness at the saucer perimeter is about 20:1. Keeping the gravity wheel thickness relatively thin reduces its mass and makes building it into a spaceship more practical. Thus, the shape of the Enterprise fits nicely with the idea of having a gravity wheel inside of it.

The gravity wheel is made up of many sections that are connected together. Each section of the wheel is autonomous in its functioning and is referred to as a pod. The pods are connected together to form a wheel so that the wheel is a kind of train of pods connected together in a continuous loop. Each pod is autonomous in the sense that it has backup power, water, food, and oxygen. This is for in the case of an emergency such as if the ship is struck by a meteor. Each atmosphere inside of each pod will be sealed and connected to other pods only through air locks.

Around the entire exterior surface of the gravity wheel that faces toward the interior of the stationary saucer hull are mounted permanent magnets. Facing these magnets, and mounted on a stationary support ring fixed to the stationary saucer hull, are rows of permanent magnets and electromagnets. These permanent magnets and electromagnets oppose the magnets mounted to the wheel in a way to accomplish magnetic suspension. The electromagnets constantly have the currents in them altered to adjust their magnetic fields. The magnetic fields are constantly adjusted to cause nearly the same gap size to be maintained between the outer wheel surface and the wall of the stationary donut-shaped cavity in the saucer hull. Sensors and computers constantly monitor this gap so that the needed current adjustments to the electromagnets can be precisely determined.

The electromagnets interact with the permanent magnets mounted on the gravity wheel to cause the wheel to be suspended but also to rotate the wheel at a constant angular velocity. This means that the wheel and the system of permanent magnets and electromagnets form a giant electric motor. And the moving part of the motor – the gravity wheel – has no friction with any part of the saucer hull as it spins. Since the wheel does not touch anything inside the saucer as it spins, this eliminates any mechanical wear on parts of the wheel. This also eliminates audile noise created by mechanical bearings in the wheel that might be constantly heard by human inhabits inside the wheel.

Superconducting materials can be considered for reducing the electrical energy consumed by the electromagnets. Since superconducting materials maintain a constant gap between themselves and permanent magnets, by simultaneously attracting and repelling the permanent magnets, they may be a good technological fit for helping implement the magnetic suspension of the gravity wheel.  Similarly, superconducting wire might also be used as the windings in the electromagnets to improve their magnetic output power and energy efficiency.

Besides the forces applied by the electromagnets to  maintain the  gap between the gravity wheel and the walls of the saucer hull cavity, a force is needed to counter the constant force pushing the ship forward as the ion propulsion engines cause the constant acceleration of the ship. Rather than use the electromagnets for this, the permanent magnets near the front of the ship can be mechanically moved forward some to nullify the force that is trying to push the ship into the gravity wheel as the ship accelerates. On the other hand, superconducting magnets may eliminate the need to move the permanent magnets since they tend to lock a fixed gap in place. Researchers will have to determine the best approach to the overall system of magnets for controlling the gravity wheel.

Regardless of how much the superconducting materials may help in suspending the gravity wheel, it is very doubtful that they could enable the elimination of the electromagnets altogether. Electromagnets will be needed to constantly fine tune the position of the gravity wheel so that it remains stable and has no vibrations that might be felt by people inside the wheel. Also, in emergency situations the electromagnets can act as brakes to stop the wheel entirely.

Since the wheel is large in diameter, the angular velocity of the rotating wheel is slow. The wheel only spins 2.0 times per minute (one rotation every 30 seconds). However, the speed that the wheel passes a person standing near it inside the stationary saucer hull is around 100 miles per hour. At this speed, wind becomes a factor for the gravity wheel. Unwanted sounds from wind rushing past the sides of the pods that form the wheel become a concern since this will be heard inside the pods. Also, wind means that more power must be applied to the electromagnets to overcome the wind resistive force for keeping the wheel spinning and stable. This is a waste of energy.

To eliminate the wind noise and the extra power to the electromagnets, the donut-shaped cavity containing the wheel will have the air pumped out of it to create a near vacuum. This will also prevent any audible noise from mechanically coupling to the wheel structure from other parts of the ship such as any hum from the ion propulsion engines as they run continuously.

A calculator is available on the internet for readers interested in trying different wheel diameters to see the angular velocity and surface speed of the spinning wheel. Below is the output from the calculator based on the plan for the Enterprise as discussed above.

The results below are a screen grab from the SpinCalc calculator by Theodore Hall.

Image credit - Theodore Hall's SpinCalc Calculator

Image credit – Theodore Hall’s SpinCalc Calculator

The wall of the donut-shaped cavity facing the vacuum has a gap of approximately one foot to the gravity wheel at all points. This large gap is to assure that after any slight drifts of the wheel from perfect alignment during rotation, the wheel will never touch the wall of the cavity. One foot of course is only an estimate, and research will be needed to find the true optimum value.

The gravity wheel will have an emergency shutdown mode where it can stop quickly, but the goal is that this will never be used over the life of the ship except during periodic testing.

The interior of each pod will look very different from any other pod. And the function of each pod will vary. In general, diversity should be the goal to keep the pods interesting to visitors as well as practical. Here are examples of nine different pods that might be in the gravity wheel:

  1. One pod will contain the command bridge (see example looks below) for the ship where the captain, key officers, and key crew members will often work. Various other compartments inside this pod will be where many other crew members work for controlling the ship and doing tasks needed during missions.
  2. One pod will be a dining hall and have restaurants as well as various entertainment.
  3. One pod will have a gym for recreational use. It can also be configured as a place where spectators can watch sports such as a basketball game.
  4. One pod will be for sleeping quarters.
  5. One pod will be for growing food.
  6. One pod will be for a relaxing nature walk among many plants and small ponds. Thus this pod is a park.
  7. One pod will house a space museum, an Imax theater, and other movie theaters.
  8. One pod will have extensive live video feeds, and recorded video, from onboard sensors including the Enterprise’s telescopes.
  9. One pod will house machining and manufacturing facilities. (These are sister facilities to the facilities within gravityless sections of the saucer hull.)

Below are two examples of how the bridge of the Enterprise might look. The first one is from the original Star Trek series on television. The second one is from the 2009 movie entitled Star Trek.

Image credit - CBS (for original Star Trek TV series)

Image credit – CBS (for original Star Trek TV series)

Image credit - Paramount Pictures (from 2009 Star Trek movie)

Image credit – Paramount Pictures (from 2009 Star Trek movie)

Each pod will be very large and will have many compartments. Walking through all pods until you arrive back at where you started is a walk of 4647 feet, nearly a mile. This comes from pi x 1480’ = 4647′. So if there are nine pods, each is 516 feet in length when walking on the floor of it. Since each pod is large, each could have more than a single purpose, unlike in the list of example pods given above. For example, a single pod could have sleeping quarters, restaurants, and recreational facilities. The final pod configurations will be a subject of considerable investigation during the Enterprise development.

All of the pods in the Enterprise will be elaborately developed. The ceilings inside of most of the pods will be 100 feet high. This gives the possibility for creating dazzling curved ceilings that visitors will enjoy seeing. It’s well known that very high ceilings, as have been long used in cathedrals, create a sense of spaciousness and wonder in people inside the rooms with these ceilings. The pod designers should take advantage of this. This builds on the theme that the Enterprise should inspire us, and certainly the gravity wheel living areas should be dazzling and inspiring.

Each pod’s internal design can range from being rather Earthlike in its architecture and living spaces to incorporating futuristic and fantasy themes as seems appropriate for the inside of Earth’s first immense and fantastic space super-ship. The goal is to make the width-limited pods, with their high curved ceilings, into interesting places to look at. People have already dreamed up many ideas for what the inside of a gravity wheel in space might look like. Below are some examples .

Image credit - NASA (called the Stanford Torus)

Image credit – NASA (called the Stanford Torus)

Image credit - NASA (called the Stanford Torus)

Image credit – NASA (called the Stanford Torus) 

Image credit - unknown (internet)

Image credit – unknown (internet)

Image credit - unknown (internet)

Image credit – unknown (internet)

Image credit - unknown (internet)

Image credit – unknown (internet) 

Each pod will be double walled as shown below.

Gravity Wheel Pod walls

The stationary donut-shaped cavity inside the saucer hull where the wheel floats and rotates will also be double walled. This means that from the inside of a pod to space outside the ship will have a total of four walls. This exceeds the requirement for a minimum of triple redundancy as required for all aspects of the Enterprise ship design. The four walls are shown below.

The floor of the pods, where people walk, cannot be only .5” thick. It is undesirable for people to have the sense that they are walking on a thin sheet of material due to the sound and feel of it as they walk. Also, a composite floor will not be too esthetically appealing over a large area. Thus the .5” thick composite floor will in most places be covered with various flooring materials to make the surface more appealing to people and also less prone to accidental puncture. Some floor surfaces may appear similar to those on Earth by using materials such as light-weight concrete, wood flooring, and dirt with grass. So for the inside of the hull, the last figure above is modified slightly to show the innermost wall as the floor of the pod as shown below.

USS Enterprise Flooring

The walls throughout the ship are kept as thin as possible to keep the ship’s mass low. With a ship this large, even a slight increase in wall thicknesses can add millions of pounds of mass to the ship. While the hulls of the Enterprise generally use .2” thick walls (excluding ribs or other structures to add strength), hull walls in or near the gravity wheel will generally have .5” thick walls as shown in the figures above. This increases the wall strength for the rotating pods which is desired so that each pod in the gravity wheel is safer. In fact, each pod could float free in space and the inhabitants would still survive. This fits with the spirit that each pod is an autonomous entity unto itself. Of course a pod will never float out in space unless a disaster happens such as a large meteor strike on the ship that causes the gravity wheel to break up. In this case some pods might get ejected from the ship and go off into space. It may be a stretch to think that a pod should be designed to be able to sustain itself in space until rescue teams bring it back to the Enterprise, but that is something for future development teams to ponder. Just how autonomous a pod should be is an open question.

The thicker double walls for the pods, compared to the .2″ thick walls for most of the ship’s walls, have a second purpose. The thicker walls increase the radiation shielding for humans and electronic equipment inside the pods. The net thickness of all walls from the inside of a pod to outside in space, when ignoring any flooring materials, is 1.625 inches (41.3mm).  This achieves 6.25 gram/cm2 for radiation shielding.

If technically possible, two pods will have active shields to significantly improve the blocking of radiation from space. One pod will be where the crew mostly works and where the bridge is located. The other will be the pod for sleeping quarters, eating, and recreation. The active shields are needed mostly for when the ship is on a long duration mission outside of Earth’s orbit and thus outside of Earth’s protective magnetic field.

Finally, we derive the number of people that can be onboard the Gen1 Enterprise within the pods of the gravity wheel. 400,000 square feet of human habitat space inside the pods will exist as derived below. Also derived below is that the pods can hold up to 1000 people.

  • (1500’ – 2×9’) x 3.14 x (75’ – 2×9’) x .80  x 1.885 = 400,000 square feet
  • 1500’ is the saucer hull diameter
  • 75’ is the saucer hull thickness
  • 3.14 is pi
  • 9’ accounts for the wall air gaps between the inside of a pod and the outside of the ship
  • .80  factor accounts for dead spaces between pods
  • 1.885 factor accounts for some pods having more than one deck
  • Assume each person at any one moment needs an average of 20’ x 20’ = 400 sq feet.
  • 400,000 / 400 = 1000 person capacity