The Orbital Convention - Design and Documentation Conventions for Rotating Habitats
How will rotating habitats be designed, and what kind of design views can be employed? I will begin by explaining how terrestrial buildings are designed and documented. Using this as an anchor, I will explore how rotating habitats are different and how they present an interesting challenge for visualizing the design views and construction documents. I will end by suggesting a convention for designing and documenting the design intent of these orbiting structures.
Background - Terrestrial Buildings
Terrestrial buildings start out as ideas. From there, a program is developed, research is conducted, sketches are made, the design is developed, and after many iterations and reviews, each with increasing detail, construction documents are submitted. While this process may change in various ways from project to project, the contractor needs the construction documents to know what to build (and to have a document that forms the basis for the contract with the client, hence the industry term "contract documents"). Apart from the specification book, these documents are largely comprised of several standard types of annotated isometric drawings that follow industry conventions. These drawing conventions help to ensure that the design information is documented in a way that is easy to build from and helps to convey design intent so there are fewer gaps in intent during construction (though there are always going to be some gaps). The types of drawings conventionally used are as follows:
Plans
The most common of the drawings, plans are formed by "cutting" the building with a plane that is perpendicular to the ground plane. This type of drawing shows the main organizational logic to a building, the structure, and most of the openings that will be accessible to people occupying the building. Plans usually show what is beyond the plane of the cut, but this is not a necessary quality.
Sections
Sections are conceptually similar to plans in that they involve "cutting" the building, but sections differ from plans in that the plane of the cut is perpendicular to the ground plane. Sections often show only what the plane is cutting through and omit components beyond the cut plane. Sections are often further distinguished from each other by being longitudinal (wherein the cut plane is parallel to the "long" axis of the building or portion thereof) or transverse (wherein the cut plane is perpendicular to that of the longitudinal section), though these distinctions are not universally applicable. The purpose of a section is to convey the vertical relationships between the various components, which represents design intent that cannot easily be documented in other types of drawings.
Elevations
Elevations are similar to sections in orientation, but they differ in that they do not usually "cut" through anything and they only show what is beyond the cut plane. Elevations are used to document the design of the vertical surfaces, such as indicating the component types and materials.
The building components drawn in the aforementioned views all have a specific location in 3-dimensional space relative to all the other components. These locations must be documented in the drawings for clarity, consistency, and to avoid catastrophic errors. Rather than referencing the real-space location of all the components, dimensions strings pull most of the weight of locating components. These dimension strings specify how far apart an element is from a known reference such as a grid line or a level. Grid lines represent a 2-dimensional plane that is always perpendicular to the ground plane, while the levels are always parallel to the ground plane. Grid lines are usually labeled with letters and numbers (letters usually perpendicular to numbers), while levels simply indicate how far from the first floor level or ground level they are. By dimensioning components to these grid lines and levels (which are all each dimensioned to each other), their location in space can be documented and correctly understood.
Rotating Habitats
From a documentation standpoint, the main quality of rotating habitats in space that distinguishes them from terrestrial buildings is that rotating habitats have no ground plane to use as a basis for orienting the drawings. Furthermore, even if the rotating habitat were located on Earth or otherwise had a ground plane to reference, such a reference would not produce helpful drawings for design and certainly not for construction. This is because the conventional drawings mentioned in the previous section rely conceptually on the presence of conventional gravity and the concept of up and down as an absolute and unidirectional constant. Rotating habitats have up and down, but they are not absolute and unidirectional across the structure.
Locating components in 3-dimensional space is an arguably greater problem. "Vertical" walls that are not perpendicular to the axis or rotation cannot be located using standard orthogonal grids. This is because the "vertical" direction is constantly changing along the circumference because the floor is curved into a circle. That is to say, most vertical walls are not all parallel to each other. The wall-to-wall dimensions change depending on how far up the wall one measures. This all makes for a design that is quite difficult to interpret correctly using terrestrial conventions, to say the least.
The Orbital Convention
With a few modifications to the basic assumptions about drawing geometry and relationships, rotating habitats can be designed and documented in a similar way to terrestrial buildings. These conceptual modifications involve the nature of and relationship between grid lines / levels, and also the extension of isometric projection to include the parallel depiction of converging planes (ie: making non-parallel elements appear parallel).
Radial Plans
Radial Plan
Locating components in 3-dimensional space is an arguably greater problem. "Vertical" walls that are not perpendicular to the axis or rotation cannot be located using standard orthogonal grids. This is because the "vertical" direction is constantly changing along the circumference because the floor is curved into a circle. That is to say, most vertical walls are not all parallel to each other. The wall-to-wall dimensions change depending on how far up the wall one measures. This all makes for a design that is quite difficult to interpret correctly using terrestrial conventions, to say the least.
When represented in full, the floor plan of a rotating habitat would appear as a long strip with matchlines (imaginary lines that represent the same location in space) at each end. For ease in digestion and formatting, this overall plan would likely be broken up into several enlarged plans. As a standard, these radial floor plans should be oriented so that the anti-spinward direction is at the top and the spinward direction is at the bottom.
This type of view also includes the inner and outer exterior faces. The inner face of the structure is the face that is closest to the axis while the outer face is the face that is furthest from the axis (fully enclosed habitats do not include an inner face. The inner face is analogous to the roof plans of terrestrial structures, and while there is no direct terrestrial analogue to the outer face (as the bottom of the floor slab is not usually a concern on Earth) it might be described as combining the qualities of sub-grade plans, reflected ceiling plans, and elevations. These faces should be cataloged under the "plan" category because they would best be represented in the same orientation. If a distinguishing descriptor must be coined, the inner exterior face plan can be referred to as the centrifugal plan, while the outer exterior plan can be referred to as the centripetal plan.
Sections
Tangential Section
Axial Section
The "vertical" cuts through rotating habitats will be slightly different from (yet analogous to) the longitudinal and transverse sections of terrestrial structures because, in contrast to their flat, terrestrial counterparts, the perpendicular sections of rotating habitats are qualitatively different from each other.
A tangential section is a section cut perpendicular to the tangent of the circumference (ie: like cutting through the side of a ring). This would appear similar to a transverse section, with the omission of the ground of course. As a standard, tangential sections should be cut looking anti-spinward, with the direction of the axis towards the top.
An axial section is a section cut perpendicular to the axis (ie: like cutting a ring into two smaller rings). In contrast to the tangential section, the axial section would show the curvature of the floors and other "horizontal" assemblies. Similar to the radial floor plans, these sections should also be divided into more easily digestible "segments". As a standard, axial sections should be oriented so that the spinward direction is to the left and the anti-spinward direction is to the right, with direction of the axis towards the top.
Elevations
The elevation views of rotating habitats include the surface that has thus far not been mentioned, that is the exterior "sides". There are two of these faces when viewed parallel to the axis. To distinguish them in an intuitive manner, they can be distinguished by their direction of rotation. One face can be referred to as the clockwise elevation, while the other can be referred to as the anti-clockwise elevation (relating to the direction of the spin). These elevations should also be broken into segments as are the axial sections. The clockwise elevation would appear much like the axial sections, while the anti-clockwise elevations would show the spinward direction to the right and the anti-spinward direction to the left. As a standard, these elevations should (as the axial sections) be oriented with the direction of the axis at the top.
Levels
In the same way that terrestrial buildings have an absolute zero point where they can base their levels (the ground plane), rotating habitats need a similar absolute zero point to reference. The central axis (the imaginary line about which the structure rotates) works wonderfully to fill this role. Instead of distinguishing levels by how far from the ground plane they are (as in terrestrial structures), "levels" can reference how far from the axis they are (ie: their radius). In this way, multiple, coaxial, cylindrical planes can be defined. These cylindrical planes can form the base point for radial floor plans.
Grid Lines
As opposed to terrestrial structures where all grid lines usually have the same characteristics as each other, there are two different kinds of grid lines that can be used on a rotating habitat: axial and tangential. Axial grids are drawn perpendicular to the axis of rotation while tangential grids are drawn perpendicular to the tangent of the circumference. These distinctions are analogous to the perpendicular grids of terrestrial habitats. In fact, angled grids sharing a centroid are sometimes employed in terrestrial structures involving circular curves.
BIM
Building Information Modeling will no doubt play a huge part (more so than in any terrestrial building) in the design and implementation of rotating habitats. Every component should be scheduled and every assembly detailed. That being said, there is no BIM software that can accomplish what is required for designing using the aforementioned conventions. Because of this, new modeling software will have to be developed for this purpose.
Conclusion
These conventions may lay the groundwork for future standardization of design drawings and construction documents for rotating habitats. As such, I will be working towards presenting these conventions formally and requesting comments from industry professionals. As the need arises, it is my hope that these conventions are considered for adoption by the relevant standardization bodies.
