4.5 Mechanisms

For the beginner structures and mechanisms engineer, this section will provide a brief overview of the various components and what they’re used for in spacecraft. These mechanisms are typically very risky as they have a significant rate of failure. As they are so risky, systems engineers prefer not to use active mechanisms unless necessary. Mechanisms may be critical for engineering solutions or science applications, which will be discussed below. Thus, deployers are rigorously tested on the ground in gravity offloading testbeds, which simulate microgravity by placing wheels underneath a structure or stringing cables from the structure to the ceiling to compensate for gravity.

Deployers

Deployers or deployment mechanisms transform a packaged spacecraft into its operational form. The common need for all deployers is the desire to achieve a different geometry than is feasible with the rocket fairing volume constraint. Deployers can achieve great lengths (booms), large surface areas (solar panels or solar sails), or immense volumes (habitation modules).

^{This is the ISS S4/S4 solar panel deployment. Video Courtesy of NASA via YouTube.}

Booms

Booms are typically used to take advantage of length extension. This length extension could offer spatial isolation, like mitigating electromagnetic noise for a magnetometer on the tip of the boom. A boom could also offer geometric placement for optics, as a shade or occluder. Booms may also be used to manipulate spacecraft dynamics. A boom can modify the moment of inertia of a spacecraft to create spin stability [Pankow et al.] or mass distribution of a spacecraft to create a gravity differential to preference an orientation [Kowalski et al.].

^{Deployable Composite Booms (DCB). Video courtesy of NASA.}

Solar Panels

Solar panels rely on the surface area to generate power. Some spacecraft, like our Artemis Cubesat kit, have solar panels on most faces of the spacecraft structure. But some spacecraft have opted to extend solar arrays away from the primary structure to get as much surface area and thus as much power as possible. This level of power generation may be critical to fulfilling mission requirements. These solar arrays can’t fit in the rocket fairing as is so the solar panels must be folded close to the primary structure and deployed once in orbit. Solar panel hinges and motors deploy these solar panels to their full extent. Vipavetz and Kraft give great lessons learned as to the reasons solar panel arrays have historically failed grouped into mechanical loading, on-orbit space environment, tribology (mechanisms and lubricants), and basic systems engineering.

^{Glory Solar Array Deployment. Video courtesy of NASA.}

^{Engineering with Origami. Video by Veritasium} 

Light Sails or Shades

Light sails or shades are deployed much like thin booms with the addition of unfurling a thin sail. The careful folding, like origami, of the sail, is ingenious. The sail is made of an incredibly thin mylar material that could risk tearing with poor fabrication or assembly. This surface area is necessary for a light sail to capture as much linear momentum from photons as possible, as the individual exchange from a single photon is not much, but the summation across a large surface area can propel a small spacecraft.

^{The full video can be seen at Raw video: LightSail solar sail deployment test. Video Courtesy of The Planetary Society.}

^{The full video can be seen at James Webb Space Telescope – Unfurls. Video courtesy of Northrop Grumman.}

Antennas

Antennas, like the radar antenna on RainCube, require a parabolic dish shape that is too large to be launched as is, thus they must be compacted and deployed after launch. A small business, named Freefall Aerospace, has created a lightweight, low-volume stowed spacecraft antenna that is inflatable, bypassing rigid deployment. MarCO-A and B are “our first and second interplanetary CubeSats”, enabled by a deployable high-gain, X-band antenna flat panel.

Inflatable Space Habitats

Inflatable space habitats are deployable modules for crewed space. The ISS has an expandable habitat called the Bigelow Expandable Activity Module that has been operational since 2016. These habitats are pressurized structures and provide a greater volume of living space [Wikipedia]. There are proposed uses of inflatable habitats on planetary surfaces but no instances yet, only use in space.

Restraints or Launch Locks

Restraints or launch locks restrain the payload and isolator during spacecraft launch. The spacecraft interfaces with an adapter or dispenser system of the launch vehicle commonly called the Mechanical Lock System [Eurockot]. The adapters and interfaces vary with the spacecraft and rocket, but there are some standards associated with the size. For example, CubeSats can rely on the PPOD deployer for mechanical interfacing.

Separation Mechanisms

^{Glory Solar Array Deployment. Video Courtesy of NASA.}

Separation mechanisms disconnect the spacecraft from the launch vehicle once in the proper orbit. There are many options for separation mechanisms: clamp bands, motorized light bands, Marmon clamps, dispensers, and custom systems [Spaceflight]. The best technical solution depends on the size of the spacecraft, launch vehicle provider, allowable shock, and typical tip-off rate. Separation mechanism characteristics include the imparted velocity in the axial and lateral direction, spin rate, umbilical connectors to supply power or data, allowable volume or length dimensions, and any applied loads.

Ordinance Devices

Ordnance is an explosive device that enables the sudden release of spacecraft. Ordnance systems initiate important discrete events, like lift-off, stage separations, spacecraft separation, and flight termination [ULA launch]. Ordnance often incorporates the use of explosive bolts, or pyrotechnic fasteners, in separating different stages of the launch vehicle and spacecraft. An explosive charge separates the bolt at a specified break plane. The explosion can be the result of explosive detonating material or a pyrotechnic pressure-generating material [Pacsci EMC].

^{Hi-Shear Explosive Bolt P/N 55-07057-1 HACL Film 554 [film] San Diego Air and Space Museum Archives. Film from the Atlas-Centaur Heritage Film Collection was donated to the San Diego Air and Space Museum by Lockheed Martin and United Launch Alliance. The Collection contains 3,000 reels of 16-millimeter film.}

Spin Bearings

Dynamics and control of dual-spin gyrostat spacecraft with changing structure. V. Aslanov, V. Yudintsev.

An example of spin bearings, or ball bearings, in space, is a dual-spin spacecraft. Two rigid bodies are connected by a bearing that allows the two bodies to rotate at different rates. The spinning of one body stabilizes the other body so that the payload may track or point. Bearings notoriously fail in space, although specifically reaction wheel bearings and not passive mechanical joints. Bearings naturally wear over time, accumulating friction and potentially jamming up. Some bearings fail due to bearing damage caused by electrical arcing [AEGIS]. Rotating joints also increase complexity in design. “When electrical power or signals must be passed across a rotating interface, a slip ring (sliding electrical contact) or twist capsule (specialized flexible harness) is required” [Honeybee Robotics].

Scan Platforms

Some payloads require sweeping across a field of view to collect swaths of information. To conduct a sweep, the payload may be mounted to a scanning platform. A famous example of a scan platform is on the Voyager spacecraft. The payloads look away from the rotation axis and collect visible UV and IR data along a horizontal plane. Another sample mission is SENTINEL-1, which carries a synthetic aperture radar instrument that scans quickly along elevation and azimuth.