Design example  

SPACE collapsible tube mast

Long booms or masts are frequently required on spacecraft for a variety of purposes, such as supports for solar arrays, as antennas, or to locate experiments remote from the spacecraft environment. Prior to, and during, launch the masts must be stored in a compact form on board the spacecraft. There may also be a need to retract and redeploy masts when in orbit, for example to minimize the angular momentum during changes in the attitude of the spacecraft. A variety of designs of extensible or foldable masts have been developed. Some are based on a series of telescopic tubes, others make use of the collapsible tube principle consisting of a thin walled tube of elastic material, split longitudinally, which is opened out flat and coiled for launch. When uncoiled, it reverts to a circular cross-section and in some designs, provision is made for the edges of the split to interlock mechanically to increase the rigidity. The collapsible tube mast (CTM) is a closed tube of lenticular cross-section that can be flattened and then reeled on a drum for storage in a relatively small volume. Collapsible masts made from thin metal sheet were explored in the United States space programme in the 1960s. 

Design details

A design for a lenticular collapsible tube mast is shown in cross-section below.

Collapsible Tube Mast section

It consists of two channel sections, each composed of circular arcs and joined at the flanges by welding or adhesive bonding. The tube shape and size are characterized by four parameters:

• The shape radius (r)
• The shape angle (f)
• The flange width (b)
• The wall thickness (t)

Variation of the shape angle between 0 and 90° produces a family of cross-sections, with small values of f yielding flat tubes, and large values giving rise to a more bulbous shape. At small angles, the bending stiffness EI_Z is higher than EI_x, and in practice the tube is often required to have approximately equal stiffness about the x and z axes. This occurs for f = 1.4 radians, but to provide equal buckling resistance about the x and z axes a slight compromise is necessary and this is given by a value of 1.3 radians (~75º). In order to maintain the flattening strain to within acceptable limits (0.4%), the wall thickness for a shape radius (r) of 20 mm needs to be approximately 0.16 mm. To keep the overall stress level low, strains should be less than 0.25% and this gives the radius of the coiling drum R = 64 mm. The values of r and t, together with the external volume of the storage compartment that can be accommodated in the spacecraft, define the number of turns and hence the length of mast that can be carried. As an example, approximately 60 m of mast can be carried in a storage volume of approximately 24 litres using the above dimensions. The width of the flanges is determined partly by the shear stress between the tube halves, partly by the need to minimize the overall storage volume, and partly by the requirement that sufficient width must be provided to enable the deployment mechanism to engage. 

Deployment Mechanism

The deployment mechanism is shown below.

CTM storage box and deployment mechanism

Material and fabrication method

The stresses induced by flattening and coiling, together with consideration of longitudinal stiffness for the mast dictate a material with orthotropic properties, and the need to fabricate masts some tens of metres in length suggests a plain weave fabric as the most convenient feedstock material. In considering methods for fabricating long lengths of mast, truly continuous processes such as pultrusion and roll-forming must be rejected because of the risk of damaging or distorting the very thin fabrics required. An alternative technique is sequential moulding where high longitudinal stresses inherent in, for example, the pultrusion process are avoided. The principle of the method, as developed for the production of continuous profiles from metal or ceramic powders, is shown below.

Principle of sequential moulding compaction

Adaptation to the formation of the mast half-profile is straightforward using the open-ended die-set illustrated below.

Experimental die setup

Production

Production of the mast takes place in two stages. In the first stage, illustrated below, preimpregnated fabric from a storage reel is sandwiched between two carrier strips of aluminium foil, 0.24 mm thick, which are treated, on their inner surfaces, with a release agent to prevent adhesion to the feedstock.

Schematic of CTM forming stage

After passage through the press, excess width is trimmed from the flanges of the moulded profile by a pair of guillotine blades mounted symmetrically about the apex of the profile. When a thermosetting matrix is used, a partial post-cure is given prior to removal of the carrier strips, whereupon the half-profile is flattened and reeled on to a storage drum. In the second stage, two identical half-profiles are bonded together at the flanges to form the mast.

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