A review on large deployable structures for astrophysics missions

Abstract

As the performance of space based astrophysics observatories is directly limited by the size of the spacecraft and the telescope it carries, current missions are reaching the limit of the launchers’ capabilities. Before considering to develop larger launchers or to implement formation flying missions or in orbit assembly, the possibility of deploying structures once in orbit is an appealing solution. This paper describes the different technologies currently available to develop deployable structures, with an emphasis on those that can allow achieving long focal lengths. The review of these technologies is followed by a comparison of their performance and a list of trade-off parameters to be considered before selecting the most appropriate solution for a given application. Additionally, a preliminary structural analysis was performed on a typical deployable structure, applied to the case of a mission requiring a 20 m focal length extension. The results show that by using several deployable masts, it is possible to build stiff deployed structures with eigen frequencies over 1 Hz. Finally, a discussion on metrology concepts is provided, as knowledge of the relative position between the telescope and the deployed focal plane instruments is critical.

Introduction

The launchers’ capability is a major constraint in spacecraft design. Designs based on large rigid and non-deployable structures are constrained in size by the fairings’ dimensions. In the case of astrophysics missions, this directly constrains the size of the telescopes we can design: it sets a theoretical limit on their configuration, including focal lengths and aperture diameters. Given the difficulty in developing launchers capable of sending larger payloads, the sizes of our telescopes are bound to fit within the available fairings, meaning that their performance (angular resolution, collecting area, etc.) is intrinsically limited by the fairings’ size.

Unfortunately, this upper limit is already being reached, as attested by the following examples:

• The Herschel telescope has a 3.5 m wide mirror, which is nearly the maximum width allowed by a 4.5 m wide Ariane 5 envelope

• The XMM-Newton telescope (X-ray multi-mirror mission) has a 7.5 m focal length, which is nearly the maximum length allowed by a 10 m long cylindrical Ariane 5 envelope

• The Chandra telescope has a 10 m focal length, which is nearly the maximum length allowed by the 18 m long shuttle payload bay, which also needs to include the inertial upper stage used to reach the final orbit

Formation flying could be used when long baselines are required. However, mission designs based on a formation flying concept are characterised not only by additional operational flexibility (e.g. capability to vary the inter-spacecraft distance and/or to change the configuration of the formation in case of more than 2 spacecrafts) but also by additional development risk, test complexity and operations challenges. More specifically, formation flying implies the development of multiple, fully functional, spacecrafts, thus increasing the programme complexity and cost. Formation flying cannot be tested on ground on a full system level scale, while specific functional and performance tests may anyhow require large and specialised test facilities. Finally operating a mission based on formation flying is likely to enhance considerably the complexity of the control centre, requiring a large degree of onboard autonomy, including specific anti-collision procedures.

On this basis, deployable structures have the potential to offer an ideal solution for large apertures (> 10 m diameter) and medium baselines (from 15 to 50 m). Telescopes using such technologies will hence no longer be limited by the size of the fairing but rather by its usable volume in which the folded structures will need to be stored.

There are 2 main improvements astrophysics missions (with large telescopes) can gain from deployable structures:

• Increased apertures (e.g. larger collecting area of an infra red/visible telescope, higher angular resolution)

• Increased baselines and focal lengths (e.g. longer focal length of an X-ray telescope for better response at higher photon energy or an interferometer baseline)

Increase in the aperture of a telescope means deploying larger surfaces. This can be done with unfold and latch mechanisms deploying smaller surfaces (segments) or by directly folding one single large surface. This latter option can be implemented with membranes or thin shape memory composite surfaces.

Unfolding and latching segments [1], [2], [3], [4], [5] is the technology baselined for the James Webb Space Telescope (JWST). The limitations associated with this concept are the areal density of the segments (the volume occupied by the segments and their mass need to be optimised), the folded configuration of the segments and the deployment scheme (best packing efficiency with space left for the deployment procedure), the impact on the telescope point spread function due to the lack of a continuous mirror surface and post-deployment stability (typically micro-dynamic stability of the mechanisms will be required).

As an alternative, a large membrane can be deployed by either inflating an enclosed volume [6], [7], [8] or using deployable booms (they shall be discussed later on) to stretch it. It should be noted that inflated structures have typically a short lifetime unless they are rigidized after inflation. Additionally, any desired curvature and surface accuracy cannot be obtained with this technique. Electrostatic curvature could be used to control the surface figure, but this technology has only been demonstrated with reflective coatings for very large radii of curvatures and small apertures [8], [9].

In principle, we could think of combining these 2 concepts, but this would mean segmenting a large membrane, unfolding and latching these segments and electro-statically controlling their curvature. This could result in large light weight apertures with good surface accuracy and stability over long periods.

In principle, shape memory composites (SMC) could also be used to deploy large mirrors. This technology has already been used for large deployable antennas as they do not require stringent surface accuracies, while shape memory composites with a thin coating of reflective material for astrophysics applications have only been demonstrated for small apertures (<1 m diameter) [10], [11]. Severe challenges would have to be faced for larger sizes.

In order to achieve an adequate level of analysis, we will not discuss these concepts further in this paper. Rather, we will focus on the case of deployable structures used to increase baselines and focal lengths.

Increasing a baseline via a deployable structure can be applied to interferometer baselines or to focal lengths. Interferometers with a baseline under the length of a typical fairing have been envisaged (as in the case of the space interferometry mission (SIM)), [12], [13], [14]) as well as formation flying interferometers with much longer baselines (Darwin and the terrestrial planet finder-interferometer TPF-I, [15], [16]). Deployable baselines would lie between those 2 concepts. Extending focal lengths has also already been envisaged (International X-ray Observatory (IXO), [17]).

For both applications, an extension mechanism is required, typically in the form of boom or mast. The only difference lies in the need for an interferometer to extremely accurately control and measure this baseline as well as to repeatedly operate it by modifying its length within a specified range. Additionally, an interferometer would use a telescope at each end of the baseline (possibly weighing several hundreds of kg or more), while applications for focal length extensions would install the payload module at the end of the baseline (with a typical mass of order 1 ton). Increased baselines for focal length extensions are hence the first challenge to be met in the near future. The different extension mechanisms already available will be reviewed in Section 2.

While aiming at increasing the scale of future astrophysics missions by using deployable structures, one must not forget that the spacecrafts in their deployed configuration will still need to meet several requirements regarding the quality of the imaging system. Table 1 gives a set of typical astrophysics missions’ requirements based on previous missions depending on the observed wavelength range. The data presented in this table were extracted from the NASA [18] and the ESA [19] websites. Future deployable astrophysics missions will most probably be constrained by similar requirements.