2.4 ADAM mast
2.4.1 Applications of ADAM
2.4.1.1 IPEX-II
IPEX-II flew on the Space Shuttle Discovery in August of 1997. Its purpose was purely the study of truss behavior in the space environment [27]. One of the primary goals was to observe thermal snap in orbit, so it was outfitted with high-sensitivity accelerometers. It demonstrated that thermal snap (slipping against static friction in a joint due to thermal expansion) occurred in an ADAM mast when it moving into and out of sunlight. The IPEX-II mast has also been used as test hardware in a number of studies on mast dynamics [34] and micromechanics. Hardaway and Peterson [18] used the IPEX-II mast to demonstrate that a jointed bay, under shear loads much lower than those that would cause frictional slippage, would produce very subtle vibrations in its shear mode. This was attributed to “the sudden release of strain energy stored in the hysteretic mechanisms and/or the materials of the structure.” [18, p. 2076]
An early version of ADAM, IPEX-II was not produced for a specific structural use and lacks certain features of later systems, such as a reliable latch backstop. It was used for some preliminary testing in this project, but the WSOA sample mast was preferred because it is a later model with a variety of performance advantages.
2.4.1.2 Shuttle Radar Topography Mission
The Shuttle Radar Topography Mission (SRTM) was flown by the Space Shuttle Endeavour as a part of STS-99 in February of 2000. SRTM used an ADAM boom to create a long 60-m separation between two parts of its interferometric synthetic aperture radar system [17]. This radar system was used to generate an elevation map of the majority of the earth’s surface in greater detail than had been previously available.
SRTM demonstrated the necessity of on-orbit monitoring of structures. With a 60-m-long boom (Figure 2.11), its normal modes could cause significant dynamics, and estimates produced before launch were expected to be accurate to only 20% [52]. Further, a damping subsystem failure damaged the dynamic behavior of the mast, requiring the use of redundant systems to maintain control of the boom. Umland [49] presented an investigation of the damping subsystem failure, which is believed to have arisen from the expansion of a component of the damping system between testing and use.
The leading explanation was that a combination of incorrect tolerances and stress relaxation lead to the closing an unintentionally small clearance in the damping canisters, locking them. A secondary theory was that the damping canister material may have expanded by absorbing silicone fluid. In
Figure 2.11: The SRTM mast (NASA image [33]). The mast is approximately 1 m wide and 60 m long when fully deployed.
either case, Umland describes a failure that resulted from minute changes in a tightly toleranced moving part during the time spent in storage.
Despite this difficulty, the SRTM experiment produced an altitude map of 80% of the earth’s surface. Calibration of the radar data [52] required simultaneous tracking, through the metrology system, of the position and attitude of the outboard antenna. GPS and ground telemetry for the shuttle orbiter were also integrated for production of the final map.
At the time of this writing, the SRTM mast is on display in the Smithsonian Air and Space Museum[44]. It is, at 60 m, the longest rigid structure ever flown in space [33].
2.4.1.3 NuSTAR
Figure 2.12: Artist’s image of NuSTAR, with the optics at left and detectors/spacecraft bus at right (NASA image [5]).
NuSTAR (Figure 2.12) is an x-ray telescope, scheduled to launch in 2012, which will dramatically improve astronomers’ ability to resolve objects in the hard x-ray spectrum. It will take observations in the range of 10-79 keV with 40 arcsecond resolution [5]. X-ray focusing optics, unlike visible light optics, rely on reflective elements oriented at grazing angles to the incoming rays [23], and the focal length of this telescope is consequently quite long in comparison to the diameter of its optics. The optics will be deployed and held 10 m away from the detectors by an ADAM boom.
NuSTAR is the highest-precision use of this technology to date, and also arguably the highest- precision application currently possible. The focusing requirements of this system are not very stringent, requiring accuracy of millimeters in the axial separation between the optics and the detectors [28].
Because the absolute deployment location of the mast is difficult to measure on the ground, due to complications associated with complete gravity offloading, an adjustment mechanism is built into the last section of the mast to enable a one-time alignment to optimize the location of the optical axes on the focal plane. This mechanism provides two angular adjustments as well as rotation. The mast is not perfectly rigid, but undergoes thermal distortions particularly when going in and out of Earth shadow (the mission is deployed in a low-Earth orbit) that translate into changes in telescope alignment of 1–2 [arcminutes].[19, p. 5]
Moving beyond the demands of NuSTAR will require even more predictable structures. Previ- ously neglected mechanical effects need to be enumerated, characterized, and included in predictive modeling. The remainder of this thesis will observe and model some of these under-characterized effects.
Chapter 3
Experimental properties of mast components
This section presents experimental measurements of the longeron ball-end joint friction, the in situ cable preload, the cable stiffness, and the latch behavior.
Characterization of the latches occupies a substantial portion of this chapter because they are the most complex single part on the mast, and because their behavior is very difficult to model from first principles. Because the latches are typically under tension during use, and because of the relatively large displacements they can undergo, the support rigging for latch measurements required special attention.