Using Femap to make parts suitable for deep-space missions

Top precision and maintenance free reliability

To boldly go where no human has gone before was the mission of the starship Enterprise. Its aim was to explore strange new worlds, to seek out new life and new civilizations. While manned space voyages of this magnitude remain fictitious, mankind is sending out space probes to achieve these goals.

During their deep-space missions, typically lasting several years, the satellites are exposed to extreme environmental conditions. These include temperatures ranging from −160 °C to more than 350 °C and acceleration forces amounting to several g as well as high levels of various kinds of radiation. At the same time, there are no gravitational effects such as thermal convection. The on-board instruments and associated fixtures require high precision and operational reliability.

“Although they are optimized for minimum weight, their stability and functionality need to be sustained over several years without any maintenance or cleaning,” says Dr. Luc Blecha, chief technical officer (CTO) of Almatech. Employing 25 scientists and engineers, the company based in Lausanne, Switzerland, develops lightweight structures and mechanical solutions for exceptional requirements such as high precision and reliability in harsh environmental conditions. Almatech is frequently involved in the design of components for spacecraft programs of the European Space Agency (ESA).

Structural components for outer space

Scheduled for launch in late 2019, the Characterising Exoplanet Satellite (CHEOPS) will observe individual bright stars that are known to host exoplanets. It uses a photometer on its telescope to measure the dimming of the starlight caused by a transiting planet. It will provide scientists with the high-precision transit signatures that are needed to measure the sizes of small planets. This data will provide key insight into the formation and evolutionary history of planets.

As part of the Swiss-managed CHEOPS project, Almatech was in charge of all the structural components. The task involved the design and construction of the tubular main structure made of carbon fiber reinforced plastics (CFRP) as well as titanium brackets and the junctions holding the primary and secondary mirrors. The mirrors cannot be adjusted in flight, so their support needs to be rigid and have high stability over the range of temperatures experienced in space.

BepiColombo, a joint European and Japanese mission to Mercury, is already on its way to our inner neighbor. Launched in October 2018, it will start orbiting the least understood planet in our solar system in late 2025. The mission comprises two spacecraft: the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO). While gathering data during its one-year mission, the MPO will endure temperatures in excess of 350 °C.

Almatech designed and optimized a baffle that protects the MPO against heating to more than 270°C. It also protects the laser receiver of a built-in altimeter against the heat coming from the sun. The components included a very fine aluminum mirror used to deflect rays of sunlight. “The shape of this mirror was given, the tolerable roughness was specified at 4 nm, regardless of any external influences,” says Blecha. “By comparison, the diameter of one aluminum atom is 0.25 nm.”

The Solar Orbiter is a collaborative mission between the ESA and the United States’ National Aeronautics and Space Administration (NASA) to study the sun and its outer atmosphere. Scheduled for launch in 2019, the spacecraft will observe the sun’s atmosphere and combine these observations with measurements taken in the environment surrounding the orbiter. It will provide insight into fundamental physical processes studied under conditions that are impossible to reproduce on Earth and unfeasible to observe from astronomical distances.

A Spectral Imaging of the Coronal Environment (SPICE) instrument aboard the Solar Orbiter will observe both the solar disk and the corona to characterize plasma properties at and near the sun. For this instrument, Almatech designed a slit-changing mechanism. It moves the shutter by deforming parts rather than sliding along guiding tracks. “This is vital because particles created by abrasion would over time disable the optical instrument, and cleaning is impossible,” says Blecha.

Testing the digital twin again and again

For many parts, Almatech’s role is to optimize existing designs. The CHEOPS telescope structure, for instance, needed to be reduced in complexity and weight while retaining its structural strength. “Because all components we create are unique and need to function for many years without any maintenance or cleaning, development cycles take longer than in terrestrial designs,” says Blecha. “Although the shape of the baffle for the BepiColombo orbiter was given, it took us four years to arrive at the final hardware.” Almatech spent a similar period developing the slit-changing mechanism for the Solar Orbiter, although they were contracted to provide the entire development from first idea to final hardware.

The main cause for these extended development cycles is the enormous number of tests performed to deliver proof that all requirements will be met in all fathomable situations over the entire lifecycle of the components. Within a development cycle, several physical prototypes are built and tested as well, but Almatech performs the vast majority of these tests in the virtual world using a digital twin of the component under scrutiny. For this purpose, the space-grade equipment designers use Femap™ software from Siemens Digital Industries Software in conjunction with the Nastran® solver to simulate performance, starting at very early phases of product development. “The various model analyses provide proof to clients and authorities that the complex devices will perform as required under the anticipated conditions,” says Blecha. “They are also instrumental in our efforts to reduce mass without compromising stability.”

Nastran is a premier finite element method (FEM) solver for computational performance, accuracy, reliability and scalability. It includes powerful solutions for linear and nonlinear structural analysis, dynamic response, acoustics, rotor dynamics, aeroelasticity, thermal analysis and optimization. “The advantage of having all these solutions available in a single solver is that input and output file formats are the same for all solution types,” says Blecha. “Femap with only one user interface for everything greatly simplifies modeling processes across several engineering disciplines.”

The single user interface is key to completing the task of calculating the effects of vibrations, noise, temperature and temperature changes on the parts and at the same time optimizing the mass. “Our aim is to get a good balance between mechanical and thermomechanical aspects, to find the best possible solution,” says Blecha. Before starting to produce real parts, the Almatech experts reiterate all types of quasi-static, frequency response and thermoelastic analyses of the software model and test physical prototypes using shakers. “We almost invariably encounter opportunities for improvement, and using Femap makes it easy to reiterate the simulation process,” Blecha adds. The fast iterations proved particularly useful in the optimization of the main structural body of the CHEOPS satellite. The temperature variations in outer space can cause dimensional changes detrimental to the function of the telescope. “Using Femap for thermomechanical simulations, we were able to identify and address these issues early,” says Blecha. “Taking advantage of Femap’s capability for a wide range of analyses allowed us to steady the focal distance of the telescope while at the same time reducing the mass of the components.”