Good vibrations for SMART-1
12 July 2001To survive the physical stresses associated with launch, a spacecraft and its payload must be solidly built and well tested. Each spacecraft built for an ESA science mission is unique and so must undergo its own 'baptism of fire' to prove that it can withstand the launch and reach its orbit in perfect shape ready to carry out its mission.
During the past few weeks the structural model of SMART-1, the first in ESA's series of Small Missions for Advanced Research in Technology, has undergone vibration testing at ESTEC - ESA's space research and technology centre at Noordwijk in the Netherlands. SMART-1 will test solar electric primary propulsion as the key technology for future deep-space missions. BepiColombo, ESA's mission to explore the planet Mercury, could be the first to benefit from the SMART-1 mission results.
The SMART-1 structural model was constructed in order to mimic the mechanical behaviour of the final flight spacecraft. Any mechanical test which must be performed to verify that the design of the spacecraft is sound is then performed on this model. The real instruments (the payload) are replaced by dummy instruments which have the same mass and dimensions as the actual payload.
Vibration testing involves shaking the 'spacecraft' in a way that simulates the possible types of vibration that the spacecraft will encounter from launch to operation. Vibrations can also arise from the noise generated by the launch rocket, which can reach up to 146 decibels. The so-called 'acoustic tests' check that these vibrations will not damage the spacecraft. Engineers are experts in designing the elements that make up a scientific satellite but since each spacecraft, and its payload, is unique, it is vital to ensure that the final spacecraft construction can survive the rigors of its journey into space.
The acoustic test is performed in the Large European Acoustic Facility (LEAF) at ESTEC. LEAF is a large chamber in which the structural model is exposed to acoustic noise comparable to several Jumbo jets at a distance of 30 m. Other vibration tests are performed by clamping the spacecraft to an electro-dynamic shaker, which loads the 42 kg SMART-1 structure with the equivalent of over 4 tonnes about 20 times per second. Lower levels of vibrations are applied up to 100 Hz.
Shock tests are also performed on the structural model. For these tests Arianespace provides a flight representative adapter with an explosive ring (called a Shogun). The spacecraft is mounted on it and then the explosives are fired, generating shock waves of 5000 g.
In parallel with these mechanical tests, electrical and software tests are performed on the bench test models - simplified versions of the final instruments. With the data accumulated from these tests, final adjustments to the spacecraft and payload fittings, if necessary, can be incorporated into the real flight spacecraft.
This autumn, the real SMART-1 will start to take shape and it too will repeat a series of mechanical, electrical and thermal tests prior to shipping to Kourou for launch in winter 2002.
The Swedish Space Corporation, as prime contractor for the SMART-1 mission, is responsible for the development and construction of the SMART-1 spacecraft.