Motivation

1.2 Objective . . . . 7 1.2.1 Literature production . . . 10 1.3 Basic concepts . . . . 12 1.3.1 Planetary and space thermal environment . . . 12 1.3.2 Spacecraft Thermal Control . . . 18 1.3.3 Thermal modelling . . . 19

1.1 Motivation

One of the biggest steps in the human history took place in 1957. The 4^thof October that year, the Soviet Union (USSR) successfully launched the world’s first artificial satellite. It was called "Sputnik" and it meant the beginning of a long trip around the Earth and beyond for years. It is really intriguing thinking about the design process of these first missions. What did they know about space? How did they performed their simulations? Nowadays, we live in a era in which we depend on the computers and the technology for almost everything. We cannot even imagine not using them for designing or testing new satellites. But more important than technology is our dependence on years and years of studies and knowledge about the space and the experience acquired in thousands of ballooning, rockets and satellite missions. The Sputnik-1 satellite, which is shown in Figure 1.1, was conceived not as a scientific satellite itself but as a the simpliest satellite (prosteishyi sputnik 1) to

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become the first orbiting the Earth [1]. However, it provided valuable information about the density of the upper atmosphere and the ionosphere characteristics.

Figure 1.1: Exploded view of Sputnik-1. NASA History Division.

By the time the Sputnik 1 was launched, many studies were carried out with the aim of understanding the upper-atmosphere, its properties and its composition.

In 1804, Gay-Lussac an Biot used a manned balloon for firstly measuring the air composition and the magnetic field at 7000 m [2]. Since the first unmanned balloon flight realized by Hermite and Besangon in 1892, the study of the atmosphere improved by the use of sounding balloons which were capable of reaching altitudes above 30000 m. Based on these researches, scientists hold their attention beyond.

First theoretical studies of the upper-atmosphere showed many fascinating phenom- ena which made reaching this nearly empty region a challenge from a technological and scientific point of view. By the time sounding rockets made their appearance in the mid-1940’s, a coherent picture of the upper-atmosphere was theoretically set forth by B. Haurwitz in his publication of 1937 about "The Physical State of the Upper Atmosphere" [3]. It provided helpful information for those who began using rockets for the study of this region.

Such were the problems to which the rocket experimenters addressed themselves [4]. Once started, the results of their research flowed in a steady stream into the

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literature, contributing to a growing understanding of upper atmospheric phenomena.

The first questions rockets experimenters addressed themselves were those ground- based scientist considered more significant. Studying the solar spectrum from ground had several limitations due to the atmospheric absorption. It was in 1946 when Richard Tousey and his colleagues firstly measured the solar spectra from above the ozonosphere [5]. This event marked the beginning of many years of intensive research on the structure and energy content of the solar spectrum, which still continues nowadays. From a thermal point of view the interesting wavelength range goes from 0.2 µm to 100 µm, where the solar spectrum can be approximated by a blackbody radiating at a 6000 K temperature. The computed solar irradiance spectrum observations from the Solar Radiation and Climate Experiment (SORCE) in the thermal radiation wavelength range are shown in Figure 1.2.

Figure 1.2: Spectral Solar Irradiance from 200 nm to 100000 nm. NRLSSI2 Daily Average 1/1/1882 [6].

Atmospheric structure, that is, the variation of pressure, temperature, and density with altitude, as well as its chemical composition, also received the early attention of the rocket experimenters. Almost every flight carried gauges to measure these fundamental parameters. As a result of many rocket observations, in the early 1950s the Rocket and Satellite Research Panel was able to issue an improved estimate of upper-atmospheric structure for use by geophysics [7]. By the time Sputnik went into orbit, the groundwork had been laid to give a considerable amount of information about both geographical and temporal variations of these quantities.

Although satellite missions allowed experimenters for a better study of all these parameters, sounding rockets continued being launched since they provided the best means of obtaining vertical cross sections of atmospheric properties up to satellite altitudes. In addition, they also were cheaper devices for testing new instrumentation or making exploratory measurements of phenomena to be studied in detail later with more expensive spacecraft. Their relatively low cost and the speed with which a sounding rocket experiment could be prepared and carried out also made sounding rockets useful for graduate research where the students needed to complete a project in a reasonable amount of time to support his dissertation.

But not only did students find sounding rockets attractive. Many professional space scientists continued to favor sounding rockets for much of their research, as opposed to the more complicated, more expensive, and more demanding satellites.

One of the biggest problems experimenters had to face when they required long-duration observations is the design of the spacecraft to carry their scientific payload on board up to space. There were some proposals for building a standardized satellite to serve experimenters as a common platform. This would have considerably reduced the development time and cost and it would have improved the reliability of space missions. However, for each satellite a great deal of tailoring was required in terms of orbit, orientation, telemetry, electrical energy supply, thermal control, etc.

Such problems defeated the efforts to produce standardized satellites. Nevertheless, a considerable level of uniformity was achieved. Similar design approaches were adopted by spacecraft based on technology used successfully by previous missions.

In addition, when the design of a new satellite starts, engineers had already in mind a lot of information about the orientation capabilities, the potential thermal issues or the required structural loads, etc. Not only did the technology used by spacecrafts acquire over the years a high level of standardization but also did the design process in terms of procedures, requirements, analyses, test, etc.

All these first missions provided valuable information for the design of the next satellites since the experience acquired and the data obtained were continuously compiled into the literature. Nowadays, the design process of a satellite is completely standardized with the aim of guaranteeing the success of the mission, reducing cost, optimizing work, etc. It seems quite obvious that if there is a lack of information when designing a satellite, an oversizing would be necessary to reduce risks and guarantee the success of the mission. This is why space missions have become more and more complex with time. Information about the satellites performance in space or the knowledge of the environment to deal with is continuously increasing allowing the experimenters to be more focused on the scientific objective of the spacecraft.

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For a successful mission, it is necessary to ensure the good performance of the scientific payload and the support equipment during the whole life of the spacecraft.

To do so, many analyses and tests are performed during the design process. In order to deal with all these tasks, satellites are usually divided into different subsystems for structuring the work into well separated disciplines. The Thermal Control Subsystem (TCS) of a satellite shall guarantee the survival of the equipment to the harsh environment in space. According to Ref [8], the TCS is the responsible of

"maintaining all spacecraft and payload components and subsystems within their required temperature and gradients limits for each mission phase". Once defined the thermal requirements and constraints of the system and the different components, the determination of the thermal environment should be performed in order to select the worst-case analyses to be done in the design process.

As pointed out before, the thermal characterization of the satellites environment is possible thanks to other many space mission that were performed before. The thermal environment criteria used for spacecraft design has evolved as the technology has done over the years. The availability of Earth-observation data has provided a valuable information which has allowed for a better characterization of the thermal environment.

Not only have the analysis capabilities hugely improved, but also has the amount of information about the space environment considerably increased. In the last decade, the growth of the internet capacities together with an increasing interest for big data strategies have led to the main organizations, the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA), to provide open access to huge satellite-based observations databases. Using this data and carefully analysing it using statistical methods, the characterization of the thermal environment could be particularized to each mission in order to reduce uncertainties and get more complex designs avoiding the over-sizing.

Nowadays, the access to space has been opened to private investments, research institutes, universities, etc, by the use of small satellites. During the last decade, the standardization has become a reality in form of CubeSat platforms [9]. These small cubic satellites provide suitable platforms for small scientific experiments or technology demonstration. The miniaturization of the electronics has allowed for improving the capabilities of this emerging technology. Nevertheless, there are cheaper alternatives for experimenters than reaching the space. The stratosphere is also a very suitable place for these purposes. Being above 99 % Earth’s atmosphere mass, wave front distortions due to atmospheric turbulence are virtually non- existent [10] providing an advantageous location compared to ground-based solar

observations. Scientific balloons also provide a platform for the demonstration of promising new instrument and spacecraft technologies [11]. Furthermore, residence time over a determined area is huge when compared to Low Earth Orbits (LEO).

For that reason, stratospheric platforms are being presented as a suitable solution to provide communication coverage to non-accessible areas [12] or just to monitor the Earth environment [13]. The capabilities and utilities of these kind of flights are constantly increasing and as a result, the design process of these systems is becoming more and more relevant.

Figure 1.3: Balloon inflation for CREAM mission launch. NASA Columbia Scientific Balloon Facility (CSBF).

From a thermal point of view, stratospheric missions have many similarities with space missions. Being in the stratosphere, radiation is the main heat transfer mechanism as in most cases, convection could be considered negligible. This is why, stratospheric balloon payloads are usually analysed using the same tools used for the space systems analysis. However, there are two big differences with respect to the satellites thermal analysis. Firstly, the ascent phase of this kind of platforms should be analysed due to the convective effects. The low temperatures in the tropopause as well as the high relative wind speed make freezing a real problem that must be avoided. Secondly, these analyses used to be performed using averaged values for the environmental conditions [14] without taking into account the local characteristics or the seasonal variability. Increasingly complex systems, such as SUNRISE III, a project the Instituto Universitario de Microgravedad "Ignacio da Riva" (IDR) from Universidad Politécnica de Madrid (UPM) is involved in, require deeper studies to ensure survival to both the ascent and float phases. Using real- data based environments, uncertainties could be reduced by defining particularized worst-cases accounting for every influencing parameter.

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