Microaccelerations as a factor of cosmic space

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Samara State Aerospace University, Russia

443026, Russia, Samara, p/b 1253

e-mail: axe_backdraft@inbox.ru

Microaccelerations as a factor of cosmic space

A.V. Sedelnikov

Annotation

microacceleration component near earth

This paper considers microaccelerations as an inalienable factor of circumterrestrial cosmic space; it represents the author's classification of microaccelerations and peculiarities of microaccelerations as an object of research.

Key words: microaccelerations, cosmic space, classification of microaccelerations, constructional component.

The term weightlessness does not describe precisely the condition of cosmic space on the circumterrestrial orbit. It is perhaps impossible to achieve absolute weightlessness as a spacecraft is affected by an uncompensated system of perturbing factors. Once we reach the circumterrestrial orbit we enter the environment that is characterized by microaccelerations instead of habitual Earth gravity field [1].

Microaccelerations are a phenomenon that can be witnessed in the internal environment of a spacecraft and that is caused by a combination of external and internal perturbing factors affecting the spacecraft. It can be said that we deal with the microaccelerations field as microaccelerations values are different in every point of the environment. Unlike Earth gravity field, this field changes constantly as factors which cause it are changeable. The term “microgravity” used in Western countries does not reveal precisely the main point of this phenomenon. [1] features situations when microaccelerations field is mostly affected by factors which have nothing to do with gravity, e.g. natural oscillations of big resilient elements of a spacecraft. Therefore the author tends to think that it would be more correct to talk about microaccelerations, not microgravity.

Peculiarities of microaccelerations as a factor of cosmic space

Some of the first Russian measurements of the influence different perturbing factors have on microaccelerations created were conducted on the orbital stations “Salut-6, 7” and the orbital complex “Mir” (see Table 1.1) [2].

Influence different perturbing factors had on miroaccelerations field on the orbital stations “Salut-6, 7” and the orbital complex “Mir”

Table 1.1

First American estimations should be dated back to the middle of 70s after the launch of the cosmic orbital station “Skylab” [3, 4]. Special division “Microgravity Science and Applications Division” (MSAD) was created in NASA in the early 90s with the group “Principal Investigator Microgravity services” (PIMS) functioning within it. It dealt with measurements and research of microaccelerations in space shuttles and provided experimentators with the information about microaccelerations in places where instruments were located. Moreover PIMS conducted serious explanatory work about the nature of microaccelerations and their influence on cosmic experiments; it showed how measurement data should be used, etc. [5]. But the group was disbanded in 2005 as National Center for Microgravity Research had already functioned.

Microaccelerations as an object of research possess a number of peculiarities. The first peculiarity is the fact that microaccelerations are a factor of cosmic space. Experiments conducted in outer space demonstrably showed that the condition of absolute weightlessness is unachievable. Because of incompensation of perturbing factors affecting the spacecraft the internal environment of this spacecraft will be represented by the microaccelerations field instead of weightlessness. Therefore it would be more correct to say that cosmic experiments are conducted in the microaccelerations field, not in weightlessness.

It should be taken into consideration that this factor is very difficult to synthesize on the Earth where gravity is present. Short-time conditions similar to those of spacecraft internal environment can be achieved when flying test-benches are nosediving or when containers with equipment are dropped in the mines of National Center for Microgravity Research (the USA) or Microgravity laboratory “Drop Tower Bremen” (ZARM, Germany). But this factor can be fully researched only in conditions of a real space flight. And the reason why researches in this field are significantly restricted is because of the high price of space programmes.

The second peculiarity is the difficulty of receiving trustworthy experimental data. It was mentioned in the previous section that starting overloads a spacecraft undergoes 7-8 times exceed values measured with accelerometers. Therefore when, for instance, microaccelerations are estimated with the help of French accelerometers BETA, components of microaccelerations located on coordinate axises on the board of the spacecraft “FOTON 11” have maximal values measuring approximately 1,6 • 10^-4 m/sec², the second one has 5,7 • 10^-3 m/sec² and the third one has 3,1 • 10^-3 m/sec² (Fig. 1.1) [6]. The authors would like to mention that software may possibly have been used incorrectly as there is a great constant displacement in the given graphical dependencies of different microacceleration components on time. However beside the reason stated by experts who processed the measurements received it can be suggested that a part of measuring equipment could break down because of serious starting overloads.

Figure 1. Estimation of microaccelerations on the board of the spacecraft FOTON 11 with the help of French accelerometers BETA (quoted from [6])

Similar case was observed during the testing of the specially designed vibration-isolating device MGIM (Microgravity Isolation Mount) that was supposed to preserve a hypothetic container with technological equipment from harmful effects of the microaccelerations field. Efficiency of measuring equipment had not been analyzed before MGIM oscillatory amplitude measuring. As a result experimentators conventionally divided all measuring series into “successful” and “unsuccessful”. MGIM oscillatory amplitudes in these series had almost twentyfold difference (Fig. 1.2 and 1.3). No one managed to explain this. Engines starting, crew's activity, etc. also could cause these drops as well as measuring equipment failure and possible resonances.

Figure 2. Oscillatory amplitude of the vibration-isolating device MGIM during the unsuccessful series on the board of the orbital complex “Mir” (quoted from [7])

Figure 3. Oscillatory amplitude of the vibration-isolating device MGIM during the successful series on the board of the orbital complex “Mir” (quoted from [7])

There are some other cases of measurements when breaking-down of measuring equipment can be stated.

The third peculiarity is the difficulty of microaccelerations measurement. Unlike temperature or angular speed, microaccelerations cannot be measured directly. Therefore reliable methods of microaccelerations estimation through experimental data are needed. Thus, for example, the convection indicator “Dacon” designed by Permian experts in cooperation with the space-rocket corporation “Energiya” was tested on the orbital complex “Mir”. The idea of such a device was proposed in [8]. Therewith a simplified model of microaccelerations designed for the station “Salut” was used for microaccelerations estimation [9]. “Dacon” monitored movements of convectional type related to the microaccelerations field thus enabling their estimation [10]. However the authors [11] believe that relations between microaccelerations and movements of convectional type are quite complicated and estimation data is therefore very approximate. This may explain the anomalous behaviour of extrinsic channel during the directional crystallization [11]. It is mentioned in [5] that small size and usage of air as working medium did not allow achieving sensitivity needed for microaccelerations field registration on the orbital complex “Mir” [10, 12].

This idea also found a response abroad. A similar device with water as working medium was created and successfully used on the spacecraft “Space Shuttle” STS-95 within the Japanese-American project JUSTSAP [13]. The device was good at measuring the spacecraft quasistatic microaccelerations field and outer perturbations but responded with a very long delay [5]. Therefore “Dacon” was upgraded despite all the hesitations. Sensitivity of the indicator named “Dacon-M” was hundredfold increased as compared with “Dacon” indicator due to the enlarged box, working medium choice and box pressure increase. Results of experiments conducted on the ISS board with the help of “Dacon-M” are shown in [14]. Therewith the authors notice that the influence microaccelerations have on the convection indicator depends on frequency qualities of the indicator and microaccelerations themselves. It means that estimations of devices based on one and the same principle can differ from each other thus characterizing this peculiarity.

It should also be mentioned that difficulty of measurements and certain unreliability of experimental data result in attempts to simplify the experiments as much as possible in order to exclude possible mistakes during the interpretation of the results received. One can consider the experiment named “Dynamics” conducted on the board of the spacecraft “Foton-M3” [15] as an example of this approach. The experiment consisted in constant video recording of pellets moving freely inside a cubic box with two transpicuous sides. Processing of results of the experiment showed that microaccelerations estimation conducted this way quadrates with that conducted with the help of accelerometers with accuracy of 1 mym/sec². This accuracy may be insufficient due to the maximum allowable level of microaccelerations for the project of the space laboratory “OKA-T' measuring 10 mym/sec² [1]. It is clear that requirements for microaccelerations will get even more stringent as space technologies develop. For example, microaccelerations measuring 20 mym/sec² [1] were allowable for the first Soviet project of the space laboratory “NIKA-T”.

Some papers [16] state that microaccelerations level in modern spacecraft significantly exceeds one necessary for effective implementation of designed gravity-sensitive processes. Therewith this 1 mym/sec² that is almost unachievable at the modern stage of space technology development is introduced as the maximal level in the wide frequency band (0-100 Hz). It will be shown later that a spacecraft supplied by solar panels practically cannot achieve this level. But these are modern requirements which cannot be overlooked.

Classification of microaccelerations

Microaccelerations can be classified according to frequency: there are quasistatic (low frequency) component of microaccelerations which frequency measures up to 0.01 Hz [14] and vibrational (high frequency) one. Quasistatic component is considered to be the most unfavourable one as it cannot be damped properly and creates quite a stable microaccelerations field. Vibrational component without a constant source of perturbation (e.g. a spacecraft flywheel “Spot-4” [17]) significantly affects the microaccelerations field only within the transient process (10-20 sec after the spacecraft orientation system control jet engines shutdown) and then its affection can be neglected [18] due to more intensive damping.

Microaccelerations constituting the field in the internal environment of a spacecraft are of different natures and can be classified as internal - external as well as the mechanical effect on the system itself. E.g. work of executive devices, control system jet engine and natural oscillations of big resilient elements of a spacecraft can be considered as internal perturbing factors while aerodynamic and gravitational perturbations belong to external perturbing factors.

The author proposes his own classification based on the way of combating microaccelerations in order to make further discourse clearer.

- Metastable component of microaccelerations.

It is created due to the affection of constant factors of cosmic space: gravitational and electromagnetic fields, aerodynamic forces and moments, light pressure, etc.

This component of microaccelerations can be effectively struggled. For example when orbital height of a space laboratory is increased, aerodynamic affection is completely neutralized and gravitational and magnetic components less contribute to the general level of microaccelerations. Every part of cosmic space is characterized by a certain level of the metastable component. Researches conducted on the board of orbital complexes “Salut-6”, “Salut-7” and “Mir” showed that contribution of these factors is significant (see Table 1.1) at the orbital height of 200-400 km. The situation will be different if we deal with a spacecraft with the orbital height over 600 km.

Metastable component of microaccelerations is excluded from consideration in estimation presented in this paper as it is entirely defined by characteristics of a space where a space laboratory is located. It is considered to be impossible to change these characteristics. However this is not needed to solve the problem of microaccelerations minimization. One should only find such parameters of the laboratory orbit so that metastable component would not significantly affect the processes and experiments conducted.

The next part:

- occasional component of microaccelerations.

It is created due to heterogeneities and fluctuations of gravitational and electromagnetic fields, solar activity changes, heterogeneity of the atmosphere, etc. on the one hand and affection of occasional factors of cosmic space such as micrometeorites, etc. - on the other hand. Off-nominal situations caused by equipment failures such as breaking-down of one of three powered gyroscopes of the cosmic orbital station “Skylab” or non-opening of one of the solar panels of the same station also can be considered as belonging to this category.

Taking into consideration the occasional character of this component it is enough to make only a probabilistic estimation of it. However this component as well as metastable one should also be excluded from consideration when it comes to fractal estimation as occasional component is smaller than metastable one and it does not directly depend on the spacecraft parameters. The author also believes that there is no point in modeling off-nominal situations in this paper as all the experiments conducted in these situations are likely to be unsuccessful. These estimations should be made separately during the check-up of systems and equipment efficiency of a spacecraft.

And the last part:

- constructional component of microaccelerations.

It is created due to internal perturbations such as spacecraft orbit correction, work of control jet engine, technological equipment, all kinds of executive devices, uncontrolled rotation of the spacecraft around the centre of mass during the passive orientation, crew's activity, etc. And this very component is of interest for making estimation in order to control its value at the early stages of building and designing a space laboratory. It is called “constructional” because these microaccelerations are entirely defined by the constructional-layout scheme of a spacecraft and depend on the way the spacecraft movement is regulated.

Engineers who design modern space laboratories should use all constructional methods available in attempt to reduce the microaccelerations level in the zone of supposed technological equipment location. Therefore the estimation made is important and essential. Of course constructional methods will depend on processes which are likely to be implemented on the laboratory board. For example, low energy intensity of the space telescope “Spot-4” enabled engineers to fix the only solar panel on the casing with the help of a resilient rod [24]. Thus the influence quasistatic component of microaccelerations had on quality of pictures received was minimized to the level that could be neglected. But the solar panel cannot be strictly directed to the Sun in this case as this method could not be implemented in the constructional-layout scheme of “NIKA-T” where cosine of the angle between the normal to the solar panel and direction to the Sun should not be less than 0.9 [1]. Otherwise power produced would not be enough to supply the consumers: 2-3.6 kW per day are needed for power consuming experiments. The author's work is dedicated to modeling this component of microaccelerations [1].

It should be taken into consideration that two absolutely identical spacecraft launched into one and the same orbit will have different microaccelerations fields in their internal environments. This happens due to the fact that launching conditions will be different and this will lead to differences in primary parameters of spacecraft's rotational movements around the centre of mass. One will observe different time of spacecraft critical disorientation, occasional character of this disorientation and, as a consequence, starting of one of orientation system control jet engines will be of an occasional character.

This peculiarity shows that we should discern between constructional component of microaccelerations that would be the same for two identical spacecraft launched into one and the same orbit and that is microaccelerations determined by the constructional-layout scheme of a spacecraft, its mass-inertia characteristics, orientation system control jet engine moment, parameters of big resilient elements and the way they are fixed as well as the way the spacecraft is controlled and the implemented microaccelerations field that would be different even in identical spacecraft moving along one and the same orbit due to all the factors mentioned above. Actually this is typical of occasional processes when constructional component is a nonrandom function and fulfills the role of process mathematical expectation. In this case the implemented microaccelerations field can be compared with the specific implementation of this occasional process. This field is occasional and it depends on a wide range of occasional factors but it is related with constructional component.

Conclusions

Microaccelerations are a complicated object of research that is one of the main factors of cosmic space. Detailed research of microaccelerations will allow future correct using of cosmic space capabilities not only in space materials science but also in exploration of outer space by the man.

Mathematical models fulfill an important function in research of this object as experimental researches of microaccelerations demand serious expenses and their results are not always reliable.

In this paper the author proposes the classification that allows selecting the component of microaccelerations that can be controlled through the constructional-layout scheme optimization. This component can be modeled during the modern space laboratories designing.

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