RU2355608C2 - Method of determining thermal resistance of spacecraft temperature control vacuum shield thermal insulation system for thermal-vacuum tests - Google Patents

Abstract

FIELD: aircraft engineering.

SUBSTANCE: invention relates to the ground simulation tests of spacecraft elements and can be used in designing and experimental development of multi-layer vacuum shield thermal insulation (VSTI). In compliance with this invention, with the temperature control system stable thermal conditions, temperatures T₁ and T₂ of outer (on both sides) layers of analysed VSTI section are measured. Given T₁ ≠ T₂, equilibrium temperatures (T_ch and T_IR) of radiation energy receivers are measured. One of aforesaid receivers features a high heat radiation absorbing capacity (A_s), approximating to A_s of ideal black body. The other one features A_s<(0.1 to 0.2) in the visible spectrum band. Aforesaid receivers are fitted in the area of analysed VSTI. Thermal resistance of VSTI (R_VSTI) is determined, via VSTI, from the formula derived from the condition of balance of heat radiant flow and heat conductivity. The proposed method allows controlling the local value of R_VSTI during ground development and testing of the spacecraft heat control system in thermal vacuum chamber, including testing on curved surfaces of spacecraft with due allowance for its configuration. The effect of residual gas amount in VSTI on R_VSTI and the process of the said gas draining, as well as defects of separate sections of VSTI, etc. can be determined during above-described tests.

EFFECT: ease of use, possibility of automation.

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Description

The invention relates to space technology, to the field of design and ground testing during thermal vacuum tests (TWI) in thermal vacuum chambers (TCEs) of a spacecraft (SC) thermal control system (CTR), and in particular, to questions of determining the thermal resistance of screen-vacuum thermal insulation (EVTI) CTP .

PAG is designed to maintain the thermal regime of all spacecraft elements within specified limits by ensuring their controlled heat transfer with the environment. The required thermal regime of the spacecraft as a whole and its individual elements is supported by active and passive means of providing thermal regimes (COTR). Passive MOTR include EVTI - a package of multilayer thermal insulation, consisting of a set of screens with high reflectivity, separated by gaskets from materials with low thermal conductivity. EVTI has unique thermal insulation characteristics. Its thermal resistance, referred to the weight of a unit of surface area, is the largest of all known types of thermal insulation. EVTI is technologically advanced, can be installed on spacecraft elements of various shapes. The unique thermal insulation properties of EVTI are manifested in their pure form only for smooth surfaces through which neither conclusions nor structural elements pass. Otherwise, significant heat overflows occur along individual EVTI layers, leading to a decrease in the total thermal resistance.

One of the most important determining parameters of EUTI is the thermal resistance of EUTI, the value of which depends on the packing density of the layers. The main criterion for indirect assessment of the thermophysical properties of EVTI is to control the height of the thermal insulation package both during their manufacture and when they are installed on the spacecraft. By their nature, multilayer thermal insulation packages are very flexible and fit freely on the structural elements of the spacecraft. At the same time, taking into account the overloads and forces arising from insulation evacuation, significant changes in the local values of the EVTI packing density and the practical impossibility of controlling the height of the packages installed on the spacecraft in real operating conditions in TCEs are possible.

Taking into account the features of the spacecraft configuration, the applied technology for the manufacture and assembly of EVTI, as well as its installation and fastening on the surface of the spacecraft, complicate the calculation methods for predicting the thermal resistance of EVTI in operating conditions. Therefore, control over the change in the thermal resistance of individual sections of the EVTI can be performed only by direct measurement during TWI of the spacecraft, carried out in the TCE with imitation of external radiation effects in the visible region of the spectrum and part of the infrared region [1, p.9].

The objective of the invention is:

- determination of the local thermal resistance of the screen-vacuum thermal insulation of the spacecraft thermal control system during thermal vacuum tests of the spacecraft in a TCE;

- determination of the thermal resistance of EVTI on curved surfaces, taking into account the features of the configuration of the spacecraft, and the simplicity of the implementation of this method.

The problem is achieved by the method of determining the thermal resistance of the screen-vacuum thermal insulation of the spacecraft thermal control system during thermal vacuum tests, including measuring the temperature of the outer layers of the screen-vacuum thermal insulation T ₁ and T ₂ , with the steady-state thermal state of the system, at T ₁ ≠ T ₂ , the equilibrium temperatures are measured T _h and T _ik working surfaces of radiant energy receivers, one of which has a high absorptivity of thermal radiation, close to bsolyutno black body, and the other with low absorbance of the visible spectrum with an absorption coefficient of the radiation A _s <(0,1-0,2), wherein radiant energy receivers installed in the vicinity of the test site screen-vacuum thermal insulation and thermal resistance ekranno- vacuum thermal insulation R _euti is determined from the expression

где ε_эвти и А_Sэвти - степень черноты и коэффициент поглощения солнечной энергии наружной поверхностью внешнего слоя ЭВТИ; ε - степень черноты рабочей поверхности приемника лучистой энергии с низкой поглощательной способностью излучения видимой области спектра; σ - постоянная Стефана - Больцмана.

where ε _euti and A _Seuti - degree of blackness and absorption coefficient of solar energy by the outer surface of the outer layer of EVTI; ε is the degree of blackness of the working surface of the receiver of radiant energy with low absorption ability of the radiation of the visible region of the spectrum; σ is the Stefan - Boltzmann constant.

Q _TCEs thermal radiation incident on the analyzed portion SVHI in TCEs determined simulators radiation space, and a solar radiation planetary [1, p.28, p.38, p.47], self-emitting elements TCEs constructions, including thermal and cryogenic screens , as well as reflected radiation from structural elements. The radiation q of the _TVC depends on the temperature and physical properties of not only the given radiating body, but also other bodies surrounding it, as well as on the shape, size and relative position of the bodies in the TCE space. Calculating the radiant heat transfer of such a complex system poses great difficulties that are proposed to be overcome with the help of two radiant energy detectors (PLE) [1, p. 130], the working surface of one of which has a high absorption capacity of thermal rays (light and infrared), and the working surface of another with a low absorption capacity of the rays of the visible region of the spectrum (light rays) and installed near the studied section of the EVTI. To distinguish from the total heat flow q _TCEs its components, namely solar (light) q _s and infrared q _IR radiation, using PLE structure where energy exchange with the environment is performed only radiation conducting measurement of equilibrium temperatures of worktops PLE T _h and T _ik [2, p. 769]. PLE can be made, for example, in the form of a thin-walled flat plate with high thermal conductivity (for example, made of silver, copper or aluminum), on the working surface of which a layer of thermal-regulating coating (TRP) is applied, and a sensitive element is installed on its back surface (for example, resistance thermometer). Moreover, on one of the PLEs on its working surface is applied TRP with a high absorption capacity of thermal radiation, close to a completely black body (for example, black enamel or black paint [2, p. 799]), and on the working surface of another PLE is used TRP with high selectivity on the spectrum of absorbed radiation with a solar absorption coefficient A _s <(0.1-0.2). The plate of the PLE, in addition to the working surface, is closed by thermal insulation and it is assumed that heat leakage from the PLE through the thermal insulation is negligible. Moreover, in order to minimize the effect of the PLE on the heat transfer of the studied EVTI section with the space environment simulated in the TCE, we select the area of the PLE S _ple much less than the area of the studied section EVTI S _euti .

As a specific example, the drawing shows a fragment of EVTI, for which the proposed method is implemented. The drawing shows: the investigated section EVTI 1 with the outer layers 2 and 3, on which temperature sensors 5 and 6 are installed; two radiant energy receivers 4 and 7, one of which has a high absorption capacity of thermal radiation, close to a completely black body, and the other with a low absorption capacity of radiation of the visible region of the spectrum with a solar absorption coefficient As <(0.1-0.2) . The studied section of EVTI 1 is exposed to external radiation from the side of infrared radiation simulators (IRI) 8 and solar radiation simulator (ICI) 9.

The method for determining the thermal resistance of the EVTI of the spacecraft thermal control system during TWI is implemented as follows.

A spacecraft with a STR (not shown), which includes EVTI 1, is placed in a TCE (not shown). Provide in the TCEs the operating conditions of the spacecraft operation, i.e. imitation of external influences of outer space. When the thermal state is established, the systems measure temperatures T ₁ and T ₂ with temperature sensors 5 and 6, mounted against each other on the inner surfaces of the outer layers 2 and 3 of EVTI 1. If the equilibrium between EVTI 1 and the radiation is disturbed, i.e. EVTI 1 emits more or less energy than it absorbs (T ₁ ≠ T ₂ ), they measure the equilibrium temperatures T _h and T _ik using sensitive elements, for example resistance thermometers, radiant energy receivers 4 and 7. Then from expression (1) determine R _euti .

Expression (1) for determining R _{euti is} obtained as follows.

Under the conditions of radiative heat transfer in the real spectrum of radiation, expression (1) was obtained on the basis of the heat flux balance for the stationary section of the EUTI 1 of interest.

As can be seen from the drawing, q _euti is the heat flux density - self radiation with EVTI 1, which is completely determined by the temperature T _{2 of the} outer layer 3 and the physical properties of the outer surface of the outer layer 3 EVTI 1 - emissivity of EVTI 1.

At the same time, from the side of bodies installed in TCEs, radiant energy in the amount of q _TCEs - incident radiation is incident on EVTI 1. Part q _TCE absorbed SVHI 1 - radiation absorption q _abs

where A _euti is the absorption capacity of EUTI.

A _SVHI depends on the spectral composition of the radiant energy q _TCEs falling on SVHI 1, and heat radiation characteristics. The remaining energy of the radiation incident on the EUTI 1 is reflected q _qr - reflected radiation

The resulting radiation q _res is the difference between the intrinsic radiation of the monitored section of the EVTI 1 q _euti and that part of the incident external radiation q _TVK that is absorbed by the considered section of the EVTI 1

q _acc . Composing the energy balance, we get

The value of q _rez determines the density of the heat flux passing through the EVTI 1 in the direction normal to its layers and transferred to the bodies around it in the process of radiant heat transfer. In this case, we consider the incident radiation energy q _TVK known, determined by measuring the equilibrium temperatures T _h and T _{ik of} the radiant energy receivers 4 and 7.

Radiation q _euti EVTI 1 is determined by the law of Stefan - Boltzmann

where ε _euti is the degree of blackness of the outer surface of the outer layer 3 EVTI 1; T ₂ - the temperature of the outer layer 3.

Since we are most interested in those rays whose occurrence is determined only by the temperature and optical properties of the emitting elements installed in the TCEs and by the design of the TCEs, namely, light and infrared rays, expression (2) for the thermal radiation absorbed by EUTI 1 is determined from the relation

wherein

Whence from (4), (5) and (6) we define q _res

будем принимать равным собственному излучению, т.е. энергетический баланс имеет видFor PLE 4 with a high absorption capacity of thermal radiation, close to a completely black body, the incident radiant heat flux

will be taken equal to its own radiation, i.e. energy balance has the form

where T _h - temperature TRP PLE 4; ε _h - emissivity coefficient of TRP PLE 4; And _h is the absorption coefficient of TRP PLE 4; q _h - heat loss PLE 4, which is neglected.

For PLE 7 with a low absorbance of visible radiation, the energy balance has the form

where T _IR - temperature TRP PLE 7; q _to are the heat losses of PLE 7, which are neglected, i.e. q _k = 0.

From (9) and (10) we determine the components of the heat flux q _s and q _ik :

Thermal resistance EVTI 1 is determined from the expression

Substituting in (13) the expression for q _res from (8), taking into account (11) and (12), we obtain the relation for determining the thermal resistance of EVTI 1

If there is no solar radiation simulator 9 in the TCE or when it does not work, relation (14) for R _euti will take the form

Here is a calculated example of the implementation of the method for determining the thermal resistance of the EVTI of the spacecraft thermal control system during TWI.

The dimensions of all parameters in the calculation examples are given in the International system of units. σ = 5.67-10 ^-8 W / (m ² · K ⁴ ).

A spacecraft with a STR, which includes EVTI 1, is placed in a TCE. Provide in the TCEs the operating conditions of the spacecraft operation, i.e. when simulating in the TCE the thermal part of external electromagnetic radiation, simulating the absorption of radiation by outer space and vacuum, providing radiation

the nature of the external heat transfer and the operating mode of the insulation (within 10 ^-3 ... 10 ^-4 Pa).

We perform the calculation examples, taking the following optical characteristics for the outer surface of the outer layer 3 EVTI 1: ε _euti = 0.6; A _Seuti = 0.3.

Suppose that on the working surfaces of the PLE 4 and 7 applied TRP with the following optical characteristics:

- PLE 4 with high thermal radiation absorption capacity close to the black body and A _h> (0.95-0.99);

- PLE 7 with low absorption of radiation in the visible region of the spectrum: ε = 0.85; A _s = 0.1.

Consider example number 1. The TCE has a solar radiation simulator 9 and an infrared radiation simulator 8.

With the steady thermal state of the system, we fix T ₁ and T ₂ with temperature sensors 5 and 6, for example, T ₁ = 293 K and T ₂ = 354 K.

We fix the readings of resistance thermometers installed on the PLE 4 and 7, for example T _h = 410 K and T _ik = 283 K.

From expression (14) we determine R _euti

Из соотношений (11) и (12) определяем составляющие внешнего теплового потока, воздействующего на исследуемый участок ЭВТИ 1 q_s и q_ик:

From relations (11) and (12) we determine the components of the external heat flux affecting the studied section of the EVTI 1 q _s and q _ir :

Consider example number 2. In the TCEs, the solar radiation simulator 9 is disabled, and only the infrared radiation from the infrared radiation simulator 8 and from the cooled walls of the TCEs is affected by the EVTI.

With the steady thermal state of the system, we fix the readings of the temperature sensors 5 and 6 T ₁ = 293 K and T ₂ = 122 K.

We fix the temperature T _h according to the resistance thermometer installed on the PLE 4, T _h = 115 K.

From the expression (15) we determine R _euti

From relation (12) we determine the external heat flux of infrared radiation acting on the studied section of EVTI 1, q _ik :

For example No. 2, we estimate the value of the limiting absolute error in determining the thermal resistance of the EVTI 1 ΔR _euti [3, p.132] for the values of the variables T ₁ , T ₂ , T _{h of the} function R _euti (15) from the following expression

We take the absolute absolute errors in the measurement of T ₁ , T ₂ , T _h with the corresponding temperature sensors equal to ΔT ₁ = ΔT ₂ = ΔT _h = 0.5 K. From (16)

In this case, the relative error in estimating the thermal resistance of EVTI 1 was ΔR _euti / R _euti = (16/107) · 100% ~ 15%.

Let's consider an example No. 3. Only infrared radiation acts on the studied section of EVTI 1, as in example No. 2.

With the steady thermal state of the system, we fix the readings of the temperature sensors 5 and 6 T ₁ = 293 K and T ₂ = 155 K.

We fix the readings of the resistance thermometer PLE 4, T _h = 115 K.

From the expression (15) we determine R _euti

From relation (12) we determine the external heat flux affecting the studied section of EVTI 1, q _ik :

, т.е. такой же, как в примере №2.

, i.e. the same as in example No. 2.

Let us estimate the magnitude of the absolute error ΔR _euti from (16), taking the maximum absolute errors when measuring T ₁ , T ₂ , T _h with the corresponding temperature sensors equal to ΔT ₁ = ΔT ₂ = ΔT _h = 0.5 K.

Moreover, the relative error in estimating the thermal resistance of EVTI 1 was ΔR _euti / R _euti = (0.34 / 10) · 100% ~ 3.4%.

A comparative analysis of examples No. 2 and No. 3 to determine the thermal resistance of the two studied sections of EVTI with the same external influences on these sections showed that in example No. 3 R _euti turned out to be an order of magnitude smaller than in example No. 2, which may be due to imperfect installation technology EVTI package on the spacecraft surface (for example, the permissible density of the EVTI screens was exceeded). Thus, these TWI results make it possible to identify defects in individual sections of the EVTI, to improve the design and assembly technology of the EVTI with the further development of the STR satellite in the TVK.

The application of the proposed method for determining the thermal resistance of the EVTI thermal control system of the spacecraft with TVI allows:

1) to control the value of the local thermal resistance of the EVTI in the conditions of ground testing of the spacecraft in the TCE;

2) to control the thermal resistance of EVTI on the curved surfaces of the spacecraft, taking into account the features of its configuration;

3) use the results of TWI in the analysis of the effect on the thermal resistance of the EVTI of the residual gas in the EVTI and the process of its drainage when the STP enters the operating mode;

4) to identify defects in individual sections of the EVTI, to improve the design, the technology of assembly of the EVTI and its installation on the spacecraft during the development of the STR of the spacecraft in a thermal vacuum chamber;

5) automate the process of determining the thermal resistance of EVTI, using the information received from the corresponding temperature sensors.

Information sources

1. O. B. Andreychuk, N. N. Malakhov. Thermal tests of spacecraft. M .: Engineering. 1982.

2. Physical quantities. Handbook Ed. I.S. Grigorieva, E.Z. Meilikhova. M .: Energoatomizdat. 1991.

3. I.N. Bronstein, K. A. Semendyaev. Handbook of mathematics for engineers and students of technical schools. M .: Science. 1986.

Claims (1)

A method for determining the thermal resistance of the screen-vacuum thermal insulation of the spacecraft thermal control system during thermal vacuum tests, including measuring the temperatures of the outer layers of the screen-vacuum thermal insulation T ₁ and T ₂ in the steady state thermal system, measuring at T ₁ ≠ T _{2 the} equilibrium temperatures T _h and T _ik working surfaces of radiant energy receivers, one of which has a high absorption capacity of thermal radiation, close to the absorption capacity of absolutely black and, the other - a low absorbance of radiation in the visible region of the spectrum with an absorption coefficient A _s <(0, l-0,2), wherein the radiant energy receivers is set in the vicinity of the test site screen-vacuum thermal insulation and thermal resistance screen-vacuum thermal insulation R _euti is determined by the formula

where ε _euti and A _Seuti - degree of blackness and absorption coefficient of solar energy by the outer surface of the outer layer of the screen-vacuum thermal insulation;
ε is the degree of blackness of the working surface of the receiver of radiant energy with low absorption of radiation in the visible region of the spectrum;
σ is the Stefan-Boltzmann constant.