Calculation and Design of the Main Equipment for Mobile Space Simulation System

The rocket and space industry plays a major role in the military and economic security of the state and in space exploration. It is obvious that all man-made satellites and spacecraft are highly complex systems, which consist of many parts. The highest requirements are placed on the parts of the spacecraft, and so all up-to-date technologies are employed in the production of space technology. High demands are also placed on modern materials, which must have the required elasticity, strength, heat resistance, electrical conductivity and other special qualities. In the current scenarios of evolving technology, one can see the increased use of composite materials, especially pertaining to advanced design solutions for most modern aircraft (Airbus A-380, Boeing B-787, SJF F-35, UAV). Owing to this, it has become possible to produce not only fragments but also the entire support structure of unmanned aerial vehicles (UAVs), civil aircraft (Airbus, Boeing) or the military aircraft of a leading manufacturer, Lockheed Martin, in the case of the 5th generation joint strike fighter (JSF) F-35 [1]. The rocket and space industry significantly impacts on the level of scientific, economic and military potential [2].

So, now there is a rapid development of space technology and, every year, the number of launches of vehicles for various purposes into space is only increasing. Satellites are expensive, they are usually more than $100 million dollars and that is without launching them. The service life of these devices is increased too – they are designed to last more than 10 years – and this is a huge plus for the customer. To achieve this goal, not only design technologies need to be developed, but also test systems need to be improved [3].

Since it is impossible for the timely repair of any of the satellite elements while it is in operation, the testers must anticipate all possible situations and conditions in which the device can be located. For this purpose, a series of tests are conducted to study the satellite’s behaviour under extreme conditions. Clearly each object launched into space and its components must be tested to confirm the safety and declared functional characteristics of space objects and their components following the procedures and standards established by space agencies [4,5].

Such testing is carried out by special testing centres, which are stationary. The services of these centres are expensive and not always available to small companies, societies, educational institutions, etc. This problem can be solved by creating a mobile ground simulator of space conditions, with the help of which it would be possible to provide testing services at the request of the customer in a place and time acceptable to them [6]. Such a type of project called Metamorphosis (Fig. 1) was developed at the Institute of Aeronautics of Riga Technical University [‘Prototype development of transportable in intermodal traffic mobile space testing facility “Metamorphosis”’ (Metamorphosis, project No. 1.1.1.1/18/A/133)] [7] in cooperation with SIA Cryogenic and Vacuum Systems. This company specialises in the development and production of vacuum and cryogenic equipment. It has extensive experience in creating technologies used in space research and the development and operation of space environment simulators [8]. Deep vacuum, low temperature of outer space and various types of radiation are the main properties of the space environment, which affect the space objects in the Earth’s orbit. The most important among them is deep vacuum. The vacuum cryogenic space simulator is the whole complex of units. The primary element of such a system for simulation of outer space environmental conditions is a vacuum chamber with a distributed hydraulic system.

Fig. 1.

The spacecraft are developed for various applications such as space science, navigation, communications, technology testing and verification, earth observation, weather observation, military applications and others.

To start the operation phase, the target elements of the spacecraft need to meet all conventional space project life cycle development phases such as Concept Studies; Concept and Technology Development; Preliminary Design and Technology Completion; Final Design and Fabrication; System Assembly Integration and Test (AIT); Launch Campaign, Operations and Sustainment; and Closeout [9]. In the integration and test phases, the spacecraft is assembled, integrated and tested in accordance with the requirements of the ESA ECSS and NASA standards systems, is the development and testing of space technology, taking place in conditions as close as possible to the real parameters of outer space.

For execution, a spacecraft testing program is required for use in different types of facilities such as environmental tests facilities (vibration test facility, thermal tests facility, acoustics test facility), mass properties test facility, space simulation chamber, EMC test facility, magnetics test facility and others [10].

The components of vacuum space simulators include such elements: vacuum chamber, vacuum pump system, vacuum measuring and/or control system, temperature control system, automated control system, piping system with, shut-off valves and other fittings (Fig. 2). The chamber is the heart of the vacuum system, creating a safe barrier between the external environment and internal processes.

Fig. 2.

The space simulation chambers are systems used to recreate as close as possible the environment conditions that the spacecraft experiences in space, as well as serve in space-components qualification and material research used in spacecraft. These systems allow the spacecraft’s thermal behaviour to be analysed.

The main factor that determines the geometric characteristics of the space environmental simulator vacuum chamber is the maximum overall dimensions of the test object. On the other hand, because the mobile space testing vacuum system will be housed in a vehicle – the maximum overall dimensions are determined by the permitted dimensions of this vehicle (Fig. 3). In this case, the structurally maximum possible diameter of the vacuum chamber lies in the range of 1,800– 2,400 mm, and the minimum dimensions are related to the sample size and it should be 360–400 mm.

Fig. 3.

Determination of the diameter and length of the cylindrical part of the vacuum chamber.

The diameter of the vacuum chamber [15] can be determined as the sum of the characteristic diameters (Fig. 4): 1 D=Ds+2Lcs+2hcs+2hm \[D={{D}_{s}}+2{{L}_{cs}}+2{{h}_{cs}}+2{{h}_{m}}\] where D is the diameter of the vacuum chamber, mm; D_s is the characteristic diameter of the test object, mm. D_{s min} = 141.4 mm; D_{s nom} = 148 mm; D_{s max} = 354 mm; L_cs is the distance from the test object to the cryogenic shrouds, mm; h_cs is the thickness of cryogenic shrouds, mm; h_m is the distance from the cryogenic shrouds to the body of the vacuum chamber, mm.

Fig. 4.

The length of the cylindrical part of the vacuum chamber can be determined as the sum of the characteristic length:

where L is the length of the vacuum chamber, mm; L_s is the characteristic length of the test object, mm. L_{s min} = 248 mm; L_{s nom} = 360 mm; L_{s max} = 472 mm.

The main requirement of the standards [17,18] for space environmental simulators is sufficient reliability of simulating real operating conditions, that is, factors of outer space such as (1) high vacuum; (2) zero reflection coefficient of electromagnetic radiation; and (3) absence of convection and heat transfer through the thermal conductivity of the environment. High vacuum is understood as a state of gas in which collisions of gas molecules with the walls of the vessel prevail over the collision of gas molecules with each other.

In standards [17,18] is accepted, that the pressure in the vacuum chamber should not exceed 10⁻⁵ Torr and in any case ensure the achievement of the molecular regime of gas flow in the vacuum chamber, characteristic of the space environment.

The criterion for determining the nature of the movement and interaction of gas molecules is the Knudsen criterion: 3 Kn=Lm/Ds \[{{K}_{n}}={{L}_{m}}\text{/}{{D}_{s}}\] where L_m is the mean free path of gas molecules; D_s is the characteristic size of the vacuum vessel. 4 Lm=1πd2n2 \[{{L}_{m}}=\frac{1}{\pi {{d}^{2}}n\sqrt{2}}\] where d is the effective diameter of gas molecule, m; n is the number of molecules (concentration) per unit volume.

If using the Botzmann equation, 5 p⋅V=n⋅k⋅T \[p\cdot V=n\cdot k\cdot T\] where P is the gas pressure, Pa. According to the standards [19,20], the minimum pressure in the vacuum chamber is 10⁵ Torr and for air pressure, it will be 1.33×10³ Pa; k is the Boltzmann constant, 1.38×10²³ J/K; T is the gas temperature, K. The temperature of the atmosphere in the test bench chamber is 20°C or 293°K.

The expression for determination of the mean free path of an ideal gas molecule (4) can be rewritten: 6 Lm=kTπd2P2 \[{{L}_{m}}=\frac{kT}{\pi {{d}^{2}}P\sqrt{2}}\]

Since the initial atmosphere in the vacuum chamber of the test complex is air – the largest effective molecular diameter of a diatomic structure (N₂ and O₂) is about 3.5×10¹⁰ m.

In the result, the length of the average free path of an air molecule as an ideal gas (6) will be: 7 Lm=(1.38⋅10−23⋅293)/(3.14⋅(3.5⋅10−10)2⋅1.33⋅10−3⋅1,414)=5.6m \[{{L}_{m}}=\left( 1.38\cdot {{10}^{-23}}\cdot 293 \right)/(3.14\cdot {{\left( 3.5\cdot {{10}^{-10}} \right)}^{2}}\cdot 1.33\cdot {{10}^{-3}}\cdot 1,414)=5.6\,m\]

The estimate of the free path for an ideal gas can be taken as the maximum. Turning to a real gas, it is necessary to introduce a correction for the non-ideal nature of the gas. First, with increasing temperature, the vibrational motions of molecules increase, which is practically demonstrated as an increase in the free path and is expressed as Sutherland correction for mean free path: 8 Lm=μPπRT2M \[{{L}_{m}}=\frac{\mu }{P}\sqrt{\frac{\pi RT}{2\text{M}}}\] where C is the Sutherland constant. The Sutherland constant value for air is 120.

Then, taking into account the correction, the free path for air at a temperature of 293 K and a pressure of 10⁵ Torr will be 4.74 m.

In a real gas, gas molecules do not behave like rigid spheres, but rather attract each other at large distances and repel each other at smaller distances, which can be described using the Lennard–Jones potential, where the radius of the molecule is rm = 21/6ϭ~1.122 ϭ. Then the value of the free path of an air molecule at a pressure of 10⁵ Torr and a temperature of 293 K will be 0.93 m.

This estimate of the mean free path can be taken as the most probable in assessing the molecular regime of gas flow.

There is also a way to estimate the bridged path length from the assumption that the real gas is pseudo-viscous. In this case, the mean free path of a molecule for a real gas is determined as 9 Lm=μPπRT2M \[{{L}_{m}}=\frac{\mu }{P}\sqrt{\frac{\pi RT}{2\text{M}}}\] where μ is the viscosity, Pa s (μ = 18.5 μPa s); P is the pressure, Pa (1.33×10³ Pa); R is the universal gas constant, 8.31446261815324 m³ · PaK¹ · mol⁻¹; T is the temperature, K (T = 293 K); M is the molecular mass g · mol⁻¹.

Then, when substituted in Eq. (9), we obtain L_m = 0.27 m. This estimate can be discarded provided that a known molecular gas flow regime is ensured. Thus, it was decided to suppose for the vacuum calculation as the main estimate of the free path of a gas molecule in the vacuum chamber of the space environmental simulator at 0.93 m. From expression (8), taking into account the correction for the Lennard–Jones potential, we obtain a value of 0.186 m. Based on the obtained value of the mean free path and expression (3), let us determine the limitations of the overall dimensions of the vacuum chamber according to the Knudsen criterion to ensure the molecular regime of gas flow in the chamber for the main mode at maximum pressure (1×10⁵ Torr): 10 Ds=Lm/Kn=0.93/10=0.093 \[{{D}_{s}}={{L}_{m}}\text{/}{{K}_{n}}=0.93\text{/}10=0.093\]

Thus, the characteristic size that is the length of the shortest distance from the wall of the vacuum vessel to the test object should not exceed 0.093 m.

According to experimental data, during thermal cycling with imitation of the shadow side of a spacecraft in a space environmental simulation facility, with liquid nitrogen cryogenic shrouds and with a temperature of 90–95°K, the temperature in the region close to the surface of the space of the spacecraft is close to 150°K.

A verification calculation needs to be made to account for the effect of viscosity at low temperatures. So, for example, when the temperature of the atmosphere in the local volume of the vacuum chamber decreases from 293°K to 150°K, taking into account the value of the Sutherland coefficient (for air 120) and initial viscosity μ₀ = 18.5 μPa s, the viscosity of air at a temperature of 150°K will be 14.1 μPa s, and, the free path of a molecule in accordance with Eq. (9) will be 0.08715 m.

Since the effect of a decrease in temperature on the mean free path of a molecule in viscous is very significant, we will take this this limitation into account. Then the characteristic size of the vacuum chamber should not exceed 0.0872 m.

Substituting into Eq. (1), the obtained values of the characteristic diameter of the test object and the double distance from the test object to the cryogenic shroud (7–9), taking the thickness of the cryogenic shields as 10 mm and the distance from the cryogenic shroud to the body of the vacuum chamber as 40 mm, we obtain the calculated diameters of the vacuum chamber: Dmin=141.4+2⋅87.2+2⋅10+2⋅40=415.8Dnom=148+2⋅87.2+2⋅10+2⋅40=422.4Dmax=354+2⋅87.2+2⋅10+2⋅40=628.4 \[\begin{array}{*{35}{l}} {{D}_{\min }}=141.4+2\cdot 87.2+2\cdot 10+2\cdot 40=415.8 \\ {{D}_{\text{nom}}}=148+2\cdot 87.2+2\cdot 10+2\cdot 40=422.4 \\ {{D}_{\max }}=354+2\cdot 87.2+2\cdot 10+2\cdot 40=628.4 \\ \end{array}\]

Taking into account the technological tolerances, we initially take the calculated inner diameter of the prototype vacuum chamber: D_min = 400⁺¹⁰ mm; D_nom = 500⁺¹⁰ mm; D_max = 800⁺¹⁰ mm.

Substituting into Eq. (2) the obtained values of the characteristic length, we obtain the calculated the total length of the cylindrical part of the vacuum chamber: ➢

for a diameter of 400 mm, the length of the cylindrical part is taken as 650 mm;

The vacuum pumping system is also part of the vacuum space simulators. The vacuum system function is to produce a desirable vacuum level in a reasonable time, and also needs to maintain such a level during the entire test.

To generate vacuum in the chamber, the system has two interconnected pumping units (Fig. 5). One dry vacuum pump is used to reduce the pressure inside the chamber from 1,013 mbar to 1 × 10⁻³ mbar, and a cryogenic pump is used to relieve pressure from 1 × 10⁻³ mbar to about 1 × 10⁻⁸ mbar. The dry pump functions as a primary pumping unit in the system and operates as backing pump for the cryogenic pump. The backing pumping necessary for the initial operation of the cryogenic pump is achieved by a connection line from the dry mechanical vacuum pump. This pumping process is necessary because of the mechanical performance limits of existing pumping units. The maximum level of vacuum that can be generated inside the chamber depends on the efficiency of pumping units, the level of conductance in lines and appropriate control of cleaning, which avoids the presence of undesirable gases. The cryogenic pump is mounted directly on the high vacuum valve, which is in turn mounted directly to the upper of the chamber, maximising conductance in the system.

Fig. 5.

The principle of operation of vacuum pumps of the condensation type is that when the molecules of a substance have a condensation temperature above the temperature of the cryogenic surface contact, they condense and are held by the latter. In the molecular flow mode (the mean free path is much greater than the distance between the surfaces), the gas flow Q to the cold surface will be equal to [19]: 11 Q=pART/2πM, \[Q=pA\sqrt{RT\text{/}2\pi M,}\] where A is the cold surface area; Q is the gas flow; R the universal gas constant is numerically equal to the work of expansion of one mole of an ideal gas in an isobaric process with an increase in temperature by 1 K; T is the gas temperature; M is the molecular weight.

The theoretical pumping speed per unit of cold surface is determined by the formula: 12 St=Q/pART/2πM=3.64T/M, \[{{S}_{t}}=Q\text{/}pA\sqrt{RT\text{/}2\pi M}=3.64\sqrt{T\text{/}M},\] where S_t is the theoretical pumping rate l/(s · cm²).

These are the designations adopted in vacuum technology. At T = 293 K, the maximum pumping rates of nitrogen and hydrogen are 11.8 and 44.2 l/(s · cm²), respectively. The limiting pressure in chamber p₀ equals the saturated vapour pressure at a cold surface temperature T_x. At pressure p = p₀, the pumping speed is zero, that is, the actual pumping rate S is equal to: 13 S=St(1−p0/p). \[S={{S}_{t}}\left( 1-{{p}_{0}}\text{/}p \right).\]

Gas flow rate is the amount of gas that has passed through the cross-section of the pipeline per unit of time. The question is as to what to consider as a measure of the amount of gas. Traditionally, the volume of gas is the amount of space that it occupies, and the resulting flow rate is called volumetric, and the gas flow rate is expressed in volumetric units (cm³ · min¹, l · min⁻¹, m³ · h⁻¹, etc.). Another measure of the amount of gas is its mass, and the corresponding flow rate is called mass. It is measured in units of mass (e.g. g · s⁻¹ or kg · h⁻¹), which are much less common in practice.

According to Eq. (12), all molecules in contact with a cold surface are held by it. It means that the accommodation (capture) coefficient a_k < 1. Simultaneously with the condensation process, the opposite process occurs, that is, evaporation, and the accommodation coefficient is always less than one a_k < 1. It depends on the coefficients of condensation and evaporation – the temperatures of the gas and the cold surface. In addition, the pumping rate and the duration of operation of cryo-condensation pumps depend on the accumulation of a layer of frost on the cold (cryogenic) surface. The thermal conductivity of this layer, as a rule, is low, and the temperature on its surface can significantly exceed the temperature of the cold surface, thus reducing the speed of pumping out of the medium from the vacuum chamber and affecting the level of the achieved vacuum.

The vacuum chamber allows to maintain a vacuum and simulate the phenomena of thermal effects and other types of radiation without being influenced by the Earth’s atmosphere; these radiations are essential characteristics during the simulation of the outer space thermal environment in the space simulator. High-precision, reliable vacuum sensors are used to control the vacuum level. They are provided with appropriate certificates and allow maintaining the vacuum level during the operation of the vacuum chamber. Parameters of the vacuum level are set from the control panel of the vacuum chamber (Fig. 6).

Fig. 6.

There are no standards or specific rules that describe criteria to build space simulation chambers; however, pressure vessels’ international design standards are generally used for reference for making the appropriate adjustments while considering a vacuum chamber operation.

While designing the MSESF, the following general provisions were taken into account: The main shape of chamber is cylindrical (Fig. 6). As the main structural material of the chamber, austenitic stainless steel grade AISI 304 EN 10088-2 was considered suitable for use at a temperature of liquid nitrogen (70–95 K/203.15°C to 178.15°C).

The use of carbon steels is allowed only for external structures of chamber supports, connected with the vacuum chamber wall through the sheets of austenitic steel.

During the manufacturing of the chamber, joint processing of parts from carbon and austenitic steel is not allowed (only assembly is allowed) to prevent particles of carbon steel falling onto the austenitic steel parts surface and further formation of corrosion sections of weakened material.

After the manufacture of the chamber, anticorrosion treatment of the outer surfaces of the chamber must be carried out by painting with a paint based on impact-resistant and elastic epoxy resin (to ensure coating resistance to temperature deformations and resistance to moisture condensation during accelerated heating of cryogenic shrouds). The vacuum chamber and its supports must be carefully grounded to reduce the corrosion effect during the formation of galvanic pairs.

Calculation has been made in accordance with ISO 16528-1:2007 and ISO 16528-2:2007 [16,17] and include: strengthen calculation for cylindrical shell, elliptical cover, saddle support, flat bottom and reinforcing rib of the flat bottom. Calculated parameters and the results are given in Tables 2–4.

The 3D CAD prototype was created and the necessary technical documentation (Fig. 7) has been prepared after the basic elements of the MSESF had been calculated and analysed in CAE programmes.

Figure 7:

The main stages of the design of the mobile space environment simulation facility ‘METAMORPHOSIS’ developed at Riga Technical University in cooperation with SIA Cryogenic and Vacuum Systems are determined. The article focuses on the fact that in the integration and test phases, the designed mobile testing facility is assembled, integrated, and tested in accordance with the requirements of the ESA ECSS and NASA standards systems. It is shown that the main challenge in designing the MSESF facility is to ensure the ability to maintain operational characteristics in the conditions of intermodal transport, that is, in conditions of vibration, shock, linear acceleration and angular displacement of the supporting structure of the vacuum chamber. In the course of the analysis of the vulnerability of a conventional thermal vacuum facility structure in the conditions of intermodal transportation, the main technical risks and the parts most vulnerable to these influences are identified. High-vacuum pumping system, vacuum seals, thin-walled pipelines of cryogenic and vacuum systems and other elements, systems and components (total number is 538 units) are included in the units list and required additional checks or design and technological measures to protect against destruction of the impact of the specified mechanical loads.

The composition of the systems and the structure of the space simulator test facility have been determined. It is shown that the vacuum chamber with the vacuum system are its essential parts. The principles of their calculations are also shown. The following dimensions of the vacuum chamber are accepted as the prototype version: diameter of the vacuum chamber is Ø500 mm and the length is 780 mm. These sizes are used as a base for further calculations. Based on the results of the prototype vacuum system and basic geometric parameters’ calculations, the development of the 3D CAD model of prototype construction elements has been completed and the design documentation has been prepared as well.

The next stage in the implementation of the Metamorphosis project is to carry out experiments and tests on the developed mobile space environment simulation facility.