The Studies of Small Space Vehicles Ammoniac Electrothermal Engine Units Design and Structural Layout

The paper studies the issues of design and structural layout of ammoniac corrective engine units (CEU) with electrothermal micromotors (ETMM) for manoeuvrable small space vehicles (SSV) using all-purpose approach. CEU and ETMM structural schemes are defined, adaptation questions of CEU to SSV are addressed, CEU mass analysis is carried out, and experimental study’s findings are given.


Introduction
The modern stage of cosmic space exploration is characterized by large-scale development and use of SSV of 30-400 kg for solving scientific and application problems launched by group and additional ways (Sadovnichy e.t al., 2011;Sadovnichy et al, 2007;. For that reason, there are relevant objectives of SSV orbital manoeuvring: liquidation of orbital injection mistakes, orbital parameters maintenance during the period of active existence, inter-orbital manoeuvring, SSV orbital groupings constructing, SSV withdrawal to the orbit of disposal, etc. Electric CEU used for the orbital manoeuvring of "larger" space devices are marked by the greater value of thrust (for example, stationary plasma engines -up to 20W/MN and more), that makes them not applicable for SSV with limited electrical power supply. CEU with thermocatalytic micromotors with low thrust value have greater thrust (100-500 MN), that is unacceptable for SSV because of prohibitive disturbances at CEU working. Developed ammoniac CEU with electrothermal micromotor (ETMM) thrust 30 MN is characterized by thrust value to 4W/MN and is successfully used in many SSV (Blinov V. N. et al., 2011, patents 2332583, 2442011, 2375267).
Ammoniac CEU operating principle is based on ammonia dissociating in ETMM with its decomposition into hydrogen and azote. It is possible to significantly increase ETMM thrust performance index due to double reduction of molecular mass of flowing out ammonia in comparison with ammonia gas. The processes of rapid high temperature dissociation of ammonia in ETMM in vacuum for various modes of functioning, project and design parameters define the effectiveness of CEU when solving the problems of orbital manoeuvring of SSV by the criterion of CEU reduced mass that uses ETMM thrust performance index and power consumed  When using ammoniac CEU with ETMM the problem of the quest for a compromise between thrust value and ETMM thrust performance index raises. Ammoniac CEU specific performances improving involves enhancement of their design and improvement of ammonia high temperature dissociating processes effectiveness for various methods, and of launching duration, power consumed.

Materials and Methods
By their structural and methodical construction ammoniac CEU are designed for low-orbiting manoeuvrable SSV (MSSV) of various applications with the use of all-purpose design methods (guaranteed result method and structural approach) and dimensions & mass method to calculate mass characteristics of CEU and their adaptation means to MSSV.
The efficiency estimation of all-purpose CEU developed according to the method of "guaranteed" result shows that best effectiveness is achieved for most "heavy" objective function from amongst feasible ones for CEU with fixed vectors of design and structural parameters. When performing all other objective functions all-purpose CEU effectiveness is always lower, even if these functions are closely approximated (Brusov, V. S., & Baranov, S. K., 1989;.
CEU effectiveness improvement can be achieved using structural access to CEU composition optimization.
Let us present the range of S * design parameter vector, defining CEU structural composition as: where, S B -"s B " vector range, which defines base structure of all-purpose CEU, used when solving all range of purpose-oriented problems; S C -"s C " vector range, which defines component structures of all-purpose CEU used when solving separate purpose-oriented problems.
When forming a new structure of CEU in solving various purpose-oriented problems, we will have a number of CEU versions, which have the same base structure, but differ among themselves with components and associated systems composition.
When applying all-purpose research method of CEU design and structural layout for MSSV one of the relevant tasks of designing is a structural parametric synthesis carried out in the presence of current and expected variety of purpose tasks and conditions of MSSV use. Finding optimal structure and main design parameters of CEU at early designing stages makes it possible to reduce expenditures for CEU development providing the high execution effectiveness of purpose-oriented problems at hand.
Let us consider an interrelation of CEU design and structural parameters. CEU structure is understood to be its breakdown to systems and structure with an indication of connections between them, unchanged (according to the all-purpose method of guaranteed result) or changeable (according to the all-purpose method of structural design) and providing the idea of CEU as a complex system.
(2) shows that all-purpose CEU base structures used for overall range of objective tasks solving are structures the mass of which does not dependent on fuel mass. Base structures are CEU -control unit; evaporator; pressure regulator valve; electro-valve; isolating valve; ETMM -electrical power supply; temperature control system; terminals containment system; heat element with insulation system; gas flow formation system. CEU component structures are fuel systems with structural parts. www.ccsenet.org/mas Modern Applied Science Vol. 9, No. 5;2015 CEU mass characteristics and its adaptation means to MSSV are defined using the dimensions & mass method according to the generalized formula: where, , -density and volume of the i-th element.
Let us examine CEU installed in MSSV in a special division with capability of movement in two orthogonally related directions for ETMM thrust vector setting ( Figure 3) (patent 2375267).
Input parameter for all constructional elements dimensions calculation is fuel reserve m F and operating pressure in fuel container p o .
Thus, the radius of fuel cylindrical container R t with egg ends R sph (R sph = R t ) is defined via fuel reserves in CEU m F (Blinov V.N. et al., 2012): where, Plate dimensions for fuel container mounting are defined based in container and automatic equipment dimensions. For rectangular plate, lateral and longitudinal dimensions l t , l l are defined by the dimensions of fuel container and coefficients k t , k l , with regard to dimensions increase for CEU automatic equipment installation: In the plate for fuel container mounting cut is made with a dimension 2R t and (k ext +2)R t . Strength elements thickness for designing initial stages is set based on analogues.
Using the given expressions, the following expression for CEU total weight is found according to dimensions & mass method (Blinov V.N. et al., 2012): -container cylindrical section generalized factor, considering density γ m and break point stress under tension σ b of construction material, fuel container coefficient k t , safety coefficient n, weight coefficient k w , considering container wall thickening in respect to minimal permissible value; = 2 + 2 − 2 + 2 -plate generalized factor for mounting fuel container, considering lateral and longitudinal plate dimensions for mounting fuel container -plate reduced thickness coefficient, considering ratio of plate weight thickness p w δ to fuel container radius; γ p -platform material density; k l = 4πF l γ l -lodgement reduced factor for mounting container with a square of F l and material specific gravity γ l .
Let us write the structural mass equation of CEU adaptation means to MSSV with movable in two orthogonally The mass of end plate with diagonal supports (we assume its dimensions as l t , l l ): where red p 1 m S F γ -the reduced area of section and density of end plate beams material.
We also assume the dimensions of plate for CEU installation without diagonal supports as l t , l l , and then its mass will be -the reduced area of plate beams section for CEU installation.
Stand mass with a length l r , cross sectional area F r and material density γ r for fastening and moving CEU: where h aut , h CU -the altitude of automatic equipment protrusion and two control units towards the fuel container; h reg -increase in stands length due to position control of DUMIT when setting ETMM thrust vector.
In order to define MSSV CEU m let us introduce the constant coefficient mass load of lateral plates -structural weight side p m of SSV for the volume they occupy in the following way: Then, if CEU as a part of MSSV occupies volume V CEU and the dimensions of CEU define cross dimensions of MSSV (what is fair for this MSSV layout): where i l i t k k , -coefficients, determining cross dimensions of MSSV owing to l t l l , extension.
The presented dependences demonstrate interaction of CEU design (m F , P sp ), structural parameters and CEU adaptation means as a part of MSSV. A preference for all-purpose design method is driven by the extent of difference of objective problems among set ones with MSSV design layout change estimation: dimensions, mass. MSSV design layout change subject to objective tasks difference degree is defined with regard to CEU accepted arrangement as a part of MSSV. s .
It's concluded based on the analysis of constructed ASP and MSSV that for a MSSV mass range of (30-40), (40-70) kg the structure, as a rule, is a component structure and it is developed according to the structural method depending on the current objective task of MSSV (Blinov V.N. et al., 2012).
When increasing MSSV mass range the method of structural decomposition of ASP is defined based on the analysis of orbital manoeuvring tasks and used methods of MSSV designing in general.
Thus for MSSV ERS with a mass of to 250 kg a preference is given to a ASP scheme wherein frame structure with plates, on which base structures of APS support systems, optoelectronic system and CEU (frame arrangement) are mounted, is used as a base structure (patent 2457157). Given this, both the guaranteed result method and the structural design method can be applied.
When designing MSSV with CEU embedded in the instrument unit, a combined approach can be used to increase the guaranteed result method effectiveness (Figure 2) (Blinov V.N. et al., 2012):  The application of structural method allows searching for optimal structural solutions without feasible alternative structures initial set previous construction ( Figure 6).  1 -ETMM; 2 -pressure switch; 3 -electro-pneumatic valve; 4 -pressure-relief valve; 5 -ammonia evaporator; 6 -filter; 7 -ammonia container; 8 -outer case; 9 -gas passage with a nozzle; 10 -electric heater; 11 -inner case; 12 -clamping nut with ammonia feeding line -directly in CEU control unit owing to control program execution in accordance with flexible cyclogram, parameters of which can be changed using input data (pre-set values), -by means of the execution of onetime temporary commands from SSV on-board control system.
To manage CEU external (transmitted to SSV board from a ground control post) and additional commands are used. Control unit external commands are formed in the form of temporary commands of SSV on-board control system at a ground control post, are transmitted to SSV board during communication session, and then in the form of CAN interface command are transmitted to connected current (main or reserve) CEU control unit at the set time.
Maximum operate time of electro valves is CEU working time limit in the mode of developing thrust for the purpose of overheating prevention. Moreover, when CEU operates SSV pointing and attitude control system identifies disturbing torques occurring at ETMM operating. CEU control unit periodically compares current kinetic momentum of SVV hand wheels control motors with given range of torques and in case of overrunning generates command for CEU shutdown.
By means of CEU control program selection the pre-set type of starting (cold and hot), CEU running time at single start, and evaporator and ETMM power consumption distribution by time are realized. Realizable control parameters of CEU control eventually defines ETMM thrust performance index.
ETMM hot launching cyclogram involves structure preheating and gasified ammonia injection in EMTT ( Figure  9). ETMM cold starting cyclogram involves gasified ammonia injection in ETMM with simultaneous switching of heating elements (Figure 10). The disadvantage of cold starting is ETMM low reliability because of heaters' possible burning due to the lack of heat pickup when heating up the structure.
Ammonia temperature at the inlet to ETMM nozzle throat section is measured using two embedded thermocouples of ТХА type, pressure -using pressure switch in CEU piping. CEU control unit measures electrical parameters (power, current, resistance).

Results and Discussion
The design method of CEU and ETMM with convertible and extensible structure was used for ammoniac CEU construction with a fuel reserve of 2.7 kg, 4.0 kg, and 0.4 kg (Figure 11-13).  CEU as a part of SSV 1 -ETMM (heat-protective casing is not shown); 2 -pressure-relief valve; 3 -evaporator; 4 -electro-pneumatic valve; 5 -filter; 6 -vent coupling; 7 -filling coupling; 8 -ETMM heat-protective casing; 9 -fuel container; 10 -thrust vector setting system -reliability support (a, b): unit control, pneumatic-hydraulic systems, ETMM heating elements, ETMM redundancy (b); cold starting use, the use of ETMM two control channel by temperature and power.
The distinctive design peculiarities of ETMM with a conic nozzle are: -conic nozzle implementing together with a gas passage; -electrical wire (nichrome) heating elements (main and reserve) location out of gas passage in two-channel ceramic tube; -gas passage location with heating elements in a ring flanged cylindrical can; -cylindrical can location with gas passage and heating elements inside cylindrical body; -installation of contact elements of two thermocouples for temperature control in gas passage; -gas flow vortex in the form of diagonal gas-feed cuts in can ring flange, which contacts casing inside surface.
For ETMM with a conic nozzle an increased heating (35-40%) of ammonia gas in comparison with analogues, 30-35% increase in fuel gas spinning are provided.
The operation of ETMM with a conic nozzle made conjointly with gas passage showed its effectiveness. However when pressure at the inlet of ETMM critical nozzle is approximately 0.05 MPa, the diameter of nozzle throat section is small (0.7 mm). Therefore, technologic difficulties arise concerning its production.
Pressure at the inlet of critical nozzle should be reduced to 0.02 MPa in order to increase nozzle throat section diameter. Given this, nozzle throat section diameter increases to 1 mm and its production and control present no technologic problems.
On the other hand, one of the ways of ETMM thrust performance index increase is the use of a bell nozzle. The optimal diameter of bell nozzle cut for nozzle throat diameter of 1 mm is 10 mm. In ETMM with a conic nozzle, the maximum diameter of nozzle section is defined by the gas passage outer diameter, which cannot be more than 3…5 mm in view of design features. At such dimensions bell nozzle will have non-optimum characteristics.
That is why using all-purpose method the ammoniac ETMM with an opportunity of bell nozzle installation was constructed ( Figure 15). 1 -nozzle; 2 -tubular heating element (main and reserve); 3 -casing; 4 -flanged can; 5 -gas passage; 6thermocouple; 7 -terminals casing Figure 15. ETMM structural scheme with a bell-shaped nozzle with T-shaped configuration of terminals Constructed ETMM allows using various bell nozzles far different by characteristics without main structure change. It is provided by means of bell nozzle executing in the form of separate removable nozzle insert put in ETMM body and contacting gas passage end surface providing gas inlet to the nozzle. The use of a bell nozzle makes it possible to increase ETMM thrust performance index by (10-15)%, and significantly improve the operational characteristics of nozzle production.
ETMM cold starting guarantees heating elements integrity maintenance and is considered main starting method of ETMM.
Cold starting efficiency upgrading can be achieved by the increase in power consumption and ETMM operating time at single starting.
At ETMM cold starting with a capacity of 60W, duration from 3 to 10 minutes, temperature in ETMM vessel hit the range from 731°K to 890°K. When increasing ETMM operation time to 20 minutes, the predicted temperature will be up to 913-923°K. At dissociating power consumption of 80W ammonia, the temperature increases by 373-393°K.
The use of hot starting of ETMM is only possible when providing heating elements reliability by means of development of heat removal system from heaters during ETMM heating.
At ETMM short switching in accordance with "hot" scheme from 2 to 5 minutes long temperature in ETMM vessel reached the range from 847°K to 999°K.  Figure 18. Ammonia temperature change in ETMM and evaporator casing temperature change at ETMM cold starting and its operation for 600s at power consumption 60W and gas-flow rate ≈12-16 mg/s Figure 19. Ammonia temperature change in ETMM and evaporator casing temperature change at ETMM cold starting and its operation for 300s at power consumption 80W and gas-flow rate ≈12-16 mg/s To analyse the specialty areas of ammoniac CEU with ETMM in accordance with figure 12 using dimensions and mass equations (1-13) mass characteristics of CEU and its adaptation means to SSV are shown in Figure 20.  Vol. 9, No. 5;2015 Figure 20. Change of dry mass of dislocating CEU to SSV, fuel, and CEU adaptation means to SSV with a mass of 150 kg depending on realizable reference speed Reference speed through CEU and SSV parameters is defined according to the proportion: where ΔV-reference speed; The analysis of similar findings for SSV with a mass of 100-400 kg shows that for the realization of reference speed 100 m/s the use of ammoniac CEU with ETMM with general energy consumption no more than 100W as a part of SSV requires (19-12)% of mass costs (fuel mass, CEU, CEU adaptation means to SSV - Figure 21).  Vol. 9, No. 5;2015