The rocket and space world at a crossroads: global trends require lower cost and increase the environmental safety of space services. Designers have to invent new liquid-propellant rocket engines (LRE) on environmentally friendly fuels, replacing expensive, extremely energy-intensive liquid hydrogen with cheap liquefied natural gas (LNG) with a methane content of 90 – 98 percent. This fuel paired with liquid oxygen allows you to create new high-performance and low-cost engines with maximum use of existing elements of design, material, technological and production reserve.
LNG is not poisonous; when it is burned in oxygen, water vapor and carbon dioxide are formed. Unlike kerosene, which is widely used in rocket technology, the LNG straits quickly evaporate without harming the environment.
The ignition temperature of natural gas with air and the lower limit of its explosive concentration is higher than that of hydrogen and kerosene vapor, therefore, in the area of low concentrations, compared to other hydrocarbon fuels, it is less explosive.
In general, the operation of LNG as a rocket fuel does not require any additional fire and explosion prevention measures that have not been applied previously.
LNG density is six times higher than liquid hydrogen, but two times lower than kerosene. Lower density leads to a corresponding increase in the size of the LNG tank compared to a kerosene tank. However, given the higher cost ratios of the oxidizer and fuel (approximately 3,5 to 1 for liquid oxygen (LN) + LNG fuel and 2,7 to 1 for LCD and kerosene fuel), the total amount of fuel, LCD + LNG, increases only percent on 20. Taking into account the effect of cryogenic hardening of the material, as well as the possibility of combining the bottom of tanks LCD and LNG weighting of fuel tanks will be relatively small.
And finally, the production and transportation of LNG has long been mastered.
The chemical engineering engineering design bureau (Khimmash Design Bureau) named after A. Isaev in Korolyov near Moscow began work (as it turned out, stretching for years due to very scarce financing) on the development of “LCD + LNG” fuel in 1994, when the design - design studies and a decision was made to create a new engine using the schematic design base of the existing oxygen-hydrogen 1 7,5 tf engine, successfully operated as part of the upper stage (Cryogenic Upper Stage) 12KRB of the Indian GSLV MkI launch vehicle (Geos ynchronous satellite launch vehicle).
In 1996, autonomous fire tests of the gas generator on the LCD and natural gas as components of the fuel took place, with the aim of mainly checking the start-up and stable operation modes - 13 inclusions confirmed the gas generator's performance and gave results that were used in the development of regenerative gas generators working on open and closed schemes.
In August-September of 1997, a chemical test of the KVD 1 engine steering block (also using natural gas instead of hydrogen) was carried out in KB Himmash, in which the chamber deflected in two planes at an angle of ± 39,5 degrees (thrust - 200 kgf, pressure in the chamber - 40 kg / cm2), start and stop fittings, pyrotechnic ignition system and electric drives - one regular steering unit KVD1 passed six starts with a total operating time of more than 450 seconds and a chamber pressure in the 42 – 36 range kg / cm2. The test results confirmed the possibility of creating a small-sized chamber using natural gas as a cooler.
In August, 1997 of the year, Khimmash Design Bureau began firing tests of a full-size engine of a closed circuit with an 7,5 vehicle mass on the LCD + LNG fuel. The basis for the manufacture was the modified KVD1 engine of a closed circuit with afterburning of reducing gas-generating gas and cooling the chamber with fuel.
The standard oxidizer pump KVD1 was modified: the diameter of the pump impeller is increased to provide the necessary ratio of the oxidizer and fuel pump pressure. Also carried out the adjustment of the hydraulic settings of the engine lines to ensure the calculated ratio of components.
The use of a prototype engine that previously passed a fire test series on “LCD + liquid hydrogen” fuel ensured a maximum reduction in research costs.
Cold tests allowed to work out the method of preparing the engine and stand for fire work in terms of providing the required LNG parameters in bench tanks, cooling the oxidizer and fuel lines to temperatures that ensure reliable operation of the pumps during the starting period and stable and steady start of the engine.
The first fire test of the engine took place on 22 on August 1997 of the year at the stand of the enterprise, which is today called the Scientific and Testing Center of the Rocket and Space Industry (SIC RCP). In the practice of Khimmash Design Bureau, these tests were the first experience of using LNG as a fuel of a full-size, closed-circuit engine.
The task of the test was to obtain a successful result by reducing the parameters and easing the engine operating conditions.
The control of the mode output and mode operation was carried out using thrust regulators and the ratio of fuel component costs using the 1 KVD algorithms, taking into account the mutual influence of control channels.
The program of the first fire test engine closed circuit was completely implemented. The engine has worked the specified time, there were no comments on the condition of the material part.
The test results confirmed the fundamental possibility of using the LNG oxygen-hydrogen engine in the aggregates as a fuel.
A lot of gas - no coke
Further tests were continued in order to more thoroughly study the processes associated with the use of LNG, test the operation of engine units under wider conditions of use, and optimize design solutions.
In total, five fire tests of two KVD 1997 engines, adapted for the use of LCD + LNG fuel, lasting from 2005 to 1 seconds, the content of methane in LNG from 17 to 60 percent, passed from 89,3 to 99,5.
In general, the results of these tests allowed to determine the basic principles of the development of the engine and its units using the “LCD + LNG” fuel and move to the next stage of research in 2006, involving the development, manufacture and testing of the С5.86 engine. The combustion chamber, the gas generator, the turbopump assembly and the regulating organs of the latter are structurally and parametrically made specifically for operation on the “LC + LNG” fuel.
By 2009, two fire tests of С5.86 engines with a duration of 68 and 60 seconds were carried out with methane content in LNG 97,9 and 97,7 percent.
Positive results were obtained on the launch and shutdown of rocket engine, operation in steady-state modes of propulsion and the ratio of fuel components (in accordance with control actions). But one of the main tasks - experimental verification of the absence of accumulation of solid phase in the cooling path of the chamber (coke) and in the gas path (soot) with sufficiently long inclusions - could not be performed due to the limited amount of bench LNG tanks (maximum activation time was 68 seconds ). Therefore, in 2010, the decision was made to retrofit a test bench for conducting fire tests with a duration of at least 1000 seconds.
As a new workplace, the SEC RCP stand was used for testing oxygen-hydrogen LPRE, which has capacities of the corresponding volume. In preparation for the test was taken into account significant experience previously obtained during the seven fire tests. During the period from June to September 2010, bench systems of liquid hydrogen were refined for use of LNG, the С5.86 engine No. 2 was installed on the stand, comprehensive inspections of measurement systems, control, emergency protection, control of the ratio of fuel consumption and pressure in the combustion chamber were conducted.
Refueling of bench containers with fuel was carried out from the tanker transport tank (volume - 56,4 м3 with refueling 16 т) with the help of a block of refueling LNG, including a heat exchanger, filters, stop valves, measuring instruments. After the filling of the tanks was completed, the bench lines for supplying the fuel components to the engine became chilled and filled.
The engine started and worked fine. Mode changes occurred in accordance with the effects of the control system. With 1100 seconds, the gas-generating gas temperature was constantly increasing, as a result of which it was decided to stop the engine. The shutdown took place on command at 1160 second without comment. The reason for the increase in temperature was the leakage that occurred during the test of the exhaust manifold of the cooling chamber of the combustion chamber - a crack in the weld of the plugged technological fitting installed on the manifold.
Analysis of the results of the fire test allowed to conclude:
-in the process of operation, the engine parameters were stable in the modes with various combinations of the ratio of fuel component costs (2,42 to 1 - 3,03 to 1) and thrust (6311 - 7340 kgf);
- confirmed the absence of solid phase formations in the gas path and the absence of coke deposits in the liquid path of the engine;
- necessary experimental data were obtained to clarify the method for calculating the cooling of the combustion chamber when using LNG as a cooler;
- the dynamics of the exit of the cooling path of the combustion chamber to the steady-state thermal regime was investigated;
- confirmed the correctness of technical solutions to ensure the launch, control, regulation, and other things, taking into account the characteristics of LNG;
5.86 developed by С7,5 can be used (alone or in combination) as a cruising engine in promising boosters and upper stages of the launch vehicles;
-positive results of fire tests confirmed the feasibility of further experiments to create an engine on the fuel "LCD + LNG".
At the next fire test in 2011, the engine was turned on twice. Before the first shutdown, the engine worked for 162 seconds. On the second start, which was carried out to confirm the absence of formations of a solid phase in the gas path and coke deposits in the liquid path, a record duration of engine operation of this dimension was achieved with a single switch-on - 2007 seconds, and the throttling potential was confirmed. The test was terminated on the development of fuel components. The total operating time of this engine instance was 3389 seconds (four starts). The performed fault detection confirmed the absence of solid and coke formations in the engine paths.
The complex of theoretical and experimental work with С5.86 No. 2 confirmed:
- the fundamental possibility of creating the engine of the required dimensions on the fuel pair of components "LCD + LNG" with the afterburning of regenerative generator gas, ensuring the maintenance of stable performance and the practical absence of the solid phase in the gas paths and coke deposits in the liquid paths of the engine;
-the possibility of multiple start and stop the engine;
-the possibility of continuous engine operation;
-the correctness of the adopted technical decisions to ensure multiple start-up, control, regulation, taking into account the peculiarities of LNG and emergency protection;
-the ability of the SEC RCP stand to conduct long-term tests.
Together with SIC RCP, the technology of transporting, refueling and thermostating large masses of LNG was developed and technological solutions practically applicable to the procedure for refueling flight products were developed.
LNG - the way to reusable flights
Due to the fact that components and assemblies of the C5.86 engine demonstrator No. 2 were not optimized due to limited funding, the number of tasks was not fully resolved, including:
refinement of the thermophysical properties of LNG as a cooler;
obtaining additional data to verify the convergence of the characteristics of the main units in the simulation of water and work on LNG;
experimental verification of the possible influence of the composition of natural gas on the characteristics of the main aggregates, including the cooling paths of the combustion chamber and the gas generator;
determination of the characteristics of the LRE in a wider range of changes in operating modes and basic parameters for both single and multiple inclusions;
optimization of dynamic processes at startup.
To solve these problems, Khimmash Design Bureau manufactured the upgraded С5.86А engine No. 2А, the turbopump unit of which was equipped for the first time with a starting turbine, upgraded with the main turbine and a fuel pump. The cooling path of the combustion chamber has been upgraded and the throttle needle of the fuel ratio has been redesigned.
The fire test of the engine was carried out on 13 on September 2013 of the year (methane content in LNG - 94,6%). The test program involved three inclusions with a total duration of 1500 seconds (1300 + 100 + 100). The engine was started and operated normally, however, at 532 second, the emergency protection system formed an emergency shutdown command. The cause of the accident was the ingress of an extraneous metal particle into the flow part of the oxidizer pump.
Despite the accident, С5.86А № XNUMHA worked for a long time. For the first time, the engine was launched for use as part of a rocket stage that requires multiple start-ups according to the implemented scheme using an onboard refillable pressure accumulator. A stable operation mode was obtained for a given mode according to the load and the maximum ratio of fuel component costs previously implemented. The possible reserves for forcing the thrust and increasing the cost ratio of the fuel components are determined.
Now Khimmash Design Bureau is completing the production of a new C5.86 instance for testing for the maximum possible resource in terms of operation time and number of inclusions. It should become a prototype of a real engine on “LCD + LNG” fuel, which will give a new quality to the upper stages of launch vehicles and breathe life into reusable transport systems. With their help, space will be available not only for researchers and inventors, but, perhaps, just for travelers.