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Satellites' Flaming and Ice Motors

Hi with you lozgain the somewhat unusual role of the author of a corporate blog post. In January, the Asteroid Safety Olympiad was held at the Chelyabinsk Boiling Point , in which Valery Bogdanov and Ruslan Peshkov, satellite builders and teachers of the South Ural State University, gave a lecture for schoolchildren on engines for spacecraft. This material is a detailed treatment of the lecture and my conversation with them.


Methane-Oxygen Fuel Orientation Engine, NASA Photo / John H. Glenn Research Center

Wealth of choice


The first written evidence of the creation of rockets dates back to the 13th century. Only four centuries later, thanks to Newton, did humanity understand how they work. Hint: the rocket does not “push off” - in a vacuum there is nothing to repel from, but accelerates in one direction due to the emission of the substance in the opposite direction. And in the 20th century, people, creating satellites, came up with many options for their movement, and there are even those that do not work on mass ejection. The main types of satellite engines can be classified as follows:



As often happens, in our world, there is no “ideal” engine for all cases, and the task of “choosing an engine for a spacecraft” can have many different conditions, making some options unacceptable or ineffective. And in order to be able to compare anything, we first need to find the criteria by which we will compare. For engines, specific impulse and thrust can be distinguished.

Specific impulse- a measure of engine efficiency. By definition, this is the time during which the engine can develop traction by spending 1 kg of fuel. If we measure the engine thrust in kilograms (more precisely, kilogram-force), then the specific impulse will be measured in seconds, and if the thrust is measured in Newtons, then the unit of measurement will be a meter per second. From seconds to meters per second and vice versa, the specific impulse can be calculated by multiplying or dividing by the acceleration of gravity. The meaning of the change in meters per second can be more easily understood if it is represented as the speed of the substance flowing out of the engine at the nozzle exit. Obviously, the larger the specific impulse, the more efficient the engine will be.

Thrust, in fact, is a service parameter or even a limitation. Very efficient engines, which, however, develop a little traction, cannot be applied in a situation where you need to make a maneuver that requires a large change in speed and cannot be stretched in time (or divided into many engine starts).

The combination of both values ​​allows us to build a graph of the most suitable engines in each case.


Source: Reactive control systems for spacecraft, Belyaev N.M., Belik N.P., Uvarov E.I. - M.: Mechanical Engineering, 1979.

Cold gas engines


If you were interested in the history of astronautics, then you know that gas engines oriented Gagarin's "Moon-3" and "East". Now that the satellites are abandoning chemical engines and are switching to electro-reactive, gas engines may seem archaic due to weak traction and low specific impulse, however, this is not so. Until now, designs are sent to space that seemingly outdated engines are better than others. First of all, compressed gas is non-toxic and is perfect for astronauts' rocket packs - a possible leak will not poison the crew, and the exhaust settled on the suit will not have to be deactivated.


Bruce McCandless tests MMU installation in 1984, NASA photo

The design of a cold gas propulsion system is simple and reliable, except for cylinders, solenoid valves and pipelines, consider that nothing is needed. If you have enough small reserves of characteristic speed (delta-V), then this design will be easier than more complex alternatives. Orientation gas engines have excellent dynamics - the valves are very fast (reaction time up to 20 milliseconds) and do not need, say, preliminary heating of the catalyst. Due to these advantages, they are perfectly suited for accurate low-thrust orientation systems, therefore, for example, the Gaia telescope went flying with gas orientation engines. The LISA Pathfindermade to test the possibility of the gravitational observatory working in space and in need of very precise movement, two engine options - cold gas and electric with field emission, in flight they were successfully tested both together and separately.


The scheme of the engine on cold gas. Red is the region of high gas pressure, blue is the region of low.

But, of course, you cannot get a lot of thrust or a big change in speed on gas engines — the specific impulse of compressed nitrogen is no more than 80 seconds, and it also decreases with decreasing pressure in the tank. Compressed hydrogen has a specific impulse of more than 270 seconds, but due to the low density, the thrust is irrationally small.

Sublimation engines


In the diagram in the lower left corner there are sublimating substances acting as a working fluid. That is, a solid body is in the tank, which, when heated, immediately passes into the gas, bypassing the liquid state. In everyday life, you could meet “dry ice” - carbon dioxide frozen to a solid state. Due to the fact that compressed gas engines can offer large thrust and specific impulse, they actually supplanted sublimation engines. But the concept is not abandoned at all - NASA research, performed in the early tenths, considers such engines to be promising, provided that the working fluid is extracted on the spot. Carbon dioxide ice on Mars, methane and carbon monoxide in the outer solar system, volatile substances of asteroids and comets - all this can theoretically be used as locally produced fuel.


The concept of a device deviating a comet from a dangerous course and using local resources as a working fluid, NASA illustration

Single component engines



This is not a fire on board, but a regular discharge of hydrogen peroxide used in the orientation system, landing of Soyuz MS-02, NASA / Bill Ingals photo

There are substances that decompose under certain conditions with the release of heat, and decomposition products are well suited for to direct them into the Laval nozzle and get traction. The design is still simple, there is only one fuel supply line, there are no problems mixing the components in the combustion chamber, the temperatures do not require complex cooling, but the specific impulse is already higher than that of cold gas engines.


Single Component Engine Diagram Concentrated Hydrogen Peroxide

Abilitydecomposed on a simple iron catalyst into water and oxygen with abundant heat release made it a popular type of single-component fuel even at the dawn of space exploration - the turbo pump of the engines of the first and second stages of the R-7 family rockets since 1957 and still works on the decomposition of peroxide . The engines of orientation of the first versions of the Soyuz spacecraft worked on it, and until now it has been used to orient the descent vehicle. Concentrated peroxide is not toxic, but causes burns if it comes into contact with the skin and is explosive if the lines are contaminated. And yet it will not work for years and decades. The specific impulse is relatively small, in the region of 150 seconds. Due to the last two properties, peroxide is rarely used now.


Hydrazine traction motor 1 Newton, manufacturedArianespace

hydrazine decomposes in the presence of a catalyst heated to 200-300 ° C. Due to the need for preheating, the engine's dynamics drop, hydrazine is very toxic, but, thanks to the highest specific impulse of 230-240 seconds and the possibility of long-term storage, it remains a popular option for a single-component engine.

Nitrous oxide is also decomposable in the presence of a catalyst, but at higher temperatures, which complicates the design of the engine. But it is good because it is not toxic, not choking, does not cause burns, is not explosive, does not cause corrosion and can be stored for a very long time. At Stanford University weresuccessfully tested engines for nitrous oxide with a thrust of up to 2 Newtons, working without destroying the catalyst for more than an hour at temperatures up to 1225 ° C. The operating temperature may be even lower, an article by the University of Surrey (Great Britain) showed that at a temperature of 520 ° C nitrous oxide decomposes without catalysts, which allows the creation of engines on self-sustaining decomposition. Unfortunately, the specific impulse of nitrous oxide is low, up to 170 seconds.


Nitrous Oxide Experimental Catalytic Chamber, Stanford University Photo

The toxicity of hydrazine leads to the study of more exotic options, for example, ammonium dinitramide (ADN) or hydroxylammonium nitrate (HAN). Having a specific impulse even slightly larger than that of hydrazine, these chemical compounds are much less toxic, but require high temperatures in the catalytic chamber.

Hydrazine classic


The spacecraft should work for a long time, so the fuel for it should be able to be stored for years. And if you need a lot of traction and a sufficiently large specific impulse, then the most developed and familiar option will be a two-component engine based on asymmetric dimethylhydrazine (as an option, monomethylhydrazine or mixtures with hydrazine, so-called aerosine) and diazotetroxide.


Tests of a 200 N thrust orientation engine in a pressure chamber, photo by Arianespace

This fuel vapor has many advantages - the fuel is in a liquid state at room temperature, stored for years, self-igniting upon contact, that is, it does not need ignition systems, has good dynamics and a good specific impulse in the region of 320 seconds. But, of course, not without drawbacks. Both components are very toxic, cause corrosion and require special materials. And, since there are now two components, now we have two tanks, two supply systems, that is, a twice as high probability of failure, and a system of mixing the components in the combustion chamber appears. For example, in 2010, the Akatsuki probe could not enter Venus’s orbit due to a failure of the boost valve. As a result, an excess of oxidizing agent appeared in the mixture, the temperature of the combustion chamber and nozzle increased, and they were irreparably damaged.The device was able to enter the orbit of Venus only five years later, having performed a very long braking maneuver on single-component orientation engines.


Akatsuki probe propulsion system diagram , source

Due to the combination of high thrust and high specific impulse, the engines on this fuel pair are practically uncontested for manned spacecraft and ISS supply trucks. Until recently, they were also a familiar classic for interplanetary missions and geostationary satellites, but now in these areas they are being crowded with electric propulsion engines.

Alternatives


The difficulties of working with cryogenic components did not stop the designers. A unique example of engines using non-toxic components is the Buran combined propulsion system, which used liquid oxygen and kerosene. The use of this fuel pair gave a great power ratio (specific impulse in the region of 358 seconds, higher than UDMH + AT), non-toxic components made the operation of the ship safer and more environmentally friendly, and also allowed the use of oxygen for power supply and life support systems.


ODU "Burana", a large tank of liquid oxygen is clearly visible, source

Special technical solutions: deep cooling of oxygen to -210 ° C before refueling, constant stirring in the tank and gasification before being fed to the orientation engines made it possible to create a propulsion system that could provide flight up to 30 days.

And in the middle of the 2000s, NASA was considering a methane-oxygen fuel pair for the lunar module and the manned spacecraft. Methane is much easier to store in liquid form than hydrogen, and the specific impulse is higher than that of UDMH-AT. The ship, which became the Orion, eventually received hydrazine engines. But the idea of ​​methane engines will not disappear anywhere, because for engines operating on locally collected resources, methane remains an interesting option.

Electric future


2020, by submissionof experts, should be the year when half of the new commercial satellites will use electric propulsion engines. Given that they are the ones on the massively launched Starlink and OneWeb satellites, now this estimate looks conservative. The geostationary satellite on UDMH + AT will reach the target orbit in a maximum of a week, but half of its initial mass will be fuel. And on electric engines, the orbit will take up to six months, but the satellite will be 40% lighter. The large solar panels installed for high-power repeaters combine beautifully with electric jet engines. A similar electric attack occurs in the camp of scientific apparatuses - the Dawn probe that set off in 2007 had a record reserve of characteristic speed, 11 km / s, unattainable for chemical engines.All these remarkable results are possible due to the fact that the specific impulse of electric jet engines is an order of magnitude greater than chemical ones and for different models is in a wide range of several thousand seconds. But, of course, everything has a price - the thrust of electric jet engines is measured in millinewtons, and it can be represented at the household level as the weight of a small coin.

Two types of electro-jet engines are most common:

Ionic engines ionize the gas by electron bombardment and emit the resulting ions in an electric field.



In a Hall effect design, also often called a plasma engine, the working fluid is fed into an annular chamber to which a potential difference is applied. Neutral gas is ionized and accelerated by an electric field, ejected from the engine at great speed.



Ion engines have a greater specific impulse, but so far suffer from problems with durability: from the large potential difference between the accelerating and focusing gratings and erosion with time, the grating breaks out. So today, Hall effect engines have a number of operational advantages.

There are also less common options. For example, an electrothermal, it is also an electric heating engine in which fuel is accelerated by heating with electric current. On Iridium satellites of the first generation (98 pieces were built in total) , seven single-component engines with a thrust of 1 Newton and one more efficient electric heating engine with a thrust of 0.369 N were installed . All engines were powered by one tank with hydrazine.


Electric heating engine productionAerojet Rocketdyne

Electric propulsion engines are a promising topic, and where you can do without a lot of engine thrust, there will be more. And when, finally, nuclear reactors fly into space, their enormous energy capabilities, combined with a specific impulse of electric propulsion, will open up new perspectives.

Too inflexible


Solid propellant engines are gradually losing popularity for spacecraft. Simplicity of design and high traction cannot compensate for a single inclusion and far from a record specific impulse - about 290 seconds. But they had a glorious past: on the first Pioneers, small solid-state engines were going to carry out path corrections, they were used as brake engines on the manned Mercury and Gemini, they were turned on before Surveyor’s automatic probes on the moon, and for a long time they were used as apogee ones engines, transferring satellites from a geo-transitional orbit close to geostationary.


First Generation Syncom Geostationary Satellite Diagram, center-left apogee engine

Exotic


It is possible to heat the working fluid not only with electricity. A lot of heat is released during a nuclear reaction, and back in the 50s the ideas of a ramjet engine arose. Hydrogen was supposed to enter the reactor core and be thrown out. The specific impulse was expected in the region of 900 seconds. Development was carried out on both sides of the ocean, in the USA - NERVA, in the USSR - RD-0410, but in the end they were phased out.


NERVA engine

There were also more exotic designs: pulsed explosive, engines based on salts of nuclear fuel, gas-phase nuclear engines, etc., but so far they have not gone beyond diagrams and drawings.

There are more environmentally friendly concepts of heating, for example, solar energy or lasers. On the surface of the moon, the regolith warms up above one hundred degrees during the day, so the principle of concentration of sunlight for heating the working fluid is real, but without very light mirrors it will lose to the total mass of the system with a conventional chemical engine.


Solar Moth Experimental Satellite Concept, Source

To change the speed, all of the above options used the release of matter in the opposite direction. But there are designs that do without it. The most famous and successful are solar sails. The IKAROS spacecraft launched to Venus together with the Akatsuki probe launched a 14x14 meter sail on June 10, 2010. The shape of the sail was supported by the rotation of the apparatus, and the orientation was controlled by LEDs at the edges changing the reflectivity. The experimental satellite successfully flew past Venus and by 2013, thanks to the sail, it changed its speed by about 400 m / s.


Reduced IKAROS layout

The solar sail blade is not an easy thing, it must be opened, ideally without creases and sagging, to ensure strength and controllability so that a random micrometeorite does not become a disaster for the mission, as well as durability. There is a potentially more reliable and efficient option - an electric sail. Instead of a fragile cloth, thin pins or cables are opened, an electron gun is placed on the satellite, which ejects electrons, which is why the satellite itself and the cables acquire a positive charge and repel solar wind ions. Unfortunately, the experimental Estonian cubate ESTCube-1 was not able to open the sail in space, and the Finnish Aalto-1 was supposed to open the sail last year, but there is no news about it.


Electric sail working principle, illustration by Alexandre Szames

Ropes can be used for braking in the Earth’s magnetic field or, conversely, used as an engine. If we unwind a conductive cable with an electronic emitter at the end, a current will appear in the cable and the satellite will begin to slow down without fuel consumption. And if you reverse the direction of the current, then you can accelerate. Unfortunately, the Japanese HTV-KITE experiment failed - the cable did not unwind.


The principle of passive braking with a conductive cable

After unwinding the cable and unwinding the resulting system, you can convert the rotation moment to speed, unhooking the load at the end at the right time. Thus, they successfully returned (even if they could not find later) the Fotino capsule from the Photon-M3 apparatus. In theory, rotating cable systems can be used to move goods between orbits, but so far such systems only work in science fiction (for example, Neil Stevenson, Semieviye).


Mutual position of "Photino" and "Photon-M3" when unwinding a cable

Epilogue


The twenty-first century for satellite engines promises to be no less interesting than the twentieth - an abundance of different concepts promises new exciting experiments and the development of space engines in many different directions.

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