Rocket fuel

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The fuel for a rocket , more precisely a rocket engine , is called rocket fuel . It creates the thrust of a rocket.

The choice of rocket fuel is the determining factor for the specific momentum ( ) of a rocket engine. The specific impulse is a measure of the efficiency of engines, i.e. the consumption of fuel per impulse.

Although a high specific impulse is always desirable, fuels with lower efficiency are often used. For example, in the first stage of rocket engines, kerosene is often used as fuel or solid fuel rockets , although engines with liquid hydrogen or electric propulsion have a much higher specific impulse and are therefore more efficient. The reason lies in the low price and the simplicity of the former engines and in the comparatively low thrust that the latter engines allow. When taking off from the surface of the earth, a high thrust is necessary because the rocket has to overcome the acceleration of gravity. In a second stage, other fuels can be used (e.g. liquid hydrogen) because the required thrust is lower. For missions beyond Earth orbit, engines with low thrust and high specific momentum can be used.

In addition to the price of rocket fuel, important properties are its density (influences the size of the tank), shelf life (decomposition, evaporation), hazard ( self-ignition , ignition behavior and environmental compatibility) and aggressiveness ( corrosion ) against the tank, lines, pumps and turbines.

The most common propellants used in rockets are chemical. The products of a chemical reaction are ejected from the engine nozzle at high speed. Both energy and supporting mass come from the chemical reaction. In contrast, many electric and nuclear propulsion systems use a dedicated support mass (e.g. hydrogen) that is not burned, but heated electrically or nuclearly and thus escapes at high speed.

Shelf life and storage

The various fuel classifications also have special properties with regard to their durability and storage. Solid propellants are the easiest to store, but certain conditions also limit their storage. Neither cracks nor shrinkage should occur. Liquid fuels, on the other hand, should neither freeze nor evaporate in the normal ambient temperature range (e.g. during start-up and storage), which means a temperature range of –20 ° C to +80 ° C.

Fuels liquefied by freezing and referred to as cryogenic in space travel are difficult to store because of their physical state, since evaporation cannot be avoided even with complex tank insulation. The use in rockets thus reduces the possible downtime between refueling and launch of the rocket and requires additional technological measures (e.g. insulation of the tanks, prevention of ice formation, continuous refueling before take-off, evaporation devices) in the design of the rocket.

Chemical fuels

In chemical fuel systems, a chemical reaction creates the rocket's thrust. A general distinction is made either according to the type of fuel in solid, liquid or hybrid fuels or according to the number of reaction substances involved in the combustion process in Monergol, Diergol or Triergol. The chemical reaction releases thermal energy and reaction products , which create high pressures and temperatures in the combustion chamber , causing the reaction products to be expelled from the engine nozzle at high speed.

Chemical rocket fuels usually require a propellant (also called fuel ) and an oxidizer . These can be in mixed (solid rocket) or unmixed form before the start. Depending on the type and area of ​​application of the missile, the following fuels are used:

Solid propellant

Solid propellants can be homogeneous or heterogeneous solids (composites) that contain other additives (stabilizers) in addition to the fuel and the oxidizer.

Homogeneous solid fuels

The homogeneous fuels are homogeneous colloid- based mixtures of cellulose nitrate or glycerine trinitrate , which may also contain additives of oxidizers, fuels and stabilizers (reduce the tendency of the nitrates to break down spontaneously, e.g. diethylphenyl urethane , diphenylamine ). If only cellulose nitrate is used, it is also referred to as single-base fuel, otherwise double-base fuels, which are more energy-rich, but therefore also require stabilizers.

Black powder is mostly used as the solid fuel for fireworks and model rockets . For military applications, black powder was largely replaced by the low-smoke cellulose nitrate powder as early as the Second World War. The homogeneous solid propellants mostly belong to the category of low-energy propellants, as they have an exit speed of less than 2200 m / s.

Heterogeneous solid fuels (composites)

Heterogeneous solid propellants (composites) are produced by mechanical mixing of fuel (s) and oxidizer (s).

For solid rockets , as are common in space travel or for some military rockets, pourable mixtures of an oxidizer such as ammonium perchlorate or sodium / ammonium nitrate and a reducing agent such as aluminum powder ( ammonium perchlorate composite propellant ) are used. The supporting substance, also a reducing agent, consists of synthetic resins such as polyurethanes or polysulphides , but mainly HTPB . Small amounts of iron oxide as a catalyst and other additives improve the properties.

The mixture is poured into molds. The propellant is then hardened, which greatly reduces the formation of cracks and cavities and thus makes transport and handling very safe. It was also investigated whether lithium , beryllium , boron or magnesium can be used instead of or in addition to aluminum . With highly developed composites, exit speeds of up to 3300 m / s can be achieved. Apart from aluminum, these (beryllium because of its toxicity, lithium because of its difficult handling, boron because of the formation of impermeable oxide layers) have so far not been used.

The Space Shuttle boosters can serve as an example of the composition . In these, the fuel consists of 69.93% ammonium perchlorate as oxidizer, 16% aluminum powder as fuel and 0.07% iron oxide powder as catalyst. The binding substance used is 12.04% polybutadiene acrylic acid acrylonitrile and 1.96% of an epoxy hardener, which also burn and thus provide additional thrust.

In 2009 it was possible to use the explosiveness of aluminum and water in the new rocket fuel Alice .

Hybrid fuel

Hybrid fuel (Lithergol) refers to a mixed drive consisting of a solid and a liquid fuel component . Most of the time the reducing fuel is solid, often a plastic , for example HTPB, or it is bound into it e.g. B. lithium hydride etc. The oxidizer is then liquid, mostly nitric acid , nitrous oxide , liquid oxygen , fluorine , oxygen difluoride , or FLOX (mixture of liquid oxygen and liquid fluorine). For example, the SpaceShipOne flew on HTPB and nitrous oxide. However, experiments have also been carried out with inverse hybrids, in which a liquid fuel is burned by a solid oxidizer. Missiles with a corresponding propulsion system are called hybrid missiles .

Liquid fuel

As liquid fuel in the operating condition of liquid fuels or oxidizers are referred to, which are used in the rocket engines. A distinction is made between Monergole (one-fuel fuels), Diergole (two-fuel fuels) and Triergole (three-fuel systems), which leads directly to the number of separate tanks required.

Monergole

The liquid fuels in this category are low-energy fuels. In the case of the so-called Katergole, monergols are made to disintegrate by adding a catalyst , other forms such as the torpedo fuel Otto 2 are converted by oxidation . An example of a katergol is hydrazine , which is used, for example, for attitude control systems of spacecraft. In this case, hydrazine is with the aid of a catalyst ( iridium or molybdenum - nitride on alumina decomposed with a large surface area) to nitrogen and hydrogen. Another example is a 70-80% solution of hydrogen peroxide . Calcium permanganate or silver-plated gauze is used as a catalyst . However, hydrogen peroxide is very dangerous because of its tendency to decompose spontaneously (even if it is slightly contaminated by metallic or organic substances). Also, ethylene oxide can be used as Monergol. It breaks down into methane and carbon monoxide depending on the reaction conditions . The resulting gas mixture can be completely oxidized to carbon dioxide and water in an afterburner.

Performance data of some monergoles
combustible
material		 catalyst Exit
speed (m / s)
N 2 H 4 Iridium on aluminum oxide 2220
H 2 O 2 Calcium permanganate 1860

Diergole

With diergol systems (two-fuel systems), with the exception of hybrid drives, both components are liquid (e.g. hydrogen / oxygen). In the case of the hybrid drive, the fuel is usually present in solid form and the oxidizer as a gas or liquid. The strongest representatives of the Diergol systems include hydrogen-oxygen mixtures, which can achieve exit speeds of up to 4500 m / s (13680 km / h) in a vacuum.

The following are commonly used as fuel: alcohol , gasoline , kerosene , hydrazine , UDMH (asymmetrical dimethylhydrazine), MMH (monomethylhydrazine), aerozin 50 (50% UDMH and 50% hydrazine), UH 25 (75% UDMH and 25% hydrazine) and liquid hydrogen . In the past, ammonia was also used before the switch to hydrazine and its derivatives or mixtures of both. Methane and hydrogen provide the largest specific impulse, but are difficult to handle because of the low storage temperatures. Syntin is another hydrocarbon that was used in the Soviet Union in the 1980s and 1990s as a fuel for the Soyuz rocket and the Buran. In practice only oxygen and fluorine or compounds containing high concentrations of one of the two substances are used as oxidizers. Almost all oxidizers with the exception of nitrous oxide are either chemically aggressive or require deep cooling. In particular, liquid oxygen (LOX: liquid oxygen ), hydrogen peroxide , fuming nitric acid (RFNA: red fuming nitric acid ), nitrous tetroxide or nitrous oxide are used . In principle, liquid fluorine is also conceivable, but practically impossible for environmental reasons .

Ignition takes place either electrically, with a solid cartridge, or with some fuel combinations by itself ( hypergol ), which is an advantage for this fuel combination, since more or less complex ignition systems can be omitted.

Theoretical performance data of some fuel combinations
Oxi
dator		 combustible
material		 Mixing
ratio		 medium
density
(g / cm 3 )		 Combustion
temperature
(° C)		 Exit
speed
(m / s)
O 2 C 2 H 5 OH 1.43 1.01 2960 2740
O 2 RP-1 2.58 1.03 3403 2941
O 2 C 3 H 4 2.05 1.08 N / A 3093
O 2 C 2 H 4 2.38 0.88 3486 3053
O 2 CH 4 3.21 0.82 3260 3034
O 2 N 2 H 4 0.90 1.07 3130 3070
O 2 H 2 4.02 0.28 2700 3830
O 2 B 2 H 6 1.96 0.74 3489 3351
O 2 W 5 H 9 2.12 0.92 3834 3124
ClF 3 C 10 H 20 3.20 1.41 3250 2530
ClF 3 N 2 H 4 2.81 1.49 3650 2885
H 2 O 2 (95%) UDMH 4.54 1.24 2650 2720
H 2 O 2 (95%) RP-1 7.35 1.30 2915 2730
H 2 O 2 (95%) N 2 H 4 2.17 1.26 2580 2760
N 2 O 4 Aerozin 2.00 2.00 3100 2820
N 2 O 4 MMH 2.17 1.19 3122 2827
N 2 O 4 N 2 H 4 1.36 1.21 2992 2862
ENT 3 C 10 H 20 4.80 1.35 2960 2630
ENT 3 N 2 H 4 1.45 1.28 2800 2830
F 2 N 2 H 4 2.30 1.31 4440 3560
F 2 H 2 7.60 0.45 3600 4020
F 2 W 5 H 9 5.14 1.23 5050 3502
F 2 CH 4 4.53 1.03 3918 3414
OF 2 H 2 5.92 0.39 3311 4014
OF 2 CH 4 4.94 1.06 4157 3485
OF 2 B 2 H 6 3.95 1.01 4479 3653
OF 2 RP-1 3.87 1.28 4436 3424
OF 2 MMH 2.28 1.24 4075 3427
OF 2 N 2 H 4 1.51 1.26 3769 3381
OF 2 W 5 H 9 4.16 1.20 4825 3539
N 2 F 4 CH 4 6.44 1.15 3705 3127
N 2 F 4 MMH 3.35 1.32 3819 3163
N 2 F 4 N 2 H 4 3.22 1.83 4214 3283
N 2 F 4 W 5 H 9 7.76 1.34 4791 3259

(Combustion chamber pressure of 7 MPa, expansion ratio 1:70, adiabatic combustion, isentropic expansion, chemical equilibrium).

Triergole

Triergol systems (three-substance systems) contain diergol systems (two components), which are additionally supplied with hydrogen or metal powder ( lithium , aluminum , beryllium ) to increase the specific impulse . Although these fuel systems have been well researched so far, they have never been used in practice because of the complex structure of the engine and rocket (three tanks!).

Theoretical performance data of some Triergole
Oxi
dator		 combustible
material		 Additional
fuel		 Exit
speed
(m / s) 		Stei-
delay
O 2 H 2 26% Be 4500 17%
O 2 H 2 29% Li 4000 04%
O 2 N 2 H 4 15% Be 3350 09%
F 2 N 2 H 4 25% Li 3700 03%
F 2 H 2 15% Be 4100 02%
F 2 H 2 20% Li 4400 09%
N 2 O 4 MMH 15% Be 3100 10%
N 2 O 4 MMH 15% Al 2900 03%
N 2 O 4 N 2 H 4 10% Be 3200 12%
H 2 O 2 N 2 H 4 13% Be 3300 17%

(Combustion chamber pressure of 7 MPa, expansion ratio 1:70, adiabatic combustion, isentropic expansion, chemical equilibrium)

Oberth effect

Space pioneer Hermann Oberth , after whom the French rocket pioneer Robert Esnault-Pelterie later named the effect, found out through empirical experimentation that when the rocket fuels hydrogen and oxygen react , the exit velocity can be increased by increasing the hydrogen content. This is because, as a result of the excess hydrogen, the dissociation is practically eliminated and pure hydrogen is lighter and can therefore flow out faster than dissociated or even undissociated water vapor . A further side effect is a slightly lower temperature with correspondingly lower demands on the drive's cooling system, so that when the oxygen weight is reduced, there is an increase in the payload .

Today hydrogen and oxygen are used in hydrogen-oxygen engines in a mass ratio of 1: 4 to 1: 6 (instead of the stoichiometrically correct mass ratio of 1: 8).

This effect must not be confused with the English use of “ Oberth Effect ”, which describes the connection that a more favorable ratio between the kinetic and potential energy of the ejected fuel is achieved at higher airspeed of the spaceship.

Chemical fuels currently in use

The following combinations are particularly common for large rockets:
For propulsion:

Only non-cryogenic substances are used for the attitude control system:

research

Two ways are currently being investigated to increase the specific momentum of chemical engines: free radicals and metastable elements. All methods are still in the experimental stage:

  • Ozone is unstable, but the tetra oxygen allotrope is said to be more stable. This would allow specific pulses of up to 564 s (5538 Ns / kg) in a vacuum.
  • Attempts are also being made to use hydrogen radicals as fuel. To increase the stability of the element, they are mixed with liquid hydrogen. If this combination (with theoretical 15.4% radicals) is burned with liquid oxygen, specific impulses of up to 750 s (7358 Ns / kg) can be achieved in a vacuum.
  • At the Université d'Orsay in Paris , metastable helium was produced on a test basis and stored as Bose-Einstein condensate . The reaction from metastable helium to helium would make specific pulses of 2825 s (27713 Ns / kg) possible, more than with nuclear drives.

Fuels in electric drives

The term fuel (but above all the term fuel) is misleading in the case of electric drives, as it only functions as a medium for transmitting impulses, but not as an actual energy source. Instead, it is generally referred to as support mass.

In the case of an ion drive , cesium, xenon or mercury serve as a supporting mass. The fuel is ionized and accelerated with the help of an electric and a magnetic field. The advantage of this design is that the necessary electrical energy can be obtained in space using solar cells, for example, and very little fuel is used, because very little mass is emitted, but at a very high speed. The thrust forces achieved are extremely small. In addition, the engine only works in a high vacuum, such as is the case in space.

In thermal arc engines working with hydrazine, ammonia or hydrogen. The arc heats the support mass, which expands and is accelerated backwards through a nozzle.

Fuels in nuclear engines

Liquid hydrogen or ammonia, which is heated to approx. 3000 ° C with the help of a reactor (project NERVA ), is used as a supporting mass in a nuclear drive .

The Orion project envisaged the use of small atomic bombs for propulsion.

Fusion drive

There are several approaches to realizing a fusion drive . One of them uses laser pulses to bring a small amount of 3 He to the temperature required for fusion. The high-energy reaction products leave the drive through a magnetic nozzle . If you ignite many such reactions in succession, a quasi continuous recoil would result.

Antimatter Propulsion

The energy for a currently hypothetical antimatter propulsion would be provided by pair annihilation of matter and antimatter. In this process, the entire rest energy of the particles is completely converted into high-energy gamma quanta, which would first have to be converted into kinetic energy through absorption in order to accelerate other matter and expel it in a directed manner.

The greatest problem from today's point of view is the generation and storage of antimatter. Since the production consumes as much energy as the reaction later supplies, production on board the spaceship is ruled out. The antimatter would have to be carried. The storage of these must be 100 percent reliable, otherwise the spaceship would be destroyed.

With the current state of the art, an antimatter drive is not possible, since there is no way of generating large amounts of antimatter. With the matter-antimatter engine, you could almost reach the speed of light. Only about 0.1 grams of antiprotons would be needed for a flight to Mars and back, but even the production of this small amount of antiprotons is currently utopian.

See also

literature

swell

  1. ↑ Jared Ledgard: The Preparatory Manual of Black Powder and Pyrotechnics. V1.4, Jared Ledgard 2007, ISBN 978-0-615-17427-3 , pp. 39, 51-52, 73, 77, 540, 549.
  2. ↑ Entry on rocket fuels. In: Römpp Online . Georg Thieme Verlag, accessed on February 6, 2012.
  3. ↑ Armin Dadieu, Ralf Damm, Eckart W. Schmidt: Raketentreibstoffe . Springer-Verlag, 2013, ISBN 978-3-7091-7132-5 , p. 496 ( limited preview in Google Book search).
  4. ↑ NASA: PROPELLANTS ( Memento from April 27, 2011 in the Internet Archive )
  5. ↑ Horst W. Köhler: Klipp und Klar: 100x space travel. Bibliographisches Institut, Mannheim, Vienna, Zurich 1977, ISBN 3-411-01707-4 , p. 30.
  6. ^ Clay Robison, William. (1953). Properties of ethylene oxide and hydrazine related to their use as propellants .
  7. ^ Frederick C. Durant, American Astronautical Society, International Academy of Astronautics: First steps toward space: proceedings of the first and second History ... AAS Publications, 1974, ISBN 978-0-87703-243-4 , pp. 134 ( limited preview in Google Book search).
  8. ↑ http://isdc2.xisp.net/~kmiller/isdc_archive/fileDownload.php/?link=fileSelect&file_id=360 (link not available)

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