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The Analysis of Use of Liquid Biofuels for Liquid Rockets Propulsion

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  • Lukasiewicz Research Network - Institute of Aviation

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Liquids are desirable as propulsion media in modern rocket techniques and have been utilized since the beginning of modern rocketry. It has been proved recently that liquid biofuels from agricultural available lands can provide safe and convenient energy for rockets. Mostly because of their reasonably high density. That allows to reduce the weight of the tanks, resulting in a high propellant structure mass ratio. Liquid rockets have been built to use cryogenic liquid fuels (such as liquid hydrogen) or liquid mixture of hydrocarbons that can be stored at room temperature (ethanol, petrochemical fuels; RP-1 or syntin). Liquid biofuels such as biodiesel or Bio-SPK (Synthetic Paraffinic Kerosene) may offer potential possibilities similar, or even superior, to traditional RP-1. Brief comparison and potential performance analysis of liquid biofuels in comparison to petrochemical ones is presented in the paper.
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The Analysis of Use of Liquid Biofuels for Liquid Rockets Propulsion
Institute of Aviation, Al. Krakowska 110/114, 02-256 Warsaw, Poland
G. Rarata, P. Surmacz
ABSTRACT
Liquids are desirable as propulsion media in modern rocket techniques and have
been utilized since the beginning of modern rocketry. It has been proved recently
that liquid biofuels from agricultural available lands can provide safe and
convenient energy for rockets. Mostly because of their reasonably high density.
That allows to reduce the weight of the tanks, resulting in a high propellant
structure mass ratio. Liquid rockets have been built to use cryogenic liquid fuels
(such as liquid hydrogen) or liquid mixture of hydrocarbons that can be stored at
room temperature (ethanol, petrochemical fuels; RP-1 or syntin). Liquid biofuels
such as biodiesel or Bio-SPK (Synthetic Paraffinic Kerosene) may offer potential
possibilities similar, or even superior, to traditional RP-1. Brief comparison and
potential performance analysis of liquid biofuels in comparison to petrochemical
ones is presented in the paper.
1. INTRODUCTION
The most popular liquid rocket fuels today are hydrogen, hydrocarbons, or molecules
rich in NH bonds. Liquid oxygen, together with HTP hydrogen peroxide, are the most
efficient and inexpensive rocket oxidants, being currently in use. Hydrocarbon derived liquid
rocket fuels, such as RP-1, have been evaluated thoroughly in a number of practical missions
performed so far. Their use, physical properties and performance is well documented.
However, despite their acceptable performance, stability, availability and low cost, there is
growing demand for more alternative or “greener” fuels. Additionally, high toxicity of some
of commonly used liquid propellants leads to technological complication and rise of the
overall costs.
Therefore, the efforts of many developers have been directed at creation of new sets of
ecological clean propellants. Well known setups, such as: LH2 + LOX, C2H5OH + 98% H2O2,
kerosene + LOX, kerosene + 98% H2O2 have been tested [1,2].
The depletion of energy resources, the reduced carbon emission, low sulphur contents
and less PAH emissions make biodiesels environmentally friendly as well as good alternative
to conventional petrochemical fuel. It is the fuel that posses suitable properties to be used in a
liquid rocket engine, and is totally derived from biological sources. It has already been used
for decades in diesel engines. Biodiesel consists of long-chain fatty acids methyl esters.
Biodiesel and other liquid biofuels are already being tested by the airline industry. Aside from
replacing fossil fuels with a renewable energy source, biodiesel powered rockets may be able
to achieve even better performance. This is due to the fact that the biofuel named “biodiesel”
is denser than traditional rocket fuel so it requires less volume in a tank. However, despite
fluctuating petroleum prices, interest in alternative fuels among rocket builders has not been
evident so far. Rocket scientific groups present rather traditional approach towards the
question of rocket propellants. Most of them perceive the use of any propellant for the
rocketry via its energetic efficiency, specific impulse or density, without any attention to the
environmental aspects. Meanwhile, in the field of internal combustion engines, the use of
fossil and renewable fuels is being a subject of a constant and thorough evaluation and
quantification towards the environmental impact.
Recent tests performed by Flometrics proved that biodiesel performance was about 4%
lower than the RP-1 (Fig. 1) [6]. However, the fire tests were based on the same hardware. As
the result, the mixture ratio with biodiesel was too rich. Thus, more testing at various mixture
ratios would show if the difference is less at other ratios. Also the effectiveness of biodiesel as
a coolant in the regenerative cooling jacket on the rocket motor was not evaluated.
Fig. 1. Flometrics Inc. recently tested burning B100 in a Rocketdyne LR-101 rocket engine [6]
The most advanced liquid biofuel, SPK (Synthetic Paraffinic Kerosene) type, in fact is
chemically very similar to the RP-1. It exceptionality relies on the novel technology that has
been adopted, to produce fuel nearly identical to the petrochemical one from oil crops [7].
Significant progress has been made in verifying the performance of SPK made from
sustainable sources of bio-derived oils, that can be used in commercial aircraft at a blend ratio
of up to 50 percent with traditional jet fuel (Jet A or Jet A-1).
2. CEMICAL FORMULA
Chemically, biodiesel is composed of monoalkyl esters of long chains and fatty acids
derived from renewable feed stock like vegetable oils and animal fats. It is produced by
transestrification, in which oil or fat is reacted with a monohydric alcohol in presence of a
catalyst. Biodiesel is used in compression ignition engines (diesel engines) or heating boilers.
The biodiesel fuel has in general, the same or similar properties of the conventional diesel fuel
and can be blended in any percentage with diesel fuel.
Rocket Propellant - 1, (RP-1) is similar to gasoline. It is a fuel constituted basically of
hydrocarbons and a small portion of oxygenized compounds. These hydrocarbons are, in
general, lighter fuels than those that compose biodiesel fuel, because they are formed by
molecules of small carbonic chains (normally has 12, or slightly less, carbon atoms). Because
of the lack of light hydrocarbons, RP-1 has a high flash point, and is less of a fire hazard than
gasoline or even some jet and diesel fuels.
Biodiesel shows a wide variety of advantages on its fossil origin partner (diesel fuel).
Biodiesel is also about 9% denser than the RP-1. Nowadays, biodiesel is growing as a serious
competitor in the energy market, which also takes into account the ecological benefits that its
use represents. The average molecular weight of soybean oil methyl ester (biodiesel) is 292.2
g/mol. This was calculated using the average fatty acid distribution for soybean methyl esters
presented in Table 1 [3].
Table 1. Typical soybean oil methyl ester [3]
2. CO2 EMISSION
The table above (see Table 1) also shows the molecular weight and chemical formula
for each of the component esters; each component was designated by a different character.
The emission rates of CO2 are close to 282 g of CO2 by 100 g biodiesel, from the
stoichiometric combustion reaction (1) with air:
In a rocket engine combustion chamber there are mostly fuel-rich conditions, which
will produce some CO instead of CO2. But this also results in incomplete combustion,
producing some organics of high molecular weight and numerous vibration modes [4]. Thus
above assumption is serves only as an approximation.
The chemical formula of RP-1 is different from the model chemical compound of
gasoline C8H18 (octane). But for the simplicity of combustion reaction for normalized air
excess, we use the latter one. The reaction (2) can be written as follows:
The CO2 emission for this kind of fuel is nearly 310 g of CO2 for 100 g of fuel which is
significantly more than in the case of biodiesel.
It is worth to remark here the fact that biomass-derived fuels decrease the net
atmospheric carbon. It happens in two ways: first, they participate in the relatively rapid
biological cycling of carbon to the atmosphere (engine emissions) and from the atmosphere
(photosynthesis). Second, they substitute fossil fuels. Fossil fuel combustion releases carbon
that took millions of years to be removed from the atmosphere. Combustion of biomass fuels
participates in a process that allows CO2 to be rapidly recycled to fuel [5].
3. THEORETICAL PERFORMANCE
As biodiesel is a complex mixture of C14, C16, C18, C20 and C22 methyl esters with
highly saturated carbon chain. Thus it is very difficult to propose a detailed kinetic scheme.
Therefore, surrogate model fuels of simple and well characterized composition have to be
used. Besides, the long chain methyl esters generate a lot of species during the combustion
process. Thus a form of a bio-diesel surrogate, in order to simplify the calculations, is
proposed. The properties of methyl oleate (C19H36O2) have been “adapted” to represent those
of the most popular biodiesel nowadays, that is Rapeseed Methyl Ester, RME. Calculations
were based on the procedures built into the NASA code CEA (Chemical Equilibrium with
Applications). The bio-diesel surrogate performance in the model rocket combustion chamber
was performed. The value of heat of formation of the biodiesel surrogate was assumed as –
626 kJ/mol. The figure below (Fig. 2) presents the theoretical performance (Isp at expansion
ratio of 100). LOX and 98% H2O2 have been utilized as oxidizers. The Isp is presented as the
function of oxidizer/fuel weight ratio and pressure ratio of combustion chamber versus outlet.
Fig. 2. The comparison of calculated values of specific impulse of RP-1 and biodiesel with LOX and
H2O2 for the frozen composition
4. CONCLUSIONS
The practical fire tests, performed in the USA recently, as well as theoretical analysis
done by the authors, shows that the use of biodiesel in a rocket engine may provide very
comparable results to the traditional RP-1. In both cases it has been found that there is slight
(about 4%) drop in engine performance. Some additional tests with different blends of fuel
and oxidizer should be run to see if engine performance can be improved.
Aside from replacing fossil fuels with a renewable energy source, biofuel rockets may
be able to deliver more cargo for the same volume of a rocket tank, as the biofuel is denser
than traditional rocket fuel.
Biofuel such as the biodiesel burns in a little cleaner way, mostly due to its organic
nature. In the automobile industry biodiesel has been used for years. The exhaust
concentrations of two key pollutants, compared to traditional petrochemical fuel, have been
found reduced significantly. Some research prove that emission of unburned hydrocarbons is
reduced by 37% while CO emissions is decreased by 12% [8]. The only exhaust constituent of
concern that is find to increase, is NOX. On the average, burning 100% biodiesel increases the
NOX in the exhaust by 9%.
REFERENCES
1. S. Frolik, B. Austin, J. Rusek, S. Heister, Development of hypergolic liquid fuels for use
with hydrogen peroxide, 36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and
Exhibit
2. D. Haeseler, A. Goetz, A. Froehlich, Non – toxic propellants for future advanced launcher
propulsion systems, 36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and
Exhibit
3. www.biodiesel.org/aboutnbb/foundation, 2007
4. G.P. Sutton, O. Biblarz, Rocket Propulsion Elements, Wiley-Interscience, 7 edition, 2000
5. J. Sheehan, V. Camobreco, J. Duffield, M. Graboski, H. Shapoiri, An Overview of
Biodiesel and Petroleum Diesel Life Cycles, National Renewable Energy Laboratory, US
Department of Energy, 1998
6. www.flometrics.com
7. J.D. Kinder, T. Rahmes, Evaluation of Bio-Derived Synthetic Paraffinic Kerosene (Bio-
SPK), Sustainable Biofuels Research & Technology Program, 2009
8. J.P. Stergar, J.P. Chastain, Engine Performance and Emissions Characteristics When
Using Biodiesel in Diesel Engines, the American Society of Agricultural and Biological
Engineers, St. Joseph, Michigan, 2008
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Conference Paper
Vegetable oil, used cooking oil, and animal fats can be converted into biodiesel through a process known as transesterification. These esters exhibit chemical properties similar to that of traditional, petroleum-based diesel fuels. The purpose of this study was to review the available data concerning the affects of using 100% biodiesel on engine performance and exhaust emissions. It was found that using 100% biodiesel reduced engine performance. Engine output power was reduced by 11%, and specific fuel consumption was increased by 14% on average. The effective engine efficiency was decreased by only 1.6%. The exhaust concentrations of two key pollutants were reduced significantly. Emission of unburned hydrocarbons was reduced by 37% while CO emissions were decreased by 12%. The only exhaust constituent of concern that was increased was NOX. On the average, burning 100% biodiesel increased the NOX in the exhaust by 9%. The data reviewed in this paper suggests that an optimal biodiesel/diesel blend should be developed that minimizes the reductions in engine performance, reduces the NOX emissions, while maintaining favorable reductions in unburned hydrocarbons and CO.
Evaluation of Bio-Derived Synthetic Paraffinic Kerosene (Bio-SPK)
  • J D Kinder
  • T Rahmes
J.D. Kinder, T. Rahmes, Evaluation of Bio-Derived Synthetic Paraffinic Kerosene (Bio-SPK), Sustainable Biofuels Research & Technology Program, 2009