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Hybrid boosters for Ariane 5
Acta Astronautica Vol. 15, No. 12, pp. 1059-1061, 1987
0094-5767/87 $3.00+0.00 © 1987PergamonJournals Ltd
Printed in Great Britain. All rights reserved
Academy Transactions Note HYBRID BOOSTERS FOR ARIANE 5 R. E. Lot and E. DARGIES DFVLR Institute of Chemical Propulsion and Engineering, D-7101 Hardthausen-Lampoldshausen, F.R.G. (Received 14 May 1987)
Abstract--Results of a systems analysis are presented which investigates replacing Ariane 5's solid rocket boosters by LOX/polyethylene hybrids. Their advantages and disadvantages are discussed. A performance and mass breakdown is presented.
Ariane 5P, like the U.S. Space Transportation System and the future Japanese launcher H II, uses a cryogenic LOX/LH2-core stage along with two solid rocket boosters (SRBs). According to the nomenclature widely used in Europe, Ariane 5P appears to converge on a configuration 2P230/H 155/H 10, where P stands for solid propellants and H designates stages with LOX/LH2, while the numbers indicate propellant masses in tons. SRBs have failed catastrophically in 1985 and 1986 during launches of Titan and the STS-Challenger. The failures were caused by defects in case bonding and a leak in the combustion chamber wall respectively. In hybrid propulsion systems, the combustion chamber contains only the solid fuel, while the liquid oxydizer is stored in a tank and injected through a feed system into combustion channels inside the solid fuel. Therefore, hybrids do not rely at all on case bonding. Hybrid combustion occurs along well oxidizerventilated surfaces only. While in solids a small leak in the chamber wall results in the immediate ignition (followed by self-sustained combustion) of all propellant-surfaces along which the leakage gases flow, the same situation in hybrids would cause an extremely fuel-rich flow with hardly any widening of the leakage flow path. Consequently, hybrids have a much higher potential safety standard than solids. There are other failure modes on system level, in which they are superior to solid as well as liquid propulsion systems. Consider a serious malfunction of the cryogenic core stage engine (explosion, turbine rupture, etc.): it is easy to imagine how severed solid or liquid boosters would join the explosion. On the other hand, there is no way how hybrid boosters could be made to tMember, International Academy of Astronautics (Engineering Sciences Section).
explode. In all likelihood, they won't even burn by inflicting external damage. It is for this reason that their TNT equivalent is essentially zero (e.g. during transportation). Another safety argument concerns shut-down capability. It is a well-known catastrophic single point failure mode of the shuttle system, if only one of the SRBs ignites. In this respect, hybrids and liquids share the advantage of being able to be shut-down. (The same is true for mission-adapted throttleability, which belongs to another category of advantages as compared with solids.) In terms of specific impulse, hybrids lie between solids and liquids. However, they can easily reach levels, which in solids would be called "very high energy propellants" (see Fig. 1 for the combination LOX/polyethylene chosen in this analysis). Another draw-back of solids is their high pollution load inflicted to the launch site by the emission of hydrochloric acid and aluminum oxide. Hybrids with LOX/solid hydrocarbon as propellants produce extremely clean combustion products. The same is true for cryogenic liquids (LOX/LH2) and more or less true for semi-cryogenic ones (liquid hydrocarbons: these have a tendency of producing soot). However, these liquid propulsion systems have a disadvantage in common with hybrids: engines of the proper size are not available. This brings us to the disadvantages of hybrids. First of all, hybrid boosters would require a completely new development with only a small base of experience to rely on. This would tend to increase the costs of development. Secondly, hybrid combustion is tricky. Combustion efficiency tends to be low and overall mixture ratio (and hence specific impulse too) is sensitive to grain geometry, which changes with time. In addition, a high degree of fuel consumption is difficult to achieve. Again, all this increases the necessary developmental effort. Thirdly, but less severe, hybrid boosters turn out quite bulky, as the present analysis shows (see Fig. 2).
1059
1060
R . E . Lo and E. DARGIES 198
350 j
330
f
f 196
3~o
.E u
194
290
192
270
2
¢~ 250 (/3
~9o
230 188
210 0
I I J L J L I I I l I I 10 20 30 40 50 60 70 80 90 100 110 120
Chamber pressure
Fig.
1.
-
20
I 30
I 50
~ 60
I 70
I 80
I 90
~ I I 100 110 120
Initial, chamber pressure (bar)
(bar)
Maximum sea level specific LOX/polyethylene.
J 40
impulse
of
Fig. 3. Hybrid booster mass as function of chamber pressure.
i
J
121-"-
2PI70IHI20/HIO
2HY157/H120/H I 0
Fig. 2. Ariane 5 HY and Ariane 5P.
Table 1. Performance data Maximum thrust
5.224 kN
Specific impulse Maximum Time average
304,5 s 298.2 s
Chamber pressure Initial value Maximum Time average
60 bar 69 bar 48 bar
Table 2. Main components included in the mass-models Low alloy carbon steel case, including the solid fuel grain Aluminium alloy oxidizer tank Oxidizer turbopump Hydrazine gas generator with bydrazine tank Cold gas pressurization system Aluminium alloy connecting structure Nose cone Gimballing system
Hybrid boosters for ARIANE 5 Table 3. Mass breakdown (in tons) Propellants 157.2 Fuel 38.I Oxidizer 115.3 Unusable rests 3.8 Case 16.7 Injection system 0.5 Igniter 0.65 Turbopump 0.18 LOX tank 1.74 Hydrazine 1.4 Pressurizationsystem 1.9 Structure 2.I Nozzle 4.9 Others 0.73 Total mass
188.0
This is a result of the low regression rate (solid fuel evaporation rate) of most hybrid fuel/oxidizer combinations. Nevertheless, the inherent safety advantages of hybrids compared with both solid and liquid boosters has prompted us to the present investigation. Our system analysis is based on the performance of the SRBs of the Ariane 5P configuration 2P170/H120/ H10, which has been the nominal configuration until recently. In other words, the hybrid boosters with 157 tons of LOX/polyethylene in the resulting Ariane 5 HY version 2 HYI57/H120/HI0 have exactly the
1061
same performance and mission profile as the SRBs of Ariane 5P. The following HY 157 data must be compared with P170 data (174.5 tons of propellant, 198 tons total mass, 25 m length, 3.1 m max. diameter). HY157 is a turbopump-fed hybrid rocket propulsion system with hydrazine gas generator and cold gas tank pressurization. Overall length is 23.5 m with 5.2 m diameter. Total mass was optimized. Figure 3 shows the masses as function of initial chamber pressure of hybrid boosters capable of performing the required mission. For each chamber pressure, nozzle area ratio was chosen according to a Mach-number dependant separation criterium. Performance data, main components and mass breakdown are shown in Tables 1-3.
CONCLUSION Hybrid boosters offer a large potential of system reliability and safety and are worth further investigations, particularly for use in manned systems. In addition and as a final remark it should be pointed out that two PI70 boosters would need propellants worth about 30 million DM, while the hybrids need less than 1 million!
0094-5767/87 $3.00+0.00 © 1987PergamonJournals Ltd
Printed in Great Britain. All rights reserved
Academy Transactions Note HYBRID BOOSTERS FOR ARIANE 5 R. E. Lot and E. DARGIES DFVLR Institute of Chemical Propulsion and Engineering, D-7101 Hardthausen-Lampoldshausen, F.R.G. (Received 14 May 1987)
Abstract--Results of a systems analysis are presented which investigates replacing Ariane 5's solid rocket boosters by LOX/polyethylene hybrids. Their advantages and disadvantages are discussed. A performance and mass breakdown is presented.
Ariane 5P, like the U.S. Space Transportation System and the future Japanese launcher H II, uses a cryogenic LOX/LH2-core stage along with two solid rocket boosters (SRBs). According to the nomenclature widely used in Europe, Ariane 5P appears to converge on a configuration 2P230/H 155/H 10, where P stands for solid propellants and H designates stages with LOX/LH2, while the numbers indicate propellant masses in tons. SRBs have failed catastrophically in 1985 and 1986 during launches of Titan and the STS-Challenger. The failures were caused by defects in case bonding and a leak in the combustion chamber wall respectively. In hybrid propulsion systems, the combustion chamber contains only the solid fuel, while the liquid oxydizer is stored in a tank and injected through a feed system into combustion channels inside the solid fuel. Therefore, hybrids do not rely at all on case bonding. Hybrid combustion occurs along well oxidizerventilated surfaces only. While in solids a small leak in the chamber wall results in the immediate ignition (followed by self-sustained combustion) of all propellant-surfaces along which the leakage gases flow, the same situation in hybrids would cause an extremely fuel-rich flow with hardly any widening of the leakage flow path. Consequently, hybrids have a much higher potential safety standard than solids. There are other failure modes on system level, in which they are superior to solid as well as liquid propulsion systems. Consider a serious malfunction of the cryogenic core stage engine (explosion, turbine rupture, etc.): it is easy to imagine how severed solid or liquid boosters would join the explosion. On the other hand, there is no way how hybrid boosters could be made to tMember, International Academy of Astronautics (Engineering Sciences Section).
explode. In all likelihood, they won't even burn by inflicting external damage. It is for this reason that their TNT equivalent is essentially zero (e.g. during transportation). Another safety argument concerns shut-down capability. It is a well-known catastrophic single point failure mode of the shuttle system, if only one of the SRBs ignites. In this respect, hybrids and liquids share the advantage of being able to be shut-down. (The same is true for mission-adapted throttleability, which belongs to another category of advantages as compared with solids.) In terms of specific impulse, hybrids lie between solids and liquids. However, they can easily reach levels, which in solids would be called "very high energy propellants" (see Fig. 1 for the combination LOX/polyethylene chosen in this analysis). Another draw-back of solids is their high pollution load inflicted to the launch site by the emission of hydrochloric acid and aluminum oxide. Hybrids with LOX/solid hydrocarbon as propellants produce extremely clean combustion products. The same is true for cryogenic liquids (LOX/LH2) and more or less true for semi-cryogenic ones (liquid hydrocarbons: these have a tendency of producing soot). However, these liquid propulsion systems have a disadvantage in common with hybrids: engines of the proper size are not available. This brings us to the disadvantages of hybrids. First of all, hybrid boosters would require a completely new development with only a small base of experience to rely on. This would tend to increase the costs of development. Secondly, hybrid combustion is tricky. Combustion efficiency tends to be low and overall mixture ratio (and hence specific impulse too) is sensitive to grain geometry, which changes with time. In addition, a high degree of fuel consumption is difficult to achieve. Again, all this increases the necessary developmental effort. Thirdly, but less severe, hybrid boosters turn out quite bulky, as the present analysis shows (see Fig. 2).
1059
1060
R . E . Lo and E. DARGIES 198
350 j
330
f
f 196
3~o
.E u
194
290
192
270
2
¢~ 250 (/3
~9o
230 188
210 0
I I J L J L I I I l I I 10 20 30 40 50 60 70 80 90 100 110 120
Chamber pressure
Fig.
1.
-
20
I 30
I 50
~ 60
I 70
I 80
I 90
~ I I 100 110 120
Initial, chamber pressure (bar)
(bar)
Maximum sea level specific LOX/polyethylene.
J 40
impulse
of
Fig. 3. Hybrid booster mass as function of chamber pressure.
i
J
121-"-
2PI70IHI20/HIO
2HY157/H120/H I 0
Fig. 2. Ariane 5 HY and Ariane 5P.
Table 1. Performance data Maximum thrust
5.224 kN
Specific impulse Maximum Time average
304,5 s 298.2 s
Chamber pressure Initial value Maximum Time average
60 bar 69 bar 48 bar
Table 2. Main components included in the mass-models Low alloy carbon steel case, including the solid fuel grain Aluminium alloy oxidizer tank Oxidizer turbopump Hydrazine gas generator with bydrazine tank Cold gas pressurization system Aluminium alloy connecting structure Nose cone Gimballing system
Hybrid boosters for ARIANE 5 Table 3. Mass breakdown (in tons) Propellants 157.2 Fuel 38.I Oxidizer 115.3 Unusable rests 3.8 Case 16.7 Injection system 0.5 Igniter 0.65 Turbopump 0.18 LOX tank 1.74 Hydrazine 1.4 Pressurizationsystem 1.9 Structure 2.I Nozzle 4.9 Others 0.73 Total mass
188.0
This is a result of the low regression rate (solid fuel evaporation rate) of most hybrid fuel/oxidizer combinations. Nevertheless, the inherent safety advantages of hybrids compared with both solid and liquid boosters has prompted us to the present investigation. Our system analysis is based on the performance of the SRBs of the Ariane 5P configuration 2P170/H120/ H10, which has been the nominal configuration until recently. In other words, the hybrid boosters with 157 tons of LOX/polyethylene in the resulting Ariane 5 HY version 2 HYI57/H120/HI0 have exactly the
1061
same performance and mission profile as the SRBs of Ariane 5P. The following HY 157 data must be compared with P170 data (174.5 tons of propellant, 198 tons total mass, 25 m length, 3.1 m max. diameter). HY157 is a turbopump-fed hybrid rocket propulsion system with hydrazine gas generator and cold gas tank pressurization. Overall length is 23.5 m with 5.2 m diameter. Total mass was optimized. Figure 3 shows the masses as function of initial chamber pressure of hybrid boosters capable of performing the required mission. For each chamber pressure, nozzle area ratio was chosen according to a Mach-number dependant separation criterium. Performance data, main components and mass breakdown are shown in Tables 1-3.
CONCLUSION Hybrid boosters offer a large potential of system reliability and safety and are worth further investigations, particularly for use in manned systems. In addition and as a final remark it should be pointed out that two PI70 boosters would need propellants worth about 30 million DM, while the hybrids need less than 1 million!