The highest specific impulse chemical rockets use liquid propellants.
The first rockets used liquid oxygen and gasoline as propellants. Both are readily available, cheap, high energy, and dense. Oxygen is a moderate cryogen -- air will not liquify against a liquid oxygen tank, so it is possible to store LOX briefly in a rocket without heroic insulation measures. Gasoline has since been replaced by RP-1, a highly refined grade of kerosene. The combination is so good that to this day it is still used in the first stages of most orbital launchers, as well as the long-range offensive missiles of China and North Korea. The Space Shuttle uses solid rocket motors for its first stage.
During the 1950s there was a great burst of activity by propellant chemists to find high-energy liquid propellants better suited to the military. Military rockets need to sit in silos for many years, able to launch at a moment's notice. Propellants requiring continuous refrigeration, and which cause their rockets grow ever-thicker blankets of ice, are not practical. As the military is willing to handle and use hazardous materials, a great number of dangerous chemicals were brewed up in large batches, virtually all of which were dead ends.
Nitric acid is a particularly good story. The acid itself (HNO3) is not stable, and gives off NO2 fumes (hence the name white fuming nitric acid). Unlike N2O fumes, which make people happy, NO2 fumes make people die. The addition of large amounts of N2O4 makes the mixture red, but keeps it from changing composition, leaving the problem that nitric acid will eat any container it is placed in, releasing gases that can build up pressure in the process. The breakthrough was the addition of a little hydrofluoric acid (HF), which forms a self-healing metal fluoride on the interior of tank walls and makes Inhibited Red Fuming Nitric Acid storable. Although the development of military propellants was treated with the greatest secrecy, the trick to inhibiting nitric acid was published in 1954 and Russian rockets with the same fuel appeared shortly afterwards, the first being the SS-1B ("Scud"). Eventually the chemists gave up stabilizing HNO3 with N2O4, and just used straight N2O4, which is a slightly better oxidizer anyway. (In the propellant table below, note that N2O4 is always in equilibrium with NO2, and so mixtures are sometimes quoted with the latter.)
All the early rocket theorists noted that hydrogen would be an ideal propellant. Hydrogen gas is very bulky, however, and the only practical storage was as a liquid, requiring either immense pressures or extreme cold. Since immense pressure requires heavy tanks, cryogenic storage was pursued, and not mastered in rockets until the 1960s as part of the Apollo and Centaur upper-stage programs. The Space Shuttle uses liquid hydrogen and liquid oxygen for the orbiter's main engines.
The highest specific impulse chemistry ever test-fired in a rocket engine was lithium and fluorine, with hydrogen added to improve the exaust thermodynamics (making this a tripropellant). The combination delivered 542 seconds (542 lbf·s/lb, 5.32 kN·s/kg, 5320 m/s) specific impulse in a vacuum. The impracticality of this chemistry highlights why exotic propellants are not actually used: to make all three components liquids, the hydrogen must be kept below -252 °C (just 21 K) and the lithium must be kept above 180 °C (453 K). Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, and hydrogen, while not hypergolic, is an explosive hazard. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which trashes the environment, makes work around the launch pad difficult, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket. Finally, both lithium and fluorine are expensive and rare, enough to actually matter.
The common liquid fuel combinations in use today are:
To approximate Isp at other chamber pressures | |
---|---|
Pressure | Multiply by |
lbf/in² absolute | |
1000 | 1.00 |
900 | 0.99 |
800 | 0.98 |
700 | 0.97 |
600 | 0.95 |
500 | 0.93 |
400 | 0.91 |
300 | 0.88 |
JANAF thermochemical data used throughout. Calculations performed by Rocketdyne, results appear in "Modern Engineering for Design of Liquid-Propellant Rocket Engines", Huzel and Huang. Some of the units have been converted to metric, but pressures have not. These are best-possible specific impulse calculations.
Assumptions:
Definitions
r | Mixture ratio: mass oxidizer / mass fuel |
Ve | Average exhaust velocity, m/s. The same measure as specific impulse in different units, numerically equal to specific impulse in N·s/kg. |
C* | Characteristic velocity, m/s. Equal to chamber pressure multiplied by throat area, divided by mass flow. Used to check experimental rocket's combustion efficiency. |
Tc | Chamber temperature, °C |
d | Bulk density of fuel and oxidizer, g/cm³ |
Optimum expansion from 1000 psia to 14.7 psia | Optimum expansion from 1000 psia to vacuum (nozzle area ratio = 40) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Oxidizer | Fuel | Ve | r | Tc | d | C* | Ve | r | Tc | d | C* |
LOX | H2 | 3816 | 4.13 | 2740 | 0.29 | 2416 | 4462 | 4.83 | 2978 | 0.32 | 2386 |
H2-Be 49/51 | 4498 | 0.87 | 2558 | 0.23 | 2833 | 5295 | 0.91 | 2589 | 0.24 | 2850 | |
CH4 | 3034 | 3.21 | 3260 | 0.82 | 1857 | 3615 | 3.45 | 3290 | 0.83 | 1838 | |
C2H6 | 3006 | 2.89 | 3320 | 0.90 | 1840 | 3584 | 3.10 | 3351 | 0.91 | 1825 | |
C2H4 | 3053 | 2.38 | 3486 | 0.88 | 1875 | 3635 | 2.59 | 3521 | 0.89 | 1855 | |
RP-1 | 2941 | 2.58 | 3403 | 1.03 | 1799 | 3510 | 2.77 | 3428 | 1.03 | 1783 | |
N2H4 | 3065 | 0.92 | 3132 | 1.07 | 1892 | 3460 | 0.98 | 3146 | 1.07 | 1878 | |
B5H9 | 3124 | 2.12 | 3834 | 0.92 | 1895 | 3758 | 2.16 | 3863 | 0.92 | 1894 | |
B2H6 | 3351 | 1.96 | 3489 | 0.74 | 2041 | 4016 | 2.06 | 3563 | 0.75 | 2039 | |
CH4/H2 92.6/7.4 | 3126 | 3.36 | 3245 | 0.71 | 1920 | 3719 | 3.63 | 3287 | 0.72 | 1897 | |
GOX | GH2 | 3997 | 3.29 | 2576 | - | 2550 | 4485 | 3.92 | 2862 | - | 2519 |
F2 | H2 | 4036 | 7.94 | 3689 | 0.46 | 2556 | 4697 | 9.74 | 3985 | 0.52 | 2530 |
H2-Li 65.2/34.0 | 4256 | 0.96 | 1830 | 0.19 | 2680 | ||||||
H2-Li 60.7/39.3 | 5050 | 1.08 | 1974 | 0.21 | 2656 | ||||||
CH4 | 3414 | 4.53 | 3918 | 1.03 | 2068 | 4075 | 4.74 | 3933 | 1.04 | 2064 | |
C2H6 | 3335 | 3.68 | 3914 | 1.09 | 2019 | 3987 | 3.78 | 3923 | 1.10 | 2014 | |
MMH | 3413 | 2.39 | 4074 | 1.24 | 2063 | 4071 | 2.47 | 4091 | 1.24 | 1987 | |
N2H4 | 3580 | 2.32 | 4461 | 1.31 | 2219 | 4215 | 2.37 | 4468 | 1.31 | 2122 | |
NH3 | 3531 | 3.32 | 4337 | 1.12 | 2194 | 4143 | 3.35 | 4341 | 1.12 | 2193 | |
B5H9 | 3502 | 5.14 | 5050 | 1.23 | 2147 | 4191 | 5.58 | 5083 | 1.25 | 2140 | |
OF2 | H2 | 4014 | 5.92 | 3311 | 0.39 | 2542 | 4679 | 7.37 | 3587 | 0.44 | 2499 |
CH4 | 3485 | 4.94 | 4157 | 1.06 | 2160 | 4131 | 5.58 | 4207 | 1.09 | 2139 | |
C2H6 | 3511 | 3.87 | 4539 | 1.13 | 2176 | 4137 | 3.86 | 4538 | 1.13 | 2176 | |
RP-1 | 3424 | 3.87 | 4436 | 1.28 | 2132 | 4021 | 3.85 | 4432 | 1.28 | 2130 | |
MMH | 3427 | 2.28 | 4075 | 1.24 | 2119 | 4067 | 2.58 | 4133 | 1.26 | 2106 | |
N2H4 | 3381 | 1.51 | 3769 | 1.26 | 2087 | 4008 | 1.65 | 3814 | 1.27 | 2081 | |
MMH/N2H4/H20 50.5/29.8/19.7 | 3286 | 1.75 | 3726 | 1.24 | 2025 | 3908 | 1.92 | 3769 | 1.25 | 2018 | |
B2H6 | 3653 | 3.95 | 4479 | 1.01 | 2244 | 4367 | 3.98 | 4486 | 1.02 | 2167 | |
B5H9 | 3539 | 4.16 | 4825 | 1.20 | 2163 | 4239 | 4.30 | 4844 | 1.21 | 2161 | |
F2/O2 30/70 | H2 | 3871 | 4.80 | 2954 | 0.32 | 2453 | 4520 | 5.70 | 3195 | 0.36 | 2417 |
RP-1 | 3103 | 3.01 | 3665 | 1.09 | 1908 | 3697 | 3.30 | 3692 | 1.10 | 1889 | |
F2/O2 70/30 | RP-1 | 3377 | 3.84 | 4361 | 1.20 | 2106 | 3955 | 3.84 | 4361 | 1.20 | 2104 |
F2/O2 87.8/12.2 | MMH | 3525 | 2.82 | 4454 | 1.24 | 2191 | 4148 | 2.83 | 4453 | 1.23 | 2186 |
N2F4 | CH4 | 3127 | 6.44 | 3705 | 1.15 | 1917 | 3692 | 6.51 | 3707 | 1.15 | 1915 |
C2H4 | 3035 | 3.67 | 3741 | 1.13 | 1844 | 3612 | 3.71 | 3743 | 1.14 | 1843 | |
MMH | 3163 | 3.35 | 3819 | 1.32 | 1928 | 3730 | 3.39 | 3823 | 1.32 | 1926 | |
N2H4 | 3283 | 3.22 | 4214 | 1.38 | 2059 | 3827 | 3.25 | 4216 | 1.38 | 2058 | |
NH3 | 3204 | 4.58 | 4062 | 1.22 | 2020 | 3723 | 4.58 | 4062 | 1.22 | 2021 | |
B5H9 | 3259 | 7.76 | 4791 | 1.34 | 1997 | 3898 | 8.31 | 4803 | 1.35 | 1992 | |
ClF5 | MMH | 2962 | 2.82 | 3577 | 1.40 | 1837 | 3488 | 2.83 | 3579 | 1.40 | 1837 |
N2H4 | 3069 | 2.66 | 3894 | 1.47 | 1935 | 3580 | 2.71 | 3905 | 1.47 | 1934 | |
MMH/N2H4 86/14 | 2971 | 2.78 | 3575 | 1.41 | 1844 | 3498 | 2.81 | 3579 | 1.41 | 1844 | |
MMH/N2H4/N2H5NO3 55/26/19 | 2989 | 2.46 | 3717 | 1.46 | 1864 | 3500 | 2.49 | 3722 | 1.46 | 1863 | |
ClF3 | MMH/N2H4/N2H5NO3 55/26/19 | 2789 | 2.97 | 3407 | 1.42 | 1739 | 3274 | 3.01 | 3413 | 1.42 | 1739 |
N2H4 | 2885 | 2.81 | 3650 | 1.49 | 1824 | 3356 | 2.89 | 3666 | 1.50 | 1822 | |
N2O4 | MMH | 2827 | 2.17 | 3122 | 1.19 | 1745 | 3347 | 2.37 | 3125 | 1.20 | 1724 |
MMH/Be 76.6/29.4 | 3106 | 0.99 | 3193 | 1.17 | 1858 | 3720 | 1.10 | 3451 | 1.24 | 1849 | |
MMH/Al 63/27 | 2891 | 0.85 | 3294 | 1.27 | 1785 | ||||||
MMH/Al 58/42 | 3460 | 0.87 | 3450 | 1.31 | 1771 | ||||||
N2H4 | 2862 | 1.36 | 2992 | 1.21 | 1781 | 3369 | 1.42 | 2993 | 1.22 | 1770 | |
N2H4/UDMH 50/50 | 2831 | 1.98 | 3095 | 1.12 | 1747 | 3349 | 2.15 | 3096 | 1.20 | 1731 | |
N2H4/Be 80/20 | 3209 | 0.51 | 3038 | 1.20 | 1918 | ||||||
N2H4/Be 76.6/23.4 | 3849 | 0.60 | 3230 | 1.22 | 1913 | ||||||
B5H9 | 2927 | 3.18 | 3678 | 1.11 | 1782 | 3513 | 3.26 | 3706 | 1.11 | 1781 | |
NO/N2O4 25/75 | MMH | 2839 | 2.28 | 3153 | 1.17 | 1753 | 3360 | 2.50 | 3158 | 1.18 | 1732 |
N2H4/Be 76.6/23.4 | 2872 | 1.43 | 3023 | 1.19 | 1787 | 3381 | 1.51 | 3026 | 1.20 | 1775 | |
IRFNA IIIa | UDMH/DETA 60/40 | 2638 | 3.26 | 2848 | 1.30 | 1627 | 3123 | 3.41 | 2839 | 1.31 | 1617 |
MMH | 2690 | 2.59 | 2849 | 1.27 | 1665 | 3178 | 2.71 | 2841 | 1.28 | 1655 | |
UDMH | 2668 | 3.13 | 2874 | 1.26 | 1648 | 3157 | 3.31 | 2864 | 1.27 | 1634 | |
IRFNA IV HDA | UDMH/DETA 60/40 | 2689 | 3.06 | 2903 | 1.32 | 1656 | 3187 | 3.25 | 2951 | 1.33 | 1641 |
MMH | 2742 | 2.43 | 2953 | 1.29 | 1696 | 3242 | 2.58 | 2947 | 1.31 | 1680 | |
UDMH | 2719 | 2.95 | 2983 | 1.28 | 1676 | 3220 | 3.12 | 2977 | 1.29 | 1662 | |
H2O2 | MMH | 2790 | 3.46 | 2720 | 1.24 | 1726 | 3301 | 3.69 | 2707 | 1.24 | 1714 |
N2H4 | 2810 | 2.05 | 2651 | 1.24 | 1751 | 3700 | 2.12 | 2645 | 1.25 | 1744 | |
N2H4/Be 74.5/25.5 | 3289 | 0.48 | 2915 | 1.21 | 1943 | 3954 | 0.57 | 3098 | 1.24 | 1940 | |
B5H9 | 3016 | 2.20 | 2667 | 1.02 | 1828 | 3642 | 2.09 | 2597 | 1.01 | 1817 | |
N2H4 | B2H6 | 3342 | 1.16 | 2231 | 0.63 | 2080 | 3953 | 1.16 | 2231 | 0.63 | 2080 |
B5H9 | 3204 | 1.27 | 2441 | 0.80 | 1960 | 3819 | 1.27 | 2441 | 0.80 | 1960 |
Definitions of some of the mixtures: