For the ancient Chinese, a high-energy rocket propellant was black powder. As rocket technology improved, better propellants were found, often by borrowing from gun propellants. Gun propellants, such as guncotton and nitroglycerin, burn rather than detonate when not highly confined, and this is a good thing, since the last thing you want your rocket to do is blow up (and I guess that technically, that would be the last thing it did).
Rockets carry their fuel and oxidizer with them. This means that they must not only accelerate the payload, but the fuel as well. This puts a premium on fuels with high energy per unit weight.
As with gun propellants, rocket fuels can be either a mix of oxidizing agent and fuel, as in black powder, or they can combine the fuel and the oxidizer on a single molecule, as with guncotton or nitroglycerine. The Apollo lunar missions used liquid hydrogen and liquid oxygen. The reactant was water vapor. The light weight of the water molecule (18 atomic mass units) is a benefit, because one of the constraints on rocket engine design is the speed of the reaction mass being thrown behind. The faster the exhaust gases, the better the performance. For a given amount of energy, lighter molecules move faster than heavier ones.
There are several problems with cryogenic liquid fueled rockets. The fuels have to be stored separately from the rocket. This is not a big problem for the Apollo program, but for systems that launch more frequently or need to be stored in an aircraft, storing the fuels in the rocket is desirable. The fuels need to be kept very cold. This means that cryogenic Dewars (and their associated weight) must be on board the rocket. The Dewars also need to be strong enough to withstand the forces the rocket undergoes during launch, but also during shipping and outfitting. The fuels are low density. This means that large tanks are needed to store them. Large tanks add to the weight. Every bit of added weight means more fuel is needed, which adds more weight.
Solid propellants solve several of these problems, but add their own costs. Solid propellants can be stored in the rocket. They don't need cooling. They are high density, so the container is smaller. On the cost side, the reactant gases are generally heavier molecules, such as carbon dioxide (44 atomic mass units), nitrogen gas (28 atomic mass units), and in some cases metal oxides or chlorides, which are even more massive. As a result, the exhaust gases are heavier, and are harder to get moving at the same speed as water molecules.
The US Titan missile project was started in 1959. The Titan I used kerosene as the liquid fuel, and liquid oxygen as the oxidizer. Each molecule of kerosene produced 12 molecules of carbon dioxide and 13 molecules of water vapor.
By changing the fuel to dimethylhydrazine and the oxidizer to nitrogen tetroxide, the need for cryogenic storage was eliminated. The new Titan II rocket could have the fuel stored in it, allowing the missile to be fired in a single minute (hence the name Minuteman Missile). The simplified maintenance and storage meant the missile could be kept in a missile silo.
Those fuels were difficult to handle without accidents, and led to a program to develop solid propellants. The Titan III missile, with solid rockets, launched in 1964. The fuel was aluminum powder, the oxidizer was ammonium perchlorate, and the two were mixed with polybutadiene acrylic acid as a binder. Aluminum powder is used because it generates a lot of energy when it burns (per unit weight), and it is denser than hydrocarbon fuels, so it takes up less space, meaning the container weighs less. It is also difficult to accidentally ignite, and it helps to stabilize the burn rate. Some mixes add iron oxide to increase the burn temperature through the thermite reaction.
The Minuteman is a three-stage rocket. The first stage uses the Titan III mix as described above. The second stage uses a similar mix, with a different binder. The third stage adds HMX, nitrocellulose, and nitroglycerin to the mix. The higher energy molecules in the third stage allow the exhaust gases to reach higher speeds, improving the performance of the rocket. At low speeds (such as at liftoff), the exhaust gases are moving much faster than the rocket is, and so their speed is of less importance. As the rocket gains speed, high-speed exhaust gases become more important. The third stage is moving so fast, the cheap propellant is not cost effective, and the higher energy mix is needed.
The Trident missile system used a fuel called XLDB-70, for "cross-linked double-base, 70% solids". The solids were HMX, aluminum, and ammonium perchlorate. The binder was polyglycol adipate, nitrocellulose, nitroglycerin, and hexadiisocryanate, making up 30% of the fuel. Cross-linking the binder means that extra chemical bonds are made in the binder, so that it is more stable, and stiffer. The polyglycol adipate was replaced by polyethylene glycol in the later Trident systems, and the new fuel was named NEPE-75. The 75 referred to the fact that with the improved binder, it could hold 75% solids.
The use of ammonium perchlorate was found to cause environmental pollution. It produces hydrogen chloride gas when it burns (which turns into hydrochloric acid when it encounters water). Exposure to perchlorates over time produces thyroid problems. This is not just a problem when the missile launches, but it is a considerable problem for the companies making the fuel.
Besides the environmental impact of the perchlorates, the use of HMX in the Minuteman third stage and the Trident is a cause of concern because it is susceptible to detonation by shock or friction. Again, not only is this a problem for the missile, it is a problem for the companies working with the fuels.
Modern impact, friction, and heat insensitive high-energy molecules can be added to solid rocket fuels to increase safety and performance. Furazan-based molecules such as diaminoazoxyfurazan (called either DAAF or DAAOF depending on the researcher) have been added to mixes, with interesting results. The molecule adds high energy content, but also modifies the reaction rate of perchlorate mixes, helping to control the rate of burning. In mixes without perchlorate, its susceptibility to shock (it is being considered as a booster explosive for less sensitive explosives) may be moderated by the plastic or rubber binder.
One way to increase rocket performance is to increase the pressure. This makes the exhaust gases move faster. With modern lightweight but incredibly strong composite materials, increasing the pressure is definitely an option. The problem has been that as the pressure rises, the solid propellant burns faster. This in turn increases the pressure, leading to a runaway situation that ends in an explosive catastrophe.
DAAF can be added to perchlorate mixtures to modify the burn rate. In a similar fashion, perchlorate itself can be used (as much as 10% of a mix) to moderate nitramine propellants, such as a mixture of RDX, nitroglycerin, trimethylolethane trinitrate, and polyurethane. In these mixes, the pressure exponent (a measure of how quickly increased pressure causes the fuel to burn faster) goes from numbers greater than 0.9 to numbers less than 0.65, a very significant drop.
The nitramine-based propellants replace nitrocellulose and nitroglycerin double-base propellants, which has the benefit of reducing the smoke from the missile, making it harder to track. This is known as a "reduced signature" propellant for this reason. Low signature missiles fired from aircraft carriers or field emplacements do not give away the position of the attacker. Ammonium perchlorate propellants produce even more smoke in the exhaust than double-base propellants, giving another reason to phase them out in favor of higher energy molecules.
Propellants using CL-20 have even higher performance than RDX based propellants, matching that of liquid fueled rockets. As newer high-energy molecules are developed, this trend is expected to continue.
Replacing the inert binders with high-energy polymers such as GAP (glycidyl azide polymer, discussed earlier) increases the performance still further. By adding one to five percent heavy metal catalysts (lead, tin, or copper compounds), the pressure exponent can be dropped by large amounts. Further drops can be had by adding carbon in the form of carbon black, or graphite, in amounts less than one percent by weight. The pressure exponent in mixes like this is generally less than 0.6, and the chemical stability improves as well.
Combinations of the insensitive high explosive FOX-7 with GAP are being used in shoulder-launched rockets, replacing RDX and HMX, resulting in munitions that are safer to transport and handle, and less sensitive to detonation in a fire, or when hit by bullets, fragments, or nearby exploding ordinance concussions. However, FOX-7 is still (at the time of this writing) expensive and in limited supply.
Rocket fuels are rated by a number called the specific impulse. Specific impulse is a way of combining the performance related metrics discussed earlier (heat of the reaction, temperature of the reaction products, and the molecular weight of the reaction products) into a single number that can be used to easily compare two fuels.
The units (Newton seconds per kilogram) are unimportant for this discussion, as we can treat the specific impulse values as simple numbers for comparisons. Fuels that burn hotter will have larger values for specific impulse. Fuels that contain more light elements such as hydrogen will have higher value than fuels that have metal oxides or chlorides in the reaction products, or fuels that produce a higher percentage of carbon dioxide.
Thrust, the force that moves a rocket, is a function of the specific impulse times the density of the propellant, times the rate of burning, times the surface area being burned. Increasing any of these factors increases thrust. Increasing the surface area is generally not feasible in a rocket due to the limited volume available (increasing surface area in a constant volume means using less propellant). Where high burn rates are more important than long burn times, such as interceptor missiles, increasing surface area by making star-shaped cavities in the propellant are a tradeoff that has benefits.
Burn rate modifiers can speed up the burn rate, or slow it down as we saw with DAAF and perchlorates. Adding iron oxides to fuels that contain aluminum raises the temperature of the reaction through the thermite process. Adding heat conductive metal whiskers can also speed up the burn rate.
Thus, another important metric for judging rocket fuels is the burn rate, and the associated pressure exponent. The rate at which a propellant burns is proportional to the pressure raised to the pressure exponent. Exponents less than one mean that as the pressure increases, the burn rate increases, but at progressively slower rates.
The lowest pressure exponent fuels are the same as those used in guns — double-base propellants containing a mix of nitroglycerin plasticized with nitrocellulose. They have pressure exponents in the range of 0 to 0.3, meaning that as the pressure increases, the burn rate increases proportionally, with no tendency towards runaway explosions. Unfortunately, they also have the lowest specific impulse (2100 to 2300 Newton seconds per kilogram), due to the relatively cool temperatures, and the carbon dioxide they produce. Their burn rates are in the 10 to 25 millimeter per second range.
Next are the ammonium perchlorate based fuels. Those containing nitramines and aluminum have specific impulses in the 2500 to 2600 range, but they produce the most smoke. The reduced signature versions without the aluminum have specific impulse values of 2400 to 2500. The pressure exponents are in the range of 0.3 to 0.5 (an exponent of 0.5 means that the burn rate goes up as the square root of the pressure). The burn rates are in the 6 to 40 millimeter per second range.
Ammonium nitrate based propellants have slightly lower specific impulse values (2200 to 2350), and pressure exponents of 0.4 to 0.6. They are low signature, and generally consist of ammonium nitrate, nitramines, and GAP, the burn rates are 5 to 10 millimeters per second.
Also in the same exponent range (0.4 to 0.6) are mainly nitramine based propellants, with GAP and some ammonium perchlorate (less than 10%) to bring the pressure exponent down. They have specific impulse values in the 2300 to 2450 range.
The latest solid rocket propellants contain ammonium dinitramide (ADN) as the oxidizer instead of ammonium perchlorate. ADN is a high-energy molecule by itself, with a respectable detonation velocity of 8,074 meters per second (close to TATB). When burned, it produces more gas than any of the other nitramides, even more than RDX. As an oxidizer, it has an oxygen balance of 25.8, which puts it in between ammonium nitrate (20.0) and ammonium perchlorate (34.0). Because it burns hotter, and has lighter reaction products, it can perform better than ammonium perchlorate. Its other benefits as a replacement for ammonium perchlorate are that it is low signature (less smoke), and it does not produce any hydrogen chloride gas (just water, CO2 and nitrogen gas).
Mixtures of ADN, RDX, GAP, and aluminum reach specific impulse values higher than any of the previous solid propellants, in the 2500 to 2700 range.
Another high-energy oxidizer used for replacing ammonium perchlorate is hydrazinium nitroformate, or HNF.
Mixtures of HNF with aluminum and GAP, polyGlyN, or polyNIMMO are showing promise as high-energy propellants with reduced signature, high specific impulse, and no chlorine in the exhaust. Chlorine, in addition to producing hydrochloric acid when burned with hydrogen containing fuels, is a contributor to ozone depletion in the upper atmosphere, which is especially important when dealing with upper stage booster fuels. The closer to the ozone layer the chlorine is released, the more pronounced the effects.
A competitor to ADN/aluminum is CL-20 mixed with energetic binders such as GAP. CL-20 is currently available in kilogram quantities, but production is expected to increase as more uses for the very energetic molecule are found. Made in batches of 50 to 100 kilograms, it is still expensive and less easily available than the ADN propellants. It is also more sensitive to heat, shock, and friction.
TNAZ is also being tested in rocket propellants. Its advantages are an energy content close to RDX, a melting point that makes it castable like TNT, and chemical compatibility with metals like aluminum and steel, as well as the common binder polymers and plasticizers.
We have been assuming in the foregoing discussion that a pressure exponent of 0.7 or above is to be avoided. This is to reduce the possibility of a runaway reaction that eventually exceeds the burst strength of the combustion chamber.
However, there is a class of rocket engine where a higher pressure exponent is desired, or even required. This is the thrust magnitude controlled class (TMC), an engine where gases or fluids are injected into a solid rocket combustion chamber to change the thrust. A propellant with a high pressure exponent allows a small change in pressure to have a larger change in the burn rate. The thrust can thus be controlled with smaller changes to the flow rate of the gas or fluid being injected.
TMC rocket engines are being used in upper stage rockets for satellite launch and spacecraft propulsion systems.
A typical TMC engine might inject the water vapor and oxygen provided by decomposing hydrogen peroxide. The decomposition can be easily controlled, and the resulting gases have enough pressure to enter the combustion chamber. There, they expand and possibly react with a low oxygen balanced propellant, increasing the pressure. The high pressure exponent causes the burn rate to quickly rise.
Since the pressure is under the control of the injection system, runaway is not an issue. The pressure can be kept below any critical limits by reducing the flow rate.
As with the move towards less sensitive explosives, safer rocket propellants are also a high priority in military rockets. A fire on an aircraft carrier deck, or enemy bullets and shell fragments, should ideally not set off rocket fuels any more than they should explode bombs.
So-called LOVA propellants (for low vulnerability ammunition) should not unintentionally burn or explode, and should not release toxic combustion products. Two of these propellants are XM39 and M43. They both consist of 76% RDX, 4% nitrocellulose, and 0.4% ethyl centralite. The latter compound is a plasticizer used in many smokeless powder gun propellants.
The remaining ingredients are 7.6% acetyl triethyl citrate, known as ATEC (a plasticizer used in cellulose derivatives and PVC plastics) in XM39, and an (unspecified) energetic plasticizer in M43 (energetic meaning it has azide or nitro groups added to make it higher energy).