In spaceflight, liquid rocket fuels like RP-1, hydrogen & methane are often mentioned and compared. Solid rocket fuel, though the oldest form of rocket propellant, remains an integral part of many modern orbital rockets.
Solid rocket fuel is defined as a rocket propellant in which the fuel and oxidizer are mixed and combined with a binder to form a solid compound with a rubberlike feel. It is combined within the rocket booster casing during manufacturing, making it easy to transport and store at room temperature.
Centuries before liquid propellants or the idea of sending a rocket into space even existed, solid-fueled rockets were already in use. The Chinese Song dynasty used “fire arrows” containing gunpowder as propellant during conflicts with the Mongols as early as 1232.
The same principles that were used in these ancient rocket-propelled arrows are still used today to power modern solid-fueled rocket boosters. Most modern orbital launch vehicles primarily use liquid propellants, but solid-fueled rocket boosters still play a crucial role.
In fact, the Space Shuttle would never have been able to lift off and reach orbit without the assistance of its two solid rocket boosters, and a modern Arianne 5 launch vehicle also needs its pair of solid-fueled boosters to assist its main hydrogen-fueled engine in reaching Space.
As the upcoming sections will illustrate, this propellant has numerous advantages over its liquid counterparts but also some serious drawbacks. However, one first needs to define what exactly solid rocket propellant is and how it is manufactured.
What is Solid Rocket Propellant?
Solid rocket fuel is a propellant in which the fuel & oxidizer are mixed and combined with a binder to form a solid compound with a rubberlike feel. It is combined with the rocket casing during manufacturing, making it easy to transport, and can be stored at room temperature.
The different chemical compounds chosen for the mixture are selected depending on the intended use of the solid booster and the amount of thrust needed. However, in all cases, the finished rubberlike propellant needs to contain a fuel and oxidizer component.
A typical mixture (as was the case with the solid rocket boosters that powered the Space Shuttle) will use ammonium perchlorate as the oxidizer, aluminum powder as fuel, and polybutadiene acrylonitrile (PBAN) as the binder.
This is just one of the multiple combinations that can be used to create the desired propellant mixture. Additional compounds like iron oxide (as a catalyst), epoxy curing agents, and other substances needed for a specific application can also be added.
There are two primary types of solid rocket propellant, namely:
- Homogeneous Mixtures
- Composites
Homogeneous Mixtures can be single-, double-, or triple-base mixtures, depending on the number of primary ingredients. The microscopically small ingredients are mixed as a liquid with binders and curing agents and turned into a solid compound.
Typical fuel types for homogeneous mixtures include RDX and nitrocellulose. RDX acts as both a fuel and oxidizer, while nitrocellulose acts as a fuel, oxidizer, and structural polymer.
Composites typically consist of fuel particles like powdered aluminum or beryllium mixed with solid oxidizer granules like ammonium perchlorate or potassium nitrate. Binding and curing agents are also added, and the mixture is set into a solid state.
Before proceeding, a clarification between “fuel” and “propellant” is required. Fuel forms one part of a propellant and needs an oxidizer to combust. As solid rocket propellant contains both the fuel & oxidizer, it will be referred to as “propellant” for the remainder of this article.
How Solid Rocket Propellant Is Made
The manufacturing of the solid rocket propellant typically used on orbital launch vehicles starts with the rocket casing. In large solid rocket boosters, the solid propellant is mixed and cured in the different casing segments that make up the booster.
(The solid boosters of the Space Shuttle were manufactured in four segments, while the new boosters that form part of the Space Launch System for NASA’s Artemis program consist of five segments.)
Before the propellant can be mixed inside the casing, the casing itself needs to be protected from the extremely high temperatures generated by the combustion process. This is done by adding a layer of insulating material around 2 inches thick on the inside of the casing.
A mold (which forms the hole that runs the length of the entire rocket and acts as the rocket’s combustion chamber) is lowered down the center of the cylindrical casing. The wet propellant mixture is then poured in between the casing and mold and allowed to dry & set.
The hole in the middle of the propellant is called the perforation, whose shape also determines the burn rate and, as a result, the amount of thrust the booster can create at specific points during the rocket’s ascent.
After manufacturing, the different booster segments are shipped to the respective launch location, where they are combined at an assembly facility.
Advantages Of Using Solid Rocket Propellant For Orbital Rockets
Solid rocket propellant offers some key advantages but also a number of drawbacks compared to their liquid counterparts. Some of the primary advantages of using solid rocket propellant include:
- Easy Transportation And Storage
- Simplicity And Reliability
- Storage At Room Temperature
- Superior Thrust Due To High Density
- Less Complicated Engines Than Liquid Equivalents
1) Easy Transportation And Handling
Liquid propellants need to be handled with extreme care since they are highly flammable, sometimes extremely toxic, and in the case of cryogenic fuels, need special insulation and refrigeration while being transported.
(Liquid hydrogen, for example, has to be cooled to temperatures below -253° Celsius or -423° Fahrenheit to remain a liquid. Learn more about liquid hydrogen, what it is, as well as the different advantages and drawbacks of this fuel in this article.)
On the other hand, solid rocket propellant becomes very stable after it has turned into a solid and cured inside its casings. It does not ignite easily, does not produce any toxic fumes, and, due to its solid nature, cannot spill or leak.
All these attributes make solid rocket propellant much safer and easier to handle and transport than any other type of rocket propellant.
2) Simplicity And Reliability
From the section explaining how solid rocket propellant is made, it is clear that its manufacturing is not that straightforward but still much simpler than the production of most liquid propellants.
RP-1 propellant, for example, is a very refined form of kerosene. After crude oil has been extracted from the Earth, the fuel still has to go through numerous steps at processing plants to remove all unwanted substances, increasing its density and making it more energetic.
(Learn more about RP-1 propellant, what it is, and its different advantages and drawbacks in this article.)
Combusting the propellant is also a much simpler process. It is almost as easy as activating the ignition motor situated in the front part of a solid rocket booster, and the solid propellant just burns and continues to burn until all the fuel is depleted.
This is in sharp contrast to liquid-fueled rockets, where tanks and combustion chambers have to be chilled and pressurized before launch, turbopumps ignited and spun up, and the flow of fuel controlled throughout the launch.
As a result of its relatively simple design and combustion process, solid rocket propellants also combust and burn more reliably than liquid propellants.
(This is also one of the primary reasons why it is so popular and widely used in military applications, where solid-fueled rockets and ballistic missiles can be stored for years and reliably used at a moment’s notice.)
3) Storage At Room Temperature
Another huge advantage of solid rocket propellant, apart from its simplicity and ease of use, is that it can be stored at room temperature for long periods, sometimes up to decades.
This adds to its appeal for military use, as discussed in the previous section. It also makes it a very useful “off the shelf” option as a solid strap-on booster, which can be stored for sustained periods and attached to orbital launch vehicles if extra thrust is needed.
(Learn more about strap-on boosters, what they are, and their different advantages and drawbacks in this article.)
4) Superior Thrust Due To High Density
One of the Holy Grails in modern spaceflight is Specific Impulse, the measurement of how efficiently a rocket burns its fuel. It is essentially the equivalent of the automotive “miles per gallon” and is measured in seconds.
Of all the major fuel types, liquid hydrogen is currently the most efficient, followed by liquid methane and RP-1 propellant. Solid rocket propellant can’t match its liquid counterparts when it comes to fuel efficiency, but it has another huge advantage up its sleeve.
Due to the high density of the solid propellant, it is more energetic and, as a result, provides more thrust (or raw power) than similarly sized liquid propellants. This makes them a very appealing choice for launching a large orbital rocket into Space.
Typically, more than 85% of a rocket’s mass consists of fuel, of which the vast majority is used simply to get a launch vehicle through Earth’s thick atmosphere while battling its strong gravitational force.
To achieve this, fuel efficiency is not as important as thrust, which is why hydrogen is mostly used in the upper stages of orbital rockets when the vehicle has already cleared the thickest part of the atmosphere and gained sufficient velocity to break free from the planet’s gravity.
To assist vehicles in generating enough thrust to successfully get a heavy launch vehicle into orbit, solid-fueled rocket propellants are often used. They provide a simple, relatively cheap, and reliable means of providing the necessary thrust.
Even if a launch vehicle’s liquid propellant has sufficient thrust to lift it into orbit like an RP-1 fueled Atlas V rocket, a much heavier payload or the required orbit can fall outside the vehicle’s ability. Adding a number of solid rockets helps a rocket overcome this obstacle.
(Some launch vehicles like the Space Shuttle and the Ariane 5 rockets could not even get off their respective launchpads without the assistance of solid rocket boosters, which provided the necessary thrust.)
5) Less Complicated Engines Than Liquid Equivalents
Liquid-fueled rocket engines are complex and require separate fuel and oxidizer tanks, additional equipment to keep the fuel lines and tanks pressurized, gas generators (or preburners) to enable sufficient propellant flow, & turbopumps for pressurized combustion.
In contrast, a solid-fueled rocket basically consists of the casing, a nose cone, nozzle, and the propellant grain inside with a hole running through it, which also acts as the combustion chamber. It clearly illustrates how much simpler the design of a solid propellant rocket is.
As previously stated, due to this relatively simple design, solid-propellant rockets are easier and less expensive to manufacture, more reliable since it has almost no moving parts, and fewer processes involved in the combustion process, which means a lot less to go wrong.
Disadvantages Of Using Solid Rocket Propellant For Orbital Rockets
Despite the advantages of solid rocket propellant, it also has several major drawbacks that prevent it from being the first choice as the main engine of most orbital rockets. The most notable disadvantages of using solid rocket propellant include:
- Can Not Be Shut Down Or Restarted
- No Thrust Control
- Lower Specific Impulse Than Liquid Propellants
- Shorter Burn Times
- Limited Means Of Cooling Nozzle
- Dirty Exhaust Plumes
1) Can Not Be Shut Down Or Restarted
Once a solid rocket is ignited, it cannot be switched off. The combustion continues until all the fuel is depleted, and the booster is ejected from the rest of the launch vehicle. It essentially means the rocket can only be used once and not restarted at a later stage.
This is where liquid-propelled rockets have a significant advantage since they have full control over their fuel flow, allowing them to shut their engines off and restart them at any point, whether it forms part of the original program or in case of an emergency.
2) No Thrust Control
Another disadvantage closely related to its inability to shut down or restart is the fact that solid propellant rockets can also not slow down or speed up. In other words, it has very little to no thrust control.
(By changing the shape of the mold during the manufacturing process, which will change the shape of the hole/combustion chamber in the center of the rocket, engineers can affect the burn rate, which can allow the vehicle to produce more or less thrust at certain points.)
This is a major drawback since it is crucial for a launch vehicle to control its thrust. During a period called Max Q (when the vehicle experiences the maximum amount of dynamic forces on it), the vehicle must reduce thrust to minimize these stresses before powering up again.
Similarly, before stage separation, the rocket also needs to reduce thrust to allow for a smooth separation and allow the first stage to fall away before igniting the second stage. This allows for a safe second-stage ignition with no danger of first-stage interference.
(You can learn more about rocket staging, what it is, and why it is so crucial for all orbital launch vehicles in this article.)
3) Lower Specific Impulse Than Liquid Propellants
In the section about superior thrust, the importance of an orbital rocket’s fuel efficiency, known as Specific Impulse, was highlighted. And although solid rocket propellant provides superior thrust, it simply cannot match liquid propellants when it comes to fuel efficiency.
Of your liquid propellants, hydrogen remains the most fuel-efficient propellant, even after more than half a century of use. It is followed by liquid methane and then RP-1 propellant, with the latter primarily used in the first stages of launch vehicles.
(Liquid methane is currently being developed as fuel for both the first and upper stages of orbital rockets like SpaceX’s Starship due to its versatility and the possibility of off-planet production. Learn more about this fuel and its advantages and drawbacks in this article.)
As a result of its lower Specific Impulse, solid rocket propellant is mostly utilized during the first part of a launch when thrust is crucial, although some vehicles do use solid rocket fuel in their upper stages, like satellites, which typically use it to place them in a higher orbit.
4) Shorter Burn Times
Due to its high density and energetic nature, solid rocket propellant burns very quickly, resulting in much shorter burn times than an equally sized liquid-fueled rocket.
Although this is very useful for getting an orbital rocket off the launchpad very quickly, it usually needs to be paired with another longer burning propellant or upper stage with a high Specific Impulse like liquid hydrogen to reach a required orbit.
5) Limited Means Of Cooling Nozzle
The temperatures inside a liquid-fueled rocket engine can reach 3 300° Celsius (6 000° Fahrenheit), enough to melt most metals. Inside solid-fueled rocket motors like the ones used in the Space Shuttle and the SLS system, they reach similar temperatures.
Liquid-fueled rocket engines use a variety of methods to keep their engines and nozzles cool. One of the most widely used methods is called regenerative cooling, where cryogenic fuel is pumped through channels in the engine and nozzle walls, which prevent them from melting.
However, due to the nature of the propellant, a solid rocket cannot use this technique. As a result, the majority of solid rocket boosters use a method called ablative cooling, where a layer of material designed to erode and burn away is applied to nozzle walls.
As the nozzle heats up, the ablative material burns away, carrying most of the heat with it. Although effective, it means that the nozzle has a limited lifespan, and refurbishment of a used solid rocket requires an extensive overhaul of the rocket nozzle.
(Learn more about the different techniques used to keep rocket engines cool and prevent them from melting in this article.)
6) Dirty Exhaust Plumes
Apart from Specific Impulse, creating a clean burning fuel is also a big priority for the space industry. The effort to reduce carbon emissions and other greenhouse gases that contribute to Global Warming has not escaped this sector of technology.
Fuels like liquid methane and liquid hydrogen are being aggressively pushed due to their clean burning characteristics with little to no harmful compounds in their exhaust gases. (Hydrogen produces nothing but water in its exhaust plumes.)
Unfortunately, the combustion of solid rocket propellant produces the dirtiest exhaust gases of all rocket fuels. Its exhaust plumes contain carbon dioxide, soot, sulfur, aluminum & nitrogen oxides, and hydrogen chlorides.
(The large exhaust plumes that were seen trailing behind the Space Shuttle as it took to the skies were all generated by its two large solid rocket boosters.)
Conclusion
Solid rocket propellant is the oldest form of fuel used to propel rockets, dating back to 1232. Some argue that they are obsolete monstrosities, which only add to air pollution with the high amounts of carbon dioxide, soot, nitrogen & aluminum oxides in their exhaust plumes.
But as this article illustrated, they still have a crucial role in orbital launch vehicles. Their simplistic design, ability to be stored for long periods at room temperature, and assisting liquid-fueled rockets in reaching orbit make them indispensable for the foreseeable future.
This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.
