The white-hot flames blasting through a rocket’s nozzles as it is launched into Space are synonymous with any orbital rocket launch. Something, though, must be used to initiate this violent combustion process.
Orbital rocket engines are primarily ignited utilizing hypergolic fuels, high-voltage sparkplugs, heat generation, or pyrotechnic detonators, depending on the type of launch vehicle, rocket stage, and engine used. Laser beam ignition is also in the early stages of development.
There are several ways to ignite or start a rocket engine and allow the propellents to combust. And there is no one-size-fits-all approach. Several factors need to be considered in order to decide which ignition method will work the best.
Depending on the type of launch vehicle, the stage involved & whether the engine will need to be restarted more than once, one of up to five different methods may be utilized to initiate the combustion between the fuel and oxidizer, as the following section will highlight.
The Five Primary Ways In Which A Rocket Engine Is Ignited
As mentioned during the introduction, up to five distinct methods can be used to start a rocket engine, depending on several factors. In reality, though, only four of these are used on a regular basis. These methods are:
- High Voltage Electrical Spark Plugs
- Generating Heat
- Hypergolic Fuels
- Pyrotechnic Detonation
- Laser Beams
One will get a better understanding of how each one works and under which conditions a specific method will be chosen by taking a closer look at each process individually.
1) High Voltage Electrical Spark Plugs
Many readers owning a conventional car with an internal combustion engine will be familiar with a spark plug, the device that allows the fuel in the engine to ignite. Some rocket engines use the same approach when it comes to igniting their propellents.
In the case of a liquid-fueled rocket engine, a more robust, reliable spark plug (which otherwise functions in the same fashion as its automotive counterpart) provides a spark when an electrical current is run through it.
Flowing the fuel and oxidizer in their liquid states across the small localized region surrounding the spark plug will result in the mixture combusting and burning. However, a small flame or ignited bit of gas is insufficient and actually dangerous for rocket engine ignition.
The pressure with which fuel & oxidizer are pumped into the main combustion chamber means a large volume of propellent is traveling at a high velocity through the relatively small space, which makes it challenging for ignition to occur, especially from a small heat source.
The cold temperature of the fuel and supercooled oxidizer (liquid oxygen) compounds the problem. Even if combustion is possible, only igniting a small portion of the propellent or a late ignition may have catastrophic results.
It means that the heat source has to be big enough to combust the fuel & oxidizer mixture, and it also needs to do it evenly across the combustion chamber. An ignition in only part of the chamber can result in an uneven spread of pressure, which can damage the engine.
Similarly, a late ignition results in a large quantity of unburned propellant igniting all at once, causing a larger pressure buildup than the engine, especially the combustion chamber, was designed to handle. The resulting uncontrolled combustion can destroy the entire engine.
It is clear that using a single spark plug or inadequately small heat source to ignite a large liquid-fueled rocket engine is impractical and dangerous. This brings us to the second method of starting a rocket engine: Generating a large enough heat source.
2) Generating Heat
It is commonly known that a heat source, both large & hot enough close to a combustible material, will cause it to ignite if the heat is sustained for a sufficient period. Rocket engines use this principle to ignite their liquid-fueled rocket engines through a torch igniter system.
The process starts with a simple spark created by a high-voltage spark plug, as described in the previous section. As the gaseous hydrogen and oxygen flow through the small confined region around the spark plug, it ignites into a very hot gas.
The hot gas creates a flame front that is relayed through a channel from where it exits through a hole in the center of the main injector into the combustion chamber. This creates a hot and big enough heat source that allows the propellant around it to combust evenly.
This form of ignition has been reliably used in engines like the RS-25 (used in the Space Shuttle Main Engines), as well as the RL10 (used in the Centaur upper stage that has been used since the 1960s and is still used on the Atlas V and Delta IV Heavy launch vehicles.)
3) Hypergolic Fuels
It would be ideal for fuel & oxidizers to be able to spontaneously combust without relying on a separate ignition system, which will make the whole process more reliable and simple. As many readers will know, such propellents already exist and are known as hypergolic fuels.
These fuels spontaneously combust when the fuel and oxidizer come into contact with each other. However, these compounds are highly toxic and can be deadly to humans who come into direct contact with or just inhale the fumes. It is also hazardous to the environment.
Despite the dangers, the fuel is sometimes used as the main propellents of launch vehicles like Russia’s Proton rocket, but also in the much safer environment of space where they are used in the upper stages of orbital spacecraft to repeatedly & reliably reignite their engines.
The fuel component of hyperbolic propellants is usually a form of hydrazine like monomethylhydrazine (MMH) or unsymmetrical dimethylhydrazine (UDMH). The oxidizer component is typically nitrogen tetroxide (NTO) or nitric acid.
The propellants are also used in a spacecraft’s Reaction Control Systems, which are crucial for allowing the vehicle to maneuver in the vacuum of space. This makes these otherwise dangerous compounds ideal to reliably reignite and allow these systems to function.
(One can learn more about Reaction Control Systems, what they are, and their various applications in this article.)
However, a small amount of hyperbolic fuel, specifically TEA/TEB (Triethylaluminium /Triethylborane), also ignites upon contact with liquid oxygen or other oxidizers, making it a suitable source for combusting the propellents in a liquid-fueled rocket engine.
Combining a small amount of this fuel with liquid oxygen (LOX) in a rocket engine’s combustion chamber provides enough hot gas/heat to ignite the main propellants. The number of times an engine can be reignited depends on the amount stored onboard.
Combusting propellants in this fashion is known as pyrophoric ignition. The massive F1 engines of the Saturn V rocket used this form of ignition, and more recently, SpaceX’s Falcon 9 uses it to start and reignite its first-stage boosters during liftoff and landing.
4) Pyrotechnic Detonation
Taking 32 wooden birch sticks, attaching pyrotechnic detonators at the end, and shoving them up the nozzles of an orbital rocket to start its engines seems like a very archaic and outdated way of launching a modern orbital launch vehicle.
Yet, this is exactly what is done to this very day to start the engines of a Soyuz rocket, the workhorse of the Russian space fleet and arguably one of the most successful launch vehicles in spaceflight history.
The pyrotechnic explosives are placed on either side of a spring-loaded sensor at the edge of an inverted T-shaped wooden object. Electrical wires run from each “stick” to a central control system monitoring all 32 devices (for each of the 32 rocket engines on the rocket).
As soon as each detonator receives the electrical signal, it starts burning through the wire connecting the spring-loaded sensor. As soon as the wire snaps, it separates the electrical connection in the sensor, which sends the signal to the central control system.
Once the central command system receives the signal from all 32 devices that the pyrotechnic detonators did go off, it opens the valves that allow the flow of propellants through the respective combustion chambers, ensuring synchronized, even combustion.
Although there are more modern and “less messy” ways of starting a rocket, this system is a cheap and reliable way of ensuring that a rocket with multiple boosters and combustion chambers starts reliably with all engines firing together.
It has the added advantage of quick turnaround times in case of a malfunction. During a recent launch, the system detected that one of the devices didn’t detonate. The launch was halted, but the rocket was launched the next day after replacing the faulty culprit.
If this happened in a more complex ignition system that was situated in the injectors or combustion chamber of a rocket engine, this type of malfunction would most certainly have led to weeks or even months of delay.
Pyrotechnic detonators were also used to ignite the large solid rocket boosters of the space shuttles during the STS Program. A small pyrotechnic charge was detonated in the top section of the rocket, which ignited a small amount of pyro booster charge.
In turn, this charge ignites a small amount of solid fuel in the solid rocket motor initiator, which fires a flame down the length of the channel inside the booster, igniting the main solid rocket propellant.
(It is clear that different ignition systems are used for different types of rocket motors, but also the different types of fuel used. One can learn more about the different types of fuel used in orbital rockets, their characteristics, and their benefits & drawbacks in this article.)
5) Laser Beams
Sometimes it may seem like rocket technology is standing still when you view fundamentally the same type of launch vehicles that have been flying for decades still being used or the same technologies and techniques/principles being applied year after year.
It makes sense that tried and tested methods and technologies are kept and integrated into new components and spacecraft. However, there is constant innovation and development in the space industry, but one only becomes aware of it once it is implemented.
Laser beam ignition is one such technology. Although none of these systems are currently used in any operational orbital rocket engine, a number of companies are hard at work developing this technology.
Essentially, the principle behind this new technology is that a laser pulse is focused on the propellents as they are injected into the combustion chamber. When the threshold for the optical breakdown of the propellants is surpassed, the laser-induced plasma ignites the fuel.
(Once a finalized version is operational in an active orbit rocket engine, this article will be updated.)
Conclusion
As illustrated, there are several ways of igniting the propellents of a rocket engine. The chosen method primarily depends on the type of engine used and its intended use (whether it forms part of the first or upper stage or whether it needs to be restarted multiple times.)
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.
