The Ins and Outs of Propulsion TestingMarch 8, 2019
The term “propulsion” comes from the Latin words “pro,” which means forward, and “pellere,” which means to drive. Propulsion, then, means to drive something forward. Propulsion is the force that causes a balloon to fly forward when the helium inside is released. It’s also the force that causes a rocket to launch into outer space.
The overarching principle that explains propulsion is Newton’s third law of motion, which states that, for every action in nature, there is an equal and opposite reaction. Since Newton developed his three laws of motion in the 17th century, we’ve come to understand thrust better and how it has many practical applications, not least of which is rocketry. Nearly three centuries after Sir Isaac Newton shared his theories about the natural world, people became obsessed with the idea of launching a rocket into space, especially in a timely manner as to beat other countries to the goal.
The space race, as it is now called, led to rapid innovations in rocketry. Many decades later, the National Air and Space Administration (NASA), the Air Force and private organizations continue to work together to push rocketry, and propulsion systems in particular, further than ever before. The space missions we aspire to today demand more from spacecraft, and innovators all over the nation and the world are rising to the challenge.
Propulsion systems are the heart of a rocket, so they deserve a great deal of focus. As we innovate and rely on current systems, testing is a crucial step. While we can always depend on the principle behind Newton’s third law of motion to work, the components of a rocket engine that produce the thrust could experience problems. Testing is critical for making sure a propulsion system is functioning correctly and is ready to propel a rocket or spacecraft into space.
In this article, we’re focusing on propulsion testing. We’ll look more closely at what a propulsion system is, why propulsion testing is necessary, what a rocket testing facility is and how propulsion testing is performed.
What Is a Propulsion System?
Rockets are made up of four main systems — structural, payload, guidance and propulsion. The propulsion system produces the thrust that moves a rocket forward. No matter how effective the other aspects of a spacecraft are, without the propulsion system, it would never get off the ground. The exact way thrust is produced depends on the propulsion system, as there are different kinds. However, all systems function based on Newton’s third law of motion by creating combustion that produces thrust.
The propulsion system is called a system for a reason — it consists of all the parts that make up the rocket’s engine. This includes components such as the propellants, injectors, combustion chamber and nozzle. Let’s take a closer look at each of these components and how they contribute to the overall propulsion system:
Propellants consist of some sort of fuel and an oxidizer. When fuel and oxidizer mix, combustion occurs, producing hot exhaust gas. In liquid rockets, the propellants are stored separately as liquids and are mixed in the combustion chamber. In solid rockets, these propellants are premixed and stored in a solid cylinder. The propellants will only burn when they’re exposed to extreme heat from the igniter. In hybrid propellant engines, one of the propellants, usually the fuel, is solid and the other is liquid. The liquid propellant gets injected into the solid’s tank, causing the combustion to occur.
Injectors are an critical component of liquid and hybrid rocket engines. They inject the propellants into the combustion chamber. To control the process, they only inject the propellants in certain proportions and at the right time. The injector also has the task of closing off the top of the combustion chamber to contain the combustion. It can do this since it is positioned at the upper end of the combustor. The injector has the most significant role of any component in a rocket engine.
3. Combustion Chamber
The combustion chamber is where the propellants mix and cause combustion. In the case of hybrid rocket engines, the tank containing the solid propellant also serves as the combustion chamber, but in liquid rockets, it is a separate entity. The combustion chamber must be strong to contain the high temperature and pressure produced by the combustion. The chamber must also be long enough to ensure combustion is complete before the resulting gases move into the nozzle.
After the combustion occurs, the gases enter into the nozzle. However the combustion occurs, the hot exhaust produced flows through the rocket nozzle and is accelerated to the back of the rocket. The nozzle’s job is to convert the slow-moving chemical-thermal energy from the combustion into high-velocity kinetic energy. A nozzle throat connects the convergent and divergent sections of the nozzle. The gas exits the nozzle at the end of the divergent section. This action causes the opposite reaction of a thrusting force that propels the spacecraft forward.
Why Is Propulsion Testing Important?
A rocket is nothing without its engine. A propulsion system containing a flaw could turn into a tragic explosion that immediately lays waste to countless hours of work, expensive materials and possibly human lives.
Propulsion testing involves examining all the different components of a spacecraft’s propulsion system and putting the system through tests to ensure that it’s in proper working order. A challenge for aerospace engineers is that they can’t simply take a rocket for a test drive to see how it works. Launching a rocket is extremely expensive, and that’s if everything goes right. If one component of the propulsion system doesn’t work properly, the entire rocket can be destroyed.
In 2015, SpaceX lost tens of millions of dollars, and NASA lost well over a million dollars worth of cargo when a Falcon 9 rocket exploded. Thankfully, the rocket was unmanned, but the financial loss was a tragedy of its own.
Research and theoretical models are necessary and helpful to prevent problems from occurring, but they are inadequate on their own. This is where testing comes in. Tests can take over where theory leaves off, either revealing unforeseen problems or confirming hypotheses and the efficacy of design models and materials.
Before an engine can be certified for flight, it must successfully go through ground testing. Testing a rocket on the ground can be costly, but it is well worth it to ensure that a rocket is ready for a real-life launch. Testing a rocket’s propulsion system before relying on it in a real launch protects the valuable asset of the rocket itself as well as any crew members who may be on board.
Testing a rocket’s propulsion system is in everyone’s best interest, but it’s not just a good idea — it’s a requirement. The U.S. military and various general and industry-specific standard-setting bodies have requirements for what tests a product must go through to be considered ready and safe for use. These standard-setting bodies don’t only specify standards for test results but also how the tests are conducted. Here are a few relevant examples:
- MIL-STD-810: This standard calls for replicating the rugged conditions of a product’s intended environment in testing. The 28 MIL-STD-810 standard identifies various testing methods that are each meant to replicate different environmental conditions. In-depth data is gathered to demonstrate how reliable the product really is under such harsh conditions.
- MIL-STD-202: This standard involves environmental tests, electrical characteristic tests and physical characteristic tests. It applies to any electrical parts that weigh under 300 pounds in a piece of equipment as well as printed circuit boards.
- RTCA DO-160: This standard comes from the Radio Technical Commission for Aeronautics. It covers a broad range of requirements for the environmental testing of avionics hardware, including testing for environmental factors like temperature, humidity, vibration and more.
When a rocket has gone through all the necessary testing, including every step of propulsion testing, and no longer presents any issues, then it is ready for launch. Testing is never an absolute guarantee that a rocket won’t experience any problems. After all, freak incidents and unexpected environmental issues can occur. On the whole, though, testing can impart a reasonable level of confidence that a rocket is ready for a successful launch.
What Are Rocket Test Facilities?
Since rockets were first being developed, propulsion test programs have been an essential means of finding and correcting issues or confirming that a rocket is ready for launch. These tests must take place in facilities that are specially designed to facilitate rocket tests. In 1957, the Rocket Engine Test Facility (RETF) was finished being built by the National Advisory Committee for Aeronautics (NACA) at the Lewis Research Center in Cleveland, Ohio.
The 10-acre site had two buildings and various support structures designed to facilitate full-scale rocket thrust. The first test in August of 1957 involved a 20,000-pound thrust rocket engine that fired successfully. At this facility, researchers were also able to test alternative forms of propellants.
In the early 1960s, the Rocket Propulsion Test Complex, or the National Space Technology Laboratories, was established in Mississippi. This facility was to be the national testing site for large rocket propulsion systems. Researchers innovated new and better designs for rocket propulsion systems. Rockets such as the Saturn V rocket, which was critical to the effort to land a man on the moon, were tested at this facility to ensure that the engine was ready before it continued its journey to eventually being launched.
In the mid-1960s, NASA established the White Sands Test Facility in New Mexico, which also became a hub for rocket testing. This facility is still in operation today as a component of the Johnson Space Center in Houston, Texas.
The countless men and women working on spacecraft propulsion decades ago, like those working today, understood the importance of propulsion testing. After lift-off, there would be no more opportunities to work on an engine or correct issues. They had to do all they could on the outset to ensure that a rocket was prepared to launch and take on outer space.
Today, rocket test facilities continue to carry out the same function as they did decades ago, but today, more sophisticated technology allows for even more advanced testing and more accurate simulations of space. These facilities have progressed technologically, and they’ve grown physically. For example, the NTS Santa Clarita site in California is more than 150 acres. This facility is equipped for a range of climatic testing, environmental simulation testing, space simulation testing and hazardous vibration and acceleration.
How Is Propulsion Testing Performed?
Propulsion testing involves a series of tests. Testing spacecraft propulsion systems begins in the early stages of development and continues until a rocket is completely ready for a real launch. At first, engineers inspect a rocket engine to ensure that the general components and subsystem assembly look correct. They can use static testing to identify defects in a spacecraft without it actually moving at all.
In addition to static tests, propulsion systems must undergo space simulations to confirm that they’ll perform under the harsh conditions of space. Other types of tests are also performed through thrust measuring systems, pressure vessels, valves, vacuum chambers and other testing components. These tests can help reveal any remaining issues that may cause a problem when the rocket is launched or can demonstrate that the rocket is ready.
Let’s look at a few different types of testing that propulsion systems undergo.
1. Thrust Stand and Exhaust Duct Rocket Testing
One of the major steps in static testing is evaluating a rocket engine’s ability to produce thrust. After all, this is the whole point of the propulsion system. A common means of testing thrust is through a thrust measuring system (TMS), which uses load cells to detect the amount of upward movement the thrust produces.
At NTS, our thrust measuring system can measure as high as 50,000-pound-force (lbf) of thrust. We fire test articles into a water-cooled duct that can handle a rocket engine with 50,000-lbf of thrust. Electric motors can monitor load cell calibration from a remote location before hot fire testing takes place.
Since thrust is such a vital aspect of a propulsion system, testing the thrust is critical. A TMS makes it possible to test thrust without actually launching a rocket, making it a valuable tool for propulsion testing.
2. Thermal Vacuum and Space Simulation
To confirm that a rocket is ready for launch, a propulsion test program must effectively recreate the conditions the rocket will be under when it’s launched and once it’s left Earth’s atmosphere. If a propulsion system works properly in a lab that bears no resemblance to the conditions of a real launch or of space, that is no guarantee that it will perform well under different conditions. Though space is a very different environment, sophisticated rocket test facilities can effectively simulate the conditions of a launch and of space.
A thermal vacuum chamber can manipulate both pressure and temperature to mimic the environment of the upper atmosphere and outer space. Thermal vacuum chambers have provided a means of testing aircraft since the beginning of the U.S. space program and continue to be an effective tool. These chambers must be enormous to accommodate spacecraft and must be secured to keep engineers safe.
Space simulation can reveal how a propulsion system will perform in extreme conditions like high vacuum, cold space and infrared sun radiation. NTS can design custom chambers and fixtures that will effectively simulate a broad range of extreme space-related conditions. When it comes to temperature, for instance, our space simulation chambers can be as cold as -320°F and as hot as 1,000°F.
3. Launch Level Acoustics
Launch level acoustic testing tests propulsion system noise. Large rocket engines can produce a sound pressure level of greater than 200 decibels. Sound waves this intense can harm a rocket during its ascent. Low-frequency waves are damaging to the whole vehicle and to any crew members, and high-frequency waves can damage more delicate components.
Suppression systems can help to mitigate this issue. Rocket exhaust is safely directed into a trench or flame bucket. A water deluge system works to reduce the noise and also cools the exhaust.
Launch-level acoustics testing can help ensure suppression systems are working as they should. A test stand surrounded by microphones provides the perfect means of collecting acoustic data. This data can inform improvements in the suppression system at the launch pad.
4. Cryogenic Fuel Storage
Cryogenic fuel storage is another aspect of a propulsion system that can be tested. Any gas stored at subfreezing temperatures and condensed to form a highly combustible liquid is cryogenic. Some common examples include liquid hydrogen (LH2) as a fuel and liquid oxygen (LO2 or LOX) as an oxidizer. These propellants must be stored at hundreds of degrees below 0°F, which makes them difficult to store for longer periods of time.
Because of this challenge, the material and design of fuel storage tanks intended to hold cryogenic propellants is critical. In the past, these tanks were made of metal, but today some are being made from composite materials, which are better equipped to store cryogenic propellants for long periods.
Like all other components in a rocket’s engine, cryogenic fuel tanks must be inspected and tested to ensure that they’re capable of properly storing cryogenic fuels for the necessary duration of time for a given mission.
Partner With NTS for Cutting-Edge Propulsion Testing
For nearly 60 years, NTS has forged a reputation as a trusted partner for engineering, inspection, testing and certification services. We provide propulsion testing services, including all of the tests we’ve discussed in this article. NTS is an exceptionally capable partner for space and satellite testing, space simulation and more.
NTS has partnered with both the Department of Defense and commercial businesses and has had a hand in every major space project since the earliest days of manned space exploration. Significant space platforms we’ve worked on include the Space Launch System (SLS), Delta IV, Falcon 9, Space Shuttle, Titan IV and many others.
In 2014, NTS acquired Wyle Laboratories’ San Bernardino location, which greatly expanded our capacity for propulsion testing. Our new astronautics and propulsion testing services at that location include some of the testing we’ve discussed in this article, as well as large run tanks and pressure vessels, propellant-run tanks and flow systems, a quiet technology test facility and more.
Our experienced aerospace engineers, technicians and project managers will work closely with your own teams to make sure we understand your goals and parameters, including any budgetary restraints. We’ll custom engineer the perfect solution to test your equipment and deliver the results you desire. Our vast physical infrastructure spread across multiple locations along with our wealth of expertise empowers us to overcome any challenge.