Blast Off! Engineering Rockets with Colin Heye, PhD
Human space exploration is a monumental accomplishment. Last year was the 50th anniversary of the Apollo 11 moon landing, a landmark achievement for humanity. With its Artemis Program, NASA is hard at work preparing for a triumphant return, aiming to put fresh boot marks on the moon’s surface in the 2020s. There are also a number of companies racing to develop private spaceflight technologies, heralding the dawn of a new era of manned space missions.
While we may get caught up in the history and majesty of space exploration, we may lose sight of the extreme amounts of technical work it takes to make all of these things happen. Fundamentally, astronauts are being propelled out of the Earth’s atmosphere by being strapped to a massive controlled explosion, with “controlled” being the key word. This “control” is dependent on using elaborate engineering to harness complex chemistry and physics.
Colin Heye, PhD
Understanding these various factors is the job of Colin Heye at Blue Origin, the aerospace company founded by Jeff Bezos in 2000. Colin is currently a propulsion analyst and is working to understand and improve the performance of the company’s rocket engines.
In his work, Colin uses physics-based simulations and models of fluid dynamics, a vital field for studying rocket propulsion. This builds on the work that he did while earning his PhD at the University of Texas in Austin. During grad school, Colin researched combustion and how it was impacted by the fluid dynamics of its components, specifically studying the flow of gases and vapors (which are technically fluids) during combustion. While his dissertation work focused predominantly on modeling aircraft and gas turbine conditions, the methodology has been translatable to the combustion seen in rocket engines. Importantly, Colin’s experiences in grad school gave him a strong understanding of the combustion models, including their limitations, a notable aspect because the body of work around rocket engine conditions is less researched. Since rockets involve significantly higher pressures and temperatures than aircraft engines, it’s important to know the limitations of the existing models and be able to identify and question data that may have been extrapolated beyond a model’s useful range.
At its most basic, rocket propulsion is the downward force that sends the rocket skyward. Ultimately, the end goal is to propel material (people, equipment, etc.) into space, so the propulsion system – the engines and propellant tanks – has to be able to lift more than its own weight.
The types of rocket engines that Colin works with are chemical rockets, meaning that they rely on mixing fuels and oxidizers for combustion. During his time at Blue Origin, Colin has mainly worked on rocket engines that use liquid propellants, which include both the fuel and an oxidizer like liquid oxygen. The specific components of the propellant are selected by a variety of considerations, including density, carbon content, and storability. Storability, for example, helps determine how the propellants will be managed and when they will be loaded onto the rocket. Storable propellants are liquid at or near atmospheric conditions, which are in contrast to cryogenic propellants that require specialized cooling and pressurization in order to remain in a liquid state during operation. Think of liquid oxygen or liquid hydrogen. Since these substances normally exist as gases, they require substantial efforts to compress and cool them into liquids, impacting how and when they are used in rockets.
Other factors, like a propellant’s carbon content and density, contribute to the calculation of the engine’s specific impulse, a number that illustrates how efficiently the engine uses a propellant. Impulse, broadly speaking, is the amount of force produced within a certain period of time. Taking into account the amount of propellant used to produce the impulse, you can calculate the specific impulse of the rocket – the impulse you get from a given mass of propellant. The specific impulse varies with different propellants, which is due in part to different levels of energy density. Similar to how the number of calories differ between a donut and a comparably sized floret of broccoli, rocket propellants also have differing amounts of energy density. Liquid hydrogen produces a higher specific impulse than RP-1 (a highly refined form of kerosene designed for use as rocket fuel), but it’s not nearly as dense as RP-1. Due to its higher density, RP-1 is actually more powerful than liquid hydrogen by volume.
These two fuels also illustrate how carbon content is an important consideration in rocket propulsion. Combustion is a chemical reaction between a fuel and an oxidizer (liquid oxygen is often used in rockets) that generally results in hot oxidized gases. Liquid hydrogen, sometimes used as a rocket fuel, contains no carbon, and a combustion reaction with liquid hydrogen produces more energy per mass than reactions with carbon-containing propellants, like RP-1 or methane. The mass of the carbon atoms is significantly greater than the mass of the hydrogen atoms in carbon-containing fuels, affecting the amount of energy per mass produced. However, while carbon content and specific impulse calculations are important, there are other considerations to take into account choosing propellants. For example, while liquid hydrogen can contribute to high energy per mass reactions, its lower density could require a larger tank size, potentially impacting the rocket’s velocity due to its aerodynamic drag.
In addition to the characteristics of the propellant components, there has to be careful attention paid to the flow and mixing of these materials, as well as to the flow of the exhaust of the combustion reaction. And, of course, doing all of these things while keeping in mind the unique structural constraints of a metal tube hurtling into space.
Colin has worked on everything from the tank through the nozzle exit, including time spent analyzing the chamber where the fuel and oxidizer mix and burn. For the propulsion system as a whole, it’s vital to get as much of the mass moving as quickly as possible at the end of the nozzle without melting, exploding, or otherwise damaging the nozzle or combustion chamber.
During this intense process, the fuel and oxidizer must flow into the combustion chamber and then mix, factors that are strongly dependent on fluid dynamics. The flow of the fuel and oxidizer into the chamber is dependent on pressure differences, with the pressure of the combustion chamber being lower than that of the liquids flowing into it from the propellant storage tanks. This is often done by specialized pumps or by using pressurized propellants.
While a car engine relies on fuel injectors to spray fuel into the combustion chamber for optimal mixing with air, rocket combustion takes that idea to the extreme. A unique aspect of rocket combustion is that the pressures and temperatures commonly reach supercritical conditions. Under these conditions, compounds can exceed their critical point, which is the specific temperature/pressure where the compound can coexist as a vapor and a liquid. With this elimination of the boundary between liquid and gas, these supercritical fluids have altered properties, affecting the dynamics within the combustion chamber. Colin has described the dynamics of these supercritical fluids as a “strange mixture of stirring creamer in coffee and putting your thumb over a garden hose.” As you can imagine, these combustion conditions are quite different than those you would find in a car engine or in the burning of alcohol during the flambéing of Bananas Foster. So special attention has to be paid to ensure that the supercritical fluids still undergo the heat-producing chemical reactions critical for proper propulsion.
Once ignition occurs, the resulting hot gases also act as fluids, behaving in ways that can be predicted and modeled with fluid dynamics, though with varying levels of complexity and uncertainty. After ignition, the resulting flame flickers and will move further and closer to the surrounding surfaces, non-uniformly heating the wall of the combustion chamber. So, understanding the dynamics of the flame and the hot gases is essential to ensure proper cooling of the chamber to prevent melting and catastrophic failure. Additionally, as the gases leave through the rocket’s exhaust nozzle, they create a boundary layer, a common feature in fluid dynamics. In this case, the boundary layer is formed by the gases slowing down as they interact with the nozzle sidewalls. This layer of slow and relatively viscous gas slowly builds up, reducing the average velocity of the flow. Friction is the driving force behind boundary layer growth, converting precious velocity (kinetic energy) into heat (thermal energy), reducing the engine’s available thrust. Understanding the dynamics of this phenomenon is vital to ensure the rocket is still able to reach its escape velocity.
Colin continues to work on these and other captivating challenges as he helps develop the cutting-edge rocket technology Blue Origin hopes to use to propel humanity to new heights.
Special thanks to:
Colin Heye, PhD, propulsion analyst, Blue Origin (CHeye@blueorigin.com)
Qiagen and Todd Festerling for sponsoring the blog