This photo is provided by NASA.
One of the most remarkable and noble demonstrations of the power of chemistry is, without question, space exploration. From the first unmanned missions in the late 1950s to the Space Shuttle and now Artemis, various rocket engine technologies have relied on the invention of powerful fuel/oxidizer packages. To provide maximum thrust efficiency, rocket propellants are made from highly reactive chemical materials, making them potentially very toxic, corrosive, and difficult to store and transport. These constraints will give each propellant package a set of benefits and drawbacks, and the selection of the best package will depend on the rocket’s mission.
The Liquid Oxygen/Liquid Hydrogen (LOX/LH2) cryogenic propellant is truly remarkable through its efficacy and reliability, but also fascinating because its combustion generates a completely green by-product: water!
Cryogenic Propellants
At room temperature, rocket propellants can come as solids, liquids, or gases.
Gaseous propellants are not practical for large-scale rocket missions because they would require very large, sturdy, and, therefore, heavy rocket tanks to safely carry the needed quantity of compressed fuel and oxidizer gases. Because liquids contain a lot more molecules than gases per volume unit, the solution is to transform the gases into their corresponding liquid phases through compression and cooling.
Some of these propellants have extremely low boiling points and, therefore, require extremely low-temperature cooling techniques to liquefy them. Gas liquefaction requiring temperatures below -150°C (-238°F) is defined as cryogenic cooling.
With boiling points of -183°C (-297°F) and –253°C (-423°F), respectively, oxygen and hydrogen need to be cryogenically cooled to be liquified. This requirement is a significant constraint when it comes to using LOX/LH2 for rocket propulsion, but the LOX/LH2 package’s benefits well outweigh its disadvantages.
Specific Impulse or How to Measure a Propellant’s Efficacy
Oxidizers and fuels are stable elements at room temperature, but they will react in a violent exothermic reaction when mixed and triggered by a heat source. In rocket engines, the two reactants will be mixed in the main combustion chamber and ignited. Fuel and oxidizers aren’t always mixed in stochiometric proportions. Engineers may decide to run the engine “fuel rich” to reduce the combustion temperature and protect the engine from overheating. The resulting explosive reaction between the oxidizer and the fuel is what will provide the rocket its thrust.
Fuel + Oxidizer + Ignition = High Heat + High Exhaust Energy + Nozzle Geometry = Thrust
Two important characteristics, sometimes confused, define a rocket propellant’s potency. The first one, thrust, is measured in Newtons (or pounds) and expresses the propellant’s reaction force potential, as defined by Newton’s Third Law. The higher the thrust, the larger the payload the rocket will be able to lift.
The second characteristic is specific impulse (Isp). Considered the most important parameter for a propellant, specific impulse defines how efficiently a propellant can convert its mass into thrust. More precisely, it measures the time (in seconds) that a certain quantity of propellant will be able to push a certain load. A high thrust propellant doesn’t mean a high specific impulse value. Engines using propellants with a high specific impulse will tend to have lower thrusts, but they will use their propellant’s mass more efficiently. In simple terms, they get greater gas mileage.
Table 1 lists some of the most common LOX and fuel combinations. LOX/LH2 exhibits the highest Isp value. That efficiency is one of the reasons why the LOX/LH2 pair has been so abundantly utilized as a rocket propellant in the last five decades of space exploration.
Table 1. Propellant packages: properties of LOX and various fuels.
Note: *RP-1 (Rocket Propellant-1) is a highly refined form of kerosene and is widely used in liquid rocket engines.
Hydrogen Combustion – Reaction Mechanisms
While other propellants release large quantities of polluting chemicals after combustion, the LOX/LH2 package is particularly special because, in addition to its outstanding specific impulse, its by-product is water. This is an extraordinary benefit Mother Nature allows us to leverage!
Hydrogen (H2) and Oxygen (O2) are stable elements, and they will not spontaneously react when mixed at room temperature. For a reaction to occur, H-H and O=O covalent bonds need to be broken. The energy necessary to break these bonds is called activation energy (Ea), as shown in Figure 1. When enough energy is supplied to overcome the H-H and O=O bonding energy, a chain reaction will occur until water is formed and a much lower, very stable energy level is reached. The transition reaction towards water’s very stable structure is what gives off large amounts of energy during H2 combustion with O2. The net energy released (DE) equals -482kJ/mol (Figure 1).
Figure 1. Energy levels for H2, O2, H2O molecules and energy changes from reactants to the products.
Despite the reaction’s apparent simplicity, H2 combustion with O2 is complex and involves several intermediary reactions involving extremely reactive H and O radicals. For this study, only the main reactions leading to the formation of water are listed (Figure 2). All reactions are reversible. The letter k represents the different reactions’ rate constants. When one radical generates two or more radicals (Figure 2 reactions numbered 3 and 4), the reactions are called chain-branching reactions. Because these reactions produce more reactive radicals than they consume, they accelerate very rapidly and explain the explosive nature of H2 combustion reaction with O2.
The radical reactions aren’t always happening in the exact order displayed in Figure 2. Other radicals, not mentioned here, may be formed through other chain reaction schemes. Propellant mixture, pressure, and temperature also influence H2 combustion kinetic mechanisms.
Figure 2. Main radical reactions involved in H2 combustion in O2.
The Turbopump – A Fundamental Element of Modern Rocket Engines
Various rocket designs with different liquid propellants have been proposed in the last century; however, WWII Germany’s V-2 (Figure 3) program probably advanced large rocket engineering in the most significant way. The V-2 design was the first iteration of what would become revolutionary large-scale rocketry capable of carrying large payloads at high altitudes and over long distances. Today’s rocket engine designs have successfully leveraged V-2 innovations, the availability of new materials, and other technological advancements to create exceptional rocket engines necessary in modern space programs.
Figure 3 (left). V-2 Rocket in the Peenemünde Historical Technical Museum © by AElFwine is licensed under CC-SA 3.0.
Figure 4 (right). The V-2 turbopump; photo of V-2 turbopump was kindly provided by Historicspacecraft.org.
To create powerful and consistent thrust, rocket engines need to be fed with astronomic volumes of liquid propellant. V-2 engineers accomplished this task by creating the first version of what equips today’s rocket engines: the turbopump (Figures 4 and 5). Because a rocket engine’s performance relies so much on the turbopump, it is without question the most complex and critical component of rocket engines.
The V-2 turbopump was revolutionary in its design and performance. One steam turbine rotating at 4,000 rpm would drive both fuel and LOX centrifugal pumps injecting 128 lbs of alcohol and 159 lbs of LOX in the V-2 combustion chamber per second. The steam for the turbine was generated through an ingenious steam plant creating superheated steam on demand, as illustrated in Figure 6.
Figure 5. Cross-section of a V-2 turbopump provided by Enginehistory.org. Some component parts identifications have been removed to provide more clarity to the reader.
Figure 6. The V-2 steam plant use NaMnO4 and concentrated H2O2 for steam generation. Photo of the V-2 power plant was kindly provided by Historicspacecraft.org.
Today’s rocket engine turbopumps operate under a similar concept of pushing extremely large amounts of propellants into the combustion chamber, but designs have evolved to produce more power, increase durability and reliability, and leverage the propellant’s energy potential with minimum energy loss.
Powering Artemis With the Iconic RS-25 Engine
Designed in the 1970s by Aerojet Rocketdyne, the RS-25 (Figure 7) was originally developed and used for NASA’s Space Shuttle program’s 135 missions. Upgraded versions of the engine were built over time, and Artemis will benefit from enhanced versions of the RS-25.
Although the RS-25 also benefited from older rocket engine concepts, it is a sophisticated cryogenic engine that results from decades of technology advancements and design optimization, making it one of the most efficient and powerful rocket engines ever produced.
Figure 7. A close-up view of a Space Shuttle Main Engine RS-25 during a test firing at the John C. Stennis Space Center in the early 1980s in Hancock County, Mississippi. Photo provided by NASA.
The RS-25 utilizes the LOX/LH2 cryogenic propellant package for its superior specific impulse; however, this fuel/oxidizer combination presents unique engine engineering challenges that had to be overcome to make the RS-25 the amazing piece of engineering it is today.
Hydrogen’s density is extremely low (71 g/L), which means that it will take very large amounts of LH2 to stoichiometrically match LOX quantity for efficient combustion to happen. Figure 8 compares various fuel/oxidizer packages. LH2 volume will need to be 2.7 times as high as LOX to feed the RS-25 engine!
Figure 8. The volume ratio of LOX and liquid fuels as propellants for rocket engines. Image rebuilt from Everyday Astronaut chart.
The 6:1 LOX/LH2 mass ratio in Figure 8 needs to be explained. Engineers chose to adjust this ratio to avoid overheating and potentially melting the RS-25 engine’s components. An O2 and H2 stoichiometric reaction would require O2 mass to be 8 times that of H2’s. Reducing it to a lower value, 6 in this case (making the propellant “fuel-rich”), solves the combustion heat challenge. See Figure 9 for detailed calculations.
Figure 9. Calculation of LH2 and LOX volumes for RS-25 fuel-rich mixture.
The significant difference in LH2 and LOX densities and flow rates prevents the RS-25 from operating on a single turbopump. Two separate turbopump sets had to be designed and tuned to accommodate these very different cryogenic liquids and their respective physical properties (Figure 10). Note that the LOX high-pressure turbopump (right) also includes a helium safety seal to prevent hydrogen-rich pre-burner hot gases from mixing with LOX main feed and potentially causing a catastrophic failure.
Figure 10. RS-25 rocket engine’s LH2 and LOX separate turbopumps and main combustion chamber (MCC). Photo provided by NASA.
Both high-pressure turbopumps are incredible pieces of engineering. Rotating between 28,000 and 35,000 rpm, they are developing tens of thousands of horsepower. Turbines from these two pumps have dozens of blades. Just one of these blades (each only the size of a quarter) develops more power than a Corvette engine!
Figure 11 shows that the RS-25’s fuel (LH2) and oxidizer (LOX) paths from the main tanks to the combustion chamber are designed to fulfill multiple auxiliary functions: cooling, external tank pressurization, turbopumps feed, and pogo suppression. A main engine controller (engine computer) attached to the RS-25 monitors propellant flows and temperatures, making adjustments as necessary through propellant valves.
Figure 11. LH2 and LOX various paths through engine parts. Image provided by NASA. Some component parts identifications have been removed to provide more clarity to the reader.
The RS-25 can be oriented through gimbal bearings (+/- 10.5°) to alter the engine’s thrust vector and make vehicle trajectory corrections as the rocket’s center of mass changes during flight.
It takes four RS-25 engines to push Artemis into space with the help of two Solid Rocket Boosters (SRBs). The four RS-25 engines mounted on the Artemis Space Launch System (SLS) will provide 2 of the 8.8 million pounds necessary to send the SLS into space. The SLS rocket will be about 15% more powerful than the revered Apollo mission’s Saturn V rocket, making Artemis the most powerful rocket ever launched.
RS-25 Engine Fun Facts
Infographic rebuilt from NASA image.
References
Remaining uncertainties in the kinetic mechanism of hydrogen combustion – Combustion and Flame – Volume 152, Issue 4, March 2008, Pages 507-528 – Alexander A.Konnov.
https://www.jhuapl.edu/Content/techdigest/pdf/APL-V02-N06/APL-02-06-Wilson.pdf
https://www.nist.gov/system/files/documents/el/fire_research/R0301124.pdf
History of Rocket Propellant Rocket Engines
U.S. Manned Rocket Propulsion Evolution, Part 2.2: V-2 Propulsion, Compiled by Kimble D. McCutcheon, Published 1 Nov 2020; Revised 3 Aug 2022
https://www.enginehistory.org/Rockets/RPE02/RPE02-2.shtml
Rocket Physics, Extra Credit: Rocket Fuels
Aerojet Rocketdyne
https://www.rocket.com/sites/default/files/documents/lr_3-26-19_RS25_data%20sheet.pdf
Monster motor breathes fire in Mississippi
Saturn V
