Designing and testing chemical propulsion systems and nuclear thermal engines for satellites and spacecraft, in support of NASA's space exploration missions.
When engineers want to move a vehicle through the air or space, they must apply a force to the vehicle. This force is known as thrust and is generated by the propulsion system of the vehicle. Different propulsion systems develop thrust in different ways, but all thrust is generated through some application of Newton’s third law of motion. In rocket engines, which are mainly used either to escape Earth’s atmosphere or in-space, there are two broad category of propulsion systems – electric and chemical. They can be further broken down into sub-categories as shown and expanded on below.
Electric Propulsion Systems typically use electric heating or electric or magnetic fields to accelerate propellants (usually gases). These systems can be very fuel-efficient but can only accelerate relatively few particles of gasses at a time, resulting in very tiny thrusts. Often, these are ideal engines for deep space exploration where transit times can be very long and rapid maneuvers are not required.
Chemical Propulsion Systems, on the other hand, uses chemical reactions to release energy and accelerate gases to generate thrust. These systems produce relatively large thrusts in relatively short periods of time. There are several kinds of chemical propulsion, including liquid/gaseous propulsion, solid propulsion, and hybrid propulsion. An example of liquid chemical propulsion is shown in the image above in the banner.
Liquid propulsion systems are typically either a monopropellant (a single propellant fluid) or a bi-propellant (two fluids, a fuel and an oxidizer). The propellants may be stored and fed from high-pressure fuel tanks (pressure-fed) or use turbopumps to move the propellant to the engine (pump-fed). Some propellants ignite on contact with one another, called hypergolic propellants. These hypergolic propellants do not require any other ignition system to burn – making them much more reliable since there are no ignition systems to possibly fail. They have good in-space long-term storability but are also very toxic and corrosive to handle. The Orion vehicle European Service Module is an example of a vehicle with a hypergolic engine system.
When the propellants are stored at temperatures below the normal freezing point of water, they are considered “cryogenic” and are typically liquids or very cold, condensed gases. Hydrogen and Oxygen are examples of cryogenic propellants, and their advantage is very high performance and safer handling than hypergolics. Their disadvantage is trying to keep them cold for long periods of time so that they don’t boil away.
The Space Shuttle Orbiter and its big orange tank or the Saturn V rocket are examples of vehicles with cryogenic liquid propulsion systems. Liquid Propulsion systems can produce a wide range of thrust, can be controlled on and off, but often must be fueled and set up just prior to their launch. Also included with Liquid propulsion systems are Nuclear Thermal Systems, which typically use a nuclear reactor to thermally heat cryogenic hydrogen gas to very high temperatures before exhausting the hydrogen through a rocket nozzle.
| Liquid Propulsion System | Typical Propellants | Advantages | Disadvantages |
| Monopropellants | Hydrazine; Green Propellants | Relatively simple | Require heavy catalyst systems to operate |
| Hypergolic (bi-propellant) | Hydrazine; Nitrogen Tetroxide | Long term, in-space storable Don’t require an ignition source | Toxic; Corrosive Lower performing than other propellants |
| Cryogenic (bi-propellant) | Hydrogen; Oxygen; Methane | High performance; Relatively benign | Need to be kept very cold; can have very large tanks |
| Nuclear Thermal | Hydrogen | Very High Performance | Requires nuclear reactor; heavy |
As opposed to liquid propellants, solid propellants are typically a solid cast material that contains both fuel and oxidizer bound in suspension that can produce thrust through chemical reactions. This “fuel” can be handled at room temperature until an ignition source is applied. When the propellants are ignited, they release both the fuel and oxidizer constituents which burn and generate thrust. The solid rocket boosters on either side of the space shuttle are examples of solid propellant rockets. These systems generate a lot of thrust, can be stored for long periods of time in a “ready-to-go” state, but generally lack the controllability of turning them off and on when thrust is no longer needed or when a series of thrust pulses is needed, with some exceptions.
Hybrid propellant systems are a combination of solid propellant systems and liquid propellant systems. Typically, there is a solid-propellant fuel but the oxidizer needed for combustion is kept separate as a liquid or gas. Only so long as the oxidizer is supplied over the solid propellant is there combustion and thrust. The advantages of this type of system is the high thrust of solid propellant systems combined with the controllability (on-off) of liquid propulsion systems.
Our research on chemical and thermal propulsion systems at Glenn Research Center primarily focuses on chemical propulsion systems, in all their forms and types, with the major emphasis on liquid propulsion systems. Our work supports the systems used for the Orion Service Module, Nuclear Thermal engine development, small monopropellant and bi-propellant thrusters for satellites and spacecraft, and propulsion systems for future NASA endeavors like the Artemis program to the Moon and beyond.
Our capabilities include studying engine performance (analysis), helping to design & build the next generation of engines (design and development) and running engines through a battery of tests (testing) to ensure they can meet the rigorous demands of spaceflight.
The NASA Glenn Research Center also supports and promotes research in advanced propulsion and propellants needed for chemical propulsion as well as other propulsion systems and a more thorough description of this work is described in Fuels and Space Propellants for Reusable Launch Vehicles. This research encompasses both air-breathing fuels and space flight rocket propulsion and propellants.
There are several components that make up chemical propulsion systems. These can be divided into three major groups: The Propellant Delivery/Feed System, the Thrust Chamber Assembly, and sometimes Thrust Vector Control systems. Each of these groups can be further broken down into specific components that make up the group, and often have dedicated branches or … Read the rest ⇢
The heart of “rocket science”, analysis is the research and engineering of examining current and future engine designs, examining test data, specification sheets, design details, as well as the anticipated conditions the spacecraft or vehicle will be subjected to. These all help to describe the overall performance of an engine system, and analysis ensures that … Read the rest ⇢
What is a Rocket? (K-4 grade) A site for K-4 students describing the basics of a rocket
What is a Rocket? (5-8 grade) A site for 5-8 grade students describing basics of a rocket
CEARRUN: A Web-Based Version of NASA’s Chemical Equilibrium with Applications (CEA) software
The following list of NASA Special Publications (SP) provides design guidance for a number or rocket propulsion components and systems. Although somewhat dated (1970’s), much of the design guidance is still used in modern applications and developments.
| Title | Author(s) | Type | Topic Area(s) | Date | Link/Report Number |
|---|---|---|---|---|---|
| Liquid rocket engine turbopump bearings - Space vehicle design criteria /chemical propulsion/ | Butner, M. F., Keller, R. B., Jr. | Special Publication | Machine Elements and Processes | 1971, March 1 | NASA SP-8048 link to be added |
| Solid rocket motor igniters. NASA space vehicle design criteria, chemical propulsion | Special Publication | Propellants | 1971, March 1 | NTRS | |
| Liquid rocket engine turbopump inducers | Jakobsen, J. K., Keller, R. B., Jr. | Book | Propulsion Systems | 1971, may 1 | NASA SP-8052 link to be added |
| Prevention of coupled structure-propulsion instability /pogo/, NASA space vehicle design criteria, structures | Special Publication | Space Vehicles | 19070, October 1 | NTRS | |
| Liquid rocket pressure regulators, relief valves, check valves, burst disks, and explosive valves | Special Publication | Propulsion Systems | 1973, March 1 | NTRS | |
| Liquid propellant gas generators | Special Publication | Propulsion Systems | 1972, March 1 | NASA SP-8081 link to be added | |
| Liquid rocket engine fluid-cooled combustion chambers | Special Publication | Propulsion Systems | 1972, April 1 | NTRS | |
| Liquid rocket metal tanks and tank components | Wagner, W. A., Keller, R. B. | Special Publication | Space Craft Propulsion and Power | 1974, May 1 | NTRS |
| Liquid rocket engine injectors | Gill, G. S., Nurick, W. H. | Special Publication | Spacecraft Propulsion and Power | 1976, March 1 | NASA SP-8089 link to be added |
| Liquid rocket actuators and operators | Special Publication | Auxuliary Systems | 1973, May 1 | NTRS | |
| Liquid rocket valve components | Special Publication | Porpulsion Systems | 1973, August 1 | NTRS | |
| Liquid rocket valve assemblies | Special Publication | Machine Elements and Processes | 1973, November 1 | NTRS |
