Launch Vehicle Aerodynamics

FIELDS OF STUDY: Aerospace Engineering; Space Technology; Astronautics

ABSTRACT: Launch vehicle aerodynamics is used in the design of space launch vehicles used to send a spacecraft into orbit or beyond. The study of aerodynamics usually involves examining several forces such as drag, thrust, and lift. Launch vehicle aerodynamics includes testing on models of launch vehicles that will be used in space travel. The National Aeronautics and Space Administration (NASA) performs such testing, which often consists of different types of wind tunnel tests. Engineers use these tests to prepare proposed launch vehicles for space missions.

A Brief History of Launch Vehicles

A space launch vehicle is a rocket that is used to carry a payload into outer space. Many different launch vehicles have been used in space exploration. The United States and Russia are two of the leading nations in the manufacture of launch vehicles. The United States began using the Delta family of launch vehicles for space missions in the 1960s. This family includes several different types of expendable, or single-use, launch vehicles, some of which were still in use into the 2020s. The US National Security Space Launch program also used launch vehicles provided by SpaceX, a private spaceflight company.

Other notable launch vehicles that have been produced by the United States include the Atlas family, Saturn V, and the reusable Space Transportation System (STS), which is commonly known as the space shuttle. As trends in space missions shifted in the early twenty-first century, the United States developed several launch vehicles that are smaller in size than previous launch vehicles. Pegasus and Taurus are examples of these smaller launch vehicles designed to carry payloads of advanced satellites and other small spacecraft. In the 2020s, the United States relied on the Atlas V, Delta IV, Falcon 9, and Falcoln Heavy. The Falcon family was developed by SpaceX. Russia’s successful launch vehicles include the widely-used Soyuz and the larger Proton (both originally developed by the Soviet Union).

All launch vehicles, whether expendable or reusable, are meant to lift a spacecraft into low-Earth orbit (LEO) or beyond. To do so they must accelerate beyond Earth’s atmosphere, working against both gravity and atmospheric friction. To achieve the required thrust, launch vehicles are powered by chemically propelled rockets. Many considerations, including rocket construction, fuel types, and launch site and conditions, are taken to minimize the energy needed to reach outer space. As launch vehicles must travel through the atmosphere as efficiently as possible, aerodynamics helps determine their optimal design.

Basics of Aerodynamics

Aerodynamics is used to study how objects such as aircraft and rockets are affected by air or other gases. Several forces come into play with aircraft aerodynamics, including that of launch vehicles. Drag is one of the main aerodynamic forces. This force acts against an aircraft’s motion as it travels through the air. Therefore, drag is generally an unwanted force, as the aircraft must counteract or overcome the force. Different forces affect drag, including the air pressure on the front of the aircraft, the suction on the back, and the friction on the sides.

A value known as the drag coefficient (c_d) can be used to identify the amount of drag on an aircraft. The drag coefficient is dependent on a number of factors, including the shape, surface roughness, and speed of the aircraft, the turbulence of the air, and the density of the air. The drag coefficient is typically calculated using either wind tunnel tests or computer models. Modern aircraft have a drag coefficient between 0.01 and 0.03.

An aircraft can counteract or overcome drag by producing thrust. Thrust is generated by an aircraft engine and is the force that actually moves the aircraft. If an aircraft is flying horizontally at a steady speed, the thrust is barely counteracting the drag. Thrust must be increased in order to accelerate.

Lift is yet another aerodynamic force. The wings of a standard airplane produce lift, which is the force that prevents the plane from falling from the sky. Lift involves the speed of the air that flows over the top of the wings compared to that along the bottom of the wings. Air flows slower along the bottom, which causes the air to push up on the wings more forcefully than the air that is pushing down on the wings. Traditional launch vehicles, however, do not use wings. Instead they rely on vertically launched rockets to reach the upper atmosphere (where drag is lower) as fast as possible without using lift. The lack of wings allows rockets to be lighter and more aerodynamic. Still, future spaceplanes may use winged designs to allow operation in the atmosphere along with the ability to reach escape velocity.

Wind Tunnel Tests

Before an aircraft such as a launch vehicle is built, aerodynamics testing of the aircraft is performed. Typically, a model of the proposed aircraft is tested using wind tunnel tests. These tests involve moving air past the model in a controlled environment such as a tunnel and determining the forces that are acting on the model. Several different types of wind tunnel tests can be used. One type directly measures aerodynamic forces and moments. A machine known as a force balance is used to mount the model in the tunnel. This force balance measures lift and drag.

In another type of wind tunnel test, pressure data is collected using pressure taps, which allows component performance to be calculated. Aircraft inlet performance and airfoil drag can also be determined in this type of wind tunnel test. A third type of test involves collecting diagnostic information through instrumentation including static pressure taps, total pressure rakes, and hot-wire velocity probes. Such instrumentation provides various kinds of flow information. Yet another type of wind tunnel test collects diagnostic information by making the flow of gases around the craft visible. The visualization methods used include laser sheet, free stream smoke, and surface oil flow.

Example of Launch Vehicle Aerodynamics Testing

NASA conducts extensive aerodynamics testing on their space launch vehicles. Typically, this testing allows engineers to predict vehicle control, trajectories, and payload performance. One example of such testing took place during the design of the Space Launch System (SLS) in 2012. The SLS is the United States’ successor to the space shuttle and first exploration-class rocket since Saturn V. It is expected to allow for human exploration outside of Earth’s orbit.

The aerodynamics testing of SLS was performed at NASA’s Langley Research Center in Hampton, Virginia. Scientists tested the aerodynamics of a model of the SLS in Langley’s Transonic Dynamics Tunnel (TDT), simulating the environment to which the full-size SLS rocket would be exposed in flight. As part of the wind tunnel testing, 360 pressure transducers were placed on the surface of the ten-foot-long model. The transducers allowed measurement of the rapidly changing forces of unsteady flow encountered in the atmosphere, providing the NASA team with a huge amount of data. The data was then used to prepare the SLS for future space missions. For example, data collected allowed the team to determine the launch vehicle’s capability to handle the bending forces and accelerations it would be exposed to and make final design tweaks to improve it. The SLS was being prepared for its first mission, Artemis 1, which was to send an uncrewed capsule around the Moon. This was the first step in sending humans back to the Moon.

The intense energy required for liftoff and the nature of Earth’s atmosphere mean that powerful forces act upon any craft launched into space. Aerospace engineers must ensure that a craft can structurally withstand these forces, with little margin for error. In this way understanding aerodynamics is crucial not only to a launch vehicle’s ability to reach orbit efficiently, but to the safety of the vehicle’s payload.

PRINCIPAL TERMS

• aerodynamics: the study of the way in which air or gases act on an object; often used when designing aircraft, automobiles, buildings, and bridges.
• payload: the carrying capacity of an aircraft or spacecraft. With spacecraft, the payload may include astronauts, equipment, a satellite, and/or a space probe.
• wind tunnel tests: aerodynamics tests that involve moving air past an object in a tunnel to study the resulting effects.

Bibliography

Blevins, John A, et al. "An Overview of the Characterization of the Space Launch System Aerodynamic Environments." Aerospace Research Central. American Inst. of Aeronautics and Astronautics, 13 Jan 2014. Web. 8 June 2015.

Chan, David T., et al. "Aerodynamic Characterization and Improved Testing Methods for the Space Launch System Liftoff and Transition Environment." American Institute of Aeronautics and Astronautics Special Session: Space Launch System Aerosciences II, 15 June 2019, DOI: 10.2514/6.2019-3398. Accessed 16 June 2022.

"Chapter 14. Launch Phase." Basics of Space Flight. Jet Propulsion Laboratory/NASA, 2013. Web. 8 June 2015.

Congiu, Sasha. "SLS Model ‘Flies’ through Langley Wind Tunnel Testing." NASA. NASA, 28 Nov. 2012. Web. 8 June 2015.

David, Leonard. "Russia Reignites Its Rocket Industry with New Angara Booster." Space.com. Purch, 19 Aug. 2014. Web. 8 June 2015.

"The Great Escape: SLS Provides Power for Missions to the Moon."National Aeronautics and Space Administration, 10 Dec. 2021, www.nasa.gov/exploration/systems/sls/to-the-moon.html. Accessed 16 June 2022.

Lucas, Jim. "What Is Aerodynamics?" Live Science. Purch, 20 Sept. 2014. Web. 8 June 2015.

"Testing on the Ground Before You Fly: Wind Tunnels at NASA Ames." National Aeronautics and Space Administration, 21 May 2020, www.nasa.gov/ames/wind-tunnels. Accessed 16 June 2022.