July 2002
Volume 10, number 4
NASA HL-20 Lifting Body Airframe Modeled with Simulink and the Aerospace Blockset
by: Stacey Gage
For most flight control designs, the airframe, or plant model, needs to be modeled, simulated, and analyzed. Ideally, this airframe would be modeled quickly, reusing blocks or model structure to reduce validation time and leave more time available for control design. This article discusses using Simulink and the Aerospace Blockset for modeling the airframe of a NASA HL-20 lifting body, which is a low-cost complement to the Space Shuttle orbiter. The Aerospace Blockset was used to rapidly assemble portions of the HL-20 airframe. The remaining portions, including the calculation of aerodynamic coefficients, were modeled with Simulink. This article examines the construction of the Simulink model of the HL-20 airframe and touches on how the aerodynamic data was used in the model.
NASA HL-20 Lifting Body
The HL-20, also known as personnel launch system (PLS), is a lifting body reentry vehicle that was designed to complement the Space Shuttle orbiter. Designed to carry up to ten people and very little cargo1, the HL-20 lifting body was to be placed in orbit either launched vertically via booster rockets or transported in the payload bay of the Space Shuttle orbiter. The HL-20 lifting body was designed to deorbit using an onboard propulsion system while its reentry was to be nose-first, horizontal and unpowered.
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Figure 1: Top view of the HL-20 lifting body. (NASA Langley photo) |
The HL-20 lifting body was developed as a low-cost solution for getting to and from low Earth's orbit. The proposed benefits of the HL-20 were reduced operating costs due to rapid turnaround between landing and launch, improved flight safety, and ability to land conventionally on runways. Potential scenarios for the HL-20 were orbital rescue of stranded astronauts, International Space Station crew exchange if the Space Shuttle orbiter was not available, observation missions, and satellite servicing missions. Although the HL-20 is not currently an active program, the aerodynamic data from the HL-20 is being used in a current NASA project2.
Modeling assumptions and limitations
There were several assumptions made when modeling the airframe. The airframe is assumed to be rigid and have constant mass, center of gravity, and inertia since the model is to represent the reentry portion of the mission. Preliminary aerodynamic data for the HL-20 lifting body was obtained from NASA TM4302. This data has additional assumptions and limitations that follow:
- HL-20 is a laterally symmetric vehicle.
- Compressibility (Mach) effects are negligible.
- Control effectiveness is assumed to vary nonlinearly with the angle of attack and linearly with the angle of deflection. Control effectiveness is not dependent on the sideslip angle.
- The nonlinear six-degrees-of-freedom (6DoF) aerodynamic model is intended for an early version of the HL-20; therefore the data is not recommended for absolute performance of HL-20.
Constructing the airframe model
The HL-20 airframe model discussed in this article, hl20airframe.mdl, is available for download from MATLAB Central at /matlabcentral/. An additional file containing model parameters and aerodynamic data, init_hl20.m, is part of the download in a zip file, hl20.zip. Product dependencies for this model are MATLAB, Simulink, and the Aerospace Blockset. After downloading the model, you can follow along with the article by stepping through levels of the model as you read about them. For those who are also interested in flight control systems, there is an example of an auto-land control for the HL-20 airframe in the Aerospace Blockset.
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Figure 2: HL-20 airframe model.
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The typical airframe model consists of a number of components, such as:
- Equations of motion
- Environmental models
- Calculations of aerodynamic coefficients, forces, and moments
When you open the example model, hl20airframe.mdl, you will see that these components are represented as subsystems in Simulink or blocks from the Aerospace Blockset. Clicking on the HL-20 airframe subsystem in hl20airframe.mdl opens the subsystem to show the five airframe components:
- 6DoF (Euler angles)
- Environment models
- Alpha, Beta, Mach
- Aerodynamic coefficients
- Forces and moments
Figure 3: HL-20 airframe subsystem. Click to enlarge image.
