Shuttle Reviews: PAL Ramp Analysis
Boeing analysis supports Space Shuttle modified External Tank
Boeing engineers are working with NASA, United Space Alliance and Lockheed Martin to determine the effect of removing foam ramps from the Space Shuttle's external fuel tank. The engineers are conducting analyses of the aerodynamic environments to determine the changes in heating and static and dynamic loads on the hardware of the external tank.
The analysis, combined with advanced wind tunnel testing at NASA facilities, will ultimately determine whether the space shuttle will be safe to fly following removal of the foam ramps from the External Tank. In July, Discovery lost a sizeable piece of foam from the "protuberance air load" (PAL) ramp, a protective ramp designed to smooth the airflow over cable trays and pressure lines during launch.
"The PAL ramps were originally placed on the tank to account for incoming airflow that was meeting the cable tray at about a 90 degree angle," said Carlos Ortiz, Boeing lead engineer for the aero sciences group for integrated vehicle analysis, Space Shuttle Systems Engineering and Integration (SE&I). "Back in the 80's, wind tunnel tests and analysis indicated some of the flow could cause an aeroelastic effect, a potential dynamic instability in the cable tray which could be destructive." The PAL ramps were installed to divert the flow of air over the cable trays and pressurization lines to preclude the flow conditions for the aeroelastic phenomena.
After the Columbia accident, the PAL ramps were identified as potential debris sources and NASA spent considerable time in 2003-2005 understanding what the effects would be if the PAL ramps were removed. Based on their analysis and testing, it was concluded that the cable trays do not have a significant aeroelastic response and this supported the decision to remove the PAL ramps. With the ramps removed, Boeing engineers were asked to define the aerodynamic environments based on this new configuration.
Engineers today have better tools to examine the air flow around the cable trays and pressure lines than they did in the 1970s and 1980s. By using computational fluid dynamics (CFD), NASA and Boeing engineers have improved their ability to analyze and understand the complex airflow around the shuttle. "We've done some post-processing of the CFD solutions to see what the effect will be to the environments with the PAL ramp removed. We are validating these findings with wind tunnel testing," said Ortiz.
A component wind tunnel test was conducted in March and April at the Glenn Research Center in Ohio that took an in depth look at the cable tray and lines. The test used 25 and 50 percent scale models of the ET section affected by the PAL ramp removal. "It was more of a local flow field environment, where we are looking at the effects of steady and unsteady flow on a localized area," said Ortiz.
"When the ramps are removed, air can travel underneath the cable tray and pressure lines. The flow under and around these components is what we are assessing," said Tom McGowen, Boeing project manager for SE&I.
Ames Research Center conducted additional wind tunnel tests with a 3 percent model of the tank, orbiter, and boosters. "They ran a range of tests from Mach 0.6 to Mach 2.5 with most of the focus on the transonic range," said Ortiz.
It has been found that the worst loads environment is in the transonic regime just before and after you reach Mach 1 during the first minute of flight. "These are speeds in the range from .6 Mach to 1.0 local Mach where we have seen the peak environments happening," said Ortiz.
The supersonic region of Mach 3 and 4 is where peak heating occurs, caused by the friction of the air molecules flowing over the ET. "As far as the PAL ramp area, we are only dealing with aerodynamic heating because it is well forward of any influence of the exhaust plumes from the boosters or engines," said Robin Taylor, lead for the heating analysis group for integrated vehicle analysis, SE&I. "We have to understand what is going on underneath the cable tray, and of course, how the heating is going to change in all affected areas," Taylor said.
"There is a significant amount of wind tunnel data with no PAL ramps installed from when the vehicle was first designed," said McGowen. And there have been recent wind tunnel tests in 2004 and 2005 that will provide data for the analysis.
In the dynamic loads area, tools like CFD don't work. "There are aerodynamic acoustic and buffet loads that need to be defined. In terms of modeling techniques, your grid densities to capture such unsteady high frequency phenomena would challenge your computational capacity, and extensive testing would still have to be done for model and simulation validation," said Steve DelBasso, integrated vehicle analysis engineer for SE&I.
"The PAL ramp removal buffet loads now under evaluation are based on conservative, simplified, semi-empirical, analytical techniques that were used as part of the initial design back in 1979. One of the key aspects of the wind tunnel tests is to confirm the conservatism and adequacy of these loads," DelBasso added. The Glenn wind tunnel test will be the primary source of data for the dynamic loads environment.
"Following the wind tunnel tests, we are going to come back to the program with a lot more knowledge than we had before." Ortiz said. He added they expect to complete their analysis in May. The environments that Boeing produces will go to the ET tank community. "They will apply these loads and heating to their hardware and will show that they can withstand that environment and that is when you get the thumbs up to go fly without the PAL ramp," McGowen added.
"The lessons we learn here will also prepare us to support future space exploration activities," said Dave Eck, manager of the integrated vehicle analysis SE&I. "The work has provided our newer folks an opportunity to be involved with a certification and design process as opposed to the operation of an existing vehicle."
The areas around the cable trays near the PAL ramp locations on the ET will also be instrumented for the next flight. "Flight data will be used to further validate the wind tunnel test results," Ortiz added.