The STS-126 mission will resume ISS operations after a break for STS-125’s flight to the Hubble Space Telescope, but one of the newest additions to the mission plan won’t take place until reentry.
The mission’s Program Freeze Point was overviewed at the PRCB (Program Requirements Control Board) meeting earlier this month, where a Boundary Layer Transition (BLT) Detailed Test Objective (DTO) was proposed.
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During re-entry, the compression of air on the leading edge of the space shuttle forms a protective ‘blanket’ around the orbiter. Temperatures within that region are a manageable 2-3,000 degrees, while just a few inches away the full force of heating results in readings of up to 10,000 degrees.
As long as the orbiter’s surface is smooth, the boundary layer keeps tile heating within the limits of their design, but any interruption in the air flow causes a boundary layer ‘trip’ where turbulence behind the trip point brings in the extreme heat to the tiles, and could cause tile failure and/or damage to the underlying surface.
Picture a smooth flowing river and water moving uniformly downstream, then place a large rock in the middle and you’d observe the water behind the rock swirling around. Everything ahead of the rock stays in a steady state, but the water past the rock is very turbulent.
When the shuttle team finds a gap filler or tile damage on the underside of the orbiter, they have to determine if that ‘change in smoothness’ will trip the boundary layer, at what speed it will be impacted, what sort of temperatures will impinge upon the tiles, and what damage (if any) will result.
Unfortunately there is very little research into this area. There are a lot of computer simulations, but very little actual experiments to validate the models.
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The orbiter involved, Endeavour, is multi-tasked with STS-326 rescue support for STS-124, will also be the last shuttle to sit on launch pad 39B as STS-400 – the rescue support for STS-125’s Hubble mission – and will be the heaviest logistics flight ever, STS-126, carrying supplies – including a new six-crew Galley – to the ISS.
As noted in the expansive 81 page presentation – available to download on L2 – STS-126 is proposed to be the first mission with a specific test installed to measure the impacts of a boundary layer transition, with the DTO utilizing a specially designed tile to ‘trip’ the boundary layer – along with instrumentation underneath tiles downstream that would measure any thermal impacts.
A specially designed ‘trip tile’ will be installed near the main landing gear door, with a roughly one-quarter inch protuberance, to trip the boundary layer at speeds less than or equal to Mach 15.
Current DAT (Debris Analysis Team) assessments have cleared the orbiter up to Mach 18. so it’s been classified as a low risk to mission safety.
Thermocouples will be installed underneath the tile on the left-hand wing and downstream of the trip tile to measure the specific temperatures during reentry.
The tile is being installed in a location where studies show that even a very large break in the boundary layer will not be a concern. If Arcjet testing of the setup – scheduled for mid-January – shows any reason for concern, the tile can be removed late in the OPF (Orbiter Processing Facility) flow if necessary.
Since Return To Flight, there have been 1093 hits to the bottom of the orbiter, but only one in this area – so additional damage during launch is not expected. Ten thousand simulations of foam debris striking the area produced no critical damage in any.
However, if something does happen, that would make the test an issue on flight, EVA tools and techniques have been tested in case the protuberance needs to be removed.
Future plans call for installing the thermocouples on Discovery as well, and beginning her testing with a half-inch protuberance on STS-127 – for a transition closer to Mach 18. The last test in Phase I is a three-quarter inch notch on STS-128, which would transition at Mach 19.
Phase II testing will add a catalytic coating material on one of the tiles which would gather catalytic / turbulent heating interaction data.
Two additional flights are on the manifest through the end of the shuttle program – based on current manifest – which are currently being penciled in as contingency flights for this experiment.
The overall goal is not just to help shuttle operations. There is little understanding of boundary layer transitions, and the data from this experiment is expected to strongly impact development of computational techniques.
The results will impact many other thermodynamic interests in the aerospace fields.
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