STS-119 BLT test a success, gains a greater understanding of reentry
STS-119’s Boundary Layer Transition (BLT) Detailed Test Objective (DTO) has been deemed a huge success, following the presentation of the test results. The tests will continue on at least two more flights, as engineers gain vital data on the conditions a spacecraft endures during reentry.
The goal of the experiment was to measure the effects of increased heating from an early boundary layer transition as the orbiter returns to Earth. The DTO was set to debut on STS-126, before being deferred to STS-119.
During a normal reentry, the orbiter compresses the air as it is entering the atmosphere, which creates a protective region (similar to insulation) with temperatures in the 2000-3000F degree range.
Those values are ably handled by the tiles and other protective systems on the shuttle, however, just inches away – outside of the boundary layer – the readings can rise to 10,000 degrees
If there is any sort of interruption in that flow – such as a protruding gap filler, or damaged tile, or other rough surface – the boundary layer will be tripped sooner than expected, and the extreme heat could cause damage to the tiles, or the orbiter itself in severe cases.
Gap fillers started to become a concern on STS-28, when they were the likely cause of an early BLT (around Mach 18). Several of the gap fillers were found protruding after landing, and more than the usual amount fell on the runway after touchdown.
However a 1989 report from NASA stated that protruding gap fillers were not a flight safety concern, only an issue for turn-around for the next flight.
The STS-119 test was proposed as a means of accurately measuring the actual conditions of an early BLT by installing a specially designed tile with a 0.25” bump to Discovery’s belly. Located near the port main landing gear door, the protrusion was expected to cause a transition around Mach 15.
Specially installed tiles with Modular Auxiliary Data System (MADS) instrumentation were installed in several spots across the orbiter’s belly with the ability to measure precise temperatures.
A Navy P3 Orion was also used, flying over the Gulf of Mexico underneath the shuttle’s path to take infrared pictures known as Hypersonic AeroTHermodynamic InfraRed Measurements (HYTHIRM).
The initial review of the data – via Program Requirements Control Board (PRCB) presentations, available on L2 – indicates that if anything, the models used to determine BLT temperatures were too conservative.
Predictions of the speed at which the 0.25” protuberance would trip the boundary layer were estimated to be around Mach 15.4. However, the initial BLT occurred at Mach 15.6, 969 seconds after the start of Entry Interface (EI).
Surface temperatures in the smooth flow region – protected by the boundary layer – were measured at around 1,600 degrees F which was consistent with flight history.
The temperatures in the turbulent wedge of the nose cone area were approximately 1,900 degrees F – which was a little lower than predictions, but consistent with observations from STS-28.
The main area of concern was heating after the trip region became established. The lack of any Reaction Cured Glass (tile coating) “melt or flow” leads to the conclusion that most temperatures were less than 2,500 degrees F – with predictions of around 2,700-2,800 degrees F.
Documentation claims the peak temperature measured was approximately 2,000 degrees F, while pre-flight predictions were nearly 3,000 degrees F.
Post flight scans of the OML (Outer Mold Line) shape of the tiles in the protuberance area show no discernible change in the surface of the tiles.
One unexpected benefit of the multiple observation methods was the measurement of what’s called an Asymmetric Boundary Layer Transition (ABLT).
This occurred on the opposite side from the protuberance at Mach 10.5 – within limits for a normal BLT at Mach 11 – from some sort of “roughness” on the vehicle.
Post-flight GN&C assessments, along with data from the additional thermocouples and imagery from the HYTHIRM project, isolated the source of that ABLT to a region near the starboard side of the nose gear door.
Such high resolution data might be able to pin down the source of airflow tripping. However, given the lower speeds that such a situation can occur, ABLT is not considered a significant issue.
Next up for BLT is to increase the size of the protuberance to 0.35” – to be installed on Discovery for her STS-128 flight. The larger wedge is expected to trip the boundary layer at around Mach 18, provided information on higher levels of heat.
HYTHIRM is now preparing for the STS-125 entry, but has indicated readiness for being involved with the STS-128 experiment.
L2 members: Documentation – from which most of the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.