Boundary Layer Transition height increase and addition noted for STS-134

by Chris Gebhardt

As NASA and Space Shuttle Program Managers continue to narrow down options for the final two missions on the flight manifest, preparations and modifications for/to Orbiters Discovery and Endeavour are continuing in their respective Orbiter Processing Facilities (OPFs) at the Kennedy Space Center – most notably the process to approve a new Boundary Layer Transition (BLT) Detailed Test Objective (DTO) protuberance height to Discovery (STS-133) and the addition of a BLT DTO to Endeavour for her STS-134 mission.

BLT DTO Protuberance Height Increase and Addition to STS-134:

A Change Request (CR) has been put forward by the Shuttle Program to increase the height of the BLT DTO protuberance tile on Discovery for the STS-133 mission to one-half inches and perform a re-flight of the Catalytic Coating DTO on a TPS (Thermal Protection System) tile downstream of the BLT DTO tile.

While the increase of the protuberance tile over time (to a potential 0.5″) and the flight of the Catalytic Coating DTO was always part of the BLT DTO plan, the addition of a BLT DTO test on any orbiter other than Discovery has not been under consideration since Discovery gained all the BLT DTO flights and since it was deemed unnecessary – given the DTO’s objectives and the time necessary to install all of the BLT DTO instrumentation and TPS protuberance tile on an Orbiter – to have more than one Orbiter undergo the modification process.

However, that position has been reevaluated now that Endeavour is tracking a rather pro-longed stay inside OPF-2 for a newly realigned launch date of February 2011 for the STS-134/ULF-6 mission.

As outlined by a PRCB (Program Requirements Control Board) presentation (available for download on L2), “The 4th & 5th Orbiter BLT DTO [flights] will attempt to gather flight-derived data regarding the effects of Mach 19.5 transition due to a 0.50″ protuberance on the Space Shuttle Orbiter during the re-entry trajectory.”

In all, for STS-133, CR approval will see Discovery fly with a 0.5″ protuberance tile and a total of 10 thermocouples located on the protuberance tile as well as downstream TPS tiles. These thermocouples will record the valuable data sought by the DTO team.

Additionally, Discovery will fly with the 2nd re-flight (3rd total flight) of the Catalytic Coating DTO. This will involve coating one TPS tile with a catalytic coating material and installing two thermocouples to measure both the heating effects of the catalytic coating as well as the “fully-turbulent heating” from the BLT DTO tile.

Furthermore, while the BLT DTO experiment on STS-134/Endeavour would be tasked with gathering more data on the effects of 0.5″ protuberance, Endeavour will only be affixed with three (3) thermocouples instead of 10 and will not carry a TPS tile coated with the catalytic coating.

Benefits and Significance of a 0.5″ BLT DTO Tile:

As noted by the PRCB presentation, there are several benefits to flying two 0.5″ BLT DTO protuberance flights that will trip the Boundary Layer at ~Mach 19, not the least of which being the benefit to future spacecraft returning to Earth from “trajectories beyond LEO (Low Earth Orbit).”

“BLT FE Flight Test Objectives planned to include three primary areas of entry data: Boundary Layer Transition, Turbulent Heating, Turbulent/Catalytic Heating.”

In all, Boundary Layer transition data has been collected on three previous Discovery flights: STS-119, STS-128, and STS-131.

However, the presentation notes that the data collected for “turbulent heating” on STS-128 and STS-131 was “compromised.”

Likewise, the two flights (STS-128 and STS-131) designed to provide data on Turbulent/Catalytic Heating both produced “compromised” data beginning at the first roll maneuver during entry.

Engineers have determined that the BLT DTO data is being compromised by a “surface thermocouple measurement anomaly;” however, the root cause for this anomaly has not yet been determined.

The presentation notes that Orbiter flight testing to “disprove some legs of root cause” are possible, as is the addition of alternate measurements for the BLT DTO flight on Endeavour that could lead to “future design changes for entry instrumentation.”

However, since BLT onset data is extremely limited in controlled environments, and obtaining “fully turbulent data at higher Mach numbers will provide more relevant data for re-entry trajectories beyond LEO,” an increase of the BLT DTO protuberance tile from 0.35″ to 0.5″ was requested for STS-133.

Moreover, the addition of a second 0.5″ BLT DTO on Endeavour/STS-134 was requested to help “reduce uncertainty from 1.06 to 0.90 times the standard deviation of the flight data,” notes the PRCB document.

“Adding BLT FE flight will allow the technical community to rigorously evaluate ground/flight scaling because the flight uncertainty will no longer be dominated by the number of flights.”

STS-134 Specific Articles:

Furthermore, highlighting the multi-faceted potential of the Space Shuttle Orbiter is its ability to gather this BLT data, a fact noted in the PRCB presentation.

“Decreasing the Uncertainty of BLT prediction with Orbiter flight data is one of the primary benefits being accomplished by the BLT Flight Experiment.”

Currently, there is a +/- Mach 2.5 uncertainty factor in all BLT onset prediction tools.

As part of the BLT DTO process, engineering teams were tasked with determining the BLT Mach increment increase from the previous flight for the progressive BLT DTO. Their analysis showed that a Mach increment of >1.0 in “free stream Mach number would provide a unique data point of magnitude similar or greater to the 1-sigma uncertainty of the current state of the art empirical BLT prediction capability.”

A BLT protuberance height of 0.5″ would translate to a trip of boundary layer at ~Mach 19, an increment increase of ~1.5 from STS-131’s Mach 17.5 trip of the boundary layer.

In all previous Boundary Layer trips during flight, no trips exceed the Mach 17.9 limit. Thus, STS-133 will be the first occurrence of a break in the BLT flight history.

BLT DTO Predictions for 0.5″ Protuberance:

A benefit of the previous BLT DTO flights in regard to STS-133 and STS-134 is the understanding that the unadjusted aeroheating analysis tools “over-predict” heating flux.

In fact, the analysis tools over-predicted the STS-119 heating environment by ~900-degrees F compared to the actual flight data.

Following this understanding, a “unique method [was] developed to account for the discrepancy between analysis and flight data.” This allowed the teams to analyze potential temperature problems downstream of the TPS protuberance on STS-128 and STS-131 while still maintaining a high degree of conservatism.

Furthermore, these “lessons learned” were applied to the pre-decisional analysis of the 0.5″ BLT protuberance.

In all, an extremely conservative Orbiter weight was assumed for each mission. (STS-133’s Nominal End Of Mission [NEOM] weight is ~204,000 lbs v. the 233,000 lbs analysis weight. Likewise, STS-134’s NEOM weight is ~203,000 lbs v. the 233,000 lbs analysis weight.)

A Mach 14.6 BLT for all areas upstream of the protuberance was used – a measurement that includes all but nine (9) recorded Orbiter flights in the history of the Space Shuttle Program. These non-included flights are believed to be a result of upstream gap fillers that would be seen and accounted for under current program on-orbit imagery inspections.

Furthermore, “Analysis for STS-133/134 used conservative & generic wing heating limits. Less-conservative location & configuration specific limits could have been used, but were not,” notes the PRCB document.

Moreover, the document notes that “trends in heating adjustments” were required to match flight data and that additional reductions in heating are required as the height of the protuberance increases, reductions that were made to properly predicted the 0.5″ protuberance heating environment.

Engineers further utilized the Arc Jet facility to understand the potential heating environment induced by the protuberance.

A protuberance height of 0.35″ (the height flown on STS-128 and STS-131) was used and the heating capacity of the Arc Jet facility was increased by “utilizing a 14-heater pack configuration.”

The document notes that during the highest temperature run the protuberance temperature reached 3,000-degrees F – at which point, 180-seconds into the test, all temperature measurements were lost “presumably due to thermocouple wire exposure to hot gas flow.”

Post-test inspection showed a change to the shape of the protuberance and a reduction to its height – which correlated to a decrease in the upstream and downstream temperatures.

According to the document, “This provides some level of validation that a protuberance that experiences over-temperature conditions and changes shape will result in a progressively more benign environment downstream of the protuberance.”

Therefore, “Analysis and results have been reviewed and approved by the Entry Aeroheating Panel, Orbiter Thermal Panel, BLT IPT, TPS PRT and members of the SLD Panel,” notes the PRCB document

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