NASA appears to be finding a balance between aerodynamic loadings on their modified External Tanks and performance issues during the ride uphill via their Low Q and High Q ascent profiles.
While Shuttle Atlantis’ STS-115 is expected to fly a High Q ascent profile due to its payload and abort options, documents show the decision to fly Low Q on STS-121 proved to be the right decision for Discovery, potentially avoiding foam liberation earlier into the ascent.
**Information collated from a 64 page document – available to download on L2**
**STS-115 LIVE UPDATE PAGES**
Atlantis’ mission has not had its ascent profile confirmed, although it is almost certain the launch will be a High Q ascent, given this is required by way of performance for the assembly missions that include large International Space Station elements, such as the P3/P4 truss that will ride with STS-115.
‘If you look at the difference between a Low Q ascent and a High Q for us, the main differences are where we pick our different abort boundaries,’ noted STS-115 commander Brett Jett. ‘So from a crew perspective, High Q gives us more abort options during the ascent.
‘The Program have looked at the margins on the tank and they are very comfortable with the tank both structurally and with the foam performance, so we believe it was the right decision (at present) to go High Q with 115. With our heavy payload, Low Q would have been very difficult for us to do.’
The Low Q approach was originally an option to fix the Orbiter Window issue for High Q mission profiles. However, it returned for Discovery’s Return to Flight mission last month to reduced the stresses on the External Tank, following concerns on how the ET would perform without its PAL ramp and partially modified Ice/Frost ramps.
Recently, NASA ‘performed a mission specific assessment of STS-121 using Low Q and High Q I-load designs based on STS-121 measured wind to produce trajectory assessments and inputs to ascent aero-heating predictions, (which) determined parameter deltas between Low Q and High Q mission designs to determine if STS-121 foam loss events could have occurred earlier in time if STS-121 had been a High Q mission design.’
The fascinating document, which took data from a number of NASA centers and major Shuttle contractors, reiterated that ET foam loss comes from a complex set of combinations, thus playing down the importance of Low Q as an absolute solution for solving liberation from the tank.
‘ET Foam loss is caused by complex combinations of heat transfer/venting/material failure processes based on initial configuration of the foam at lift-off – Cracks, Voids, Delamination, Damage, Thickness, Shape, velocity, dynamic pressure, etc,’ noted the document, which went on to show that most of the elements noted are not affected by the Low or High Q ascent profile.
Interestingly, the document goes on to back up the decision to move to a Low Q ascent profile for STS-121, noting – inside a red-highlighted page: ‘Trajectory indications are that a STS-121 High Q mission design may have caused earlier foam loss based on velocity trends and releases from 30-80 seconds may have caused more damage if impacts occurred.’
This information backs up the Low Q decision, given the foam loss that was seen on STS-121. While the liberation mass was within limits, it was also an important factor that the loss happened later into the ascent, lowering the risk to the orbiter, had the foam loss struck the vehicle.
Another set of interesting data finds are based around ‘Aeroheating’ – one of the contributors to ‘Cryo-ingestion,’ which is listed as part of the complex mix that can lead to foam loss.
‘STS-121 High Q mission design would have produced heating rates prior to the peak heating time frame ~5-10 seconds earlier than Low Q mission designs,’ noted the evaluation of Discovery’s ascent. ‘STS-121 High Q mission design would have produced ~4-10% higher peak at STS-121 foam loss locations.
‘STS-121 High Q mission design would have produced ~4-6% higher heat load at STS-121 foam loss locations. STS-121 Low Q mission design produced on average ~57% of the design heating rate and heat load while High Q mission design would produce ~63% of the design values.’
From these findings, the document noted: ‘Aeroheating indications are that a STS-121 High Q mission design may have produced ET foam loss ~5-10 seconds earlier for failures driven by external temperature or foam temperature distribution and that an increase in peak heating rate from 90-110 seconds and heat load may have caused additional ET foam loss for failures driven by external temperature.’
While Discovery’s ascent was still one of the cleanest – if not cleanest ever – launches, NASA still has work to do – which it has acknowledged on many occasions – to fully understand the science behind foam liberation.
What appears to be the case with this document is STS-121’s ascent has provided engineers with a host of new data, to aid their understanding as they head into the final salvo of missions before the Shuttle is retired in 2010.
This is highlighted by a host of new data, driven from the newly installed DFI (Developmental Flight Instrumentation) – which makes up a number of pages in the document. Ironically, Discovery’s tank (ET-119) did not fly with the full suite of DFI, which is still around four tanks away from being realised, with funding currently being requested to achieve that goal.
Early results from the DFI have proved to be encouraging, as it was noted: ‘Preliminary MAF (Michoud Assembly Facility) analysis has shown that base drive acceleration dominates the analytical response predictions. Based on STS-121 and STS-114 LO2 cable tray accelerometer data PAL ramp removal does not cause significant differences in the cable tray response.
‘Based on STS-121 and STS-114 LO2 cable tray accelerometer data, the cable tray response produced expected sensitivity to dynamic pressure. STS-121 LH2 cable tray response does not indicate any design load issues.’
All the findings noted in the document will also prove to be useful when a similar batch of data is collated post STS-115, showing any issues between to the two ‘Q’ ascent profiles, allowing NASA to build yet more confidence in achieving a smooth completion of the International Space Station.
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