STS-126 will see the flight debut of two new sets of instrumentation, aimed at gathering more detailed data on RSRM (Reusable Solid Rocket Motor) behaviour during the first stage of launch. The resulting data will benefit the efforts to understand and mitigate Thrust Oscillation on Ares I.
The main objective is to record pressure variations inside the boosters at higher fidelity than has been achieved in any previous shuttle flight.
These initiatives are in support of the Ares I program and will provide valuable input to the ongoing efforts to overcome problems the design teams have encountered with Thrust Oscillation (TO).
Modelling carried out on the Ares I vehicle predicts two major structural resonances at about 10 Hz and 12 Hz. Natural oscillations and variations in thrust – which the RSRMs generate – are predicted to produce significant excitation of these resonances. Higher frequency vibrations are also of interest for their potential impact on sub-components.
Analysis so far has relied heavily on data gathered from extensive static ground firing tests of SRMs and there is a need to validate results against real flight data. This requirement for comparative flight data will be addressed in STS-126 and several following missions.
Two methods have been chosen to gather pressure data – the first makes use of existing Operational Pressure Transducers (OPT), and the second uses a new Intelligent Pressure Transducer (IPT), according to RSRM and SRB Flight Readiness Review presentations – available to download via L2.
Both transducer types fit to the forward dome of the RSRM, and are located within the double ring of bolts of the igniter adapter ring.
This adapter holds the igniter inside the combustion chamber and caps/seals the forward segment of the motor.
Four special bolt holes provide access to the combustion chamber gas pressure inside the motor through narrow connecting channels.
Operational Pressure Transducers (OPT) occupy three of these positions and a special sealing bolt normally blanks off the fourth. However, for STS-126, it will, instead, carry the IPT.
In normal operations, the triply redundant OPTs provide a critical cue for first-stage booster separation. Pressure readings are also recorded during flight for later download from the recovered booster – these are used for analysis of flight performance.
Pressure data recorded by the standard SRB systems only captures low frequency variations (a few Hz at most) with low resolution. Design teams are interested in pressure variations in the frequency range 10 to 100 Hz, and this requires additional instrumentation.
Modifications for STS-126 involve the use of Extended Data Acquisition System (EDAS) units, a special electronic Signal Conditioning (SC) module, and additional wiring to tap into OPT output signals at the SRB Integrated Electronics Assembly (IEA).
EDAS units have a 10-year history of frequent flight use in the Shuttle program. They have four general purpose data recording channels and are fitted inside SRB forward skirts. First flown on STS-91 to record high quality data on water impact forces during SRB splashdown (using strain gauges and accelerometers), they are now used to capture other vehicle dynamic behaviour.
The EDAS units are capable of capturing signals at 1200 samples-per-second (sps) and digitising with 12-bit resolution (4096 discrete levels). This means they can faithfully capture signals in the range DC to approximately 500 Hz.
Low frequency pressure content of the OPT signals (DC to just under 10 Hz) is stripped out by electronic filters in the SC module and the signal connected to one input channel of the EDAS.
By removing the large “DC” background pressure (up to 900 psia) and amplifying the smaller residual AC variations (-20 to +20 psi) a resolution of 0.01 psi is achieved. This compares with only 0.24 psi possible without the filtering.
The result is capture of very high resolution pressure data in the frequency range 10 – 100 Hz.
Other EDAS channels on this flight capture signals from two strain gauges and one accelerometer fixed to the inside SRB forward skirt walls. The four signals are time-synchronised within a single EDAS unit allowing engineers to correlate pressure fluctuations with acceleration and strain variations.
For STS-126 two EDAS/SC units are fitted to the RH SRB, but only one in the LH booster due to lack of sufficient mounting holes on the existing EDAS adaptor plate.
Intelligent Pressure Transducer:
The IPT is a self-contained pressure sensor, signal conditioner, and data recorder designed to capture pressure data at high sampling rates (300 sps) with 12-bit resolution
Unlike the EDAS/OPT instrumentation, the IPT will capture pressure oscillation frequencies from DC to approximately 100 Hz but with a lower resolution of ~0.3 psi. Full scale range is nominally 0 to 1000 psia. This gives continuous coverage well below and above the frequency band in which Ares-I structural resonances are predicted to lie.
Stellar Technology Inc. (STI) produces the IPT and also the latest generation of OPTs, and both share safety critical elements in their design. They also shared their first full scale static firing demonstration tests in November 2001 on Engineering Test Motor 2.
Size is similar to the STI OPT with an identical body height of 5 inches but slightly larger diameter (1.625 inches versus 1.00 inch). It is powered by a single internal non-rechargeable 2/3 AA sized 3.6V lithium thionyl chloride battery.
There is no electrical connection to any flight hardware – it has a single connector which is an interface to a laptop PC for data download after flight. Data collection starts by “launch window programmed activation”.
Engineers will be able to make direct one-to-one comparisons of flight data with IPT recordings made during several full-size motor static firing tests.
As with all new flight equipment, extensive and detailed assessments of possible hazards and risks were presented to the PRCB for approval. Two main areas addressed were: loss of pressure containment of the RSRM, and potential impacts on other critical hardware in the forward skirt.
The transducer provides structural integrity of the RSRM pressure vessel, and mechanical failure of the IPT could cause structural failure of the igniter.
“Structural failure of the igniter components could result in forward dome burn through, loss of capability to separate during the separation phase, thrust imbalance between SRB’s, loss of vehicle control, and structural breakup.”
Structural failure modes considered were: Failure of the (sensing) diaphragm and secondary containment chamber; Failure of the transducer pressure cap (to which the diaphragm assembly is welded); and battery explosion causing shrapnel to damage other critical hardware in the forward skirt (including the IPT).
Rationale for accepting the first two hazard causes as “Controlled” are that relevant critical structures in the IPT are identical to flight qualified STI OPTs, (12 of which have flown on four STS flights) and so have been given the same classification.
Battery explosion has been classified as “Catastrophic x Remote” leading to an “Accepted Risk”. This is on the basis of extensive tests and applied controls/verifications.
“IPT main battery conforms to safety standards in UL-1642 – Testing includes room- and high-temperature short-circuit, oven heating, impact, crush, and abnormal charging – UL testing bounds all flight conditions, including worst-case short circuit within IPT.”
Demonstration tests of 14 IPTs on 9 full-scale static SRM firings have shown that hazardous battery conditions are not created under normal motor operation.
The final major hazard considered was “loss of igniter sealing functions” by causes related to O-rings, contamination, and sealing surfaces. Again this is classified the same as for STI OPTs (Controlled) since they share identical designs in these areas.
L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.