Low Q option for STS-121

by Chris Bergin

Documents, acquired by this site and dated the April 18, have recommended that STS-121 becomes a “Low Q” mission – the first since STS-103 – to reduce aerodynamic loading on the Shuttle by around seven percent.

Although the introduction notes there are no restraints being implemented into STS-121 flow at present – following the latest series of wind tunnel tests on External Tank modifications – recommendations are being laid out, including the timeline for crew and Mission Control simulation training.

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**You can view the documents by clicking here**

The Low Q approach was originally an option to fix the Orbiter Window issue for High Q mission profiles. However, that is no longer required, given that SE&I has shown that new Orbiter Window constraints hold no issues to Launch Probability for any season (Summer, Transition & Winter) – the document states in its introduction.

However, it continues, the Space Shuttle Program (SSP) still has the option to fly a Low Q design for STS-121 to reduce stress on the overall vehicle.

A few weeks ago, CBS’ Bill Harwood mentioned the possibility of a Low Q mission.

Such an option would be useful in reducing the aerodynamic loading on the modified External Tank, bringing down dynamic pressures (noted by the value psf) to below 735 psf – which was the valve observed during recent wind tunnel testing, resulting in a crack/divot appearing on a modified ice/frost ramp test article, at ‘Station/STA’ xt1528.

According to further information acquired by this site, during that test, one divot formed at the inboard area of the thin wall region of the ramp, causing a 0.2 inch ‘crack’. No problems were noted below the 735 psf number, although, on a further test, during a 60 second run at higher psf values, the ramp failed, liberating a large 3 inch section of foam, exposing one of the two feedlines the ramps surround.

Further tests are required, with the ‘cut down’ ice/frost ramp being reduced in size over the Easter weekend.

STS-121, currently slated for July, falls under the bracket of a ‘High Q Summer Design’ profile, with High Q psf values reaching 760 psf at around mach 1.5. A Low Q mission profile at the same point of ascent is valued at 695 psf.

The Shuttle achieves a Low Q ascent profile by lowering the ‘throttle bucket’ to 67 percent, from 72 percent, at an earlier time of throttle down – holding the lower setting for a longer period as the Shuttle climbs through Max Q. This – the document notes, causes a new thrust vector, and a change in the trajectory.

A 1100 lb performance impact was noted for a Low Q mission profile, as well as a change in abort times: Additional 10 seconds RTLS exposure versus TAL. Additional 15 seconds TAL versus PTA.


On the plus side, the document notes that the Q value is reduced through mach regions. Aerodynamic loads environments are reduced by seven percent. Launch probability for both High and Low Q is 100 percent with all constraints added (Window, High Q & Roll Manoeuvre). All I-loads changes can be uplinked on DOL so additional design cycle not required.

Most importantly, there are no constraints to launch with this change.

As part of the recommendation, it was noted that the orbiter is certified for low Q trajectory designs, should the program desire to fly this option, and the low Q option is more benign from a structural perspective, increasing margins for many parts.

The document also made reference to the on-going tests to the modified External Tank, as it is in the ‘midst of assessing updated PDR environments for LO2 feed line, cable trays, and press-lines so impacts can not be stated at this time but low Q could result in reduced loads.’

Other elements that will need to be worked to change STS-121 into a Low Q profile include SSME Controller Software (S/W) dates. Change flight specific ‘Single Command Channel Shutdown Timer’ I-Load, which are required L-53 days (May 9).

The Crew of STS-121 and controllers at the MCC (Mission Control Center) at JSC, will also require simulation training, currently pencilled in for May 4. Uplink Requirements would follow on May 15.

In conclusion to the recommendations noted, the document rounds up its finding with the claim that Low Q reduces aerodynamic stress on Orbiter, ET area
where PAL Ramp removed and Ice/Frost Ramp redesign, obviously a positive option to aid a clean STS-121.

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