STS-125/400 Single Pad option progress – aim to protect Ares I-X

by Chris Bergin

Managers are closing in on facilitating a July launch date for Ares I-X, via a Single Pad option for STS-125’s mission to service Hubble and the supporting STS-400 LON (Launch On Need) shuttle. The plan would free up Pad 39B for transition to Constellation, but likely delay STS-125 by a few weeks as a result.


The plan was previously proposed and rejected last year, before a series of delays impacted on STS-125’s launch date. Currently targeting mid May, STS-125 is waiting on test results and schedule timelines for the delivery of additional replacement Hubble hardware to Florida, due to last year’s anomalies on the aging space telescope.

NASA have the contingency of advancing STS-127 into the May launch slot on the schedule, should the replacement hardware fail to achieve delivery milestones – which is the only scheduling issue for STS-125.

Atlantis, tasked with carrying out the final servicing mission to Hubble, was rolled back from Pad 39A, following on orbit issues with Hubble. After being de-stacked, she has been patiently waiting in her Orbiter Processing Facility (OPF-1) for the realigned schedule, which will soon undergo evaluations.

“Looking forward to hosting manifest discussion next Thursday,” noted NASA HQ on the latest Shuttle Stand-up/Integration report on L2. “Will be an open discussion among themselves, ESMD (Exploration Systems Mission Directive) and science community concerning all leans and activities putting demands on Shuttle manifest.”

No realigned launch dates will come out of the meeting, with the STS-125 launch date decision still believed to be a number of weeks away.

“Not looking to make any decisions,” added the note, “just laying everything out for Bill Gerstenmaier (NASA Associate Administrator for Space Operations).”

The importance of the launch date relates to the mission’s rescue support – which will be provided by Endeavour as STS-400 – should the May target hold – and the ‘disruption’ to Constellation’s plans for Pad 39B, which Endeavour is currently expecting to take residence of in May.

As seen before the previous delay to the STS-125 mission, Endeavour – as the LON shuttle – was rolled on to Pad 39B, for the contingency of launching within days of a serious problem with Atlantis on orbit.

This is required due to the unique nature of a Hubble servicing mission, where Atlantis would not be able to provide her crew with the “safe haven” of the International Space Station (ISS).

Instead, Atlantis would undego a level of powerdowns, and become a lifeboat for her crew, prior to the launch of Endeavour with a four crew subset – which would head to rendezvous with Atlantis, where the two orbiters would grapple via their robotic arms, prior to a transfer of the STS-125 crew to the rescue orbiter.

The single issue with Endeavour being sat on Pad 39B, for the highly unlikely requirement of launching on a rescue mission, relates to the pad needing to remain active for the Shuttle Program, as opposed to pushing forward with its modifications for the Constellation Program (CxP), for the summer test launch of Ares I-X.

Work has already been taking place for CxP at 39B, with nearly a half of the construction work on the three 600ft Lightning Protection Towers now completed for Ares I. Ares I-X requirements include the use of a Mobile Launch Platform (MLP), but also – specific to the pad – the construction of a extension tower on top of the Fixed Service Structure (FSS).

That extension will provide protection against lightning strikes for Ares I-X when sat on the pad, but is not required for any other test launch – with Ares I-Y not scheduled until 2011. Ares I-Y will use the three giant towers as protection.

Ares I-X pad modifications cannot take place at the pad until after STS-400 rolls off the pad, ahead of Endeavour’s change to her primary STS-127 mission, which will launch off Pad 39A. 

This current scenario, based on a May launch date for STS-125 – and STS-400’s supporting role from Pad 39B – is likely to result in making July an impossible target for Ares I-X. An October schedule is deemed more realistic, which is undesirable for Constellation, due to their requirement to gain test data from Ares I-X as part of their development cycle as early as possible.

To aid Ares I-X, and potentially bring the schedule back to July, the Space Shuttle Program (SSP) managers are looking to help CxP by re-evaluating the use of the “Single Pad” option, previously known as the “P-t-P” (Pad to Pad) plan.

The Single Pad plan is complicated, based on tight processing timelines and requirements, mini pad flows, and a large amount of planning to ensure no short cuts are risked with the vehicles.

Previously thrown out as an option, managers are taking a second look at the plan – available to download on L2 – which would follow similar requirements, but with a number of get-wells to make the option “more feasible”.

The plan would see Endeavour – as STS-400 – rolling out to Pad 39A, conducting a pad flow up to  and including the loading of hypergolics into the orbiter’s systems, rolling back to the Vehicle Assembly Building (VAB), and then allowing Atlantis STS-125 to rollout out to 39A to conduct a full pad flow towards launch.

In the event of a LON emergency, Endeavour would roll out as soon as possible – based on pad turnaround – and prepare to launch for a link-up with her striken sister.

“KSC plan would be to process half of the pad operations for the rescue flight,” noted the original presentation. “(Endeavour) can roll out before HST and process half of it pad operations stopping at Hyper Loads. It would then return to the VAB freeing up the pad for HST.”

Such a scenario would result in a two week delay to the STS-125 launch date, based on the cryo capability of Atlantis in lifeboat (power down) mode and when Endeavour could launch on a rescue mission.

“This is less efficient for the overall manifest since both the rescue flight and HST will be waiting on a pad for processing,” added the presentation. “The parallel work for Pad OPS that is possible with two pads is not possible. The pad flows become a serial impact, resulting in a 14 day slip to HST.”

The gap between Atlantis launching, finding a serious issue – most likely via serious Thermal Protection System (TPS) damage during ascent or a serious MMOD (Micrometeoroid Orbital Debris) strike on orbit – powering down to conserve cryos (the lifeblood of an orbiter), and Endeavour arriving to rescue the crew is the most important consideration.

This was one of the major reasons the Single Pad plan was previously dismissed, with scenarios of an “Apollo 13 style extreme powerdown” to allow Atlantis to ‘stay alive’ until a viable limit of Flight Day 25 if required, should Late Inspections near the end of the mission discover a serious, unrepairable MMOD strike.

Numerous mitigation options to protect an orbiter from MMOD strikes on a Hubble Servicing Mission are in place, and recent External Tank performance from a foam/ice liberation threat have been hugely successful, which has decreased the likelihood a rescue mission would be required. That will likely weigh heavily on the re-evaluation of the Single Pad option.

“For the scenario where Late Inspection identifies MMOD damage that cannot be repaired, an “Extreme Powerdown” is required to extend Orbiter stay out to 25 days (from FD10 to FD25) in support of Single Pad Operations,” added the original presentation.

“There are concerns about the viability Of the extreme powerdown configurations (Apollo 13 scenario) and the capabilities lost. Loss of Attitude control/disposal capability due to Propellant Systems that have been powered off:

“HST Orbiter Attitude control (for simpler grapple operations). Eliminates near term disposal options. Compounds Rescue Mission challenges. Loss of required Orbiter cooling (Freon Loop, Water Loop, Interchanger HX) to LRU (Fuel Cell, etc.). Fuel Cell operations at the very low power levels that may be required are not “certified” (voltage, cooling, etc.).”

From a timeline standpoint, the difference between Single Pad and Dual Pad options – according to the original presentation – changes the ability to launch Endeavour on a rescue from three days from LON call-up (Dual Pads) to 15 days after LON call up (Single Pad).

“Dual Pad: Pad 39B retained in configuration that supports Shuttle Launch Capability till post STS-125. Provides capability for LON launch as early as three days after call-up,” listed the presentation.

“Single Pad: Pad 39B not required for support, and is transitioned to Constellation per current plans. LON ready to be launched from pad 39A 15 days after SM4 launch.”

Numerous other factors also have to be taken into account and deemed as acceptable risk, should managers decide to go with the Single Pad option. Examples such as scrubs of the rescue mission launch, and the reasons for the serious damage to Atlantis – such as Pad related debris that may risk similar damage to Endeavour – are shown as examples.

Delays to the launch of the LON rescue mission for reasons such as unacceptable weather were also included.

“88 percent of launch attempts have delays of 6 days or less (92 percent for 7 days) Longest scrub was 6 months (STS-35 H2 leak). Pad Abort (Scrub after SSME start) leads to a 21 day delay (4 occurrences). 95 percent of launch attempts have delays of 11 days or less due to combined weather and non-weather

“99 percent of launch attempts have delays 6 days or less due to weather (i.e. almost all of the delays > 6 days are non-weather delays).”

Back to the limits of keeping a damaged Atlantis alive on orbit during various requirements of powerdown, the presentation outlined LON call up times based on Flight Day 4 (TPS inspections) and Flight Day 10 or 11 Late Inspections, noting plans are in place that will allow for a viable crew rescue up to Flight Day 25 of STS-125 during a powerdown emergency situation in the “best case” scenarios.

“The LON call-up can occur on Flight Day 4, Flight Day 10 or Flight Day 11 depending upon the failure scenario,” added the 37 page presentation on L2.

No matter when the call-up the HST Orbiter has 100 percent success out to 25 days – Best Case. For FD4 HST Orbiter has 100 percent success out to 25 days, For FD10 HST Orbiter has 100 percent success out to 20 days, For FD11 HST Orbiter has 100 success out to 16 days

“For FD10 scenario initial powerdown is assumed to occur on FD4, with further powerdown on FD10. For FD11 scenario initial powerdown does not occur until FD11. Launch opportunities available every day.

“Following a successful launch and ascent, it takes two days to be in position to begin crew transfer operations. Crew transfer operations require one day crew transfer can start as late as the end of the 25th day.”

The current stage of evaluations into approving a Single Pad option – solely designed for the purpose of protecting Ares I-X – is focusing on the impacts of Payload Bay Door (PLBD) drying requirements, which is an area for discussion due to the difference between an orbiter being housed in the VAB, as opposed to the environmentally controlled comforts of an OPF, and protection at the pad.

This becomes a factor when two stacks are in the VAB, as will be the case in the STS-125/STS-400 scenario, waiting for their rollout order for Pad 39A, because when the stack is at the pad the orbiter payload bay doors are protected by the humidity controlled environment of the Payload Changeout Room (PCR). This would be the situation for the dual-pad LON option.

For single pad operations, the LON stack will initially go out to the pad for the first part of pad processing, but return to the VAB and wait whilst STS-125 is rolled out, processed, and launched, meaning less time under the environment controls of the OPF and pad.

“A discussion concerning single versus dual pad operations was held, along with the topic on how PLBD drying requirements may be impacted,” noted a Shuttle Stand-up/Integration report note last week.

“Regarding LON needs for upcoming HST mission, the doors must be at a relative humidity level of 60 percent at launch. Analysis of the mission trajectory thermal values may allow levels up to 70 percent, making Single Pad operations much more feasible.”

Should all safety and engineering requirements continue to provide confidence in a final Single Pad option being taken, work would start almost immediately with the handover of Pad 39B to Constellation.

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.

Related Articles