SSPTS allows for STS-118 mission extension – Arcjet to aid tile tests

by Chris Bergin

The successful utilization of the Station-to-Shuttle Power Transfer System (SSPTS) has allowed shuttle managers to officially extend STS-118 by three extra docked days.

The news came on Flight Day 5, which also saw the use of the OBSS (Orbiter Boom Sensor System) on a Focused Inspection, to gain extra data on whether the damaged area of Endeavour’s TPS (Thermal Protection System) requires on orbit repair, ahead of test article testing at the Arcjet facility.

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Boeing’s SSPTS provides electrical power to the orbiter main busses from the International Space Station (ISS), allowing the fuel cells to operate at lower power levels, thereby consuming less reactant from the PRSD (Power Reactant Storage and Distributation) storage tanks.

Only half of the capability of SSPTS is being utilized during STS-118, its debut flight, as it becomes a permanent fixture on both Endeavour and Discovery. Prior to flight, Fuel Cell Output Limits, were amended to allow a fuel cell to operate at a power level below 2 kW when operating with an SSPTS, and at 0 kW load when the fuel cell is disconnected from the main bus and in standby.

‘SSPTS activation was also completed yesterday with three of four OPCUs (Orbiter Power Converter Units) checked out successfully per the plan,’ noted an ISS On Orbit Status report. ‘For the first time in history, the ISS is transferring power to the orbiter to allow it to remain docked longer.

‘OPCUs 1A, 1B, and 2A are currently active, transmitting up to 6.45 kW of power to the Shuttle PTUs (Power Transfer Units). Two of the OPCUs were deactivated Saturday morning, prior to locking the S4 BGA (Beta Gimbal Assembly) on EVA-1, to maintain ISS power balance.’

**UPDATED: Ride home through the fire, sparks and plasma of re-entry with Atlantis, Discovery and Endeavour. FOUR Stunning high quality 2hr, 355-400mb Camcorder and HUD videos – from payload bay closure to post landing – several more videos showing landing from 90,000 ft also available**

The successful checkout of the SSPTS is a major milestone of the mission, especially in regards to the planning of future missions involving Endeavour and Discovery, with the added flexibility of mission extensions.

‘Once again, another great day in space,’ said Matt Abbott, STS-118 lead shuttle flight director. ‘Both vehicles continue to perform very, very well. Things are going great.

‘The SSPTS, that we’ve been operating for the past couple of days, is still behaving just perfectly. We’ve been operating three of the four power converters (OPCUs) for the past couple of days and today (Sunday) we tested out the fourth converter – and that one checked out perfectly.

‘As a result of that, the Mission Management Team (MMT) gave the go-ahead to extend the mission by three days – so we’re now one third of the way through our mission on Flight Day 5.’

The mission extension for STS-118 automatically adds a fourth EVA to the mission, while also removing timeline constraints for Endeavour astronaut Barbara Morgan, who will now be able to conduct live link ups with a number of American schools, an ‘educator in space’ legacy was initially tasked to Christa McAuliffe – who Morgan was the back-up to – prior to the loss of Challenger.

‘After the MMT reviewed the consumables status, and the status of the orbiter, we had a unanimous agreement that executing the pre-flight plan by the operations team to extend the mission by three days – and add a fourth EVA – was acceptable,’ added deputy shuttle manager John Shannon.

‘I have to say that the team that put together the SSPTS really did an amazing job. There have been no glitches, no concerns at all. It’s performed better than advertised.

‘As a matter of fact it has provided the shuttle program with a capability to support the station even better, by not only having the full mission we expected to have, but we also anticipate that we’ll be able to transfer additional oxygen that we will not need for the Fuel Cells – so we’ll transfer that over to the station’s airlock tank.

‘It has given us a new tool. It will provide us with additional on orbit capability while docked to the station, while being able to replenish their oxygen and water systems.’

Flight Day 5 also completed the objective of a Focused Inspection on the damaged tile area near Endeavour’s starboard main landing gear door.

The use of the OBSS on the end of the shuttle’s robotic arm allowed the gathering of data that gave the MMT a better insight into the depth of the impact – caused by a liberated piece of foam from th 17 inch LOX feedline, that bounced off the External Tank’s strut and hit the orbiter during ascent.

Five different targets were looked at in the region of the damage, with the OBSS creating a 3D map for NASA to evaluate on the ground. That data will be placed into a computational model, that can accurately predict thermal boundary levels for the areas of damage during re-entry, thus aiding the decision on whether a repair needs to be carried out.

All other areas have been cleared as no concern, with further evaluations on the main area – the actual impact point of the foam at T+58 seconds during ascent – which will include the utilization of Johnson Space Center’s Arcjet facility – otherwise known as NASA’s Atmospheric Reentry Materials and Structures Evaluation Facility, successfully used during STS-117 for evaluating the correct repair method on Atlantis’ OMS Pod’s protruding blanket.

‘We’ve looked at the early Focused Inspection data, and as we thought, it might be a fairly deep gouge,’ added Shannon. ‘The tile is 1.2 inches thick and the gouge goes pretty much through the entire thickness of the tile. (We can also see a) small area of the filler bar – which is about 0.2 inches thick.

‘So this is the data we were looking for. Not only did we get really great imagery, but we got a good laser scan of it as well. The process now is to make a point cloud map of the exactly what that damage is, and we’ll do two things with that map.

‘The first is we’ll be able to model it on our thermal analysis models to understand what the actual thermal impacts will be for damage of this type. The second is a test method, using a two foot by two foot sections of tiles, and we’ll take that cloud map and actually mimic that same damage on the test articles, (before using) them in the Arcjet facility here at Johnson Space Center and simulate re-entry conditions.’

Thermal analysis models are expected to be completed on Monday, with the simulated re-entry testing on the test articles to take place over both Monday and Tuesday. The MMT will make a decision on whether to repair the tile around Tuesday evening.

Meanwhile, Flight Day 6 will involve the second spacewalk of the mission (EVA 2) with Rick Mastracchio and Dave Williams, installing the new Control Moment Gyroscope 3, as a replacement on Z1 Truss – along with the deactivation of the External Stowage Platform 3 (ESP 3) power connection. Cargo transfers between Endeavour and the ISS will also take place.

L2 members: Vast amounts of unreleased NASA documentation for STS-118 available to download in the L2 STS-118 Special Section, updated live.


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