Engineers begin removing orbiter MPS components for donation to SLS

by Chris Bergin

Kennedy Space Center engineers have begun removing Main Propulsion System (MPS) hardware from the aft of the retired orbiters. The flight-flown hardware – a natural match to the RS-25Ds the SLS will initially fly with – will live on with the monster rocket, as much as some of the orbiter hardware will be focused on the test program side of SLS’ development.


The MPS relates to the powerhouse in the aft compartment of the vehicle, aiding the acceleration from lift-off of an orbiter to Main Engine Cutoff (MECO) – the phase of ascent referred to as “powered flight”. As such, the Integrated MPS consists of the three RS-25Ds, the External Tank (ET), a propellant management system used to transport fuel and oxidizer from the tank to the engines, and a multi-purpose helium system.

For the Heavy Lift Launch Vehicle (HLV) – currently named the SLS – a large section of the External Tank design will be translated into the core stage, becoming part of the in-line rocket. All of this heritage is being fed directly from the Space Shuttle Program (SSP) into the SLS program.

The SLS will utilize between three and five RS-25s, initially those which have flown with the Shuttle – known as RS-25Ds. Once the 15 engines – which are currently enroute to Stennis Space Center (SSC) – have been used, SLS will begin using RS-25Es – an expendable version of the super-reliable Pratt and Whitney Rocketdyne (PWR) system.

As revealed by this site, the all-powerful Program Requirements Control Board (PRCB) concurred with the SLS program’s wish of using the hardware from the orbiter’s MPS, which will provide several benefits, from a flight-proven standpoint, through to saving money for the test phase of the HLV’s development.

“There has been a request to removed the entire MPS (Main Propulsion System) from the orbiters for SLS (Space Launch System),” noted PRCB and KSC Processing notes at the time (L2 Link to MPS-SLS presentations).

“Potential Cost and Schedule mitigation option for SLS. MPS component development can pace the overall core stage schedule. Retaining and utilizing SSP MPS hardware can have large initial cost savings.

“Beyond the existing Transition & Retirement (T&R) plan: Acquire high value items from the Orbiters, LRU (Line Replacement Unit) spares, GSE, Tooling and Documentation. SLS is needing hardware to support a test program and first two flights. Core Stage Option Utilizing Shuttle MPS. RS25 Engines. MPS. TVC (Thrust Vector Control) and Avionics.”

The Orbiter MPS includes major hardware items such as the Propellant Management System (PMS). The MPS PMS consists of manifolds, distribution lines, and valves that transport propellants from the tanks to the three main engines for combustion, and gases from the engines to the tank for pressurization purposes.

In addition to its primary function of feeding propellants from the External Tank to the engines during powered flight, the PMS also controls the loading of propellants before launch, the post-MECO propellant dump and vacuum inerting.

The removal of this hardware inside the aft compartments of the orbiters involves disconnecting major hardware – such as the Propellant Feedline Manifolds, which consists of 17-inch and 12-inch piping – through the three spaces left vacant by the removed SSMEs and access doors on the side of the aft.

This work has now begun on Discovery, as confirmed by Orbiter Flow manager Stephanie Stilson during an interview with’s Philip Sloss, during Atlantis’ move from OPF-2 to the Vehicle Assembly Building (VAB).

“I don’t like seeing her in pieces, because that is not how I envisioned her to be,” opened Ms Stilson, as Atlantis was pushed out of OPF-2 with all her propulsive elements missing. “But it’s all part of the process.”

That process – known as Transition and Retirement (T&R) – is mainly focused on preparing the orbiters for their retirement homes. However, as confirmed by Ms Stilson, even Atlantis’ vacation in the VAB will include the opening work on removing parts of Atlantis’ MPS, a process which has since begun on Discovery.

“We have started (removing MPS hardware) from Discovery just recently, but that is one of the major tasks we’ll be doing inside the Vehicle Assembly Building with Atlantis, which is pulling those major components out of the aft.”

Items being removed include the feedlines – vacuum jacketed for H2, insulated for O2 – Fill and Drain (F&D) lines, recirculation lines (H2), and gaseous H2 and O2 lines – which are used to maintain pressure in the ET – via more well known items of hardware such as the Flow Control Valves (FCVs).

The FCVs were highlighted during an investigation into a small liberation from one of the valve’s poppet’s during STS-126.

Mitigation procedures – which included screening of flown valves post-flight at the fabricator Vacco – resulted in no further issues.

Click here for numerous articles on the FCV issue since STS-126.

These are all natural elements of hardware which would provide both the SLS core and the SLS engines with the role they had previously enjoyed with the orbiter, such as the FCV-related Ullage Pressure System (UPS) – which deals with the volume in the LH2 and LO2 tanks not occupied by liquid propellant.

The ullage pressure system consists of the sensors, lines, and valves that are used to collect gaseous propellants (gaseous hydrogen and gaseous oxygen) from the three main engines; the system supplies the gaseous propellants to the External Tank to maintain propellant tank pressure during engine operation, as well as maintaining tank structural integrity.

Propellants must be supplied to the SSME with adequate head pressure for proper engine operation.

Also being removed is the MPS helium system, which consists of storage tanks, distribution lines, regulators, and valves that supply helium to the main engines and the MPS PMS.

The helium supply tanks consist of three large (17.3-cubic-foot) and seven small (4.7-cubic-foot) helium tanks known as Composite Overwrap Pressure Vessels (COPV).

Each large tank is plumbed to two of the small tanks to form three clusters. Each cluster provides helium to one of the main engines. The remaining small tank is the pneumatic helium supply.

Part of the removal process was based on plans created in the event Atlantis and Endeavour had continued flying – as much as these plans were prior to the recent private investment-driven effort – with Discovery retiring to become a parts donor to her younger sisters.

Discovery was already certain of retirement after STS-133, given she was due for her lengthy Orbiter Modification Down Period (OMDP), something which wouldn’t of been viable without her looking forward to numerous follow-on missions.

A large amount of evaluation – along with a high level of protection – will be provided on all MPS hardware being removed, which had previously been specific to Atlantis and Endeavour, but now sees – as initially hoped – all three orbiters are officially donating to the SLS program.

The removed hardware, which will mainly be taken out of the orbiter via side access hatches on the aft of the vehicles, will be treated as flight hardware, and carefully handled and stored as a result.

“All SLS-hardware is to be maintained ‘flight like’. The term ‘flight like’ is defined as follows: Parts will be maintained visibly clean, appropriately handled/transported, and maintained in good working condition. Part cleanliness shall be maintained using best shop practices: Cleanliness will be maintained via work in an environmentally controlled atmosphere,” added PRCB documentation.

This relates both work in the OPF – requiring the maintaining of purges as appropriate, using double clean bags/tape/caps/plugs to secure open ports/lines, and undertaking a best effort to maintain an acceptable level of cleanliness using field expedient techniques if work is to be performed in an uncontrolled environment, such as the Vehicle Assembly Building (VAB), where Atlantis will have the opening work performed on her.

All of the removed MPS hardware is being placed into storage at the Assembly & Refurbishment Facility (ARF) – which also includes the large GSE (Ground Support Equipment) used on the orbiter MPS – currently located in the NSLD (NASA Shuttle Logistics Depot) at KSC.

The ARF is under the control of the Marshall Space Flight Center (MSFC), while some hardware will also take up residency at KSC’s Logistics Warehouse.

“Remove high value MPS, TVC and Avionics hardware from the aft compartment of orbiters prior to sending them to museums. Package, transport and store the hardware in the Assembly Refurbishment Facility. Floorspace is available to store hardware in an environmentally controlled area,” the PRCB overview continued.

While the opening SLS missions will launch with RS-25D which had previously flow with the orbiters, it has not yet been decided if SLS will launch with new – albeit very similar – MPS components.

Documentation shows interest in launching SLS-1 and SLS-2 – both set for missions to the Moon – with orbiter MPS hardware. However. sources note a lot of the hardware will be used by the test program, which will utilize hardware on the ground.

However, with all three orbiters donating, there remains the possibility that SLS will make its debut launch not only with RS-25Ds that had previously powered an orbiter safely uphill, but also with part of an orbiter’s guts, which served the vehicles so well during their careers.

Images: Via L2 content, driven by L2’s fast exapanding SLS specific L2 section, which includes, presentations, videos, graphics and internal updates on the SLS and HLV, available on no other site. Other images via Philip Sloss, and NASA.)

(L2 is – as it has been for the past several years – providing full, exclusive SLS coverage, available no where else on the internet. To join L2, click here:

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