NASA’s three retired Space Shuttle orbiters are set to donate their entire Main Propulsion Systems (MPS) to the opening salvo of Space Launch System (SLS) Heavy Lift Launch Vehicles (HLV). The work to remove the MPS’ array of plumbing, tanks and valves from the aft of the orbiters would result in a delay of six to nine months to the scheduled arrival at their museums.
Shuttle to SLS MPS:
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 Space Shuttle Main Engines (SSMEs), an external propellant tank (ET), a propellant management system used to transport fuel and oxidizer from the tank to the engines, and a multipurpose helium system.
For SLS, the External Tank will be translated into the core stage, becoming part of the in-line HLV. The second stage will ride on top of the core, with the Orion (Multi-Purpose Crew Vehicle) riding on top of the second stage on the crewed version of SLS, replaced by a payload on the cargo version.
Two Solid Rocket Boosters (SRBs) will be attached to either side of the core stage, not unlike the Shuttle stack, as much as these boosters will be larger five segment motors, potentially changing to a liquid version later in SLS’ lifetime.
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As has been noted previously, SLS will utilize SSMEs – otherwise known as RS-25s. The stock of SSME (RS-25D) engines are currently being preserved at the Kennedy Space Center (KSC), after being donated by the Space Shuttle Program (SSP). There are at least three sets of three engines, removed from each orbiter, along with a spare set of three engines (12 engines in total).
Prior to the orbiters going on display at their respective exhibition sites, each vehicle will be fitted with three Replica Shuttle Main Engines (RSMEs) – made from previously scrapped nozzles and installed via an adaptor – which are being fabricated by SSME contractor Pratt & Whitney Rocketdyne (PWR).
Given the adaptor will be installed inside the SSME domes shields, visitors to the orbiters at their exhibitions won’t notice the difference, especially as the nozzles will be real hardware, repaired from their previous scrap designation.
The recommendation to preserve the flown SSMEs for future use was called for by NASA’s MSFC (Marshall Space Flight Center), KSC and JSC (Johnson Space Center), providing free engines for SLS to utilize on its opening flights.
All of the reusable engines will be destroyed when the unrecoverable core stage burns up in the designated disposal corridor, not unlike the shuttle ET.
As seen in the latest SLS documentation, the plan is to move to a cheaper, non-reusable version of the SSME – known as the RS-25E – once the RS-25D stock is exhausted by the SLS flights.
Now SLS managers have requested the use of the major plumbing inside the orbiter’s own MPS system, which is a natural match for the RS-25s.
“There has been a request to removed the entire MPS (Main Propulsion System) from the orbiters for SLS (Space Launch System),” noted KSC processing information (L2), adding such an operation would delay the shipping dates of the orbiters by over half a year.
“This would be six to nine month impact to the T&R (Transition and Retirement) schedule (for the orbiters).”
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.
The PMS is the lifeline of the integrated MPS. 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 would involve 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.
Items which would be removed includes 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 frabricator Vacco – resulted in no further issues.
Click here for NASASpaceflight.com 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.
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.
Although this would all result in a large amount of hardware being removed from the aft of the orbiters, visitors to the exhibitions would not have been able to see – or likely have any access – to the aft compartment, meaning their visual appearance will not be altered.
A final decision on proceeding with this work is expected later this year.
(Images: Via Larry Sullivan, Chris Gebhardt (MaxQ Entertainment/NASASpaceflight.com) orbiter engineering tour video (1000mb) available on L2, plus L2 content – driven by L2′s new SLS specific L2 section, which includes, presentations, videos, graphics and internal updates on the SLS and HLV available no where else on the internet).
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