SLS Program pressing forward with engine heat shield design change
The Space Launch System (SLS) heat shields around its RS-25 engines will not follow the heritage of the Space Shuttle Program (SSP), following a decision to move away from the “eyelid and dome” style design. Instead, SLS will have a lighter flexible blanket installed, similar to that used on other vehicles and on the aft skirt of the Solid Rocket Boosters (SRBs).
SLS Design Change:
NASA’s new Heavy Lift Launch Vehicle (HLV) remains on track for its debut on Exploration Mission -1 (EM-1) to the Moon in 2017, after successfully transitioning from the joint System Requirements Review (SRR) and System Definition Review (SDR) to the key PDR stage of development.
While the design of the vehicle – such as the appearance, or Outer Mold Line (OML) – are all-but baselined into what spectators can expect to see roaring off Pad 39B in the second half of this decade, a large amount of fine-tuning is taking place, via the NASA and contractor engineering teams at the Marshall Space Flight Center (MSFC).
The prime contractor is Boeing, who’s involvement with the monster rocket ranges back to prior to the official announcement by NASA, confirming the SLS configuration as a Shuttle Derived (SD) HLV.
As part of the fine-tuning process, Boeing engineers recently recommended a design change relating to the heatshields surrounding the four engines on the core of the SLS. The HLV will initially launch with four Pratt & Whitney Rocketdyne (PWR) RS-25Ds, donated by the Space Shuttle Program (SSP), prior to moving to the non-reusable version, known as the RS-25E.
As seen with the Shuttle orbiters, a dome of Thermal Protection System (TPS) tiles and a ring of white thermal blankets surrounded each of the SSMEs on the aft, with only the Nozzle visible from the outside. This was known as the “eyelid and dome” hardware, something that had remained relatively unchanged during the life of the SSP.
The “eyelid” is the rigid spherical portion attached to the engine nozzle, whereas the “dome” is the portion attached to the orbiter. The dome has a rigid conical portion with TPS tiles on it and a flexible blanket portion with a seal that is pressed against the eyelid with 48 spring cans.
This was a very complex piece of hardware, requiring a lot of refurbishment between flights, one reason it was usually seen undergoing work on the Orbiter Processing Facility (OPF) floor between flights, during the period the SSMEs had been removed from the vehicle.
While one obvious reason for the change could be assumed to the fact SLS core will not be returning home for reuse – at least removing the re-entry considerations – even the orbiter’s engine heat shield design was driven not by the vehicle’s return, but instead by the launch environments.
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As such, there are a lot of factors that allow this change to take place. Primarily, it is due to the improvement in flexible thermal protection materials over the 40 years since the orbiter was developed, while the move will also save some weight on the aft of the vehicle.
“The Boeing team is recommending replacement of the heritage orbiter ‘eyelid and dome’ engine heat shields with a flexible blanket similar to what is used for the Solid Rocket Booster nozzles and other expendable launch vehicles,” noted one of the numerous updates on L2’s SLS rolling updates section, confirming the change.
“The blanket design will save approximately 700 pounds in weight while being easier to produce, assemble, and install. The blanket will use the same attachment scheme on the engine nozzle and core base heat shield as the heritage design.”
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The flexible thermal blankets will be similar to those used around the RS-68 nozzles on the Delta IV, with additional knowledge available from their use on the aft skirt of the Solid Rocket Booster.
As such, engineers will have a database of understanding on how these blankets perform during launch, including any potential challenges – such as that observed during STS-116’s ascent.
Discovery’s launch was nominal. However, long range trackers – used by NASA’s Systems Engineering and Integration (SE&I) group – provided an amazing close up video (available on L2 – LINK) of the two booster nozzles with their blankets vibrating under the intense atmosphere of the exhaust.
The left booster, however, showed its “curtain blanket” was “flapping” during ascent, after becoming partially detached.
It has been noted by sources that the behavior of the blankets is something that will be assessed to make sure the ignition overpressure (IOP) does not cause the blanket to contact the engine powerhead components.
Testing may also be required if the environment at the base of the SLS core is worse than what the existing materials have been certified for. Even if the thermal environment is not worse, the debris or pressure environment may be a relevant area of evaluation.
The design change will debut during the test firings at the NASA Stennis Space Center (SSC). These firings will be known as “Green Run” testing – given they are carried out with the first two flight articles. Since they are the flight articles, using real RS-25 engines, they will have these blankets installed.
(Images: Via L2′s SLS specific L2 section, which includes, presentations, videos, graphics and internal – interactive with actual SLS engineers – updates on the SLS and HLV, available on no other site. Lead image SLS rendering used used with permission from terrabuilder.com and Boeing.)
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