SLS on track to CDR as engineers work though a baffling issue

by Chris Bergin

As the Space Launch System (SLS) heads on to the path towards its Critical Design Review (CDR), engineers are close to mitigating an issue relating to the core stage’s slosh baffles. The work on the baffle design and placement is part of the stability analysis for the HLV’s ride uphill.


In the latest milestone to end 2012 – the Preliminary Design Review (PDR) – SLS passed all 12 key criteria, according to information provided to L2, with only five “yellow” risks, all of which are expected to be addressed. In other words, SLS is suffering from zero critical design or schedule threats at this time.

This is a refreshingly positive overview when compared to the numerous “red” issues suffered by Ares 1 at this same stage of development.

One would hope for such an outcome, with the vehicle only just beginning its second full year of development. However, these early years are always the most critical, as teams literally draw the templates that will go to the giant machines at Marshall and Michoud for the physical cutting, bending and welding of metal that will piece together the monster rocket.

SLS Core via TerraBuilder, Inc and L2Marking the current state of progress for the end of 2012, the vehicle’s core stage PDR brought together NASA, Boeing and other contractors, with the complete concurrence SLS is ready to proceed towards the next key stage – known as the Critical Design Review (CDR).

“Passing a preliminary design review within 12 months of bringing Boeing on contract shows we are on track toward meeting a 2017 launch date,” said Tony Lavoie, manager of the SLS Stages Element at Marshall.

“We can now allow those time-critical areas of design to move forward with initial fabrication and proceed toward the final design phase – culminating in a critical design review in 2014 – with confidence.”

Issues during development are expected, not least because SLS will be the most powerful rocket set to leave the planet. However, the highly experienced SLS team are yet to come across a problem that mirrored those that blighted the Constellation Program.

Slosh Baffles on Shuttle ET - via L2One such example of an issue being mitigated is the slosh baffle design on the core stage – an element of hardware that helps alleviate the liquid propellants from “sloshing” around in the tanks as they feed the engines, a condition that can lead to instability of the rocket during its launch.

Boeing – the contractor for SLS’ core – completed the slosh baffle design for the core stage tanks in the latter part of 2012. Based on past experience, Boeing’s design featured a relatively low number of baffles to save production time and cost, and to reduce weight.

However, due to the complex nature of how slosh baffles impact on flight, Boeing requested SLS’ Guidance, Navigation and Control (GNC) group to look at the effect, if any, on the stability of the control system with their design in place. The result was a significant drop in stability margins, resulting in Boeing having to go back and rework their slosh baffle design.

At the time, the concern relating to a redesign was the potential impact on SLS’ schedule, resulting in the delay on delivery of the first flight core stage to KSC by around two months – eating into the six months or so of current margin.

“The Design Analysis Cycle-2 (DAC-2) Flight Control System (FCS) design and stability analysis for ascent flight has been completed, and was presented for internal review,” noted SLS information via L2’s rolling update section. “Included in the analysis was the re-evaluation of slosh damping in the Core Stage Liquid Oxygen (LOX) tank required to achieve the desired stability margins.”

SLS Block 1 via L2The notes add that the potential for easing the usual conservatism that NASA engineers tend to use in their analysis may allow for some middle ground for the baffle design in the final area of interest at the top of the LOX tank dome – allowing for Boeing’s goal of reduced complexity and weight to be realized after all.

“The damping profile for the LOX tank was updated from previous, post-DAC-1 analyses, which was used by Boeing to place and size seven ring baffles. The damping profile update differs slightly from the previous one, but remains within damping capability of the baffles, except for a small region in the upper dome area.

“The damping requirement in this area is under review to see if some conservatism can be safely removed in order to meet the capability. If so, this may allow Boeing to move the uppermost baffle back to the upper barrel section and out of the dome, which would save weight and reduce design complexity.”

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Other areas of concern that have been highlighted over recent months include a mis-match of an upper umbilical plate between Boeing’s design and the Ground Systems Development and Operations (GSDO) Program. However, a quick decision was made on the go-forward design for this element of hardware ahead of the PDR.

There was also a note of a last-minute concern relating to PDR mass properties coming in above the requirement. This was resolved when a SLS team opted to work through a weekend to identify enough mass savings options to get back in the box, resulting in no impact to the positive PDR overview.

(Images: Via NASA and L2 content from 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.)

(L2 is – as it has been for the past several years – providing full exclusive SLS  and Exploration Planning coverage.  To join L2, click here:

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