Advanced Boosters progress towards a solid future for SLS
The giant boosters that will help shove NASA’s new monster rocket uphill are hitting milestones on two fronts. As Orbital ATK prepare for next month’s Qualification Motor -1 (QM-1) test firing of the five segment solid motor, teams are also testing the technologies required for the Advanced Booster – a motor that is set to provide SLS with extra upmass capability from the mid 2020s onwards.
The previously delayed QM-1 test firing now remains on track for March 11, still well within the critical path schedule for the debut launch of SLS in 2018.
The five segment version is a direct descendant of the four segment motor that pushed Space Shuttles through their first stage flight – boosters that were continually upgraded and improved right through to their final mission with STS-135.
With the QM-1 motor now anchored into the ground at Orbital ATK’s test facility in Promontory, Utah – preparations for its big, noisy day included final checks on the flight-like avionics system that will be tested during the firing.
While the test will produce a huge amount of data on the booster’s performance, the validation of the avionics system – responsible for igniting, steering and jettison of the boosters – will be critical.
“We are designing a system for a human-rated vehicle that has to be at a minimum single-fault tolerant, which means no one failure on a critical system can result in a big problem for the mission,” noted Eric Corder, avionics system manager for the SLS Booster Element at MSFC.
“We don’t want the rocket to just operate the way it’s supposed to. Our team intentionally implements failure scenarios to the electronics to make sure, for example, a shorted circuit or faulty box doesn’t compromise mission success. That’s even an issue that may have a one-in-10,000 chance of occurring.”
The heritage of mitigating potential issues during first stage flight was seen right through to the final Shuttle missions – such as the redesign of the Thrust Vector Control (TVC) Auxiliary Power Unit (APU) fuel pump ahead of STS-133.
This design change eliminated the highest – albeit extremely unlikely to occur – “critical 1” failure scenario in the system, which held the potential of a LOV/C (Loss of Vehicle/Crew) event.
Ultimately, in tandem with improvements to numerous elements of the Space Shuttle system – such as the External Tank modifications, the final flights were the “safest” in Shuttle history. Post Flight IFA (In Flight Anomaly) documentation (L2) confirmed the final flights of the Shuttle Reusable Solid Rocket Motors (RSRM) performed extremely well.
With the Space Launch System (SLS) utilizing a large amount of Shuttle hardware, the heritage of the safety and performance improvements will naturally migrate into the monster rocket.
The five segment booster – a direct descendant of the four segment motor – is expected to fly into at least the mid-2020s, with both the 70 mT Block 1 and the 105 mT Block 1B variant – with the latter expected to become the SLS workhorse.
Early SLS manifests pointed to a switch to using the Advanced Booster in 2024, on a cargo mission – although it is likely this will now take place in the second half of the 2020s.
The Advanced Booster drive became a competition, at the request of political powers, leading to NASA’s SLS Advanced Booster Engineering Demonstration and/or Risk Reduction (ABEDRR) procurement process.
That in turn resulted in some interesting options for providing SLS with additional power for its eventual role of lofting large payloads into deep space.
In 2012, Pratt & Whitney Rocketdyne (PWR) – now Aerojet Rockeydyne – went straight into “beast mode” with a proposal to utilize F-1 powerhouses from the heritage of the Saturn era, resulting two liquid boosters that could enable SLS to power 150 mT to orbit.
Alabama company Dynetics was brought on board to mature the design that saw two modernized F-1 engines – known as the F-1B – paired on each booster.
Internally, SLS engineers cited they were having issues with all versions of liquid boosters, claiming high acceleration forces were problematic for the stack, while a completely new Mobile Launcher design would be required for such a vehicle.
It was also claimed that KSC Ground Operations were informed in the middle of 2014 that the boosters for SLS will be solids throughout the evolution of the vehicle. All work to revamp SLS’ Pad 39B pad – including the new flame deflection and trench design – have the solids at the center of their evaluations.
As such, Orbital ATK’s Advanced Booster proposal is the clear favorite to take over from the current five segment motor.
These new super boosters – nicknamed “The Dark Knights” – would be a large evolution on the current motors, both internally and externally.
Moving away from the traditional steel casings, a switch to composite materials, fabricated from low cost, high strength fibers, provide a documented payload capability improvement of 4,128lbm.
With the move back to four segments, the simplified stage assemblies comparison cites this would become part of a saving of 480 man hours – resulting in a 50 percent reduction in the man hours compared to that required for the five segment motor.
ATK claimed in their documentation that the booster would be 40 percent less expensive and 24 percent more reliable than the current SLS booster.
Testing of the composite materials picked up the pace in 2013, with ATK successfully completing the filament winding of a pathfinder Advanced Booster composite case – resulting in a 92-inch-diameter, 27-foot-long composite structure.
At the time, ATK – as they were then known – confirmed they overcame challenges related to the affordability and performance required of an Advanced Booster during the case winding operations for the test article.
The composite testing achieved another milestone this year, when engineers used a 25 feet long and 92 inches in diameter composite case in a “burst test” – subjecting it to 3,000 pounds per square inch of pressure.
*Click here for video of the test*
“The test is very dramatic,” noted Angie Jackman, of the SLS Spacecraft/Payload Integration and Evolution (SPIE) office at MSFC. “When composites fail, it’s the glue or the resin that fails first – not the fiber that fails. There’s a big boom, and it’s all spaghetti.”
The test showed the casing performed as expected, failing well above the pressures associated with its role during a launch. It was even purposely damaged at multiple points on the casing to study what effect it would have on how the casing performed.
Internal to the booster, technicians that are part of the Propellant/Liner/Insulation (PLI) Integrated Product Team (IPT) have been working on the all-important propellant mix, cooking up recipes for increased performance to aid SLS during first stage flight.
Back in 2013, work involved testing 66 unique propellant mixes to test candidate propellant formulations for burn rate performance and mechanical property characteristics.
The team worked on selecting the best formulation within each design of experiments (DOE) matrix family to pursue further testing within larger scale mixes.
Details of the favored mix is likely to have been made proprietary – although previous schedules have shown this winning mix is set for a test firing as earlier as this year.
Also under evaluation is a nose cone design change, with wind tunnel testing as early as 2013 showing engineers have been looking into solutions for an aeroacoustic loading issue (vibrations) – in the transonic flight region – impacting on the booster/core attach points.
Initial information pointed to potentially changing the nose cone design to that seen on boosters used by Ariane 5, or indeed the cone design currently portrayed by ATK’s Advanced Boosters, in order to mitigate the issue.
Currently, representations of the evolved Block 2 SLS show the nose cones as portrayed on all variants of the monster rocket. However, they also show an all-white vehicle, which is not accurate. The core of SLS won’t be painted white.
However, Orbital ATK’s fully evolved Block 2 SLS renderings show the nose cose design in place, albeit via graphics that are tagged as notional.
It is also unclear whether SLS will require to switch back to an original plan of adding a fifth RS-25 engine on the core, in order to achieve the 130 mT requirement laid out by lawmakers. Currently, all variants of SLS have been baselined with four RS-25s on their cores.
Also, SLS evaluations continue to show “over performance” against the baseline 70 mT and 105 mT ratings, which have been further aided by the future incorporation of the powerful Exploration Upper Stage (EUS).
It has been claimed internally, that the Block 1B, with advanced boosters and EUS could achieve most – if not all – of NASA’s deep space exploration goals, potentially removing the need to evolve to the Block 2.
There is also an interesting future scenario based on when the Block 2 is expected to come on line, which is not until the 2030s, by which time SpaceX’s more powerful Exploration Class rocket for its Mars Colonial Transporter (MCT) system should already be flying.
(Images: NASA, Orbital ATK and L2 artist Nathan Koga via L2’s SLS sections, 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: http://www.nasaspaceflight.com/l2/)