Thrust Oscillation options outlined for Ares I PDR

by Chris Bergin

Ares I engineers have created a “Plan A” option – which combines three mitigation techniques – to attempt to solve Thrust Oscillation issues during first stage flight.

Plan A involves an active Tuned Mass Absorber (TMA) to the aft skirt, dampers on the interstage, and the crew seat isolators – though during a teleconference to the media, Constellation managers claimed crew seats is a fluid option, referencing only an active TMA system and Interstage dampers as the current forward plan.

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Thrust Oscillation Update:

Thrust Oscillation is not a unique condition in relation to Ares I – but a recognized characteristic of solid rocket motors – in Ares I’s case, the five segment first stage.

Also known as ‘Resonant Burning’ – described as vortices that shed within the solid rocket motors during combustion due to the shearing of internal flow at propellant discontinuities – the issue relates to when the frequency of thrust oscillations is coincident with the acoustic modes of the motor cavity.

The problem relates to the crew riding Ares I inside their Orion vehicle – when pressure oscillations drive resonant modes in the vehicle structure. These oscillations are capable of rendering the astronauts incapacitated, or worse.

Currently, astronauts riding in Ares I would be subjected to G forces in the range of 5-6 G at around 115 seconds into first stage flight. Engineers have been tasked with reducing Thrust Oscillation to the point at which astronauts would not suffer degraded operational capability.

The reduction target is based on historical data that dates back to the Gemini program, which had a crew specification – that allowed for coarse visual, manual tasks and speech to be unaffected – of 0.25 Gs. Between 0.3 and 0.5 G, the crew are restricted to simple visual and manual tasks only – deemed as performance degradation.

Above 3.7 G, and the jack-hammer properties of Thrust Oscillation enters the realm of health risks to the crew. Riding on the space shuttle currently sees the crew endure only 0.1 G.

Many options – some simple, some exotic – have been evaluated over the previous year, with favored options changing throughout the effort. The effort itself is based around the potential to eliminate or reduce Forcing Function, cancel or isolate Forcing Function, or detune stack from Forcing Function.

Ruled out from the mitigation process were concepts that called for a change to the Ares I configuration, including the shortening of the stage to a four segment first stage booster, and changes to the internal ballistics of the solid.

Plan A:

As of this month, ‘Plan A’ involves the interstage receiving what is know as the ‘Passive SoftRide Isolator Concept’ – as seen before with ELV approaches to protecting the payload from the vibrations of launch and ascent, such as with GLAST.

The SoftRide Isolator decouples vehicle structural response from the Forcing Function, thus mitigating Thrust Oscillation on the first stage travelling through the vehicle stack to Orion.

However, there are concerns over the structural integrity of the interstage even prior to the spring dampers being installed. Failure of a few isolators could lead to rapid failure at the interface, sources note, with the system introducing vehicle bending.

Some mitigation of this threat has come via a small design change to the ends of the springs that will be attached in the interstage.

Notes also mention that this concept has a performance impact deemed to be red – though no explanation is given.

The main mitigation approach to Thrust Oscillation on Plan A is called the Aft Skirt TOA (Tuned Oscillating Arrays) Concept, which consists of 16 actuators around the circumference of the bottom end of the vehicle.

This system is replacing the Parasorber TMA concept, which was designed to damp Forcing Function in the area of the first stage parachutes.

However, Plan A lists this as the passive system, whereas CxP manager Jeff Hanley noted an electrically driven system – via 600V Li-Poly batteries – although the concept could still work passively, should a failure occur during ascent.

The Active Aft Skirt RMA (Reaction Mass Actuator) Concept is the specific name for the battery-driven TMA version. With the appearance of a car shock absorber, multiple RMAs are combined in a single control loop known as ‘virtual actuator’ controlling. The RMA concept is shown as the second option on ‘Plan B’ (B2).

Interestingly, a third mitigation technique is called for on Plan A, noted as Crew Seat Isolators – the final approach to finally bring down the Thrust Oscillation X-axis acceleration from 1 G to 0.25G.

While it is possible this concept is only required in tandem with the passive version of the Aft Skirt TOA, the concept may still be required as fallback mode during a potential failure of the electronic RMA system.

Crew seat isolators – which is a 500lb system that calls for a seat/pallet interface into the Orion vehicle – is still on the table, but is currently classed as an immature design, with further work required.

This work would not be completed in time for the Thrust Oscillation status report to the PDR (Preliminary Design Review), which is due to be completed in around a month’s time.

It was noted on the teleconference that there is a desire to remove such a concept from the vehicle – given it will only be used for a few seconds of the flight, and an unnecessary mass impact for missions in total.

Plan B:

Plan B concepts work on several mixed approaches, including the very exotic system that called for multiple thrusters firing in the direction of flight.

Known as the Active Pulsing RCS Concept, this closed loop, active control propulsion system, is designed to provide an order of magnitude oscillation amplitude reduction. This is still active as a potential system – as noted on ‘Plan B’ as the first option (B1).

However, serious problems with implementation and design removed this option from being an option on Plan A, notably with over four years of development time estimated, and a concern with the aft end of the first stage being enveloped by plume induced flow separation during the time the thrusters need to be active.

Work had been carried out to reduce the drag factor by around half, via the installation of fairings around the pods. This also solved the potential issue of the thrusters being torn away from the vehicle during the aerodynamic environment during first stage flight.

Interestingly, the second option on ‘Plan B’ (B2) is the option that was referenced by the CxP managers, which works on two systems of TO mitigation – namely the Active Aft Skirt RMA and SoftRide Isolators.

Due to the immaturity of Plan A’s crew seat isolator option, Plan B2 allows for a smoother transition through the PDR process.

‘Plan B3’ is listed as an extension of B2, with a second set of SoftRide Isolators installed into the area between the second stage of Ares I and the service module of Orion.

A large amount of work remains, notably due to the lack of flight data. This will begin to collate during November’s STS-126 flight with Endeavour, with also some data to be gained from next year’s Ares I-X test flight.

Currently, CxP have to rely on ground test data from four segment firings of the Solid Rocket Motor, and computational models. New data will also be gained via NASA Ames later this year.

Notably, this phase of mitigation appears to be aimed mainly at the Orion that will fly to the International Space Station (ISS).

A target block upgrade for the Lunar Orion has been referenced on information as an effort to initiate cold flow/subscale testing of internal motor physics.

However, the over-riding priority is to fine-tune a workable solution to TO, without causing any further pressure on the 2015 manned flight for Orion 2.

L2 Members: Refer to TAG keywords ‘Ares I’ and ‘TO’ for updates and expansive presentations.

Selection of L2 Resources For Ares I, V and Constellation (some not listed): 

Ares I TO Mitigation Plans and Notes. Ares V Frabrication Hardware Presentations and Photos. Ares V Point of Departure Presentation. Orion WEST in NBL Hi Res and Notes. 8th Floor News Updates. Ares V Alternative Engine Arrangement Presentation. Hi Res CxP Logos. Full (and very expansive) set of presentations on Ares/Orion mass and status report. Ares I ‘Parasorber’ TO migitation hardware presentation and animation. Ares I TO Risk Slides. Ares I Risk Status.

Hi Res Images of Ares I FS Parachute Test Vehicle (JDTV). 20mb of new Lunar Images of Chariot, Athlete and Lunar Crane. Ares I KSC Processing Master Book – 184 pages. Changes for Ares I-X (Images). PRCB Transition Presentation (Shuttle to Ares). Ares V (5.5/6xRS-68) Presentation (and more). Orion Parachute Vehicle Images. Latest Risk Matrix for Ares. SI Unit Directive Document. CxP PMR08 Manifest. PDR and associated notes from CPCB meeting. Orion Parachute Test Vehicle (PTV) Photographs.

110mb worth of Ares I-X Weekly Test Presentations (Ares I-X, J2-X etc. up to end of April) The Orion LIDS (Low Impact Docking System) Section (Images, Videos, Engineering Notes). Hi Res Images of Ares I in the VAB. Ares I-X Integrated Milestone Charts. Ares I Thrust Oscillation Focus Team Status Presentations (over 50mb – includes DTO on Shuttle missions), Ares I-X Global Buckling Status Presentation, Ares I – Launch Pad Stabilization and Damping Presentation, Ares I: Purge/Vent/Drain and Vehicle Access Presentation.

Ares Tilt Up Umbilical Arm (TUUA) Test – Video, Ares/Orion Comm and Tracking Presentation, Ares I Nozzle Extension Update Presentation, Ares/Orion Integrated Stack TIM Summary (Major Issues) Presentation, Orion Land vs Water Landing Update + Crew Survival (post 36 hrs) Presentations.

Altair Overview Presentation. Ares I Risks and Status. Ares I-X Booster Recovery Images and Video. Ares I-X Pad Images. Ares I-Y Mission Overview Video (50mb – Superb).  Orion Lunar Transit CGI Video. (Several more videos, including first video of Orion splashdown). 

Orion Rendezvous with the ISS CGI Video, plus AERCam Inspections. Ares I Thurst Oscillation Update Section.  Images of completed PA-1 boilerplate Command Module at LaRC.  CxP Planning for Architecture Closure – Feb 19. Ares V Overview Presentations. Other Major CxP Updates for Feb (List restricted to L2).

Orion 607 Overview Presentation (Jan 08), Constellation Program Status/Budget and new Manifest to Orion 20 Presentation (Jan, 08). Michoud Transition to Ares I/V (Jan 17, 08). Several MLAS (Max Launch Abort System) Presentations. Over 60 Hi Res Images of Orion Mock-up at JSC (Hatch, Seats, Flight Deck) – December.

Lunar Habitat Assembly. PRCB Presentations on hardware and infrastruction transition (from Palmdale to MLP Park) ‘Follow live’ Lightning Towers Construction images. Latest Mobile Launcher details. Orion/Ares I/Delta IV Heavy NEO Feasibility Study (Video). Constellation EVA Study Presentation. Superb Gene Kranz address to CxP workforce (Apollo to Orion feature) video. MOD ‘LEO to Mars’ presentations.

Superb Ares I Launch Ascent, Pad Abort Test CGI Videos (three). Integrated Stack (IS) Technical Interchange Meeting (TIM) notes – Nov 6 to Nov 15. The full ‘8th Floor News’ – Constellation Update (performance issues) – Nov 5. Ares I Mobile Launcher PMR.

‘Proposed’ Ares I SRBSF (Mini VAB) and graphic. LSAM (LDAC-1) Video and Images. Several Constellation All Hands Videos and Presentations. Ares I Pad Rollercoaster (Old and New presentation and slides – the very cool ‘CGI ride on the Ares pad coaster’ video. Ares I VAB ‘In-Line’ Stacking presentation slides.

Presentation of Ares/Orion impacts relating to Shuttle manifest acceleration. Ares I Interstage diagrams. Ares V Super Crawler. Ares I Launch Pad images (ML etc.) Hi Res images of Ares I-X Upper Stage. Orion 606-7 Data Updates. Updates Constellation launch schedule through to Orion 15. Orion Seat test photos. New ML Graphic and info. New Ares V graphic and baseline data. Large collection of hi res Orion paracute drop tests. SIX Part Series of Ares I Upper Stage Graphical Overviews. DAC-1C DDD Vast Slides on Vehicle Design. ATK First Stage Presentation. 39B Lightning Towers Slides. DAC-1C Departure points to DAC-2 Upper Stage Graphcs (Many Changes).

Orion/CEV Display Layout Presentation (40 pages). ATK figures on the 5-Seg Booster weight for CLV. Weather Shield (Rain Shield) for Orion on the pad. New Super hi-res images of Ares I. ATK Cutaway graphics of Ares I – perspective and axonometric. Ares I/Orion CxP 72031 Requirements Validation Matrix Information. CEV Paracute Assembly System (CPAS) Presentation.

Orion Launch Abort System (LAS) overview presentation. Changes to Ares I Upper Stage – expansive details and data. Ares I/Orion CxP 72031 Requirements Validation Matrix Information. CLV Umbilical Trade Matrix XLS. Vehicle interfaces for the DAC 1C version of Orion Ares. Ares I-X Test Flight Plan (full outline) Presentation. Ares I-X timeline and modification expanded info. Ares I Reference Trajectory. Boeing’s STS to Ares – Lessons Learned Presentation. CLV DAC-1C (Changes to CLV Upper Stage).

Ares I-X: Four Seg+Dummy ‘Tuna Can’ stage. Ascent Developmental Flight Test Presentation. CLV Pad 39B Handover Info and Latest. New images of CLV on top of new MLP and LUT. Lockheed Martin CEV/Orion Updates. ATK figures on the 5-Seg Booster weight for CLV.

90 Minute Video of Constellation all hands meeting. Escape System Trade Study Presentation. CEV-CLV Design Analysis Cycle Review (DAC-2) Presentation. Flight Design and Dynamics Division CEV update. CLV Mono-propellant RCS system. CEV pressurisation system review. CLV/CEV Configuration Images. The 2×3 Seg SRB Crew Launch Vehicle Option Presentation…

….plus much more (L2 Constellation over 200gb in size).

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