Thrust Oscillation: Parasorber and Thrusters lead Ares I options

by Chris Bergin

Engineers tackling one of the biggest issues with Ares I, namely Thrust Oscillation, are understood to be currently favoring either adding the “Parasorber” concept, or a set of pulsing thrusters – firing in either direction during first stage flight – to the aft skirt, along with Crew Seat Isolators.

With the July status of the engineering efforts showing the issue to be an across the board high “RED” risk to Ares I’s development, the mitigation process is likely to continue until at least the end of the decade.

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Ares I TO Latest:

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.

Since the condition with Ares I was found last year, a Tiger/Focus team have been working on down selecting mitigation options. However, the latest documentation shows the main focus remains on ‘researching’ the condition’s relation to Ares I and associated processes towards mitigation. 

‘Mitigation Plan: Detailed research (this is research, not mitigation) plan for the TO risk is managed by the MSFC (Marshall Space Flight Center) TOFT (Thrust Oscillation Focus Team),’ noted documentation dated July 10.

‘The plan contains multiple parallel tasks, with complicated interdependencies. Plan does not ‘buy down’ risk because it is research, not mitigation. Assessment of parallel paths on risk level is ongoing and may result in reduced risk as plan is executed.’

The documentation shows 10 elements being worked on research, all of which are 5×4 (likelihood x consequence) ‘RED’ risks. These 10 ‘siblings’ are unable to breach 5×4 on the risk matrix, as the parent element – in this case Thrust Oscillation itself – is always one level higher, meaning it is currently seen as a full 5×5 risk.

Sources claim 5×5 program risk is only managed by Level 1 Management, due to its risk to the program overall. However, this will have to be reduced as data is gathered to finalize a mitigation option.

Data gathering via actual flight hardware is set to begin with STS-125 – with sensors believed to have been added to Atlantis’ boosters, though this is still unconfirmed. Three downstream flights will have TO related data gathering DTOs (Detailed Test Objectives) – as confirmed in mission baseline documentation.

At present, two mitigation options are gaining a large amount of attention (Active Thrusters and the Parasorber concept) – likely added in combination with a third (Crew Seat Isolators).

Liquid Thrusters On A Solid First Stage:

Originally noted as Active Pulse Thrusters (ATP) – see image of ATP left hand side – this system would lead to a very different appearance of the aft skirt on the bottom of the first stage.

‘Active Pulse Thrusters (RCS TO Damper) – could provide 10X reduction in TO. Relatively mature thruster design. Self contained. Relatively mature control system,’ noted the first appearance of the concept on TO mitigation documented.

Now matured to an even more elaborate system, the concept calls for four 7,500lb thrust Orbital Maneuvering – type – Engines (OME) firing upwards, with another four more firing downwards.

The outcome would be 30,000lbs thrust, in both directions, creating a reduction in what is understood to be around 38,000lbs of Thrust Oscillation related forces, reduced to an acceptable level for the astronauts on board.

The current concerns noted with this system relates to the 1.17m diameter thrusters – protruding out of the aft skirt – being impacted by supersonic flows during first stage. Sources note that they could be sheared off during ascent.

The OMEs would also require a large scale redesign to their valve systems to allow for the required reaction and pulsing rate. They would also – based on their mass alone – add a major mass impact to Ares I’s payload capability.

‘Performance and aft skirt design challenge: (around) 500 lbm (pounds mass) payload impact,’ added documentation on the original APT design – which has since been refined and likely now heavier. ‘Trade required for separation and booster deceleration. Add failure modes. Must survive aft skirt environments.’

Added to the impact on Ares I performance – which is struggling regardless – additional failure modes would hit Ares I’s LOC/M (Loss of Crew/Mission) numbers.

Other options based around the same concept – though not documented – speak of various designs, one of which is noted to consist of a larger number of smaller thrusters – up to 16 to 20 in total – firing through the worst part of TO concerns (around 11 seconds of first stage flight).

That concept is still undecided on whether the thrusters would fire in either direction, or – given its small role during over two minutes of first stage flight – it would be used in tandem with another mitigation option.

Parasorber Concept:

One of the early favorites as a mitigation option has since been refined into what is being called the ‘Parasorber’. This concept appears to be a progression on the Tuned Mass Absorber (TMA) concept shown in earlier documentation.

Based around the Ares I First Stage recovery parachutes, this concept’s main negative points – as with the TMA – relate to the added weight that will make recovery of the first stage more difficult – added to the center of gravity moving farther forward.

‘Three main parachute packs (green) with hard covers (blue) connected to MPSS (Main Parachute Support System) via frames (grey),’ outlined the design in one United Space Alliance (USA) presentation.

‘Packs are free to travel up and down on linear bearings connecting the frames to the connecting the frames to the MPSS panels (light blue). The frames are suspended by springs as well as damped.’

The concept assumes a 4 inch gap between MPSS panels and Main Parachute pack. However, testing on Ares I-X doesn’t appear viable, as the test vehicle has no such gap in its MPSS design.

The concept is reasonably simple, with a frame suspension – consisting of guide rails, springs and a damper system – mounted on a square frame connected to the MPSS panel. The concept currently shows – via an animation – 8 inches of travel during first stage flight.

‘Damper and spring form will be determined by spring and damping constants derived from analysis,’ added the presentation. ‘Frame form will be determined by predicted loads.’

How far down the line the Parasorber has matured from the TMA is not noted. However, given it is a passive system, the mass impacts will be far less than the other option of an active (powered) system.

It will also have to achieve higher than the previously documented 2x reduction on TO, which was originally classed as a reduction in loads good enough for the health of the astronauts, but not below the limits required for crew performance.

‘Tuned Mass Absorber: Use existing mass to counterbalance TO: 2X reduction for passive system, 3X reduction for active system. Payload impact less than other concepts,’ noted the previous TMA documentation.

‘Immature design: May create problems for First Stage recovery system. Active control design is immature. Reduces loads to well below human health limit, but not performance limit. Adds failure modes.’

Crew Seat Isolators:

With the likely addition of Crew Seat Isolators in tandem with any down selected mitigation approach, further gains in reducing the loads will be forthcoming.

Aiding both data collation and how much isolation can be gained, STS-119 – Discovery’s flight to the International Space Station (ISS) in February 2009 – is carrying out a special DTO for the Constellation program.

‘Constellation Program (CxP) has requested OPO (Orbiter Project Office) to implement a DTO to gather launch vibration data. DTO will be done in conjunction with a DSO to record astronaut visual performance to aid in CxP unimpeded crew performance specification development,’ noted the documentation.

To perform this DTO, three Mission Specialist seats will be outfitted with triaxial accelerometers. After recording data during launch, the accelerometers will be removed and stowed during the post-insertion timeline.

Graphics associated with the DTO also appear to show that astronauts will carry out an eye sight test – placed in front of them on the wall of the middeck – during the ride uphill.

‘Astronaut records visual function from middeck locker mounted card and other symptoms during launch profile,’ noted the text byline associated with the graphic.

The Space Shuttle Program (SSP) is currently planning two additional Thrust Oscillation DTOs on STS-127 and STS-128, though the Constellation Program have been quick to send out a memo to disassociate this as a test for Ares, even though it is.

‘Removing DTO wording regarding that it’s for Constellation. Will just say that it’s a DTO, what it’s for, who is responsible for funding, and what safety boards the DTO will go through for certification,’ noted the memo, pointing to Constellation avoiding the costs of the DTOs.

No documentation on the Isolators themselves has been created at this time, with data gathering the primary requirement prior to a design for the seats on Orion.

Other non-hardware options are believed to still be under evaluation – such as the Castletop Inhibitor – though no mention has been made of these other options for some time now.

Selection of L2 Resources For Ares I, V and Constellation: 

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 190,000mb in size).

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