Ares I Thrust Oscillation mitigation – the current state of play

by Chris Bergin

One of the top “RED” concerns for Ares I – Thrust Oscillation – has received an array of potential mitigation techniques, varying from detuning the vehicle, adding modifying fuel grain inhibitors, to a configuration change to a four segment first stage.

An array of unclassified documentation has been acquired by that shows the huge engineering effort that is being conducted on solving the issue.

**The most comprehensive collection of Shuttle, Ares, Orion and ISS related presentations and mission documentation, plus expansive daily processing documentation and updates are available to download on L2 **

All of this article is based on documented L2 information. For an overview of how L2 works, **click here for sample**




The problem:

Thrust Oscillation is a recognized characteristic of solid rocket motors – in Ares I’s case, the five segment first stage – with concerns arising when pressure oscillations drive resonant modes in the vehicle structure. It is not clear as to exactly how bad the TO numbers are, due to lack of flight data.

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.

Last year, preliminary structural analyses on Ares I indicated that a potential resonant concern resulted in high dynamic G force levels in the upper stage, and more importantly, in the Orion capsule – a serious threat to the health of the astronauts during ascent.

The immediate problem at hand – the lack of flight data available, with only a four segment booster database consisting mainly from ground testing and some flight data from DFI (Development Flight Instrumentation) which only flew with Columbia during STS-1 through to STS-4. There were some additional flights with SRB instrumentation on STS-9.

Interestingly this could be both positive and negative for the state of Ares I/Orion’s impacts, due to the uncertainties involved.

This led to a Focus Team – or Tiger Team – being formed to review the analyses and to determine a path forward. This has now reached the ‘checkpoint’ stage of evaluations.

The Focus/Tiger Team:

The team tasked with finding a solution to the problem is collated from several centers and contractors.

Members from MSFC (Marshall Space Flight Center) Ares and Orion systems engineering and vehicle integration were joined by NESC (NASA Engineering and Safety Center), The Aerospace Corporation, ATK, Pratt and Whitney Rocketdyne, and Boeing among others.

One name that stands out is Scott Horowitz – the former Constellation chief – who was then hired as a consultant by the Langley Research Center (LaRC), and has been working on the MLAS (Max Launch Abort System). The former US Air Force Colonel is classed as an NESC consultant on the Tiger Team.

Early Conclusions:

Engineers found that the frequency of the motor pressure oscillation was well understood to be a characteristic of 1L standing (acoustic) wave and a function of hot gas properties and length.

However, the team was tasked with finding out how these characteristics affected Ares I and Orion, given their understanding was specific to the shuttle’s four segment SRB.

‘The Space Shuttle RSRM (Reusable Solid Rocket Motor) exhibits pressure oscillations, but the Shuttle Stack is insensitive to the four segment motor frequencies,’ noted one of the unclassified presentations on early conclusions.

‘The immaturity of the Ares and Orion designs restrict any assessment of the impacts to the sensitive subsystems to a quantive assessment.’

Interestingly, the presentation notes that the engineers were already aware of this issue in relation to other vehicles, such as Ariane, Taurus and Pegasus, though any data gained would not be a direct relation to Ares I’s specifics.

‘Other launch vehicles may provide valuable information on thrust oscillation and mitigation,’ confirmed the early conclusions. However, it is not clear if the team have been in contact with the operators of those other vehicles for the purpose of gaining data.

The early findings also noted that the crew health limit for TO is around 0.6 Gs at these frequencies (early conclusions) ‘and may be much lower – at around 0.25 Gs for crew performance.’

Actions and Analysis:

The Focus/Tiger team were first tasked with an action summary, which would determine the appropriate nominal and dispersed Forcing Function (FF) – used to clarify the effects of the nozzle and internal geometry. Interestingly, this already called for the need for flight data.

Flight data was also called for the need to ‘verify and validate the vehicle loads model on force application, loads combinations and appropriate application of damping and axial stiffness.’

The team was also given the action of conducting near-term evaluations of element subsystems sensitivity to dynamic loads, and ‘the evaluation of candidate global mitigation and local mitigation – as sensitive subsystems are identified.’

The analysis status at the time of the opening findings noted “initial preliminary assessment of the element impacts of “conservative” loads indicated “RED” status for crew, Service Module structure and the Upper Stage Main Propulsion System. At this point, Thrust Oscillation officially became the number one concern for the new vehicle.

Immediately, the team called for shuttle flight data to help their evaluations, classed as a priority action to close the TO problem. This is understood to have been added to Atlantis’ STS-125 flight – currently set to launch later this year (September/October – TBA).

Set of Mitigation Options:

Soon the Tiger Team had a set of mitigation options to counter Thrust Oscillation. Interestingly, amid rumors of a configuration change in the offing, the first documented note of switching to a four segment Ares I with two J2-X upper stage engines gained a mention as a direct possibility of solving the TO problem.

“First Stage: Reduce Forcing Function (FF) – Review of Internal Ballistics, Review of Inert Components. Damp FF – Isolator at Forward Skirt, Chute Damper,” noted the NASA presentation on Global Mitigation Options.

Reducing or eliminating the FF is classed as “probably unachievable,” and has a “high risk due to schedule impact and late verification of design changes.”

Detuning the stack from FF: Requires a 10 percent frequency shift. However, the risks associated relate to multiple small design solutions that are currently unidentified.

“Upper Stage: Damp FF – Enhance Damping at Upper Stage Prop Isolator joint at Interstage. Separate FF and CLV (Crew Launch Vehicle) Frequency – Stiffen Upper Stage Structure, Isolate Vulnerable Components.

“Orion: Damp FF – Isolator Joint at SMA. Isolate Crew Seats.

“Integrated Ares: Revise Baseline Ares Configuration – Active RCS (Reaction Control System) Damping of FF. NEW Ares Configuration – 4 Segment RSRM with Multi-Use.”

However, the amount of mitigation options were soon to be refined further by the checkpoint presentation which outlines the current state of play.

Extensive overviews of each mitigation option will follow in future articles as they are finalized.

March Checkpoint – Current State Of Play:

The most striking note at the start of the Checkpoint overview of the TO mitigation effort is the note that “Over-Conservatism (has been) reduced” – pointing to what appears to be good news that initial data points were over estimated in their seriousness. However, this is not clarified in the presentation, which immediately goes on to note an opening set of findings and requirements.

“First Stage, Upper Stage, and Upper Stage engine: No major impacts from Thrust Oscillation loads, but LOX tank aft dome may need to be strengthened and more analysis is needed on the MPS effects at staging.

“Orion and Crew require mitigation: Orion will have to make changes to stiffen Service Module tanks (TO is a drive – current design can handle 4 + / – 0.5 G requirement.

“First Stage internal motor physics is much better understood, but cold flow and sub-scale hot fire testing is required prior to motor design changes.

“Shuttle Flight Data is needed to: Clarify differences in flight and ground test data. Quantify Forcing Function (FF) and vehicle response analysis. Determine crew seat environments.”

The Checkpoint presentation goes on to confirm that the team has found mitigation options that are available to reduce Thrust Oscillation by at least 10 times, with the following options continuing through to further evaluations.

“Active counter pulsing – 500 lbs to payload and additional testing and verification.

“Internal motor changes – FS schedule impact due to cold flow and sub scale testing: ‘Castle Top’ inhibitors, Receding inhibitors, plus fairing of aft-facing steps.

“Vehicle Configuration Change: 4 Segment, 3-J2Xd5 – significant reliability hit, but detunes system.” This vehicle configuration change is understood to be undesirable, according to sources. A 5.5 segment first stage was also rumored at one point.

Also listed are mitigation options to reduce the loads/accelerations on Orion, which involves the use of isolators and absorbers.

“Mechanical isolation at Ares/Orion and FS/US (First Stage/Upper Stage). Tuned mass absorber (2x to 3x reduction). Local isolations in Command Module will be required in combination with global mitigation to meet performance spec of 0.14 to 0.3 Gs (0.25 G from Mercury/Gemini). All options require integrated analyses.”

While the near term goals state that validation models and associated forcing function is complete, along with multi-center evaluations – such as MSFC’s Time Domain tuning, validated independently by JSC/Boeing – further work in the short term is continuing on the mitigation possibilities.

“Continue analysis of internal motor options: Recommendation to be delivered on internal motor mods to eliminate pressure oscillations and ‘castle top’ inhibitor. Continue to look at the feasibility of the tuned mass absorber and to provide a design for the isolating of the Upper Stage/Orion.”


A lot of work is required until this issue can go away, not least via flight data that may still be required all the way up to Ares I-Y in 2011. The sum total of mitigation processes is likely to be from a collaboration of detuning the vehicle and additional hardware, plus stiffening of certain Ares I structure elements.

This then adds mass, which Ares I and Orion can ill afford, thus a balance will need to be created over the coming months – or even years – to ensure Ares I can still perform its tasks for lofting Orion to the ISS, and to the moon/Mars.

“The natural maturing of the vehicle design and the explicit management of critical design parameters can detune the system and reduce loads,” added Constellation’s summary pages.

“Frequency will naturally move lower with stiffness and increasing mass. Damping will increase due to both knowledge of the current configuration, and through explicit design decisions.

“The probability that the maturing design will move the system response below the crew spec is not known today. There are limits to the design changes that can be proposed.

“Integrated structural design is a complex balance of all loads, controls and performance and the current margins are not sufficiently robust to allow any design change to avoid TO loads. Test data will be required to verify the design and implementation of global design changes.”

This admission on the current margins clearly points to the performance concerns that have been challenging engineers for some time with Ares I. While this is a fluid situation, it means only small mitigation efforts that involve design changes can be considered at this time.

“Design and requirements changes that can be implemented now with a relatively low impact (scar) would ensure margin in the design that could be traded for performance in the future as the design matures,” added the summary.

Long-term, instrumentation will be used on both shuttle and Ares I-X to verify internal pressure to force transfer functions. With Ares I-Y and Orion 1 to define internal pressure to force transfer functions and dispersions.

This issue has a long path in front of it before it can no longer be described as a major concern for the vehicle.

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