A trade study has begun on three leading candidates to mitigate Ares I’s Thrust Oscillation problem, as the Tiger Team work through design immaturity and mass constraints.
Active Pulse RCS (Reaction Control System) – mounted on the aft skirt, Isolation Mounts between the First Stage and the Upper Stage, and a Tuned Mass Damper are three concepts that have made the cut as the most favorable options.
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Thrust Oscillation Latest:
Thrust Oscillation (TO) – also known as ‘Resonant Burning’ – is a recognized characteristic of solid rocket motors. The concern relates resonant modes in the vehicle structure which are capable of harming the astronauts riding in Orion.
For months now, the Focus Team – or Tiger Team – have been working on potential mitigation options, and have been successful in finding multiple approaches that – in theory – could solve the issue, or at least reduce the vibration environment low enough for acceptable crew performance.
Active Pulse RCS:
The anti-Thrust Oscillation RCS would be a totally new system, located on the aft skirt of the Ares I booster. Known as Active Pulse Thrusters (APT), documentation shows this system to hold the potential of reducing Thrust Oscillation by around 10 times that which is currently expected.
‘Active Pulse Thrusters (RCS TO Damper): First Stage carries most of the design changes (Orion Service Module tanks change required),’ noted associated documentation on this concept. ‘Could provide 10X reduction in TO. Relatively mature thruster design. Self contained. Relatively mature control system.’
However, it would – as with most of the mitigation options – hold a mass impact on the vehicle, something Ares I has been struggling with since its early design cycles.
‘Performance and aft skirt design challenge: (around) 500 lbm (pounds mass) payload impact. Trade required for separation and booster deceleration. Add failure modes. Must survive aft skirt environments.’
The system consists of four pods, located around the aft skirt on the Ares I First Stage. Early graphics of a system – that are bound to mature if accepted as the way forward – show each pod will have a fuel tank, an oxidizer tank, a pressurant tank, and seven thrusters.
The downside of this concept – which is a completely separate system than the roll control system on the interstage – is the addition of failure modes, which would hit Ares I’s LOC/M (Loss of Crew/Mission) numbers.
Also on the downside, the concept is a retro thrusting system (negative thrusting) – which would impact on Ares I’s performance figures.
Isolation Mounts between the First Stage and the Upper Stage:
This concept isn’t likely to survive the trade study, should the issue of the Ares I Interstage structural strength come into question – given it adds flexibility to that area of the vehicle.
Other negative impacts are noted in associated documentation, and the concept itself is not deemed as capable of eliminating the issue of Thrust Oscillation to within required limits for the crew performance.
‘Most of the isolation carried on the Ares Upper Stage side of the interface. Would reduce loads to well within crew health limits, (but) still would not meet performance limit,’ added the documentation on the first review of this concept.
‘May reduce payload by 1000 lbm. Reduces lateral stiffness unless mitigated in the design. Adds failure modes. Changes system modes for loads and control.’
Tuned Mass Damper (First Stage Parachutes):
This concept is understood to be the favored option on its potential for solving the problem of Thrust Oscillation.
Based around the Ares I First Stage recovery parachutes, this concept’s main negative points relate to the added weight that will make recovery of the first stage more difficult – added to the center of gravity moving farther forward.
Working as either a passive or active system, this concept will still require maturity, given associated documentation shows it is projected to reduce Thrust Oscillation down to acceptable crew health limits, but not enough for crew performance requirements.
‘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 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.’
The system weight is classed as 1000 lbm, which consists of rails and springs under the top plate of the parachute platform on the First Stage. The active system would require a control system and associated battery power supply – all located under the aeroshell that houses the drogue parachute.
The passive system has a rail attachment on the forward skirt extension of the First Stage providing lateral support. Damping would be provided by springs attached through the ancillary ring.
All three concepts noted (on April 23) as heading through the trade study would require maturing to mitigate their negative issues, thus the eventual winner is likely to undergo a large element of refinement.
Current Ares I Mass Figures:
Also, the resulting mitigation concept for Thrust Oscillation will affect the current mass figures for Ares I, which remain tight.
Currently, end of April documented figures for Ares I places the Upper Stage dry weight as “Allocated 30,800 pounds – Estimated 31,526 pounds.” Interstage dry weight is documented as “Allocated 7,270 pounds – Estimated 7,398 pounds.”
Residual mass – only recently added – is classed as “Allocated 0 pounds (due to it being a new factor) – Estimated 2,687 pounds,” while the Upper Stage Engine’s figures come in at: “Allocated 5,450 pounds – Estimated 5,676 pounds.”
The above figures “do not take into account any mass gain caused by thrust oscillation mitigation.”
Ares I related testing:
ATK and NASA will test fire another Space Shuttle Reusable Solid Rocket Motor (RSRM) on May 1, which – while checking into booster age-life certification, will also gain more valuable data on the motor for Ares I.
“Another test objective will help in the development of the Ares I crew launch vehicle and its launch pad acoustics, or sound measurements, will be taken using microphones placed outside the motor,” noted ATK. “The data will be used to make predictions of Ares I acoustics during lift-off.”
ATK remain on track to test fire a fully developed five-segment five segment motor in April, 2009. The Utah-based company are also on track for a full-scale launch abort motor test, using reverse flow motors – the four nozzles on the LAS (Launch Abort System) – in a vertical test stand.
The alternative abort system, known as MLAS (Max Launch Abort System) was believed to be winding down as a project recently due to mass and complexity issues. However, information points to this concept remaining alive, via special funding from what is described as “Manager’s Discretionary Funding” – which may allow for a test firing later this year.
One other related test has been postponed until the end of the year – namely the pad abort test. “Orion pad abort test slated for September has been pushed to December,” noted a NASA HQ memo recently.