Constellation Finalizes Thrust Oscillation Fix

NASA and contractor engineers have developed multiple options for “de-tuning” the Ares I rocket to prevent any problematic thrust oscillations from originating in its solid-rocket main stage to sync up with the natural resonance of the rest of the vehicle. 

The Constellation Program Control Board set a formal baseline for thrust oscillation mitigation during a meeting Dec. 17.  Moving forward, the Ares I vehicle design will be updated to include the addition of upper plane C-Spring isolator module and the upper stage fuel tank LOX damper.

While evaluations of data from the DM-1 motor test and Ares I-X test flight to date show no problematic thrust oscillation vibrations occurred, the Constellation team concluded incorporating the upper plane C-Spring isolators and LOX damper at this phase of design is a sensible addition.

“When we discover an engineering risk, like thrust oscillation, we tackle it with full rigor,” said Jeff Hanley, Constellation Program manager. “That’s what this team has done with thrust oscillation. We assumed the worst when the problem was first discovered. The good news is there is no empirical evidence of problematic oscillations from our ground test of the first stage development motor or during the Ares I-X first test flight.”

“The isolators work like shock absorbers to de-tune the vehicle and the LOX damper will counter the vehicle acoustic response by absorbing and disrupting the oscillation. Together these options will give us added confidence in the tuning of the vehicle as we mature the Ares and Orion designs,” added Hanley.

The NASA team, along with the prime contractors, has worked this issue carefully, understanding and minimizing any effects of the integrated vehicle response by introducing new thrust oscillation hardware into the design. The team will “scar,” or prepare, the upper stage design to accommodate the addition of this mitigation hardware at a later time, if desirable.

“The options approved today puts us on a robust foundation as we move forward,” said Hanley. “Finalizing the thrust oscillation design now allows us to keep to our schedule and provides contractors specific requirements about what we need them to build.”

Dual-Plane Isolators Emerge as Most Promising Thrust Oscillation Fix

Engineers and rocket scientists love data. So no surprise the NASA thrust oscillation mitigation team has been gathering reams of data to best understand how to design an integrated vehicle that avoids thrust oscillation. This week at Ames Research Center, Moffett Field, Calif.  NASA and industry engineers reviewed the latest progress to qualify and validate our understanding of thrust oscillation problems and solutions. 

For those new to this issue, thrust oscillation is a phenomenon that can appear in all solid rockets where pressure created during launch conditions creates an up-and-down vibration at a frequency that could impact crew situational awareness or health. For Ares I, engineers expect a smooth ride up from liftoff to 115 seconds, but as the first stage nears burnout, thrust oscillations could pose a problem for a few seconds impairing the crew’s ability to read displays and respond to what they see.    

Since the previous technical interchange meeting ( several things have changed. Orion and Ares designs have matured and very helpful measurements have been captured during liftoff of the STS-126, STS-119 and STS-125 space shuttle missions which validate assumptions about how the solid rocket pressure oscillations occur in-flight.  Mathematical modelers have an improved understanding of vehicle responses to candidate hardware designs. And finally, with the conclusion of the crew situational awareness testing, a new requirement has been proposed based on that work.

Constellation engineers have been pouring over new data to pinpoint several important factors that will drive optimal thrust oscillation fixes. 

First, two important numbers to keep in mind: 12.3 Hz and .7g. 

The thrust oscillation frequency of Ares I five-segment solid rocket motor is predicted to be approx. 12.3 Hz. By comparison, the shuttle’s four-segment solid rocket motor thrust oscillation frequency is 15 Hz. Ares I is a bit lower because it is longer. Think of an organ pipe: the longer the pipe, the lower the natural frequency.  The goal of any mitigation is to minimize the effects on the crew due to the first stage thrust oscillation. There are two basic ways to do this: “de-tune” the vehicle stack or increase damping in the system.  “De-tuning” is another way to say frequency separation — moving the natural frequencies of the Ares I vehicle and the Orion spacecraft away from 12.3 hz.  Damping absorbs the extra energy in the system and can be targeted to specific frequencies.  The goal of any mitigation system, or combination of systems, is to de-tune the vehicle approximately 1.5-2 Hz away from the vehicle’s natural resonance and avoid any problematic thrust oscillations with 99% certainty.

Since the Gemini era, NASA spacecraft designers used a limit of .25g peak as a safe threshold against these problematic longitudinal pressure oscillations. Based on increased fidelity gained through the crew situational awareness test series, the Constellation Program expects to set a new threshold, limiting the maximum peak to .7g, with a mean vibration level to not exceed .21g’s rms (root mean square) for any five second period during first stage flight. 

Keeping these figures in mind, the team scrutinized proposed hardware solutions and how well each system, or combination of systems, will impact the integrated vehicle. Each thrust oscillation simulation includes over 10,000 analysis points including variations in forcing function, structural frequency response, and mode shape to provide an accurate assessment of how mitigation solutions will actually work in flight.

Design solutions under active development include passive single and multi-plane C-spring isolators, and mass absorbers called a Tuned Oscillation Array (or TOA).  Work also continues on a LOX damper, which uses the slave mass of the Upper Stage liquid oxygen propellant to dampen out vibrations. Subscale hardware for two LOX damper designs — a bellows and diaphragm — have also been built and tested in the lab. All candidate solutions are being worked full force, and full steam ahead, to meet these updated parameters.

Initially, a dual plane C-spring isolator system was too heavy to incorporate into the overall vehicle design.  The updated designs use titanium, not steel for the isolator springs, improving overall system performance while reducing the weight of the system. The weight reduction made a dual-plane C-spring isolator system much more attractive as a design solution and it is out-performing the other passive systems. The next step is to make a decision about how best to implement a dual plane solution into integrated designs.  Nothing is off the table yet, as the team continues to refine which fix is most robust.

The team’s analysis during this session reemphasized that thrust oscillation is not just a first stage or Ares problem. It’s a technical challenge that impacts the entire vehicle and can be solved by an integrated team of Ares and Orion engineers. Because of this, final decisions about which solution is optimal will be incorporated as an issue into the Constellation Preliminary Design Review scheduled for late this year. The team also looks forward to capturing data from the upcoming five segment development motor test (DM-1) and Ares I-X flight which will further characterize how the in-line vehicle responds.

Reported  by Jennifer Morcone, NASA MSFC public affairs

Engineers Review Solutions to Thrust Oscillation on Ares I

This week, engineers from across NASA and partner contractors gathered at Marshall Space Flight Center to analyze designs to minimize vibrations in the Ares I rocket. During this three-day meeting of the minds, participants showcased tremendous progress toward understanding the physics of thrust oscillation, updates to several candidate mitigation solutions, and results from early subscale testing of candidate hardware. This was not a decision-making session, but an opportunity for engineers and managers to scrutinize proposed solutions.

Thrust oscillation may be felt for a few seconds at the end of first-stage powered flight. Also called “resonant burning,” thrust oscillation is a phenomenon in all solid propellant rockets forcing vibrations through the entire structure, in the case of Ares I, that includes the Orion crew module. 

To date, several promising mitigating systems have been identified to counteract vibrations stemming from thrust oscillations. Two primary options are actively under development:

Isolators are C-shaped springs that could be placed between the Ares I frustum and interstage to “detune” the vehicle resulting in less vibration for the crew while maintaining vehicle control stability. The design is based on an existing “soft-ride” technology developed by CSA Engineering, Mountain View, Calif. “Soft ride” technology, which has flown on 17 spacecraft, has been typically placed inside the payload shroud to protect payloads from oscillations.  The current  Ares design incorporates a ring of 136 C-shaped springs and attach hardware into an isolator module which measures 18.5 inches in height.  ATK Launch Systems, located near Brigham City, Utah, is the Ares I prime contractor and is working aggressively with CSA Engineering to mature the isolator design. Moving forward, engineering teams will continue to evaluate the performance of the C-shaped springs and supporting hardware. Engineering design units have been tested on a “shaker stand” which simulates the thrust oscillation loads and demonstrated functionality and effectiveness of this system. 

Tuned Oscillation Arrays:
An earlier active mitigation concept called Reaction Mass Actuators (RMA), has matured into a passive solution known as Tuned Oscillating Arrays (TOA). This system will be mounted inside the first stage aft skirt and includes an array of boxes that contain masses suspended on springs which absorb or soak up the vibration oscillation produced during first stage flight. Analysis of the aft skirt has indicated  that the existing skirt design can support the TOA approach. Following the recent STS-119 shuttle mission, engineers conducted a fit check with TOA volume simulators and found the solution to be feasible in the existing aft skirt design. Next steps include finalizing  bracket concepts to connect TOA boxes to the aft skirt and examining handling processes and equipment needed for ground support. The active RMA concept, which includes powered springs that actively cancel out the vibration, is on hold but available for restart if required later. 

Two other alternative thrust oscillation strategies under study as risk mitigation to the baseline include:

A “dual plane” solution:
A dual plane solution would employ two rings of isolators, one located at the interstage/frustum interface and another between Orion and the Ares upper stage. Having redundancy of isolator rings may provide increased “detuning” capability to ensure the Orion does not respond to the oscillations of the first stage motor.

LOX damper:
Engineers are also evaluating a concept called a LOX damper, which uses the fundamental physical properties of liquids to leverage the kinetic energy in the movement of the existing liquid oxygen in the upper stage tank to dampen out vibrations. The devices, installed within the liquid oxygen tank, can engage the mass of the liquid propellant to generate momentum in the fluid itself to counter the vehicle acoustic response and disrupt oscillation. Engineers are evaluating the effectiveness and applicability of this design.

Data analysis:
In addition to discussing specific design solutions, the thrust oscillation team is pouring over existing ground and new flight test data captured from recent shuttle missions STS-126 and STS-119.  During recent shuttle flights, sensors placed on both ATK-produced solid rocket boosters measured pressure oscillations, in addition to vibration measurements, on crew seats.  Measurements are helping engineers anticipate the magnitude of thrust oscillations forces that may be expected on future Ares I flights. 

NASA engineers and astronauts are also evaluating crew situational awareness under various vibration conditions in a simulator at the Ames Research Center. NASA is working to set the final requirements for acceptable crew vibrations – currently a 0.25g requirement that was developed during the Gemini era.

Next steps:
Testing of the isolators and TOA candidate mitigation hardware will march forward.  NASA teams will capture additional data from future shuttle flights and from the upcoming test flight of Ares I-X to better understand the risk to the Ares I vehicle and the Orion capsule. Considering all information, NASA will finalize vehicle designs in a thrust oscillation preliminary design review which will define which system, or combination of systems, works best to minimize vibrations on the Ares I vehicle.