segunda-feira, 21 de janeiro de 2008

A NASA arrisca-se a estar a criar o maior vibrador do Mundo!

Surgem os primeiros problemas com o foguetão Ares I, que deverá ser o foguetão que substituirá o Space Shuttle como meio de lançamento para naves tripuladas dos Estados Unidos. Estes problemas prendem-se com vibrações perigosas de toda a estrutura, que serão agora estudadas pela NASA durante os próximos anos. Creio ser um problema grave e de difícil solução, uma vez que nunca foram usados foguetões de combustível sólido para lançar astronautas no espaço. É uma área onde não há experiência, e assim o potencial de risco e de ocorrência de desastres é maior, contrariando o objectivo da NASA de criar um meio de lançamento mais seguro do que o Space Shuttle.

NASA Moon Rocket May Shake Too Much

The Associated Press
Saturday, January 19, 2008; 1:25 AM

WASHINGTON -- NASA is wrestling with a potentially dangerous problem in a spacecraft, this time in a moon rocket that hasn't even been built yet.

Engineers are concerned that the new rocket meant to replace the space shuttle and send astronauts on their way to the moon could shake violently during the first few minutes of flight, possibly destroying the entire vehicle.

"They know it's a real problem," said Carnegie Mellon University engineering professor Paul Fischbeck, who has consulted on risk issues with NASA in the past. "This thing is going to shake apart the whole structure, and they've got to solve it."

If not corrected, the shaking would arise from the powerful first stage of the Ares I rocket, which will lift the Orion crew capsule into orbit.

NASA officials hope to have a plan for fixing the design as early as March, and they do not expect it to delay the goal of returning astronauts to the moon by 2020.

"I hope no one was so ill-informed as to believe that we would be able to develop a system to replace the shuttle without facing any challenges in doing so," NASA administrator Michael Griffin said in a statement to The Associated Press. "NASA has an excellent track record of resolving technical challenges. We're confident we'll solve this one as well."

Professor Jorge Arenas of the Institute of Acoustics in Valdivia, Chile, acknowledged that the problem was serious but said: "NASA has developed one of the safest and risk-controlled space programs in engineering history."

The space agency has been working on a plan to return to the moon, at a cost of more than $100 billion, since 2005. It involves two different rockets: Ares I, which would carry the astronauts into space, and an unmanned heavy-lift cargo ship, Ares V.

The concern isn't the shaking on the first stage, but how it affects everything that sits on top: the Orion crew capsule, instrument unit, and a booster.

That first stage is composed of five segments derived from the solid rocket boosters that NASA uses to launch the shuttle and would be built by ATK Launch Systems of Brigham City, Utah.

The shaking problem, which is common to solid rocket boosters, involves pulses of added acceleration caused by gas vortices in the rocket similar to the wake that develops behind a fast-moving boat, said Arenas, who has researched vibration and space-launch issues.

Those vortices happen to match the natural vibrating frequencies of the motor's combustion chamber, and the combination causes the shaking.

Senior managers were told of the findings last fall, but NASA did not talk about them publicly until the AP filed a Freedom of Information Act request earlier this month and the watchdog Web site submitted detailed engineering-oriented questions.

The response to those questions, given to both Nasawatch and AP, were shared with outside experts, who judged it a serious problem.

NASA engineers characterized the shaking as being in what the agency considers the "red zone" of risk, ranking a five on a 1-to-5 scale of severity.

"It's highly likely to happen and if it does, it's a disaster," said Fischbeck, an expert in engineering risks.

The first launch of astronauts aboard Ares I and Orion is set for March 2015.

Do site NASAWatch

NASA's Exploration Systems Mission Directorate Responds to Ares 1 and Orion Questions

Keith Cowing
Thursday, January 17, 2008

Editor's note: Earlier this month I submitted a series of questions to NASA's Exploration Systems Mission Directorate (ESMD) regarding the Ares 1 and Orion projects. The following was provided to me by ESMD PAO today:
Questions from Keith Cowing, NASA Watch

Q: Do launch vibroacoustics, oscillations, etc. generated by the current Ares I design exceed acceptable baselined limits for the Orion spacecraft? Do these oscillations exceed limits and/or pose a risk to the crew inside the Orion spacecraft? If so, to what extent to they exceed acceptable limits?

Q: Using a standard NASA 5x5 risk management matrix, what is the current risk rating of Ares I vibroacoustics and oscillations issue by the Constellation and Ares programs?

Q: Have any presentations given by ATK, NASA or other contractor personnel in November or December 2007 - or at any other time - characterized the risk (using a standard NASA 5x5 risk management matrix) to Ares I first stage development as being 4x5 due to incomplete requirements and/or insufficient performance from the DAC-1 Ares I Design?

Q: Are studies underway at NASA to reduce the weight of Orion and associated hardware so as to allow additional weight to be added to the Ares I to alleviate oscillation issues and/or conform to lower performance (payload capacity) by the Ares I?

Q: Is NASA looking at stiffening the Ares I structure so as to pass vibrations on to the Ares I upperstage and payloads i.e. Orion? If so what loads will be transferred to upper stages and/or payloads (Orion)?

Q: Is NASA looking to use a dampening system to handle these vibroacoustic or oscillation loads? If so, what is the weight of such this system? How much would the weight of such a system affect Ares I payload capacity? What would be the added cost of such a dampening system?

Q: Is a tiger team or working group working to report back to Ares and/ or Constellation program managers on these Ares I vibration and oscillation issues? Is this tiger team due to report its findings in March 2008? If not March 2008, when are these results due to be reported and to whom will these results be reported?

Q: Is any portion of the current tiger team's deliberations or proposed solutions considered to be restricted information due to ITAR concerns?

Q: Was NASA Administrator Griffin made aware of these Ares I vibration issues in 2007? If so, when was he made aware of these issues? Who told him about these issues? What specifically was Griffin told about these issues? What was Griffin's response to these issues?

Q: Is NASA currently working on any plans for alternate ways to launch the Orion spacecraft? Do these alternate approaches involve the use of EELVs? Do they involve other commercially available launch vehicles? Do they involve use of shuttle-derived launch systems? Do they involve the use of any launch vehicle design or concept with the working name of "Jupiter" or "Direct"?

Q: If Administrator Griffin was ware of these design issues in 2007 did he inform Congress of these issues? If so, when? Has information on these design issues been presented to members of Congress or to Congressional staff by anyone at NASA?

Q: Has any information on these Ares I design issues been presented to the Government Accountability Office? If so, when?

NASA's Exploration Systems Mission Directorate responds:

Thrust oscillation, also called resonant burning, is a phenomenon characterized by increased acceleration pulses that may be felt during the latter stages of first-stage powered flight. Depending on the amplitude of these pulses, the impact on the vehicle structure and astronauts may be significant.

Thrust oscillation is a characteristic of all solid rocket motors including the first stage of the Ares I launch vehicle. Vortices, created inside the solid rocket motor by the burning propellant or other flow disturbances, can coincide, or tune, with the acoustic modes of the motor combustion chamber, generating longitudinal forces. These longitudinal forces may increase the loads experienced by the Ares I during flight, and may exceed allowable loads on various portions of the vehicle and allowable forces on the astronaut crew.

During any new development program, program risks must be identified and resolved prior to hardware development. Thrust oscillation is such a risk. It is being reviewed, and a mitigation plan is being developed. NASA is committed to resolve this issue prior to the Ares I Project's preliminary design review, currently scheduled for late 2008.

By March 2008, the space agency anticipates having:

* Characterized the potential impact of thrust oscillation (sensitivities to the crew and vehicle components)
* Assessed design feasibilities
* Formulated a plan to manage sensitive design parameters (tests, trade studies and analyses required)

NASA has given careful consideration to many different launch concepts (shuttle-derived, evolved expendable launch vehicle, etc.) over several years. This activity culminated with release of the Exploration Systems Architecture Study in 2005. Since then, the baseline architecture has been improved to decrease life cycle costs significantly.

NASA's analysis backs up the fact that the Ares family enables the safest, least expensive launch architecture to meet requirements for missions to the International Space Station, the moon and Mars. NASA is not contemplating alternatives to the current approach.

NASA is studying the phenomenon of resonant burning of the Ares I first stage in order to improve prediction of the phenomenon and its associated impacts. Thrust oscillation forces may be reduced by vehicle structures, as is the case with the space shuttle and Titan IV.

NASA is working to understand how thrust oscillation may impact the entire stack - Ares first stage, upper stage, and Orion crew vehicle -- and to determine how to minimize the impact. NASA is assessing all vehicle system impacts and defining potential solutions. This includes reviewing thrust oscillation impacts on the Orion, upper stage, J-2X engine, reaction control system, first stage, and avionics. Such a thorough approach -- working with all systems to identify all scenarios and their corresponding sensitivities -- is crucial for successful mitigation.

The Constellation Program has brought in experts from NASA and industry to review these issues and lessons already learned from similar induced responses - namely, affects on launch vehicle hardware and allowable amplitudes of thrust oscillation. The Orion and Ares teams are holding detailed discussions and developing a plan to fully characterize Ares I thrust oscillation, assess any design changes that may be proposed, and manage sensitive design parameters with additional tests, trade studies and analyses.

An integrated thrust oscillation focus team is reviewing the various components of the Ares I and Orion integrated vehicle and the potential impacts of thrust oscillation on the motor, loads and controls, first stage, upper stage, and Orion.

All "at risk" items are being defined (including structural, performance, and human risk items), and a resolution matrix is being established. The goal is to have risks defined and quantified and a mitigation path determined by March 2008.

After the Ares I system design review in late October 2007, thrust oscillation was identified as a risk by the Ares Project and assigned a risk of four-by-five (out of five-by-five) on the NASA risk matrix. NASA uses the risk matrix as a way to track the probability that a risk may manifest itself and the overall impact if the risk does manifest itself. Risks are scored from 1 (low) to 5 (high) for both probability and overall impact.

The thrust oscillation risk is not directly associated with launch vehicle performance or first-stage development. The thrust oscillation risk is associated with the integrated stack, meaning the assembled Ares I first stage, upper stage and Orion crew vehicle. Ares I performance is tracked on a monthly basis, and Ares I consistently has met its performance requirements with margin.

The administrator makes it a practice to be fully informed on all matters concerning shuttle operations and Ares and Orion development. Details on the thrust oscillation issue were communicated to senior agency management first in October 2007 at the Exploration Systems Mission Directorate quarterly program review and again in mid-November 2007 at the Constellation integrated stack status meeting, which the NASA administrator attended.

At the November meeting, the Ares I thrust oscillation issue was addressed as a very small portion of a much broader briefing to the NASA administrator. Steve Cook, manager of the Ares Projects Office at NASA's Marshall Space Flight Center, led the discussion. In response, the administrator stated that he understood the challenge and that he was aware that a team had been working on the issue since early November.

Because it is early in the process, NASA is still working to characterize the potential impact, assess design feasibilities and formulate a plan to address the technical analysis on the thrust oscillation issue. Therefore, NASA has not held a formal briefing for congressional staff or Government Accountability Office staff but has been open about this issue since first learning about it.

Thrust oscillation is a new engineering challenge to the developers of Ares - but a challenge very similar to many NASA encountered during the Apollo Program and development of the space shuttle. Every time NASA faces an engineering challenge - and it faces many - agency engineers examine all the options for addressing the issue. NASA has an excellent track record of resolving technical challenges. NASA is confident it will solve this one as well.

Sem comentários: