Memo From SOFIA Chief Engineer to NASA Chief Engineer Chris Scolese regarding SOFIA Technical Issues

Status Report From: NASA HQ
Posted: Friday, March 3, 2006

Editor's note: Rep. Ralph Hall made specific mention of this memo in a question to NASA Science Mission Directorate Associate Administrator Dr. Mary Cleave during a House Science Committee hearing on NASA's FY 2007 science budget on 2 March 2006.

Date: Fri, 24 Feb 2006 10:49:59 -0800
To: Chris Scolese
From: Nans Kunz
Subject: SOFIA Technical Issues
Cc: Joel K Kearns, "Carol W. Carroll"


I am the Chief Engineer of the SOFIA Program and under the current implementation of the iTA I am also the Systems-Technical Warrant Holder (STWH). This email is in regard to the SOFIA technical issues that were raised during the House Committee Hearing for NASA's 2007 Budget Proposal last week (February 16th).

Although I did not have the opportunity to watch the hearings live, because of my position on the program, several people have asked me about the technical issues that were raised and I have since seen video clips from a portion of the hearings. In any case, I thought because of your position, that you too may be asked questions with regard to SOFIA's technical issues. Therefore, perhaps I can provide some information that might be helpful in answering questions that may come your way.

First, the program status in regard to "cutting the hole and putting in the telescope". The structural modification of the fuselage is complete (the hole was not actually cut) and the telescope is completely installed and is functional. We have already performed ramp operations where the aircraft was rolled out onto the ramp and the telescope was operated to track Polaris and perform various coordinate system calibrations. Further, the cavity door is installed over the cavity opening and is locked in place and ready for the door closed flight tests (we are not planning to open the door during the first test flights).

With regard to "laminar flow over the opening" being an issue. Currently there are no outstanding issues in this area that are preventing us from going into the flight test phase. The technical challenges and risks related to the flow over our open port cavity was recognized early in the program and therefore significant efforts have been expended on analyses and testing to development the shear layer control configuration that we have implemented on SOFIA. As a recognized risk, this issue has been scrutinized by external independent review teams on a regular basis throughout the Program life starting well before approval to proceed into development. The most recent review related to this issue was conducted by an NESC blue ribbon panel of experts chaired by Mike Kehoe. The final report for this review was just delivered last October (2005). This report finds that the program has been prudent in it's approach, that the configuration implemented is technically sound and that the program should proceed with the plans to go full scale flight test.

A little more background on this issue:

First, since our cavity opening is located in the aft fuselage we will not have laminar flow over the opening. The flow over this part of the fuselage is a fully developed (turbulent) boundary layer. Measurements & analyses have been completed to assess the degradation to the incoming light that will be caused by the shear layer and these estimates were evaluated by the science community well before we were allowed to proceed into development. In short, this degradation only effects the science in the visible and near IR wavelengths, and was found to be compatible in regard to being able to perform outstanding science in these wave lengths. Further, this degradation does not effect the longer wavelength portion of the spectrum in which SOFIA will also operate allowing outstanding science in this portion of the spectrum as well.

With regard to the technical risks of flying with a very large hole in the side of the fuselage including cavity acoustic resonances or aircraft controllability issues. These risks were identified as some of the key technical risks early on during the Phase A/B feasibility and conceptual studies. As a result we believe appropriate attention, analyses, testing has been conducted throughout the program to address and mitigate these risks. We do have the heritage of SOFIA's predecessor the KAO which contained a 36" telescope in a similar open port cavity design which operated successfully for over 20 years. Because the SOFIA telescope and the associated hole in the fuselage is significantly larger than what was done previously, we started wind tunnel testing well before Phase C/D development began. As a result, we have had five different wind tunnel test series that addressed the shear layer control and cavity acoustics issues starting in 1990 and concluding in 1997. This was followed by a couple more wind tunnel tests using a smaller 3% scale model to verify the effects (or lack of) of the cavity opening on the overall handling characteristics of the Boeing 747-SP aircraft. Although the risk of encountering a technical problem when we proceed into the open door flight testing is not zero, we believe that we have good rationale to be optimistic and expect the SOFIA cavity to behave as predicted. Furthermore, in the small chance we do encounter some cavity acoustics performance issues, we have back-up solutions that are ready to implement.

I hope you find this email helpful with the appropriate level of detail. Of course if you have any questions please feel free to call me and I'd be happy to discuss any of the technical issues associated with SOFIA. We are planning to get to our first test flights this year so you are likely to see more of SOFIA in the coming months.


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