National Aeronautics and Space Administration

www.nasa.gov

Mechanical Design of Principal Investigator Rack Drawer (PIRD) for SOFIA

Airworthiness criteria provides a potential safety of the aircraft during emergency

situation. Typically, FAA has to confer the certificate of airworthiness for the design and

construction of any aircraft part or assembly to be put into operation in flight. At NASA,

AFSRB performs similar roles & responsibilities as FAA does for commercial airlines. The

design of PIRD must comply with FAA/ AFSRB airworthiness requirements. In order to

comply with FAA/ AFSRB, NASA/ USRA requires the design of PIRD to meet the load

requirement of 9g in forward; 6g in downward; and 3.5g in lateral directions, where g is the

acceleration due to gravity. The specified design load for PIRD is 50 lbs. This implies that

PIRD shall be designed for loads: 9 x 50 = 450 lbs.; 6 x 50 = 300 lbs. and 3.5 x 50 = 175

lbs.

An example of usefulness of airworthiness is the survival of almost everyone during a

recent deadly crash of Boeing 777 aircraft of Asiana Airlines at San Francisco

International Airport. Many lives were saved partly due to 9g interior design, and 16g seat load design criteria used in the construction of this aircraft.

This material is based upon work supported by the S.D. Bechtel, Jr. Foundation, the National Marine Sanctuary Foundation, the

Carnegie Corporation of New York, and/ or the National Science Foundation under Grant Nos. 0952013 and 0833353. Any opinions,

findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the

views of the funders. The STAR program is administered by the Cal Poly Center for Excellence in Science and Mathematics

Education (CESaME) on behalf of the California State University.

SOFIA (Stratospheric Observatory For Infrared Astronomy) , the world’s largest airborne

observatory with 2.5-meter diameter infrared telescope is equipped with 7 Science

Instruments (SI): EXES, FIFI-LS, FLITECAM, FORCAST, GREAT, HAWC, and HIPO.

Flying at altitudes between 39,000 and 45,000 feet, SOFIA, a 747-SP (Special

Performance) aircraft, avoids 99% of the atmospheric water vapor and is able to observe

the occultation of stars by solar system objects. By determining the size, compositions,

and atmospheric structures of these objects, SOFIA can help answer the questions on

creation and evolution of the universe, formation of the stars and planets, and nature of

black hole at the center of Milky Way galaxy.

Hari P. Shetty, STEM Teacher & Researcher *, Murali Krishna R. Kandlagunta, Mechanical Engineer **,

John Miles, SI Development Manager **, Zaheer Ali, SM Operations Lab Supervisor **

References: 1. Gima, Mike. Interface Control Document: SI-AS-01. Honeywell TSI Inc. 4 June 2004.

2. Kandlagunta, Murali Krishna R. Auxiliary PI Rack Arrangement Design & Analysis: SCI-US-REP-SE07-2042 Rev. B.

USRA. 2 Nov. 2012.

3. Middle Atlantic Product Catalog (www.MiddleAtlantic.com). D & TD Series Drawers 96-006 Rev. 3c. 31 July 2012.

4. Young, Warren C. Roark’s Formulas for Stress & Strain. McGraw Hill Inc. 6th Edition.

5. Khurmi, R. S., Gupta, J. K. Machine Design: Chand S. Publications.

6. Mahadevan, K., Reddy Balveera K. Design Data Handbook for Mechanical Engineers: KREC. 1987. 3rd Edition.

7. VOI-SHAN. Aerospace Fastener Design Manual.

8. SREWCORP. Aircraft & Aerospace Fastener Manual: Government & Industry Standards & Specifications.

9. Department of Defense. Military Handbook: Metallic Material and Elements for Aerospace Vehicle Structures. MIL-

HDBK-5H: 1 Dec. 1998.

10. www.McMaster.com for fasteners.

SOFIA with Telescope

Location of PIRD inside SOFIA

Acknowledgements: Special thanks to the following for their knowledge sharing, guidance, and support.

Chris McCarthy, Science/University Liaison (STAR); Greg Stoehr, Master Teacher (STAR), Erick Young, SMO Director

USRA; Dr. John Keller, Director, CESaME, Dr. Bryan Rebar former Director STAR, Dr. Dimitri R.D-Frazer, Director STAR,

Russ Billings, NASA/Aero Institute.

PIRD is made of CS (Carbon Steel) while its assembly parts such as Latch, C-

brackets, L-brackets, and Front plate are designed with Aluminum (AL) 6061. AL has

a density of 0.098 lbs./ inch3 and CS has a density of 0.284 lbs./ inch3. Thus, AL weighs

about 2.9 times lighter than CS. Thus, it minimizes the load on the PI Rack. In terms of

tensile strength, AL (tensile strength = 42,000 lbs./ inch2) provides about 76% of tensile

strength of CS (55,000 lbs./ inch2). Thus, AL becomes more suitable in terms of weight

and tensile strength combination for the critical load and bending moment design of

PIRD. In order to achieve maximum tensile strength at joints, Stainless Steel (SS 18-8,

SS 304/ 316) is used for fasteners & pins.

3D model of the conceptual design of the PIRD is created and developed using Creo-

Parametric 3D Modeling computer application tool by PTC. This application also helps to

perform the stress analysis. However, the stress analysis is mostly done manually to

ensure the accuracy of the calculations and to compare with the computer application

results.

Weight or Load of PIRD and its assembly parts are calculated as: Weight = Density of

the material (lbs./ inch3) x Volume (inch3). Load of each equipment on PI Rack is added

to ensure that total load of the PI rack falls below the maximum allowed payload of 600

lbs. as per ICD guidelines.

Vendor supplied Drawer is analyzed and modified for its ability to sustain the 9g loads.

Corners of the Drawer are reinforced with newly designed L-brackets. Front plate is

redesigned and reinforced with fasteners to take 9g load and support the Latch assembly.

Principal Investigator (PI) rack installed on SOFIA houses various electronic and utility

equipment required to control or monitor the SIs. A total of 3 PI racks can be installed on

SOFIA. USRA/ NASA requires a mechanical design for a PI Rack Drawer (PIRD) that can

be used for storing laptop, books, or tools. Mechanical design deliverables include:

conceptual design, 3D assembly model, stress analysis calculations, engineering

drawings, ICD (Interface Control Documentation) compliance documentation and

associated vendor documents/ drawings. These documentations are necessary to obtain

approvals from USRA, NASA, and FAA (Federal Aviation Administration) / AFSRB

(Airworthiness & Flight Safety Review Board) before PIRD is fabricated, tested, and

installed on SOFIA.

SOFIA

PIRD

Airworthiness Design

Maximum Payload Criteria

Newly Designed Parts & Assembly for PIRD

Design Modifications

New Designs & Stress Analysis

Material for PIRD Assembly

* California State University, Sacramento, CA, ** USRA / NASA Ames Research Center, Mountain View, CA

Overturning Moment is calculated as: Bending Moment (inch-lbs.) = Weight of each

equipment (lbs.) x vertical distance (inch) of its center of gravity from the reference

point, which is the base of the PI Rack. Sum of the Overturning Moments of all equipment

installed on the PI Rack is calculated to ensure that it falls below the maximum allowed

overturning moment of 12,000 inch-lbs. as per ICD guidelines.

Maximum Overturning Moment Criteria SOFIA & Hari

C-Brackets (2) are designed in order to install the PIRD on the PI Rack and are analyzed

for their load bearing capacity under shear and bending stress using sufficient thickness

and suitable material.

L-Brackets (4) are designed to reinforce the corners of PIRD and are analyzed for their

shear, tensile, and bending stress.

Front Plate made of AL 6061, T6 is designed and fastened to the bottom and sides of the

drawer’s inside using flat head bolt & nuts.

Latch Assemblies (2) is designed to have the PIRD operate under airworthiness load

conditions. 3 different designs were proposed, and one of them has been approved for the

stress analysis calculations.

Fasteners and Pins of aircraft / aerospace industry quality are selected from the fastener

manuals to meet the size requirements and are analyzed for their shear stress and pull

out/ tensile strength capacities.

Conceptual Design & 3D Models

Newly designed PIRD

Drawer by Vendor

Modified Drawer Parts

Front of PI Rack

at Lab Rear of PI Rack

at Lab

Assembly Parts of

PIRD

Exploded View of

Latch Assembly PIRD on PI Rack