Tag Archives: drones

Episode 75 — Vectored Thrust

f22Mechanical engineer Herb Roberts shares his stories of developing an advanced jet engine for the US military in this episode of The Engineering Commons podcast.

  • Jeff admits he’s never torn apart an engine, although he spent a lot of time repairing his 1968 Pontiac Firebird convertible, which had an incredible knack for discovering inopportune times to break down by the side of the road.
  • Instead of rebuilding engines, Jeff was far more likely to be found fooling around with his father’s “Trash 80” computer (from soon-to-be-defunct Radio Shack).
  • Our guest for this episode is Herb Roberts, a professional engineer who helped develop a Pratt & Whitney turbofan engine to power the U.S. military’s F22 stealth fighter.
  • Herb seemed to exhibit “The Knack” from an early age, and enjoyed building Heathkit devices.
  • While wandering around his college campus one day, Herb came across a booth run by the American Institute of Aeronautics and Astronautics (AIAA). This led him to read the books “Herman the German” and “Not Much of an Engineer.”
  • Brian mentions Ben Rich’s book, “Skunk Works: A Personal Memoir of My Years at Lockheed.”
  • Herb once worked at the Raspet Flight Research Laboratory (RFRL), named for aeronautic innovator Gus Raspet. The facility, located on the campus of Mississippi State University, is well-known for developing advanced composite materials.
  • While a student at MSU, our guest worked on some of the earliest unmanned aerial vehicles (UAVs). Herb also worked with engineers from Honda as they began their development of the HondaJet.
  • Two teams competed for the F22 contract in 1986: Team A was Lockheed/Boeing/General Dynamics, and Team B was Northrop/McDonnell-Douglas.
  • The Lockheed team produced the YF22 prototype fighter, while the Northrop team built the YF23 demonstration vehicle.
  • Herb’s employer at the time, Pratt & Whitney, developed an after-burning turbofan (the F119 engine) capable of powering either the YF22 or YF23.
  • Specific fuel consumption (SFC) describes the amount of fuel used by an engine in producing a unit of thrust.
  • A turbofan’s bypass ratio divides the mass flow rate of air drawn through the fan disk (thus bypassing the engine core) by the mass flow rate of air passing through the engine’s combustion chamber.
  • When work on the F119 engine began, Pratt & Whitney engineers didn’t know what the associated airframe would look like, or even how many engines would be required.
  • While Pratt & Whitney used advanced materials to lower engine weight, General Electric increased efficiency by implementing a variable-bypass design.
  • Herb describes a 1992 accident at Edwards Air Force Base, where an F22 crashed following a low-altitude pass over the airfield.
  • Prepared to engage in computer-aided design, our guest was mildly surprised to be assigned a drafting board on his first day at Pratt & Whitney.
  • Prior to the advent of finite element analysis (FEA), engineers would calculate stress and strains using “Roark’s Handbook.”
  • Despite its limitations, beam and plate analysis worked sufficiently well for Pratt & Whitney engineers to design the J58 engine that powered the Lockheed SR-71 Blackbird.
  • Herb worked with his colleagues to develop model elements, based on the “rule of mixtures,” that permited three-dimensional structural analysis.
  • As a result of being moved from “non-exempt” to “exempt” employment status after his first year on the job, Herb actually took a pay cut despite receiving a six percent raise, as his overtime income was substantially reduced.
  • Our guest is obviously an extroverted engineer, as he recognizes other engineers by their shoes!
  • Herb recommends that engineers try to be 500 percent better, rather than just five percent better.
  • Our guest can be reached by email: herbertroberts -=+ at +=- gmail.com. He can occassionally be found on Twitter as @too_many_rules.

Thanks to Robert Pernett for use of the photo titled “Lockheed F22 Fighter in Flight.” Podcast theme music by Paul Stevenson.