Which face mask to make?

During the coronavirus pandemic, face masks have become an important tool for preventing transmission of the virus and protecting health-care workers, but commercial face mask supplies are low. Members of the public are mobilizing to make face masks, but there are many different designs and options that have different purposes. I've created a web page to help one determine which face mask to make.

(Image credit: UnityPoint Health, Cedar Rapids, Iowa.)

The Risk of Space Junk

Image credit: NASA


The November 2019 issue of Prism (published by ASEE) includes an article by Thomas Grose about the risks associated with space junk in low earth orbit (LEO).  The likelihood of a collision continues to increase: 4000 satellites and the International Space Station (ISS) travel in LEO, which has 128 million bits of junk; 20,000 pieces have a diameter greater than 10 cm.  The consequences of a collision could be more debris, a damaged satellite (which could interrupt communication or other services), or a casualty on the ISS.   A possible worst case is the Kessler syndrome (a chain reaction of debris-generating collisions).

The current mitigation efforts include rules to reduce the growth of space junk and a system for detecting possible collisions (so that spacecraft can be moved out of the way).  NASA has a technical standard, "Process for Limiting Orbital Debris," that requires space programs to assess and limit the likelihood of generating debris during normal operations, explosions, intentional breakups, and collisions.  "Orbital debris" is defined as follows:

Artificial objects, including derelict spacecraft and spent launch vehicle orbital stages, left in orbit which no longer serve a useful purpose. In this document, only debris of diameter 1 mm and larger is considered.

The NASA standard also discusses reducing the likelihood of collision by reducing a spacecraft's cross-sectional area.

New systems for tracking more space junk more precisely (e.g., the Space Fence) could lead to an automated "traffic control" system that warns a spacecraft operator when a collision is imminent while reducing the likelihood of false alarms.  An alarm is costly because it disrupts normal operations, and the spacecraft must burn fuel to move away from the space junk and then return to its normal position.

Researchers are also developing spacecraft that can capture space junk, which could reduce the likelihood of a collision.

The article mentions few efforts to reduce the consequences of a collision.  Astronauts in the ISS can head to a shelter if a close call is imminent.  But hardening a satellite would require more mass, which makes it more expensive.  Perhaps we need "shatterproof" materials or designs for spacecraft.

Robust Multiple-Vehicle Route Planning

Planning problems with multiple vehicles occur in many settings, including the well-known vehicle routing problem (VRP) and drone delivery operations, including the flying sidekick problem.   Most approaches to these problems assume that the vehicles are reliable and will not fail.

In the real world, however, a vehicle could fail, in which case the other vehicles would have to change their routes to cover the locations that the failed vehicle did not visit.  In recent research done at the University of Maryland, Ruchir Patel studied this challenge and developed approaches for generating robust routes for multiple-vehicle operations.  His thesis, Multi-Vehicle Route Planning for Centralized and Decentralized Systems, describes the results of his research.  The key idea is to consider the worst-case performance of a solution instead of the best-case performance.  A solution with better worst-case performance is more robust and will perform well even if a vehicle fails.

He found that a genetic algorithm (GA) could find high quality solutions, but the computational effort was substantial because evaluating the robustness of a solution required evaluating all possible failure scenarios and generating a recovery plan for each one.  His approach used Prim's Algorithm to generate a minimum spanning tree and construct a recovery plan quickly.  Although the computational effort may be acceptable for pre-mission planning, faster approaches could be useful for real-time, online planning.

Managing the risk of wildfires

In the August 25, 2019, issue of the The Washington Post, Steven Pearlstein wrote a column about how San Diego Gas & Electric (SDG&E) is pro-actively managing the risk that their power grid will cause a wildfire.  Some wildfires are caused when trees or branches fall and hit a transmission line and then the energized power line sparks a fire on the ground.  The power company's approach addresses two potential problems: damage to the transmission line, and the transmission line sparking a wildfire.

For the first risk, SDG&E is burying transmission lines in high-risk areas in the mountains and backcountry.  This is a preventive action because buried transmission lines can not be damaged by falling trees and branches.

For the second risk, SDG&E has installed a system that monitors transmission lines in remote areas, detects when a transmission line is damaged, and turns off the power to that transmission line instantly.  This contingency plan (turn off the power if the line is damaged) prevents the lines from sparking a wildfire.

Moreover, SDG&E is monitoring the conditions in the high-risk area and calculating the conditional probability that a spark would lead to a large wildfire in those locations.  If that conditional probability becomes too high, then the risk has increased to an unacceptable level, and the power company cuts the power to that transmission line (even though it is not damaged).  From Pearlstein's column:

“We are doing with wildfires what the National Weather Service has done with hurricanes,” said Scott Drury, the utility’s president. ... The wildfire model gives SDG&E the confidence to shut off power because it can pinpoint precisely where and when the risks are at levels that have resulted in disastrous fires in the past.

Pearlstein's article also discusses how people in the area are managing the risks associated with a power outage: parking outside their garages, storing water, and buying generators.

Too Many Warnings?

At 5:35 P.M. on September 17, 2018, the University of Maryland Police Department (UMPD) sent a tornado warning by email and text message to university students, faculty, and staff: "A Tornado Warning has been issued for the UMD campus. The sirens will be activated. Seek shelter immediately, avoid windows."  This message did not mention that the warning was issued by Accuweather.  The National Weather Service (NWS) did not issue a tornado warning.  At 6:04 P.M., the UMPD sent the following message: "The Tornado Warning that was issued by AccuWeather is now cancelled as of 6:00 PM."

Part of Accuweather's business is generating severe weather alerts for its clients. Its website touts the benefits of Accuweather's warnings and its "null tornado notifications" that a NWS tornado warning will not affect a client's facility.  The website discusses the benefits of knowing that a tornado warning is not a direct threat, so a shutdown is not necessary, and claims that Accuweather's false alarm rate is lower than the NWS false alarm rate (10% to 80%).

When one organization issues a warning while the other does not, the inconsistency could decrease trust in warnings, increasing the likelihood of ignoring warnings.
An article in The Washington Post about the false alarm (Angela Fritz and Sarah Larimer, "Red flags at U-Md. over false alarm for tornado," September 19, 2018) quoted Gary Szatkowki, a former NWS meteorologist, who asked "What do we want to do about a weather organization issuing a tornado warning when the Weather Service does not?"

Although the University of Maryland is a large institution (approximately 40,000 students), its cost of a false alarm is smaller than the same cost for a factory (where a shutdown directly impacts throughput and revenue).  Moreover, many students and others are outdoors, and the university has a strong desire to be viewed as a safe place, so the potential impact is large.

In that case, the university should be willing to tolerate more warnings.  Too many warnings?  No, in this case.

Improving Decision-Making Processes

The WomenCorporateDirectors Foundation’s Thought Leadership Commission and the KPMG Board Leadership Center issued a report that describes some of the problems that can cause poor decision making and recommends improving decision-making processes.  The report discusses incomplete information, groupthink, overconfidence, and other poor practices as causes of decision-making failures. 

The report presents five decision-making styles (first presented in an article by Dan Lovallo and Olivier Sibony):

• Visionary,
• Guardian,
• Motivator,
• Flexible, and
• Catalyst.

Each style has its strengths and weaknesses.  I would add that a decision-maker needs to use the right decision-making style for the decision-context that is present (see more about this at this blog post). 

The report also discusses ways to create a more inquisitive, risk-based decision-making process by considering multiple viewpoints, identifying the pros and cons of every alternative, and discussing the associated risks (what could go wrong).  The resulting process will resemble the "discovery decision-making process" described by Paul Nutt.

Finally, the report recommends evaluating the decision-making process, not its outcomes, as a way to identify opportunities for improvement.

HT: ISE Magazine

Scientists describe. Engineers decide.

The May, 2018, issue of Mechanical Engineering magazine included the article "Without Engineering, Civilization Does Not Exist" by Adrian Bejan.  I wrote the following letter about this article; my letter appeared in the July 2018 issue.

To the Editor:

I wholeheartedly concur with Adrian Bejan's conclusion that engineering has transformed our society for the better.  Unfortunately, Professor Bejan's definition of engineering as type of science was inaccurate, which contributes to the public's misunderstanding of our profession.

The article stated that "engineering is a science of what is useful" and "engineering is [a] body of science."  It concluded by repeating that engineering is "the science of useful things."

Although engineers need to learn science and some scientists do some engineering, engineering is not a type of science, for it has a different objective.  Scientists study the world in order to understand it, but engineers design new products and systems.  Scientists describe; engineers decide.  Henry Petroski discussed this distinction and provided more ways to view it in An Engineer's Alphabet.  Professor Bejan was closer to the truth when he described engineers as those who "are developing new contrivances and improving old ones."

Jeffrey W. Herrmann