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1.  Why does the author call engineering 'contradictory'?  What does this mean for design, and design for reliability in particular? Can you give an illustrative example? (at least 200 words)

 

The author describes engineering as contradictory because at once “design requires creativity, yet not the unbridled creativity of the artist, for the engineer is confined by physical, legal, financial, and other constraints”. This contradiction arises especially in cutting edge and extremely innovative designs which push such limitations. When designing for reliability, reliability often comes at a cost. So a fine balance is sought as contrasting requirements must be met. Sometimes, the designs that best meet requirements are so radically different that they have an almost art-like character.

 

An example is the Voyager designed by Burt Rutan. His designs have a unique look and some may consider him something of an artist-engineer. His aircraft designs frequently pushed the boundaries and challenged conventions. The Voyager was no different. The Voyager was designed for the sole purpose of being the first aircraft to fly around the world without stopping or refueling. The design pushed the limits of endurance, reliability, and even human comfort.

 

One of the requirements is that the plane must be capable of carrying enough fuel to go around the world without refueling [1]. So this meant that the structure had to be optimized with the airframe weight being as low as possible to maximize performance. The result is that the unfueled Voyager weighed 250 lbs but when fueled, weighed 9694.5 lbs. However, this fuel had to stored somewhere and asides from the fuselage, the only other place was the wings. During takeoff on the record setting flight, the wings were so heavy that the wing tips scraped the ground and were damaged. This demonstrated that the fine balance between fuel capacity and structural integrity was met but with only a small margin. Another example is that to be as aerodynamically efficient as possible, aerodynamic stability had to be compromised. The result was a plane that was nearly unstable in pitch (up and down motion). Additionally, due to weight minimization, the structure was rather fragile and the pilots were forced to detour around a poor weather which consumed crucial fuel. After a tenuous flight, the plane landed with only 1.5% of its starting fuel.

 

The success demonstrated that the aircraft met all its requirements and to do so, only was designed with as much as necessary and not more but no less. Such is the fine art of multi-disciplinary optimization that Burt Rutan somehow mastered even at a time when modern engineering computing tools have yet to be developed. The best designs meet as much of the requirements as possible but don't waste resources trying to do better especially at the expense of other requirements.

 

[1] https://www.centennialofflight.net/essay/Explorers_Record_Setters_and_Daredevils/rutan/EX32.htm

 


 2.  Space X just had a successful launch of their "Falcon Heavy" rocket.  What engineering risks did the designers have to consider when designing the rocket and its launch system? Can you describe at least four? (I am sure there are many).  Can you find any comments from the engineers (or from Elon Musk, the head of Space X) about whet they learned from past failures? Please include any references used. (at least 300 words)

 

One risk is due to bird-strikes. Even slight damage to various parts of a launching rocket may have severe implications. [1] Potential solutions may involve using noise makers to scare birds away and also shielding vulnerable areas.

 

Another concern is engine failure which may lead to catastrophic loss of control. For that reason, the Falcon Heavy with three cores had each core have nine rocket engines. If a single engine failed, the remaining engines would give the rocket some semblance of stability both during launch and recovery. Elon Musk compared this to the current paradigm of distributed servers over a wide geographic area rather than the use of a single, large mainframe. [2] This desire for redundancy is evident in the use of at least two engines in nearly all large jetliners.

 

Another concern is icing build up and breakup during launch. The very low temperatures of liquid oxygen in the rockets results in ice build up on the outside. This ice may break up due to aerodynamic forces and vibrations during launch and fall to the side towards the smaller boosters. Though this is a concern, icing breakup is not a major problem and mitigation primarily involves concealing sensitive equipment from falling ice. Elon Musk has stated that it is one of the risks but from his tone, it the risk can be treated as minimal though not entirely negligible. Alternatively, the side boosters can have their tops shielded. De-icing systems for rockets are not, to my knowledge, in use.

 

Finally, another concern is electronical failure. A space vehicle of the scale of the Falcon Heavy is likely to have very complex electrical systems. Because of this complexity, there are many more points of failure. Thus, the Falcon Heavy is certain to have redundant systems as well as multiple backups and fail safes [3]. An example might be triple redundant sets of inertial measurement units which sense motion in 6 axes. “Decisions” are made through “votes” in which if a disagreement between the sensors exist, the two sensors that indicate the most similar results are assumed to be correct and the abnormal measuring sensor is assumed to be faulty.

 

[1] https://www.space.com/6323-bird-strikes-threaten-space-shuttles.html

[2] https://arstechnica.com/science/2018/02/musks-inspiration-for-27-engines-modern-computer-clusters/

[3] https://spaceflight101.com/falcon-heavy-demo/falcon-heavy-launches-on-inaugural-flight/

 

 

3. Please answer the "For Further Exploration" problem on page 61 of "Lessons Amid the Rubble". Please include any references used. (at least 300 words)

 

In June 28th, 2016, Facebook’s solar powered drone called Aquila crashed in the Yuma, Arizona desert.

 

In December 2016, the National Transportation Safety Board completed its investigation reported that during an autopilot landing following a successful 90 minute test flight, the aircraft encountered winds in excess of the test limit of 7 kts. These wings then increased to 10 kts at ground level and 12 to 18 knots at flight altitude. During this turbulence, the NTSB states that a post-flight analysis of telemetry data indicates aircraft motion consistent with turbulence. On final approach, at 20 ft above ground level, the right wing saw structural failure with a downward deflection resulting in a crash and catastrophic damage. The NTSB found that the structural failure was due to the autopilot trying to fight the an upwards gust by trying to nose the plane downwards, resulting in increased airspeed then in leveling back onto the glide path, had to pull the plane back up while in excess speed. This combination plus the uncommon flying wing structure produced torsional effects on the wing that causes structural failure. In conclusion the NTSB determined that the crash was a result in the aircraft’s structural limits being exceed after being disturbed by gusts beyond the autopilot’s capabilities. The NTSB’s reporting can be described as methodical and scientific.

 

The Verge’s reporting on these findings are in contrast and features writing expecting of a popular publication. For one, they stated that “Elated [by the success of test flight], they decided to keep it [the plane] aloft three times longer than planned.” This writing can be described as sensationalizing because the FAA reported that during the 90 minute test flight, no anomalies were observed and weather was favorable. However, The Verge’s writing seems to indicate that Facebook engineers pushed the plane to some limit. Of course, it is possible that if the plane landed earlier, gusts may have been avoided; However, these gusts were not forecast for the time even at landing. Unlike the NTSB which only describe the results of their investigation, The Verge also reported on the aftermath and quoted Facebook engineers as now claiming that the autopilot flaws have been corrected. The Verge also provided comparisons to Google’s crashed drone which failed under similar conditions.

 

Overall, the NTSB’s reporting is described as very focused, detailing only the crash and the resulting investigation reports. The Verge, however, writes in a more popular manner and also describes the context of the flight and made comparisons to Google’s drone crash. Their coverage could be described as being less rigorous and slightly sensationalizing. However, their reporting is still valuable in that unlike the NTSB, they discuss the aftermath of the crash.

 

[1] https://www.theverge.com/2016/12/16/13983868/facebook-drone-crash-aquila-wing-failure-ntsb-report

[2] https://www.ntsb.gov/_layouts/ntsb.aviation/brief.aspx?ev_id=20160701X62525&key=1

 

 

 

 

 

 

DRAFT: This module has unpublished changes.