System Development Approaches and Human Factors in Software Engineering

Waterfall Approach for Complex Systems

Answers are expected to two questions. Answers to only two questions will be marked.  Attempted solutions that you do not wish to submit should be crossed out. If you do attempt more than two questions, and do not identify which two you want to be marked, only the first two in the answer book will be marked. For each question, the distribution of marks out of 20 is indicated in brackets.

1. Question 1 

a.Define the waterfall approach and briefly discuss whether it is suitable for complex system where the requirements are not well-understood and subject to refinement and change.  

b.Describe any two alternative system development approaches that could be used in circumstances where requirements are not well-understood and subject to change.

c.i) Explain the V- diagram, commonly used for verification and validation of the quality of a system. Use a figure to support your answer.

ii)Briefly discuss the differences between verification and validation.

d.With respect to system engineering, define the term quality and also explain how this is different from the meaning of this word in common usage.

2. Question 2

a.You are part of a team involved in the development of a new railway locomotive control room system. What human factor - related issues are likely to occur while preparing the requirements and design of the instruments panels and control of the system? Define and clearly distinguish physical and cognitive human factor issues.  

b.Preliminary mass budget has been prepared for a pre-phase A study of a moon atmospheric probe. The science payload sub-system consists of six instruments of five different types, labelled A-E. The instruments measure different parameters, some are new or modified designs and some have been flown on previous missions. A sub-system mass budget for the science payload is given in Table Q2.1. The mass budget for the whole probe system is given in Table Q2.2.

i)Briefly discuss why different percentage mass margins are commonly applied to different elements of a sub-system. Use the margins given in Table Q2.1 to inform your answer.

ii)Complete the missing numbers of the science payload sub-system and the probe system-level mass budget to calculate an overall budget mass for the system and the overall margin held at system level. You should apply a 10% sub-system margin for the science payload and a 20% system-level margin.

iii)Discuss whether and why? the value of the overall margin calculated in (b) (ii) is likely to be appropriate for this type of spacecraft at this stage of the system’s life cycle.

Table Q2.1- Science payload sub-system data for a moon probe

Element Unit Mass (kg) Number off % Margin

Instrument A 1.2 2 5

Instrument B 4.5 1 5

Instrument C 2.1 2 20

Instrument D 1.8 1 15

Instrument E 2.5 1 10

Question continues overleaf…

Table Q2.2- System Mass data for a moon probe

Sub-system Best Estimate Mass Budget mass (kg)

Structure 87.0 107.0

Communications 3.3 3.9

Data handling 5.1 5.9

Thermal 76 93.5

Mechanisms 2.5 2.9

Power 35 39.6

Science payload - -

Harness 5.1 5.7

3. Question 3 

a.A novel re-useable launch vehicle uses a hydraulic system to move flight controls and other equipment. It is split into three sub-systems with pressure being provided from a number of different sources to aid safety and reliability. A schematic diagram of the system is given in Figure Q3.1.

The sub-systems are called Green (G), Blue (B) and Yellow (Y). Any one of these sub-systems can power the primary flight controls required for safe flight.

The hydraulic pumps are powered from various independent sources: the auxiliary turbine generators, electric motors or by a Ram Air Turbine. A Pressure Transfer Unit (PTU) can pressurise the Green Sub-System using pressure from the Yellow sub-system.  

The vehicle will be uncontrollable if hydraulic power to the primary flight controls is lost, so a Fault-Tree has been prepared (Figure Q3.2) to calculate the probability for this event to occur during a flight.

Complete this fault tree by calculating the following missing event probabilities indicated in Figure Q3.2, stating your assumptions:

i.No pressure available from the Ram Air Turbine.

ii.No pressure in the yellow (Y) sub-system

iii.No hydraulic pressure in all hydraulic sub-systems

iv.No hydraulic fluid in all sub-systems

v.No hydraulic supply to the primary flight controls during atmospheric flight.

(b) Define the terms redundancy, diversity and separation in the context of reliability assurance. Give two examples of how they have already been applied in applied in the system architecture of Figure Q3.1 or how the system architecture could be enhanced.

c) Sketch and label a ‘bathtub’ curve representing a component’s reliability. Also briefly discuss the kind of products which for the ‘bath tub’ curve is appropriate and not appropriate.