Recent events in the Middle East and elsewhere point to a need for a new Gunship aircraft to respond to rapidly changing situations. According to the RFP, the number one general design requirement is to achieve high survivability versus low cost threats, such as Anti-Aircraft Artillery (AAA) and Man-Portable Air Defense Systems (MANPADS).

The design mission of the Gunship consists of takeoff from home base, climb to best cruise altitude, cruise at best cruise speed to the target area at least 500 nautical miles from takeoff, loiter at 20,000 feet for a minimum of four hours with a sustained manuever of 1.5g's, descend to 10,000 feet for target identification and to expend payload, climb to cruise altitude, cruise back to home base, loiter, and land.

The Gunship will be at risk of being shot down by enemy/terrorist (threat) weapons while on any of the mission segments. Of interest in this design competition is the segment where the Gunship descends to 10,000 feet and attempts to identify the target and deliver its ordnance. The primary threat weapons at this altitude are the AAA and the MANPADS.

Your Gunship will have a better chance of surviving a mission into a man-made hostile area when it is designed and operated to avoid being hit by any of the threat weapons in that area (referred to as low susceptibility - don't get hit) and to withstand any hits that do occur (referred to as low vulnerability - don't go down if hit). (Ref. 1, Sec. 1.1.1, What is Aircraft Combat Survivability?)

Low susceptibility is achieved by using the six susceptibility reduction (SR) concepts listed below (Ref. 1, Sec. 1.1.8, How is Survivability Enhanced? and Sec. 4.4, Task III: Design for Low Susceptibility Using Susceptibility Reduction Technology)

• Threat warning (look out for the threat weapons),
• Noise jamming and deceiving (e.g. electronic countermeasures equipment),
• Signature reduction (e.g. stealth),
• Expendables (e.g. chaff and flares),
• Threat suppression (eliminate/degrade the enemy's air defense), and
• Weapons and tactics, flight performance, and crew training and proficiency

Low vulnerability is achieved by using the six vulnerability reduction (VR) concepts listed below (Ref. 1, Sec. 1.1.8, How is Survivability Enhanced? and Sec. 5.4, Task III: Design for Low Vulnerability Using Vulnerability Reduction Technology)

• Component redundancy (with separation) (e.g. two widely spaced engines),
• Component location (e.g. no fuel near ignition sources),
• Passive damage suppression (e.g. nitrogen enriched air in fuel tank ullages),
• Active damage suppression (e.g. fire detection and extinguishing system),
• Component shielding (e.g. armored pilot seat), and
• Component elimination or replacement (e.g. electrically powered actuators)

In this design competition, both vulnerability reduction and susceptibility will be considered.

Note: The design exercise described here has been simplified for use by the student, and should not be considered as representative of any particular aircraft, weapon, or scenario. The numerical examples and functional relationships given are more complex in reality and often classified. Furthermore, there are many more design options to consider when designing an aircraft to be survivable in combat. The intent of the exercise is to provide the student with an opportunity to learn about the survivability discipline and how it affects the design and operation of aircraft. It is not to provide information and details from which the student may possibly draw erroneous conclusions regarding the impact of any particular feature on the survivability of actual aircraft.

There are many different ways to enhance an aircraft's survivability, as illustrated in Table P.2 in the Preface of Ref. 1 and on the page What is this discipline called survivability?. A trade study is conducted to determine the benefits (e.g. fewer aircraft shot down and people killed over the life of the aircraft) and impacts (e.g. increase in dollar cost due to purchase, installation, maintenance, and operations) associated with any particular survivability enhancement feature or combination of features. The trade study process is illustrated below:

The mission-threat analysis (Ref. 1, Sec. 1.1.12.1 Mission-threat analysis) identifies the weapons that are the primary threat to the Gunship on each mission. For the Gunship on the design mission described above, the threats in the target area that are estimated to have a maximum altitude of at least 10,000 feet are (Ref. 1, Sec. 3.4.1, Types of Threats):

• a light or medium caliber AAA (a gun), and
  

• a MANPADS (an infrared (IR) guided missile).

Both the gun projectile and the MANPADS missile are assumed to have a high-explosive (HE) warhead with a contact fuze -- the projectile or missile must hit the aircraft in order to damage and possibly kill it. (Ref.1, Sec. 3.4.2, Types of Warheads and Their Damage Mechanisms)

The aircraft description (Ref. 1, Sec. 1.1.12.2 Aircraft description) is your Gunship design.

The susceptibility assessment (Ref. 1, Sec. 1.1.12.3 Susceptibility assessment) determines the probability the aircraft is hit by the projectile or missile, P_H, when it is engaged by the gun or the missile system.

The vulnerability assessment (Ref. 1, Sec. 1.1.12.4, Vulnerability assessment) determines the probability the aircraft is killed when hit, P_K|H, by either the gun projectile or the guided missile.

The survivability assessment (Ref. 1, Sec. 1.1.2, How Do We Measure Survivability? and Ref. 1, Sec. 1.1.12.5, Survivability assessment) determines the probability the aircraft is killed in an encounter with either the AAA or the MANPADS (a one-on-one scenario), P_K (denoted as P_K|E in the RFP), where

     P_K = P_K = P_H•P_K|H           (Ref. 1, Eq. 1.2)

The trade study (Ref. 1, Sec. 1.1.12.6 Trade Studies and Ref. 1, Sec. 6.3, Survivability Enhancement Trade Studies) determines the benefits and impacts associated with each survivability feature considered. The benefits include the number of aircraft and lives saved by the feature over the lifetime of the aircraft (usually converted to a $ figure), and the impacts include the additional weight, acquisition and operations dollar costs, degraded flight performance, schedule delay, etc. When a specific numerical requirement is imposed on the maximum value of P_K for a particular threat weapon, the trade study determines the optimum combination of survivability enhancement features that satisfy that requirement.

Once the design is finalized, test articles are built and tested to determine the ability of the design to achieve the levels of survivability stipulated in the design requirements.

In the case of the AAA threat, the P_K (the P_K|E in the RFP) without any survivability enhancements is assumed to be 0.18. This can be reduced to the P_K < 0.10 design requirement by use of up to four of the vulnerability reduction features listed below (Ref.1, Sec. 5.4, Task III: Design for Low Vulnerability Using Vulnerability Reduction Technology):

• Fuel tank ullage protection (prevents a fuel tank explosion or catastrophic fire and subsequent loss of structural integrity)
• Dry bay fire detection and suppression (prevents the loss of surrounding structure and other critical components that are located within the bay, e.g., flight control, electrical, and hydraulic components)
• Engine bay fire detection and suppression (prevents loss of an engine)
• Redundancy (with separation) of flight critical controls (prevents loss of control of the aircraft)

Each of these VR features is designed to prevent the loss of one or more critical components on the aircraft (a critical component is a component that is essential for continuous safe flight).

In the case of the MANPADS threat, the overall P_K without any survivability enhancements is assumed to be 0.45. This can be reduced to the P_K < 0.10 requirement through use of up to four of the vulnerability reduction features listed above and one of two susceptibility reduction features listed below (Ref.1, Sec. 4.4, Task III: Design for Low Susceptibility Using Susceptibility Reduction Technology):

• a flare dispenser (the flares draw the IR missile away from the aircraft)
• a MANPADS IR Countermeasures (IRCM) system (a modulated electromagnetic signal confuses the missile tracking system)

Each of the above survivability enhancement features can achieve part of the overall P_K < 0.10 design requirement for each threat. Your team must decide which measures to incorporate in the design to meet the design requirements for both the AAA and the MANPADS and to determine the total system cost and weight with the selected features included.

The data for each of the four vulnerability reduction features and the two susceptibility reduction features are given below.

Vulnerability Reduction Features for Both the AAA and the MANPADS Threats

Susceptibility Reduction Features for the MANPADS Threat

(1) Determine the VR and SR features that you want to add to the design of your Gunship to satisfy the survivability design requirement of P_K < 0.1 for both the AAA and the MANPADS threats. Use the methodology described below to determine the value of P_K for your selected design.

For the AAA threat and the baseline design with no VR features

     P_K (baseline) = [P_H (baseline)]•[P_K|H (baseline)] = 0.18

For the more survivable aircraft with one or more SR or VR features

     P_K (more survivable) = [P_H (more survivable)]•[P_K|H (more survivable)]

Dividing the second equation by the first equation and solving for the P_K (more survivable) results in

     P_K (more survivable) = 0.18•SR•VR

where

     SR = the relative change in susceptibility = [P_H (more survivable)]/[P_H (baseline)]

and

     VR = the relative change in vulnerability = [P_K|H (more survivable)]/[P_K|H (baseline)]

For example, suppose you select fuel tank inerting in all of the fuel tanks as one VR feature to use. Accordingly, the reduction in the P_K|H is 30%, and hence VR = (1 - 0.3) = 0.7. Therefore, the new P_K is

     P_K (more survivable) = 0.18•[SR = 1]•[VR = 0.7] = 0.126          (fuel tank inerting)

This value is still above the design requirement of less than 0.10, so another VR feature must be added, such as dry bay fire protection in 50% of the dry bay volumes. The dry bay protection provides a P_K|H reduction of 25%, and consequently the P_K is further reduced to

     P_K = 0.18•[SR = 1]•[VR = 0.7•0.75] = 0.095       (fuel tank inerting and dry bay protection)

Although this value of P_K is less than the design requirement, this particular combination of VR features may not be the best combination when the associated weight and cost impacts are considered. Furthermore, the MANPADS threat must also be considered using this same methodology with a reduction in both susceptibility (SR) and vulnerability (VR). Thus, you must examine all possible combinations of the VR and SR features to determine the 'best' combination of features that satisfies the design requirement of P_K < 0.1 for both threats and has the 'least' impact on the final weight and cost figures.

(If only it were this easy in 'real life'! In reality, we don't have the luxury of simple vulnerability reduction relationships like those given above that can be applied to our vulnerability measures. Instead, we use detailed models to simulate the effects of thousands of different shots at and hits on an aircraft. However, because our purpose here is to educate you on the potential value of these (and other) susceptibility and vulnerability reduction features, we hope you'll take the next step to learn more about survivability as described in Ref. 1.)

(2) Answer the question 'Which features, if any, would you add to your Gunship design if there was no survivability requirement?' and explain your answer. (Hint: System Safety and Combat Survivability have a common goal -- see Ref. 1, Sec. 1.1.15, Relationship between the Combat Survivability Discipline, .... , and the System Safety Discipline for Military and Civilian Aircraft)

If you have any questions, please send a message to Prof. Robert E. Ball at reball@redshift.com