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The four-hour loiter is conducted at a best endurance speed (Vbe) of 269 kts at 20,000 ft., and
attack, at a best endurance speed of 225 kts. As shown in Figure 3.24 above, Firefox has ample thrust to
sustain the recoil force of the 105mm and 40mm cannons. The maximum speed at attack altitude is 465 kts,
which allows Firefox to quickly leave an engagement area if necessary.
The rate-of-climb performance was determined for a range of aircraft speeds and altitudes, as
shown in Figure 3.24.
Figure 3.24: Rate of climb and maximum rate of climb
The maximum rate-of-climb from each altitude was used to determine the cruise, service, and
absolute ceilings (also shown in Figure 3.24), and was used to compile a flight envelope. The flight
envelope in Figure 3.25 shows that all key points called for by the RFP fall well within Firefox’s operating
region.
Figure 3.25: Flight envelope
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3.3.2 Takeoff and Landing
The RFP balanced field length takeoff requirement is 5,000 ft. over a 50 ft. obstacle on a standard
day, while the landing requirement is 5,000 ft. over a 50 ft. obstacle on a wet runway on a standard day with
60% fuel burned. The RFP also requires that the maximum landing weight be equal to 80% of the TOGW.
As illustrated in Figure 3.26, both landing and takeoff distances were met without excessively exceeding the
requirements and increasing costs.
Figure 3.26: Takeoff and landing distances
The BFL distance was 4,556 ft. and the total landing distance for a wet runway was 4,551 ft.
Landing distances for other surfaces such as grass and dirt runway surfaces are presented below in Table
3.4.
Table 3.4: Takeoff and landing distances
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These surfaces were analyzed to account for the various conditions encountered when operating
out of austere and forward airbases, and show that Firefox meets these BFL requirements as well.
3.4 Structures, Materials, and Weights
3.4.1 Velocity-Load Diagram
The velocity-load (V-n) diagram in Figure 3.27 shows the maximum loads the pilot can induce at
various airspeeds. The RFP required a maximum positive load factor of 3.5g and a maximum negative load
factor of 1.2g. A 1.5 factor of safety resulted in ultimate load factors of 5.25g and -1.8g, positive and
negative respectively.
Figure 3.27: V-n diagram
The diagram shows the maneuver envelope in green and the gust envelope wiith dashed lines. The
gust envelope is represented for vertical gusts of 66, 50, and 25 fps. As illustrated by Figure 3.27, the gust
envelope is the limiting load factor for the structure. The cruise speed of 325 knots equivalent airspeed
(KEAS) gave a limit speed or maximum dive speed of 410 KEAS.
3.4.2 Landing Gear
A tricycle configuration was selected with dual wheel layout on both the main and nose gears,
because it provides excellent stability on all types of runways, and stability during braking and ground
maneuvers. At a typical landing angle, the relative location of the main assembly to the aircraft center-ofgravity (CG) produces a nose-down pitching moment upon touchdown. This moment helps to reduce the
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angle of attack of the aircraft and thus limits the lift generated by the wing. In addition, the braking force,
which acts behind the aircraft CG, has a stabilizing effect, enabling the pilot to make complete use of the
brakes.
The longitudinal location of the front and main landing gear was determined primarily from the
forward and aft CG limits. While vertical travel of the CG was also examined, only the upper boundary was
used, as it represented the most adverse condition.
The spanwise location of the main landing gear was determined by Figure 3.28, and the tire-ground
contact point had to fall within the design space, with shock struts fully-extended, to fulfill all landing gear
placement constraints.
Figure 3.28. Main landing gear location constraints
The main gear was placed to efficiently transmit the loads to the wing spar as well as the structure
supporting the 105mm cannon. As pictured in Figure 3.29, the retractable gears swing inboard cleanly into
landing gear bays behind the spars, eliminating the need for bulky, drag inducing fairings.
Figure 3.29: Main landing gear retraction
The nose gear retracts into compartment, just forward of the cockpit, and is protected by spectra
armor to prevent an exploding tire from injuring the crew. The longitudinal location was driven by the
location of the main gear, and a final load ratio of 8:92 was achieved with the main landing gear supporting
92% of the aircraft weight.
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Figure 3.30: Forward landing gear
Tipover angle was examined to size landing gear strut lengths, and Firefox has a tipover angle of
17º, approximately equal to the recommended 15º design point used for most aircraft.
Figure 3.31: Aircraft tipover angle
The next step was to analyze the load distribution with gears in down and locked position, with the
wheel center three inches aft of the strut centerline to provide adequate shimmy prevention. Raymer’s
method was used to calculate main landing gear and nose landing gear loads of 20,000 lbs and 10,000 lbs
per tire, respectively.24
The austere field requirement put important constraints on tire pressures. As a measure of
validation and also to determine the number of allowable passes on austere fields, California Bearing Ratio
(CBR) tables were referenced and analyzed in conjunction with the tire pressure limitation. Based on the
footprint area of the tires selected, distance between adjacent tires, and CBR tables, the allowable number
of aircraft passes was calculated to be 1,400 passes for the nose gear and 2,000 passes on the main gear
for operations on hard sand fields, placing Firefox within maneuvering limits for austere field requirements.
For the nose gear, B.F. Goodrich, Goodyear, and Bridgestone manufacture sets which met both the
pressure and loading requirements however, only Goodyear Type III 15.00-12 tires were compact enough
to fit in the nose gear bay. Only Goodyear Type III 17.00-16 tires met the required loading while falling
within the 60 max psi limit.
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3.4.3 Fuselage Structure
One of the primary structural concerns was the 20,000 lb recoil force from the 105mm cannon. To
ameliorate this issue, the cannon was tied directly in-line with the wing spars, thus directing the recoil into
the strongest structural element of the aircraft.
The A-10 Thunderbolt, which also carries a 20,000 lb recoil force gun, provided guidance for
spacing the fuselage rings around the 105mm cannon, which were spaced 12 inches apart. Additionally,
historical data from Roskam29 of aircraft with similar TOGW and mission profiles, led to the selection of a
fuselage ring spacing of 20 inches apart. Structural features can be seen in Foldout 2, page 54.
3.4.4 Wing Structure
The wing structure of the aircraft is shown in Figure 3.32, and is composed of ribs spaced 18 inches
apart along the span of the wing. Integrally machined leading edge, center, and trailing edge spars are
located at 17%, 45%, and 65% chord, respectively.
Figure 3.32: Wing structure
The third spar was implemented due to increase the survivability under heavy fire. A fourth spar
was analyzed; however, it would have increased the structural weight of the aircraft, and decreased wing
volume for fuel, and made it impossible to retract the landing gear into the wing-root.
54
Nose Gear
Main Landing Gear
Forward 40mm Cannon
Structure
•
•
•
3rd Spar for increased
survivability
Rib Spacing: 18 inches
Average Frame
Spacing: 20 inches
Aft 40mm Cannon
Structure
Foldout 2
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3.4.5 Materials Selection
Composite Materials
In recent years, there has been an increasing trend in the use of composite materials, especially
combat aircraft, which rely on them for ballistic protection. The F/A-18 Hornet, Joint Strike Fighter, the X45A, along with Lockheed Martin’s F-35, all employ extensive use of ballistic-resistant composites for skin
panels and structures. As progression continues toward more extensive use of composites in manufacturing
aircraft, the tools and equipment required to create composites will become readily available, lowering
costs.
Thermosets and next generation composites
In the past, composites have proven to be difficult to manufacture and repair however, current
development have improved these issues. Traditional composites use thermoset resins which possess low
fracture toughness, leading to the brittle behavior that complicates maintenance and repairing procedures.
Moreover, proper servicing of damaged thermoset composites requires a high temperature environment,
and adds further to the level of complexity33. A new generation of composites feature thermoplastic resins,
which in contrast to thermosets, exhibit a much higher degree of toughness and are capable of withstanding
impact without adverse de-lamination or fracture characteristics33. Also, thermoplastic resins have a much
lower servicing temperature, making them easier to repair and allowing composite structures to be serviced
in the field with portable equipment33.
Aramid fibers can be integrated with thermoplastic resins, resulting in extraordinary ballistic
resistance, with tensile strengths nearly ten times that of aluminum with half the density.33 However, despite
its performance in tension, aramid fiber composites fare poorly in compression. To enhance the
compressive ability, a hybrid composite of aramid and carbon fibers produces a material combining the
high-impact resistance and tensile strength of aramid along with the high-compressive strength of carbon.
Improvements in aramid fibers has led to the development of Spectra, a third-generation ballistic
composite armor patented and produced by Allied Signal. Unlike Kevlar or other aramid fibers, Spectra
Shield material is unwoven and is instead constructed from thousands of strands of unidirectional fiber
layered in 0° and 90° angles, bonded with resin and sealed between thin sheets of polyethylene film. The
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elasticity of the Spectra fiber eliminates the de-lamination upon ballistic impact, a characteristic of traditional
composite materials. Additionally, the unwoven bi-directional pattern keeps high velocity metal jacketed
rounds from pushing the fibers apart and transfers energy away from the point of impact in all directions.
The result is improved performance in fragmentation, multiple-hit, angle shot, and high velocity impact
situations. At $1.00 pound per square ft., Spectra Shield costs about twice as much as traditional aramid
fibers, however results in a 30% improvement in RHA penetration. Spectra shield is currently used to
protect AC-130U Spooky cockpits.
Materials Breakdown
Firefox employs aramid-carbon fiber thermoplastic composite panels on the wing skin to enhance
the damage tolerance and survivability of the gunship. Research shows that new technologies in fabricating
composite material provide a natural ability for the composite panels to induce crack arrest in the event that
it sustains penetration, decreasing maintenance costs. Moreover, separate panels allow for easier repair
processes in the event that significant damage is sustained since only the damaged panels needs to be
serviced.
Shielding of components is achieved through the use of plated materials concentrated near the
cabin as shown in Figure 3.34. The pressurized cabin is protected with Spectra armor as used on the AC130U.
Figure 3.34: Materials usage
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The titanium alloy armored area directly below the crew cabin, performs a number of roles,
including protection from enemy fire, and shielding from the 105mm cannon’s blast overpressure, which can
be as high as 5,000 psi. The titanium plate also functions as a load-bearing structure, distributing loads to
the surrounding skin.
3.5 Weight and Balance
3.5.1 Weight Breakdown
TOGW was initially estimated using the fuel fractions method from the preliminary design process,
and then refined as Firefox progressed. The equations used to calculate the primary structural weight of the
aircraft were provided by Raymer24, and did not reflect the increased use of composite materials in modern
aircraft. They also neglected to address the additional structure necessary to support the weapons systems
and enhance the overall survivability of the aircraft was. For these reasons, a worst-case scenario was
calculated and compiled, by piecing together the most conservative elements from each weight estimation
method.
The propulsion and fuel system groups were calculated using Nicolai34, and were updated as
weights for survivability features such as engine-bay fire suppression were selected. Engine installed weight
was based on physical data provided by General Electric, and calculated using Raymer’s24 method26, while
fixed equipment weights were estimated using Torenbeek35. Known component weights, such as avionics,
and countermeasure systems were used in place of estimates whenever possible.
The weapon system weights were determined from published values for both cannons and
ammunition. The 5,090 CTA 40mm rounds weigh 2.2lbs each and the 100 CTA 105mm rounds weigh 44
lbs each. Crew weights were based on an average pilot weight of 190 lb plus an additional 30 lbs to account
for flight gear. Table 3.5 shows the complete weight breakdown of the aircraft, as well as an illustration of
the weight distribution.
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Table 3.5: Weight breakdown
A weight history, shown in Figure 3.35, was kept throughout the design process, to recognize the
trends and benefits with each evolving configuration. Some of the weight fluctuation was due to design
improvements, while others were due to improved weight estimates.
Figure 3.35: Weight History
The initial drop in TOGW was due to a wing redesign, and the weight increase because of improved
fuel estimates based on a detailed analysis of mission fuel burn. After the Configuration Downselect Review
at Lockheed Martin, the crew was reduced from 8 to 5 members resulting in a total weight savings of about
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3,000 lbs. The large increase in weight immediately after the Preliminary Design Review (PDR) at Northrop
Grumman was the result of improved weight estimates for the weapons systems, based on actual
manufacturer values. The other slight weight fluctuations after PDR are due to wing refinements. The final
reduction in weight was achieved by reconfiguring the interior of the aircraft to eliminate unused space and
excess structure, resulting in a weight savings of 5,000 lbs.
3.6 Stability and Control
3.6.1 Tail Type Selection
Conventional, T-tail, and H-tail configurations were analyzed during the initial configuration for
Firefox. T-tail was eliminated for structural reasons, because the vertical stabilizer has to be stiffened to
support the increased load on the horizontal stabilizer. A conventional tail addressed the structural concerns
by mounting the stabilizers directly to the fuselage, and also involved less complex control-surface linkages.
However, the H-tail configuration shown in Figure 3.36 was selected based on stability, performance, and
survivability reasons.
Figure 3.36: Tail Configuration
The H-tail configuration affords better stability and accuracy during attack, because it can deflect
the rudder and compensate for yaw without rolling the aircraft. Additionally, the end-plate effects reduce the
size and weight of the horizontal stabilizer, and keep induced drag to a minimum. The redundant vertical tail
surfaces increases survivability, and reduces the overall height, leading to a smaller aircraft that fits more
easily into hangars. The size of the vertical stabilizer were minimized while meeting OEI requirements with
one rudder surface inoperable.
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3.6.2 Tail Sizing
Tail sizing was conducted using methods from Roskam29 and Schaufele25, which were checked with
each other to ensure accuracy. Schaufele25 was based on empirical trends, and gave a horizontal tail
surface area based on desired CG shift in percent chord, resulting in a horizontal tail volume coefficient of
1.0 based on the 30% travel. Roskam29, based on static margin and CG travel, yielded a value of 1.10, as
shown in Figure 3.37. The vertical surfaces were sized using the average of the two methods, which
provided comparable results of 0.090 and 0.10.
For the horizontal and vertical stabilizers, a NACA 0012 airfoil was selected based on the maximum lift
coefficient and the thickness required for structural supports. A thickness of 1.8 feet was adequate for the
spar sizes needed to sustain the recoil impulse, and a symmetrical airfoil was desired since the range of
necessary deflections would be evenly matched.
Horizontal Stabilizer Sizing
Evaluation of the forward and aft CG limits was performed using Figure 3.37. The nose wheel liftoff
constraint was based on Firefox’s ability to rotate the nose for liftoff, and was calculated by summing the
pitching moments of the aircraft. The aft CG limit was evaluated similarly.
Figure 3.37: Static longitudinal stability
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A final horizontal tail volume coefficient of 1.00 contained the most forward and aft adverse loading
conditions and gave Firefox a static margin factor of safety at the takeoff CG location.
3.6.3 Center-of-Gravity Excursion
Since Firefox’s CG varies greatly with ammunition expenditure and loading, a plot of CG excursion was
needed for tail sizing and stability purposes. The most adverse loading conditions were determined by
loading the aircraft for a typical mission. Starting with the empty weight; fuel, crew, and ammunition weights
and locations were added. Finally the 5% reserve fuel as stated by the RFP was used to obtain the
maximum and minimum loading constraints shown by the dashed lines in Figure 3.38.
Figure 3.38: Center-of-gravity excursion
Firefox was run through a standard gunship mission to develop the CG excursions shown in Figure
3.38. The solid green line accounts for all aft 40mm ammunition onboard and no fuel, while the solid blue
line represents only forward 40mm and 105mm ammunition loaded, with no fuel. Operation of the gunship
falls within the designed forward and aft CG limits, and all ammunition expenditure occurs after the
maximum landing weight so that Firefox is capable of aborting the missions.
Based on the CG locations at mid-mission cruise, takeoff and landing, a set of three moment curves
in Figure 3.39 were developed to demonstrate static aircraft stability. The horizontal tail was given a 3.1°
upward incidence to provide zero pitching moment about the CG during cruise, enabling the pilot to fly a
trimmed aircraft.
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Figure 3.39: Pitching moment
Finally, elevator angle to trim was examined in order to ensure the aircraft was capable of trimming
at all flight conditions. The cruise curve was corrected for Mach number to demonstrate stability throughout
the entire flight envelope. A final design point of ±20° was capable of trimming up to stall for each scenario
allowing the pilot to maintain complete control of the aircraft at all times and regain control of the aircraft
when necessary.
3.6.4 Dynamic Stability
MIL-F-8785C was consulted in order to ensure that Firefox met established military specified
dynamic stability requirements. Based on the gunship mission description, Firefox was classified using the
definitions for non-terminal Flight Phases as Class A-IV highly maneuverable, for use in ground attack (GA)
and in-flight refueling (RR).
The first dynamic stability requirements examined was the longitudinal axis for the short period and
phugoid mode responses. Using both the Pamadi and Nelson to derive approximations, a longitudinal state
space model was created in Equation 2, and broken into phugoid and short period modal approximations.
These were evaluated using the aircraft’s cruise, loiter, and attack steady state conditions.
Xu
⎡ Δu ⎤ ⎡
⎢ Δw ⎥ ⎢
Zu
⎢ ⎥=⎢
⎢ Δq ⎥ ⎢ M u + M w Z u
⎢ ⎥ ⎢
0
⎣ Δθ ⎦ ⎣
Xw
Zw
M w + M w Z w
0
0
uo
M q + M w uo
1
− g ⎤ ⎡ Δu ⎤ ⎡
Xδe
⎤
⎥
⎢
⎥
⎢
⎥
0 ⎥ ⎢ Δw⎥ ⎢
Zδ e
⎥ [ Δδ e ] Equation 2
+
0 ⎥ ⎢ Δq ⎥ ⎢ M δ e + M w Zδ e ⎥
⎥⎢ ⎥ ⎢
⎥
0 ⎦ ⎣ Δθ ⎦ ⎣
0
⎦
After assessing the basic traits of these modes, the values were then checked against the MIL-F8785C specification in order to establish the handling qualities rating. Takeoff and landing performance
were also included in this evaluation to validate the chosen flap configuration. This was process was
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performed for both the short period and phugoid mode characteristics, and the results are shown in the
following tables.
Table 3.6: Phugoid & short period evaluations
In these evaluations, the gunship received a consistent rating of Level 1 flying qualities stating that
it was clearly adequate for the mission Flight Phases defined. Next, the lateral aircraft dynamics were
examined to evaluate the roll, spiral, and dutch roll behavior during flight. Control derivatives for the lateral
dynamics were estimated using the Pamadi, Nelson, and Roskam design texts, and used to develop the
state space model shown in Equation 3.
⎡ Δβ ⎤ ⎡Yβ / uo Yp / uo
⎢ Δp ⎥ ⎢
Lp
⎢ ⎥ = ⎢ Lβ
⎢ Δr ⎥ ⎢ N β
Np
⎢ ⎥ ⎢
1
⎣ Δφ ⎦ ⎣ 0
− (1 − Yr / uo ) − g cos θ o / uo ⎤ ⎡ Δβ ⎤ ⎡ 0 Yδ r / uo ⎤
⎥⎢ ⎥ ⎢
⎥
Lr
0
⎥ ⎢ Δp ⎥ + ⎢ Lδ a Lδ r ⎥ ⎡ Δδ a ⎤
⎥ ⎢ Δr ⎥ ⎢ Nδ a Nδ r ⎥ ⎢⎣ Δδ r ⎥⎦
Nr
0
⎥⎢ ⎥ ⎢
⎥
0 ⎦
0
0
⎦ ⎣ Δφ ⎦ ⎣ 0
Equation 3
This was used to examine the roll, spiral, dutch roll characteristics of the gunship during cruise,
loiter, and attack phases of the mission.
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Table 3.7: Roll mode & spiral stability evaluation
Meeting the dutch roll mode requirements stated in the Milspec was a necessity for Firefox, since
this particular mode was not only called out for ground-attack aircraft, but would directly affect tracking and
firing of the 105mm cannon.
Table 3.8: Lateral oscillation evaluation
Essentially, the Level 1 dutch roll ratings indicate that the only displacement Firefox experiences
when it fires the 105mm cannon, is straight back, in-line with the flight path, which is critical, because it
ensures precision accuracy shot after shot. A Level 2 rating was obtained for cruise due to the emphasis on
low-speed performance however, does not inhibit the gunship in its primary role.
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3.7 Systems
3.7.1 Fire Control
Sensor and targeting devices for Firefox were selected with regard to its over-fly engagement
pattern, which meant targets had to be acquired on either side of the gunship. Active electronic beam
scanning offers this capability, by allowing the radar beam to be steered at near the speed of light. This
enables the radar’s multiple target tracking modes to interleave in near-real time, allowing the pilot and crew
to use both modes simultaneously. The AN/APG-79 AESA radar made by Raytheon, is composed of
numerous solid state transmit and receive modules to virtually eliminate mechanical breakdown, which
increases its reliability, and reduces operation costs. Moreover, as demonstrated in Figure 3.40, the APG79 radar provides state-of-the-art high-resolution ground-mapping superior to that of any current fire control
system. 38
Figure 3.40: AN/APG-79 high-resolution ground mapping38
This advanced imaging aids to increase situational awareness, provide precision fire capability, and
expedites target acquisition. The AESA radar is located at the nose of the aircraft to provide initial ground
mapping as the aircraft enters the battlefield.
In addition to the APG-79, two AN/AAS-52 Multi-Spectral Targeting Systems (MTS) are
incorporated into Firefox. Pictured in Figure 3.41, Raytheon's AN/AAS-52 is a multi-spectral system
designed to provide surveillance, targeting, range finding, tracking and laser designation for laser-guided
munitions. Generally, it consists of a spot laser, IR and EO sensors. 16
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Figure 3.41: AN/AAS-52 Multi-Spectral Targeting
The system is combat proven and used in the Predator UAV. Growth potential allows the addition of
TV camera, spot tracker, avionics and other multiple wavelength sensors, which provides Firefox with full
nighttime capability. Two MSTs positioned behind the 105mm tank gun and on the tail of the aircraft provide
full view of targets at any position regardless of orientation as well as a measure of redundancy, reducing
the chances of an attrition kill due to damage to the externally mounted MSTs. Sensor positions are
displayed in Foldout 3, page 88.
Performing in conjunction with the APG-79 and MST is the AN/APX-114 Interrogator, which
represents the most technically advanced, compact, lightweight interrogator available for airborne, highpower applications. The AN/APX-114 Airborne Interrogator initiates an Identify Friendly or Foe (IFF) inquiry
and provides the advanced processing capability to positively identify friendly targets. The system is
designed with a Versa Module Eurocard (VME) open architecture that enables maximum system flexibility
with cost-effective upgrades. An IFF signal processing card incorporates Raytheon’s Very Large Scale
Integration (VSLI) reply decoder chipset, which uses leading-edge pulse detection techniques. The APX
114 platform interfaces include Mil-STD-1553B, RS-232, Ethernet and RS-422. The 2.5-kilowatt transmitter
and receiver sensitivity of -83 dB will meet or exceed the link margin required for airborne combat
applications.40 Furthermore, the APX-114 offers significant advantages in acquisition and life cycle cost,
technical superiority, reliability, and growth potential.
All three components are from Raytheon, which provides reduced cost benefits in the long run as
supply and maintenance are all contracted from the same company. This also insures maximum
compatibility, for example, the APX-114 was designed specifically to talk to the APG-79 class of radars.
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3.7.2 Weapons
Recoil Force
The greatest limiting factor associated with the tank gun was recoil force. To address this concern,
Jane’s Armour & Artillery was used as a primary source for a historical study into the relationship between
recoil stroke and recoil force14. A contact from Rheinmetall provided additional 105mm recoil data to
construct the scalable curve displayed in Figure 3.42.
Figure 3.42: Recoil force vs. recoil strength
A 3.15 ft. recoil stroke would reduce the recoil force to 20,000lbs; equivalent to the A-10’s 30mm
GAU 13. Discussion with Rheinmetall confirmed that modifying a current 105mm tank gun to this slightly
longer recoil stroke would not be difficult.
3.7.3 Ammunition
The compact, cylindrical geometry of CTA rounds allows the gun loading mechanism to be
simplified resulting in a small, lighter weapon as shown in Figure 3.43.
Figure 3.43: Cannon size comparison
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CTA International reports that the gun system has been designed to increase reliability by
eliminating over 50% of a conventional gun’s most unreliable parts. The simplified rotating loading
mechanism shown in Figure 3.44 will be employed in both the 40mm and 105mm weapon systems.32
Figure 3.44: CTA 40mm reloading system
After the round is spent, the cartridge remains in the loading mechanism, until the next round enters
the chamber, ejecting the previous cartridge from the bottom of the aircraft. Advanced concept engineers
from Lockheed, Boeing and Northrop Grumman, and Western all recommended ejecting spent cartridges
rather than trying to collect them. The loading systems achieves 200 SPM for the 40mm cannons, however
the 105mm rate of fire is limited to 30 SPM by the recoil recovery time.
The small size and simple compact geometry (smooth cylinders) of the rounds means they take up
less than half the storage space of a traditional 40mm. As shown in Figure 3.45, the CTA 40mm rounds are
currently available in High Explosive (HE) air bursting rounds, and armor penetrating rounds, though future
variations are in development.
Figure 3.45 CTA 40mm HE and APFSDS-T, and HE penetrating concrete wall32
The HE rounds cover roughly twice the Pk area as traditional 40mm rounds, and the Armor Piercing
Fin Stabilized Discarding Sabot (APFSDS-T) CTA 40mm rounds achieve higher armor penetration (125mm
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versus 90mm RHA). Because the 40mm HE rounds are not ballistically matched with the APFSDS-T
rounds, the ammunition cannot be mixed with a 4:1 ratio as in the AC-130U. However, the ammunition
magazines developed by CTA International are selectable between two different types of rounds, and take
only three seconds to switch. 32
Figure 3.46 CTA 40mm automatic magazine
Ammunition for the 105mm CTA cannon is currently being developed by Esterline Armtec in HE,
and APFSDS-T varieties (Figure 3.47), although High-Explosive Anti-Tank (HEAT) and other varieties are
also planned.
Figure 3.47: Cutaway of 105mm APFSDS-T, 105mm magazine32
The HE rounds are standard all-around cost-effective ammunition for engaging a variety of targets,
while the HEAT and APFSDS rounds are designed for heavily armored vehicles.
Since the range for air-launched CTA 40mm and CTA 105mm cannons is unavailable, a trajectory
analysis was conducted using a code developed in Java. The physical size and geometry of the projectiles
was used to calculate a bullet coefficient, which was used along with altitude, muzzle velocity and aircraft
attack velocity to generate the curves shown in Figure 3.48.
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Figure 3.48: Trajectory analysis of projectiles
When expended at an angle of 0°, the CTA 40mm rounds have a range of 15.2 miles, and the CTA
105mm a range of 18.7 miles, which greatly compromises the accuracy of the projectiles. The solution lies
in the development of Cased Telescoped Guided Projectiles (CTGP) in Figure 3.49, which integrates
technology from the Thales aerospace Starstreak MANPADS, into CTA cartridges.
Figure 3.49: CTGP 40mm HE with discarding sabot32
CTA International is currently developing laser and GPS guided projectiles, although similar
projects such as the 155mm GPS guided Excaliber howitzer round are already operational. Firefox will use
laser-guided CTA 40mm rounds for smaller, more mobile targets, while GPS-guided CTA 105mm rounds
will be required for long-range structural engagements. CTGP are expected to be 2.5 times the cost of
standard CTA rounds, however the accuracy at long range afforded by the guidance system, puts Firefox
easily out of reach of even the most sophisticated long-range MANPADS or AAA systems.
Both forward CTA 40mm magazines contain 2,000 rounds each, the rear CTA 40mm cannon 1,090
rounds, and the CTA 105mm cannon 100 rounds. For a typical mission, all three CTA 40mm magazines will
be loaded with an unmixed and selectable 4:1 ratio of HE to APFSDS-T, while the CTA 105mm magazine
will be filled exclusively with HE. However for high threat situations, long range GPS guided 105mm HE
CTGP rounds, and laser guided 40mm CTGP rounds can be swapped in for the 40mm APFSDS-T, and half
of the 105mm rounds.
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Reloading Replenishing, and Access
Reloading for both the CTA 40mm and CTA 105mm rounds is accomplished through a Linkless
Ammunition Loading System (LALS), similar to the one currently used on the AC-130U. Figure 3.50 shows
the LALS ammunition conveyer attaching to the magazine through access doors through the bottom of the
aircraft.
Figure 3.50: 40mm replenishing with LALS
Unlike missiles and bombs, which require 30 minutes each to install, the LALS system replenishes
the ammunition magazines in only 15 minutes, ensuring Firefox can match the AC-130U’s 30-minute
turnaround time.
Firefox maintains a very small logistics train, by first avoiding a UAV configuration, and secondly by
its selection of weapons. Rounds for cannons are much more compact and easier to transport than missiles
and bombs, and the two-caliber approach further simplifies re-supply. Additionally, as illustrated in Figure
3.51, CTA style ammunition saves on volume for transport.
Figure 3.51: CTA reduces the logistics train32
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Compact CTA ammunition saves greatly facilitates the transport of ammunition to austere fields,
requiring half the throughput volume of traditional 30mm rounds.
Access to the 105mm gun is easily achieved through panels opening from the bottom of the
fuselage. The barrel simply drops down for barrel repair and gun replacement as shown in Figure 3.52.
Figure 3.52: 105mm barrel access
3.8 Survivability
Survivability considerations for Firefox began in the initial configuration stage with the selection of
such primary features such as the H-tail, top-mounted engines, and minimal wing area. In subsequent
iterations, survivability features were broken down into categories of susceptibility and vulnerability
reduction and examined more rigorously.
3.8.1 Susceptibility Reduction
Susceptibility reduction is generally designated by the probability of being struck, PH36, and is
achieved through six concepts: signature reduction, threat warning, threat suppression, noise jamming and
deceiving, expendables, and weapons/tactics/crew proficiency.36
Infrared Signatures Sources
An aircraft’s IR signature is composed of radiation emitted by and reflected from the aircraft in the
0.77- to 1000μm band of the electromagnetic wave spectrum. However, most of the IR signature of interest
lies within the 1-3 μm short-wave infrared (SWIR) band, the 3-5 μm mid-wave infrared (MWIR) band, and
the 8-12 μm long-wave infrared (LWIR) band. Figure 3.53 illustrates some examples of these IR sources
and the general radiation phenomena.
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Figure 3.53: Infrared Sources 36
As illustrated in Figure 3.54, the four general sources of this signature are the airframe and
propulsion system, the exhaust gas or plume from the engines, and the reflected radiation incident on the
aircraft.36
Figure 3.54: Primary IR sources
In general, the surfaces of the aircraft emit and reflect IR radiation with an intensity that varies as a
function of the radiation wavelength λ, surface temperature T, surface emissivity ε, and the viewing angle
relative to the surface. IR detectors can often only sense radiation within a narrowband of wavelengths. The
vertical gray bands in Figure 3.55 show the extent of the SWIR, MWIR, and LWIR bands.
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Figure 3.55: Radiant exitance: SWIR, MWIR, & LWIR bands36
The 1-3 μm and 3-5 μm bands are typical detection center-bands for early and later generation IR
missiles. As shown, the 1-5 μm bands also describe the center-band of an engine tailpipe. While various
other sources discussed earlier also contribute to an aircraft’s total IR signature, the major provider of IR
trace for missiles originates from the engines.
Various gases in the atmosphere absorb the IR radiation propagating away from the aircraft at
several discrete wavelengths. The CO2 and H2O bands are where the greatest atmospheric absorption
occurs, and are also the bands that have high emissivity. Because these two gases are also present in the
exhaust plume from an aircraft engine, a large amount of energy is radiated into the atmosphere and
absorbed by these bands. However, the radiation from the hot CO2 and H2O molecules in the exhaust
plume is not completely absorbed by the CO2 and H2O molecules in the surrounding atmosphere because
of the significant differences in temperature and pressure. 36 Thus, their emission and absorption values are
as illustrated in Figure 3.56, which shows both the radiation from the plume (the dashed line) and the
atmospheric absorption (the grey area) in the vicinity of the 4.3 μm CO2 band. The spikes of radiation from
the plume on both sides of the 4.3 μm wavelength, are known as the blue spike and red spike, and are not
absorbed as a result of differences between the plume and atmosphere.36
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Figure 3.56: Exhaust plume IR radiation36
The radiant exitance of a typical engine along the center-band of an IR missile’s detection band in
addition to the large spikes of IR radiation resulting from exhaust plume serves to illustrate the impact of the
engine on the overall IR of an aircraft. Consequently, to properly address IR reduction of the gunship, focus
was placed on the engine location, design, and housing. In subsequent design stages, engagement
patterns determined aspects of the gunship which would be most vulnerable.
A typical infrared radiation signature at several locations around a turbojet-engine aircraft at some
arbitrary distance away may appear as shown by the grey line in Figure 3.57. As seen by the red line, the
partially buried engine configuration employed by Firefox reduces exhaust plume temperatures by
approximately 50% and hot engine part visibility by approximately 70%.
Figure 3.57. IR signature around an aircraft36
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Most important to note is the change in scale of intensity in Figure 3.56 for the different aspects.
When the gunship is viewed from the front and sides, as is the case when Firefox is entering the
engagement zone in an over-fly attack pattern, the plume radiation would be the primary source of the IR
signature in the MWIR band. The radiation is most intense at the two spikes of the CO2 radiation line
centered at 4.3 μm. When viewed from the tail, the engine hot parts become the major IR source in the
SWIR and MWIR bands, and the red and blue spikes of radiation from the plume are dwarfed by the hot
part emissions, as shown in the 180° view in Figure 3.57. While the minor spikes of a front and side view
can be easily addressed with infrared countermeasures, the large IR emission visible from the rear as
Firefox exits the engagement area, is a notable problem, as the base signature is five times the magnitude
of any other aspect. Accordingly, the particular engine configuration employed by Firefox was integrated as
a means to address this issue.
Infrared Signature Reduction
Reduction of the IR signature is accomplished by reducing the temperature and surface emissivity
of hot components, exhausts, observable surface radiating area, and surface reflectivity for the reflecting
surfaces.36
Cooling hot engine components and exhaust plume, was addressed in section 3.2.2: Nacelle
Design, and as illustrated in Figure 3.55, is the most effective technique for controlling the IR radiation level
in the bandwidth of an IR detector.
Radiation from the airframe consists of the emissions from the relatively cool surfaces in the LWIR
band; from aerodynamically heated surfaces, hot metal spots, aircraft lights in the LWIR and MWIR bands;
and the reflection of incident radiation or sun glint in all three bands36. Aerodynamic heating effects are only
a significant source of IR signature for supersonic aircraft, and for aircraft flying low altitudes at very high
subsonic speeds, neither of which matches the RFP mission profile. IR radiation from hot spots caused by
sources other than the propulsion system and aerodynamic heating, such as oil coolers and heat
exchangers, are dealt with by reducing the surface emissivity through insulation between the hot part and
the skin and a cooling flow system.
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3.8.2 Vulnerability Reduction
Vulnerability reduction refers to the use of any design technique or piece of equipment that controls
or reduces either the amount or the consequence of damage to the aircraft caused by the damage
mechanisms. In general, any method for reducing vulnerability is a specific application of one of six
vulnerability concepts: component redundancy with separation; component location; passive damage
suppression; active damage suppression; component shielding; and component elimination or
replacement39.
Passive Damage Suppression
Passive damage suppression is any design technique that reduces vulnerability by incorporating a
feature that, after the impingement of a damage mechanism, tends to either contain the damage or reduce
its effect. These passive features have been integrated into Firefox throughout the design process.
Features such as the tri-spar wings for redundant load paths were included during initial configuration, while
material and component selection were integrated later on. Furthermore, large-diameter, thin-wall control
rods that can function with perforations caused by projectiles and fragments are installed on the gunship.
Dual, tandem hydraulic power actuators are made with rip-stop construction to prevent cracks in one power
cylinder from propagating to the other power cylinder and causing a loss of both power-control hydraulic
subsystems.
Another subcategory of passive damage suppression is ballistic resistance, which is a design
technique involving the use of high-strength materials in components for the purpose of preventing the total
penetration of an impacting penetrator or fragment. The casing around hydraulic actuators and gearboxes
installed in Firefox are such areas that have been made ballistically resistant.
Component Redundancy
Nearly all modern aircraft, both commercial and military class, feature flight control redundancy to
some degree. The obvious benefit to military aircraft is decreased vulnerability in the event damage is
incurred during wartime. The added benefit of redundant flight control loops is safety of flight during noncombat operations. The particular difference between flight control redundancy for a gunship as opposed to
a commercial aircraft deals with component separation.
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Component Elimination or Replacement
Vulnerability can be reduced by completely eliminating a particular critical component or by
replacing the component with something less vulnerable component that accomplishes the same function36.
A conventional fuel-feed boost pump is replaced with a fuel-feed suction device. This reduces Firefox’s
vulnerability by eliminating the possibility of pumping fuel through damage-caused holes in fuel transfer and
feed lines and into void spaces where a fire can start.
Active Damage Suppression
Figure 3.58 shows a trade study investigating a number of active damage suppression devices for
Firefox. Fire detectors and extinguishers are mounted inside the engine cowlings around the core of the
engine and are manually set off by the pilot when a “fire” warning light is indicated. Also, an on-board inert
gas generation system (OBIGGS) is employed to inhibit the oxygen level in the tank ullage space to
increase beyond a certain level preventing hot missile fragments or incendiary bullets from igniting the fuel
vapors if these spaces are penetrated.
3.8.3 Gunship Survivability Enhancement
The AIAA sub-branch, Survivability Technical Committee (STC), published a supplemental
resource listing six survivability enhancing features, shown in Figure 3.58. These were provided along with
a brief description of each, an approximate cost and weight value, as well as benefits of the feature in terms
of a probability of kill per encounter (PK|E) reduction.39
Figure 3.58: Survivability enhancing features
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Additionally, a baseline PK|E versus MANPADS and AAA was provided to be 0.45 and 0.18
respectively. As mentioned previously, redundant flight controls were included in Firefox as a baseline
feature. The remaining five features were assembled into various packages, generating a possible 31
different combinations, which produced six viable packages that met the PK|E requirement of less than 0.10
for both MANPADS and AAA. Each of the six packages was examined against five categories emphasizing
survivability and cost-reduction.
Weighting of Categories
Each of the five categories carried a weighted percentage that contributed to the total measure of
merit awarded each of the packages. Weight minimizing was attributed a weight of five, while every other
category was given a weighting of ten. Figure 3.59 shows a chart indicating the weighting as well as the five
categories each package was measured by.
Figure 3.59. Survivability features trade study summary
Of the six packages which met both MANPADS and AAA PK|E requirements, none differed by more
than 200 lbs whereas cost differences ranged to nearly $2 million. By inspection, 200 lbs in equipment
weight referenced to a 95,000 lb aircraft was much less significant than $2 million referenced to a $70
million aircraft. Integrating a weighting scale served to alleviate this discrepancy in emphasis, producing
more pertinent results for Firefox.
Categories of Measurement
Addressing cost issues also inherently implicated weight as cost is effectively increased with
weight. Accordingly, both cost and weight minimizing were included as two of the five categories of
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measurement. Low cost and weight values attributed a high measure of merit to the package in those
categories.
The measure of merit attributed to the categories labeled MANPADS and AAA were scored based
on the proximity of the PK|E value of the package to the required 0.10 value. Lower values of PK|E incur
unneeded costs despite increased survivability
Cost and weight minimizing dealt primarily with a low cost for the gunship while MANPADS and
AAA chiefly concerned the survivability of the aircraft. Susceptibility reduction was included as a fifth
category of measurement as a means of addressing two issues. Insuring low susceptibility decreases
maintenance and operations costs because the gunship will incur less damage during engagements. An
aircraft that is consistently sustaining damage also unnerves the crew, so low susceptibility also introduces
psychological benefits. Even the hardiest of veterans suggest that vulnerability reduction is the least
desirable feature they would like to see in their aircraft38. Fact remains that susceptibility reduction is by far
the more desirable route. In providing increased ride quality, crew proficiency is enhanced as well,
subsequently improving survivability and lethality
Based on these five categories of measurement, package B was attributed the highest measure of
merit. Figure 3.60 shows a summary of the components loadout of package B along with a comparison to
the AC-130 based on a similar analysis. Firefox satisfies the PK|E requirement for both MANPADS and AAA
at a lower cost and weight.
Figure 3.60: Firefox vs AC-130 survivability enhancements
Countermeasures
The IR countermeasures equipment selected for Firefox is the Suite of Integrated Infrared
Countermeasures (SIIRCM)/Common Missile Warning System (CMWS). SIIRCM contributes to the Joint
Vision 2010 concept of full-dimensional protection by improving individual aircraft's probability of survival
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against an increasing worldwide proliferation of advanced IR guided missiles. The Advanced Threat Infrared
Countermeasures (ATIRCM) is part of the U.S. Army's SIIRCM concept of IR protection including the
Advanced Infrared Countermeasures Munitions and passive IR features. These features include host
platform modifications such as engine exhaust/heat suppression and special coatings, intended to reduce
the platform IR signature38. ATIRCM is a subset of the SIIRCM program and is specifically comprised of an
active IR jammer and the passive Common Missile Warning Receiver. Figure 3.61 displays the combined
SIIRCM suite used aboard Firefox.
Figure 3.61: SIIRCM suite
CMWS is a software reprogrammable system intended to provide automatic passive missile
detection, threat declaration, positive warning of a post launch missile that is homing on the host platform,
countermeasures effectiveness assessment, false alarm suppression, and cues to other onboard systems
such as expendable countermeasures dispensers. Additionally, the ATIRCM adds active directional
countermeasures via an arc lamp and laser.
The SIIRCM suite is already employed in the Air Force F-16s, insuring readily available servicing
equipment at airbases where Firefox would be stationed. Particular attention was given to IR reduction and
suppression in the SIIRCM suite, making it the most advanced and readily available countermeasure
package to the Air Force that would address the specific MANPADS threat called out in the RFP.
While AAA was also a chief consideration, the nature of AAA threats common to the gunship
platform are primarily visually based weapons41. Consequently, the countermeasures suite focused on
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evasion of MANPAD threats while the passive damage suppression features provided the most effective
means of addressing AAA threats.
Fuel Tank Ullage Protection
To prevent fuel vapors from igniting the fuel tanks, the OBIGGS system pictured in Figure 3.62 was
selected. The OBIGGS approach has significant advantages, in that provide an aircraft fuel system with a
continuous and unlimited supply of nitrogen gas and there are virtually no maintenance related activities
required. The only maintenance activity is associated with an OBIGGS system, is the occasional
replacement of an inlet air filter approximately every 1,000 hours, depending upon the cleanliness of the
engine bleed air that is being utilized as the OBIGGS source air.
Figure 3.62: OBIGGS System36
The integral OBIGGS oxygen monitor provides the pilot and/or maintenance crew with notification
of required maintenance actions. Repair of the actual system is typically performed by the original
equipment manufacturer as the system reliabilities for this type of system, including health monitoring, is
greater than 10,000 hours mean time between failures38. To further illustrate this level of reliability, an
aircraft would encounter only three system failures per year, based on a 100 aircraft fleet, where the total
average flight time per aircraft is 20 hours per month. OBIGGS is a proven solution that has minimal aircraft
integration impact and negligible long-term maintenance and lifecycle costs. An OBIGGS incorporated with
self-sealing fuel tanks meets the survivability needs of avoiding damage by providing protection against
multiple hit scenarios.
Sensor and Threat Detector/Suppression Placement
To locate the positions of sensor and threat detector/suppression devices, the aircraft was analyzed
for probability of kill due to fragmentation (PK|F) from proximity fuzing of a missile in the vicinity of the
gunship. The PK|F was determined from two qualities of proximity fuzing missile: the immediate blast zone
as well as fragmentation travel as shown in Figure 3.63. The lethality of the proximity warhead is presented
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in a discrete form by determining the PK|F for an array of detonation locations around the aircraft with the
same endgame conditions depicting the gunship entering the engagement zone and leaving. Vulnerability
of the gunship was assessed for a baseline model of Firefox’s most critical components: crew location, fuel
tanks, and engines.
Figure 3.63: PK|F values, entering & exiting engagement zone
The array of PK|F values locates the optimal fuzing region, along which the IR jammer heads (IRJH)
included in the SIIRCM suite are located on the center-band and the CMWS threat sensors are located
around the center-band. The array generated as the gunship enters the engagement zone locates the IRJH
near the forward section of the fuselage along with three of the six CMWS provided in the SIIRCM suite.
Similarly, the array generated as the aircraft exits the engagement region positions the IRJH in the aft
portion of the aircraft along with the remaining three threat sensors as shown in Foldout 3, page 88.
3.9 Systems
3.9.1 Hydraulic System
Two independent pumps drive a dual 8,000 psi system which is used to power the primary flight
control actuators. Electrical lines lead to the pumps, (one at the wing root and one at the tail), with the
actual hydraulic lines beginning at the pumps to minimize weight. The risk of flammable fueling leaking is
minimized since the only portion of the primary control system containing carrying hydraulic fuel lines is
located in the wing. Moreover, in the event of a fire due to hydraulic line leakage, the present fire
suppression system in the wing can be employed while jettison valves at the wingtip aid to quickly eject
hydraulic fluid. A secondary mechanical/electrical system acts as a backup if the primary hydraulic system
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fails. Firefox’s 8,000 psi hydraulic system yields a 25% weight savings over the lower pressure 4,000 psi
systems seen in many of the older military aircraft such as the C-17. For survivability, hydraulic lines are
physically separated by at least 20 inches in order to minimize the possibility that a direct hit would sever
lines from both systems. The hydraulic systems from the F-4 Phantom and A-10 Thunderbolt, were used to
guide the layout shown in Foldout 3, page 88.
3.9.2 Electrical System
An investigation into utilizing an independently APU powered weapons system was conducted.
While engine wear would be lowered, the resulting increase in acquisition and maintenance as well as
weight to the gunship proved too costly for the benefits. Furthermore, the use of an APU would notably
increase Firefox’s IR signature regardless of the location of the APU exhaust port.
For these reasons, a generator system was selected, with the engines driving two 60kVA
generators to power the primary electrical system. Each engine supplies 125 hp for a combined 250 hp or
186 kW power, meeting the 150 kW peak power demand during the attack segment of the flight profile. The
excess 36 kW provides a buffer against surge power loading from weapons usage. Additionally, the dual
system provides an extra measure of survivability and allows Firefox to retain primary offensive and
defensive capabilities in the event that one generator becomes inoperable in flight.
3.9.3 Fueling System
Firefox conducts aerial refueling via a standard boom system, such as carried by the KC-135
Stratotanker. MIL-STD 1797A requires that refueling operations be conducted at 1.4 times the stall speed.
For Firefox at a standard refueling altitude of 30,000 ft., this corresponds to a minimum refueling speed of
210 kts. The KC-135 typically refuels fighter aircraft at 250-300 kts and 30,000 ft., which falls easily within
Firefox’s flight envelope. The Universal Aerial Refueling Receptacle Slipway Installation (UARRSI) is
located on the centerline behind the cockpit to minimize CG shifts during refueling, and feeds directly into
Firefox’s wing tanks.
While on the ground, Firefox is refueled by high-pressure refueling ports on the underside of each
wing. Engine fuel pumps are located in the engine nacelles, and fuel jettison lines are located in both wings.
All fuel lines are redundant, as shown in Foldout 3, page 88, to maintain survivability during combat.
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3.9.4 Anti-Icing
Research by the National Aeronautics and Space Administration (NASA) Glenn Research Center
into ultrasonic technology, will provide a lightweight, cost-effective solution for aircraft ice-protection.
As shown in Figure 3.64, the compact device attaches directly to the inner surface of the wing
leading edge similar to traditional anti-icing systems. Sound waves generated at the tip of the device create
a stress field at the interface between the ice and wing, and cause de-bonding to occur. Current research is
focused on de-bonding ice from aluminum and has shown that the de-bonding of a 0.032-in. aluminum plate
and ice occurs within one second. Future investigations will include composites, glass, and steel.
Figure 3.64: Ultrasonic anti-icing device42
Aside from the immediate cost savings and reduced maintenance requirements compared to
traditional thermal anti-icing units, ultrasonic anti-icing is also environmentally friendly as there is no cooling
fluid needed. Moreover, heating at the wing edge is avoided, eliminating thermal fatigue and consequently,
unnecessary material degradation as well as decreased infrared signature. With a TRL of 7, the ultrasonic
anti-ice system will be available for use in Firefox by the operation date of 2015.
3.9.5 Oxygen System
A liquid oxygen system is used in case of a loss in cabin pressurization. Using liquid oxygen saves
weight and volume, and is delivered to the flight deck via hoses and masks that drop down from the ceiling.
3.9.6 Cockpit Layout
The armored crew cabin is the only pressurized portion of the aircraft, and was kept as small as
ergonomically possible to reduce the structural costs associated with pressurization. The reduction also
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minimized the vulnerable area the aircraft, and the close proximity of the crew to each other increased
moral and efficiency, as illustrated in Foldout 4, page 96
Five multi-function liquid crystal displays (LCDs) bring real-time performance and navigation
information to the pilot and flight engineer with minimal effort.
Figure 3.65: Instrument panel
Because more women are joining the armed services, aircraft must be designed to accommodate
the shortest woman as well as the tallest man. MIL-STD-1333B guidelines dictate that the workstations are
designed for 5th and 95th percentile crewmembers21, which means the cockpit is designed for a variety of
reach and viewing angles. Firefox’s cockpit features adjustable seats, with long rudder pedals travel
adjustments, and a side-mounted flight stick, which offers the pilot an enhanced, uncluttered view of the
flight instruments.
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Hydraulic Lines
Mechanical
Hydraulic Pumps
Fuel Lines
Fuel Tanks
AN/AAR-57
AAS-52 MST
ALQ-212 IRJH
APG-79 AESA
Foldout 3
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3.9.7 Maintenance
Mechanics can spend 40-50% of their maintenance man-hours retrieving information. Portable
maintenance aids (PMAs) help reduce the amount of time mechanics spend looking for maintenance
information, ordering parts, recording part usage, and getting technical data. By 2015 technology advances
should provide smaller, cheaper, and more reliable devices to complement the enhanced communications
systems that will be available. These advances coupled with historical data lead to an estimated 13
maintenance man-hours per flight hour (MMH/FH), which is equivalent to the current A-10 Thunderbolt.
3.10 Manufacturing
Firefox is designed with a traditional aluminum alloy frame, however the extensive use of composite
skin and armor, necessitates the need for lay-up facilities and autoclaves. To reduce costs, these elements
will be sub-contracted to aerospace companies such as Vought, and Honeywell, who have extensive
facilities and experience with composite materials. Use of thermoset resin based composites in Firefox
allow for injection molded pieces around the wing-fuselage junction. Injection molding is a cost effective
means of producing precision replications of the complex curvature of the wing-fuselage blend as well as
the nacelle design. These sub-assemblies will be brought together and integrated into the central assembly
line as shown in Figure 3.67.
Figure 3.67: Central assembly line
Manufacturing will begin with the main assembly, with wing, and empennage and sub-assemblies
and finally skin panels and armor added as the airframe proceeds through the assembly line.
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Table 3.9 shows an initial production timeline for Firefox. Static and dynamic testing will ensure the
structure meets the RFP loading requirements in addition to identifying possible fatigue issues from cyclic
gust loading from low altitude flight. Due to the large stress loads created by the weapon fire, extensive
testing with a prototype aircraft will help identify any load transmission issues overlooked in the design
phase.
Table 3.9: Firefox production timeline
Once the design modifications driven by the static, dynamic, and weapon fire test results are
implemented, the part tooling will commence, followed by the aircraft subassembly construction. The final
set of design modifications will then be applied before beginning the full-scale production of Firefox.
3.11 Costs
One of the key RFP requirements was to minimize acquisition costs and life-cycle costs. To begin
analysis, the total life cycle cost was broken down into four major categories: Research Development Test
and Evaluation (RDTE), Acquisition, Operating, and Disposal.
Two primary factors when estimating program costs are production run size and service life. Firefox
is designed to replace the AC-130U, so the target production run size is at least 100 aircraft. Since only 21
AC-130 gunships are currently in service, total production is not expected to exceed 200 units, unless the
same airframe can support alternative configurations and missions. Firefox’s configuration is predisposed to
accommodate new technology and alternative configurations, which will extend the production run, and
expected service life for at least 20 years. The modular avionics can be upgraded instead of replaced, and
the guns, removed and upgraded as needed.
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3.11.1 Research Development Testing and Evaluation
RDTE cost was estimated using the method for military aircraft outlined in Roskam41, and as shown
in Figure 3.68, decreases greatly by the time production reaches 100 aircraft.
Figure 3.68: RDTE cost for varied production runs
Acquisition cost is the sum of the flyaway, or production, cost and the RDTE cost. Methods from
Raymer24 and Roskam41 independently estimated costs within $5 million of each other, and were averaged
to determine the flyaway cost shown in Figure 3.69.
Figure 3.69: Flyaway cost for varied production runs
The flyaway cost of $70 million for 21 units is slightly higher than the $60 million C-130H to AC130U conversion, and is due to the more advanced avionics and state-of-the-art weapon systems that
Firefox will use. The cost of a new C-130H airframe will be $47 million by Firefox’s IOC date of 2015, and
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the cost to acquire a new AC-130 will be $122 million. Figure 3.69 shows that producing 400 aircraft will cut
the initial flyaway cost by over one-third to $18 million.
The operating cost for Firefox is primarily influenced by the cost to run and maintain the aircraft and
its weapons systems, as shown in Figure 3.70. Operating cost methods provided by Raymer1 and
Roskam14 did not account for ammunition expenditure, so the operating cost was determined by modifying
Raymer’s DAPCA IV based method24. The breakdown in Figure 3.70 assumes that, on average, only half
the ammunition is spent, and includes ammunition in the miscellaneous category.
Figure 3.70: Operating cost breakdown
Due to the large contribution of ammunition costs, the operating cost per hour was analyzed based
on the amount expended, as shown in Table 3.10. Because operating cost is the largest part of the lifecycle cost, as shown by Figure 3.71, producing 100-400 aircraft will significantly reduce the operating cost
after the 20-30 year service life envisioned for Firefox. An average mission would have Firefox using
between none and all of its ammunition.
Table 3.10: Operating cost varies with ammunition expenditure
Life-cycle cost is the sum of disposal, operating, acquisition, and RDT&E cost, as given by Equation
4. Disposal cost is generally estimated to be about 1% of the life-cycle cost for aircraft using standard
materials and not utilizing any nuclear weapons or power, which is costly to dispose of.41
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C LCC = CRDTE + CAcquisition + COPS + CDisposal
C LCC = CRDTE + (CMAN + CProfit ) + COPS + 0.01CLCC
Equation 4
Figure 3.71: Breakdown of and average life cycle cost
Consumable materials like fuel and ammunition, as well as personnel salaries increase the overall
lifecycle cost for a longer service life, as shown in Figure 3.70. However, as Figure 3.71 presents, the
acquisition, and RDT&E costs are minimized by extending service life. Because the military will not have to
redesign a new aircraft every decade to replace Firefox, the additional cost incurred by extending service
life is justified.
3.12 Aircraft Variants
The size and performance characteristics built into Firefox give many secondary configuration and
mission capabilities. Advanced weapon systems currently in development can directly replace the 105mm
position with little modification.
3.12.1 Railgun
Railguns were the highest scoring alternative system in the weapon selection trade study, and with
a TRL of seven, these powerful yet capable weapons will soon be realized. A notional railgun was scaled to
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the length and recoil force of the 105mm cannon, using target specifications from published NAVSEA and
Batelle presentations. These values are presented below in Table 3.11.7
Table 3.11: Scaled rail gun comparison
Power is supplied by a Bugatti 4MW Super Compact Turbine power generator, while the 3.9 foot
recoil stroke reduced the recoil force to 20,000 lbs. As illustrated in Figure 3.72, because of the size
matching, the railgun cannon can directly replace the 105mm with minimum modification.
Figure 3.72: Railgun configuration
The power generator and pulsed power supplies occupy the area once housing the 105mm
ammunition, and the new 100 round ammunition box is sandwiched between the power supply and the
forward 40mm magazines. The Mach 5 velocity reduces ballistic effects, increasing accuracy, and reducing
flight time from the 105mm howitzer’s 9.2 seconds, to a mere 2.2 seconds at a range of 18,000 ft. 7
3.13 Conclusions
Paladin Aerospace proposes a cost-effective, high performance solution to the AIAA 2004/2005
Undergraduate Team Aircraft Design RFP. The advanced next-generation gunship, Firefox, will take full
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advantage of the developing battlefield communications network to refocus its role from hunter/killer, to pure
killer. Powerful, yet persistent 40mm CTA cannons and CTA 105mm tank gun provide lethal firepower at a
low cost-per-kill. Through the use of state-of-the-art avionics and weapon systems, the crew is reduced
from the AC-130U’s thirteen to four which, as illustrated in Table 3.12, greatly minimizes the size, weight
and cost of the aircraft.
Table 3.12: AC-130 Comparison
Aerodynamically, Firefox is optimized for maximum fuel efficiency, while structural features such as a
large H-tail and a third spar, enhance survivability. The location of the twin, shoulder-mounted engines, hides
them from ground fire as well as infrared (IR) seeking devices, while the sophisticated yet cost-effective
countermeasure system guards against AAA and MANPADS threats. As displayed in Foldout 4, page 96,
Firefox meets all RFP specified requirements with a flyaway cost of $70 million, and an IOC date of 2015.
95
Gunship Mission
Requirement
Non-traditional Weapons
Cruise Speed
Loiter Altitude
Payload Expenditure Altitude
Balance Field Length
Landing Distance
Limit Load Factor
Maneuver at 20,000 ft
Payload
AAA PK|E
MANPADS PK|E
Unrefueled Time on Station
Mission Radius
Initial Cruise Ceiling
Maximum Landing Weight
Aerial Refueling
RFP
≥
=
=
≤
≤
≥
=
≥
<
<
≥
≥
≥
=
Investigate
400
kts
20,000 ft
10,000 ft
5,000
ft
5,000
ft
3.5
g's
1.5
g's
15,000 lbs
Best: 380
20,000
10,000
4,556
4,551
3.5
1.5
15,600
0.1
0.096
0.1
4
hrs
500
n.mi.
30,000 ft
80% TOGW
Capable
Firefox
Met
?
Utilized
kts
ft
ft
ft
ft
g's
g's
lbs
9
9
9
9
9
9
9
9
9
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
Fuel Tanks - 24,348 pounds total
Engine: CF34-10
Fuel Line
UARRSI Refueling Port
Hydraulic Line and Pump
40mm CTA Cannon
40mm Ammunition
105mm Ammunition
Folding Stairway
Single Fowler Flaps
Cabin Layout
11: Battle Manager
12: Mission Systems
13: Pilot
14: Flight Engineer
9
0.072
4 hrs
500 n.mi.
38,000 ft
80 % TOGW
Capable
9
9
9
9
9
9
Ferry Mission
Requirement
Cruise Speed
Range
Landing Distance
RFP
=
≥
=
Best Speed
2,600 n.mi.
Austere Base
Firefox
400 kts
2,600 n.mi.
Capable
Met
?
9
9
9
Foldout 4
Page 96
Paladin Aerospace
Cal Poly
3.15 References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
MacDicken, R., Dinich, P., “Historical Gunships”, Cal Poly State University, 2004.
“US Air Force Airplanes”, Community Webshots [online database], URL:
http://community.webshots.com/album/63976037MYLqpK [cited 15 Jan 2005].
“USAF Presentation on Next Generation Gunship”, USAF, 2003.
“C17 Globemaster III”, Boeing [online database], URL:
http://www.boeing.com/companyoffices/gallery/images/c17/c1704.htm [cited 7 June 2005].
“Distributed Aperture Semi-Active Laser Seeker (DASALS)”, BAE Systems [online database], URL:
www.na.baesystems.com [cited 15 Jan 2005].
Leslie, David, “40mm Cased Telescoped Weapons System”, CTA International [online database],
URL: www.dtic.mil/ndia/2003gun/cta.pdf. 2003 [cited 15, Jan 2005].
David, Allen Adams, “Naval Rail Guns are Revolutionary”, Proceedings, 2003.
“Rail Guns”,Military.com [online database], URL:
www.military.com/soldiertech/0,14632,Soldiertech_RailGuns,00.html [cited 15 Jan 2005].
“Key Missile Defense Systems Programs”, All Systems Go: Journal of Boeing Integrated Defense
Systems [online database], URL: www.boeing.com/ids/allsystemsgo/issues/vol1/num7 [cited 15 Jan
2005].
“Advanced Tactical Laser (ATL)”, GlobalSecurity.org [online database], URL:
www.globalsecurity.org/military/systems/aircraft/systems/atl.htm [cited 15 Jan 2005].
“Vehicle Mounted Active Denial System (V-MADS)”, GlobalSecurity.org [online database], URL:
www.globalsecurity.org/military/systems/ground/v-mads.htm [cited 15 Jan 2005].
“Active Denial System”, Air Force Research Laboratory [online database], URL:
www.de.afrl.af.mil/factsheets/activedenial.html [cited 15 Jan 2005].
Braybrook, Roy. “Hard Nut Crackers”. Armada International. [online database] URL:
www.armada.ch/03-6/fullarticle.cfm [cited 15 Jan 2005]/
Foss, Christopher, Jane’s Armour and Artiller, 10th ed., Jane’s Information Group, London, August
1997.
Liss, Steven D., “Advanced Light Armament for Combat Vehicles”, Tank-automotive & Armaments
Command, US Army. 16 April 2002.
“Military Aircraft Systems”, Global Security.org [online database], URL:
www.globalsecurity.org/military [cited 10 April 2005].
Valdes, Robert., “How the Predator UAV Works.” How Stuff Works [online], URL:
http://science.howstuffworks.com/predator.htm [cited 10 April 2005].
Fulghum, David A., John D. Morrocco, Robert Wall, “Strikes Hit Old Targets, Reveal New Problems”.
Aviation Week and Space Technology. February 2001. p. 25.
Jones, Christopher A., “Unmanned Aerial Vehicles (UAVs): An Assessment of Historical Operations
and Future Possibilities”, Maxwell AFB, AL. March 1997.
Powers, Rod, “United States Military Pay Charts”. About. [online database] URL:
http://usmilitary.about.com [cited 10 April 2005].
Roskam, J., Airplane Design Part I: Preliminary Sizing Of Airplanes, DARcorporation, Lawrence, KS,
1997.
Roskam, J., Airplane Design Part VI: Preliminary Calculation of Aerodynamics, Thrust, and Power
Characteristics, 3rd ed., DARcorporation, Kansas, 2000. pp. 214-280
Steve’s Nasa paper ~
Raymer, D.P., Aircraft Design: A Conceptual Approach, 3rd ed., AIAA, Reston, VA, 1999.
Schaufele, R.D., The Elements of Aircraft Preliminary Design, 1st ed., Aries Publications, Santa Ana,
California, 2000. pp.89-153, 195-277
“CF34”, General Electric [online database] URL: www.geae.com/cf34 [cited 5 Jan 2005].
Waters, Mark. AERO 541 Class Notes. California Polytechnic State University. 2005.
“Aircraft Landing Gear Design: Principles and Practices,” AIAA Education Series, WA, 1988
Roskam, J., Airplane Design Part III: Layout and Design of Cockpit, Fuselage, Wing and
Empennage: Cutaways and Inboards, 3rd ed., DARcorporation, Lawrence, KS, 2002. pp.164-214,
249-273
97
Paladin Aerospace
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
Cal Poly
Pustam, Anil, “Next Generation Gunship”, Military Aerospace Technology [online database],
http://www.military-aerospace-technology.com/article.cfm?DocID=852 [cited 28 March 2005].
Spick, Mike, “F/A-18 Hornet (Modern Fighting Aircraft)”, Salamander Books, UK, 1992.
CTAI, “40mm CTWS”, 2005.
U.S. Department of Commerce, “Critical Technology Assessment of the U.S. Advanced
Composites Industry”, Washington, DC: U.S. Government Printing Office, December 1993.
Nicolai, L. M., Fundamentals of Aircraft Design, 2nd ed., METS, Inc., San Jose, CA, 1984.
Torenbeek, E., Synthesis of Subsonic Airplane Design, Kluwer, Academic Press, 1982.
Ball, Robert E., “The Fundamentals of Aircraft Combat Survivability Analysis and Design, 2nd
Edition,” AIAA Education Series, VA, 2003.
“Boeing News Release,” Boeing [online database], URL:
http://www.boeing.com/news/releases/2000/news_release_000323n.htm. 2000.
“Survivability Technical Committee Supplemental Webpage” [online database], URL:
www.aiaa.org/tc/sur/Pages/Design_Comp_Frame%202004_05.html [cited 15 Jan 2005].
Aircraft Survivability,” Joint Aircraft Survivability Program Office, Fall 2003.
“Raytheon Homepage”. Raytheon [online database], URL: www.raytheon.com [cited 15 Jan 2005].
Roskam, J., Airplane Design Part VIII: Airplane Cost Estimation: Design, Development,
Manufacturing, and Operating, 2nd ed., DARcorporation, Kansas, 2002. pp. 3-14, 21-41,45-56, 145177,188-189
“Aircraft Anti-Icing and Deicing using Ultrasound Technology” [online database], URL:
http://globalspec.com/goto/PDFViewer?pdfURL=http%3A%2F%2Fmastersonic%2Ecom%2Fdocume
nts%2Fmmm%5Fapplications%2Fdifferent%5Fmmm%5Fapplications%2Ftop3%5F00119%2Epdf
[cited 6 June 2005].
3.15 Design Tools
Company
Software
Description
Utilization
Solidworks
Solidworks
Parametric, feature-based
three dimensional modeling
and computer aided drafting
Aircraft solid model
generation
Solidworks
Floworks
Fluid flow analysis for
Solidworks
Validation of engine nacelle
design
Microsoft
Excel/Visual
Basic
Spreadsheet calculation for
complex numerical analysis
Used in conjunction with
Matlab to conduct numerical
analysis
Microsoft
Word
Word Processor
Report generation
Adobe
Photoshop
Image editor
Touch-up of figures and
illustrations
Mathworks
Matlab
High-level technical computing
language and interactive
development environment.
Aerodynamics and
performance trade studies
and analysis
Mark Drela
(public license)
XFOIL
Design and analysis of
subsonic isolated airfoils
Airfoil selection and analysis
PDAS
Tranair
Transonic Panel Method
CFD Analysis of configuration
Autodesk
Autodesk Map
Two dimensional computer
drafting tool
Large scale three view
drawing and annotation
98