F-35 Bay Presents Challenges to Weapons

 

F-35 Bay Presents Challenges to Weapons

By Amy Butler
Aviation Week & Space Technology

September 17, 2012
Amy Butler Washington

Developers of the multinational F-35 are finally embarking on a multi-year
campaign to demonstrate the single-engine stealthy fighter's ability to
dispatch weapons. But uncertainties loom about the impact of
internal-carriage requirements on those weapons' effectiveness.

The Lockheed Martin F-35, and the F-22 before it, have introduced a new
level of complexity into the air-launched-weapons world by demanding that
munitions long anchored on external wing and belly pylons of legacy fighters
be carried in small, stealthy internal bays. The Air Force decided decades
ago to forgo large payloads -- epitomized by the F-15 Strike Eagle -- in
pursuit of a significantly reduced radar cross section, allowing for
fighters to evade air defenses and penetrate into enemy air space.

While the F-22 was a step in this direction, the F-35 is expected to carry
far more weapon types in its bay, which has a challenging thermal and
acoustic environment. Although the bay has not presented developers with
conditions beyond the specifications of weapons slated for use in the F-35,
engineers acknowledge there is little margin. "We are within about 10-12
degrees in most cases. But it is close [to the design specifications of some
weapons]", says Charlie Wagner, weapons integrated project team lead for the
F-35 Joint Program Office. "It is not that simple, though. Maybe I can get
[a weapon] that hot. But can I get it hot for an hour? Or can it be that hot
for two days?"

Wagner says experts in the military are studying the potential prolonged and
cumulative effects of operating so close to margin for weapons such as the
1,000-lb. GBU-32 Joint Direct Attack Munition (JDAM) and GBU-31 2,000-lb.
version. Both ground-attack weapons are slated for early use on the
aircraft. A thornier issue, perhaps, will be the environmental impact of the
bay on weapons employing more sophisticated electronics, sensors, and
motors, such as the Raytheon AIM-120C7/D Advanced Medium-Range Air-to-Air
Missile (AMRAAM) or British AIM-132 Advanced Short-Range Air-to-Air Missile
(ASRAAM). "I don't know if the weapons people know that for sure," says
Wagner. "We are pushing what they designed to....If I'm going to the extreme
of what they tested [a weapon] to, they may not have a real good
understanding of how that is going to affect the weapon over the next 20
years."

Operating near or at the margins is not new for the F-35 program, which has
suffered problems keeping the F-35B to the required weight. Though the
weight issues have been resolved -- with roughly 300 lb. of margin now on
the aircraft, according to Marine Corps Commandant Gen. James Amos --
developers are loath to run into similar problems with the operating margins
of weapons in the F-35 bay. This is especially true as the Pentagon faces
major funding cuts in the coming years; shortcomings in programs could make
them vulnerable for reductions or terminations by Congress.

As scientists and engineers continue to study these issues, the
flight-testing program is moving forward. The first jettison test took place
Aug. 8, when a GBU-32 was dropped from BF-03, a
short-takeoff-and-vertical-landing F-35B at NAS Patuxent River, Md. The
trial was designed only to ensure safe separation and the weapon was inert.
This is the first in a short series of such jettison tests. At least two are
slated for next month, including the first drop of a GBU-31 from a
conventional-takeoff-and-landing F-35A.

The F-35 maintained a speed of roughly Mach 0.65 at level flight for the
first drop; higher speeds are slated for future demonstrations. However,
Wagner notes that it is not urgent to test high bank angles for JDAM drops
because, in the field, the F-35 will likely be employed primarily in level
flying conditions to maintain the lowest radar cross section possible for
ground-attack missions.

Varied angle-of-attack maneuvers will come into play when engineers begin
testing the use of AMRAAM on the F-35. As pilots will want to "fire and
forget" their air-attack weapons, they will need to employ them in a much
larger flight envelope than the ground-attack munitions.

Unlike the JDAM series, AMRAAM is powered by a solid-rocket-fuel motor and
is dispatched from its position on the door of the F-35 weapon bay. JDAMs
are carried on a bomb rack mounted inside the belly.

Initially, developers plan only to demonstrate safe separation of the
AMRAAM, using test bodies lacking a rocket motor. They hope to actually fire
an AMRAAM from the F-35 by the end of January, Wagner says.

Although the three F-35 types share some elements, each weapon model must be
tested separately on each fighter version. Those trials will be conducted
in parallel, he says. The program prioritizes tests of internal-carriage
weapons, as those are required for the Block II and Block III F-35 releases;
Block IIB software is what the U.S. Marine Corps will use for declaring
initial operational capability (IOC) with its F-35Bs.

The first external weapons will be used in Block III, which is the software
version required for IOC for Air Force and Navy models; Block III will be
released no earlier than 2017. Introduction of the Boeing GBU-39
Small-Diameter Bomb, a 250-lb. glide weapon, is slated for the Block IV
software release at the end of the decade. This weapon was developed by the
Air Force specifically to maximize the number of ground targets that the
F-35, with its limited internal-bay space, can attack on a single mission;
four SDBs can be mounted in place of each JDAM position, allowing for the
F-35 and F-22 to each carry eight of the weapons internally. The SDB II, a
version incorporating a tri-mode seeker for all-weather, day/night
engagements, is now being developed by Raytheon. The first units will be
delivered for operational use in 2016.

The F-35 also is being developed to carry nuclear weapons. Testing of
nuclear munitions on the fighter is not slated to begin until after the
aircraft development phase is complete.

In the meantime, F-35 developers are sharing the thermal and acoustic data
being collected from the bay with the weapon manufacturers. While the bay
environment is close to the design specifications of weapons already in the
arsenal, the developers hope that sharing this information will help
engineers crafting next-generation munitions to design them with those
environmental factors in mind, Wagner says.

The Aug. 8 weapon separation test was a major milestone for the Pentagon-led
F-35 program. To view a video of it, check out the digital edition of AW&ST
on leading tablets and smartphones, or visit www.AviationWeek.com/jdamdrop

Integrating a number of diverse weapons onto the F-35 will be critical to
the stealthy fighter's utility for its global customers. To view an
interactive chart of weapons slated for use on the F-35, check out the
digital edition of AW&ST on leading tablets and smartphones, or visit
www.AviationWeek.com/jsfweapons