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 SKUNKS
WORKING
Lockheed Martin Advanced Development Projects is making perhaps
the first realistic tests of a hybrid airship--a concept that dates back
many decades but that is just now being tried at a significant scale.
The Skunk Works had secretly built the craft and hoped for a quiet
first flight at its Palmdale, Calif., facility, but a few passers-by
noticed the strange object in the sky.
The Defense Dept. is showing interest in two categories of
airships--those that can carry large cargo at low altitude, exemplified by
the Defense Advanced Research Projects Agency (Darpa) Walrus program, and
those that can operate in high-altitude low-wind conditions and remain on
station for long periods of time. The configuration of the Skunks Works
ship indicates it is the former--a hybrid heavy-load carrier.
The interest is across the services and the notional applications
are diverse, ranging from logistics--delivery of an integrated fighting
unit within theater, for example--to sensor, communications and even
laser-weapon relay platforms.
But airships aren't there yet. Major unresolved issues could
derail the airship dream, such as their traditional delicate ground
handling, and possibly prohibitive economics and vulnerability. These
issues have been debated endlessly on paper, and now Lockheed Martin, a
prime airship proponent, is investing to seek real answers.
A hybrid airship derives most of its lift by being filled with a
lighter-than-air gas such as helium. Overall, it is heavier than air and
gains the final 20% or so of lift by flying like an aircraft, but with slow
takeoff and landing speeds that allow operations from short unprepared
strips.
The Skunk Works made the first flight of its "P-791"
test-bed on Jan. 31 at its facility on the Palmdale Air Force Plant 42
airport. The manned flight was about a 5-min. circuit around the airport in
the morning and appeared to be successful. The company did not announce or
want to discuss the flight.
The P-791 is not part of a government contract, but rather an
independent research and development project by the Skunk Works to better
understand airship capabilities and technologies, such as materials, a
company official says. However, it may also be a quarter-scale prototype of
a heavy-lifter.
TO GAIN MORE SPAN TO ACT LIKE a wing, the P-791 is three
pressurized lobes joined together. An observer of the first flight says it
was about the size of three Fuji blimps blended together. The Fuji blimp, a
Skyship 600 model, is 206 ft. long. That suggests the P-791 would have a
gross lift of roughly 3-5 tons.
The observer saw the craft performing very tight 360-deg. turns
while taxiing. It made a brief takeoff roll, climbed to a low altitude,
made a few banks--including a long sweeping turn--then came back and
landed. The landing approach had a nose-down body attitude that leveled for
the flare. The flight was very smooth, the observer says. The craft was
flown by P-791 Chief Test Pilot Eric P. Hansen.
The speed of the test bed was estimated at about 20 kt. A
full-scale version would be able to go much faster, over 100 kt. Lockheed
Martin has long proposed a large transport airship, at one time called the
Aerocraft, which was halted around 2000 (AW&ST Feb. 22, 1999, p. 26).
That design was about 800 ft. long and was to carry 1-1.2 million lb. at
125 kt. The Skunk Works was one of two contractors to receive one-year,
$3-million Darpa contracts in August 2005 to study Walrus. The second
Walrus phase would be a three-year demonstration effort.
Hybrid airships have a long history. The Aereon Corp. in New
Jersey started experiments in the late 1950s, but they were small scale
(see www.aereoncorp.com). The company tested the "deltoid aero
body" shape, also called a deltoid pumpkinseed, with a 1,200-lb.
manned demonstrator in 1970-71. That was followed by several studies funded
by the military at less than $1 million. In the U.K., the Advanced
Technologies Group built a 40-ft.-long unmanned SkyKitten hybrid airship
and flew it in 2000 (AW&ST Sept. 23, 2002, p. 30). Nothing in the field
has progressed to the size or apparent sophistication of the Skunk Works test
bed.
The P-791 uses four air cushions as landing gear, located on the
outer lobes. Taxiing the vehicle could be like flying a hovercraft, except
one with greater exposure to winds. An advantage of the air cushions is
they could be reversed to suck the aircraft onto the ground to resist winds
for cargo operations. Air pressure may also be the best way to spread
landing loads into the inflatable structure. It's not clear if there are
any devices, such as wheels, to keep the airship from sliding sideways when
taxiing in crosswinds. The craft has a special towing system.
GROUND HANDLING IS A MAJOR ISSUE facing hybrid airships.
Conventional lighter-than-air craft require large ground crews and, because
they are especially sensitive to winds on the ground, the airstrip is an
area ripe for accidents. Hybrids are only slightly heavier than air, and a
hybrid must show large improvements in ground handling over a standard
blimp to be successful. The P-791's current limits are to remain in the
hangar if winds are above 5 kt., and there is a 10-kt. limit for taxiing
and flight. That could restrict its flight test in windy Palmdale. It's not
clear how the pilot was performing the balletic spins on his
taxi-out--whether purely with vectored thrust, or by spinning around one
sucked-down air cushion, or other means.
The P-791 appears to have four propellers--two at the tail and two
on the sides. The tail units appear to be able to pivot for yaw vectoring,
and it's unclear if the ones on the sides can move. One knowledgeable
individual says there are four vectored propulsors used for ground
handling, but it's not clear if these are the main propellers, or separate
units perhaps connected with the air cushion system. The rings around the
motors may be shrouds for the propellers and/or gimbal rings for vectoring.
Vectored thrust can be useful for lighter-than-air blimps, which lose
conventional control authority as they approach zero airspeed while
landing, but a hybrid airship lands with some airspeed that may keep the
tail control surfaces effective. But for control during low-speed air
cushion taxiing, vectoring would seem essential.
The P-791 appears similar to the proposed full-scale version of
the British SkyKitten, called the SkyCat. They have similar overall
shapes--though the Skunk Works design is wider--and similar propulsion
layouts, and both use air cushion landing gear. Perhaps the two programs
have people in common.
One of the partner names on the side is TCOM, which makes
aerostats and envelopes for airships.
"Hybrid airships have been the subject of studies and
questions for half a century," one expert says. "Now it stops
being hype and they will meet reality."
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