Hydrogen, the Combustible: Not a gas to take lightly

Information is coming out now about the Fukushima Dai-ichi building blasts. Engineers and executives struggled about when, and how to vent the flammable mixture from the reactor buildings.

People might well wonder why Units 1, 3, and 4 all suffered big hydrogen explosions. How could hydrogen achieve the conditions necessary for explosion so easily? Many accidental, combusible mixtures of fuel gas and air in everyday life never catch fire or blow up.

It helps to know that hydrogen-air mixtures are remarkable in their ability to ignite. Here's a NASA hazard analysis for hydrogen as a gas and a cryogenic liquid.  

Flammability range: Its lower and upper combustibility limits, when mixed with air at standard pressure, are 4% and 75% by volume. Compared to other fuel gases like propane and methane, the lower limit isn't that remarkable, but the upper limit is. Among common fuel gases, only acetylene has a higher upper flammable limit. Hydrogen mixtures with air are explosive in the 18-59% range. 

Energy of ignition: The minimum ignition energy for hydrogen-air mixtures is only a tenth of that needed to start gasoline, methane and propane fires. A spark not visible to the human eye -- less than you feel after walking across a carpet on a winter's day and touching a doorknob -- is enough. 

Permeability through flaws in pipes and fittings: High.

Is it burning? It can be hard to tell. Often the human eye can't detect a hydrogen fire in daylight, if nothing else like plastic or grease is burning. Industrial firefighters use infrared flame detectors.

What's it add up to? The fact that hydrogen needs special attention doesn't mean it's "unsafe" compared to other fuel gases. It's got a safety edge, in fact, when it comes to hazards posed by leaks in open air. As readers of Hindenpunk literature know, hydrogen has the lowest density of all gases. It rises and therefore disperses rapidly into the open air when released from an outside source like a leaking pipe flange. That's safer than a gas that spreads along the surface, like chilly methane boiling off a liquefied natural gas tanker spill. 

The Big Muddy in flood: When loose ships can sink cities

The Coast Guard is off again, then on again, when it comes to allowing ship traffic through the lower Mississippi. The USCG reopened the river near Natchez to a trickle of the usual traffic volume yesterday: vessels must be spaced and stick to the middle.

Most reports explain the traffic slowdown and occasional shutdown as driven by the need to keep wakes down, because ripples can overtop the levees and start erosion. Right now the river level at New Orleans's Carrollton Gauge is 17.23 feet, four feet below the record and up a bit from yesterday.

Not mentioned as prominently in news reports, but equally worrisome, is the danger that a vessel will bash a hole through a levee. That would be particularly ironic to the New Orleans Hyatt, which is to reopen in October after being closed ever since Katrina. The water is so high that in some locations a ship's hull could override the bank and strike a levee. This can happen if a ship loses power or if a barge breaks loose from a tow following a collision or wire break.

During the New Orleans flood of 2005, a storm surge traveling up the Industrial Canal threw a barge against a levee, helping to flood the Lower Ninth Ward.  This from the Wall Street Journal, September 8, 2005:

"As storms approach New Orleans, owners of ships, tugboats and freight barges that populate the city's port and waterways attempt to secure their craft.... As the hurricane rolled into New Orleans, scores of boats broke free or sank. In the Industrial Canal, the gush of water broke a barge from its moorings. It isn't known whose barge it was. The huge steel hull became a water-borne missile."

Other allisions of interest:

In 1996 the outbound MV Bright Field lost power while laden with 56,000 tons of corn and drifted into the Riverwalk Marketplace, injuring dozens of mall-goers. Here tugs are taking it away from the scene:

An NTSB report identified a clogged oil filter, and unheeded engine alarms, as the main causes. (As word-lovers know, the verb for crashing into a stationary object is allide, not collide.) Here's a link to a short video clip of the crash. 

In 1998 and again in 2004, St. Louis saw runaway barges breaking loose and threatening other vessels along the riverbank. This academic paper looked at the lower Mississippi River and identified stretches most likely to have collisions, allisions, and groundings.

This recent report from the Times-Picayune described risks from tied-up barges and tugs: 

"Barges and tugboats are required to stay 180 feet from the levees, for fear that they will crash into the banks and cause damage. But after the Coast Guard prohibited ships from navigating the river at night because of the flooding, barges and tugboats began mooring themselves to the levees so they would not have to keep their engines running at night … .Inspectors on their night-time rounds are on the lookout for the ships. They found nine tied up on Tuesday night and seven on Monday, including some triple-wide barges that could have caused massive damage if they slammed into the levees … Some were close enough that inspectors standing on top of the levees could grab the side of the boat."

Helicopter, Hush Thyself

We've seen many articles the last two days about the stealthy helicopter allegedly used by the 160th SOAR in the Bin Laden raid, said to have been wrecked against a wall in a mishap called settling with power, and left behind as an intentionally demolished wreck with a mostly intact tailboom. Remarkable to me is the fact that the tail rotor blades are essentially undamaged ... normally in a tail-strike incident these are turned into small fragments. So the tail rotor must have been at a full stop before it encountered any hard object. 

Taking it at face value, two qualities are claimed in the Abbottabad stories: low radar cross section, and low noise.

How do such noisy machines hush themselves? I wrote about the original low-noise helo in this article for Air&Space. It was the 500P, the "Quiet One," a highly modified variant of the OH-6A Cayuse. It was financed by the CIA and built by the aircraft division of Hughes Tool. 

 

The 500P went into action to place a wiretap on an enemy telephone line near Vinh, North Vietnam, in December 1972. It flew out of a secret base called PS-44 near Pakse, Laos. The operation was successful, and I interviewed the participants.

The story begins in 1968, when Hughes Tool Aircraft Division sold two piston-powered Model 269 helicopters to an affluent Los Angeles suburb for police patrols. Citizens soon called to complain about the noise, and the city told Hughes to either make them quieter or take them back. An emerging market for police patrols was at stake. Engineers at Hughes identified the tail rotor as one of the biggest noisemakers. By doubling the number of blades to four, Hughes was able to cut the speed of the rotor in half, which reduced the helicopter's noise.

Coincidentally, the Advanced Research Projects Agency was hunting for contractors who could cut noise from military helicopters of all sizes. After hearing about Hughes' work on the police helicopters, ARPA offered the company $200,000 in 1968 to work similar magic on a Hughes OH-6A light helicopter. Hughes Tool made a short movie about the modifications, which included a new set of gears to slow the tail rotor, and showed it to ARPA.  ARPA approved money for an all-out quietizing effort, Phase II, and gave the project the code name Mainstreet. Even before work was fully under way, the CIA ordered two (later registered as N351X and N352X) for use in the field. Test flights began at Culver City, California, in 1971, followed by a brisk training program for the U.S. instructor-pilots who would later train mission pilots.

Flights of the Quiet One included low-level work at the secret Air Force base Area 51 in Nevada and touchdowns on peaks in California to familiarize pilots with close-quarters maneuvering and landing in darkness. Pilots needed at least eight hours to get comfortable with steering by sole reference to the comparatively narrow view of the forward-looking infrared (FLIR) camera, which was mounted just above the skids.

The slapping noise that some helicopters produce, which can be heard two miles away or more, is caused by "blade vortex interaction," in which the tip of each whirling rotor blade makes tiny tornadoes that are then struck by oncoming blades. The Quiet One's modifications included an extra main rotor blade, changes to the tips on the main blades, and engine adjustments that allowed the pilot to slow the main rotor speed, making the blades quieter. The helicopter also had extra fuel tanks in the rear passenger compartment, an alcohol-water injection system to boost the Allison engine's power output for short periods, an engine exhaust muffler, lead-vinyl pads to deaden skin noise, and even a baffle to block noise slipping out the air intake.

The extensive alterations were not to eliminate all noise -- an impossibility -- but to damp the frequencies of noise that people associate with a helicopter. 

How quiet was it? I'm told that because of the quieting gear, the helicopter couldn't be heard from the porch of the PS-44 main building unless it was flying overhead. Even then, at night, it sounded like a far-off airplane. The helicopter had its own hangar so Soviet spyplanes and satellites could not get a look at the peculiar profile produced by the extra main rotor blade, a tail rotor with blades in an odd scissored configuration, and big muffler on the rear fuselage. More technical information is available in this unclassified report. 

Did the remarkable 500P continue to fly by night? Officially, the Vinh wiretap was the first and only wartime mission of the original black helicopter. Its remarkable qualities were offset by the reduced flight performance.

Hunting for Bin Laden? Take a SOAR helicopter

News reports have it that Seal Team 6 and CIA operatives rode from Afghanistan to Abbottabad on helicopters of the Night Stalkers, aka 160^th Special Operations Regiment, or SOAR. 

Those reports make no mention of using the MH-6 Little Bird, which is interesting because old hands at SOAR consider the single-engine, egg-shaped helicopter well-suited to putting operatives on urban rooftops. The other two machines that SOAR uses, variants of the Black Hawk and Chinook, have their good points if a large team is needed, but are extremely loud and offer bigger targets for ground fire. 

While researching The God Machine I visited the 160^th SOAR headquarters at Fort Campbell, Kentucky, talked to pilots, trainers, and old hands, and hopped aboard an MH-60 for a night-time training flight. (Yes, it was a black helicopter ... or at least a very dark gray.) The following is adapted from my book.
 

The origins of SOAR can be found in the southwest corner of Fort Campbell. It's a tan, concrete-walled building and in 1981 housed the super-secret Task Force 160, the helicopter unit preparing for a second try at spiriting 53 American hostages from hostile Iran.

The reason for forming Task Force 160 was the humiliating failure of an earlier helicopter-dependent rescue attempt in 1980. It was code-named Operation Eagle Claw, and popularly known as the “Desert One” mission. The origin of Eagle Claw was a time of high tension between the United States and the new Islamic regime over the fate of Shah Reza Pahlevi, the deposed ruler. Less than two weeks after the United States allowed the shah to enter the country for surgery, armed revolutionaries broke into the U.S. embassy in Tehran and took dozens of prisoners.

A few Americans were released but that still left 53 hostages. President Jimmy Carter authorized an elaborate plan in April 1980 in which forces from all branches of the armed services would go deep into Iran to retrieve them. As part of this eight Navy RH-53 Sea Stallion helicopters, normally used for minesweeping, would depart from an aircraft carrier and fly 600 miles to a remote landing strip called Desert One. Forty-four aircraft would participate in six elaborate phases.

Things started going wrong early, when three of eight helicopters dropped out with real or suspected mechanical problems. That violated minimums and the rescue was scrubbed. Still the story was not over. While one helicopter was hovering in a dust cloud, its pilot struggling to maintain his visual fix on the ground, its rotor blades cut into a C-130 transport. The shrapnel ignited fuel and ammunition on both aircraft. At that point it was a scramble for survival. In the departure, the task force left eight bodies and much wreckage behind.

Eagle Claw generated many official recriminations and reports. it also led to a high-risk program, code-named Honey Badger, to go in one more time with more helicopters and commandos, but different tactics.

For Honey Badger the Army gathered new H-60 Black Hawks, Vietnam-era Loach OH-6 helicopters, and troops from the 101st Airborne. The task would be enormously difficult. The Iranians had dispersed the hostages across multiple locations and kept them moving.

The Loaches trained by night, and evaded notice during daylight hours by sliding into garage stalls in the back of the building each dawn.  “The Huey and OH-58 didn’t suit,” recalled Clif O’Brien, a retired command sergeant-major, and a participant in the preparations. “The MH-6 is easier to work on, rapidly deployable, and crash-worthy. The deployability is excellent. You can put six MH-6’s off a C-141 [transport], and in six or seven minutes you can have them running.” The troops of SOAR call the MH-6 and its armed cousin, the AH-6, the Little Bird.

Preparations for Honey Badger stood down after the release of all hostages in 1981, but the Army decided to retain a permanent, commando-style helicopter force. The existence of Task Force 160 (now the 160th Special Operations Aviation Regiment) remained secret until its cover was blown during Persian Gulf operations in 1987. By then Army helicopter pilots had come to regard it as the prime billet among combat units.

Night-time training accidents were the first obstacle. After four helicopters crashed during training in 1983, a regimen was set up to train pilots about navigating and approaching a target with the early model of night-vision goggles then available; these were originally intended for use by truck drivers and had a narrow field of view.

Task Force 160 did battle for the first time in the island nation of Grenada, supporting an American action to oppose a Marxist movement that was receiving Cuban support and to evacuate American citizens. Instead of its preferred night approach, however, the helicopters had to come in during the day. They took heavy anti-aircraft fire on approaching one target, a prison. One helicopter crashed, for the unit’s first combat fatality. Later the unit shot up Iranian gunboats and a minelayer in the Persian Gulf, used two of its MH-47 Chinooks to haul back a Soviet Mi-24 helicopter gunship abandoned in Chad, and fought troops loyal to Gen. Manuel Noriega in Panama.

The latter action led to SOAR’s second and third combat fatalities, when an armed Little Bird was shot down with a rocket-propelled grenade at the Colon harbor. The helicopter had been covering the exit of SEALs following a commando raid on “high value targets” at a beach house.

Little publicity arose from that, but the reclusive regiment became headline news on October 3, 1993, because of a battle in the narrow streets of Mogadishu, Somalia. That afternoon a U.S. special forces raid arrived via SOAR helicopters at a building across from the Olympic Hotel. The action initially captured two dozen of Mohammad Farah Aidid's assistants but went bad after a rocket-propelled grenade hit the tail of one orbiting Black Hawk, which spun out of control and crashed a few hundred yards away. A second Black Hawk caught an RPG round in the cockpit and crashed a mile further off. The battle to recover bodies and wounded men lasted well into dark, then resumed before dawn. In one of the most dramatic moments, an MH-6 "Little Bird" helicopter with the codename of Star 41 made a high-risk landing in a narrow street in the midst of the gun battle, gathered up Sergeants Daniel Busch and Jim Smith from the crashed Super 61 Black Hawk, and launched safely. The toll for SOAR aviators was five killed and one captured.

The copilot on Star 41, the Little Bird that touched down in the Mogadishu alleyway, was Chief Warrant Officer 3 Karl Maier. At the time I interviewed him, Maier’s job at Fort Campbell was operations officer for the training arm of the SOAR unit, known as the Green Platoon. In warrior style, Maier made no claim to heroism that day: “We were unarmed and afraid,” he said, noting that the combined action of all the gunfire from armed Little Birds overhead was so fearsome that the Somali fighters stayed back and made the rescue possible.

The process to prepare Army helicopter pilots for SOAR work takes three months, followed by two years of additional preparation for those who want the authority to plan and lead a mission. Trainees in Green Platoon stay very busy. After two weeks of individual combat training, pilots spend three weeks planning and flying low-level night missions to unmarked landing zones at least 60 miles from the base. All navigation must be done with map, clock, and compass; no other gadgets are permitted.

The MH-6 and AH-6 helicopters now used in battle by SOAR pilots are modified MD530F models. The MH-6 helicopter weighs 2,100 pounds empty; fully loaded and fueled it weighs more than twice as much. Four fully armed soldiers can ride on the outside of a Little Bird, seated on fold-down planks attached to the landing-skid struts.

“The guys in the MH-6 work close in, up to the front doors and to the top of the building,” said Maier. “It’s very good at urban warfare. Compare that to regular army aviation – their urban guys get you to the outside of town. They’re not dropping you off downtown.” The Little Bird is preferred for dropping off troops because it small and nimble, which makes it hard to hit from the ground. “They [the enemy fighters] don’t know where we’re going to land, and at night we’re all blacked out,” Maier said. “They’re shooting at the noise and that’s behind us.”

After graduating from the Green Platoon, pilots selected for the AH-6 gunship learn to use the trusty, 2.75-inch folding-fin rocket. “This is direct fire on a target, not standoff like the Apache [helicopter],” Maier said. “The good guys identify themselves and you shoot around ‘em.” At a distance of 200 yards Little Bird pilots can put a full load of rockets into an standard garage door.

By the time an AH-6 Little Bird pilot is ready to graduate from Fort Campbell he has fired a small mountain of rockets, and a truckload of machine-gun rounds. Most of all, he's had the benefit of learning from the world’s best assault-helicopter instructors.