When jets take off from an aircraft carrier, dont they have a wall that comes up behind the plane to help it take off? How does this increase the thrust? It’s not like the jet is actually pushing off the wall. Right?
From what I know about propulsion is that the jet is sending trillions of molecules out at high speed, and this causes the jet to move in the opposite direction. Nothing is needed to push off us, this is why the shuttle can steer in space. It is usually compared to a person throwing a baseball on while ice skates or something. He throws the ball, and is propelled backwards.
How does the wall fit into this? If the person threw a ball at a barn or something, he wont be propelled any further or faster. So how does this wall help?
From my extensive knowledge of carrier ops (I read Flight of the Intruder ) the wall is called a Jet Blast Deflector and it’s there so that jets can line up behind the one already on the catapult without getting fried.
It’s designed to protect flight deck crew and other valuable equipment from jetwash. Having someone blown overboard while launching aircraft is considered gauche.
Correct answers to the question. I’ll add that JBDs on modern carriers are water cooled so that afterburning engines don’t melt the metal. Early JBDs, Korean war vintage, were just a small vertical panel with slats arranged like a Venetian blind to redirect exhaust gas upward.
The bigger risk now comes from the JBD is someone is foolish enough to walk under one. The area beneath must be inspected for FOD and I remember one incident of somone being killed benath one on the Constellation after going underneath with no safety struts in place.
A bit off-topic, but I heard that carriers have on average one death per day from accidental causes like being sucked into intakes etc. That can’t be right, can it?
From what I read, a carrier will suffer some deaths during peacetime cruises, but not one every day. That would be ~50,000 deaths per year, assuming 12 carriers at sea for 12 months. (Of course, the actual number we could have at sea is lower, as is the duration of their cruises.)
A carrier can be a hazardous place to work buy come on, who told you one per day? I’ve heard all kinds of claims that working on a flight deck is the most hazardous job in the military but even I don’t believe that. It is reasonably hazardous for work that doesn’t involve anyone shooting at you. I’m sure the trops in Iraq and Afghanistan would love to trade the death and injury rate they have for that of an operational carrier.
I was fortunate to have been on two cruises in the early '80s where no one died from a flight deck accident despite such events as losing an F-14 with an arresting gear failure and a collision with a fuel ship. Unfortunately that is a technicality as the Ranger lost seven men in an engine room fire and the Constellation lost two men in flight deck accidents not during the cruise, one each immediately before and just after. One was crushed under a JBD as I mentioned before and one was a new guy on the flight deck who panicked and tried to hide in a niche in the island. The exhaust from an A-7 Corsair burned him to death. Shore accidents happen too. A fire control tech (my specialty, weapons radar and control) was killed when testing a system. He neglected to check for CADs, cartridge actuated device used to eject missles, and when he did a jettison check the steel cover for the CAD blew through his chest. I know of one incident where someone survived being sucked into an F-14 intake. No minor miracle because the intake is the size of the friggling Holland tunnel but his belt caught on a teperature probe so he didn’t go through the engine but he was seriously messed up from the dramatic pressure change. I worked in a hangar at NAS Mirimar with a red circle painted on the overhead to mark where a tech was killed in a seat ejection.
No, the navy would never tolerate an accident rate of one person a day being sucked into an intake. Do you realize how much those engines cost?
Say, does anyone know what current hazardous duty, combat and flight deck pay are? In the early '80s we got $85 a month for flight deck pay.
Here’s a video of someone getting sucked down an A-6 intake in 1991. (The video streams, but you can right-click and “Save Target As…”. It’s a 1.7Mb mpeg).
A question about jet blast deflectors. And be gentle, as I’m not even qualified as a layman when it comes to planes, aerodynamics, jets, etc.
Obviously JBDs are not needed for the plane to “push” against, as Newton’s 3rd law gives the plane plenty of thrust even with nothing around it (except air to use in burning the fuel).
But what about ground effects? It seems to me I’ve heard of ground effects giving a little extra “lift” to planes when they are very close to the ground. If I understand correctly, this is kind of a cushion of higher density air that helps support the plane.
So is there any similar effect between a plane and the jet blast deflector? Is the JDB big enough to hold an air cushion that would add a small amount to the total?
Again, I’m pretty much completely clueless here, but wouldn’t it be more like ~4400 deaths per year? 365 deaths per year per carrier, multiplied by 12 carriers = 4380. Or have I missed something else?
Hmm… seems to me it would be rather small – and coming from aft it could disturb the airflow around the airplane more than it helps anything.
As it is, an airplane at launch position will generally have the ship turned into the wind and going at a pretty nice clip, say 25kt, so it already takes off into a headwind of that plus whatever the natural wind was.
In the case of conventional Fleet Carriers of the type used currently by the US, French and Brazilian navies (and planned to return to the British RN) the jets/heavier FWAC don’t take off on their own power, they are catapulted.
You seem to be making the assumption that only one person has ever been sucked into a jet intake. Sadly this is not the case. The incident I’m referring to happened on shore in the early '80s. I’ve seen the A-6 video and I was astonished that it could suck a person in so easily as the intake is relatively high off the deck and small. The A-6 incident happend on a ship, that’s why it was recorded by the PLAT camera. The incident I was referring to happened on shore.
I never saw that clip until a few years ago and it scared the holy crap out of me even though I’ve not been on a flight deck in almost 20 years.
The F-14’s intakes are huge and very close to the ground. That’s considered one of the poor elements of the Tomcat’s design as it is extremely prone to sucking up bolts, tools, chunks of concrete, people and other things that will FOD (foreign object damage) an engine. One of the first things I was taught was that when walking under the belly while engines are turning stay exactly in the center so you won’t be sucked into either engine. Fortunately the closest I came was crouching between the intakes inadvertantely putting a hand on the intake lip. Engines were at idle but it nearly sucked the glove off my hand. It took about an hour for my heart rate to get back to normal.
That is true for wings because the high pressure bubble undeath makes for a bigger pressure differential between top and bottom surfaces of the wing.
I won’t go as far to say that you would not add zero thrust by placing a jet in front of a vertical wall but any effect would be minute at best. Pushing against the back end of an airplane with a high pressure air bubble isn’t the same as a ground effect bubble pushing against the bottom surface of the wings. A plane is designed to be streamlined in the longitudinal axis. Any tiny amount of thrust you did gain would be lost as soon as the plane moved away from the wall. Good question though.
Even if you could add a significant amount of thrust this way it wouldn’t be useful. The catapult is powerful enough to put the plane at flight speed all by itself. The engines have to be at max power to make sure they keep flying. I have witnessed a cat shot where the pilot, the C.O. of our sqadron, had the parking brake set. A 65,000 airplane being dragged to 150 knots on wheels that don’t turn. Tires pumped to 300psi for shipboard use. After that cat shot we called the skipper “Boom Boom Bertsch.” Fortunately he was able to bingo to the beach and landed safely.
All that is a moot point because the JBD is angled to redirect the hot exausts gasses up, and away from everyone around. People have to stand really close to an F-14 on launch. As a troubleshooter I would sometimes have to position myself about even with the engine nozzle and about 25 feet away. The flame at zone 5 afterburner is about three feet in diameter and twenty feet long. Even with it aimed 90º away it’s really @&$%ing hot. I’ve had my beard singed when I was a little too close. A mistake I didn’t repeat.
Since we’re on the topic I witnessed an afterburner blowout near the Philippines from an engine full of rainwater. As the engine went to afterburner it started running erratically and then started chucking fireballs out the back. Big flaming balls of JP5 about 10-12 feet in diameter. Remember I’m 25 feet away. I recall thinking that it really looked cool and wishing I had a camera. I turned my head and saw that everyone else was running for their lives. “Fuck, do those guys know something I dont?” More experienced people though it was a fod and the engine was blowing chunks of hot metal. In a FOD most will come out the back but it will also shoot out the front and if the turbine starts shedding blades flaming metal will start shooting out to the side. Notice that a lot of militay jets have a vertical red stripe over the midsection of the engines. That’s where you don’t want to stand if the engine comes apart.
I really don’t know the accidental death rate on a carrier but the conventional wisdom when I was in that you would lose at least one person on the flight deck per cruse.
Sorry about the sea stories but I’m enjoying this thread. I’ll save the one about CAG leaving the fuel dump switch on for later.
In the neighborhood of 3-5 million dollars apiece depending, sometimes more. I know you’re not joking. One story that’s been passed around the jet-engine manufacturer world is about a crew ingestion event on the S-3. There have been several of those - the intake is almost 4 feet across, and at chest level. SOP is to replace the engine and send it back to the overhaul facility intact, where it is assigned to trainees for disassembly and cleaning (they are not told what happened, so they don’t suspect it - in theory, anyway). One carrier captain is rumored to have ended his career by having the affected engine covered with a flag and slid overboard with the full burial-at-sea ceremony - he got nailed for destruction of government property. Or so the story goes.
The F-8 and A-7, with large, single inlets down low, were notorious for killing almost more crewmen than enemies.
I was taught that an additional danger of those planes was the fact that the intake was at the extreme nose, you had nothing on the plane to help gauge how close you were getting. With an F-14 you knew the nose gear was a safe limit. Draw an imaginary arc with the intake root at the center and nose gear to mark the circumference for the most severe danger area. The same was somewhat true of S-3s and props on hummers. Plane captains would have to form a human chain to keep people from walking into engines.
