Ladakh Flash Point

[lcloo]

If a jet with full load needs 1,000m length runway and 200km/hr speed to take off on a lowland airfield, what is the requirement to take off on a plateau airfield with 50% less oxygen? Can it take off with full payload at 300km/hr speed with runway of 4,000m length?

Consider these, (1) density volume of air flow over the wings per second and (2) density volume of oxygen per second entering the combustion chamber of the jet engine. Any jet propulsion engineer care to answer?

 

[plawolf]

I've heard this idea before, but wouldn't this only work on J-15? And new J-15 intended for 003 no less not the current ones.

Depends on how they do it. But personally, I think if you are setting off to design a high-altitude aircraft launch system, maglev sleds would be a far better starting point than catapults.

China already have Maglev train tech, so it should be fairly straight forwards to modify that to make a sled that is magleved above the runway, with a carrier style wheel stop, which released once the sled gets up to speed, they could build a track to loop back around to slow the sled for the next launch and/or recover airframe in the event of a failed launch.

The benefits of this is that it will not require catapult specific landing gears and structural mods; can accelerate the plane much more gradually over a much longer distance to avoid stress damage to the airframe; can accelerate airframe to speeds beyond what their landing gear wheels are rated for, and reduce wear and tear on landing gear and wheels; can safely recover aircraft in the event of a failed launch, for example as a result of engine failure; the maglev rails can be built flush with the runway to not affect normal use.

 

[lcloo]

Depends on how they do it. But personally, I think if you are setting off to design a high-altitude aircraft launch system, maglev sleds would be a far better starting point than catapults.

China already have Maglev train tech, so it should be fairly straight forwards to modify that to make a sled that is magleved above the runway, with a carrier style wheel stop, which released once the sled gets up to speed, they could build a track to loop back around to slow the sled for the next launch and/or recover airframe in the event of a failed launch.

The benefits of this is that it will not require catapult specific landing gears and structural mods; can accelerate the plane much more gradually over a much longer distance to avoid stress damage to the airframe; can accelerate airframe to speeds beyond what their landing gear wheels are rated for, and reduce wear and tear on landing gear and wheels; can safely recover aircraft in the event of a failed launch, for example as a result of engine failure; the maglev rails can be built flush with the runway to not affect normal use.

That is a very good idea, but if the airfield is attacked and the maglev launch system is damaged or destroyed, the airfield would be rendered out of commission for a long time.

 

[Nobonita Barua]

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Oka folks, they have a "homegrown"(!!!!) mizzile now which can strike Tibet. Though i admit, i have no idea what would they be targeting at Tibet.

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And they are prepared.

 

[[witty username]]

Not confirmed but it is believed that Swiss Air Force used F-18A Hornets with arrestor hooks for short landings on high altitude mountain airstrips.
Rocket assisted take offs have been used to get heavily loaded aircraft into the air on shorter airstrips (or no airstrips), or airstrips at altitudes as you can see from the dramatic Soviet era video:

There is also rocket assisted landing...

I believe the US actually planned to use this during the Iranian hostage crisis, but decided against it as the technology was not very reliable.

 

[crash8pilot]

If a jet with full load needs 1,000m length runway and 200km/hr speed to take off on a lowland airfield, what is the requirement to take off on a plateau airfield with 50% less oxygen? Can it take off with full payload at 300km/hr speed with runway of 4,000m length?

Consider these, (1) density volume of air flow over the wings per second and (2) density volume of oxygen per second entering the combustion chamber of the jet engine. Any jet propulsion engineer care to answer?

I'm not an engineer but a pilot, so I'll take a stab at answering.

There's no such thing as "50% less oxygen", the atmosphere will always be composed of 78% nitrogen, 21% oxygen, with the rest made up of CO2, argon, and other noble gases. That that said the density of oxygen decreases (oxygen molecules are further apart) with an increase in altitude, and as such an aircraft and engine's performance reduces.

A better way of explaining might be to look at the lift formula (lift = coefficient of lift x half of air density x speed² x wing surface area), as we know the force of lift opposes that of weight. With the value of air density in the formula decreasing, other values in the formula will have to increase in order to produce the required lift for a given aircraft weight. Increasing coefficient of lift can be achieved by increasing the camber of aerofoils, such as more slats or flaps. Increasing the takeoff speed would also work as you mentioned, however you're gonna need a loooooooooong runway to allow the plane/engine to accelerate, which is why high altitude airports such as Mexico City, Quito in Ecuador and Santiago in Chile feature longer than usual runways (might be challenging to produce a massive runway in mountainous terrain). Jet assisted takeoff bottles (or afterburners) could also help the aircraft accelerate to the required takeoff speed, but don't forget these bottles produce parasite drag (which will need to be overcome by additional thrust), and also increasing the weight of the aircraft (which is the problem we're trying to solve in the first place). The aircraft's wing could also be redesigned to increase the surface area.

I must admit the catapult idea might be a work around to increase a fighter jet's acceleration momentum, and thereby reaching the required takeoff speed at a shorter distance. I'm no structural engineer, but I'm pretty sure catapult aircraft require their landing gears (and perhaps other critical components?) to be reinforced to withstand the force of the catapult. So on top of installing catapults in high altitude air bases, the PLA's fleet assigned to the area will have to be overhauled? That sounds fairly expensive and inefficient. I've mentioned in a previous post, but perhaps a better work around to the problem might be to launch a fighter with it's required weapons loadout as well as sufficient contingent fuel, then have the fighter meet a tanker enroute to top up on gas before proceeding to their objective?

 

[Bright Sword]

That is a very good idea, but if the airfield is attacked and the maglev launch system is damaged or destroyed, the airfield would be rendered out of commission for a long time.

If a jet with full load needs 1,000m length runway and 200km/hr speed to take off on a lowland airfield, what is the requirement to take off on a plateau airfield with 50% less oxygen? Can it take off with full payload at 300km/hr speed with runway of 4,000m length?

Consider these, (1) density volume of air flow over the wings per second and (2) density volume of oxygen per second entering the combustion chamber of the jet engine. Any jet propulsion engineer care to answer?

I believe it has less to do with aerodynamics rather than pure thermodynamics. The calorific value of any fuel is constant but the thermal energy and (therefore the propulsive thrust) depends on the complete combustion of the fuel. If there is insufficient oxygen then the fuel is only partially consumed. This is the old fashioned concept of the carburetor where air instead of fuel is controlled. Less air means less oxygen. Fuel injection of course controls the fuel while maintaining full turbocharged air intake.
At high altitudes there is a natural carburetor effect because the volume of air being sucked in contains less oxygen per unit volume. An undesirable loss of engine power (due to any factor) is technically called de-rating. The derating problem was inherent with piston engined air craft as well when before and during World War 2 aero engine designers struggled to produce engines that could perform at altitudes above 11000 meters.Very large turbo chargers and compressors were used to suck in as much air as possible as can be seen in the late model fighter designs of late World War 2 ( Hawker Typhoon, Bf-109G, P-51 ). Ultimately a dead end was reached when a piston engine could be enhanced no further and even if that was possible the propellers could not produce the required screw and pitch forward motion in the rarified air.
Jet engines were the answer because they were self feeding with the air sucked in with large rotary compressors and the propulsion was by a reactive thrust instead of a screw and pitch motion of the propeller.
But jet engines consume enormous amounts of air during start up and there is a "warm up" time when the rotation of the compressor is sufficient to deliver the required amount of air to burn the fuel and produce temperatures which expands the air and then produce the thrust. The "warm up" time is considerable at high altitudes.
Early jet engines had frustratingly long "warm up" times, a fact exploited by piston engine fighters when tackling their jet engine adversaries, often catching them on the runway when getting ready for action.
Which is why there was extensive research on rocket powered aircraft. Rockets are not limited by altitude or availability of oxygen since they carry their own oxidizing elements liquid ( example: Hydrogen Peroxide) or solid (
Nitrate compounds). There is no "warm up" time. But rocket engines cannot be switched off once started and will continue burning till the fuel
is exhausted, Rocket motors cannot be controlled or throttled. So rocket powered aircraft other than a few unique German and Japanese designs a failure. The Rocket Assisted Take Off however uses a combination of conventional aircraft engine and rocket power to get a heavy aircraft in the air in the shortest possible take off distance ( or even no distance). This method circumvents the de-rating of the engines due to altitude related oxygen limitations.

 

[Sardaukar20]

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Oka folks, they have a "homegrown"(!!!!) mizzile now which can strike Tibet. Though i admit, i have no idea what would they be targeting at Tibet.

Please, Log in or Register to view URLs content!

And they are prepared.

So India is now deploying the Nirhbays? Interesting. Are these weapons ready? They have been testing it for 7 years. Can't deploy it years ago, but ok to deploy them now? How many are available now?

In any case, the Nirbays are not gonna scare the Chinese one bit. They have prepared against mass Tomahawk strikes. I wonder what is India's preparation against CJ-10s or Baburs? Its one thing to launch Nirhbays, its another when the enemy can shoot back.

 

[broadsword]

I'm not an engineer but a pilot, so I'll take a stab at answering.

There's no such thing as "50% less oxygen", the atmosphere will always be composed of 78% nitrogen, 21% oxygen, with the rest made up of CO2, argon, and other noble gases. That that said the density of oxygen decreases (oxygen molecules are further apart) with an increase in altitude, and as such an aircraft and engine's performance reduces.

A better way of explaining might be to look at the lift formula (lift = coefficient of lift x half of air density x speed² x wing surface area), as we know the force of lift opposes that of weight. With the value of air density in the formula decreasing, other values in the formula will have to increase in order to produce the required lift for a given aircraft weight. Increasing coefficient of lift can be achieved by increasing the camber of aerofoils, such as more slats or flaps. Increasing the takeoff speed would also work as you mentioned, however you're gonna need a loooooooooong runway to allow the plane/engine to accelerate, which is why high altitude airports such as Mexico City, Quito in Ecuador and Santiago in Chile feature longer than usual runways (might be challenging to produce a massive runway in mountainous terrain). Jet assisted takeoff bottles (or afterburners) could also help the aircraft accelerate to the required takeoff speed, but don't forget these bottles produce parasite drag (which will need to be overcome by additional thrust), and also increasing the weight of the aircraft (which is the problem we're trying to solve in the first place). The aircraft's wing could also be redesigned to increase the surface area.

I must admit the catapult idea might be a work around to increase a fighter jet's acceleration momentum, and thereby reaching the required takeoff speed at a shorter distance. I'm no structural engineer, but I'm pretty sure catapult aircraft require their landing gears (and perhaps other critical components?) to be reinforced to withstand the force of the catapult. So on top of installing catapults in high altitude air bases, the PLA's fleet assigned to the area will have to be overhauled? That sounds fairly expensive and inefficient. I've mentioned in a previous post, but perhaps a better work around to the problem might be to launch a fighter with it's required weapons loadout as well as sufficient contingent fuel, then have the fighter meet a tanker enroute to top up on gas before proceeding to their objective?

How about taking off from negative gradient runways?

 

[Sardaukar20]

I like this article:

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Its so deluded in its analysis. It makes for excellent comedy. When General Winter does his work on IA troops. Or when PLA puts IA in its place. Every one of those points of "India > China" would become meme or troll material.