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Chris32
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 Posted: Fri Apr 15, 2011 6:40 pm Post subject: |
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| TuppyTrucker wrote: | | Chris32 wrote: | I can't for the life of me see how it works.
If it is a standard mortar shell then how can it control its trajectory once it is out of the launching tube? |
"The GPS and an accompanying computer control the tail fins and can steer the mortar to ensure it follows its pre-programmed trajectory."
Same as a JDAM bomb. Dumb standard bomb with GPS and control bolted on in a steerable tail.
Of course it does require someone to enter the correct target grid. Best not let a Merkin do that. |
Unless the laws of physics have changed, or it is not a standard mortar bomb, that doesn't do it for me. Altering the angle of attack of the tail fins will change the attitude of the device and may change its range, but the device will continue on a ballistic trajectory through its centre of mass. If you want to steer it, it either needs support surfaces (in addition to the steerable tail) placed in such a way as to achieve lift through its centre of mass, or it needs an auxiliary power system to provide thrust in flight. |
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M8TJT
The Other Tired Old Man
Joined: Apr 04, 2006
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Location: Bexhill, South Sussex, UK
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| Posted: Fri Apr 15, 2011 8:02 pm Post subject: |
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| Chris32 wrote: | | Altering the angle of attack of the tail fins will change the attitude of the device and may change its range, but the device will continue on a ballistic trajectory through its centre of mass. If you want to steer it, it either needs support surfaces (in addition to the steerable tail) placed in such a way as to achieve lift through its centre of mass, or it needs an auxiliary power system to provide thrust in flight. |
Then how on earth do laser guided smart bombs with steerable tail fins work then? Wouuld you be prepared to stand in the target area to support your theory? I think not. Surely if you 'change the attitude of the device' the aerodynamic forces acting on it will change, and thus change its trajectory. |
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Duddy
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Location: Manchester UK
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| Posted: Fri Apr 15, 2011 8:33 pm Post subject: |
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It works just like the rudder on a ship.
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HUAWEI P30 Pro (new edition) with Speedtrap Alert & alcatel1 for SatNav
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Retired but want to go back to work for a rest. |
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Darren
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| Posted: Fri Apr 15, 2011 8:47 pm Post subject: |
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| Chris32 wrote: | | Unless the laws of physics have changed, or it is not a standard mortar bomb, that doesn't do it for me. |
Yeah, of course it's all an elaborate hoax. 
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Chris32
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| Posted: Fri Apr 15, 2011 8:49 pm Post subject: |
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| M8TJT wrote: | | Then how on earth do laser guided smart bombs with steerable tail fins work then? |
By having support control surfaces on either side of the centre of mass. |
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mikealder
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Location: Blackpool , Lancs
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| Posted: Fri Apr 15, 2011 8:55 pm Post subject: |
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Steerable nose section combined with steerable fins just like the Paveway this gives the required control at either end of the package, the image in post one probably looks very little like whats being used in theatre.
Sticking some controlled fins at either end of a mortor shell and combining it with GPS for accuracy is a logical adaptation of modern technology - Mike |
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Darren
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| Posted: Fri Apr 15, 2011 8:58 pm Post subject: |
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From: Defence Studies Blog
| Quote: | | In the nose, a GPS receiver and computer controlled aerodynamic directional fins keep the round on its programmed trajectory. Folding fins in the tail provide stability. |
My post was a summary of the info provided in the official PR which was slightly inaccurate. The detailed analysis isn't really relevant to the wider audience of this site but I trust this clarifies the technical minutia.
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M8TJT
The Other Tired Old Man
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| Posted: Fri Apr 15, 2011 11:02 pm Post subject: |
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| Darren wrote: | From: Defence Studies Blog
| Quote: | | In the nose, a GPS receiver and computer controlled aerodynamic directional fins keep the round on its programmed trajectory. Folding fins in the tail provide stability. |
My post was a summary of the info provided in the official PR which was slightly inaccurate. The detailed analysis isn't really relevant to the wider audience of this site but I trust this clarifies the technical minutia. |
But according to Chris32, fins in the tail will not do the trick, you need fins or some other aerodynamic feature around the centre of mass.
I dont think this is true because the centre of pressure changes with the angle of attack (caused by the steerable tail or nose fins). If the centre of pressure varies about the centre of gravity (mass) the object will experiences a pitching torque about its centre of mass, thus change its direction.
Duddy seems to have summed it up fairly succinctly above. Where are the necessary fins around the centre of mass of a ship? |
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DennisN
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| Posted: Fri Apr 15, 2011 11:06 pm Post subject: |
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Wot? 
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Dennis
If it tastes good - it's fattening.
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Andy_P
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| Posted: Sat Apr 16, 2011 3:33 am Post subject: |
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| If it has the same "pinpont accuracy" of a lot of the smart bombs in Iraq and Afghanistan that we DIDN'T see on the news, then what the tail fins do is slightly academic. |
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Chris32
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| Posted: Sat Apr 16, 2011 12:14 pm Post subject: |
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[quote="Darren"]From: Defence Studies Blog
| Quote: | | In the nose, a GPS receiver and computer controlled aerodynamic directional fins keep the round on its programmed trajectory. Folding fins in the tail provide stability. |
That makes perfect sense and is the kind of information that I was looking for. It's good to know that Newton's first law still works . . . . 
Thanks. |
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M8TJT
The Other Tired Old Man
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| Posted: Sat Apr 16, 2011 1:28 pm Post subject: |
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| Part of Newton's first law states that 'An object that is in motion will not change its velocity unless an unbalanced force acts upon it'. Surely it is because the steerable front or rear fins cause an unbalanced force that causes a change of velocity, in this case changing the direction of flight (as well as probably slowing its speed slightly)? |
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Chris32
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| Posted: Sat Apr 16, 2011 3:05 pm Post subject: |
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| M8TJT wrote: | | Part of Newton's first law states that 'An object that is in motion will not change its velocity unless an unbalanced force acts upon it'. Surely it is because the steerable front or rear fins cause an unbalanced force that causes a change of velocity, in this case changing the direction of flight (as well as probably slowing its speed slightly)? |
I wasn't really wanting to start a debate about basic physics, but since you ask. The object has a mass which is centred at a point. If you want to change the velocity of that point (velocity is a vector quantity so direction is involved) you must apply an external force through that point. If you apply a force other than through the centre of mass then a new velocity will be created that is the result of the couple formed by the momentum of the object in the direction of the initial velocity and the distance and angle of the new force. The effect will cause rotation of the object (tumbling) but (of itself) no change in the initial velocity.
Changing the attitude of the axis of of a moving body does not of itself change its direction of travel, (otherwise all aeroplanes would have to land on their noses).
Having support/control surfaces on either side of the centre of mass allows a force to be applied through the centre of mass and hence to change the direction of travel. |
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Andy_P
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| Posted: Sat Apr 16, 2011 3:42 pm Post subject: |
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Thinking back to my gliding days, where you have very simple control surfaces - rudder on the tail and ailerons on the wings.
If you tried to turn using the rudder alone, you pretty much just carried on in the same straight line, just crabbed sideways. |
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M8TJT
The Other Tired Old Man
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Location: Bexhill, South Sussex, UK
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| Posted: Sat Apr 16, 2011 3:43 pm Post subject: |
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Ah, I see it all now  Simplistically, the control surface on the front provides (say) lift and the fins at the back hold the back down causing a rotation force through the centre of mass thus changing the trajectory. I still have a problem understandin how a ships rudder works though  |
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