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The Fleet Ship designs, strategies, and tactics.

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  #1  
Old July 8th, 2005, 12:38 PM
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I have been trying to remember my physics. (It has been a few years.)

As I recall using the following abbreviations.
M=mass (in kilograms)
m=meter
s=second
G=9.81m/s^2
v=velocity (meters per second)
a=accelleration (meters per second per second)
t=time (in seconds)
F=force
KE=Kinetic Energy
d=distance

KE = .5mv^2
assuming an initial velocity of 0.
v=at
t=(d/(.5a))^.5 (plus or minus, but you can't have negative time.)

Low Earth Orbit is between 500Km and 1500Km. (Lets call it 1000Km.)

If we reserve 1g for maneuver and/or overcoming friction. Firing a 6G missile at the planet from 1000km, means that it will hit the target in about 184 seconds. (Or just over 3 minutes, plenty of burn time left on the missile, regardless of the ruleset you are using.)

Missile Velocity at impact will be about 10830 meters per second. (9.81*6*184)

Kinetic Energy just before impact for a 100Kg steel bar (or equivalent) would be 5,864,445,000 Joules.

1 Kilo of TNT is 4.6MegaJoules. So without a warhead this 100KG steel bar is equivalent to approxmately 1.2 tons of TNT. And the Imperium is worried about nukes being used?

Fire the same missile from geostationary orbit.
(36,000 KM) It takes 1106 seconds (just over 18 minutes, again plenty of burn time on the missile left.)
Velocity is now 65099 m/s.
Kinetic energy is now 211,895,032,000 Joules(Same steel bar.)or about 46 tons of TNT. Still not quite a nuke, but not bad for a steel bar with a short range missile engine strapped to it.

Are we sure that damage from Naval gunfire is sufficient in the rules?
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Old July 8th, 2005, 01:05 PM
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BTW 1000kg of steel is approximately .13 cubic meters. (or about .0095 Traveller Displacement Tons.) A Missile is .05 displacement tons. so if the warhead is one fifth of the missile, and you hit your target from geostationary orbit you hit it, without an explosive charge, with the equivalent of 460 tons of TNT, or the equivalent of a baby nuke. (And a Triple missile turret is almost a 1.5 kilotons yield.)
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Old July 8th, 2005, 01:19 PM
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Well I daresay my physics is no less rusty and perhaps more but I'm thinking a couple problems with your outline leap out at me.

Terminal velocity for one, dependant on atmosphere, is going to limit your impact velocity. Those numbers are all for vacuum right?

Another is the size of the impactor. Depending on the rule set you won't have a 100kg warhead (your steel bar). T20 for one has the whole missile massing only 50kg and I think the actual warhead is 10kg or less. Depending on the type of propulsion your missile will actually lose mass as it accelerates but that will unduly complicate things for a game.

I think all things considered being able to use space missiles for ground targets is fair enough in trade offs for the combat/damage rules.

As far as the Imperial edict of No Nukes, I agree it seems silly in light of just about everything and as a rule of war should probably be dropped or modified. Maybe it's just the long lasting radioactive fallout that irks the rulers so "clean" use of nukes might be fine.
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Old July 8th, 2005, 02:06 PM
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Quote:
Originally posted by far-trader:
Well I daresay my physics is no less rusty and perhaps more but I'm thinking a couple problems with your outline leap out at me.

Terminal velocity for one, dependant on atmosphere, is going to limit your impact velocity. Those numbers are all for vacuum right?

Another is the size of the impactor. Depending on the rule set you won't have a 100kg warhead (your steel bar). T20 for one has the whole missile massing only 50kg and I think the actual warhead is 10kg or less. Depending on the type of propulsion your missile will actually lose mass as it accelerates but that will unduly complicate things for a game.

I think all things considered being able to use space missiles for ground targets is fair enough in trade offs for the combat/damage rules.

As far as the Imperial edict of No Nukes, I agree it seems silly in light of just about everything and as a rule of war should probably be dropped or modified. Maybe it's just the long lasting radioactive fallout that irks the rulers so "clean" use of nukes might be fine.
Dan,
I considered several factors in my numbers. the first is the fact that such a missile would actually have 7G accelleration. (1 G for gravity and 6 for the engine.) So aerodynamic braking (ie friction) would be compensated for.

Terminal velocity is based on a falling object, not a powered one. (Again deals with atmospheric braking.)

Note that in Traveller the 6G 6 turn burn missile as described in Mayday is virtually useless, in pursuit scenarios unless you are close to the target or have a large overtake advantage at the time of launch. (The Pursured on the other hand can shower you with missiles throughout the pursuit.)
Missiles have to have an accelleration advantage over their targets to be effective. So the accelleration numbers are definitely on the conservative side.

Volume in Traveller has always been the overriding consideration, not mass. 1000kg of steel is less than 1/5th of the volume of a T20 missile.

It isn't a relativistic rock, but it is close enough. [img]smile.gif[/img] (Give a man a relativistic rock and he'll shatter a planet today, teach him the physics and he'll shatter planets for the rest of his life. [img]smile.gif[/img] )
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Old July 8th, 2005, 02:21 PM
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Right you are, terminal velocity is not the term I was looking for. I was meaning not a simple falling body but the maximum velocity for the powered body due to air resistance.

I would argue that mass has in fact always (or at least originally) been the overriding consideration in Traveller, not volume. Volume seems to have been a handwave attempt to "correct" certain problems that just created more. But that is an entirely different pickle.
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Old July 8th, 2005, 02:55 PM
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There isn't really a maximum for a powered body based strictly on air resistance. (Especially with technically advanced materials, bonded superdense, for example.) You can calculate at what velocity the object will break up or depart controlled descent, or in this case aimed descent, but as long as you have fuel, your weapon doesn't melt, breakup or start flying, then you get to max velocity based on relativity. You accelleration may slow but it will continue. And like I said 6G is extremely conservative.
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Old July 8th, 2005, 05:53 PM
Piper Piper is offline
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hmmmm .... try this link and see if it works any better. Page 18 or so; Kinetic weapons.
The main points are atmospheric heating and (surprisingly) aerodynamic effects. Apparently, the Rand people calculate that even ablation can cause serious accuracy problems. They think an almost vertical drop would be needed.
Their 100kg tungsten penetrator (free-falling, so the velocity is much lower) can penetrate about 1.5 meters of concrete, but they figure that most of the force will be in a 30 degree cone in the direction of travel.
Compare that with the main battery of an Iowa-class battleship.

You do have a good point, Bhoins. Ship missiles might deserve some enhancement in the ortillery role.
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Old July 8th, 2005, 06:27 PM
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My apologies, sir. I had the same problem with the UBB code and elected to post the link as I wasn't sure the pdf would work.

I agree totally; the extra width is annoying.
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Old July 8th, 2005, 06:47 PM
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Thank you very much Piper, apology accepted though not truly neccessary. You tried which counts, and more to the point even found a link that does fix it and work. Off to tidy up my rants here now [img]smile.gif[/img]
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Old July 8th, 2005, 06:57 PM
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Quote:
Originally posted by far-trader:
Right you are, terminal velocity is not the term I was looking for. I was meaning not a simple falling body but the maximum velocity for the powered body due to air resistance.
In the automotive world, that's know as "Maximum Speed (Drag Limited)"
...as opposed to "Maximum Speed (Electronically Limited)" (The car's ECU has an RPM limit in the highest gear)
...or "Maximum Speed (Traction Limited)" (The car's body has insufficent downforce, and steering control is lost above this speed.) (!!!)

I think that it's likely that you can only push an object through an atmosphere so fast (vertically or horizontally) before you reach it's drag limit, since (simply put) drag induces heat, and eventually - above that limit, you'll put more heat into the object than you can radiate away, and melt the object.

...though the thought of raining drip-drops of molten whatever on the bad guys is interesting in and of itself...

[img]graemlins/file_23.gif[/img]
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