Orbital Firesupport

[BetterThanLife]

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?

 

[BetterThanLife]

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.)

 

[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.

 

[BetterThanLife]

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.

(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.

)

 

[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.

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.

 

[BetterThanLife]

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.

 

[Piper]

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.

 

[Piper]

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.

 

[far-trader]

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

 

[jonathonbarton]

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...

 

[far-trader]

Originally posted by Jonathon Barton:
...though the thought of raining drip-drops of molten whatever on the bad guys is interesting in and of itself...

So when Elijah was confronted and said "If I am a man of God, let fire come down from heaven and destroy you and your fifty men!" He was actually calling for an ortillary strike and acting as a forward observer

 

[clementk]

One of my favorite RPG quotes from Mark Urbin's game quotes files is:

"Here at Ortillery Command we have at our disposal hundred megawatt laser beams, Mach 20 titanium rods and guided thermonuclear bombs. Some people say we think that we're God. We're not God. We just borrowed his SMITE button for our fire control system"

It's been too long since I cranked up Akira...orbital SMITE laser goodness.

 

[Piper]

Originally posted by Jonathon Barton:
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.

Wikipedia has some formulae for the number crunchers among us.

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

It would sure make a believer out of me.

 

[BetterThanLife]

In Shadowrun it was called a Thor Shot.

From low earth orbit the velocity at impact would be about Mach 31. The Heat Tiles on the Shuttle are designed to handle that. We aren't talking about being at that speed or temperature for any length of time. the Atmosphere isn't that deep in the first place. You don't begin to get a real atmosphere until you get below 60 Km Altitude. From low Earth Orbit you are only dealing with atmosphere for the last 10 seconds of the flight. From Stationary Orbit you are dealing with atmosphere for about a second. Now at that speed, especially the higher one, you are going to get some serious atmospheric compression and shockwaves. The heat will work in your favor at the impact sight.

It might break up but it won't have time to melt.

 

[mike wightman]

Long rods might be used to penetrate through earth to hard or deeply buried targets. However, the physics of high-velocity impact limits penetration depth as shown by high-speed photography of a bullet impacting steel at just above 1 kilometer per second. A copper-jacketed lead bullet fragments against the hardened steel, but in the process produces a pressure sufficient to leave a small crater.
Very strong projectiles impacting earth or rock at similar speed can penetrate to depths several times their length.
Tests done by Sandia laboratory confirm predictions that, even for the hardest rod materials, penetration is maximum around 1 km/s. Above that speed, the rod tip simply liquefies, and penetration depth falls off, becoming effectively independent of impact speed. Therefore, for maximum penetration, such rods would need to be orbited at very low altitudes, and could only deliver one ninth the destructive energy per gram as a conventional bomb. The effort is entirely mismatched to the results.

Richard L. Garwin, Columbia University physics professor.

 

[Whipsnade]

Bhoins wrote: It might break up but it won't have time to melt.'

Richard L. Garwin, Columbia University physics professor, explained: Tests done by Sandia laboratory confirm predictions that, even for the hardest rod materials, penetration is maximum around 1 km/s. Above that speed, the rod tip simply liquefies, and penetration depth falls off, becoming effectively independent of impact speed. and The effort is entirely mismatched to the results.

Damn that nasty Reality! Always getting in the way of our fun!

I guess an honest-to-ghu warhead and not just some lump of bonded superdense is the way to go.


Have fun,
Bill

 

[mike wightman]

Here's a link to Richard L. Garwin's full discussion paper

 

[Piper]

So what's a reasonable way to add Thor shots to Traveller?

What about treating it as a indirect fire mission using the effective range KEAP effects for a given bore size?

(all figures are from Striker)
A 100 ton missile bay has 50 25cm launchers. This gives a beaten zone multiplier of 7. If we use the max burst area of 6, we get a 42cm X 42cm beaten zone. Roll for contact hits only, and using the maximum modifiers for a TL10 KEAP round as an example, we get a penetration of 54. This is sufficient to blow through a meter of concrete.

Thoughts?

 

[Piper]

At currently attainable muzzle velocities the melting occurs during impact, not in flight.

SEFOP ammunition is still largely experimental, but holds a lot of promise for extreme velocity penetration.
This link has some rather interesting field results for kinetic penetrators.

 

[parmasson]

Hmmm… seems like it would be easier to just zap your target with a meson gun and be done with it.
We now return to the actual discussion...