-Z- (Z@Gundam.Com)
Tue, 24 Aug 1999 12:58:01 -0700


At 20:26 8/23/1999 -0400, you wrote:
>Escape velocity of earth is something like 11 m/sec (right?), so
> E = (1/2) m V^2 You have 7.7e13 J.

Escape velocity is closer to 11,000 m/sec. But what you want is the
orbital velocity, because anything traveling faster than that won't enter
the Earth's atmosphere FROM orbit (or, as they so ungrammatically put it
now, "deorbit"). This will be your unretarded re-entry velocity. I don't
have lunar orbital velocity figures handy, but re-entry from the Apollo
lunar missions involved velocities of 35,000 to 36,000 ft/sec. Taking the
top of the regime gives us:

36,000 ft/sec = 24,545.5 mph = 10.9728 km/sec = 39,502.1 kph = Mach 33

The minimum unretarded re-entry velocity, coming in from low Earth orbit, is:

25,000 ft/sec = 17,045.5 mph = 7.62 km/sec = 27,432 kph = Mach 22.9892

(By the way, I got my Mach number conversion wrong in my last post. I
thought Mach was on the order of 1,000 MILES per hour, when it's actually
around 1,000 KILOMETERS per hour. The actual value is Mach 1 = 331 m/sec =
1,193.26 kph = 1,087.47 ft/sec = 741.455 mph.)

>Now for the life of me, I can't
>remember or find out how many J there is to the Megaton, so I do some
>searching to try to figure it out. The heat of combustion for TNT is 821
>"kg calories per gram molecular weight", I have no idea what that really
>means, if it's 821 calories per gram, than it's 3.4e15 J per Megaton.

Megatons are usually expressed in ergs, not joules, which may be why you
didn't find a direct conversion factor.

One megaton (TNT) = 4.184e22 ergs = 4.184e15 joules = 4.184e15 wattseconds
= 3.96567e12 BTU (international) = 1e15 calories (thermal).

One calorie (thermal) = 4.184e7 ergs = 4.184 joules = 1e-9 ton (TNT).

>Hmm so a colony drop is conservative only 0.02 Megaton or 22 kiloton. Hmm
>how big was Hiroshima again? About the same?

The Little Boy dropped by the B-29 Enola Gay was about 15 kilotons and
destroyed about two-thirds of Hiroshima. The Fat Man dropped by the B-29
Bock's Car was about 20 kilotons and destroyed about half of Nagasaki.

-Z-

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