Jules Verne Forum

<jvf@Gilead.org.il>

JanFebMarAprMayJunJulAugSepOctNovDec 19961997199819992000200120022003200420052006200720082009

Re: No cannon

From: James Keeline <keeline~at~yahoo.com>
Date: Sat, 5 Apr 2008 05:59:14 -0700 (PDT)
To: Jules Verne Forum <jvf~at~gilead.org.il>

--- David McCallister <mystery1881~at~verizon.net> wrote:

> I wonder how that would fit in with the canon-launcher proposed in "The
> Begum's Fortune"? Wasn't that supposed to send a shell only one "orbit" of
> the Earth - back to Oregon and Frankville (or Stahlstadt). Wouldn't it still
> need to achieve escape velocity, even for a low- or sub-orbital "trajectory"?

Escape velocity (http://en.wikipedia.org/wiki/Escape_velocity) is a
sufficiently large speed and direction necessary to let an object leave orbit.
Objects going to the Moon need to exceed this. The page linked above gives the
velocity in km/sec. In terms of miles/sec the value is about 7 mi/sec or a
little more than 25,000 miles/hour. Orbital velocity is much lower and
suborbital velocity lower still.

As noted before, the relevance this has for a cannon shot is that all of the
velocity must be imparted to the object in the length of the cannon. Hence, to
get to escape velocity in a 200 ft cannon requires some serious acceleration.

The effect of high acceleration on passengers is obviously an issue. My
understanding is that Verne knew this but accepted the flaw in an effort to
write a good story which was plausible with the technology of the day. He
later criticized the moon story by H.G. Wells as pure fantasy in its mode of
propulsion.

Another problem would be the effect of a metal object leaving the cannon muzzle
at 25,000 miles per hour in a dense portion of the Earth's atmosphere would
generate a lot of heat due to friction.

There have been some proposals to use long electromagnetic accelerator track to
launch objects into orbit with more reasonable acceleration rates. This would
be even easier from the Moon with its lower gravity -- about 1/6 that of the
Earth.

There's some interesting material on this in the ANNOTATED FROM THE EARTH TO
THE MOON.

James Keeline
Received on Sat 05 Apr 2008 - 15:59:34 IDT

JV.Gilead.org.il
Copyright © Zvi Har’El
$Date: 2009/02/01 22:36:11 $$