[ExI] question re "honkin' big cannon" space launch

[Jeff Davis]

On Sat, Feb 28, 2009 at 12:57 PM, Keith Henson <hkeithhenson at gmail.com> wrote:

> I don't understand what you are asking here.  Lowest orbit?

Thanks for the numbers, Keith.  And my apologies for not being clear.

Hall's "space pier" suggests that ballistic launch is feasible at an
altitude of 100km, implying that feasibility is "limited" by
atmospheric density, (as well as, intuitively, vehicle strength, and
shape).  So, if the launch vehicle exits the end of a tubular launch
mechanism, and in so doing abruptly encounters a non-zero atmospheric
density, what is the greatest density/lowest altitude at which this
launch scheme is feasible.

That's the question: lowest altitude for the end of the launch tube.

No doubt other factors come into play. A discussion of vehicle
strength and shape(pointy, etc) is reasonable, but of only modest
interest to me at this point.  Similarly the launch velocity (the
velocity at which the vehicle exits the launch "tube"), since that
directly determines the force of "impact" with what remains of the
atmosphere at altitude.  This relates to the issue that Stefano
brought up regarding circularizing the orbit to avoid reentering the
atmosphere after the first half orbit.  The vehicle's on board
thrusters have only half an orbital period to accomplish this, which
is out to apogee and back again -- a function of launch velocity.

It's only fair to mention why I want to know this minimum altitude.  I
have an alternate design notion for a structure to elevate the end of
the launch tube, and the size of that structure is dependent on the
altitude it needs to reach.

Again, Keith, thanks for your help.

Best, Jeff Davis

   "Everything's hard till you know how to do it."
                           Ray Charles