Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: A dumb question about escape velocity
Date: Thu, 28 Dec 2000 19:08:20 GMT

In article <20001227193424.08822.00000094@ng-ff1.aol.com>,
Navigaiter <navigaiter@aol.com> wrote:
>   So what is the need for an orbit in the first place? [besides letting the
>personel catch their breaths.]

There have been launches to the Moon (and beyond) which have not gone via
parking orbit, and it does improve efficiency *slightly*.  The price you
pay for it is very narrow and infrequent launch windows, because Earth's
rotation puts your launch site in precisely the right place only once a
day.  With a stop in parking orbit, it suffices to arrange that the
parking orbit passes through the right place, which is much easier.

Note that a lunar launch without parking orbit still looks nothing like
Navigaiter's slow vertical ascent.  The rocket tips over to horizontal as
quickly as possible, given the need to clear the atmosphere first, and
accelerates as hard as it can; it's just that it doesn't stop accelerating
as it reaches orbital velocity.  The trajectory differs only slightly from
what you'd get with a very brief stop in parking orbit.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: A dumb question about escape velocity
Date: Fri, 29 Dec 2000 05:21:23 GMT

In article <978051436.424300@shelley.paradise.net.nz>,
Don Stokes <don@news.daedalus.co.nz> wrote:
>>...very narrow and infrequent launch windows, because Earth's
>>rotation puts your launch site in precisely the right place only once a
>>day.  With a stop in parking orbit, it suffices to arrange that the
>>parking orbit passes through the right place, which is much easier.
>
>It also decouples the trans-lunar injection from the vagaries of weather
>and booster performance in the atmosphere.  Launch and low altitude
>flight is pretty rough on the vehicle, and it's useful to park in LEO,
>check that nothing important fell off and correct any discrepancies.

While this is undoubtedly useful, it's given too much weight in many
discussions of the matter, because the launch-window issue is harder to
explain.  The dominant issue is wider launch windows; having some time for
orbital checkout etc. is usually a helpful side effect rather than a
primary motive.  Unmanned deep-space launches via parking orbit, in
particular, tend to run on tight preprogrammed schedules with little
opportunity for tinkering in parking orbit, because of limited
communications and inflexible control systems.  The parking orbit is
primarily a way of reaching a tightly constrained departure path without
severely restricting the launch window.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)