NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/sci/space/shuttle/3184

Internet Message Format | 1993-01-22 | 3.6 KB

Path: sparky!uunet!cs.utexas.edu!sun-barr!decwrl!csus.edu!netcom.com!netcomsv!matrix!george.gassaway
From: [e-mail] (George Gassaway)
Newsgroups: sci.space.shuttle
Subject: Re : To the moon.....
Message-ID: <[e-mail]>
Date: 22 Jan 93 02:03:00 GMT
Distribution: world
Organization: The MATRIX BBS - Birmingham, AL - 205-323-2016
Reply-To: [e-mail] (George Gassaway)
Lines: 57

In article <[e-mail]> [e-mail] (Brian Stuart
Thorn) writes:

>     There is very definitely something wrong here. Apollo got to the
>     Moon and back from an Earth parking orbit not much different than
>     Shuttles. It did this on the power of a partially spent S-IVB
>     Third Stage and the Service Propulsion System of the Apollo
>     spacecraft. No matter how you calculate this, that does not add
>     up to three times the energy of the SSMEs and SRBs.

>------------------------------------

I've noted several replies in other messages related to this subject which
leave out a key element, one Brian Thorn was (IMO) correctly basically
addressing though not necessarily the following point (I didn't have the
original message he replied to to comment on). Some people are confusing
THRUST with ENERGY. Thrust is how much force is expended at the moment of
firing. But, energy in this situation is how long the thrust is continued
(potential energy, converted to kinetic energy).

A very basic example, say you have two engines:
 Engine 1 produces 1 pound of thrust.
 Engine 2 produces 10 pounds of thrust.
If they are built to the same level of fuel use efficiency, then the fuel
required to fire engine 1 for 10 seconds will only be able to run engine 2
for 1 second. BUT, both engines will have produced the same total energy,
10 pound-seconds in this example (usually rated in Newton-seconds), which
would mean they would both produce the same delta-V velocity change.

Or, for those of you who have done any model rocketry, you ought to be
familiar with how model rocket engines rated for the same power class
often are available in different thrust levels and corresponding burn
times (i.e. 5 newton-second "B" engines, B4 and B6 types. B4's have lower
thrust but burn longer than B6's, same total energy expended).

So the fun fantasy problem discussed here is not tied directly to having
shuttle type "thrust" levels, it's a matter of how much fuel you have in
the tank(s) and thus how much potential energy (delta-V) is available to
run through those engines.
  That's why a single SSME's thrust would be adequate for the job, as long
as somebody filled the tank with enough fuel (hydrogen/oxygen).....

BTW - another possible problem on reentry if it came back at 25,000
instead of being slowed to 17,500. The orbiter is built to withstand only
about 5 G's (maybe as low as 4.5, would have to look it up). An Apollo
type reentry profile might be so steep to create G forces to break up the
orbiter. A shallower profile to keep the G's low would likely cause it to
skip off the atmosphere and bounce too far to re-enter again (would not
seem ideal for aerobraking, and remember there ws always that small fear
that a too shallow Apollo entry would indeed make them skip off and never
come back into the atmosphere).
  Yet another reason to keep the ET on the way back and do a deceleration
burn into a 17,500 or so LEO parking orbit then do a normal deorbit burn.

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