Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Heavy Lift Capacity Boosters
Message-ID: <1988Sep24.215753.27239@utzoo.uucp>
Organization: U of Toronto Zoology
References: <677@eplrx7.UUCP> <2240@ssc-vax.UUCP> <6871@ihlpl.ATT.COM>
Date: Sat, 24 Sep 88 21:57:53 GMT

In article <6871@ihlpl.ATT.COM> knudsen@ihlpl.ATT.COM (Knudsen) writes:
>Well, this is good thinking and the first time I've seen it spelled
>out this way.  Use SSMEs as the base (literally and figuratively)
>for a new series of heavy launchers.
>
>One suggestion:  Seems that what distinguishes the SSMEs from
>earlier liquid engines is their longevity -- designed for re-use,
>and test-fired for over 30 minutes.

And their enormous price.  Don't forget that.  One significant problem
in throwing SSMEs away is that they are awesomely expensive; this is why
the current Shuttle-C plan is counting on using time-expired shuttle
engines, not newly-built SSMEs.  I'm not sure about the more recent Boeing
studies, but almost everybody who has talked about serious use of SSMEs in
expendables has also talked about trying to change the design to make it
cheaper.

>Are there any applications where a single engine that burns for
>over 20 minutes would be especially helpful?  Like a Mars or
>deep-space probe (a big one), or something really huge
>into Clarke orbit?

Almost any in-space propulsion application is probably going to prefer
using fewer engines but running them longer.  Assuming that individual
engines weigh the same either way, the results will be similar but the
smaller number of engines will weigh less, and the lower acceleration
will mean lower structural weights.

There are limits to this, since for efficient trajectories one wants
accelerations that are not dramatically lower than the local acceleration
of gravity.  There is also a complication in that your structure may
need to stand higher accelerations earlier in its history, e.g. getting
into orbit for the first time.  (The shuttle is a particularly bad case
since a shuttle payload has to be rated to take a 9G crash load *at right
angles* to the usual thrust vector.)  But on the whole lower thrusts are
often desirable.
-- 
NASA is into artificial        |     Henry Spencer at U of Toronto Zoology
stupidity.  - Jerry Pournelle  | uunet!attcan!utzoo!henry henry@zoo.toronto.edu