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