From: eder@hsvaic.boeing.com (Dani Eder)
Newsgroups: sci.space
Subject: Re: Sizing of launch vehicles (was Saturn Class)
Date: 1 Sep 92 18:34:40 GMT
Distribution: na
Organization: Boeing AI Center, Huntsville, AL

fcrary@ocf.berkeley.edu (Frank Crary) writes:

>In article <1579@hsvaic.boeing.com> eder@hsvaic.boeing.com (Dani Eder) writes:

>>Now, for the space station delivery job alone, you have a need to
>>deliver 75 tons per year (5 Shuttle loads), for say 10 years.  Thus
>>you have a total delivery requirement of 750 tons, or 750,000 kg.
>>To find total cost, we need to add recurring and development costs,
>>with the development cost spread over the launched mass:

>This neglects  the use of pre-existing systems (e.g. ones where the
>development costs have already been paid for.)

>But more importantly, you are treating the Space Station as a constant-mass
>payload, which need only be delivered to orbit. Assembling the pieces
>will also be required, and clearly assembly of 75 10-tonne pieces is
>much harder than assembly of (say) 15 50-tonne pieces. 

>I suspect the difference in costs (which was only around a 10% difference
>between 10 and 25 tonne payloads) would be overshadowed by the changes
>in on orbit assembly costs.

I'm sorry I wasn't clear enough in my original posting.  The mass of the
Space Station at assembly complete is about 200 tons.  The rest of the
mass represents supplies, experiments, and orbit maintenance propellant.
The five flights per year is the figure being planned for by the space
station logistics people.

My intent in the posting was was to show that the development cost has
to be considered in any launch vehicle selection.  At the traffic rates
for the space station support job, the optimal new vehicle size is
pretty small.  Note that in the 1986 study I worked on, we considered
F1 and F1-A engines as booster engines.  They turned out to be un-economic
for the job we were designing for - launching a combined SDI and civilian
traffic model.  The actual answer we got was that for mostly eastern
launches you actually want a fully reuseable two stage shuttle, sized
so it flies about 20-25 times per year (take your total annual traffic
and divide by 20-25, and that gives you the payload size).  This rule
holds across a range of vehicle sizes for this reason:

As you go to larger and larger rockets, you still have to develop them,
build them and launch them.  The test facilities, assembly buildings,
and launch pads all get bigger the larger the rocket gets, and thus cost
more.  You want to spread these fixed costs over a large number of 
launches to minimze them.  On the other hand, launch operations costs
tend to go with the number of launches, not the size of the vehicle.
The point where the incremental facilities cost balances the incremental
operations cost comes out to 200-250 launches.  If you project a 10
year operating life, then you size for 20-25 launches per year.

Another reason for getting to rates of 2 a month is production efficiency.
The 737 production line takes 20-30 days to run the aircraft from delivery
of sections (fuselage sections, wings, landing gear), through installation
of systems (engines, avionics), to rolling a near-complete aircraft out
the door (it can fly, but painting, and customer-specific items get put
in elsewhere).  During this time the aiplane moves through several 
'stations' in the assembly line, spending a few days at each station.

So, the assembly stations, which are all needed, have natural rates
of several a month.  If you build at a much lower rate, like 6 times
a year, your factory is not being used efficiently.  I maintain that
a rocket is inherently simpler than an airplane to build, and therefore
the efficient factory production rate will be even higher.

I agree with your comment about simplicity of assembly.  It is even
stronger than you state. A Station built of larger pieces actually 
will mass less than one built of smaller pieces, since you eliminate
the connectors (electrical, hatches, etc) between sections.  This tends
to make larger launch vehicles look more attractive than otherwise.
To my knowlege, no one has ever taken a combined look at Space Station
and launch vehicle costs to look for a global optimum (after all, the
programs are under different associate administrators).  The Space
Station has mandated from the beginning that they will use the Space
Shuttle for delivery, so the question never was asked.

Dani Eder

-- 
Dani Eder/Boeing/Advanced Civil Space/(205)464-2697(w)/232-7467(h)/
Rt.1, Box 188-2, Athens AL 35611/Member: Space Studies Institute
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***THE ABOVE IS NOT THE OPINION OF THE BOEING COMPANY OR ITS MANAGEMENT.***