Re: Google Cindy Sheehan (was: Re: Peace Mom)

On Mon, 22 Aug 2005 19:08:53 -0500, "w.d.greene" <bil64@xxxxxxxxxxx>
wrote:

>
>"Josh Hill" wrote:
>> "w.d.greene" wrote:
>>>"Josh Hill" wrote:
>>
>>>> Estimated cost of building the DC-Y single stage to take off vehicle
>>>> -- a reusable spaceship that would slash the cost of putting stuff in
>>>> orbit -- $6 billion.
>>>
>>>Um, no.
>>>
>>>Single-stage to orbit (SSTO) does not work. It cannot work. Any
>>>fledgling
>>>aerospace engineer with just a passing knowledge of the rocket equation
>>>understands this. Space Shuttle is 1.5 stages and it can only pull it off
>>>by being the highest performing system ever built.
>>
>> I'm not sure I understand why you say that. The Shuttle was
>> state-of-the art in the 70's, but it's hardly that now -- both engine
>> design and structural materials have improved, with further
>> improvements on the horizon.
>
>This is a common misunderstanding that is unforutnately repeated over and
>over in the mass media. With regards to the engines, the space shuttle main
>engines are the highest performance, high thrust engines ever built. Could
>they be higher in performance? Yes. Theoretically, you might be able to
>squeeze out another one or two percent in specific impulse. But that's it.
>Why? It has nothing whatsoever to do with 1970's technology or 2010
>technology or even 2000 BCE technology. It has to do with the fundamental
>properties of hydrogen and oxygen and the rest of the periodic table.

You're oversimplifying, I think. According to what I've been reading
(which is a fair amount since I logged on last), the SSME's were
originally supposed to have had an expanding nozzle, but that was
scrapped as part of a cost cutting move. So their specific impulse
drops from 454 to 368 secs at sea level. An expanding nozzle or better
yet an aerospike engine would be more efficient.

Also, the sea-level F/W ratio of the Block II SSME's is 51; the
shortened-nozzle Block II+ was (is?) supposed to increase that to 58,
and the Block III to 70 by incorporating a channel-wall nozzle, jet
boost pumps, electrical valves, an extra-large throat combustion
chamber, and a new controller. That's significantly closer to the
75-80 F/W ratio required by an SSTO.

And then there are full-flow engines, hydrostatic bearings, thrust
vectoring using carbon vanes or engine throttling as in the X-33,
improvements in the manufacturing process (""Production costs of the
current engines are also high because of their complexity, including
the large number of parts needed and the manufacturing technology that
was available when the SSME was developed") which don't affect
performance but do affect cost and lead time, and tripropellant
engines.

So it seems to me that as things now stand, there's lots of room for
improvements in engines for a new design, and some room for
improvements in engines for the current one.

>The solid boosters could be of higher performance, but again probably only
>marginally without radical changes (like maybe going to liquid propellant
>boosters).

Sure, but those solid fuel boosters were a serious compromise. One
article I read claims that replacing the solid fuel boosters with
recoverable H2O2/Kerosene boosters would increase payload mass by
almost a third, from 24,950 kg to 33,140 kg.

>Yes, there have been materials improvements. However, some of them have
>already been incorporated into the orbiter, the external tank, the engines,
>and the solid motors. See, these things are regularly and routinely updated
>and upgraded.

>The Shuttle is, in fact, in many ways 2000's technology with only a few
>exceptions here and there.

Seems to me there are more than a few exceptions. The ceramic tiles,
the aluminum frame, the fuel cells, the hydrazine-powered APU's, for
example. And then there were design decisions that looked good at the
time, such as the side-mounted fuel tank, that were in retrospect
mistakes, and in a sense, those represent 70's technology too, because
we didn't know better and now we do. As well as political/cost
compromises, e.g., the SRB's.

Which isn't to say that we could do all that much better with the
original cost constraints . . .

>> I found a few papers that rough out SSTO proposals, including this
>> one:
>>
>> http://www.ssdl.gatech.edu/main/ssdl_paper_archive/iaf-st-87-07.pdf
>>
>> Short on details and I don't know enough to fill them in, but I don't
>> see any assumptions there that seem outlandish, e.g., they posit a 10%
>> mass reduction for the vehicle.
>
>In order for SSTO to work, the propellant mass fraction has to be on the
>order of 91% to 93% of the net liftoff weight (post engine ignition and
>hold-down). That doesn't leave much for the rest of the vehicle as in tanks
>and structure and engines and wings and landing gear and, oh yeah, payload.

The figures I've seen are 89-90% but either way, it's a challenge.
Judging by what I've been reading, current materials technology may
not be quite up to the task. But then, how will we get there if we
don't do the research, linear aerospike engines, collapsing fuel
tanks, and all?

>Yes, the assumptions are outlandish once they're stacked up one on top of
>another. It's interesting to note that the gross liftoff weight is roughly
>the same as Shuttle and yet it delivers less half of the payload.

Well, that's to be expected from an SSTO -- no one has ever suggested
that staging wasn't efficient in that regard. But the extra fuel costs
a lot less than prepping a Shuttle or throwing away the equivalent of
a 747 with every launch.

--
Josh

"You know I could run for governor but I'm basically
a media creation. I've never done anything. I've
worked for my dad. I worked in the oil business. But
that's not the kind of profile you have to have
to get elected to public office." - George W. Bush
.

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