Adrian Tymes wrote:
>
> The Silent Observer wrote:
>
> > Randall Clague wrote:
> >>We're on the same page. What I'm asking you is, to which capability
> >>number shall we apply Moore's Law? Altitude? Delta-v? Mass
> >>fraction? Total impulse? Each will yield a different answer.
> >
> > To disagree with Ian a bit, ISTM that Delta V is the factor that can be
> > increased by actual engineering; Isp is a matter of propellant choice
> > and making the engine last long enough for your burn (or your reuse
> > cycle), and has pretty well known limits for any given propellant
> > chemistry.
>
> Well known theoretical limits. In practice, it's not so easy to achieve
> 100% of the theoretical ISP limit; engineering is required to realize a
> good (and then better) percent. Higher delta V can be realized by
> simply taking along more fuel and accepting a poorer mass ratio (and
> higher costs): a 100 kg payload on a rocket with a 10,000 kg fuel tank
> vs. a 100,000 kg fuel tank is still the same payload.
Then engineering limits are well known, also, though the techniques of
working up to them are less well distributed, having been state secrets
related to ballistic missiles for much of the past sixty years. My
point was that the change from, say, peroxide monopropellant to
peroxide/kerosene, even with a relatively low level of development of
the bipropellant engine, is a much bigger step in performance than
anything you can get by improving on current peroxide monoprop motors
you can buy OTS.
> > Altitude increases wildly with relatively modest increases
> > in Delta V. Mass fraction is good, but translates very directly into
> > Delta V; in fact, it's Delta V that gets you anywhere. A huge mass
> > fraction with peroxide monopropellant still won't get you to orbit, and
> > an Isp equivalent to 10 km/s effective exhaust velocity won't help you
> > if the engine is so heavy you only have a 30% mass fraction.
>
> Doesn't the ISP - at least, as measured in practice - account for the
> weight of the engine?
No, though there are figures that take that into account. Isp, Specific
Impulse, is just another way of stating exhaust velocity; it's strictly
impulse (thrust integrated over time) per unit propellant mass, and for
chemical rockets is about 90% determined by propellant chemistry. You
can get a fantastic Isp with a heat sink motor that's too heavy to lift
itself and its tankage, or you can get fairly clunky Isp and still have
a good load lifting engine (witness the F1 that boosted the Saturn V).
To give an extreme example, the NERVA nuclear engine, essentially a
submarine reactor cooled with hydrogen, was to have an ISP of something
like 3000 seconds (to use the bastard units found in historical
literature) -- about 28 km/s exhaust velocity, 7-8 times that of the
best chemical rockets. It would need less than a 70% mass ratio to make
orbital velocity -- except that it could never have developed enough
thrust to lift the engine, tanks, pumps, and shielding against gravity
from the Earth's surface.
--
Love wealth above life itself, and starve in splendor.
-- Elvish proverb
Donald Qualls, aka The Silent Observer NAR # 70141-SR Insured
Rocket Pages http://silent1.home.netcom.com/launches.htm
Telescope Pages http://silent1.home.netcom.com/astronomy.htm
Lathe Pages http://silent1.home.netcom.com/HomebuiltLathe.htm
Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.
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