Re: Spaceship One stepping-stone or dead-end?

From: Jake McGuire (jamcguir_at_yahoo.com)
Date: 10/06/04 

Date: 6 Oct 2004 12:17:11 -0700

greg@see-web-page.edu (Greg Kuperberg) wrote in message news:<cjv71i$55d$1@conifold.math.ucdavis.edu>...
> So I'll grant you that these passenger planes are an outlier for my rule
> of thumb that cost should scale as at least v^2. Indeed, I'm surprised
> that the Caravan costs almost as much as a much faster business jet like
> the CJ1. I'm not sure what the remaining attraction is - maybe that they
> are easier to fly or that (in one configuration) they can land on water.

You're not thinking it through. The Caravan does cost less, and can
carry more payload over a shorter range with less fuel and for less
money. Direct Operating Costs are somewhere around $800/hr for the
CJ1 and $200/hr for the Caravan. Ignoring climb, descent, and
landing, a 700 mile trip will take two hours and cost $1600 in the CJ1
and take four hours and cost $800 in the Caravan. Which is better
depends entirely on the situation.
 
> It is a flaw in my data, but not necessarily my argument, and it certainly
> isn't a *fatal* flaw in anything. If you actually cared for a sincere
> victory, you would have shown me where I'm wrong with the vehicles that
> are designed to maximize top speed. Race cars are closer to that, for
> instance.

Not particularly. Race cars are almost always designed to minimize
lap times, which usually involves trading off top speed. Compare lap
times and top speeds of F1 vs. MotoGP for a prime example. In fact,
there are very few vehicles that are designed to maximize top speed,
which makes trying to derive some sort of statistical scaling law a
futile, or at least misguided, effort.

> I doubt that my rule is really wrong, because, for example,
> 300 mph is much harder for cars than 150 mph.

Think about the reasons this is the case (hint: installed power vs.
top speed is a big one), and ask how that would apply to a
rocket-powered craft.

> And mach 6 is much harder for jets than mach 3.

Think about the reasons this is the case (hint: airbreathing engine
performance vs. speed), and then think about how that would apply to a
rocket-powered craft.
 
> For that matter, you haven't provided any good comparison either.
> You provided one comparison, which you then turned around and said
> was irrelevant.

My assertion is that there is no simple scaling rule relating cost to
top speed that will have any useful predictive power in extrapolating
from the cost of SpaceShip One to some hypothetical follow-on vehicle.

> And why are you so hot under the collar about all of
> this anyway? This is just an idle discussion. It's not like I poured
> salt on your birthday cake.

I'm not hot under the collar. I was staying out of the silly
back-and-forth between you and Rand, but posted a counterexample in
response to your request for one. You responded with factually
incorrect information and incomplete reasoning, which I pointed out.
In fairness, nothing you posted was nearly as silly as Rand's claim
that it was easy to buy a bigger engine for a car and thereby double
its top speed - I'm surprised you didn't hammer him on that one.

Aerospace vehicle design (particularly for high performance and novel
vehicles) is an incredibly complicated subject, and depends on
integrating many separate and complicated disciplines. Statistical
formulae and rules of thumb are really only useful for designing
something that doesn't depart much from past experience - if you're
doing something wholly new you really need to go through all of the
grunt work. It is without a doubt frustrating, because it makes it
hard for amateurs (of which I am one) to contribute, but it's
nonetheless the case.

-jake

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