speed record set by scramjet - fair?

Look up Newton's first law of motion, the law of inertia.

The law of inertia has nothing to do with this.

The law of inertia has nothing to do with this? It has everything to
do with it. It is usually stated thusly: An object at rest tends to
stay at rest and an object in motion tends to stay in motion with the
same speed and in the same direction unless acted upon by an
unbalanced force.

In this case, if you dropped anything at all out of that rocket at
Mach 9.5, it would contine to move at Mach 9.5 forever unless acted
upon. The jet would never have to fire its engines and it would
maintain Mach 9.5 if it weren't for the effect of air friction, the
unbalanced force. In a vacuum, the thrust required to accelerate from
Mach 0.5 to 1.0 is exactly the same force required to accelerate from
Mach 9.5 to Mach 10.0.


To accererate the
jet from Mach 9.5 to Mach 10 takes exactly the same amount of power as
accerating from 0 mph to Mach 0.5, not very much.

You are absolutely wrong on this point. The drag at Mach 9.5 is vastly
larger than the drag at 0 mph, and as such requires vastly greater amounts
of power to accomplish any acceleration. Nearly all of the power invested
is used to overcome drag, not inertia.

I see part of the problem. You, like many non-technical people, think
that inertia is only something to overcome. Inertia is as much about
the difficulty in slowing something down as it speeding something up.
I understand why you were confused about that, though, because in
common non-technical usage, the word is almost only used to mean hard
to get going, not hard to stop. But it is also inertia that keeps
objects moving.

For what it is worth, there isn't a lot of air at 100,000 feet. If I
am not mistaken, the density of air at 100,000 feet is 1/400 the air
density at 5000 feet. It is pretty thin, so that also has to be taken
into consideration when evaluating the accomplishment.



But there IS friction. In this scenario, the friction dominates the physics
completely. Your frictionless scenario is completely irrelevant.

The frictionless scenario is the starting point for understanding the
problem. Once you undersand that the plane would fly at the rocket's
speed without an engine if there were no air resistance, you can limit
the problem to analyzing the power it takes to overcome friction.
However, I admit that I did not know the velocity cubed rule. I don't
think it is basic high school physics like the first law of motion is,
but I didn't know it. I was under the impression that the relationship
between velocity and drag was linear. I never studied fluid dynamics
and made a wrong assumption. My bad. That made my comparison between
accelerating from 0.5 to 1.0 versus 9.5 to 10.0 incorrect. It makes
the achievement of the scramjet more impressive than I thought.
Thanks for educating me.


i.

This is elementary physics, a subject that it seems fewer and fewer
people have a grasp of these days.

Yes, you are demonstrating that quite well.

Well, I wasn't trying to personalize my statement and I don't think
you really needed to either. My statement is in fact true. Less and
less people have a grasp of physics these days. In point of fact, I
have a very old bachelor's degree in physical chemistry, which is not
physics per se, but I did study mechanics, if not fluid dynamics.

-- Don French


Pete