Re: Rocket efficiency

From: Mordecai (admin1_at_vwtools.net)
Date: 07/17/04 

To: sci-space-tech@moderators.isc.org
Date:  17 Jul 2004 08:49:11 -0700

> Take ion engines. They use massive amounts of power to produce very low
> thrust, but since they do not consume much mass they can keep producing
> thrust almost indefinitely, so the total delta-v can be quite high.
>
> The whole mass efficiency/energy efficiency tradeoff can be applied to many
> more things than rocket engines. For example airplanes. They work by
> accelerating air downwards to produce a force to compensate gravity. If you
> want a very energy efficient airplane you need to accelerate a lot of air
> downward a little bit. That is why sailplanes have such high wing aspect
> ratios: to have as much air as possible to work with.
>
> Or a propeller. All else being equal, a large propeller will be more energy
> efficient in producing thrust at low velocities than a small propeller
> because it has more air to work with and thus needs to accelerate that air
> less.
>
> Hope that helped.

To summarize, maximum thrust is *not* what one is primarily after. In
the vacuum of space, maximum exhaust velocity is the determinant of a
rocket final velocity. In going from ground to orbit, the rocket
*does* have to produce enough thrust to counteract the forces acting
on it; however, the exhaust velocity will *still* be the most
inmportant factor in determining the final velocity of the rocket, as
long as the rocket produces enough thrust to overcome air resistance
and gravity.

The basic rocket equation follows:

V(t) = Vxln(dM/dt) -gt

where V is the velocity of the rocket, Vx the exhaust velocity, dM/dt
the change in the weight of the rocket between liftoff and the current
weight (ie the fuel used), and gt is gravity times the the time passed
since liftoff.

Note: while my basic premise was correct, Last night I also posted
some goblygook for examples. I was going to come on and flame
*myself* this morning for posting nonsense, but I see the moderater
never allowed my post through. I guess I should thank him; now I
won't have to flame myself :)

Relevant Pages

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