Saturday, March 22, 2008

Notes from Zubrin

ENTERING SPACE, Creating a Spacefaring Civilization by Robert Zubrin.

(Many thanks to Mr. Zubrin for a wonderful book, and many apologies for any mistrakes which might be unintentionally placed in following material which freely extracts from his text. Any such misteaks are entirely my fault and not the fault of Mr. Z or anyone else.)


Newton's law of reation. By expelling mass at high velocity to rear of space vehicle, an equal amount of momentum contributes to the forward direction of travel. Resultant thrust (T) is simply product of propellant mass flow times exhaust velocity.

Eq. 1: Thrust = propellant mass flow * exhaust velocity.
T = m C

I'll take the liberty of restating same concept with different terms which I've previously used.
For propellant mass flow, I'll use fuel flow per sec, ffsec.
For exhaust velocity, use vExh.
Thus, very same equation, restated as follows
T = ffsec * vExh

The higher the exhaust velocity, the greater the thrust. Thus, rocket engineers use exhaust velocity to rate rocket engines by "specific impulse" (Isp) defined as number of seconds a pound of propellant can deliver a pound of thrust.
Stated another way:
Isp is the number of seconds a quantity of propellant produces its own weight in thrust.

A common factor between thrust and weight is the force of gravity.
Force equals mass times acceleration, f = m a.
Force of gravity equals mass times acceleration due to gravity, f = m g.
Numerous observations result in the axiom that free fall due to near Earth gravity is 9.8 meters per second per second (g=9.8 m/sec2). For convenience, I'll subsequently use g=10 m/sec2.

The term, g, can be a common factor between thrust and weight, because thrust is often rated as a weight related unit such as "pounds". Term, g, can also be a common factor between specific impulse and propulsion particle velocity,

Eq. 2.
vExh = g * Isp.

Isp = vExh / g

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