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From: lmc@cisden.UUCP (Lyle McElhaney)
Newsgroups: net.columbia
Subject: Re: SRB vs liquid
Message-ID: <485@cisden.UUCP>
Date: Fri, 7-Feb-86 13:45:35 EST
Article-I.D.: cisden.485
Posted: Fri Feb  7 13:45:35 1986
Date-Received: Tue, 11-Feb-86 06:15:14 EST
References: <615@uwvax.UUCP>
Organization: ConTel Information Systems, Denver
Lines: 59

> On the news last night (McNeal, Leherer (sp)) two gentlemen 
> were disagreeing on the safty of a SRB vs. liquid fuel rocket. It
> was implied that due to NASA budget restrictions, SRBs were used
> when liquid fuel was better/safer.
> 
> Whats the scoop on this? Is there any truth to it?

There are several good reasons for using SRB's rather than liquid fueled
rockets for the shuttle. The safety factors aside (its very hard to proove
anything pro/con there) take a look at the shuttle itself. 75% of the
liftoff weight is liquid fuel, used in the most complex and effecient
large engines ever made, that represent a large percentage of the cost of
both the orbiter and the maintenence/operations budgets. However, at the
moment of liftoff, these liquid rockets develop less than 10% of the total
thrust requirements of the shuttle; the rest comes from the SRB's. There
are several problems with solids; how can you turn them off (you can't,
you can only destroy them or let them run out), you can't control closely
the thrust developed, they are polluting (compared to the liquids rockets
in the shuttle, anyway). But they do deliver, powerfully, reliably, and
cheaply.

A few other comments about stuff I've read in a marathon session here:
The SRBs are slightly maneuverable, are blown away from the ET after
separation (after burn-out) by small rockets, and then return to the water
on three of the largest parachute systems ever made (I think that the
parachute package in the nose of each SRB weighs ~5 tons). Each is filled
with a mixture containing aluminum, a latex-like compound, and a
perchloride oxidizer. This mixture burns steadily in the open, nothing
violent, until it is burned under high pressure. The burning surface of
the solid fuel runs nearly the entire length of the SRB, burning from the
center outwards toward the shell. The destruct device on each SRB is an
externally mounted strip running the length of the SRB on each side,
designed to split the casing into two parts lengthwise upon use, thereby
destroying both the SRB's aerodynamics (such as it is) and dropping the
internal pressure drastically throughout the rocket, effectively killing
the thrust of any remaining fuel (which will continue burning).

The top one-third of the external tank is a liquid oxygen tank; the bottom
two thirds is for liquid hydrogen. Two 17 inch pipes, one from each tank,
exit the body of the ET at the bottom of each tank, travel externally down
the side of the tank, and then enter the body of the shuttle near the
engine pod. This is the *only* source of fuel for the three large rockets;
after jettisoning the ET, they are of no use. After jettison, the ET
disintegrates in the atmosphere over the Indian Ocean, not reused. There
have been studies concerning what it would take to deliver the tank to
orbit, for any of several uses.

All the other rockets on board the orbiter use hypergolic fuels, which are
dangerous (as spelled out here in great detail) but are ideal for use in
vacuum; they don't need to be ignited, or forced under pressure, or kept
*cold*. Starting and stopping such rockets is simple. The hypergolics
aren't as poisonous as many organic poisons, but are very caustic.

Challenger was carrying the second tracking and data relay satellite
(TDRS), and the Colorado University's Spartan Halley satellite; not
Galileo or the telescope.

Lyle McElhaney
...hao!contel!lmc