Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Galileo Gravity Boost
Message-ID: <1989Oct24.164919.1134@utzoo.uucp>
Organization: U of Toronto Zoology
References: <12027@eerie.acsu.Buffalo.EDU> <34700003@uxh.cso.uiuc.edu> <18630@pasteur.Berkeley.EDU> <10150@thorin.cs.unc.edu> <36082@srcsip.UUCP>
Date: Tue, 24 Oct 89 16:49:19 GMT

In article <36082@srcsip.UUCP> rogers@src.honeywell.com (Brynn Rogers) writes:
>   I think that the best (most efficient) time to make a course 
>correction or add delta-V is when a spacecraft is closest to a planet
>(moon,star,asteroid, or gravity well).  Any small changes made
>in course or speed of the spacecraft are much more effective than
>changes made in deep space.

Correct.  How much gain there is depends on how deep the gravity well is;
you don't gain a *lot* unless you're talking about something like Jupiter.

(For those who aren't up on this, you gain because on leaving the planet's
gravity well at higher speed, the gravitational field has less time to slow
you down, so you don't lose all the speed you gained going in.  The "extra"
energy comes from the fuel you carried in but aren't carrying out again.)

>   Why doesn't Galileo (or future craft) use this fact to shorten its
>trip time?   (Other than the fact that it dumps the IUS close to
>Earth.)  If there were a small kick motor on Galileo that fired on the
>closest approach to Venus (apogee or perogee, I forget which), wouldn't it
>give a great improvement to the trip time, at small cost?

I suspect the answer is (a) it wouldn't help that much in such a relatively
shallow gravity well, and (b) the IUS is pushed to its limit just getting
Galileo out, and the extra mass of a kick motor would make it impossible.
There are *lots* of ways the trip could be speeded up if more fuel or an
extra kick motor could have been added.

>   Isn't the reason Galileo goes to Venus, then back to Earth, then to
>Jupiter, still taking 6 years due to the fact that the IUS has a
>significantly smaller amount of specific impulse than the Centaur upper
>stage?  ...

Yes.  Actually the deficiency is in delta-V (total velocity change), although
lower specific impulse is the reason for that.

> How long would the trip take with
>a Centaur upper stage as originally planned?

I think it was a couple of years shorter.  No numbers on hand.

>Did that trajectory bring it around Venus?

No; there wasn't any need for either the Venus flyby or the Earth flybys.
-- 
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry henry@zoo.toronto.edu