```Date: 4 Jun 87 20:05:38 GMT
From: fluke!ssc-vax!eder@beaver.cs.washington.edu  (Dani Eder)
Subject: Re: shuttle aerobatics???

In article <12402@topaz.rutgers.edu>, josh@topaz.rutgers.edu (J Storrs Hall) writes:
> A friend tells me that the shuttle performs a diving maneuver as part
> of its ascent (yes, ascent) to gain speed.  I feel that this is off
> the wall but couldn't convince him.  I could see this as a possibility
> for air-breathing craft with an operating ceiling, and I know all
> about the planet-diving trick for interplanetary acceleration: but
> that the shuttle would do it during ascent is completely unbelievable.

> Can someone say something authoritative on the subject?

> --JoSH

I asked Marc Martin, who is our trajectory / performance person why this
happens.  He gave me several reasons: (1) the Shuttle has a 'Max-Q'
constraint.  Q is the dynamic air pressure in flight.  The Shuttle is
limited to about 650 pounds/square foot dynamic pressure.  Dynamic
pressure is a function of air pressure and velocity, so the Shuttle
flies a high trajectory to get to lower air pressure.  This trajectory
is too high for what comes later. (2) The shuttle does a 'depressed
trajectory' later in the flight to make sure the External Tank reenters
in a well defined area.  The way they do this is by flying to a 57x2
mile orbit at main engine cut-off.  The apogee of this orbit is 57
miles, about where the orbiter is, and the perigee is 2 miles, halfway
around the earth.  The tank follows this orbit until it gets to thick
air somewhere above 2 miles, and burns up.  The orbiter fires it's OMS
engines and raises its' orbit before it follows the ET in.  (3) In
general, an efficient launch trajectory want you to spend as little time
firing downwards as possible, and as much time firing sideways
(horizontal velocity is what is needed to stay in orbit).  If you are
outside the atmosphere, you can point your engines in the most efficient
direction, without having to worry about aerodynamic forces caused by
your velocity (wind).  In the atmosphere you want to point your vehicle
along its long axis to minimize drag.  So if you are out of the
atmosphere quick, you can fly a more efficient trajectory.  If you are
thrusting purely sideways, it is possible that you will start to fall
vertically before reaching orbit.