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    Re: Cel nav in space
    From: Derrick Young
    Date: 2005 Jan 5, 15:13 -0500

    Cliff,
    
    I was not talking about intentional steering - just what happens when
    you are trying to boost a large object into space.  You are correct in
    the statement about the intentional steering being done to gain the
    additional rotational speed - but not from the Cape - if you have ever
    noticed, the shuttle launches from the Cape are mainly (not always) low
    altitude missions.  When they want the shuttle to go into a higher
    orbit, with minimal additional fuel expenditure, they launch from
    Vandenburg AFB in California.  Launching from California allows the
    shuttle to launch in a westerly direction - there by taking advantage of
    the earth's rotation. This results in a higher orbit for less fuel.
    
    When launching from the Cape, part of the intentional turn east is to
    avoid over flights of Cuba - by turning east, they are actually burning
    more fuel than needed - but then they don't have to deal with the
    political ramifications.
    
    As I said in my post, the degree of the curve that you will see is
    directly related to the total mass of the object (the rocket) and the
    amount of thrust (which determines how quickly it gets up to max speed).
    This is why you see "ballistic" type of trajectories with the smaller
    rockets.  But when you increase the total mass to where it more closely
    matches the total thrust available, you will see the "gravity turn"
    trajectory.
    
    True, you can influence the total amount of the gravity turn by
    intentional steering, using gimbaled motor exhausts, adding fins, or
    starting the vehicle to spin about the long axis (like the rifling spins
    a bullet).  But you will still see a gravity turn being followed
    whenever the total mass being boosted is close to the instantaneous
    thrust that can be put out by the engine.
    
    Please, don't misunderstand - I am not saying one pound of payload over
    one pound of thrust - remember that the launch weight of the vehicle
    will be the largest mass.  As fuel is burned, there is less and less
    total mass.  Also, if you are using solid rocket propellant, since the
    fuel burns only on the face, as it burns, the total ignition chamber
    gets larger, which decreases the thrust (during flight we measure this
    by keeping track of the head cap pressure).  You have to have sufficient
    thrust available at the end of the burn to insure that there is enough
    speed departed to the payload.
    
    The SCOUT was about 75 feet long, with a total launch weight of about
    47,000(+) pounds.  With that configuration, it would boost a satellite
    weighing about 100 pounds into a 110 NM orbit.  There were other
    configurations, some weighing as much as 60,000 pounds - that would
    boost satellites as large as 350 pounds into that orbit.  We had to be
    constantly aware of the total vehicle weight, payload weight (satellite
    plus the external shroud and separation mechanism).  If we went heavy is
    anything, we may not reach orbit.  Somewhere, I have records of the
    motor tests that we did to determine total motor thrust as well as the
    actual thrust curves that occurred during the burn.  That was
    interesting - you see different curves when the air pressure changed.
    So for the first stage, we were ok - upper stages had to be conducted in
    a vacuum.
    
    derrick
    
    
    

       
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