NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Cel nav in space
From: Derrick Young
Date: 2005 Jan 5, 15:13 -0500
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