Author Topic: Falcon 9 Q&A  (Read 4932 times)

Offline drunyan8315

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Re: Falcon 9 Q&A
« Reply #20 on: 04/16/2017 05:25 AM »
I believe that Elon Musk mentioned the angle of attack required of the rocket body cylinder during the press conference after the most recent launch.

Rockets that are going to orbit cannot follow a purely ballistic trajectory like an artillery rocket intended to come back down to impact the ground. Prior to obtaining orbital velocity, an orbit-bound rocket must maintain a slight "nose high" attitude to develop a thrust vector upward, preventing it from descending below the desired trajectory, unless it has wings!

Offline Jim

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Re: Falcon 9 Q&A
« Reply #21 on: 04/17/2017 02:53 PM »
  A little-appreciated fact is that the rocket must generate aerodynamic lift

Not true at all. 


Offline Jim

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Re: Falcon 9 Q&A
« Reply #22 on: 04/17/2017 02:58 PM »
I believe that Elon Musk mentioned the angle of attack required of the rocket body cylinder during the press conference after the most recent launch.


It is not "required" but minimized


B. There is nothing inherently improper with wings or fins located forward on the body of a missile, as they are on virtually every air-to-air missile in existence. The fact that they are located on the aft end of the MOAB is a design choice appropriate to their application in a minimalistic guidance system of a gravity bomb.


Look at your words:  missile, air to air, etc.  Launch vehicles are not missiles, especially ones that operate at less than 50kft.

Launch vehicles spend most of the time out of the atmosphere.

Prior to obtaining orbital velocity, an orbit-bound rocket must maintain a slight "nose high" attitude to develop a thrust vector upward, preventing it from descending below the desired trajectory, unless it has wings!

Wrong.  Launch vehicles don't flight with "canted" thrust vector to counter gravity.  The rocket is in free flight, there is no side gravity force.  An astronaut only feels the thrust of the engine and not a pull to the center of the earth and not to a floor like an airplane.  If an airplane had strong enough engines, it wouldn't need wings.
« Last Edit: 04/17/2017 03:06 PM by Jim »

Offline Manabu

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Re: Falcon 9 Q&A
« Reply #23 on: 04/28/2017 02:44 AM »
I believe that Elon Musk mentioned the angle of attack required of the rocket body cylinder during the press conference after the most recent launch.
That angle of attack and aerodinamic lift from the rocket body cylinder were for minimizing the boostback fuel in RTLS manouvers, not to help in the ascending phase.
« Last Edit: 04/28/2017 02:45 AM by Manabu »

Online envy887

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Re: Falcon 9 Q&A
« Reply #24 on: 05/23/2017 02:41 PM »
...
Launch vehicles spend most of the time out of the atmosphere.

Prior to obtaining orbital velocity, an orbit-bound rocket must maintain a slight "nose high" attitude to develop a thrust vector upward, preventing it from descending below the desired trajectory, unless it has wings!

Wrong.  Launch vehicles don't flight with "canted" thrust vector to counter gravity.  The rocket is in free flight, there is no side gravity force.  An astronaut only feels the thrust of the engine and not a pull to the center of the earth and not to a floor like an airplane.  If an airplane had strong enough engines, it wouldn't need wings.

Launch vehicles are absolutely under the pull of gravity - they must either fly with some component of the thrust vector countering gravity, or they must accelerate under gravity. What they don't do is counter gravity with aerodynamic lift, because (as Jim noted) there is no atmosphere for most of the vehicle's flight.

The idea that rockets need aerodynamic lift for anything is totally misguided. It's trivial to have a thrust component that is countering gravity but is not "canted" relative to the vehicle or its direction of travel -  you simply point the rocket varying amounts of "up" while always thrusting and traveling in the direction the rocket is pointed (i.e. angle of attack is always zero).

Any thrust component that is opposing gravity is not accelerating the vehicle downrange, and so is contributing to gravity losses.  At liftoff the rocket is entirely vertical, so a large fraction of the booster's thrust is countering gravity, with the rest accelerating the vehicle upward. So most gravity losses are incurred near liftoff.

To reduce gravity losses (or more precisely, to absorb them primarily with the heavy booster(s) and not a light upper stage) many vehicles loft the upper stage far outside the atmosphere, where it slows vertically and eventually starts falling back without attempting to thrust to counter gravity. But as it slows vertically, it is accelerating downrange so to reach orbital velocity before falling back into the atmosphere. Orbit, of course, is a free-fall that never reaches the ground.