Author Topic: Falcon 9 Q&A  (Read 12115 times)

Offline BeamRider

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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 »

Offline 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.

Offline nicp

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Re: Falcon 9 Q&A
« Reply #25 on: 12/21/2017 08:05 PM »
Forgive me if this is covered elsewhere, couldn't find anything.
I've just noticed that the SpaceX coverage of NROL-76 uses m/s (metres per second), but CRS-13 uses km/h (unit of the antichrist).

The former is a proper SI unit of speed (yes I know some of you want feet per second or furlongs per fortnight, but SpaceX went metric, don't blame me).

My problem is km/h. Really? The only time I have ever seen that is the speedometer in a car.
Ok, so I'm British and less than 60 so I use m/s for engineering. When I'm driving, it's miles/hr.

I would be willing to bet SpaceX use SI units (kg/seconds/metres) internally. So why on earth would they switch the coverage to use km/h?
Where's my Guinness?

Offline envy887

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Re: Falcon 9 Q&A
« Reply #26 on: 12/21/2017 08:10 PM »
Forgive me if this is covered elsewhere, couldn't find anything.
I've just noticed that the SpaceX coverage of NROL-76 uses m/s (metres per second), but CRS-13 uses km/h (unit of the antichrist).

The former is a proper SI unit of speed (yes I know some of you want feet per second or furlongs per fortnight, but SpaceX went metric, don't blame me).

My problem is km/h. Really? The only time I have ever seen that is the speedometer in a car.
Ok, so I'm British and less than 60 so I use m/s for engineering. When I'm driving, it's miles/hr.

I would be willing to bet SpaceX use SI units (kg/seconds/metres) internally. So why on earth would they switch the coverage to use km/h?

Km/h are what 90% of the world uses for measuring speed, and is only used on the hosted webcast targeted at a general audience.

The technical webcast (when it's broadcast, which is not every flight) only shows m/s.
« Last Edit: 12/21/2017 08:17 PM by envy887 »

Online nacnud

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Re: Falcon 9 Q&A
« Reply #27 on: 12/21/2017 08:12 PM »
They're either Canadian (or Mexican) or trolling all the KSP players out there.

PS I'd vote for m/s if I had a vote
« Last Edit: 12/21/2017 08:19 PM by nacnud »

Offline nicp

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Re: Falcon 9 Q&A
« Reply #28 on: 12/21/2017 08:17 PM »
So all I'm seeing is a difference between the technical and hosted (non-technical) webcasts? I've watched many (all?) of both, never realized.

I do not get on with km/h (as you may have guessed). For some reason I have trouble converting that in my head to m/s, whereas feet per second, miles per hour are no problem.

Sigh. Another conversion constant to try to remember...
Where's my Guinness?

Online ugordan

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Re: Falcon 9 Q&A
« Reply #29 on: 12/22/2017 07:40 AM »
The technical webcast (when it's broadcast, which is not every flight) only shows m/s.

Well, not really. Not consistently, anyway. The CRS-13 technical webcast showed km/h.

Offline deruch

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Re: Falcon 9 Q&A
« Reply #30 on: 12/22/2017 09:35 AM »
For some reason I have trouble converting that [km/h] in my head to m/s,

Divide by 4, then add 10% of your new total.  quick conversion.  It's obviously not perfectly accurate.  But the actual conversion is to multiply by (1000/3600)=0.277| (sorry that's supposed to be 7s repeating, not sure how to type a bar) and my way is the same as multiplying by 0.275.  If you need additional accuracy, you can keep taking and adding 10% of your previous 10%.  So, say the speed is 8000 km/h.  That gives 2000 +200 +20 +2 +..... and just keep adding 10% of the previous adjustment until you reach enough accuracy.
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Offline cebri

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Re: Falcon 9 Q&A
« Reply #31 on: 12/27/2017 08:45 AM »
One question can't seem to find the answer for. Does F9 use Draco engines as RCS or just nitrogen thrusters? Thanks.

Online ugordan

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Re: Falcon 9 Q&A
« Reply #32 on: 12/27/2017 08:47 AM »
Nitrogen.

Offline koshvv

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Re: Falcon 9 Q&A
« Reply #33 on: 12/29/2017 12:47 PM »
My problem is km/h. Really? The only time I have ever seen that is the speedometer in a car.

I would be willing to bet SpaceX use SI units (kg/seconds/metres) internally. So why on earth would they switch the coverage to use km/h?
I would like they use km/s (or m/s, conversion is trivial). Something like 18000 km/h is absurd number. I never encountered such long distances in my life and have no slightest idea how fast 18000 km/h is.
In contrast, 5000 m/s (or 5 km/s) is the distance from my home to Auchan, covered in just one second. Wow, it's pretty fast!

Offline deruch

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Re: Falcon 9 Q&A
« Reply #34 on: 12/29/2017 01:03 PM »
My problem is km/h. Really? The only time I have ever seen that is the speedometer in a car.

I would be willing to bet SpaceX use SI units (kg/seconds/metres) internally. So why on earth would they switch the coverage to use km/h?
I would like they use km/s (or m/s, conversion is trivial). Something like 18000 km/h is absurd number. I never encountered such long distances in my life and have no slightest idea how fast 18000 km/h is.
In contrast, 5000 m/s (or 5 km/s) is the distance from my home to Auchan, covered in just one second. Wow, it's pretty fast!

I provided a quick method of estimating a conversion from km/h to m/s in response to @nicp's original complaint.  It's very easy.

http://forum.nasaspaceflight.com/index.php?topic=39481.msg1762532#msg1762532
Shouldn't reality posts be in "Advanced concepts"?  --Nomadd

Offline Hominans Kosmos

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Re: Falcon 9 Q&A
« Reply #35 on: 02/22/2018 02:38 PM »
My problem is km/h. Really? The only time I have ever seen that is the speedometer in a car.

I would be willing to bet SpaceX use SI units (kg/seconds/metres) internally. So why on earth would they switch the coverage to use km/h?
I would like they use km/s (or m/s, conversion is trivial). Something like 18000 km/h is absurd number. I never encountered such long distances in my life and have no slightest idea how fast 18000 km/h is.
In contrast, 5000 m/s (or 5 km/s) is the distance from my home to Auchan, covered in just one second. Wow, it's pretty fast!

km/h is the perfect unit of measurement for communicating with the general public. Suggestions of using any other unit of measurement is demanding the company adopt the policy of communicating with the general public in gibberish.

Offline Oberon_Command

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Re: Falcon 9 Q&A
« Reply #36 on: 02/22/2018 02:50 PM »
For some reason I have trouble converting that [km/h] in my head to m/s,

Divide by 4, then add 10% of your new total.  quick conversion.  It's obviously not perfectly accurate.  But the actual conversion is to multiply by (1000/3600)=0.277| (sorry that's supposed to be 7s repeating, not sure how to type a bar) and my way is the same as multiplying by 0.275.  If you need additional accuracy, you can keep taking and adding 10% of your previous 10%.  So, say the speed is 8000 km/h.  That gives 2000 +200 +20 +2 +..... and just keep adding 10% of the previous adjustment until you reach enough accuracy.

Actually, there's an easier way - the conversion factor is 3.6. There are 3600 seconds in an hour; a kilometer is 1000 meters. Therefore, an object travelling 1m/s travels 3.6km in an hour. So, 18000km/h = 5000m/s, because 18000 / 3.6 is 5000, while 7000m/s = (7000 * 3.6)km/h = 25,200km/h.

Easy conversions like that are one of the advantages of the metric system. :P
« Last Edit: 02/22/2018 02:51 PM by Oberon_Command »

Offline deruch

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Re: Falcon 9 Q&A
« Reply #37 on: 03/01/2018 02:38 PM »
For some reason I have trouble converting that [km/h] in my head to m/s,

Divide by 4, then add 10% of your new total.  quick conversion.  It's obviously not perfectly accurate.  But the actual conversion is to multiply by (1000/3600)=0.277| (sorry that's supposed to be 7s repeating, not sure how to type a bar) and my way is the same as multiplying by 0.275.  If you need additional accuracy, you can keep taking and adding 10% of your previous 10%.  So, say the speed is 8000 km/h.  That gives 2000 +200 +20 +2 +..... and just keep adding 10% of the previous adjustment until you reach enough accuracy.

Actually, there's an easier way - the conversion factor is 3.6. There are 3600 seconds in an hour; a kilometer is 1000 meters. Therefore, an object travelling 1m/s travels 3.6km in an hour. So, 18000km/h = 5000m/s, because 18000 / 3.6 is 5000, while 7000m/s = (7000 * 3.6)km/h = 25,200km/h.

Easy conversions like that are one of the advantages of the metric system. :P
Maybe you missed the part where nicp said, "in my head"?  Sure, multiplying and dividing by 3.6 is going to get you an accurate result and is simple enough with a calculator.  But the method I mentioned is much better for "in your head" math.
Shouldn't reality posts be in "Advanced concepts"?  --Nomadd

Offline IanThePineapple

Re: Falcon 9 Q&A
« Reply #38 on: 11/12/2018 05:53 PM »
Is it known which core F9R Dev2 is/was? I thought I read somewhere that it was core 1007, but I couldn't find any of this info again.

Thanks for the help!
« Last Edit: 11/12/2018 05:54 PM by IanThePineapple »