Falcon 9 flight trajectory

[cambrianera]

the 1st stage, INCLUDING the engines, has about the same wet/dry weight ratio of a standard beer can.

Also if it has a different length/diameter ratio, try to throw an empty can from a skyscraper and you get an idea of the dynamic involved in the reentry...

Not exactly, mass= proportional to dimension^3, air resistance=proportional to dimension^2,  therefore  scaling up gives larger terminal velocity.
By the way, this kind of discussion went out when I noted that second stage is approximately 50:1 scale of 500cc beer can (lenght, diameter and thickness)
http://forum.nasaspaceflight.com/index.php?topic=32180.msg1121913#msg1121913.

[pagheca]

Not exactly, mass= proportional to dimension^3, air resistance=proportional to dimension^2,  therefore  scaling up gives larger terminal velocity.
By the way, this kind of discussion went out when I noted that second stage is approximately 50:1 scale of 500cc beer can (lenght, diameter and thickness)
http://forum.nasaspaceflight.com/index.php?topic=32180.msg1121913#msg1121913.

My point is totally different.

I'm talking about the wet/dry mass ratio, not the shape. This ratio can't be scaled up and has no direct relation to the terminal velocity. I used this analogy "to get an idea" of how light is the first stage of a descending Falcon 9, despite the mass of the 9 engines and the much more stringent design constraints. To me it looks really amazing that you can design a "beer can" to leave the atmosphere, reach Mach 6 and re-enter it without crashing everything.

[cambrianera]

Not exactly, mass= proportional to dimension^3, air resistance=proportional to dimension^2,  therefore  scaling up gives larger terminal velocity.
By the way, this kind of discussion went out when I noted that second stage is approximately 50:1 scale of 500cc beer can (lenght, diameter and thickness)
http://forum.nasaspaceflight.com/index.php?topic=32180.msg1121913#msg1121913.

My point is totally different.

I'm talking about the wet/dry mass ratio, not the shape. This ratio can't be scaled up and has no direct relation to the terminal velocity. I used this analogy "to get an idea" of how light is the first stage of a descending Falcon 9, despite the mass of the 9 engines and the much more stringent design constraints. To me it looks really amazing that you can design a "beer can" to leave the atmosphere, reach Mach 6 and re-enter it without crashing everything.

Realy hate to be pedantic, but part of what you wrote is "try to throw an empty can from a skyscraper and you get an idea of the dynamic involved in the reentry".
That's not correct, hence my post.

 

[rockettrey]

Because we are discussing launch loft/trajectory here and there in this thread, I am emboldened to ask a question that I have wondered about for years.  Because SpaceX is involved with and doing something similar to my thoughts on this matter (with this launch), I now absolutely have to ask the question.  I just wished I took orbital mechanics in college...

Shuttle (and others) execute a roll program a few seconds after launch where they would negate some of their vertical acceleration for horizontal acceleration presumably to help reach orbital velocity and altitude together- possibly a compromise for something like fuel (weight) or time.  Why wouldn't a launch vehicle go vertical at first to get out of the atmosphere (and drag) as fast as possible, and THEN work on the horizontal component (escape velocity) where drag losses are much lower? 

My question for you smart rocket scientists out there- is SpaceX possibly thinking along these lines?  This would also have the side benefit of reducing the boost back distance.  I am ABSOLUTELY not qualified to speculate on that- I can only ask the question....but eagerly await informed answers!

[NovaSilisko]

Because we are discussing launch loft/trajectory here and there in this thread, I am emboldened to ask a question that I have wondered about for years.  Because SpaceX is involved with and doing something similar to my thoughts on this matter (with this launch), I now absolutely have to ask the question.  I just wished I took orbital mechanics in college...

Shuttle (and others) execute a roll program a few seconds after launch where they would negate some of their vertical acceleration for horizontal acceleration presumably to help reach orbital velocity and altitude together- possibly a compromise for something like fuel (weight) or time.  Why wouldn't a launch vehicle go vertical at first to get out of the atmosphere (and drag) as fast as possible, and THEN work on the horizontal component (escape velocity) where drag losses are much lower? 

My question for you smart rocket scientists out there- is SpaceX possibly thinking along these lines?  This would also have the side benefit of reducing the boost back distance.  I am ABSOLUTELY not qualified to speculate on that- I can only ask the question....but eagerly await informed answers!

Gravity.

Every second you go straight vertical is 9.81 m/s/s delta-v lost*. Atmospheric drag tapers off relatively quickly, but you still have nearly 1g of downward force in LEO.


_{*somebody please inform me how wrong I am here, I feel like I might be ;_;}

[Enceladus]

Because we are discussing launch loft/trajectory here and there in this thread, I am emboldened to ask a question that I have wondered about for years.  Because SpaceX is involved with and doing something similar to my thoughts on this matter (with this launch), I now absolutely have to ask the question.  I just wished I took orbital mechanics in college...

Shuttle (and others) execute a roll program a few seconds after launch where they would negate some of their vertical acceleration for horizontal acceleration presumably to help reach orbital velocity and altitude together- possibly a compromise for something like fuel (weight) or time.  Why wouldn't a launch vehicle go vertical at first to get out of the atmosphere (and drag) as fast as possible, and THEN work on the horizontal component (escape velocity) where drag losses are much lower? 

My question for you smart rocket scientists out there- is SpaceX possibly thinking along these lines?  This would also have the side benefit of reducing the boost back distance.  I am ABSOLUTELY not qualified to speculate on that- I can only ask the question....but eagerly await informed answers!

Gravity.

Every second you go straight vertical is 9.81 m/s/s delta-v lost*. Atmospheric drag tapers off relatively quickly, but you still have nearly 1g of downward force in LEO.


_{*somebody please inform me how wrong I am here, I feel like I might be ;_;}

You're on the right track. It's called gravity drag and the concept is a little hard to grasp. But basically, yes. You want to spend as little time as possible boosting straight up, but for aerodynamic purposes, you want to maximize this time. So the real flight plan is a complex compromise.

[baldusi]

I believe that the gravity loss is the integral of sin(alpha(t)) x g(altitude) where alpha is the angle wrt to horizontal.

[e of pi]

Because we are discussing launch loft/trajectory here and there in this thread, I am emboldened to ask a question that I have wondered about for years.  Because SpaceX is involved with and doing something similar to my thoughts on this matter (with this launch), I now absolutely have to ask the question.  I just wished I took orbital mechanics in college...

Shuttle (and others) execute a roll program a few seconds after launch where they would negate some of their vertical acceleration for horizontal acceleration presumably to help reach orbital velocity and altitude together- possibly a compromise for something like fuel (weight) or time.  Why wouldn't a launch vehicle go vertical at first to get out of the atmosphere (and drag) as fast as possible, and THEN work on the horizontal component (escape velocity) where drag losses are much lower? 

My question for you smart rocket scientists out there- is SpaceX possibly thinking along these lines?  This would also have the side benefit of reducing the boost back distance.  I am ABSOLUTELY not qualified to speculate on that- I can only ask the question....but eagerly await informed answers!

Perhaps better belongs in one of the public-side Q&A threads, but basically it boils down to the fact that there's more than one type of loss. Aerodynamic drag, of course, is one of them. However, long and skinny rockets are pretty low-drag (not, of course, by coincidence, but it's a good shape). Another, though, is what's often called "gravity drag" or "gravity losses." This is basically the effect of gravity pulling the rocket "down" for the duration of the engine burn. It's a lot more complicated than this, but think about it this way: take a rocket that can do a delta-v of 9800 m/s. It can, basically, make orbit. Now, have it sit on the pad, and throttle the engines so that it burns through all the fuel while hovering right over the pad, over a period of about 17 minutes. The rocket hasn't made orbit, it's still hovering right over the pad when the tanks run dry. So...where'd all the performance go? Fighting these "gravity losses" or "gravity drag."

A post here lists some sample values for these two terms for some real launch vehicles:
http://forum.nasaspaceflight.com/index.php?topic=9959.msg189860#msg189860
As you can see, gravity losses are much larger than the aerodynamic drag. While there may be some benefit to a more "lofted" trajectory (that is, steeper initial trajectory) to reduce boostback requirements, the effect isn't really coming from lower drag--you're increasing the gravity losses, but if the decreases in the first stage's required fuel for boostback is enough, it might pay. That'd be some interesting optimization, but it's not really as much related to aerodynamics.

[edkyle99]

There is an optimal ascent trajectory (pitch-over plus pitch-rate) for every direct insertion orbit altitude for a specific rocket/payload combination.  It balances the need to climb out of the atmosphere quickly to minimize drag and to maximize vacuum specific impulse with the need to pitch over and accelerate while minimizing gravity losses.  The better question for F9-10 might be this.  Was a steeper than optimal ascent trajectory purposefully flown to move the first stage landing zone closer to Florida?

 - Ed Kyle

[deruch]

Because we are discussing launch loft/trajectory here and there in this thread, I am emboldened to ask a question that I have wondered about for years.  Because SpaceX is involved with and doing something similar to my thoughts on this matter (with this launch), I now absolutely have to ask the question.  I just wished I took orbital mechanics in college...

Shuttle (and others) execute a roll program a few seconds after launch where they would negate some of their vertical acceleration for horizontal acceleration presumably to help reach orbital velocity and altitude together- possibly a compromise for something like fuel (weight) or time.  Why wouldn't a launch vehicle go vertical at first to get out of the atmosphere (and drag) as fast as possible, and THEN work on the horizontal component (escape velocity) where drag losses are much lower? 

My question for you smart rocket scientists out there- is SpaceX possibly thinking along these lines?  This would also have the side benefit of reducing the boost back distance.  I am ABSOLUTELY not qualified to speculate on that- I can only ask the question....but eagerly await informed answers!

Since you specifically ask in relation to the Shuttle, an additional concern that the trajectories of manned craft may also need to deal with is ballistic reentries in the event of a failure.  A flatter than optimal (based solely on the physics) trajectory may be required when needing to ensure the survivability of a crew in the event of a launch failure.

[aero]

Did anyone get an accurate time for MECO?

We know that a full burn is 183 seconds and Stage 1 consumes 850 klb, or 386,364 kg of prop. That is 2111.27 kg/s. I'd like to guess how much prop was reserved for boost back and landing.

[Proponent]

For anyone who has a little math under his belt, the short piece attached may shed a little light on velocity losses suffered by a rocket ascending to orbit.  Criticism welcome.

[fatjohn1408]

Did anyone get an accurate time for MECO?

We know that a full burn is 183 seconds and Stage 1 consumes 850 klb, or 386,364 kg of prop. That is 2111.27 kg/s. I'd like to guess how much prop was reserved for boost back and landing.

It was over 9000.

[Paul_G]

....... Why wouldn't a launch vehicle go vertical at first to get out of the atmosphere (and drag) as fast as possible, and THEN work on the horizontal component (escape velocity) where drag losses are much lower? 

I always had difficulty in trying to understand why rockets didn't go straight up. Then I came across things like Kerbal Space Programme, or SimpleRocket on iPhone/Andriod, and these tools let you visualise what happens if you try ti go straight up, and once you see the resulting trajectories from a 'straight up' launch, you get to understand the concept (if not the maths) of gravity loss, and how increasing the horizontal aspect of your trajectory gets you into orbit quicker, and with less fuel.

Paul

[Jim]

There is an optimal ascent trajectory (pitch-over plus pitch-rate) for every direct insertion orbit altitude for a specific rocket/payload combination.  It balances the need to climb out of the atmosphere quickly to minimize drag and to maximize vacuum specific impulse with the need to pitch over and accelerate while minimizing gravity losses.  The better question for F9-10 might be this.  Was a steeper than optimal ascent trajectory purposefully flown to move the first stage landing zone closer to Florida?

 - Ed Kyle

There is also the vehicles' ability to handle aeroloads. More "fragile" vehicles would have lofted trajectories vs a robust vehicles like ICBM's with SRM's which quickly go almost horizontal.

[Icepilot]

From watching the video & assuming the speed/height/downrange commentary marked the minutes, we have:
            km altitude   km/sec    km downrange
T+1m     13                0.45         2.5
T+2m     54                1.4         21
T+3m   112                1.7         51

MECO was at T+2m41sec. So the first stage was 100 km high, 1.6 km/sec total speed, about 45 km downrange with a horizontal component of about 0.5 km/sec.
So SpaceX needs a single engine to provide an empty first stage a delta-v of maybe 0.6 km/sec?
How well do long cylinders perform as lifting bodies?

[Space Ghost 1962]

There is an optimal ascent trajectory (pitch-over plus pitch-rate) for every direct insertion orbit altitude for a specific rocket/payload combination.  It balances the need to climb out of the atmosphere quickly to minimize drag and to maximize vacuum specific impulse with the need to pitch over and accelerate while minimizing gravity losses.  The better question for F9-10 might be this.  Was a steeper than optimal ascent trajectory purposefully flown to move the first stage landing zone closer to Florida?

 - Ed Kyle

There is also the vehicles' ability to handle aeroloads. More "fragile" vehicles would have lofted trajectories vs a robust vehicles like ICBM's with SRM's which quickly go almost horizontal.

Been thinking that one way to "optimize" reuse cost affecting GLOW would be to intentionally reduce structural mass and retropropulsive propellant losses. You'd fly more lofted and might get a lower bound on what you might get away with RLV vs ELV performance hit.

That would drive both costing/margins as well as give the designers a new threshold with, say, composite inserts, to trade improvements against.

[modemeagle]

Here are some graphs from my simulation of this flight.

[LouScheffer]

There is an optimal ascent trajectory (pitch-over plus pitch-rate) for every direct insertion orbit altitude for a specific rocket/payload combination.  It balances the need to climb out of the atmosphere quickly to minimize drag and to maximize vacuum specific impulse with the need to pitch over and accelerate while minimizing gravity losses.  The better question for F9-10 might be this.  Was a steeper than optimal ascent trajectory purposefully flown to move the first stage landing zone closer to Florida?

 - Ed Kyle

There is also the vehicles' ability to handle aeroloads. More "fragile" vehicles would have lofted trajectories vs a robust vehicles like ICBM's with SRM's which quickly go almost horizontal.

The Apollo astronauts commented on how the launch from the moon pitched over right away, accelerating almost horizontally, in contrast to the launch from Earth

[woods170]

There is an optimal ascent trajectory (pitch-over plus pitch-rate) for every direct insertion orbit altitude for a specific rocket/payload combination.  It balances the need to climb out of the atmosphere quickly to minimize drag and to maximize vacuum specific impulse with the need to pitch over and accelerate while minimizing gravity losses.  The better question for F9-10 might be this.  Was a steeper than optimal ascent trajectory purposefully flown to move the first stage landing zone closer to Florida?

 - Ed Kyle

There is also the vehicles' ability to handle aeroloads. More "fragile" vehicles would have lofted trajectories vs a robust vehicles like ICBM's with SRM's which quickly go almost horizontal.

The Apollo astronauts commented on how the launch from the moon pitched over right away, accelerating almost horizontally, in contrast to the launch from Earth

No atmosphere, and thus no aeroloads, to worry about. Only considerations for early pitch over is lunar mountains nearby, like at Apollo 15 and 17 landing sites.