Skipjack wrote:
RTLS is much simpler, all it costs is fuel and that is extremely cheap compared to the rest of the launch costs (Musk once said about 200k USD for a launch).
I don't think anyone questions that RTLS is ideal if you can do it. My question, at least, is HOW is this done.
Skipjack wrote:
So the only additional fuel you need is for the burn that flies you back. So the difference is not that huge.
This is the part I'm having a hard time with... if I had to guess, by the time the first stage separates, I'd think ~90% of the momentum is along the x-axis. Gravity, in this case, doesn't help you much in getting back to the launch site. Reversing course (even on an empty 1st stage) strikes me as impractical/impossible... though I'm certainly open to the fact that my hunch is wrong. Someone earlier suggested it might be easier to actually accelerate further to make it once around... this strikes me as somewhat more possible. Even then, still surprised it only means sacrificing 30% payload, though.
Actually the stage still has a lot of upwards momentum after staging and it continues up during the braking burn. Someone did a really solid calculation of that and posted it at NSF.
Note that this graph is an older version with data available in 2012. I know it was updated later, but it should give you a general idea.
the first stage separation would occur at a velocity of approximately 2.0 km/s (6,500 km/h; 4,100 mph; Mach 6) rather than the 3.4 km/s (11,000 km/h; 7,000 mph; Mach 10) for an expendable Falcon 9, to provide the residual fuel necessary for the deceleration and turnaround maneuver and the controlled descent and landing.
This statement does not include a "fly back" stage. What I see is a desciption for a pitch over, burn, descent and land.
Also, by limiting the first stage boost to 4100mph, vice 7000mph, how much does that limit the achievable orbital profile and payload? That is like almost cutting the boost in half.
The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)
What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)
the first stage separation would occur at a velocity of approximately 2.0 km/s (6,500 km/h; 4,100 mph; Mach 6) rather than the 3.4 km/s (11,000 km/h; 7,000 mph; Mach 10) for an expendable Falcon 9, to provide the residual fuel necessary for the deceleration and turnaround maneuver and the controlled descent and landing.
This statement does not include a "fly back" stage. What I see is a desciption for a pitch over, burn, descent and land.
Also, by limiting the first stage boost to 4100mph, vice 7000mph, how much does that limit the achievable orbital profile and payload? That is like almost cutting the boost in half.
The image at the link shows an estimated trajectory of the stage flying all the way back to the launch site from the staging point (it is in line with projections made by others that wanted to do RTLS). The current F9 1.1 is already staging at mach 6 with the trajectory that goes with that. According to Gwynne Shotwell, the 13 tonnes payload quoted on SpaceX site also already include the reuse penalty (otherwise the payload would have been a lot more) for the first stage.
mvanwink5 wrote:bottom left graph? Top only shows return to ground, not to origination, right? Sorry I am so slow at understanding the graphing context here..
They all show the same thing. In each graph the x and y axis represent different aspects of the flight envelope. The bottom left graph gives you the best idea of the actual trajectory.
Skipjack,
To me the lower left graph indicates the down range termination of flight to be 80 miles from origination, that is what has me hung up, and further, that graph seems to be the only one that shows more than velocity acceleration, and altitude, and the only one to include the downrange position. Thanks for attempting to clear my understanding though. I just must be just missing the context.
Counting the days to commercial fusion. It is not that long now.
mvanwink5 wrote:Skipjack,
To me the lower left graph indicates the down range termination of flight to be 80 miles from origination, that is what has me hung up, and further, that graph seems to be the only one that shows more than velocity acceleration, and altitude, and the only one to include the downrange position. Thanks for attempting to clear my understanding though. I just must be just missing the context.
The graph STARTS at stage separation that is at 80 miles altitude, it does not end there. The top graphs show seconds into the launch on X axis and altitude, dynamic pressure and velocity on the Y axis.
The bottom left graph shows range on the X axis (distance from launch site) and altitude on the Y axis. The bottom right graph shows seconds into the launch on the X and acceleration on the Y axis.
mvanwink5 wrote:Thanks! I had the path backwards, no wonder!
Don't worry! It is easy to get confused. First time I looked at it, I just had the bottom left one and I thought the same thing. I needed the context of the other graphs to understand what was going on.
That is a long way back for stuff to go wrong. It still speaks for over water, vice land.
Thanks for the graphs. I would like to see SpaceX's actuals vice a fanboy estimate. But these will do for now.
Still seems like a stiff penalty in lost lift capacity. But I do see the benefit of getting the primary back for recycle.
Too bad they don't get the secondary back as well. Another engine and tank cheaper.
The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)
What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)
