One of my questions regarding the May 11 launch is Why no boostback burn? I assume that without such a burn, Stage One would continue on at about 8,000 km/hr in an EASTWARD direction, away from the drone ship. (I noticed that during the 'Tess' launch they TALKED ABOUT a boostback burn but did not do one. So the same question applies....)
Landing on the properly-positioned barge is a great advantage for the launch vehicle's performance because it requires no boostback burn. That's why you didn't see one, and that's why the barge wasn't at Cape Canaveral, it was more than 300 km downrange.
For a barge landing the first stage can burn more of its propellant load during primary boost, allowing a larger payload, or higher ∆V out of the 1st stage. For a payload asking for GTO, this is good.
I did a rough estimate of the downrange distance at BECO: ~150 km. At that point the 2nd stage began decelerating at ~3.5 m/s^2 (calculated from numbers taken by the display on the upper right of SpaceX's video), and the 1st stage was decelerating faster than that. That was while the 1st stage was oriented pretty much along the flight path. Once the 1st stage turns side-on to the flight path that deceleration gets much larger, roughly comparable to the acceleration during the boost burn, and will decrease as the stage's speed decreases. So it roughly parallels, in reverse, the boost phase velocity profile; from that you'd expect a downrange target point of ~300 km.
But the 1st stage was going upward at ~930 m/s at staging (again, calculated from the numbers given in the video), so the return phase would take a bit longer than the boost phase and cover a little more ground, so you expect the target point to be more than 300 km downrange.
On one of the coverage web sites the (non-SpaceX) announcer mentioned the downrange distance and I don't remember the precise number, but it was in the 300's of km. I haven't been able to find that particular video again to verify the precise number.
As someone who grew up knowing that all those big, expensive 1st stages (Atlas, Titan, Saturn V) became junk only ~2 minutes after liftoff, these recoverable 1st stages are so cool!!
1$\begingroup$ The summary is here: "Landing on the barge is a great advantage for the launch vehicle's performance because it requires no boostback burn." Landed on barge -> no boostback burn. $\endgroup$ May 13, 2018 at 4:14
Why no boostback burn?
For the May 11, 2018 SpaceX launch, you didn't see the boostback burn because there was no boostback burn!
This is because the first stage did not need to "boost" back to anywhere.
Instead, it continued on in the same direction, along it's roughly parabolic arc, and eventually landing on a ship in the ocean.
At the video time code of
24:17in the Bangabandhu Satellite-1 Mission which corresponds to about
T+ 00:06:20 there is the first of only two burns in the descent and landing. If you look at the bottom of the screen or the screen shot here, you can see that it is called the Stage 1 Entry Burn.
The second is called the Stage 1 Landing (burn).
Using the excellent simulator on Flightclub.io you can see this graphically. Red segments are burns; for the first stage there are three, Ascent, Entry, and Landing.
$\begingroup$ I've just asked for clarification in Understanding Flightclub.io simulation of SpaceX's Bangabandhu launch $\endgroup$ May 13, 2018 at 4:13
$\begingroup$ Okay, your answers sound good but my problem lies in believing that stage one can steer itself from 60 - 70 miles high to a target the size of a manhole cover without any real physical guidance system. The grid fins are useless in a vacuum and can AT BEST only slightly affect trajectory -- they are for attitude control, having no airfoil for lift. The jet thrusters likewise are near useless in thick air and are only useful for attitude control in a vacuum. $\endgroup$– AllancwMay 14, 2018 at 20:03
$\begingroup$ Jeez, it didn't let me edit it properly. Can I have another shot? $\endgroup$– AllancwMay 14, 2018 at 20:12
$\begingroup$ @Allancw It's quite a technological feat I agree! If you ask a new question, such as "What are all of the aspects of trajectory control used land a first stage within a few meters of target after hundreds of kilometers of flight?" that would provide space for people to compose a suitable answer. There are in fact plenty. $\endgroup$ May 14, 2018 at 21:49
$\begingroup$ @Allancw I don't understand what "...it didn't let me edit it properly. Can I have another shot?" means. You can always ask another question. You can edit your current question, though once there's an answer it's discouraged to make any big changes. Comments can only be edited for five minutes though. It takes a little time to get used to the Stack Exchange system but it works really great once you get used to it. Welcome by the way, great first question! $\endgroup$ May 14, 2018 at 21:52