SpaceX May 11, 2018 Falcon 9 launch had no boostback burn

Question

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....)

Answer 1

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!!

Answer 2

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.