SpaceX, the private rocket launching company owned by Elon Musk, has had successes lately with commercial satellite launches. On 1 May 2017, they launched the military satellite NROL-76 from historic Launch Complex 39A at the Kennedy Space Center in Florida. Another major accomplishment by SpaceX is the successful landings and re-use of the stage 1 booster of the Falcon 9 rocket. This has never been done before, not even by NASA. The way I see it, that story deserves some explanation.
Watch this compilation of clips from the most recent booster landing, their 4th. The landing takes place on an unused pad not far from launch complex 39A a short distance up the coast. I wondered what flight path the stage 1 booster took to allow it to return back to this spot near the launch site. Most of its fuel had been used to get it and the stage 2 payload to high altitude, far downrange, and going very fast. The flight needed to be very efficient.
Most Rocket Launches
Rockets launched from Cape Canaveral head out over the Atlantic generally in an easterly direction so as not to endanger populated areas. Plus, the eastward rotation of the Earth gives the rocket an assist in speed of over 900 mph from Florida. This graphic depicts a typical launch from the Cape to the east. Note the coastline of Florida and the Bahamas to the right. The red zones are areas where boats have been cleared.
Below is the same graphic looking to the south. Stage 1 boosters typically follow the light colored path that descends back down to the ocean into the rectangular red zone. Boosters might be recovered and reused as they were with the space shuttle program. The red line that goes off the left edge of the graphic is the path of stage 2 with the payload headed for orbit. SpaceX was the first to successfully place a barge in the red zone and land the stage 1 booster on it.
After the SpaceX stage 1 booster separates from stage 2, the booster turns 180˚ around and fires 3 of its 9 engines. This reverse thrust slows it down for the descent to the surface. It continues in reverse to the barge floating in position for the landing.
Landing Back Near The Launch Site
Here are two graphics showing the flight path of the SpaceX rockets when the booster returns to land near the launch site. Notice the loop-back path that comes back near the launch site. That is the path of the stage 1 booster. The long curving path in red is the that of the stage 2 with the payload headed for orbit.
What are the mechanics needed to get the stage 1 booster to make such a loopback maneuver? This next graphic should help show what needs to happen. After about two and a half minutes from launch, the 9 booster main engines turn off. Stage 1 separates from stage 2. Stage 1 begins a turn-around and starts firing 3 of its 9 engines for a time in what is called a boostback burn. It slows down from over 1600 m/s to the east to about 600 m/s to the west. The engine burns direct the booster upward and back west toward coastline. Meanwhile, the stage 2 engine burn continues to place the payload into orbit.
After the stage 1 boostback burn, the booster coasts up and over the high point of its arc. It continues to turn around to put the engines in the leading end, directed toward the ground. After going over the apex of the arc, it continues in free-fall toward Earth picking up speed as it re-enters the atmosphere.
As the stage 1 booster falls faster, it nears a red section in the flight path below the label in this next graphic. That red section is where the booster does a re-entry engine burn to slow it down from about 1300 to 360 m/s. It is still about 25-30 miles up when that takes place. The engines are off and the fall continues but at a much slower rate.
At an altitude of about 4 miles, the stage 1 booster fires 1 of the 9 engines to slow it for the landing.
One final video from the May 1 launch shows the separation of stage 1 from stage 2. The booster turns and ignites the boostback burn. The video then shows the booster doing the re-entry burn described above. The next part of the video shows a close-up of the falling booster just before the final landing burn takes place. Last is the successful landing.
I want to acknowledge the help of a web page called Flight Club maintained by Declan Murphy. The images used in this post are from screenshots off of his simulator. I thank him for his work.
The site is a fabulous simulator of rocket flights. Declan has built the launch scenarios for many of the SpaceX flights such as this one. The link to that flight takes you to an Earth view. In a few seconds, the view shifts to a close-up of the launch pad. Lift-off occurs soon after. Zoom in or out, drag to rotate views, shift-drag to move laterally, play with the clock device in the lower left. You can’t break anything. Users can also build their own scenario from the online tools he provides.