SpaceX Nailed Its Most Difficult Landing Yet

Contributed by
May 7, 2016
<?xml encoding="utf-8" ?>

On Friday morning  at 05:21 UTC a SpaceX Falcon 9 rocket roared away from its launch pad in Florida, carrying a Japanese satellite into orbit.

Eight minutes and 40 seconds later, the first stage booster set down on a barge in the Atlantic Ocean, marking the third time the company had managed to land a booster after launch, and the second time at sea.

One of my favorite parts of all this is hearing the team at SpaceX cheering as the booster comes in. Watch:

So cool. And this was a very big deal. Here’s why.

First off, getting a first stage booster to come back to Earth in a controlled landing is an amazing feat of technology all by itself. These flights are not just going straight up and back down; to go into orbit you need to achieve a velocity of about 8 kilometers per second (about 18,000 miles per hour) sideways, more or less parallel to the Earth’s surface.

When the Falcon 9 launched, it starts to head eastward, along the direction of Earth’s rotation (that gives an added boost to the rocket’s velocity of 1,000 km/hr due to Earth’s spin).* Two minutes later the rocket is moving about 8,000 km/hr relative to Earth’s surface. The first stage booster engines cut off, the second stage separates away from it, the second stage engines fire, and then that upper stage takes the payload up to orbit. Dumping the first stage gets rid of a lot of dead weight, saving a huge amount of fuel; that’s why big rockets like this have separate stages.

For this launch, the payload was a Japanese communication satellite. The final orbit for that is geosynchronous, about 36,000 km above Earth’s surface; at that height, one orbit takes 24 hours, so the satellite stays over the same part of the Earth. That has advantages for weather and communications satellites; the former see the same part of Earth all the time while the latter can be in 24-hour contact with ground receivers.

But it takes a lot of energy to put a satellite in that orbit, and for rockets, energy = velocity = fuel. To get the satellite on the correct trajectory, the first stage had to use almost all its fuel, and was moving very rapidly eastward relative to the ground. After separation, it had to kill much of its sideways velocity, drop back down to the planet, then kill its vertical velocity with the fuel it had left ... and all of this facing higher velocities and therefore far more difficult conditions then the successful landing last month. It was still moving at 2 km/s when it dropped back down into the atmosphere for this landing, twice as fast as usual. That means four times as much energy was needed to slow it down, and it generated eight times as much heat due to compression as it rammed through the air like a meteor.

Before the launch, Elon Musk gave the odds of a successful landing at 50/50. Earlier launches had been lower energy, making it easier—though by no means easy—to land the booster. Still, as you saw, they made it. The booster landed right in the center of the drone ship. Bull’s-eye.

If SpaceX can refurbish the booster and use it again (a big if, to be sure) then it will save a lot of money; it costs very little to go through that compared with building a first stage booster from scratch.

And that’s why the successful booster touchdown on the floating drone ship Of Course I Still Love You was such a big deal. Many launches will be to high orbit, or be on otherwise energy-intensive trajectories. Being able to return the booster in those circumstances saves a lot of money.

But it’s more than that. The Falcon Heavy is scheduled for its first test flight sometime later this year; it’s essentially three Falcon 9s strapped together. Many of the Heavy launches will be for payloads with a lot of mass into low orbit, or lower mass into high-energy orbits. This most recent booster landing means that they’re capable of bringing back those FH boosters from more launches as well.

This also plays into the effort to launch a Dragon 2 capsule to Mars. That will take a fully expendable Falcon Heavy launch. Getting a high-mass payload to Mars takes lot of energy, and all three boosters will be run to exhaustion to get the second stage on its way with the payload. But as I wrote before, the maneuvering to slow the booster in Earth’s upper atmosphere is very much like what’s needed to slow any payload in Mars’s atmosphere, too. So there’s that connection at least.

And don’t discount the sheer technical capability of SpaceX to do complex and difficult missions. I saw this tweet after the booster landing:

Well, I think the goal of launching a Dragon V2 on its way to Mars as soon as 2018 is ambitious and may only have a 50/50 chance of happening … but SpaceX is showing that, sometimes, those odds are good enough to bet on.

*Orbital speed is measured from the Earth’s center, because that’s how gravity works. That speed doesn’t matter if the Earth is rotating or not; to orbit Earth you need to be moving at about 8 km/sec. The Earth is spinning, though, so at the equator the ground is moving eastward relative to the Earth’s center at roughly 1,675 km/hr (making one full spin of 40,000 km every 24 hours). That’s free velocity for the rocket! The closer to the equator, the faster that motion is, which is why launches are done as far south as possible.

Update, May 7, 2016: To be clear, to save fuel the booster used atmospheric drag to slow its horizontal velocity; it didn't perform the usual "boostback burn" to slow down. I wasn't aware of this for this launch; I thank Thanos Tour for tweeting me about it. You can read more about this on the Reddit page for this mission.