Why SpaceX's 'Next Few Missions' Will Attempt to Land a Rocket at Sea

SpaceX has confirmed it’s not going to retire the drone boat anytime soon.

Jan 13 2016, 7:15pm

Image: SpaceX

You wouldn't be crazy to assume that, after last month's successful rocket landing, SpaceX would ditch its giant drone boats and focus solely on terrestrial landing attempts. For the foreseeable future, however, the company will return to trying to land its rocket at sea.

A barge landing attempt has already been announced for Sunday's Jason 3 NASA satellite launch from California's Vandenberg Air Force Base, but SpaceX tells me drone boats will likely continue to be a regular part of the company's reusable rocket recovery program, and that "the next few missions will all be drone ships."

That's primarily because landing a rocket at sea requires less fuel than landing a rocket back at the launch pad.

SpaceX has come very close to successfully landing its Falcon 9 rocket on its autonomous boat platforms, but it hasn't done so yet. Safely landing a rocket that has been to space is hard enough—add in the possibility of choppy waves and the platform's smaller size compared to a landing pad, and, well, the choice to attempt all future landings on an earthbound landing pad seems to be an easy one.

Image: SpaceX

But it's not that simple. The decision to land at sea instead of on land comes down to a number of factors, including Federal Aviation Administration launch licensing (the first few landings were attempted at sea in part because there's less chance of, say, landing on a house), Federal Communications Commission licensing, the customer, payload, and expected weather.

The most important deciding factor in making the decision, however, is the fuel requirement for any given launch. SpaceX's Falcon 9 rocket has to carry its payload into orbit and, depending on the mass of that payload (and the amount of fuel loaded into any given rocket), it might not be feasible for the first stage of the rocket to boost itself all the way back to the launch site. You can see what this looks like, more or less, in this diagram designed by a Redditor—at-sea landings require less fuel than land ones.

"Fuel requirements are the big thing (which stems from payload and trajectory of each particular mission)," a SpaceX spokesperson told me in an email. "The next few missions will all be drone ships. This is good practice for future high velocity launches that don't have enough of a delta velocity budget [acceleration] to return to the launch site."

As SpaceX CEO Elon Musk explained in a blog post late last month, massive amounts of acceleration are needed to actually get a satellite into orbit. Returning the rocket to Earth in a controlled fashion necessarily means that the rocket then boost back against that initial acceleration. Two things affect how difficult this is: The payload's mass and the acceleration it needs. An imperfect comparison: You are running and need to quickly run in the exact opposite direction. Going back in the opposite direction is harder the faster you are going. Similarly, if you weigh more, more overall force is necessary to get you to change direction.

But back to rockets: Larger payloads or satellites that are going into higher altitude orbits require higher-energy launches and thus will burn more fuel on their way into orbit. For those types of launches, landing pad recovery attempts might not ever be an option.

"In the case of the Falcon 9 rocket, the boost stage is able to accelerate a payload mass of 125 metric tons to 8,000 km/h and land on an ocean platform or to 5,000 km/h and land back at the launch site," Musk wrote. "The second one is lower because the rocket is moving super fast away from the launch site, so it has to a screeching U-turn with nitrogen attitude thrusters, then fire the engines to create a reversed ballistic arc, then reorient again for atmospheric entry and have the engines pointed in the right direction for the landing burn."

Musk's and SpaceX's statements line up with everything that's been spotted by SpaceX fans over the past few weeks. Permit records for both its planned January 17 Jason 3 NASA satellite launch from California's Vandenberg Air Force Base and its January 23 SES-9 satellite launch from Cape Canaveral both suggest a barge landing (though SpaceX often applies for both barge and landing pad permits to keep its options open). Some back-of-the-napkin math by Redditors suggested that it might be technically feasible to boost the Jason 3 rocket back to a landing pad (even though it's the last launch with a less powerful Falcon 9 rocket), but it's not entirely clear that the landing pad at Vandenberg is in any shape to have a rocket land there.

Photos of Vandenberg Air Force Base Space Launch Complex 4 show that the landing pad there was, at least very recently, occupied by a giant tent (giant as in, there is a cherry picker sitting under it and massive construction equipment looks very small next to it).

SpaceX enthusiasts on Reddit and forums are speculating that the tent may serve as a sort of makeshift control center or a storage facility for rocket cores.

In any case, it looks like SpaceX is headed back to sea for the foreseeable future.