Yesterday at MIT's AeroAstro Centennial Symposium, SpaceX CEO Elon Musk said his company will make a first attempt to land the booster stage of its Falcon 9 rocket on a floating platform during the upcoming ISS resupply mission. If the attempt is successful, the company plans to refurbish and reuse the booster stage, making spaceflight history and paving the way for a significant reduction in the cost of access to space.

Achieving a practical, largely reusable spacecraft has been the holy grail of spaceflight for the past few decades, and for very good reasons. Propellant only makes up a tiny percentage (in the case of a Falcon 9 rocket, about 0.3 percent) of the cost of the craft, so being able to reuse all the hardware for multiple flights could potentially slash the cost of spaceflight by a factor of 10 or more.

NASA has been keeping a close eye on SpaceX's progress on the rocket reusability front, partly to investigate how the space agency might be able to land very heavy payloads on the surface of Mars in the coming years. As SpaceX was contracted to resupply the International Space Station, the space agency had a front seat as it witnessed the first stage of the Falcon 9 rocket attempt several boost-backs through the atmosphere leading to, so far, two successful soft landings in water – during the third and fourth ISS resupply missions, respectively.

Now, SpaceX and Tesla CEO Elon Musk has said that, on the fifth resupply mission planned for December 9th, the reusable rocket program is ready to go one step further: instead of a soft water landing, the first stage will attempt for the first time to propulsively land on a floating platform in the middle of the Atlantic Ocean.

"Before we boost back to the launch site and try to land there, we need to show that we can land with precision over and over again," said Musk. "So for the upcoming launch we've got a chance of landing on a floating platform. We have a huge platform that is being constructed at a shipyard in Louisiana right now which is 300 feet long by 170 feet wide (90 by 50 meters)."

That might seem like a massive target when you consider that the goal for SpaceX is to propulsively land "with the accuracy of a helicopter," but there's a number of adverse factors. First of all, the legspan of the rocket is quite large, at 60 feet (18 m). Secondly, the platform will be out in the Atlantic Ocean, well out of harm's way should anything not go to plan, and therefore it won't be anchored to the seafloor. Instead, several motors will try and keep it level and at specific GPS coordinates, waiting for the stage to land at that precise spot a few minutes after launch.

"If we land on that [platform], I think we'll be able to refly that booster," Musk continued. "It's probably not more of a 50 percent chance of landing it on the platform [on the first try], but there's a lot of launches that will occur over the next year, at least a dozen, so I think it's quite likely, probably 80 or 90 percent likely, that one of those flights will be able to land and refly."

All Falcon 9 booster stages come with the hardware allowing them to attempt a soft splashdown, but this isn't always possible depending on the payload of the rocket. For instance, launching a satellite in geostationary orbit requires a larger amount of propellant to be drawn from the booster stage, not leaving enough in the tanks to attempt a landing.

In his talk, Musk also gave more insights into the advantages of propulsive landing as opposed to the more familiar approach of recovering hardware with a parachute-driven soft landing in the ocean. As it turns out, the landing legs equipped in the reusable version of the Falcon 9 also double as giant body flaps that are able to generate a large amount of drag as the stage travels through the atmosphere, cutting terminal velocity – and therefore the propellant needed to boost back to a soft landing – in half.

Introduced in September last year, the latest iteration of the Falcon 9 rocket is approximately 60 percent heavier than the initial expendable version, with the bigger tanks in the first stage containing the extra fuel needed for powered landing.

But, while SpaceX might finally be closing in on reusing the booster stage (which reportedly accounts for about 70 percent of the cost of the rocket), building a reusable second stage won't be nearly as easy as storing more propellant in the second stage of the Falcon 9. In fact, Musk said yesterday, the second stage in the Falcon family will never be reusable, due to the low specific impulse of the engine and the fact that the mission profile often involves launching satellites in geostationary orbit, which is way too far for an easy boost-back.

So, for a reusable second stage from SpaceX, we will have to wait for the next generation of rockets.

"With the next generation of vehicles, which is a methane-oxygen system in which the propellants are cooled to close to their freezing temperatures to increase the density, we can definitely do full reusability," said Musk. "We're talking about a much bigger vehicle, and we're also going to be upgrading to a full-flow staged combustion. That system is intended to be a fully reusable transportation system all the way to Mars and back."

Musk said that we could start seeing the first test flights for that system as early as five or six years from now.

Source: MIT

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