SpaceX CRS-5 Mission
Landing A Rocket On A Boat
Early tomorrow morning, with a planned launch time of 4:47 AM EST, SpaceX will make its second attempt to launch the CRS-5 International Space Station resupply mission, the first attempt having been aborted due to a faulty actuator in the second stage of the launch vehicle. SpaceX has launched four previous such missions successfully. They're simple in principle: SpaceX launches their Dragon cargo capsule, full of supplies, parts, consumables, and science experiments that the ISS needs, lifting it into orbit with the Falcon 9 rocket. It is placed into an orbit that brings it to rendezvous with the ISS. The astronauts dock the capsule and unload the cargo, replacing it with waste products and trash, and undock the capsule, which deorbits and eventually lands in the Pacific Ocean for recovery.
What makes this fifth mission special is SpaceX's intention to test the centerpiece strategy of their re-usability efforts -- the controlled landing of the mostly-spent first stage of the Falcon 9 rocket on an automated platform ship in the Atlantic Ocean. If successful, it will be a monumental step towards reducing the cost of Earth orbit missions.
SpaceX has proven the concept in a few ways. On a small scale, they've developed and launched the Falcon 9 Reusable Development Vehicle, or "Grasshopper" prototype, which they've launched several times from New Mexico and Texas test sites to altitudes of several hundred meters, testing its ability to guide itself back to the ground under engine power safely.
Operationally, SpaceX has taken the opportunity to "land" the first stage of the Falcon 9 during satellite launches and previous CRS missions on the waters of the Atlantic ocean. They were generally successful in achieving the proper velocity and orientation at touchdown, though of course the waters quickly tipped the rocket over.
This is a challenge of a different magnitude. Unlike the Grasshopper, the Falcon 9 first stage, when it separates from the launch vehicle, will be 80 kilometers high and traveling at about 3.4 kilometers per second. And unlike previous soft water landings during orbital launches, the target for this stage is not "anywhere in the ocean" but on this very specific 300 foot by 175 foot target:
As I mentioned in the post 2015 In Spaceflight, hitting that size of a target from an 80 kilometer starting altitude is roughly equivalent to dropping a basketball into a hoop from the top of the second-tallest building in New York City. The feat will be attempted using some careful engine burns, and engineering savvy.
The first stage separates from the vehicle with around 15% of its propellant remaining, reserved for using the engine to achieve the deceleration and landing. The engines are re-lit, and first perform a minor burn that adjusts the trajectory of the stage to the desired landing point. The stage then orients itself against the direction of its flight, and fires the engines to slow itself to a manageable velocity for atmospheric maneuvering. The stage then deploys these:
They are called grid fins, a technology that first found use on missile systems from the Soviet Union in the 1970s and 1980s. Traditional missiles and rockets use planar fins, wedges of solid metal that are aligned parallel to the airflow over the rocket, like this:
As the vehicle flies, these fins, or hinged surfaces on the fins, will rotate to re-direct the air passing over it, and alter the vehicle's trajectory, in the same way that the elevators or ailerons on an airplane can be used to steer it. But for vehicles that are traveling faster than the speed of sound, as the Falcon 9 first stage will be for much of its descent, the force of the extremely fast-moving air around the vehicle means it would take a lot more power to rotate the fins and re-direct the airflow -- larger hinges, more powerful actuators, and in general more mass. Grid fins solve this by sitting perpendicular to the airflow, and allowing most of the air to pass through them unperturbed. When the vehicle needs to be steered, they can rotate, causing more of the interior surfaces of the grid fin to impact the air and re-direct it. Each of the Falcon 9's grid fins can rotate independently of one another for precise steering in the atmosphere.
As the vehicle slows and steers itself through the air, the barge (SpaceX is calling it an "Autonomous Spaceport Drone Ship") will be holding itself at a fixed position in the Atlantic Ocean, using powerful thrusters that can fight the push and pull of ocean currents. The deck of the ship is stabilized to counter the rocking motion of waves and provide the rocket with a level, motionless platform to land on. The rocket re-lights an engine for one final burn, which reduces its rate of descent to about 2 meters per second, sufficiently low to safely touch down on the drone ship . Since most of the mass of the almost entirely empty rocket stage is in the engines and landing legs at the bottom, the center of mass should be low enough that the rocket is stable on the platform, and won't tip over.
At least, that's the plan. SpaceX is understandably trying to manage expectations for this first attempt. Elon Musk predicted a 50% chance of success, but later admitted in a Reddit AMA that he made the figure up.
Right now, it costs an estimated $435 million for one launch of the Delta IV Heavy, which can lift 28.8 tons into orbit -- that's about $6,851 per pound. A Falcon 9 launch, which can put around 13 tons into orbit, costs $61.2 million, or about $2,135 per pound. The goal of landing and reusing first stages is to drive this cost down even further; Musk recently speculated that a reusable configuration of the planned Falcon Heavy could drive costs to under $100 per pound. Cheaper access to space means more opportunities for space exploration, crewed and uncrewed. It also bolsters the feasibility of accomplishing a large mission using multiple launches and on-orbit assembly.
T-0 is at 4:47 AM tomorrow. The first stage will burn out and separate about two and a half minutes into the flight, and begin its landing attempt. The webcast will follow the second stage as it continues to accelerate to orbit for rendezvous with the ISS, which, after all, is the primary mission. But we may hear call outs on the mission control loop of important milestones in the first stage return flight. If the landing attempt is successful, pictures or video may be available some time afterwards. The webcast will be viewable here, and keep an eye on Twitter (#spX5 will likely be a good hashtag) for landing developments.
Image credits: Cover - SpaceX; Autonomous Spaceport Drone Ship at sea - Elon Musk via Twitter; Falcon 9 grid fins in deployment - Elon Musk via Twitter; Conventional missile fins - Outside The Square Gallery.