How and Why SpaceX Lands Its Rockets
25 July 2018
A jumbo jet costs about the same as one Falcon 9 rocket, but airlines don't junk a plane after a one-way trip from LA to New York. Yet when it comes to space travel, rockets fly only once, even though the rocket itself represents the majority of launch cost.
The Space Shuttle was technically reusable, but its giant fuel tank was discarded after each launch, and its side boosters parachuted into corrosive salt water every flight, beginning a long and involved process of retrieval and reprocessing.
Historically, most rockets have needed to use all of their available fuel in order to get their payload into space. SpaceX rockets were built from the beginning with reusability in mind, they have enough built-in fuel margin to deliver a Dragon to the space station and return the first-stage to Earth. That extra fuel is needed to reignite the engines a few times to slow the rocket down and ultimately land the first stage after it has sent the spacecraft on its way.
In addition to extra fuel, SpaceX has added a few critical features to their Falcon 9 first stage for reusability’s sake. The rocket has small, foldable heat-resistant wings called grid fins needed for steering the first-stage as it plummets from the edge of space through Earth’s atmosphere, cold-gas thrusters on the top of the first-stage that are used to flip the rocket around as it begins its journey back to Earth, and strong but lightweight carbon fiber landing legs that deploy as it approaches touchdown. All of these systems, while built and programmed by humans, are totally automated once the rocket is launched—and are reacting and adjusting their behavior based on incoming, real-time data.
Launching a Falcon 9 rocket into space and then bringing back the first stage booster to Earth in a vertical landing is incredibly hard. The whole process takes about 9 miutes, and it all starts off with a blast.
A SpaceX mission is comprised of several different steps, but while the whole “launch” and “flight” parts are pretty conventional, its after the separation of the first and second stage rocket boosters where the fun of the landing process begins. Here are the steps behind a Falcon 9 booster landing.
After the Falcon 9 leaves the atmosphere, the first stage will separate from the second stage. This happens at least 50 miles high, but can happen much later too if the mission payload is being delivered to a higher orbit. The higher the orbit, the harder the landing process becomes. Although the first stage is separated, it doesn’t immediately begin to descend, coasting for another 50 miles or so as it slows down.
While it’s coasting, cold gas thrusters are turned on the flip the first stage 180 degrees, so that it descends with the bottom facing the Earth.
3. Boostback Burn
Three of the nine booster engines turn on to begin guiding the rocket back down to Earth towards the landing site or one of the company’s two ocean droneships. It’s traveling at about 3,000 mph at this point.
4. Entry Burn + Grid Fins Deploy
The supersonic retropulsion burn begins with the center engine turning on, as the booster starts to enter the Earth’s atmosphere once again. The grid fins come out to stabilize the rocket and slow it down as it makes its descent. The rocket starts to become completely vertical, and finagle itself into a more elegant fall back to the surface of the planet. It starts to slow down to about 560 mph.
5. Final Burn
One last engine burn ignites as the rocket slows to a crawl of about 5 mph. Four legs made of carbon fiber and aluminum unfold to allow the rocket some stability upon touchdown, augmented by the ejection of compressed helium.
The Falcon 9 first stage booster makes it back down to Earth. Engineers come to rocket (either onboard the droneship or sitting at a solid landing site) and secure it. The whole thing takes from launch to landing, takes about nine minutes.
Falcon 9 is a family of two-stage-to-orbit medium lift launch vehicles, named for its use of nine Merlin first-stage engines, designed and manufactured by SpaceX. Variants include the initial v1.0 (expendable), v1.1 (partially-reusable), and current "Full Thrust" v1.2 (partially-reusable). Falcon 9 is powered by rocket engines utilizing liquid oxygen (LOX) and rocket-grade kerosene (RP-1) propellants.
The current "Full Thrust" version can lift payloads of up to 22,800 kilograms (50,300 lb) to low Earth orbit, and up to 8,300 kg (18,300 lb) to geostationary transfer orbit (GTO), when flying in expendable mode.
The first stage can be recovered and reused for GTO payloads up to 5,500 kg (12,100 lb) automatically landing after disconnection of the second stage. The Falcon 9 has been subsequently upgraded and uprated; on July 22, 2018 a Falcon 9 launched the Telstar 19V 7,075 kg (15,598 lb) satellite into a sub-synchronous transfer orbit, also called a GTO- orbit.
In 2008, SpaceX won a Commercial Resupply Services (CRS) contract in NASA's Commercial Orbital Transportation Services (COTS) program to deliver cargo to the International Space Station (ISS) using the Falcon 9 and Dragon capsule. The first mission under this contract launched in October 2012.
SpaceX intends to certify the Falcon 9 to be human-rated for transporting NASA astronauts to the ISS as part of the Commercial Crew Development program.
The initial Falcon 9 version 1.0 flew five times from June 2010 to March 2013; version 1.1 flew fifteen times from September 2013 to January 2016. The "Full Thrust" version has been in service since December 2015, with several additional upgrades within this version. The latest version, Block 5, introduced in May 2018, featured increased engine thrust, improved landing legs, and other minor improvements to help recovery and reuse.
The Falcon Heavy derivative, introduced February 2018, groups three Falcon 9 first stages together side by side.
SpaceX had predicted that its launches would have high reliability based on the philosophy that "through simplicity, reliability and low cost can go hand-in-hand" by 2011. As of 22 July 2018 Falcon 9 has achieved 56 out of 58 primary missions, with one rocket destroyed in flight and one on the launch pad during fueling for an engine test, yielding a success rate of 96.6%. For comparison, present industry benchmark, the Russian Soyuz series has more than 1,700 launches with a success rate of 97.4%.
As with the company's smaller Falcon 1 vehicle, Falcon 9's launch sequence includes a hold-down feature that allows full engine ignition and systems check before liftoff. After first-stage engine start, the launcher is held down and not released for flight until all propulsion and vehicle systems are confirmed to be operating normally. Similar hold-down systems have been used on other launch vehicles such as the Saturn V and Space Shuttle. An automatic safe shut-down and unloading of propellant occurs if any abnormal conditions are detected. Prior to the launch date, SpaceX always completes a test of the Falcon 9 culminating in a firing of the first stage's Merlin 1D engines for three-and-a-half seconds to verify performance.
Falcon 9 has triple redundant flight computers and inertial navigation, with a GPS overlay for additional orbit insertion accuracy.
SpaceX intended to recover the first stages of several early Falcon flights to assist engineers in designing for future reusability. They were equipped with parachutes but failed to survive the aerodynamic stress and heating during atmospheric re-entry following stage separation.
Although reusability of the second stage is more difficult, SpaceX intended from the beginning to make both stages of the Falcon 9 reusable. Both stages in the early launches were covered with a layer of ablative cork and had parachutes to land them gently in the sea. The stages were also marinized by salt-water corrosion resistant material, anodizing and paying attention to galvanic corrosion.
Musk said that if the vehicle does not become reusable, "I will consider us to have failed."
In late 2011, SpaceX announced a change in the approach, eliminating the parachutes and going with a propulsively-powered-descent approach. Included was a video said to be an approximation depicting the first stage returning tail-first for a powered descent and the second stage, with heat shield, reentering head first before rotating for a powered descent. Design was complete on the system for "bringing the rocket back to launchpad using only thrusters" by February 2012.
A reusable first stage was then flight-tested by SpaceX with the suborbital Grasshopper rocket. Between 2012 and 2013, this low-altitude, low-speed demonstration test vehicle made eight VTVL test flights, including a 79-second round-trip flight to an altitude of 744 m (2,441 ft). In March 2013, SpaceX announced that, beginning with the first flight of the Falcon 9 v1.1 (the sixth flight overall of Falcon 9), every first stage would be instrumented and equipped as a controlled descent test vehicle. SpaceX continued their propulsive-return over-water tests, saying they "will continue doing such tests until they can do a return to the launch site and a powered landing. ... [SpaceX] expect several failures before they 'learn how to do it right.'"
At the time of the rocket's maiden flight in 2010, the price of a Falcon 9 v1.0 launch was listed from $49.9 to $56 million.
By 2012, the listed price range had increased to $54–$59.5 million. In August 2013, the initial list price for a Falcon 9 v1.1 was $56.5 million; it was raised to $61.2 million by June 2014. Since May 2016, the standard price for a Falcon 9 Full Thrust mission (allowing booster recovery) is published as $62 million.
Dragon cargo missions to the ISS have an average cost of $133 million under a fixed price contract with NASA, including the cost of the capsule. The DSCOVR mission, also launched with Falcon 9 for NOAA, cost $97 million.
In 2004, Elon Musk stated, "long term plans call for development of a heavy lift product and even a super-heavy, if there is customer demand. Ultimately, I believe $500 per pound ($1100/kg) [of payload delivered to orbit] or less is very achievable." At its 2016 launch price and at full LEO payload capacity, a Falcon 9 FT launch costs just over $2,700 per kilogram ($1,200/lb) for the expendable version.
In 2011, Musk estimated that fuel and oxidizer for the Falcon 9 v1.0 rocket cost a total of about $200,000. The first stage uses 245,620 L (64,885 US gal) of liquid oxygen and 146,020 L (38,575 US gal) of RP-1 fuel, while the second stage uses 28,000 L (7,300 US gal) of liquid oxygen and 17,000 L (4,600 US gal) of RP-1.
By 2018, the Falcon 9's decreased launch costs has led to competitors developing new rockets. Arianespace is working on Ariane 6, ULA on Vulcan, and ILS on Proton Medium.
SECONDARY PAYLOAD SERVICES
Falcon 9 payload services include secondary and tertiary payload connection via an EELV Secondary Payload Adapter ring, the same interstage adapter first used for launching secondary payloads on US DoD missions that use the Evolved Expendable Launch Vehicles (EELV) Atlas V and Delta IV. This enables secondary and even tertiary missions with minimal impact to the original mission. In 2011, SpaceX announced pricing for ESPA-compatible payloads on the Falcon 9.
|Written by: Luis Clement|