Less than two days now remain before the long-awaited flight of SpaceX’s Falcon 9 v1.1, carrying the fifth dedicated Dragon cargo mission to the International Space Station (ISS). As outlined in yesterday’s AmericaSpace preview article, the CRS-5 flight is being executed under the language of the $1.6 billion Commercial Resupply Services contract, signed between NASA and SpaceX back in December 2008, and will involve the delivery of 3,700 pounds (1,680 kg) of experiments, technology demonstrations, and supplies for use by the incumbent Expedition 42 crew. However, the opening minutes of tomorrow’s mission will be monitored with particular closeness, as SpaceX makes its first effort to land the first stage of the Falcon 9 v1.1 on a floating platform in the Atlantic Ocean. The endeavor—which SpaceX admits carries only a marginal likelihood of success on this inaugural attempt—is a dramatic endorsement of SpaceX CEO Elon Musk’s pledge to make his launch vehicles partially or fully reusable in the coming years.
Known as the Autonomous Spaceport Drone Ship (ASDS), the vast, steel-hulled platform—whose appearance was revealed by Musk in a tweeted photograph from November 2014—is a Marmac 300 Freight Barge, recommissioned and specially modified for use by SpaceX. Built by Gulf Coast Fabrication in 1998 and now listed by McDonough Marine Services, the barge measures 288 feet (87.8 meters) in length, 100 feet (30.5 meters) in diameter, and 19.8 feet (6 meters) deep, and has a gross mass in excess of 8.8 million pounds (3.9 million kg). Despite being unanchored during Falcon 9 v1.1 recovery operations, the ASDS is reportedly capable of holding its oceanic position to within 10 feet (3 meters), “even in a storm,” and utilizes Differential Global Positioning System (GPS) hardware and four, diesel-powered azimuth thrusters, repurposed from oil rigs.
Yet the return of a Falcon 9 v1.1 first stage from the edge of space and ensuring it remains stable atop an oceanic platform until its return to dry land is entirely new territory. During operations, the ASDS will be stationed between 186-250 miles (300-400 km) to the northeast of the launch facilities at Cape Canaveral Air Force Station. “Depending on where we port,” SpaceX told AmericaSpace, “the drive takes half a week or so to get to the landing location.”
Supporting the emplacement of the ASDS for Tuesday morning’s launch are two vessels: the 82-foot-long (25-meter) Elsbeth III tug and the 164-foot-long (50-meter) Go Quest support boat. With a maximum speed of around 5.6 knots, the Elsbeth III is responsible for bringing the gigantic ASDS to its oceanic position, whilst the faster Go Quest—capable of up to 12.1 knots—takes up a support position, laden with communications and tracking equipment. As of Saturday afternoon, both vessels were recorded as being in the Port of Jacksonville, but the Elsbeth III departed at 8:46 p.m. EST and was followed by the Go Quest at 10:08 p.m. EST. No exact destination was recorded for the tug, but the Go Quest was reportedly headed for Blake Plateau, which lies between the continental shelf and deep ocean basin, off the southeastern coastal states of North Carolina, South Carolina, Georgia, and Florida.
In view of the nature of Dragon’s destination—the ISS—the launch of the Falcon 9 v1.1 will occur during an “instantaneous” window at 6:20:29 a.m. EST Tuesday, 6 January, almost an hour before local sunrise. According to data published by the 45th Space Wing at Patrick Air Force Base, mariners are required to avoid the proscribed Launch Hazard Area between 4:15-7:00 a.m. EST. However, present estimates predict a 40 percent chance that the Thick Cloud Rule may violate SpaceX’s opening launch attempt. “The next frontal boundary will move into the area by Monday, bringing another bout of upper-level clouds and increased rain chances,” it was noted. “With the boundary overhead, the primary weather concern will be thick clouds.” Since the instantaneous launch window offers no scope for the Falcon 9 v1.1 to fly beyond 6:20:29 a.m., if weather conditions are not satisfactory at T-0, Tuesday’s attempt will be scrubbed and the countdown clock recycled to track a backup opportunity at 5:09 a.m. EST Friday, 9 January.
Patrick Air Force Base meteorologists anticipate that the frontal boundary will move to the south of the Space Coast by Wednesday, with gusty winds and a slight risk of coastal showers by Friday, presenting Flight Through Precipitation as the main concern if the opening launch date is missed and the backup opportunity is taken. If SpaceX flies on Tuesday or Friday, it will be well before local sunrise at 7:16 a.m. EST, thereby marking CRS-5 as the third Dragon mission—after CRS-1 in October 2012 and CRS-4 in September 2014—to launch in the hours of darkness.
The Falcon 9 v1.1 will fly from Space Launch Complex (SLC)-40 at Cape Canaveral Air Force Station, Fla., with the nine Merlin 1D engines of its first stage powering it uphill for the first three minutes of ascent. After clearing the tower, the booster will execute a combined pitch, roll, and yaw program maneuver to establish it onto the proper flight azimuth to inject its Dragon payload into low-Earth orbit. Eighty seconds into the climb, the vehicle will exceed the speed of sound and experience a period of maximum aerodynamic duress—colloquially known as “Max Q”—on its airframe. At T+130 seconds, two of the Merlins will throttle back, under computer command, in order to reduce the rate of acceleration at the point of Main Engine Cutoff (MECO). Finally, two minutes and 58 seconds after departing the Cape, the seven remaining Merlins will shut down and the first stage will separate from the rapidly ascending stack.
As the second stage and the Dragon payload continue their voyage into orbit, the first stage will head for a powered touchdown atop the ASDS in the Atlantic Ocean. However, traveling at a velocity of 2,900 mph (4,670 km/h), the stabilization of the 150-foot-tall (46-meter) stage has been likened to someone balancing a rubber broomstick on their hand, in the middle of a fierce wind storm. Three Merlin engine firings will be executed in order to steadily reduce this velocity and stabilize the first stage: an initial “boost-back” burn will adjust the vehicle’s impact point, after which a “supersonic retro-propulsion” burn will slow it to about 560 mph (900 km/h) and a final “landing” burn will bring this down still further to just 4.5 mph (7.2 km/h). During the final burn, the first stage will deploy its four extendable landing legs and a quartet of lattice-like hypersonic grid fins—configured in an “X-wing” layout—will be unfurled to control the lift vector and, together with engine gimbaling, will enable a precise touchdown on the ASDS.
“Grid fins perform well in all velocity ranges, including supersonic and subsonic speeds, with the exception of the transonic regime, due to the shockwave enveloping the grid,” Spaceflight101 noted in June 2014. “These properties make them ideally suitable for the Falcon 9 first stage that starts out at supersonic speeds and returns to subsonic velocity as it travels through the atmosphere, en-route to the landing site.” With two degrees of freedom, the fins have the capacity to rotate and tilt, thereby enabling them to eliminate roll rates and maintaining control during flight.
Of course, in addition to the technical difficulty of bringing a rocket stage back from the edge of space to land on an unanchored barge, there remains the reality that SpaceX may need to deal with large ocean swells and GPS positioning errors. Unsurprisingly, Elon Musk has explained that he anticipates no higher than a 50 percent likelihood of the first stage making a successful landing on the ASDS on its first attempt. That said, the relatively low center-of-gravity of the first stage, coupled with the 60-foot (18-meter) spread of its landing legs, should make it fairly stable from tipping over, although it has been remarked that “sliding” across the barge’s deck remains an inherent risk.
However, if it works as advertised, this will mark the first occasion on which SpaceX has returned its Falcon 9 v1.1 flight hardware from the high atmosphere and landed it smoothly onto a solid surface. The company has performed several water splashdowns between September 2013 and September 2014, with mixed results. Equipped with four fold-out landing legs—made from carbon-fiber and aluminum honeycomb, weighing 4,400 pounds (2,000 kg), and deployed by means of a telescoping piston on an A-frame—the first stage made its initial attempt to return softly to water during the maiden voyage of the Falcon 9 v1.1 in September 2013. Unfortunately, it experienced an uncontrolled roll during descent, which overwhelmed the capabilities of its attitude-control system to compensate. This forced a planned final “burn” of the center Merlin 1D engine to be shortened, due to the centrifugal effect on propellant against the tank walls, which damaged the baffles and allowed for debris intrusion.
A second attempt during the ascent of the CRS-3 Dragon cargo mission in April 2014 met with greater success, experiencing no spin and zero vertical velocity, as planned. This marked the first successful controlled oceanic touchdown of liquid-fueled orbital booster, but due to rough seas the first stage was not recovered. SpaceX’s third attempt took place during the Orbcomm OG-2 ascent in July 2014, and was again successful, although the first stage’s hull integrity was breached when it toppled from vertical to horizontal in the Atlantic and, like its predecessor, it was not retrieved. Most recently, during the CRS-4 Dragon launch in September 2014, another successful splashdown was logged, but again no attempt was made to recover the first stage, as it was known that it would not survive the tip-over from a vertical to a horizontal orientation into the ocean.
Tuesday’s landing attempt follows on the heels of several years of Vertical-Takeoff-Vertical-Landing (VTVL) technology demonstrations by SpaceX, involving its Grasshopper and Falcon 9 Reusable Development Vehicle (F9R Dev). The 106-foot-tall (32-meter) Grasshopper—based upon the first stage of a first-generation Falcon 9 v1.0—flew on eight occasions from SpaceX’s test facility in McGregor, Texas, between September 2012 and October 2013. It remained airborne for durations of up to 79 seconds, performed lateral maneuvers of up to 330 feet (100 meters), evaluated new navigational sensors, and achieved a peak altitude of 2,440 feet (744 meters).
Upon its retirement, the 160-foot-tall (48.8-meter) F9R Dev was brought into service, based upon a qualification-testing first stage of the larger Falcon 9 v1.1, and on 17 April 2014 it performed its first flight, equipped with landing legs. Its initial two missions allowed the vehicle to demonstrate its hovering and lateral maneuvering capabilities, whilst a third flight on 17 June reached a peak altitude of 3,300 feet (1,000 meters) and exercised the same kind of steerable hypersonic grid fins which the CRS-5 Falcon will boast. Unfortunately, on 22 August 2014, on its fifth flight, the F9R Dev was lost when a blocked sensor caused a loss of control and the Flight Termination System (FTS) commanded the remote destruction of the vehicle. “Rockets are tricky” tweeted Elon Musk in the aftermath of the accident. A third test vehicle, the F9R Dev-2, is presently under construction and will initially fly low-altitude missions out of McGregor, before attempting high-altitude flights of up to 300,000 feet (91,000 meters) from Spaceport America, north of Las Cruces, N.M.