As SpaceX’s second Dragon cargo flight to the International Space Station – the first under its $1.6 billion Commercial Resupply Services (CRS) contract with NASA – closes in on a rendezvous and berthing at the outpost tomorrow morning, further details have arisen over an ‘engine-out’ anomaly just 80 seconds into Sunday night’s ascent. According to SpaceX, one of the nine Merlin-1C engines on the Falcon 9 rocket’s first stage experienced a sudden loss of pressure and a shutdown command was automatically issued. During this process, panels designed to relieve engine pressure were jettisoned to protect the first stage and the other eight engines burned an additional 30 seconds to compensate for the reduced thrust. Although Dragon was inserted into its proper orbit, the fate of the small Orbcomm ‘piggyback’ satellite remains unclear. The incident raised questions about the reliability of the Falcon 9 on only its fourth flight, but also served to vindicate SpaceX’s assertion that the rocket can recover from as many as two engine ‘flame-outs’.
Seven minutes ahead of launch, the Dragon craft in the Falcon’s nose transitioned to its internal power supplies, followed, two minutes later, by the booster itself. As personnel within the SpaceX control room watched pensively – none more so than CEO Elon Musk himself – the final ‘Go’ for launch was received at 8:33 pm EDT. All of the Falcon’s propellant tanks were verified to be at their correct flight pressures by the T-30-second point and the 227-foot-tall vehicle roared spectacularly into a darkened Florida sky at 8:35 pm. The nine first-stage Merlin-1Cs, producing a combined 1.1 million pounds of thrust, provided the impulse for the first three minutes of the climb to orbit. It was during this initial boost that the ‘Engine 1 Anomaly’ appeared.
“Falcon 9 detected an anomaly on one of the nine engines and shut it down,” Musk noted after the launch. “As designed, the flight computer then recomputed a new ascent profile in real time to reach the target orbit, which is why the burn times were a bit longer.” He added that Falcon is the only rocket currently in service which can endure an engine ‘flame-out’ and still successfully complete its assigned mission. The Merlin-1Cs on the first stage – each capable of 125,000 pounds of thrust at sea level – are powered by a mixture of rocket-grade kerosene (RP-1) and liquid oxygen. No previous anomalies of this nature have been experienced on the vehicle’s previous missions in June 2010, December 2010 or the triumphant first Dragon cargo flight to the station last May.
The flame-out caused some consternation when launch video revealed the highly-visible ‘incident’ and speculation arose that an explosion might have occurred in the aft compartment of the Falcon. However, SpaceX were quick to verify that Engine 1 “did not explode, because we continued to receive data from it”. Engineering analyses are currently ongoing, although speculation has centred on the possible fracture-related failure of the engine’s fuel dome. It would appear that debris seen falling from the rocket into the exhaust plume was related to the jettisoned pressure-relief panels.
Its secondary passenger, the Orbcomm, appears not to have been quite so lucky. Original plans called for the Falcon’s second stage to execute a short second burn of its single Merlin-1C engine to raise its orbit, ahead of deploying the Orbcomm, about 62 minutes after launch. This burn, however, was dependent upon the stage being sufficiently healthy and a propellant mass check shortly after the first engine cutoff failed to pass the requirements to ensure a safe injection of Orbcomm into its correct orbit. Consequently, no second engine burn was conducted and the small satellite was deployed into an insertion orbit of 125 x 200 miles, far lower than the planned 220 x 470 miles. SpaceX President Gwynne Shotwell initially told journalists at Sunday’s first post-launch conference that the Orbcomm had been successfully released, but it later became clear that the insertion orbit is “unworkable”.
Although the 313-pound Orbcomm is alive and functional, and presently occupies a low elliptical orbit, its planned five-year lifespan is expected to be seriously impaired. Built by Sierra Nevada Corporation, the satellite is the first in a new second-generation class of Orbcomms, officially designated ‘O2G’, to be launched. It is intended to serve as a prototype testbed for future missions and, indeed, two further satellites in the series are currently scheduled to ride another Falcon 9 into space next year. Both Orbcomm and Sierra Nevada Corporation have established contact with the satellite and are presently working to determine if the orbit can be raised with its small on-board propulsion system.
As for Dragon itself, a successful insertion into a preliminary orbit of 122 x 203 miles was confirmed at 8:45 pm EDT. Two minutes later, the cargo ship parted company from the second stage of the Falcon 9 and at 8:51 pm its twin solar arrays were deployed, followed later by the opening of the critical Guidance, Navigation and Control (GNC) bay door. Aboard the Dragon are over 1,000 pounds of usable items, including equipment, experiments and supplies for the International Space Station’s Expedition 33 crew – currently at its three-member level of Commander Sunita Williams and Flight Engineers Yuri Malenchenko and Aki Hoshide – and arrival in the vicinity of the multi-national research facility is scheduled in the early hours of Wednesday.
Grappling of the cargo ship by the Canadarm2 robotic arm will occur under the control of Hoshide and the Dragon will be berthed for almost three weeks at the Earth-facing (‘nadir’) port of the Harmony node. Measuring 19.3 feet long and 12 feet wide, the Dragon has the capability to ferry pressurised and unpressurised cargo to the station. Although no unpressurised payloads are being carried on this flight, subsequent operational missions under the CRS contract – which requires SpaceX to fly 12 cargo deliveries and truck 44,000 pounds of research and other supplies by 2015 – will exploit this capability. Assuming that tomorrow’s berthing goes well, Dragon is expected to return to a parachute-assisted splashdown in the waters of the Pacific Ocean, off southern California, on 28 October.
Despite the mixed results of the launch, it must be borne in mind that the accomplishments of the Falcon 9 and the Dragon are truly astonishing. They have restored the United States’ capacity to launch its own cargo-delivery missions, from its own soil, for the first time since last year’s retirement of the Space Shuttle fleet. When Dragon returns to Earth in three weeks’ time, it will bring back over 1,670 pounds of cargo, including frozen samples from space station experiments. Moreover, the Falcon 9 rocket performed exactly as it was designed to do in the event of an off-nominal ‘engine-out’ scenario.
After all, this is rocket science.
Or as the charismatic Elon Musk put it: Rocket science is “super-frickin’ damn hard.”