SpaceX and Orbcomm are pressing ahead with plans to launch eleven OG-2 satellites tonight from Cape Canaveral AFS after deciding to call off Sunday’s launch attempt; a move that was made after determining, “an additional day prior to launch will allow for more analysis and time to further chill the Falcon-9 rocket’s liquid oxygen in preparation for launch” said Orbcomm. SpaceX CEO Elon Musk took to Twitter as well, adding, “Just reviewed mission params w/ SpaceX team. Monte Carlo runs show tomorrow night has a 10% higher chance of a good landing. Punting 24 hours.”
SpaceX is now targeting a window of 8:29 p.m. – 8:34 p.m. EST for launch, with a landing at the Cape expected 10 minutes later. If SpaceX cannot launch tonight, they will not be able to try again until no earlier than (NET) Dec. 28.
The decision was made after further review of static test fire data from a couple days before, when the Falcon-9 underwent a dress rehearsal / practice countdown at its launch pad and ignited its nine Merlin 1D+ engines for a short duration. The Static Test Fire was completed Dec. 18, after 48 hours of frustrating delays caused by a number of technical issues since the Falcon-9 stack was initially raised vertical last Wednesday.
As outlined in a previous AmericaSpace article, this mission marks the maiden voyage of the “Full Thrust” (FT) variant of the rocket, also known as the “Upgraded Falcon 9”, whose nine Merlin 1D+ first-stage engines are expected to deliver a 33-percent performance gain over their predecessors. This will be achieved by running the engines at 100-percent rated thrust, as opposed to the approximately 80-percent thrust previously used, together with structural enhancements—such as increased tank volumes, a lengthened second stage and an improved “Octaweb” support structure for the first-stage powerplants—and the “densification” of the liquid oxygen load (which SpaceX CEO Elon Musk described as “deep cryo oxygen”) to meet an extra 13 percent of performance.
“SpaceX had a good run through of pad operations,” Orbcomm noted on Thursday, 17 December, “and is looking to static fire Falcon-9 this afternoon.” By mid-afternoon Thursday, SpaceX CEO Elon Musk tweeted on RTF progress: “Working towards static fire. Deep cryo liquid oxygen presenting some challenges.”
As of Friday morning, Orbcomm explained, the vehicle remained vertical on the pad, as teams continued “working towards a static fire today” to verify its readiness “and aim towards launching a few days after the static fire is completed”. The prospect of a successful Static Test Fire seemed to brighten, with Musk noting in the early afternoon that SpaceX came within 5 minutes of conducting the test, only to suffer an abort, “triggered by flight computer on upper stage throttle valve”.
He added that SpaceX engineers were “adjusting thresholds and restarting count at T-10 minutes”. A second attempt on Friday came within 2 minutes of ignition, when it too was frustrated “due to slow ground side valve”. The ignition sequence was adjusted, Musk explained, by 0.6 seconds for a third attempt.
That third attempt appeared to go well. “Static fire test looks good,” Musk tweeted at 7:09 p.m. EST. “Pending data review, will aim to launch Sunday.”
In the final hours prior to launch, Falcon-9 will be fueled with liquid oxygen and a highly refined form of rocket-grade kerosene, known as “RP-1.” The cryogenic nature of the oxygen—whose liquid state exists within a range from -221.54 degrees Celsius (-368.77 degrees Fahrenheit) to -182.96 degrees Celsius (-297.33 degrees Fahrenheit)—requires the fuel lines of the engines to be chilled, in order to avoid thermally shocking and potentially fracturing them.
However, Musk noted, “-340 F in this case. Deep cryo increases density and amplifies rocket performance. First time anyone has gone this low for O2,” when addressing followers on his Twitter account.
All propellants should be fully loaded within one hour, and the countdown will reach its final “Go/No-Go” polling point of all stations at T-13 minutes. Assuming it passes through the poll of flight controllers, the Terminal Countdown will get underway at T-10 minutes.
During this phase, the Merlin 1D+ engines will be chilled, ahead of their ignition sequence. All external power utilities from the Ground Support Equipment (GSE) will be disconnected and the roughly 90-second process of retracting the “strongback” from the vehicle will get underway. The Flight Termination System (FTS)—which is tasked with destroying the Falcon 9 in the event of a major accident during ascent—will be placed onto internal power and armed. By T-2 minutes and 15 seconds, the first stage’s propellant tanks will attain flight pressure. The Merlin 1D+s will be purged with gaseous nitrogen, and, at T-60 seconds, the SLC-40 complex’s “Niagara” deluge system of 53 nozzles will be activated, flooding the pad surface and flame trench with 30,000 gallons (113,500 liters) of water per minute to suppress acoustic energy radiating from the engine exhausts.
At T-3 seconds, the nine Merlins will roar to life. Following computer-commanded health checks, the stack will be released from SLC-40. Immediately after clearing the tower, the booster will execute a combined pitch, roll, and yaw program maneuver, which is designed to establish it onto the proper flight azimuth to inject OG-2 into orbit. Eighty seconds into the uphill climb, the vehicle will exceed the speed of sound and experience a period of maximum aerodynamic duress—colloquially dubbed “Max Q”—on its airframe. At about this time, the Merlin 1D+ Vacuum engine of the second stage will undergo a chill-down protocol, ahead of its own ignition later in the ascent. Two of the first-stage engines will throttle back, under computer command, in order to reduce the rate of acceleration at the point of Main Engine Cutoff (MECO).
Finally, at T+2 minutes and 58 seconds, the seven remaining engines will shut down and, a few seconds later, the first stage will separate from the rapidly ascending stack, at which point the booster will go into landing mode, and the turn will then come for the restartable second stage, whose Merlin 1D+ Vacuum engine will come to life to continue boosting the OG-2 stack into orbit.
At this point SpaceX will attempt to land their booster’s first stage back at Cape Canaveral. SpaceX has attempted to land a first stage Falcon-9 on the company’s Autonomous Spaceport Drone Ship (ASDS) offshore twice previously. Each time SpaceX hit the mark, but both times the booster tipped over. An attempt to land during the CRS-5 launch in January achieved partial success, reaching the deck, but impacted hard at a 45-degree angle and exploded, primarily due to a premature exhaustion of hydraulic fluid in its hypersonic grid fins. A second attempt was planned for the DSCOVR launch in February, but was called off due to rough sea conditions, and the most recent attempt during April’s CRS-6 launch reached the deck, but touched down with excessive lateral velocity and toppled over upon impact.
SpaceX has confirmed that, although the ASDS was offshore this week, it will NOT serve as an alternative landing pad. SpaceX is committed to LZ-1; the booster either falls in the ocean or lands at LZ-1.
The Falcon’s first stage will touch down at LZ-1 approximately 10 minutes after lift-off, and preliminary trajectory analysis from the “Environmental Assessment for SpaceX Vertical Landing of Falcon-9 and Construction at Launch Complex 13” indicates that a point directly beneath the vehicle at stage separation falls approximately 16 nautical miles from the launch site.
Stabilizing the 150-foot-tall rocket stage in flight—traveling at a velocity of 2,900 mph at separation—has been likened to someone balancing a rubber broomstick on their hand in the middle of a fierce wind storm. After first stage engine cutoff, exoatmospheric cold gas thrusters are triggered to flip the first stage into position for retrograde burn. Three of the rocket’s nine Merlin engines are then restarted to conduct the retrograde burn in order to reduce the velocity of the booster and to place it in the correct angle to land. Once the first stage is in position and approaching its landing target, two of the three engines will be shut down to end the boost-back burn.
Utilizing compressed helium to deploy its four extendable landing legs, the booster and a quartet of lattice-like hypersonic grid fins, configured in an “X-wing” layout, will then be unfurled to control the rocket’s lift vector, and a final single engine burn will slow the stage to a velocity of zero for a stable landing at LZ-1.
An approximately 200 foot by 200 foot concrete landing pad was designed to support the weight and thrust energy of the booster landing, and it is surrounded by an approximately 750 foot diameter of compressed soil and gravel. Four additional, 150 foot diameter concrete “contingency” pads serve as backups, and will only be utilized in order to enable the safe landing of a single vehicle should last-second navigation and landing diversion be required.
Although propellants are burned to depletion during flight, there is a potential for some LOX and RP-1 to remain in the Falcon first stage upon landing, as well as a small amount of ordnance, such as small explosive bolts and on-board batteries. It is very possible that the booster won’t land perfectly, as it is still a developing vehicle and these landing attempts are tests, so to be safe SpaceX will have an established emergency response team standing by to contain any unexpected spills (if, for example, the booster tips over upon landing and cracks open, as previous boosters toppled over on the ASDS).
Safing activities will begin upon completion of all landing activities and engine shutdown. The LOX oxidizer system will be purged, and any excess fuel will be drained into a suitable truck-mounted container or tanker. Any remaining pressurants (i.e., helium or nitrogen) will be vented, and any FTS explosives will also be rendered “inert” prior to declaring the vehicle safe. The vehicle will then be lifted and placed on to the stand, followed by the landing legs being removed or folded back into place. Falcon-9 will then be lowered into a horizontal position, placed on a transport vehicle, and taken to a SpaceX facility.
The current launch weather forecast predicts partly cloudy conditions, with an 80-percent probability of acceptable conditions expected for a launch and landing attempt tonight. Primary concerns are for cumulus cloud and thick cloud rule violations.
– Article Written by Ben Evans and Mike Killian.