Fine-tuning Falcon 9 landing focuses on throttle valve response

by Chris Bergin

SpaceX is getting ever closer to successfully returning the first stage of its Falcon 9 v1.1, with the latest attempt showing the effort is now focused on fine-tuning. The return of the first stage used during the CRS-6 Dragon launch was the best landing attempt to date, with a slower than expected throttle valve response parameter cited as the main reason the stage failed to nail its touchdown.

Getting Closer:

The ongoing efforts to advance its reusability aspirations is not to the detriment of SpaceX’s primary mission objectives.

The CRS-6 Dragon was successfully lofted into orbit and has since arrived at the International Space Station (ISS). The “quick look” assessment of the Falcon 9 v1.1’s performance noted zero performance issues with the rocket as it pushed the latest Dragon uphill.

The ascent profile called for the first and second stages to part ways during staging, marked with the cut off of the Merlin 1D engines at two minutes and 39.31 seconds before separating 1.92 seconds later.

Staging usually results in everyone’s attention switching to the second stage and passenger as they sprint away into space, leaving the first stage to head towards its eventual demise. However, SpaceX aims to change the game, with ambitions to not only return the stages for a landing, but to eventually reuse them, thus reducing costs.

While the business case – and future technology applications, such as landing on Mars – drives SpaceX forward, the effort has also captured the public’s imagination.’s forum already has several million views for discussion threads relating to SpaceX reusability development while the CRS-6 mission broke the site record for the number of visitors during a launch.

2015-04-19-153901Testing the technology required to achieve that goal has been ongoing for a number of years, involving test rockets at its McGregor test site in Texas, through to incremental testing with the v1.1 version of its Falcon 9.

Numerous additions, from landing legs to grid fins, have helped refine the return of the stage via lessons learned from the initial “boost back” data gained during the early v1.1 missions.

The goal is to bring the stages back to landing pads near the launch sites. However, SpaceX is initially testing the technology via landings on the Autonomous Spaceport Drone Ship (ASDS) located off the Florida coast.

2015-04-03-172515Successfully landing on the ASDS will prove the stage’s return capability, before switching to landings on terra firma.

Ironically, the ASDS landings have been classed as more challenging, with numerous additional considerations to take into account, such as the stability of the ship in the ocean at the point of touchdown.

While the first stage involved with the CRS-5 mission came close to a landing, the CRS-6 first stage proved to be the best ASDS landing attempt to date.

2015-04-19-154115Per a landing attempt, numerous tasks have to be successfully conducted by the stage post-staging, the first of which was observed via the live coverage of the launch, as the stage used its Reaction Control System (RCS) thrusters to rotate the core to allow its engines to face the direction of travel.

In total, the stage was tasked with three burns to reduce its velocity and begin the dive towards the ASDS.

The first descent burn lasted 27.68 seconds, beginning at four minutes and 29.19 seconds after liftoff to reduce the distance the stage was travelling downrange.

Six minutes and 31.63 seconds after launch, grid fins on the first stage were deployed to stabilize its descent, with a second burn beginning 12.17 seconds later to slow the vehicle during reentry.

2015-04-19-151447The third and final burn took place as the rocket approached its landing target, beginning at eight minutes and 0.12 seconds mission elapsed time.

Almost twenty-four seconds into the burn the rocket deployed its landing legs with touchdown six seconds later; eight minutes, 30.01 seconds after liftoff.

As seen in released videos, the stage conducted all of its tasks successfully, as it closed in on the deck of the ASDS. However, the stage failed to nail its landing due to a “slower than expected throttle valve response,” per comments made by SpaceX CEO Elon Musk, resulting in a hard landing.

2015-04-19-151609While the landing was far more accurate and smoother than the CRS-5 attempt, the stage failed to land safely on all of its legs – despite a valiant attempt by the stage’s Attitude Control System (ACS) thrusters to correct the angle.

The stage eventually toppled over and exploded on the deck.

The ASDS, as designed, did not suffer serious damage and will be cleaned up and ready to set sail in June.

That next landing attempt will come via the CRS-7 mission to send another Dragon on a resupply mission to the ISS. While the chances of a successful landing, securing of the stage on deck via chains and arrival back in port will likely remain “50/50”, Mr. Musk believes SpaceX is getting closer.

2015-04-19-151659“A stable landing is no problem with proper throttle response,” he added, showing a video of a smooth landing conducted by the F-9R Dev-1 test vehicle that was housed the SpaceX’s McGregor test center.

A successful landing during the CRS-7 mission may also allow for the first projected attempt to return a stage to land, with the rocket tasked with lofting the Jason-3 satellite provisionally pencilled for a debute landing at SLC-4 at Vandenberg Air Force Base.

SpaceX also plans for a second ASDS on the West Coast and has confirmed the goal of landing stages on the East Coast, at Cape Canaveral’s SLC-13.

The first stage to safely return will be put through its paces at its test base at Spaceport America in New Mexico.

2015-04-19-151319Those tests will be used to find hardware limits, such as how many cycles can be put on a stage while the second successfully recovered booster would provide the role of qualification testing.

Should the recovery efforts progress, the first launch of a reused booster is likely to occur in late 2016.

Next up for SpaceX is the April 27 launch of TurkmenÄlem communications satellite. Due to the high-performance requirements of this mission, a landing attempt won’t be made with the first stage.

This launch was to be conducted ahead of the CRS-6 mission. However, issues with the helium pressurization bottles (COPVs) resulted in the decision to launch the ISS mission first.

The rocket set to loft the TurkmenAlem52E/MonacoSat 1 (TurkmenSat 1) mission uphill is already in SpaceX’s Cape hanger undergoing processing, with the key Static Fire test currently set to take place next week, with a NET (No Earlier Than) date of April 22.

SpaceX then plans to launch the Dragon 2 test vehicle on the Pad Abort Test per their Commercial Crew development milestones, before the CRS-7 – and next landing attempt – on June 19.

(Images: via L2’s SpaceX Section, including renderings created by L2 Artist Nathan Koga – Click here for full resolution F9, F9-R, FH and BFR renderings and more – these are not official SpaceX images. Other images from SpaceX)

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