SpaceX’s latest attempt to land a first stage rocket on a barge at sea took place on Tuesday evening following a perfect launch for the CRS-6 mission, delivering supplied to the International Space Station on behalf of NASA.
Just under three minutes after launch the Falcon 9’s first and second stages separated, and the first stage performed a successful controlled descent before it attempted a precision landing on what SpaceX are terming an ‘autonomous spaceport drone ship’ – named ‘Just Read the Instructions’. Unfortunately the landing wasn’t exactly successful.
The stage made it to the drone ship and landed, but excess lateral velocity caused it to tip over.
A Vine clip released by SpaceX, followed by some HD video footage, tells the story. The rocket comes in on target and landing legs deploy. But it’s clear that the rocket was travelling fast – by at least one estimate at around 35.8 m/s or 86mph (Wired). While descent speed would have been slowed considerably just before touchdown, it may have contributed to the problems that subsequently arose. You can see that the rocket only establishes a vertical alignment moments before touchdown; as a result there’s a horizontal movement of the lower portion of the rocket just as it reaches the platform – what Musk noted as adding unwelcome ‘lateral velocity’. Upper nitrogen thrusters engage – working to keep the rocket vertical – but it’s not enough. That lateral motion at the rocket’s base results in tilting in the opposite direction at the top, which ultimately leads to the rocket slowly collapsing over to one side. Explosion follows duly.
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So what can be taken from this event. Firstly, it should be re-itterated that this effort is only the second attempt in an ongoing endeavour to land and recover a first stage rocket. It’s also fair to say that this was an great improvement over the first attempt in February. That attempt ended somewhat more drastically after the lattice fins – which are crucial in guiding final moments of controlled descent – failed due to a loss of hydraulic fluid powering them. The hard collision and immediate explosion seen back then was considerably worse what we see happening on Tuesday.
To SpaceX’s credit, on both occasions the first stages successfully navigated their way to Just Read the Instructions. Reaching such a small ship, in the middle of the ocean, in itself represents a significant accomplishment. What the videos fail to give an impression of is the complexity and astonishing engineering feat that takes place after separation of the rocket stages. Re-igniting its engines, and re-orienting itself into a trajectory fit for navigation and landing from a speed of some 3000mph is ground-breaking.
To consider relative success of that feat and the failures that followed we need some context – for that we can look toward the profile of the mission.
Profile of Launch, Separation & Landing
SpaceX have always been a remarkably open company in many ways. They maintain an engaging presence on social media, live-stream their launches, and publish profiles of all their activities. This is especially the case with respect to their mission to recover and re-use rockets, where they’ve taken steps to bring the public into the fold on how this is being done, and the significance of rapid reusability in space flight (SpaceX). And of course there is Elon Musk himself, founder and CEO of SpaceX – his Twitter feed offers straight-forward insights on the latest news of SpaceX activities (and of Tesla and SolarCity for that matter), often with a characteristic sense of wry humour which, ironically enough, is exemplified even in the face of his $60 million rockets exploding.
Beneath is a schematic that SpaceX published recently. It illustrates the various stages of their commercial missions. Notably though it depicts something no other rocket companies in the world are attempting – controlled descent and landing of a first stage rocket.
Shortly after launch, at an altitude of 50 miles, the main engine cuts off just prior to separation of first and second stages. While the second stage ignites its engine, and continues onward with its payload stored in the Dragon spacecraft, the first stage coasts to an altitude of 100 miles or so.
The first phase of the recovery then occurs with the so-called boostback burn – when three of the nine Merlin 1D engines on the first stage re-ignite. The rocket has an on-board guidance-control system, which orients (or ‘gimbals’) the Merlin engines to rotate the rocket to between 120 and 180 degrees, an angle it maintains through its descent trajectory toward the drone ship. During this phase the rocket is traveling at around 3000 mph.
Next is the supersonic retro-propulsion burn. The center engine ignites to slow the descent further still, and gimbals itself to help the rocket attain a fully vertical position. Four lattice fins extend to stabilise and further decelerate the rocket – each moving independently to control roll, pitch, and yaw. These actions drastically lower the rocket’s speed, from 3000 mph to about 500 mph.
Finally, as the rocket reaches the landing pad, there’s the landing burn. The engines initiate this final burn to slow the craft to around 5 mph, while four carbon-fiber / aluminium landing legs unfold for touchdown.
All things considered it seems as though the controlled descent (and boostback burn) performed as intended once again. Aside from successful navigation; the final approach was considerably improved over February’s attempt when the rocket collided hard after coming in fast at a sheer angle. Modifications made to the hydraulic fuel capacities of the first stage seem to have resolved the root problem of the previous attempt’s failure. Indeed, apart from fateful lateral motion, the rocket was nearly vertical and looked to be steady before momentum took it’s toll.
The precise cause for Tuesday’s failure remains to be understood. Perhaps the supersonic retro propulsion burn didn’t slow the rocket enough, and it really was coming in too fast for gimbaling and thrusters to compensate adequately. Perhaps it was the final landing burn, or problems relating to gimbal manoeuvres, or that there wasn’t enough power in the nitrogen thrusters, any of which could reduce the stability of the rocket in its final seconds before touchdown.
In any event, we can count on SpaceX to release some information that will shed light on what went wrong. Whatever it was, understanding precisely what happened will be the first step to fixing any problem – something that you can count on SpaceX to do. Whether it’s software tuning, or more substantial modifications to the structure, engines or other features of the first stage, that are required, we’ll have to wait to find out.
But It Looked So Easy!?
Maybe it crossed your mind that earlier video released by SpaceX (and published here too) covering their reusability test programme sort of made things look easy. Well, if it did it’s probably because, relatively speaking, that was easier.
The test programme began with ‘Grasshopper’ – a 10-story Vertical Takeoff Vertical Landing (VTVL) vehicle consisting of a Falcon 9 first stage, but with a single Merlin 1D engine. It landed from altitudes of up to 744 meters. Then there was the F9R development test vehicle – a more advanced prototype of a recoverable first stage. It landed after controlled descents from 1000 meters.
Critically though, these tests were fundamentally different from the February and latest attempt at soft-landing. For a start, neither Grasshopper nor F9R were commercial launches, involving descent from outside of Earth’s atmosphere. Consequently, there were differences in fuel load and in the mass of the vehicles – something has a large bearing on the physics of stabilising the descent. It’s also been speculated that the testing vehicles had additional ballast weight to aid stability – something the actual first stage rocket does not have. On top of this, the test vehicles weren’t travelling anything near as fast on their way down as the first stage is after a commercial launch.
So while the reusability test programme certainly laid foundations, especially for particular techniques necessary for soft-landings (for instance engine throttling, gimbaling and aspects of precision landing) it was never intended as a completely analogous testing platform. For that, there really is only way. That’s to try, try and try again. So here’s hoping for success the third time around.
So, the prize of recovering a first stage rocket remains elusive. Still, no one ever said it was going to be easy. SpaceX’s next landing and recovery attempt will occur during the CRS-7 mission, currently scheduled for June 2015.
The Primary Mission Objective
What of the primary mission? Yes, well that was payload delivery to the International Space Station (ISS), and it was the sixth such Commercial Resupply Services (CRS) mission SpaceX have undertaken on behalf of NASA. The mission saw the Dragon spacecraft loaded with some 1950kg (4,300 pounds) of supplies on route for the ISS.
The Dragon spacecraft is currently over half way through its two-and-a-half day journey to reach the ISS. Arrival is anticipated on Friday, April 17 at approximately 7:00am EDT. At that time the Dragon spacecraft will be secured using the station’s robotic arm. Full berthing and hatch opening are scheduled for Saturday. A custom built Italian espresso machine is included in the payload manifest. It’s called ‘ISSpresso’… clever.
If you haven’t already, be sure to check out the previous Phlebas article considering SpaceX’s endeavours for a multi-use rocket: Successes and Delays: SpaceX’s Pursuit of Re-useable Rockets. The article outlines technical features of the Falcon 9 rocket, highlighting those that have been developed to allow for controlled descent and landing, and takes a look at the programme of research that has gotten SpaceX to where it now is, including analysis of the February attempt at soft-landing.
This article will be updated once new information is released about events of Tuesday.
A video of the attempted soft-landing recorded using a GoPro camera fixed to Just Read the Instructions has been released. Seen from this close up, it’s almost painful how close it seems the rocket was to success (17 April 2015).
On 19 April, Musk tweeted that the “Cause of hard rocket landing confirmed as due to slower than expected throttle valve response. Next attempt in 2 months”. That attempt will come after another resupply mission to the ISS, scheduled for June 19. Just like the most recent resupply, ISS’ position in low Earth orbit means there will be enough fuel to try another first stage landing attempt (5 May 2015).