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Reusable space rockets: how close are we?
In the 1960s, rocketry appeared simple: Thunderbird 3 tripped into space without the tiresome dropping of stages, returned intact to an island launch pad and refuelled for another mission. How close are we today to Gerry Anderson’s vision of the reusable rocket?
Successful rocket launches are often described as ‘controlled explosions’, in which tonnes of highly energetic propellant are brought together in combustion chambers to produce thrust.
Europe’s Ariane 5, for example, carries up to 200 tonnes of liquid propellant in its tanks and another 240 tonnes in its solid propellant boosters. That’s enough to lift 800 tonnes vertically off a launch pad (exceeding the top speed of a Bugatti Veyron within seven seconds) and deliver a 20-tonne space station supply vehicle to orbit a few minutes later. Anyone lucky enough to experience such an impressive defeat of the Earth’s gravitational pull would agree that the workhorses of the commercial space industry are spectacular in their operation.
The problem is that - as the term ‘expendable launch vehicle’ admits - every part of these carefully and expensively-built rockets is thrown away during each delivery operation. This is akin to flying a brand new Boeing 737 from San Francisco to Los Angeles, offloading the passengers, and then dumping the aircraft in the Pacific Ocean, which is pretty much what the space launch industry has been doing since Sputnik bleeped its way into the Space Age.
It’s not that rocket designers have been blind to the issues; it’s more a case of it actually being ‘rocket science’. One of the key aspects that makes launching payloads into space difficult is the challenging environment created by the functioning rocket engine itself. At the very least, the delivery, combustion and management of either cryogenic or corrosive propellants, coupled with the operation of fine-tolerance rotating machinery at high temperatures and pressures, to produce vertical motion through the atmosphere at supersonic speeds, makes for an interesting engineering exercise. Plus, unlike an aircraft, if the power plants fail there is no glide-back option: you are committed to reach orbit as soon as you leave the pad. Astronaut John Glenn famously summarised the experience as “sitting on top of two million parts, all built by the lowest bidder on a government contract”.
With the caveat that statistics should be treated with caution, the figures for rocket reliability do not make encouraging reading. Of the 5000-plus space launches since Sputnik, only 92 per cent are classed as successful, which implies that almost one in ten launches are abject failures. That said, technology has improved somewhat since the late 1950s, so has reliability and, of course, statistics differ between launch vehicles. For example, although the Ariane 5 suffered problems in its early career, as of 27 January 2016 it had conducted 70 successive successful launches over a period of 13 years. For this reason, commercial users consider it a ‘reliable’ launch vehicle, with the proviso that you are ‘only as good as your last launch’.
For decades, with all this in mind, launch system designers have sought a solution to reducing the cost of delivering payload to space while, at the same time, maintaining current levels of reliability. Unsurprisingly, the discussion often hinges on not throwing away the rocket on each and every launch. Thus, the reusable rocket has become the holy grail of the space-launch industry.
The design of the US space shuttle fleet, retired in 2011 after 135 missions, went some way towards this goal by reusing parts of the system. Firstly, the orbiters themselves were designed for multiple missions, in that key subsystems would undergo considerable refurbishment after each flight. This included, most notably, the space shuttle main engines (SSMEs) attached to the orbiter, and the strap-on solid rocket boosters (SRBs) which were recovered from the Atlantic.
By design, the only discarded element was the external tank (ET), though at $27m a pop (based on a $465m Nasa contract for 17 ETs) they were not what most would regard as ‘disposable items’. Indeed, even this partial reusability came at a cost which - depending whether one includes R&D, ground facilities, etc - was typically estimated to be anywhere between $450m and $1.5bn per flight. Although the shuttle did launch a number of communications satellites on its early missions, it was never really an option as a commercial launch vehicle.
So is that it? Is this rocket engineering stuff just so difficult that reusing hardware at an ‘affordable cost’ is impossible? Not so, if recent developments are anything to go by. In late 2015, two private space companies - SpaceX and Blue Origin - gave the industry hope that at least partial reusability was back on the table.
Since its inception in 2002 by former PayPal entrepreneur Elon Musk, SpaceX has become known in the space industry as a source of disruptive technologies, promising (and eventually delivering) launch services at a cost that incumbent providers thought unachievable. Without attempting a full-blown financial analysis, it is worth noting that, in a little over a decade SpaceX has gone from a start-up with a dream to a company that most commercial satellite operators will consider - alongside Arianespace and International Launch Services (ILS) - when they need to launch a new satellite.
In parallel, the company has fielded its Dragon space station supply-capsule and signed contracts with Nasa to develop a crew-carrying version to reduce the Agency’s reliance on Russia’s Soyuz. The message is that, following decades of programmes run on the government model, private space companies are now accepting the mantle of ‘expanding the envelope’.
A key part of the mantle is the quest for that holy grail of rocketry and, as soon as SpaceX started bolting legs to the base of its Falcon 9 first stage, it became clear that the company had embarked on the search for the reusable rocket. In addition to the carbon-fibre legs, which deploy as touchdown approaches, the stage is equipped with cold-gas thrusters to reorient the stage for the return phase and small, foldable heat-resistant fins for steering. Of course, the critical item is the flight control system that makes a landing possible: according to SpaceX, the flight is “totally automated once the rocket is launched…based on incoming, real-time data”.
Its first attempt at a landing - on what SpaceX called a “drone ship” in the Atlantic - occurred in January 2015, but failed because, according to the company, “the first stage prematurely ran out of the hydraulic fluid” used to steer the fins that help control the rocket’s descent.
A second attempt in April was closer, but about 10 seconds before landing a propellant valve stopped responding to commands and the vehicle momentarily lost control. The stage reached the landing platform, but tipped over and burst into flames as its remaining propellant ignited.
However, on 21 December 2015, having deployed 11 Orbcomm communications satellites into low Earth orbit, SpaceX landed a Falcon 9 first stage back in Florida, not far from the pad at Cape Canaveral Air Force Station from which it had launched minutes earlier. It was dark, so the landing itself was hard to discern on the video feed, but when the smoke dispersed it was clear that SpaceX had succeeded. Musk said: “I do think it’s a revolutionary moment. No-one has ever brought an orbital class booster back intact…This is a fundamental step-change compared to any other rocket that’s ever flown”. A few days later, when engineers had made an initial examination of the stage, Musk tweeted that it was “ready to fire again”.
Arianespace chief executive Stephane Israel was understandably less upbeat about a competitor’s success. Speaking at a briefing in Paris in early January, he cautioned that a single demonstration does not prove the economic viability of the concept of reusable stages. Israel’s order book showed that, while Arianespace tied with SpaceX in 2014 with nine commercial orders each, in 2015 the European company signed 11 or 12 commercial Ariane 5 launch contracts (one customer remained undisclosed) compared with nine for SpaceX, and one each for two other providers.
Commenting on his company’s view of reusable rockets, Israel said: “When we launch Ariane 5 we try to fill it to capacity. We’re not cutting performance by saving fuel for a return to the launch base”. Musk addressed the economics of reusability by pointing out that “it costs $60m to build a Falcon 9, but only $200,000 to fuel”.
Naturally, SpaceX will have to make a thorough engineering assessment of the mechanical stresses incurred by the stage throughout its flight profile before judging it qualified to make another launch. Indeed, it may well turn out that some customers will refuse to sanction the use of a ‘second-hand stage’ for their precious payloads.
On the other hand, the ability to examine a rocket stage that has been to the fringes of space and back offers something that dumping it unceremoniously in an ocean does not. No-one really knows which parts of a stage might be overdesigned (and therefore heavier than they need to be) and how margins might safely be decreased, because they can’t X-ray the structure and inspect the engine plumbing with a borescope post-launch. In truth, the economics of stage reuse will not be proven until a used stage has flown again, and even then questions will remain: how many times can a stage be reused, how much does it cost to refurbish and, crucially, will reusing hardware reduce overall reliability?
The search continues
Though SpaceX has undoubtedly been disruptive, it won’t have things all its own way in future. Apart from a European proposal for engine recovery, a successful flight test by Blue Origin’s New Shepard launch vehicle on 23 November 2015 and a second controlled landing on 22 January 2016 shows promise for reusable launch systems. Established in 2000 by Amazon founder Jeff Bezos, Blue Origin has been media-averse since its existence became public in 2003. Yet its second development flight, which succeeded in crossing the notional 100km threshold to ‘space’, could change all that. It was the first time a commercial capsule and its carrier rocket had been launched into space and returned to the Earth intact.
For most viewers, the mission video has a greater visual impact than that of the December SpaceX flight, being shot in the clear blue skies of West Texas. In fact, the dynamic, almost surreal, terminal descent and landing phase could be straight out of ‘Thunderbirds’. However, Elon Musk was keen to point out that, unlike the Falcon, New Shepard is not designed to deliver payloads to orbit. Its raison d’être is to take scientific instruments and fare-paying uber-tourists on a short suborbital hop into the microgravity environment and parachute them back safely to Earth. As such, it is in competition with Virgin Galactic.
However, the differences are only as significant as those between cars and trucks in a world of horse-drawn carriages. The recent successful demonstrations of these two reusable rockets mark a tipping point in launch-vehicle development: in future, whether or not to incorporate reusability will be there on the list of design decisions.
Fast-forward to the 2060s and the centenary of Gerry Anderson’s ‘Thunderbirds’, the dream of full reusability could even be achieved by a British company. As development of its air-breathing hybrid rocket engine continues, the promise of the UK’s Skylon spaceplane teeters on the cutting edge of feasibility. ‘Thunderbirds’ may have gone, but Anderson’s vision remains.
The path from launch to touchdown
The launch ascent of the recoverable Falcon 9 first stage is standard until separation of the second stage, which continues to boost its payload into orbit. Now close to being in space itself, but on a ballistic (unpowered) trajectory, the first stage performs a ‘flip manoeuvre’ using cold gas thrusters to reorient its engines to fire against the direction of motion.
At the appropriate point of the trajectory, three of the nine engines are re-lit in a ‘boostback burn’ to reduce the stage’s velocity, effectively acting as a retro-rocket. As the stage re-enters the atmosphere the ‘grid fins’ deploy to allow steering and the engines light again in an ‘entry burn’ to slow the stage for a controlled entry.
A combination of grid fins, cold gas thrusters and steerable rocket engines control the vehicle, keeping the stage within a few metres of its target trajectory throughout the landing burn. The vehicle’s legs are deployed just before touchdown to avoid affecting the aerodynamics and to obviate potential damage.
Europe’s answer to reusability
In June 2015, leading European aerospace company Airbus Defence and Space announced “a five-year R&D programme” for a recoverable Ariane first-stage engine and avionics package called Adeline (mercifully short for Advanced Expendable Launcher with Innovative Engine Economy). The philosophy is to recover and refurbish only the expensive and complicated parts of the rocket, while discarding empty tanks and related structures that represent only 20 per cent by value. The design involves a cylindrical propulsion bay that lands on a runway using conventional wings and aero-engines.
Although there is often more than one engineering solution to a problem, the announcement by Airbus appeared to be a reaction to SpaceX’s landing tests and predicted Adeline’s availability “by 2025”, by which time SpaceX could be boasting ten years of experience with stage reusability.
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