The machine is actually just one of three, the first stage of the US Air Force’s payload workhorse, the Delta IV rocket. But now, four minutes after it sprung into action, it is mere debris falling back to Earth.
That’s because the Delta IV is designed to be discarded. The rocket is relatively heavy, but also relatively simple to build; it has 80% fewer pieces than, for example, the main engine that powered the Space Shuttle. And instead of an expensive cooling mechanism it uses simple technologies such as dissipating heat by burning away the nozzle’s lining. No harm done, so to speak, as it’s a single-use machine.
Ultimately, though, it begs a question: Is a throwaway rocket really cheaper than one that can be reused multiple times? And, anyway, what if we want to do more than put a few GPS satellites into orbit? “The only way we’re going to be exploring the solar system, and be able to return, is if these systems are reusable. Otherwise it’s a one-way trip,” said Gwynne Shotwell, speaking at the 33rd Space Symposium in Colorado Springs in early April.
Reusing rockets may help space companies save huge amounts of money (Credit: iStock)
Of course, Shotwell is somewhat biased. She is the president of SpaceX, the rocket manufacturing start-up owned by electric car pioneer Elon Musk, of Tesla fame. And just a few weeks ago, on 30 March, SpaceX for the first time managed to send a rocket into space that had been flown before. “This is a huge day,” Musk told the media after the launch. “My mind’s blown.”
The Falcon 9 rocket with its ‘reused’ booster lifted off from Nasa’s Kennedy Space Center in Florida to carry a satellite, SES-10, to orbit. This first stage then returned once more to Earth – touching down on SpaceX’s droneship (which carries the somewhat twee name ‘Of Course I Still Love You’) somewhere in the Atlantic Ocean. Musk called the feat a “milestone in space,” while Shotwell said at the symposium that the cost of reusing the first stage of the rocket was “substantially less than half” that of creating a new first stage.
That’s partly explained by the fact that the Falcon uses a rather complex rocket design, not just for reusability, but also to achieve a reliability that is potentially “human-rated,” in other words would qualify it to carry humans into space. That’s why the Falcon 9’s first stage has nine engines, can fly even if two of them fail, and uses highly durable aluminium-lithium alloy tanks to store liquid oxygen and propellant. Still, “looking forward for reusability, we don’t believe it really, really counts unless you can turn it around rapidly, or almost as rapidly, as you turn around an aircraft,” Shotwell said.
So how realistic is this goal? After all, the Space Shuttle flew back and forth many times – so it can be done. Even the shuttle’s solid-fuel rocket boosters were reused, although they did not land upright, but splashed down in water at the end of a parachute. 
Space Shuttle Atlantis launched on 8 July 2011 and came back to Earth 13 days later (Credit: iStock)
However, ever since the Space Shuttle retired, with the landing of Atlantis on 21 July 2011 after three decades of ferrying astronauts and missions into space, we have been back to dumping heaps of burning metal into the ocean, or allowing ultra-expensive rocket parts to burn up in the Earth’s atmosphere. So is reusing rockets really sensible, and what are the challenges to make it happen?
Countless heat tiles
Let’s look first at how this rocket recycling business started – in the US, with the shuttle programme, and in the Soviet Union, with Buran. Both spacecraft looked suspiciously similar, but Buran flew only one – unmanned – mission, in 1988. The shuttle, in contrast, was a truly reusable low-Earth orbit launch system. Or, rather, a partially reusable one – the main external fuel tank was jettisoned to crash and burn, and had to be replaced after each lift-off. And reusability came at a price. The cost of flying and repairing the shuttle was so big that it took up most of Nasa’s budget. After every flight, every single one of the 35,000 heat-resistant tiles protecting the shuttle during re-entry into Earth’s atmosphere had to be painstakingly inspected for damage and replaced if necessary, alongside the inspection of the RS-25 engines and the refurbishment of the solid-fuel rocket boosters.
“The RS-25 engines had to be rebuilt if they exceeded a set maximum time of operation or experienced any issues with the components, such as turbo pumps, valves, nozzle tube leaks, and so on,” says Mark Rogers, manager of the Engineering Directorate’s Advanced Concepts Office at Nasa’s Marshall Center.
The solid rocket booster had to be completely rebuilt and re-poured with propellant, he adds. “Remember, we were flying crew on every launch, so crew safety was the top priority and still is today,” says Rogers. “So there were a series of tests and inspections after each shuttle flight – the specifications of structural crack propagation, bearing noise, tile damage, and so on.”
Rockets must be reusable to allow humans to visit other planets (Credit: iStock)
The Space Shuttle made more than 100 flights before it finally retired. All its flight hours have provided valuable data for those who came next in line, among them SpaceX. Musk’s venture analysed the data and adapted current technology alongside the introduction of several new features. “They apply a step-by-step development approach to get a working system,” says Hendrik Weihs at the German Aerospace Centre (DLR). “That way, the reusable first stage is an important step to reduce cost compared to a ‘classical’ expendable rocket, but I’m sure it is not the last step towards a fully reusable system.”
Jerome Breteau, programme manager at the Future Launchers Preparatory Programme at the European Space Agency, says that the main change is the shift of focus from upper to lower stage reuse. The lower rocket stage has a maximum speed of around 2km/s, thus staying below the thermal barrier – rocket surface heating due to air friction. This means it does not require costly high-tech reusable thermal protection like on the shuttle, “but rather low-cost materials such as sprayed silicon-based foam, cork or composite panel – which is the choice of SpaceX for their Falcon 9,” says Breteau. “There is no high-tech material development for this reusability, because the objective is to be cheap.”
On the other hand, re-entry vehicles and upper rocket stages require advanced high-
performance – and very high-cost – heat shields, he adds, able to withstand re-entry temperatures up to 1,800°C due to the orbital speed of 7km/s and more. “The materials are either reusable, such as carbon and silicon carbide-based materials like on the Space Shuttle and the ESA IXV Demonstrator, or expendable, like silica phenolic-based pyrolytic materials that are very fragile – hence the shuttle failure,” says Breteau.
So far, there are few plans to reuse the upper stages because it’s very costly to equip them with the right thermal shielding, he says. But there could always be some workaround approach in future, he adds,“with upper stage reuse in orbit for other use, such as for observation platforms, propellant storage, or even manned habitats”.
Cutting-edge materials
All this reusability, current and future, comes at a price. During a press conference after Falcon 9’s March launch, Musk said that the company had so far spent at least $1 billion on reusable launch vehicle technologies. “We do have to figure out some way to pay off the development costs of reusability,” he said. SpaceX is now also working on ways to recover and re-fly the payload fairing.
To make sure that a material in a rocket that has just come back is reliable to fly again, a visual inspection is not enough, says Weihs. So engineers use the data about the applied loads from multiple sensors inside the craft, and thoroughly check the condition of the vehicle after flight.
Manufacturing process is also different. For reusable systems, it’s crucial to know precisely the fatigue behaviour, or tolerable lifetime, for each component, says Weihs. Modern rocket engines are commonly made out of metal, he adds, so, to extend the lifetime, engineers must lower the stresses in the material, thus reducing the engine’s performance. “The engine will then last longer, but we’ll be getting less thrust out of it,” he says.
_(cropped).jpg?sfvrsn=3145de12_0 "Buran was very similar to the Space Shuttle, but flew only once (Credit: Creative Commons)")
Buran was very similar to the Space Shuttle, but flew only once (Credit: Creative Commons)
Metal engines are also heavy. Shaving structural weight saves a lot of pounds of fuel, says Weihs, making the system more efficient. So to make lighter, longer-lasting and at the same time very efficient engines researchers are looking into new, fibre-reinforced ceramic and plastic materials with low thermal expansion.
One problem with such materials, though, is that they are usually limited when it comes to applied temperatures, says Weihs. There is ongoing research to enhance the temperature limit, he adds, or to apply an effective, robust and reusable thermal protection.
Pulickel Ajayan, a materials scientist at Rice University in Houston, Texas, is working with Nasa to do just that. His team has created so-called “fuzzy fibres” of silicon carbide, able to withstand temperatures of up to 1,600°C. To make it work, the scientists embedded silicon carbide nanotubes and nanowires into the surface of the existing fibres, to make their exposed parts act like the hooks and loops of Velcro, but on the nanoscale. “The fuzzies on the fibres would grab onto each other and onto the ceramic surrounding it, to help it resist breakage,” says Chandra Sekhar Tiwary, a colleague of Ajayan. “Also, the fuzzies would help the fibre survive longer in oxidation, as oxygen makes the ceramic break down in high temperatures.”
Multiple efforts
SpaceX hopes to use its boosters at least 10 times before retiring them. Musk’s firm is not the only one working on reusable rockets, though. Airbus has recently proposed a reusable system for the European Space Agency’s Ariane rocket. And then there is another rocket start-up: Blue Origin, the firm privately funded by Jeff Bezos, the founder of e-commerce giant Amazon. On 23 November 2015, Blue Origin flew its sub-orbital New Shepard spacecraft to just over 100km, crossing the Kármán line – the altitude where space begins. The rocket then detached from its payload and returned to its landing pad. This is significantly lower than SpaceX’s exploits, but in contrast Blue Origin’s rocket has been launched and landed five times.
It’s a feat in itself, but SpaceX and Blue Origin have somewhat different goals: while Bezos wants to launch rockets into sub-orbit, Musk eyes orbital space flights, which require the craft to stay in Earth’s orbit at orbital velocity, roughly 17,400mph at a 200km circular orbit. Having said that, Bezos is now also planning a reusable orbital rocket dubbed New Glenn. The idea is that its engines will be returned to Earth via parachute.
Blue Origin has already reused a rocket on several sub-orbital flights (Credit: Blue Origin)
SpaceX’s Shotwell argues that reusable boosters will be able to shave 30% off the cost of sending payloads into space, but not everybody agrees. Tory Bruno, the head of United Launch Alliance, a joint venture between Boeing and Lockheed Martin that builds the Delta IV rocket, said in an interview at the 33rd Space Symposium that reusing rockets would cut launch costs by around 10% at best. That’s because making a rocket is about half the total cost of a launch, and the lower stage accounts for half of that, he said. Reusing first-stage boosters is expensive in itself, from the cost of building and actually lifting the equipment to land them safely back on Earth all the way to the safety tests and refurbishing of the rocket.
Given the huge cost of launching payloads and humans into space, such a small return on investment may well pay off in the long term. After all, recoverable rockets are welcome news for satellite operators that currently have to wait in line for months to hitch a ride to space. For any future astronauts, however, it may come down to whether they are confident to entrust their lives to a second-hand rocket. But, if we are ever to go to Mars and beyond, recyclable rockets will probably be our only option anyway.