Wednesday, 30 April 2014

Spaceflight via Russian Trampolines

You can't get off Earth using a trampoline, but you might just be able to escape Mars biggest moon, Phobos. Image: NASA
Since the Space Shuttle was retired in 2011, NASA has had to rely on Russian Soyuz rockets to get its astronauts into space. This hasn't come cheap, costing $70million a seat. But they've had to grin and bear it, as the next American spacecraft capable of carrying humans (probably the SpaceX Dragon) wont be ready until at least 2017.

As you may have noticed in the news, US-Russian relationships are rather strained at the moment, and the Ukraine sanctions are beginning to affect the Russian space industry. Yesterday Dmitry Rogozin, the Russian deputy prime minister, tweeted that:

“After analysing the sanctions against our space industry, I suggest to the USA to bring their astronauts to the International Space Station using a trampoline,”

Now this could have lead to a very interesting long blog post about the politics of space exploration, but there's a far more important question the ask: Could you actually get into space using a trampoline?

The key factor in getting off a planet is the escape velocity, how fast you need to go to escape a given object's gravity. For Earth this value is about 11.2 kilometres per second, which is fairly fast.

How fast can you go using a trampoline?

The Guinness Book of World Records lists the highest height achieved by a team on a trampoline as 6.73 meters, using two people to provide the bounce for a third. With a bit of maths, that means that the speed they were going as they left the trampoline was about 11.5 meters per second.

Not fast enough for Earth, but the Solar System is full of smaller objects. Which is the largest with an escape velocity of less than 11.5 meters per second?

It turns out to be Phobos, one of the moons of Mars, which comes in with an escape velocity of 11.4 metres per second. Its a small world, a lumpy rock with an average diameter of eleven kilometres and a mass nearly two billion times smaller than that of the Earth.

This might be a practical question, as Phobos has been mentioned as a possible destination for Mars-bound astronauts. It takes much less fuel to get there than to land on Mars, and could be used as a base to remotely control Mars rovers without the annoying time delay caused by the signals having to go from Mars to Earth and back.  

So if in 20 years time you're an adventurer stranded on a desolate Martian moon, make sure you've got a trampoline.

Note: Discussions over lunch resulted in the decision that the speed you could jump on the trampoline wouldn't be affected by the lower gravity, as the key factor is the energy you're  producing in your legs. You'd jump higher, but accelerate less slowly down towards the trampoline, affects which would cancel out. If you can do some maths showing this is wrong, please let me know. 

New blogs will be posted on Twitter, as always.

Thursday, 24 April 2014

Falcon 9, Skylon and the Future of Spaceflight

It costs over £1000 to send a single kilogram of anything into space. In that one figure lies the ruin of many predictions of the future.

In 1968 Arthur C Clarke and Stanley Kubrick presented the world with their vision of the future in 2001: A Space Odyssey. They imagined a future of mankind in space, complete with bases on the Moon, space planes, voyages to the gas giants and a vast circular space station.
A PanAm space plane approaches the huge Space Station V in 2001: A Space Odyssey...
As far as we've got: The International Space Station is much smaller than Space Station V. Although its still just about the most incredible thing we've ever built.

None of this has happened. Neither have countless other predictions of the future. Wernher Von Braun imagined a complex space infrastructure; Nasa planned to have humans on Mars in the 1980s; The Space Shuttle promised easy access to space for all.

Many factors contributed to this, not least the changing requirements of politics at the end of the Space Race, but the major one has been cost. No one predicted just how ridiculously expensive space flight would be.

One of the main reasons behind this expense is that every time a rocket is launched, at a cost of around $100 million, it is thrown away. These high launch costs lead to even more expensive spacecraft. Without the chance to launch repair missions or backups, satellites have to be built to never fail, with multiple redundant systems. Communications satellites cost hundreds of millions, and the price tags of science missions are often measured in billions.   

It can be argued then that the key to the future of space flight lies in reusability. And true reusability, not like that of the Space Shuttle which ended up costing over a billion dollars per launch.

However getting a rocket back to use again turns out to be rather tricky. Rockets are built in stages to save on fuel. Each part burns through its own stock of propellant before dropping off, leaving the next stage(s) to get the now much lighter rocket into space.

When the first stage detaches it is going very fast, travelling through very thin air. With no thrust remaining, the stage follows a parabolic arc downwards, at which point it smashes into the thicker air near the ground and turns into tiny pieces.

Even if it survives this, the next stop is the ocean or ground, both of which are rather bad for you at high speed. Amazon founder Jeff Bezos recently recovered parts of the Saturn V moon rockets from the Atlantic. There wasn't much left. The space shuttle boosters, which parachuted down into the ocean from relatively low heights, required months of refurbishment after being soaked in salty water.

That's just the first stage. Recovering the later parts of the rocket from space means surviving the extreme temperatures of reentry, impossible without a heavy heat shield. And every kilogram you use on reusability is one kilogram less put into space. 

With these challenges, it isn't much of a surprise that it's taken fifty years for someone to have a go.

With landing legs folded up around its base, the latest SpaceX Falcon 9 rocket blasts off from Cape Canaveral. The first stage would later be successfully landed  into the ocean. Photo Credit: SpaceX 
Last Friday a SpaceX Falcon 9 rocket blasted off on an otherwise routine mission to the International Space Station, (accidentally drenching itself in dirty water as it did so). What made this rocket unique was its choice of optional extras: Landing legs.

The Falcon 9 version 1.1 is deliberately more powerful than it needs to be. So when the first stage separated it had more than enough fuel left to fire up three of its nine engines, slowing it down enough to safely fall back down to Earth. As it reached the ocean with landing legs unfolded, one engine fired again to lower it gently down into the Atlantic. The first stage of a rocket has, for the first time, successfully landed in one piece. 

Stormy seas have meant that SpaceX have been unable (at time of writing) to recover the stage, but that doesn't matter too much. Just getting the rocket down has provided them with the knowledge they need for the next step: Flying the stage all the way back to the pad, ready to be used again.

Being able to reuse a rocket has huge advantages. Currently a Falcon 9, the most cost affective rocket available in the world at the moment, has a price of $56.6million. (I find it amusing that SpaceX list the prices of their rockets online- do they take Paypal?)

However, the fuel for a launch comes to about $200 thousand. Being able to reuse the first, largest stage of a rocket, turning it around with no more maintenance than an airliner gets between flights, could dramatically slash the cost of flying into space. SpaceX already has long term plans to bring back not just the first stage, but the whole rocket.

Using some of the rocket's fuel to retun it to Earth comes with a price, of course: A 30% hit to the amount of payload the Falcon 9 can carry to space. This is where, I think, reusable rockets will benefit all of us. If SpaceX start saying rockets capable of lifting 4 ton satellites cost half a million dollars each, but lifting 5 tons will cost fifty million, then there's going to be a huge pressure to miniaturise technology, much of which will probably end up in your pocket.

Concept art of the Reaction Engines Skylon, a huge space plane scheduled to being flying at the end of this decade
While SpaceX are trying make rockets work like aeroplanes,  across the Atlantic a plan is coming together to turn planes into rockets. The British company Reaction Engines is working on bringing to reality one of the staples of science fiction: The space plane

Known as Skylon, the 85 metre long behemoth will take what is both an old fashioned and radically different way into space. Taking off from a runway, its unique hybrid SABREs (Synergetic Air-Breathing Rocket Engine) will run like jet engines at first, taking in air from the atmosphere and burning it with on board hydrogen fuel.

Then, when the plane reaches five and a half times the speed of sound, the SABREs will switch to rocket mode to take the Skylon and fifteen tons of payload into low Earth orbit. Cargo delivered to space, the Skylon will then fly back to Earth, landing on a runway and ready to fly again in a couple of days.

Whilst space planes have been imagined for years (see the first image), Reaction Engines have only just managed to develop the key piece of technology needed to actually make one fly. At Mach 5.5 the air hitting the engines will be hugely and quickly compressed, causing it to heat up to temperatures of thousands of degrees. To avoid melting the engine, the air must be cooled to well below zero degrees Celsius in a fraction of a second.

The solution is to use a precooler, sending the air between a series of tiny pipes through which the plane's cold hydrogen fuel is being pumped, taking away the tremendous heat. Last year Reaction Engines successfully demonstrated this technology, securing funding to work towards a full scale prototype. If all goes to plan, the first Skylons could begin flying at the end of the decade.
Funding is Skylon's main challenge. Reaction Engines have raised only a small amount of the roughly $10 billion they need to bring the plane to life, mainly from private industries. This is a deliberate policy, as it will only be worth having space planes that can fly every day if there will be something to launch every day. If the space industry thinks not, they simply won't fund it. 

SpaceX's reusable Dragon spacecraft becomes the first privately-operated vehicle to dock with the International Space Station in 2012. Photo Credit: SpaceX

If SpaceX and Reaction Engines succeed in their aims to create reusable spacecraft, and it's a big if, then it could completely change the paradigm of space flight. SpaceX are talking about sending people to Mars for half a million each, whilst Reaction Engines plan to launch dozens of people ta a time in Skylon. Asteroid mining, space tourism, huge space telescopes and more would all become more affordable.

The challenges ahead are huge. SpaceX has yet to recover, let alone re-fly, a single rocket stage. Skylon's mighty SABREs are at this point no more than small prototypes in an Oxfordshire backyard. Both companies may yet fail in ther dreams of reusability, a subject so complex that I've only been able to touch on the basics here.

But if they succed, then we may just get 2001's dream of a future in space. Just a little bit later than pannned.

New blogs and science news on Twitter, as always