Rocketing to reality

Materials World magazine
10 Jun 2014

How do you change the face of space travel? Eoin Redahan asks Reaction Engine’s Alan Bond to trace the journey of his air-breathing rocket engine.  

Florentino Ariza carried his unrequited love with him. Like a badge, he wore it for a lifetime. He finally found favour with Fermina Daza, the woman he adored. By that stage, he was deep into old age. Alan Bond could probably empathise with the main character from Love in the Time of Cholera, for he lugged his epiphany with him for years. The thermodynamic principles for his space engine first came to mind in 1982, but it has taken more than 30 years to see the fruit of his labour. Having secured UK Government and commercial backing, Bond’s Synergetic Air-Breathing Rocket Engine (SABRE) is now close to realisation. So, what does it take to make the dream of a re-usable, hydrogen-fuelled spacecraft a palpable reality?

Capture an idea
Bond set out to make space travel leaner. The industry needed to move from fuelheavy, single-use launch vehicles to lighter, re-usable spacecraft. To do this, he conceived an engine with an ultralight heat exchanger that removed the need to carry 250t of on-board oxidant on its way to orbit.
In 1989, he formed Reaction Engines, based in Oxford, to develop SABRE – a closed-cycle rocket engine with a pre-cooled turbo-compressor that provided a high-pressure air supply to the combustion chamber. The engine would use oxygen from the atmosphere to burn with the on-board liquid hydrogen fuel instead of carrying huge volumes of liquid oxygen. The heat exchanger would be responsible for cooling the 1,000°C airstreams into a usable -150°C.

If Bond and his colleagues were to receive the financial support to make SABRE happen, they believed it could revolutionise the approach to space travel. He notes, ‘This could increase launch volumes by up to 15 times. Once grounded, the craft could be back up in space within two days.’  

Take your cap in hand
While Reaction Engines has had a major technology programme running on the heat exchanger technology for 15 years, the financial support to build the engine proved elusive for a long time. Bond says, ‘Convincing the commercial world there is something worth putting its money into takes the lion’s share of my time. Basically I’m an engineer, but I’ve spent 70% of my career trying to find ways of bringing what I design into reality.’

Nevertheless, after years of frustration, the company gained traction. About £10bln will be needed to develop the engine and the Skylon plane, which will house it. Several tens of millions have been found (85% from private investors) to date and the UK Government has offered 20% of the programme’s next step. The first stages of full-scale development will proceed for the next four years, though as Bond says, ‘We’ve got technology programmes on each of the engine components, and we’re at a good readiness level with all of them. The next steps are to get these into a full engine and test it.’

Know thy process
In order to make a heat exchanger that is 100 times lighter than existing offerings, Bond has relied more on manufacturing processes than innovative materials. He says, ‘One of the things I find frustrating is that everyone assumes we need magical new wonder-materials. What we need are new ways of working with extremely good existing materials. We use Inconel 718, which is a superb nickel alloy. It has been around for more than 40 years, so it’s just a question of turning that into very thin, extremely effective heat exchanger surfaces.’

The basis for creating the heat exchanger relies on a centuries-old drawing process. Bond says, ‘We start with a billet of material with a hole in it, and you pull it through a die to push its dimensions down. And that gives us a basic tube.’

The engineers are at the limit of what can be achieved with standard tube drawing, but they are using other, unnamed processes to further reduce the walls to a thickness of about 20 microns. Once manufactured, the tubes will be brazed to the heat exchanger and then assembled into the engine. The aim is to make components that are as robust and reliable as those used in standard passenger planes.
After that, it is a case of finding a space plane that can house the SABRE. To that end, the company is bringing the mountain to Muhammad. ‘We’re fixing the size of the engine,’ Bond explains. ‘We won’t go on to develop the Skylon vehicle ourselves, but what we will be doing is giving suitable air framers a specification. So, if you build a vehicle to this broad specification, you will have an engine to power it. With a space plane, the two have got to be very precisely matched.’

And dream the good dream
The re-usability and fuel efficiency of such spacecraft has the potential to put space travel on a whole new plane. Bond notes, ‘I’d like to see competing commercial operators around the world operating their own systems. You could have a situation where somebody wants to fly a mission, let’s say a scientist at the Rutherford Lab who has bolted a couple of spectrometers to an aerospace-style laboratory bench and wants to have it in orbit by Friday. He’ll be able to phone an operator to find the best price and within a couple of weeks, his experiment is in space.’ By the time the project finishes in 2024, Bond hopes to have reached a situation whereby SABRE-powered spacecraft can be put into orbit every few weeks. ‘As for the other things that will enable – you can put as much imagination into that as you wish. Once you’re in Earth’s orbit, you’ve solved 90% of the problem of getting into the rest of the solar system. I can see this leading to the growth of a major space transportation infrastructure in the latter part of this century. Then we will be looking at exploiting the resources of the moon and the exploration of the outer solar system.’

Sadly, most of us won’t be alive to see the moon mined for all its delicious cheese. However, we may yet live to sit in SABRE-propelled high-speed aeroplanes. Bond says, ‘These engines could certainly result in very rapid terrestrial transport. My little quote is, anywhere on Earth in four hours.’

Bond on…
The problem with UK innovation

‘When we’ve got projects of this sort, the problem is getting the funding and support to make them happen. That’s got to change. We’re not short of the innovation, the people or the intellect in the UK, but we are short of the mechanisms to bring them out.’

Using SABRE in commercial air travel
‘There is considerable interest if the price is right, but that depends on getting a hydrogen fuel infrastructure in place, and that’s not going to come overnight. If somebody decided to go ahead and get Mach Five terrestrial transport, the biggest problem they’d face would be making sure liquid hydrogen is available at airports when required. One advantage of these engines is that they burn liquid hydrogen, so we’re not dealing with hydrocarbon issues. The change to a liquid hydrogen infrastructure would need to be very large, but by 2030 or 2040, there could be high-speed aircraft serving the commercial civilian transportation market.’  

Advances in testing technology
‘Back in the days of Blue Streak (a satellite launch vehicle), we used the same film and cameras that recorded the data from nuclear explosions, but nowadays we can get framing of millions of frames per second if we want to. A lot of our instrumentation is still the boring old pressure tappings and that sort of thing, but the cost of instrumentation has fallen dramatically.’   

For more information on SABRE, click here.