Title image credits: Gravity Industries

Episode 7 of Talking Engineering: An Interview with Richard Browning

Since their launch in March 2017, Richard and his team at Gravity have invented, built and patented an ‘iron man’ flight system, the realisation of Richard’s vision to develop an entirely new and authentic form of human flight. Gravity has to date been experienced by over a billion people globally, chiefly through the incredible videos on its YouTube channel.

The current suit consists of a large back-mounted turbine and four arm-mounted turbines, two per arm.

In its first 24 months, Gravity executed 96 flight events across 30 countries, including 5 TED talks. Richard continues to work with the rest of the team to develop Gravity into a world-class aeronautical engineering business and to continually challenge perceived boundaries in human aviation.

JH: What are the primary challenges in shrinking an engine down to a suitable size for human ‘jet-pack’ flight?

RB: The engines we have developed are centrifugal compressor turbo jets – a very simple in-line compressor with a small cross-sectional area and very aggressive compression. Essentially, the engines put high pressure air into a chamber and then combust the turbo fuel to create a high-pressure jet at the back, which bypasses the turbine wheel and in turn helps drive the compressor. It really is a micro-gas turbine engine.  

However, there’s no real cooling or lubrication system, except for oil being sprayed onto the ceramic bearing, which is doing around 120,000 rpm. And that is how you get away with such simplicity and smallness and end up with the engine – in the case of our latest oil engines – being no more than 2kg in mass, despite putting out 25kgs of thrust.

JH: Am I right that the first engines you experimented with had been originally developed in Germany for drones? 

RB: Yes, indeed (of course there are several manufacturers out there now making these kinds of engines). They started out originally, to be honest, on model aircraft. The creators were kind of model enthusiasts, who tended to be retired aero-engineers anyway and they would go out and do all the things they had wanted to do during their careers without any of the scrutiny of having to deal with passenger airliners. So they built these micro-gas turbines for model aircraft and they got so good that they eventually could be used as target drones (for target practice) for fast jets and cruise missiles.

JH: And when you first started modifying these micro-gas turbines to make them suitable for a human jet suit, what would you say were the big technical challenges?

RB: First of all, we worked with the manufacturer of the engines, we didn’t build them ourselves. We worked with them and they have now got a specific design just for us that we use. I mean the challenges they went through are obviously building a gas turbine that is extremely light, extremely small, reliable and stable. 

You will know of Frank Whittle, who invented the first jet engine, and he had this constant problem of when using too much fuel the engine overaccelerated and would blow, or when using too little fuel it would flame out. So, you’re controlling a very, very unstable constant explosion of fuel. Whereas a little micro-processor regulating fuel is perfectly capable with its processing speed of managing that challenge. Using the large in-line compressor of a normal gas turbine, you know, with a sequence of compression stages, has been avoided by having these centripetal compressors which are able to compress a huge amount of air with a very small torque-factor. If you were to put an in-line compressor on our engine to do that it would need to be 2ft long. 

JH: And, you just talked about the control system that manages that very fine balance of fuel. Could you speak about other any other computation going on during a flight?

RB: Yes, for the engines to constantly put down the required on-demand power, you have to hold a certain RPM. The micro-processor in the engine is simply looking at RPM with a magnetic sensor, looking at the exhaust gas temperature (using a little probe in the exhaust) and looking at the fuel pump setting, in other words what the fuel pump is doing in terms of how much fuel is going in. Depending on the engine, they also sometimes have ambient pressure sensors and even a combustion chamber pressure sensor as well. So they’re relatively simple inputs, but then the processor has to balance all of those things. So if the RPM is dropping off, it puts more fuel in, until the RPM picks up to the desired level and it also has to monitor the effects of that. If it’s just seeing an increase in exhaust cap temperature and not an increase in RPM then you probably have a problem with the air flow. Maybe not enough air. So it’s a relatively simple thing to optimize, dare I say, or at least an awful lot easier when you’ve got a high-processing-speed processor.

In terms of flying, what we do is we have a throttle control system – a power node system which can incrementally close the power up and down. When you’re flying, you’re flying by just squeezing that trigger in the arm grip, bringing the engines up very quickly to the desired power, and then never letting go of the trigger. And then just as you land, you let go and drop the engines back to idle. All of the flying control and maneuverability helps works through vectored thrust. In other words, its down to where you point the turbines. If you point them down you go up; if you flare them out to the side, gravity is able to bring you down again. And that is why you get that ludicrous degree of control that you’ve seen in the videos. If you are banking hard, for instance, or if it’s a particularly hot day, then you can nudge the power up a bit. Consequently, if you’re going fast in a straight line, you’re developing aerodynamic lift over your body and therefore you don’t need quite as much power, and you can nudge the power down a bit. So, its really just a trigger function. 

Our latest CAN (Controller Area Network) bus control system, which we now use rather than what was traditionally a PWM (Pulse Width Modulation) throttle control system, is the same system that you get in cars in terms of sophistication. So that’s the control system!

JH: You mentioned the configuration of two engines per hand and one on your back. Did it take you a while to settle on that configuration and do you think there is an alternative configuration that is better, and just waiting for further technical innovation? 

RB: Yes, the first flights were two engines on each arm and one engine on each leg. And it did work, but your legs are quite poorly controlled compared to your arms, and with the engines on your legs, you end up being very, very restricted on where you can take off and land. That makes it pretty unusable for anything other than stunt flights. Now, we can take off and land from many surfaces, especially because you can accelerate the engines while you vector thrust them to the side, and then throw the thrust downwards and jump at the same time, and then you flash the heat on the surface you are taking off from. Even if it’s a thermo-plastic deck on a Royal Navy patrol boat, it doesn’t cause any problem at all in terms of damage to the surface!

The way to think of the configuration is really like a camera tripod of support. The only reason you’ve got two on each arm is that it’s not like you feel huge pressure going up your arm, it just feels like you’re leaning on a kitchen work-top when you’re flying. The result is that you have this 3-legged camera tripod of support. One of the later generations of suits that we’ve got (and we’ve flown the prototype) – which will be coming soon entails swapping the rear single engine out for three turbines identical to the arm ones, so you have all of only one engine type. We have actually gained about 20kg of thrust by doing this. But its fundamentally the same idea. Thrust on each arm and thrust on the back. There’s an ongoing debate about this online, but what people don’t realise is that there is no military, search and rescue, or entertainment reason for having your hands free. Most aviation devices entail using your hands to fly them because they are really good at controlling them. 

And to give you a military example, there is no tactical reason why you should be hovering above ground trying to take shots at anything, because you have no cover from fire or sight, so that’s pointless. There are other suits that have been built already which allow you to put down covering fire while you are airborne and that keep your hands free, so we’re happy with the core lay-out.

JH: So you don’t think that is going to be one of the things that changes, whereas you are saying other things might?

RB: Well, never say never. We are very happy to change our ideas if we find evidence to suggest we have better ways to do things. That lay-out has served us very well and it’s hard to beat it. Believe me, we have tried many other options. For now, it’s fine.

JH: How did you decide on the balance that you currently have in your suits between maximum power output and fuel consumption or endurance? 

RB: The biggest weakness with this whole concept is efficiency, because as I’m sure you know, one of the clever things about a helicopter is that you’ve got an enormous rotor blowing a large cross-section of air relatively slowly. That’s the efficient way to go and as an example, I think it was in Canada there was a science project where they had a cyclist connected to a very light-weight structure the size of 2 or 3 tennis courts, powering 4 enormous very slow-moving rotor blade configurations, and that actually got off the ground. That’s crazy! And the reason that is possible is because of the ludicrously huge areas of air that are being moved. 

If you go to the other end of the spectrum, our tiny areas, our tiny rotors on the inside of our engines, they are inches across, right, so you are blowing a very, very small cross-section of air, at an extremely high velocity and that is not efficient. That’s why turbofans on airliners get ever bigger, because they’re getting more efficient the bigger they get. So, it’s never in the near term going to be that efficient. But the bonus is that it is so small and light – just 2kg. I just walked out of the exhibition center earlier with one on my back because they’re just tiny. So, this is never going to be an efficient long-range traveling device. We can do 3 or 4 miles quite easily, and for tactical, medical, search-and-rescue and entertainment that will be fine. The system we’ve got will do about 4 or 5 minutes and you travel at 60 mph so you can do approximately 5 miles. We can easily hop a few hundred meters here or there and its actually turned out to be fine. 

JH: Do you think there will ever be a truly green alternative to jet fuel and how might that impact the design of your engine?

RB: Well if you’re talking about bio-jet fuels, we’ve run it before and we are trying to get a longer-term solution or a long-term supply source of it. When it comes to the electric version, which you might have seen me launch at Goodwood a few weeks back, it flew, but it’s got all the problems everybody is familiar with in terms of the weight of the batteries and the lack of retained energy in batteries compared to fossil fuels. So we have proved that it works but it is pretty marginal as it weighs twice what a jet suit does and lasts for a fraction of the time. So as the world gets better batteries, which it inevitably will, we are ready to hoover them up and one day maybe the electric suit will overtake the jet suit in terms of range and capability. 

JH: Obviously that’s one way things can go into the future. What are some of the other technical developments that you foresee in the years to come? What are the next steps? 

RB: To be honest, a lot of it is just fine-tuning or adapting the core design for specific purposes. So, for the military and search-and-rescue, people obviously need something that is reliable and robust and as simple to use as possible. It needs to be something that will start the first time you ignite it, whether it’s been dropped in a puddle or dragged over a rocky surface and smashed around and abused. So, making it robust and as water-proof as you can is one avenue. That’s what is going on right now. The military version is in the back of my car right now and we are pretty well along that journey. 

Overall, it’s adapting the core concept to specific uses. We have another version that is ready to fly in the next two weeks in my lab. We call it the light suit, and nobody has really seen it yet. It’s just a far more sophisticated CAD version of the core design where everything is a lot more compact. The fuel bladder is tucked around the back of the engine in a much neater way. It’s tiny,  and yet it has more than enough fuel and power to be able to do most of our training and most of our events. It really looks like a 25-litre day-pack!

When it comes to entertainment, this new suit looks all the more ridiculous. You can barely spot the jet suit on the person; we’ve got the electronics out of the way now. In the old days we used to – and I still fly these suits sometimes – have the electronics all visible on the front and they are sort of flappy, wrapped around you. In the very early days, it allowed you to fix and re-set electronics while wearing the suit. All of that is now incorporated into the back-pack. So it’s much neater and smaller and more tidy. So all of that kind of stuff is keeping us pretty busy for now. But there are longer term things – we’re looking at the wing systems and other stuff we’ve played with – but we’ve got a long way to go with that and it’s not as easy as people might imagine. You have to go pretty fast with a decent-size wing to generate helpful aerodynamic lift and it can be pretty difficult flying those. In a way, we need to just focus on building out the businesses, the entertainment business and the professional business (military and search-and-rescue) to properly scale into the demand for those, and then the revenue from there will help to accelerate the progress in other areas. 

A note from the writer: Many thanks to Mr Richard Browning for putting aside the time to discuss his awesome line of work with me. Sadly, this will be the last article that I will publish while at Eton, as I begin to focus on my A-Level exams. I have really enjoyed interviewing and talking to all of the engineers who played a part in my ‘Talking Engineering’ series and I look forward to continuing my research and tech-related discussions at Cambridge and beyond!

– Jasper Hersov

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