Roger Sloman is perhaps the kind of inventor Urban Air Mobility (UAM) needs: a man who wants to save lives, who wants you to walk away from a serious helicopter or electric air taxi emergency forced landing. His tiny rockets fit in your hand — lightly — but mounted under a helicopter’s skids, or tucked into the cables of an eVTOL’s (electric Vertical Takeoff and Landing) ballistic parachute, they pack a reverse-thrust punch that could make a real difference the day an (inevitable) incident occurs. Roger deals in milliseconds to solve the zero-altitude/zero-speed equation, and he has physics on his side [see our report on the AVCP system]. We met on May 16 following his technical presentation at the Vertical Flight Society‘s annual convention.
Q: With Roger Sloman of ABBS Group at Forum 75, Philadelphia. Perhaps you could tell us a little bit about your work these past few years, which has led you to develop the Active VTOL Crash Prevention system?
Roger Sloman: It actually began in 2008, when I was involved in doing a mine blast test on an armored vehicle. When we looked at the high-speed video of that test, what I noticed was that when the mine goes off, the vehicle doesn’t move for about 10 milliseconds. I saw that there was a time gap in which you could do something to counteract the vehicle being blown into the air. So that was the original observation that led to everything that’s been going on for the past 10 years and cost about £6 million pounds and 10 years of my life. One of the key developments that we’ve made to give effect to that is the linear rocket motor, which is a different format rocket motor. The efflux doesn’t come out at the end of the tube, it comes out of the side of a square section tube. And this enables you to initiate the whole motor very, very quickly and burn all the propellant very quickly, like 20 milliseconds.
So we initiate in about two or three milliseconds and burn the motor out in about 20 milliseconds, which is necessary to counteract the force of the mine underneath the vehicle. Obviously, when I invented the linear rocket motor, I thought of all the other applications I could develop. One of them was stopping helicopters crashing and I envisaged using the linear rocket motors under the skids for the helicopter. You don’t need to deliver all the impulse very quickly in that case, you deliver it over about a second and the object is to slow the impact velocity with the ground. So that’s how it all began. Essentially the the concept for the helicopter crash prevention, the invention of the linear rocket motor, was in 2012. And then of course the eVTOLs started to appear about 2016, so we’ve really preempted it. But there’s an obvious requirement for the same system for eVTOLs.
My opinion is that there is no way that the certifying authorities will certify these aircraft to carry passengers above an urban environment without a “zero-zero” system. A lot of people say they will fit a ballistic parachute, but a ballistic parachute doesn’t work below about 250 feet. So there’s a 250 foot safety gap, in which if any emergency occurs — loss of control, or power, or bird strike — there’s a strong possibility of crashing to the ground and actually killing people. Which would be disastrous for the industry.
Some people in the industry recognize it, other people are basically ignoring it and relying on redundancy to cover the situation. But EASA, the European Aircraft Safety Agency, has already indicated that they will specify that in any power or control emergency situation, the aircraft has to perform a controlled landing. That means that you can’t use a normal ballistic parachute, which is a circular parachute, because you can’t steer it. So they’re insisting on a steerable parachute, if you have one. And they are also specifying criteria of no injuries to the occupants. No injuries to the occupants means a controlled landing. You can’t be landing it 10 meters per second, which is an FAA [Federal Aviation Administration] helicopter standard. And that will not suffice to meet the no injury criteria.
So it has to be a fully controlled landing in a one meter per second, maybe two if you’re lucky, if you’ve got stroking seats and crushable structure. But otherwise you have to have the rocket motors. Physics dictates it’s the only way to do it. And talking to NASA Langley people and FAA people, they are very aware of this. The eVTOL industry seems to want to ignore that at this point in time [laughter]. I think they’ll find that they have to do something about a zero-zero system in due course of time. That essentially is it.
Q: Nobody likes to think about a crash landing scene, but somebody has to! In such a scenario, passengers will exit the aircraft as quickly as they can. First responders will approach the aircraft, perhaps to free trapped passengers. There may be damaged or flammable batteries about. Will the retro rockets pose any danger in that context?
Roger Sloman: No. They are designed to burn out completely. The way we do that is to rotate the motors from the vertical position to the horizontal position, which is also necessary to control the landing. And if you fit in the motors underneath the aircraft, it’s necessary to control the motors fast, individually. If you have four motors under the aircraft, you have to control all of them faster to get the right attitude. Just bear in mind that the motors only burn for the last second of the descent. So it’s not such a big issue in that case, but there is the capability, potentially, to control the attitude on landing. The key issue is that you’ve got to fit twice the thrust as the weight of the aircraft to deal with the momentum of the aircraft and gravity at the same time. So if you land the aircraft, then if you don’t do something about stopping the thrust, then you’re going to take off again. Which is not the idea. So we just rotate the motors, and the linear rocket motor is perfect for that because you can simply rotate it about his long axis. So that’s very simple.
Q: Now, in the scenario where there’s a total loss of power on an aircraft, how would the sensors and logic board and ignition of the system be powered? Perhaps an eVTOL manufacturer would have the choice between integrating with aircraft power or keeping it isolated?
Roger Sloman: Yes, that’s absolutely correct. The advantage of making the system completely independent is that obviously if the aircraft systems go down, then the system will still work. So it is a choice for the airframe companies to decide whether they want to integrate the system into the aircraft sensor control system, whether it also has an independent system that can cut in if the aircraft system fails or whether it’s set up completely independently anyway.
Q: Let’s talk about software for a second. The logic board that takes information from the sensors needs to take decisions about exactly when to fire the retro rocket system. How complex does that software need to be? I read that you have already made some effort with optimization, with lookup tables and so forth. Can you talk about the software for a second?
Roger Sloman: Yes. Ideally you want to avoid software. Safety-critical software is not good if you can avoid it. The point about the system is that at any aircraft weight, altitude and current descent weight, there’s a certain number of total impulse required to deliver the aircraft to the ground at landing speed as long as you haven’t got continuing thrust input from the aircraft rotor system. So the key to managing that is, as soon as you initiate the rocket motors, at the right height, you then cut the power to the rotors. So there’s no interference between the two systems working. The key issue is actually sensing the actual height. It’s fairly critical to a meter or so. The initiation height of the motors is anything from 3 meters to 15, depending on the velocity of the descent. So, that is a key issue and the sensors to accurately determine the height is an interesting question that is to be resolved.
Q: One last question. How do you see the eVTOL industry developing in the next few years? You mentioned how crashes could have an impact on public acceptance of urban air mobility.
Roger Sloman: Yes. Well, they say they going to evolve and develop and come into service, probably in the cargo drone space initially in Alaska or Siberia, et cetera, where transport is an issue or communications are an issue. Emergency disaster relief is another obvious one. Those applications will precede urban air mobility because the regulation certification for those activities are going to be much less onerous. So that will set the ball rolling. But urban air mobility is a much more complex issue in terms of controlling the airspace and avoiding accidents. Having emergency landing points is an issue. Currently, all the architects doing designs for the Uber vertiports draw them with nice surroundings of trees and people and car parks, and that is not going to be acceptable. There’s a reason why airfields are big, open, clear spaces without anything in, and that’s going to be required for the vertiports and possibly even emergency landing points between the vertiports. So this is something that needs to be addressed as well as the basic aircraft safety.
Q: All right Roger, thank you so much for sitting down with us today.
Roger Sloman: Thank you very much.