In conversation with Scramspace director Russell Boyce
The University of Queensland’s (UQ) Scramspace project hopes to launch its unmanned scramjet vehicle from a test range in Norway early next week. We caught up with Scramspace Director and Chair for Hypersonics at UQ, Professor Russell Boyce, who is in Norway for the test, to talk about Scramspace, the test flight, and the future of scramjet technology.
Gizmag: I understand that the tests depend on the weather. Is rain a problem for launching?
Russell Boyce: We’re not too worried about clear skies when it comes to collecting data from the experiment, but for our payload it’s not a benefit. It needs some holes inside because when it’s flying through the atmosphere the pressure is falling quite rapidly and we need vents on the side of the scramjet to even that out. Despite that, we should be good to fly on Sunday.
Gizmag: Why is the test being conducted in Norway? Why not at the RAAF Woomera Test Range in Australia?
RB: Woomera is extremely busy and Australian defense needs have top priority, particularly when there are active troops engaged in things in the vicinity. It’s also quite hard to get a spot on the range. Another issue, and this is in public domain, is that the rocket motors we use have a small amount of asbestos in them and regulations don’t permit the import of asbestos into Australia. The third reason is the launch service provided by Germany’s DLR Mobile Rocket base. They routinely launch from Norway and a rocket base in Sweden.
Gizmag: Why is Australia carrying out a hypersonic test like this?
RB: Australia’s been doing hypersonic work since the 1960s. It started with re-entry research and it employed a special kind of shock tunnel that was invented in Australia, which enabled to do research at high speed that other people couldn't. And so from that, this particular shock tunnel enabled us to do hypersonic research back in the ‘80s. We have the capability in Australia to do high Mach number scramjet research at Mach eight, ten, twelve – that sort of number. In the United States and the rest of the world, scramjets are operating at typically Mach four, five, six, or up to seven. In Australia, we are able to work with a different class of scramjet and we've done it for quite some time.
Gizmag: How does a scramjet work? It looks like an empty pipe, not an engine.
RB: If you think of a jet engine underneath the wing of a Boeing 777, for example, it requires moving compressor blades to compress the air to the temperature and pressure required for ignition and combustion. It’s a bit like a diesel cycle in that you compress and then add fuel that ignites. It’s the same with scramjets, except at supersonic speed and the compression is done by shockwaves instead. So, that’s where the “ram” comes from. The air comes in the inlet and it’s compressed to the desired conditions to produce the shockwaves and then, in the combustion chamber, there are the conditions that we need so that we can carefully inject fuel in such a way that it doesn't scrap the whole cycle and combustion takes place and you release a vast amount of energy, which you convert to thrust with a nozzle and you have a working engine.
Gizmag: It sounds like a ramjet. Is a scramjet a more advanced version of a ramjet or is it different?
RB: In most ways, it’s completely different. A ramjet has an inlet with shockwaves and the air that passes through that inlet, by the time it passes into the combustion chamber, it’s reduced to subsonic speed. As a result of the conservation of energy, the temperature of that combustion chamber is extremely high.
The problem with the combustion reaction is that if the temperature is too high, you might start those combustion reactions, but you never complete them. Instead of combining hydrogen and oxygen into water, you might split the hydrogen and oxygen up, but they never recombine into the water molecules. Therefore, a lot of energy simply dribbles out the back of the engine in the form of chemical energy rather than producing thrust. So, as you go up in speed with a ramjet and the temperature in the combustion chamber gets higher and higher, the ramjet gets less efficient.
In a scramjet, there’s supersonic combustion and you design your inlet and your internal chamber in such a way that that you never get subsonic flow inside the engine. Therefore, the temperature remains low, and you have a chance of good combustion efficiency, but that brings a whole new class of problems because the air through the scramjet doesn't get much below 90 percent of flight velocity. It’s screaming through the engine and then you have the challenge of injecting fuel and mixing it so it won’t ignite before it all goes to the back.
Gizmag: Other hypersonic designers say that the incoming air is heated so much it could melt the engine. Is heat a problem for Scramspace?
RB: Yes, it’s definitely a problem. Dealing with it and making sure that the materials have structural integrity is a real challenge. So, for example, at the leading edge of the Mach 8 scramjet the temperature of the gas is exceeding 3,000 K, which is extremely hot, so you need to have a fuel that can withstand that environment. Ideally, the engine materials won’t reach that temperature as they receive the heat from that air flow and manage it in some way.
You can have high-temperature ceramics, which can survive at those temperatures, but has poor mechanical properties, you can have thermal conducting material, which operate at reasonably high temperatures, but you try to conduct as much heat away from the hot areas as possible as quickly as possible. There’s a variety of games that can be played. On Scramspace, we’re only conducting our experiment for about three seconds, so we've designed the engine from aluminum with sufficient mass to absorb that heat and manage structural integrity for the duration of the test.
Gizmag: What do you hope to accomplish with this test flight?
RB: We’re looking at a particular concept for an engine that we've been working on in Australia for about a decade for very high Mach number flight and it involves injecting fuel into the inlet rather than the combustion chamber and then using shockwaves to ignite the fuel. We've done this in ground tests and supercomputer simulations, where we can get very nice combustion and we can understand what’s taking place from the point of view of physics and chemistry. What we need to do is get some flight data of the same configuration as the simulations and ground tests to supply more answers and beyond that, this is the first high Mach flight experiment that Australia has done, which is a free flyer.
Prior to this, there have been a number of experiments where the payload has been a captive carry on a rocket motor where the rocket flies it up into the atmosphere at the Mach numbers its needs during the reentry, but its still attached to the rocket. What we want to do is measure the performance of this scramjet, so for this flight we are pushing off from the rocket as soon as we leave the atmosphere.
We will have a free-flying 1.8-meter (5.9-ft) long aircraft with the operational concept for a scramjet engine with sensors on board to measure how much acceleration is produced during combustion. I should say that this flight is going to be net drag. We never designed this engine to produce thrust. We designed it to modify the drag as it operates, and the data we collect will help us to evaluate our simulations.
Gizmag: It sounds like you’re using the Earth’s atmosphere as a giant wind tunnel.
RB: That’s exactly what we’re doing. When doing high Mach research, you have a common action by three approaches: To have ground-based wind tunnel testing, which gives you great data, but doesn't always represent the reality of flight. The second is the computational simulation and that can be extremely precise, but not necessarily accurate. And the third is the reality of flight, but that’s quite expensive and don’t take on lightly or do very often.
The process is to be able to take all three and couple them together. That’s the significant part of this Scramspace program; doing all three simultaneously in a highly complex fashion. From that, we hope to get the best possible understanding of the figures and what is taking place.
Gizmag: What’s the next for Scramspace after the test?
RB: It will take quite some time to get through this flight’s data. That will be the focus in the short term. Also, reconstructing the flight in computational simulations, comparing it to data testing, and writing publications. There are many flight experiments that I can think of to do to take the science forward.
Unfortunately, it has not been possible at this stage to get funding and most of the Scramspace flight team will be dispersing at the end of this project and that’s quite a shame. The reason for the funding in the first place is to build a talent pool for the Australian space effort and we've done that quite successfully. Ideally, we’d keep this team together and proceed to another flight experiment, but the talent we generated will be dispersing into other opportunities in the aerospace sector.
Gizmag: Looking further forward, what do you see as the practical application of this technology?
RB: If you were to replace the middle stage of a launch vehicle with an air-breathing system, such as a scramjet, particularly if it was reusable, then the efficiency gains that you achieve over a full rocket configuration are so significant and the reliability is so great that the ultimate outcome could be the reduction of the price per kilo for sending a payload into orbit of 50 percent or greater.
Whether it would be Australia doing this or not is another matter. The Australian government recently launched our first national space policy and the focus is more on pragmatic, satellite-based applications rather than developing a launch capability. However, it was the same part of the government that funded this particular project that now recognizes that Australia now has a new capability, so the hope would be that we would either plug into a bigger international project and hopefully see a capability for putting nano satellites and pico satellites into orbit cost efficiently in the coming decades. And that, in turn will open to Australia other specialized technologies later on.
Gizmag: Are scramjets going to be used on manned aircraft or is the heat too much of a problem for more than payload launches?
RB: The reality is that if you can manage the heat for a few minutes, then you can manage the heat for a long time.Once the material’s heated up and you have an equilibrium there’s no reason why we can’t keep cruising. The difficulty that I see is that it’s a bigger step to go from an unmanned system to a man-rated system. Certainly NASA has seen that when it was looking at returning to the Moon. It takes a lot of time and a lot of money.
The problem I see it of scramjet-powered vehicles is that I believe that we may know how to fly these, how to get them working – it’s just a matter of financial resources. We have the talent and the knowhow. But getting it to the point where the reliability has been demonstrated, that’s going to make many decades. I don’t think I’ll see it in my lifetime.
Gizmag: So, we could see a hypersonic Concorde one day?
RB: That’s certainly been desire for many people around the world for many years. At the moment, there are many concepts being worked on in the US and in Europe and possibly in Asia. It’s something that’s going to happen, it’s just going to take some time before it’s at the stage where we can safely and reliably put passengers in these things.
Project website: Scramspace