The HyShot Scramjet
Published on Mar 27, 2006 at 2:25 pm.
11 Comments.
Filed under aeronautics, rockets.
The Australians are doing some good work with scramjets. They recently had a successful test of the HyShot scramjet, being developed by the University of Queensland. This is in marked contrast to NASA, whose X-43A scramjet was doomed by budget considerations.
Now, for those of you who are not familiar with the terminology, let me explain. Jet engines provide thrust by hurling hot gasses out the back of the engine at substantially higher velocities than they take in gasses at the front. This means that there is more gas momentum out the back than in the front, and so conservation of momentum requires a forward push on the engine. In physics, force is defined as the rate of change of momentum, so the addition of momentum out the back of the engine must be ballanced by an addition of momentum forward on the engine itself. I derive all this in my introductory physics classes. Actually, I derive it for rockets, and then apply it to jet engines. I imagine that in an aviation course, they’d derive it for jet engines and apply it to rockets. Basically, both provide thrust using the same physics. In fact, that is one reason that the Jet Propulsion Laboratory got its name, even though it was designing rockets.
The big difference between rockets and jet engines is that jets take in air, using oxygen in that air to burn the fuel injected into the engine. Rockets, however, are entirely self contained. They carry both fuel and oxidizer (usually, but not always, oxygen itself). Now, you might suppose that it would be more efficient to scoop oxygen out of the air instead of carrying it along with you. Well, yes and no. Clearly, if you are trying to operate your engine in space, where there is no air, or at extremely high altitudes, where the air is too thin, or underwater, where the oxygen is hard to get to, then you need to have your own supply of oxidizer. However, at lower altitudes, there is plenty of oxygen, right? Yes, that is true, but there is a catch when you try to go too fast.
There are several basic categories of jet engines. The simplest is a ramjet. The ramjet operates by having a scoop on the front end. The scoop takes in air simply by moving the jet forward. The opening then rapidly narrows, compressing the air. Compressing a gas makes it hot. In this case, it is compressed enough to make it extremely hot. Fuel is then injected into the hot air stream, and if the engine is moving fast enough to compress the air enough, then it is hot enough to ignite the fuel. The fuel then expands, and heats the air even further. The hot gasses then spew out the back of the engine at much higher velocity than the air goes into the front of the engine, providing thrust. This is a very simple in that there are no moving parts! However, since forward motion provides the compression, the ramjet has no thrust at all if it is stationary or moving slowly.
Other forms of jet engines, such as you find on commercial aircraft involve turbines in the engines. Turbojets are like ramjets, except a turbine is placed in the after part of the engine. The high speed exhaust then drives the turbine blades, which in turn are connected by a shaft to compressor blades at the front of the engine. These act like impellors to such air into the engine at slow speed or stationary operation and as compressors to compress the air, much as a turbocharger would do. Turboprops are similar, except that the shaft continues onward and is connected to a propellor (usually forward) of the engine. The propellor slicing through the air actually provides more thrust than does the jet exhaust, whose main function is to drive the turbines that power the propellor. Turboprops tend to be more efficient at low speeds or altitudes. Turbofans are similar, in that a shaft extends through the engine. The back end of the shaft is driven by hot exhaust gasses passing through the turbine, and the foward part of the shaft connects to compressor blades that compress the gas going into the engine. Forward of that is a set of turbine blades that act as a fan to suck air into the engine. Actually, most of the air blown by the fan blows past the combustion portion of the engine. This has the effect of increasing thrust at low speeds and making the engine operate with much less noise as heard from the outside. Most commercial jets use turbofans. The fan, though, does not work well at speeds near or above Mach 1, so many military aircraft use turbojets. All of the above are very broad generalizations. It is actually a bit more complex than that, but after all, this isn’t an aviation blog.
At higher and higher speeds, though, the moving parts of the engine begin to offer resistance of their own, and they have difficulty keeping up with the inflow of air. You rapidly start to lose efficiency, except for the ramjet. But, even the ramjet has problems. Above about Mach 5, the air tends to be moving so fast that the fuel injected into the airstream doesn’t start to burn until it is already carried out the back of the engine. Then, all it does is make a pretty fiery trail, but it provides absolutely no thrust. This limits the speed of even ramjets. Aeronautical engineers refer to speeds above Mach 5 as hypersonic (speeds above Mach 1 are supersonic). This is the realm of something called a scramjet. A scramjet is basically a modified ramjet that can operate at these hypersonic speeds. The biggest difference between the two is where the combustion takes place. In a normal ramjet, there is a combustion chamber just aft of the air intake. The combustion chamber has a larger cross sectional area than the space around the intake at its narrowest. From the narrow portion of the intake to the combustion chamber the cross sectional area gradually opens up, effectively slowing the gasses within the engine to near the same speed as the gasses that flow through an ordinary turbojet. With a scramjet, this is not practical, so the hot gas is injected at the narrowest portion of the engine, and then burns as the cross section opens up. This has an effect of creating an elongated combustion chamber, and for all practical purposes makes the back end of the engine basically like that of a high efficiency rocket engine. Often higher energy and faster burning fuel is used in scramjets, too. Ordinary jet engines use refined kerosine as fuel. Scramjets often are designed to burn hydrogen. Such an arrangement is expected to work up to speeds of near Mach 15.Â
Scramjets have a limitation, though, in that they only work when moving forward very fast. Thus, but the NASA X-43 and the Australian HyShot are carried to speed by conventional rockets before the scramjet engine fires on what would normally be an upper stage of the rocket. So far, the X-43 has the scramjet speed record, but the Australians are likely to soon match that. Since NASA does not seem to be seriously moving forward with scramjet technology, the Australians are likely to pass us up pretty soon.
So, why do we even care about developing scramjets? As I said earlier, rockets have to carry fuel and oxydizer with them. Scramjets can take oxygen from the atmosphere. This means that less propellant is needed (in rocket terminology, propellant is the sum of the fuel and oxydizer). Less propellant means less weight. Less weight means either less fuel needed to do the job, and thus less expense, or else a greater payload capacity, also reducing expense. For suborbital flights, most of the boost phase is in the atmosphere, so scramjets could get the job done with only an assist from rockets at the highest altitudes. At lower altitudes, either rockets or other jet engines could be used on a booster to get the scramjet up to speed. Scramjets are also economical in that they are 100% reusable.Â
But is suborbital spaceflight really what we want? Yes! There are many wonderful uses for suborbital spaceflight. As I already mentioned in early blog entries, private companies are already working on tourist dollars for suborbital flights. A larger rocket assist at the end of the scramjet flight might even let a payload reach low earth orbit. Beyond tourism, there is also the courier service industry, in which packages could be send for same day delivery to anywhere in the world. Rockets could do the job, but if scramjets are more economical, then they would be preferred. So, this is very useful and profitable research being done.
-Astroprof
James M. Essig on December 30, 2006 at 10:10 pm: 1
I am curious as to what is the ultimate velocity obtainable by a scramjet using hydrogen as fuel. As the technical art of scramjets progresses, perhaps the terminal velocities of scramjets can actually exceed low earth orbital velocity. This would obviously require further advances in high temperature/high stress capable materials as well as optimizing the effeciency of the combustion process in terms of extracting energy from the fuel combustion and converting it to air craft kinetic energy.
One can imagine that at somepoint, scramjets may be able to reach suborbital altitudes with velocities greater than that for low earth orbit, but use aerodynamics to “hold the aircraft down” so that it does not go flying off into space. If such flight is not possible with the combustion of hydrogen, perhaps some other exotic form of chemically combustable fuel may do the trick. I do not know if any such fuels are in development not to mention whether or not they would be toxic, but I can imagine that some exotic, currently yet-to-be developed fuel that has twice the heating value of hydrogen might do the trick.
Anyhow, launch vehicles comprising scramjets, whether or not they can ever reach orbital velocity in scramjet mode, may allow for much cheaper access to Earth orbit. This could open up a new realm of spacebased construction projects such as larger more capable spacestations, the assembly of manned spacecraft capable of traveling about the solar system and perhaps beyond, and the fielding of zero gravity manufacturing facilities and/or laboratories for federal, public, and/or commercial ventures.
James M. Essig on February 2, 2007 at 11:07 am: 2
About a half a year ago, I read a publically available online article regarding some alledgedly “Black” space plane like propulsion program in which a combustable jet fuel based on the element Boron was developed which provided a heating value 150% greater than kerosine. Supposedly, a key issue with the use of this fuel was toxic exhaust, but that it allows for much easier hypersonic flight and near Earth orbit insertion for jet aircraft.
James M. Essig on March 9, 2007 at 6:10 pm: 3
I wonder if anyone has considered molten or liquid paraffin for jet propulsion wherein the atmospheric combution of the paraffin would be supplemented with a little supply of pure liquid oxygen so that the spacecraft could operate as sort of a jet and rocket combination simultaneously. I here that paraffin combustion is highly exothermic in the presence of pure oxygen and that paraffin based rocket fuels oxidized with pure oxygen are now being considered for rocket propulsion. One benefit of using paraffin as a oxidant is that it is environmentally safe with little or no dangerous exaust fumes.
Anybody reader have any comments, please feel free to post them. Unfortunately, I am beginning to feel that I own this blog site.
Lowcost and safe access to orbit is very important and is a next big step in our manned exploration of space.
Astroprof on March 10, 2007 at 9:18 am: 4
Hey. I think that perhaps not so many people are reading the older postings. I don’t know the specifics of the latest research on fuel technology, but one issue with scramjets is that the fuel has to burn very fast, and I’m not sure paraffin can burn fast enough. Really, you need a chemist to comment on the fuel issues. As for the first comment, if you are going to reach suborbital velocities, actually hopping out of the atmosphere should be the best thing. That way there is not so much drag.
James M. Essig on March 10, 2007 at 4:39 pm: 5
Hi Astroprof;
Thanks for the critique. Hopping in and out of the atmosphere seems like the better approach to suborbital velocities. I wonder what the latest research is, if any is ongoing, on suborbital saucer or disk shaped craft that would sort of hop along the upper atmosphere with an aerodynamic effieciency of a Freezby toy flying disk. I use to occasionally hear reports of such research in the open literature, but I have heard very little credible mantion of such recently.
Thanks;
Jim
James M. Essig on March 11, 2007 at 6:38 am: 6
Regarding solid rocket fuels, I’d like to mention the promise of paraffin being used as a rocket fuel and the work that NASA is doing to develope paraffin based solid rocket fuels. According to one NASA website, paraffin as it has been incorporated into small scale rocket combustion tests, appearently burns rapidly in the presence of pure oxygen and according to a NASA website, it is being seriously considered for use in space shuttle rocket boosters that may optionally have the ability to fly back to the launch site so that they can be easily reused. It turns out that the only combustion products of paraffin are carbon dioxide and water. Therefor, it is much safer in terms of combustion products than currently used solid rocket fuels. Also, paraffin is much cheaper, safer to store, and safer to handle than other solid rocket fuels.
Another benefit of paraffin based solid rocket fuels is, unlike other types of solid rocket fuels, a paraffin based solid rocket can be shut down and restarted thus permitting safer managment of emergency situations involving rocket launch and flight of paraffin based rockets.
Now the primary web-based source that I used to learn more about this fuel is about 4 years old, however, according to open literature on this subject, the testing of paraffin based solid rocket concepts is well underway.
Paraffin based rockets could be critical to Earth orbit access and perhaps to other nearterm manned missions to other planetary bodies within the solar system. Because paraffin is so cheap and safe, carrying it to spacecraft in low Earth orbit or in lunar orbit would be safe. I can imagine the liquid oxygen for oxidizing the paraffin could also be brought up from Earth or perhaps a lunar liquid oxygen production facility.
Low cost and safe access to Earth and lunar orbit could eventually be very instrumental to the assembly of large manned interstellar spacecraft, what ever form they might take. Granted that we are not ready to launch a manned interstellar mission to nearby stars yet, any development of precursor systems for cheap access to Earth and/or lunar orbit cannot come a moment to soon.
Anyhow, that’s all for now. I plan to be back at the site later this week.
Thanks;
Jim
Astroprof on March 11, 2007 at 9:15 am: 7
Now that you mention, I might have heard something several years ago about paraffin as rocket fuel, but then NASA was testing out all sorts of things as rocket fuel. Most anything burns well in pure oxygen! But, good rocket fuel may or may not be good scramjet fuel.
Sadly, I think that the biggest problem is that NASA has little money right now to work on researching new fuels. I think that such research is going to have to be done primarily in the private sector.
James M. Essig on March 26, 2007 at 7:07 pm: 8
I would like to mention the almost success of SpaceDev’s attempt to launch a payload into orbit from a commercially owned facility a few days ago. It appears that the rocket had a fairly normal ascent until about 4 1/2 minutes into flight at which time it developed some form of aerodynamic instability which caused the failure of the vehicle. The good news is that if they had succeeded, they would have had the first successful flight of a vehicle that can lift a payload into orbit at a cost of approximately 10 times cheaper than the next best current alternative for the payload mass range that SpaceDev anticipates for this rocket model. Good news indeed! Cheaper access to Earth orbit will hopefully enable the construction of more elaborate manned spacecraft which can travel about the solar system, and in the not too distant future, perhaps to our nearby stellar neighbors. The greater the number of aerospace companies trying to crack the problem of safe and low cost access to Earth orbit, probably the better.
James M. Essig on April 1, 2007 at 4:41 am: 9
Now that the Airbus A-380 has proven its flight worthiness, perhaps it can be used as a platform to launch scramjets specifically designed to mate with the super-jumbo-jet wherein the scramjets would be drop released at 40,000 feet or so and then take off and where they could then carry humans and/or cargo into low Earth orbit. I am sure this idea has almost certainly already been considered regarding the A-380, but since it is such a big plane and since presumably, a large number of units are going to be produced, I just thought that I would bring the idea to any reader’s mind this morning. Personally, I think this idea is rather neat.
James M. Essig on June 9, 2007 at 1:52 am: 10
It is interesting to note the ongoing development by the U.S. in a hypersonic scramjet powered cruise missle otherwise known as the X-51A. The idea is to launch the cruise missle from high high flying subsonic platform where it will be boosted to approximately Mach 4.5 at which point the scramjets will take over to accellerate the craft even further for an estimated target speed of Mach 6 to Mach 7. If such a system is sucessfully developed and fielded as a real world scramjet system, I can imagine that doing such will do a big part in lending creadence of the concept of the scramjet for other purposes as well. Upon further development of the scramjet for this and other military purposes, I can see a transference of the technology for research in low cost access to Earth orbit through the implementation of scramjet technologies for peaceful purposes. Note that the X-51 will be powered by hydrocarbon fuels.
James M. Essig on December 17, 2007 at 1:17 pm: 11
Last month’s issue of I believe Popular Science (or was it Popular Mechanics? I forget.) has an excellent article on the likely relatively near term development of scramjets capable of pushing a craft to Mach 15. The article states that such a system incorporated into an airliner would allow for non stop trips from distances equivalent to roughly half the radius of the Earth in under 2 hours. Wow! Imagine a nonstop flight between New York and Tokyo in under 2 hours while flying high enough such that the passengers would be at the edge of space.
The article states that many of the technical hurdles that kept significant progress on workable scramjets from being realized have been overcome. Such scramjets would not only have excellent military applications in terms of recallable manned fighter planes and manned strategic bombers as well as very rapid deployment systems for military personel, but would be great for the commercial airline industry and air cargo carriers such as UPS, FedEX, and the like. One can imagine same afternoon delivery of parcels between say China and the U.S. or between Japan or African Nations and the U.S.. The catering industry for expensive freshly cooked ethnic quisuine cooked by indeginous peoples in their homelands could be delivered to customers in the U.S. and served in just a few hours from any point of origin on the Globe. Thus, such a technology has the ability to permit greater cultural dialogue among nations which can go a long way toward promoting global peace initiatives.