Mach > X – The flight industry in the hypersonic range

Popular Science artist impression of the mysterous warplane over southern cal

Introduction

Ever since Concordes were abandoned, there hasn’t been much talk in daily society about aircraft traveling high speed. Most of the time, the aviation industry is overshadowed by the computer industry. Really, who can ignore Apple and Microsoft shoving gadgets in one’s face. Nevertheless, the industry has been very active. Since World War 2, nations have realized the benefits of air superiority. Critically important to air superiority is the ability to go fast. If you can move faster than your opponent can shoot, you can hit him quicker or get away from him more easily. That’s the thought behind some of the recent developments in aviation.

This article will discuss supersonic airplanes and technology, including recent developments, design, and considerations. This is a technical article and thus will not discuss the political implications of such aircraft.

In the News

As reported by the BBC and LA times, the Pentagon released details of their most recent failure, ahem, project. Really cool stuff: this jet can reach speeds of mach 20 (20 times the speed of sound). The Falcon Hypersonic Technology Vehicle 2, as they call it, flew over the Pacific for a test run. 9 minutes into the flight, signal to the craft was lost… but it kept transmitting flight data after awhile. What happened to the craft? I’m sure the government is cleaning it up, but it does remind me of a few things.

Back in the Reagan era, it was said that the U.S. could replace the aging SR-71 Blackbirds with something faster – think mach 5 speed. Given the huge defense budget and the administration’s agenda (anyone think of “Star Wars“?), no doubt a tons of funds could easily be shifted to aerodynamic technologies without much notice. Except that people were bound to notice the plane, and this resulted in the rumored Aurora project. It’s kinda hard to hide very loud sonic booms over southern California.

Back in 2006, Popular Science magazine had reported several mysterious aircraft projects that may be under development at Area 51. Some of the evidence for these craft consist of gaps in the defense budget and needs in the current U.S. arsenal. One such need (or rather, want) is a quick strike-anywhere bomber, which they say is the Aurora.

The crash over the Pacific by the Falcon (it was supposed to crash, don’t worry) is reminiscent of another aircraft that nearly hit mach 10: the X-43A designed an built by NASA (why is NASA building hypersonic jets?). The plane hit mach 9.6, about 3 times what the SR-71 did and also setting the world record. Similarly, Boeing created a cruise missile, the X-51, that utilized scramjets to hit mach 6.

NASA’s X-43A hypersonic aircraft

Design Considerations

If you’ve got an engineering background like me (no, you don’t have to have a degree), you may start to think of the design issues you will be having when you make these kinds of jets. But for those who don’t, allow me to lay out a very brief list.

  • Flight control (stability during flight / balance / etc – Go ask a pilot)
  • Speed
  • Temperatures
  • Engines
  • Guidance
  • Communication

Flight control – How on earth do you control something going this fast? – You let a computer do it. There has been a steady shift towards making UAVs (unmanned aerial vehicles) for various reasons. It’s hard to control things moving this fast or flying wings for that matter. Early experimental planes such as the XB-35 flying wing and YB-49 demonstrated the inherent instability of flying wings, hence, computers had to be used to help stabilize craft like the B2 bomber. Another experimental, the Grumman X-29, was promising in some respects – it provided excellent control at supersonic speeds and didn’t lose lift, but it too had inherent instability needing computer control and the had the problem of the wings possibly ripping off in flight due to the extreme pressure. Popular science concept art

Speed – With computers keeping the plane under control, the next issue is speed. Speed can rip off your wings if not your plane’s front end. Both are serious design considerations when building hypersonic aircraft. The technology for protection for handling speeds like this has been around since the space shuttles, but it isn’t designed for engines and, as we discovered with the Space Shuttle Columbia incident, it isn’t always reliable. Another issue is how to actually attain that speed, which brings me to the next bullet point.

Engines – The quest for higher speed has usually been fulfilled through rocketry until jets came around. Even then, rockets have dominated. The Bell X-1, despite being shaped like a plane, was a rocket. However, jet engines are now approaching rocket speeds. The aforementioned Falcon test speed was about 13,000 mph. In comparison, the Saturn V hit speeds of around 15,700 mph. How do we get that fast? Developments in aviation led to ramjet and scramjet engines. Based on the way these two engines function, neither can move the airplane forward from a standstill, but once the plane is moving fast (usually from another jet), these types of engines can move the plane to incredible speeds. I recall a Popular Science article mentioning that, for the Aurora project, the government planned on making a single engine to transition to different engines, either ramjet to scramjet or another jet to ramjet to scramjet, though I believe it was the former. The former is already tough to design mechanically. Now how do these work? I’ll explain in the section (Design: Ramjets and Scramjets) or you could read this summary NASA article.

Temperature – The result of going fast is an incredible amount of air pressure and specifically air friction. Friction causes heat. Heat can melt your aircraft both externally and internally. While aircraft engineers need to consider materials for ensuring the aircraft won’t burn up, they also have to consider the devices within the aircraft (don’t worry about the pilot – there probably won’t be one). Electronics, for instance, work at certain temperature ranges (surprise!). Electrons moving through a printed circuit board produce heat and yet this increases electronic resistance in the board, which makes it harder for the electrons to move through the board. At some point, your board may melt (I’m also considering the solder on the boards). And hey, if the electronics melt, your electronic communications will stop. Never mind the internal problems in the engine you need to consider, like the fact that your propeller will burn up if you try to use one (another reason to use ramjets and scramjets).

Communication -I mention communication as the last thing because recently that seems to be an issue, despite NASA and the government having had experience with high speed ships (like the Saturn V for instance). For one thing, if you tell your ship to fly to a specific location and then realize that was a bad idea, you’re ship may have already traveled a hundred miles in those few minutes of debate. That’s not a great thing for a diplomat trying to negotiate peace or a mechanic who realizes he put a bolt on wrong before the plane takes off (I can’t see this happening given the number of safety checks and such they perform on these planes but hey, Murphy’s Law lingers with all of us). Those examples seem alittle too complex to explain. Hm… Actually, the recent lack of communication with the aforementioned Falcon is probably a better example.

Design: Ramjets and Scramjets

If you elected not to read this simple NASA article or find the more specific one, then you’re welcome to read my abridged version I give here. First, let’s present some diagrams:

ramjet design

scramjet design

In both engines (ramjets and scramjets), the air isn’t sucked into the inlet, as in other plans. Rather, it freely flows in. This is critical to the design, and it is also the reason why ramjets and scramjets cannot move the plane starting from a standstill: the engines take advantage of the air’s own natural pressure generated at high speeds (the air acts like a liquid at these speeds). The air flowing into the inlet is compressed against the sides by the cone. Here’s where the difference between the types of engines comes in: at this stage, the ramjet brings the compressed air into a chamber where it ignites it like a rocket. However, it slows it down to subsonic speeds (freaking weird when you are moving at mach 3-6, but remember that this speed is with respect to the aircraft, not the ground). The fact that the ramjet has to slow the air down the air to these speeds is an inherent inefficiency, but its design does allow it to operate at lower speeds than scramjets. Scramjets, on the other hand, keep the air at supersonic speeds (making them more efficient) and simply ignite the fuel as the air passes by. Actually, it’s more complicated than it sounds, but that’s the idea.

You might be wondering about the nozzles. Ramjets are like open-ended rockets, just that their air source happens to come from outside instead of from the interior. Being like rockets, it appears they take advantage of the De Laval nozzle, a type of nozzle that accelerates air at subsonic speeds by the way it is shaped and the way gases move out of it.

Terminology

  • Mach – a measure of speed equal to that of sound (sonic), about 343 m/sec or 768 mph.
  • Sonic – speed of sound
  • Supersonic – greater than the speed of sound
  • Hypersonic – greater than mach 10

Major References

Other links

Images

About chronologicaldot

Just a Christ-centered, train-loving, computer geek.
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