Yesterday, an Atlas V and an Antares rocket both roared off of the United States eastern coast and into space. However, in a remote area of Norway, another rocket launch didn’t go as planned. ScramSpace, a scramjet developed by researchers at the University of Queensland, disappeared over the Atlantic Ocean without sending back the hoped-for data. Scientists were left to clean up the two rocket sections that launched ScramSpace skyward and head home.
What’s the difference? How can complex orbital missions like the Atlas V and Antares be successful when a scramjet flight so easily goes awry? Part of the difference is the physics, which makes scramjets harder to test. While both rockets and scramjets are means of achieving high speeds, they differ in design. Scramjets squeeze high speed (greater than Mach 5) air into a small tube, compressing and heating it. Then a fuel is injected into the airstream. They only work above Mach 5. In contrast, rockets supply their own fuel and oxidizer (air is an example of an oxidizer, oxygen is another), mixing them to cause the explosive reaction that gets them moving. The physics of rockets is fairly well understood after years of launch. Scramjets have not had the luxury of so many tests, mainly because testing scramjets is difficult.
First of all, the testing setup for scramjets is tricky. Unlike rockets, which can be ignited at any speed, scramjets only work when they are already traveling at Mach 5 or greater. Since no research planes currently achieve these speeds, rockets are required. ScramSpace, the University of Queensland explains, uses a two-stage rocket to send the scramjet into space. Then the test is completed as the scramjet accelerates toward Earth (see graphic at top). Other projects, like the X-43 and X-51 projects in the US, launch the scramjet from a B-52 bomber, accelerate it with a rocket and then perform the experiment, according to NASA. These tests are not simple or easy to do.
Plus, once you get to Mach 5, the conditions are nasty. At high speed, aerodynamic heating causes the scramjet to experience extreme temperatures. This means that scramjets require high temperature alloys to simply maintain their structural integrity. The physics also get complicated at hypersonic (above Mach 5) speeds. Shockwaves bounce around inside the scramjet tube and must be “swallowed” correctly in order to achieve maximum thrust. In an ABC Catalyst special, a ScramSpace engineer explains that the air must also rush into the inlet at the right angle. Extensive ground tests help scientists determine these conditions, but they don’t depict how the vehicle will react in actual flight.
The test setup also leaves very little room for error. In the same ABC short video, a ScramSpace engineer explains how vital proper controls are to the experiment, demonstrating how reaction thrusters are used to properly align the craft in space. If improperly aligned, the ScramSpace vehicle would not work correctly. U.S. projects have also had difficulties controlling scramjets at high speeds. An X-51 WaveRider scramjet failed to ignite when one tiny fin (of four) became loose before the ignition, according to Zach Rosenburg. Controlling the vehicles is difficult and the conditions at Mach 5 and above leave little room for error.
Despite all these difficulties, scramjets have accomplished some amazing test flights. The third test of the X-43 still holds the world record for an air breathing engine (as opposed to a rocket engine, which carries its own “air” or oxidizer). According to NASA, it achieved a speed of Mach 9.6, and, theoretically, scramjets can go even faster, reaching Mach 14 or 15. That’s almost fast enough to reach space! The last test of the X-51 Waverider flew successfully for over 6 minutes, according to Space Ref. This was huge accomplishment for a test period that is typically 5-15 seconds.
Overcoming the difficulties of scramjet flight is difficult, not impossible. Perfecting scramjets is challenge worth taking on. Since scramjets use atmospheric air as the oxidizer, instead of carrying big oxygen tanks as rockets often do, they offer great weight savings over rockets. Yet, funding for scramjets is often limited. The ScramSpace team is heading back the University of Queensland, according to a recent report, not to begin building again, but to be disbanded. Once perfected, scramjets can become the transportation of the future, streaking through the skies at speeds above Mach 10, but they face many physical and budgetary challenges in the near future.